Introduction
5.1
Marine works for the proposed
reclamation at Tai Lam Kok and road widening from Ka Loon Tsuen to Tai Lam Kok
may adversely impact the water quality of the neighbouring waters. During the operation phase, run-off across
paved surfaces of the proposed widened carriageway would be the key water
quality issue.
5.2
The objective of the present study is
to identify and evaluate the potential water quality impacts associated with
the construction and operation of the proposed Castle Peak Road (CPR)
improvement works, and to recommend appropriate mitigation measures to
alleviate any identified adverse impacts to acceptable levels, wherever
necessary.
5.3
Legislation and associated guidance
relevant to the water quality impacts assessment of the proposed CPR
improvement works are described below.
Environmental Impact Assessment Ordinance
5.4
This water quality impact assessment
will be carried out following the criteria and guidelines as stated in Annexes
6 and 14 of the Technical Memorandum on
Environmental Impact Assessment Process (EIAO-TM) as required in the EIA
Study Brief.
Water Pollution Control Ordinance
5.5
The Water Pollution Control Ordinance
(WPCO) (Cap. 358) provides the major statutory framework for the protection and
control of water quality in Hong Kong.
According to the Ordinance and its subsidiary legislation, the whole
Hong Kong waters are divided into ten Water Control Zone (WCZs). Corresponding statements of Water Quality Objective
(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 proposed site of the improvement works
is located within the North Western WCZ and the corresponding WQOs are
summarized in Table 5.1.
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 discoluration
(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 or damage of material
(f) not to
contain substances which settle to form objectionable deposits
|
Whole Zone
Whole Zone
Whole Zone
Whole Zone
Whole Zone
Whole Zone
|
|
E. coli
|
annual geometric
mean not to exceed 610/100 mL
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
geometric
mean of the most recent 5 consecutive samples taken at intervals of between 7
and 21 days not to be less than 1/100 mL
geometric
mean 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
fish culture
subzones
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; 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
|
|
Parameter
|
Objective
|
Part(s)
of Zone
|
|
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 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
other inland
waters
|
|
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
other inland
waters
|
|
Un-ionized ammonia
|
annual mean not to exceed 0.021 mg/L
|
whole
zone
|
|
Nutrients
|
not to be
present in quantities that cause excessive 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
|
Bathing Beach
Subzones
|
|
Turbidity
|
No changes in
turbidity or any other factors shall reduce light transmission substantially
from the normal level
|
Bathing Beach
Subzones
|
Technical Memorandum for Effluent Discharges
5.6
Besides setting the WQOs, the WPCO
controls effluent discharging into the WCZ through a licensing system. The Technical
Memorandum on Standards for Effluents
Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters
(TM-DSS) issued under Section 21 of the WPCO gives guidance on the permissible
effluent discharges based on the type of receiving waters (foul sewers, stormwater
drains, inland and coastal waters). The
limits control the physical, chemical and microbial quality of effluents,
discharging into the marine waters.
ProPECC Practice Notes
5.7
A Practice Note for Professional
Persons, Construction Site Drainage
(ProPECC PN 1/94) was issued by the EPD to provide guidelines for handling and
disposal of construction site discharges.
The ProPECC PN 1/94 provides good practice guidelines for dealing with
ten types of discharge from a construction site. These include surface runoff, groundwater, boring and drilling
water, bentonite slurry, water for testing and sterilization of water retaining
structures and water pipes, and wastewater from site facilities. Practices given in the ProPECC PN 1/94
should be followed as far as practicable during construction to minimize the
water quality impact due to construction site drainage.
Description of the Environment
Baseline Conditions
5.8
The proposed work area is located
between Siu Lam and Ka Loon Tsuen. At
Tai Lam Kok, some landing facilities may cause some local turbidity but the
extent is limited. These facilities will probably be removed under the proposed
road improvement works. Since there is
no industrial nor densely populated residential development in the adjacent
areas, sources of water pollutants mainly arise from run-off of existing roads
and domestic discharge into Tai Lam Chung Nullah from the isolated huts located
up the valley.
5.9
Routine water quality monitoring of
marine waters has been carried out by the EPD and the findings are published
annually. A summary of EPD’s monitoring
data (for 1998) at station NM1 and
NM2 (Figure 5.1) which is located in the vicinity of the works area is given in
Table 5.2.
5.10
The data show that the water quality
at NM1 and NM2 was by and large similar and generally complied with the WQO
with the exception of NM1 which failed to comply with the depth-averaged DO
objective. Long-term water quality
trends indicate that there is a significant rise in the E.coli level at NM1 between 1989 and 1998. This may be related to the increased sewage
input from nearby unsewered areas in Sham Tseng, Tsing Lung Tau and Tai Lam
Chung where sewerage will be provided in phases under the relevant Sewerage
Master Plan Implementation Work.
Table 5.2 Summary Statistics of Marine Water Quality
at NM1 and NM2 for 1998
|
Determinand
|
NM1
|
NM2
|
|
Number of samples
|
6
|
6
|
|
Temperature (ºC)
|
23.6
|
24.9
|
|
(18.2-26.8)
|
(19.0-29.6)
|
|
Salinity (ppt)
|
29.3
|
24.4
|
|
(21.9-32.5)
|
(11.0-31.7)
|
|
Dissolved Oxygen (mg/L)
|
Surface
|
5.5
|
6.2
|
|
|
(3.2-8.4)
|
(42.-7.9)
|
|
Bottom
|
5.0
|
5.6
|
|
|
(2.7-8.3)
|
(5.1-6.2)
|
|
Dissolved Oxygen (% Saturation)
|
Surface
|
79
|
82
|
|
|
(55-106)
|
(62-100)
|
|
Bottom
|
72
|
80
|
|
|
(38-105)
|
(71-90)
|
|
pH
|
8.0
|
8.1
|
|
(7.9-8.2)
|
(7.9-8.3)
|
|
Secchi Disc Depth (m)
|
2.2
|
1.4
|
|
(1.5-3.0)
|
(1.0-1.5)
|
|
Turbidity (NTU)
|
6.5
|
7.3
|
|
(3.1-12.0)
|
(4.7-12.5)
|
|
Suspended Solids (mg/L)
|
4.0
|
6.3
|
|
(1.1-6.7)
|
(3.5-10.4)
|
|
5-day Biochemical Oxygen Demand (mg/L)
|
0.9
|
0.9
|
|
(0.2-2.0)
|
(0.3-2.7)
|
|
Ammoniacal Nitrogen (mg/L)
|
0.13
|
0.14
|
|
(0.03-0.30)
|
(0.05-0.030)
|
|
Unionized Ammonia (mg/L)
|
0.005
|
0.006
|
|
(0.002-0.007)
|
(0.004-0.009)
|
|
Nitrite Nitrogen (mg/L)
|
0.03
|
0.05
|
|
(0.02-0.06)
|
(0.02-0.06)
|
|
Nitrate Nitrogen (mg/L)
|
0.27
|
0.38
|
|
(0.10-0.52)
|
(0.11-0.94)
|
|
Total Inorganic Nitrogen (mg/L)
|
0.43
|
0.57
|
|
(0.24-0.66)
|
(0.28-1.07)
|
|
Total Kjeldahl Nitrogen (mg/L)
|
0.84
|
0.79
|
|
(0.51-1.23)
|
(0.22-1.20)
|
|
Total Nitrogen (mg/L)
|
1.14
|
1.22
|
|
(0.84-1.55)
|
(0.89-1.70)
|
|
Orthophosphate Phosphorus (mg/L)
|
0.03
|
0.03
|
|
(0.01-0.04)
|
(0.02-0.05)
|
|
Total Phosphorus (mg/L)
|
0.05
|
0.06
|
|
(0.03-0.07)
|
(0.05-0.07)
|
|
Silica (as SiO² ) (mg/L)
|
2.0
|
2.9
|
|
(0.4-4.0)
|
(0.9-6.4)
|
|
|
3.2
|
4.0
|
|
(1.5-6.1)
|
(1.3-11.0)
|
|
Phaeo-pigment
(µg/L)
|
0.9
|
2.0
|
|
(0.2-2.7)
|
(0.2-9.3)
|
|
E.coli (cfu/100mL)
|
110
|
300
|
|
(6-570)
|
(120-1900)
|
|
Faecal Coliforms (cfu/100mL)
|
600
|
1300
|
|
(20-1400)
|
(360-4200)
|
Note: 1. Except as specified, data presented
are depth-averaged data.
2. Data
presented are annual arithmetic means except for E.coli and faecal coliforms which are geometric means.
3. Data
enclosed in brackets indicate the ranges.
5.11
In accordance with the EIAO-TM, no
water quality sensitive receivers have been identified within the Study Area
(defined in the EIA Study Brief as a distance of 300m from the proposed road
alignment). The nearest receivers are
the Ma Wan Fish Culture Zone (FCZ) and nursery areas in bays on north Lantau to
the west of Kap Shui Mun Channel, which are located at about 2 km and 2.5 km to
the south-east of the works area boundary, respectively. In addition, no gazetted and non-gazetted
bathing beaches are found within the Study Area. The nearest gazetted beaches are Golden Beach and Anglers’ Beach
which are about 2.5km to the west and 2km to the east of the works area
boundary, respectively.
Construction Phase
Tai Lam Kok Reclamation and Seawall Construction
5.12
As the rates of dredging for the
seawall construction and reclamation at Tai Lam Kok are expected to be low and
small scale (3,000 m3 day-1), a near field model of
sediment dispersion [R.E. Wilson 1979)]
has been adopted to assess the impacts from suspended sediment plumes generated
during dredging activities.
5.13
In this model, the depth-averaged
suspended sediment concentrations are calculated at varying distances from the
source of the sediment release. This
model has been used to predict suspended sediment concentrations with distance
from the dredging works and hence determine at what distance the elevations in
suspended sediment concentrations would be acceptable according to the WQO. The WQO for suspended sediments for Western
Buffer and North Western WCZs states that the marine activities during the
construction works must not cause the natural ambient level to be raised by
more than 30% nor give rise to accumulation of suspended sediments.
5.14
For the purpose of the assessment,
monitoring data for the past 4 years (1997-2000) for stations NM1 and NM2 was
obtained from EPD to establish the ambient values and allowable increases in SS
concentration under the WQO for the Study Area and Golden Beach. For other sensitive receivers, the ambient
values and allowable increases in SS concentration were taken from the EPD
monitoring station WM4 (Figure 5.1).
Table 5.3 shows the allowable elevations in SS concentration.
Table 5.3 Ambient and Allowable Elevation in
Suspended Sediment Concentrations (mgL-1)
|
Sensitive Receiver
|
90 Percentile
|
30% Elevation
|
|
Ma Wan Fish Culture Zone / Kap Shui Mun /Anglers Beach (WM4)
|
15.0
|
4.5
|
|
Golden Beach (NM2)
|
10.9
|
3.3
|
|
Study Area (NM1)
|
10.2
|
3.1
|
|
|
|
|
Notes: All values are depth-averaged
Land Based Construction Activities
5.15
The potential water quality impact
associated with the land based construction activities has been assessed
qualitatively. Consideration has been
given to controlling potentially harmful impacts from works sites and to the
use of ‘best’ practice measures to minimise the potential for discharges of
pollutants to nearby receiving waters.
Operation Phase
5.16
According to the latest available
layout of the proposed road, only small-scale reclamation of 0.8 ha is required
at Tai Lam Kok. The reclamation will
follow the shape of the existing shoreline and the maximum extent of the new
seawall will only be around 50m from the existing shoreline. Although works will be required in the
sub-tidal areas for earth embankment along the existing coastline, the general
shape of the existing shoreline within the Study Area is expected to remain
unchanged. Hence, significant
alteration to the tidal flows and flushing effects of the nearby waters and
Victoria Harbour is not anticipated.
Identification of Impacts
Construction Phase
5.17
Potential sources of water quality
impacts associated with the proposed construction works are identified as
follows:
·
Seawall and reclamation at Tai Lam Kok
·
Seawall construction between Ka Loon
Tsuen and Tai Lam Kok
·
Construction runoff and drainage
·
General construction activities
·
Sewage effluent
5.18
Construction of the seawall and
reclamation at Tai Lam Kok and seawall between Ka Loon Tsuen and Tai Lam Kok
will involve dredging activities which will cause disturbance to the marine
sediments and release of fine sediments into suspension. The suspended fine sediment may then be
transported away from the work area by tidal currents to form sediment plumes. The extent of the fine sediment lost to
suspension will primarily depend on dredging rates and methods. As indicated in the programme, filling will
only commence when the seawall construction has been completed and will take
place behind the seawalls. Any losses
of fine sediment to suspension during filling will thus be retained by the
seawalls and are not expected to affect the receiving waters beyond the
reclamation areas. Hence, only the
losses of fine sediment to suspension from dredging activities are considered
to have the potential to impact on sensitive receivers. Furthermore, as the proposed seawall from Ka
Loon Tsuen to Tai Lam Kok is in close proximity to the existing seawall,
rockfill rather than sandfill will be used to fill the gap between the old and new
seawalls.
5.19
During dredging, contaminants such as
heavy metals and other toxic substances may be released from sediment when
seabed is disturbed. The potential for adverse effects on water quality through
mobilization and release of contaminants into the surrounding water column
during dredging will depend on the level of contamination of the marine
sediment. As part of the project, a
sediment sampling and testing survey within the Study Area was undertaken to
provide an indication of the sediment quality for the purpose of this EIA
study. The sediment testing results
indicate that the majority of the sediment is classified as Category L. Sediment sub-samples at three sampling
locations (VB5 at depth 1.9-2.1m, VB9 at depth 0.9-1.9m, & GS1 surface sediment)
were classified as contaminated (Category M, H and H, respectively), according
to WBTC No. 3/2000 Management of
Dredged/Excavated Sediment. The
sediment contamination is primarily due to the levels of arsenic and TBT
exceeding the Lower Chemical Exceedance Levels (LCEL). Further discussion of the sediment quality
and classification of dredged sediment is given in Section 6. It should be noted that VB5 and VB9 are
located outside the area where dredging for the foundation of the new seawall
for TLK to KLT is identified to be required.
5.20
During land based construction
activities, runoff and drainage from construction sites would be the major
sources of potential water quality impacts to the nearby water bodies. Site run-off and drainage may contain increased
loads of suspended solids and contaminants and may enter the adjacent coastal
waters if uncontrolled. In addition,
sewage arising from the on-site construction workforce will have the potential
to cause water pollution if it is discharged directly into adjacent waters
without any appropriate treatment.
Cumulative Impacts
5.21
If the suspended sediment plumes
arising from the dredging activities of the proposed project were to interface
with those from other concurrent projects, there would be the potential of
cumulative impacts on nearby sensitive receivers. Projects which may have marine works undertaken concurrently with
the dredging works of the proposed project have been identified and are listed
below:
·
Container Terminal 9 reclamation
·
Backfilling of South Tsing Yi and
North Lantau Marine Borrow Areas with uncontaminated mud
·
Tsuen Wan Bay Further Reclamation
·
Sham Tseng Further Reclamation
·
Tang Lung Chau Dangerous Goods
Anchorage
·
Lamma Extension power station
reclamation
·
Tung Chung and Tai Ho Further
Development
·
Yam O Reclamation for Theme Park.
5.22
The marine works for the Penny’s Bay
Reclamation will be completed in June 2002.
The cumulative impacts from the proposed project and the above-mentioned
concurrent projects have been assessed in terms of elevated suspended sediment
(SS) concentrations. Increases in SS
concentrations from these concurrent projects have been determined by reviewing
the results of the water quality assessments undertaken in the EIA studies for
these projects.
Operational Phase
5.23
During the operation phase, the
potential water quality impact on the coastal waters will arise primarily from
run-off across paved surfaces of the proposed widened carriageway. Such runoff particularly in the first flush
following a prolonged dry period, may contain minimal amount of oil, grease and
grit.
Predication and Evaluation of Impacts
Construction Phase
Release of Suspended Sediment during Dredging
5.24
The preliminary construction programme
shows the timings and duration of dredging and filling for the reclamation at
Tai Lam Kok and for the road widening from Ka Loon Tsuen to Tai Lam Kok The dredging will be undertaken using two
open grab dredgers, with a total dredging rate of 3,000m3 day-1
(assuming 24-hour working per day).
5.25
With respect to rate of sediment loss
during dredging, two previous studies reviewed world-wide data on loss
rates from dredging operations and concluded that for grab dredgers working in
areas with significant amounts of debris on the sea bed that the sediment loss
rate would be 25kgm-3, while the loss rate in areas where debris is
less likely to hinder the operations would be 17kgm-3. Since there are no existing mooring buoys or
port facilities in the vicinity of the proposed dredging areas, there are
unlikely to be significant quantities of debris on the seabed. The lower figure of 17kgm-3 has
therefore been taken for this study.
Based on the daily volume rate of dredging, the loss rate is calculated
to be 0.59kgs-1.
5.26
The concentrations of suspended
sediment plumes has been calculated based on a simple model that is used to
calculate the depth-averaged suspended sediment concentrations along the
centreline of a plume by solving the advection-diffusion equation for a
continuous line source. This model is
appropriate for areas where the tidal current is uni-directional for each phase
of the tidal cycle (i.e. the ebb and flood phases). Since the currents at the proposed dredging area generally follow
the coastline, it is considered the model is appropriate for the calculation. In addition, the model is applicable for
suspended sediment plumes of length no greater than the maximum tidal
excursion. According to the Northshore
Lantau Development Feasibility Study
the maximum tidal current speed in the study area is approximately 0.5 ms-1
and a representative period for each phase of the tidal cycle in Hong Kong is 6
hours. The tidal excursion is
calculated to be 6.9 km based on the following equation.
Tidal
excursion = maximum speed * period * 2/p
5.27
Owing to the low dredging rate, the
extent of the plumes is limited and is expected to be within the tidal
excursion. The following formula is
used to estimate the concentration of suspended sediment (SS) at a certain
distance from the source:
C(x) =
q/(D*x*w*Öp)
Where C(x)
= SS concentration at distance x from the source
q =
sediment loss rate = 0.59 kgs-1
D =
water depth = 10.5 m
x =
distance from source
w
= diffusion velocity = 0.01 ms-1
For
calculation of SS concentrations, a water depth of 10.5m, which is the minimum
depth at the dredging areas has been selected to give a worst-case assessment
as SS concentration is inversely proportional to depth. The diffusion velocity represents reductions
in the centre-line concentrations due to lateral spreading. As suggested by Wilson (1979), a diffusion
velocity of 0.01ms-1 has been used for the calculation.
5.28
Based on the above equation, the
suspended sediment concentrations have been predicted and are presented in
Table 5.4.
Table 5.4 Predicted Elevation in Suspended Sediment
Concentrations (Non-cumulative and Unmitigated
Scenario)
|
Distance from Source
(m)
|
Concentration of
Suspended Sediment (mgL-1)
|
|
100
|
31.7
|
|
200
|
15.9
|
|
300
|
10.6
|
|
400
|
7.9
|
|
500
|
6.3
|
|
600
|
5.3
|
|
700
|
4.5
|
|
800
|
4.0
|
|
900
|
3.5
|
|
1000
|
3.2
|
|
1100
|
2.9
|
|
1200
|
2.6
|
|
1300
|
2.4
|
|
1400
|
2.3
|
|
1500
|
2.1
|
|
1600
|
2.0
|
|
1700
|
1.9
|
|
1800
|
1.8
|
|
1900
|
1.7
|
|
2000
|
1.6
|
5.29
As described in Section 5.11, the
nearest receivers include Ma Wan FCZ, nursery areas in bays on north Lantau to
the west of Kap Shui Mun Channel, Golden Beach and Anglers’ Beach, which are
all located outside the Study Area and far away from the works area of the
proposed project (at a distance of greater than 2 km). As shown in the above table for the
unmitigated scenario, the SS concentration at a distance of more than 0.7 km
from the dredging site will be below the allowable elevations in SS
concentration given in Table 5.3. The
elevated SS concentrations at Ma Wan FCZ and Anglers’ Beach are predicted to be
1.6 mg/L, which is well below the allowable elevated SS concentration.
5.30
It is known that the water quality at
Ma Wan FCZ, which is particularly sensitive to water quality is already heavily
influenced by other ongoing marine works in the vicinity of the area. Although the elevated SS concentrations from
the proposed project would be well below the allowable elevated SS
concentration, there is the potential for cumulative impacts from marine works
of the proposed project and the concurrent projects as listed in Section
5.21. Hence, cumulative impact
assessment for the Ma Wan FCZ has been carried out.
5.31
Since the maximum elevated SS
concentrations arising from these concurrent projects on the Ma Wan FCZ were
predicted to occur in the dry season, the predicted elevations in SS for the
dry season have been used for the cumulative impact assessment as a worst case
scenario. The potential cumulative
impacts from the proposed project and the concurrent projects on the Ma Wan FCZ
in terms of elevated SS concentrations are presented in Table 5.5.
Table 5.5 Maximum
Predicted Cumulative Elevations in Suspended Sediment Concentration at Ma Wan
Fish Culture Zone (Unmitigated Scenario)
|
Project
|
Elevations in Suspended
Sediment Concentration, mg/l
|
|
CT9 Reclamation
|
3.7
|
|
Backfilling North of Lantau and South
Tsing Yi MBAs
|
2.9
|
|
Tang Lung Chau Dangerous Goods Anchorage
|
0.9
|
|
Further Development in Tung Chung &
Tai Ho
|
<1
|
|
Tsuen Wan Bay Further Reclamation
|
0.0
|
|
HEC Lamma Extension Reclamation
|
0.0
|
|
Sham Tseng Further Reclamation
|
1.5
|
|
Yam O Reclamation for Theme Park
|
0.0
|
|
Reclamation at Tai Lam Kok of the
proposed project
|
1.6
|
|
Total
|
11.6
|
5.32
As shown in the above table, the total
cumulative elevation in SS concentrations is predicted to be 11.6 mg/l,
exceeding the allowable elevated SS concentration of 4.5 mg/l for the Ma Wan
FCZ. The results indicate that the
dredging works of the proposed project contribute 1.6mg/l (approximately 14%)
to the total cumulative elevation in SS concentration. Hence, appropriate water quality mitigation
measures are considered necessary to minimize the potential cumulative impact
on the Ma Wan FCZ and are discussed in Section 5.48 – 5.54.
Release of Contaminants during Dredging
5.33
Since the sediment testing results
show that the levels of PCB and PAH in local sediments are less than the
respective LCEL, these potential contaminants would not be of concern. However, the testing of tributyltin (TBT) in
interstitial water indicates that the TBT level at GS1 has exceeded the LCEL,
whereas no TBT exceedances were found at the other grab sampling. GS1 is
located close to the Brothers Point Landing Facilities at Tai Lam Kok, which
could be one reason for this localized TBT exceedence. In order to assess the likelihood of release
of TBT from the marine mud during dredging activities, elutriation tests were
conducted. Since there is no existing
local legislative standards or guidelines for the concentration of TBT in
marine waters, reference is made to the Japanese Standard .
A standard of 0.1 mg TBT/L
is recommended for shipyards, harbours and fisning port and areas with
pollution sources. This standard is
considered applicable to the study area as a barging point is located at Tai
Lam Kok and the Tai Lam Chung Nullah carried domestic discharges from unsewered
areas.
5.34
Table 5.6 shows the results of
elutriate test for the vibrocores VB2 – VB4, which were located within the
dredging area of the proposed project.
Table 5.6 Marine
Site Investigation Sediment Elutriate Test Results
|
Vibrocore
|
Sampling Depth (m)
|
TBT Content (mg-TBTL-1)
|
|
VB2
|
0
– 0.9
|
<0.05
|
|
0.9
– 1.9
|
<0.05
|
|
VB3
|
0
- 1.37
|
<0.05
|
|
1.37
– 2.37
|
<0.05
|
|
VB4
|
0
– 0.9
|
<0.05
|
|
0.9
– 1.9
|
<0.05
|
|
1.9
– 2.9
|
<0.05
|
|
Blank
|
-
|
<0.05
|
5.35
As indicated in the above table, the
measured TBT concentrations in the elutriate samples were not higher than the
Japanese Standard and the background values recorded in the seawater sample
from the site (blank results).
Furthermore, there are no water sensitive receivers (WSR) identified in
the vicinity of the proposed dredging area (the closest WSR is located at a
distance of more than 2km , Section refers 5.11). With the implementation of the recommended mitigation measures,
including closed grab dredgers to minimize sediment resuspension (Section 5.48
– 5.54), it is therefore concluded that adverse water quality impacts due to
the potential release of TBT from the sediment are not anticipated during the
dredging activities.
5.36
The comparison of the elutriate test
results for nutrients with the seawater sample from the site indicates that the
concentrations of total inorganic nitrogen (TIN) and ammoniacal nitrogen
(NH3-N) in the elutriate samples are higher than the background values recorded
in the seawater sample. Therefore, it
can be concluded that these nutrients are likely to be released from the
sediment into the marine waters when the seabed is disturbed during dredging
activities.
5.37
An assessment of nutrients release
during dredging has been made in relation to the results of the predicted
elevation in SS concentrations and the sediment quality data for the study
area. The predicted elevations in SS
concentrations for the unmitigated scenario at the nearest sensitive receivers
(Ma Wan FCZ and Anglers’ Beach) which are located within the Western Buffer WCZ
will be used to calculate the effects of these increased suspended sediment concentrations
on NH3-N and TIN concentrations. In the
calculation, it is assumed that all of the TIN and ammonia concentrations in
the sediment are released to the water.
These are conservative assumptions and will likely result in an
over-prediction of the potential impacts.
5.38
In order to determine compliance with
the water quality criteria, the background water quality data at the sensitive
receivers is based on EPD’s monitoring data (for 1998). The calculated NH3-N released from the
sediment will result in a concentration of total ammonia in the receiving
waters that must be converted to unionised ammonia to compare with the WQO of
Western Buffer WCZ (unionized ammonia: annual mean not to exceed 0.21
mg/L). The WQO for TIN states that the
annual mean depth average inorganic nitrogen not to exceed 0.4 mg/L. The data at EPD monitoring station WM4
indicates that on average the unionised ammonia constitutes 4.2% of the total
ammonia concentration. The results of
the analysis for TIN and NH3-N are contained in Tables 5.7 and 5.8 below.
Table 5.7 Calculation of
the Effects of Increased Suspended Sediment Concentrations on Total Inorganic
Nitrogen Concentrations
|
Sensitive Receiver
|
Elevation in SS
concentration
(mg L-1)
|
TIN in Sediment
(mg kg-1) (1)
|
TIN Increase (mg L-1)
|
Background
TIN (mg L-1)(2)
|
Total TIN (mg L-1)
|
|
Ma Wan Fish Culture Zone/ Anglers Beach
(WM4)
|
1.6
|
28.5
|
4.6
x 10-5
|
0.29
|
0.290
|
Notes: (1) Sediment quality
data from marine site investigation works
(2) Taken from EPD routine water quality monitoring data (1998)
Table 5.8 Calculation
of the Effects of Increased Suspended Sediment Concentrations on Unionised
Ammoniacal Nitrogen Concentrations
|
Distance from source
|
Elevation in SS
concentration
(mg L-1)
|
NH3-N in Sediment
(mg kg-1)(1)
|
Increase in Unionised
Ammonia
(mg L-1)
|
Background Unionised
Ammonia
(mg L-1)(2)
|
Total Unionised Ammonia
(mg L-1)
|
|
Ma Wan Fish Culture Zone/ Anglers Beach
(WM4)
|
1.6
|
2.23
|
1.5
x 10-7
|
0.005
|
0.005
|
Notes: (1) Sediment quality
data from marine site investigation works
(2) Taken from EPD routine water quality monitoring data (1998)
5.39
The data in Tables 5.7 and 5.8 indicate
that there will be no exceedance of the WQOs for TIN and unionised ammonia.
5.40
As described in Section 5.19, the
sediment testing results indicate that all the samples collected within the
proposed dredging area are classified as Category L (Uncontaminated). Hence, it is considered that the potential
release of metals into the water column is not anticipated to result in adverse
impacts on water quality during the dredging works.
Construction Runoff and Drainage
5.41
During road widening and associated
improvement works for CPR from Siu Lam to Ka Loon Tsuen, soil surfaces would be
exposed and an elevated level of suspended particles would be present in the
surface run-off. Sediment laden runoff
may carry pollutants (adsorbed onto the particle surfaces) into the stormwater
drainage system. Sources of water
pollution include release of cement materials with rain wash, wash water from
dust suppression sprays, and fuel, oil and lubricants from maintenance of
construction vehicles and mechanical equipment.
5.42
Mitigation measures should be
implemented to control construction site runoff, and to minimise the chances of
introducing sediment and pollutants into the stormwater drainage system and the
receiving coastal waters.
General Construction Activities
5.43
On-site construction activities may
cause water pollution from the following:
·
Uncontrolled discharge of debris and
rubbish such as packaging, construction waste and refuse.
·
Spillages of liquids stored on-site,
such as oil, diesel and solvents etc, are likely to result in water quality
impacts if they enter adjacent drains or coastal waters.
5.44
Good construction practices and site
management should be observed to ensure that litter, fuels and solvents do not
enter the nearby coastal waters and storm water drains.
Sewage Effluent
5.45
Domestic
sewage would be generated from the workforce during the construction
phase. It is unlikely that sewage
generated from the site would have a significant water quality impact, provided
that sewage is not discharged directly into stormwater drains or coastal waters
adjacent to the construction site and temporary sanitary facilities, such as
portable chemical toilets, are used on-site and properly maintained.
Operation Phase
5.46
During the operation phase, the
potential for water quality impact on the coastal waters will arise primarily
from run-off across paved surfaces of the proposed widened carriageway. Such runoff, particularly in the first flush
following a prolonged dry period, may cause water quality impacts to the
receiving waters if uncontrolled. A
surface water drainage system will be provided to collect the road runoff.
5.47
Accidental spillage may be a concern
during the operation phase of the project.
In the event of a spill, it is the responsibility of appropriate
authorities to take immediate action to cordon the area of spillage and
implement clean-up actions. As there
are no water sensitive receivers located in the vicinity of the proposed road
works, the impact associated with the discharge of this runoff or accidental
spillage is not anticipated to be adverse.
Mitigation of Adverse Environmental Impacts
Construction Phase
Dredging
5.48
As discussed in Section 5.30 – 5.32, the
cumulative elevation in SS concentration is predicted to exceed the allowable
elevated SS concentration of 4.5mg/l at the Ma Wan FCZ. The dredging works of the proposed project
would contribute approximately 14% of the total predicted exceedance without
the implementation of mitigation measures for the project. In order to minimize the potential
cumulative impact from dredging works of the proposed project, the use of
closed grab dredgers and silt curtains is recommended.
5.49
With the implementation of the
recommended closed grab dredgers and silt curtains, the sediment loss rate
would be reduced to 0.1 kgs-1 (with respect to rate of sediment loss
during dredging, the Contaminated Spoil Management Study (Mott MacDonald, 1991, Table 6.12)
reviewed relevant literature and concluded that losses from close-grab dredgers
with silt curtain were estimated at 3
kgm-3 of mud removed). The
elevation in SS concentrations from the mitigated dredging works of the project
has been predicted and is shown in Table 5.9.
Table 5.9 Predicted
Elevation in Suspended Sediment Concentrations (Non-cumulative and Mitigated
Scenario)
|
Distance from Source
(m)
|
Concentration of
Suspended Sediment (mgL-1)
|
|
100
|
5.37
|
|
200
|
2.69
|
|
300
|
1.79
|
|
400
|
1.34
|
|
500
|
1.07
|
|
600
|
0.90
|
|
700
|
0.77
|
|
800
|
0.67
|
|
900
|
0.60
|
|
1000
|
0.54
|
|
1100
|
0.49
|
|
1200
|
0.45
|
|
1300
|
0.41
|
|
1400
|
0.38
|
|
1500
|
0.36
|
|
1600
|
0.34
|
|
1700
|
0.32
|
|
1800
|
0.30
|
|
1900
|
0.28
|
|
2000
|
0.27
|
5.50
As indicated in Table 5.9, the
elevation in SS concentrations from the mitigated dredging works of the project
is predicted to be 0.27 mg/l at the Ma Wan FCZ. The total increase in SS concentrations at the Ma Wan FCZ due to
the cumulative effect of the proposed project and concurrent projects would
thus be reduced to 10.3 mg/l, of which the dredging works of the proposed
project contributes less than 3%. This
indicates that the dredging works of the proposed project would not contribute
significantly to the total SS concentrations at the Ma Wan FCZ. Further constraints on the dredging
activities of the proposed project could not reduce the predicted cumulative
impact to below the WQO and therefore it would be the responsibility of the
concurrent projects to apply mitigation measures if unacceptable water quality
impacts are detected during construction.
5.51
In order to determine whether the
potential exceedance of the WQO at the Ma Wan FCZ will adversely impact on the
fish stocks, the total SS concentration (i.e. ambient SS concentration plus the
predicted elevation) has been predicted.
As shown in Table 5.3, the ambient SS concentration at the Man Wan FCZ
is 15mg/l. Hence, the total SS
concentration at the Ma Wan FCZ is predicted to be 26.3mg/l. Literature reviews show that lethal responses
had not been reported in adult fish at SS concentration below 125 mg/l and that sublethal effects were only observed
when levels exceeded 90 mg/l . It should be noted that these tolerance
thresholds were taken from international literature as there are no sediment
tolerance data specific to Hong Kong species.
Since the predicted total SS concentration at the Ma Wan FCZ is well
below the tolerance levels for fish species, adverse cumulative impacts to
culture fisheries resources at the Ma Wan FCZ are not anticipated.
5.52
It should be noted that the cumulative
impact assessment has adopted the maximum elevated SS concentrations predicted
at the Ma Wan FCZ from each of the concurrent projects and is therefore
considered conservative. For instance,
the elevated SS concentration from CT9 has adopted the value predicted during the
construction of the major dredging works.
However, according to the construction program of CT9, all the major
dredging works will be completed prior to the commencement of the construction
of the proposed project and only maintenance dredging works will be carried out
concurrently with the project.
Furthermore, it has been assumed that the maximum elevations in SS
concentrations at the Ma Wan FCZ from the concurrent projects occur at the same
time within the tidal cycle, which may not necessarily be the case. Therefore, the actual total elevated SS
concentration from the proposed project and the concurrent projects would
likely be much lower than the predicted value.
It should also be noted that the dredging works for the project will be
short-term comprising 5 months in the year 2003 and only 2 months in the year
2005. Since the dredging works will be
short-term, it may be possible to schedule the dredging works to be undertaken
during the periods of least marine work in Western Harbour and this should be
examined during the detailed design stage.
Hence, the cumulative impact would probably be further reduced.
5.53
It is known that the beaches at north
of Ma Wan, such as Gemini Beach, are located on the fast-flowing Ma Wan Channel
which is a major tidal channel connecting Northwest Waters and Western Harbour
and would likely be more exposed to SS compared with the more sheltered Ma Wan
FCZ. However, as these beaches are
situated at a distance of more than 4km from the dredging area of the project,
no water quality impact on these beaches is anticipated from the proposed
project with the recommended closed grab dredgers and silt curtains in place.
5.54
In order to alleviate any potential
water quality impact from the proposed project, the following mitigation
measures shall be implemented during the construction of the seawalls and
reclamation:
·
Dredging should be undertaken using
closed grab dredgers with a maximum total production rate of 3,000 m3
day-1;
·
Deployment of silt curtain around the
immediate dredging area while dredging works are in progress;
·
Filling should commence only after the
completion of seawall construction and should be undertaken behind the
seawalls;
·
Mechanical grabs should be designed
and maintained to avoid spillage and seal tightly while being lifted;
·
All vessels should be sized so that
adequate clearance is maintained between vessels and the seabed in all tide
conditions, to ensure that undue turbidity is not generated by turbulence from
vessel movement or propeller wash;
·
All hopper barges and dredgers should
be fitted with tight fitting seals to their bottom openings to prevent leakage
of material;
·
Construction activities should not
cause foam, oil, grease, scum, litter or other objectionable matter to be
present on the water within the site or dumping grounds; and
·
Loading of barges and hoppers should
be controlled to prevent splashing of dredged material into the surrounding
water. Barges or hoppers should not be
filled to a level that will cause the overflow of materials or polluted water
during loading or transportation.
Construction Runoff and Drainage
5.55
The Contractor shall observe and
comply with the Water Pollution Control Ordinance and its subsidiary
regulations. The Contractor shall carry
out the Works in such a manner so as to minimise adverse impacts on the water
quality during the road improvement works.
5.56
Proper site management measures shall
be implemented to control site runoff and drainage, and thereby prevent high
sediment loadings from entering the nearby coastal waters and stormwater
drains. The Contractor shall follow the
practices, and be responsible for the design, construction, operation and
maintenance of all the mitigation measures 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. These mitigation measures shall include the
following practices to minimise site surface runoff and the chance of erosion,
and also to retain and reduce any suspended solids prior to discharge:
·
Before commencing any site formation
work, all sewer and drainage connections shall be sealed to prevent debris,
soil, sand etc. from entering public sewers/drains.
·
Provision of perimeter channels to
intercept storm-runoff from outside the site.
These shall be constructed in advance of site formation works and
earthworks.
·
Temporary ditches such as channels,
earth bunds or sand bag barriers shall be included to facilitate runoff
discharge into the stormwater drain, via a sand/silt basin/trap.
·
Works programmes shall be designed to
minimise works areas at any one time, thus minimising exposed soil areas and
reducing the potential for increased siltation and runoff.
·
Sand/silt removal facilities such as
sand traps, silt traps and sediment basins shall be provided to remove the
sand/silt particles from run-off. These
facilities shall be properly and regularly cleaned and maintained. These
facilities shall be carefully planned to ensure that they would be installed at
appropriate locations to capture all surface water generated on site.
·
Careful programming of the works to
minimise excavation works during the rainy season.
·
Temporary access roads shall be
protected by crushed gravel and exposed slope surfaces shall be protected when
rainstorms are likely.
·
Open stockpiles of construction
materials on-site shall be covered with tarpaulin or similar fabric during
rainstorms to prevent erosion.
General Construction Activities
5.57
Debris and rubbish generated on-site
should be collected, handled and disposed of properly to avoid entering the
nearby coastal waters and stormwater drains.
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. Open drainage channels and culverts near the
works areas should be covered to block the entrance of large debris and refuse.
Sewage Effluent
5.58 Portable toilets shall be provided by the Contractors, where
necessary, to handle sewage from the workforce. The Contractor shall also be responsible for waste disposal and
maintenance practices.
Operation Phase
5.59
A surface water drainage system will
be provided to collect road runoff. The
following measures are recommended to ensure road runoff will comply with the
standards stipulated in the TM-DSS for discharges into storm water drains:
·
The road drainage should be directed
through silt traps in the gully inlets to remove silt and grit before entering
the public storm water drainage system; and
·
The silt traps should be regularly
cleaned and maintained in good working condition.
Evaluation of Residual Environmental Impacts
5.60
With the adoption and incorporation of
the recommended mitigation measures for both the construction and operation
phases, no residual impacts on water quality impact are anticipated to occur
due to the marine works of the proposed project.
5.61
The use of the recommended closed grab
dredgers and silt curtains could greatly reduce the elevated SS concentrations
contributed from the proposed project at the Ma Wan FCZ from 1.6mg/l to
0.27mg/l. Nevertheless, due to the
cumulative effect from the concurrent projects, the cumulative elevation in SS
concentrations would still exceed the allowable elevated SS concentration by
6.8mg/l. However, the results indicate
that the dredging works of the proposed project would only contribute less than
3% to the total elevated SS concentrations at the Ma Wan FCZ. The duration of the contribution of the
project construction to the predicted elevated SS concentrations would be very
short. Further constraints on the
dredging activities of the proposed project could not reduce the predicted
cumulative impact to below the WQO and therefore it would be the responsibility
of the concurrent projects to apply mitigation measures if unacceptable impacts
are detected during construction.
Environmental Monitoring and Audit
5.62
Monitoring and auditing for marine
water quality is considered necessary during the dredging activities of the
proposed project to ensure that the released SS concentrations from the
dredging activities would not adversely affect the sensitive receivers. This monitoring programme will be required
to ensure the implementation of the recommended water quality mitigation
measures and to assess the effectiveness of these measures during the
construction works. If monitoring
results indicate that the dredging activities have exceeded the predicted
elevated SS concentrations even after the implementation of the recommended
mitigation measures, the construction program should be carefully reviewed to
slow down production rates. Details of
the water quality monitoring procedures will be given in the stand-alone
EM&A Manual.
Conclusion
Construction Phase
5.63
The marine works for Tai Lam Kok
reclamation and seawall construction from Ka Loon Tsuen to Tai Lam Kok are
identified to be the key water quality issues for this project. The potential water quality impacts
associated with the dredging activities during the construction of the
reclamation and seawalls have been assessed by predicting the increases in
suspended sediment concentrations due to fine sediment lost to suspension.
5.64
There are no sensitive receivers
identified within the Study Area as specified in the EIA Study Brief. The nearest receivers include Ma Wan FCZ,
nursery areas in bays on north Lantau to the west of Kap Shui Mun Channel,
Golden Beach and Anglers’ Beach, which are all located outside the Study Area
and are far away from the works area of the proposed project. The assessment results show that the
unmitigated elevations in SS concentration generated from the dredging
activities of the proposed project at these sensitive receivers would be well
below the allowable elevations in SS concentrations. However, the cumulative elevated SS concentrations from the
proposed project and the identified concurrent projects are predicted to exceed
the allowable elevated SS concentration at the Ma Wan FCZ. The use of closed grab dredgers and silt curtains
is therefore recommended to minimize the water quality impact from the dredging
works for the project.
5.65
The potential impact arising from the
release of contaminants from sediment disturbed during the dredging works has
also been assessed. It was predicted
that adverse water quality impacts arising from the release of heavy metals,
nutrients and organic compounds are not anticipated during the dredging works.
5.66
The potential water quality impacts
arising from construction runoff and drainage, and other land-based
construction activities, have been assessed qualitatively.
5.67
It is concluded that minimal impacts
on the receiving waters are expected provided that mitigation measures are
implemented during the construction phase. With the adoption of the recommended
mitigation measures, no unacceptable residual impacts on water quality impact
are anticipated to be associated with the project.
Operation Phase
5.68
The only source of potential impact on
water quality during the operation phase will be runoff from the road
surfaces. With the implementation of
the recommended mitigated measures for the surface water drainage system, it is
anticipated that the water quality impacts associated with the operation phase
would be minimal and acceptable.