Table
of Contents
5. WATER
QUALITY IMPACT.. 5-1
5.1 Introduction. 5-1
5.2 Environmental Legislation, Standards and Guidelines. 5-1
5.3 Description of the Environment 5-3
5.4 Identification of Water Sensitive Receivers. 5-5
5.5 Assessment Approach and Methodology. 5-5
5.6 Identification of Potential Impacts. 5-5
5.7 Prediction and Evaluation of Potential Impacts. 5-6
5.8 Cumulative Impact 5-7
5.9 Recommended Water Quality Mitigation Measures. 5-7
5.10 Evaluation of Residual Impacts. 5-10
5.11 Environmental Monitoring and Audit Requirements. 5-10
5.12 Conclusion. 5-10
LIST OF TABLES
Table 5-1 Summary of WQOs for Victoria Harbour (Phase One) WCZ.. 5-2
Table
5-2 WSD Standards at Flushing
Water Intakes. 5-3
Table
5-3 Marine Water Quality for
Victoria Harbour (Phase One) WCZ in 2012 at VM12, VM14 and VT8 5-4
LIST OF FIGURES
Figure 5.1
Water Control Zones and Water
Sensitive Receivers
General
5.2.1
The EIAO-TM specifies the
assessment method and criteria that are needed to be followed in the EIA
study. It also provides details of the
assessment criteria and guidelines that are relevant to the water quality impact
assessment, including:
·
Annex
6 Criteria for Evaluating Water Pollution; and
·
Annex
14 Guidelines for Assessment of Water Pollution.
5.2.2
Furthermore, the Water
Pollution Control Ordinance (Cap. 358) (WPCO) is also applicable to the water
quality impact assessment of this Project.
5.2.3
Other relevant guidelines
include:
·
Water
Supplies Department (WSD) Water Quality Criteria;
·
Technical
Memorandum on Standards for Effluents Discharged into Drainage and Sewerage
Systems, Inland and Coastal Waters (TM-DSS); and
·
Practice
Note for Professional Persons on Construction Site Drainage (ProPECC PN 1/94).
Water Pollution Control Ordinance
5.2.4
The WPCO is the principal
legislation to protect and control the water quality in Hong Kong. Under the ordinance, Hong Kong waters are classified
into 10 Water Control Zones (WCZs), in which the corresponding statements of
Water Quality Objectives (WQOs) are stipulated.
The Project area is adjacent to the Victoria Harbour (Phase One) WCZ as
indicated in Figure 5.1 and the corresponding WQOs are summarised in Table 5-1.
Table 5-1 Summary of WQOs for
Victoria Harbour (Phase One) WCZ
|
Parameter
|
Water Quality Objective
|
Sub-Zone
|
|
Offensive Odour, tints
|
Not to be present
|
Whole zone
|
|
Visible Foam, Oil Scum, Litter
|
Not to be present
|
Whole zone
|
|
E. coli
|
Not to exceed 1,000 per 100mL, calculated
as the geometric mean of the most recent 5 consecutive samples taken at
intervals between 7 and 21 days
|
Inland waters
|
|
Colour
|
Change due to human activity not to exceed 50 Hazen units
|
Inland waters
|
|
Depth-Averaged Dissolved
Oxygen (DO)
|
Not less than 4.0mg/L for
90% of samples at a year
|
Marine waters
|
|
DO within 2m of the
Seabed
|
Not less than 2.0mg/L for
90% of samples at a year
|
Marine waters
|
|
DO
|
Not less than 4.0mg/L
|
Inland
waters
|
|
pH
|
To be in the range of 6.5 to 8.5, change
due to human activity not to exceed 0.2
|
Marine waters
|
|
Not to exceed the range of 6.0 to 9.0 due
to human activity
|
Inland
waters
|
|
Temperature
|
Change due to human
activity not to exceed 2.0°C
|
Whole zone
|
|
Salinity
|
Change due to human
activity not to exceed 10%
|
Whole zone
|
|
Suspended Solids (SS)
|
Not to raise the ambient level by 30% due to caused by human activity and shall not affect aquatic communities
|
Marine waters
|
|
|
Annual median not to exceed 25mg/L due to
human activity
|
Inland waters
|
|
Un-ionised
Ammonia (UIA)
|
Annual mean not to exceed
0.021mg/L as un-ionised form
|
Whole zone
|
|
Nutrients
|
Shall not cause excessive
algal growth
|
Marine waters
|
|
Total Inorganic Nitrogen
(TIN)
|
Annual mean depth-averaged
inorganic nitrogen not to exceed 0.4mg/L
|
Marine waters
|
|
5-day Biochemical
Oxygen Demand (BOD5)
|
Not to exceed 5mg/L
|
Inland waters
|
|
Chemical Oxygen
Demand (COD)
|
Not to exceed 30mg/L
|
Inland waters
|
|
Toxic
substances
|
Should not attain such
levels as to produce significant toxic effects in humans, fish or any other
aquatic organisms
|
Whole
zone
|
|
Human
activity should not cause a risk to any beneficial
use of the aquatic environment
|
Whole
zone
|
Water Supplies Department (WSD) Water Quality Criteria
5.2.5
Besides the WQOs set under the
WPCO, the WSD specifies a set of water quality objectives at flushing water
intakes as shown in Table 5-2.
Table 5-2 WSD Standards at
Flushing Water Intakes
|
Parameter (in
mg/l unless otherwise stated)
|
WSD Target Limit
|
|
Colour (Hazen Unit)
|
< 20
|
|
Turbidity (NTU)
|
< 10
|
|
Threshold Odour Number (odour unit)
|
< 100
|
|
Ammoniacal Nitrogen (NH3-N)
|
< 1
|
|
Suspended Solids
|
< 10
|
|
Dissolved Oxygen
|
> 2
|
|
Biochemical Oxygen Demand
|
< 10
|
|
Synthetic Detergents
|
< 5
|
|
E.coli (no. per 100 ml)
|
< 20,000
|
Technical Memorandum on Standards for Effluents Discharged
into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS)
5.2.6
Besides setting the WQOs, the
WPCO controls effluent discharge into the WCZs through a licensing system. The TM-DSS provides guidance on permissible
effluent discharges based on the type of receiving waters (foul sewers, inland
/ coastal / inshore / marine waters).
The limits control the physical, chemical and microbial quality of
effluents. Any sewage from the proposed
construction and operation activities should comply with the standards for
effluents discharged into the inshore waters of the Victoria Harbour (Phase
One) WCZ.
ProPECC Notes
5.2.7
The Practice Note for
Professional Persons on Construction Site Drainage (ProPECC
PN 1/94) was issued by the EPD to provide environmental guidelines for handling
and disposal of construction site discharges.
It provides good practice guidelines for dealing with various types of
discharge from a construction site.
Practices given in the ProPECC PN 1/94 should
be followed during the construction phase in order to minimise the water
quality impact due to construction site drainage.
5.3.1
According to the “Marine Water
Quality in Hong Kong” in 2012, which is the latest available information from
EPD at the moment of preparing this Report, after the commissioning of the Stonecutters Island Sewage Treatment
Works (SCISTW) under the Harbour Area Treatment
Scheme (HATS) Stage 1 in 2002, about 75% of the sewage around Victoria Harbour now receives chemically enhanced primary treatment,
resulting in a 70% reduction of the pollution load (in terms of organic
pollutants) into the harbour. In the Victoria Harbour WCZ, the 2012 WQOs compliance rate was 77%
as compared with 50% in 2011. Victoria Harbour Water
Control Zone had a higher overall compliance rate in which the improvement in
2012 was mainly due to high compliance rates with the Dissolved Oxygen (DO)
objective. Compliance with the Total Inorganic
Nitrogen (TIN) objective in 2012 remained at 30% as in 2011 and non-compliance
are mainly observed at stations in the central and western parts of the Harbour. The overall annual average level of TIN in
Victoria Harbour increased could be due to a higher
background TIN level under the influence of Pearl River discharge, the
year-to-year normal range of fluctuation of the discharge from the Tolo Harbour Effluent Export
Scheme and surface run-off, as well as the gradual increase in effluent
discharged from the four preliminary treatment plants located in Hong Kong Island during the period.
5.3.2
The representative EPD’s marine
water quality monitoring stations within the Victoria Harbour (Phase One) WCZ
are in vicinity of the Project area, including VM12 (Rambler Channel (South)),
VM14 (Rambler Channel (North)) and VT8 (Rambler Channel for Typhoon
Shelter). The location of these stations
is shown in Figure 5.1 and the recent monitoring data are summarised in Table 5-3. Further to description of
marine water environment in Section 5.3.1, the water quality of Victoria Harbour has been on an improving
trend during the last decade after implementation of the HATS Stage 1 at the
end of 2001, in particular shown by consistent decreasing patterns of ammoniacal-nitrogen and BOD5.
Table 5-3 Marine Water Quality
Statistics for the Victoria Harbour (Phase One) WCZ in 2012 at VM12, VM14 and
VT8
|
Parameter
|
EPD’s Monitoring Station
|
|
VM12
|
VM14
|
VT8
|
|
Temperature (°C)
|
23.3
(15.9 - 28.3)
|
23.5
(16.0 – 28.5)
|
23.8
(17.4 – 28.6)
|
|
Salinity (ppt)
|
30.0
(26.0 – 32.6)
|
28.9
(21.2 – 32.5)
|
27.6
(18.2 – 32.0)
|
|
Dissolved Oxygen (mg/L)
|
6.0
(3.8 – 8.4)
|
6.2
(3.6 – 8.1)
|
6.3
(4.9 – 7.4)
|
|
Bottom Dissolved Oxygen
(mg/L)
|
6.0
(3.5 – 8.4)
|
6.2
(3.3 – 8.1)
|
6.1
(4.2 – 7.3)
|
|
BOD5 (mg/L)
|
0.4
(<0.1 – 1.0)
|
0.4
(<0.1 – 1.0)
|
0.6
(0.3 – 1.0)
|
|
SS (mg/L)
|
8.3
(2.5 – 16.0)
|
5.8
(2.1 – 13.3)
|
8.2
(3.0 – 16.0)
|
|
TIN (mg/L)
|
0.51
(0.34 – 0.85)
|
0.56
(0.32 – 1.08)
|
0.69
(0.31 – 1.15)
|
|
NH3-N (mg/L)
|
0.190
(0.113 – 0.247)
|
0.159
(0.075 – 0.273)
|
0.155
(0.047 – 0.270)
|
|
Un-ionised
Ammonia (mg/L)
|
0.004
(0.002 – 0.007)
|
0.003
(0.001 – 0.006)
|
0.003
(0.001 – 0.005)
|
|
Chlorophyll-a (µg/L)
|
1.3
(0.4 – 3.5)
|
1.4
(0.4 – 4.2)
|
3.4
(0.8 – 9.0)
|
|
E.coli (count/100mL)
|
410
(75 – 2,100)
|
360
(51 – 5,200)
|
710
(430 – 890)
|
Notes:
[1] Data
presented are depth averaged (except as specified) and annual arithmetic means
except for E. coli (geometric
mean).
[2] Data in brackets indicate ranges.
[3] Underlined indicates occurrence of
non-compliance with that parameter of WQOs.
5.3.3
To further improve the water
quality of Victoria Harbour, the construction of HATS Stage 2A has been proceeded with a view to commissioning by the end of
2014. Upon commissioning, the sewage
tunnels will collect the remaining 25% of the sewage currently generated daily
from Hong Kong Island region, and convey the sewage to the SCISTW for
treatment. Therefore, it is anticipated
that the marine water environment would be further improved.
5.4.1
To evaluate the potential water
quality impacts from the Project, areas within 300m from the proposed Project site, and the adjacent water sensitive receivers (WSRs) within the Victoria
Harbour (Phase One) WCZ are considered. No inland
watercourse such as river or natural stream located within 300m from the Project site. The WSRs within the
Victoria Harbour (Phase One) WCZ are identified as below and shown in Figure 5.1:-
·
F1 – Kwai Chung Hospital Flushing Water Intake
·
C1 – Kwai Chung Hospital Cooling Water Intake
· C2 –
Tsuen Wan Cooling Water Intake
·
TS1 – Rambler Channel Typhoon Shelter
5.5.1
The criteria and guidelines for
assessing water quality impacts as stated in Annexes 6 and 14 of the EIAO-TM
have been followed.
5.5.2
The specific construction
methods and operational activities of the Project have been reviewed and the
potential pollution sources including pollutants from point discharges and
non-point sources to surface water run-off, sewage from workforce and polluted
discharge generated from the Project have been identified.
5.5.3
The identified pollution
sources have been evaluated to determine the significance of impact to the
adjacent water system and their representative sensitive receivers.
5.5.4
The potential cumulative
impacts due to other related concurrent and planned projects activities or
pollution sources within the assessment area have been assessed and mitigation
measures proposed where required to control any water
quality impacts to acceptable levels.
Construction Phase
5.6.1
The
Project would involve construction of flyover as described in Section 2. Since no marine construction works
are required, potential water pollution sources during the construction phase
would be those generated from the land-based works activities including foundation
piling, footings, columns, bridge decks, utilities work, general cleaning,
wheel washing, dust suppression, etc.
·
Construction site run-off during foundation piling;
·
Accidental chemical spillage; and
·
Sewage effluent produced by on-site workforce.
Construction
Site Run-off
5.6.3
Construction site run-off may
contain increased loads of sediments, other suspended solids and
contaminants. Release of uncontrolled
site run-off would increase the suspended solid levels and turbidity in the
nearby water environment.
5.6.4
Potential pollution sources of
site run-off comprise:
· Run-off
and erosion from bare soil and earth, drainage channels, earth working areas
and stockpiles;
· Wastewater
from dust suppression sprays and wheel washing facilities at site entrances;
· Fuel,
oil, solvents and lubricants from maintenance of construction machinery and
equipment.
5.6.5
With a
construction site area of approximately 23,500m2 and return period
of 50 years, the estimated peak hour flow would be 0.59m3/s for
rainfall duration of 3 minutes. Effluents including any
site runoff into the drainage channels, are controlled
to comply with the WPCO.
Accidental
Chemical Spillage
5.6.6
The
on-site general construction activities may cause contamination of the surface
soils due to accidental spillage of chemicals used in construction works, which
includes spent lubrication oil, diesel and solvents, etc. The contaminated soil particles may be washed
away by construction site run-off or stormwater
drainage which in turn causes water pollution.
Sewage
Effluent
5.6.7
Potential impacts may arise
from wastewater generated from eating areas, temporary sanitary facilities and waste disposal areas
provided for the on-site construction workforce. The characteristics of the wastewater
may include high levels of organics (ie. BOD5), ammonia and E. coli. Assuming 150
workers on site with a daily sewage generation of 0.06 m3 per capita
in accordance with Sewerage Manual by Drainage Services Department (DSD), the
estimated sewage would be 9m3/day.
Operation Phase
Construction Phase
5.7.1
Potential
sources of water quality impacts associated with the construction phase of the
Project include:
· Construction
site run-off;
· General
construction activities and accidental spillage of chemicals; and
· Sewage
generated from on-site construction workers.
Construction
Site Run-off
5.7.2
Site run-off and
drainage from the Site may contain suspended solids and other contaminants. Potential sources of water pollution from
site run-off would comprise:
·
Run-off from exposed bare soil and earth, drainage
channels and stockpiles;
·
Release of grouting and cement materials with rain
wash;
·
Wash water from dust suppression sprays and vehicle
wheel washing; and
·
Fuel, oil and lubricant from maintenance of
construction vehicles and mechanical equipment.
5.7.3
Perimeter drains could be
applied along the Site boundary to collect site run-off and also intercept
run-off from outside.
5.7.4
Mitigation measures of good
site practice shall be implemented to control construction site
run-off, and to minimise the chance of introducing silt and other pollutants
into the storm water drainage system. Silt removal facilities shall be provided
and discharge license under WPCO shall be obtained prior to any discharge.
5.7.5
Since all construction works
would be land-based in nature, with the implementation of adequate site
drainage and provision of silt removal facilities, as well as control under
WPCO, no unacceptable water quality impacts to WSRs are anticipated.
General
Construction Activities and Accidental Spillage of Chemicals
5.7.6
General construction activities
have the potential to cause water pollution as a result of stockpiling,
accumulation of debris and rubbish, concrete dust, etc. Spillage of chemicals, such as oil and diesel
from construction plant and equipment and paints, could also result in water
quality impacts.
5.7.7
Nevertheless, it is considered
that the impact of these activities to WSRs will be minimal provided that the
site boundaries are well maintained with the aforesaid perimeter drains. Good construction and site management
practices, such as sediment barriers, site drainage and waste disposal, will also
limit the sediment and pollutants to acceptable levels.
Sewage
Generated from On-Site Construction Workers
5.7.8
Sewage is characterised by high
levels of BOD, ammonia and E.coli. Water quality impacts from sewage
generated by the on-site workforce will be avoided if adequate sewage
collection and disposal facilities, such as portable chemical toilets, are
properly installed and maintained.
Operation
Phase
5.7.9
The
road surface runoff will be discharged to decked nullah
leading to Rambler Channel via public storm drain. As incorporated in the design, the runoff will be
collected by surface water drainage system and discharged to storm
drains. The surface
runoff can also be controlled by best management practice, etc
(e.g. properly designed silt traps with appropriate spacing, sufficient
cleaning frequency for silt traps and road gullies, etc).
With adequate capacity
to cater for treating all surface water and best management practice, it is envisaged that the water
quality impact from the land-based operation will be minimal and further
mitigation measure would be not be required.
5.8.1
There are no major concurrent
projects nearby and therefore it is envisaged that the cumulative impact is
minimal.
Construction
5.9.1
General good site practice as
described in the following is to be implemented as measures to minimise
potential water quality impact during construction phase.
Construction
Site Run-off and General Construction Activities
5.9.2
In accordance with the ProPECC PN 1/94, construction
phase mitigation measures should include the following:
·
At the establishment of works site, perimeter
drains to direct off-site water around the Site should be constructed with
internal drainage works and erosion and sedimentation control facilities
implemented. Channels
(both temporary and permanent drainage pipes and culverts), earth bunds or sand
bag barriers should be provided to divert the stormwater
to silt removal facilities. The design
of the temporary on-site drainage system will be undertaken by the Contractor
prior to the commencement of construction;
·
Dikes or embankments for flood protection should be
implemented around the boundaries of earthwork areas. Temporary ditches should be provided to
facilitate the run-off discharge into an appropriate watercourse, through a
silt / sediment trap. Silt / sediment traps should also be incorporated in the
permanent drainage channels to enhance deposition rates;
·
The design of efficient silt removal facilities
should be based on the guidelines in Appendix A1 of ProPECC
PN 1/94, which states that the retention time for silt / sand traps should be 5
minutes under maximum flow conditions. A
sedimentation basin would be required when necessary. The detailed design of
the silt / sand traps should be undertaken by the Contractor prior to the
commencement of construction;
·
The construction works should be programmed to
minimise surface excavation works during rainy seasons (April to September), as
possible. All exposed earth areas should
be completed and vegetated as soon as possible after the earthworks have been
completed. If excavation of soil cannot
be avoided during the rainy season, or at any time of year when rainstorms are
likely, exposed slope surfaces should be covered by tarpaulin or other means;
·
The overall slope of works sites should be kept to
a minimum to reduce the erosive potential of surface water flows, and all
trafficked areas and access roads should be protected by coarse stone
ballast. An additional advantage
accruing from the use of crushed stone is the positive traction gained during
the prolonged periods of inclement weather and the reduction of surface sheet
flows;
·
All drainage facilities and erosion and sediment
control structures should be regularly inspected and maintained to ensure their
proper and efficient operation at all times particularly following
rainstorms. Deposited silts and grits
should be removed regularly and disposed of by spreading evenly over stable,
vegetated areas;
·
Measures should be taken to minimise the ingress of
site drainage into excavations. If the
excavation of trenches in wet season is inevitable, they should be dug and
backfilled in short sections wherever practicable. The water pumped out from trenches or
foundation excavations should be discharged into storm drains via silt removal
facilities;
·
All open stockpiles of construction materials (for
example, aggregates, sand and fill material) should be covered with tarpaulin
or similar fabric during rainstorms.
Measures should be taken to prevent the washing away of construction
materials, soil, silt or debris into any drainage system;
·
Manholes (including newly constructed ones) should
always be adequately covered and temporarily sealed so as to prevent silt,
construction materials or debris being washed into the drainage system and
storm run-off being directed into foul sewers;
·
Precautions to be taken at any time of the year
when rainstorms are likely, actions to be taken when a rainstorm is imminent or
forecasted and during or after rainstorms, are summarised in Appendix A2 of ProPECC PN 1/94.
Particular attention should be paid to the control of silty surface
run-off during rainstorm events;
·
All vehicles and plant should be cleaned before
leaving the Site to ensure no earth, mud, debris and the like is deposited by
them on roads. An adequately designed
and sited wheel washing facilities / bay should be provided at the exit of the
Site where practicable. Wash-water
should have sand and silt settled out and removed at least on a weekly basis to
ensure the continued efficiency of the process.
The section of access road leading to, and exiting from, the
wheel-washing bay to public roads should be paved with sufficient backfall toward the wheel-washing bay to prevent vehicle
tracking of soil and silty water to public roads and drains;
·
Oil interceptors should be provided in the drainage
system downstream of any oil / fuel pollution sources. Oil interceptors should be emptied and
cleaned regularly to prevent the release of oil and grease into the storm water
drainage system after accidental spillage.
A bypass should be provided for oil interceptors to prevent flushing
during heavy rain;
·
The construction solid waste, debris and rubbish
on-site should be collected, handled and disposed of properly to avoid causing
any water quality impacts. The
requirements for solid waste management are detailed in Section 6 of this EIA
report; and
·
All fuel tanks and storage areas should be provided
with locks and sited on sealed areas, within bunds with adequate storage
capacity to prevent spilled fuel oils.
5.9.3
Site drainage will be
implemented according to the requirement and findings from Drainage Impact
Assessment which has been submitted to and approved by DSD.
5.9.4
By adopting the above
mitigation measures with good management practices, it is anticipated that the
impacts of construction site run-off will be reduced to an acceptable level.
Effluent
Discharge
Sewage Generated from On-Site
Construction Workers
Accidental Spillage of Chemicals
5.9.8
Any maintenance facilities
should be located on hard standings within a bunded area, and sumps and oil
interceptors should be provided. Maintenance of vehicles and equipment involving activities with
potential for leakage and spillage should only be undertaken within the areas
appropriately equipped to control these discharges.
Operation
5.9.9
The
surface runoff will be controlled by best management practice. Surface runoff collection system with silt traps and road gullies should be provided. The road should be designed to direct all surface runoff
to the system. The capacity should be properly
designed to cater for all surface water. Drainage will
be implemented in accordance with the requirement and findings from Drainage
Impact Assessment which has been submitted to and approved by DSD.
5.9.10
The
system should be properly maintained and cleaned regularly to ensure good
service condition
in which Highways
Department will be responsible for the maintenance of road drains.
5.10.1
Adverse residual impacts during
the construction and operation phases of this Project would not be anticipated,
provided that the above mitigation measures are implemented.
5.11.1
With the proper implementation
of recommended mitigation measures, the adverse impact during construction and
operation of the Project would be insignificant and hence environmental
monitoring for water quality is not considered necessary but regular site
inspection should be conducted at the construction and work area in order to ensure
the mitigation measures are adequately implemented.
5.12.1
Potential water pollution
sources have been identified as construction site run-off, sewage from
workforce, and potential risk of chemical spillage. Adverse residual impacts would not be anticipated with the
implementation of adequate mitigation measures including the construction site
practices in accordance with the EPD’s ProPECC PN 1/94
Construction Site Drainage, provision and management of portable toilets
on-site, and preventive measures to avoid accidental chemical spillages. Furthermore, implications arising from the surface runoff during the operation
phase would be insignificant. Adverse water quality impact would not be
anticipated with the implementation of the recommended mitigation measures
based on the findings of this EIA study.