5.1 This section presents the assessment of the potential water quality impacts associated with the construction and operation of the proposed TM54SPS.
5.2 The main issue associated with water quality impacts during the operation phase is the potential of sewage overflow from TM54SPS in case of electricity shortage or pump failure. The design of the TM54SPS has incorporated a number of precautionary measures to minimize the risk of electricity shortage and pump failure. These measures will be described in this assessment.
5.3 Potential impacts on water quality resulting from site run-off during the construction phase are also addressed in this section. Good site practices and mitigation measures are recommended with reference to applicable legislation and guidelines pertinent to water quality.
5.4
The Water Pollution Control
Ordinance (Cap. 358), in existence since 1980, is the major legislation
relating to the protection and control of water quality in
Table 5.1 Water Quality Objectives for Suspended Solids and Dissolved Oxygen at North Western Water Control Zone
|
Parameter |
Part or
Parts of Zone |
Water
Quality Objective |
|
Suspended
Solids (SS) |
Tuen Mun (A, B and C) Subzones |
Waste discharges shall not cause the annual
medium of suspended solids to exceed 20 mg/L |
|
Marine Waters |
Waste discharge 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 |
|
|
Other Inland
Waters |
Water
discharges shall not cause the annual median of suspended solids to exceed 25mg/L |
|
|
Dissolved
Oxygen (DO) |
Tuen Mun (A, B and C) Subzones |
Waste discharges shall not cause the level of
dissolved oxygen to be less than 4 mg/L |
|
Marine Waters |
The level
of DO shall not fall below 4 mg/L for 90% of the sampling occasions during
the whole year. In addition, the
DO level shall not be less than 2 mg/L within |
5.5
Besides
setting the WQOs, the WPCO controls effluent discharging into the
WCZs through a licensing system. The
Technical Memorandum (TM) on Standards for Effluents Discharged into Drainage
and Sewerage Systems, Inland and Coastal Waters issued under Section 21 of the
WPCO gives guidance on the permissible effluent discharges based on the type of
receiving waters (foul sewers, storm water drains, inland and coastal waters).
The limits given in the TM control the physical, chemical and microbial quality
of effluents. Under the TM,
effluents discharged into the sewerage system and the inshore and marine waters
of the WCZ are subject to standards for particular volumes of discharge. These standards are defined by EPD
and specified in licence conditions for any new discharge within a WCZ. For this Project, the TM standards for
effluents discharged into the sewerage system and the inshore waters of the
North Western WCZ will apply to the construction and operation phases, as shown
in Table 1 and Table
5.6
A
practice note (PN) for professional persons was issued by the EPD to
provide environmental guidelines for handling and disposal of construction site
discharges. The ProPECC PN 1/94
“Construction Site Drainage” provides good practice guidelines for dealing with
various types of discharge from a construction site. Practices outlined in the PN should be
followed as far as possible during construction to minimize the water quality
impact due to construction site drainage.
5.7
The
only watercourse within the vicinity of the site is a nullah along
5.8
According
to EPD’s Annual Report titled “2006 River Water Quality in
Table 5.2 Summary of Water Quality
Monitoring Results for
|
Parameter |
Unit |
TN1 |
TN2 |
TN3 |
TN4 |
TN5 |
TN6 |
|
Dissolved oxygen |
mg/L |
4.1 (2.0-7.1) |
8.1 (6.5-9.6) |
4.9 (3.1-6.9) |
5.5 (3.6-9.5) |
5.0 (3.4-7.6) |
4.7 (2.3-7.1) |
|
pH |
|
7.6 (7.4-8.4) |
7.5 (7.1-8.5) |
7.6 (7.2-8.2) |
7.6 (7.4-8.0) |
7.6 (7.5-8.1) |
7.4 (7.3-7.7) |
|
Suspended solids |
mg/L |
15 (6-330) |
16 (3-370) |
11 (2-130) |
10 (2-70) |
7 (3-100) |
3 (1-17) |
|
5-day Biochemical Oxygen Demand |
mg/L |
23 (6-45) |
1 (1-33) |
4 (2-8) |
4 (1-7) |
4 (2-7) |
3 (1-4) |
|
Chemical Oxygen Demand |
mg/L |
35 (21-60) |
9 (3-36) |
20 (15-34) |
18 (9-33) |
20 (12-30) |
19 (8-32) |
|
Oil & grease |
mg/L |
15 (0.5-5.1) |
0.5 (0.5-1.1) |
0.5 (0.5-0.5) |
0.5 (0.5-1.0) |
0.5 (0.5-0.9) |
0.5 (0.5-0.8) |
|
Faecal coliforms |
cfu/100mL |
1,000,000 (430,000-5,900,000) |
35,000 (370-560,000) |
140,000 (7,100-1,300,000) |
110,000 (4,600-830,000) |
170,000 (29,000-600,000) |
130,000 (16,000-720,000) |
|
E.coli |
cfu/100mL |
180,000 (110,000-250,000) |
13,000 (30-300,000) |
18,000 (440-260,000) |
20,000 (900-130,000) |
28,000 (2,900-120,000) |
15,000 (3,900-52,000) |
|
Ammonia-nitrogen |
mg/L |
5.7 (0.51-10.00) |
0.40 (0.16-5.00) |
0.54 (0.14-1.00) |
0.38 (0.15-1.00) |
0.57 (0.13-0.85) |
0.49 (0.12-0.90) |
|
Nitrate-nitrogen |
mg/L |
0.77 (0.01-4.70) |
0.81 (0.55-2.00) |
0.36 (0.11-0.66) |
0.36 (0.27-0.73) |
0.35 (0.22-0.83) |
0.27 (0.01-0.58) |
|
Total Kjeldahl nitrogen, SP |
mg/L |
7.05 (1.90-14.00) |
1.00 (0.27-5.80) |
0.98 (0.53-1.60) |
0.83 (0.43-1.50) |
0.99 (0.53-1.40) |
0.79 (0.43-1.30) |
|
Ortho-phosphate |
mg/L |
0.74 (0.27-1.00) |
0.11 (0.03-0.72) |
0.05 (0.01-0.12) |
0.06 (0.01-0.13) |
0.05 (0.02-0.10) |
0.06 (0.03-0.10) |
|
Total phosphorus, SP |
mg/L |
1.05 (0.79-1.90) |
0.19 (0.04-1.00) |
0.12 (0.07-0.22) |
0.13 (0.06-0.39) |
0.13 (0.08-0.17) |
0.10 (0.06-0.15) |
|
Sulphide, SP |
mg/L |
0.04 (0.02-0.09) |
0.02 (0.02-0.07) |
0.02 (0.02-0.04) |
0.02 (0.02-0.03) |
0.02 (0.02-0.09) |
0.02 (0.02-0.02) |
|
Aluminium |
µg/L |
185 (100-1,600) |
145 (50-1,300) |
90 (50-540) |
140 (80-470) |
105 (80-510) |
70 (50-130) |
|
Cadmium |
µg/L |
0.1 (0.1-0.2) |
0.1 (0.1-0.2) |
0.1 (0.1-0.4) |
0.1 (0.1-0.2) |
0.1 (0.1-0.2) |
0.1 (0.1-0.1) |
|
Chromium |
µg/L |
1 (1-2) |
1 (1-1) |
1 (1-2) |
1 (1-3) |
1 (1-2) |
1 (1-3) |
|
Copper |
µg/L |
4 (3-17) |
3 (1-9) |
4 (1-10) |
4 (3-10) |
4 (2-12) |
4 (3-7) |
|
Lead |
µg/L |
3 (1-86) |
3 (1-80) |
1 (1-13) |
1 (1-14) |
1 (1-11) |
1 (1-1) |
|
Zinc |
µg/L |
30 (20-190) |
30 (10-110) |
15 (10-90) |
15 (10-100) |
20 (10-120) |
20 (10-20) |
|
Flow |
L/s |
119 (36-6,576) |
79 (11-804) |
NM |
NM |
NM |
NM |
Notes:
1.
Data presented are in annual medians of monthly samples,
except those for faecal coliforms and E.coli
which are in annual geometric means. Figures in brackets are annual ranges.
2.
SP – soluble and particulate fractions (i.e. total value).
3.
NM indicates no measurement taken
5.9
The
only identified water sensitive receiver (WSR) within the assessment area of
5.10
In
accordance with the EIA Study Brief, WSR downstream of the emergency sewage
discharge location shall be included.
As advised by CEDD, the emergency overflow pipe would be connected to
the existing nullah south of the TM54SPS or to the proposed box culvert to be
constructed under the Phase 2 works of the Formation, Roads and Drains in Tuen
Mun Area 54. The future box
culvert would be connected to the Tsing Lun Nullah along
5.11
In
accordance with the EIA Study Brief, the assessment area for the water quality
impact assessment covers the area within
5.12
The
water sensitive receivers that may be affected by the construction and
operation activities for the Project have been identified. Potential sources of water quality
impact that may arise during the construction and operation phases of the
Project are as described in the following section. This task included identifying
pollutants from point discharges and non-point sources that could affect the
quality of surface water run-off.
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
determined.
5.13
Potential
sources of water quality impact associated with the proposed
construction activities at the works areas of the Project have been identified
and include:
•
construction site runoff and drainage
•
general construction activities
•
sewage effluent produced by on-site workforce.
5.14
The
assessment of operational stage water quality impacts of the Project has
addressed the potential issue of sewage bypass into the nearby future box
culvert which would be connected to the box culvert to be constructed and finally be
connected to the existing box culvert along Siu Hong Road which extends to the
Tuen Mun River Channel (
5.15
Runoff
from the surface construction works areas may contain increased loads of
sediments, other suspended solids and contaminants. Potential sources of pollution from site
drainage include:
•
runoff from and erosion of exposed soil surfaces, earth
working areas and stockpiles
•
release of cement and other materials with rain wash
•
wash water from dust suppression sprays
•
fuel and lubricants from maintenance of construction
vehicles and mechanical equipment.
5.16
Sediment
laden runoff particularly from works areas subjected to excavation or earth
works, if uncontrolled, may carry pollutants (adsorbed onto the particle
surfaces) into the nullah along
5.17
As
a good site practice, mitigation measures should be implemented to control
construction site runoff and drainage from the works areas, and to prevent
runoff and drainage water with high levels of suspended solids from entering the
Tsing Lun Nullah. With the
implementation of adequate construction site drainage and provision of sediment
removal facilities as described in Section 5.26, it is anticipated that
unacceptable water quality impacts would not arise. The construction phase discharge would
be collected by the temporary drainage system installed by the Contractor and
then treated or desilted on-site before discharging to the storm water
drain. The contractor would be required to obtain a license from EPD for
discharge to the public drainage system.
5.18
No
adverse water quality impacts would be expected at the Tuen Mun Typhoon Shelter
and gazetted bathing beaches at
5.19
On-site
construction activities may cause water pollution from the following:
•
uncontrolled discharge of debris and rubbish such as
packaging, construction materials and refuse
•
spillages of liquids stored on-site, such as oil, diesel and
solvents etc.
5.20
Good
construction and site management practices should be observed, as detailed in Section 5.27 - 5.28, to ensure that
litter, fuels and solvents do not enter the public drainage system
5.21
Domestic
sewage would be generated from the workforce during the construction phase.
However, portable chemical toilets can be installed within the construction
site. The Contractor will have the responsibility to ensure that chemical
toilets are used and properly maintained, and that licensed Contractors are
employed to collect and dispose of the waste off-site at approved locations. Therefore, adverse water quality impacts
would not be expected.
5.22
The
potential for water quality impacts in the operation phase is mainly associated
with the possibility of sewage overflow from TM54SPS due to the events of pump
failure and power supply failure. Various
precautionary measures are proposed to be incorporated in the design of TM54SPS.
These measures include:
•
A standby pump will be provided to cater for breakdown and
maintenance of the duty pump in order to avoid sewage bypass
•
Backup power supply in the form of dual power supply will be
provided to secure electrical power supply
•
An alarm would be installed to signal emergency high water
level in the wet well
5.23
With
the incorporation of the above preventive measures into the design of TM54SPS,
the possibility of emergency sewage bypass would be extremely remote.
5.24
The
proposed discharge point of emergency bypass from the TM54SPS is the nearby
future box culvert to be constructed under Phase 2 of the proposed development
at Tuen Mun Area 54 or the existing nullah south of the TM54SPS. The overflow bypass from TM54SPS would
discharge into the box culvert to reduce any significant aesthetic impact. The future box culvert connects to the
box culvert to be constructed under the infrastructural works of Site 2 and finally
connects to the existing box culvert along
5.25
Proposed
mitigation measures for containing and minimizing water quality impacts are
summarised below.
5.26
The
site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” should
be followed as far as practicable in order to minimise surface runoff and the
chance of erosion. The following
measures are recommended to protect water quality of the nearby Tsing Lun Nullah,
and when properly implemented should be sufficient to adequately control site
discharges so as to avoid water quality impacts:
•
Sand/silt removal facilities such as sand/silt traps and
sediment basins should be provided to remove sand/silt particles from runoff to
meet the requirements of the Technical Memorandum standard under the Water
Pollution Control Ordinance. The
design of efficient silt removal facilities should be based on the guidelines
in Appendix A1 of ProPECC PN 1/94, which states that the retention time for
silt/sand traps should be 5 minutes under maximum flow conditions. The detailed design of the sand/silt
traps shall be undertaken by the contractor prior to the commencement of
construction.
•
All drainage facilities and erosion and sediment control
structures should be regularly inspected and maintained to ensure proper and
efficient operation at all times and particularly during rainstorms. Deposited silt and grit should be
regularly removed, at the onset of and after each rainstorm to ensure that
these facilities are functioning properly at all times.
•
Measures should be taken to minimize the ingress of site
drainage into excavations. Water
pumped out from foundation excavations should be discharged into storm drains
via silt removal facilities.
•
If surface excavation works cannot be avoided during the wet
season (April to September), temporarily exposed slope/soil surfaces should be
covered by a tarpaulin or other means, as far as practicable, and temporary
access roads should be protected by crushed stone or gravel, as excavation
proceeds. Interception channels should be provided (e.g. along the crest/edge
of the excavation) to prevent storm runoff from washing across exposed soil
surfaces. Arrangements should always
be in place to ensure that adequate surface protection measures can be safely
carried out well before the arrival of a rainstorm. Other measures that need to be
implemented before, during and after rainstorms are summarized in ProPECC PN
1/94.
•
All vehicles and plant should be cleaned before leaving a
construction site to ensure no earth, mud, debris and the like is deposited by
them on roads. An adequately
designed and sited wheel washing facility should be provided at every
construction site exit where practicable.
Wash-water should have sand and silt settled out and removed at least on
a weekly basis to ensure the continued efficiency of the process. The section of access road leading to,
and exiting from, the wheel-wash bay to the public road should be paved with
sufficient backfall toward the wheel-wash bay to prevent vehicle tracking of
soil and silty water to public roads and drains.
•
Open stockpiles of construction materials or construction
wastes on-site should be covered with tarpaulin or similar fabric during
rainstorms.
5.27
Debris
and refuse generated on-site should be collected, handled and disposed of
properly to avoid entering any nearby storm water drain. Stockpiles of cement and other
construction materials should be kept covered when not being used.
5.28
Oils
and fuels should only be used and stored in designated areas which have
pollution prevention facilities. To
prevent spillage of fuels and solvents to any nearby storm water drain, all
fuel tanks and storage areas should be provided with locks and be sited on
sealed areas, within bunds of a capacity equal to 110% of the storage capacity
of the largest tank. The bund
should be drained of rainwater after a rain event.
5.29
Temporary
sanitary facilities, such as portable chemical toilets, should be employed
on-site where necessary to handle sewage from the workforce. A licensed contractor would be
responsible for appropriate disposal of waste matter and maintenance of these
facilities.
5.30
As
discussed in Section 5.22, various precautionary measures are proposed to be
incorporated in the design of TM54SPS to avoid emergency bypass of sewage to
the maximum practicable extent. These measures include:
•
A standby pump will be provided to cater for breakdown and
maintenance of the three duty pumps in order to avoid sewage bypass
•
Backup power supply in the form of dual power supply will be
provided to secure electrical power supply
•
An alarm would be installed to signal emergency high water
level in the wet well
•
Regular maintenance and checking of plant equipment to
prevent equipment failure
5.31
A
hand-held screen should be provided to cover the lower half of the opening of
the overflow bypass. This would
prevent the discharge of floating solids into receiving waterbodies (e.g. Tuen
Mun River Channel) as far as practicable whilst ensuring flooding at the
facilities would not occur even if the screen is blocked.
5.32
A
contingency plan to deal with the emergency discharges that may occur during
the operation of the TM54SPS should be developed before commissioning of the
sewage pumping station, including the following items:
•
Locations of the sensitive receivers in the vicinity of the
emergency discharges at North Western waters and Tuen Mun River Channel;
•
A list of relevant government bodies to be informed and to
provide assistance in the event of emergency discharge, including key contact
persons and telephone numbers;
•
Reporting procedures required in the event of emergency
discharge; and
•
Procedures listing the most cost-effective means in
rectifying the breakdown of the TM54SPS in order to minimize the discharge
duration.
5.33
With
the full implementation of the recommended mitigation measures for the
construction phase of the proposed Project, no unacceptable residual impacts on
water quality are expected. It is
recommended that regular audit of the implementation of the recommended mitigation
measures at the works area be carried out during the construction phase. With the incorporation of all the
proposed precautionary measures in the design of TM54SPS, minimal residual
operational water quality impact is expected.
5.34
Since
there would not be any direct impact on the identified WSR arising from the
construction activities of the Project, water quality monitoring is not
considered necessary during the construction phase. However, it is recommended that regular
site inspections be undertaken to inspect the construction activities and works
area in order to ensure the recommended mitigation measures are properly
implemented.
5.35
The
key issue in terms of water quality during the construction phase of the
Project would be the potential for release of sediment-laden run-off from
surface works areas. Minimisation
of water quality deterioration could be achieved through implementing adequate
mitigation measures such as control measures on site run-off and drainage from
the works areas to minimise construction site run-off. Proper site management and good
housekeeping practices would also be required to ensure that construction
wastes and other construction-related materials would not enter the Tsing Lun Nullah. Sewage effluent arising from the
construction workforce would also be handled through provision of portable
toilets.
5.36
With
the implementation of these recommended mitigation measures, the construction
works for the Project would not be expected to result in unacceptable impacts
on water quality. Site inspections
should be undertaken routinely to inspect the works areas in order to ensure
the recommended mitigation measures are properly implemented.
5.37
In
view of the potential adverse effect of emergency
sewage bypass on the quality of the receiving waters of the Tuen Mun River
Channel, various precautionary measures are proposed to be incorporated in the
design of TM54SPS to avoid emergency bypass of sewage to the maximum
practicable extent. With the
incorporation of these proposed precautionary measures, the possibility of
emergency sewage bypass would be extremely remote.