5
water quality Impact
5.1.1
This section presents an assessment of the
potential water quality impacts associated with construction and operation of
the Project. Recommendations for mitigation measures have been provided to
minimize the identified water quality impacts.
Environmental Impact
Assessment Ordinance (EIAO)
5.2.1
The Technical Memorandum on Environmental Impact
Assessment Process (EIAO-TM) was issued by Environmental Protection Department
(EPD) under Section 16 of the EIAO.
It specifies the assessment method and criteria that are to be followed
in an EIA Study. Reference sections in the EIAO-TM provide the details of
assessment criteria and guidelines that are relevant to the water quality
impact assessment, including:
·
Annex 6 – Criteria for Evaluating Water
Pollution
·
Annex 14 – Guidelines for Assessment of Water
Pollution
Water Quality
Objective
Table 5.1 Summary
of Water Quality Objectives for North Western WCZ
|
Parameters
|
Objectives
|
Sub-Zone
|
|
Offensive Odour, Tints
|
Not to be present
|
Whole zone
|
|
Visible foam, oil scum, litter
|
Not to be present
|
Whole zone
|
|
Dissolved Oxygen (DO) within 2 m of the seabed
|
Not less than 2.0 mg/L for 90% of the sampling
occasions during the whole year
|
Marine waters
|
|
Depth-averaged DO
|
Not less than 4.0 mg/L
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones,
Water Gathering Ground Subzones and other inland waters
|
|
Not less than 4.0 mg/L for 90 % of the sampling
occasions during the whole year
|
Marine waters
|
|
pH
|
To be in the range of 6.5 - 8.5, change due to human
activity not to exceed 0.2
|
Marine waters excepting Bathing Beach Subzones
|
|
To be in the range of 6.5 – 8.5
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones
and Water Gathering Ground Subzones
|
|
To be in the range of 6.0 –9.0
|
Other inland waters
|
|
To be in the range of 6.0 –9.0 for 95% samples
collected during the whole year and waste discharges shall not cause the
natural pH range to be extended by more than 0.5 units
|
Bathing Beach Subzones
|
|
Salinity
|
Change due to human activity not to exceed 10% of
ambient
|
Whole zone
|
|
Temperature
|
Change due to human activity not to exceed 2 oC
|
Whole zone
|
|
Suspended solids (SS)
|
Waste discharge not to raise the natural ambient
level by 30%, nor cause the accumulation of suspended solids which may
adversely affect aquatic communities
|
Marine waters
|
|
Not to cause the annual median to exceed 20 mg/L
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones
and Water Gathering Ground Subzones
|
|
Not to cause the annual median to exceed 25 mg/L
|
Inland waters
|
|
Unionized Ammonia (UIA)
|
Annual mean not to exceed 0.021 mg/L as unionized
form
|
Whole zone
|
|
Nutrients
|
Shall not cause excessive algal growth
|
Marine waters
|
|
Total Inorganic Nitrogen (TIN)
|
Annual mean depth-averaged inorganic nitrogen not to
exceed 0.3 mg/L
|
Castle Peak Bay Subzone
|
|
Annual mean depth-averaged inorganic nitrogen not to
exceed 0.5 mg/L
|
Marine waters excepting Castle Peak Bay Subzone
|
|
Bacteria
|
Not exceed 610 per 100ml, calculated as the geometric
mean of all samples collected in one calendar year
|
Secondary Contact Recreation Subzones
|
|
Should be less than 1 per 100 ml, calculated as the
running median of the most recent 5 consecutive samples taken between 7 and
21 days.
|
Tuen Mun (A) and Tuen Mun (B) Subzones and Water
Gathering Ground Subzones
|
|
Not exceed 1000 per 100 ml, calculated as the running
median of the most recent 5 consecutive samples taken between 7 and 21 days
|
Tuen Mun (C) Subzone and other inland waters
|
|
Not exceed 180 per 100 ml, calculated as the
geometric mean of all samples collected from March to October inclusive.
Samples should be taken at least 3 times in one calendar month at intervals
of between 3 and 14 days.
|
Bathing Beach Subzones
|
|
Colour
|
Not to exceed 30 Hazen units
|
Tuen Mun (A) and Tuen Mun (B) Subzones and Water
Gathering Ground Subzones
|
|
Not to exceed 50 Hazen units
|
Tuen Mun (C) Subzone and other inland waters
|
|
5-Day Biochemical Oxygen Demand (BOD5)
|
Not to exceed 3 mg/L
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones
and Water Gathering Ground Subzones
|
|
Not to exceed 5 mg/L
|
Inland waters
|
|
Chemical Oxygen Demand (COD)
|
Not to exceed 15 mg/L
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones
and Water Gathering Ground Subzones
|
|
Not to exceed 30 mg/L
|
Inland waters
|
|
Toxins
|
Should not cause a risk to any beneficial uses of the
aquatic environment
|
Whole zone
|
|
Waste discharge shall not cause the toxins in water
significant to produce toxic carcinogenic, mutagenic or teratogenic effects
in humans, fish or any other aquatic organisms.
|
Whole zone
|
|
Phenol
|
Quantities shall not sufficient to produce a specific
odour or more than 0.05 mg/L as C6 H5OH
|
Bathing Beach
Subzones
|
|
Turbidity
|
Shall not reduce light transmission substantially
from the normal level
|
Bathing Beach
Subzones
|
Source: Statement
of Water Quality Objectives (North Western Water Control Zone)
Technical Memorandum on Effluents Discharge Standards
5.2.3
Discharges of effluents are subject to control
under the WPCO. The Technical
Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems,
Inland and Coastal Waters (TM-DSS), issued under Section 21 of the WPCO, gives
guidance on 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 effluent. Any sewage from the proposed
construction and operational activities must comply with the standards for
effluent discharged into the foul sewers, inshore waters and marine waters of
the North Western WCZ provided in the TM-DSS.
Practice Note
5.2.4
A practice note for professional persons has
been issued by the EPD to provide guidelines for handling and disposal of
construction site discharges. The ProPECC PN 1/94 “Construction Site Drainage”
provides good practice guidelines for dealing with ten types of discharge from
a construction site. These include
surface runoff, groundwater, boring and drilling water, bentonite slurry, water
for testing and sterilisation of water retaining structures and water pipes, wastewater
from building construction, acid cleaning, etching and pickling wastewater, and
wastewater from site facilities.
Guidelines given in the ProPECC PN 1/94 should be followed as far as
possible during construction to minimize the water quality impact due to
construction site drainage.
Water Sensitive
Receivers
5.3.1
The key Water Sensitive Receivers (WSRs) near
the Project site have been identified as follows:
·
Existing Tai Ho Wan (WSR 1);
·
Inland watercourses in the vicinity (WSR 3);
·
Channelized watercourse/ditch near Siu Ho Wan
Sewerage Treatment Works (SHWSTW) (WSR 4), Tai Ho Offtake and Pigging Station
(WSR 5) and Siu Ho Wan Water Treatment Works (WSR 6); and
·
The Brothers Marine Park (WSR 2).
5.3.2
The indicative locations of these WSRs are shown
in Figure No. NEX1062/S/SHD/ACM/Z10/170. Some watercourses / channelized
watercourses / ditches are generally draining from the hillside in the
southeast towards the Project site.
Together with the drainage within the Project site, these watercourses /
channelized watercourses / ditches will eventually discharge to the marine
water via the existing box culvert.
Marine Water
5.4.1
North Western WCZ is situated in the Pearl River
Estuary and is strongly influenced by the freshwater flows from Guangdong
Province. Ebb tide currents are flowing to the southeast whilst the flood tide
currents are to the northwest.
5.4.2
A summary of the marine water quality data
collected at the monitoring stations, namely NM1, NM2 and NM3 in the central
area of North Western WCZ which are located in the vicinity of the Project, is
given in Table 5.2. These monitoring stations are indicated
in Figure No. NEX1062/S/SHD/ACM/Z10/170. According to the EPD’s publication
“Marine Water Quality in Hong Kong 2015”, full compliance of the depth-averaged
DO levels was recorded at these stations, except NM1. Non-compliance of TIN
levels at NM2 and NM3 might be associated with the higher background level of
the discharges from Pearl River, and other local discharges. The bottom DO and UIA levels recorded in
these monitoring stations fully complied with the relevant WQOs.
Table 5.2 Summary
Statistics of Marine Water Quality of North Western Water WCZ Collected by EPD
in 2015
|
Parameter
|
Lantau
Island
|
Pearl Island
|
Pillar Point
|
WPCO WQO
(in marine waters)
|
|
NM1
|
NM2
|
NM3
|
|
Temperature
(oC)
|
23.7
(16.9
– 27.4)
|
24.0
(16.9 – 27.8)
|
24.2
(17.0 – 27.7)
|
Not more than 2oC in daily temperature range
|
|
Salinity
|
29.9
(25.2 – 31.7)
|
28.1
(18.9 – 31.3)
|
27.9
(21.0 – 31.3)
|
Not to cause more than 10% change
|
|
Dissolved
Oxygen (DO)
(mg/l)
|
Depth average
|
5.4
(3.7 – 7.2)
|
5.7
(4.3 – 7.4)
|
5.6
(4.3 – 7.3)
|
Not less than 4 mg/l for 90% of the
samples
|
|
Bottom
|
5.2
(2.5
– 7.3)
|
5.4
(3.2 – 7.3)
|
5.3
(3.0
– 7.4)
|
Not less than 2 mg/l for 90% of the
samples
|
|
Dissolved Oxygen (DO)
(% Saturation)
|
Depth
average
|
75
(53
– 90)
|
79
(62 - 92)
|
78
(62 – 91)
|
-
|
|
Bottom
|
72
(36 – 92)
|
75
(46 - 91)
|
74
(44 – 91)
|
-
|
|
pH
|
7.8
(7.7 – 8.0)
|
7.8
(7.6 - 8.0)
|
7.8
(7.6 – 8.0)
|
6.5
- 8.5 (± 0.2
from natural range)
|
|
Suspended
Solids (SS)
(mg/L)
|
8.0
(1.3
– 22.0)
|
6.6
(2.7 – 20.3)
|
10.8
(4.2 – 30.3)
|
Not
more than 30% increase
|
|
Ammonia
Nitrogen (NH3-N) (mg/L)
|
0.107
(0.016
– 0.177)
|
0.106
(0.014 – 0.180)
|
0.110
(0.013
– 0.203)
|
-
|
|
Unionised
Ammonia (UIA) (mg/L)
|
0.003
(<0.001
– 0.006)
|
0.003
(<0.001 - 0.009)
|
0.003
(<0.001
– 0.008)
|
Not
more than 0.021 mg/l for annual mean
|
|
Nitrite
Nitrogen (NO2-N) (mg/L)
|
0.052
(0.016
– 0.130)
|
0.066
(0.017 - 0.131)
|
0.070
(0.018 – 0.153)
|
-
|
|
Nitrate
Nitrogen (NO3-N) (mg/L)
|
0.286
(0.127
– 0.723)
|
0.421
(0.133 - 1.230)
|
0.429
(0.137 – 0.963)
|
-
|
|
Total
Inorganic Nitrogen (TIN) (mg/L)
|
0.44
(0.27
– 0.84)
|
0.59
(0.29 - 1.36)
|
0.61
(0.31 – 1.09)
|
Not
more than 0.5 mg/l for annual mean
|
|
Total
Nitrogen (TN)
(mg/L)
|
0.71
(0.56
– 1.06)
|
0.86
(0.52 - 1.61)
|
0.88
(0.55 – 1.28)
|
-
|
|
Orthophosphate
Phosphorus (PO4) (mg/L)
|
0.022
(0.008 – 0.031)
|
0.024
(0.013 - 0.039)
|
0.025
(0.011 – 0.040)
|
-
|
|
Total
Phosphorus (TP)
(mg/L)
|
0.05
(0.03
– 0.07)
|
0.05
(0.03 - 0.08)
|
0.06
(0.04 – 0.09)
|
-
|
|
Chlorophyll-a
(µg/L)
|
1.6
(0.4
– 4.6)
|
1.8
(0.4 – 5.8)
|
1.7
(0.7 – 4.1)
|
-
|
|
E. coli
(cfu/100
mL)
|
97
(7 – 510)
|
31
(2 - 220)
|
59
(10 – 220)
|
-
|
|
Faecal Coliforms
(cfu/100mL)
|
200
(48 – 650)
|
70
(4
– 340)
|
130
(23 – 580)
|
-
|
Notes:
(1)
Data source: Marine Water Quality in Hong Kong in
2015 (EPD).
(2)
Data presented are annual arithmetic means of
depth-averaged results except for E.coli and faecal coliforms that are annual
geometric means.
(3)
Data in brackets indicate the ranges
Tai Ho Wan and
Inland Watercourse /Channelized Watercourse/Ditch
5.4.3
The catchments of existing Tai Ho Wan and inland
watercourses / channelized watercourses / ditches located in the vicinity of the
Project site are shown in Figure No. NEX1062/S/SHD/ACM/Z10/170. The upstream sections
of these watercourses are mainly collecting the runoff from the hillside with
no major pollution source. In dry season, most of these watercourses are
expected to be dry, whereas in the wet seasons, the water flows in these
watercourses mostly would be rainwater with low pollution levels. Site
inspections and water sampling at these watercourses were conducted in 21 July
2016 representing the wet season.
It was observed during the site inspections that most of these
watercourses were dried out or had a very low flow (with water depth of less
than 3 cm). Since there were steep
slopes and construction sites for Tuen Mun Chek Lap Kok Link within the study
area, only limited accesses are available and practical for conducting water
sampling. Due to the low water flow
and site constraints, water sampling was only practical to be carried out at
three stations (namely Stations S1, S2 and S3 respectively) as shown in Figure No. NEX1062/S/SHD/ACM/Z10/170. The shortest distance of
S1 to the Scheme Boundary is about 73m. The associated sampling results are
shown in Table
5.3.
Table 5.3 Water
Quality Monitoring Data for Inland Watercourses
|
Parameter
|
Unit
|
Averaged
Value of 2 Consecutive Measurements
|
|
Station
S1
|
Station
S2
|
Station
S3
|
|
pH Value
|
-
|
7.73
|
7.43
|
7.93
|
|
Temperature
|
°C
|
30.80
|
27.65
|
30.15
|
|
Salinity
|
ppt
|
0.10
|
0.00
|
15.50
|
|
Turbidity
|
NTU
|
3.33
|
5.01
|
4.42
|
|
Dissolved Oxygen
|
mg/L
|
7.67
|
9.03
|
8.30
|
|
Dissolved Oxygen - % Saturation
|
%
|
102.85
|
113.60
|
107.15
|
5.5.1
The assessment area includes all areas within
500 m from the Project boundary, and covers relevant WSRs that have a bearing on
the environmental acceptability of the Project within the North Western WCZ. Since the Project involved only
land-based works and no marine works would be required, a 500m assessment area
is therefore considered adequate. The assessment area was further extended to
cover other areas such as stream courses, existing and planned drainage system
as well as other water system(s) in the vicinity, which may be affected by the
Project and have a bearing on the environmental acceptability of the Project.
5.5.2
The WSRs that may be affected by the Project
have been identified. Potential
sources of water quality impact that may arise during the construction works
and operational activities were described and quantified, where applicable,
including point discharges and non-point sources to surface water runoff,
sewage from workforce and future users as well as industrial wastewater in
accordance with the requirement stipulated in Section 4 (vii) of Appendix D of
the Study Brief. This task included
identifying pollutants from point discharges and non-point sources that could
affect the quality of surface water runoff. All the identified sources of potential
water quality impact were then evaluated and their impact significance
determined. The need for mitigation measures to reduce any identified adverse
impacts on water quality to acceptable levels was also determined.
5.6.1
The Project will only involve land-based
construction works with neither marine works nor works at streams and/or
watercourses to be undertaken. Also, no diversion, reconstruction and
re-alignment of streams and watercourses, as well as existing box culvert will
be required for the Project.
Potential sources of water quality impact associated with the
construction of the Project have been identified and include:
·
General construction activities
·
Construction site runoff;
·
Construction works near channelized
watercourse/ditch;
·
Accidental spillage;
·
Sewage effluent from construction workforce; and
·
Groundwater from Contaminated Areas,
Contaminated Site Runoff and Wastewater from Land Decontamination.
General
Construction Activities
5.6.2
The land-based construction works could have the
potential to cause water pollution.
Various types of construction activities may generate wastewater. These
include general cleaning and polishing, wheel washing, dust suppression and
utility installation. These types of wastewater would contain high
concentrations of suspended solids (SS). Various construction works may also
generate debris and rubbish such as packaging, construction materials and
refuse. Uncontrolled discharge of
site effluents, rubbish and refuse generated from the construction works would
lead to deterioration in water quality.
5.6.3
Considering that excavation and piling works
involved for the Project would generate approximately 21,826m3 land-based
sediment, which require disposal of at the designated disposal areas. As
mentioned in Section 7.5.19, the
excavated sediments will be loaded onto the barge at public barging point,
where the exact location will be determined by the contractor(s). Therefore, no installation of
temporary support facility and dredging or excavation is expected.
Notwithstanding the above, accidental release of excavated sediment during
transport to the disposal areas by barges, will increase the SS and contaminant
level of receiving water and deteriorate water quality.
5.6.4
Adoption of the guidelines and good site
practices for handling and disposal of construction discharges as specified in Section 5.8 would minimize the potential impacts.
Construction Site Runoff
5.6.5
Surface runoff generated from the construction
site may contained increased loads of SS and contaminants. Potential pollution
sources of site runoff may include:
·
Runoff and erosion of exposed bare soil and
earth, drainage channel, earth working area and stockpiles;
·
Release of any bentonite slurries, concrete
washings and other grouting materials with construction runoff or storm water;
·
Wash water from dust suppression sprays and
wheel washing facilities; and
·
Fuel, oil and lubricants from maintenance of
construction vehicles and equipment.
5.6.6
During rainstorms, site runoff would wash away
the soil particles on work areas and areas with the topsoil exposed. The construction runoff is generally
characterized by high concentrations of SS. Release of uncontrolled site runoff
would increase the SS levels, turbidity and cause depletion of dissolved oxygen
levels in the nearby water environment.
Site runoff may also wash away contaminants and therefore cause water
pollution.
5.6.7
Windblown dust would be generated from exposed
soil surfaces in the works areas. It
is possible that windblown dust would fall directly onto the nearby water
bodies when a strong wind occurs.
Dispersion of dust within the works areas may increase the SS levels in
surface runoff causing a potential impact to the nearby sensitive receivers.
5.6.8
Total construction area of about 0.3087 km2,
involving SHD Replanning Works, construction of SHO, western access and local
assesses outside the proposed podium deck would generate surface runoff during
construction phase. For worst case scenario assessment, it is assumed that all
works area is 100% active and paved.
5.6.9
According
to the “DSD Stormwater Drainage Manual”, annual rainfall in Hong Kong is around
2,200mm. However, the EPD study namely "Update on Cumulative Water Quality
and Hydrological Effect of Coastal Developments and Upgrading of Assessment
Tool (Update Study)” suggested that only rainfall events of sufficient
intensity and volume would give rise to runoff and that runoff percentage is
about 44% and 82% for dry and wet season, respectively. Therefore, only 1,386mm of 2,200mm
annual rainfall would be considered as effective rainfall that would generate
runoff (i.e. 1386mm=2200mm×(82%+44%)/2).
5.6.10
More
surface runoff would be generated from the paved area than
from the unpaved area. Making reference
to the “DSD Stormwater Drainage Manual”, about 0.9
as the runoff coefficient for paved areas is assumed. The average daily runoff
generated from the construction area is estimated to be
about 1,055 m3/day (= 0.9 × 1386mm/year × 0.3087km2).
Given that SHD, with an area of about 0.3 km2 is
an existing facility and any runoff generated would be discharged to existing
drainage system, hence adverse water quality impact is not expected.
5.6.11
It is also anticipated that with the
implementation of appropriate measures to control runoff and drainage from the
construction site, disturbance of water bodies would be avoided and
deterioration in water quality would be minimal. Thus, unacceptable impacts on the water
quality are not expected, provided that the relevant mitigation measures as
specified in Section 5.8 are properly implemented.
Construction Works
near Channelized Watercourse / Ditch
5.6.12 As
mentioned in Section 6.6.1,
upgrading and reprovision of sewerage network would be required for this
Project, with a rising main size of 200mm dia. which is the same size of the
existing rising main and a standby 200mm dia. rising main for conveying the flow
from the on-site pumping station to public sewerage system. Pollution of inland waters may occur due
to potential release of construction wastes and wastewater from the adjacent
works area. Construction wastes and wastewater are generally characterized by
high concentration of SS and elevated pH.
Adoption of good housekeeping and mitigation measures would reduce the
generation of construction wastes and potential water quality impact. The
implementation of measures to control site run-off will be adopted for the
works adjacent to the inland channelized watercourse/ditch in order to prevent
run-off with high levels of SS from entering the water environment. With the
implementation of Best Management Practices and provision of mitigation
measures as specified in ProPECC Note PN 1/94 “Construction Site Drainage” and
ETWB TC (Works) No. 5/2005 “Protection of natural streams / rivers from adverse
impacts arising from construction works” where applicable (as detailed in Section 5.8) , it is anticipated that water quality impacts would be
minimised.
Accidental
Spillage
5.6.13
The use of chemicals such as engine oil and
lubricants, and their storage as waste materials has the potential to create
impacts on the water quality if spillage occurs and enters adjacent water
environment. Waste oil may
infiltrate into the surface soil layer, or runoff into the nearby water
environment, increasing hydrocarbon levels. The potential impacts could however be
mitigated by practical mitigation measures and good site practices (as given in
Section 5.8).
Sewage Effluent
from Construction Workforce
5.6.14
During the construction of the Project, the
workforce on site will generate sewage effluents, which are characterized by
high levels of BOD, ammonia and E. coli counts. Based on the DSD Sewerage
Manual, the sewage production rate for construction workers is estimated at
0.35 m3 per worker per day. For every 100 construction workers
working simultaneously at the construction site, about 35 m3 of
sewage would be generated per day. Potential water quality impacts upon the
local drainage and fresh water system may arise from these sewage effluents, if
uncontrolled.
5.6.15
However, this temporary sewage can be adequately
treated by interim sewage treatment facilities, such as portable chemical
toilets. Provided that sewage is not discharged directly into storm drains or
inland waters adjacent to the construction site, temporary sanitary facilities
would be used and properly maintained, and mitigation measures as recommended
in Section 5.8 are adopted as far as practicable, it is unlikely that sewage
generated from the sites would have a significant water quality impact.
Groundwater from
Contaminated Areas, Contaminated Site Runoff and Wastewater from Land
Decontamination
5.6.16 It
is identified that some of the construction works areas might have land
contamination issues. Proper land
contamination remediation and mitigation measures are proposed in Section 8. Any contaminated material disturbed, or
material which comes into contact with the contaminated material, has the
potential to be washed with site runoff into drainage system. Any wastewater discharge from land
decontamination processes could also adversely affect the nearby water
environment. Excavated contaminated
materials would be properly stored, housed and covered to avoid generation of
contaminated runoff. Open stockpiling of contaminated materials will not be
allowed. Any contaminated site runoff and wastewater from land decontamination
activities will be properly treated and disposed in compliance with the
requirements of the TM-DSS.
Mitigation measures for contaminated site runoff and wastewater from
land decontamination are recommended in Section
5.8. With proper
implementation of the recommended mitigation measures, the potential water
quality impacts arising from the land decontamination works would be
minimised.
Operational
Phase
5.6.18
Potential sources of water quality impact
associated with the operation of the Project include:
·
Sewage and wastewater discharge;
·
Emergency Discharge from Sewage Pumping Station;
·
Surface and road runoff from SHO and
Reprovisioned SHD; and
·
Accidental spillage of chemicals.
Sewage and Wastewater Discharge
5.6.19
Sewage effluent generated from the operation of
SHO and Reprovisioned SHD and wastewater discharged from the train washing,
heavy cleaning and maintenance facilities in Reprovisioned SHD would be the key
potential sources of water pollution. With reference to the assessment of
sewerage and sewage treatment implications for the Project provided in Section 6, the average sewage flow from
the SHO and Reprovisioned SHD is estimated to be about 1,229m3 per
day. Adequate sewerage and sewage
treatment facilities will be provided for the Project development to avoid
direct discharge of sewage and wastewater to the nearby drainage system and
water environment. Assessment on the sewerage and sewage treatment implications
for the Project is provided in Section 6.
Emergency Discharge from Sewage Pumping
Station
5.6.20 New
Sewage Pumping Station (SPS) including two duty and one standby pump is
proposed under the Project as shown in Drawing
No. NEX1062/S/SHD/ACM/Z10/181. The normal operation of this new SPS
would actually have beneficial effect through the enhancement of the efficiency
of the sewerage system. In order to
minimize the consequences of pump and/or power failure, various precautionary
measures as recommended in Sections
6.6.3 to 6.6.5 will be
incorporated in the design of the proposed SPS. The recommended precautionary measures
is summarized below:
·
Provision of twin rising mains (Section 6.6.1 refers);
·
Provision of two duty pumps and one standby pump
at SPS (Section 6.6.3 refers);
·
Provision of dual-feed power supply (Section 6.6.3 refers);
·
Provision of emergency storage tank with 1-hour
ADWF capacity (Section 6.6.3
refers);
·
Provision of either ductile iron (DI)
pipes or high-density polyethylene pipe (HDPE) and fittings with sufficient
strength to resist the pumping pressure and external load. Thrust blocks,
supports or relevant fittings should also be provided to avoid pipe bursting
and leakage (Section 6.6.4 refers);
·
Provision of Main Control System (MCS) to
provide real-time notification of alert signal in emergency situation (Section 6.6.5 refers);
·
Provision of regular inspection, routine maintenance
and repair from qualified personnel (Section
6.6.5 refers);
·
Provision of on-site staff for 24-7 emergency
repair service in case of emergency situations (Section 6.6.5 refers) to allow for resumption of pumping system
operation within 1 hour after emergency repair in the event of any incident;
·
Tankering
away, if necessary, would be conducted as a
last resort for proper disposal at public sewage treatment works to maximize
buffer for emergency storage as far as practicable, in cases where the broken-down
plant cannot be recovered in time to prevent the outflow of raw sewage (Section 6.6.5 refers); and
·
Preparation of Operation and Maintenance
(O&M) Manual to standardize operation and maintenance procedures (Section 6.6.5 refers).
5.6.21
With incorporation of the recommended preventive
measures and no emergency discharge outlet at the sewage pumping station, no emergency
sewage discharge during the operation of SHO and Reprovisioned SHD is
anticipated, and thus the potential water quality impact in the extremely
remote event that emergency sewage discharge does occur would be minimized.
Surface and Road
Runoff from SHO and Reprovisioned SHD
5.6.23
The
ultimate Reprovisioned SHD and partial SHO will be located underneath the
podium deck of SHD Topside Development.
Upon completion for each phase of SHD Topside Development, only
positive impact with no worsen situation than current condition, in term of
surface runoff production is expected.
The surface
runoff collected from the podium deck of SHD Topside Development would be conveyed
by its drainage system, then to downpipes
in the utilities riser and subsequent manholes and gravity drains and finally be discharged to the
existing box culverts to the marine waters.
Surface runoff generated from the podium deck has been
assessed
in the SHD Topside Development EIA, while surface
runoff associated with the operation of this Project would only be generated
from partial SHO, western access and local accesses outside the podium deck. As such, an area of about 0.0087 km2
is anticipated to generate surface runoff when in operation.
5.6.24 The average daily runoff
generated from the area of SHO and Reprovisioned SHD is
estimated to be about 30 m3/day (= 0.9 × 1386mm/year × 0.0087km2). Pollution loading was calculated and
tabulated in Table
5.4 below. The water quality as well as
hydrology and flow regime impacts, if any, would be limited and localized due
to only about 30 m3/day surface runoff with insignificant addition
pollution loadings will be generated, and hence no adverse impacts would be
expected.
Table 5.4 Pollution
Loading from Surface Run-off
|
Paved Area
(km2)
|
Rainfall Intensity
(mm/year)
|
Average Daily Runoff (m3/day)
|
1-Silt Trap Removal
Efficiency
|
Pollutant Concentration
(mg/L)(1)
|
Loading (kg/day)
|
|
BOD5
|
TN
|
TP
|
BOD5
|
TN
|
TP
|
|
0.0087
|
1386
|
30
|
0.8
|
22.5
|
2.0
|
0.2
|
0.54
|
0.048
|
0.0048
|
Notes:
(1)
Data source: Update on Cumulative Water Quality and
Hydrological Effect of Coastal Developments and Upgrading of Assessment Tool – Pollution
Loading Inventory Report (EPD).
5.6.25
Storm
water control measures including Best Management Practices (BMPs), would be
implemented within the development site as described in Section 5.8.
It is expected that with proper implementation of the recommended
measures, the water quality impact due to the non-point source pollution from
the Project would be minimized.
Accidental
Spillage of Chemicals
5.6.26
A number of chemicals such as paints, lubricants
and used batteries would be stored onsite and be used for maintenance
activities in SHO and Reprovisioned SHD. Adverse water quality impacts arising
from chemical spillage can be minimised by appropriate storage management and
drainage system design as recommended in Section
5.8.
General Construction Activities and
Construction Site Runoff
·
prevent or minimise the likelihood of pollutants
(generated from construction activities) being in contact with rainfall or
runoff; and
·
abate pollutants in the stormwater surface
runoff prior to the discharge of surface runoff to the nearby water bodies.
Construction
Site Runoff
5.8.2
It is important that Best Management Practices (BMPs)
of mitigation measures in controlling water pollution and good site management,
as specified in the ProPECC PN 1/94 “Construction Site Drainage” are followed,
where applicable, to prevent runoff with high level of SS from entering the
surrounding waters.
5.8.4
Surface and road runoff from construction sites
should be discharged into storm drains via adequately designed sand/silt
removal facilities such as sand traps, silt traps and sedimentation
basins. Channels or earth bunds or
sand bag barriers should be provided on site during construction works to
properly direct stormwater to such silt removal facilities. Perimeter channels
should be provided on site boundaries where necessary to intercept storm runoff
from outside the site so that it will not wash across the site. Catchpits and
perimeter channels should be constructed in advance of site formation works and
earthworks.
5.8.5
Silt removal facilities, channels and manholes
should be maintained and the deposited silt and grit should be removed
regularly, at the onset of and after each rainstorm to prevent local
flooding. Any practical options for
the diversion and re-alignment of drainage should comply with both engineering
and environmental requirements in order to provide adequate hydraulic capacity
of all drains.
5.8.6
Construction works should be programmed to
minimize soil excavation works in rainy seasons (April to September). If soil excavation cannot be avoided in
these months or at any time of year when rainstorms are likely, for the purpose
of preventing soil erosion, temporary exposed slope surfaces should be covered
e.g. by tarpaulin, and temporary access roads should be protected by crushed
stone or gravel, as excavation proceeds.
Intercepting channels should be provided (e.g. along the crest / edge of
excavation) to prevent storm runoff from washing across exposed soil
surfaces. Arrangements should
always be in place in such a way that adequate surface protection measures can
be safely carried out well before the arrival of a rainstorm.
5.8.7
Earthworks final surfaces should be well
compacted and the subsequent permanent work or surface protection should be
carried out immediately after the final surfaces are formed to prevent erosion
caused by rainstorms. Appropriate
drainage like intercepting channels should be provided where necessary.
5.8.8
Measures should be taken to minimize the ingress
of rainwater into trenches. If excavation of trenches in wet seasons is
necessary, they should be dug and backfilled in short sections. Rainwater pumped out from trenches or
foundation excavations should be discharged into storm drains via silt removal
facilities.
5.8.9
If bentonite slurries are required for any
construction works, they should be reconditioned and reused wherever
practicable to minimise the disposal volume of used bentonite slurries. Temporary enclosed storage locations
should be provided on-site for any unused bentonite that needs to be
transported away after the related construction activities are completed.
Requirements as stipulated in ProPECC Note PN 1/94 should be closely followed
when handling and disposing bentonite slurries.
5.8.10 Open
stockpiles of construction materials (e.g. aggregates, sand and fill material)
on sites should be covered with tarpaulin or similar fabric during
rainstorms.
5.8.11 Manholes
(including newly constructed ones) should always be adequately covered and
temporarily sealed so as to prevent silt, construction materials or debris from
getting into the drainage system, and to prevent storm runoff from getting into
foul sewers. Discharge of surface
runoff into foul sewers must always be prevented in order not to unduly
overload the foul sewerage system.
·
Loading of the excavated sediment to the barge
shall be controlled to avoid splashing and overflowing of the sediment slurry
to the surrounding water.
·
The barge transporting the sediments to the
designated disposal sites shall be equipped with tight fitting seals to prevent
leakage and shall not be filled to a level that would cause overflow of
materials or laden water during loading or transportation.
·
Monitoring of the barge loading shall be
conducted to ensure that loss of material does not take place during
transportation. Transport barges or vessels shall be equipped with automatic
self-monitoring devices as specified by the Director of Environmental
Protection (DEP).
Water
for Bored Piling Works
5.8.14
Water used in ground boring and drilling for
site investigation or rock / soil anchoring should be re-circulated as far as
practicable after sedimentation. When there is a need for final disposal, the
wastewater should be discharged into storm drains via silt removal facilities.
Wheel
Washing Water
5.8.15
Wash-water from wheel washing facility should
have been treated by silt removal facilities before discharging into storm
drains. Treated wash-water could be used as dust suppression measures as far as
practicable. The section of access road between the wheel washing bay and the
public road should be paved to reduce vehicle tracking of soil and to prevent
silty water from entering public road and drains.
Construction Works near Channelized
Watercourse/Ditch
·
The use of less or smaller construction plants
may be specified in works area close to the inland water bodies.
·
Temporary storage of material (e.g. equipment,
filling materials, chemicals and fuel) and temporary stockpile of construction
materials should be located well away from watercourse/ditch when carrying out
of the construction works.
·
Stockpiling of construction materials and dusty
materials should be covered and located away from any watercourse/ditch.
·
Construction debris and spoil should be covered
up and / or disposed of as soon as possible to avoid being washed into the
nearby water receivers.
·
Construction activities, which generate large
amount of wastewater, should be carried out in a distance away from the
watercourse/ditch, where practicable.
·
Construction effluent, site run-off and sewage
should be properly collected and / or treated.
Accidental Spillage of Chemicals
5.8.18 Any
service shop and maintenance facilities should be located on hard standings
within a bunded area, and sumps and oil interceptors should be provided. Maintenance of vehicles and equipment
involving activities with potential for leakage and spillage should only be
undertaken within the areas appropriately equipped to control these discharges.
·
Suitable containers should be used to hold the
chemical wastes to avoid leakage or spillage during storage, handling and
transport.
·
Chemical waste containers should be suitably labelled,
to notify and warn the personnel who are handling the wastes, to avoid
accidents.
·
Storage area should be selected at a safe
location on site and adequate space should be allocated to the storage area.
Sewage Effluent from Construction Workforce
Groundwater from Contaminated Areas,
Contaminated Site Runoff and Wastewater from Land Decontamination
5.8.23
No direct discharge of groundwater from
contaminated areas should be adopted.
Prior to any excavation
works within the potentially contaminated areas, the baseline groundwater
quality in these areas should be reviewed based on the past relevant site
investigation data and any additional groundwater quality measurements to be
performed with reference to Guidance
Note for Contaminated Land Assessment and Remediation and 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, this contaminated groundwater should be either
properly treated or properly recharged into the ground in compliance with the
requirements of the TM-DSS. If wastewater treatment is to be deployed for
treating the contaminated groundwater, the wastewater treatment unit shall
deploy suitable treatment processes (e.g. oil interceptor / activated carbon)
to reduce the pollution level to an acceptable standard and remove any
prohibited substances (such as total petroleum hydrocarbon) to an undetectable
range. All treated effluent from the wastewater treatment plant shall meet the
requirements as stated in the TM-DSS and should be either discharged into the
foul sewers or tankered away for proper disposal.
Operational
Phase
Sewage and wastewater discharge
5.8.25
Sewage effluents generated from the operation of
Reprovisioned SHD and SHO should be properly
collected and diverted to public sewers for proper treatment and disposal. Wastewater
generated by train washing, heavy cleaning and maintenance facilities should be
collected and diverted to the depot sewerage system. Standard oil/grit
interceptors/chambers should be provided where necessary to remove the oil,
lubricants, grease, silt and grit from wastewater generated from train washing,
heavy cleaning and maintenance facilities before discharge to the depot
sewerage system. Discharge
of any effluent to the public sewers will be subject to control under the WPCO
and the relevant dischargers shall apply to EPD for a discharge licence for
discharge of commercial and industrial effluent and the discharge quality must
satisfy all the standards listed in the TM-DSS and meet the requirements
specified in the discharge licence. Assessment of the sewage and sewerage
implications for this Project is presented in Section 6.
5.8.26
The practices outlined in ProPECC PN 5/93 for
handling, treatment and disposal of operational stage effluent should also be
adopted where applicable.
Emergency Discharge from Sewage Pumping
Station
·
Provision of twin rising mains;
·
Provision of two duty pumps and one standby pump
at SPS;
·
Provision of dual-feed power supply;
·
Provision of emergency storage tank with 1-hour
ADWF capacity;
·
Provision of either ductile iron (DI)
pipes or high-density polyethylene pipe (HDPE) and fittings with sufficient
strength to resist the pumping pressure and external load. Thrust blocks, supports
or relevant fittings should also be provided to avoid pipe bursting and
leakage;
·
Provision of Main Control System (MCS) to
provide real-time notification of alert signal in emergency situation;
·
Provision of regular inspection, routine
maintenance and repair from qualified personnel;
·
Provision of on-site staff for 24-7 emergency
repair service in case of emergency situations to allow for resumption of
pumping system operation within 1 hour after emergency repair in the event of
any incident;
·
Tankering
away, if necessary, would be conducted as a
last resort for proper disposal at public sewage treatment works to maximize
buffer for emergency storage as far as practicable, in cases where the
broken-down plant cannot be recovered in time to prevent the outflow of raw
sewage; and
·
Preparation of Operation and Maintenance
(O&M) Manual to standardize operation and maintenance procedures.
Best Storm Water Management Practices and
Storm Water Pollution Control Plan
Design
Measures
5.8.30
The drainage system should be designed to avoid
flooding.
Devices
and Facilities
5.8.31
Screening facilities such as standard gully
grating and trash grille, with spacing which is capable of screening large
substances such as fallen leaves and rubbish should be provided at the inlet of
drainage system.
5.8.32
Road gullies with standard design and silt traps
and oil interceptors should be incorporated during the detailed design to
remove particles present in stormwater runoff, where appropriate.
Administrative
Measures
5.8.33
Good
management measures such as regular cleaning and sweeping of road surface
are suggested. The road surface cleaning should also be carried out prior to
occurrence rainstorm.
Accidental Spillage of Chemicals
5.8.35
Chemical should be properly stored on site
within bunded area and separate drainage system should be provided as
appropriate to avoid any spilled chemicals from entering the storm drain in
case of accidental spillage. Also, adequate tools for cleanup of spilled
chemicals should be provided on site and appropriate training shall be provided
to staffs to further prevent potential adverse water quality impacts.
Construction
Phase
5.11.1 The key issue from the
land-based construction activities would be the potential release of construction site
runoff
from surface works areas, wastewater from general construction activities, pollution
to nearby watercourse/ditch, accidental spillage and sewage from construction
workforce, as well as the potential groundwater from contaminated areas, if
any, contaminated site runoff and wastewater from land decontamination. Minimisation of water quality deterioration could be achieved through implementing
adequate mitigation measures. Regular site inspections should be undertaken
routinely to inspect the construction activities and works areas in order to
ensure the recommended mitigation measures are properly implemented.
Operational
Phase
5.11.2
The key source of potential impact on water
quality during the operational phase will be the sewage and wastewater
generated from the SHO and Reprovisioned SHD. However, no adverse water quality
impact associated with the operational phase would be anticipated, provided
that adequate sewerage and sewage treatment facilities are properly implemented to
accommodate all the sewage effluents.
5.11.3 In
view of the potential adverse effect of emergency discharge from sewage pumping
station, various precautionary measures are proposed to be incorporated in the
design of the SPS to avoid consequences of pump and/or power failure to the
maximum practicable extent.
With the incorporation of the precautionary measures, no emergency sewage discharge during the
operation of SHO and Reprovisioned SHD is anticipated and the potential
water quality impact in the unlikely event that discharge does occur would be
minimised.
5.11.4 Another
source of potential impact during the operational phase will be the runoff or
non-point source pollution from the partial SHO, western access and local
accesses outside the podium deck as well as accidental spillage of
chemical. Storm water control
measures including adequate storm drainage system with suitable pollutant
removal devices and best storm water management practices are recommended for
the Project to minimise the non-point source pollution. Proper storage and
management, as well as the provision of a separate drainage system are
recommended to avoid pollution from chemical spillage. With proper
implementation of the recommended mitigation measures, it is anticipated that
the water quality impacts associated with the non-point source discharge and
chemical spillage would be minimised.