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 were
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
|
|
Aesthetic appearance
|
(a)
Waste discharges shall cause no
objectionable odours or discolouration of the water
|
Whole zone
|
|
(b)
Tarry residues, floating wood,
articles made of glass, plastic, rubber or of any other substances should be
absent
|
Whole zone
|
|
(c)
Mineral oil should not be visible
on the surface. Surfactants should not give rise to a lasting foam
|
Whole zone
|
|
(d)
There should be no recognisable
sewage-derived debris
|
Whole zone
|
|
(e)
Floating, submerged
and semi-submerged objects or a size likely to interfere with the free
movement of vessels, or cause damage to vessels, should be absent
|
Whole zone
|
|
(f)
Waste discharges shall not cause
the water to contain substances which settle to form objectionable deposits
|
Whole zone
|
|
Dissolved Oxygen (DO) within 2 m of the seabed
|
Not less than 2mg/L for 90% of the sampling occasions
during the whole year
|
Marine waters
|
|
Depth-averaged DO
|
(a)
Waste discharges shall not cause
the level of dissolved oxygen to fall below 4mg/L for 90% of the sampling
occasions during the whole year
|
Marine waters
|
|
(b)
Waste discharges shall not cause
the level of dissolved oxygen to be less than 4mg/L
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones, Water Gathering Ground Subzones and other inland waters
|
|
pH
|
(a)
Within the range of 6.5-8.5 units.
Waste discharge shall not cause the natural pH range to be extended by more
than 0.2 unit.
|
Marine waters excepting Bathing Beach Subzones
|
|
(b)
Waste discharges shall not cause
the pH of the water to exceed the range of 6.5-8.5 units.
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
|
|
(c)
Within the range of 6.0-9.0 units
|
Other inland waters
|
|
(d)
Within the range of 6.0-9.0 units
for 95% of samples collected during the whole year. Waste
discharges shall not cause the natural pH range to be extended by more than
0.5 unit.
|
Bathing Beach Subzones
|
|
Salinity
|
Waste discharges shall not cause
the natural ambient salinity level to change by more than 10%
|
Whole zone
|
|
Temperature
|
Waste discharges shall not cause
the natural daily temperature range to change by more than 2.0 degree
Celsius.
|
Whole zone
|
|
Suspended solids (SS)
|
(a)
Waste discharges shall neither
cause the natural ambient level to be raised by more than 30% nor give rise
to accumulation of suspended solids which may adversely affect aquatic
communities
|
Marine waters
|
|
(b)
Waste discharges shall not cause
the annual median of suspended solid to exceed 20mg/L
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
|
|
(c)
Waste discharges shall not cause
the annual median of suspended solid to exceed 25mg/L
|
Other inland waters
|
|
Unionized Ammonia (UIA)
|
Arithmetic mean should not be
more than 0.021 mg/L
|
Whole zone
|
|
Nutrients
|
(a)
Shall not be present in quantities
sufficient to cause excessive or nuisance growth of algae or other aquatic
plants
|
Marine waters
|
|
(b)
Without limiting the generality of objective
(a) above, the level of inorganic nitrogen should not exceed 0.3 mg/L,
expressed as annual mean
|
Castle Peak Bay Subzone
|
|
(c)
Without limiting the generality of
objective (a) above, the level of inorganic nitrogen should not exceed 0.5
mg/L, expressed as annual mean
|
Marine waters excepting Castle Peak Bay Subzone
|
|
Bacteria
|
(a)
The level of E.coli
should not exceed 610 per 100 mL, calculated as the geometric mean of all
samples collected in a calendar year.
|
Secondary Contact Recreation Subzones
|
|
(b)
The level of E.coli
should be less than 1 per 100 mL, calculated as the running median of the
most recent 5 consecutive samples taken at intervals of between 7 and 21
days.
|
Tuen Mun (A) and Tuen Mun (B)
Subzones and Water Gathering Ground Subzones
|
|
(c)
The level of E.coli
should not exceed 1000 per 100 mL, calculated as the running median of the
most recent 5 consecutive samples taken at intervals of between 7 and 21
days.
|
Tuen Mun (C) Subzone and other inland waters
|
|
(d)
The level of E.coli
should 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 a calendar month at intervals of between 3 and 14 days.
|
Bathing Beach Subzones
|
|
Colour
|
(a)
Waste discharges shall not cause
the colour of water to exceed 30 Hazen units
|
Tuen Mun (A) and Tuen Mun (B)
Subzones and Water Gathering Ground Subzones
|
|
(b)
Waste discharges shall not cause
the colour of water to exceed 50 Hazen units
|
Tuen Mun (C) Subzone and other inland waters
|
|
5-Day Biochemical Oxygen Demand (BOD5)
|
(a)
Waste discharges shall not cause to
exceed 3 mg/L
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
|
|
(b)
Waste discharges shall not cause to
exceed 5 mg/L
|
Other inland waters
|
|
Chemical Oxygen Demand (COD)
|
(a)
Waste discharges shall not cause to
exceed 15 mg/L
|
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
|
|
(b)
Waste discharges shall not cause to
exceed 30 mg/L
|
Other inland waters
|
|
Toxins
|
(a)
Waste discharges shall not cause
the toxins in water to attain such levels as to produce significant toxic,
carcinogenic, mutagenic or teratogenic effects in
humans, fish or any other aquatic organisms
|
Whole zone
|
|
(b)
Waste discharges shall not cause a risk
to any beneficial use of the aquatic environment
|
Whole zone
|
|
Phenol
|
Shall not be present in such
quantities as to produce a specific odour, or in concentration greater than
0.05 mg/L as C6H5OH
|
Bathing Beach Subzones
|
|
Turbidity
|
Waste discharges 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
The Professional Persons
Environmental Consultative Committee Practice Note on “Construction Site
Drainage” (ProPECC PN 1/94) published by EPD
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 ProPECC PN 1/94 should be followed as far as possible
during construction to minimize the water quality impact due to construction
site drainage.
5.2.5
The “Drainage Plans Subject
to Comment by the EPD” (ProPECC PN 5/93) provides
guidelines and practices for handling, treatment and
disposal of various effluent discharges to stormwater drains and foul sewers
during the operational phase. The
design of site drainage and disposal of various site effluents generated within
the new development area should follow the relevant guidelines and practices as
given in the ProPECC PN 5/93.
Technical Circular
5.2.6
Environment, Transport and
Works Bureau (ETWB) Technical Circular (Works) No. 5/2005 (ETWB TC (Works) No.
5/2005) provides an administrative framework to better protect all natural
streams/rivers from the impacts of construction works. The procedures promulgated under this
Circular aim to clarify and strengthen existing measures for protection of
natural streams/rivers from government projects and private developments. The guidelines and precautionary
mitigation measures given in the ETWB TC (Works) No. 5/2005 should be followed
as far as possible to protect the inland watercourse at or near the Project
area during the construction phase.
Hong Kong Planning
Standards and Guidelines
5.2.7
The Hong Kong Planning
Standards and Guidelines (HKPSG), Chapter 9 (Environment), provides additional
guidelines against water pollution for sensitive use such as aquaculture and
fisheries zones, bathing waters and other contact recreational waters.
WSD Water Quality
Criteria for Flushing Water Intakes
5.2.8
Water Supplies Department (WSD)
has established a set of criteria for seawater uptake at flushing water intake
points. These requirements are listed in Table 5.2. These water quality
criteria were used as the water quality assessment criteria for the Project.
Table 5.2 WSD Standards at
Flushing Water Intakes
|
Parameters
(in mg/L unless otherwise stated)
|
Water
Quality Criteria
|
|
Colour (H.U.)
|
< 20
|
|
Turbidity (N.T.U.)
|
< 10
|
|
Threshold Odour No. (T.O.N.)
|
< 100
|
|
Ammoniacal Nitrogen
|
< 1
|
|
Suspended Solids
|
< 10
|
|
Dissolved Oxygen
|
> 2
|
|
Biochemical Oxygen Demand
|
< 10
|
|
Synthetic Detergents
|
< 5
|
|
E. coli (no./100mL)
|
< 20, 000
|
Assessment Area
5.3.1
The Assessment Area for this
water quality impact assessment includes areas within 500 m from the Project
boundary including works sites and works areas. The Assessment Area fall within the North Western WCZ as designated under the WPCO (Cap. 358).
The baseline condition of water bodies in the Study Area were established with
reference to routine river and marine water quality monitoring data collected
by EPD. Descriptions of the baseline conditions provided in the subsequent
sections are extracted from the EPD’s reports River Water Quality in Hong
Kong in 2020 and Marine Water Quality in Hong Kong in 2020 which
contain the latest information published by EPD on river and marine water
quality.
Marine Water
5.3.2
The EPD monitoring data
collected in 2020 are summarized in Table 5.3 for Pearl Island (NM2) and Tuen Mun Typhoon Shelter (NT1) and the respective locations
of these monitoring stations are illustrated in Figure No. C1502/C/TME/ACM/M53/101.
Descriptions of the baseline water quality conditions provided in the
subsequent section are based on the EPD’s report Marine Water Quality in Hong
Kong in 2020.
5.3.3
The North
Western WCZ attained an overall WQO compliance rate of 67%, with the DO
and NH3-N WQOs fully met.
The TIN level, however, could not meet the WQO under the influence of
high background level in the Pearl River Estuary. Notwithstanding, the red
tides occurrence in these waters remained very low even in the presence of rich
nutrients, likely owing to strong tidal flushing hindering the proliferation of
phytoplankton community in the area.
Table 5.3 Summary
Statistics of Marine Water Quality of North Western
WCZ Collected by EPD in 2020
|
Parameters
|
Pearl
Island
|
Tuen Mun
Typhoon Shelter
|
WPCO
WQO
(in marine waters)
|
|
NM2
|
NT1
|
|
Temperature
(℃)
|
25.9
(20.5 - 29.4)
|
26.1
(20.9 - 29.8)
|
Not cause more than ±2℃ in daily temperature range
|
|
Salinity
|
26.6
(19.8 - 31.6)
|
26.1
(16.8 - 31.7)
|
Not to cause more than ±10%
change
|
|
Dissolved Oxygen
(mg/L)
|
Depth Average
|
5.8
(4.7 - 7.2)
|
6.4
(4.8
- 8.2)
|
Not less than 4 mg/L
|
|
Bottom
|
5.6
(4.0 - 7.5)
|
6.1
(4.8
- 9.1)
|
Not less than 2 mg/L
|
|
Dissolved Oxygen
(% Saturation)
|
Depth Average
|
82
(68 - 104)
|
91
(69
- 119)
|
-
|
|
Bottom
|
79
(58 - 108)
|
87
(69
- 131)
|
|
|
pH
|
7.9
(7.7 - 8.1)
|
8.0
(7.7
- 8.3)
|
Marine waters excepting Bathing
Beach Subzones: 6.5 - 8.5 (±0.2 from natural range)
Bathing Beach Subzones: 6.0 - 9.0 (±0.5 from natural range)
|
|
Secchi Disc Depth (m)
|
2.1
(1.7 - 2.6)
|
1.8
(1.6
- 1.9)
|
|
|
Turbidity (NTU)
|
4.3
(1.9 - 7.8)
|
5.4
(3.3
- 7.5)
|
-
|
|
Suspended Solids
(mg/L)
|
7.1
(2.4 - 12.7)
|
12.1
(4.4
- 21.0)
|
Not more than 30% increase
|
|
5-day Biochemical Oxygen Demand
(mg/L)
|
0.6
(0.3 - 1.0)
|
1.1
(0.5
- 3.0)
|
-
|
|
Ammonia-nitrogen
(mg/L)
|
0.076
(0.031 - 0.177)
|
0.110
(0.055
- 0.195)
|
-
|
|
Unionised Ammonia
(mg/L)
|
0.003
(0.001 - 0.007)
|
0.005
(0.002
- 0.007)
|
Not more than 0.021 mg/L
|
|
Nitrite-nitrogen
(mg/L)
|
0.077
(0.023 - 0.150)
|
0.062
(0.022
- 0.135)
|
-
|
|
Nitrate-nitrogen
(mg/L)
|
0.469
(0.157 - 0.757)
|
0.384
(0.150
- 0.580)
|
-
|
|
Total Inorganic Nitrogen
(mg/L)
|
0.62
(0.28 - 0.90)
|
0.56
(0.29
- 0.69)
|
Castle Peak Bay Subzone: Not
more than 0.3 mg/L
Marine waters excepting Castle
Peak Bay Subzone: Not more than 0.5 mg/L
|
|
Total Kjeldahl
Nitrogen
(mg/L)
|
0.32
(0.07 - 0.62)
|
0.43
(0.28
- 0.68)
|
|
|
Total Nitrogen
(mg/L)
|
0.87
(0.57 - 1.08)
|
0.88
(0.59
- 1.03)
|
-
|
|
Ortho-phosphate
(mg/L)
|
0.022
(0.015 - 0.033)
|
0.014
(0.012
- 0.015)
|
-
|
|
Total phosphorus
(mg/L)
|
0.08
(0.03 - 0.24)
|
0.06
(0.05
- 0.07)
|
-
|
|
Silica (as SiO2)
(mg/L)
|
2.59
(0.54 - 4.27)
|
2.28
(0.51
- 4.25)
|
|
|
Chlorophyll-a
(µg/L)
|
3.8
(0.5 - 22.3)
|
7.9
(1.1
- 33.0)
|
-
|
|
E. coli
(cfu/100mL)
|
36
(13 -
140)
|
110
(12
- 730)
|
Secondary Contact Recreation
Subzones: Annual geometric mean not to exceed 610 cfu/100mL
Bathing Beach Subzones:
Geometric mean of March to October samples not to exceed 180 cfu/100mL
|
|
Faecal Coliforms
(cfu/100mL)
|
87
(24 - 590)
|
850
(110
- 18000)
|
-
|
|
|
|
|
|
|
|
Notes:
1.
Data source: EPD Marine Water
Quality in Hong Kong in 2020
2.
Except as specified, data
presented are depth-averaged values calculated by taking the means of three
depths: surface, mid-depth and bottom.
3.
Data presented are annual
arithmetic means of depth-averaged results except for E.coli and faecal coliforms that
are annual geometric means.
4.
Data in brackets indicate the
ranges.
Inland Water
5.3.4
The water quality monitoring
results at EPD’s river water quality monitoring stations at Tuen
Mun River, TN3, TN5 and TN6, are shown in Table 5.4 and the locations of these
stations are illustrated in Figure No. C1502/C/TME/ACM/M53/101. According to the EPD’s publication River
Water Quality in Hong Kong in 2020, the compliance rate of Tuen Mun River was 84% in 2020.
5.3.5
All industrial discharges are
controlled under the WPCO. Discharges from domestic and commercial premises are
connected to public sewers. Construction of a trunk sewer to the west of Tuen Mun River was completed in 2015. The Water Quality
Index (WQI) gradings for the upper Tuen Mun River was
rated “Bad”, mainly due to discharges from unsewered rural areas. The WQI
gradings for the middle and lower river sections maintained at “Good” in 2020.
Given that Tuen Mun River is still affected by
effluents discharged from unsewered villages especially in the upstream areas
and expedient connections, sewerage works for villages are being planned and
progressively implemented.
Table 5.4 Summary
Statistics of River Water Quality of Tuen Mun River
Collected by EPD in 2020
|
Parameters
|
Tuen Mun River
|
WPCO WQO
(in
inland waters)
|
|
TN3
|
TN5
|
TN6
|
|
Dissolved Oxygen
(mg/L)
|
4.4
(3.1 - 6.6)
|
4.9
(2.8 - 6.3)
|
4.9
(3.3 - 9.1)
|
Not less than 4 mg/L
|
|
pH
|
7.5
(7.3 - 7.8)
|
7.6
(7.4 - 7.9)
|
7.6
(6.2 - 8.4)
|
Tuen Mun (A), Tuen
Mun (B) and Tuen Mun (C) Subzones and Water
Gathering Ground Subzones: Within the range of 6.5-8.5
Other inland waters: Within the range of 6.0-9.0
|
|
Suspended Solids
(mg/L)
|
8.3
(2.5 - 19.0)
|
8.0
(2.2 - 25.0)
|
6.7
(2.6 - 9.0)
|
Tuen Mun (A), Tuen
Mun (B) and Tuen Mun (C) Subzones and Water
Gathering Ground Subzones: Annual median not to exceed 20 mg/L
Other inland waters: Annual median not to exceed 25
mg/L
|
|
5-Day Biochemical Oxygen Demand (mg/L)
|
4.5
(2.2 - 15.0)
|
3.3
(1.8 - 9.4)
|
3.1
(2.1 - 6.8)
|
Tuen Mun (A), Tuen
Mun (B) and Tuen Mun (C) Subzones and Water
Gathering Ground Subzones: Not to exceed 3 mg/L
Other inland waters: Not to exceed 5 mg/L
|
|
Chemical Oxygen Demand (mg/L)
|
15
(12 - 32)
|
12
(8 - 23)
|
12
(4 - 35)
|
Tuen Mun (A), Tuen
Mun (B) and Tuen Mun (C) Subzones and Water
Gathering Ground Subzones: Not to exceed 15 mg/L
Other inland waters: Not to exceed 30 mg/L
|
|
Oil & Grease (mg/L)
|
<0.5
(<0.5 - <0.5)
|
<0.5
(<0.5 - <0.5)
|
<0.5
(<0.5 - <0.5)
|
-
|
|
E.coli (counts/100mL)
|
9,300
(1,100 - 60,000)
|
7,000
(300 - 57,000)
|
3,400
(10 - 55,000)
|
Tuen Mun (A), Tuen
Mun (B) Subzones and Water Gathering Ground Subzones: Not to exceed 1 cfu/100mL
Tuen Mun (C) Subzones and other
inland waters: Not to exceed 1000 cfu/100mL
|
|
Faecal Coliforms (counts/100mL)
|
91,000
(6,400 - 3,000,000)
|
50 000
(2,000 - 550,000)
|
36,000
(490 - 630,000)
|
-
|
|
Ammonia-Nitrogen (mg/L)
|
0.495
(0.160 - 1.100)
|
0.585
(0.220 - 0.930)
|
0.470
(0.110 - 1.000)
|
-
|
|
Nitrate-nitrogen
(mg/L)
|
0.320
(0.160 - 0.650)
|
0.340
(0.180 - 0.680)
|
0.350
(0.120 - 0.700)
|
-
|
|
Total Kjeldahl Nitrogen
(mg/L)
|
1.05
(0.43 - 1.50)
|
1.25
(0.45 - 2.20)
|
0.95
(0.27 - 1.70)
|
-
|
|
Ortho-Phosphate
(mg/L)
|
0.037
(0.007 - 0.057)
|
0.035
(0.007 - 0.063)
|
0.035
(0.017 - 0.054)
|
-
|
|
Total phosphorus
(mg/L)
|
0.11
(0.07 - 0.17)
|
0.12
(0.06 - 0.13)
|
0.11
(0.05 - 0.19)
|
-
|
|
Sulphide
(mg/L)
|
<0.02
(<0.02 - <0.02)
|
<0.02
(<0.02 - <0.02)
|
<0.02
(<0.02 - 0.02)
|
-
|
|
Aluminium
(µg/L)
|
<50
(<50 - 173)
|
<50
(<50 - 81)
|
<50
(<50 - 254)
|
-
|
|
Cadmium
(µg/L)
|
<0.1
(<0.1 - <0.1)
|
<0.1
(<0.1 - 0.1)
|
<0.1
(<0.1 - <0.1)
|
-
|
|
Chromium
(µg/L)
|
2
(<1 - 3)
|
2
(<1 - 3)
|
2
(<1 - 3)
|
-
|
|
Copper
(µg/L)
|
4
(1 - 8)
|
4
(1 - 7)
|
5
(2 - 7)
|
-
|
|
Lead
(µg/L)
|
<1
(<1 - <1)
|
<1
(<1 - <1)
|
<1
(<1 - <1)
|
-
|
|
Zinc
(µg/L)
|
11
(<10 - 25)
|
13
(<10 - 19)
|
11
(<10 - 27)
|
-
|
|
Flow
(m3/s)
|
NM-
|
NM
|
NM
|
-
|
Remarks:
1.
Data source: EPD River Water
Quality in Hong Kong in 2020
2.
Data presented are in annual
medians of monthly samples; except those for faecal coliforms and E. coli
which are in annual geometric means
3.
Figures in brackets are annual
ranges
4.
cfu – colony forming unit
5.
Values at or below laboratory
reporting limits are presented as laboratory reporting limits
6.
Equal values for annual medians
(or geometric means) and ranges indicate that all data are the same as or below
laboratory reporting limits.
7.
NM indicates no measurement
taken
5.4.1
Water sensitive receivers
(WSRs) within 500m from the Project site boundary are identified and listed
below:
·
Tuen Mun
River Channel (WSR1);
·
Tuen Mun
Typhoon Shelter (WSR2);
·
Secondary contact recreation subzone (WSR3); and
·
Tuen Mun
flushing water intake (WSR4).
5.4.2
The indicative locations of
these WSRs are shown in Figure No. C1502/C/TME/ACM/M53/101.
The WSRs of the offsite Mong Wing Street works area
site boundary are shown in Figure No. C1502/C/TME/ACM/M53/102. The offsite works area is
currently used as site office for the project “Tuen
Mun - Chek Lap Kok Link
(TM-CLKL)”, to support the construction of the Project. This works area would be handed over by
Highways Department in 2024 tentatively. This offsite works area will only
be used as site office, material storage for pre-cast unit (non-dusty material)
and temporary tree nursery, and there would be no excavation works, earthworks
and stockpiling of dusty materials to be conducted at this works area.
5.5.1
The assessment area includes
all areas within 500 m from the Project boundary including works site and works
area, and covers relevant WSRs that have a bearing on the environmental
acceptability of the Project within the North Western WCZ. The
methodology employed to assess potential water quality impacts associated with
the construction and operation of the Project followed the detailed technical
requirements given in Appendix D of the Study Brief. The WSRs that may be affected by the
Project were identified. Potential
sources of water quality impact that may arise during the construction works
and operation activities were described and quantified, where applicable,
including point discharges and non-point sources to surface water runoff,
sewage from workforce and polluted discharge generated from the Project. 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.
·
Construction works at Tuen Mun River Channel;
·
General construction activities;
·
Construction site runoff;
·
Construction works in close
proximity to inland water;
·
Accidental spillage of chemicals;
·
Sewage effluent from
construction workforce; and
·
Groundwater from Contaminated
Areas, Contaminated Site Runoff and Wastewater from Land Decontamination.
Construction Works at Tuen Mun River
Channel
5.6.2
The construction of pier and
viaduct structure across Tuen Mun River would require
the installation of new piers in Tuen Mun River. Pier installation would involve pilling
and minor excavation activities in Tuen Mun River,
which may lead to elevated suspended solids (SS) levels and dispersion of SS to
the downstream section of the river if in contact with river water. However, according to the proposed design
as discussed in Section 2, concrete cofferdams will be erected at Tuen Mun River and thereafter dewatering will be conducted
inside the cofferdam to create a dry working zone for the construction of
temporary steel platform and/or watertight casing. Piles will then be constructed within
the watertight casing with piling rigs on barge or temporary steel platforms. Upon the completion of pile cap
construction, the cofferdams will be removed to maintain the water flow within Tuen Mun River.
Water pumps would be used to collect any construction site runoff and
ingress/seepage water within the concrete cofferdam/watertight casing. The collected construction site runoff
and ingress/seepage water would be diverted to the on-site wastewater treatment
facilities for treatment to satisfactory levels before discharge. Adoption of the proposed construction
method not only enables safe and secured working area on water, but also
provides cleaner, dry and tidier working environment.
5.6.3
The pier construction would be
conducted in phases at the embankment during dry seasons. Since the cross-section area of the
river channel to be blocked by the pier installation was anticipated localized
and temporary, the change in flow regime would be localized and any local water
quality impact would expect to be transient. As a result, the potential impact on
water quality and hydrology of Tuen Mun River would
be insignificant, no unacceptable water quality would be expected if the
mitigation measures detailed in Section 5.8 are
implemented properly.
General
Construction Activities
5.6.4
Wastewater generated from
construction activities, including general cleaning
and polishing, wheel washing, dust suppression, concreting works and utility
installation may contain high SS concentrations. It may also contain a certain amount of
grease and oil. Potential water
quality impacts due to the wastewater discharge can be minimised
if construction and site management practices are implemented to ensure that
litter, fuels, and solvents do not enter public drainage systems. It is expected that if the good site
practice suggested in Section 5.8 are followed as far as
practicable, the potential water quality impacts associated with construction
activities would be minimal.
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 unpaved lands 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 and turbidity 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 sites. 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 sites
may increase the SS levels in surface runoff causing a potential impact to the
nearby sensitive receivers.
5.6.8
However, it is 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 in Close Proximity to Inland
Water
5.6.9
Pollution of inland waters may
occur due to potential release of construction wastewater from the adjacent
construction works area. Construction 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 and drainage water will be adopted for the construction works
adjacent to the inland water in order to prevent
run-off and drainage water with high levels of SS from entering the water
environment. With the
implementation of adequate construction site drainage and Best Management
Practices as described in Section 5.8 and
provision of mitigation measures as specified in ETWB TC (Works) No. 5/2005 “Protection of natural streams / rivers from
adverse impacts arising from construction works” as detailed in Section 5.8, it
is anticipated that water quality impacts would be minimised.
Accidental
Spillage of
Chemicals
5.6.10
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
5.6.11
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.
5.6.12
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
5.6.13
It is identified that some of
the works sites might have land contamination
issues. Proper land contamination
remediation and mitigation measures are proposed in Section 7. Any
contaminated material disturbed, or material which comes into
contact with the contaminated material, has the potential to be washed
with site runoff into watercourses and 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
5.6.15
Potential sources of water
quality impact associated with the operation of the Project include:
·
Sewage and wastewater effluents
from stations;
·
Non-point source surface
run-off from new impervious areas including viaduct; and
·
Hydrodynamic and water quality
impact on Tuen Mun River Channel.
Sewage and Wastewater Effluents from stations
5.6.16
Sewage effluent from the staff
and passengers at A16 and TMS Stations would be connected to the existing foul
sewerage system. Runoff from
cleaning activities at the stations which would enter floor drains would also
be connected to the foul sewer. Hence, no adverse water quality impact is
anticipated to arise from sewage and wastewater effluents generated during the
operation of the stations.
Non-point source
surface run-off from new impervious areas
5.6.18
An additional paved area of about 2.71
ha is
anticipated to generate surface runoff when in
operation
and the surface
runoff collected from the paved areas of viaduct and stations would be conveyed to the
existing public drainage system.
5.6.19
According to the “DSD Stormwater Drainage Manual (5th edition)”, annual rainfall in Hong Kong is around 2,400mm. 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 1,512mm
of 2,400mm annual rainfall would be considered as effective rainfall that would
generate runoff (i.e. 1,512mm=2,400mm×(82%+44%)/2).
5.6.20
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 additional paved area is estimated to be about 101.0m3/day (= 0.9 × 1,512mm/year × 2.71ha). The hydrology and flow regime impacts,
if any, would be limited and localized due to about 101.0 m3/day
surface runoff will be generated, and hence no adverse impacts would be
expected.
5.6.21
Storm water control measures including Best Management Practices (BMPs),
would be implemented within the development site as described in Sections 5.8.28 to 5.8.33. 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.
Hydrodynamic and water quality impact on Tuen
Mun River Channel
5.6.22
The permanent pier structures of the Project in Tuen
Mun River Channel has potential to obstruct the river
flow. Based on preliminary design,
the total area of permanent structures including pile caps and piers would be about
340 m2 in Tuen Mun River Channel. The percentage of the total area of
permanent structures within the 500m assessment area of the Tuen
Mun River Channel is estimated to be less than 0.2%. It is therefore considered that the area
of the river channel to be blocked by the piers was anticipated small, the
change in flow regime would be insignificant and any local water quality impact
would expect to be acceptable.
Construction of Piers in Tuen
Mun River
5.8.1
The pilling works should be
conducted by phases. The method and
sequence of the proposed pier works in Tuen Mun River
should be carefully designed so that wastewater and sediment laden water
generated from the pilling works would be confined and physically separated
from the watercourse.
5.8.2
All pilling, the associated excavation
works and construction of pile caps in river should be fully enclosed by casing/concrete
cofferdam/watertight precast pile cap shells. Concrete cofferdam and watertight
precast pile cap shells should be constructed to isolate the construction
activities from the river water.
The detail design of the concrete cofferdams and watertight precast pile
cap shells will be conducted by the Contractor during the construction phase to
fulfil the requirements in DSD Technical Circular No. 1/2017 "Temporary
Flow Diversions and Temporary Works Affecting Capacity in Stormwater System"
for DSD approval in order to formulate feasible
options of these temporary structure.
5.8.3
Water pumps should be used to
collect any construction site runoff and ingress/seepage water within the
concrete cofferdam and watertight precast pile cap shells. The collected construction site surface
runoff and ingress/seepage water should be diverted to the on-site wastewater
treatment facilities for treatment to satisfactory levels before
discharged. Discharge licence issued by the EPD for discharging effluent from the
construction site under the WPCO is needed. The discharge quality and quantity must
meet the requirements specified in the discharge licence
and follow the TM-DSS.
5.8.4
To further minimize any adverse
water quality impact during the pilling and excavation works, silt curtains
should be deployed to completely enclose the concrete cofferdam/watertight precast
pile cap shells prior to setting up piling works and installation of concrete
cofferdam/watertight precast pile cap shells. Silt curtains should only be removed
after completion of pilling works and removal of concrete cofferdam/watertight precast
pile cap shells. The Contractor
should be responsible for the design, installation and
maintenance of the silt curtain to minimize the impacts on water quality. The design and specification of the silt
curtains should be submitted by the Contractor to the Project Engineer for
approval.
Construction Site Runoff and General
Construction Activities
·
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.
5.8.8
Surface 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.
5.8.9
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.
5.8.10
Construction works should be
programmed to minimize soil excavation works in rainy seasons (April to
September) as far as practicable.
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 that adequate surface
protection measures can be safely carried out well before the arrival of a
rainstorm.
5.8.11
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.
5.8.12
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.
5.8.13
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.
5.8.14
Open stockpiles of construction
materials (e.g. aggregates, sand and fill material) on
sites should be covered with tarpaulin or similar fabric.
5.8.15
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.
5.8.16
Good site practices should be
adopted to remove rubbish and litter from construction sites so as to prevent the rubbish and
litter from spreading from the site area.
It is recommended to clean the construction sites on a regular
basis. Also, as discussed in Section 6.4.21 and 6.4.22, the following mitigation measures related to the transportation of the sediment should be
implemented to minimize the potential water quality impact:
·
Loading of the excavated
sediment to the barge should 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 should be equipped with tight
fitting seals to prevent leakage and should not be filled to a level that would
cause overflow of materials or laden water during loading or
transportation.
·
Monitoring of the barge loading
should be conducted to ensure that loss of material does not take place during
transportation. Transport barges or vessels should be equipped with automatic
self-monitoring devices as specified by the Director of Environmental
Protection (DEP).
5.8.17
Discharge licence issued by the EPD for
discharge of effluent from the construction site under the WPCO is needed. The discharge quality and quantity must
meet the requirements specified in the discharge licence. All the runoff and wastewater generated
from the works areas should be treated so that it satisfies all the standards
listed in the TM-DSS. The
beneficial uses of the treated effluent for other on-site activities such as
dust suppression, wheel washing and general cleaning etc., can minimise water consumption and reduce the effluent
discharge volume. If monitoring of
the treated effluent quality from the works areas is required during the
construction phase of the Project, the monitoring should be carried out in
accordance with the relevant WPCO licence.
Construction Works in Close
Proximity to Inland Water
·
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
watercourses when carrying out of the construction works.
·
Stockpiling of construction
materials and dusty materials should be covered and located away from any
watercourses.
·
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 watercourses, where practicable.
·
Construction effluent, site
run-off and sewage should be properly collected and / or treated.
·
Proper shoring may need to be
erected in order to prevent soil / mud from slipping
into the inland water bodies.
Accidental Spillage of Chemicals
5.8.20
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 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
5.8.25
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 should
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 should 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 Effluents discharge
5.8.27
All sewage and wastewater effluents
generated from operation of the stations should be properly collected and
diverted to public sewers for proper treatment and disposal. No direct discharge of sewage and
wastewater effluents into the storm drains or inland/marine waters will be allowed.
Stormwater Runoff
Design
Measures
5.8.29
Exposed surface should be
avoided to minimise soil erosion. The Site
should be either hard paved or covered by landscaping area and plantation where
appropriate.
5.8.30
The drainage system should be
designed to avoid flooding. The drainage system will be designed to avoid any case of flooding based
on at least 1 in 50 year return period.
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
Silt traps and oil interceptors
should be incorporated as appropriate during the detailed design to remove
particles and oil present in stormwater runoff, where appropriate.
Administrative
Measures
Hydrodynamic and water quality impact on Tuen
Mun River Channel
5.8.34
No significant change in flow
regime and water quality associated with operation of the Tuen
Mun South Extension is anticipated.
No adverse hydrodynamic and water quality impacts would therefore be
expected during the operational phase and no mitigation measures are considered
necessary.
Construction
Phase
5.11.1
The potential water quality impacts from the
construction works are associated with the construction works at Tuen Mun River and in close proximity to
inland water, wastewater generated from general construction activities,
construction site run-off, groundwater from contaminated areas and contaminated
site runoff, accidental spillage of chemicals and sewage effluent from
construction workforce. Provided
that proper site management practices and the mitigation
measures including adequate construction site drainage, provision of sediment
removal facilities, practical groundwater control measures and chemical toilets
would be implemented, no adverse water quality impact during construction phase
would be anticipated.
Operational
Phase
5.11.2
The key potential source of water quality impact
during operational phase would be related to non-point source stormwater runoff,
sewage and wastewater effluents from stations, and potential hydrodynamic and
water quality impact on Tuen Mun River due to the
installation of pier structure. Adequate
drainage system should be provided to collect the stormwater runoff. All sewage and wastewater generated from
the Project should be properly collected and diverted to public sewers for
proper treatment and disposal. With
proper design of drainage and sewerage systems and implementation of the
recommended mitigation measures, the associated water quality impacts are
expected to be minimal and acceptable.
No significant change in flow regime and water quality associated with
the proposed piers is expected. Therefore,
no adverse water quality impact during operational phase would be anticipated.