4.1 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.
4.2
The mitigation measures recommended
for the construction phase of the Project are summarised below. The implementation
schedule of the recommended water quality mitigation measures is presented in Appendix A.
4.3 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 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 of more than 50m3 should be covered with tarpaulin or
similar fabric during rainstorms.
4.4 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.
4.5 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.
4.6 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.