Table of Contents
6.2 Legislation Requirement & Guidelines
6.4 Water Quality Sensitive Receivers
6.7 Potential Sources of Impact
6.12 Environmental Monitoring and Audit
This Section describes the baseline conditions and
potential impacts on marine water quality from the construction and operation
of the Project at Tseung Kwan O (TKO). Mathematical modelling was used to predict
potential impacts to water quality, and the predictions were assessed with
reference to the relevant environmental legislation, standards and tolerance
criteria.
The following relevant legislation and associated
guidance are applicable to the evaluation of water quality impacts associated
with the Project:
The Water
Pollution Control Ordinance (WPCO) is the primary legislation for the
control of water pollution and water quality in Hong Kong. Under the WPCO,
Hong Kong waters are divided into 10 Water Control Zones (WCZs). Each WCZ has a designated set of statutory
Water Quality Objectives (WQOs).
The Study Area will cover Junk Bay, Eastern Buffer,
Port Shelter, Mirs Bay and Southern WCZs. The applicable WQOs for the WCZs are
presented in Table 6.1 to
Table 6.5 and used in the following water quality impact
assessment.
Table 6.1 Water
Quality Objectives Applicable to the Junk Bay WCZ
|
Water Quality Objective
|
Applicable Area |
A |
AESTHETIC APPEARANCE |
|
a) |
Waste discharges shall cause no objectionable odours or discolouration of the water. |
Whole zone |
b) |
Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent. |
Whole zone |
c) |
Mineral oil should not be visible on the surface. Surfactants should not give rise to lasting foam. |
Whole zone |
d) |
There should be no recognisable sewage-derived debris |
Whole zone |
e) |
Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent. |
Whole zone |
f) |
Waste discharges shall not cause the water to contain substances which settle to form objectionable deposits. |
Whole zone |
B |
BACTERIA |
|
a) |
The level of Escherichia coli should not exceed 610 per 100 milligrams per litre, calculated as the geometric mean of all samples collected in one calendar year. |
Secondary Contact Recreation Subzone & Fish Culture Zones |
b) |
The level of Escherichia coli should not exceed 1000 per 100 ml, calculated as the running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days. |
Inland waters |
C |
COLOUR |
|
|
Waste discharges shall not cause the colour of water to exceed 50 Hazen units. |
Inland waters |
D |
DISSOLVED OXYGEN |
|
a) |
Waste discharges shall not cause the level of dissolved oxygen to fall below 4 mg per litre for 90% of the sampling occasions during the year; values should be calculated as water column average. In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Marine waters excepting Fish Culture Subzones |
b) |
The dissolved oxygen level should not be less than 5 milligrams per litre for 90% of the sampling occasions during the year; values should be calculated as water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Fish Culture Subzones |
c) |
Waste discharges shall not cause the level of dissolved oxygen to be less than 4 milligrams per litre. |
Inland waters of the Zone |
E |
pH |
|
a) |
The pH of the water should be within the range of 6.5 - 8.5 units. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units. |
Marine waters |
b) |
The pH of the water should be within the range of 6.0 - 9.0 units. |
Inland waters |
F |
TEMPERATURE |
|
|
Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 °C. |
Whole zone |
G |
SALINITY |
|
|
Waste discharges shall not cause the natural ambient salinity level to change by more than 10%. |
Whole zone |
H |
SUSPENDED SOLIDS |
|
a) |
Waste discharges shall neither cause the natural ambient level to be raised by 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities. |
Marine waters |
b) |
Waste discharges shall not cause the annual median of suspended solids to exceed 25 milligrams per litre. |
Inland waters |
I |
AMMONIA |
|
|
The un-ionized ammoniacal nitrogen level should not be more than 0.021 milligram per litre, calculated as the annual average (arithmetic mean), as unionized form. |
Whole zone |
J |
NUTRIENTS |
|
a) |
Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants. |
Whole zone |
b) |
Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.3 milligram per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1m below surface, mid-depth and 1m above seabed). |
Whole zone |
K |
5-DAY BIOCHEMICAL OXYGEN DEMAND |
|
|
Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 milligrams per litre. |
Inland waters |
L |
CHEMICAL OXYGEN DEMAND |
|
|
Waste discharges shall not cause the chemical
oxygen demand to exceed 30 milligrams per litre. |
Inland waters |
M |
DANGEROUS SUBSTANCES |
|
a) |
Waste discharges shall not cause the dangerous substances in marine waters to attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to interactions of toxic substances with each other. |
Whole zone |
b) |
Waste discharges of dangerous substances shall not put a risk to any beneficial uses of the aquatic environment. |
Whole zone |
Note: (a) Reference: Junk Bay Water Control Zone Statement
of Water Quality Objectives |
Table 6.2 Water Quality Objectives
Applicable to the Eastern Buffer WCZ
|
Water Quality Objective
|
Applicable Area |
A |
AESTHETIC APPEARANCE |
|
a) |
Waste discharges shall cause no objectionable odours or discolouration of the water. |
Whole zone |
b) |
Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent. |
Whole zone |
c) |
Mineral oil should not be visible on the surface. Surfactants should not give rise to lasting foam. |
Whole zone |
d) |
There should be no recognisable sewage-derived debris |
Whole zone |
e) |
Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent. |
Whole zone |
f) |
Waste discharges shall not cause the water to contain substances which settle to form objectionable deposits. |
Whole zone |
B |
BACTERIA |
|
a) |
The level of Escherichia coli should not exceed 610 per 100 milligrams per litre, calculated as the geometric mean of all samples collected in one calendar year. |
Fish Culture Zones |
b) |
The level of Escherichia coli should be less than 1 per 100 mL, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days. |
Water Gathering Ground Subzones |
c) |
The level of Escherichia coli should not
exceed 1000 per 100 mL, calculated as the geometric mean of the most recent 5
consecutive samples taken at intervals of between 7 and 21 days. |
Other inland waters |
C |
COLOUR |
|
a) |
Human activity should not cause the colour of water to exceed 30 Hazen units. |
Water Gathering Ground Subzones |
b) |
Human activity should not cause the colour of water to exceed 50 Hazen units. |
Other inland waters |
D |
DISSOLVED OXYGEN |
|
a) |
Waste discharges shall not cause the level of dissolved oxygen to fall below 4 mg per litre for 90% of the sampling occasions during the year; values should be calculated as water column average. In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Marine waters excepting Fish Culture Subzones |
b) |
The dissolved oxygen level should not be less than 5 milligrams per litre for 90% of the sampling occasions during the year; values should be calculated as water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Fish Culture Subzones |
c) |
The level of dissolved oxygen should not be less than 4 mg per litre. |
Water Gathering Ground Subzones and other inland waters |
E |
pH |
|
a) |
The pH of the water should be within the range of 6.5 - 8.5 units. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units. |
Marine waters |
b) |
Human activity should not cause the pH of the water to exceed the range of 6.5-8.5 units. |
Water Gathering Ground Subzones |
c) |
Human activity should not cause the pH of the water to exceed the range of 6.0-9.0 units. |
Other inland waters |
F |
TEMPERATURE |
|
|
Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 °C. |
Whole zone |
G |
SALINITY |
|
|
Waste discharges shall not cause the natural ambient salinity level to change by more than 10%. |
Whole zone |
H |
SUSPENDED SOLIDS |
|
a) |
Human activity should neither cause the natural ambient level to be raise by more than 30 % nor give rise to accumulation of suspended solids which may adversely affect aquatic communities. |
Marine waters |
b) |
Human activity should not cause the annual median of suspended solids to exceed 20 mg per litre. |
Water Gathering Ground Subzones |
c) |
Human activity should not cause the annual median of suspended solids to exceed 25 mg per litre. |
Other inland waters |
I |
AMMONIA |
|
|
The un-ionized ammoniacal nitrogen level should not be more than 0.021 milligram per litre, calculated as the annual average (arithmetic mean). |
Whole zone |
J |
NUTRIENTS |
|
a) |
Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants. |
Whole zone |
b) |
Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.4 milligram per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1m below surface, mid-depth and 1m above seabed). |
Whole zone |
K |
5-DAY BIOCHEMICAL OXYGEN DEMAND |
|
a) |
The 5-day biochemical oxygen demand should not exceed 3 mg per litre. |
Water Gathering Ground Subzones |
b) |
The 5-day biochemical oxygen demand should
not exceed 5 mg per litre. |
Other inland waters |
L |
CHEMICAL OXYGEN DEMAND |
|
a) |
The chemical oxygen demand should not exceed
15 mg per litre. |
Water Gathering Ground Subzones |
b) |
The chemical oxygen demand should not exceed
30 mg per litre. |
Other inland waters |
M |
DANGEROUS SUBSTANCES |
|
a) |
Waste discharges shall not cause the dangerous substances in marine waters to attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to interactions of toxic substances with each other. |
Whole zone |
b) |
Waste discharges of dangerous substances shall not put a risk to any beneficial uses of the aquatic environment. |
Whole zone |
Note: (a) Reference: Statement of Water Quality
Objectives (Eastern Buffer Water Control Zone) |
Table 6.3 Water Quality Objectives
Applicable to the Port Shelter WCZ
|
Water Quality Objective
|
Applicable Area |
|
A |
AESTHETIC APPEARANCE |
|
|
a) |
Waste discharges shall cause no objectionable odours or discolouration of the water. |
Whole zone |
|
b) |
Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent. |
Whole zone |
|
c) |
Mineral oil should not be visible on the surface. Surfactants should not give rise to lasting foam. |
Whole zone |
|
d) |
There should be no recognisable sewage-derived debris |
Whole zone |
|
e) |
Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent. |
Whole zone |
|
f) |
Waste discharges shall not cause the water to contain substances which settle to form objectionable deposits. |
Whole zone |
|
B |
BACTERIA |
|
|
a) |
The level of Escherichia coli should not exceed 610 per 100 milligrams per litre, calculated as the geometric mean of all samples collected in one calendar year. |
Secondary Contact Recreation Subzone & Fish Culture Zones |
|
b) |
The level of Escherichia coli should not exceed 180 per 100 milligrams per litre, calculated as the geometric mean of all samples collected from March to October inclusive in one calendar year. Samples should be taken at least 3 times in a calendar month at intervals of between 3 and 14 days. |
||
Bathing Beach Subzones |
|||
C |
COLOUR |
|
|
|
Human activity should not cause the colour of water to exceed 50 Hazen units. |
Inland waters |
|
D |
DISSOLVED OXYGEN |
|
|
a) |
Waste discharges shall not cause the level of dissolved oxygen to fall below 4 mg per litre for 90% of the sampling occasions during the year; values should be calculated as water column average. In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Marine waters excepting Fish Culture Subzones |
|
b) |
The dissolved oxygen level should not be less than 5 milligrams per litre for 90% of the sampling occasions during the year; values should be calculated as water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Fish Culture Subzones |
|
c) |
Waste discharges shall not cause the level of dissolved oxygen to be less than 4 milligrams per litre. |
Inland waters of the Zone |
E |
pH |
|
a) |
The pH of the water should be within the range of 6.5 - 8.5 units. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units. |
Marine waters excepting Bathing Beach Subzones |
b) |
The pH of the water should be within the range of 6.0 - 9.0 units for 95% of samples. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.5 units. |
Bathing Beach Subzones |
c) |
Waste discharges shall not cause the pH of the water to exceed the range of 6.5-8.5 units. |
Ho Chung (A) Subzone |
d) |
The pH of the water should be within the range of 6.0 - 9.0 units. |
Other inland waters |
F |
TEMPERATURE |
|
|
Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 °C. |
Whole zone |
G |
SALINITY |
|
|
Waste discharges shall not cause the natural ambient salinity level to change by more than 10%. |
Whole zone |
H |
SUSPENDED SOLIDS |
|
a) |
Waste discharges shall neither cause the natural ambient level to be raised by 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities. |
Marine waters |
b) |
Waste discharges shall not cause the annual median of suspended solids to exceed 25 milligrams per litre. |
Inland waters |
I |
AMMONIA |
|
|
The un-ionized ammoniacal nitrogen level should not be more than 0.021 milligram per litre, calculated as the annual average (arithmetic mean). |
Whole zone |
J |
NUTRIENTS |
|
a) |
Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants. |
Marine waters |
b) |
Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.1 milligram per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1m below surface, mid-depth and 1m above seabed). |
Marine waters |
K |
5-DAY BIOCHEMICAL OXYGEN DEMAND |
|
|
Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 milligrams per litre. |
Inland waters |
L |
CHEMICAL OXYGEN DEMAND |
|
|
Waste discharges shall not cause the chemical
oxygen demand to exceed 30 milligrams per litre. |
Inland waters |
M |
DANGEROUS SUBSTANCES |
|
a) |
Waste discharges shall not cause the dangerous substances in marine waters to attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to interactions of toxic substances with each other. |
Whole zone |
b) |
Waste discharges of dangerous substances shall not put a risk to any beneficial uses of the aquatic environment. |
Whole zone |
Note: (a) Reference: Port Shelter Water Control Zone
Statement of Water Quality Objectives |
Table 6.4 Water Quality Objectives
Applicable to the Mirs Bay WCZ
|
Water Quality Objective
|
Applicable Area |
A |
AESTHETIC APPEARANCE |
|
a) |
Waste discharges shall cause no objectionable odours or discolouration of the water. |
Whole zone |
b) |
Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent. |
Whole zone |
c) |
Mineral oil should not be visible on the surface. Surfactants should not give rise to lasting foam. |
Whole zone |
d) |
There should be no recognisable sewage-derived debris |
Whole zone |
e) |
Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent. |
Whole zone |
f) |
Waste discharges shall not cause the water to contain substances which settle to form objectionable deposits. |
Whole zone |
B |
BACTERIA |
|
a) |
The level of Escherichia coli should not exceed 610 per 100 milligrams per litre, calculated as the geometric mean of all samples collected in one calendar year. |
Secondary Contact Recreation Subzone & Fish Culture Zones |
b) |
The level of Escherichia coli should be zero per 100 ml, calculated as the running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days. |
Water Gathering Ground Subzones |
(c) |
The level of Escherichia coli should not exceed 1000 per 100 ml, calculated as the running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days. |
Other inland waters of the Zone |
C |
COLOUR |
|
a) |
Waste discharges shall not cause the colour of water to exceed 30 Hazen units. |
Water Gathering Ground Subzones |
b) |
Waste discharges shall not cause the colour of water to exceed 50 Hazen units. |
Other inland waters of the Zone |
D |
DISSOLVED OXYGEN |
|
a) |
Waste discharges shall not cause the level of dissolved oxygen to fall below 4 mg per litre for 90% of the sampling occasions during the year; values should be calculated as water column average. In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Marine waters excepting Fish Culture Subzones |
b) |
The dissolved oxygen level should not be less than 5 milligrams per litre for 90% of the sampling occasions during the year; values should be calculated as water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Fish Culture Subzones |
c) |
Waste discharges shall not cause the level of dissolved oxygen to be less than 4 milligrams per litre. |
Inland waters of the Zone |
E |
pH |
|
a) |
The pH of the water should be within the range of 6.5 - 8.5 units. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units. |
Marine waters |
b) |
Waste discharges shall not cause the pH of the water to exceed the range of 6.5-8.5 units. |
Water Gathering Ground Subzones |
c) |
The pH of the water should be within the range of 6.0 - 9.0 units. |
Other inland waters of the Zone |
F |
TEMPERATURE |
|
|
Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 °C. |
Whole zone |
G |
SALINITY |
|
|
Waste discharges shall not cause the natural ambient salinity level to change by more than 10%. |
Whole zone |
H |
SUSPENDED SOLIDS |
|
a) |
Waste discharges shall neither cause the natural ambient level to be raised by 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities. |
Marine waters |
b) |
Waste discharges shall not cause the annual median of suspended solids to exceed 20 milligrams per litre. |
Water Gathering Ground Subzones and Other inland waters of the Zone |
I |
AMMONIA |
|
|
The un-ionized ammoniacal nitrogen level should not be more than 0.021 milligram per litre, calculated as the annual average (arithmetic mean). |
Whole zone |
J |
NUTRIENTS |
|
a) |
Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants. |
Whole zone |
b) |
Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.3 milligram per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1m below surface, mid-depth and 1m above seabed). |
|
K |
5-DAY BIOCHEMICAL OXYGEN DEMAND |
|
a) |
Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 3 milligrams per litre. |
Water Gathering Ground Subzones |
b) |
Waste discharges shall not cause the 5-day
biochemical oxygen demand to exceed 5 milligrams per litre. |
Other inland waters of the Zone |
L |
CHEMICAL OXYGEN DEMAND |
|
a) |
Waste discharges shall not cause the chemical
oxygen demand to exceed 15 milligrams per litre. |
Water Gathering Ground Subzones |
b) |
Waste discharges shall not cause the chemical
oxygen demand to exceed 30 milligrams per litre. |
Other inland waters of the Zone |
M |
DANGEROUS SUBSTANCES |
|
a) |
Waste discharges shall not cause the dangerous substances in marine waters to attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to interactions of toxic substances with each other. |
Whole zone |
b) |
Waste discharges of dangerous substances shall not put a risk to any beneficial uses of the aquatic environment. |
Whole zone |
Note: (a) Reference: Statement of Water Quality
Objectives (Mirs Bay Water Control Zone) |
Table 6.5 Water Quality
Objectives Applicable to the Southern WCZ
|
Water
Quality Objective |
Applicable
Area |
|
A |
AESTHETIC APPEARANCE |
|
|
a) |
Waste discharges shall cause no objectionable odours or discolouration of the water. |
Whole zone |
|
b) |
Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent. |
Whole zone |
|
c) |
Mineral oil should not be visible on the surface. Surfactants should not give rise to lasting foam. |
Whole zone |
|
d) |
There should be no recognisable sewage-derived debris |
Whole zone |
|
e) |
Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent. |
Whole zone |
|
f) |
Waste discharges shall not cause the water to contain substances which settle to form objectionable deposits. |
Whole zone |
|
B |
BACTERIA |
|
|
a) |
The level of Escherichia coli should not exceed 610 per 100 milligrams per litre, calculated as the geometric mean of all samples collected in one calendar year. |
Secondary Contact Recreation Subzone & Fish Culture Zones |
|
b) |
The level of Escherichia coli should not exceed 180 per 100 milligrams per litre, calculated as the geometric mean of all samples collected from March to October inclusive in one calendar year. Samples should be taken at least 3 times in a calendar month at intervals of between 3 and 14 days. |
||
Bathing Beach Subzones |
|||
C |
DISSOLVED OXYGEN |
|
|
a) |
Waste discharges shall not cause the level of dissolved oxygen to fall below 4 mg per litre for 90% of the sampling occasions during the year; values should be calculated as water column average. In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Marine waters excepting Fish Culture Subzones |
|
b) |
The dissolved oxygen level should not be less than 5 milligrams per litre for 90% of the sampling occasions during the year; values should be calculated as water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. |
Fish Culture Subzones |
|
c) |
Waste discharges shall not cause the level of dissolved oxygen to be less than 4 milligrams per litre. |
Inland waters of the Zone |
D |
pH |
|
a) |
The pH of the water should be within the range of 6.5 - 8.5 units. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units. |
Marine waters excepting Bathing Beach Subzones; Mui Wo (A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo (F) Subzones. |
b) |
The pH of the water should be within the range of 6.0 - 9.0 units. |
Mui Wo (D) Sub-zone and other inland waters. |
c) |
The pH of the water should be within the range of 6.0 - 9.0 units for 95% of samples. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.5 units. |
Bathing Beach Subzones |
E |
TEMPERATURE |
|
|
Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 °C. |
Whole zone |
F |
SALINITY |
|
|
Waste discharges shall not cause the natural ambient salinity level to change by more than 10%. |
Whole zone |
G |
SUSPENDED SOLIDS |
|
a) |
Waste discharges shall neither cause the natural ambient level to be raised by 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities. |
Marine waters of the whole zone |
b) |
Waste discharges shall not cause the annual median of suspended solids to exceed 20 milligrams per litre. |
Mui Wo (A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo (F) Subzones. |
c) |
Waste discharges shall not cause the annual median of suspended solids to exceed 25 milligrams per litre. |
Mui Wo (D) Sub-zone and other inland waters. |
H |
AMMONIA |
|
|
The un-ionized ammoniacal nitrogen level should not be more than 0.021 milligram per litre, calculated as the annual average (arithmetic mean). |
Whole zone |
I |
NUTRIENTS |
|
a) |
Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants. |
Whole zone |
b) |
Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.1 milligram per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1m below surface, mid-depth and 1m above seabed). |
|
J |
5-DAY BIOCHEMICAL OXYGEN DEMAND |
|
|
Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 milligrams per litre. |
Inland waters of the zone |
K |
CHEMICAL OXYGEN DEMAND |
|
|
Waste discharges
shall not cause the chemical oxygen demand to exceed 30 milligrams per litre. |
Inland waters of the zone |
L |
DANGEROUS SUBSTANCES |
|
a) |
Waste discharges shall not cause the dangerous substances in marine waters to attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to interactions of toxic substances with each other. |
Whole zone |
b) |
Waste discharges of dangerous substances shall not put a risk to any beneficial uses of the aquatic environment. |
Whole zone |
Note: (b) Reference: Statement of Water Quality
Objectives (Southern Water Control Zone) |
6.2.2
Technical Memorandum for Effluents Discharged into Drainage and Sewerage
Systems, Inland and Coastal Waters (TM-ICW)
All discharges during both the construction and
operation phases of the Project are required to comply with the TM-ICW issued under Section 21 of the WPCO.
The TM-ICW
defines acceptable discharge limits to different types of receiving
waters. Under the TM-ICW, effluents discharged into the drainage and sewerage
systems, inshore and coastal waters of the WCZs are subject to pollutant
concentration standards for specified discharge volumes. These are defined by the Environmental
Protection Department (EPD) and are specified in licence conditions for any new
discharge within a WCZ.
The TM-ICW
also prohibits new effluent (1) within 100m of the boundaries of a gazetted
beach in any direction, including rivers, streams and storm water drains, (2)
within 200m of the seaward boundaries of a marine fish culture zone or a site
of special scientific interest, and within 100m of the landward boundaries (3)
in any typhoon shelter, (4) in any marina and (5) within 100m of a seawater
intake point. As shown in Figure
6.1, there is no gazetted beach, site of special scientific interest,
typhoon shelter, marina and existing seawater intake within 100m from the
proposed submarine outfall. The nearest
fish culture zone at Tung Lung Chau is more than 500 m away. The proposed seawater intake under this
project would also be over 150 m away from the proposed submarine outfall.
6.2.3
Technical Memorandum on Environmental Impact Assessment Process
(EIAO-TM)
Annexes 6 and 14 of the EIAO-TM provide general guidelines and criteria to be used in
assessing water quality impacts.
The EIAO-TM
recognizes that, in the application of the above water quality criteria, it may
not be possible to achieve the WQO at the point of discharge as there are areas
which are subjected to greater impacts (which are termed by the EPD as the
mixing zones) where the initial dilution of the discharge takes place. The definition of this area is determined on
a case-by-case basis. In general, the criteria
for acceptance of the mixing zone are that it must not impair the integrity of
the water body as a whole and must not damage the ecosystems.
6.2.4
Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN 1/94)
Apart from the above statutory requirements, the ProPECC PN 1/94, issued by EPD in 1994, also provide useful
guidelines on water pollution associated with construction activities.
6.3.1
Overview
The Study Area for water quality, which consists of
marine waters within 7 km from the boundary of the proposed TKO Desalination
Plant and the associated marine intake and outfall, covers the Junk Bay WCZ,
Eastern Buffer WCZ, Port Shelter WCZ, Mirs Bay WCZ
and Southern WCZ. The Study Area is
located at the eastern end of the Victoria Harbour and is less affected
seasonally by the Pearl River flow when compared with the Western and Central
Waters. Water depth varies widely over
the Study Area, from relatively shallow waters in Tai Miu
Wan and Junk Bay WCZ to the deeper Mirs Bay and
Southern Waters.
6.3.2
Marine Water Quality
Baseline marine water quality of the Study Area has
been determined through a review of EPD routine monthly water quality
monitoring data collected in 2009 to 2013.
Six EPD monitoring stations are identified within the Study Area and the
locations of these stations are presented in Figure 6.1.
Water quality monitoring data from stations JM3,
JM4, EM1, EM2, EM3 and MM19 were used to provide the baseline water quality
condition in the vicinity of the Project Site.
The monitoring results in 2009 to 2013 at the selected monitoring
stations are summarized in Table 6.6. According
to the EPD’s Marine Water Quality in Hong Kong in 2013, full compliance at all
monitoring stations in both the Eastern Buffer and Junk Bay WCZs was achieved
since the implementation of HATS Stage 1 in end 2011. The overall WQO compliance rate of the Mirs Bay WCZ was 100% in 2013. The water quality of the Mirs
Bay WCZ was good in 2013 with high DO and low TIN levels. Moreover, the Mirs Bay WCZ also complied with the bacteriological WQO of ≤
610 E. coli cfu
/ 100 mL (annual geometric mean) for secondary contact recreation. For Port Shelter, the 2013 the water quality
of the Port Shelter WCZ was good and the overall WQO compliance rate was 100%,
with full compliance with the WQOs for DO, E.
coli, TIN and NH3-N.
Although the TIN level in the Port Shelter WCZ complied with the WQO in
2013, long term data still showed an increasing trend during the period from
2009 to 2013. Because a similar pattern of TIN elevation was also noted at both
the inner and outer parts of the Port Shelter WCZ as well as the Mirs Bay and Eastern Buffer WCZs, it is observed that the
elevated TIN level in recent years is a regional phenomenon which could be
related to a higher background TIN level in the eastern waters of Hong Kong
brought about by an increase in shipping and port activities in the region.
6.3.3
Sediment Quality
Baseline marine sediment quality in the Study Area
has been determined through a review of EPD routine biannual sediment quality
monitoring data collected between 2009 and 2013. Locations of EPD sediment monitoring stations
within the Study Area are presented in Figure 6.1.
Sediment monitoring data from stations JS2, ES1,
ES2 and ES4 were reviewed to represent the sediment quality adjacent to the
Project Site (Table 6.7). The
levels for metals, Polycyclic Aromatic Hydrocarbons (PAHs) and Polychlorinated
Biphenyls (PCBs) were compared to the relevant sediment quality criteria
specified in ETWB TC(W) No. 34/2002 Management of Dredged/Excavated
Sediment.
The EPD routine monitoring data indicate that mild
level of sediment contamination is identified in the vicinity of the Project
Site. Exceedance of the Lower Chemical
Exceedance Level (LCEL) of copper and silver are identified at Sediment Station
JS2. Exceedance of the Upper Chemical
Exceedance Level (UCEL) for silver and exceedance of LCEL for copper, zinc and
high molecular weight polycyclic aromatic hydrocarbon are identified at
Sediment Station ES4.
Sediment sampling and testing was conducted under
this Study to identify the level of sediment contamination within the marine
construction works area. Sediment
sampling locations are shown in Figure 6.3. The testing results are compared against the
LCEL and UCEL below in Table 6.8. Sediment testing and the associated elutriate
testing results from the laboratory are enclosed in Annex 6H. As shown, no
exceedance in LCEL is identified for all sediment samples for all
parameters. Furthermore, sediment
elutriate test was conducted using sediment samples from these sampling
stations to identify the potential of dissolution of sediment-bounded
nutrients, heavy metals and trace organic pollutants due to disturbance from
marine works under this Project. The
sediment elutriate test results are shown in Table 6.9 below.
Table 6.6 Summary of EPD Routine
Water Quality Monitoring Data from Selected Stations of the Junk Bay, Eastern
Buffer and Mirs Bay Water Control Zones from 2009 to
2013
Parameter |
JM3 |
JM4 |
EM1 |
EM2 |
EM3 |
MM19 |
Temperature (°C) |
23.1 |
22.9 |
22.9 |
22.9 |
22.8 |
22.7 |
(15.0-29.1) |
(14.8-29.1) |
(15.0-29.2) |
(14.9-29.0) |
(14.9-28.9) |
(15.0-29.1) |
|
Salinity (psu) |
32.1 |
32.3 |
32.3 |
32.3 |
32.6 |
32.8 |
(27.7-33.8) |
(29.5-33.9) |
(29.7-33.9) |
(25.7-33.9) |
(30.1-33.9) |
(31.2-34.0) |
|
Dissolved Oxygen (mg/L) - Depth Averaged |
6.5 |
6.3 |
6.2 |
6.4 |
6.5 |
6.5 |
(4.1-9.6) |
(3.9-9.7) |
(3.7-9.7) |
(4.2-9.7) |
(4.1-9.7) |
(3.6-8.3) |
|
Dissolved Oxygen (mg/L) - Bottom |
6.1 |
6.0 |
6.0 |
6.0 |
6.1 |
5.9 |
(2.9-9.7) |
(2.7-9.7) |
(2.6-9.6) |
(3.1-9.7) |
(2.5-9.7) |
(2.2-8) |
|
Suspended Solids (mg/L) |
2.9 |
3.3 |
3.4 |
3.4 |
3.4 |
2.7 |
(0.8-7.9) |
(0.8-8.6) |
(0.6-7.5) |
(0.7-10.2) |
(0.6-11.2) |
(0.8-7.5) |
|
5-day Biochemical Oxygen Demand (mg/L) |
0.8 |
0.8 |
0.7 |
0.7 |
0.7 |
0.6 |
(0.2-3.6) |
(0.1-4.0) |
(0.2-2.1) |
(0.1-4.9) |
(0.1-4.9) |
(0.1-1.5) |
|
Unionised Ammonia (mg/L) |
0.002 |
0.002 |
0.003 |
0.002 |
0.002 |
0.001 |
(0.001-0.009) |
(0.001-0.008) |
(0.001-0.047) |
(0.001-0.005) |
(0.001-0.005) |
(0.001-0.005) |
|
Total Inorganic Nitrogen (mg/L) |
0.18 |
0.16 |
0.17 |
0.14 |
0.11 |
0.09 |
(0.05-0.34) |
(0.04-0.33) |
(0.04-0.45) |
(0.02-0.47) |
(0.01-0.28) |
(0.01-0.27) |
|
Orthophosphate Phosphorus (mg/L) |
0.014 |
0.013 |
0.014 |
0.012 |
0.011 |
0.009 |
(0.003-0.035) |
(0.004-0.029) |
(0.003-0.029) |
(0.004-0.027) |
(0.002-0.030) |
(0.003-0.026) |
|
Total Phosphorus (mg/L) |
0.032 |
0.031 |
0.032 |
0.030 |
0.029 |
0.027 |
(0.020-0.047) |
(0.020-0.047) |
(0.020-0.100) |
(0.020-0.047) |
(0.020-0.047) |
(0.020-0.045) |
|
Chlorophyll-a (mg/L) |
4.0 |
3.3 |
3.2 |
2.7 |
2.2 |
1.8 |
(0.2-29.7) |
(0.5-20.9) |
(0.2-24.3) |
(0.5-21.6) |
(0.4-12.8) |
(0.5-11.1) |
|
Escherichia coli (cfu/100ml) |
59 |
56 |
60 |
26 |
6 |
2 |
(4-817) |
(1-697) |
(1-1563) |
(1-1000) |
(1-474) |
(1-5) |
Notes:
(a)
Data presented are depth-averaged values calculated by taking
the means of three depths, i.e. surface (S), mid-depth (M) and bottom (B),
except as specified.
(b)
Data presented are annual arithmetic means except for E. coli, which are geometric means.
(c)
Data enclosed in brackets indicate the ranges regardless of
the depths.
(d)
Bold and underlined figures indicate non-compliance with
the WQOs: TIN = 0.1 mg/L for South WCZ; TIN = 0.4 mg/L for Western Buffer WCZ.
Table 6.7 Summary of EPD Routine
Marine Sediment Quality Monitoring Data from Selected Station of for the Junk
Bay and Eastern Buffer Water Control Zone (2009-2013)
Parameter |
ETWB TC(W) No. 34/2002 Guideline |
Junk
Bay WCZ |
Eastern
Buffer WCZ |
|||
LCEL |
UCEL |
JS2 |
ES1 |
ES2 |
ES4 |
|
Arsenic (mg/kg) |
12 |
42 |
7.8 |
5.5 |
5.5 |
5.5 |
|
|
(6.8-8.6) |
(4.4-6.2) |
(4.4-7.3) |
(3.4-7.0) |
|
Cadmium (mg/kg) |
1.5 |
4 |
0.1 |
<0.1 |
0.1 |
0.1 |
|
|
(0.1-0.2) |
(<0.1-<0.1) |
(<0.1-0.2) |
(<0.1-0.4) |
|
Chromium (mg/kg) |
80 |
160 |
43 |
24 |
22 |
30 |
|
|
(29-48) |
(18-30) |
(17-30) |
(24-39) |
|
Copper (mg/kg) |
65 |
110 |
78 |
27 |
16 |
52 |
|
|
(55-92) |
(18-36) |
(9-54) |
(32-100) |
|
Lead (mg/kg) |
75 |
110 |
48 |
28 |
24 |
35 |
|
|
(35-55) |
(21-35) |
(18-38) |
(26-61) |
|
Mercury (mg/kg) |
0.5 |
1 |
0.22 |
0.12 |
0.07 |
0.21 |
|
|
(0.17-0.29) |
(0.08-0.21) |
(<0.05-0.17) |
(0.12-0.50) |
|
Nickel (mg/kg) |
40 |
40 |
21 |
14 |
14 |
16 |
|
|
(15-23) |
(11-18) |
(12-19) |
(13-21) |
|
Silver (mg/kg) |
1 |
2 |
1.6 |
0.5 |
0.2 |
1.3 |
|
|
(0.9-2.0) |
(0.3-0.6) |
(<0.2-0.4) |
(0.7-4.2) |
|
Zinc (mg/kg) |
200 |
270 |
130 |
69 |
61 |
110 |
|
|
(97-150) |
(56-87) |
(40-120) |
(69-260) |
|
Total Polychlorinated Biphenyls
(PCBs) (mg
/ kg) |
23 |
180 |
18 |
18 |
18 |
18 |
|
|
(18-18) |
(18-18) |
(18-18) |
(18-19) |
|
Low Molecular Weight Polycylic Aromatic Hydrocarbons (PAHs) (μg/kg) |
550 |
3,160 |
120 |
120 |
96 |
110 |
|
|
(93-250) |
(90-210) |
(90-120) |
(90-140) |
|
High Molecular Weight Polycylic Aromatic Hydrocarbons (PAHs) (μg/kg) |
1,700 |
9,600 |
300 |
220 |
84 |
400 |
|
|
(130-430) |
(68-840) |
(25-300) |
(110-1800) |
|
Chemical Oxygen Demand (mg/kg) |
-- |
-- |
13700 |
10800 |
8790 |
12400 |
|
|
(11000-16000) |
(9100-14000) |
(6700-13000) |
(9300-24000) |
|
Total Kjeldahl Nitrogen
(mg/kg) |
-- |
-- |
500 |
380 |
390 |
440 |
|
|
(320-600) |
(220-460) |
(280-490) |
(290-530) |
|
Ammonia Nitrogen (mg/kg) |
-- |
-- |
4.5 |
4.6 |
4.6 |
5 |
|
|
(0.10-8.20) |
(0.30-14.00) |
(0.10-19.00) |
(0.80-12.00) |
|
Total Phosphorus (mg/kg) |
-- |
-- |
200 |
190 |
200 |
190 |
|
|
(150-230) |
(110-230) |
(170-240) |
(120-230) |
Table 6.8 Summary of Marine Sediment Quality at Geophysical Survey Stations conducted under this Study
Parameter
(Unit) |
LOR |
LCEL |
UCEL |
SD5-1 |
SD5-2 |
SD5-3 |
SD5-4 |
SD5-5 |
SD5-6 |
GS5 |
GS6 |
GS7 |
Reference Sample |
0.0-0.9m |
0.9-1.9m |
1.9-2.9m |
4.9-5.9m |
7.9-8.9m |
10.9-11.9m |
||||||||
Silver (mg/kg) |
0.1 |
1 |
2 |
<0.1 |
<0.1 |
<0.1 |
<0.1 |
<0.1 |
0.1 |
0.5 |
0.4 |
0.5 |
0.2 |
Arsenic (mg/kg) |
1 |
12 |
42 |
4 |
3 |
4 |
5 |
8 |
8 |
7 |
6 |
7 |
9 |
Cadmium (mg/kg) |
0.2 |
1.5 |
4 |
<0.2 |
<0.2 |
<0.2 |
<0.2 |
<0.2 |
<0.2 |
<0.2 |
<0.2 |
<0.2 |
0.4 |
Chromium (mg/kg) |
1 |
80 |
160 |
6 |
27 |
33 |
29 |
38 |
43 |
43 |
38 |
40 |
14 |
Copper (mg/kg) |
1 |
65 |
110 |
7 |
8 |
8 |
8 |
12 |
15 |
44 |
37 |
38 |
16 |
Nickel (mg/kg) |
1 |
40 |
40 |
4 |
20 |
26 |
21 |
28 |
32 |
25 |
23 |
24 |
10 |
Lead (mg/kg) |
1 |
75 |
110 |
6 |
19 |
16 |
17 |
24 |
26 |
37 |
33 |
36 |
14 |
Zinc (mg/kg) |
1 |
200 |
270 |
30 |
61 |
73 |
60 |
72 |
79 |
106 |
96 |
101 |
72 |
Mercury (mg/kg) |
0.05 |
0.5 |
1 |
<0.05 |
<0.05 |
<0.05 |
<0.05 |
<0.05 |
<0.05 |
0.11 |
0.13 |
0.16 |
<0.05 |
Total PCBs (µg/kg) |
18 |
123 |
180 |
<18 |
<18 |
<18 |
<18 |
<18 |
<18 |
<18 |
<18 |
<18 |
<18 |
LMW PAHs (µg/kg) |
550 |
550 |
3160 |
<550 |
<550 |
<550 |
<550 |
<550 |
<550 |
<550 |
<550 |
<550 |
<550 |
HMW PAHs (µg/kg) |
1700 |
1700 |
9600 |
<1700 |
<1700 |
<1700 |
<1700 |
<1700 |
<1700 |
<1700 |
<1700 |
<1700 |
<1700 |
TBT (µg/kg) |
0.015 |
0.15 |
0.15 |
<0.015 |
<0.015 |
<0.015 |
<0.015 |
<0.015 |
<0.015 |
<0.015 |
<0.015 |
<0.015 |
IS |
Ammonia (mg/kg) |
0.1 |
- |
- |
4 |
3.6 |
4.6 |
12.8 |
25.7 |
30.3 |
11.2 |
8.6 |
9.2 |
26.7 |
Reactive Phosphorous (mg/kg) |
0.1 |
- |
- |
0.5 |
0.2 |
0.2 |
0.2 |
0.6 |
1.3 |
1.1 |
1.1 |
1.2 |
1.3 |
Total Kjeldahl
Nitrogen (mg/kg) |
50 |
- |
- |
980 |
760 |
870 |
860 |
670 |
740 |
1330 |
1410 |
1480 |
1520 |
Total Phosphorous (mg/kg) |
10 |
- |
- |
492 |
414 |
488 |
544 |
332 |
419 |
573 |
551 |
566 |
611 |
Nitrate (mg/kg) |
1 |
- |
- |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
Nitrite (mg/kg) |
1 |
- |
- |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
<1.0 |
Notes:
(a) LOR: Limit of Reporting; LCEL: Lower Chemical Exceedance Level;
UCEL: Upper Chemical Exceedance Level
Table 6.9 Summary of Sediment Elutriate Test Results at Geophysical Survey Stations conducted under this Study
Parameter (Unit) |
WQC |
LOR |
SD5-1 |
SD5-2 |
SD5-3 |
SD5-4 |
SD5-5 |
SD5-6 |
SD5-6 |
GS5 |
GS6 |
GS7 |
GS5 |
GS6 |
GS7 |
Reference Sample |
Reference Sample |
0.0-0.9m |
0.9-1.9m |
1.9-2.9m |
4.9-5.9m |
7.9-8.9m |
10.9-11.9m |
Water Blank |
Water Blank |
Water Blank |
Water Blank |
Water Blank |
|||||||
Mercury (µg/L) |
0.3 |
1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
Arsenic (µg/L) |
25 |
10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
Cadmium (µg/L) |
2.5 |
1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
Chromium (µg/L) |
15 |
10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
<10 |
Copper (µg/L) |
5 |
1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
Lead (µg/L) |
25 |
1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
Nickel (µg/L) |
30 |
5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
<5 |
Silver (µg/L) |
1.9 |
1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
<1 |
Zinc (µg/L) |
40 |
25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
<25 |
Total PCBs (µg/L) |
0.03 |
0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
<0.18 |
Total PAHs (µg/L) |
3 |
0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
<0.16 |
TBT (µg/L) |
0.1 |
0.015 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
<0.15 |
Ammonia (mg/L) |
- |
0.01 |
0.47 |
0.54 |
0.31 |
1.82 |
3.29 |
2.93 |
0.01 |
0.62 |
0.83 |
0.75 |
<0.01 |
<0.01 |
<0.01 |
1 |
0.04 |
Reactive Phosphorous (µg/L) |
- |
10 |
130 |
100 |
40 |
110 |
110 |
60 |
<10 |
70 |
120 |
70 |
<10 |
<10 |
<10 |
70 |
<10 |
Total Kjeldahl
Nitrogen (mg/L) |
- |
0.1 |
0.9 |
0.9 |
0.7 |
1.9 |
4.2 |
3.6 |
0.3 |
0.9 |
1.2 |
1.1 |
0.3 |
0.2 |
0.3 |
1.4 |
0.3 |
Total Phosphorous (mg/L) |
- |
0.1 |
0.1 |
0.1 |
<0.1 |
0.1 |
0.1 |
<0.1 |
<0.1 |
<0.1 |
0.1 |
<0.1 |
<0.1 |
<0.1 |
<0.1 |
0.1 |
<0.1 |
Nitrate (mg/L) |
- |
0.01 |
<0.01 |
0.06 |
0.02 |
0.02 |
0.02 |
0.04 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
0.02 |
Nitrite (mg/L) |
- |
0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
<0.01 |
Notes:
(a) WQC: Water quality assessment
criteria (stipulated under Section 6.5.6);
LOR: Limit of Reporting
The water quality sensitive receivers (WSRs) have
been identified in accordance with Annex
14 of the EIAO-TM as well as
Clause 3.4.4.2 of the Environmental Impact Assessment Study Brief for
Desalination Plant at Tseung Kwan O (No.
ESB-266/2013). These WSRs are
illustrated in Figure 6.1 and listed
in Table 6.10. The
representative WSRs as discrete modelling output points are shown in Figure 6.2.
Table 6.10 Water Quality Sensitive Receivers (WSRs) in the Vicinity of the Project Site
Description |
Location |
Model Output Location |
Distance from Seawater Intake |
Distance from Submarine Outfall |
Fisheries Sensitive
Receivers |
|
|
|
|
Fish
Culture Zones |
Tung
Lung Chau |
SR2 |
1371 |
1147 |
Po Toi O |
SR27 |
1776 |
2124 |
|
Ecological Sensitive
Receivers |
|
|
|
|
Coral
Communities |
Junk Bay |
SR20 |
4161 |
4220 |
Junk Bay |
SR21 |
3659 |
3692 |
|
Junk Bay |
SR22 |
4109 |
4114 |
|
Fat Tong Chau |
SR16 |
2084 |
2097 |
|
Tai Miu Wan |
SR4 |
915 |
1233 |
|
Cape Collinson |
SR10 |
2089 |
2000 |
|
Cape Collinson |
SR11 |
2446 |
2320 |
|
Cape Collinson |
SR12 |
3036 |
2808 |
|
Tung Lung Chau |
SR1 |
2047 |
1597 |
|
Tung Lung Chau |
SR3 |
1353 |
1321 |
|
Tung Lung Chau |
SR6 |
1876 |
1973 |
|
Tung Lung Chau |
SR7 |
2396 |
2301 |
|
Tung Lung Chau |
SR8 |
2843 |
2641 |
|
Tung Lung Chau |
SR9 |
3298 |
2863 |
|
Tai Long Pai |
SR32 |
3727 |
3309 |
|
Coral Survey Locations D-W |
SR33 |
400 |
723 |
|
Coral Survey Locations D-T |
SR34 |
784 |
1134 |
|
Coral Survey Locations D-L |
SR35 |
1231 |
946 |
|
Kwun Tsai |
SR36 |
90 |
246 |
|
Tit Cham Chau |
SR37 |
256 |
75 |
|
Water Quality Sensitive
Receivers |
|
|
||
Gazetted
Bathing Beaches |
Big
Wave Bay |
SR12 |
3036 |
2808 |
|
Clear
Water Bay |
SR29 |
2506 |
2780 |
|
Clear Water Bay |
SR30 |
2738 |
3017 |
|
Shek O |
SR15 |
4979 |
4620 |
|
Rocky Bay |
SR14 |
4350 |
4007 |
Coastal
Protection Areas |
Clear Water Bay |
SR28 |
2430 |
2780 |
|
Clear Water Bay |
SR29 |
2506 |
2780 |
|
Clear Water Bay |
SR30 |
2738 |
3017 |
|
Clear Water Bay |
SR31 |
2863 |
3178 |
|
Po Toi O |
SR27 |
1776 |
2124 |
|
Tai Miu
Wan |
SR4 |
915 |
1233 |
|
Tai Miu
Wan |
SR5 |
1466 |
1749 |
|
Cape Collinson |
SR10 |
2089 |
2000 |
|
Cape Collinson |
SR11 |
2446 |
2320 |
|
Big Wave Bay |
SR13 |
3785 |
3554 |
|
Tai Tam |
SR25 |
2704 |
2983 |
|
Tai Tam |
SR26 |
2041 |
2317 |
Seawater
Intakes |
Tseung Kwan O
New Town |
SR19 |
3539 |
3617 |
|
Siu Sai Wan |
SR16 |
2084 |
2097 |
|
Siu Sai Wan |
SR17 |
3001 |
2955 |
|
Heng Fa Chuen |
SR23 |
4472 |
4456 |
|
Heng Fa Chuen |
SR24 |
3957 |
3928 |
Noted: There are seven coral sites identified and
surveyed in the coral survey conducted under this Study. Two of them (D-O and D-I) are located at Tit
Cham Chau and Kwun Tsai and would be assessed as SR36
and SR37 respectively. Two of them (D-F
and D-E) are previously known coral sites and are already represented as SR16
and SR4 respectively. The remaining three
coral sites are included as SR33-SR35.
6.5.1
Water Quality Objectives
Suspended Solids
Specific water quality criteria for SS level
allowed at fish culture zones and Water Supplies Department (WSD) Flushing
Water Intakes are stipulated below. For
all other WSRs with no specific water quality criteria, elevation in SS
concentrations resulting from the Project’s construction and operational
activities were assessed against the WQO.
The WQO for SS is defined as not to raise the natural ambient level by
30%, nor cause the accumulation of SS which may adversely affect aquatic
communities. The assessment criterion is
hence defined as the WQO allowable increase in SS concentrations within the
corresponding WCZs.
SS data from EPD’s routine water quality monitoring
programme in 2009-2013 have been analyzed to
determine the WQO allowable increase at the WSRs. This is calculated as 30% of the ambient
level (90th percentile value) from the 2009-2013 baseline marine water quality
data. For each WSR, ambient level was
derived from the closest EPD water quality monitoring station. The assessment criterion for SS at each WSR
is summarized in Table 6.11.
Dissolved Oxygen
Oxygen depletion resulting from the Project’s
construction and operational activities were assessed against the WQO. The assessment criterion is defined as the
WQO allowable depletion in DO levels at the WSRs.
DO data from EPD’s routine water quality monitoring programme from 2009-2013 have been analyzed to determine WQO allowable depletion at the WSRs. Allowable DO depletion is calculated as the ambient DO level minus the WQO, i.e. 4 mg/L for depth-averaged, surface and middle layers, and 2 mg/L for bottom layer. Ambient level is calculated as the 10th-percentile value from the 2009-2013 marine water quality data. For each WSR, ambient level was derived from the closest EPD water quality monitoring station. The assessment criterion for DO at each WSR is summarized in Table 6.12.
Table 6.11 Water Quality Assessment
Criteria for Suspended Solids (mg/L) at WSRs
Sensitive
Receivers |
Name
|
Model
Output Location |
EPD Station |
Relevant Depth for Assessment |
Dry Season |
Wet Season |
|||
Ambient Level (a) |
WQO Allowable Change |
Ambient Level (a) |
WQO Allowable Change |
||||||
Fisheries
Sensitive Receivers |
|||||||||
Fish Culture Zones |
Tung Lung Chau |
SR2 |
EM2 |
Depth-averaged |
7.27 |
42.73(AFCD) |
5.13 |
44.87(AFCD) |
|
|
Po Toi
O |
SR27 |
MM19 |
Depth-averaged |
5.37 |
44.63(AFCD) |
3.69 |
46.31(AFCD) |
|
Ecological
Sensitive Receivers |
|||||||||
Coral Communities |
Junk Bay |
SR20 |
JM3 |
Bottom |
8.70 |
2.61 |
5.80 |
1.74 |
|
|
|
SR21 |
JM4 |
Bottom |
8.28 |
2.48 |
9.24 |
2.77 |
|
|
|
SR22 |
JM4 |
Bottom |
8.28 |
2.48 |
9.24 |
2.77 |
|
|
Fat Tong Chau |
SR16 |
EM1 |
Bottom |
7.34 |
2.20 |
6.85 |
2.06 |
|
|
Tai Miu
Wan |
SR4 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
|
Cape Collinson |
SR10 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
|
|
SR11 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
|
|
SR12 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
|
Tung Lung Chau |
SR1 |
EM3 |
Bottom |
11.80 |
3.54 |
6.92 |
2.08 |
|
|
|
SR3 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
|
|
SR6 |
MM19 |
Bottom |
9.84 |
2.95 |
6.76 |
2.03 |
|
|
|
SR7 |
MM19 |
Bottom |
9.84 |
2.95 |
6.76 |
2.03 |
|
|
|
SR8 |
MM19 |
Bottom |
9.84 |
2.95 |
6.76 |
2.03 |
|
|
|
SR9 |
EM3 |
Bottom |
11.80 |
3.54 |
6.92 |
2.08 |
|
|
Tai Long Pai |
SR32 |
EM3 |
Bottom |
11.80 |
3.54 |
6.92 |
2.08 |
|
|
D-W |
SR33 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
|
D-T |
SR34 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
|
D-L |
SR35 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
|
Kwun
Tsai |
SR36 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
|
Tit Cham Chau |
SR37 |
EM2 |
Bottom |
12.60 |
3.78 |
8.20 |
2.46 |
|
Water
Quality Sensitive Receivers |
|||||||||
Gazetted Bathing Beaches |
Big Wave Bay |
SR12 |
EM2 |
Depth-averaged |
7.27 |
2.18 |
5.13 |
1.54 |
|
|
Clear Water Bay |
SR29 |
MM19 |
Depth-averaged |
5.37 |
1.61 |
3.69 |
1.11 |
|
|
|
SR30 |
MM19 |
Depth-averaged |
5.37 |
1.61 |
3.69 |
1.11 |
|
|
Shek O |
SR15 |
EM3 |
Depth-averaged |
7.17 |
2.15 |
5.54 |
1.66 |
|
|
Rocky Bay |
SR14 |
EM3 |
Depth-averaged |
7.17 |
2.15 |
5.54 |
1.66 |
|
Coastal Protection Areas |
Clear Water Bay |
SR28 |
MM19 |
Depth-averaged |
5.37 |
1.61 |
3.69 |
1.11 |
|
|
|
SR29 |
MM19 |
Depth-averaged |
5.37 |
1.61 |
3.69 |
1.11 |
|
|
|
SR30 |
MM19 |
Depth-averaged |
5.37 |
1.61 |
3.69 |
1.11 |
|
|
|
SR31 |
MM19 |
Depth-averaged |
5.37 |
1.61 |
3.69 |
1.11 |
|
|
Po Toi
O |
SR27 |
MM19 |
Depth-averaged |
5.37 |
1.61 |
3.69 |
1.11 |
|
|
Tai Miu
Wan |
SR4 |
EM2 |
Depth-averaged |
7.27 |
2.18 |
5.13 |
1.54 |
|
|
|
SR5 |
EM2 |
Depth-averaged |
7.27 |
2.18 |
5.13 |
1.54 |
|
|
Cape Collinson |
SR10 |
EM2 |
Depth-averaged |
7.27 |
2.18 |
5.13 |
1.54 |
|
|
|
SR11 |
EM2 |
Depth-averaged |
7.27 |
2.18 |
5.13 |
1.54 |
|
|
Big Wave Bay |
SR13 |
EM3 |
Depth-averaged |
7.17 |
2.15 |
5.54 |
1.66 |
|
|
Tai Tam |
SR25 |
MM19 |
Depth-averaged |
5.37 |
1.61 |
3.69 |
1.11 |
|
|
|
SR26 |
MM19 |
Depth-averaged |
5.37 |
1.61 |
3.69 |
1.11 |
|
WSD Flushing Intake |
Tseung Kwan O New Town |
SR19 |
JM3 |
Mid-depth |
4.50 |
5.50 (WSD) |
4.78 |
5.22 (WSD) |
|
|
Siu Sai Wan |
SR17 |
EM1 |
Mid-depth |
4.10 |
5.90 (WSD) |
5.24 |
4.76 (WSD) |
|
|
Heng Fa Chuen |
SR23 |
EM1 |
Mid-depth |
4.10 |
5.90 (WSD) |
5.24 |
4.76 (WSD) |
|
Cooling Water Intake |
Chai Wan Godown |
SR16 |
EM1 |
Mid-depth |
4.10 |
1.23 |
5.24 |
1.57 |
|
|
Pamela Youde
Nethersole Eastern Hospital |
SR24 |
EM1 |
Mid-depth |
4.10 |
1.23 |
5.24 |
1.57 |
|
Notes:
(a) Ambient level is calculated as 90th percentile of the EPD routine monitoring data of 2009-2013 at respective EPD station close to the WSRs;
(b) This table is applicable for those WSRs which were assessed against the WQO. Assessment of some WSRs should also refer to the specific assessment criterion of SS for this type of WSR. The allowable elevation of SS for Fish Culture Zones is 50 mg/L minus ambient level;
(c) There is a specific requirement for the seawater intake and the SS should be maintained at below 10 mg/L. Allowable SS increase is calculated as 10 mg/L minus 90th percentile of the ambient SS level.
Table 6.12 Water Quality Objectives for Dissolved Oxygen (mg/L) at WSRs
Sensitive
Receivers |
Name
|
Model
Output Location |
EPD Station |
Relevant Depth for Assessment |
Dry Season |
Wet Season |
|||
Ambient Level (a) |
WQO Allowable DO Depletion |
Ambient Level (a) |
WQO Allowable DO Depletion |
||||||
Fisheries
Sensitive Receivers |
|||||||||
Fish Culture Zones |
Tung Lung Chau |
SR2 |
EM2 |
Depth-averaged |
6.25 |
1.25 |
4.63 |
- |
|
|
Po Toi
O |
SR27 |
MM19 |
Depth-averaged |
6.40 |
1.40 |
4.68 |
- |
|
Ecological Sensitive Receivers |
|||||||||
Coral Communities |
Junk Bay |
SR20 |
JM3 |
Bottom |
6.08 |
4.08 |
3.90 |
1.90 |
|
|
|
SR21 |
JM4 |
Bottom |
6.14 |
4.14 |
3.24 |
1.24 |
|
|
|
SR22 |
JM4 |
Bottom |
6.14 |
4.14 |
3.24 |
1.24 |
|
|
Fat Tong Chau |
SR16 |
EM1 |
Bottom |
6.14 |
4.14 |
3.22 |
1.22 |
|
|
Tai Miu
Wan |
SR4 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
|
Cape Collinson |
SR10 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
|
|
SR11 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
|
|
SR12 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
|
Tung Lung Chau |
SR1 |
EM3 |
Bottom |
6.38 |
4.38 |
3.32 |
1.32 |
|
|
|
SR3 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
|
|
SR6 |
MM19 |
Bottom |
6.24 |
4.24 |
3.20 |
1.20 |
|
|
|
SR7 |
MM19 |
Bottom |
6.24 |
4.24 |
3.20 |
1.20 |
|
|
|
SR8 |
MM19 |
Bottom |
6.24 |
4.24 |
3.20 |
1.20 |
|
|
|
SR9 |
EM3 |
Bottom |
6.38 |
4.38 |
3.32 |
1.32 |
|
|
Tai Long Pai |
SR32 |
EM3 |
Bottom |
6.38 |
4.38 |
3.32 |
1.32 |
|
|
D-W |
SR33 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
|
D-T |
SR34 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
|
D-L |
SR35 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
|
Kwun
Tsai |
SR36 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
|
Tit Cham Chau |
SR37 |
EM2 |
Bottom |
6.34 |
4.34 |
3.32 |
1.32 |
|
Water Quality Sensitive Receivers |
|||||||||
Gazetted Bathing Beaches |
Big Wave Bay |
SR12 |
EM2 |
Depth-averaged |
6.25 |
2.25 |
4.63 |
0.63 |
|
|
Clear Water Bay |
SR29 |
MM19 |
Depth-averaged |
6.40 |
2.40 |
4.68 |
0.68 |
|
|
|
SR30 |
MM19 |
Depth-averaged |
6.40 |
2.40 |
4.68 |
0.68 |
|
|
Shek O |
SR15 |
EM3 |
Depth-averaged |
6.34 |
2.34 |
4.56 |
0.56 |
|
|
Rocky Bay |
SR14 |
EM3 |
Depth-averaged |
6.34 |
2.34 |
4.56 |
0.56 |
|
Coastal Protection Areas |
Clear Water Bay |
SR28 |
MM19 |
Depth-averaged |
6.40 |
2.40 |
4.68 |
0.68 |
|
|
|
SR29 |
MM19 |
Depth-averaged |
6.40 |
2.40 |
4.68 |
0.68 |
|
|
|
SR30 |
MM19 |
Depth-averaged |
6.40 |
2.40 |
4.68 |
0.68 |
|
|
|
SR31 |
MM19 |
Depth-averaged |
6.40 |
2.40 |
4.68 |
0.68 |
|
|
Po Toi
O |
SR27 |
MM19 |
Depth-averaged |
6.40 |
2.40 |
4.68 |
0.68 |
|
|
Tai Miu
Wan |
SR4 |
EM2 |
Depth-averaged |
6.25 |
2.25 |
4.63 |
0.63 |
|
|
|
SR5 |
EM2 |
Depth-averaged |
6.25 |
2.25 |
4.63 |
0.63 |
|
|
Cape Collinson |
SR10 |
EM2 |
Depth-averaged |
6.25 |
2.25 |
4.63 |
0.63 |
|
|
|
SR11 |
EM2 |
Depth-averaged |
6.25 |
2.25 |
4.63 |
0.63 |
|
|
Big Wave Bay |
SR13 |
EM3 |
Depth-averaged |
6.34 |
2.34 |
4.56 |
0.56 |
|
|
Tai Tam |
SR25 |
MM19 |
Depth-averaged |
6.40 |
2.40 |
4.68 |
0.68 |
|
|
|
SR26 |
MM19 |
Depth-averaged |
6.40 |
2.40 |
4.68 |
0.68 |
|
WSD Flushing Intake |
Tseung Kwan O New Town |
SR19 |
JM3 |
Mid-depth |
5.98 |
3.98 |
4.58 |
2.58 |
|
|
Siu Sai Wan |
SR17 |
EM1 |
Mid-depth |
6.08 |
4.08 |
3.80 |
1.80 |
|
|
Heng Fa Chuen |
SR23 |
EM1 |
Mid-depth |
6.08 |
4.08 |
3.80 |
1.80 |
|
Cooling Water Intake |
Chai Wan Godown |
SR16 |
EM1 |
Mid-depth |
6.08 |
- |
3.80 |
- |
|
|
Pamela Youde
Nethersole Eastern Hospital |
SR24 |
EM1 |
Mid-depth |
6.08 |
- |
3.80 |
- |
|
Notes:
(a) Ambient level is calculated as 10th percentile of the EPD routine monitoring data of 2009-2013 at respective EPD station close to WSRs;
(b) This table is applicable for those WSRs which were assessed against the WQO. WQO may not be applicable for the assessment all the WSRs and it should refer to the specific assessment criterion of DO for this type of WSR. The value for Fish Culture Zones is 5.0 mg/L;
(c) There is a specific requirement for the seawater intake and the DO
should be maintained at above 2 mg/L. Allowable DO
depletion is calculated as average of the ambient DO
level minus 2 mg/L.
(d) EPD water quality monitoring data indicated
that 10th percentile DO level of fish culture zone at Tung Lung Chau
and Po Toi O is below the corresponding WQO criteria
of 5 mg/L in wet season.
Criteria for Nutrients for Project
Construction
Elevation in the levels of nutrients, if any, as a
result of the Project’s construction activities were assessed in accordance
with the respective WQO and WSD criteria summarized in Table 6.13 and Table 6.14. Other
water quality criteria for the Project operation are provided below in Section 6.5.6.
Table 6.13 Other
Relevant WQO Criteria for Construction Phase
Parameters
(unit: mg/L) |
Assessment
Criteria |
|||||
|
|
|||||
Total Inorganic
Nitrogen |
|
|||||
Unionized Ammonia |
< 0.021 mg/L for all WCZs |
6.5.2
Water Quality Criteria from Water Supplies Department
The WSD has a set of standards for the quality of
abstracted seawater (Table 6.14). Water
quality of WSD’s seawater intakes have been assessed against an SS criterion of
< 10 mg/L. The maximum allowable SS
elevation and DO depletion is provided in Table 6.11 and Table 6.12 above.
Table 6.14 WSD Water
Quality Criteria for Flushing Water Intakes
Parameter |
Criterion |
Colour (HU) |
< 20 |
Turbidity (NTU) |
< 10 |
Threshold Odour No. |
< 100 |
Ammoniacal Nitrogen (mg/L) |
< 1 |
Suspended Solids (mg/L) |
< 10 |
Dissolved Oxygen (mg/L) |
> 2 |
5-day Biochemical Oxygen Demand (mg/L) |
< 10 |
Synthetic Detergents (mg/L) |
< 5 |
E. coli (cfu/100mL) |
< 20,000 |
6.5.3
Sediment Deposition Rate and Suspended Solids Criteria for Corals
Impacts to coral communities, if any, have been
assessed with regard to sediment deposition.
The assessment criterion of 100 g/m2/day, which represents an
indicative level above which sustained deposition could harm sediment sensitive
hermatypic corals, has been used in approved EIA
Reports ([1])
([2])
and was adopted here.
Coral communities have been identified along the
coastline of Tung Lung Chau, Fat Tong Chau, Tai Miu
Wan, Junk Bay and Cape Collinson. There are
no established legislative criteria for water quality at coral communities;
however, information on hard coral tolerances to SS indicates that a 20%
reduction in annual growth rate corresponds to a 30% increase in average
long-term background SS levels (2).
WQOs of SS (30% increase) at the identified coral communities are
presented in Table 6.11 and have been utilized for determining the
acceptability of impacts on corals ([3])
in this EIA.
6.5.4
Criteria for Fish Culture Zones
The Agriculture, Fisheries and Conservation
Department (AFCD) have identified a guideline for the protection of water
quality at Fish Culture Zones (FCZs) and a maximum SS value of 50 mg/L is
recommended. This criterion has been
adopted in previous approved EIA Reports ([4])
([5]) ([6]). Thus, for the purposes of assessment, the
AFCD criterion is considered to be applicable for the Tung Lung Chau and Po Toi O FCZs.
For FCZs, in accordance with the WQO, the DO
criterion is set at > 5 mg/L measured as water column average. The maximum allowable SS elevation and DO
depletion is provided in Table 6.11 and Table 6.12 above. As
shown in Table 6.12, the 10th-percentile ambient DO level
at FCZs of Tung Lung Chau and Po Toi O are both below
the proposed criteria of 5 mg/L in wet season.
6.5.5
Criteria for Dissolved Metals and Organic Compounds
There are no existing regulatory standards or
guidelines for dissolved metals and organic contaminants in the marine waters
of Hong Kong. It is thus proposed to
make reference to relevant international standards and this approach has been
adopted in previous approved EIAs, i.e., EIA for Decommissioning of Cheoy Lee Shipyard at Penny’s Bay ([7]),
EIA for Disposal of Contaminated Mud in the East Sha
Chau Marine Borrow Pit ([8]),
EIA for Wanchai Development Phase II ([9]), EIA for Liquefied Natural
Gas (LNG) Receiving Terminal and Associated Facilities ([10]),
and EIA for Hong Kong Offshore Wind Farm in Southeastern
Waters ([11]). Table 6.15 shows the assessment criteria for dissolved metals
and organic compounds for this Study.
Table 6.15 Summary of Assessment Criteria
for Dissolved Metals and Organic Compounds
Parameter |
Unit |
Assessment Criteria for
this Study |
Metals |
|
|
Cadmium (Cd) |
mg/L |
2.5 (a) (b) |
Chromium (Cr) |
mg/L |
15 (a) (b) |
Copper (Cu) |
mg/L |
5 (a) (b) |
Nickel (Ni) |
mg/L |
30 (a) (b) |
Lead (Pb) |
mg/L |
25 (a) (b) |
Zinc (Zn) |
mg/L |
40 (a) (c) |
Mercury (Hg) |
mg/L |
0.3 (b) |
Arsenic (As) |
mg/L |
25 (a) (b) |
Silver (Ag) |
mg/L |
1.9 (d) |
Total PAHs |
mg/L |
3.0 (f) |
PCBs |
|
|
Total PCBs |
mg/L |
0.03 (d) |
Organotins |
|
|
Tributyltin (TBT) |
mg/L |
0.1 (e) |
Notes:
(a) UK Environment Agency, Environmental Quality Standards (EQS) for List 1 & 2 dangerous substances, EC Dangerous Substances Directive (76/464/EEC) (http://www.ukmarinesac.org.uk/activities/water-quality/wq4_1.htm).
(b) Annual average dissolved concentration (i.e. usually involving filtration a 0.45-um membrane filter before analysis).
(c) Annual average total concentration (i.e. without filtration).
(d) U.S. Environmental Protection Agency, National Recommended Water Quality Criteria, 2009. (http://www.epa.gov/waterscience/criteria/wqctable). The Criteria Maximum Concentration (CMC) is an estimate of the highest concentration of a material in surface water (i.e. saltwater) to which an aquatic community can be exposed briefly without resulting in an unacceptable effect. CMC is used as the criterion of the respective compounds in this study.
(e) Salazar MH, Salazar SM (1996) Mussels as Bioindicators: Effects of TBT on Survival, Bioaccumulation, and Growth under Natural Conditions. In Organotin, edited by M.A. Champ and P.F. Seligman. Chapman & Hall, London.
(f) Australian and New Zealand Environment and Conservation Council (ANZECC), Australian and New Zealand Guidelines for Fresh and Marine Water Quality (1992)
There are no existing regulatory standards or
guidelines for total PCBs, total PAHs and tributyltin (TBT) in water and hence
reference has been made to the United States Environmental Protection Agency
(USEPA) water quality criteria, Australian water quality guidelines, and
international literature, respectively.
The assessment criteria for total PCBs, total PAHs and TBT are 0.03 μg/L, 3.0 μg/L and 0.1 μg/L respectively.
6.5.6
Criteria for Saline Water Discharges
During Project operation, reverse osmosis (RO)
concentrate from desalination plant would be discharged into marine water via a
submarine outfall. RO concentrate would
be high in salinity and other constituents which are originally present in
marine water (such as suspended solid).
Some chemicals would also be added to desalination system and be
discharged together with the RO concentrate during normal operation and
bio-growth control works. Notable
constituents and the corresponding proposed assessment criteria for water
quality assessment are given in Table 6.16 below.
Given
that non-metal and corrosion resistance materials, such as super-duplex
stainless steel or equivalent corrosive resistance material, would be adopted
in the proposed desalination plant, leaching of corrosion products into the RO
stream, namely copper and nickel, is not anticipated and hence further
assessment on corrosion products is not necessary.
Table 6.16 Assessment
Criteria for Constituents of Reverse Osmosis Concentrate
Parameters
(unit: mg/L) |
Assessment
Criteria |
|||||
|
|
|||||
Salinity |
Change not to exceed 10% for all WCZs |
|||||
Iron |
< 0.3 mg/L (a) |
|||||
pH |
6.5-8.5 |
|||||
Total Inorganic
Nitrogen |
|
|||||
Suspended Solids |
Not to raise the natural ambient level by 30% |
|||||
Sodium Metabisulphite |
Oxygen depletion due to sodium metabisulphite not to exceed WQO criterion (5 mg/L for
fish culture zones, 4 mg/L for other WSRs) |
|||||
Total Residual Chlorine |
0.02 mg/L (b) |
|||||
Anti-scalant |
0.362 mg/L (One thousandth of the levels
indicated by the available EC50 data (c)) |
Notes:
(a) Australian and New Zealand Environment and Conservation Council (ANZECC), Australian and New Zealand Guidelines for Fresh and Marine Water Quality (2000) – Interim indicative working level. (http://www.mincos.gov.au/publications/australian_and_new_zealand_guidelines_for_fresh_and_marine_water_quality).
(b) Tender Ref. WP 98-567 Provision of Service for Ecotoxicity Testing of Marine Antifoulant – Chlorine in Hong Kong Final Report January 2000. Submitted to Environmental Protection Department by the Centre for Coastal Pollution and Conservation, City University of Hong Kong.
(https://echa.europa.eu/documents/10162/16960216/tgdpart2_2ed_en.pdf)
6.6.1
General Methodology
The methodology employed to assess potential water
quality impacts associated with the construction and operation of the Project
is presented in the Water Quality Method Statement (Annex 6A) and has been based on the information presented in the
Project Description (Section 3). Full details of the scenarios examined in the
modelling works are provided in Annex 6A. As discussed previously, the WSRs as well as
the water quality modelling output points in the vicinity of the Project are
presented in Figure 6.1 and Figure 6.2 respectively.
Impacts due to the dispersion of fine sediment in
suspension and the subsequent sedimentation during the construction of the
proposed submarine outfall and seawater intake have been simulated using Delft3D
PART module. Mitigation measures, as
proposed in Section 6.8.1 such as the use of silt curtain, were assumed to be absent for modelling
of the unmitigated scenario.
The depletion of DO and the elevation in nutrient
levels associated with the release of SS are calculated using the modelled
maximum SS concentrations.
Dissolved Oxygen Depletion
The degree of DO depletion exerted by a sediment
plume is a function of the sediment oxygen demand of the sediment, its
concentration in the water column and the rate of oxygen replenishment. The impact of the sediment oxygen demand on
DO concentrations has been calculated based on the following equation ([12]):
DO (mg
O2/L) = DO ( g O2/m3) = SS (g DW/m3)
×
fraction of organic matter in sediment (g C/g DW) × 2.67 (g O2/gC)
The assumption behind this equation is that all the
released organic matter is eventually re-mineralized within the water column. This leads to an estimated depletion with
respect to the background DO concentrations.
This DO depletion depends on the quality of the released sediments, i.e.
on the percentage of organic matter in the sediment. The fraction of organic matter in sediment
(Chemical oxygen demand in Table 6.7) was taken as 14,000 mg/kg based on maximum data
from EPD Sediment Monitoring Stations ES1 located near the Project Site from
2009-2013. It should be noted that other
EPD Sediment Monitoring Stations are much further away from the dredging site
(>1 km) and therefore the adoption of EPD Sediment Monitoring Station ES1 is
deemed the most appropriate.
This is a conservative prediction of DO depletion
since oxygen depletion is not instantaneous and will depend on tidally averaged
suspended sediment concentrations. It is
worth noting that the above equation does not account for re-aeration which
would tend to reduce impacts of the SS on DO concentrations in the water
column. The proposed analysis, which is
on the conservative side, will not, therefore, underestimate the DO
depletion. Further, it should be noted
that, for sediment in suspension to exert any oxygen demand in the water column
will take time and, at the same time, the sediment will be transported and
mixed or dispersed with oxygenated water.
As a result, the oxygen demand and the impact on DO concentrations will
diminish as the suspended sediment concentrations decrease.
Nutrients
An assessment of nutrient release during marine
dredging for submarine outfall and seawater intake has been carried out based
on the predicted SS elevation and the testing results of EPD sediment
monitoring station. In the calculation
it is assumed that all total kjeldahl nitrogen (TKN)
concentrations in the sediments are released to the water. This is a highly conservative assumption and
will result in the overestimation of the potential impacts.
Ammonia nitrogen is the sum of ionized ammonia and
unionized ammonia (UIA). Under normal
conditions of Hong Kong waters, more than 90% of the ammonia nitrogen would be
in the ionized form. EPD marine water
quality monitoring data at EM2 from 1986 to 2013 indicated that on average 5.7%
of ammonia nitrogen exists as UIA. For
the purpose of assessment, this average value would be adopted for estimation
of UIA from disturbance of marine sediment due to marine construction. In view that the mineralization of the
organic nitrogen will also contribute to ammonia, the calculations of NH3-N
are based on maximum total TKN concentrations from the sediment sampling and
testing exercise conducted under this Study (1480 mg/kg at station GS7 under Table 6.8), which is higher than the maximum level of TKN in
the sediment at EPD station ES1 from 2009-2013.
TKN is the total of ammonia nitrogen and organic nitrogen. Note that it
is a highly conservative approach since it is assumed that 100% of organic
nitrogen will be mineralized to ammonia but this is unlikely to occur in
reality.
The maximum SS concentration at each WSR is
multiplied by the following factors to predict the maximum UIA elevations:
Max
UIA (mg/L) = Max SS (mg DW/L) × Max TKN (mg N/ kg DW) × 10-6 × 5.7%
The maximum predicted SS concentration at each WSR
is multiplied by the maximum concentration of TKN in sediment (mg/kg) at
sediment sampling station GS7 conducted under this Study to give the maximum
elevation in TIN (mg/L). In accordance
with sediment test results conducted under this Study (TKN, assume 100% of
organic nitrogen will be mineralized to ammonia), maximum TKN concentrations in
the sediment is 1480 mg/kg. While
nitrate and nitrite may also be constituent of TIN in marine water, they are
generally in negligible concentration in view of low electrochemical potential
of marine sediment. Under low
electrochemical potential (i.e. reducing environment), oxidized forms of
nitrogen (e.g. nitrate and nitrite) tend to be reduced to their reduced form (e.g.
ammonia), and thus is considered as part of TKN. The concentration these two oxidized nitrogen
species are both found to be below the detection limit of 1 mg/kg, making them
negligible when compared with the contribution by TKN. The calculations of maximum elevation in TIN
(from TKN) at WSRs are shown below:
Max
TIN (mg/L) = Max SS (mg DW/L) × Max TKN (mg N/ kg DW) × 10-6
Heavy Metals and Micro-Organic Pollutants
Sediment elutriate test was conducted under this
Study to determine the level of potential leaching of sediment-bounded
pollutants during the construction of submarine outfall and seawater
intake. The level of pollutant presence
in elutriate would be compared against the water quality assessment criteria
stipulated under Section 6.5.6. In case the level pollutants
presence in the elutriate exceeds the proposed water quality assessment
criteria, the level of pollutants would be estimated based on the maximum
predicted level of suspended solid at the most impacted WSR and the maximum
level of pollutants identified in the geophysical survey conducted under this
Study. The appropriate adopted method for the estimation of TIN from maximum
TKN level would be followed, assuming 100% release of sediment-bounded
pollutants.
The simulation of operational impacts on
hydrodynamic regimes and water quality due to the discharge of RO concentrate
and other associated chemicals from the Project have also been studied by means
of computational modelling.
Hydrodynamic Change
Delft3D-FLOW was first carried out to simulate the
hydrodynamic conditions without Project to provide ambient conditions for near
field dispersion modelling using the CORMIX suite of model. The CORMIX model predicted the typical
vertical profile of the effluent plume of the RO concentrate in the water
column. The predicted vertical profile
of the effluent plume was then used to carry out the second iteration of the
Delft3D-FLOW modelling to provide ambient conditions for second iteration of
CORMIX near field modelling. This
ensures the potential change in hydrodynamic due to the saline discharge would
be taken into account in both the far field and near field model. To ensure the robustness of the hydrodynamic
simulation, the vertical profile of effluent plume predicted by CORMIX in the
first and second iteration was compared to see if convergence has been
reached. It was observed that the
predicted vertical profile of effluent plume in the first iteration is similar
to the second iteration in both seasons under the conservative 10th
percentile flow scenario. The Delft3D
FLOW simulations (second iteration for both seasons) were then used for the
water quality modelling assessment of RO concentrate discharge.
Water Quality Impacts
Delft3D PART was used for simulation of discharge
of various constituents in RO concentrate from the Project. Three kinds of tracers, namely conservative
tracer, decayable tracer and sediment, were used for
simulation of various constituents in the RO concentrate. The use of these three kinds of tracers is
stipulated below in Table 6.17.
Table 6.17 Use of
Delft3D PART Tracers for Various RO Concentrate Constituents
Type of Tracer |
Properties |
Constituents Represented |
Conservative Tracer |
Non-decayable and non-settling |
Iron, sodium metabisulphite, TIN, Anti-scalant |
Decayable Tracer |
Decayable (T90 = 8289s) but non-settling |
Total residual
chlorine |
Sediment |
Non-decayable but settling |
Suspended solid |
In the modelling process, it is assumed that iron
(Fe), sodium metabisulphite (SMBS), TIN and anti-scalant are conservative substance and no adsorption /
absorption process occurs. This is a
rather conservative method to assess the impacts from Fe and SMBS. For SMBS, the maximum oxygen depletion due
to the presence of SMBS would be compared against the allowed level by
WQO. For total residual chlorine (TRC),
the chlorine decay value (T90 = 8289s), which were adopted in both EIAs of HATS
2A ([13])
and Express Rail Link ([14]),
would be used under this Study. This T90
factor is the most conservative value upon our review of relevant past EIA
studies.
6.6.4
Uncertainties in Assessment Methodology
Uncertainties in Sediment Transport Assessment
Uncertainties in the assessment of the impacts from
sediment plumes will be considered when drawing conclusions from the
assessment. In carrying out the
assessment, the worst case assumptions have been made in order to provide a
conservative assessment of environmental impacts. These assumptions are as follows:
·
The
calculations of loss rates of sediment to suspension are based on conservative
estimates for the methods of working;
·
The
modelled dredging period covers a whole tidal spring-neap cycle (15 days) for
both dredging at submarine outfall and seawater intake, which is much longer
than the required actual dredging period.
This ensures the worst case tidal condition within a spring-neap cycle
could be captured in the modelling exercise; and
·
The
assessment is based on peak dredging rate for submarine outfall and seawater
intake construction. In reality these
will only occur for a short period of time.
Uncertainties arising from Operations
To ensure robustness of modelling assessment,
conservative assumptions have been made to address the uncertainties from
future operation. These assumptions
include:
·
Constituents
in the RO concentrate, including salinity, SS, anti-scalant,
TIN and Fe under normal operation as well as TRC and SMBS under bio-growth
control works period, were assumed to be released at their maximum level from
the plant. This ensures any fluctuation
in effluent quality of RO concentrate within specified limit will not result in
water quality impact beyond the modelled worst case;
·
Continuous
release was assumed for modelling all constituents, including TRC and SMBS
which would only be released at specific time period when bio-growth control
works is being conducted. This ensures
any fluctuation in effluent discharge time period will not result in water
quality impact beyond the modelled worst case; and
·
Model
spin up was conducted for multiple tidal spring-neap cycles to ensure
sufficient background build-up of constituents, if any, could be captured in
the modelling.
Potential sources of impacts to water quality
arising from the Project may occur during both the construction and operation
phases. Each is discussed in turn below.
6.7.1
Construction Phases
Marine-based and land-based construction activities
of the Project have the potential to affect water quality through:
·
Changes
in water quality, including suspended sediment dispersion, sediment deposition,
DO depletion, and elevated concentrations of nutrients, heavy metals and
micro-organic pollutants, due to marine dredging at submarine outfall and
seawater intake;
·
Other
marine construction which would not involve significant sediment disturbance;
·
Vessel
discharges;
·
Land-based
site runoff from construction workforce; and
·
Water
from cleaning and sterilization of the new fresh water main.
6.7.2
Operation Phase
The potential impacts to water quality arising from
the operation of the proposed desalination have been identified as follows:
·
Changes
in the hydrodynamic regime and water quality due to the discharge of RO
concentrate from the Project; and
·
Generation
of sewage effluent.
Elevation of Suspended Solids and Sedimentation
Based on the latest design information, it is
understood that the intake structures and outfall diffusers of the proposed
desalination plant will be installed by trenchless method. As such dredging is limited to an area of the
seabed for the installation of the above-seabed portion of the outfalls and
intake structures. The estimated dredged
volumes are about 1,740 m3 (seawater intake) and 4,590 m3
(submarine outfall). Working hours are
assumed to be 12 hours per day with a maximum dredging rate of 3,500 m3/day
(i.e. 0.081 m3/s). It is
assumed that dredging for seawater intake and submarine outfall would not be
conducted concurrently and the same dredging rate applies to dredging at both
locations. Only one closed grab dredger
would be working and spillage of mud from closed grab dredgers is assumed to
take place uniformly over the water column.
Loss rates have been taken from previously accepted
EIAs in Hong Kong ([15])([16])([17])
and have been based on a review of worldwide data on loss rates from dredging
operations undertaken as part of assessing the impacts of dredging areas of Kellett Bank for mooring buoys ([18]). The assessment concluded that for 8 m3
(minimum) grab dredgers working in areas with significant amounts of debris on
the seabed (such as in the vicinity of existing mooring buoys) that the loss
rates would be 25 kg per cubic metre dredged, while the grab dredger bucket
size in areas where debris is less likely to hinder operations could be 12 or
16 m3, with a loss rate of 17 kg/ m3. It is assumed there is little debris based on
the fact the area is away from marine works and heavy marine traffic /
industry. The value of 17 kg/ m3,
for dredgers with grab size of 12 or 16 m3, will therefore be used
for this Study. The maximum sediment
loss rate is calculated from the maximum dredging rate (0.081 m3/s) and
the unit loss rate of sediment (17 kg/m3):
Loss
Rate (kg /s)
=
Dredging Rate (m3/ s) * Loss Rate (kg/ m3)
=
0.081 m3 /s * 17 kg /m3
= 1.3773 kg/ s
Therefore a continuous release rate of 1.3773 kg/s for
one dredger will be adopted in the model for release throughout the whole water
column. Given the small extent of marine
dredging area, one stationary source at the outfall discharge point is assumed
in the model to represent the grab dredger.
It should be noted that while the whole dredging would last for only 2
days at the assessed peak dredging rate (6,330 m3 ÷ 3,500 m3/day
= 1.81 day), the modelled sediment release would last for a whole 15-day
spring-neap cycle to ensure all possible worst-case tidal conditions are
included in the simulation. The sediment
plume modelling period would last for one more spring-neap cycle (total
simulation period: 30 days) to allow sufficient time for any suspended solids
(SS) release to reach the potential WSRs.
Other detail settings for the sediment plume modelling are provided in Annex 6A and would not be provided in
detail below.
Suspended Solids Dispersion
Simulation results of SS elevation arising from the
marine dredging at seawater intake and submarine outfall in both the dry and
wet seasons are presented below in Table 6.18. It should
be noted that the values presented in Table 6.18 are the
maximum levels predicted at the specific WSRs during the specified season (dry
/ wet season) with the results of both dredging at seawater intake and
submarine outfall. It should be
highlighted that the values presented in Table 6.18 is the instantaneous maximum ([19]) of SS elevation at the specified water depth for assessment at the WSRs
throughout the simulation period. The predicted
SS elevation is compared to the proposed assessment criteria stipulated under Section 6.5.1 and Table 6.11. The
predicted percentage time of compliance to assessment criteria for SS elevation
is also shown in Table 6.18 for each WSR. Contour plots of
instantaneous SS elevation at surface, middle and bottom layer of the water
column on typical working day for dredging at submarine outfall and seawater
intake are also provided in Annex 6B
for both seasons. Please refer to Annex 6B-1 to Annex 6B-34 for contour plots showing the instantaneous SS
elevation of unmitigated scenario for wet season and Annex 6B-69 to Annex 6B-102 for contour plots showing the same of
unmitigated scenario for dry season.
Maximum SS elevation throughout the whole tidal cycle of SS elevation
for dredging at seawater intake and submarine outfall are shown in Annex 6B-137, 138, 149 and 150.
Tidal average of sedimentation flux for dredging at seawater intake and
submarine outfall are shown in Annex
6B-145, 146, 157 and 158. Maximum sedimentation flux for dredging at
seawater intake and submarine outfall throughout the modelled tidal period are
shown in Annex 6B-147, 148, 159 and 160.
The results presented in Table 6.18 indicate that elevation in SS is generally
confined within a short distance from the sediment sources. Elevation in SS is limited in most WSRs and
compliance to the corresponding assessment criteria is expected. As shown in both Table 6.18 and Annex 6B-1 to 34 and 69 to 102, the sediment
plume is highly localized and is bounded within Tai Miu
Wan and southern corner of Junk Bay by the fast-moving current in the Tathong Channel.
Elevation of SS and sedimentation experienced on the other side of the
channel (i.e. near Hong Kong Island) is negligible. The SS plume tends to drift towards the Junk
Bay during flood tide and drift toward Tung Lung Chau during ebb tide.
Elevation in SS is predicted to be above the
corresponding assessment criteria for coral communities at Fat Tong Chau (SR16)
and Tai Miu Wan (SR4) for dredging at submarine
outfall and at Kwun Tsai (SR36) and Tit Cham Chau
(SR37) for dredging at seawater intake under unmitigated scenario of wet season
(Figures
6.6 & 6.7). SS elevation
predicted at SR16 and SR4 would be in compliance to the WQO criteria for over
99% of the simulation time. The
predicted maximum SS elevation at SR16 is 3.42 mg/L in wet season and 2.89 mg/L
in dry season. Exceedance in WQO SS
criteria is only predicted at SR4 in wet season, with maximum SS elevation
predicted to be 4.42 mg/L. For coral
identified at SR36 and SR37, the predicted maximum level of SS elevation as
well as the sedimentation flux would be higher than the corresponding
assessment criteria in both seasons. As
shown in Table 6.18 and Annex 6B,
marine dredging at the proposed seawater intake (Annex 6B-1 to 17 and 69 to 85) is the source which contributes to the exceedance of SS level
at the most impacted SR36 and SR37. The
sediment plume from dredging (shown in Annex
6B-18 to 34 and 86 to 103) at the proposed submarine outfall would not encroach to SR36
and SR37. For other coral communities
identified under this Study, it is predicted that the sediment deposition would
be in compliance with the proposed sedimentation criteria (100 g/m2/day)
under both dredging at submarine outfall and seawater intake in both seasons.
Table 6.18 Predicted
Maximum Elevation in Suspended Solid and Sediment Deposition at WSRs from
Marine Dredging at Seawater Intake and Submarine Outfall – Unmitigated Scenario
WSR
(ID) |
SS
Elevation (mg/L) |
Sediment
Deposition (g/m2/day) |
|||||||
Wet Season |
Dry Season |
Criteria |
Wet Season |
Dry Season |
|||||
Criteria |
Max |
Compliance
Time % |
Criteria |
Max |
Compliance
Time % |
Max |
Max |
||
Dredging at Seawater Intake |
|||||||||
Fish Culture Zone
(Depth-averaged) |
|||||||||
Tung Lung Chau (SR2) |
44.87 |
0.03 |
100.00% |
42.73 |
0.05 |
100.00% |
- |
- |
- |
Po Toi O (SR27) |
46.31 |
0.00 |
100.00% |
44.63 |
0.00 |
100.00% |
- |
- |
- |
Coral Communities (Bottom) |
|||||||||
Junk Bay (SR20) |
1.74 |
0.05 |
100.00% |
2.61 |
0.00 |
100.00% |
100 |
0.13 |
0.00 |
Junk Bay (SR21) |
2.77 |
0.49 |
100.00% |
2.48 |
0.01 |
100.00% |
100 |
3.48 |
0.87 |
Junk Bay (SR22) |
2.77 |
0.10 |
100.00% |
2.48 |
0.00 |
100.00% |
100 |
0.66 |
0.43 |
Fat Tong Chau (SR16) |
2.06 |
1.25 |
100.00% |
2.20 |
0.01 |
100.00% |
100 |
3.69 |
6.03 |
Tai Miu Wan (SR4) |
2.46 |
0.52 |
100.00% |
3.78 |
0.01 |
100.00% |
100 |
1.19 |
0.22 |
Cape Collinson (SR10) |
2.46 |
0.02 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.05 |
0.05 |
Cape Collinson (SR11) |
2.46 |
0.01 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.03 |
0.00 |
Cape Collinson (SR12) |
2.46 |
0.01 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.03 |
0.00 |
Tung Lung Chau (SR1) |
2.08 |
0.00 |
100.00% |
3.54 |
0.00 |
100.00% |
100 |
0.00 |
1.11 |
Tung Lung Chau (SR3) |
2.46 |
0.32 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.95 |
2.44 |
Tung Lung Chau (SR6) |
2.03 |
0.28 |
100.00% |
2.95 |
0.00 |
100.00% |
100 |
0.24 |
0.67 |
Tung Lung Chau (SR7) |
2.03 |
0.00 |
100.00% |
2.95 |
0.00 |
100.00% |
100 |
0.00 |
0.00 |
Tung Lung Chau (SR8) |
2.03 |
0.00 |
100.00% |
2.95 |
0.00 |
100.00% |
100 |
0.00 |
0.00 |
Tung Lung Chau (SR9) |
2.08 |
0.02 |
100.00% |
3.54 |
0.00 |
100.00% |
100 |
0.03 |
0.05 |
Tai Long Pai (SR32) |
2.08 |
0.01 |
100.00% |
3.54 |
0.00 |
100.00% |
100 |
0.03 |
0.09 |
D-W (SR33) |
2.46 |
0.00 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.00 |
0.00 |
D-T (SR34) |
2.46 |
0.00 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.00 |
0.00 |
D-L (SR35) |
2.46 |
0.12 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.47 |
0.67 |
Kwun Tsai (SR36) |
2.46 |
14.42 |
90.98% |
3.78 |
13.62 |
91.96% |
100 |
168.30 |
115.10 |
Tit Cham Chau (SR37) |
2.46 |
58.74 |
88.63% |
3.78 |
86.74 |
92.93% |
100 |
382.12 |
309.78 |
Gazetted
Bathing Beaches (Depth-averaged) |
|||||||||
Big Wave Bay (SR12) |
1.54 |
0.00 |
100.00% |
2.18 |
0.00 |
100.00% |
- |
- |
- |
Clear Water Bay (SR29) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Clear Water Bay (SR30) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Shek O (SR15) |
1.66 |
0.00 |
100.00% |
2.15 |
0.00 |
100.00% |
- |
- |
- |
Rocky Bay (SR14) |
1.66 |
0.00 |
100.00% |
2.15 |
0.00 |
100.00% |
- |
- |
- |
Coastal Protection Areas
(Depth-averaged) |
|||||||||
Clear Water Bay (SR28) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Clear Water Bay (SR29) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Clear Water Bay (SR30) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Clear Water Bay (SR31) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Po Toi O (SR27) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Tai Miu Wan (SR4) |
1.54 |
0.10 |
100.00% |
2.18 |
0.02 |
100.00% |
- |
- |
- |
Tai Miu Wan (SR5) |
1.54 |
0.00 |
100.00% |
2.18 |
0.00 |
100.00% |
- |
- |
- |
Cape Collinson (SR10) |
1.54 |
0.00 |
100.00% |
2.18 |
0.01 |
100.00% |
- |
- |
- |
Cape Collinson (SR11) |
1.54 |
0.00 |
100.00% |
2.18 |
0.00 |
100.00% |
- |
- |
- |
Big Wave Bay (SR13) |
1.66 |
0.00 |
100.00% |
2.15 |
0.00 |
100.00% |
- |
- |
- |
Tai Tam (SR25) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Tai Tam (SR26) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Seawater Intakes (Mid-depth) |
|||||||||
Tseung Kwan O New Town (SR19) |
5.22 |
0.00 |
100.00% |
5.50 |
0.00 |
100.00% |
- |
- |
- |
Siu Sai Wan (SR17) |
4.76 |
0.02 |
100.00% |
5.90 |
0.01 |
100.00% |
- |
- |
- |
Heng Fa Chuen (SR23) |
4.76 |
0.00 |
100.00% |
5.90 |
0.00 |
100.00% |
- |
- |
- |
Chai Wan Godown
(SR16) |
1.57 |
0.03 |
100.00% |
1.23 |
0.01 |
100.00% |
- |
- |
- |
Pamela Youde
Nethersole Eastern Hospital (SR24) |
1.57 |
0.01 |
100.00% |
1.23 |
0.02 |
100.00% |
- |
- |
- |
Dredging at Submarine Outfall |
|||||||||
Fish Culture Zone (Depth-averaged) |
|||||||||
Tung Lung Chau (SR2) |
44.87 |
0.10 |
100.00% |
42.73 |
0.45 |
100.00% |
- |
- |
- |
Po Toi O (SR27) |
46.31 |
0.00 |
100.00% |
44.63 |
0.00 |
100.00% |
- |
- |
- |
Coral Communities (Bottom) |
|||||||||
Junk Bay (SR20) |
1.74 |
0.22 |
100.00% |
2.61 |
0.00 |
100.00% |
100 |
0.53 |
0.00 |
Junk Bay (SR21) |
2.77 |
1.89 |
100.00% |
2.48 |
0.43 |
100.00% |
100 |
9.39 |
6.50 |
Junk Bay (SR22) |
2.77 |
0.83 |
100.00% |
2.48 |
0.09 |
100.00% |
100 |
4.69 |
1.62 |
Fat Tong Chau (SR16) |
2.06 |
3.42 |
99.17% |
2.20 |
2.89 |
99.45% |
100 |
30.84 |
18.62 |
Tai Miu Wan (SR4) |
2.46 |
4.42 |
99.72% |
3.78 |
0.12 |
100.00% |
100 |
15.17 |
0.67 |
Cape Collinson (SR10) |
2.46 |
0.03 |
100.00% |
3.78 |
0.01 |
100.00% |
100 |
0.08 |
0.10 |
Cape Collinson (SR11) |
2.46 |
0.01 |
100.00% |
3.78 |
0.01 |
100.00% |
100 |
0.03 |
0.09 |
Cape Collinson (SR12) |
2.46 |
0.01 |
100.00% |
3.78 |
0.02 |
100.00% |
100 |
0.03 |
0.08 |
Tung Lung Chau (SR1) |
2.08 |
0.28 |
100.00% |
3.54 |
0.86 |
100.00% |
100 |
0.71 |
6.67 |
Tung Lung Chau (SR3) |
2.46 |
0.43 |
100.00% |
3.78 |
0.90 |
100.00% |
100 |
1.90 |
14.89 |
Tung Lung Chau (SR6) |
2.03 |
1.43 |
100.00% |
2.95 |
0.14 |
100.00% |
100 |
3.79 |
1.33 |
Tung Lung Chau (SR7) |
2.03 |
0.00 |
100.00% |
2.95 |
0.05 |
100.00% |
100 |
0.00 |
0.22 |
Tung Lung Chau (SR8) |
2.03 |
0.15 |
100.00% |
2.95 |
0.00 |
100.00% |
100 |
0.24 |
0.00 |
Tung Lung Chau (SR9) |
2.08 |
0.12 |
100.00% |
3.54 |
0.12 |
100.00% |
100 |
0.23 |
0.30 |
Tai Long Pai (SR32) |
2.08 |
0.04 |
100.00% |
3.54 |
0.25 |
100.00% |
100 |
2.63 |
0.15 |
D-W (SR33) |
2.46 |
0.00 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.00 |
0.00 |
D-T (SR34) |
2.46 |
0.00 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.00 |
0.00 |
D-L (SR35) |
2.46 |
0.38 |
100.00% |
3.78 |
0.88 |
100.00% |
100 |
0.71 |
2.44 |
Kwun Tsai (SR36) |
2.46 |
0.58 |
100.00% |
3.78 |
0.24 |
100.00% |
100 |
9.96 |
2.67 |
Tit Cham Chau (SR37) |
2.46 |
0.08 |
100.00% |
3.78 |
0.00 |
100.00% |
100 |
0.00 |
0.00 |
Gazetted
Bathing Beaches (Depth-averaged) |
|||||||||
Big Wave Bay (SR12) |
1.54 |
0.00 |
100.00% |
2.18 |
0.01 |
100.00% |
- |
- |
- |
Clear Water Bay (SR29) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Clear Water Bay (SR30) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Shek O (SR15) |
1.66 |
0.00 |
100.00% |
2.15 |
0.00 |
100.00% |
- |
- |
- |
Rocky Bay (SR14) |
1.66 |
0.00 |
100.00% |
2.15 |
0.00 |
100.00% |
- |
- |
- |
Coastal Protection Areas
(Depth-averaged) |
|||||||||
Clear Water Bay (SR28) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Clear Water Bay (SR29) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Clear Water Bay (SR30) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Clear Water Bay (SR31) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Po Toi O (SR27) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Tai Miu Wan (SR4) |
1.54 |
0.93 |
100.00% |
2.18 |
0.05 |
100.00% |
- |
- |
- |
Tai Miu Wan (SR5) |
1.54 |
0.00 |
100.00% |
2.18 |
0.04 |
100.00% |
- |
- |
- |
Cape Collinson (SR10) |
1.54 |
0.01 |
100.00% |
2.18 |
0.01 |
100.00% |
- |
- |
- |
Cape Collinson (SR11) |
1.54 |
0.00 |
100.00% |
2.18 |
0.01 |
100.00% |
- |
- |
- |
Big Wave Bay (SR13) |
1.66 |
0.00 |
100.00% |
2.15 |
0.00 |
100.00% |
- |
- |
- |
Tai Tam (SR25) |
1.11 |
0.00 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Tai Tam (SR26) |
1.11 |
0.01 |
100.00% |
1.61 |
0.00 |
100.00% |
- |
- |
- |
Seawater Intakes (Mid-depth) |
|||||||||
Tseung Kwan O New Town (SR19) |
5.22 |
0.00 |
100.00% |
5.50 |
0.00 |
100.00% |
- |
- |
- |
Siu Sai Wan (SR17) |
4.76 |
0.04 |
100.00% |
5.90 |
0.02 |
100.00% |
- |
- |
- |
Heng Fa Chuen (SR23) |
4.76 |
0.23 |
100.00% |
5.90 |
0.05 |
100.00% |
- |
- |
- |
Chai Wan Godown
(SR16) |
1.57 |
0.02 |
100.00% |
1.23 |
0.01 |
100.00% |
- |
- |
- |
Pamela Youde
Nethersole Eastern Hospital (SR24) |
1.57 |
0.02 |
100.00% |
1.23 |
0.02 |
100.00% |
- |
- |
- |
Note: |
|||||||||
Bold and
underlined figures indicate exceedance of WQO. |
To minimize potential water quality impacts from
elevation of SS due to the proposed dredging works at submarine outfall and
seawater intake, mitigation measure in form of silt curtain around grab dredger
would be adopted for the dredging at both the proposed submarine outfall
and seawater intake. As discussed in the previous section, the
contribution of SS elevation from dredging at seawater intake would be much
higher than that of dredging at submarine outfall for the nearby SR36 and SR37 Furthermore, the rate of dredging at the
seawater intake would also be reduced from 3,500 m3/day (in the
unmitigated scenario) to 750 m3/day.
Silt curtains would be highly effective in areas where current speeds
are low, as the effectiveness of the silt curtains will be reduced in areas of
high current speeds. Hydrodynamic
simulation by Delft3D indicated that tidal current near the proposed submarine
outfall and seawater intake would be in general below 0.2 m/s, which is deemed
appropriate for the application of silt curtain. According to the Contaminated Spoil
Management Study ([20]),
the implementation of silt curtain around the closed grab dredgers will reduce
the dispersion of SS by a factor of 4 (or about 75%). This SS reduction factors have been adopted
in a number of past studies involved in release of marine sediment from marine
works, including the approved SCL Hung Hom to
Admiralty Section, IWMF, WDII & CWB EIA, CT Dredging EIA as well as the
Western Coast Road EIA study. The
combined use of different types of silt curtain can further reduce SS
dispersion. The combined use of a
floating type silt curtain (described above with 75% sediment dispersion
reduction) and a cage type silt curtain is known to reduce sediment dispersion
by 95% according to the approved EIAs of Tuen Mun – Chek Lap Kok Link, Hong
Kong – Zhuhai – Macau Bridge Boundary Crossing Facilities and Hong Kong Link
Road. Under the mitigated scenario of
this Project, a single layer of floating type silt curtain would be used for
dredging at marine outfall and the combined used of one floating type and one
cage type silt curtain would be adopted at the seawater intake to ensure
protection of nearby WSRs. The layout
for the proposed use of silt curtain is indicatively shown in Figures
6.4 and 6.5. The combined use of two types of silt curtain
and reduction of dredging rate would mainly reduce the level of SS elevation at
SR16, SR36 and SR37 since the maximum SS elevations at these receivers are all
contributed by the dredging at seawater intake. For SR4, the effect of
mitigation measure would be less significant, as only single layer of silt
curtain would be used for dredging at submarine outfall. The predicted SS elevations with the
implementation of silt curtain are presented in Table 6.19. The
corresponding instantaneous SS elevation at surface, middle and bottom layer of
the water column on typical working day for dredging at submarine outfall and
seawater intake for mitigated scenario are also provided in Annex 6B for both seasons. Please refer to Annex 6B-35 to Annex 6B-68
for contour plots of mitigated scenario for wet season and Annex 6B-103 to Annex 6B-136
for contour plots of mitigated scenario for dry season. Contour plots for maximum instantaneous SS
elevation under mitigated scenarios for dredging at seawater intake and
submarine outfall are shown in Annex
6B-139, 140 for wet season and Annex 6B-151, 152 for dry season.
Table 6.19 Predicted
Maximum Elevation in Suspended Solid and Sediment Deposition at Coral
Communities (SR4, SR16, SR36 and SR37) from Marine Dredging at Seawater Intake
and Submarine Outfall – Mitigated Scenario
WSRs |
Criteria |
Maximum
SS Elevation (mg/L) |
Compliance
Time % |
Maximum
Sediment Deposition (g/m2/day) |
Compliance
Time % |
|
Dredging at Seawater Intake (Dredging at 750
m3day, with combined use of floating type and cage type silt
curtain) |
|
|||||
Wet Season |
|
|
|
|
|
|
SR4 (B) |
2.46 |
0.01 |
100.00% |
0.01 |
100.00% |
|
SR16 (B) |
2.06 |
0.01 |
100.00% |
0.04 |
100.00% |
|
SR36 (B) |
2.46 |
0.15 |
100.00% |
1.80 |
100.00% |
|
SR37 (B) |
2.46 |
0.63 |
100.00% |
4.09 |
100.00% |
|
Dry Season |
|
|
|
|
|
|
SR4 (B) |
3.78 |
0.00 |
100.00% |
0.00 |
100.00% |
|
SR16 (B) |
2.20 |
0.00 |
100.00% |
0.06 |
100.00% |
|
SR36 (B) |
3.78 |
0.15 |
100.00% |
1.23 |
100.00% |
|
SR37 (B) |
3.78 |
0.93 |
100.00% |
3.32 |
100.00% |
|
Dredging at Submarine Outfall (Dredging at
3,500 m3day, with floating type silt curtain) |
|
|||||
Wet Season |
|
|
|
|
|
|
SR4 (B) |
2.46 |
1.11 |
100.00% |
3.79 |
100.00% |
|
SR16 (B) |
2.06 |
0.85 |
100.00% |
7.71 |
100.00% |
|
SR36 (B) |
2.46 |
0.15 |
100.00% |
2.49 |
100.00% |
|
SR37 (B) |
2.46 |
0.02 |
100.00% |
0.00 |
100.00% |
|
Dry Season |
|
|
|
|
|
|
SR4 (B) |
3.78 |
0.03 |
100.00% |
0.17 |
100.00% |
|
SR16 (B) |
2.20 |
0.72 |
100.00% |
4.65 |
100.00% |
|
SR36 (B) |
3.78 |
0.06 |
100.00% |
0.67 |
100.00% |
|
SR37 (B) |
3.78 |
0.00 |
100.00% |
0.00 |
100.00% |
|
Note: |
||||||
(B): Bottom SS
level, which is the relevant water depth for assessment |
As shown in Table 6.19, full compliance to assessment criteria is
predicted at SR4, SR16, SR36 and SR37 with the implementation of silt curtain
around grab dredger (Figures 6.6 & 6.7). It is also shown in Annex 6B that the extent of the SS plume would be reduced
significantly with the implementation of silt curtain(s) around the grab
dredger and reduction of dredging rate for dredging at the proposed seawater
intake. No adverse water quality impact
from SS elevation due to marine dredging at submarine outfall and seawater
intake is anticipated.
DO Depletion
Dispersion of SS may release sediment-bounded
pollutant into the water column.
Readily-biodegradable organic compounds could be taken up by
micro-organism and result in depletion of dissolved oxygen (DO). An assessment of DO depletion during dredging
is made in relation to the results of the sediment plume modelling of dredging
activities and the sediment quality data near the Study Area. The predicted maximum elevations in SS
concentrations at various WSRs under unmitigated scenario (given in Table 6.18 above) were used to estimate the effects of
increased SS concentrations on DO based on the methodology stipulated under Section 6.6.2 above. The allowed level of DO
depletion is provided in Table 6.12 above. The
predicted maximum DO depletion from marine dredging under this Project is
presented in Table 6.20 below.
Table 6.20 Predicted
Maximum Depletion in Dissolved Oxygen at WSRs from Marine Dredging at Seawater
Intake and Submarine Outfall – Unmitigated Scenario
WSR
(ID) |
Wet
Season |
Dry
Season |
||||
Max SS Elevation (mg/L) |
Max DO Depletion (mg/L) |
Allowed DO Depletion (mg/L) |
Max SS Elevation (mg/L) |
Max DO Depletion (mg/L) |
Allowed DO Depletion (mg/L) |
|
Fish Culture Zone
(Depth-averaged) |
||||||
Tung Lung Chau (SR2) |
0.10 |
0.0013 |
- |
0.45 |
0.0064 |
1.25 |
Po Toi O (SR27) (a) |
0.00 |
0.0000 |
- |
0.00 |
0.0000 |
1.40 |
Coral Communities (Bottom) |
||||||
Junk Bay (SR20) |
0.22 |
0.0030 |
1.90 |
0.00 |
0.0000 |
4.08 |
Junk Bay (SR21) |
1.89 |
0.0265 |
1.24 |
0.43 |
0.0060 |
4.14 |
Junk Bay (SR22) |
0.83 |
0.0116 |
1.24 |
0.09 |
0.0013 |
4.14 |
Fat Tong Chau (SR16) |
3.42 |
0.0478 |
1.22 |
2.89 |
0.0405 |
4.14 |
Tai Miu Wan (SR4) |
4.42 |
0.0619 |
1.32 |
0.12 |
0.0017 |
4.34 |
Cape Collinson (SR10) |
0.03 |
0.0004 |
1.32 |
0.01 |
0.0002 |
4.34 |
Cape Collinson (SR11) |
0.01 |
0.0002 |
1.32 |
0.01 |
0.0002 |
4.34 |
Cape Collinson (SR12) |
0.01 |
0.0001 |
1.32 |
0.02 |
0.0003 |
4.34 |
Tung Lung Chau (SR1) |
0.28 |
0.0039 |
1.32 |
0.86 |
0.0121 |
4.38 |
Tung Lung Chau (SR3) |
0.43 |
0.0061 |
1.32 |
0.90 |
0.0125 |
4.34 |
Tung Lung Chau (SR6) |
1.43 |
0.0200 |
1.20 |
0.14 |
0.0019 |
4.24 |
Tung Lung Chau (SR7) |
0.00 |
0.0000 |
1.20 |
0.05 |
0.0007 |
4.24 |
Tung Lung Chau (SR8) |
0.15 |
0.0021 |
1.20 |
0.00 |
0.0000 |
4.24 |
Tung Lung Chau (SR9) |
0.12 |
0.0017 |
1.32 |
0.12 |
0.0017 |
4.38 |
Tai Long Pai (SR32) |
0.04 |
0.0006 |
1.32 |
0.25 |
0.0035 |
4.38 |
D-W (SR33) |
0.00 |
0.0000 |
1.32 |
0.00 |
0.0000 |
4.34 |
D-T (SR34) |
0.00 |
0.0000 |
1.32 |
0.00 |
0.0000 |
4.34 |
D-L (SR35) |
0.38 |
0.0054 |
1.32 |
0.88 |
0.0123 |
4.34 |
Kwun Tsai (SR36) |
14.42 |
0.2019 |
1.32 |
13.62 |
0.1907 |
4.34 |
Tit Cham Chau (SR37) |
58.74 |
0.8223 |
1.32 |
86.74 |
1.2144 |
4.34 |
Gazetted
Bathing Beaches (Depth-averaged) |
||||||
Big Wave Bay (SR12) |
0.00 |
0.0000 |
0.63 |
0.01 |
0.0001 |
2.25 |
Clear Water Bay (SR29) |
0.00 |
0.0000 |
0.68 |
0.00 |
0.0000 |
2.40 |
Clear Water Bay (SR30) |
0.00 |
0.0000 |
0.68 |
0.00 |
0.0000 |
2.40 |
Shek O (SR15) |
0.00 |
0.0000 |
0.56 |
0.00 |
0.0000 |
2.34 |
Rocky Bay (SR14) |
0.00 |
0.0000 |
0.56 |
0.00 |
0.0000 |
2.34 |
Coastal Protection Areas
(Depth-averaged) |
||||||
Clear Water Bay (SR28) |
0.00 |
0.0000 |
0.68 |
0.00 |
0.0000 |
2.40 |
Clear Water Bay (SR29) |
0.00 |
0.0000 |
0.68 |
0.00 |
0.0000 |
2.40 |
Clear Water Bay (SR30) |
0.00 |
0.0000 |
0.68 |
0.00 |
0.0000 |
2.40 |
Clear Water Bay (SR31) |
0.00 |
0.0000 |
0.68 |
0.00 |
0.0000 |
2.40 |
Po Toi O (SR27) (a) |
0.00 |
0.0000 |
0.68 |
0.00 |
0.0000 |
2.40 |
Tai Miu Wan (SR4) |
0.93 |
0.0130 |
0.63 |
0.05 |
0.0007 |
2.25 |
Tai Miu Wan (SR5) |
0.00 |
0.0000 |
0.63 |
0.04 |
0.0006 |
2.25 |
Cape Collinson (SR10) |
0.01 |
0.0002 |
0.63 |
0.01 |
0.0001 |
2.25 |
Cape Collinson (SR11) |
0.00 |
0.0000 |
0.63 |
0.01 |
0.0001 |
2.25 |
Big Wave Bay (SR13) |
0.00 |
0.0000 |
0.56 |
0.00 (b) |
0.0001
(b) |
2.34 |
Tai Tam (SR25) |
0.00 |
0.0001 |
0.68 |
0.00 |
0.0000 |
2.40 |
Tai Tam (SR26) |
0.01 |
0.0002 |
0.68 |
0.00 |
0.0000 |
2.40 |
Seawater Intakes (Mid-depth) |
||||||
Tseung Kwan O New Town (SR19) |
0.00 |
0.0000 |
2.58 |
0.00 |
0.0000 |
3.98 |
Siu Sai Wan (SR17) |
0.04 |
0.0006 |
1.80 |
0.02 |
0.0003 |
4.08 |
Heng Fa Chuen (SR23) |
0.23 |
0.0032 |
1.80 |
0.05 |
0.0007 |
4.08 |
Chai Wan Godown
(SR16) (c) |
0.02 |
0.0003 |
- |
0.01 |
0.0001 |
- |
Pamela Youde
Nethersole Eastern Hospital (SR24) (c) |
0.02 |
0.0003 |
- |
0.02 |
0.0003 |
- |
Note: |
||||||
(a)
EPD
water quality monitoring data indicated that 10th percentile DO
level of fish culture zone at Po Toi O is below the
corresponding WQO criteria of 5 mg/L in wet season. (b)
Maximum
SS elevation at SR13 is 0.0042 mg/L (i.e. below level of significant of two
decimal places). The corresponding
maximum DO depletion is predicted to be 0.000059 mg/L. (c)
Cooling
water intakes for Chai Wan Godown and Pamela Youde Nethersole Eastern
Hospital are not sensitive to DO depletion.
The predict values of DO depletion are for reference only and there is
no corresponding assessment criteria. |
As discussed previously under section 6.5.1, the baseline 10%-percentile DO level is already below the WQO criteria
(5 mg/L) at the two fish culture zones (SR2 and SR27) in wet season. The maximum potential DO depletion at the
Tung Lung Chau Fish Culture Zone (SR2) due to marine dredging is predicted to
be only 0.0013 mg/L in wet season, which is already negligible. Review of EPD water quality monitoring data
indicated under baseline condition, DO level at SR2 would be equal to or above
5 mg/L for 83.8240% of time in wet season.
With the 0.0013 mg/L DO depletion, the time for DO compliance is reduced
to 83.7948% (i.e. reduction by 0.0292%).
It should be highlighted that WQO requires the level of DO be above 5
mg/L for 90% of sampling incidents within a year, regardless of the season of
the sampling event. Since baseline DO in
general is lower in wet season (refer to Table 6.12) while SS elevation (also
DO depletion) at WSRs is also generally higher in wet season as well, the above
predicted of DO depletion would be too conservative for WSRs with low DO level
in wet season. Routine EPD monitoring
data from 2009 – 2013 indicated that annual depth-average 10th-percentile
DO at EM2 is 5.05 mg/L (without considering the seasons). As the predicted maximum DO at the Tung Lung
Chau FCZ is only 0.0013 mg/L, the marine construction works would not reduce
the 10th-percentile DO at SR2 to below 5 mg/L. It should also be highlighted that the above
calculation based on continuous DO depletion resulted by maximum SS elevation
from the dredging operation. In fact the
actual time required for the dredging works would be very short based on the
recommended working rate (2 days for dredging at submarine outfall, 3 days for
dredging at seawater intake). Also, the
predicted maximum SS elevation occurs at WSRs for short period of time (one
hour) out of the whole dredging period.
This means the maximum DO depletion presented above is very transient.
As such, the change in
absolute level of DO (0.0013 mg/L) and the induced reduction of time of DO
above 5mg/L in wet season are both minimal and not significant. Also, there will be no non-compliance in DO
expected on an annual basis, it is therefore considered no unacceptable adverse
water quality impact from the potential change in DO level is expected from the
proposed dredging works under this Project.
It should also be highlighted that the WQO criteria for DO requires DO
level at fish culture zone be above 5 mg/L for 90% of the samples without
making reference on the seasons. The
10th-percentile DO level at EM2 within the same baseline period (all samples
from 2009 to 2013) is 5.05 mg/L which is slightly above the criteria and thus
complies with the WQO DO criteria in annual basis. Taken into account the above, the 0.0013 mg/L
maximum DO depletion would not results in exceedance in WQO criteria.
It should be highlighted that the Po Toi O Fish Culture Zone (SR27) is far away from the Project
and is beyond the envelope of impact from this Project. The DO condition at SR27 would not be
adversely affected by this Project. No
significant DO depletion at all other non-fish culture zone WSRs is identified
under the unmitigated scenario. No
additional mitigation measure is required for DO depletion. No adverse water quality impact from DO
depletion due to marine dredging at submarine outfall and seawater intake is
anticipated.
Elevation of Nutrients
Other than readily-biodegradable organic compounds
(which are assessed in the previous section), sediment-bounded nitrogenous
compounds could also be released into the water column and result in an
increase of total inorganic nitrogen (TIN) and UIA. Review of sediment quality data (provided in Table 6.7 above) indicated that level of TKN is low near the Project site
(represented by EPD sediment monitoring station ES1) when compared to the level
at other EPD sediment quality monitoring stations in the Victoria Harbour. Nevertheless, sediment elutriate test results
(provided in Table 6.9 above) indicated that there will be dissolution of ammonia nitrogen as
well as organic nitrogen from sediment disturbance, which could results in
change in water quality. Maximum
elutriate level of ammonia (ionized + unionized) is 3.29 mg/L while the seawater
blank ammonia level is 0.01 mg/L. As
discussed in Section 6.6.2, EPD marine water quality monitoring data at EM2 from 1986 to 2013
indicated that on average 5.7% of ammonia nitrogen exists as UIA. This means for the area of initial
disturbance of sediment (which can be very small though), the maximum level of
UIA could potentially be (3.29 mg/L × 5.7%) = 0.18753 mg/L, which is higher than the
corresponding WQO criteria. For TKN, the
maximum elutriate level and seawater blank level are 4.2 and 0.3 mg/L
respectively. Assuming all TKN turns
into TIN upon release into the water column (which is a very conservative
assumption), there would be a potential increase in TIN over the corresponding
WQO at the area of initial disturbance of sediment. Further detailed assessment of potential
elevation of TIN and UIA at WSR is therefore conducted based on the method
stipulated in Section 6.6.2 above. Based on the conservative
prediction of TIN and UIA using the maximum predicted SS elevation at SR4 (1.11
mg/L for dredging at submarine outfall in wet season, mitigated) and the maximum
TKN level in sediment sampled under this Study, the predicted maximum elevation
in TIN and UIA is calculated as follow:
Max TIN (mg/L) = Max SS (mg DW/L) ×
Max TKN (mg N/ kg DW) × 10-6
= 1.11 × 1480
× 10-6 mg/L = 0.001643 mg/L
Max UIA (mg/L) = Max SS (mg DW/L) ×
Max TKN (mg N/ kg DW) × 10-6 × 5.7%
= 1.11 × 1480 × 10-6 × 5.7% mg/L
=
0.0000941 mg/L = 0.0941 µg/L
As shown in the calculation above, only minimal
elevation of TIN and UIA is expected at the SR4, which is the most adversely
impacted by the proposed marine dredging.
The predicted maximum elevation of TIN and UIA is below 3% of the
allowed level of elevation (i.e. WQO criteria minus background level) as shown
in Table 6.21 below. Potential elevation of
TIN and UIA at other WSRs are expected to be even lower. No exceedance of WQO TIN and UIA criteria is
expected at SR4 and any other WSRs from the proposed dredging operation. No adverse water quality impact from the
release of nutrient due to marine dredging at submarine outfall and seawater
intake is anticipated.
Table 6.21 Predicted
Worst Case Elevation of TIN and UIA and its comparison with the corresponding
WQO Criteria
Unit: mg/L |
TIN
(mg/L) |
UIA
(µg/L) |
||||
WQO Criteria |
||||||
|
Total |
Background Level |
Allowed Elevation |
Total |
Background Level |
Allowed Elevation |
EM1 |
0.4 |
0.23 |
0.17 |
21 |
2 |
19 |
EM2 |
0.4 |
0.19 |
0.21 |
21 |
2 |
19 |
EM3 |
0.4 |
0.17 |
0.23 |
21 |
1 |
20 |
JM3 |
0.3 |
0.24 |
0.06 |
21 |
2 |
19 |
JM4 |
0.3 |
0.22 |
0.08 |
21 |
2 |
19 |
MM19 |
0.3 |
0.12 |
0.18 |
21 |
1 |
20 |
Predicted Elevation
at Most Adversely Impacted Receivers |
||||||
Tai Miu Wan (SR4) |
0.001643 |
0.0941 |
||||
Note: Annual mean of TIN
and UIA given in Table 6.6 were taken as the background level at each
EPD monitoring stations. |
Elevation of Heavy Metals and Micro-organic
Pollutants
The proposed dredging operation may disturb bottom sediment
and results in release of sediment-bounded pollutants, namely heavy metals,
metalloid (arsenic) and trace organic compounds (PAHs, PCBs and TBT). As shown in Table 6.9, the sediment elutriate test results indicated
that the levels of all heavy metals, metalloid and trace organic compounds in
sediment samples analyzed under this Study are below
the corresponding limit of reporting (LOR) adopted for the laboratory
tests. With the exception of mercury and
total PCB, the limit of reporting adopted for the laboratory tests are all
below the proposed water quality assessment criteria for heavy metals,
metalloid and trace organic compounds.
This means there will not be any mixing zone for heavy metals, metalloid
and trace organic compounds, except mercury and total PCB. To assess the potential water quality impacts
from the release of mercury and total PCB from disturbed sediment, further
estimation would be required according to the methodology stipulated in Section 6.6.2. Based on the conservative
prediction of released bounded using the maximum predicted SS elevation at SR4
(1.11 mg/L in wet season, mitigated) and the maximum pollutant level in
sediment, the predicted maximum elevation of mercury and total PCB is
calculated and presented in Table 6.22 below.
Table 6.22 Predicted
Worst Case Elevation of Pollutants and its comparison with the corresponding
Assessment Criteria
Parameter |
Assessment Criteria (mg/L) |
Maximum
Sediment Pollutant Level from Geophysical Survey (mg/kg) |
Predicted
Maximum SS Elevation (mg/L) |
Predicted
Maximum Pollutant Elevation (mg/L) |
Metals |
|
|
|
|
Mercury (Hg) |
0.3 |
0.16 |
1.11 |
0.177 |
Total PCBs |
0.03 |
0.018 |
1.11 |
0.019890 |
As shown, only minimal elevation of mercury and
total PCB is expected at SR4, which is the most adversely impacted by the
proposed marine dredging. The predicted
maximum elevation of pollutants is below the proposed assessment criteria as
shown in Table 6.22 above below.
Potential elevation of mercury and total PCB at other WSRs are expected
to be even lower. No exceedance of the
proposed assessment criteria for pollutants is expected at SR4 and any other
WSRs from the proposed dredging operation.
No adverse water quality impact from the release of pollutants due to
marine dredging at submarine outfall and seawater intake is anticipated.
Other Marine
Construction Works
As
discussed in Section 3, marine
construction works (other than sediment dredging assessed in the previous
sections) would be required for the construction of the proposed submarine
outfall and seawater intake. These
marine construction works includes:
· Construction of submarine outfall diffuser and submerged intake using prefabricated structures;
· Backfilling using rock fill with low fines content.
These
marine construction works would be conducted at the seabed level of the outfall
and intake site and is expected to result in limited disturbance to the
seabed. The adoption of prefabricated
structure for the construction of submarine outfall and seawater intake would
avoid any concreting works and also shorten the duration required for marine
construction, thus limiting the potential water quality impact. The use of rock fill with low fines content
would also minimize any loss of fine to the water column during the backfill
works. In view of the above, no
unacceptable water quality impact is expected from these non-dredging marine
construction works.
It should
be noted that horizontal directional drilling (HDD) would be used to construct
the pipeline from the outfall diffuser and seawater intake to the desalination
plant. Drilling would be starting on the
landside of the Project site and proceed outward to the locations of the
proposed outfall diffuser and seawater intake.
As the HDD works would be conducted within the bedrock, disturbance of
marine sediment is not expected.
Generally, marine dredging at the submarine outfall and seawater intake
would only be conducted after the completion of HDD so drilling fluid would not
be exposed to seawater. Used drilling
fluid and other wastewater from HDD would only be collected on the landside
works area. Rock debris contained in the
drilling fluid from the HDD would be removed from the fluid and be collected as
fill material onsite. The drilling fluid
would then be reconditioned (if necessary) and be reused as much as
practicable. When the drilling fluid is
no longer suitable for reuse or after the completion of the HDD process, the
drilling fluid would be dewatered and disposed to the landfill
accordingly. Any wastewater generated
from the drilling process would be collected in the landside works area and
treated properly before disposal.
General Construction Activities
Discharges and runoff from the site during the
construction phase, particularly during site formation, excavation and
backfilling works, will contain SS which could be a source of water
pollution. Uncontrolled disposal of
debris and rubbish such as packaging, construction materials and refuse and
spillages of chemicals stored on-site, such as oil, diesel and solvents would
also result in contamination of construction site runoff. However, it is anticipated that no
unacceptable water quality impacts would arise from the land-based works if
standard site practices and mitigation measures, described in Section 6.9.1, are in place and properly implemented.
Sewage Effluents from Construction Workforce
Appropriate numbers of portable toilets shall be
provided by a licensed contractor to serve the construction workers over the
construction site to prevent direct disposal of sewage into the water
environment. The Contractor shall also
be responsible for waste disposal and maintenance practices.
Sterilization of Water Mains Prior to Commissioning
The proposed fresh water mains would be cleaned and
sterilized prior to commissioning.
Typically, water mains are sterilized by chlorination. The purpose of chlorination is to eliminate
potential pathogens and maintain a sterile / aseptic condition for potable
water transfer.
The cleaning and sterilization procedures of fresh
water mains would be carried out according to the latest version of WSD Departmental Instruction No. 805: Mainlaying - Cleaning and Sterilization of Fresh Water
Mains (Section 3.11). General
procedures for cleaning and sterilization of potable water mains are outlined
below:
·
The
internal of the pipeline should be cleaned as thoroughly as possible to ensure
that no foreign matter remains inside the water mains. For 1200 mm diameter mains, a CCTV survey
should be carried out to inspect and assess the internal conditions of the
pipeline.
·
After
cleaning, the pipeline should be disinfected by completely filling with water
that has been dosed with a homogeneous solution of chloride of lime (or sodium
hypochlorite solution) such that the final concentration of free chorine in the
water is at least 30 ppm. The water
shall be left in the pipeline for at least 24 hours. The pipeline should be drained down and
flushed thoroughly with potable water, until the concentration of the remaining
chlorine is less than 1 ppm.
·
Water
samples will then be taken for bacteriological and chemical analyses. The pipeline can only be put into service
after the test results are found to be satisfactory.
Based on the preliminary design, a total of 11,000
m3 of chlorinated water would be required for sterilization of the
new fresh water main and an additional 22,000 m3 of potable water
would be used for cleaning and flushing.
The sterilization water would be dechlorinated
with TRC level below 1 mg/L before discharge.
The cleaning and flushing water would also be treated and desilted to the relevant discharge requirement stipulated
in TM-DSS before discharging into public sewer.
With the implementation of appropriate treatment processes and
facilities to control water discharge from sterilized water mains, adverse
impact on water quality is not expected.
To ensure sterilization water is sufficiently dechlorinated,
monitoring of the TRC concentration before discharge is recommended.
Spillage of Chemicals
Site drainage should be well maintained and good
construction practices should be observed to ensure that oil, fuels, solvents,
drilling fluid and other chemicals are managed, stored and handled properly and
do not enter the nearby water streams.
No adverse water quality impacts are expected with proper implementation
of the recommended mitigation measures.
Plant Testing and Commissioning
A brief period of plant testing and commissioning
would be required before the actual operation of the Project. Works involved would be similar to that of
normal operation (which would be assessed in the following sections) but would
be conducted at smaller scale. In view
of the similar nature and smaller scale, the assessment of potential water
quality impact from plant testing and commissioning would be covered under the
operation assessment provided below.
6.8.2
Operation Phase
The design capacity (after expansion) of the
Project is 270 Mld (about 250,000 m3/day),
with maximum saline discharge of 464,000 m3/day. The normal operation of the proposed
desalination plant is outlined below. A
number of chemicals would be added during the desalination process to improve
the quality of potable water from RO permeate.
Chemicals present in the RO concentrate may include
most types of chemicals which are dosed upstream to the RO process, together
with the concentrated constituents of marine water. Based on the preliminary design of the Desalination
Plant, the proposed effluent standards upon commissioning of the Project are
summarized in Table 6.23. Since no
heating is involved in the desalination process, the temperature of the RO
concentrate would be similar to the ambient water temperature. There would also be no cooling water
discharge in the proposed desalination plant.
The effluent will also have DO level that is similar to ambient DO level
in the seawater. Chemicals added for
coagulation (ferric chloride), flocculation (polymer) and pH adjustment
(sulphuric acid and hydrochloric acid) would be removed during the
pre-treatment process or neutralized and would not present in the RO
concentrate at significant level.
Polymer, which is a high molecular weight compound (up to 200,000 atomic
mass unit per molecule) and carries multiple charges, tends to adsorb onto
particle surfaces. Through interparticle bridging coalesce with other particles to
form floc, polymer enhances the removal of suspended
solids from the treated stream. Most of
the polymer dosed would be removed in the pretreatment
process together with the floc and would not be
present in the RO concentrate at notable level.
Residual polymer in the RO concentrate, if any, would also be adsorbed
to suspended solids in the water column and be removed from the water column
quickly. Therefore, assessment on the
use of polymer is not required.
Anti-scalant would also
be added during the desalination process and is expected to present in the RO
concentrate. Based on the material
safety data sheet (MSDS) of typical anti-scalant
products (provide in Annex 6F), anti-scalant is known to be low in toxicity to aquatic fish and
human. Toxicity studies indicated that
EC50 for most test organisms would be in the order of hundreds or
thousands of mg/L, which would be well above the expected discharge
concentration and is not considered a major concern on water quality. As such, there is no commonly used water
quality standard for anti-scalant. For the purpose of deriving a Predicted
No-Effect Concentration (PNEC) from the toxicology data of the anti-scalant, reference would be made to the method proposed
under the Technical Guidance Document on
Risk Assessment by the European Chemical Bureau of the Institute for Health and
Consumer Protection under the European Commission. An assessment factor of 1000 would be applied
to the available EC50 data of the proposed anti-scalant,
i.e. the proposed assessment criteria for the anti-scalant
would be one thousandth of the EC50 data available, for conservative
assessment. The lowest EC50
value (362 mg/L for algae) from the MSDSs of the potential anti-scalant candidates would be used for the calculation and
the assessment criteria for anti-scalant under this
Study would be 0.362 mg/L.
With reference to the characteristics of effluent
from the desalination plant (see Table 6.23), the operation phase modelling for normal
operation will consider five key parameters, namely salinity, Fe, anti-scalant, TIN and SS, for a focused assessment. The addition of other chemicals during
routine maintenance such as bio-growth control and chemical cleaning of RO unit
is also assessed in detail in the following sections.
Table 6.23 Proposed
Effluent Discharge Standards of TKO Desalination Plant upon Commissioning
Parameters |
Upper Limit |
Discharge to
Marine Water from Normal Operation |
|
Discharge Flow Rate |
464,000 m3/day (5.3704 m3/s) |
Fe |
0.6 mg/L |
Total Phosphorus |
1 mg/L |
Total Inorganic Nitrogen |
2 mg/L |
Salinity |
65,000 mg/L |
pH |
6-9 |
SS |
13 mg/L |
Anti-scalant |
8 mg/L |
Discharge to
Marine Water during Bio-growth control |
|
pH |
6-9 |
Sodium Metabisulphite (SMBS) |
0.5 mg/L |
Total Residual
Chlorine (TRC) |
0.1 mg/L |
Discharge to Public
Sewer during Chemical Cleaning |
|
Maximum Amount Discharged per Event |
514.8 m3 |
Fe |
0.6 mg/L |
pH |
6-9 |
SS |
13 mg/L |
Citric acid |
10-30 mg/L |
Sodium lauryl sulphate |
10-30 mg/L |
Change in Hydrodynamic due to Discharge of RO
Concentrate
The discharge of RO concentrate, which is high in
salinity, may alter the flow regime near the submarine outfall. Salinity is also a water quality criterion
stipulated under the WQO and any change due to discharge should not exceed 10%
of natural ambient level. To assess the
potential change in flow regime, computation modelling has been conducted using
a combination of CORMIX suite of model (for near field dispersion modelling)
and Delft3D suite of model (for far field dispersion modelling). The detailed approach is summarized in Annex 6A and Section 6.6.3 above. The assessment criteria
stipulated under Section 6.5.6 would be followed.
As stated in Section
6.6.3 above, continuous discharge of RO concentrate is simulated using CORMIX
suite of model (using ambient conditions provided by Delft3D FLOW base case scenario)
to determine the near field behaviour of the discharge plume. Detailed predictions from CORMIX simulation
are presented in Annex 6C.
Contour plots showing the mixing zone of relative
salinity elevation ([21])
at surface, middle and bottom level of the water column at selected tidal
condition (mid-flood, high water, mid-ebb and low water for both spring tide
and neap tide) near the outfall in both seasons are presented in Annex 6D. As shown in Annex 6D, the effluent plume of RO concentrate discharge is
confined closely to submarine outfall in both seasons. A small mixing zone where relative salinity
elevation >0.1 (i.e. salinity elevation by 10% as stated in the WQO) could
be observed at the bottom level. Localized
plume of minor elevation relative salinity elevation >0.05 (i.e. salinity
elevation by 5%) is observed near the outfall for some of the tidal conditions
at bottom level. The maximum percentage
increase in salinity at the outfall is predicted under low water condition in
spring tide of dry season, where a 13.4% increase of salinity above ambient is
predicted at the bottom level. Maximum
percentage increase in salinity at other occasions (i.e. wet season, neap tide,
etc), other locations further away from the outfall
or other level of the water column (surface or middle) would be lower. Yet the mixing zone predicted under all
occasion is very small (less than 30 m × 30 m), highly localized and away from any WSRs
identified (Figures 6.8a to 6.8d).
No unacceptable change in flow regime from the elevation in salinity due
to the discharge of RO concentrate is expected.
The predicted elevation in salinity at the nearest WSR at Tit Cham Chau
(SR37) would be the highest in low water condition (i.e. slack water after ebb
tide) in both seasons. The predicted
percentage increase in salinity at SR37 would be about 1.65% and 1.36%
respectively in low water condition in dry and wet season respectively. The predicted increase at other WSRs would be
even lower. Compliance to the WQO
criteria for salinity is expected at all WSRs identified under this Study.
Change in Water Quality due to Discharge of RO
Concentrate during Normal Operation
As discussed in the previous section, a number of
chemicals would be continuously dosed to the seawater extracted during the
normal operation. Extracted seawater
would be first screened to remove large objects. The screened seawater would then be dosed
with ferric chloride (to enhance coagulation), polymer (to enhance flocculation)
and sulphuric acid (to adjust pH to allow optimum coagulation and flocculation)
and then subjected to clarification.
Large particles would be removed through this process.
The clarified water would then be filtered to
remove suspended particles of smaller size.
Anti-scalant would be added to inhibit scaling
of the RO membrane. pH adjustment in
form of sodium hydroxide may be needed to optimize the working pH of anti-scalant. The RO
permeate (water which passes through the RO membrane) would be diverted for
further downstream treatment and the RO concentrate would be neutralized to pH
6-9 and then discharged via the submarine outfall.
The discharge of Fe, anti-scalant,
TIN and SS is modelled using the Delft3D PART suite of model. Inert, non-settling tracer (i.e. conservative
tracer) was assumed for Fe, anti-scalant and TIN,
while inert, settling tracer (i.e. sediment) was assumed for SS. The release is assumed to be continuous and
located at the vertical layers predicted by the CORMIX near field simulation
(the second iteration for both seasons given in Annex 6C). Assessment
criteria stipulated in Table 6.16 are adopted. Based on the field survey conducted
under this Project, the level of Fe in ambient seawater is around 0.19
mg/L. The allowed elevation for Fe at
WRSs is therefore 0.3 mg/L – 0.19 mg/L = 0.11 mg/L. Similarly, allowed elevation of TIN is [0.4
mg/L (WQO for TIN at Eastern Buffer WCZ) – 0.17 mg/L (long term average TIN
from 1986-2013 at EPD Water Quality Monitoring Station EM2)] = 0.23 mg/L.
The predicted maximum depth-averaged elevation of
Fe, anti-scalant, TIN and SS from RO concentrate
discharge over a spring-neap tidal cycle for dry and wet season are presented
in Annex 6E-1 to Annex 6E-4. The corresponding plots for bottom level of
these parameters are provided in Annex
6E-1a to Annex 6E-4a for assessment of impact to coral sites. Based on the results of the near field
dispersion modelling presented in Annex
6C, the effluent plume of the RO concentrate would be mostly concentrated
at the bottom level of the water column, thus the elevation and plume extent of
these parameters would be bigger than that of the depth-averaged plots. It should be highlighted the predicted values
and plots are the elevation above baseline level. For water quality parameters including TRC and
SMBS, their corresponding baseline level in marine water should be negligible
due to their tendency to decay and the lack of nearby source, the predicted
elevation should be similar or equal the absolute level.
Iron
As shown in Annex
6E-1, no mixing zone of exceedance of the allowed elevation of 0.11 mg/L of
Fe is predicted in both seasons for depth-averaged results. Small effluent plumes, highly localized and
confined near the submarine outfall, are observed in both seasons at a level
10% of the proposed assessment criteria of 0.3 mg/L. The predicted bottom level of iron is higher
since the RO concentrate stay low in the water column. As shown a mixing zone with iron level above
allowed elevation of 0.11 mg/L is predicted near the outfall without
encroaching into nearby coral sites. The
predicted maximum iron elevation level at the bottom level of the nearest WSR
at Tit Cham Chau (SR37) is 0.0066 mg/L in dry season and 0.0152 mg/L in wet
season, which is lower than the assessment criteria. The predicted iron elevation level would be
even lower at other WSRs.
Anti-scalant
As shown in Annex
6E-2 and 2a, small mixing zone
with maximum anti-scalant concentration higher than
the proposed assessment criteria of 0.362 mg/L is predicted near the proposed
submarine outfall under wet season for the depth-averaged and bottom level
results. Yet the depth-averaged mixing
zone is highly localized and does not encroach to any WSR identified under this
Project (as shown in Figure 6.8f). Whilst the mixing zone predicted at bottom
level is bigger, it is not predicted to encroach into the nearby coral sites
SR36 and 37 (Figure 6.8f). It should
be noted that the predicted level shown in Annex
6E-2 and 2a is the instantaneous
maximum predicted throughout the 15-day spring-neap cycle and does not reflect
the actual extent of effluent plume at any incident. The prediction is made based on conservative
assumptions that (1) the RO concentration would be continuously discharged at
the maximum capacity of proposed plant and (2) the concentration of anti-scalant would be maintained at the proposed maximum
level. These multiple levels of
conservative assumptions would result in overestimation of impacts. The predicted maximum level of anti-scalant at the bottom level of the nearest WSR at Tit Cham
Chau (SR37) is 0.1939 mg/L in dry season and 0.3486 mg/L in wet season, which
is lower than the assessment criteria.
The predicted maximum level of anti-scalant at
the bottom level of the nearest WSR at Kwun Tsai
(SR36) is 0.0648 mg/L in dry season and 0.1539 mg/L in wet season, which is
also lower than the assessment criteria.
The predicted level of anti-scalant would be
lower at other WSRs. For example,
maximum anti-scalant level predicted at the nearby
Tung Lung Chau Fish Culture Zone (SR2) is 0.0416 mg/L in wet season and 0.1399
in dry season while mean anti-scalant level at SR2 is
predicted to be 0.0089 mg/L in wet season and 0.0271 mg/L in dry season.
Anti-scalant is in
general a mixture of polymers of different molecular size and biodegradation is
expected as stated in the corresponding MSDSs (provided in Annex 6F). The degradation
of these polymers is slow and the degradation product would unlikely be present
in significant concentration in view of the significant flushing at the
submarine outfall and the nearby waters.
Degradation products of the anti-scalant are
therefore not considered a water quality concern. It is, however, understood that certain type
of anti-scalant may contain phosphorus. As stated in Annex 6F, degradation of anti-scalant is
only about 23% in 28 day, which means about 0.929% ([22]) of anti-scalant degrade every day. As given in Table 6.23, the percentage of mass of phosphorus in anti-scalant would be below 12.5%. This means phosphate of mass less than
(0.929% × 12.5% =) 0.12% of anti-scalant would be
released each day, which is unlikely to be of concern considering the low level
of anti-scalant presented in the marine water (0.0881
mg/L in dry season and 0.2032 mg/L in wet season at the most impacted WSR as
stated in the previous section). It is
therefore considered the discharge of anti-scalant would
not contribute to significant level of biologically available phosphorus before
leaving the HK waters in view
of the slow degradation rate and relatively high flushing near the
outfall. No secondary impact, such as eutrophication or algal bloom, is therefore
expected.
Total Inorganic Nitrogen
Contour plot of predicted maximum elevation of TIN
from the RO discharge is provided in Annex
6E-3 and 3a. As shown, there is no mixing zone with TIN
elevation higher than the 0.23 mg/L assessment criterion predicted in both
seasons for depth-averaged results. Very
small effluent plume of 0.13 mg/L is predicted close to the proposed submarine
outfall, but that neither exceeds the proposed TIN assessment criterion nor
encroaches to any WSRs nearby. For
bottom level of TIN, predicted level is higher and a localized mixing zone is
predicted in wet season around the proposed submarine outfall. The predicted maximum TIN level at the bottom
level of the nearest WSR at Tit Cham Chau (SR37) is 0.0485 mg/L in dry season
and 0.0872 mg/L in wet season, which is lower than the assessment criteria. The predicted TIN level would be even lower
at other WSRs.
It should be highlighted that there will not be
addition of nitrogen loading to seawater during the desalination process. The TIN level is predicted to be in
compliance with the relevant WQO level at the nearby WSRs and mixing zone is small
and confined near the outfall. No
secondary impact, such as eutrophication or algal bloom, is therefore expected.
Suspended Solids and Sedimentation
For SS, no mixing zone with exceedance of the WQO
criteria of 30% elevation above ambient level is predicted in dry season for
both depth-averaged and bottom results.
Maximum depth-averaged SS elevation is predicted to be higher in wet
season, reaching 1.70 mg/L near the submarine outfall, as shown in Annex 6E-4. The predicted maximum SS level in wet season
is slightly above the 30% elevation above ambient level of the nearby EPD
marine water monitoring station of EM3.
Yet the mixing zone is very small, confined near the submarine outfall
and does not reach any identified WSRs under this Study. Slightly bigger zone of mixing is predicted
for bottom SS level. Yet the predicted
maximum mixing zone of bottom SS is still away from the nearby receivers. The predicted maximum bottom SS level at the
bottom level of the nearest WSR at Tit Cham Chau (SR37) is 0.15204 mg/L in dry
season and 0.32677 mg/L in wet season, which is lower than the assessment
criteria of 30% above ambient level. The
predicted SS level would be even lower at other WSRs.
It is observed that the maximum sediment plume
predicted is not located right at the proposed submarine outfall as shown in Annex 6E-4. It is because the sediment release rate is
very low ([23]) that any increase in SS near the outfall would be below the assessment
criteria of 30% increase above the ambient level. On the other hand, sediment released from the
outfall would settle around the outfall (as shown in Annex 6E-5), and be re-suspended (together with the existing
sediment) when the tide condition becomes favourable (i.e. high current
velocity during spring tide). Such
re-suspension event would occur in short period of time in a tidal-cycle and
the effect would be very localized depending on the ambient flow
condition. It should be highlighted that
such re-suspension event would be naturally occurring (if the sediment
conditions are suitable) and is not a result of this Project.
The
maximum sedimentation flux from the RO concentrate discharge is shown in Annex 6E-5. A small area where maximum sedimentation rate
exceeds the 100 g/m2/day assessment criteria for coral is
predicted. Yet the area is predicted to
be confined close to the submarine outfall and would not encroach to any of the
coral WSRs identified under this Study.
It should be highlighted that the extent of maximum sedimentation flux
is predicted based on continuous discharge at the maximum design capacity of
the plant with maximum SS concentration in RO concentrate, which represents a
significant overestimation of sedimentation rate. No adverse impact from sedimentation would be
expected. Also, no maintenance dredging
near submarine outfall would be expected. The
predicted maximum seabed (i.e. bottom) sedimentation flux at the nearest WSR at
Tit Cham Chau (SR37) is 30.94 g/m2/day in dry season and 46.10 g/m2/day
in wet season, which is lower than the assessment criteria. The predicted sedimentation flux would be
even lower at other WSRs.
In view of the above, no adverse water quality
impact from the elevation of Fe, anti-scalant, TIN,
SS and sedimentation flux due to the discharge of RO concentrate would be
expected.
Change in Water Quality due to Discharge of RO
Concentrate during Bio-growth Control Works
Control of bio-growth at seawater inlet as well as
the pre-treatment unit (clarification and filtration) would be conducted on a
regular basis. Base on the latest design
information available, bio-growth control works would be conducted for every 5
days. Chlorine at a level of 15 mg/L
would be dosed to the system for a period of 6 hours. For the pre-treatment unit, bio-growth
control would be conducted every 24 hours using chlorine at 5 – 10 mg/L level
for 30 minutes. No other biocide is
required based on the latest design information available. As a measure to avoid damage to the membrane
of the RO unit, SMBS would be dosed to the extracted water before reaching the
RO unit for dechlorination. The level of total residual chlorine (TRC)
and SMBS is expected to be minimal in the RO concentrate. For conservative reason, computational
modelling using Delft3D PART model was conducted to study the dispersion of TRC
and SMBS from the submarine outfall at a maximum discharge level of 0.1 mg/L
and 0.5 mg/L respectively. Inert,
non-settling tracer (i.e. conservative tracer) was assumed for SMBS, while decayable, non-settling tracer was assumed for TRC. Similar to the modelling assessment for Fe
and SS, the release is assumed to be continuous and located at the vertical
layers predicted by the CORMIX near field simulation (the second iteration for
both seasons given in Annex 6C). It should be noted that this is a very
conservative approach to assume continuous release of TRC and SMBS. In fact the release of TRC and SMBS would
only last for a brief period (30 minutes for bio-growth control at
pre-treatment unit and 6 hours for bio-growth control at inlet) when the
bio-growth control works is being conducted.
By assuming continuous release, the modelling simulation takes into account
the worst case scenario that may arise from different tide condition as well as
the potential for background buildup, thus ensuring
conservative assessment. Assessment
criteria stipulated in Table 6.16 are adopted.
The predicted maximum depth-averaged concentration
of TRC and SMBS from RO concentrate discharge over a spring-neap tidal cycle
for dry and wet season is presented in Annex
6E-6 and Annex 6E-7.
Total Residual Chlorine
As shown in Annex
6E-6, no mixing zone with exceedance of the proposed assessment criteria of
0.02 mg/L of TRC is predicted in both seasons.
Effluent plumes are observed within the Tai Miu
Wan and Tathong Channel in both seasons at a level
one-tenth of the proposed assessment criteria.
The predicted level of TRC at the bottom level is slightly higher and
small and localized mixing zone with TRC level above 0.02mg/L is predicted
within the Tai Miu Wan in wet season. Yet the bottom mixing zone does not encroach
into the nearby coral sites. The
predicted maximum TRC level at the nearest WSR at Tit Cham Chau (SR37) is
0.00160 mg/L in dry season and 0.00398 mg/L in wet season, which is lower than
the assessment criteria. The predicted
TRC level would be even lower at other WSRs.
Sodium Metabisulphite and
associated Oxygen Depletion
No criterion is proposed to directly assess the
acceptable level for SMBS in view of lack of / nil in toxicity to human and
aquatic animals at the maximum discharge concentration given in Table 6.23. Yet as a
reducing dechlorination agent, the presence of SMBS
would contribute to an increase in chemical oxygen demand in the water
column. The product of hydrolysis of
SMBS, sulphite ion, may also be present in the RO concentrate. Sulphite, which is slightly reducing, acts
similar to SMBS and reacts with DO.
Since the amount of DO being consumed by each mole of SMBS would be the
same as that consumed by the amount of sulphite produced by the one mole of
SMBS, the potential water quality impact (DO depletion) would be assessed
together in this section for simplicity ([24]). The maximum potential DO
depletion from SMBS is calculated from the predicted maximum SMBS level from RO
concentrate discharge. This approach is
considered appropriate and is the same as the approach adopted for calculating
DO depletion from release of sediment-bounded pollutant from construction phase
marine dredging under Section 6.8.1.
SMBS reacts with DO according to the following
stoichiometric equation:
Na2S2O5 (aq) + O2 (aq) + H2O
(l) à 2 NaHSO4 (aq)
One molecule of SMBS reacts with one molecule of
DO. Taken into account the molecular
mass of SMBS and oxygen, 1 mg/L of SMBS would react with 0.16832 mg/L of DO
(assuming complete reaction). The level
of maximum potential DO depletion is calculated from the maximum predicted SMBS
level accordingly.
Contour plots of SMBS and the corresponding plots
for depth-averaged DO depletion are provided in Annex 6E-7 and Annex 6E-8.
As shown in the contour plots for DO depletion (Annex 6E-8), DO depletion would be below 0.016832 mg/L (corresponds
to 0.1 mg of SMBS). As discussed under section 6.8.1, the 10th-percentile
DO at EM2 is 5.05 mg/L (all samples without dividing into two seasons). This means the allowed DO depletion would be
0.05 mg/L. The predicted maximum bottom
DO depletion at the nearest WSR at Tit Cham Chau (SR37) is 0.0020 mg/L in dry
season and 0.0037 mg/L in wet season.
The level of DO depletion is also predicted to be below 0.01 mg/L at the
nearby Tung Lung Chau Fish Culture Zone (SR2), which indicates compliance to
the assessment criteria for DO. It is
therefore expected that any depletion of DO due to the discharge of SMBS from
RO concentrate would not result in exceedance of the relevant WQO criteria.
It is also known that SMBS releases odorous sulphur
dioxide gas when it first dissolves in water.
Such emission of sulphur dioxide would only be at notable level near its
saturation concentration (SMBS saturates in water at 540 g/L at room
temperature, which is 106 times of the maximum residual level in RO
concentrate). Yet the level of SMBS
present in the RO concentrate would only be 0.5 mg/L or below, which is well
below the concentration of SMBS dosing solution when it is first prepared. Therefore, significant release of sulphur
dioxide gas from RO concentrate is not expected.
Chlorination By-products
With the use of chlorination as a measure for
bio-growth control, the generation of chlorination by-products may potentially
be a concern. A review of nearby water
quality data indicated that the level of organics in marine water near the
Project (indicated by the average 5-day biochemical oxygen demand at EPD
monitoring station EM2) is quite low (below 1.1 mg/L in 2013). For comparison, such level of organics is
about 1/80 of the effluent from Stonecutters Sewage Treatment Works (as given
in Figure 4.6 of the approved EIA of HATS Stage 2A) and about 1/4 of the raw
water from Dongjiang in 2013 ([25]). The amount of organics available from
seawater for reaction with chlorine from bio-growth control would be limited
and the generation of chlorination by-products is expected to be limited as
well.
Polymer, also known as polyelectrolyte, is a class
of high molecular mass (inorganic and organic) compounds which generally carry
multiple (positive and / or negative) charges.
Based on the preliminary design information, polymer would be dosed to
the seawater drawn into the desalination plant after the preliminary filtration
on an as-needed basis with dosing level at maximum 1 mg/L. As a precautionary measure, the dosing of
polymer will be controlled to avoid any concurrent dosing during chlorination
for bio-growth control. This
precautionary measure will be incorporated in the operation manual of the
plant. As such, no formation of
chlorination by-products would be expected from the use of polymer for pre-treatment.
The addition of anti-scalant,
which is organic in nature, would be conducted after the removal of chlorine
from the bio-growth control exercise.
Monitoring of TRC level of the dechlorinated
bio-growth control effluent would be routinely conducted for every bio-growth
control exercise to ensure sufficient dechlorination. The risk of formation of chlorination
by-products would also be low. No
unacceptable water quality impact from chlorination by-products is expected.
In view of the above, no adverse water quality
impact from the potential discharge of TRC, SMBS and chlorination by-products
due to the bio-growth control works would be expected.
Change in Water Quality due to Discharge of RO
Concentrate during Chemical Cleaning
Chemical cleaning is a process of removal of
mineral scale, biological matter, colloidal particles and insoluble organic
constituents deposited on the RO membrane by chemical method. The deposited substance, which deposited on
the RO membrane due to the local increase of solute concentration during the RO
process, would be dissolved, detach or re-suspended in the cleaning solution in
response to the change in pH, allowing easy removal. Chemical cleaning would be conducted once a
month for the RO membrane. Based on the
latest design information, acids (hydrochloric acid and citric acid, both are
commonly used acids in household cleaning products), base (sodium hydroxide,
commonly used as drain cleaner) and surfactant (sodium lauryl sulphate, used in
liquid soap and detergent) would be used to remobilize the deposited substance
from the RO unit. These chemicals are
common and non-hazardous at the proposed discharge concentration. The amount of wash water is expected to be
about 514.8 m3 for each cleaning event, which would last for 4 to 8
hours. After the cleaning event, the
wash water would be neutralized to pH of 6-9 and treated to appropriate quality
stipulated in Table 1 of Cap 358AK
Technical Memorandum Standards for Effluents Discharged into Drainage and
Sewerage Systems, Inland and Coastal Waters before disposal to public sewer
which leads to the Tseung Kwan O Preliminary
Treatment Works. No adverse water
quality impact from the discharge of effluent from chemical cleaning to public
sewer would be expected.
Desalination Plant Failure, Power Failure and Other
Contingency
In case of plant failure or power failure, water
extraction and the desalination process would be discontinued. Limited discharge of RO concentrate, which is
generated before the plant / power failure would be expected, with effluent
quality similar to that stipulated in Table 6.23. No
emergency discharge from the submarine outfall is expected. No adverse water quality impact would be
expected from plant failure or power failure.
Also, there will not be any discharge from the plant when it is put into
its standby / dormant mode.
Maintenance Dredging
Based on the current design information, there is no
need for maintenance dredging at the submarine outfall. While Annex 6E-5 shows that
sedimentation near the proposed outfall is predicted, it should be highlighted
that the extent of maximum sedimentation flux is predicted based on continuous
discharge at the maximum design capacity of the plant with maximum SS
concentration in RO concentrate, which represents a significant overestimation
of sedimentation rate. Any need for maintenance dredging would be
reviewed and confirmed upon commencement of the Project, subject to field
measurement after the commencement of the Project. Should there be a need for maintenance dredging
at the proposed submarine outfall, the potential water quality impact should be
assessed in a separate environmental study.
Generation of Sewage Effluent
The sewage generated from human activities of the
Project would include the sewage from the operation staff and visitors. Sewage effluent and neutralized chemical cleaning water from the Reverse Osmosis (RO) system
would be disposed to Tseung Kwan O Preliminary
Treatment Works for this Project. No
adverse water quality impact from the generation of sewage effluent from
Project operation is expected (see Section
7).
Accidental Spillage of Chemicals
A number of chemicals would be stored onsite and be
used during the desalination process.
Based on the latest design information available, chemicals would be
stored on site at bunded area and separate drainage
system will be provided as appropriate to avoid any spilled chemicals from
entering the storm drain in case of an accidental spillage. Adequate tools for cleanup
of spilled chemicals should be stored on site.
Appropriate training shall also be provided to staff. No adverse water quality impacts are expected
with proper implementation of the recommended mitigation measures.
Marine Construction
To minimize potential water quality impacts from
elevated SS due to the proposed dredging works at submarine outfall and
seawater intake, mitigation measure in form of (1) floating type silt curtain
around grab dredger would be adopted for dredging at the proposed submarine
outfall and (2) combine used of one floating type silt curtain and one cage
type silt curtain for dredging at seawater intake. Silt curtains shall be formed from tough,
abrasion resistant, permeable membranes, suitable for the purpose, supported on
floating booms in such a way as to ensure that the sediment plume shall be
restricted to within the limit of the works area. The silt curtain shall be formed and
installed in such a way that tidal rise and fall are accommodated, with the
silt curtains always extending from the surface to the bottom of the water
column and held with anchor blocks.
Based on the results of construction phase water quality modelling
assessment, the maximum allowed dredging rate at the seawater intake should be
limited to 750 m3/day while the maximum allowed dredging rate at the
submarine outfall is 3,500 m3/day.
The contractor shall regularly inspect the silt curtains and check that
they are moored and marked to avoid danger to marine traffic. Regular inspection on the integrity of the
silt curtain should be carried out by the contractor and any damage to the silt
curtain shall be repaired by the contractor promptly. Relevant marine works shall be stopped until
the repair is fixed to the satisfaction of the engineer.
To ensure the maximum allowed dredging rate of 750
m3/day is implemented properly for dredging at seawater intake,
specific measures stipulated below will be followed:
·
Closed
grab of 3 to 6 m3 will be used for dredging at seawater intake;
·
Number
of cycle shall be limited to (750 m3 ÷ 3 m3/grab
÷ 12 hours) = 20.83, i.e. 20 to 21 grabs
per hour for 3 m3 closed grab;
·
Number
of cycle shall be limited to (750 m3 ÷ 6 m3/grab
÷ 12 hours) = 10.42, i.e. 10 to 11 grabs
per hour for 6 m3 closed grab;
·
Specific
work staff will be assigned the responsibility for monitoring the number of
grab dredged per hour; and
·
The
grab should be operated in slow and controlled manner such that the impact to
seabed by the grab when being lowered could be minimized. Also, the operator should ensure the grab be
properly closed before lifting the grab.
Furthermore, a number of standard measures and good
site practices should be implemented to avoid / minimize the potential impacts
from marine construction. These
measures include:
·
All
vessels should be well maintained and inspected before use to limit any
potential discharges to the marine environment;
·
All
vessels must have a clean ballast system;
·
No
discharge of sewage/grey wastewater should be allowed. Wastewater from potentially contaminated area
on working vessels should be minimized and collected. These kinds of wastewater should be brought
back to port and discharged at appropriate collection and treatment system; and
·
No
soil waste is allowed to be disposed overboard.
General Construction Activities
Standard site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” will be
followed as far as practicable in order to reduce surface runoff, minimize erosion,
and also to retain and reduce any SS prior to discharge. These practices include the following:
·
Silt
removal facilities such as silt traps or sedimentation facilities will be
provided to remove silt particles from runoff to meet the requirements of the
TM standard under the WPCO. The design
of silt removal facilities will be based on the guidelines provided in ProPECC PN 1/94. All
drainage facilities and erosion and sediment control structures will be
inspected on a regular basis and maintained to confirm proper and efficient
operation at all times and particularly during rainstorms. Deposited silt and grit will be removed
regularly.
·
Earthworks
to form the final surfaces will be followed up with surface protection and
drainage works to prevent erosion caused by rainstorms.
·
Appropriate
surface drainage will be designed and provided where necessary.
·
The
precautions to be taken at any time of year when rainstorms are likely together
with the actions to be taken when a rainstorm is imminent or forecasted and
actions to be taken during or after rainstorms are summarised in Appendix A2 of
ProPECC PN 1/94.
·
Oil
interceptors will be provided in the drainage system where necessary and
regularly emptied to prevent the release of oil and grease into the storm water
drainage system after accidental spillages.
·
Temporary
and permanent drainage pipes and culverts provided to facilitate runoff
discharge, if any, will be adequately designed for the controlled release of
storm flows.
·
The
temporary diverted drainage, if any, will be reinstated to the original
condition when the construction work has finished or when the temporary
diversion is no longer required.
As the Project site is next to the shoreline,
infiltration of seawater during excavation is anticipated. Appropriate infiltration control, such as
cofferdam wall, should be adopted to limit groundwater inflow to the excavation
works areas in the Project site.
Groundwater pumped out from excavation area should be discharged into
the storm system via silt removal facilities.
Appropriate numbers of portable toilets shall be
provided by a licensed contractor to serve the construction workers over the
construction site to prevent direct disposal of sewage into the water
environment.
Sterilization of Water Mains Prior to Commissioning
The sterilization water should be dechlorinated with TRC level below 1 mg/L before discharge
to public sewer. In situ testing of TRC
should also be conducted for the discharge of chlorinated water for pipeline
disinfection to ensure sufficient dechlorination before discharge to public sewer. The cleaning and flushing water should also be
treated and desilted to the relevant discharge
requirement stipulated in TM-DSS before discharging.
Spillage of Chemicals
Site drainage should be well maintained and good
construction practices should be observed to ensure that oil, fuels, solvents
and other chemicals are managed, stored and handled properly and do not enter
the nearby water streams.
Modelling
assessment for operation phase effluent discharge indicated that no adverse
water quality impact from the RO concentrate discharge and other relevant
discharge. To ensure compliance to the
effluent discharge standards stipulated in Table 6.23, effluent quality monitoring is recommended. Furthermore, regular water quality monitoring
at nearby WSRs is recommended for operation phase environmental monitoring and
audit. Details are provided in the
standalone EM&A Manual.
Elevation of Suspended Solids and Sedimentation
It is predicted that the maximum SS elevation at all
WSRs would be in compliance with the corresponding WQO SS criteria with the
implementation of silt curtain around the dredging area. No unacceptable residual water quality impact
from SS elevation due to marine dredging at submarine outfall and seawater
intake is expected.
No exceedance of the proposed sedimentation
criteria is predicted at all coral communities in the mitigated scenario. No adverse water quality impact from the
elevation in sedimentation due to marine dredging at submarine outfall and
seawater intake is expected.
Depletion of Dissolved Oxygen
No exceedance of the WQO and WSD DO criteria is
predicted at all WSRs in the unmitigated scenario. No adverse water quality impact from the
potential oxygen depletion due to marine dredging at submarine outfall and
seawater intake is expected.
Release of Nutrients, Heavy Metals and
Micro-organic Pollutants
No exceedance of the proposed assessment criteria
for nutrients, heavy metals and micro-organic pollutants is predicted at all
WSRs in the unmitigated scenario. No
adverse water quality impact from the potential elevation in nutrients, heavy
metals and micro-organic pollutants due to marine dredging at submarine outfall
and seawater intake is expected.
Modelling assessment for operation phase effluent
discharge indicated that no unacceptable adverse water quality impact from the
RO concentrate discharge and other relevant discharge. Wastewater from chemical cleaning of RO
membrane would be disposed to public sewer.
Chemicals would be stored on site at bunded area and separate drainage system will be provided
as appropriate to avoid any spilled chemicals from entering the storm drain in
case of an accidental spillage. Adequate
tools for cleanup of spilled chemicals should be stored
on site. Appropriate training shall also
be provided to staff.
Based on the latest design information available,
the construction of submarine outfall and seawater intake under this Project
would be conducted from early 2018 to mid-2019.
As discussed in the previous section, the duration of marine dredging
required under this Project would be quite short (around 2 days for submarine
outfall at 3,500 m3/day and around 3 days for seawater intake at 750
m3/day) and the extent of potential water quality impact is very
limited. Significant overlap in impacted
area with other projects nearby is not expected. Nevertheless, review of publicly available
information was conducted and a number of projects are identified with the
Study Area. These projects are
summarized below in Table 6.24:
Table 6.24 Nearby
Projects Identified
Project |
Duration |
Location |
Major Marine Activity |
Project Asia Pacific Gateway – Tseung Kwan O |
2014 Q1-Q3 |
Junk Bay, Eastern
Buffer, Southern and Mirs Bay WCZ |
Submarine cable installation |
Truck Road T2 |
Late 2015 - Late 2020 |
Kwun Tong Typhoon Shelter |
Construction of
submarine tunnel |
Backfilling Marine Borrow Areas at East Tung Lung Chau |
Oct - Dec 2014 |
Marine Waters east of Tung Lung Chau |
Marine dumping |
Cross Bay Link |
Jan 2017 - Jul 2020 |
Junk Bay |
Construction of bridge piers |
Tseung Kwan O – Lam Tin Tunnel and Associated Works |
Feb 2016 - Apr 2020 |
Junk Bay |
Reclamation |
PROJECT ASIA PACIFIC GATEWAY – TSEUNG KWAN O
According to project profile of direct-to-permit
application DIR-233/2013, this project involves laying of cable in marine
waters from Junk Bay, Eastern Buffer, Southern and Mirs
Bay WCZ. The marine works would last for
around 60 days and is scheduled from Q1 to Q3 of 2014. No overlap in marine construction with works
under DIR-233/2013 is expected. No
cumulative water quality impact is expected from this project.
TRUNK ROAD T2
Based on the approved EIA of Truck Road T2
(AEIA174/2013), the project is to construct a dual two-lane trunk road of
approximately 3 km long with about 2.7 km of the trunk road in form of tunnel
between Cha Kwo Ling and South Apron of the former
Kai Tak Airport. The project is located
at > 6 km from the proposed desalination plant and submarine pipelines. The preliminary construction programme is
anticipated between late 2015 and late 2020.
According to the approved EIA and the associated environmental permit,
non-dredge method using tunnel boring machine would be adopted for the
construction of Truck Road T2 and there will be no dredging and reclamation
allowed. No cumulative water quality
impact is anticipated.
BACKFILLING OF MARINE BURROW AREA AT EAST TUNG LUNG
CHAU
Based on our communication with the Fill Management
Committee of the Civil Engineering and Development Department, the captioned
project is solely used for the disposal of the uncontaminated dredged sediment
generated from the formation of new mud pits to the South of The Brothers from
October to December of 2014. No overlap
in project operation of the captioned project is expected. As such, there will not be cumulative water
quality impact from the captioned project.
Based on the approved EIA of the captioned project,
this project involves the construction of an approximately 1.8 km long dual
two-lane road mainly on viaduct with a footpath and a cycle track. The construction of the Cross Bay Link (CBL)
is scheduled from January 2017 to July 2020 while the marine dredging and
foundation working would be completed in 2018.
It is expected that the marine works under the CBL may potentially be
conducted concurrently with marine working under this Project. To ensure consistency of modelling results
with the approved EIA, the maximum SS elevation predicted at nearby WSRs under
the approved EIA has been taken into account in assessing the cumulative water
quality impact. Further assessment on
the cumulative water quality impact is provided below in Section 6.11.2.
TSEUNG KWAN O – LAM TIN TUNNEL AND ASSOCIATED WORKS
Based on the approved EIA of the captioned project,
this project involves the construction of a dual two-lane highway connecting
TKO at Po Yap Road in the east with Trunk Road T2 in Kai Tak Development in the
west and Lei Yue Mun Road Underpass. The project involves a 4.8 km long highway
with about 3 km of the highway in the form of a tunnel. The Tseung Kwan O –
Lam Tin Tunnel (TKO-LTT) will connect CBL to form a new external road link to
meet the anticipated traffic flow in connection with further population intake
and development in TKO New Town. The
construction of the TKO-LTT and Associated Works is scheduled from February
2016 to April 2020 while the marine dredging and foundation working would be
conducted between in May 2018 to August 2018.
It is expected that the marine works under the TKO-LTT and Associated
Works may potentially be conducted concurrently with marine working under this
Project. To ensure consistency of
modelling results with the approved EIA, the maximum SS elevation predicted at
nearby WSRs under the approved EIA would be taken into account in assessing the
cumulative water quality impact. It
should be noted that the water quality modelling of the EIAs for the CBL and
the TKO-LTT were conducted together and the cumulative impact from both
projects would be considered at the same time under this Project. Further assessment on the cumulative water
quality impact is provided below in Section
6.11.2. The
reclamation of TKO-LTT has also been taken into account in both the
construction phase and operation phase water quality modelling assessment.
It is expected that the marine construction under
the CBL and the TKO-LTT would potentially be conducted within the same time
period of the marine dredging under the Project. To ensure conservative and consistent
assessment, the predicted maximum SS elevation among all WSRs from the approved
EIAs of the captioned projects (i.e. 0.5 mg/L predicted at bottom level of CC2
in wet season) would be taken into consideration. It should be noted that this approach is
extremely conservative, since the predicted maximum SS elevation among all WSRs
identified under the CBL and the TKO-LTT is used to represent the WSRs which is
furthest away from the sediment sources under the two modelling simulations
(under the approved EIA of CBL + TKO-LTT and under this Project). Simply adding the two maximum values
predicted would very likely be way above the “actual” maximum, even if the
marine construction works are indeed conducted concurrently. Similarly, the maximum cumulative
sedimentation rate at the WSRs identified under this Project would be estimated
based on same approach, by simple addition of maximum sedimentation rate
predicted by computational modelling under this Study and the maximum
sedimentation rate predicted among all WSRs under the EIA study for the CBL and
the TKO-LTT (20 g/m2/day predicted at CC2 and CC3 in wet
season). As shown in Appendix 8.2 of the
approved CBL EIA, the predicted level of SS elevation and sedimentation rate
would be highly localized within the Junk Bay.
The level of 0.5 mg/L of SS elevation and 20 g/m2/day are
both confined near the marine works area.
This means the approach proposed above using simple addition of maximum
SS elevation and sediment rate would overestimate the potential cumulative,
thus would be conservative. The maximum
SS elevation predicted under the two captioned EIAs are provided below in Table 6.25.
Table 6.25 Cumulative SS Elevation at
Selected WSRs, under Mitigated Scenario of this Project
WSR (ID) |
SS
Elevation (mg/L) |
Sediment
Deposition (g/m2/day) |
|||||||
Wet Season |
Dry Season |
Criteria |
Wet Season |
Dry Season |
|||||
Criteria |
Max |
Compliance
Time % |
Criteria |
Max |
Compliance
Time % |
Max |
Max |
||
Fish Culture Zone (Depth-averaged) |
|||||||||
Tung
Lung Chau (SR2) |
44.87 |
0.52 |
100.00% |
42.73 |
0.61 |
100.00% |
- |
- |
- |
Po Toi O (SR27) |
46.31 |
0.50 |
100.00% |
44.63 |
0.50 |
100.00% |
- |
- |
- |
Coral Communities (Bottom) |
|||||||||
Junk
Bay (SR20) |
1.74 |
0.51 |
100.00% |
2.61 |
0.50 |
100.00% |
100 |
20.13 |
20.00 |
Junk
Bay (SR21) |
2.77 |
0.60 |
100.00% |
2.48 |
0.63 |
100.00% |
100 |
22.35 |
21.63 |
Junk
Bay (SR22) |
2.77 |
0.64 |
100.00% |
2.48 |
0.54 |
100.00% |
100 |
21.17 |
20.41 |
Fat
Tong Chau (SR16) |
2.06 |
0.80 |
100.00% |
2.20 |
1.07 |
100.00% |
100 |
27.71 |
24.65 |
Tai Miu Wan (SR4) |
2.46 |
1.61 |
100.00% |
3.78 |
0.53 |
100.00% |
100 |
23.79 |
20.17 |
Cape
Collinson (SR10) |
2.46 |
0.50 |
100.00% |
3.78 |
0.50 |
100.00% |
100 |
20.02 |
20.02 |
Cape
Collinson (SR11) |
2.46 |
0.50 |
100.00% |
3.78 |
0.50 |
100.00% |
100 |
20.01 |
20.02 |
Cape
Collinson (SR12) |
2.46 |
0.50 |
100.00% |
3.78 |
0.50 |
100.00% |
100 |
20.01 |
20.02 |
Tung
Lung Chau (SR1) |
2.08 |
0.53 |
100.00% |
3.54 |
0.73 |
100.00% |
100 |
20.18 |
21.67 |
Tung
Lung Chau (SR3) |
2.46 |
0.55 |
100.00% |
3.78 |
0.86 |
100.00% |
100 |
20.47 |
23.72 |
Tung
Lung Chau (SR6) |
2.03 |
0.57 |
100.00% |
2.95 |
0.58 |
100.00% |
100 |
20.95 |
20.33 |
Tung
Lung Chau (SR7) |
2.03 |
0.50 |
100.00% |
2.95 |
0.50 |
100.00% |
100 |
20.00 |
20.06 |
Tung
Lung Chau (SR8) |
2.03 |
0.51 |
100.00% |
2.95 |
0.50 |
100.00% |
100 |
20.06 |
20.00 |
Tung
Lung Chau (SR9) |
2.08 |
0.52 |
100.00% |
3.54 |
0.52 |
100.00% |
100 |
20.06 |
20.07 |
Tai
Long Pai (SR32) |
2.08 |
0.51 |
100.00% |
3.54 |
0.54 |
100.00% |
100 |
20.04 |
20.18 |
D-W
(SR33) |
2.46 |
0.50 |
100.00% |
3.78 |
0.50 |
100.00% |
100 |
20.00 |
20.00 |
D-T
(SR34) |
2.46 |
0.50 |
100.00% |
3.78 |
0.50 |
100.00% |
100 |
20.00 |
20.00 |
D-L
(SR35) |
2.46 |
0.60 |
100.00% |
3.78 |
0.72 |
100.00% |
100 |
20.71 |
22.44 |
Kwun
Tsai (SR36) |
2.46 |
0.65 |
100.00% |
3.78 |
0.65 |
100.00% |
100 |
22.49 |
21.23 |
Tit
Cham Chau (SR37) |
2.46 |
1.13 |
100.00% |
3.78 |
1.43 |
100.00% |
100 |
24.09 |
23.32 |
Gazetted Bathing
Beaches (Depth-averaged) |
|||||||||
Big
Wave Bay (SR12) |
1.54 |
0.50 |
100.00% |
2.18 |
0.50 |
100.00% |
- |
- |
- |
Clear
Water Bay (SR29) |
1.11 |
0.50 |
100.00% |
1.61 |
0.50 |
100.00% |
- |
- |
- |
Clear
Water Bay (SR30) |
1.11 |
0.50 |
100.00% |
1.61 |
0.50 |
100.00% |
- |
- |
- |
Shek
O (SR15) |
1.66 |
0.50 |
100.00% |
2.15 |
0.50 |
100.00% |
- |
- |
- |
Rocky
Bay (SR14) |
1.66 |
0.50 |
100.00% |
2.15 |
0.50 |
100.00% |
- |
- |
- |
Coastal Protection Areas (Depth-averaged) |
|||||||||
Clear
Water Bay (SR28) |
1.11 |
0.50 |
100.00% |
1.61 |
0.50 |
100.00% |
- |
- |
- |
Clear
Water Bay (SR29) |
1.11 |
0.50 |
100.00% |
1.61 |
0.50 |
100.00% |
- |
- |
- |
Clear
Water Bay (SR30) |
1.11 |
0.50 |
100.00% |
1.61 |
0.50 |
100.00% |
- |
- |
- |
Clear
Water Bay (SR31) |
1.11 |
0.50 |
100.00% |
1.61 |
0.50 |
100.00% |
- |
- |
- |
Po Toi O (SR27) |
1.11 |
0.50 |
100.00% |
1.61 |
0.50 |
100.00% |
- |
- |
- |
Tai Miu Wan (SR4) |
1.54 |
0.73 |
100.00% |
2.18 |
0.51 |
100.00% |
- |
- |
- |
Tai Miu Wan (SR5) |
1.54 |
0.50 |
100.00% |
2.18 |
0.51 |
100.00% |
- |
- |
- |
Cape
Collinson (SR10) |
1.54 |
0.50 |
100.00% |
2.18 |
0.50 |
100.00% |
- |
- |
- |
Cape
Collinson (SR11) |
1.54 |
0.50 |
100.00% |
2.18 |
0.50 |
100.00% |
- |
- |
- |
Big
Wave Bay (SR13) |
1.66 |
0.50 |
100.00% |
2.15 |
0.50 |
100.00% |
- |
- |
- |
Tai
Tam (SR25) |
1.11 |
0.50 |
100.00% |
1.61 |
0.50 |
100.00% |
- |
- |
- |
Tai
Tam (SR26) |
1.11 |
0.50 |
100.00% |
1.61 |
0.50 |
100.00% |
- |
- |
- |
Seawater Intakes (Mid-depth) |
|||||||||
Tseung
Kwan O New Town (SR19) |
5.22 |
0.50 |
100.00% |
5.50 |
0.50 |
100.00% |
- |
- |
- |
Siu
Sai Wan (SR17) |
4.76 |
0.51 |
100.00% |
5.90 |
0.51 |
100.00% |
- |
- |
- |
Heng Fa Chuen (SR23) |
4.76 |
0.51 |
100.00% |
5.90 |
0.50 |
100.00% |
- |
- |
- |
Chai Wan Godown
(SR16) |
1.57 |
0.56 |
100.00% |
1.23 |
0.51 |
100.00% |
- |
- |
- |
Pamela Youde Nethersole Eastern Hospital (SR24) |
1.57 |
0.51 |
100.00% |
1.23 |
0.51 |
100.00% |
- |
- |
- |
As shown in Table 6.25, no exceedance in WQO assessment criteria at all
selected WSRs is predicted, with the consideration of cumulative impact from
the marine construction of CBL and TKO-LTT.
Other WSRs not presented in Table 6.25 are further away from the marine works of CBL and
TKO-LTT and are predicted to be even less affected.
In view of the above, it is considered the
potential cumulative water quality impacts from other marine construction works
within Study Area to be adequately addressed and no unacceptable cumulative
water quality impact would be expected.
It
is noted that there is no known major discharge from the projects considered
above. Road run-off would be expected
from the Cross Bay Link and the Lam Tin – Tseung Kwan
O Tunnel but appropriate collection system would be in place to avoid direct discharge
into marine waters. Cooling water system
could be found on the both sides of the Victoria Harbour
but cumulative impact is not expected in view of (1) there is no thermal
discharge / cooling water discharge from the Project and (2) the limited plume
extent for TRC and other constituents as shown in Annex 6E. Therefore, no
cumulative water quality impact due to other cooling water discharge would be
expected.
Marine water quality monitoring at selected WSRs is
recommended for marine dredging at submarine outfall and seawater intake under
this Project. Site audit would also be
conducted throughout the marine and land-based construction under this Project. In situ testing of TRC should also be conducted
for the discharge of chlorinated water for pipeline disinfection to ensure sufficient dechlorination. The marine
works contractor is required to complete a silt curtain efficiency test for the
combined use of floating silt curtain type and cage type silt curtain for
dredging at seawater intake to confirm the silt curtain reduction efficiency
assumptions of the assessment.
Details of the environmental monitoring procedures
and audit requirements are provided in the standalone EM&A manual.
To ensure
compliance to the effluent standard specified Table 6.23 (or other standard stipulated in the WPCO Discharge Permit), regular
monitoring of effluent quality is recommended during normal operation. Furthermore, marine water monitoring at
selected nearby WSRs during the first year of project commission are
recommended to ensure compliance to WQO or other water quality criteria. Details environmental monitoring procedures
and audit requirements are provided in the standalone EM&A manual.
Computational modelling has been conducted to
predict various potential water quality impacts from the proposed marine
dredging operation under this Project, including SS elevation, sedimentation,
DO depletion, release of nutrient, heavy metal and trace organic
pollutants. Full compliance is predicted
at all identified WSRs for all parameters in both seasons, with the exception
of exceedances in SS elevation predicted at four coral WSRs including SR4,
SR16, SR36 and SR37. With the implementation
of silt curtain around grab dredger and reduced dredging rate at seawater
intake, the predicted SS elevation at these WSRs would be significantly reduced
and would be in compliance to the WQO SS criteria. To ensure environmental compliance, marine
water monitoring for the marine dredging works is recommended.
Other potential water quality impacts from marine
and land-based construction were also addressed. Appropriate preventive and mitigation
measures are recommended to minimize the potential water quality impact from
these works. Environmental monitoring
and audit is recommended to ensure the proper implementation of these measures.
The discharge of RO concentrate from the
desalination process is the main environmental concern for the Project
operation. The potential change in
hydrodynamic and water quality from Project operation was assessed in detail
using a combination of near field (CORMIX) and far field (Delft3D) modelling
tools. The change in salinity, elevation
in SS (and sedimentation flux), anti-scalant, TIN and
Fe during normal operation, as well as the discharge of TRC and SMBS during
bio-growth control works, were all assessed using the near field and far field
modelling tools specified above. The
effluent plume of the RO concentration is predicted to be highly localized in
both seasons. The elevation in salinity
and other chemicals are predicted to be diluted soon after leaving the
submarine outfall. No observable mixing zone
is predicted for most discharge scenarios for most chemicals constituents. Where mixing zone is predicted, it would be
closely confined near the submarine outfall and would not encroach to any
nearby WSRs. No adverse water quality
impact from the discharge of RO concentrate would be expected. To ensure environmental compliance,
monitoring of discharge effluent quality and marine water quality at nearby
selected WSRs are recommended.
Other potential water quality impact from regular
chemical clean, sewage effluent and chemical spillage has been addressed. Appropriate measures are recommended to
minimize any potential water quality impact associated with the daily operation
of the Project. No adverse water quality
impact from the daily operation of the Project is expected.
([1]) ERM – Hong Kong, Ltd (2007) EIA for Development of a Bathing Beach at Lung Mei, Tai Po. For Civil Engineering Department, Hong Kong SAR Government.
([2]) ERM - HK Ltd (2010). Development of an Offshore Wind Farm in Hong Kong. Final Environmental Impact Assessment. For the Hong Kong Electric Company
([3]) Maunsell Consultant Asia Ltd, (2007). Wan Chai Development Phase II and Central-Wan Chai Bypass. EIA-141/2007.
([4]) ERM – Hong Kong, Ltd (2002) EIA for the Proposed Submarine Gas Pipeline from Cheng Tou Jiao Liquefied Natural Gas Receiving Terminal, Shenzhen to Tai Po Gas Production Plank, Hong Kong. Final EIA Report. For the Hong Kong and China Gas Co., Ltd.
([5]) Maunsell (2001) EIA for Tai Po Sewage Treatment Works - Stage V. Final EIA Report. For Drainage Services Department, Hong Kong SAR Government.
([6]) ERM - Hong Kong, Ltd (2007) Liquefied Natural Gas (LNG) Receiving Terminal and Associated Facilities. For CAPCO. Final EIA Report. December 2006
([7]) Maunsell (2002). EIA for Decommissioning of Cheoy Lee Shipyard at Penny's Bay. For Civil Engineering Department, Hong Kong SAR Government.
([8]) ERM – Hong Kong (1997). EIA for Disposal of Contaminated Mud in the East Sha Chau Marine Borrow Pit. For Civil Engineering Department, Hong Kong SAR Government.
([9]) Maunsell (2001). EIA for Wanchai Development Phase II - Comprehensive Feasibility Study. For Territory Development Department, Hong Kong SAR Government.
([11]) BMT Asia Pacific Ltd (2009). EIA for Hong Kong Offshore Wind Farm in Southeastern Waters. For HK Offshore Wind Limited
([12]) ERM - HK Ltd (2010). Development of an Offshore Wind Farm in Hong Kong. Final Environmental Impact Assessment. For the Hong Kong Electric Company
([13]) ENSR Asia (HK) Ltd (2008). Harbour Area Treatment Scheme Stage 2A EIA Study – Investigation. Environmental Impact Assessment Report.
([14]) AECOM Asia Co. Ltd (2009). Environmental Impact Assessment of Hong Kong Section of Guangzhou-Shenzhen-Hong Kong Express Rail Link. Environmental Impact Assessment Report.
(15) ERM - Hong Kong, Ltd (2006) EIA Study for Liquefied Natural Gas (LNG) Receiving Terminal and Associated Facilities. For CAPCO. Register No.: AEIAR-106/2007, http://www.epd.gov.hk/eia/register/report/eiareport/eia_1252006/html/index.htm
(16) ERM (2005). Detailed Site Selection Study for a Contaminated Mud Disposal Facility within the Airport East/East of Sha Chau Area. EIA and Final Site Selection Report. For CEDD. Approved on 1 September 2005. Register No.: AEIAR-089/2005, http://www.epd.gov.hk/eia/register/report/eiareport/eia_1062005/index.htm
(17) ERM (2000). Construction of an International Theme Park in Penny’s Bay of North Lantau together with its Essential Associated Infrastructures – Final EIA Report. For CEDD. Approved on 28 April 2000. Register No.: AEIAR-032/2000 http://www.epd.gov.hk/eia/register/report/eiareport/eia_0412000/index.html
(18) ERM (1997). EIA: Dredging an Area of Kellett Bank for Reprovisioning of Six Government Mooring Bays. Working Paper on Design Scenarios. For CEDD.
([19]) Since the actual dredging operation would be completed in a few day (i.e. shorter than a spring-neap cycle), mean values of SS elevation are cannot provide representative indication of overall SS impact at the WSRs.
([20]) Mott MacDonald (1991). Contaminated Spoil Management Study, Final Report, Volume 1, for EPD, October 1991.
([21]) Relative salinity elevation is calculated by: (Salinity with discharge – Salinity without discharge) / Salinity without discharge. The corresponding WQO salinity criteria would be 0.1.
([22]) Assuming first order decay (according to OCED Guideline 302b). Decay rate = 1-(1-23%)(1/28) = 0.9291%
[23] Maximum release rate for SS =
464000 m3/day ÷ 24 hr/day ÷ 3600 s/hr
×13 g/m3 = 69.8 g/s. For
comparison, the sediment loss rate assumed under the unmitigated scenario for
dredging operation is 3500 m3/day × 17 kg/m3 ÷ 12hr/day ÷
3600 s/hr = 1.3773 kg/s = 13773 g/s., which is nearly 20 times of the loading
from operation.
([25]) Water Supplies Department, Dongjiang Raw Water (http://www.wsd.gov.hk/en/water_resources/raw_water_sources/dongjiang_raw_water/index.html). Latest revision date: 25 Sep 2014.