This Section describes the impacts
on water quality associated with the construction and operation of the proposed
East of Sha Chau Facility.
Computer modelling of sediment dispersion has been used to determine the
impacts of the proposed development.
Impacts have been assessed with reference to the relevant environmental
legislation and standards. A
review of baseline information (Part 1, Section 4) in the Study Area has
determined that there are a series of water quality sensitive receivers present
as follows:
Ecological: Sha Chau and Lung Kwu Chau Marine Park;
Seagrass and Horseshoe Crab Habitats; and the critical habitats of the
Indo-Pacific Humpback dolphin.
Fisheries: Ma Wan Fish Culture Zone; Artificial Reefs; and
Spawning Ground of Commercial Fisheries species.
Water
Quality: Beaches
at Lung Kwu Tan and around Tuen Mun; Intakes at the Airport, Tuen Mun Area 38;
and Castle Peak Power Station.
A desktop literature review
(presented in Part 1, Section 4) was conducted in order to establish the
water quality conditions of the area within and surrounding the East of Sha
Chau Facility. Potential impacts
due to the construction and operation of the East of Sha Chau Facility have
been assessed (following the EIAO-TM Annex 14 guidelines) and the impacts
evaluated (based on the criteria in EIAO-TM
Annex 6).
The proposed East of Sha Chau
Facility will consist of four purposely dredged seabed pits. The pits will be dredged sequentially
prior to backfilling with contaminated mud and capping with uncontaminated
mud. Impacts associated with the
East of Sha Chau Facility are thus divided into those occurring during the
dredging of pits and those during backfilling with contaminated mud and capping
with uncontaminated mud. Following
this assessment the potential for residual impacts and cumulative impacts
associated with concurrent projects, or through the combination of the above
works, are discussed.
Impacts from the dispersion of
sediment in suspension arising from backfilling operations have been assessed
using computer modelling.
Impacts from suspended sediment may be
caused by the transport of sediment plumes to sensitive receivers such as fish
culture zones, marine parks etc.
Sediment plumes will cause the
ambient suspended sediment concentrations to be elevated and the level of the
elevation will determine whether the impact is adverse or not. The determination of the acceptability
of any elevations is based on the criteria defined in Part 1, Section 4.
The modelling
simulated the release of sediment during backfilling operations in the wet and
dry seasons. The results have been
presented as contours of maximum and 90th percentile suspended
sediment concentrations above ambient in the surface, middle and bed layers of
the water column (Annex A). Depth averaged contour plots
illustrating the maximum and mean values recorded over the 15 day tidal cycle
modelling period are presented in Annex A. In addition, maximum elevations at the sensitive receivers
are presented in Tables 2.1a and 2.1b of Annex A.
As discussed in Annex
A, modelling of backfilling operations has been conducted for trailer
disposal (Scenario 1) and through barge disposal (Scenario 3). Due to the greater loss rates
associated with trailer disposal backfilling works, predicted concentrations calculated
for these works are discussed below as they thus represent the worst-case
scenario.
The results of
trailer disposal backfilling activities appear to indicate that sediment plumes
stay relatively close to the seabed, with no elevations > 15 mg L-1
recorded in the surface layer. In
general, SS increases appear to be confined within the pit boundaries for the
surface layer. Horizontal
dispersion is increased in the middle layers, with the maximum dispersion
recorded in the bottom layer. Nevertheless, this dispersion stays within
relatively close proximity to the pit boundaries with limited horizontal spread
following the Urmston Road and around the existing disposal pits in the East of
Sha Chau Area. Wet season contours
appear to indicate a similar pattern; however, during this season plumes appear
to have less vertical spread throughout the water column, with little or no
elevations in SS predicted in the middle and surface layers. The horizontal spread of SS at the
seabed increases, with elevations at the seabed of < 10 mg L-1
recorded on the boundary of the Sha Chau and Lung Kwu Chau Marine Park. 90th percentile
concentrations appeared to demonstrate a similar pattern to that described
above. The maximum depth average
contour plots for SS indicate that elevations of < 10 mg L-1
cover a relatively small area that is restricted to open waters and does not
affect any of the sensitive receivers (Annex A). The mean depth average plots indicate
that the < 10 mg L-1 contour does not extend beyond the boundary
of the active pit.
The potential impact at each of the water quality
sensitive receivers as a result of backfilling operations is discussed
below.
Marine Parks: The maximum depth averaged
elevations of SS concentrations at the Marine Park as a result of backfilling
operations are predicted to be 2.2 mg L-1 and 1.6 mg L-1
in the dry and wet seasons, respectively.
As such, these elevations are compliant with the WQO. It is noted that these predicted
elevations are similar in range to those predicted in the EIA for CMP IV ([1]).
Artificial Reef Deployment Areas:
Predicted elevations of SS concentrations at the ARs
within the Marine Park and at the Airport Exclusion Zone as a result of
backfilling operations are very low and compliant with the WQO (maximum = 2 mg
L-1 (dry season) and 3 mg L-1 (wet season)). As such, impacts are not expected to
occur.
Habitat of the
Indo-Pacific Humpback Dolphin: High
elevations of SS concentrations appear to only be recorded within close
proximity to the boundary of the East of Sha Chau Facility. Long term monitoring data indicates
that disposal of contaminated mud in the East of Sha Chau area does not appear
to be having an adverse affect on Sousa chinensis.
Fish Culture Zones: The maximum SS elevation at the FCZ as a
result of backfilling operations has been predicted to be < 1 mg L-1. Impacts to water quality at the Ma Wan
FCZ as a result of the backfilling works are thus unlikely to occur as the
increases in SS are expected to be negligible.
Beaches: Beaches at Lung Kwu Tan and Tuen Mun are located
remotely from the East of Sha Chau Facility (Part 1, Section 4). As such, impacts from backfilling works
were not expected. This statement
has been confirmed by the modelling work that indicates that there are no
detectable increases in SS concentrations at each of these sensitive receivers
and is therefore acceptable.
Intakes: Modelling results
indicate that the maximum elevations at these intakes are negligible (< 1 mg
L-1). As this elevation
is within the allowable increase with regard to the WQO, no unacceptable
impacts to intakes as a result of backfilling operations are expected to occur.
Spawning Area:
Maximum elevations of SS concentrations have been identified in the both
the wet and dry seasons to remain close to the seabed, with little or no
elevations recorded in the surface later in the wet season. As most fish larvae,
eggs and fry are likely to be found in the surface layer post-spawning, the
predicted impacts to water quality will not result in impacts to spawning
areas.
The information
presented in the contour plots illustrates that SS concentrations decrease
relatively rapidly outside the pit boundary of the East of Sha Chau Facility (Annex
A). This implies that whilst
there is a degree of horizontal dispersion of sediment plumes, the majority of
suspended sediments settle in close proximity to the works. The modelling exercise generated
contour plots of sediment in the Study Area as a result of backfilling
operations (Annex A). As
expected, the majority of sediment settles either within, or within relatively
close proximity to, the East of Sha Chau Facility. Sediment deposition has been predicted within the Marine Park due to backfilling operations,
however, maximum deposition has been determined to be no greater than < 25 g
m-2 day-1. The significance of these elevations is discussed in Part
3, Section 3, which has determined that levels such as those predicted are
not considered to be a concern.
Thus, with the
exception of those within the Marine Park that are not considered to be a
concern, deposited sediments will not reach water quality sensitive
receivers. As such, adverse
impacts to water quality, marine and fisheries sensitive receivers by deposited
sediments as a result of backfilling operations at the East of Sha Chau
Facility are not expected to occur.
The loss of sediment through
backfilling operations at the East of Sha Chau Facility may impact the quality
of the receiving waters. The
modelling approach has simulated the release of nutrients into the water column
and examined the subsequent effects on levels of dissolved oxygen, biochemical
oxygen demand and nutrients (as unionised ammonia).
The results of the modelling are
presented in Annex A and indicate that backfilling operations at the
East of Sha Chau Facility are not expected to cause adverse impacts to water
quality. The results indicate that
levels of dissolved oxygen, biochemical oxygen demand and nutrients do not
change appreciably from background conditions and are compliant with the
relevant WQOs.
The results of modelling suspended
sediments released from the disposal of dredged material are presented in Annex
B and are discussed above.
Using partitioning coefficients it has been possible to predict the
maximum potential release of contaminants (see Methodology in Annex A).
Maximum predicted concentrations of
contaminants have been estimated for backfilling operations at the East of Sha
Chau Facility. These predicted
concentrations have been used in the bioaccumulation assessment (Annex B)
to determine the potential uptake of contaminants into the food chain. Based on bioconcentration factors
determined from the bioaccumulation assessment, the predicted contaminant
concentrations in marine water and sediments have been assessed to calculate
the risks to humans and marine mammals associated with consuming fish and
shellfish collected from the vicinity of the East of Sha Chau Facility. The results of this assessment are
presented in Part 3, Section 5 and in Annex C.
It is also important to investigate
the potential for these desorbed contaminants to impact the identified water
quality sensitive receivers.
However, for the basis of this assessment, only those water quality
sensitive receivers considered to have the potential to be adversely impacted
by increases in contaminants in the water column have been assessed ([2]). These selected water quality sensitive
receivers are as follows:
·
Airport Exclusion Zone Artificial Reef;
·
Sha Chau and Lung Kwu Chau Marine Park;
·
San Tau Beach SSSI;
·
Tai Ho Bay; and,
·
Yam O Bay.
Maximum concentrations of
contaminants predicted at these sensitive receivers in both the dry and wet seasons
are presented in Tables 2.1 and 2.2, respectively and have been
evaluated against European Community (EC) Water Quality Standards. The EC standards which have been used
in the absence of quantitative water quality objectives for these contaminants
in Hong Kong.
Comparison to EC water quality
standards, which are presented as dissolved concentrations, requires summation
of predicted dissolved concentrations arising from backfilling operations with
ambient (soluble) concentrations (see Part 1, Section 4, Table 4.2). As no EC water quality standards or
ambient values are available for PAHs, PCBs and TBT, no comparison between
predicted concentrations and these values was possible.
Predicted concentrations of
contaminants resulting from a representative operational scenario (Scenario 1 –
Trailer disposal) at the East of Sha Chau Facility are extremely low in
comparison to EC water quality standards.
As the modelled contaminants represent a range of chemical compounds
with varying partitioning coefficients and input values (ie UCELs), the range
of results is likely to be representative of other contaminants of
concern. As predicted contaminant
concentrations are extremely low (maximum = Chromium, 2.9% of Allowed Levels
(wet season)), and modelling results for other operational scenarios are very
similar, modelling of contaminants for other operational scenarios at the East
of Sha Chau Facility is unlikely to produce detectably different results. In summary, the predicted contaminant
concentrations resulting from operations at the East of Sha Chau Facility are
negligible when compared to international water quality standards and thus no
unacceptable impacts are anticipated.
Table 2.1 Dissolved
Concentrations of Contaminants of Concern through Backfilling Operations at the
East of Sha Chau Facility (Dry Season)
|
COC |
Kd |
Unit |
Max. Sediment Conc |
Unit |
Eq. Dissolved Conc. (mg L-1) |
Dissolved Concentration (mg L-1) |
Alloweda (mg L-1) |
Minimum Ambient Conc. (mg L-1) |
|||||
|
AR1_3b |
MP2(5)b |
SG1b |
SG2b |
SG3b |
|||||||||
|
Metals |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Ag |
200 |
l/g |
2 |
mg/kg |
0.0100 |
1.1E-03 |
1.6E-03 |
1.7E-08 |
1.3E-06 |
9.6E-07 |
- |
1 |
- |
|
As |
130 |
l/ge |
42 |
mg/kg |
0.3231 |
1.5E-02 |
2.2E-02 |
9.8E-05 |
7.6E-03 |
5.5E-03 |
- |
0.5 |
- |
|
Cd |
100 |
l/g |
4 |
mg/kg |
0.0400 |
1.1E-03 |
1.6E-03 |
3.4E-08 |
2.7E-06 |
1.9E-06 |
2.5 |
1 |
0.07% |
|
Cr |
290 |
l/g |
160 |
mg/kg |
0.5517 |
1.3E-01 |
1.9E-01 |
8.3E-04 |
6.5E-02 |
4.7E-02 |
15 |
0.5 |
1.3% |
|
Cu |
122 |
l/g |
110 |
mg/kg |
0.9016 |
3.6E-02 |
5.5E-02 |
2.4E-04 |
1.9E-02 |
1.3E-02 |
5 |
0.5 |
1.1% |
|
Hg |
700 |
l/g |
1 |
mg/kg |
0.0014 |
1.9E-03 |
2.9E-03 |
8.6E-09 |
6.7E-07 |
4.8E-07 |
0.3 |
1 |
1% |
|
Ni |
40 |
l/g |
40 |
mg/kg |
1.0000 |
4.3E-03 |
6.6E-03 |
2.9E-05 |
2.2E-03 |
1.6E-03 |
30 |
0.5 |
0.02% |
|
Pb |
130 |
l/g |
110 |
mg/kg |
0.8462 |
3.9E-02 |
5.9E-02 |
2.6E-04 |
2.0E-02 |
1.4E-02 |
25 |
0.5 |
0.2% |
|
Zng |
100 |
l/g |
270 |
mg/kg |
2.7000 |
7.3E-02 |
1.1E-01 |
4.8E-04 |
3.8E-02 |
2.7E-02 |
40 |
5 |
0.3% |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Organics |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
L PAH |
0.075 |
l/g |
3.19 |
mg/kg |
42.1333 |
6.4E-07 |
9.7E-07 |
2.7E-08 |
2.1E-06 |
1.5E-06 |
- |
- |
- |
|
H PAH |
1.14 |
l/g |
9.6 |
mg/kg |
8.4211 |
3.0E-05 |
4.5E-05 |
8.2E-08 |
6.4E-06 |
4.6E-06 |
- |
- |
- |
|
PCBs |
1,585 |
l/gOC (c) |
180 |
mg/kg |
0.0095 |
9.3E-06 |
1.4E-05 |
6.1E-05 |
4.8E-03 |
3.4E-03 |
- |
- |
- |
|
TBTf |
40 |
l/gOC (c) |
0.15 |
mg/kg |
0.0003 |
1.9E-10 |
2.9E-10 |
1.3E-09 |
1.0E-07 |
7.2E-08 |
- |
|
- |
|
Notes: a Environmental
Quality Standards and Assessment Levels for Surface Water (from HMIP (1994)
Environmental and BPEO Assessment Principles for Integrated Pollution
Control) b AR1_3
= Airport Exclusion Zone Artificial Reef; MP2(5) = Sha Chau and Lung Kwu Chau
Marine Park; SG1 = San Tau Beach SSSI; SG2 = Tai Ho Bay; and SG3 = Yam O Bay c Converted
to l/g using the OC content of the sediments d Sediment
concentration equal to max. observed value at Kellett Bank e Value
is not available, lowest value of other metals has been used, in this case
about 10 for Cd f US
EPA Aquatic Life Advisory Concentration for Seawater cited in Lau MM (1991)
Tributyltin Antifoulings: A Threat to the Hong Kong Marine Environment. Arch. Environ. Contam. Toxicol. 20:
299-304. g Wen
LS, Santschi PH, Paternostro CL, Lehman RD, 1997. Colloidal and Particulate Silver in River and Estuarine
Waters of Texas. Environ Sci
Technol 31: 723-731. |
|||||||||||||
Table 2.2 Dissolved Concentrations of Contaminants of
Concern through Backfilling Operations at the East of Sha Chau Facility (Wet
Season)
|
COC |
Kd |
Unit |
Max. Sediment Conc |
Unit |
Eq. Dissolved Conc (mg L-1) |
Dissolved Concentration (mg L-1) |
Alloweda (mg L-1) |
Minimum Ambient Conc. (mg L-1) |
Maximum Predicted Diss. Conc. as % of
Allowed |
||||
|
AR1_3b |
MP2(5)b |
SG1b |
SG2b |
SG3b |
|||||||||
|
Metals |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Ag |
200 |
l/g |
2 |
mg/kg |
0.0100 |
8.8E-04 |
2.1E-03 |
2.4E-07 |
9.1E-06 |
4.8E-07 |
- |
1 |
- |
|
As |
130 |
l/ge |
42 |
mg/kg |
0.3231 |
1.2E-02 |
2.9E-02 |
1.4E-03 |
5.2E-02 |
2.7E-03 |
- |
0.5 |
- |
|
Cd |
100 |
l/g |
4 |
mg/kg |
0.0400 |
8.8E-04 |
2.1E-03 |
4.9E-07 |
1.8E-05 |
9.6E-07 |
2.5 |
1 |
0.1% |
|
Cr |
290 |
l/g |
160 |
mg/kg |
0.5517 |
1.0E-01 |
2.4E-01 |
1.2E-02 |
4.4E-01 |
2.3E-02 |
15 |
0.5 |
2.9% |
|
Cu |
122 |
l/g |
110 |
mg/kg |
0.9016 |
3.0E-02 |
7.1E-02 |
3.4E-03 |
1.3E-01 |
6.7E-03 |
5 |
0.5 |
2.5% |
|
Hg |
700 |
l/g |
1 |
mg/kg |
0.0014 |
1.5E-03 |
3.7E-03 |
1.2E-07 |
4.5E-06 |
2.4E-07 |
0.3 |
1 |
1.2% |
|
Ni |
40 |
l/g |
40 |
mg/kg |
1.0000 |
3.5E-03 |
8.4E-03 |
4.1E-04 |
1.5E-02 |
8.0E-04 |
30 |
0.5 |
0.1% |
|
Pb |
130 |
l/g |
110 |
mg/kg |
0.8462 |
3.1E-02 |
7.5E-02 |
3.7E-03 |
1.4E-01 |
7.2E-03 |
25 |
0.5 |
0.5% |
|
Zng |
100 |
l/g |
270 |
mg/kg |
2.7000 |
5.9E-02 |
1.4E-01 |
6.9E-03 |
2.6E-01 |
1.4E-02 |
40 |
5 |
0.6% |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Organics |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
L PAH |
0.075 |
l/g |
3.19 |
mg/kg |
5.2E-07 |
1.2E-06 |
3.9E-07 |
1.4E-05 |
7.6E-07 |
5.2E-07 |
- |
- |
- |
|
H PAH |
1.14 |
l/g |
9.6 |
mg/kg |
2.4E-05 |
5.8E-05 |
1.2E-06 |
4.4E-05 |
2.3E-06 |
2.4E-05 |
- |
- |
- |
|
PCBs |
1,585 |
l/gOC (c) |
180 |
mg/kg |
7.5E-06 |
1.8E-05 |
8.8E-04 |
3.3E-02 |
1.7E-03 |
7.5E-06 |
- |
- |
- |
|
TBTf |
40 |
l/gOC (c) |
0.15 |
mg/kg |
1.6E-10 |
3.8E-10 |
1.8E-08 |
6.8E-07 |
3.6E-08 |
1.6E-10 |
- |
|
- |
|
Notes: As
above |
|||||||||||||
Impacts due to the dispersion of
sediment in suspension arising from dredging operations have been assessed
using computer modelling.
The modelling simulated the release of
sediment during dredging operations in the wet and dry seasons. The results have been presented as
contours of maximum suspended sediment concentrations above ambient (Annex A). In addition, tables of elevations at the sensitive receivers
are presented in Tables 2.1a and 2.1b of Annex A.
As discussed in Annex A, modelling
of dredging operations have been conducted for grab dredging (Scenario 3)
and through trailer dredging (Scenario 4). The results appear to indicate that grab dredging results in
higher elevations in SS concentrations, they thus represent the worst-case
scenario and are discussed below.
The
results indicate that sediment plumes stay in relatively close proximity to the
pit boundaries. Plumes that extend
beyond the boundary of the facility are predicted to remain within the main
flow channel of the Urmston Road.
Wet season contours appear to indicate a similar pattern to those
predicted for the dry season.
Horizontal spread marginal increases on the boundary of the Sha Chau and
Lung Kwu Chau Marine Park.
The potential impact at each of the
water quality sensitive receivers as a result of dredging operations is
discussed below.
Marine Parks: The results of the water quality
modeling indicate that dredging operations are not predicted to increase SS
concentrations within the Marine Park as no detectable concentrations have been
identified.
Artificial Reef Deployment Areas:
Predicted elevations of SS concentrations at ARs as a
result of dredging operations are very low and compliant with the WQO (Marine
Park AR: maximum = < 1 mg L-1 (dry season); Airport Exclusion
Zone AR: maximum = 3 mg L-1 (wet season)). The significance of these elevations is discussed in Part
2, Section 4.
Seagrass Beds,
Mangroves, Horseshoe Crab Areas: Sediment
dispersion results based on dredging operations predict that elevations of SS
concentrations are expected to stay relatively close to dredging
operations. As such, elevations at
the San Tau Beach SSSI are non-detectable.
Habitat of the
Indo-Pacific Humpback Dolphin: Elevations
of SS concentrations appear to only be recorded within close proximity to the
boundary of the East of Sha Chau Facility. Long term monitoring data indicates that operations in the
East of Sha Chau area does not appear to be having an adverse affect on Sousa
chinensis.
Fish Culture Zones: Water
quality modelling results have shown that the maximum SS elevations at the FCZ
as a result of dredging operations is < 1 mg L-1, which is well
within the acceptable range and is not expected to cause adverse impacts.
Beaches: There are no detectable increases in SS concentrations
at each of these sensitive receivers due to dredging operations, therefore, no
unacceptable impacts are expected to occur.
Intakes: Modelling results indicate that there are no
detectable increases at the intakes through dredging operations, therefore, no
unacceptable impacts expected to occur.
Spawning
Area: Elevations of SS concentrations have been identified to
remain close to the seabed. As most
fish larvae, eggs and fry are likely to be found in the surface layer
post-spawning, it appears that the predicted impacts to water quality will not
result in impacts to spawning areas.
Predictions of sediment deposition as a result of
dredging operations indicate that the majority of sediment settles either
within or within relatively close proximity to the East of Sha Chau Facility (Table
2.1, Annex A). A similar
pattern of deposition is predicted for the wet and dry seasons. In
terms of deposition of sediments, the maximum deposition of SS within the
Marine
Park
due to dredging operations has been determined to be no greater than 63 g m-2
day-1. The significance
of these elevations is discussed in Part 3, Section 3, which has
determined that levels such as those predicted are not considered to be a
concern.
Thus, with the
exception of those within the Marine Park that are not considered to be a
concern, deposited sediments will not reach water quality sensitive
receivers. As such, adverse
impacts to water quality, marine and fisheries sensitive receivers by deposited
sediments as a result of dredging operations at the East of Sha Chau Facility
are not predicted to occur.
Impacts from the dispersion of
sediment in suspension arising from capping operations have been assessed using
computer modelling.
The modelling simulated the release of
sediment during capping operations in the wet and dry seasons. The results have been presented as
contours of maximum suspended sediment concentrations above ambient (Annex A). In addition, tables of elevations at the sensitive receivers
are presented in Tables 2.1a and 2.1b of Annex A.
The results of capping operations
indicate a similar pattern to barge disposal backfilling operations at the East
of Sha Chau Facility in that sediment plumes stay relatively close proximity to
the pit boundaries, particularly during the dry season. Plumes that extend beyond the boundary
of the facility appear confined within the main flow channel of the Urmston
Road. In comparison to dredging
and backfilling operations, horizontal and vertical spreads of plumes are predicted
to be lower.
The potential impact at each of the
water quality sensitive receivers as a result of capping operations is
discussed below.
Marine Parks: The results of the water
quality modeling indicate that capping operations are not predicted to increase
SS concentrations within the Marine Park as no detectable concentrations have
been identified.
Artificial Reef
Deployment Areas: Predicted
elevations of SS concentrations at the ARs within as a result of capping
operations are very low and compliant with the WQO (Marine Park AR: maximum =
< 1 mg L-1 (dry season); Airport Exclusion Zone AR: maximum =
< 1 mg L-1 (wet season)).
No unacceptable impacts are therefore expected to occur.
Seagrass Beds,
Mangroves, Horseshoe Crab Areas: Sediment
dispersion results based on capping operations predict that elevations at the
San Tau Beach SSSI sensitive receiver are non-detectable, as such no exceedance
of the WQO would occur.
Habitat of the
Indo-Pacific Humpback Dolphin: Elevations
of SS concentrations appear to only be recorded within close proximity to the
boundary of the East of Sha Chau Facility. Long term monitoring data indicates that operations in the
East of Sha Chau area do not appear to be having an adverse affect on Sousa
chinensis.
Fish Culture
Zones: Water
quality modelling results have shown that the maximum SS elevations at the FCZ
as a result of capping operations is < 1 mg L-1, which is well
within the acceptable range and is not expected to cause adverse impacts.
Beaches: There are no detectable increases in SS concentrations
at each of these sensitive receivers due to dredging operations, therefore, no
unacceptable impacts are expected to occur.
Intakes: Modelling results indicate that there are no
detectable increases at the intakes through dredging operations, therefore, no
unacceptable impacts expected to occur.
Spawning
Area: Elevations of SS concentrations have been identified to
remain close to the seabed. As most
fish larvae, eggs and fry are likely to be found in the surface layer
post-spawning, it appears that the predicted impacts to water quality will not
result in impacts to spawning areas.
Predictions of
sediment deposition as a result of dredging capping operations indicate
that the majority of sediment settles either within or within relatively close
proximity to the East of Sha Chau Facility (Table 2.1, Annex A). A similar pattern of deposition is
observed between the wet and dry seasons.
In terms of
deposition of sediments, the maximum deposition of SS within the Marine Park
due to cappingdredging
operations has been determined to be no greater than 24 g m-2 day-1. The significance
of these elevations is discussed in Part 3, Section 3, which has
determined that levels such as those predicted are not considered to be a
concern.
Deposited sediments will not reach water quality
sensitive receivers. As such,
adverse impacts to water quality, marine and fisheries sensitive receivers by
deposited sediments as a result of capping operations at the East of Sha Chau
Facility are not predicted to occur.
The water quality modelling works
have indicated that for both the dry and wet seasons, the works can proceed at
the recommended working rates without causing unacceptable impacts to water
quality sensitive receivers through either elevations of suspended sediment or
deposition of sediment. Changes to
other water quality parameters have been demonstrated to be minor, compliant
with applicable standards and therefore not of concern.
Unacceptable impacts to water
quality sensitive receivers have largely been avoided through the adoption of
the following measures:
·
Siting: A number of siting options were studied
and the preferred location avoids direct impacts to sensitive receivers.
·
Reduction
in Indirect Impacts: The East
of Sha Chau Facility is located at a sufficient distance from water quality
sensitive receivers so that the dispersion of sediments from the construction
and operation works does not affect the receivers at levels of concern (as
defined by the WQO and tolerance criteria).
·
Adoption
of Acceptable Working Rates: The
modelling work has demonstrated that the selected working rates for the
dredging and backfilling and capping of the East of Sha Chau Facility will not
cause unacceptable impacts to the receiving water quality.
Aside from the above pro-active
measures that have been instituted for the Project, the following operational
constraints should also be applied.
It should be noted that there is no requirement for constraints on
timing or sequencing, as all scenarios have been demonstrated to be acceptable
with the required mitigation measures in place.
1. Dredging
operations within the East of Sha Chau Facility do not exceed 100,000 m3
week-1.
2. Backfilling
operations within the East of Sha Chau Facility do not exceed a disposal rate
of 26,700 m3 day-1.
3. Capping
operations within the East of Sha Chau Facility do not exceed a disposal rate
of 26,700 m3 day-1.
4. No
overflow is permitted from the trailer suction hopper dredger but the Lean
Mixture Overboard (LMOB) system will be in operation at the beginning and end
of the dredging cycle when the drag head is being lowered and raised.
5. Dredged
marine mud shall be disposed of in a gazetted marine disposal area in
accordance with the Dumping at Sea
Ordinance (DASO) permit
conditions.
The following good practice measures
shall apply at all times:
1. All
disposal vessels should be fitted with tight bottom seals in order to prevent
leakage of material during transport.
2. All
barges should be filled to a level, which ensures that material does not spill
over during transport to the disposal site and that adequate freeboard is
maintained to ensure that the decks are not washed by wave action.
3. After
dredging, any excess materials should be cleaned from decks and exposed
fittings before the vessel is moved from the dredging area.
4. The
contractor(s) should ensure that the works cause no visible foam, oil, grease,
litter or other objectionable matter to be present in the water within and
adjacent to the dredging site.
5. If
installed, degassing systems should be used to avoid irregular cavitation
within the pump.
6. Monitoring
and automation systems should be used to improve the crew’s information
regarding the various dredging parameters to improve dredging accuracy and
efficiency.
7. Control
and monitoring systems should be used to alert the crew to leaks or any other
potential risks.
8. When
the dredged material has been unloaded at the disposal areas, any material that
has accumulated on the deck or other exposed parts of the vessel should be
removed and placed in the hold or a hopper. Under no circumstances should decks be washed clean in a way
that permits material to be released overboard.
9. All
dredgers should maintain adequate clearance between vessels and the seabed at
all states of the tide and reduce operations speed to ensure that excessive
turbidity is not generated by turbulence from vessel movement or propeller
wash.
No residual environmental impacts,
in terms of exceedances of applicable standards (ie Water Quality Objectives
and marine ecology and fisheries tolerance criterion), were predicted to occur
as a result of the construction and operation of the East of Sha Chau Facility,
provided that the mitigation measures, described in Section 2.4 are implemented.
The mitigation measures were specified in the form of operational
constraints and as a series of ‘best practice’ methods.
Cumulative impacts to water quality
may arise from concurrent dredging, backfilling or development projects in the
area. In addition, cumulative
impacts through the combination of dredging, backfilling and capping operations
within the East of Sha Chau Facility have the potential to occur. A number of planned projects have the
potential to result in cumulative impacts with the construction and operation
of the proposed East of Sha Chau Facility. Water quality modelling of the cumulative impacts of these
projects has been presented in Annex A. The findings indicated that no adverse impacts would be
expected to water quality sensitive receivers when compared the allowable
increases as defined by the WQO.
It should be noted, however, that the assessment has been conducted on
maximum operations without the use of operational controls.
Unacceptable cumulative impacts as a
result of concurrent project construction and operational activities are,
therefore, unlikely to occur and hence cumulative impacts to water quality are
not anticipated.
The construction and operation of
the proposed East of Sha Chau Facility has been defined at rates that maintain
environmental impacts to within acceptable levels. Actual impacts during the works will be monitored by through
a detailed Environmental Monitoring and Audit (EM&A) programme. Full details of the EM&A programme
are presented in the EM&A Manual which has been based on the on-going and
previous monitoring programmes conducted at the Contaminated Mud Disposal
Facility at East of Sha Chau. This
programme will provide management actions and supplemental mitigation measures
to be employed should impacts arise, thereby ensuring the environmental
acceptability of the East of Sha Chau Facility.
This Section has described the
impacts to water quality arising from the construction and operation of the
East of Sha Chau Facility. The purpose
of the assessment was to thoroughly evaluate the East of Sha Chau Facility in
terms of the acceptability of predicted impacts to water quality from dredging,
backfilling and capping of the pits and also concurrent activities.
Computer modelling was used to
simulate the loss of sediment to suspension during dredging, backfilling and
capping operations.
The assessment concluded that any
sediment disturbed by the works would settle rapidly back onto the seabed and
the suspended sediment elevations would be of short duration. This means that there would be little
transport of suspended sediment away from the pits and that the sediment would
not impact upon sensitive receivers.
The findings of the modelling works are comparable to the elevations
predicted during the modelling works for the CMP IV EIA. The CMP IV EIA predictions have since
been verified through the environmental monitoring and audit works.
An EM&A programme has been
devised to confirm that the works would be environmentally acceptable.