5.1
This section presents an assessment of
the potential water quality impacts associated with the construction and
operation phases of the proposed improvement works to San Tin Interchange. The key environmental issue in respect of
water quality impacts associated with the Project would be the proper control
of construction site runoff and drainage during site formation and foundation
works for the proposed slip roads.
Recommendations for mitigation measures have been made, where necessary,
to reduce the identified water quality impacts to an acceptable level. The project area would be located in-land
and thus direct impacts on marine water quality would not arise. Supports for the slip roads and Bridge A
would not be located within the San Tin River nor the San Tin Eastern Main
Drainage Channel.
Water Pollution Control Ordinance (WPCO)
5.2
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). Corresponding statements of Water Quality
Objectives (WQO) are stipulated for different water regimes (marine waters,
inland waters, bathing beaches subzones, secondary contact recreation subzones
and fish culture subzones) in the WCZ based on their beneficial uses. The study area is located within the Deep
Bay WCZ.
5.3
River water quality is ranked
according to its compliance with the WQOs.
The annual compliance of the monthly water quality data with each of the
water quality parameters is indicated by a percentage, with 100% compliance
indicating that all 12 months’ data are within the WQO. Details of the WQOs statement for the inland
waters of the Deep Bay WCZ are listed in Table 5.1.
Table
5.1 Water Quality Objectives for
Inland Waters
|
Parameters
|
Water Quality
Objectives (WQOs)
|
Water Control Zones
(WCZs) or sub-zones to which the WQOs apply
|
|
pH
|
6.5 – 8.5
|
Sub-zones of Deep
Bay WCZ:
Yuen Long & Kam
Tin sub-zones; Beas, Indus & Ganges sub-zones
|
|
6.0 – 9.0
|
Remaining parts of
WCZs
|
|
Suspended solids
(SS)
|
Annual median £ 20 mg/L
|
Sub-zones of Deep
Bay WCZ:
Yuen Long & Kam
Tin sub-zones; Beas, Indus & Ganges sub-zones
|
|
Annual median £ 25 mg/L
|
Remaining parts of
WCZs
|
|
Dissolved oxygen
(DO)
|
³ 4 mg/L
|
All WCZs
|
|
Chemical oxygen
demand (COD)
|
£ 15 mg/L
|
Sub-zones of Deep
Bay WCZ:
Yuen Long & Kam Tin
(Upper) sub-zones; Beas, Indus & Ganges sub-zones
|
|
£ 30 mg/L
|
Remaining parts of WCZs
|
|
5-day biochemical
oxygen demand (BOD)
|
£ 3 mg/L
|
Sub-zones of Deep
Bay WCZ:
Yuen Long & Kam Tin
(Upper) sub-zones; Beas, Indus & Ganges sub-zones
|
|
£ 5 mg/L
|
Remaining parts of
WCZs
|
Technical Memorandum
5.4
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. Under the TM, effluents discharged into
inland waters are subject to standards for particular volumes of
discharge. The discharge standard will
vary with the use of downstream water.
The majority of the inland water bodies within the study area are used for
pond fish culture, which are classified as Group C inland waters. Any new effluent discharge during the
construction phase would be required to comply with the terms and conditions of
a discharge licence, issued by EPD, under the WPCO.
Practice Notes
5.5
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.
5.6
There is an existing streamcourse, the
San Tin River, which runs north and to the immediate west of the line of the
San Tin Eastern Main Drainage Channel (EMDC).
This river will be diverted under the EMDC project before the construction
works commence for the slip roads.
5.7
River water quality data are not
available for the San Tin River from the EPD routine river water quality
monitoring programme. The San Tin River
drains agricultural, village and road areas.
Data available from the consultancy study for the Expansion of Kiosks
and Other Facilities at Lok Ma Chau Boundary Crossing indicates that the
quality of the river water is poor, with low dissolved oxygen, high BOD and
high ammoniacal-nitrogen concentrations (Table 5.2). Water quality was also measured in two inactive fishponds on the
east side of the Lok Ma Chau Boundary Crossing (Figure 5.1). Water quality was
generally good, with low nutrient levels.
Table 5.2 Water
quality in watercourses and unmanaged ponds alongside Lok Ma
Chau Boundary Crossing
|
Parameter
|
Location
|
|
Fishpond 1
(inactive)
|
Fishpond 2
(inactive)
|
Stream
W1
|
Stream W2
(Upstream)
|
Stream W2
(Downstream)
|
|
pH
|
9.2
|
9.0
|
7.4
|
7.6
|
7.6
|
|
Turbidity (NTU)
|
76.0
|
28.0
|
43.0
|
3.6
|
1.2
|
|
DO
|
13.0
|
13.0
|
0.5
|
9.5
|
8.5
|
|
BOD5
|
28
|
14
|
24
|
<2
|
2
|
|
SS
|
-
|
-
|
59
|
7
|
18
|
|
Ammonia as N
|
<0.01
|
<0.01
|
44.0
|
0.35
|
5.50
|
|
NO2 + NO3
as N
|
0.02
|
0.02
|
0.78
|
2.00
|
1.40
|
|
Total Inorganic N
|
0.02
|
0.02
|
45.0
|
2.40
|
6.9
|
|
Total Phosphorous
|
0.2
|
0.1
|
12.0
|
<0.1
|
0.4
|
|
E.Coli (cfu/100 ml)
|
60
|
150
|
-
|
-
|
-
|
Notes: 1)
Date of sampling: January 1999
2) All values in
mg/l unless otherwise indicated.
5.8
The water quality in active fishponds
is expected to be higher in nutrient levels than fishponds that have been
inactive for a long time. This is due
to the addition of fish feed and waste discharges from fish, as shown by data
collected for the Phase 1 Monitoring of Off-site Compensation Works at Mai Po
(Compensation for Regulation of Shenzhen River, Stage II, Phase I works).
5.9
There are no fish ponds within the
Project site boundary. The nearest
fishponds are located adjacent to the intersection of Lok Ma Chau Road and
Castle Peak Road – Chau Tau to the east of the proposed slip roads E and F, at
a distance of approximately 140m from the site boundary (also refer to Section
8.12). The location and extent of fish
ponds in the Assessment Area is included in the habitat map presented in Figure
7.2. An existing nullah located to the
south of the San Tin Interchange and running north-west adjacent to San Sham
Road is the closest water body to the site.
An existing nullah runs parallel to Castle Peak Road – Chau Tau, to the
north of Slip Road B, and is located approximately 35m to the north of the
eastern site boundary. The location of
these two nullahs is also presented in Figure 7.2. The water quality was observed to be very poor in these nullahs
during the terrestrial ecology surveys, resulting from untreated domestic and
agricultural wastes, as well as run-off from open storage and industrial areas.
5.10
The section of the proposed slip roads
north of Castle Peak Road connecting with the existing VHA would lie
immediately adjacent to the future San Tin EMDC. This drainage channel will be located on the western side of San
Sham Road, and be aligned adjacent to San Sham Road from Castle Peak road to
Shenzhen River. The construction of the
San Tin EMDC is scheduled to be completed in the fourth quarter 2006.
5.11
The assessment area for the water
quality impact assessment is defined by a distance of 500m from the project
boundary.
5.12
The water sensitive receivers that may
be affected by the construction works for the road improvement 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 impact
significance determined. The need for
mitigation measures to reduce any identified adverse impacts on water quality
to acceptable levels was determined.
Construction Phase
5.13
Potential sources of water quality
impact associated with the construction of the proposed road improvement works
have been identified and include:
·
construction
site runoff and drainage;
·
general
construction activities; and
·
sewage
effluent produced by on-site workforce.
Construction Runoff and Drainage
5.14
Runoff from the construction works
areas during site clearance and site formation activities may contain increased
loads of sediments, other suspended solids and contaminants. Potential sources of pollution from site
drainage include:
·
runoff
and erosion from exposed soil surfaces, earth working areas and stockpiles;
·
release
of grouting and cement materials with rain wash;
·
wash
water from dust suppression sprays; and
·
fuel
and lubricants from maintenance of construction vehicles and mechanical
equipment.
5.15
Sediment laden runoff may carry
pollutants (adsorbed onto the particle surfaces) into the nearby nullahs. Associated effects which may arise include
increased suspended solids concentrations in receiving waters and blockage of
storm water drains. The construction of
the San Tin EMDC would be in progress during the site clearance and formation
activities for the slip roads, and therefore the San Tin EMDC would not be
regarded as a water sensitive receiver.
5.16
Mitigation measures should be
implemented to control construction site runoff and drainage from the works
areas, and to prevent runoff and drainage water with high levels of suspended
solids from entering the nearby nullahs or local storm water drainage
system. With the implementation of
adequate construction site drainage and provision of sediment removal
facilities as described in Section 5.25, it is anticipated that unacceptable
water quality impacts would not arise.
General Construction Activities
5.17
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 the nearby nullahs or local storm
water drainage system.
5.18
Good construction and site management
practices should be observed, as detailed in Sections 5.26 & 5.27, to
ensure that litter, fuels and solvents do not enter the surrounding nullahs or
storm water drains.
Sewage Effluents
5.19
Domestic sewage would be generated
from the workforce during the construction phase. However, this temporary
sewage can be adequately treated by interim sewage treatment facilities, such
as portable chemical toilets, which can be installed within the construction
site. It is unlikely that sewage generated from the site would have a
significant water quality impact, provided that sewage is not discharged
directly to the adjacent watercourse, and chemical toilets are used and
properly maintained.
Cumulative Impacts
5.20
The identified concurrent projects
which involve construction activities during the construction period of the
road improvement works are:
·
San
Tin Eastern Main Drainage Channel
5.21
The construction works of the above
two projects are scheduled to commence in the third quarter 2002 and fourth
quarter 2002, respectively. The
excavation works for the San Tin EMDC would be completed before the commencement
of construction activities for the Project, and therefore there is limited
potential for cumulative water quality impacts to result from the construction
activities of the EMDC project. With
the implementation of mitigation measures to control construction site runoff
and drainage, potential impacts on water quality would be controlled to within
the site boundary.
5.22
The site formation and foundation
works for the Spur Line would be completed before the commencement of
construction activities for the Project.
There is very limited potential for cumulative impacts on water quality
to result from the construction activities of the Spur Line as the works areas
would not be located in close proximity to the project site boundary.
Operation Phase
5.23
The only identified potential source
of impact on water quality during the operation phase would be runoff from the
road surfaces. A surface water drainage
system would be provided to collect the road runoff. It is recommended that the HyD standard road gullies be installed
along the drainage system to trap silt and grit in the first flush of
runoff. The road drainage system will
facilitate drainage of runoff during rainstorms directly into the nullahs and
thus prevent run-off entering any fishponds in the San Tin area.
5.24
Proposed mitigation measures for
containing and minimizing water quality impacts are summarised below.
Construction Phase
Construction run-off and Drainage
5.25
The site practices outlined in ProPECC
PN 1/94 “Construction Site Drainage” should be followed as far as practicable
in order to minimise surface runoff and the chance of erosion, and also to
retain and reduce any suspended solids prior to discharge. These practices include the following items
:
·
Silt
removal facilities such as silt traps or sedimentation facilities should be
provided to remove 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.
·
Careful
programming of the works to minimise surface excavations for the road
improvement works during the wet season.
If excavation of soil cannot be avoided during the wet season, 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 surfaces should be protected by paving or fill material as soon as
possible to reduce the potential of soil erosion.
·
Open
stockpiles of construction materials or construction wastes on-site of more
than 50m3 should be covered with tarpaulin or similar fabric during
rainstorms. These materials should not
be placed near water courses.
·
A
Drainage Management Plan (DMP) should be prepared by the Contractor and
submitted to EPD before the commencement of any construction works to detail
the procedures for control of construction site runoff. The DMP should at least cover the
construction works areas for slip roads E and F and for Bridge A, and the local
widening of the existing Castle Peak Road.
No site run-off nor drainage should enter fishponds.
General Construction Activities
5.26
Debris and refuse generated on-site
should be collected, handled and disposed of properly to avoid entering the
nearby nullahs or local stormwater drainage system. Stockpiles of cement and other construction materials should be
kept covered when not being used.
5.27
Oils and fuels should only be used and
stored in designated areas which have pollution prevention facilities. All fuel tanks and storage areas should 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 from Construction Workforce
5.28
Temporary sanitary facilities, such as
portable chemical toilets, should be employed on-site. A licensed contractor would be responsible
for appropriate disposal and maintenance of these facilities.
Operation Phase
5.29
It is recommended that the HyD
standard road gullies be installed along the surface water drainage system for
the slip roads to trap any silt and grit in the first flush of runoff.
5.30
With the full implementation of the
recommended mitigation measures for the construction and operation phases of
the proposed 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.
5.31
There are no fish ponds located within
close proximity of the site limit. In
addition to auditing the implementation of the recommended mitigation measures,
a specific water quality EM&A programme is recommended during the site
clearance and formation of the at-grade road sections for slip roads E and F
and for Bridge A and the local widening of the existing Castle Peak Road, in
view of these construction works being located in the vicinity of the San Tin
River. Details of the EM&A are provided in a stand-alone EM&A
Manual.