This chapter presents the assessment of
potential water quality impacts, which may arise during the construction and
operation of the SCL - Tai Wai to Hung Hom Section.
Construction runoff, sediment dredging, sewage from site workforce, drainage
diversion are potential water pollution sources during the construction phase.
Operational water quality impact includes track run-off and tunnel seepage.
Mitigation measures have been proposed to
alleviate the potential water quality impact. Adverse residual impacts during
the construction and operational phases are not anticipated.
10.2
Legislation, Standards and Guidelines
The
relevant legislation and associated guidance applicable to the present study
for the assessment of water quality impacts include:
·
Water Pollution Control Ordinance (WPCO)
CAP 358, Water Quality Objectives (WQOs) for the
·
Technical Memorandum for Effluents Discharged
into Drainage and Sewerage Systems Inland and Coastal Waters (TM-Water),
Effluents discharge limits for the Tolo Harbour Water
Control Zone and Victoria Harbour Water Control Zone;
·
Environmental Impact Assessment Ordinance
(EIAO) (Cap. 499), Technical Memorandum on Environmental Impact Assessment
Process (TM-EIAO);
·
ProPECC
PN 5/93 “Drainage Plan subject to Comment by the Environmental Protection
Department”;
·
ProPECC PN 1/94 “Construction Site
Drainage”;
·
Guidance Notes of Contaminated Land
Assessment and Remediation;
·
“Recommended Pollution Control Clauses for
Construction Contracts” issued by EPD;
·
“Water Quality Standards for Seawater
Flushing Points” issued by the Water Supply Department; and
·
Recommended Sedimentation Rate for Coral
Sites, Pastorok & Bilyard
(1985) [10-2]
10.3.1 Marine
Water Quality Monitoring Stations Near to Project Site
The
representative marine water quality monitoring stations in the vicinity of the
project site are TM2 and TM4 at
Table 10.1a: Marine Water
Quality of
|
Parameter |
WQO |
Monitoring Station |
|
|
|
|||
|
TM2 |
TM4 |
||
|
Temperature
(°C) |
Change
due to waste discharge not to exceed 1°C. The
rate of temperature change shall not exceed 0.5°C per hour at any location,
unless due to natural phenomena. |
24.5 (16.6 –
30.7) |
24.2 (16.2
– 30.5) |
|
Salinity
(ppt) |
Change
due to waste discharge not to be greater than ±3 ppt |
30.9 (28.3
– 32.6) |
31.4 (29.0 – 32.6) |
|
Dissolved Oxygen (mg/L) |
Surface to 2m above bottom: Not less than 4 mg/L |
6.1 (4.6 – 9.0) |
6.2 (4.4 – 8.0) |
|
Dissolved
Oxygen, Bottom (mg/L) |
Bottom Not less than 2 mg/L |
6.0 (3.6 –
8.9) |
5.5 (2.2 –
8.4) |
|
SS (mg/L) |
N/A |
2.0 (1.4 – 3.5) |
1.9 (0.5 – 4.7) |
|
BOD5 (mg/L) |
N/A |
1.2 (0.5 – 1.9) |
1.1 (0.5 – 1.8) |
|
NH3-N
(mg/L) |
N/A |
0.047 (0.015
– 0.11) |
0.037 (0.006
– 0.073) |
|
Unionised
Ammonia (mg/L) |
N/A |
0.003 (<0.001-0.012) |
0.002 (<0.001
– 0.007) |
|
TIN (mg/L) |
N/A |
0.07 (0.02 – 0.15) |
0.06 (0.01
– 0.15) |
|
ChlorophyII-a
(µg/L) |
Not to
exceed 20 µg/L calculated as running arithmetic mean of 5 daily measurements
for any location and depth |
5.6 (1.6 –
9.6) |
5.1 (1.5 –
14.2) |
|
E. coli (cfu 100mL) |
Annual
geometric mean not to exceed 610 cfu/100 ml for secondary
contact recreation subzones and fish culture subzones |
7 (1 – 200) |
4 (1 – 18) |
Notes:
[1] Data
presented are depth averaged, except as specified.
[2] Data
presented are annual arithmetic mean except for E. coli, which are geometric
mean values
[3] Data enclosed
in brackets indicate the ranges
[4] Bolded cells
indicate non-compliance with the WQOs for a parameter
Table 10.1b: Marine Water Quality of
|
Parameter |
WQO |
Monitoring Station |
|||
|
|
|||||
|
VM2 |
VM4 |
VM5 |
VM6 |
||
|
Temperature
(°C) |
Change
due to waste discharge not to exceed 2°C |
23.8 (18.7
– 28.5) |
23.8 (18.6
– 28.6) |
24 (18.7
– 28.6) |
24 (18.7
– 28.6) |
|
Salinity
(ppt) |
Change
due to waste discharge not exceed 10% of natural ambient level |
31.7 (22.5
– 33.5) |
31.8 (24.9 – 33.6) |
31.2 (21.4
– 33.4) |
31.4 (23.6 – 33.3) |
|
Dissolved Oxygen (mg/L) |
Depth
average: ³ 4 mg/L for 90% of samples |
5.6 (4.1 – 7.0) |
5.3 (4.1 – 6.7) |
5.2 (4.5 – 6.8) |
5.1 (4.5 – 6.3) |
|
Dissolved
Oxygen, Bottom (mg/L) |
Bottom: ³ 2 mg/L for 90% of samples |
5.5 (4.2 –
7.0) |
5.1 (2.6 –
6.8) |
5.2 (4.4 –
6.8) |
5.0 (3.4 –
6.6) |
|
SS (mg/L) |
Waste
discharge not to raise the natural ambient level by 30% nor cause the
accumulation of suspended solids which may adversely affect aquatic
communities |
5.2 (2.7 – 8.3) |
5.8 (3.5 – 7.5) |
5.7 (3.3 – 9.1) |
6 (3.2 – 10.7) |
|
BOD5 (mg/L) |
N/A |
0.7 (<0.1 – 1.2) |
0.7 (0.2 – 1.2) |
0.8 (0.3 – 1.5) |
0.8 (0.1 – 1.7) |
|
NH3-N
(mg/L) |
N/A |
0.08 (0.041
– 0.200) |
0.1 (0.049
– 0.203) |
0.12 (0.062
– 0.203) |
0.14 (0.069
– 0.227) |
|
Unionised
Ammonia (mg/L) |
Annual
mean not to exceed 0.021 mg/L |
0.003 (0.002
– 0.006) |
0.004 (0.001
– 0.007) |
0.005 (0.002
– 0.011) |
0.005 (0.001
– 0.009) |
|
TIN (mg/L) |
Annual
mean depth-averaged TIN not to exceed 0.4 mg/L |
0.21 (0.07
– 0.60) |
0.24 (0.08
– 0.57) |
0.29 (0.12
– 0.63) |
0.32 (0.15
– 0.6) |
|
ChlorophyII-a
(µg/L) |
N/A |
3.1 (0.7 –
9.1) |
3.3 (0.7 –
8.3) |
3.9 (0.7 –
10.1) |
3.7 (0.8 –
11.4) |
|
E. coli (cfu 100mL) |
N/A |
710 (100 –9400) |
2000 (510 – 8700) |
3900 (160 – 19000) |
2500 (200 – 11000) |
Notes:
[1] Data
presented are depth averaged, except as specified.
[2] Data
presented are annual arithmetic mean except for E. coli, which are
geometric mean values
[3] Data enclosed
in brackets indicate the ranges
[4] Bolded cells
indicate non-compliance with the WQOs for a parameter
Table 10.1c: Marine Water Quality of Eastern Buffer in Year 2009
|
Parameter |
WQO |
Monitoring Station |
|
Eastern Buffer |
||
|
EM2 |
||
|
Temperature
(°C) |
Change
due to waste discharge not to exceed 2°C |
23.4 (17.5
– 28.5) |
|
Salinity
(ppt) |
Change
due to waste discharge not exceed 10% of natural ambient level |
32.2 (25.7 – 33.9) |
|
Dissolved Oxygen (mg/L) |
Depth
average: ³ 4 mg/L for 90% of samples |
5.8 (4.5 –
7.3) |
|
Dissolved
Oxygen, Bottom (mg/L) |
Bottom: ³ 2 mg/L for 90% of samples |
5.4 (3.1 –
7.2) |
|
SS
(mg/L) |
Waste
discharge not to raise the natural ambient level by 30% nor cause the
accumulation of suspended solids which may adversely affect aquatic
communities |
4.0 (2.8 – 6.6) |
|
BOD5
(mg/L) |
N/A |
0.6 (<0.1
– 1.6) |
|
NH3-N
(mg/L) |
N/A |
0.029 (0.008
– 0.055) |
|
Unionised
Ammonia (mg/L) |
Annual
mean not to exceed 0.021 mg/L |
0.001 (<0.001
– 0.003) |
|
TIN (mg/L) |
Annual
mean depth-averaged TIN not to exceed 0.4 mg/L |
0.1 (0.02
– 0.34) |
|
ChlorophyII-a
(µg/L) |
N/A |
3.4 (0.6 –
10.7) |
|
E. coli (cfu 100mL) |
Annual
geometric mean not to exceed 610 cfu/100 ml for
fish culture subzones |
19 (3 – 240) |
Notes:
[1] Data
presented are depth averaged, except as specified.
[2] Data
presented are annual arithmetic mean except for E. coli, which are
geometric mean values
[3] Data
enclosed in brackets indicate the ranges
[4] Bolded
cells indicate non-compliance with the WQOs for a parameter
According
to Marine Water Quality Report 2009 which is the best available information,
compliance of water quality at
10.3.2 River
Water Quality Monitoring Stations Near to Project Site
The
representative river water quality monitoring stations in the vicinity of the
project site are TR19, TR19C, TR20B, TR23A and TR23L at
Table 10.2: River Water
Quality of
|
Parameter |
WQO |
Monitoring Station |
||||
|
Tai Wai Nullah |
Tin Sam Nullah |
Siu Lek Yuen Nullah |
||||
|
TR19C |
TR19 |
TR20B |
TR23L |
TR23A |
||
|
Dissolved Oxygen (mg/L) |
³ 4 |
9.0 (7.9-9.9) |
9.4 (8.1 – 12.6) |
8.2 (7.4 – 9.8) |
8.6 (7.5 – 11.5) |
6.1 (3.1 – 8.9) |
|
pH |
6.5-8.5 |
7.8 (7.3 –
8.3) |
7.6 (7.3 – 8.8) |
8 (7.8 – 8.2) |
8.8 (8.1 – 9.3) |
7.8 (7.4 – 8.0) |
|
SS (mg/L) |
< 20 |
5 (3 – 37) |
5 (4 – 55) |
2 (<1 – 8) |
4 (1 – 100) |
4 (2 – 19) |
|
BOD5 (mg/L) |
< 3 (Siu Lek Yuen) < 5 (Others) |
1 (<1 – 10) |
2 (<1 – 15) |
<1 (<1 – <1) |
1 (<1 – 3) |
2 (<1 – 4) |
|
COD (mg/L) |
< 15 (Siu Lek Yuen) < 30 (Others) |
6 (3 – 12) |
7 (2 – 11) |
2 (<2 – 4) |
3 (2 – 7) |
7 (3 – 9) |
|
E. coli (cfu 100m/L) |
£ 1000 |
6300 (1500 – 28000) |
7300 (1200 – 91000) |
<1 (<1 – 1) |
1100 (150 – 8000) |
2600 (400 – 42000) |
|
NH4-N
(mg/L) |
£ 0.5 |
0.09 (0.02 – 0.24) |
0.05 (0.02 – 0.33) |
0.02 (<0.01 – 0.12) |
0.01 (<0.01 – 0.08) |
0.32 (0.11 – 0.74) |
Notes:
[1] Data
presented are annual median except for E. coli, which are annual
geometric mean values.
[2] Data
enclosed in brackets indicate the ranges.
Table 10.3: River Water
Quality of Kai Tak Nullah in Year 2009
|
Parameter |
WQO |
Kai Tak Nullah |
|||||
|
Monitoring Station |
|||||||
|
KN1 |
KN2 |
KN3 |
KN4 |
KN5 |
KN7 |
||
|
Dissolved
Oxygen (mg/L) |
³ 4 mg/L |
6.6 (5.1 – 7.5) |
7.0 (6.3 – 7.7) |
7.2 (7.1 – 8) |
7.9 (6.8 – 8.5) |
7.9 (7.1 – 8.7) |
7.4 (7.0 – 8.4) |
|
pH |
Not to exceed the range of 6.0-9.0 units |
7.1 (6.9 –
7.6) |
7.3 (7.0 –
7.6) |
7.3 (7.1 –
7.7) |
7.3 (7.0 –
7.6) |
7.3 (6.9 – 7.5) |
7.2 (6.9 – 7.4) |
|
SS
(mg/L) |
Annual median not to exceed 25 mg/L |
4 (3 – 32) |
8 (3 – 24) |
6 (4 – 19) |
11 (3 – 38) |
5 (3 – 12) |
5 (2 – 11) |
|
BOD5
(mg/L) |
≤ 5 mg/L |
4 (<1 – 6) |
3 (2 – 6) |
4 (2 – 8) |
6 (2 – 31) |
3 (1 – 8) |
3 (1 – 10) |
|
COD (mg/L) |
≤ 30 mg/L |
26 (18 – 40) |
28 (23 – 34) |
29 (23 – 39) |
32 (19 – 50) |
27 (19 – 34) |
31 (19 – 33) |
|
E. coli (cfu /
100m/L) |
≤ 1000
cfu/100mL, geometric mean of the most recent
5 consecutive samples taken at intervals of between 7 and 21 days |
85000 (8000 – 880000) |
35000 (6000 – 120000) |
56000 (11000 – 240000) |
87000 (7800 – 1300000) |
21000 (8500 – 52000) |
23000 (8000 – 40000) |
|
NH4-N
(mg/L) |
N/A |
0.74 (0.33 – 2.80) |
0.49 (0.1 – 1.6) |
0.48 (0.13 – 1.5) |
0.57 (0.09 – 2.20) |
0.27 (0.08 – 1.70) |
0.26 (0.08 – 1.5) |
Notes:
[1] Data presented are annual median except for E. coli,
which are annual geometric mean values.
[2] Data enclosed in brackets indicate the ranges
Kai Tak Nullah’s catchment includes some
of the most densely populated areas of
According to River Water Quality 2009,
10.4.1 SCL
– Tai Wai to Hung Hom Section
There
are only a few water receiving bodies in the vicinity of the project site, and
they are mainly located in Tai Wai and former
Shing Mun River Channel, which is located at a distance of about
400m away from the Tai Wai Depot, is one of the river monitoring stations for
the
Although
the nullah next to Shatin Water
Treatment Works is not connected to any major river, it is regarded as a WSR
for the construction of SCL (TAW-HUH), as it is just about 50m away. The
natural stream at Tei Lung Hau is also as part of a
WSR.
The
upper end of Kai Tak nullah at former
There
is no marine biological sensitive receiver such as fish culture zone, shellfish
culture zone, marine park/reserve and commercial fishing ground in the vicinity
of SCL (TAW-HUH).
There
are a number of sensitive receivers in the vicinity of the off-site works areas
and they are discussed below:
·
Barging points including Kai Tak Runway and Freight Pier at Hung Hom (shared with Kwun Tong Line Extension).The proposed barging works at Kai Tak Runway are located within the
Victoria Harbour Control Zone, with To Kwa Wan water and sediment monitoring stations (VT11 and
VS20) nearby.
·
The TKO Area 137 works area is located in vicinity of Tung Lung Chau Fish Cultural Zone.
Dredging works will only be carried out at Kai Tak Runway Barging Facility. No dredging work will be conducted at Freight Pier Barging Facility at Hung Hom (see Figure 1.2). Hence water quality assessment will only cover the dredging work at Kai Tak (see Section 10.5.1.7). The representative Water Sensitive Receivers (WSRs) in the vicinity of the project site are summarized in Table 10.4 and shown in Figure 10.3.
Table 10.4: Water sensitive receivers
|
WSR No. |
WSRs Description |
Works Area |
|
WSR 1 |
Shing Mun Main Channel, |
SCL - Hin Keng to Tai Wai |
|
WSR 2 |
Freshwater Stream at Tei Lung
Hau |
SCL - Hin Keng to Tai Wai |
|
WSR 3 |
Nullah next to Shatin Water Treatment Works |
SCL - Hin Keng to Tai Wai |
|
WSR 4 |
Kai Tak Nullah |
SCL - Tai Wai to Hung Hom |
|
WSR 5 |
Victoria Harbour WCZ |
Dredging at Kai Tak Runway Barging Facility |
|
WSR 6 |
Eastern Buffer WCZ, |
TKO Area 137 |
|
WSR 7 |
Tung Lung Chau Fish Cultural Zone |
TKO Area 137 |
|
WSR 8 |
Seawater intakes for flushing at Tai Wan |
Dredging at Kai Tak Runway Barging Facility |
|
WSR 9 |
Seawater intakes for flushing at North Point |
Dredging at Kai Tak Runway Barging Facility |
|
WSR 10 |
Seawater
intakes for flushing at |
Dredging at Kai Tak Runway Barging Facility |
|
WSR 11 |
Cooling water intakes for Dairy Farm Ice Plant |
Dredging at Kai Tak Runway Barging Facility |
|
WSR 12 |
Coral sites at Chiu Keng Wan |
TKO Area 137 |
|
WSR 13 |
To Kwa Wan Typhoon Shelter |
Dredging at Kai Tak Runway Barging Facility |
Although
there are hard corals Oulastrea crispata
located at Kai Tak Seawalls near the dredging location at Kai Tak Runway
Barging Facility, these corals are isolated and only common species. According
to the approved EIA for the Cruise Terminal, these corals are not considered as
WSRs.
According
to WSD’s information, there are water gathering grounds located at
The
site will be maintained by good site practices and there will be no direct
discharge of wastewater into
10.5.1 Pollution
Sources from Construction Activities
Potential
water pollution sources during construction phase will include sources mainly
from land-based activities as follows:
·
Construction runoff;
·
Runoff from tunnelling activities and
underground works;
·
Sewage effluent due to workforce on site;
·
Drainage diversion;
·
Groundwater seepage;
·
Discharge of groundwater pumped out from
potential contaminated area;
·
Dredging works for Kai Tak Runway Barging
Facility (N.B. Dredging is not required for other barging facility); and
·
Accidental Spillage.
10.5.1.1
Construction Runoff
Construction
site runoff comprises:
·
Runoff and erosion from site surfaces,
drainage channels, earth working areas and stockpiles;
·
Runoff from the proposed barging facility;
·
Wash water from dust suppression sprays
and wheel washing facilities; and
·
Fuel, oil, solvents and lubricants from
maintenance of construction machinery and equipment.
Construction
runoff may cause physical, biological and chemical effects. The physical
effects include potential blockage of drainage channels and increase of Suspended
Solid (SS) levels in THCWCZ and VHWCZ.
Local
flooding may also occur in heavy rainfall situations. The chemical and
biological effects caused by the construction runoff are highly dependent upon
its chemical and nutrient content.
Runoff
containing significant amounts of concrete and cement-derived material may
cause primary chemical effects such as increasing turbidity and discoloration,
elevation in pH, and accretion of solids. A number of secondary effects may
also result in toxic effects to water biota due to elevated pH values, and
reduced decay rates of faecal micro-organisms and photosynthetic rate due to
the decreased light penetration.
10.5.1.2
Tunnelling and Underground Works
During
tunnelling work, rainfall, surface runoff and groundwater seepage pumped out
from the tunnel would have high SS content. The situation would be worse during
wet seasons.
Surface
runoff may also be contaminated by bentonite and
grouting chemicals that would be required for the construction of bored tunnels
(for tunnel boring and ground treatment) and diaphragm walls for
cut-&-cover tunnel sections. In addition, wastewater from tunnelling works
will also contain a high concentration of SS.
10.5.1.3
Sewage Effluent
Sewage
effluents will arise from the amenity facilities used by the construction
workforce and site office’s sanitary facilities. The characteristics of sewage
would include high levels of BOD5, Ammonia and E. coli counts.
Overnight sewage
from chemical toilets will also be generated. The sludge needs to be properly managed
to minimize odour and potential health risks to the workforce by attracting
pests and other disease vectors.
The number of
construction workers to be employed on site is not available at this stage, but
is anticipated to be over 1,000 staff in the peak period. As the workers will be scattered within
the construction site, the most effective solution will be to provide adequate
number of portable toilets within the site to ensure that sewage from site
staff is properly collected. Depending on site conditions, land availability
and site activities, the locations and number of portable toilets will be
determined in the Environmental Management Plan (EMP) to be submitted by the
Contractor. No adverse waste impact
is envisaged provided that maintenance by licensed contractors is conducted
regularly.
10.5.1.4
Drainage Diversion
Drainage
infrastructure is available along the proposed alignment from Tai Wai to Hung Hom. A separate Drainage Impact Assessment will be prepared
and submitted by the Project Proponent.
The assessment will identify the diversion or upgrading of the existing
drainage infrastructure. The potential water quality impact associated with the
drainage diversion or upgrading will be from the run-off and erosion from site
surfaces and earth working areas. Small amount of wastewater may be released
during the disconnection of various drainage systems.
10.5.1.5
Groundwater Seepage
The
stations and some of the tunnel sections will be constructed by cut-&-cover
and method. Construction methodology using diaphragm wall techniques can
minimise the intrusion of groundwater during excavation. It involves excavation
of a narrow trench that is kept full of slurry, which exerts hydraulic pressure
against the trench walls and acts as a shoring to prevent collapse. Slurry trench excavations can be
performed in all types of soil, even below the ground water table.
The
construction usually begins with the excavation of discontinuous primary panels
of typically up to 6m long and down to the rockhead. In order to provide an effective cut-off
to ground water flow, the walls will need to be toe grouted. Once the excavation of a panel is
completed, a steel reinforcement cage will be placed in the centre of the
panel. Concrete is then poured in
one continuous operation. Once the
primary panels are set, secondary panels will be constructed between the
primary panels and the process then repeats to create a continuous wall. It should be noted that this slurry
trench method will reduce the gap between the panels to the practicable minimum.
After this, soil excavation will be commenced. The intrusion of groundwater through
D-wall panels during soil excavation is therefore considered insignificant.
For
those sections that may require bored tunnelling and / or drill-&-blast,
some ground treatment (e.g. grouting) will be carried out prior to bored tunnelling. The intrusion of groundwater during bored
tunnelling would therefore be insignificant.
10.5.1.6
Groundwater from Contaminated Area
Site
investigation (SI) works were commenced in February 2009 and carried out by the
GI Contractor. The SI program
includes 5 trial trenches and
24 drillholes at 10
identified potentially contaminated sites (i.e. refer to Section 12 for details). According
to Section 12 for Land Contamination
Impact Assessment, groundwater was encountered at 22 sampling locations and
none of the groundwater samples exceed the RBRGs levels for industrial
purpose. However, as soil
contamination issues were identified at Site L4 -
former
10.5.1.7
Dredging Works
No
dredging work will be conducted at Freight Pier Barging Facility at Hung Hom.
For
Kai Tak Runway Barging Facility, sediment removal is required in order to
maintain the seabed level and provide adequate draft for barging. The location
plan of Kai Tak Runway Barging Facility and the indicative dredging location is
shown in Figure 10.4.
Since
the total dredging volume will be only 56,000m3, which is a
relatively small amount compared to other dredging works (such as those in
Kowloon Bay, refer to Table
10.8), change of hydrodynamic regime due to sediment removal is
not anticipated. Furthermore, the
removed sediment is not located in major waterways and is likely to be
re-formed by natural sedimentation after the barging operation.
The
closest WSR to the dredging area at Kai Tak is the To Kwa
Wan Typhoon Shelter and WSD flushing water intake at Tai Wan, which is
about 280m and 1.2 km away. There are also seawater intakes at North Point,
Quarry Bay and Yau Tong, however, these WSRs are considered far away (> 2
km) from the dredging area and the expected water quality impact would be
negligible.
The
key water quality concerns during the dredging works include the following:
·
Dredging works will disturb the marine
bottom sediment, causing an increase in SS levels in the water column forming
sediment plumes. The increase in SS levels may lead to reduction of DO levels
and increase of nutrient levels.
·
On the other hand, organic and inorganic
contamination such as heavy metals, etc., bounded in the sediment, would be
released into the water columns via suspension and dredging works. During
dredging and transport of dredged materials, inappropriate handling and
overflow from barges would also lead to leakage and spillage of sediment.
The quantities of
fine sediment lost to suspension during dredging will primarily depend on
dredging rate and methods. Impact
from suspended solids may be caused by sediment plumes being transported to
sensitive areas.
Sediment Loss Rates
The dredging works will be conducted at a production rate of 500 m3/day and by using closed grab dredgers for 12 hours per day. Thus, the hourly production rate would be about 41.7 m3/hour.
According to the Contaminated Spoil Management Study, the sediment loss rate for closed grab dredgers were about 11 to 20 kg/m3 of dredged materials. As a conservative approach, the upper range of 20 kg/m3 was adopted in this study. Therefore, the total sediment loss rate would then be 0.23 kg/s.
Assessment Criteria
The allowable elevation in SS concentration as defined by the WQO corresponds to the 30% tolerance level. The calculated maximum SS concentrations from the dredging have been compared with the 30% tolerance values to determine the acceptability of the impacts. The relevant EPD data and allowable elevations in suspended sediment concentration are 2.5 mg/l and 2.3 mg/l for dry and wet seasons respectively and summarised in Table 10.6. Apart from the WQO, WSD also specifies the SS criteria for the flushing water intakes and the criterion value is 10 mg/l.
Table 10.6: Ambient and Tolerance Values for
Suspended Solids Concentrations in the Vicinity of Sensitive Receivers
|
Sensitive
Receiver (Relevant EPD Monitoring Station) |
Dry Season |
Wet Season |
All Season |
||
|
90th
Percentile |
30 %
Tolerance |
90th
Percentile |
30 %
Tolerance |
Absolute
Value |
|
|
WSD flushing
water intakes at Tai Wan (VM2 and VM4, 2009 data), about 1.2 km away from the
dredging area |
8.3 mg/l |
2.5 mg/l |
7.5 mg/l |
2.3 mg/l |
10 mg/l |
|
To Kwa Wan Typhoon Shelter (VM2 and VM4, 2009 data), about
280 m away from the dredging area |
8.3 mg/l |
2.5 mg/l |
7.5 mg/l |
2.3 mg/l |
- |
Impact Assessment
A two dimensional
steady-state Gaussian Dispersion Model[10-3][10-4][10-5][10-8]
has been applied to the near field water quality impact prediction. The
application of this near field model was also adopted in the Approved EIA
report of Relocation Yiu Lian Floating Dock No. 3[10-5]
and Peng Chau Helipad[10-7]. The line source
solution of advection and diffusion equation is as follows:-
C(x,y) = M/[uH(4πDx/u)0.5] exp(-y2u/4Dx)
where, C = SS
concentration (kg/m3)
x
= distance in the direct flow direction (m)
y
= distance in the lateral flow direction (m)
M
= sediment loss rate (= 0.23 kg/s)
H
= water depth (= 7m)
u
= flow velocity (= 0.24 to 0.28m/s, take 0.24 m/s in this study)[10-10])
D
= dispersion coefficient (= 1m2/s[10-5][10-7][10-9])
The water depth is
about 7m to 11m in the area. In a conservative design, 7m water depth is
assumed.
The details of the
water quality modeling results for proposed dredging works are provided in Appendix 10.1 and
the calculation
results of elevated suspended sediment concentrations are given in Table
10.7a.
Table 10.7a: Calculated
Suspended Sediment Concentrations Elevation for Dredging of Sediment
(Unmitigated)
|
Centreline Distance from Source (m), |
Elevated Suspended Sediment Concentration (mg/l) |
|
100 |
1.9 |
|
200 |
1.3 |
|
300 |
1.1 |
|
400 |
1.0 |
|
500 |
0.9 |
|
600 |
0.8 |
|
700 |
0.7 |
|
800 |
0.7 |
|
900 |
0.6 |
|
1000 |
0.6 |
|
280 (at WSR 13 - To Kwa Wan Typhoon Shelter) |
1.1 |
|
1200 (at WSR 8 - Tai Wan |
0.5 |
Note
[1] WSR13
& WSR8 are closest to the proposed dredging works.
Other WSR listed in Table 10.4 are considered not affected by dredging
activities.
The modelling results in the
above table show that without any mitigation measures at more than 200m from
the dredging operation, the suspended sediment concentrations would be below
1.3 mg/l. Since the nearest sensitive receiver is about 280m away from the
dredging location, it is concluded that sediment plumes generated from the
dredging works are expected to be localized and acceptable.
Mitigation Measures
To minimise the potential impact
due to SS, deployment of silt curtains around the closed grab dredgers is
recommended for the dredging works to minimize any significant cumulative water
quality impact with other possible concurrent marine works in the
The implementation of silt curtain around the closed grab dredgers
will reduce the dispersion of SS by 75% and the sediment loss rate within the
dredging area would be about 0.06 kg/s after deploying the silt curtain around
the works area. Thus, the predicted SS elevation at To Kwa
Wan Typhoon Shelter and Tai Wan Flushing Water Intakes would be 0.3 mg/l and
0.1 mg/l respectively, which is negligible to the background variations and
well within the criteria as stipulated in Table
10.6. The calculation results of elevated suspended sediment concentrations are
given in Table
10.7b. Details of mitigation measures during dredging operations
are presented in Section 10.7.1.5.
Table 10.7b:
Calculated Suspended Sediment Concentrations Elevation for Dredging of Sediment
(Mitigated)
|
Centreline Distance from Source (m), |
Elevated Suspended Sediment Concentration (mg/l) |
|
100 |
0.5 |
|
200 |
0.3 |
|
300 |
0.3 |
|
400 |
0.2 |
|
500 |
0.2 |
|
600 |
0.2 |
|
700 |
0.2 |
|
800 |
0.2 |
|
900 |
0.2 |
|
1000 |
0.2 |
|
280 (at WSR 13 - To Kwa Wan Typhoon Shelter) |
0.3 |
|
1200 (at WSR 8 - Tai Wan |
0.1 |
Note
[1] WSR13
& WSR8 are closest to the proposed dredging works.
Other WSR listed in Table 10.4 are considered not affected by dredging
activities.
Cumulative Impacts
for Concurrent Dredging
The dredging activities at Kai Tak Runway Barging Facility will be carried out in Jul 2012 and the duration will be six months. The possible concurrent dredging projects are listed below:-
·
SCL – Hung Hom
to Admiralty Section (SCL(HUH-ADM))
·
Submarine Gas Pipelines (SGP)
·
Cruise Terminal (CT)
·
Central
There are also marine works for Wan Chai Development II, Central-Wan Chai Bypass and Trunk Road T2. However, these projects are considered far away from the SCL (TAW-HUH) (more than 2 km away) and cumulative impact is not anticipated.
Dredging activities will be carried out at SCL(HUH-ADM), which would be commenced in 2013. Therefore, cumulative impact is not expected. Dredging is not required for SCL(MKK-HUH).
According to the approved EIA Report for the Installation
of Submarine Gas Pipelines and Associated Facilities from To Kwa Wan to North Point (EIA-SGP) (ref EIA-182/2010), the dredging rate
and sediment loss rate for the installation of submarine gas pipeline will be 4,000
m3/day and 1.39 kg/s respectively, compared with 500 m3/day
and 0.06 kg/s for that in SCL(TAW-HUH). Furthermore, the dredging works
of gas pipeline will be commenced in April 2012 for completion in December
2012, and hence cumulative impact is expected.
According to the approved EIA Report for the
Dredging Works for Proposed Cruise Terminal at Kai Tak (EIA-CT) (ref : EIA-138/2007), there are 2 stages of dredging. The
first stage of dredging would involve a total dredging volume of about
1,022,300m3. The second stage of dredging would involve a lesser
amount of about 680,000m3. Further liaison has been made with CEDD
and the website of Tourism Commission has been reviewed. According to the
information available, the dredging works for the cruise terminal has been
commenced in June 2010 and is anticipated to complete in 2015. Maintenance
dredging will be carried out regularly during the operational period.
According to the latest design for SCL(TAW-HUH), some
dredging is required for the Kai Tak Barging Facility which is anticipated to
commence in mid 2012 (see Section 3
for more details). The first stage of dredging for the Cruise Terminal, Submarine,
Gas Pipelines and Associated Facilities may
therefore overlap with the dredging work for the barging facility for Kai
Tak. Cumulative construction phase
water quality impacts are therefore anticipated.
The EIA-CT has assessed the
cumulative water quality impact due to the superposition effect from the
proposed dredging activities for Cruise Terminal (CT), Central Kowloon Route
(CKR) and Truck Road T2. It should be noted that the dredging works for CKR
project will be conducted from 2015 to 2020. Cumulative impact with CKR is not
expected. Therefore, adopting the prediction results in EIA-CT would be in a conservative
side.
Apart from the EIA-CT, the EIA Report of EIA-SGP which was approved on August 2010, has also assessed the cumulative water quality impact including
A common WSR, i.e. Tai Wan
Flushing Water Intake, was identified between the concurrent dredging works of
Kai Tak Runway Barging Facility and that of CT and SGP. Comparing the
assessment scenarios of EIA-CT and EIA-SGP to the proposed dredging works at
Kai Tak Runway Barging Facility for SCL, the dredging volume for Kai Tak
Runway Barging Facility is small (500 m3/day) and the distance
between dredging location and the common WSR is rather far away. Therefore, the
water quality impact due to Kai Tak Runway Barging Facility should be less than
that of EIA-CT and EIA-SGP. Since the EIA-CT and EIA-SGP have concluded an
acceptable water quality impact, SCL should have insignificant SS elevations
due to the much smaller scale dredging. A comparison between the assessment
scenario of EIA-CT and EIA-SGP and the proposed dredging works at Kai Tak
Runway Barging Facility for SCL is summarised in Table 10.8 and the results of cumulative
impacts with CT and SGP is presented in Table 10.8a.
Table 10.8: Comparison of Dredging Scenario for EIA-CT,
EIA-SGP and SCL(TAW-HUH)
|
Dredging Scenario Assumptions |
EIA-CT |
EIA-SGP |
SCL (TAW-HUH) |
|
Dredging Rate |
4,000 m3/day [1] |
4,000 m3/day [2] |
500 m3/day |
|
Dredging Volume |
1,380,000 m3 |
260,665 m3 |
Around 56,000 m3 |
|
Dredging Period |
P1: 2011 to 2012 (1 years, 12 hours per day) P2: 2013 to 2014 (1 years, 12 hours per day) |
Apr 2012 to Dec 2012 (8 months, 16 hours per day) |
Jul 2012 to Dec 2012 (6 months, 12 hours per day) |
|
Dredging Location |
CT, CKR [3] |
CT, SGP |
Kai Tak Runway Barging Facility |
|
Distance from dredging area to WSR |
To Kwa Wan Typhoon Shelter CT: ~1 km Tai Wan CT: ~1 km |
To Kwa Wan Typhoon Shelter SGP: ~0.48 km Tai Wan SGP: ~0.45 km |
To Kwa Wan Typhoon Shelter ~0.28 km Tai Wan ~1.2 km |
|
Mitigation Measures |
Closed Grab Dredgers with Silt Curtain |
||
Note
[1] In
Scenario 2b of EIA-CT, the assumed total dredging volumes are 4000, 2000 and
8000 m3/day for CT, CKR and T2 respectively. Since T2 is rather far
away from site, it was not considered in the comparison.
[2] In
Scenario 2a of EIA-SGP, the assumed total dredging volumes are 4000, 4000 and
6000 m3/day for CT, SGP and T2 respectively. Since T2 is rather far away from
site, it was not considered in the comparison.
[3] It should be noted that the
CKR project will be conducted on 2015 to 2020 and cumulative impact with CKR is
not expected. Therefore, adopting the prediction results in EIA-CT would be in
a conservative side.
Table
10.8a: Impact Comparison on SS
Results of CT, SGP, SCL(TAW-HUH) and the Cumulative
Impacts
|
|
SCL (TAW-HUH) |
CT and SGP |
Cumulative |
|
WSR 8 -
Tai Wan |
|||
|
Background [1] |
SS (Absolute) 5.7 mg/l (Dry Season) 5.4 mg/l (Wet Season) |
SS (Absolute) 5.7 mg/l (Dry Season) 5.4 mg/l (Wet Season) |
SS (Absolute) 5.7 mg/l (Dry Season) 5.4 mg/l (Wet Season) |
|
Project Contribution (Mitigated) |
SS (Elevation) [2] 0.1 mg/l (All Season)- |
SS (Absolute) [3] 7.5 mg/l (Dry Season) 6.9 mg/l (Wet Season) |
SS (Absolute) [4] 7.6 mg/l (Dry Season) 7.0 mg/l (Wet Season) SS (Elevation) [5] 1.9 mg/l (Dry Season) 1.6 mg/l (Wet Season) |
|
Criteria |
- |
- |
SS (Absolute) 10 mg/l SS (Elevation) 2.5 mg/l (Dry Season) 2.3 mg/l (Wet Season) |
|
WSR 13 -
To Kwa Wan Typhoon Shelter |
|||
|
Background [1] |
5.7 mg/l (Dry Season) 5.4 mg/l (Wet Season) |
5.7 mg/l (Dry Season) 5.4 mg/l (Wet Season) |
5.7 mg/l (Dry Season) 5.4 mg/l (Wet Season) |
|
Project Contribution (Mitigated) |
SS (Elevation) [2] 0.3 mg/l (All Season)- |
SS (Absolute) [6] 7.5 mg/l (Dry Season) 6.9 mg/l (Wet Season) |
SS (Absolute) [4] 7.8 mg/l (Dry Season) 7.2 mg/l (Wet Season) SS (Elevation) [5] 2.1 mg/l (Dry Season) 1.8 mg/l (Wet Season) |
|
Criteria |
- |
- |
SS (Absolute) - SS (Elevation) 2.5 mg/l (Dry Season) 2.3 mg/l (Wet Season) |
Note
[1] Background
conditions are adopted from the average SS level of Station VM2 and VM4 for
Year 2009
[2] Refers
to Table 10.7b
[3] Adopted
from EIA-SGP, which includes the cumulative SS modeling results of from CT and
SGP. No SS elevation is presented for the WSR - Tai Wan Flushing Water Intakes.
[4] The
absolute values for SCL(TAW-HUH) are calculated from
the summation of model result of CT and SGP and the modeling result in Table
10.7b.
[5] The
cumulative SS elevations are backward calculated from absolute SS minus the “With Project” Scenario.
[6] SS
levels on WSR 8 is not presented in both EIA-CT and/or
EIA-SGP. However, it is anticipated the SS level due to CT and SGP in this WSR
is less than or comparable to that on WSR13. As a conservative approach, it is
assumed the SS levels due to CT and SGP on WSR13 is also applicable to that of
WSR 8.
The cumulative impacts for
dredging works at Kai Tak Runway Barging Facility are 7.0 to 7.8 mg/l and the
SS levels would be well within the ambient variations and well below the WQO
and/or WSD’s requirements.
Release of contaminants
In the Kowloon Bay and To Kwa Wan area, there are several existing elutriate test
carried out in other previously approved EIA studies such as South East Kowloon
Development (SEKD) [10-10] and EIA-CT. The location plan of existing
elutriate test point are extracted from these approved EIA studies and
presented in Appendix
10.2.
The elutriate results in EIA-CT showed the
concentrations of
cadmium, copper, nickel, arsenic and mercury exceeded the assessment criteria.
The maximum concentrations of these parameters are 3.6 μg/L,
29 μg/L, 75 μg/L, 52 μg/L and 0.5 μg/L
respectively. Although exceedances were recorded in the elutriate results in
EIA-CT, it should be
noted that all these results in EIA-CT were located at the south end of runway
(Appendix 10.2),
in which the proposed dredging area for Kai Tak Runway Barging Facility is
located in the mid-way side (recall Figure 10.4). Therefore, the elutriate
results in EIA-CT are not applicable to this study.
By overlaying the locations of elutriate
sampling points in EIA-SEKD and our dredging area, it is noted that Sampling
Point KB7 in EIA-SEKD is located within the proposed dredging area [10-10].
Therefore, the elutriate results at this sampling point is considered
appropriate to represent the proposed dredging activities in this study. The same approach is also
adopted in the EIA Study of Kai Tak Development (KTD) [10-11]. Table 10.9
present the elutriate test results and the comparison with the international
standards.
Table 10.9:
Elutriate Test Result as Adopted in the EIA Study of SEKD
|
Station KB7, located at dredging area of Kai Tak Runway Barging Facility |
Elutriate Test Result |
Relevant International Standard / Background Concentration |
|
Cadmium |
< 0.2 mg/l |
2.5 mg/l [1] |
|
Chromium |
< 10 mg/l |
15 mg/l [1] |
|
Copper |
< 2 mg/l |
5 mg/l [1] |
|
Mercury |
0.025 mg/l |
0.3 mg/l [1] |
|
Nickel |
2 mg/l |
30 mg/l [1] |
|
Lead |
< 1 mg/l |
25 mg/l [1] |
|
Zinc |
< 10 mg/l |
40 mg/l [1] |
|
Silver |
< 1 mg/l |
2.3 mg/l [3] |
|
Arsenic |
10 mg/l |
25 mg/l [1] |
|
TBT |
< 0.015 mg/l |
0.1 mg/l [2] |
|
Nitrate Nitrogen |
0.23 mg/l |
- |
|
Nitrite Nitrogen |
< 0.01 mg/l |
- |
|
Ammonia Nitrogen |
7.15 mg/l |
- |
|
Unionized Ammonia (UIA) |
0.4 mg/l [4] |
0.021 mg/l |
|
Total Inorganic Nitrogen (TIN) |
7.39 mg/l [5] |
0.4 mg/l |
Note:
[1]
[2] Michael
H. Salazar and Sandra M Salaar (1996) Mussels as Bioindicators: Effects of TBT on Survival, Bioaccumulation,
and Growth under Natural Conditions in Organotic,
edited by M A Champ and P F Seligman. Chapman & Hall
[3] USEPA Standards
[4] [UIA]
= 5.62 x 10-10 x [NH4-N] / 10-pH, taking
pH of 8.0 according to the average pH value of EPD’s monitoring station VM2 in
Year 2009
(http://www.epd.gov.hk/epd/tc_chi/environmentinhk/water/marine_quality/files/Marinereport2009Chiv1.pdf)
[5] [TIN]
= [NO3-N] + [NO2-N] + [NH4-N]
According to the impact
evaluation of EIA-KTD, the heavy metals levels in the elutriate samples would
comply with the relevant international standards. In addition, the heavy metal
levels at the WSR would be further diluted after the contaminants are released
into the water column. Thus, the residue impacts of heavy metals at the WSRs
would be insignificant.
According to the EIA-KTD, the
levels of TBT, PCBs and PAHs in the elutriate samples at the dredging location
were all below the detection limits. Therefore, the adverse water quality
impact due to release of organic compounds would be insignificant.
The nutrient levels (UIA and TIN)
at the dredging point exceeded the WQO. A dilution calculation was conducted by
using the following equation [10-6]:
C(X) = q/DXωπ0.5
where:
C(x) = concentration at
distance x from the source
q =
sediment loss rate (assume 1 kg/s to calculate dilution factor)
D =
water depth (= 7m)
X =
distance from source
ω =
diffusion velocity (= 0.01 m/s)
The radius of initial release was
assumed to be 10m and the dilution factor is presented in Table 10.9a.
Table 10.9a:
Dilution Factor for Nutrients
|
Centreline Distance from Source (m), |
Dilution Factor |
|
100 |
10 |
|
200 |
20 |
|
300 |
30 |
|
400 |
40 |
|
500 |
50 |
|
600 |
60 |
|
700 |
70 |
|
800 |
80 |
|
900 |
90 |
|
1000 |
100 |
|
280 (at To Kwa
Wan Typhoon Shelter) |
28 |
|
1200 (at Tai Wan |
120 |
By applying the
dilution factors in Table
10.9a, the annual means of UIA and TIN concentrations are presented in Table 10.9b.
Table 10.9b:
Impact Evaluations on Calculated Annual Mean of UIA and TIN Concentrations
|
Centreline Distance from Source (m) |
UIA (mg/l) [1] |
TIN (mg/l) [2] |
||
|
During Dredging Period |
Annual Mean |
During Dredging Period |
Annual Mean |
|
|
100 |
0.043 |
0.023 |
0.949 |
0.580 |
|
200 |
0.023 |
0.013 |
0.580 |
0.395 |
|
300 |
0.016 |
0.010 |
0.456 |
0.333 |
|
400 |
0.013 |
0.008 |
0.395 |
0.302 |
|
500 |
0.011 |
0.007 |
0.358 |
0.284 |
|
600 |
0.010 |
0.006 |
0.333 |
0.272 |
|
700 |
0.009 |
0.006 |
0.316 |
0.263 |
|
800 |
0.008 |
0.006 |
0.302 |
0.256 |
|
900 |
0.007 |
0.005 |
0.292 |
0.251 |
|
1000 |
0.007 |
0.005 |
0.284 |
0.247 |
|
280 (at WSR 13 - To Kwa Wan Typhoon Shelter) |
0.017 |
0.010 |
0.474 |
0.342 |
|
1200 (at WSR 8 - Tai Wan |
0.006 |
0.005 |
0.272 |
0.241 |
|
Background |
- |
0.003 |
- |
0.210 |
Note:
[1] Background
= 0.003 mg/l according to Station VM2 in Table 10.1b, Total dredging period = 50%
of whole year (6 months dredging period).
[2] Background
= 0.21 mg/l according to Station VM2 in Table 10.1b, Total dredging period =
50% of whole year. (6 months dredging period)
According to Table 10.9b,
the predicted annual mean of UIA and TIN were well below the WQO at all WSRs. A
mixing zone of exceedance for the predicted annual mean of UIA and TIN will be
less than 200m and there is no sensitive receiver within the mixing zone.
According to the EIA-SGP[10-12], a
negligible impact of UIA and TIN was predicted due to concurrent dredging
activities of CT and SGP. Therefore, there will be no additional elevations of
UIA and TIN at the WSR. Thus, adverse cumulative water quality impact due to
release of nutrient is not anticipated.
DO Depletion
The degree of oxygen depletion
exerted by a sediment plume is a function of the sediment oxygen demand of the
sediment, its concentration in the water column and the rate of oxygen
replenishment. For the purposes of this assessment, the impact of the sediment
oxygen demand on dissolved oxygen concentrations has been calculated based on
the following equation:
DODep =
C * SOD * K * 0.001
where:
DODep =
Dissolved Oxygen depletion (mg/L)
C =
Suspended Solids concentration (kg/m3)
SOD = Sediment Oxygen Demand
K =
Daily oxygen uptake factor (set at 1.0/day for worse case estimate)
A Chemical Oxygen Demand (COD) of
22,000 mg/kg (in the range of 16000 mg/kg to 26000 mg/kg) has been taken with reference
to EPD Marine Monitoring data (VS3, i.e. VM2 for water quality sampling point)
in Year 2005 to 2009[10-13] as a suitably representative value for
sediment oxygen demand[10-6] in
Kowloon Bay and To Kwa Wan area.
The analysis using the above equation
does not allow for re-aeration which would tend to reduce any impact of the
suspended sediment on the water column DO concentrations. The analysis,
therefore, errs on the conservative side so as not to underestimate the extent
of DO depletion. Further, it should be noted that, for sediment in suspension
to exert any oxygen demand on the water column will take time and, in that
time, the sediment will be transported and mixed/dispersed with oxygenated
water. As a result, the oxygen demand and the impact on dissolved oxygen
concentrations will diminish as the suspended sediment concentrations decrease.
The calculation of predicted DO
depletion is presented in Table 10.9c.
Table 10.9c:
Calculated DO Depletion
|
Centreline Distance from Source (m) |
SS elevation[1] (mg/l) |
DO Depletion (mg/l) |
|
100 |
1.9 |
0.04 |
|
200 |
1.3 |
0.03 |
|
300 |
1.1 |
0.02 |
|
400 |
1.0 |
0.02 |
|
500 |
0.9 |
0.02 |
|
600 |
0.8 |
0.02 |
|
700 |
0.7 |
0.02 |
|
800 |
0.7 |
0.02 |
|
900 |
0.6 |
0.01 |
|
1000 |
0.6 |
0.01 |
|
280 (at WSR 13 - To Kwa Wan Typhoon Shelter) |
1.1 |
0.02 |
|
1200 (at WSR 8 - Tai Wan |
0.5 |
0.01 |
Note:
[1] Refer to
Table 10.7a, unmitigated SS elevation
According to Table 10.9c, while the
unmitigated DO depletion is less than 0.04 mg/l. According to EPD Marine Water
Quality Report 2009[10-13], the reporting limit of DO is 0.1 mg/l,
which is greater than the predicted unmitigated DO depletion. The predicted DO
depletion for mitigated scenario will be even lesser. The DO depletion due to
the dredging works, in both unmitigated and mitigated scenarios, will have a
negligible effect to the background values (see Table 10.9d). The absolute DO levels at WSR 8 and 13 from
background, SCL(TAW-HUH), concurrent projects as well
as the cumulative impact are presented in Table
10.9e.
Table 10.9d: 10 Percentile of DO Concentrations in
the Vicinity of Sensitive Receivers
|
Sensitive
Receiver (Relevant EPD Monitoring Station) |
Depth
Average |
Bottom |
|
WSD flushing
water intakes at Tai Wan (VM2 and VM4, 2009 data), about 1.2 km away from the
dredging area |
5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) |
5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
|
To Kwa Wan Typhoon Shelter (VM2 and VM4, 2009 data), about
280 m away from the dredging area |
5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) |
5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
Table 10.9e:
Impact
Comparison on Calculated DO
Concentrations of CT, SGP, SCL(TAW-HUH) and the Cumulative
Impacts
|
|
SCL (TAW-HUH) |
CT and SGP |
Cumulative |
|
WSR 8 - Tai Wan |
|||
|
Background [1] |
Minimum DO level 10%ile
Depth Averaged 5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
Minimum DO level 10%ile
Depth Averaged 5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
Minimum DO level 10%ile
Depth Averaged 5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
|
Project Contribution (Mitigated) |
DO depletion [2] 0.01 mg/l (All Season) |
DO depletion [3] Depth
Averaged 0.1 mg/l (Dry Season) 0.0 mg/l (Wet Season) Bottom 0.1 mg/l (Dry Season) 0.1 mg/l (Wet Season) |
DO depletion Depth
Averaged 0.11 mg/l (Dry Season) 0.01 mg/l (Wet Season) Bottom 0.11 mg/l (Dry Season) 0.11 mg/l (Wet Season) |
|
Minimum DO level 10%ile
Depth Averaged 5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
Minimum DO level 10%ile
Depth Averaged 5.2 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.5 mg/l (Dry Season) 3.6 mg/l (Wet Season) |
Minimum DO level 10%ile Depth
Averaged 5.2 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.5 mg/l (Dry Season) 3.6 mg/l (Wet Season) |
|
|
Criteria |
DO level 10%ile
Depth Averaged 4 mg/l 10%ile
Bottom 2 mg/l |
DO level 10%ile
Depth Averaged 4 mg/l 10%ile
Bottom 2 mg/l |
DO level 10%ile
Depth Averaged 4 mg/l 10%ile
Bottom 2 mg/l |
|
WSR 13 - To Kwa Wan Typhoon Shelter |
|||
|
Background [1] |
Minimum DO level 10%ile
Depth Averaged 5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
Minimum DO level 10%ile
Depth Averaged 5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
Minimum DO level 10%ile
Depth Averaged 5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile Bottom 5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
|
Project Contribution (Mitigated) |
DO depletion [2] 0.02 mg/l (All Season) |
DO depletion [4] Depth
Averaged 0.1 mg/l (Dry Season) 0.0 mg/l (Wet Season) Bottom 0.1 mg/l (Dry Season) 0.1 mg/l (Wet Season) |
DO depletion Depth
Averaged 0.12 mg/l (Dry Season) 0.02 mg/l (Wet Season) Bottom 0.12 mg/l (Dry Season) 0.12 mg/l (Wet Season) |
|
Minimum DO level 10%ile
Depth Averaged 5.3 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.6 mg/l (Dry Season) 3.7 mg/l (Wet Season) |
Minimum DO level 10%ile
Depth Averaged 5.2 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.5 mg/l (Dry Season) 3.6 mg/l (Wet Season) |
Minimum DO level 10%ile
Depth Averaged 5.2 mg/l (Dry Season) 4.0 mg/l (Wet Season) 10%ile
Bottom 5.5 mg/l (Dry Season) 3.6 mg/l (Wet Season) |
|
|
Criteria |
DO level 10%ile
Depth Averaged 4 mg/l 10%ile
Bottom 2 mg/l |
DO level 10%ile
Depth Averaged 4 mg/l 10%ile
Bottom 2 mg/l |
DO level 10%ile
Depth Averaged 4 mg/l 10%ile Bottom 2 mg/l |
Note:
[1] Refer
to Table 10.9d
[2] Refer
to Table 10.9c
[3] Adopted
from EIA-SGP, which includes the cumulative DO depletion results of from CT and
SGP.
[4] DO
depletion on WSR8 is not presented in both EIA-CT and/or EIA-SGP. However, it is
anticipated the DO depletion due to CT and SGP in this WSR is less than or
comparable to that on WSR13. As a conservative approach, it is assumed the DO
depletion due to CT and SGP on WSR13 is also applicable to that of WSR8.
According to Table 10.9d, the 10 percentile DO concentration will comply with
the WQO and adverse water quality impact due to DO depletion is not anticipated
for the Project.
10.5.1.8
Operation of Barging Facilities
The construction materials would
be loaded or unloaded onto barges at the barging points. In order to avoid
disturbance to marine water during transportation and loading/unloading
operation, good site practices and mitigation measures have been recommended in
Section 10.7.1.6. With the implementation of these
recommended mitigation measures, adverse water quality impact due to operation
of barging points are not expected.
10.5.1.9
Accidental Spillage
The
site coverage would be rather large during the construction phase. The soil of
site area may be potentially contaminated by accidental spillage of grouting
materials, surplus adhesives, lubrication oil, grease, acidic/alkaline
solutions, petroleum products, chemical solvents, etc. Site runoff may wash the
contaminated soil into stormwater drains or watercourses and cause water quality impact.
There
will be no direct discharge of wastewater into the
10.6.1 Pollution
Sources & Prediction of Impacts
Potential
water pollution sources during the operational phase are summarised below:
·
Runoff from rail track and operational tunnel
drainage;
·
Station runoff; and
·
Sewage from station operation.
10.6.1.1
Run off from Rail Track
Since
all tracks are contained in concrete tunnel box, there will be no rainwater
runoff. The tunnel wall will be
equipped with water-tight liner and design for no seepage. The amount of
groundwater seepage into the tunnel will be insignificant. Any tunnel run-off could be contaminated
with limited amount of grease from passing trains or from maintenance
activities. Standard designed silt
trap and oil interceptor will be provided to remove the oil, lubricants,
grease, silt, grit and debris from the wastewater before discharging into foul
drainage. The waste will then be
disposed of as general refuse and industrial wastes as described in Section 11 of the EIA Report. No adverse impact on marine environment
is anticipated.
10.6.1.2
Station Runoff
Rainwater
runoff from station structure (e.g. ventilation building, entrance etc) is not
contaminated and hence has no adverse water quality impact.
10.6.1.3
Discharge from Fresh Water Cooling
Facility
Individual
seawater cooling system would not be adopted and hence there would be no
associated impacts.
10.6.1.4
Sewage from Station
A
separate consultant will be appointed by the Project Proponent to conduct the
detailed design of sewer for station and he will be responsible for carrying
out the Sewerage Impact Assessment and submit to the relevant government
departments for approval. The
typical Average Dry Weather Flow (ADWF) for a train station (without top-side
properties) would be about 0.8l/s, which would be equivalent to about 55m3/day,
assuming 19 hours of operation. It
is therefore anticipated that the ADWF for each station would be of similar
order and probably in the order of 50-100m3 /day. Given the small quantity of the ADWF for
each station, the capacity of the foul sewer is adequate for the proposed
sewage discharge. Hence, no water
quality impact is anticipated.
In
accordance with the Practice Note for Professional Persons on Construction Site
Drainage, Environmental Protection Department, 1994 (ProPECC
PN 1/94), construction phase mitigation measures shall include the following:
10.7.1.1
Construction Runoff and Site Drainage
·
At the start of site establishment,
perimeter cut-off drains to direct off-site water around the site should be
constructed with internal drainage works and erosion and sedimentation control
facilities implemented. Channels
(both temporary and permanent drainage pipes and culverts), earth bunds or sand
bag barriers should be provided on site to direct stormwater
to silt removal facilities. The
design of the temporary on-site drainage system will be undertaken by the
contractor prior to the commencement of construction.
·
The dikes or embankments for flood protection
should be implemented around the boundaries of earthwork areas. Temporary
ditches should be provided to facilitate the runoff discharge into an
appropriate watercourse, through a site/sediment trap. The sediment/silt traps
should be incorporated in the permanent drainage channels to enhance deposition
rates.
·
The design of efficient silt removal
facilities should be based on the guidelines in Appendix A1 of ProPECC PN 1/94, which states that the retention time for
silt/sand traps should be 5 minutes under maximum flow conditions. Sizes may vary depending upon the flow
rate, but for a flow rate of 0.1 m3/s a
sedimentation basin of 30m3 would be required and for a flow
rate of 0.5 m3/s the basin would be 150 m3. The detailed design of the sand/silt
traps shall be undertaken by the contractor prior to the commencement of
construction.
·
All exposed earth areas should be
completed and vegetated as soon as possible after earthworks have been
completed, or alternatively, within 14 days of the cessation of earthworks
where practicable. Exposed slope surfaces should be covered by tarpaulin or
other means.
·
The overall slope of the site should be
kept to a minimum to reduce the erosive potential of surface water flows, and
all traffic areas and access roads protected by coarse stone ballast. An additional advantage accruing from
the use of crushed stone is the positive traction gained during prolonged
periods of inclement weather and the reduction of surface sheet flows.
·
All drainage facilities and erosion and
sediment control structures should be regularly inspected and maintained to
ensure proper and efficient operation at all times and particularly following
rainstorms. Deposited silt and grit
should be removed regularly and disposed of by spreading evenly over stable,
vegetated areas.
·
Measures should be taken to minimise the
ingress of site drainage into excavations.
If the excavation of trenches in wet periods is necessary, they should
be dug and backfilled in short sections wherever practicable. Water pumped out from trenches or
foundation excavations should be discharged into storm drains via silt removal
facilities.
·
Open stockpiles of construction materials
(for example, aggregates, sand and fill material) of more than 50m3
should be covered with tarpaulin or similar fabric during rainstorms. Measures should be taken to prevent the
washing away of construction materials, soil, silt or debris into any drainage
system.
·
Manholes (including newly constructed
ones) should always be adequately covered and temporarily sealed so as to
prevent silt, construction materials or debris being washed into the drainage
system and storm runoff being directed into foul sewers.
·
Precautions be taken at any time of year
when rainstorms are likely, actions to be taken when a rainstorm is imminent or
forecasted, and actions to be taken during or after rainstorms are summarised
in Appendix A2 of ProPECC PN 1/94. Particular attention should be paid to
the control of silty surface runoff during storm
events, especially for areas located near steep slopes.
·
All vehicles and plant should be cleaned
before leaving a construction site to ensure no earth, mud, debris and the like
is deposited by them on roads. An
adequately designed and sited wheel washing facilities should be provided at
every construction site exit where practicable. Wash-water should have sand and silt
settled out and removed at least on a weekly basis to ensure the continued
efficiency of the process. The
section of access road leading to, and exiting from, the wheel-wash bay to the
public road should be paved with sufficient backfall
toward the wheel-wash bay to prevent vehicle tracking of soil and silty water to public roads and drains.
·
Oil interceptors should be provided in the
drainage system downstream of any oil/fuel pollution sources. The oil
interceptors should be emptied and cleaned regularly to prevent the release of
oil and grease into the storm water drainage system after accidental spillage.
A bypass should be provided for the oil interceptors to prevent flushing during
heavy rain.
·
Construction solid waste, debris and
rubbish on site should be collected, handled and disposed of properly to avoid
water quality impacts. Requirements
for solid waste management are detailed in Section
11 of this EIA Report.
·
All fuel tanks and storage areas should be
provided with locks and sited on sealed areas, within bunds of a capacity equal
to 110% of the storage capacity of the largest tank to prevent spilled fuel
oils from reaching water sensitive receivers nearby.
·
By adopting the above mitigation measures
with Best Management Practices (BMPs) it is anticipated that the impacts of
construction site runoff from the construction site will be reduced to an
acceptable level before discharges.
·
All the earth works involving should be
conducted sequentially to limit the amount of construction runoff generated
from exposed areas during the wet season (April to September) as far as
practicable.
10.7.1.2
Tunnelling Works
·
Cut-&-cover/ open cut tunnelling work
should be conducted sequentially to limit the amount of construction runoff
generated from exposed areas during the wet season (April to September) as far
as practicable.
·
Uncontaminated discharge should pass
through sedimentation tanks prior to off-site discharge
·
The wastewater with a high concentration
of SS should be treated (e.g. by sedimentation tanks with sufficient retention
time) before discharge. Oil
interceptors would also be required to remove the oil, lubricants and grease
from the wastewater.
·
Direct discharge of the bentonite slurry (as a result of D-wall and bored
tunnelling construction) is not allowed.
It should be reconditioned and reused wherever practicable. Temporary storage locations (typically a
properly closed warehouse) should be provided on site for any unused bentonite that needs to be transported away after all the
related construction activities are completed. The requirements in ProPECC
PN 1/94 should be adhered to in the handling and disposal of bentonite slurries.
10.7.1.3
Sewage Effluent
Adequate numbers of
portable toilets should be provided for handling
the construction sewage generated by the workforce. The portable toilets should be
maintained in a reasonable state, which will not deter the workers from
utilizing these portable toilets.
Overnight sewerage should be collected by licensed collectors regularly.
10.7.1.4
Groundwater from Contaminated Areas
No
direct discharge of groundwater from contaminated areas should be adopted.
Prior to the excavation works within these potentially contaminated areas, the
groundwater quality should be reviewed with reference to the site investigation
data in this EIA report for compliance to the Technical Memorandum on Standards
for Effluents Discharged into Drainage on Sewerage Systems, Inland and Coastal
Waters (TM-Water) and the existence of prohibited substance should be
confirmed. The review results should be submitted to EPD for examination If the
review results indicated that the groundwater to be generated from the
excavation works would be contaminated, the contaminated groundwater should be
either properly treated in compliance with the requirements of the TM-Water or properly
recharged into the ground.
If
wastewater treatment is deployed, the wastewater treatment unit shall deploy
suitable treatment process (e.g. oil interceptor / activated carbon) to reduce
the pollution level to an acceptable standard and remove any prohibited
substances (e.g. TPH) to undetectable range. All treated effluent from
wastewater treatment plant shall meet the requirements as stated in TM-Water and
should be discharged into the foul sewers.
If
groundwater recharging wells are deployed, recharging wells should be installed
as appropriate for recharging the contaminated groundwater back into the
ground. The recharging wells should be selected at places where the groundwater
quality will not be affected by the recharge operation as indicated in the
Section 2.3 of TM-Water. The baseline groundwater
quality shall be determined prior to the selection of the recharge wells, and
submit a working plan (including the laboratory analytical results showing the
quality of groundwater at the proposed recharge location(s) as well as the
pollutant levels of groundwater to be recharged) to EPD for agreement. Pollution levels of groundwater to be
recharged shall not be higher than pollutant levels of ambient groundwater at
the recharge well. Prior to recharge, any prohibited substances such as TPH
products should be removed as necessary by installing the petrol interceptor.
The Contractor should apply for a discharge licence under the WPCO through the
Regional Office of EPD for groundwater recharge operation or discharge of
treated groundwater.
10.7.1.5
Dredging Works
The
following good practice shall apply for the dredging works:
·
Install efficient silt curtains, i.e. at least
75% SS reduction, at the point of seawall dredging to control the dispersion of
SS;
·
Water quality monitoring should be
implemented to ensure effective control of water pollution and recommend
additional mitigation measures required;
·
The decent speed of grabs should be
controlled to minimize the seabed impact and to reduce the volume of
over-dredging; and
·
All vessels should be sized so that
adequate clearance is maintained between vessels and the seabed in all tide
conditions, to ensure that undue turbidity is not generated by turbulence from
vessel movement or propeller wash.
10.7.1.6
Operation of Barging Facilities
The following good practice shall apply for the
barging facilities operations:
·
All barges should be fitted with tight
bottom seals to prevent leakage of materials during transport;
·
Barges or hoppers should not be filled to
a level that will cause overflow of materials or polluted water during loading
or transportation;
·
All vessels should be sized so that
adequate clearance is maintained between vessels and the seabed in all tide
conditions, to ensure that undue turbidity is not generated by turbulence from
vessel movement or propeller wash; and
·
Loading of barges and hoppers should be
controlled to prevent splashing of material into the surrounding water.
·
Mitigation measures as outlined
in Section 10.7.1.1 should be
applied to minimise water quality impacts from site runoff and open stockpile
spoils at the proposed barging facilities where appropriate.
10.7.1.7
Accidental Spillage
In
order to prevent accidental spillage of chemicals, proper storage and handling
facilities should be provided. All the tanks, containers, storage area should
be bunded and the locations should be locked as far
as possible from the sensitive watercourse and stormwater
drains. The Contractor should register as a chemical waste producer if chemical
wastes would be generated. Storage of chemical waste arising from the
construction activities should be stored with suitable labels and warnings.
Disposal of chemical wastes should be conducted in compliance with the
requirements as stated in the Waste disposal (Chemical Waste) (General)
Regulation.
Mitigation
measures are only required to mitigate runoff from track during the operational
phase. The following mitigation
measures during operational phase are recommended:
·
Track drainage channels discharge should
pass through oil/grit interceptors/chambers to remove oil, grease and sediment
before discharging
into public storm drainage/
foul sewerage system;
·
The silt traps and oil interceptors should
be cleaned and maintained regularly; and
·
Oily contents of the oil interceptors
should be transferred to an appropriate disposal facility, or to be collected
for reuse, if possible.
Adverse
residual impacts during the construction and operational phases are not
anticipated provided that the above mitigation measures are implemented.
Potential
water pollution sources have been identified as construction runoff, sewage
from site workforce, drainage diversion and groundwater contamination. Mitigation measures including covering
excavated materials and providing sedimentation tanks on-site etc are
recommended to mitigate any adverse water quality impacts.
To minimise the potential impact
due to SS, deployment of silt curtains around the closed grab dredgers is
recommended for the dredging works at Kai Tak Runway to minimize any
significant water quality impact in the
According to the quantitative
assessment for dredging activities, the cumulative water quality impact due to
concurrent dredging activities from CT were well within the acceptable level.
The
operational water quality impact for track run-off and tunnel seepage will have
no adverse water quality impact provided that mitigation measures are
incorporated in the design.
All
proposed mitigation measures are clearly defined in the Environmental
Mitigation Implementation Schedule.
[10-1] Agreement No. CE35/2006 (CE) Dredging Works for Proposed
Cruise Terminal at Kai Tak, EIA Report (EIA138/2007), Civil Engineering &
Development Department (2007)
[10-2] Pastorok, R. A. and Bilyard, G.R.
(1985) Effects of sewage pollution on
coral-reef communities. Marine Ecology Progress Series 21: 175-189
[10-3] CIRIA
(2000). Scoping the Assessment of Sediment Plumes from Dredging, CIRIA
Publication C547
[10-4] Fischer,
Hugo B. (1979) Mixing in inland and coastal waters
[10-5] Yiu
Lian Dockyard (2006) Environmental Impact Assessment
Report Relocation of Yiu Lian Floating Dock No. 3
[10-6] Ove
Arup & Partners (2009) Environmental Impact Assessment Report
[10-7] CEDD (2005) EIA
Study Peng Chau Helipad
[10-8] USEPA (2005)
Performance Standard for Dredging Re-suspension
[10-9] EPD (1999) Update
on Cumulative Water Quality and Hydrological Effect of Coastal Developments and
Upgrading of Assessment Tool, Report on Calibration and Verification of the
Hydrodynamic Model
[10-10] TDD (2001) EIA Study Comprehensive
Feasibility Study for the Revised Scheme of South
[10-11] CEDD (2008) EIA Study Kai Tak
Development Engineering Study cum Design and Construction of Advance Works –
Investigation, Design and Construction
[10-12] Mott (2009) EIA Study
Installation of Submarine Gas Pipelines and Associated Facilities from To Kwa Wan to North Point for
[10-13] EPD (2009) Marine Water
Quality 2009