6.1 This section presents the water quality impact assessment for the construction phase of the Project. As the construction activities of the Project will be land-based, direct impacts on water bodies will not arise. Nevertheless, the construction activities at the works area may generate surface run-off which may cause adverse water quality impacts if not properly controlled or mitigated. Recommendations for mitigation measures have been made, where necessary, to reduce the identified water quality impacts to an acceptable level.
Environmental Legislation And Standards
Water Pollution Control Ordinance (WPCO)
6.2
The Water Pollution
Control Ordinance (Cap. 358), in existence since 1980, is the major legislation
relating to the protection and control of water quality in
Table 6.1 Summary of Water
Quality Objectives for
|
Parameters |
Objectives |
Sub-Zone |
|
Offensive odour, tints |
Not to be present |
Whole zone |
|
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
|
Dissolved oxygen (DO) within 2 m of the seabed |
Not less than 2.0 mg/L for 90% of samples |
Marine waters |
|
Depth-averaged DO |
Not less than 4.0 mg/L for 90% of samples |
Marine waters |
|
PH |
To be in the range of 6.5 - 8.5, change due to human activity not to exceed 0.2 |
Marine waters |
|
Salinity |
Change due to human activity not to exceed 10% of ambient |
Whole zone |
|
Temperature |
Change due to human activity not to exceed 2 oC |
Whole zone |
|
Suspended solids (SS) |
Not to raise the ambient level by 30% caused by human activity |
Marine waters |
|
Unionised ammonia (UIA) |
Annual mean not to exceed 0.021 mg/L as unionised form |
Whole zone |
|
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
|
Total inorganic nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen not to exceed 0.4 mg/L |
Marine waters |
|
Toxic substances |
Should not attain such levels as to produce significant toxic, carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms. |
Whole zone |
|
Human activity should not cause a risk to any beneficial use of the aquatic environment. |
Whole zone |
Source:
Statement of Water Quality Objectives (
Technical Memorandum
6.3 Besides setting the WQOs, the WPCO controls effluent discharging into the WCZs through a licensing system. Guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, storm water drains, inland and coastal waters) is provided in the Technical Memorandum (TM) on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters. The limits given in the TM cover the physical, chemical and microbial quality of effluents. Any effluent discharge during the construction stage should comply with the standards for effluents discharged into the inshore waters of the Victoria Harbour WCZ, as shown in Table 9a of the TM.
Practice Notes
6.4 A practice note (PN) for professional persons was issued by the EPD to provide environmental guidelines for handling and disposal of construction site discharges. The ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with various types of discharge from a construction site. Practices outlined in the PN should be followed as far as possible during construction to minimize the water quality impact due to construction site drainage.
6.5 The study area for the water quality impact assessment covers the Victoria Harbour WCZ and all areas within 500m from the works site boundary.
6.6 The water sensitive receivers that may be affected by the construction activities for the Project have been identified. Potential sources of water quality impact that may arise during the construction phase of the Project were described. This task included identifying pollutants from point discharges and non-point sources that could affect the quality of surface water run-off. All the identified sources of potential water quality impact were then evaluated and their impact significance determined. The need for mitigation measures to reduce any identified adverse impacts on water quality to acceptable levels was determined.
Identification of Water Sensitive Receivers
6.7
Water-bodies identified
within the study area include a drainage culvert on the hillslope
between Po Shan Mansions and
Evaluation of Potential Water Quality Impacts
6.8 Potential sources of water quality impact associated with the proposed construction activities at the works areas of the Project have been identified and include:
· construction site runoff and drainage
· general construction activities
· sewage effluent produced by on-site workforce.
Construction Runoff and Drainage
6.9 During site clearance, runoff and drainage from the works area would be the main sources of potential water quality impact. Site runoff and drainage may contain increased loads of suspended solids and contaminants. Potential sources of pollution from site drainage include: runoff and erosion from exposed soil surfaces, earth working areas and stockpiles; release of grouting and cement materials with rain wash; wash water from dust suppression sprays; and fuel and lubricants from maintenance of construction vehicles and mechanical equipment. Site runoff and drainage from the works area on the upper hillslope, if uncontrolled, could enter the drainage culvert.
6.10 The installation of the soil nails and raking drains would not involve any pumping or discharge of groundwater, and hence no alteration of the existing ground water level would be expected during these construction activities. Also, air would be used as the flushing medium. Hence, the groundwater would not be affected by the flushing medium of the drilling equipment. In addition, permanent casing may be provided to the drillhole of soil nail within the permeable colluvium layer as instructed by the Engineer. This extra measure would also minimize the impact to the groundwater table situated at the permeable soil stratum.
6.11 Potential sources of pollution from the construction and installation of the soil nails may include: effluent discharge during grouting and wash water from dust suppression sprays. An outlet pipe extending above the slope surface would be installed to facilitate collection of discharge of air, water and grout from the drillhole inserted with soil nail during grouting. With this appropriate setup, spillage during the grouting operation would be effectively controlled.
6.12 As a good site practice, mitigation measures should be implemented to control site runoff and drainage from the works area, and to prevent runoff and drainage water with high levels of suspended solids from entering the drainage culvert. With the implementation of adequate construction site drainage and provision of sediment removal facilities as described in Section 6.22, it is anticipated that unacceptable water quality impacts would not arise. The construction phase discharge would be collected by the temporary drainage system installed by the Contractor and then treated or desilted on-site before discharge to the licensed point near the tunnel portal of the concurrent landslide preventive works at Po Shan under Agreement No. CE28/2004 (GE).
6.13 The Contractor has been granted a license from the Environmental Protection Department (EPD) for the discharge of site run-off and drainage water after sedimentation process. As authorized by EPD, the treated wastewater shall be discharged into the storm water drain via the catch-pit located adjacent to the tunnel portal. The quantity and composition of the treated wastewater from the premises shall not exceed the following discharge limits specified by EPD:
· Maximum flow rate is 30 m3/day.
· Maximum concentration of suspended solids is 30 mg/L.
· Maximum concentration of Chemical Oxygen Demand is 80 mg/L.
· The range of pH level is 6 to 9 units.
6.14 Given that the works would be concurrent with the landslide preventive works under Agreement No. CE28/2004 (GE), it will be important that the above recommended mitigation measures to control site runoff and drainage water are properly implemented. The identified water sensitive receivers lie outside the site boundary, and therefore there is very limited potential for cumulative impacts on water quality to result. It is recommended that regular site inspections be undertaken to inspect the works area in order to ensure the recommended mitigation measures are properly implemented.
General Construction Site Activities
6.15 On-site construction activates may cause water pollution from the following:
· Uncontrolled discharge of debris and rubbish such as packaging, construction materials and refuse etc; and
· Spillages of liquid stored on-site, such as oil, diesel and solvents etc.
6.16 However, the effects on water quality from the construction activities are likely to be minimal, provided that site boundaries are well maintained and good construction practices are implemented to ensure that litter, fuel and solvents are stored and handled properly, as detailed in Section 6.22.
Sewage Effluent
6.17 Domestic sewage would be generated from the workforce during the construction phase. Portable chemical toilets will be provided for the site workers. The Contractor will have the responsibility to ensure that chemical toilets are used and properly maintained, and that licensed contractors are employed to collect and dispose of the waste off-site at approved locations. Therefore, no adverse water quality impacts are anticipated.
Cumulative Impact from Concurrent Projects
6.18 A designated project “Agreement No. CE 28/2004 (GE) Landslide Preventive Works at Po Shan, Mid-levels – Design and Construction” (under Environmental Permit EP-235/2005/A) will still on-going during the construction period of this Project. As detailed in Section 6.13, the cumulative impact at the water sensitive receivers is expected to be minimal given that the recommended mitigation measures to control site runoff and drainage water are properly implemented.
6.19 Another designated project “Drainage Improvement in Northern Hong Kong Island – Hong Kong West Drainage Tunnel” (under Environmental Permit EP-272/2007) proposed by Drainage Services Department will be carried out during the construction period of this Project. A small portion of the proposed drainage tunnel is beneath the subject site of this Project at about +20mPD (or more than 200m below the subject site of this Project). As this concurrent tunnelling works will be carried out underground and the vertical distance is far away from the works area of this Project, cumulative water quality impact due to the concurrent tunnelling works is not anticipated.
6.20 According to the latest available information of this concurrent drainage improvement project (DSD Contract No. DC/2007/10), tunnelling works and two portal formation works (one at Cyberport and one at Tai Hang) will begin in 2008. All other shaft works will be carried out not earlier than second quarter of 2009. As the portal works are far from the subject site of this Project and other works will be carried out after the completion of this Project, no cumulative water quality impact is expected from this concurrent project.
6.21 Recommended mitigation measures for containing and minimizing potential water quality impacts during the construction activities are described below.
Construction Site Runoff and Drainage
6.22 Construction site runoff and drainage should be prevented or minimized in accordance with the guidelines stipulated in ProPECC PN 1/94 “Construction Site Drainage”. 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 would 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 would 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.
6.23 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 Site Activities
6.24 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.
6.25 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 Workforce
6.26 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.
Evaluation Of Residual Impacts
6.27 With the full implementation of the recommended mitigation measures for the construction phase of the proposed Project, no unacceptable residual impacts on water quality are expected. It is recommended that regular audit of the implementation of the recommended mitigation measures at the work area be carried out during the construction phase.
Environmental Monitoring And Audit
6.28 Water quality monitoring is not considered necessary during the construction phase as no unacceptable water quality impact is expected. However, it is recommended that regular site inspections be undertaken to inspect the construction activities and works area in order to ensure the recommended mitigation measures are properly implemented. Any effluent discharges from the site would be required to comply with the terms and conditions of a discharge licence, issued by EPD, under the WPCO.
6.29 The key issue in terms of water quality during the construction phase of the Project would be the potential for release of sediment-laden water from works areas during site clearance and the proposed soil nailing and rock slope stabilization works. Minimisation of water quality deterioration could be achieved through implementing adequate mitigation measures such as control measures on site runoff and drainage from the works areas to minimise construction runoff, and on-site treatment of site runoff and drainage prior to discharge. Proper site management and good housekeeping practices would also be required to ensure that construction wastes and other construction-related materials would not enter the drainage culvert. Sewage effluent arising from the construction workforce would also be handled through provision of portable toilets.
6.30 With the implementation of these recommended mitigation measures, the construction works for the Project would not be expected to result in unacceptable impacts on water quality. Site inspections should be undertaken routinely to inspect the works areas in order to ensure the recommended mitigation measures are properly implemented.