4                    WATER QUALITY

 

Introduction

 

4.1              The water quality assessment in the EIA Report for the Project identified that the key issue in terms of water quality would be related to earthworks for the proposed extension of Siu Ho Wan WTW, and the proposed Siu Ho Wan Raw Water Booster Pumping Station, and trench excavation for the mainlaying works at Pui O, and demolition works for Pui O Raw Water Pumping Station.  The EIA concluded that the identified water quality impacts could be minimized by implementing the recommended mitigation measures for the construction works, such as control measures on runoff and drainage from the works areas to minimise construction run-off and proper site management and good housekeeping practices.  No unacceptable residual water quality impact was expected.  It was recommended that regular site inspections be undertaken to inspect the construction activities and works areas in order to ensure the recommended mitigation measures are properly implemented.

Water Quality Mitigation Measures

 

4.2              Mitigation measures recommended for the construction phase of the Project are summarized below and are common to both the designated project and non-designated project elements.   Specific mitigation measures applicable to the mainlaying works at Pui O are also summarized below.  The implementation schedule of the recommended water quality mitigation measures for the designated project and non-designated project of Siu Ho Wan WTW Extension is presented in Appendix A.

 

Construction Site Run-off and Drainage

 

4.3              Proper site management measures should be implemented to control site runoff and drainage, and thereby minimize potential adverse impacts on water quality.  The site practices outlined in ProPECC PN 1/94 Construction Site Drainage should be followed as far as practicable in order to minimize surface runoff and the chance of erosion, and also to retain and reduce any suspended solids prior to discharge.  These practices include the following measures:

 

·      Before commencing any site formation work, all sewer and drainage connections should be sealed to prevent debris, soil, sand etc. from entering public sewers/drains.

 

·      Sand/silt removal facilities such as sand traps, 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 silt removal facilities should be based on the guidelines provided in ProPECC PN 1/94.  All drainage facilities and erosion and sediment control structures should be inspected monthly and maintained to ensure proper and efficient operation at all times and particularly during rainstorms. 

 

·      Water pumped out from foundation excavations should be discharged into silt removal facilities.

 

·      Open trench excavation for the mainlaying works at Pui O near the stream courses should be avoided in the wet season as far as practicable (i.e. during April to October), and the trench should be excavated and backfilled in short sections.

 

·      Exposed soil surfaces should be protected by paving or fill material as soon as possible to reduce the potential of soil erosion.

 

·      Open stockpiles of construction materials or construction wastes on-site of more than 50m3 should be covered with tarpaulin or similar fabric during rainstorms.

 

General Construction Activities

 

4.4              Debris and refuse generated on-site should be collected, handled and disposed of properly to avoid entering the nearby watercourses and storm water drains.  Stockpiles of cement and other construction materials should be kept covered when not being used.   In particular, care should be exercised during the mainlaying works at Pui O at the locations where the proposed raw water mains would be constructed over the top of the streams.

4.5              Oils and fuels should only be used and stored in designated areas which have pollution prevention facilities.  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

 

4.6              Temporary sanitary facilities, such as portable chemical toilets, should be employed on-site.  A licensed contractor would be responsible for appropriate disposal and maintenance of these facilities.