5.6.1
The
assessment of the impacts to water quality during the construction phase has
been divided into two aspects, formation of the reclamation and land based
construction activities, including those for the Chok Ko Wan Link Road.
Reclamation Formation
5.6.2
The
assessment of impacts from the construction of the Northshore Lantau
Development reclamations is split into three aspects; suspended sediment
concentrations, sediment deposition and chemical effects on water quality. Potential impacts on ecological resources as
a result of reclamation construction activities are covered under Section 8.
Suspended
Sediment Concentrations
5.6.3
A total of
three scenarios were simulated for the impacts from dredging of the four phases
of the Northshore Lantau Development reclamations. A summary of the scenarios is given below.
·
Scenario 1 - dredging of the seawall foundations
for the Siu Ho Wan and Northshore Reclamation, simulated for the wet and dry
season spring and neap tides;
·
Scenario 2 - dredging of the sea wall foundations
for the Fa Peng reclamation, simulated for the wet and dry season spring tides;
and
·
Scenario 3 - dredging of the seawall foundations for
the Theme Park Extension reclamation, simulated for the wet and dry season
spring tides.
5.6.4
The
predicted maximum elevations in suspended sediment concentrations at sensitive
receivers are presented in Table 5.6a
for Scenario 1 and in Table 5.6b for
Scenario 2.
Table
5.6a - Maximum Predicted Elevations in Depth Averaged Suspended Sediment
Concentrations at Sensitive Receivers (mg L-1) for Scenario 1
|
Sensitive Receiver
|
Tide Type
|
|
|
Wet Spring
|
Wet Neap
|
Dry Spring
|
Dry Neap
|
|
Ma
Wan Fish Culture Zone N
|
0.3
|
0.4
|
0.4
|
0.4
|
|
Ma
Wan Fish Culture Zone S
|
0.2
|
0.2
|
0.4
|
0.4
|
|
Tung
Wan Beach, Ma Wan
|
0.3
|
0.4
|
0.3
|
0.2
|
|
Sze
Pak Wan
|
0.2
|
0.1
|
0.2
|
0.2
|
|
Discovery
Bay Beach
|
0.2
|
0.1
|
0.2
|
0.1
|
|
Kau
Yi Chau
|
0.3
|
0.2
|
0.3
|
0.2
|
|
Silvermine
Bay Beach
|
0.1
|
0.1
|
0.2
|
0.1
|
|
Brothers
|
0.5
|
0.3
|
0.7
|
0.7
|
|
Tung
Chung Bay
|
0.4
|
0.4
|
1.0
|
0.8
|
|
Chek
Lap Kok Intake
|
0.8
|
0.2
|
0.7
|
0.8
|
Table
5.6b - Maximum Predicted Elevations in Suspended Sediment Concentrations at
Sensitive Receivers (mg L-1) for Scenarios 2 and 3
|
Sensitive Receiver
|
Scenario 2
|
Scenario 3
|
|
|
Wet Spring
|
Dry Spring
|
Wet Spring
|
Dry Spring
|
|
Ma
Wan Fish Culture Zone N
|
0.2
|
0.5
|
0.2
|
0.3
|
|
Ma
Wan Fish Culture Zone S
|
0.2
|
0.5
|
0.1
|
0.4
|
|
Tung
Wan Beach, Ma Wan
|
0.2
|
0.4
|
0.1
|
0.3
|
|
Sze
Pak Wan
|
0.4
|
0.6
|
0.5
|
1.2
|
|
Discovery
Bay Beach
|
0.4
|
0.5
|
0.4
|
1.0
|
|
Kau
Yi Chau
|
0.3
|
0.7
|
1.1
|
1.4
|
|
Silvermine
Bay Beach
|
0.3
|
0.3
|
0.3
|
0.6
|
|
Brothers
|
0.3
|
0.4
|
0.1
|
0.3
|
|
Tung
Chung Bay
|
0.1
|
0.3
|
0.1
|
0.2
|
|
Chek
Lap Kok Intake
|
0.2
|
0.3
|
0.1
|
0.2
|
5.6.5
The data in
the above tables show that there are predicted to be no exceedances of the WQO
for suspended sediments at sensitive receivers for any of the three
scenarios. There are therefore
predicted to be no adverse impacts at sensitive receivers due to elevations in
suspended sediment concentrations.
5.6.6
Contour
plots of maximum depth averaged suspended sediment concentrations are contained
in Figures D2a to D2f.
The contours for Scenario 1 (Figures
D2a and D2b) show that the
modelling predicts that the majority of the suspended sediments will be spread
out along the northern coastline of Lantau Island. In the dry season, which has the highest increases in suspended
sediment concentrations, the concentrations along the coast between Siu Ho Wan
and the eastern end of the Tung Chung New Town are predicted to be less than 3
mg L-1, while further offshore the concentrations are generally less
than 1 mg L-1. Around the
works area suspended sediment concentrations are predicted to be less than 5 mg
L-1 and are only higher in the immediate vicinity of the dredging
works. The WQO in this area may be best
defined with respect to Station NM3, which has an allowable increase in
suspended sediment concentrations in the dry and wet seasons of 8.6 mg L-1
and 2.6 mg L-1 respectively.
In the dry season there are only predicted to be exceedances of the WQO
in the immediate vicinity of the dredging works for the Siu Ho Wan
reclamation. In the wet season there
are predicted be an exceedance of the WQO along a 5 km stretch of the northern
coastline of Lantau Island. This area,
however, does not contain any sensitive receivers and has a limited offshore
extent of the exceedance and as such could be considered to be a ‘mixing zone’,
which would not be an adverse impact.
5.6.7
The
contours for Scenarios 2 and 3 (Figures
D2c to D2f) show that the
majority of the suspended sediments are predicted to be transported in a
southerly direction from the dredging activities. In Scenario 2 for the dry season the majority of the suspended
sediments remain close to the northern headland of Lantau Island in the
vicinity of Fa Peng and the Penny’s Bay reclamation for the International Theme
Park, where concentrations are predicted to be less than 1.5 mg L-1. Offshore of this area suspended sediment
concentrations are predicted to be less than 1 mg L-1. In the wet season concentrations in the
vicinity of Fa Peng and the International Theme Park are predicted to be less
than 1 mg L-1. Further
south, in the West Lamma Channel, there are ‘hot spots’ of suspended sediment
with concentrations of less than 3 mg L-1 shown. In Scenario 3 for the dry season elevated
suspended sediment concentrations are predicted cover an area from the Theme
Park Extension reclamation to Hei Ling Chau.
Within this area suspended sediment concentrations are predicted to be
less than 3 mg L-1, with the majority of concentrations less than
1.5 mg L-1. In the wet
season isolated ‘hot spots’ are shown in the West Lamma Channel with concentrations of less than 5
mg L-1, which shows a similar pattern to Scenario 2. The WQO for the area to the south of the
dredging works in Scenarios 2 and 3 is best defined by using EPD routine
monitoring station SM9, which defines allowable increases in suspended sediment
concentrations in the wet and dry seasons of 5.4 mg L-1 and 3.8 mg L-1
respectively. The contour plots show
that there are predicted to be no exceedances of the WQO in the receiving
marine waters.
5.6.8
Following
dredging of the seawall foundations, the impacts from which have been assessed
using computer modelling, the trenches will be backfilled with a firm material
suitable for providing a stable base for the seawalls. Suitable materials could be sandfill or
rockfill. The placement of rockfill
material would not result in the loss of fine sediment to suspension, provided
that the rockfill was free of such material as soil and organic debris, and as
such would not cause an adverse impact to water quality. Sandfill material is likely to contain a
small percentage of fine sediment, which could be lost to suspension during
placement. The losses of sediment to
suspension could be controlled by placing the sandfill by pumping down the arm
of a trailing suction hopper dredger, which is positioned within the
trench. This will mean that the losses
of sediment to suspension would be below the surrounding sea bed in the trench,
where the tidal currents would be lower and as such would settle out into the
trench. This would ensure that the fine
sediment would not be transported beyond the immediate vicinity of the works
area and hence adverse impacts are unlikely.
Sediment
Deposition
5.6.9
The
contours of net deposition over a tidal cycle are shown on Figures D2g to D2n . For Scenario 1 the maximum rates of
deposition outside of the works area are predicted to be less than 0.01 kg m-2
day-1, while in the vicinity of the dredging works the deposition
rates are predicted to be greater than 0.05 kg m-2 day-1.
In both Scenarios 2 and 3 the maximum rates of deposition are predicted to be
less than 0.025 kg m-2 day-1 in offshore areas. The predicted maximum rates of deposition
are therefore less than the critical value of 0.2 kg m-2 day-1,
except in the immediate vicinity of the dredging operations for Scenario 1
where the threshold value may be exceeded. However, this area is not expected
to contain corals and as such could not be considered an adverse impact. There are thus predicted to be no adverse
impacts due to sediment deposition from the dredging works for the Northshore
Lantau Development reclamation.
Water
Quality
5.6.10
A review of
the results of the sediment plume modelling has determined that the maximum
suspended sediment concentration outside of the immediate vicinity of the
dredging works is 5 mg L-1 for Scenarios 1 and 3 and 3 mg L-1
for Scenario 2. These values will be
used to calculate the effects of these increased suspended sediment
concentrations on dissolved oxygen, unionised ammonia and total inorganic
nitrogen. The calculations are based on
the sediment quality data presented in Table
5.5b for chemical oxygen demand, ammoniacal nitrogen and total nitrogen
(used here to conservatively represent total inorganic nitrogen). In the calculation it is assumed that all
of the chemical oxygen demand is exerted and that all of the total nitrogen and
ammonia are released to the water.
These are conservative assumptions and will likely result in an
over-prediction of the potential impacts.
5.6.11
In order to
determine compliance with the WQOs the background water quality data is
referenced, as presented in Section 5.5.1. The calculated of ammonia release from the
sediment will result in a concentration of total ammonia in the receiving
waters which must be converted to unionised ammonia to compare with the WQO. For the sediment plumes from Scenario 1 the
EPD routine monitoring data at Station NM1 has been reviewed and the average
proportion of unionised ammonia in total ammonia found to be 3.8%. The sediment plumes from Scenarios 2 and 3
pass closest to Stations SM10 and SM9 and the data at these stations shows that
on average the unionised ammonia constitutes 3.3% of the total ammonia
concentrations. The results of the
analysis for dissolved oxygen, total inorganic nitrogen and unionised ammonia
are contained in Tables 5.6d, 5.6e
and 5.6f.
Table
5.6d - Calculation of the Effects of
Increased Suspended Sediment Concentrations on Dissolved Oxygen Concentrations
|
Scenario
|
Suspended Sediment (mg L-1)
|
COD
(mg kg-1)
|
DO Depletion (mg L-1)
|
Background DO (mg L-1)
|
Resultant DO (mg L-1)
|
|
1
|
5
|
36,500
|
0.18
|
4.6
|
4.42
|
|
2
|
3
|
34,100
|
0.10
|
4.9
|
4.80
|
|
3
|
5
|
36,500
|
0.18
|
4.9
|
4.72
|
Table
5.6e - Calculation of the Effects of
Increased Suspended Sediment Concentrations on Total Inorganic Concentrations
|
Scenario
|
Suspended Sediment (mg L-1)
|
TIN in Sediment
(mg kg-1)
|
TIN Increase (mg L-1)
|
Background TIN (mg L-1)
|
Total TIN (mg L-1)
|
|
1
|
5
|
1,398
|
0.007
|
0.43
|
0.437
|
|
2
|
3
|
1,512
|
0.005
|
0.27
|
0.275
|
|
3
|
5
|
1,698
|
0.008
|
0.27
|
0.278
|
|
Notes:
1. Shaded
cells indicate exceedance of the WQO.
|
|
|
|
|
|
|
|
|
Table
5.6f - Calculation of the Effects of
Increased Suspended Sediment Concentrations on Unionised Ammonia Concentrations
|
Scenario
|
Suspended Sediment
(mg L-1)
|
Ammonia in Sediment
(mg kg-1)
|
Unionised Ammonia Increase
(mg L-1)
|
Background Unionised Ammonia (mg L-1)
|
Total Unionised Ammonia
(mg L-1)
|
|
1
|
5
|
108
|
0.00002
|
0.005
|
0.00502
|
|
2
|
3
|
152
|
0.00002
|
0.003
|
0.00302
|
|
3
|
5
|
108
|
0.00002
|
0.003
|
0.00302
|
5.6.12
The data in
the above tables predict that there will be no exceedances of the WQOs for
dissolved oxygen and unionised ammonia.
For total inorganic nitrogen there are, however, predicted to be
exceedances of the WQOs for Scenarios 2 and 3.
For these scenarios the existing conditions already breach the WQO for
total inorganic nitrogen. The dredging
works are predicted to increase the existing background concentrations by less
than 3%, which is a small amount. The
dredging works are therefore not responsible for the breach of the WQO and are
not contributing significantly to the breach in the WQO. Also, the dredging works will only increase
the background concentrations during the construction works and will thus be of
short duration and will not prevent recovery of the water body in the
future. It is therefore concluded that
the dredging works for the Northshore Lantau Development reclamations will not
cause adverse impacts to water quality in terms of dissolved oxygen, total
inorganic nitrogen and unionised ammonia.
5.6.13
The
prediction of the release of copper, arsenic, silver, PAHs and PCBs has been
based on the following equation, which has been used on previous approved
projects in Hong Kong
and represents the partitioning of pollutants between the adsorbed and
desorbed phases. This equation is
applicable to determine partitioning of pollutants associated with cohesive
sediments and is thus suitable for this Study.
The equation describing the partitioning is as follows:
Ct =
Cs + (CsxKdxSS)
Where Cs = concentration of metal
in water (desorbed)
Kd
= partitioning coefficient
SS
= suspended sediment concentration
Ct
= SS x Csed
Csed
= concentration of metal in sediment
5.6.14
The value
of the partitioning coefficient for copper was derived from the a previous
study which assessed the environmental impacts of dredging Kellett Bank .
The value for the partitioning coefficient for arsenic was obtained from
chemical databases in the Netherlands.
There are, however, no quoted values for silver and it is proposed to
assume that all of the silver desorbs from the suspended sediment, which will
give a conservative assessment. The
values of the partitioning coefficients for PAHs and PCBs were derived from a
previous study which assessed the environmental impacts of the disposal of
contaminated mud at East Sha Chau .
There is, however, no partition coefficient for TBT and it is proposed
to assume that all of the TBT is released, which will give a very conservative
assessment. The partition coefficients
are summarised in Table 5.6k.
Table
5.6g - Partition Coefficients (L g-1)
for Pollutants in Sediment
|
Parameter
|
Partition Coefficient
|
|
Copper
|
122
|
|
Arsenic
|
10
|
|
Silver
|
Assume
all released
|
|
Total
PAHs
|
0.075
|
|
Total
PCBs
|
1000
|
|
TBT
|
Assume
all released
|
5.6.15
The
concentrations of copper, arsenic, silver, PAHs, PCBs and TBT in the sediment
was derived from sediment quality monitoring data collected for this Study, as
presented in Table 5.5b. It should be noted that for PAHs and PCBs
the values were all below the detection limit and so the detection limit has
been used in order to provide a conservative estimate of the release to the
water column. A review of the results of the sediment plume modelling has
determined that the maximum suspended sediment concentration outside of the
immediate vicinity of the dredging works is 5 mg L-1 for Scenarios 1
and 3 and 3 mg L-1 for Scenario 2 and these values were used to
calculated the increases in pollutants in the receiving waters. The results of the analyses are shown in Table 5.6h.
Table
5.6h - Desorbed Concentrations of Pollutants
|
Scenario
|
SS
(mg L-1)
|
Copper
(mg L-1)
|
Arsenic
(mg L-1)
|
Silver
(mg L-1)
|
Total PAHs
(mg L-1)
|
Total PCBs
(mg L-1)
|
TBT
(mg L-1)
|
|
1
|
5
|
0.25
|
0.157
|
0.006
|
0.040
|
0.00008
|
0.0020
|
|
2
|
3
|
0.23
|
0.035
|
<LCEL
|
0.024
|
0.00008
|
0.0009
|
|
3
|
5
|
0.32
|
0.057
|
<LCEL
|
0.040
|
0.00008
|
0.0015
|
5.6.16
The
calculated increases in pollutant concentrations are now added to the
background concentration, presented in Tables
5.5c and 5.5d, in order to
compare with the relevant assessment standards, as shown in Tables 5.5c and 5.5d. The results of this
analysis are shown in Table 5.6i
Table
5.6i - Total Concentrations of Pollutants
|
Scenario
|
Copper
(mg L-1)
|
Arsenic
(mg L-1)
|
Silver
(mg L-1)
|
Total PAHs
(mg L-1)
|
Total PCBs
(mg L-1)
|
TBT
(mg L-1)
|
|
1
|
2.76
|
2.257
|
0.506
|
0.040
|
0.00008
|
0.0110
|
|
2
|
2.74
|
2.135
|
<LCEL
|
0.024
|
0.00008
|
0.0099
|
|
3
|
2.83
|
2.157
|
<LCEL
|
0.040
|
0.00008
|
0.0115
|
5.6.17
The data in
the above table predict that the increase in concentrations of copper, arsenic,
silver, total PAHs and total PCBs as a result of desorbtion from suspended
sediments will not cause the total concentrations to exceed the assessment
standards. There are therefore not
predicted to be adverse impacts to water quality due to the release of these
contaminants.
5.6.18
The release
of TBT from the sediments is predicted to cause increases of 22%, 10% and 15% compared with the
background concentrations for Scenarios 1, 2 and 3 respectively. There would thus be the potential for
impacts from the release of TBT from the sediment suspended during
dredging. However, despite the very
conservative assumption regarding the release of TBT from the suspended
sediment the elevation in concentrations from the release of TBT does not
result in a significant increase in background concentrations. In order to gain better estimates of the
release of TBT from the dredged sediment it is recommended that elutriate
testing be carried out during further, more detailed studies for each of the
reclamations. Such testing should be
carried out on representative samples of the sediment to be dredged and would
determine the proportion that could be released to the water column. The results from this testing would then be
used to determine whether measures would be required to reduce the release of
TBTs.
5.6.19
The
potential sources of impacts, described in Section
5.5.2, may be readily controlled by appropriate on-site measures to
minimise potential impacts and, as such, no further assessment of impacts has
been carried out.