5.1.1
This
Section deals with the assessment of the impacts on water quality of the
construction and operation of the Northshore Lantau Development.
5.1.2
The
construction phase assessment has considered the following aspects.
·
the
potential impacts to marine water quality from the construction of the
reclamations associated with the Northshore Lantau Development, which are those
at Siu Ho Wan, Northshore, Fa Peng and Theme Park Extension; and
·
the
potential impacts to marine water quality from land based construction works
associated with the development, including the Chok Ko Wan Link Road.
5.1.3
The
operation phase assessment has considered the following aspects.
·
the
potential impacts to hydrodynamics from the Northshore Lantau Development reclamations;
·
the
potential impacts to marine water quality from changes in hydrodynamics and
sewage effluent and stormwater discharges from the Northshore Lantau
Development;
·
the
potential impacts to marine water quality from the operation of the Chok Ko Wan
Link Road.
5.1.4
The overall
aim of the above assessment work was to determine the acceptability of any
predicted impacts to water quality from the construction and operation of the
Northshore Lantau Development.
Predicted impacts have been assessed with reference to the relevant
environmental legislation and standards, and suitable measures devised to
mitigate any potential adverse impacts.
The need for construction and operation Environmental Monitoring and
Audit has been assessed and recommendation made where necessary.
5.2.1
The
following relevant pieces of legislation and associated guidance are applicable
to the evaluation of water quality impacts associated with the construction and
operation of the Northshore Lantau Development.
·
Water Pollution Control Ordinance (WPCO);
·
Technical Memorandum for Effluents
Discharged into Drainage and Sewerage Systems Inland and Coastal Waters; and
·
Environmental Impact Assessment Ordinance
(Cap. 499. S.16), Technical Memorandum on Environmental Impact Assessment
Process (EIAO TM), Annexes 6 and 14.
5.2.2
Apart from
the above statutory requirements, the Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN
1/94), issued by ProPECC in 1994, also provides useful guidelines on the
management of construction site drainage and prevention of water pollution
associated with construction activities.
5.2.3
The Water Pollution Control Ordinance (WPCO) is the legislation for the control
of water pollution and water quality in Hong Kong. Under the WPCO, Hong
Kong waters are divided into 10 Water Control Zones (WCZs). Each WCZ has a designated set of statutory
Water Quality Objectives (WQOs). The WQOs set limits for different parameters
that should be achieved in order to maintain the water quality within the
WCZs. Elements of the Northshore Lantau
Development will be located within the Southern, Western Buffer and North
Western WCZs. The locations of the WCZs
are shown on Figure 5.2a.
5.2.4
The WQOs
for the Western Buffer, the Southern and the North Western WCZs, which are
presented in Tables D1a, D1b and D1c respectively, are applicable as
evaluation criteria for assessing compliance of any effects from the
construction and operation of the Northshore Lantau Development.
5.2.5
All
discharges during both the construction the operational phases of the Project
are required to comply with the Technical
Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland
and Coastal Waters (TM) issued under Section 21 of the WPCO. The TM defines
discharge limits to different types of receiving waters. Under the TM, effluents discharged into the
drainage and sewerage systems, inshore and coastal waters of the WCZs are
subject to pollutant concentration standards for particular discharge
volumes. Any new discharges within a
WCZ are subject to licence conditions and the TM acts as a guideline for
setting discharge standards for the licence.
5.2.6
Annexes 6 and
14 of the EIAO TM provide general guidelines and criteria to be used in
assessing water quality issues.
5.3.1
Elements of
the Northshore Lantau Development will be contained with the Southern (the
Theme Park Extension), the Western Buffer (Fa Peng Reclamation) and the North
Western (Northshore Lantau and Siu Ho Wan reclamations) WCZs. Sewage effluent from the Northshore Lantau
Development will be conveyed to the Siu Ho Wan Sewage Treatment Works for
treatment and discharge via a submarine outfall.
5.3.2
Kap Shui
Mun, which is the channel between the northern tip of Lantau Island and Ma Wan
Island, forms one of the main flow channel between the waters of the North West
New Territories and the Western Harbour. The other flow channel is on the northern side of Ma Wan Island,
the Ma Wan Channel. Tidal current
speeds through Kap Shui Mun are generally high, greater then 1 ms-1
for the ebb phase of the tidal cycle for spring tides. In the vicinity of the reclamation at Yam O
Wan, tidal currents are lower due to the sheltering effect of the headlands to
north east and south west. Further
offshore, currents are stronger, up to 0.6 ms-1, with the main flows
from the Pearl Estuary converging in this area before bifurcating around Ma Wan
Island.
5.3.3
On the
Penny's Bay side tidal currents are much lower, less than 0.3 ms-1. This is because the main flows to and from
Kap Shui Mun are along the East Lamma Channel and Victoria Harbour.
5.3.4
The areas
potentially affected by the Project will exhibit seasonal differences in terms
of salinity and temperature stratification.
To the west of Ma Wan in the wet season strong salinity and temperature
stratification is to be found due to the outflow from the Pearl Estuary. To the east of Ma Wan the stratification
will be less strong due to the turbulence in the Ma Wan Channel and Kap Shui
Mun causing a partial degradation in the stratification. In the sheltered bay along the southern
Lantau Island coastline, stratification may be present due to localised run-off
and heating of the relatively stagnant surface waters.
5.3.5
The Project
area is within three WCZ’s: the Southern WCZ, the Western Buffer and North
Western WCZs. There are three routine
EPD water quality monitoring stations (one for each WCZ) located in the
vicinity of the Project area; the locations of these stations are shown in Figure 5.2a. A summary of water quality data for each of the stations is
presented in Table 5.3a. These data were measured in 1998, the most
recently published data.
Table
5.3a - EPD Routine Water Quality Monitoring Data in the Vicinity of the Project
Area
|
WQ Parameter
|
WM4
|
SM10
|
NM1
|
|
Temperature
(°C)
|
23.4
(16.7 - 27.7)
|
23.4
(16.3 - 27.2)
|
23.6
(18.2 - 26.8)
|
|
Salinity
(ppt)
|
30.9
(25.9 - 33.6)
|
30.2
(26.9 - 33.4)
|
29.3
(21.9 - 32.5)
|
|
Dissolved
Oxygen (mg L-1)
|
3.2
(2.9 - 8.1)
|
4.0
(3.2 - 7.2)
|
3.5
(3.2 - 8.4)
|
|
Dissolved
Oxygen Bottom (mg L-1)
|
2.6
(2.2 - 8.1)
|
4.3
(3.9 - 7.2)
|
2.9
(2.7 - 8.3)
|
|
5-Day Biochemical
Oxygen Demand
(mg L-1)
|
0.7
(0.2 - 1.5)
|
0.9
(0.1 - 1.8)
|
0.9
(0.2 - 2.0)
|
|
Suspended
Solids (mg L-1)
|
7.6
(1.2 - 17.4)
|
6.9
(4.1 - 9.9)
|
4.0
(1.1 - 6.7)
|
|
Total
Inorganic Nitrogen (mg L-1)
|
0.29
(0.17 - 0.42)
|
0.27
(0.14 - 0.43)
|
0.43
(0.24 - 0.66)
|
|
Unionised
Ammonia (mg L-1)
|
0.005
(0.002 - 0.007)
|
0.003
(0.001 - 0.009)
|
0.005
(0.002 - 0.007)
|
|
E.coli
(cfu 100mL-1)
|
510
(42 - 1,900)
|
9
(2 - 300)
|
110
(6 - 570)
|
|
Notes:
1. Data
presented are depth averaged, except as specified.
2. Data
presented are annual arithmetic mean except for E. coli which are geometric means.
3. Data
enclosed in brackets indicate the ranges.
4. Shaded
cells indicate non-compliance with the WQOs.
|
5.3.6
The data
indicates that there were non-compliances with the WQOs for depth averaged
oxygen at all three stations, although compliance is achieved with the bottom
dissolved oxygen WQO at the stations.
This is a deterioration from data collected in 1997, which showed
compliance with both the depth averaged and bottom dissolved oxygen WQO .
A review of unpublished data for 1999 determined that compliance with the
depth averaged dissolved oxygen WQO was achieved. It may therefore be concluded that it is premature to assess that
the dissolved oxygen concentrations in the study area are deteriorating based
on the 1998 data alone.
5.3.7
The total
inorganic nitrogen WQO is exceeded at Station SM10. The exceedance of the total inorganic nitrogen WQO at Station
SM10 has been recorded for the last ten years.
It is worth noting that the WQO for total inorganic nitrogen is not
exceeded at Station WM4, even though the average concentration is higher than
that at Station SM10. This is because
of the different WQO for total inorganic nitrogen in the Western Buffer WCZ,
which is higher compared with that of the Southern WCZ.
5.3.8
The WQO for
E. coli at Station SM10, which is in
a Secondary Contact Recreation Subzone, is achieved and the levels are low
enough to satisfy the WQO for bathing beaches.
Stations WM4 and NM1 are somewhat influenced by sewage effluent
discharges, as shown by the higher E.
coli concentrations. This is
possibly because these stations are in one of the main flow paths between the
waters of the North West New Territories and Victoria Harbour and will
therefore receive dilute discharges of sewage from these areas.
5.3.9
The data
for temperature, salinity and dissolved oxygen show a wide variation, which
indicates seasonal changes. These are
most pronounced at Station NM1, which is the station most influenced by the
discharges from the Pearl River estuary.
5.3.10
The
construction and operation of the Northshore Lantau Development will have the
potential to directly affect water quality in the waters along the southern and
northern sides of Lantau Island. The
various reclamation will have the potential to change tidal current patterns
around Ma Wan, which could in turn cause water quality effects in the East and
West Lamma Channels, the Rambler Channel and in the western end of Victoria
Harbour. Sensitive receivers have been
identified in these potentially affected areas under the broad designations of
gazetted and non-gazetted bathing beaches, water intakes, fish culture zones,
sites of ecological interest and recreational areas. The identified sensitive receivers in each of these categories
are as follows:
·
Gazetted Bathing Beaches: Butterfly, Castle Peak, Kadoorie,
Cafeteria Old, Cafeteria New, Golden, Gemini, Hoi Mei Wan, Casam, Lido, Ting
Kau, Approach, Tung Wan (Ma Wan), Silvermine Bay, Tung Wan (Cheung Chau), Kwun
Yam Wan, Hung Shing Yeh and Lo So Shing;
·
Non-Gazetted Beaches:
Dragon and Discovery Bay;
·
Water Intakes:
Castle Peak Power Station cooling water intake, Chek Lap Kok cooling
water intake, Tsuen Wan Water Supplies Department (WSD) intake, Tsing Yi WSD
intake, Cheung Sha Wan WSD intake, Yau Ma Tei WSD intake, Sheung Wan WSD
intake, Kennedy Town WSD intake, Queen Mary Hospital/Sha Wan Drive intake and
Wah Fu Estate intake ;
·
Fish Culture Zones:
Ma Wan (South and North), Cheung Sha Wan, Lo Tik Wan and Sok Kwu Wan;
·
Sites of Ecological Interest: Sha Chau, Tung Chung Bay, The Brothers,
Yam O Wan, Kau Yi Chau, Green Island, Pak Kok, Shek Kok Tsui and Luk Chau; and
·
Recreational Uses:
Discovery centre at Sze Pak Wan.
5.3.11
In addition
to the identified sensitive receivers, there are a number of open water
monitoring stations which have been considered in this study to assess water
quality in the marine waters potentially affected by Project construction and operational
activities. The locations of the above
sensitive receivers and open water monitoring stations are shown in Figure 5.3a.
5.3.12
In addition
to the above defined sensitive receivers there are three future planned WSD
seawater intakes at Tai Ho, Sham Shui Kok and Fa Peng. The locations of the intakes are shown on Figure
5.3b
. The quality of the abstracted
water will be required to comply with WSD standards, which are defined below.
5.3.13
It should
be noted that the Anglers Beach, which is currently a gazetted bathing beach,
is not included as a sensitive receiver.
This is because the beach will be lost when the construction of the
proposed Sham Tseng Further Reclamation commences in 2004.
5.3.14
The WQOs
presented in Annex D1 are considered
to be suitable as assessment criteria at the identified sensitive receivers and
monitoring stations. A number of the
sensitive receivers are Water Supplies Department (WSD) sea water intakes. The WSD has a set of standards for the
quality of abstracted water (see in Table
5.3b). Water quality at the WSD sea
water intakes has been assessed against these standards, in addition to the
WQOs.
Table
5.3b - WSD Water Quality Criteria for Abstracted Seawater
|
Parameter
|
Criterion
|
|
Colour
(HU)
|
<20
|
|
Turbidity
(NTU)
|
<10
|
|
Threshold
Odour No.
|
<100
|
|
Ammoniacal
Nitrogen (mg L-1)
|
<1
|
|
Suspended
Solids (mg L-1)
|
<10
|
|
Dissolved
Oxygen (mg L-1)
|
>2
|
|
5-day
Biochemical Oxygen Demand (mg L-1)
|
<10
|
|
Synthetic
Detergents (mg L-1)
|
<5
|
|
E. coli
(cfu 100mL-1)
|
<20,000
|
5.4.1
The
assessment of impacts to water quality during the construction phase has been
divided into two aspects, formation of the reclamations and land based
construction activities, including those for the Chok Ko Wan Link Road.
Description
of Sediment Dispersion Model
5.4.2
The
dispersion of fine sediment in suspension from the construction of the
Northshore Lantau Development reclamations has been simulated using the
Delft3D-PART particle tracking model.
The model simulates a dynamic concentration distribution of a substance
by following the tracks of thousands of discrete particles. The dominant transport process is advection
by tidal currents, while turbulent diffusion is simulated using a random walk
technique. Hydrodynamic data for the
model are provided by the delft3D-FLOW model.
At intervals the concentration of the substance being simulated is
calculated by summing the number of particles in a pre-defined grid. The size of the grid is independent of the
grid used in the hydrodynamic model and is therefore capable of representing
plumes narrow than the finest resolution in the flow model.
5.4.3
The
Delft3D-PART is ideally suited to simulating narrow suspended sediment plumes
generated by reclamation construction activities. The model includes the processes of sediment settling through the
water column and of sediment exchange between the water column and the sea
bed. Settling of sediment particles is
described by settling velocity. The
exchange of sediment between the water column and the sea bed is governed by
the processes of deposition and erosion, which are functions of bed shear
stress.
Derivation
of Scenarios
5.4.4
The
construction of the reclamations for the Northshore Lantau Development will
involve dredging of the existing marine sediments along the line of the
seawalls to provide suitable foundations and filling of the reclamations using
sand and public fill. The reclamations
will be constructed using the drained method of construction, which means that
the existing sea bed sediments under the reclamations will be left in place,
except under the seawalls. The seawall
foundation trenches may be backfilled with either sand or rockfill. The potential impacts to water quality from
this operation are discussed qualitatively and measures devised to minimise the
loss of fine sediment to suspension.
5.4.5
The
programme for reclamation construction is shown in Figure 5.4a, which shows the timings and durations of dredging,
filling and surcharging for the four reclamations (Northshore, Siu Ho Wan, Fa
Peng and the Theme Park Extension). A
key feature of the programme is that filling will not commence until sections
of seawall have been completed. This
means that any losses of fines to suspension during filling will be retained by
the seawalls and not transported beyond the works areas. It will therefore only be necessary to
simulate the losses of fine sediment to suspension from dredging
activities. Examination of the
reclamation programme shows that dredging for the Northshore and Siu Ho Wan
reclamations will be carried out concurrently.
The dredging for the Fa Peng and Theme Park Extension reclamations will
occur later and will not occur concurrently with the dredging for any other
reclamations. There are therefore three
scenarios to be simulated for the construction of the Northshore Lantau
Development reclamations as follows.
·
Scenario 1 - concurrent dredging for the Siu Ho Wan
and Northshore Reclamations;
·
Scenario 2 - dredging for the Fa Peng reclamation;
and
·
Scenario 3 - dredging for the Theme Park Extension
reclamation.
5.4.6
According
to the proposed construction programme the dredging for the seawalls will be
undertaken using varying numbers of grab dredgers, each with a rate of dredging
of 2,000 m3 day-1.
It should be noted that this is a slow rate of dredging and is able to
be accommodated due to the long construction periods for each of the
reclamations. For studies assessing the
impacts for dredging areas of Kellett Bank to provide depths suitable for
mooring buoys, reviews of available international data were made to determine
the loss rates of sediment to suspension from grab dredging .
For these two previous studies, estimates of loss rate from grab
dredging were made based on an extensive review of world wide data on loss
rates from dredging operations. The
review concluded that for 8 m3 grab dredgers working in areas with
significant amounts of debris on the sea bed (such as in the vicinity of
existing mooring buoys) that the loss rate would be 25 kg m3
dredged, while the loss rate in areas where debris is less likely to hinder the
operations would be 17 kg m3 dredged. For this Study it is proposed that the loss rate of 17 kg m3
dredged be used, as there are unlikely to be significant quantities of debris
on the seabed in the vicinity of the dredging works due to the fact that there
are no existing mooring buoys or port facilities. The loss rate of sediment to suspension for each grab dredger
will therefore be 34,000 kg day-1, or 0.39 kg s-1
assuming continuous (ie 24 hour) working.
5.4.7
The number
of grab dredgers working on each of the four reclamation areas and the
resultant total loss rates of sediment to suspension are summarised in Table 5.4a.
Table
5.4a - Summary of Dredging for the Four
Reclamation Areas and the Rates of Loss of Sediment to Suspension
|
Scenario
|
Reclamation
|
No of Grab Dredgers
|
Total Loss Rate (kg s-1)
|
|
1
|
Northshore
Siu Ho Wan
|
1
2
|
0.39
0.78
|
|
2
|
Fa
Peng
|
3
|
1.17
|
|
3
|
Theme
Park Extension
|
3
|
1.17
|
5.4.8
Representative
locations along the lengths of the seawalls were chosen for each of the grab
dredgers. Each location was taken to
represent a single grab dredger. At
each point the sediment lost to suspension was entered into the model as a line
source over the whole water depth to represent the manner in which sediment is
lost to suspension during grab dredging.
The locations of each of the points are shown in Figures 5.4b, 5.4c and 5.4d.
5.4.9
The model
simulates the loss of sediment to suspension by introducing discrete particles
into the model domain, each particle representing a finite mass of
sediment. For each simulation a total
of 1,000,000 model particles were introduced into the model area.
5.4.10
Sediment
settling was simulated by dividing the sediment in suspension into a coarse
fraction and a fine fraction. A grading
curve for the fine fraction of sediment (ie less than 63 mm) for samples collected from Penny’s Bay
was obtained from the Geotechnical Engineering Office of the Civil Engineering
Department. The grading curve is shown on Figure
5.4e
and has been produced as an average of a number of samples. The division into coarse and fine fractions
was based on the D50 grain size, (ie into the finest 50% and the
coarsest 50%). The D50 gain
size is shown on the graph to be 0.003 mm, which is taken to be the size at
which the sediment is divided into coarse and fine fractions. In order to calculate the settling
velocities of the coarse and fine fractions it is necessary to determine
representative grain sizes for each of these fractions. The representative grain size for the coarse
fraction was taken to be the D75 of the whole curve, which is shown
to be 0.012 mm. The settling velocity
was then determined using Stoke’s Law and was found to be 0.128 mm s-1. The representative grain size for the fine
fraction would be represented by the D25, which by extrapolation
would be approximately 0.0004. The
settling velocity for such a grain size would be extremely small and would be
offset by localised turbulence, which means that the fine fraction would
effectively have zero settling velocity.
The total suspended sediment concentration at any location was
calculated as the sum of the coarse and fine fractions.
5.4.11
Impacts to
water quality during the construction of a reclamation at Penny’s Bay and
around the Fa Peng headland were assessed as part of the studies for Container
Terminals 10 and 11 .
In this study computer modelling of the dispersion of fine sediment in
suspension from dredging and filling works was carried out. The modelling studies determined that the
worst case impacts to sensitive receivers were caused by wet and dry season spring
tides. Therefore, for this Study the
sediment plume modelling for Scenarios 2 and 3 was undertaken for wet and dry
season spring tides to determine the worst case impacts. There are, however, no previous computer
modelling studies for either the Northshore or Siu Ho Wan reclamation. Therefore, Scenario 1 was simulated for all
four representative tide types (ie wet and dry season spring and neap tides),
as there is no means of identifying the worst case tide types.
5.4.12
Hydrodynamic
data for input into the sediment plume model have been provided by the Delft3D
hydrodynamic model, which has been used to simulate the situation in 2012
immediately prior to the start of construction of the Northshore Lantau
Development reclamations, which included the reclamations for the proposed
Theme Park in Penny’s Bay and Yam O. A
detailed description of the hydrodynamic modelling is contained in Section 5.8.1.
5.4.13
The results
of the sediment plume modelling have been processed to determine maximum
elevations of suspended sediment concentrations at the identified sensitive
receivers and in the receiving waters.
The results have also been analysed to show rates of sediment deposition
on the sea bed from the dredging works.
Impacts
to Water Quality
5.4.14
The loss of
fine sediment to suspension during the dredging works would have the potential
to cause impacts to dissolved oxygen and nutrient levels within the receiving
waters. If there are any contaminants
(such as heavy metals, PAHs, PCBs etc) within the sediment to be dredged then
they could be released to the water column by disassociating from the
sediments. The potential impacts to
water quality have been assessed by calculation based on the results of the
sediment plume modelling for suspended sediment concentrations and the quality
of the sediments to be dredged.
Cumulative
Impacts
5.4.15
The first
phase of the construction of the reclamations for the Northshore Lantau
Development, which is the Northshore reclamation, is currently proposed to
commence in early 2012 and the completion of all four reclamations is
programmed for mid 2028 (see Figure 5.4a). There are a number of projects planned for
the vicinity of the Northshore Lantau Development reclamations, such as the
Tung Chung and Tai Ho Further Development, Sham Tseng Reclamation, Tang Lung
Chau Dangerous Goods Anchorage, Contaminated Mud Disposal at East Sha Chau,
Backfilling of North Lantau and South Tsing Yi Marine Borrow Areas. However, these projects will have all been
completed prior to 2012. There are no
other confirmed projects beyond 2012 in the vicinity of the Northshore Lantau
Developments and as such there are not anticipated to be any cumulative
impacts. If, however, information to
contradict this assessment is available at the time of further detailed studies
of the Northshore Lantau Developments then it will be necessary to review the
necessity of carrying out cumulative impact assessments.
Uncertainties
in Assessment Methodology
5.4.16
Quantitative
uncertainties in the sediment plume modelling should be considered when making
an evaluation of the modelling predictions.
Worst case conditions were adopted as model input in order to provide a
conservative prediction of environmental impacts. It is therefore possible that the input parameters may cause an
overestimation of the environmental impacts.
Some examples of the conservative nature of the input parameters are
given below.
·
The flow
model simulations used for generating the flow data for the sediment dispersion
modelling represent the case prior to the construction of any of the seawalls
for the reclamations and would therefore result in greater dispersion of the
sediment than would occur when seawalls had been at least partially
constructed;
·
The
assessment is based on the peak dredging rates, which will only occur for under
optimum conditions; and
·
The
calculations of loss rates of sediment to suspension are based on conservative
estimates for the types of plant and methods of working.
5.4.17
The
assessment of the potential impact of land based construction activities on
water quality has been undertaken in a qualitative manner. Consideration has been given to controlling
potentially harmful impacts from site works and to the use of ‘best’ practice
measures to minimise the potential for discharges of pollutants to the marine
waters in the vicinity of the Project sites.
The land based construction activities which have been considered
include those for the developments on the reclaimed land and those for the Chok
Ko Wan Link Road.
5.5.1
The
identification of potential water quality impacts during the construction phase
has been divided into two aspects, formation of the reclamation and land based
construction activities, including those for the developments on the reclaimed
land and for the Chok Ko Wan Link Road.
Suspended
Sediment
5.5.2
During
dredging for the construction of the reclamations, fine sediment will be
suspended into the water column which may then be transported away from the
works area by tidal currents to form sediment plumes. The quantities of fine sediment lost to suspension during
dredging will primarily depend on dredging rates and methods. Impacts from suspended sediment may be
caused by sediment plumes being transported to sensitive areas, such as fish
culture zones, bathing beaches, water intakes, areas of ecological interest and
recreational areas.
5.5.3
A review of
the modelling results has determined that only a small number of the sensitive
receivers identified in Section 5.3.3
are predicted to experience significant
elevated suspended sediment concentrations as a result of the dredging works
for the Northshore Lantau Development reclamations. These sensitive receivers, the locations of which are shown in Figure 5.3a, are defined as follows.
·
Ma Wan Fish
Culture Zone (North and South);
·
Tung Wan
Beach, Ma Wan;
·
Sze Pak
Wan;
·
Discovery
Bay Beach;
·
Kau Yi
Chau;
·
Silvermine
Bay Beach;
·
Chek Lap
Kok Intake;
·
Tung Chung
Bay; and
·
Brothers.
5.5.4
The closest
identified sensitive receiver to the Northshore and Siu Ho Wan reclamations is
the point marked as Yam O Bay. This
point has been positioned to determine the operational water quality within the
bay following completion of the Northshore Lantau Development, as concerns
regarding potential stagnation of the bay have been identified. This location has not therefore been
considered as a sensitive receiver for the assessment of reclamation
construction impacts.
5.5.5
Suspended
sediment plumes passing over a sensitive receiver will cause the ambient
suspended sediment concentrations to be elevated; the level of elevation will
determine whether the impact is adverse.
The determination of the acceptability of elevations in suspended
sediment concentrations has been based
on the Water Quality Objectives. The
WQO for suspended sediments for the Southern, Western Buffer and North Western
WCZs is defined as being an allowable elevation of 30% above the
background. The Environmental
Protection Department (EPD) maintains a flexible approach to the definition of
ambient levels, preferring to allow definition on a case-by-case basis rather
than designating a specific statistical parameter as representing ambient. As agreed in a previous study of the
environmental impacts of released suspended sediments , the ambient value has been taken to be
represented by the 90th percentile of reported concentrations. EPD routine monitoring data has been used as
the source of the reported concentrations, with the monitoring station nearest
to each of the identified sensitive receivers being defined as representative
of that location. EPD monitoring data
and allowable elevations in suspended sediment concentrations are summarised in
Table 5.5a.
Table
5.5a - Ambient and Tolerance Values for Suspended Sediment Concentrations (mg L-1) in the Vicinity of
Sensitive Receivers
|
Sensitive Receiver
|
Dry Season
|
Wet Season
|
|
(Relevant EPD Monitoring Station)
|
90th Percentile
|
30% Tolerance
|
90th Percentile
|
30% Tolerance
|
|
Ma Wan Fish Culture
Zone
Tung Wan Beach, Ma
Wan
(WM4)
|
15.0
|
4.5
|
10.7
|
3.2
|
|
Sze Pak Wan
Discovery Bay Beach
(SM10)
|
12.5
|
3.8
|
11.0
|
3.3
|
|
Kau Yi Chau
(SM9)
|
12.6
|
3.8
|
35.2
|
10.6
|
|
Silvermine Bay Beach
(SM11)
|
12.6
|
3.8
|
18.0
|
5.4
|
|
Brothers
Tung Chung Bay
(NM1)
|
8.8
|
2.6
|
5.4
|
1.6
|
|
Chek Lap Kok Intake
(NM3)
|
28.5
|
8.6
|
8.5
|
2.6
|
|
Notes :
1. All
values are depth averaged.
|
5.5.6
The
allowable elevation in suspended sediment concentration as defined by the WQO
for a particular site corresponds to the 30% tolerance level. The
maximum predicted suspended sediment concentrations from the modelling
of the losses to fine sediment to suspension during dredging have been compared
with the 30% tolerance values in the above table to determine the acceptability
of the impacts.
Sediment
Deposition
5.5.7
Impacts
from the formation of suspended sediment plumes may also be related to the
settling of sediment onto the seabed and smothering any organisms present. A deposition rate of 0.2 kg m-2
day-1 has been determined as a level of concern for corals. Further discussion of the selection of this
value is contained in Section 8 (marine
ecological impact assessment). The
predicted rates of deposition have been compared to this level of concern at
those sensitive receivers at which it is applicable to determine whether the
predicted impacts would be acceptable.
Water
Quality
5.5.8
The loss of
sediment to suspension during dredging and filling may have chemical effects on
the receiving waters. This is because
the sediment may contain organic and chemical pollutants.
5.5.9
As part of
this Project laboratory testing of sediment samples was undertaken. A full
description of the sediment quality testing and the classification of the
sediment according to levels of contaminants and methods of disposal of dredged
sediments is contained in Section 6,
which assesses waste management. A
summary of the results of the sediment quality testing, which have relevance to
the water quality assessment are presented in Table 5.5b. For parameters
other than chemical oxygen demand, ammonia and total kjeldahl nitrogen only
those parameters which exceeded the Lower Chemical Exceedance Level under the
proposed sediment assessment framework have been assessed. This is because the release of pollutants
from sediments containing concentrations of pollutants below the Lower Chemical
Exceedance Level is not of concern. It
should be noted that the testing of low and high molecular weight PAHs, PCBs
and TBT was not undertaken in a manner suitable for comparison with the
sediment assessment framework and further samples should be collected and
testing undertaken during detailed studies of each of the proposed reclamations
for the Northshore Lantau Development.
Table
5.5b - Summary of Sediment Quality Testing
|
Parameter
|
Northshore
|
Siu Ho Wan
|
Fa Peng
|
Theme Park Extension
|
|
Chemical
Oxygen Demand (mg kg-1)
|
36,500
|
6,590
|
34,100
|
36,500
|
|
Ammoniacal
Nitrogen (mg kg-1)
|
108
|
<2
|
152
|
108
|
|
Total
Kjeldahl Nitrogen (mg kg-1)
|
1,290
|
186
|
1,360
|
1,590
|
|
Copper
(mg kg-1)
|
81
|
<
LCEL
|
103
|
103
|
|
Arsenic
(mg kg-1)
|
33
|
<LCEL
|
12
|
12
|
|
Silver
(mg kg-1)
|
1.2
|
<LCEL
|
<LCEL
|
<LCEL
|
|
Total
PAHs (mg kg-1)
|
<8,000
|
<8,000
|
<8,000
|
<8,000
|
|
Total
PCBs (mg kg-1)
|
<100
|
<100
|
<100
|
<100
|
|
TBT
(mg kg-1)
|
405
|
1
|
294
|
294
|
|
Notes:
1.
<LCEL means that the concentrations were
less than the Lower Chemical Exceedance Level.
2.
No sediment samples were taken from the area
of the Theme Park Extension reclamation.
Sediment quality was determined as the maximum of the values for the
Theme Park and Fa Peng reclamations.
|
|
5.5.10
The
chemical oxygen demand was used to determine the reductions in dissolved oxygen
concentration, based on the predicted of increases in suspended sediment
concentrations for the three scenarios.
The reductions were then compared with the ambient levels at Stations
SM10, WM4 and NM1 from the EPD routine monitoring data to determine the
relative effects of the increases in suspended sediment concentrations on
dissolved oxygen.
5.5.11
At Stations
SM10, WM4 and NM1 the WQO for depth averaged dissolved oxygen concentrations
was breached in 1998, with values of only 3.9 mg L-1, 3.2 mg L-1
and 3.5 mg L-1 being exceeded on 90% of the sampling occasions
respectively. An examination of the
routine EPD water quality monitoring data for the years 1999 to 1995
found that the values measured in 1998 represented abnormally low
dissolved oxygen values and in 1999 the values had increased to be in
compliance with the WQO, although they were still lower than in previous
years. It is proposed that the
assessment of impacts be based on the background values in 1999 as these are
more likely to be representative of the area surrounding the Northshore Lantau
Development reclamations, rather than the unrepresentative values measured in
1998. In 1999 the concentrations of
dissolved oxygen, which were exceeded on 90% of the sampling occasions, were
4.2 mg L-1, 4.9 mg L-1 and 4.6 mg L-1 at
Stations WM4, SM10 and NM1 respectively.
5.5.12
The
assessment of nutrient impacts from increased suspended sediment concentrations
has been based on the water quality parameters of total inorganic nitrogen and
unionised ammonia. The increases in
these parameters in the receiving waters have been calculated from the
modelling predictions of suspended sediment concentrations and the sediment
quality data for total nitrogen and ammoniacal nitrogen. There is no available data on total
inorganic nitrogen from the sediment quality data and as such total nitrogen
has been used, although this will result in a conservative prediction as a
fraction of total nitrogen is organic nitrogen. The release of ammoniacal nitrogen has been calculated and then
these results converted to unionised ammonia.
The predicted increases have then been compared with the ambient levels
at Stations SM10, WM4 and NM1 to determine the relative effects of the
increases in suspended sediment concentrations on total inorganic nitrogen and
unionised ammonia concentrations.
5.5.13
Based on
EPD routine water quality monitoring data for 1998 , the mean depth averaged total inorganic
nitrogen concentrations at Stations SM10, WM4 and NM1 were 0.27 mg L-1,
0.29 mg L-1 and 0.43 mg L-1 respectively. It should be noted that the WQO for total
inorganic nitrogen is already breached at Station SM10 has been breached for
the last 10 years of monitoring; concentrations at Station WM4 are within the
WQO. The mean depth averaged unionised
ammonia concentrations at Stations SM10, WM4 and NM1 were 0.003 mg L-1,
0.005 mg L-1 and 0.005 mg L-1 respectively, determined
from EPD routine water quality monitoring data for 1998 .
5.5.14
The
sediment to be dredged for the various phases of the Northshore Lantau
Development would be categorised as Class M, that is exceeding the Lower
Chemical Exceedance Level, for three metals and metalloids - copper, arsenic
and silver. It is therefore proposed
that only the release of these three metals and metalloids to the water column
be considered. All three pollutants are
known to desorb from sediments and dissolve into the receiving waters. The European Union Water Quality Water
Quality Standards define values of 5 mg L-1 and 25 mg L-1 as being the allowable concentration in
coastal waters for copper and arsenic respectively .
The European Community Water Quality Standards do not define allowable
concentrations of silver and it is proposed to use the USEPA standard of 2.3 mg L-1. In order to determine whether the increases
in copper, silver and arsenic concentrations in the receiving waters due to
release from sediment plumes it is necessary to obtain an estimate of the
ambient levels in the marine water in the vicinity of the reclamations. As part
of a recent study of dredging an area of Kellett Bank a review was made of data collected as
part of the SSDS Stage I Baseline
Monitoring and Performance Verification.
It was determined that the mean copper concentration in the marine
waters was 2.51 mg L-1. This data represents concentrations in the
vicinity of the SSDS Stage I outfall, but in view of a lack of such data in the
vicinity of the Northshore Lantau Development reclamations this value has been
used in this assessment. As part of the
monitoring programme for disposal of contaminated mud at East Sha Chau
monitoring of silver and arsenic concentrations in the surrounding marine
waters has been undertaken as part of the study Environmental Monitoring and Audit for Contaminated Mud Pit IV at East
Sha Chau. The monitoring has
determined that average concentrations for arsenic and silver are 0.5 mg L-1 and 2.1 mg L-1 respectively, based on
data collected over the last two years.
These values represent concentrations in the vicinity of the East Sha
Chau contaminated mud disposal facility, but in view of the lack of alternative
data are considered to be appropriate for use in this study. The relevant
assessment standards and background concentrations are summarised in Table 5.5c.
Table
5.5c - Summary of Assessment
Standards in Water (mg L-1) and Background
Concentrations for Metals and the Metalloid (mg L-1)
|
Parameter
|
Assessment Standard
|
Background Concentration
|
|
Copper
|
5.0
|
2.51
|
|
Arsenic
|
25.0
|
2.1
|
|
Silver
|
2.3
|
0.5
|
5.5.15
As for
copper, silver and arsenic there are no standards in Hong Kong waters for PAHs
and TBT and so reference has been made to the European Community water quality
standards .
It is noted that the European Community standards for TBT are stringent
and may not necessarily be applicable to Hong Kong waters but are used here to
maintain consistency with the other quoted standards. The European Community standards do not make reference to a level
of concern for PCBs and so an assessment standard has been derived from work
published in the US .
In order to determine whether the potential increases in PAHs, PCBs and
TBT concentrations in the receiving waters are acceptable it is necessary to
obtain an estimate of the ambient levels in the marine waters. Data on
background levels in marine water of TBT has been obtained from the study A Study of Tributyltin Contamination of the
Marine Environment of Hong Kong .
In this study data was obtained in the vicinity of Yam O and North Tsing
Yi. These data were obtained in the
vicinity of shipyard works and as such will be applicable to this study, given
the proximity of the Theme Park Extension reclamation to the Cheoy Lee Shipyard
and the proximity of the Northshore and Fa Peng reclamations to the floating
dry docks moored offshore. The
monitoring data determined representative concentrations of TBT in the marine
waters of 0.01 mg L-1
at North Tsing Yi and 0.009 mg L-1
at Yam O. The North Tsing Yi data will
be used in the assessment of the impacts of the Theme Park Extension
reclamation, as this is the closest monitoring station, while the Yam O data
will be used in the assessment of the Northshore and Fa Peng reclamations. There is currently no data on the background
levels of PAHs, PCBs in marine waters and as such it will only be possible to
compare the predicted increase in concentrations with the relevant
standards. The relevant standards and
background concentrations are summarised in Table
5.5d.
Table
5.5d - Summary of Assessment
Standards in Water (mg L-1) and Background
Concentrations (mg L-1)
|
Parameter
|
Assessment Standard
|
Background Concentration
|
|
Total
PAHs
|
0.2
|
No
data
|
|
Total
PCBs
|
0.014
|
No
data
|
|
TBT
|
0.002
|
0.01 (Theme Park Extension)
0.009 (Northshore
and Fa Peng)
|
|
|
|
|
5.5.16
It should
be noted that the data in the above table show that the background concentrations
of TBT already exceeds the stringent European Community criteria. However, the background levels in Hong Kong
are likely to be typical of those measured in comparable marine waters around
the world, such as major ports. It may
be that the European Community criteria are not entirely applicable to Hong
Kong, but are nevertheless used in this Study to provide a reference on the
relative magnitudes of the release of TBT from sediments suspended during the
dredging works.
Cumulative
Impacts
5.5.17
As discussed
above in Section 5.4.1 there are no
identified concurrent project which could give rise to cumulative impacts.
5.5.18
During land
based construction activities for the developments on the reclaimed land and
for the Chok Ko Wan Link Road, the primary sources of potential impacts to
water quality will be from pollutants in site run-off, which may enter marine
waters directly or enter the storm drain system which discharges into these
waters. Pollutants, mainly suspended
sediments, may also enter the receiving waters if pumped groundwater is not
adequately controlled.
5.5.19
Wastewater
from temporary site facilities should be controlled to prevent direct discharge
to marine waters adjacent to the reclamation.
Such wastewater may include sewage effluent from toilets and discharges
from on-site kitchen facilities. Water
from plant servicing facilities may be contaminated with oil and other
petroleum products and would have the potential to discharge to surface waters
if spillages are not contained.