This Section describes the impacts on water quality
associated with the construction and operation of the proposed South Brothers
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, 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 South Brothers Facility. Potential impacts due to the
construction and operation of the South Brothers 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 South Brothers Facility will consist of three
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 South Brothers 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, elevations at the sensitive receivers are
presented in Tables 2.1a and 2.1b of Annex A.
As discussed
above, modelling of backfilling operations has been conducted for trailer
disposal (Scenario 2) and through barge disposal (Scenario 4). 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 a 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 > 20 mg L-1 recorded in
the surface layer outside the boundary of the Pits. 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 during the dry
season, with no plumes entering the Tung Chung Sea Channel and maximum
elevations of < 10 mg L-1 recorded along the north Lantau
development seawall. 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 >15 mg L-1 on the north Lantau seawall
and limited elevations (< 35 mg L-1) in the vicinity of the
seawall of the Tung Chung Phase 3 Developments. 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 remains offshore and does not impinge on the coastal areas
(including Tai Ho Bay) (Annex A).
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 < 1 mg
L-1 in both the dry and wet seasons (Annex A , Contour Plots and Tables 2.1a and 2.1b). These elevations are compliant with the
WQO.
Artificial Reef
Deployment Areas: Predicted elevations of SS
concentrations at the ARs within the Marine Park as a result of backfilling
operations are low (< 1 mg L-1 in both seasons) and compliant
with the WQO. Elevations at the
Airport Exclusion Zone AR have been identified as a maximum of 21 mg L-1
in the wet season and 11 mg L-1 in the dry season (Annex A, Contour Plots and Tables 2.1a & 2.1b). These elevations are in the bed layer
whereas the depth average values are < 5 mg L-1 and compliant
with the WQO. The significance of
these elevations is discussed in Part 2, Section 4.
Seagrass Beds,
Mangroves, Horseshoe Crab Areas: Sediment dispersion results
predict that maximum depth average elevations in SS concentrations are
predicted to be at < 5 mg L-1 at the Seagrass Beds, Mangroves, Horseshoe Crab Areas in both seasons and
therefore compliant with the WQO.
Elevations in the bed layer for SS at the San Tau and Yam O sensitive
receivers were 1 mg L-1 in both seasons.
Tai Ho Bay: The depth average contour plots of SS dispersion
do not extend as far as Tai Ho Bay at appreciable concentrations (ie elevations
are < 5 mg L-1) and are compliant with the WQO. In the bed layer concentrations of SS
at the mouth of Tai Ho Bay were predicted to be were 3 mg L-1 in the
dry season and 10 mg L-1 in the wet season. Given that the current velocities
within Tai Ho Bay have been demonstrated in recent field investigations to be
extremely low (0.08 ms-1 median velocity at the landward side of the
box culvert at the mouth of the bay decreasing to 0.02 ms-1 within
300m of the box culvert ([1])) it is expected that the small amount of
SS that does reach the mouth of the bay will settle out very quickly and not
reach the sensitive receivers located further inside the bay. The Tai Ho stream will not be affected
by the development.
Habitat of the Indo-Pacific Humpback
Dolphin: The waters both within and
surrounding the South Brothers Facility do not appear to be an important
habitat for Sousa chinensis as sightings are infrequent, particularly in
comparison to other waters in the north and west of Lantau. It is thus expected that as elevations in
SS appear to be confined to the immediate area of the South Brothers Facility,
unacceptable impacts to marine mammals arising from elevated SS levels will not
occur. It should be noted, that 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. Impacts to the Indo-Pacific Humpback
Dolphin are discussed in Part 2, Sections 3 & 5.
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 South Brothers 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.
Intakes: Modelling results indicate that the
maximum elevation at these intakes has been identified on the northern seawall
of the Chek Lap Kok International Airport (I1) at 2.2 mg L-1 (wet
season). 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, it appears that 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 South Brothers
Facility (Annex A). This implies
that the majority of suspended sediments settle in close proximity to the
works. The modelling exercise
generated contour plots of sediment deposition 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 South Brothers Facility. Sediment deposition is
therefore not expected to affect any nearby submarine
utilities. A similar pattern of deposition is observed in the wet and
dry seasons.
The plots indicate, that with the
exception of the Airport Exclusion Zone AR, 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 South Brothers Facility are not expected to occur. The deposition levels at the AR are
predicted to be in the range of < 75 g m-2 day-1 in
the wet season and < 25 g m-2 day-1 in the dry
season. These levels are
considered as low and not expected to cause unacceptable impacts to the ARs.
The loss of sediment through backfilling operations at the
South Brothers 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
(contour plots and Tables 2.2a - 2.2c) and indicate that backfilling
operations at the South Brothers 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 A 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 South Brothers 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 South Brothers Facility.
The results of this assessment are presented in Part 2, Section 5
and in Annex C.
As part of the water quality assessment, it is important to
also 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 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.
This discussion has shown that predicted concentrations of
contaminants resulting from a representative operational scenario (Scenario 2 –
Trailer down pipe disposal) at the South Brothers 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 broadly representative of other contaminants of
concern. In addition, as predicted
contaminant concentrations are extremely low (maximum = Chromium, 6.4% of
allowed (wet season)), and modelling results for other operational scenarios
are very similar, modelling of contaminants for other operational scenarios at
the South Brothers Facility is unlikely to produce detectably different
results. In summary, the predicted
contaminant concentrations resulting from operations at the South Brothers
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
South Brothers 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/gf |
2 |
mg/kg |
0.0100 |
4.39E-03 |
3.16E-04 |
3.82E-08 |
2.90E-06 |
1.17E-06 |
- |
1 |
- |
|
As |
130 |
l/gd |
42 |
mg/kg |
0.3231 |
5.99E-02 |
4.31E-03 |
2.18E-04 |
1.65E-02 |
6.66E-03 |
- |
0.5 |
- |
|
Cd |
100 |
l/g |
4 |
mg/kg |
0.0400 |
4.39E-03 |
3.16E-04 |
7.64E-08 |
5.79E-06 |
2.33E-06 |
2.5 |
1 |
0.02% |
|
Cr |
290 |
l/g |
160 |
mg/kg |
0.5517 |
5.09E-01 |
3.67E-02 |
1.86E-03 |
1.41E-01 |
5.66E-02 |
15 |
0.5 |
3.39% |
|
Cu |
122 |
l/g |
110 |
mg/kg |
0.9016 |
1.47E-01 |
1.06E-02 |
5.37E-04 |
4.07E-02 |
1.64E-02 |
5 |
0.5 |
2.94% |
|
Hg |
700 |
l/g |
1 |
mg/kg |
0.0014 |
7.68E-03 |
5.53E-04 |
1.91E-08 |
1.45E-06 |
5.83E-07 |
0.3 |
1 |
0.02% |
|
Ni |
40 |
l/g |
40 |
mg/kg |
1.0000 |
1.76E-02 |
1.26E-03 |
6.40E-05 |
4.85E-03 |
1.95E-03 |
30 |
0.5 |
0.06% |
|
Pb |
130 |
l/g |
110 |
mg/kg |
0.8462 |
1.57E-01 |
1.13E-02 |
5.72E-04 |
4.33E-02 |
1.74E-02 |
25 |
0.5 |
0.63% |
|
Zng |
100 |
l/g |
270 |
mg/kg |
2.7000 |
2.96E-01 |
2.13E-02 |
1.08E-03 |
8.18E-02 |
3.29E-02 |
40 |
5 |
0.74% |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Organics |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
L PAH |
0.075 |
l/g |
3.19 |
mg/kg |
2.7E-06 |
2.60E-06 |
1.87E-07 |
6.04E-08 |
4.57E-06 |
1.84E-06 |
- |
- |
- |
|
H PAH |
1.14 |
l/g |
9.6 |
mg/kg |
1.20E-04 |
1.20E-04 |
8.65E-06 |
1.83E-07 |
1.39E-05 |
5.60E-06 |
- |
- |
- |
|
PCBs |
1,585 |
l/gOC (c) |
180 |
mg/kg |
4.0E-05 |
3.76E-05 |
2.70E-06 |
1.37E-04 |
1.04E-02 |
4.18E-03 |
- |
- |
- |
|
TBTe |
40 |
l/gOC (c) |
0.15 |
mg/kg |
8.3E-10 |
7.86E-10 |
5.66E-11 |
2.87E-09 |
2.17 E-07 |
8.74E-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 Value
is not available, lowest value of other metals has been used, in this case
about 10 for Cd e 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. f 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 South Brothers
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/gf |
2 |
mg/kg |
0.0100 |
8.24E-03 |
2.28E-04 |
6.21E-07 |
9.75E-06 |
5.83E-07 |
- |
1 |
- |
|
As |
130 |
l/gd |
42 |
mg/kg |
0.3231 |
1.12E-01 |
3.11E-03 |
3.55E-03 |
5.57E-02 |
3.33E-03 |
- |
0.5 |
- |
|
Cd |
100 |
l/g |
4 |
mg/kg |
0.0400 |
8.24E-03 |
2.28E-04 |
1.24E-06 |
1.95E-05 |
1.17E-06 |
2.5 |
1 |
0.03% |
|
Cr |
290 |
l/g |
160 |
mg/kg |
0.5517 |
9.56E-01 |
2.64E-02 |
3.02E-02 |
4.73E-01 |
2.83E-02 |
15 |
0.5 |
6.37% |
|
Cu |
122 |
l/g |
110 |
mg/kg |
0.9016 |
2.76E-01 |
7.65E-03 |
8.72E-03 |
1.37E-01 |
8.19E-03 |
5 |
0.5 |
5.53% |
|
Hg |
700 |
l/g |
1 |
mg/kg |
0.0014 |
1.44E-02 |
3.99E-04 |
3.11E-07 |
4.87E-06 |
2.91E-07 |
0.3 |
1 |
0.04% |
|
Ni |
40 |
l/g |
40 |
mg/kg |
1.0000 |
3.30E-02 |
9.12E-04 |
1.04E-03 |
1.63E-02 |
9.76E-04 |
30 |
0.5 |
0.11% |
|
Pb |
130 |
l/g |
110 |
mg/kg |
0.8462 |
2.95E-01 |
8.15E-03 |
9.30E-03 |
1.46E-01 |
8.72E-03 |
25 |
0.5 |
1.18% |
|
Zng |
100 |
l/g |
270 |
mg/kg |
2.7000 |
5.56E-01 |
1.54E-02 |
1.76E-02 |
2.75E-01 |
1.65E-02 |
40 |
5 |
1.39% |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Organics |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
L PAH |
0.075 |
l/g |
3.19 |
mg/kg |
5.2E-07 |
4.88E-06 |
1.35E-07 |
9.81E-07 |
1.54E-05 |
9.21E-07 |
- |
- |
- |
|
H PAH |
1.14 |
l/g |
9.6 |
mg/kg |
2.4E-05 |
2.25E-04 |
6.24E-06 |
2.98E-06 |
4.68E-05 |
2.80E-06 |
- |
- |
- |
|
PCBs |
1,585 |
l/gOC (c) |
180 |
mg/kg |
7.5E-06 |
7.05E-05 |
1.95E-06 |
2.23E-03 |
3.49E-02 |
2.09E-03 |
- |
- |
- |
|
TBTe |
40 |
l/gOC (c) |
0.15 |
mg/kg |
1.6E-10 |
1.48E-09 |
4.08E-11 |
4.66E-08 |
7.31E-07 |
4.37E-08 |
- |
|
- |
|
Note: 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 (Pit C - Scenario 4). 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 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 do not 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 (Airport Exclusion Zone AR: maximum = < 1 mg L-1 (wet
season)).
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 the dredging operations. As such, elevations at the San Tau
Beach SSSI and Yam O are non-detectable.
At the mouth of Tai Ho Bay the predicted elevations during dredging of
Pit C area < 6.5 mg L-1 during the wet season and < 1 mg L-1
in the dry season. Although the
level in the wet season exceeds the WQO by 1.8 mg L-1 such a minor
exceedance is not expected to cause impacts to the sensitive receivers within
Tai Ho Bay. It is expected that
the small amount of SS that does reach the mouth of the bay will settle out
very quickly and not reach the sensitive receivers located further inside the
bay.
Habitat of the Indo-Pacific Humpback
Dolphin: Sightings of Sousa chinensis
are infrequent in the waters of the proposed South Brothers Facility, thus
adverse impacts are unlikely
as sediment dispersion results based on dredging operations predict that
elevations of SS concentrations are expected to stay close to the
facility. Plumes that do leave the
boundary of the site remain in areas where few dolphin sightings have been
recorded. It should be noted, that
a 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: Water quality modelling results have
predicted that there are no detectable SS elevations at the FCZ as a result of dredging
operations, therefore, no unacceptable impacts are expected to occur.
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 South Brothers
Facility (Table 2.1, Annex A).
A similar pattern of deposition is predicted for the wet and dry
seasons. The highest deposition
values at the sensitive receivers were 6 g m-2 day-1 at
the AR in the Airport Exclusion Zone and 2 g m-2 day-1 at
the mouth of Tai Ho Bay. These
values are considered to be very low. Sediment deposition is therefore not expected to affect
any nearby submarine utilities.
As
such, adverse impacts to water quality, marine and fisheries sensitive
receivers by deposited sediments as a result of dredging operations at the
South Brothers Facility are not predicted to occur.
Impacts from the dispersion of sediment in suspension
arising from capping operations have been assessed using computer modelling (Pit
A – Scenario 4).
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 at the bed layers of the water
column (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 South Brothers Facility in that
sediment plumes stay in relatively close proximity to the pit boundaries,
particularly during the dry season.
Maximum elevations on the North Lantau seawall are < 5 mg L-1. Wet season contours appear to indicate
a similar pattern.
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 do not appear 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 (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,
Yam O Bay and Tai Ho Bay sensitive receivers are non-detectable, as such no
exceedance of the WQO would occur.
Habitat of the Indo-Pacific Humpback
Dolphin: Sightings of Sousa chinensis
are infrequent in the waters of the proposed South Brothers Facility, thus
adverse impacts are unlikely
as sediment dispersion results based on capping operations predict that
elevations of SS concentrations are expected to stay close to the
facility. It should be noted, that
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: 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 capping operations indicate that the majority of sediment settles
either within or within relatively close proximity to the South Brothers
Facility (Table 2.1, Annex A).
A similar pattern of deposition is observed between the wet and dry
seasons. With the exception of the
Airport Exclusion Zone AR (3 g m-2 day-1 in the dry
season and 2 g m-2 day-1 in the wet season), deposited
sediments will not reach water quality sensitive receivers. The significance of deposition at the
AR is discussed in Part 2, Section 4. Sediment deposition is therefore not expected to
affect any nearby submarine utilities.
As such, adverse impacts to water
quality, marine and fisheries sensitive receivers by deposited sediments as a
result of capping operations at the South Brothers 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 South Brothers Facility is located at a sufficient
distance from water quality sensitive receivers so that the dispersion of
sediments from the construction and operation works do 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
South Brothers 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 of the
works, as all scenarios have been demonstrated to be acceptable with the
required mitigation measures in place.
1.
Dredging
operations within the South Brothers Facility do not exceed 100,000 m3
week-1.
2.
Backfilling
operations within the South Brothers Facility do not exceed a disposal rate of
26,700 m3 day-1.
3.
Capping
operations within the South Brothers 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 South Brothers 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 (refer to Annex
A for a full list of the projects considered). In addition, cumulative impacts through the combination of
dredging, backfilling and capping operations within the South Brothers 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 South Brothers
Facility. Water quality modelling
of the cumulative impacts of these projects has been presented in Annex A.
It is noted that the main contributor of suspended sediment
in the cumulative modelling scenarios (Scenario 4) is disposal at the North
Brothers facility. The findings
indicated that no adverse impacts would be expected to water quality sensitive
receivers when compared to 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 South
Brothers 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 South Brothers Facility.
This Section has described the impacts to water quality
arising from the construction and operation of the South Brothers
Facility. The purpose of the
assessment was to thoroughly evaluate the South Brothers 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 indicates that there would be
little transport of suspended sediment away from the pits and that the sediment
would not impact upon sensitive receivers. In general, the sediment plumes
generated by the works remain in open waters.
No residual environmental impacts, in
terms of exceedances of applicable standards were predicted to occur as a
result of the dredging, backfilling and capping of the South Brothers Facility,
provided that the recommended mitigation measures are implemented. An EM&A programme has been devised
to confirm that the works would be environmentally acceptable.
Contents
2.1 Introduction 1
2.2 Water
Quality Impact Assessment Methodology 1
2.3 Water
Quality Impact Assessment 1
2.4 Water
Quality Mitigation Measures 12
2.5 Residual
Environmental Impacts 13
2.6 Cumulative
Impacts 1314
2.7 Environmental
Monitoring & Audit 1314
2.8 Conclusions 1314