3.
WATER
QUALITY IMPACT ASSESSMENT
3.1.1
This Chapter details the current condition of the water
bodies within the Study Area, and describes the potential impacts
on water quality during the construction phase of the Project. Mitigation
measures are proposed and the residual impacts after implementation of these
measures are evaluated.
3.1.2
No water quality impact is envisaged for the fresh water main
during the operational phase.
3.2
Environmental Legislation, Policies, Standards and
Criteria
3.2.1 The assessment has been based on the criteria and guidelines for evaluation and assessment of water quality impact stated in Annexes 6 and 14 of the Technical Memorandum on EIA Process (EIAO-TM) and the EIA study brief.
3.2.2
The Water Pollution Control
Ordinance (WPCO) (Cap.358) enacted in 1980 is the principal legislation controlling water
quality in Hong Kong along with Annexes 6 and 14 of the EIAO-TM. Under the
WPCO,
Water Quality Objectives for
|
Water Quality Objectives |
Part or Parts of Zone |
|
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 substance should be absent. |
Whole Zone |
|
(c)
Mineral oil should not be visible on the surface.
Surfactants should not give rise to a 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 1000 mL, calculated as the geometric mean
of all samples collected in one calendar year. |
|
|
(b)
The level of Escherichia coli should not exceed 180 per 100
mL, 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 milligrams per litre for 90% of the sampling occasions during
the year; values should be calculated as the 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. |
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), Miu 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 degrees Celsius. |
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 |
|
(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) Subzone and other inland waters |
|
H. AMMONIA |
|
|
The ammonia nitrogen level should not be more than
0.021 milligram per litre, calculated as the annual average (arithmetic
mean), as unionised form. |
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. |
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 1 metre below surface, mid-depth and 1 metre above seabed). |
Marine waters |
|
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 concentrations of
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
toxicant interactions 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 |
Source: Cap. 358I, Southern
Water Control Zone Statement of Water Quality Objectives.
3.2.3 The other relevant guideline is the Practice Note for Professional Persons (ProPECC Note) PN 1/94 – “Construction Site Drainage” which provide guidelines for the handling and disposal of construction discharges. This ProPECC Note is generally applicable for control of site runoff and wastewater generated during the construction phase of the Project.
3.2.4
The Technical Memorandum on Standards for Effluents Discharged into
Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS) under the
WPCO defines acceptable discharge limits to different types of receiving water
in
3.3.1
According to the EIA study brief, the Study Area for the
water quality impact assessment should cover the
3.4
Baseline
Water Quality Conditions of the
Study Area
Desktop
Review and Field Observations
3.4.1
Site visit took place in January
2009. General descriptions and field observations of the water bodies in the
Study Area are provided below.
3.4.2
The Southern WCZ stretches from
Adamasta Channel
3.4.3
The Adamasta Channel is a
designated navigation fairway which is used by high speed vessels plying to and
from
Cheung Chau
(Tai Kwai Wan)
3.4.4
Tai Kwai Wan is located immediately
to the north of the landfall location at Cheung Chau. Based on site observation,
there are no natural stream courses near the Project area at Cheung Chau.
Several concrete drainage channels which collect stormwater and village
effluent from nearby villages discharge into Tai Kwai Wan via two outfalls. A
sandy beach (non-gazetted) and sitting out area can be found at Tai Kwai Wan.
The beach is found to be generally clean.
3.4.5
The proposed landfall location at
Cheung Chau is currently occupied by two contractor works compound for
temporary storage of construction materials. The works compound is already
formed and covered with concrete. A vegetated area is located between the two
works compound.
3.4.6
To the south, rocky and sandy
shores and artificial seawall can be found with
3.4.7
Further southwest of the landfall
location is the Cheung Chau sewage treatment works with a 200 m long submarine
outfall discharging treated effluent into Adamasta Channel.
Lantau (Chi Ma Wan
3.4.8
The proposed works area at Lantau
is located on a rocky shore near Ha So Pai in Chi Ma Wan Peninsula. The works
area is within
Bathing Beaches
3.4.9
The entire inland waters of Cheung
Chau and the southern coast of
Baseline
Conditions
Marine Water Quality
3.4.10 Water
quality in southern WCZ is monitored regularly as part of EPD’s marine water
quality monitoring programme. The EPD’s monitoring locations near the Study
Area is shown in Figure 3.1.
3.4.11 A
summary of EPD water quality data in 2008 for the monitoring station closest to
the Project at
3.4.12 In
2008, the
Summary of
|
Parameters |
Unit |
Values |
|
Temperature |
oC |
23 |
|
Salinity |
|
29.5 |
|
Dissolved Oxygen |
mg/L |
6.8 |
|
Dissolved Oxygen (Bottom) |
mg/L |
6.6 |
|
Dissolved Oxygen |
% saturation |
93 |
|
Dissolved Oxygen
(Bottom) |
% saturation |
91 |
|
pH |
- |
8.1 |
|
Secchi Disc
Depth |
m |
1.8 |
|
Turbidity |
NTU |
13.5 |
|
Suspended
Solids |
mg/L |
11.6 |
|
5-day
Biochemical Oxygen Demand |
mg/L |
0.7 |
|
Ammonia
Nitrogen |
mg/L |
0.07 |
|
Unionised
Ammonia |
mg/L |
0.004 |
|
Nitrite
Nitrogen |
mg/L |
0.041 |
|
Nitrate
Nitrogen |
mg/L |
0.194 |
|
Total Inorganic
Nitrogen |
mg/L |
0.31 |
|
Total Kjeldahl
Nitrogen |
mg/L |
0.27 |
|
Total Nitrogen |
mg/L |
0.51 |
|
Orthophosphate
Phosphorus |
mg/L |
0.016 |
|
Total
Phosphorus |
mg/L |
0.03 |
|
Silica |
mg/L |
1.3 |
|
Chlorophyll-a |
mg/L |
3.5 |
|
E. coli |
count/100mL |
21 |
|
Faecal
Coliforms |
count/100mL |
43 |
Notes:
1.
Unless otherwise specified, data presented are depth-averaged
values calculated by taking the means of three depths: surface, mid-depth,
bottom.
2.
Data presented are annual arithmetic means of the
depth-averaged results except for E. coli
and faecal coliforms which are annual geometric means.
3.
Data in brackets indicate the ranges.
Source: EPD (2009b) Marine Water Quality Monitoring in
3.4.13 An
updated baseline water quality should
be established before the commencement of the construction phase of the Project
to allow appropriate action and limit levels to be determined for impact
monitoring and audit.
3.5.1
The Project works area comprises
mainly of two sites, the launching site at Cheung Chau and the reception site
at Lantau. The proposed launching site at Cheung Chau is located at a headland
near Tai Kwai Wan at the north-western part of Cheung Chau. It is currently
occupied by two works compound. The proposed reception site at Lantau is
located at a rocky shore in the south-eastern coast of
3.5.2
The Water Sensitive Receivers (WSRs) likely to be impacted by the Project
include the waters of
3.6
Potential
Impacts During Construction Phase
Introduction
3.6.1
With the use of Horizontal
Directional Drilling (HDD) in laying the submarine water main, there will be no
marine works and the seabed will not be disturbed, hence the potential water
quality impact is expected to be greatly reduced. Potential water quality
impacts during the drilling works and construction of the land-based water main
are discussed below.
Site
Clearance / Preparation
3.6.2
Preparation of the land for
construction of the launching site at Cheung Chau will involve removal of
surface vegetation and excavations, demolition of temporary structures, removal
and import of construction materials. The launching site at Cheung Chau will
utilize the existing works compound as such major site formation works can be
minimized. Construction of some temporary infrastructure may be necessary such
as temporary drainage systems and site offices.
3.6.3
The permeability characteristics of
the site may change, reducing infiltration into surface layer. This may lead to
increased site surface runoff with high suspended solids loadings during the
wet season. Erosion of soil enriched in organic matter may also release
nutrients into the nearby water bodies.
3.6.4
In view of the sensitive natural
environment at the reception site in Lantau which is within the
Drilling
Works
3.6.5
It is anticipated that the drilling
works will be carried out continuously and could be completed within 12 months.
3.6.6
For the laying of the submarine
water main, drilling fluid will be required to facilitate the directional
drilling during pilot bore, forward reaming and ultimately pipe laying by
pushing. Bentonite with or without additives is commonly used as a drilling
fluid. Although inadvertent release of bentonite slurry through accidental
spillage, site runoff, or migration to the surface (‘frac-out’) can increase
sediment loads and therefore have an impact on water quality, it is non-toxic
and accidental release can be effectively managed and mitigated.
3.6.7
It is estimated that approximately
980 m3 of bentonite will be required. The bentonite will generally
be left within the borehole to stabilize the hole when drilling is suspended.
Bentonite is reconditioned and recycled throughout the drilling process.
3.6.8
The recycling process begins with a
mud pump delivering drilling fluid under high pressure from the holding tank to
the drill head nozzle. Excavated spoil (drill cuttings) are then transported as
a slurry with the drilling fluid to emerge at Cheung Chau into a mud tank for
temporary storage. A submersible pump delivers the drilling fluid and slurry
with cuttings to the cleaning systems consisting of shaker, desander and
desilter. The shaker helps to separate large drill cuttings for removal.
Lighter density liquid is then brought to the desander and desilter where finer
solids are discarded. The drilling fluid is then discharged back to the holding
tank ready for conditioning and reuse. Wastewater generated during the
recycling process, from the cleaning systems along with wastewater from wheel
washing facility and site runoff will be treated (e.g. sedimentation) and will
be reuse for washing within the site or re-circulated back to the bore hole. No
discharge of wastewater during the drilling fluid treatment and recycling
process is expected.
3.6.9
Solids which are filtered out from
the slurry are divided into coarse and fine particles. Coarse particles
(crushed rocks) that are settled out in the shaker are transfer to the washing
tank for cleaning and draining then for storage and reuse before disposal to
the designated public fill reception facilities. Similarly, finer particles
will be stored in containers for reuse on-site before off site disposal of
excess materials.
3.6.10 Based
on preliminary estimation, size of the folding tank is about 2m x 5m x 5m with
volume of about 50,000 litres (~50 m3). Mud tank of size 6m x 5m x
3m with capacity of about 90,000 litres (~ 90 m3) and 150 m3/hr
circulation (2,500 litres/min) will need to be provided. Standby tank of
similar capacity should also be provided. Notwithstanding, the Contractor will
be required to submit a Drilling Fluid Management Plan for Engineer to approve
prior to commencement of construction.
3.6.11 The
works area at Cheung Chau will be paved with a slight fall towards land to
prevent site runoff from directly flowing to the sea. The 500 mm high concrete
bund around the site will provide about 100 m3 in volume storage for
the site runoff and/or in the event of spillage / leakage of drilling fluid.
This is adequate for spillage / leakage of drilling fluid from either the
holding tank or the mud tank.
3.6.12 The
pilot hole and reaming will terminate approximately 50 m short of the final
exit point at Chi Ma Wan. Prior to drilling the final 50 m, the reamed hole
will be thoroughly cleaned with water to remove all cuttings and drilling
fluid. The final 50 m of drilling will be completed using water instead of
bentonite in order to prevent the release of bentonite at the exit pit.
Approximately 0.3 m high concrete bund will be constructed around the exit pit
to contain the drilling fluid, which is mainly silty water and generated when
the drill head bores through the rock at Lantau during the final drilling
process. The exact level of the concrete bund will be determined on site to
ensure that it will not be lower than the ground level of the launching site at
Cheung Chau.
3.6.13 Approximately
1,780 m3 of water will be required for flushing the entire drill
hole after the final drill before installing the casing and water pipe. The
water (likely to have high silt content) will undergoes sedimentation and
treatment at the Cheung Chau launching site before being re-circulated back to
the hole for flushing. Adequate number of tanks with sufficient capacity will
be required to handle and treat the water. The estimated capacity of the
proposed sedimentation tank to treat the flushing water is about 20 m3
(assuming flush rate is 3 m3/min, with a retention time of 6
minutes).
3.6.14 If not controlled properly, silty runoff from site
areas around the drilling rig, entry pit, stockpiling area and the associated
drilling fluid delivery, recovery and containment systems may contaminate the
nearby water bodies but this can be controlled by proper site containment and
effective site operations management.
Concreting Work
3.6.15 Concreting
work will be required for reinstatement of the road surface after laying of the
land-based water main along
3.6.16 However, the amount of concrete to be used is
expected to be minimal and can be controlled through good site management
practices.
Site
Workshop or Depot
3.6.17 Works
areas and site offices will be provided for site staff and contractors at the
proposed launching site in Cheung Chau only. It may include workshop or depot
to provide maintenance and repair services for the equipment on site. The
workshop may store engine oil, lubricants, chemicals and waste oil/materials which may create impacts if accidental spillage
occurs. Waste oil may infiltrate the surface topsoil and contaminate the ground.
Stormwater runoff from the site may wash the oil spillage into adjacent water
bodies. No workshop or depot will be located at the proposed reception site in
Lantau.
General
Construction Works
3.6.18 General
construction works will be continuous throughout both wet and dry seasons.
During the wet season,
silty runoff will be generated from the works area. Spillage, hydraulic leakage
and runoff from the surface of standby construction equipment during rainy
conditions may release oil and lubricants to the surrounding environment. If
the site runoff is not adequately managed, it may enter the water bodies and
increase the suspended solids, turbidity and oil & grease levels of these
water bodies.
Presence
of Additional Population (Workers)
3.6.19 The presence of site staff and construction
workers will generate sewage and domestic refuse. Impacts include waste and
wastewater generated from eating areas, toilets and waste disposal areas.
Sewage arising from on-site construction workforce would have the potential to
cause water pollution if it is discharged directly into adjacent water bodies
without any appropriate treatment.
Sterilization
of Water Main Prior to Commissioning
3.6.20 All new fresh water mains must be cleaned and
sterilized before being put into operation. Typically, water mains are
sterilized by chlorination. The purpose of chlorination is to disinfect the
water main, resulting in an absence of coliforms as confirmed by laboratory
analysis, before they are placed in service. Chlorine solutions are normally
used as the sterilizing agent.
3.6.21 The
cleaning and sterilization procedures of fresh water mains according to the General Specification for Civil Engineering
Works Volume 2, 2006 Edition (Section 22.73) and WSD Departmental
Instruction No. 805: Mainlaying -
Cleaning and Sterilization of Fresh Water Mains are listed below.
Notwithstanding the procedures below, the Contractor should also adhere to the
Project contract document with regards to sterilization of the water main.
1.
All
extraneous materials should be removed before the fresh water pipeline is laid
and connected.
2.
The
pipeline should be filled slowly with water and tested to the required
pressure.
3.
After
the pipeline has been successfully pressure tested, it should be cleaned
internally and flushed through with potable water. For long length of small
diameter mains which are inaccessible for cleaning (size below DN600), swabbing
should be carried out to remove dirt and materials inadvertently left in the
pipeline.
4.
The
pipeline should then be completed filled with water that has been dosed with a
homogeneous solution of sterilising chemicals (e.g. chloride of lime) such that
the final concentration of free chlorine in the water is at least 30 ppm. The
water should be left in the pipeline for at least 24 hours.
5.
After
the 24-hour period, the pipeline should be drained down and the sterilizing
water should be flushed out using potable water until the remaining chlorine level
in the pipeline is less than 1 ppm. Bacteriological and chemical samples should
be taken and submitted to the Waterworks Chemist for analysis in accordance
with the contract requirements.
6.
To
minimize or avoid any possibility of contamination, sterilization of the
pipeline should take place not more than 7 days before putting it into
operation.
3.6.22 Approximately
590 m3 of water will be required to clean and flush the completed
water main. Any solids flushed out will be settled out through the
sedimentation tank. Approximately 295 m3 of chlorinated water will
be used to sterilize the water main.
3.6.23 If
high chlorinated water reaches an aquatic system, it has the potential to kill
fish and other aquatic organism because of the chlorine concentration. One mode
of action is likely through damaging the gills, thus preventing the fish from
breathing. Therefore, it is necessary to dechlorinate the water in order to
make it safe for discharge to the environment.
3.6.24 Currently,
sodium bisulfite, sodium sulfite and sodium thiosulfate are most frequently
used for dechlorination. Sodium thiosulfate is preferred since it is less
hazardous and consumes less oxygen than sodium bisulfite and sodium sulfite.
The Contractor is required to submit for the Engineer approval, details of the
processes and chemicals including dosage to be used in the chlorination and
dechlorination of the sterilizing water.
3.6.25 The
sterilization water must be treated to the relevant discharge requirement
stipulated in TM-DSS before discharging. With the implementation of appropriate
treatment such as dechlorination to control water discharge from sterilization
of water main, adverse impact on water quality is not expected. Monitoring of
the total residual chlorine concentration at the discharge point is
recommended.
3.6.26 As
the site at Cheung Chau is designated for secondary recreational contact, the
sterilization of the main should be scheduled during the dry season.
3.7
Mitigation
Measures for Construction Phase Impacts
3.7.1
The main potential impacts from the
construction phase of the Project include an increase in suspended solids, pH
value, oil & grease, general site effluent and chlorinated water entering
adjacent water bodies. The following sections discuss the proposed mitigation
measures for the potential water quality impacts identified above.
General
3.7.2
The Contractor shall observe and
comply with the Water Pollution Control Ordinance (WPCO) and its subsidiary
regulations. The Contractor shall carry out the works in such a manner as to
minimise adverse impacts on the water quality during execution of the works. In
particular the Contractor shall arrange his method of working to minimise the
effects on the water quality within and outside the site and on the transport
routes.
3.7.3
The Contractor shall follow the practices, and be
responsible for the design, construction, operation and maintenance of all the
mitigation measures below and as specified in ProPECC PN 1/94 – “Construction
Site Drainage”. The design of the mitigation measures shall be submitted by the
Contractor to the Engineer for approval. All discharge from the construction
works should meet the discharge standards stipulated under the WPCO “Technical
Memorandum on Standards for Effluents Discharged into Drainage and Sewerage
Systems, Inland and Coastal Waters”.
Site
Clearance / Preparation
3.7.4
Proper construction site drainage
management measures should be implemented to control site
runoff and drainage, and thereby prevent high sediment loadings from reaching
nearby water bodies. Site runoff and wastewater should not be discharged into
nearby water bodies without proper treatment.
3.7.5
Turbid water from construction
sites must be treated to minimise the solids content before being discharged.
Advice on the handling and disposal of site discharge is given in the ProPECC
Note PN 1/94 – “Construction Site Drainage”.
3.7.6
In general, surface runoff from
construction sites should be discharged into water bodies via adequately
designed silt removal facilities such as sand traps, silt traps and sedimentation
tanks. Channels or earth bunds or sand bag barriers should be provided on site
to properly direct stormwater to such silt removal facilities. Sufficient
numbers of pumps and tanks of adequate capacity should be provided on-site.
Perimeter channels at site boundaries should be provided to intercept storm
run-off from outside the site so that it will not wash across the site.
Catchpits and perimeter channels should be constructed in advance of
earthworks.
3.7.7
Silt removal facilities and
diversion channels should be maintained and the deposited silt and grit should
be removed regularly, especially at the onset of and after each rainstorm to
ensure proper functioning of these facilities at all times.
3.7.8
Measures should be taken to prevent
the washing away of construction materials, soil, silt or debris into the
nearby water bodies. Open stockpiles susceptible to erosion should be covered
with tarpaulin or similar fabric and provided with containment such as bunds,
sand bag barriers or equivalent measures, especially during the wet season (April – September) or when heavy rainstorm is
predicted. Runoff to watercourses should be intercepted by
minimising flat exposed areas of permeable soil, and by forming pits or
diversion channels into which runoff can flow to suitable treatment facilities
before discharge.
Drilling
Works
Launching Site at Cheung Chau
3.7.9
The proposed launching site at
Cheung Chau should be paved with a slight fall towards land to prevent site
runoff from directly flowing to the sea. The launching site should be provided
with an adequately designed site drainage system to contain and treat polluted
site runoff and wastewater. Adequate numbers of tank with sufficient capacity
should be provided on-site to collect, store and treat drilling fluids,
cuttings and/or chemicals. These tanks should be surrounded by bunds and
regularly inspected and maintained to avoid leakage.
3.7.10 During directional drilling, excavated spoil
(cuttings) will be carried as a slurry with the drilling fluid to emerge at
Cheung Chau. The slurry should be treated to remove the cuttings and recycled
as drilling fluid. Cuttings should be stored in containers prior to removal and
disposal as construction & demolition material to public fill reception
facilities.
3.7.11 If
temporary storage of cuttings is required for later reuse, this should be
undertaken in Cheung Chau. Care should be taken in the storage of cuttings especially
during the wet season and the storage area should be covered and bunded to
prevent silty runoff
entering water bodies.
3.7.12 A schematic layout plan of the proposed launching
site and drainage measures at Cheung Chau is shown in Figure 3.3 to
demonstrate the launching site has adequate land area to setup the necessary
construction equipment, auxiliary equipment, site offices and drilling fluid
containment, recovery and treatment systems. In general, the site will be
divided into clean area and dirty area. The clean area will be used to house
the site offices, storage area and other non-polluting uses. The dirty area
will be the main drilling works site consisting of the drilling rig, entry pit,
mud tank, drilling pipes storage, drilling fluid recycling and treatment
systems, stockpiling area and wastewater treatment systems.
3.7.13 The entire dirty area should be surrounded by a 0.5
m tall concrete bund to fully contain and prevent site runoff from discharging
into the marine waters untreated as well as to prevent surface runoff from
entering the site. The entry pit should be further surrounded by a 0.3 m tall
concrete bund for added protection to contain and avoid spillage of drilling
fluid. All site runoff should be diverted to an adequately designed wastewater
treatment system for treatment before discharge.
3.7.14 The perimeter of the clean area should be
surrounded with 300 mm u-channel to collect and divert all site runoff for
treatment before discharge. Bund should be provided to prevent surrounding
surface runoff from entering the site.
3.7.15 After
separating the cuttings, bentonite slurry should be re-circulated for reuse in
the drilling works. By adopting a
forward reaming approach all slurry treatment and drilling fluid recycling
systems should be located and managed on Cheung Chau.
Reception Site at Lantau
3.7.16 The
pilot hole and reaming should be stopped approximately 50 m short of the final
exit point at Chi Ma Wan in order to prevent any release of slurry in to the
marine environment. Prior to drilling the final 50 m, the reamed hole should be
thoroughly cleaned with water to remove all cuttings and drilling fluid. The
final 50 m of drilling should be completed using water instead of bentonite in
order to prevent the release of bentonite at the exit pit. Approximately 0.3 m
high concrete bund should be constructed around the exit pit to contain the
drilling fluid, which is mainly silty water and generated when the drill head
bores through the rock at Lantau during the final drilling process. The exact
level of the concrete bund should be determined on site to ensure that it will
not be lower than the ground level of the launching site at Cheung Chau.
3.7.17 The
use of containment structure such as earth bund, sand bag barriers wrapped with
geotextile fabric or similar material, diversion channels or other similar
techniques should be installed surrounding the site boundary at Lantau during
the wet season to intercept storm runoff from outside the site so that it will
not wash across the site (or into the exit pit).
3.7.18 To
provide further protection, silt curtain should be installed before
commencement of works at Lantau to minimize the water quality impact. The
design and installation should be certified by the Environmental Team (ET), verified by the Independent Environmental Checker (IEC)
and approved by the Engineer to ensure the intended protection can be achieved.
The Contractor should regularly inspect and maintain the silt curtain to ensure
its effectiveness.
3.7.19 A schematic layout plan of the proposed reception
site and drainage measures at Lantau is shown in Figure 3.4.
Management Plans and Monitoring
3.7.20 The Contractor should submit a Drainage Management Plan
detailing the temporary drainage measures along with the proposed measures to
ameliorate the potential water quality impacts to the Environmental Team (ET)
for certification, the Independent Environmental Checker (IEC) for verification
and to the Engineer for approval before commencement of the construction works.
3.7.21
The Contractor should also submit a Drilling
Fluid Management Plan outlining expected volume of water, the type of soil and
an estimated quantity of bentonite, full details of plans for monitoring
drilling fluid flow (out and return) and pressure, wastewater discharge, slurry
containment, treatment, recycling and transport, and the approved disposal
site. The plan should also contain
details of contingency plans for dealing with frac-out or other inadvertent
release of slurry. Contingency plans should include the provision of standby
pumps, containment
booms, vacuum unit and tanks on site to contain and remove turbid, muddy water
should incidental frac-outs occur.
3.7.22 Regular
monitoring of suspended solids and turbidity
should be conducted during construction works. Any exceedance of water quality
in the nearby water bodies caused by site runoff should be rectified in
accordance with EM&A programme for this Project.
Concreting
Work
3.7.23 Runoff
should be carefully channelled and treated to prevent concrete-contaminated
water from entering water bodies. Adjustment of pH can be achieved by adding a
suitable neutralising reagent to wastewater prior to discharge.
3.7.24 Any
exceedance of the acceptable range of pH levels in nearby water bodies caused
by site runoff containing concrete should be monitored and rectified under the
EM&A programme for this Project.
Site
Workshop or Depot
General
Construction Works
3.7.25 Site
workshop or depot including storage of chemicals and chemical waste should be
located at the proposed launching site in Cheung Chau only. No workshop or
depot should be located at the proposed reception site in Lantau.
3.7.26 Any
Contractor generating waste oil or other chemicals as a result of his activities
should register as a chemical waste producer and provide a safe designated storage
area for chemicals on site. The storage site should be located away from existing water bodies.
3.7.27 All compounds in works areas should be located on areas
of hard standing surface with provision of diversion channels and settlement
ponds where necessary to allow interception and controlled release of settled /
treated water. Hard standing compounds should drain via an oil
interceptor. The oil
interceptor should
be regularly inspected and cleaned to avoid wash-out of oil during storm
conditions. A bypass should be provided to avoid overload of the interceptor's
capacity. To prevent spillage of fuels or other chemicals to water bodies, all
fuel tanks and storage areas should be sited on sealed areas within a bund of a
capacity equal to 110% of the storage capacity of the largest tank. Where temporary storage of chemicals or fuel drums
outside the storage area is necessary, drip tray should be provided. Disposal
of the waste oil should be carried out by a licensed collector. Good
housekeeping practices should
be implemented to minimise careless spillage and to keep the storage and the work space
in a tidy and clean condition. Appropriate training including safety codes and
relevant manuals should be given to the personnel who regularly handle the
chemicals on site.
Emergency Contingency Plan
3.7.28 The Contractor should prepare an Emergency
Contingency Plan (spill response plan) for the Project to contain and remove
accidental spillage of drilling fluids, chemicals and all hazardous materials
on-site including fuels at short notice and to prevent or to minimize the
quantities of contaminants from entering the nearby water bodies and affecting
the sensitive habitats. The Contractor should submit the Emergency Contingency
Plan to the ET for review & comment and to the Engineer for approval. The
Plan should include, but not limited to, the following:
(i)
potential
emergency situations
(ii)
chemicals
or hazardous materials used on-site (and their location)
(iii)
emergency
response team
(iv)
emergency
action plans and procedures
(v)
list of
emergency telephone hotlines
(vi)
locations
and types of emergency response equipment
(vii)
training
plan and emergency drill
(viii)
schedules
for review and audit.
General Guidance for Handling of Spillage / Leakage
3.7.29 In the event that accidental spillage or leakage of
hazardous substances / chemical wastes takes place, the response procedures as
listed below should be followed. It should be noted that the procedures below
are not exhaustive. The Contractor should propose other response procedures in
the emergency contingency plan based on actual site conditions as well as the
particular types and quantities of chemicals or hazardous substances used,
handled and stored on-site.
·
Contact person in charge or
nominated person immediately and initiate action plans based on the emergency
contingency plan.
·
Oil
leakage or spillage should be contained and cleaned up immediately. Waste oil
should be collected and stored for recycling or disposal in accordance with the
Waste Disposal Ordinance.
·
Instruct
untrained personnel to keep at a safe distance well away from the spillage
area.
·
If the
spillage / leakage is severe or is causing hazard to life, initiate emergency
evacuation and call the emergency services.
·
Only
trained persons equipped with suitable protective clothing and equipment should
be allowed to enter and clean up the waste spillage / leakage area.
·
Where
the spillage / leakage is contained in the enclosed storage area, the waste can
be transferred back into suitable containers by suitable handheld equipment,
such as hand operated pumps, scoops or shovels. If the spillage / leakage
quantity is small, it can be covered and mixed with suitable absorbing
materials such as absorbent tissue paper, pads, dry soft sand or vermiculite.
The resultant slurry should be treated as chemical waste and transferred to
suitable containers for disposal.
·
For spillage / leakage into nearby
water bodies, immediate action is required to contain the spillage / leakage.
Appropriate structural, physical barrier or secondary containment (e.g.
containment booms) should be deployed to contain the spill and if possible to
prevent contaminated water from dispersing away from the source. Suitable
liquid absorbing materials such as absorbent tissue paper, pads or rolls should
be used to recover the spilt substances. The resultant slurry should be treated
as chemical waste and transferred to suitable containers for disposal.
·
For
spillage / leakage in other areas, immediate action is required to contain the
spillage / leakage. Suitable liquid absorbing materials such as absorbent
tissue paper, pads, dry soft sand or vermiculite should be used to cover the
spill. The resultant slurry should be treated as chemical waste and transferred
to suitable containers for disposal.
·
Areas
that have been contaminated by chemical waste spillage / leakage should be
cleaned. While water is a soluble solvent for aqueous chemical wastes and water
soluble organic waste, kerosene or turpentine should be used for organic
chemical wastes that are not soluble in water. The waste from the cleanup
operation should be treated and disposed of as chemical waste.
·
In
incidents where the spillage / leakage may result in significant contamination
of an area or risk of pollution, the Environmental Protection Department and
other relevant departments should be informed immediately.
Presence of Additional Population (Workers)
3.7.30 Sewage
arising from construction workers on site should be collected in a suitable
storage facility, such as portable
chemical toilets. An
adequate number of portable toilets should be provided for the construction workforce.
The portable toilets should be maintained in a state that will not deter the
workers from using them. Wastewater collected should be discharged into foul
sewers and collected by licensed collectors.
3.7.31 The
collected wastewater from sewage facilities and also from eating areas or
washing facilities of site offices should be disposed to foul sewer. If there
is no foul sewer in the vicinity, a septic tank and soakaway system or for
larger flow, a sewage treatment plant should be provided. All domestic sewage
discharges (except into foul sewer) are controlled under the WPCO. The
Contractor must apply for a discharge licence from EPD and must comply with the
terms and conditions of a valid WPCO licence.
Sterilization
of Water Main Prior to Commissioning
3.7.32 Effluent
from the sterilization of water main should be treated including dechlorination
by physical process e.g. adsorption by activated carbon filter, or chemical
process e.g. neutralisation by dechlorination agent dosing to ensure compliance
with the discharge requirements stipulated in the TM-DSS. Adequate tanks with
sufficient capacity should be provided to allow proper treatment of the
effluent prior to discharge.
3.7.33 The
sterilization of the water main and the dechlorination process should be
conducted at the launching site at Cheung Chau. As the site at Cheung Chau is
designated for secondary recreational contact, the sterilization of the main
should be scheduled during the dry season. The Contractor should submit for the
Engineer approval, details of the dechlorination process and any chemicals
including dosage to be used.
3.7.34 The
Contractor should provide a dechlorination plant of sufficient capacity to
contain and treat the sterilising water with high chlorine concentration before
discharge. Details of the specification of the dechlorination plant should be
submitted to the Engineer for approval. They should include but not limited to
the following:
·
the nominal flow rate shall not be
less than 36 cubic meter per hour;
·
the plant shall be capable of dechlorinating
water with chlorine level up to 40 mg/litre;
·
the maximum chlorine level of
dechlorinated water shall be <0.2 mg/litre;
·
the recommended contact time if
dechlorination by adsorption with activated carbon filter is proposed;
·
a water meter or other measuring
device shall be installed at the inlet pipe of the dechlorination plant to
measure the amount of water treated, in cubic metres; and
·
sensors and data loggers for
continuous measurement and record of chlorine level shall be installed at the
inlet and outlet pipe of the dechlorination plant.
3.7.35 In-situ testing of
total residual chlorine should be conducted every hour at the discharge point
to ensure the chlorine concentration does not exceed the stipulated maximum
level when dechlorinated water is being discharged.
3.7.36 If
the dechlorinated water exceed the allowed concentration, discharge must be
suspended and the water should be circulated to a standby tank for further
dechlorination and testing. The location of the sampling point will depends on
the discharge point to be proposed by the Contractor. The Contractor should
submit details of the proposed sampling location, frequency and equipment for
verification by the ET Leader and approval by the Engineer before commencement
of the sterilization work of the completed water main. The proposed sampling
location(s) should be submitted to EPD for reference.
3.8
Residual
and Cumulative Impacts
3.8.1
With the
effective implementation of mitigation measures as described above, residual
impacts on water quality of the receiving water bodies due to the
implementation of this Project are expected to be negligible.
3.8.2
The DSD project “Agreement No. CE
31/2007(DS) – Upgrading of Cheung Chau and Tai O Sewage Collection, Treatment
and Disposal Facilities (PWP Item 354DS)” will likely coincide with the
land-based water main laying at Cheung Chau. As the sewer laying works is
relatively small scale and the interfacing works area is very small, cumulative
water quality is not anticipated as long as the recommended mitigation measures
are properly implemented by the Contractor.
3.9
Monitoring
and Audit Requirement
3.9.1
Potential water quality
impacts during the construction phase can be mitigated by the implementation of
the recommended site practices and other measures to control runoff and to
minimise water pollution.
3.9.2
A water quality monitoring and audit
programme is recommended to check for compliance with relevant water quality standards,
to ensure the proper implementation of the mitigation measures and to verify
the effectiveness of the recommended mitigation measures.
3.9.3
Monitoring of
pH, dissolved oxygen, suspended solids and turbidity will need to be carried out
during the construction stage to ensure that any deterioration in water quality
can be readily detected and prompt action taken to rectify the situation. In addition, residual chlorine will need to be
monitored during sterilization of the completed water main. Impact and control
monitoring stations should be provided.
3.9.4
No monitoring is required during the operational phase.
3.10.1 With
the use of HDD technique in laying the submarine water main, there will be no
marine works and no impact to the seabed. The preferred construction option of
using HDD will significantly reduce potential water quality impacts to the
sensitive receivers within the marine waters of the
3.10.2 The
construction works could impact the water bodies through silt-laden site
runoff, inadvertent release of drilling fluids, runoff from workshops &
depot, sewage effluent from the construction workforce and discharge of
chlorinated water during sterilization of the water main. These impacts can be
readily mitigated by provision of suitable temporary site drainage systems with
treatment facilities, installation of drilling fluid recycling and treatment
systems, implementation of the recommended good site management practices, and
proper sewage collection and disposal systems.
3.10.3
With the implementation of the recommended mitigation
measures, no adverse residual water quality impact is expected during
construction phase of the Project.
EPD (2009a) Beach Water Quality in
EPD (2009b) Marine Water Quality Monitoring in