5.1.1
This section presents the potential
water quality impacts associated with the construction and operation phases of
the proposed roads D1, D8 and D10 and the extended road P2. The majority of the construction works will
be land-based, on existing reclamation, and thus direct impacts on marine water
quality will not arise. Marine based
construction works will only be required for the elevated section of Road D10
across the Eastern Drainage Channel.
Water
Pollution Control Ordinance (WPCO)
5.2.1
The Water Pollution Control Ordinance
(Cap. 358), in existence since 1980, is the major legislation relating to the
protection and control of water quality in Hong Kong. According to the Ordinance and its subsidiary legislation, Hong
Kong waters are divided into ten water control zones (WCZ). Corresponding statements of Water Quality
Objectives (WQO) are stipulated for different water regimes (marine waters,
inland waters, bathing beaches subzones, secondary contact recreation subzones
and fish culture subzones) in the WCZ based on their beneficial uses. The study area is located within the TKO Bay
(Junk Bay) WCZ (Figure 5.1). Details of
the WQOs for the TKO Bay (Junk Bay) WCZ are reproduced in Table 5.1.
Technical
Memorandum
5.2.2
Besides setting the WQOs, the WPCO
controls effluent discharging into the WCZs through a licensing system. A Technical
Memorandum (TM) on Standards for Effluents Discharged into Drainage and Sewerage
Systems, Inland and Coastal Waters was issued under the WPCO which gives
guidance on the permissible effluent discharges based on the type of receiving
waters (foul sewers, storm water drains, inland and coastal waters). The limits
control the physical, chemical and microbial quality of effluents. Sewage from the proposed construction
activities should comply with the TM standards for effluent discharged into the
marine waters of TKO Bay (Junk Bay) WCZ (Table 10b of the TM).
Practice
Notes
5.2.3
A practice note (PN) for professional
persons was issued by the EPD to provide environmental guidelines for handling
and disposal of construction site discharges.
The ProPECC PN 1/94 “Construction
Site Drainage” provides good practice guidelines for dealing with various
types of discharge from a construction site.
Practices outlined in the PN should be followed as far as possible
during construction to minimize the water quality impact due to construction
site drainage.
WSD
Seawater Intake Water Quality Standards
5.2.4
Beside the WQOs set under the WPCO,
the Water Supplies Department has also specified a set of criteria for water
quality at their seawater intakes for flushing purposes. These criteria are shown in Table 5.2. The criteria of particular relevance to this
water quality assessment is the 10 mg/L target limit for suspended solids. A tolerable limit is also specified for
suspended solids of 20 mg/L.
Environmental
Impact Assessment Ordinance (EIAO)
5.2.5
The criteria and guidelines for
evaluating impacts on marine water quality are given in Annex 6 and Annex 14 of
the Technical Memorandum on Environmental
Impact Assessment Process and have been adopted in this project.
Table 5.1 Water Quality Objectives Statement for TKO Bay (Junk Bay) WCZ
|
Parameters
|
Objectives
|
Part(s) of
Zone
|
|
Objectionable odours,
colour, oil, lasting foams, debris, litter
|
Should be absent
|
Whole Zone
|
|
E.
coli
|
Annual geometric mean not to exceed 610 per
100 mL
|
Secondary Contact Recreation Subzone &
Fish Culture Subzones
|
|
Dissolved Oxygen
|
Discharges shall not cause depth averaged DO
to fall below 4 mg/l for 90% of total samples;
DO level not less than 2 mg/l for 90% of
total samples within 2 metres of the seabed;
Discharge shall not cause depth averaged DO to
fall below 5 mg/l for 90% of total samples;
Waste discharges shall not cause DO <
4 mg/l
|
Marine waters except Fish Culture Subzones
Whole zone
Fish Culture Subzones
Inland waters
|
|
pH
|
Within the range of 6.5-8.5 units,
discharge shall not cause the natural range to be extended by more than
0.2 units;
Within the range of 6.0-9.0 units
|
Marine waters
Inland waters
|
|
Temperature
|
Change caused by discharge not to exceed 2.0
°C from natural daily temperature
|
Whole Zone
|
|
Salinity
|
Change caused by discharge not to exceed 10%
of natural ambient salinity level
|
Whole Zone
|
|
Suspended Solids
|
Discharges not to cause the natural ambient
level to be raised by 30%, nor lead to accumulation of solids;
Discharges not to lead to annual median of
suspended solids to exceed 25 mg/l
|
Marine waters
Inland waters
|
|
Ammonia
|
Annual average of unionized ammoniacal-N level
not to exceed 0.021 mg/l
|
Whole Zone
|
|
Nutrients
|
Quantities not to cause excessive or nuisance
growth of algae or other aquatic plants;
Annual water column average of inorganic
nitrogen not to exceed 0.3 mg/l
|
Marine waters
Marine waters
|
|
Biochemical Oxygen
Demand
|
Discharge not to cause BOD5 to exceed
5 mg/l
|
Inland waters
|
|
Chemical Oxygen Demand
|
Discharge not to cause COD to exceed
30 mg/l
|
Inland waters
|
|
Dangerous Substances
|
Discharges not to cause the concentrations of
dangerous substances in the water to attain a level which produce significant
toxic effects, nor to put a risk to any beneficial uses of the aquatic
environment
|
Whole Zone
|
Table 5.2 WSD Standards for
Sea Water for Flushing Supply (at intake point)
|
Parameters (mg l-1 unless otherwise
stated)
|
Target Limit
|
|
Turbidity
(N.T.U.)
|
< 10
|
|
Ammoniacal
Nitrogen
|
< 1
|
|
Suspended
Solids
|
< 10
|
|
Dissolved
Oxygen
|
> 2
|
|
Biochemical
Oxygen Demand (5 days)
|
< 10
|
|
Synthetic
Detergents
|
< 5
|
|
Threshold
Odour no. (TON)
|
<100
|
|
Colour
(H.U)
|
<20
|
|
E. coli
(count / 100 mL)
|
< 20,000
|
5.3.1
EPD has been carrying out routine
monitoring of the quality of Hong Kong waters for a long period. There are two water quality monitoring stations
within TKO Bay (Figure 5.2):
·
JM3 inner TKO Bay
·
JM4 near the mouth of TKO Bay
5.3.2
A summary of EPD monitoring data for
1997 is given in Table 5.3.
According to the EPD’s Publication Marine
Water Quality in Hong Kong for 1997, full compliance with the WQOs for
unionised ammonia was achieved at both JM3 and JM4. Total inorganic nitrogen at station JM3 exceeded the WQO.
Ammoniacal nitrogen was reported to show a rising trend at station JM4. Full compliance with the WQO for dissolved
oxygen was achieved at both JM3 and JM4.
5.3.3
A review of the latest monitoring data
for nutrients for 1998 indicates full compliance with the WQO for both
unionised ammonia and total inorganic nitrogen at JM3 and JM4. On comparison with the monitoring data for
the previous year, the concentration of ammoniacal nitrogen was shown to
decrease at station JM3, whilst JM4 showed a slight increase.
5.3.4
Marine water quality monitoring data
have been obtained from the monitoring programme carried out by the contractor
during the restoration works period at TKO 1 Landfill. There are only two off-shore monitoring
stations (M1 and M2) in TKO Bay.
Station M1 is at the same location as the EPD routine monitoring station
JM3 in inner TKO Bay. Marine water
samples are not taken near the seawall boundary of the landfill. All water samples from stations M1 and M2
had ammoniacal nitrogen levels below the limit of detection of 0.5 mg/l. In addition, all water samples had nitrate
nitrogen and nitrite nitrogen levels below the detection limit of 0.5 mg/l and
0.05mg/l respectively. As the detection
limits are higher than those adopted in the EPD marine water quality monitoring
programme, it is considered that the data recorded at station JM3 under the EPD
monitoring programme gives a more accurate representation of the existing water
quality in inner TKO Bay.
Table 5.3 Summary statistics of marine water quality
of TKO Bay (Junk Bay) WCZ for 1997
|
Determinand
|
|
JM3
Inner TKO Bay
|
|
JM4
Mouth of TKO Bay
|
|
Number of
samples
|
|
12
|
|
12
|
|
Temperature (ºC)
|
|
22.5
|
|
22.33
|
|
|
|
(16.7-26.9)
|
|
(16.7-26.9)
|
|
Salinity (ppt)
|
|
31.7
|
|
32.0
|
|
|
|
(28.3-34.3)
|
|
(28.7-34.9)
|
|
Dissolved Oxygen
(mg/L)
|
Surface
|
5.3
|
|
5.5
|
|
|
|
(4.3-6.8)
|
|
(4.6-7.5)
|
|
|
Bottom
|
5.0
|
|
5.6
|
|
|
|
(3.9-7.1)
|
|
(3.7-7.6)
|
|
Dissolved Oxygen
(% Saturation)
|
Surface
|
73
|
|
77
|
|
|
|
(63-85)
|
|
(67-94)
|
|
|
Bottom
|
69
|
|
76
|
|
|
|
(55-89)
|
|
(53-100)
|
|
PH
|
|
7.8
|
|
7.9
|
|
|
|
(7.1-8.2)
|
|
(7.6-8.3)
|
|
Secchi Disc
Depth (m)
|
|
3.0
|
|
3.3
|
|
|
|
(1.7-5.0)
|
|
(1.8-5.0)
|
|
Turbidity (NTU)
|
|
4.0
|
|
3.9
|
|
|
|
(1.4-6.9)
|
|
(1.2-7.2)
|
|
Suspended Solids
(mg/L)
|
|
3.7
|
|
3.0
|
|
|
|
(1.4-7.8)
|
|
(1.4-4.4)
|
|
5-day
Biochemical Oxygen Demand (mg/L)
|
|
0.5
(0.3-0.9)
|
|
0.5
(0.3-0.8)
|
|
Ammoniacal
Nitrogen (mg/L)
|
|
0.23
|
|
0.20
|
|
|
|
(0.09-0.34)
|
|
(0.09-0.32)
|
|
Unionized
Ammonia (mg/L)
|
|
0.006
|
|
0.007
|
|
|
|
(0.002-0.014)
|
|
(0.003-0.020)
|
|
Nitrite Nitrogen
(mg/L)
|
|
0.02
|
|
0.02
|
|
|
|
(<0.01-0.04)
|
|
(<0.01-0.04)
|
|
Nitrate Nitrogen
(mg/L)
|
|
0.08
|
|
0.08
|
|
|
|
(0.02-0.20)
|
|
(0.02-0.21)
|
|
Total Inorganic
Nitrogen (mg/L)
|
|
0.33
|
|
0.30
|
|
|
|
(0.14-0.46)
|
|
(0.17-0.46)
|
|
Total Kjeldahl
Nitrogen (mg/L)
|
|
1.09
|
|
1.05
|
|
|
|
(0.45-1.37)
|
|
(0.42-1.40)
|
|
Total Nitrogen
(mg/L)
|
|
1.19
|
|
1.15
|
|
|
|
(0.52-1.46)
|
|
(0.50-1.45)
|
|
Orthophosphate
Phosphorus (mg/L)
|
|
0.04
|
|
0.03
|
|
|
|
(0.02-0.06)
|
|
(0.01-0.05)
|
|
Total Phosphorus
(mg/L)
|
|
0.08
|
|
0.08
|
|
|
|
(0.05-0.14)
|
|
(0.05-0.13)
|
|
Silica (as SiO²
) (mg/L)
|
|
1.0
|
|
1.0
|
|
|
|
(0.5-2.0)
|
|
(0.4-1.8)
|
|
Chlorophyll-a
(µg/L)
|
|
2.2
|
|
2.4
|
|
|
|
(0.9-5.8)
|
|
(0.6-9.7)
|
|
Phaeo-pigment
(µg/L)
|
|
0.8
|
|
1.3
|
|
|
|
(0.2-3.6)
|
|
(0.2-6.1)
|
|
E.coli
(cfu/100mL)
|
|
1054
|
|
1281
|
|
|
|
(397-4567)
|
|
(480-4967)
|
|
Faecal Coliforms
(cfu/100mL)
|
|
1910
|
|
2476
|
|
|
|
(503-7700)
|
|
(770-9733)
|
Note: 1. Except
as specified, data presented are depth-averaged data.
2.
Data presented are annual arithmetic means
except for E.coli and faecal coliforms which are geometric means.
3.
Data enclosed in brackets indicate the ranges.
§
5.3.5
Marine sensitive receivers (Figure
5.1) identified within TKO Bay include:
·
A
secondary contact recreation subzone is declared alongside the western side of
TKO Bay. The proposed port development
at Area 131 is located within this secondary contact recreation subzone and
would cause a permanent change of beneficial use of the area.
·
WSD
saltwater pumping station intake located at Area 86.
5.4.1
The evaluation criteria and guidelines
presented in Annexes 6 and 14 of the Technical
Memorandum (TM) on Environmental
Impact Assessment Process were followed where applicable for the present
study.
5.4.2
Potential sources of water quality
impact that may arise during the construction of the Project will be
described. This task will include
identifying pollutants from point discharges and non-point sources to surface
run-off. All the identified sources of potential water quality impact will then
be evaluated and their impact significance determined. The need for mitigation measures to reduce
any identified adverse impacts on water quality to acceptable levels will be
determined. Preliminary assessment on
the change in hydrodynamic regime within the Eastern Drainage Channel as a
result of pile installation was carried out as described below. This assessment approach was adopted as no
significant reduction in the flow within the drainage channel was anticipated
due to the small scale of the works, and thus water quality modelling was not
considered to be warranted.
5.4.3
The 2D Hydrodynamic model used for
examining flow within Eastern Drainage Channel is set up based upon the MIKE
model suite set up for 3D water quality modelling with the addition of the
Eastern Drainage Channel. The 3D model covers the whole of the Victoria Harbour
with a 75m grid model (see Figure 5.3).
The TKO area is modelled by a 25 m grid model. The 75 m grid and 25 m models are hydrodynamic coupled by putting
the 25 m grid inside the 75 m grid model as a nested model (see Figure
5.3). Only the 25 m grid model is used
for the assessment. The 25 m grid model
was extended to cover the Eastern Drainage Channel for this modelling exercise
(see Figure 5.3). The actual width of
the channel varies and typical sections are shown in this figure. Given the resolution of the model is 25m
grids, such model configuration is considered to be a fair representation of
the channel layout. Whole of the TKO
Bay was modelled so that the area of interest (Eastern Drainage Channel) was
far away from the boundary of the model and more realistic conditions at the
mouth of the channel could be established.
5.4.4
To model the effect of piled
structures of Road D10, the resistance to the flow due to the piers was
modelled by calculating the current induced drag force on each individual pier
and equating the force with shear stress contribution compatible with Mike21
momentum formulation. Such calculation
was carried out by a bridge pier module built inside the Mike 21 HD
module. The drag force was estimated by
this module based on the pier effective geometry, shape and height of pier
exposed to current and the current speed.
Figure 5.4 shows the locations of the four piers being modelled and the
modelling input data for each pier.
5.4.5
Boundary conditions were extracted
from the 3-D Hydrodynamic and Water Quality model. For hydrodynamic conditions boundary, velocity is used as the
principle boundary for the southern boundary and water level is used as the
principle boundary for western boundary.
5.4.6
A period of 15 days spanning across a
spring-neap tide cycle has been carried out for both dry season and wet season
modelling. Similar to the 3-D water
quality modelling, minimum of 3 days spinning up time is allowed prior to the
15 days simulation period. This is considered as adequate based on the
observation in the Tonggu Waterway Model, in which this model based, that
similar salinity field distribution can be achieved after 3 days of simulation
with different initial salinity field distribution (Study on Tonggu Waterway –
Calibration of Wet Season Hydrodynamic Model).
Further tests were conducted during the 3-D water quality modelling
study and concluded that 3 days spinning up time is adequate.
construction
phase
5.5.1
Potential sources of water quality
impact associated with the construction of the roads have been identified and
include:
·
site
runoff and drainage;
·
general
construction activities; and
·
sewage
effluent produced by on-site workforce.
5.5.2
These potential water quality impacts
are common to Roads D1, D8 and D10 and the extended road P2. Specific to Road D10 is the construction of
the pier foundations for the elevated crossing near the mouth of the Eastern
Drainage Channel. Other issues specific
to Road D10 are possible blockage of the leachate seepage flow from the TKO
Stage 1 landfill by the pile supports, and the discharge of potentially
contaminated groundwater during excavation works due to the proximity of the
works to the landfill site boundary at Area 77.
Site Runoff and Drainage
5.5.3
Runoff from the construction site may
contain increased loads of sediments, other suspended solids and
contaminants. Potential sources of
pollution from site drainage include:
·
runoff
and erosion from exposed soil surfaces, drainage channels and stockpiles;
·
release
of grouting and cement materials with rain wash;
·
wash
water from dust suppression sprays; and
·
fuel
and lubricants from maintenance of construction vehicles and mechanical
equipment.
5.5.4
Sediment laden runoff may carry
pollutants (adsorbed onto the particle surfaces) into the receiving
waters. Associated effects which may
arise include blockage of storm water drainage channels and increased suspended
solids concentrations in receiving waters.
5.5.5
Mitigation measures should be
implemented to control construction site runoff, and to minimise the chances of
introducing sediment and pollutants into the storm water drainage system and
into the receiving waters of TKO Bay.
The implementation of measures to control runoff and drainage will be
important for the sections of Road D1 and D10 adjacent to the Eastern Drainage
Channel in order to prevent runoff and drainage water with high levels of
suspended solids entering the drainage channel. In addition, the implementation of such measures will be required
for the section of Road D10 in the vicinity of the WSD saltwater pumping station
at Area 86 and to prevent sediment and pollutants discharging into the
rectangular drainage channel at the south-west of TKO Stage 1 Landfill. With the implementation of adequate
construction site drainage and the provision of sediment removal facilities as
described in Section 5.6.2, it is anticipated that unacceptable water quality
impacts would not arise.
General Construction
Activities
5.5.6
On-site construction activities may
cause water pollution from the following:
·
Uncontrolled
discharge of debris and rubbish such as packaging, construction waste and
refuse; and
·
Spillages
of liquids stored on-site, such as oil, diesel and solvents etc, are likely to
result in water quality impacts if they enter storm water drains or the open
drainage channels.
5.5.7
Good construction and site management
practices should be observed to ensure that litter, fuels and solvents do not
enter nearby storm water drains or open drainage channels. Open culverts near
the works areas should be covered to block the entrance of large debris and
refuse.
Sewage Effluents
5.5.8
Domestic sewage would be generated
from the workforce during the construction phase. However, this temporary
sewage can be adequately treated by interim sewage treatment facilities, such
as portable chemical toilets, which can be installed within the construction
site. It is unlikely that sewage generated from the site would have a
significant water quality impact, provided that sewage is not discharged
directly to marine waters, and chemical toilets are used and properly
maintained.
Eastern Drainage Channel
Road Crossing
5.5.9
The road crossing over the Eastern
Drainage Channel is approximately 160 m in length and will be supported by four
piers. Two piers will be located within
the channel itself and there will be one pier on either side of the channel
perimeter. Each pier will comprise four
pile supports within the water body of approximately 1 m diameter each. The width of the pile cap on top of the pile
supports is approximately 8 m. Further
details of the road crossing are shown in Figure 5.5.
5.5.10
Non-displacement type bored piles will
be adopted for the marine pile foundation works. The piling will involve driving temporary steel casing through
the soft mud prior to excavation of the material within the casing. Once the pile excavation is embedded into
firm stratum, permanent casings and reinforcement cages will be lowered into
the temporary casing prior to placing of concrete. No dredging of marine sediment will be required for the marine
pier foundations.
5.5.11
As minimal disturbance will result to
the seabed during the driving of casings and the excavation of sediment will be
confined within the casing, the potential for impacts on the water quality
within the channel from resuspended sediment is anticipated to be very
localized and short term. It is
anticipated that any elevations in suspended solids concentrations during the
driving of the temporary steel casing would be limited to the bottom depth of
the water column and that the resuspended sediment would settle quickly on the
seabed, close to the piling location.
The disposal of slurry water will need to comply with the TM on Effluent
Discharge Standards under the WPCO.
During the construction works for the road crossing, no waste nor
wastewater should be permitted to be discharged into the Eastern Drainage
Channel nor the waters of inner TKO Bay.
As the identified sensitive receiver of the WSD saltwater pumping
station at Area 86 is not located in the vicinity of the proposed road crossing
(Sheet 2 of 3 Figure 2.3),
no adverse water quality impacts are anticipated at the intake point of the
pumping station during the piling works.
Road D10 Adjacent to TKO
Stage 1 Landfill at Area 77
5.5.12
During the construction of Road D10,
groundwater pumped out during foundation works will not be permitted to be
directly discharged into the nearby Eastern Drainage Channel or coastal
waters. It is recommended that the
potentially contaminated groundwater be held in storage tanks. The quantity of
groundwater requiring disposal is anticipated to be small, with an estimated
total volume of 500 m3.
Options for the treatment and disposal of the groundwater are given in
Sections 5.6.6 – 5.6.9. Available
monitoring data for the TKO Stage 1 landfill has been obtained from EPD and are
presented in Table 5.4 below to give an indication of groundwater quality. B133 is a combined gas and groundwater
monitoring borehole located in the soil margin outside the waste boundary near
the western seawall of the TKO Stage 1 landfill.
Table 5.4 Monitoring
data of groundwater quality at TKO Stage 1 Landfill (for monitoring borehole
B133)
|
Parameter (mg/L)
|
Oct 1999
|
Apr 2000
|
Oct 2000
|
TM Standards*
|
|
pH
|
7.37
|
7.08
|
7.28
|
6-10
|
|
Biological Oxygen Demand
|
38
|
18
|
35
|
1000
|
|
Chemical Oxygen Demand
|
650
|
98
|
850
|
2500
|
|
Iron
|
3.1
|
1.8
|
19
|
25
|
|
Cadmium
|
<0.010
|
<0.010
|
<0.010
|
0.15
|
|
Copper
|
<0.020
|
<0.020
|
0.11
|
4
|
|
Nickel
|
<0.030
|
<0.030
|
0.05
|
3
|
|
Chromium
|
<0.020
|
<0.020
|
0.06
|
2
|
|
Zinc
|
0.04
|
0.02
|
0.36
|
5
|
|
Sulphate
|
83
|
160
|
21
|
1000
|
|
Total Nitrogen
|
210
|
160
|
730
|
200
|
* Table 1 of TM on Effluent Discharge
Standards titled “Standards for effluents discharged into foul sewers leading
into Government sewage treatment plants” and assuming a flow rate of >10 and
£
100 m3/day
Source of
data : EPD’s routine monitoring works at TKO Stage 1 Landfill
5.5.13
The monitoring data in Table 5.4
indicates that the measured Total Nitrogen content exceeded the TM effluent
discharge standard of 200 mg/L in the month of October 2000 and marginal
exceedance was recorded in October 1999.
From Table 5.4, it can be seen that the measured levels of metals,
sulphate, BOD and COD in the groundwater are well below the respective TM
discharge standards. The monitoring
data indicates that pre-treatment of the collected groundwater may be required
to meet the TM on Effluent Discharge Standards prior to the groundwater being
discharged to the TKO PTW. It should be
noted, however, that monitoring borehole B133 is not located within the
proposed alignment for Road D10.
Therefore it is recommended that the site investigation works at the
detailed design stage include groundwater sampling and analysis to determine
the composition of groundwater at the proposed pile locations (refer to Section
5.6.6).
5.5.14
The agreed alignment of Road D10 falls
inside the boundary of the TKO Stage 1 landfill at Area 77, although the
proposed pile locations will not encroach on the waste boundary at Area 77
(refer to Figure 5.4), and hence there will not be any leachate spill outbreaks
during the piling works for Road D10.
However, as a precautionary measure, trial pits will be dug during the
construction of the piers for Road D10 to ensure that there is no waste
material at the piling foundation.
Remedial measures are recommended to be implemented in the event of any
contaminated groundwater outbreaks at the surface during the piling works and
are described in Section 5.6.9.
5.5.15
An issue specific to Road D10 is the
possible obstruction to the leachate seepage flow at the landfill by the
installed pile structures for the elevated road. Road D10 would be on piles and the pile supports would cause a
very localised blockage, at intervals of approximately 40m, and would not form
a barrier to leachate seepage. The
proposed construction method of non-displacement type bored piling would
minimize impacts on the leachate migration profile as open cut is avoided. In order to prevent leachate inflow, the
piling will involve driving temporary steel casing through the soft material
prior to excavation of the material within the casing. This method of piling will minimise to the
greatest degree possible any movement of the soft material surrounding the
piling works. It is therefore
considered that there will not be any significant change to the leachate
migration profile at the landfill. The
monitoring of marine water quality during the construction works is therefore
not considered to be warranted.
Cumulative Impacts
5.5.16
The identified concurrent projects
which involve major construction activities during the construction period of
the work packages of the road project are:
·
Water
supply infrastructure
·
Construction
activities for Area 86
·
Construction
activities in Area 65D and Area 67
·
Minor
road junction improvement works at the junctions of Road D9 / Wan Po Road, Road
D6 / Wan Po Road, and road junctions along D4 and P2
5.5.17
The above projects comprise all
land-based activities. It is
anticipated that with the implementation of mitigation measures to control
construction site runoff and drainage, potential impacts on water quality would
be controlled within the site boundary of each individual project. Therefore there is very limited potential
for cumulative impacts on water quality to result from the land-based
construction activities for these concurrent projects. The implementation programme of Cross Bay
Link, Road D6, seawall construction for Road P2 and possible future development
to the south of Road D1 have yet to be established. However, they are not likely to commence before the completion of
Road D1, D8, D10 and extended road P2.
5.5.18
Marine based construction works will
only be required for the Project for the elevated section of Road D10 (Phase 2)
across the Eastern Drainage Channel. As
discussed in Section 5.5.11, impacts on water quality from the piling works are
predicted to be very localised and short-term, and hence no cumulative water
quality impacts are anticipated to result from other concurrent construction
works in TKO Bay. Potential cumulative
impacts during the operational phase will be addressed at a later stage by the
project proponent, TDD, when other proposed developments in TKO Bay are known
or confirmed.
Operation
Phase
5.5.19
The main identified source of
potential impact on water quality during the operation phase will be runoff
from the road surfaces. The road runoff
may contain minimal amounts of oil, grease and grit that may cause downstream
water quality impacts to the receiving waters of TKO Bay if uncontrolled. A surface water drainage system will be provided
to collect the road runoff. The runoff
would be discharged into the local stormwater drainage system and is considered
unlikely to produce any quantifiable adverse effects on the receiving waters of
TKO Bay.
5.5.20
The road crossing of the Eastern
Drainage Channel is in an oblique direction, therefore reducing the impact on
the tidal flows within the channel. Table 5.5 shows the impact of piers on flow
across the channel near the mouth and middle of the channel (as detailed in
Figure 5.3). This table details the
discharge flow in the channel both in terms of what is flowing in to the
channel and what is flowing out. It
covers three scenarios; both with and without the piers for the road crossing
and a comparison case being the relative change of the with and without
scenarios. This comparison is
determined as a percentage of ‘with’ minus ‘without’ divided by ‘without’. The maximum and mean inflow and outflow and
the cumulative net discharge in and out of the channel, are all below 5%. From this it can be concluded that placing
Road D10 bridge piers in the channel is likely to have an insignificant
reduction in the flushing capacity of the channel. As the impact on the flushing capacity of the channel is minimal
there will not be any change in the ability of the channel to disperse
pollutants and hence there is also a minimal impact on the water quality.
Table 5.5 Discharge across
Control Lines at the Mouth and Middle of the Eastern Drainage Channel
|
Scenarios
|
Location
|
Discharge [m3/s]
|
Cumulative Discharge at the End of Simulation Period
[m3]
|
|
In
|
Out
|
In
|
Out
|
|
Maximum
|
Mean
|
Maximum
|
Mean
|
|
With Piers
|
Dry Season
|
Mouth of Channel
|
22.82
|
3.22
|
38.46
|
3.00
|
6,453,432
|
6,051,256
|
|
Middle of Channel
|
15.20
|
1.91
|
22.33
|
1.66
|
4,139,756
|
3,352,921
|
|
Wet Season
|
Mouth of Channel
|
216.64
|
11.50
|
183.77
|
12.18
|
19,516,742
|
20,566,708
|
|
Middle of Channel
|
116.46
|
6.29
|
97.11
|
6.45
|
10,659,808
|
10,978,394
|
|
Without Piers
|
Dry Season
|
Mouth of Channel
|
22.86
|
3.22
|
38.52
|
3.00
|
6,459,962
|
6,056,445
|
|
Middle of Channel
|
15.20
|
1.91
|
22.38
|
1.66
|
4,144,029
|
3,356,267
|
|
Wet Season
|
Mouth of Channel
|
219.02
|
11.87
|
184.37
|
12.51
|
20,336,078
|
21,289,878
|
|
Middle of Channel
|
115.99
|
6.52
|
99.11
|
6.62
|
11,139,497
|
11,327,220
|
|
Comparison
|
Dry Season
|
Mouth of Channel
|
-0.20%
|
-0.05%
|
-0.14%
|
-0.01%
|
-0.10%
|
-0.09%
|
|
Middle of Channel
|
-0.01%
|
-0.09%
|
-0.21%
|
-0.05%
|
-0.10%
|
-0.10%
|
|
Wet Season
|
Mouth of Channel
|
-1.09%
|
-3.09%
|
-0.33%
|
-2.71%
|
-4.03%
|
-3.40%
|
|
Middle of Channel
|
0.40%
|
-3.41%
|
-2.02%
|
-2.57%
|
-4.31%
|
-3.08%
|
Note: For
the defined location of the mouth and middle of channel, refer to Figure 5.3.
5.6.1
Proposed mitigation measures for
containing and minimizing water quality impacts are summarised below.
construction
phase
Construction run-off and
drainage
5.6.2
The site practices outlined in ProPECC
PN 1/94 “Construction Site Drainage” shall be followed as far as practicable in
order to minimise surface runoff and the chance of erosion, and also to retain
and reduce any suspended solids prior to discharge. These practices include, inter
alia, the following items :
·
Provision
of perimeter channels to intercept storm-runoff from outside the site. These
shall be constructed in advance of site formation works and earthworks.
·
Sand/silt
removal facilities such as sand traps, silt traps and sediment basins shall be
provided to remove sand/silt particles from runoff to meet the requirements of
the Technical Memorandum standard under the Water Pollution Control
Ordinance. These facilities shall be
properly and regularly maintained.
·
Careful
programming of the works to minimise soil excavation works during rainy
seasons.
·
Exposed
soil surface shall be protected by paving as soon as possible to reduce the
potential of soil erosion.
·
Temporary
access roads shall be protected by crushed gravel and exposed slope surfaces
shall be protected when rainstorms are likely.
·
Trench
excavation shall be avoided in the wet season as far as practicable, and if
necessary, these shall be excavated and backfilled in short sections.
·
Open
stockpiles of construction materials on site shall be covered with tarpaulin or
similar fabric during rainstorms.
5.6.3
All stormwater discharge locations
from the works site shall be located at a distance of over 100 m from the
intake point for the TKO Bay WSD saltwater pumping station. Silt screens should be provided around the
intake as a precautionary measure during construction works in the vicinity for
Road D10. The disposal of slurry water
will need to comply with the TM standards under the WPCO and the discharge
point shall be located at a distance of over 100 m from the intake point for
the TKO Bay WSD saltwater pumping station.
During the construction works for the Road D10 crossing over the Eastern
Drainage Channel, no waste nor wastewater should be permitted to be discharged
into the Eastern Drainage Channel nor the waters of inner TKO Bay.
General Construction
Activities
5.6.4
Debris and rubbish generated on-site
shall be collected, handled and disposed of properly to avoid entering the
nearby stormwater drains and open drainage channels. All fuel tanks and storage areas shall be provided with locks and
be sited on sealed areas, within bunds of a capacity equal to 110% of the
storage capacity of the largest tank. Open storm water drains and culverts near
the works areas shall be covered to block the entrance of large debris and
refuse.
Sewage Effluent
5.6.5
Temporary sanitary facilities, such as
portable chemical toilets, shall be employed on-site. A licensed contractor would be responsible for appropriate
disposal and maintenance of these facilities.
Remedial Measures for Road D10 Construction Adjacent to TKO Stage 1
Landfill at Area 77
5.6.6
Groundwater pumped out during
foundation works for the construction of Road D10 shall not be permitted to be
directly discharged into the Eastern Drainage Channel or coastal waters of TKO
Bay. The groundwater may potentially be
contaminated by leachate seepage from the TKO Stage 1 landfill and shall
therefore be held in storage tanks at the works site. As EPD have advised that the leachate pre-treatment works at TKO
Stage II/III landfill site does not have any spare capacity to treat
contaminated groundwater from other sources, an identified option for the
treatment and disposal of the collected groundwater is tankering of the
groundwater to the TKO Preliminary Treatment Works (PTW) at Area 85. DSD have indicated that they have no
objection to this proposed option of disposal of the collected groundwater to
the TKOPTW provided that samples of groundwater are analyzed and the results
submitted to DSD for approval and that the TKOPTW shall not be overloaded. The site investigation works at the detailed
design stage of the Project will therefore include groundwater sampling and
analysis to determine the composition of groundwater. The monitoring results of groundwater quality would be submitted
to DSD for their approval. It is noted
that the effluent discharged from the TKOPTW is required to comply with the
discharge licence issued by EPD. A more
accurate estimate of the volume of groundwater would also be made at the
detailed design stage based on the site investigation data.
5.6.7
A leachate monitoring and contingency
plan should be developed and agreed with EPD before commencement of the
construction phase. This plan should
cover construction works at the Eastern Drainage Channel and the fringes of the
TKO Stage 1 landfill. A copy of the
plan should be submitted to EPD together with the results of the detailed site
investigation in respect of groundwater quality and leachate seepage potential
at work sites. A tentative framework of
the leachate monitoring and contingency plan is given below:
§
Procedures
for collection and storage of groundwater pumped out during foundation works;
§
Monitoring
of groundwater quality;
§
Review
of monitoring data;
§
Proposal
of suitable treatment and disposal method(s) of contaminated groundwater
(should the groundwater quality be found to exceed the TM standard for
discharge into foul sewers);
§
Site
inspections of works areas;
§
Recommended
remedial measures in event of groundwater spillage (see below).
5.6.8
During the foundation works for the
construction of Road D10, the pumped out groundwater would be stored
temporarily on site as the volume is anticipated to be small. Samples of groundwater would be taken from
the storage tanks for laboratory measurement to determine the quality of the
groundwater before tankering to the TKO PTW for treatment and disposal. The monitoring data would then be reviewed
to determine if pre-treatment of the collected groundwater would be required to
meet the TM on Effluent Discharge Standards prior to the groundwater being
discharged to the TKO PTW. It is
proposed that a packaged treatment unit be installed at the site should
pre-treatment be required. All the
required on-site pre-treatment of the collected groundwater should be
implemented to the satisfaction of the Director of Environmental Protection
prior to its discharge to the TKO PTW.
It is recommended that daily inspections of the works areas be
undertaken during the piling works to identify any contaminated groundwater
seepage or ponding at the surface.
5.6.9
Remedial measures shall also be
implemented in the event of any contaminated groundwater spillage at the
surface during the piling works, and WSD should be informed as soon as possible
whenever there is an outbreak of leachate spill (i.e. contaminated
groundwater). Spilled groundwater
ponded on the surface shall be collected and held in the storage tanks at the
site. The collected groundwater shall
be disposed as described above. If the spill is small, it may be possible to
cover the spillage using soil or sand as an adsorbent, and this waste material
would then need to be properly disposed of to landfill.
5.6.10
Careful control shall be exercised
during the construction phase to protect the existing facilities at the
landfill from any damages resulting from construction activities, such as
passive vent trench and monitoring wells DG5 and DG6 (Figure 7.4).
Operation
Phase
5.6.11
A surface water drainage system will
be provided to collect road runoff. The
following measures are recommended to minimize impacts on the receiving waters:
·
The
road drainage shall be directed through oil and grit interceptors to remove
oil, grease and silt before entering the public storm water drainage system.
·
Grit
and oil interceptors shall be regularly cleaned and maintained in good working
condition.
5.7.1
With the full implementation of the
recommended mitigation measures for the construction and operation phases of
the proposed road developments, no unacceptable residual impacts on marine
water quality are anticipated.
5.8.1
The potential has been identified for
impacts on marine water quality to result from the construction works for the
proposed road developments. These
potential sources of impact comprise construction runoff and drainage; debris,
refuse and liquid spillages from general construction activities; and sewage
effluents from the construction workforce.
Minimisation of water quality
deterioration can be achieved through implementing adequate mitigation measures
such as control measures on the runoff and drainage from the site to minimise construction run-off. Proper site management and good housekeeping
practices will also be required to ensure that construction wastes will not
enter the nearby open drainage channels.
Sewage effluent arising from the construction workforce would also
require appropriate treatment through provision of portable toilets. As such, with the implementation of these recommended mitigation measures, the
construction works are not anticipated to result in any unacceptable impacts on
marine water quality.
5.8.2
During the construction of Road D10
adjacent to the TKO Stage 1 landfill at Area 77, groundwater pumped out during
foundation works will not be permitted to be directly discharged into the
nearby drainage channel or coastal waters.
It is proposed that the groundwater be stored temporarily on site as the
volume is anticipated to be small. The
discharge of potentially contaminated groundwater and construction effluent
shall comply with the TM under WPCO. An
identified option for the treatment and disposal of the collected groundwater
is tankering of the groundwater to the TKO PTW at Area 85. A leachate monitoring and contingency plan
will be developed during the detailed design stage so as to avoid or minimize
potential adverse impacts on water quality due to groundwater contaminated by
leachate seepage.
5.8.3
The pile supports for the elevated
section of Road D10 would cause a very localised blockage, at intervals of
approximately 40 m, and would not form a barrier to leachate seepage. It is
therefore considered that there would not be any significant change to the
leachate migration profile at the landfill from the pile supports for Road
D10. The monitoring of marine water
quality during the construction works is therefore not considered to be
warranted.
5.8.4
The only source of potential impact on
water quality during the operation phase will be runoff from the road
surfaces. A surface water drainage
system will be provided to collect the road runoff.
5.8.5
Assessments have shown that the placement
of Road D10 bridge piers in the Eastern Drainage channel is likely to have
insignificant reduction in flushing capacity of the channel.