8.1
The water quality impact assessment in the EIA Report
identified that no adverse impact would occur during the construction of the Project
provided the recommended mitigation measures were correctly implemented. It is recommended that regular site
inspections (at least weekly) be undertaken to inspect the construction
activities and works areas in order to ensure the recommended mitigation
measures are properly implemented.
8.2
The site inspection should be conducted at least weekly to
inspect the construction activities and works areas in order to ensure the
recommended mitigation measures are properly implemented.
8.3
Mitigation measures recommended for the construction phase
of the Project are summarized below.
The implementation schedule of the recommended water quality mitigation
measures is presented in Appendix A.
Construction Site Run-off
and Drainage
§
At the start of site establishment (including the barging
facilities), perimeter cut-off drains to direct off-site water around the site
should be constructed with internal drainage works and erosion and
sedimentation control facilities implemented. Channels (both temporary and permanent
drainage pipes and culverts), earth bunds or sand bag barriers should be
provided on site to direct stormwater to silt removal facilities. The design of the temporary on-site
drainage system would be undertaken by the contractor prior to the commencement
of construction.
§
The dikes or embankments for flood protection should be
implemented around the boundaries of earthwork areas. Temporary ditches should be provided to
facilitate the runoff discharge into an appropriate watercourse, through a
site/sediment trap. The
sediment/silt traps should be incorporated in the permanent drainage channels
to enhance deposition rates
§
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 TM standards under the WPCO. The design of efficient silt removal
facilities should be based on the guidelines in Appendix A1 of ProPECC PN 1/94,
which states that the retention time for silt/sand traps should be 5 minutes
under maximum flow conditions.
Sizes may vary depending upon the flow rate, but for a flowrate of
§
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. If
excavation of trenches in wet periods is necessary, they should be dug and
backfilled in short sections wherever practicable. 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.
§
The overall slope of the site should be kept to a minimum to
reduce the erosive potential of surface water flows, and all trafficked areas
and access roads protected by coarse stone ballast. An additional advantage accruing from
the use of crushed stone is the positive traction gained during prolonged
periods of inclement weather and the reduction of surface sheet flows
§
All 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. Measures should be
taken to prevent the washing away of construction materials, soil, silt or
debris into any drainage system.
§
Manholes (including newly constructed ones) should always be
adequately covered and temporarily sealed so as to prevent silt, construction
materials or debris being washed into the drainage system and storm runoff
being directed into foul sewers.
§
Precautions be taken at any time of year when rainstorms are
likely, actions to be taken when a rainstorm is imminent or forecasted, and
actions to be taken during or after rainstorms are summarised in Appendix A2 of
ProPECC PN 1/94. Particular
attention should be paid to the control of silty surface runoff during storm
events, especially for areas located near steep slopes.
§
Bentonite slurries used in piling or slurry walling should
be reconditioned and reused wherever practicable. Temporary enclosed storage locations
should be provided on-site for any unused bentonite that needs to be
transported away after all the related construction activities are
completed. The requirements in
ProPECC PN 1/94 should be adhered to in the handling and disposal of bentonite
slurries.
General Construction
Activities
8.5
Construction solid waste, debris and refuse generated
on-site should be collected, handled and disposed of properly to avoid entering
any nearby stormwater drain.
Stockpiles of cement and other construction materials should be kept
covered when not being used.
Requirements for solid waste management are detailed in Section 6 of
this Manual.
8.6
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 stormwater 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.
Sewage from Construction
Workforce
8.7
Temporary sanitary facilities, such as portable chemical
toilets, should be employed on-site where necessary to handle sewage from the
workforce. A licensed contractor
should be employed to provide appropriate and adequate portable toilets and be
responsible for appropriate disposal and maintenance.
Wastewater Discharge from
Tunnelling and Open Cut Excavation
8.8
Wastewater with a high level of SS should be treated before
discharge by settlement in tanks with sufficient retention time. Oil interceptors would also be required
to remove the oil, lubricants and grease from the wastewater. In case of very high levels of SS, an
on-site pre-packaged treatment plant may be required with the addition of
flocculants to improve the settlement of solids. A discharge license under the WPCO would
be required for discharge to the stormwater drain. It may be a stipulation of the WPCO
licence to require the Contractor to monitor the quality/quantity of the
discharge to show compliance with the conditions of the licence.
8.9
Impact on tree walls at
Runoff from Rail Track
and operational tunnel drainage
8.10
The tunnel wall would be equipped
with water-tight liner and designed for no seepage.
8.11
Standard designed silt trap or grease trap (if necessary)
and oil interceptor would be provided to remove the oil, lubricants, grease,
silt and grit from the tunnel runoff before discharge into stormwater drainage.
8.12
Track drainage channels discharge should pass through
oil/grit interceptors/chambers to remove oil, grease and
sediment before being pumped to the public stormwater drainage system.
8.13
The silt traps and oil interceptors should be cleaned and
maintained regularly.
8.14
Oily contents of the oil interceptors should be transferred
to an appropriate disposal facility, or to be collected for reuse, if possible.
Sewage from Station
8.15
Sewage and wastewater effluents generated from the staff at
stations and food and beverage outlets, if any, would be connected to the
existing foul sewerage system.
8.16
Runoff from cleaning activities at the stations which would enter
floor drains would also be connected to the foul sewer.