5.1 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.
5.2 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.
Construction Phase
Construction Site Run-off and Drainage
5.3 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 specified mitigation measures and practices include the following:
· Provision of perimeter drains to intercept storm-runoff from outside the works area. These shall be constructed in advance of site formation works and earthworks. Earth bunds or sand bag barriers should be provided on-site to direct storm water to silt removal facilities. The design of the temporary on-site drainage system will be undertaken by the Contractor prior to the commencement of construction.
· 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 will be undertaken by the Contractor prior to the commencement of construction.
· Air should be used as the flushing medium of the drilling equipment to avoid the groundwater being affected by the flushing medium. In addition, permanent casing may be provided to the drillhole of soil nail within the permeable colluvium layer as instructed by the Engineer to minimize the impact to the groundwater table situated at the permeable soil stratum.
· An outlet pipe extending above the slope surface should be installed to facilitate collection of discharge of air, water and grout from the drillhole inserted with soil nail during grouting.
· 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.
· Exposed slope/soil surface should be covered by tarpaulin as soon as possible to reduce the potential of soil erosion. 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.
· Open stockpiles of construction materials or construction wastes on-site of more than 50m3 should be covered with tarpaulin or similar fabric during rainstorms.
5.4 The Contractor should ensure that all site runoff and drainage arising from the works area are properly treated by the use of sedimentation tank, and that the discharge standards as stipulated in the “Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters” are met. The treated site runoff and drainage water should be discharged to the licensed point near the tunnel portal of the concurrent landslide preventive works at Po Shan under Agreement No. CE28/2004 (GE).
General Construction Activities
5.5 Debris and rubbish generated on-site should be collected, handled and disposed of properly to avoid being flushed or blown by wind into the drainage culvert. Stockpiles of cement and other construction materials should be kept covered when not being used.
5.6 Oils and fuels should only be used and stored in designated areas which have pollution prevention facilities. To prevent spillage of fuels and solvents, 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
5.7 Temporary sanitary facilities, such as portable chemical toilets, should be employed on-site where necessary to handle sewage from the workforce. A licensed contractor would be responsible for appropriate disposal of waste matter and maintenance of these facilities.
5.8 The implementation schedule for the recommended water quality mitigation measures is presented in Appendix B.