8.                     WATER QUALITY




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.


Site Inspection


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.


Mitigation Measures


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 Phase

Construction Site Run-off and Drainage


8.4               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 and sensitive uses of the coastal area i.e. WSD flushing water intakes along the harbour front, and when properly implemented should be sufficient to adequately control site discharges so as to avoid water quality impacts:


§         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 0.1m3/s a sedimentation basin of 30m3 would be required and for a flow rate of 0.5m3/s the basin would be 150m3.  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.  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.


Monitoring of Groundwater Level


8.9               Impact on tree walls at Forbes Street due to the potential for groundwater drawdown induced by tunneling would be controlled, as a precautionary measure, through monitoring of groundwater level.  Typically the maximum allowable draw down of ground water table would be about 1m.  Recharge wells would be installed at relevant locations of tree walls at Forbes Street where water levels would be expected to drop by more than 1m.  The monitoring would be conducted during construction phase of the Project on a weekly basis.

Operational Phase

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.