5.1.1
This section
presents the potential water quality impacts associated with the construction
and operation phases of the Project.
The key environmental issue in respect of water quality impacts
associated with the Project would be proper control of construction site runoff
and drainage during road works and foundation works for proposed noise
barriers. Recommendations for
mitigation measures have been made, where necessary, to reduce the identified
water quality impacts to an acceptable level. The construction works would be land-based and thus direct
impacts on marine water quality would not arise. The road works, slope works and construction of noise
barriers would not be located within Tuen Mun River Channel.
Water Pollution Control Ordinance (WPCO)
5.2.1
The Water
Pollution Control Ordinance (Cap. 358), in existence since 1980, is the major
legislation relating to the protection and control of water quality in Hong
Kong. According to the
Ordinance and its subsidiary legislation, Hong Kong
waters are divided into ten water control zones (WCZ). The study area for the Project is
located within the Northern Western WCZ.
Technical Memorandum
5.2.2
Besides
setting the WQOs, the WPCO controls effluent discharging into the WCZs through
a licensing system. A Technical Memorandum (TM) on Standards for
Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal
Waters was issued under the WPCO which gives guidance on the permissible
effluent discharges based on the type of receiving waters (foul sewers, storm
water drains, inland and coastal waters). The limits control the physical,
chemical and microbial quality of effluents. Sewage from the proposed construction activities should
comply with the TM standards for effluent discharged into the inshore waters of
North Western WCZ (Table 10a
of the TM).
Table 5.1 Water
Quality Objectives Statement for North Western WCZ
|
Parameter
|
Objective
|
Part(s) of
Zone
|
|
Aesthetic
Appearance
|
(a) discharge
shall not cause objectionable odour or discolouration
(b) no tarry
residue, floating wood, articles made of grass, plastic, rubber or any other substance
(c) Mineral oil
not visible on the surface. Surfactants shall not give rise to a lasting
foam.
(d) no
recognizable sewage-derived debris
(e) no floating,
submerged or semi-submerged subjects likely to interfere with the free
movement of, or cause damage to vessels
(f) not to
contain substances which settle to form objectionable deposits
|
Whole Zone
Whole Zone
|
|
E. coli
|
(a) annual
geometric mean not to exceed 610/100 mL
(b) geometric
mean not to exceed 180/100 mL during March to October inclusive in 1 year;
sample should be taken at least 3 times in 1 calendar month at intervals of
between 3 to 14 days
(c) running
median of the most recent 5 consecutive samples taken at intervals of between
7 and 21 days to be less than 1/100 mL
(d) running
median of the most recent 5 consecutive samples taken at intervals of between
7 and 21 days not to exceed 1000/100 mL
|
secondary contact recreation subzones
Bathing beach subzones
Tuen Mun (A) and Tuen
Mun (B)
Subzones and Water
Gathering Ground
Subzones
Tuen Mun (C)
and other Inland Waters
|
|
Colour
|
Not to exceed
30 Hazen units
Not to exceed
50 Hazen units
|
Tuen Mun (A)
and Tuen Mun
(B) Subzones
and Water Gathering
Ground Subzones
Tuen Mun (C) and other Inland Waters
|
|
Dissolved Oxygen within 2
m of bottom
|
not less than
2 mg/L for 90% samples
|
marine waters
|
|
Depth averaged Dissolved Oxygen
|
not less than
4 mg/L for 90% samples
not less than 4mg/L at any point within the
water column
|
marine waters
Tuen Mun (A) and Tuen
Mun (B) Tuen
Mun (C) Subzones,
Water Gathering
Ground Subzones
and other Inland
waters
|
|
pH value
|
within the range 6.5 to 8.5; change due to
waste discharge not to exceed 0.2
within the
range 6.5 - 8.5
within the
range 6.0 - 9.0
within the range 6.0 - 9.0 for 95% of samples;
change due to waste discharge not to extend by 0.5
|
marine waters except
bathing beach subzones
Tuen Mun (A) and Tuen
Mun (B) Tuen
Mun (C) Subzones
and water gathering ground subzones
other inland waters
Beach Bathing Subzones
|
|
Salinity
|
change due to waste discharge not to exceed
10% of natural ambient level
|
whole zone
|
|
Temperature
|
change due to waste discharge not to exceed
the natural daily temperature by 2oC
|
whole zone
|
|
Suspended solids
|
waste discharge not to raise the natural
ambient level by 30%, nor cause the accumulation of suspended solids which
may adversely affect aquatic communities
annual median
not to exceed 20 mg/L
annual median
not to exceed 25 mg/L
|
marine waters
Tuen Mun (A) and Tuen
Mun (B) Tuen
Mun (C) Subzones
and water gathering ground subzones
|
|
5-Day Biochemical Oxygen Demand
|
not to exceed
3 mg/L
not to exceed
5 mg/L
|
Tuen Mun (A) and Tuen
Mun (B) Tuen
Mun (C) Subzones
and water gathering ground subzones
other inland waters
|
|
Chemical Oxygen Demand
|
not to exceed
15 mg/L
not to exceed
30 mg/L
|
Tuen Mun (A) and Tuen
Mun (B) Tuen
Mun (C) Subzones
and water gathering ground subzones
|
|
Un-ionized ammonia
|
annual mean
not to exceed 0.021 mg/L
|
whole zone
|
|
Nutrients
|
not to be present in quantities that cause
excessive or nuisance growth of
algae or other aquatic plants
annual mean depth average inorganic nitrogen
not to exceed 0.3 mg/L
annual mean depth average inorganic nitrogen
not to exceed 0.5 mg/L
|
marine waters
Castle Peak
Bay Subzone
marine waters except Castle
Peak Bay Subzone
|
|
Toxins
|
not to be present at levels producing
significant toxic effect, carcinogenic, mutagenic or teratogenic 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
not to cause a risk to any beneficial use of
the aquatic environment
|
whole zone
whole zone
|
|
Phenol
|
not to produce a specific odour or
concentrations greater than 0.05 mg/L as C6H5 OH
|
|
|
Turbidity
|
Waste discharges shall not reduce light
transmission substantially from the normal level
|
|
Practice Notes
5.2.3
A practice
note (PN) for professional persons was issued by the EPD to provide
environmental guidelines for handling and disposal of construction site
discharges. The ProPECC PN 1/94 “Construction Site Drainage” provides
good practice guidelines for dealing with various types of discharge from a
construction site. Practices
outlined in the PN should be followed as far as possible during construction to
minimize the water quality impact due to construction site drainage.
5.3.1
The Project
work site would be located at the northern part of Town Centre Section of TMR
near the Tuen Mun River Channel.
In accordance with the “2006 River
Water Quality Monitoring in Hong Kong”, the water quality at lower part of
the Tuen Mun
River which is located
within the Study Area had a ‘Good’ WQI.
The river quality monitoring stations in the Tuen
Mun River
closer to the Project are TN4 and TN5 (Figure
5.1) and the river water quality monitoring data in 2006 are summarized in Table 5.2.
Table 5.2 Water
Quality in Tuen Mun
River in 2006
|
Parameter
|
Location
|
|
|
TN4
|
TN5
|
|
pH (pH units)
|
7.6
|
7.6
|
|
Dissolved Oxygen
|
5.5
|
5.0
|
|
BOD5
|
4
|
4
|
|
Suspended Solids
|
10
|
7
|
|
Ammonia-nitrogen
|
0.38
|
0.57
|
|
Nitrate-nitrogen
|
0.36
|
0.35
|
|
Total Kjeldahl Nitrogen, SP
|
0.83
|
0.99
|
|
Total Phosphorous
|
0.13
|
0.13
|
|
E. Coli (cfu/100ml)
|
20,000
|
28,000
|
Note: All values in mg/L unless
otherwise indicated.
5.4.1
Water quality
sensitive receivers are the existing surface channels within the works areas
and Tuen Mun River Channel.
5.5.1
The assessment area for the water quality impact assessment is defined
by a distance of 500m
from the Project boundary and up to 500m
along the Tuen Mun River Channel from the boundary of the Project.
5.5.2
The
water sensitive receivers that may be affected by the construction works were
identified. Potential sources of water quality impact that may arise
during the construction and operation phases of the Project were
described. This task included identifying pollutants from point
discharges and non-point sources to surface run-off. All the identified
sources of potential water quality impact were then evaluated and their impacts
were determined. The need for mitigation measures to reduce any
identified adverse impacts on water quality to acceptable levels was
determined.
Construction Phase
5.6.1
Potential
sources of water quality impact associated with the construction works were
identified and included:
·
site runoff
and drainage;
·
general
construction activities; and
·
sewage
effluent produced by on-site workforce.
Site
Runoff and Drainage
5.6.2
Runoff from
the construction site may contain increased loads of sediments, other suspended
solids and contaminants. Potential
sources of pollution from site drainage would include:
·
runoff and
erosion from exposed soil surfaces and stockpiles;
·
release of
cement materials with rain wash;
·
wash water
from dust suppression sprays; and
·
fuel and
lubricants from maintenance of construction vehicles and mechanical equipment.
5.6.3
Sediment
laden runoff may carry pollutants (adsorbed onto the particle surfaces) into
the receiving waters. Associated
effects which may arise include blockage of storm water drainage channels and
increased suspended solids concentrations in receiving waters.
5.6.4
Mitigation
measures should be implemented to control construction site runoff and
drainage, and to prevent runoff and drainage water with suspended solids from
entering the local storm water drainage system. With the implementation of adequate construction site
drainage and the provision of sediment removal facilities, it was anticipated
that unacceptable water quality impacts would not arise.
General Construction Activities
5.6.5
On-site
construction activities may cause water pollution from the following:
·
uncontrolled
discharge of debris and rubbish such as packaging, construction materials and
refuse; and
·
spillages
of liquids stored on-site, such as oil, diesel and solvents etc, are likely to
result in water quality impacts if they enter storm water drains.
5.6.6
Good construction
and site management practices would be required to ensure that litter, fuels
and solvents do not enter nearby storm water drains.
Sewage Effluents
5.6.7
Domestic
sewage would be generated from the workforce during the construction phase.
However, this temporary sewage would be adequately treated by interim sewage
treatment facilities, such as portable chemical toilets, which would be
installed within the construction site. It is unlikely that sewage generated
from the site would have significant water quality impact, provided that sewage
is not discharged directly to the WCZs, and chemical toilets are used and
properly maintained.
Operational Phase
5.6.8
The potential
water quality impacts during the operation phase would be discharges of surface
runoff from the road during periods of rain. The surface runoff may contain minimal amounts of oil,
grease and grit that may cause water quality impacts to the stream if
uncontrolled. Surface water on the
road would be collected by gullies that would trap silt in the surface water
runoff. The runoff would be
discharged into the existing local stormwater drainage system and is considered
unlikely to produce any quantifiable adverse effects on WCZs. There are no ecologically sensitive
receivers or particularly sensitive aquatic receivers located in the vicinity
of the work area and therefore the impact associated with the discharge of
surface runoff would not be anticipated to be adverse.
5.7.1
Proposed
mitigation measures for minimizing water quality impacts are summarized below.
Construction Phase
Measures
for Tuen Mun
River Channel
5.7.2
Mitigation
measures would be implemented to minimize the direct impacts to the nearby Tuen
Mun River Channel by controlling construction site runoff and drainage from the
proposed works area, thus, prevent any runoff and drainage water with suspended
solids from entering the River Channel. Site runoff would be directed towards
regularly cleaned and maintained sand traps, silt traps and where appropriate,
oil/grease separators to minimize risk of sedimentation and pollution to the
River Channel. Debris and rubbish generated on-site would be collected, handled
and disposed of properly. The
stockpile or temporary storage area and chemical waste storage area shall be
located at least 30m
away from Tuen Mun River Channel.
Construction run-off and Drainage
5.7.3
The site
practices outlined in ProPECC PN 1/94 “Construction Site Drainage” shall be
followed as far as practicable in order to minimize surface runoff and the
chance of erosion, and also to retain and reduce any suspended solids prior to
discharge. These practices
include, inter alia, the following
items:
·
Provision
of perimeter channels to intercept storm-runoff from outside the site. These
shall be constructed in advance of site formation works and earthworks.
·
Sand/silt
removal facilities such as sand/silt traps and or sediment basins shall be
provided to remove sand/silt particles from runoff to meet the requirements of
the Technical Memorandum standard under the Water Pollution Control
Ordinance. The design of silt
removal facilities should be based on the guidelines provided in ProPECC PN
1/94. All drainage facilities and
erosion and sediment control structures should be inspected monthly and
maintained to ensure proper and efficient operation at all times and
particularly during rainstorms.
·
Water
pumped out from foundation excavations should be discharged into silt removal
facilities.
·
Careful
programming of the works to minimise surface excavations during the rainy
season. 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 a tarpaulin
or other means. Other measures
that need to be implemented before, during, and after rainstorms are summarized
in ProPECC PN 1/94.
·
Exposed
soil surface shall be protected by paving as soon as possible to reduce the
potential of soil erosion.
·
Open
stockpiles of construction materials on site shall be covered with tarpaulin or
similar fabric during rainstorms.
General
Construction Activities
5.7.4
Debris and
rubbish generated on-site shall be collected, handled and disposed of properly
to avoid entering the nearby nullah and stormwater drains. Stockpiles of cement and other
construction material should be kept covered when not being used.
5.7.5
Oils and
fuels should only be used and stored in designated areas which have pollution
prevention facilities. All fuel
tanks and storage areas shall be provided with locks and be sited on sealed
areas, within bunds of a capacity equal to 110% of the storage capacity of the
largest tank. The bund should be
drained of rainwater after a rain event.
Sewage Effluent
5.7.6
Temporary
sanitary facilities, such as portable chemical toilets, shall be employed
on-site. A licensed contractor
would be responsible for appropriate disposal and maintenance of these
facilities.
Operational Phase
5.7.7
Road drainage
should be provided with adequate designed silt trap to trap any silt and grit
in the runoff.
5.8
Residual Environmental Impact
5.8.1
With
the full implementation of the recommended mitigation measures for the
construction and operation phases of the Project, no unacceptable residual
impacts on water quality are anticipated. It is recommended that regular
audit of the implementation of the recommended mitigation measures be carried
out during the construction phase.
5.9.1
No
off-site water quality impact would be expected from the construction
activities of the Project. All the
site effluents and runoff generated from the construction works would be
treated and their quality be monitored before discharge under the requirement
of WPCO discharge licence. Water
quality monitoring is not considered necessary. It is recommended that regular site inspections be
undertaken to inspect the construction activities and works areas in order to
ensure the properly implementation of proposed mitigation measures.
5.10.1 Water quality impacts from the
land-based construction works can be controlled to acceptable levels by
implementing the recommended mitigation measures. All the effluents and runoff generated from the works areas
shall be treated and their quality be monitored before discharged under the
requirements of WPCO discharge licence.
No unacceptable water quality impacts would be expected from the
land-based construction activities.
Site inspections should be undertaken routinely to inspect the works
areas in order to ensure the recommended mitigation measures are properly
implemented.
5.10.2 For the operation phase, a surface
water drainage system would be provided to collect runoff from the road during
periods of rain.