9. WATER QUALITY IMPACT

Introduction

9.1 This section presents an assessment of the potential water quality impacts associated with the construction and operation phases of the proposed West Island Line (WIL). Recommendations for mitigation measures have been made, where necessary, to reduce the identified water quality impacts to an acceptable level.

Environmental Legislation, Standards and Guidelines

Water Pollution Control Ordinance (WPCO)

9.2 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 Hong Kong. According to the Ordinance and its subsidiary legislation, Hong Kong waters are divided into ten water control zones (WCZ). Corresponding statements of Water Quality Objectives (WQO) are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in each of the WCZ based on their beneficial uses. The study area is located within the Victoria Harbour WCZ and Western Buffer WCZ and the corresponding WQO are listed in Tables 9.1 and 9.2, respectively.

Table 9.1 Summary of Water Quality Objectives for Victoria Harbour WCZ

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
Toxic substances	Human activity should not cause a risk to any beneficial use of the aquatic environment.	Whole zone

Source: Statement of Water Quality Objectives (Victoria Harbour (Phases One, Two and Three) Water Control Zone).

Table 9.2 Summary of Water Quality Objectives for Western Buffer WCZ

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 excepting fish culture subzones
	Not less than 5.0 mg/L for 90% of samples	Fish culture subzones
	Not less than 4.0 mg/L	Water gathering ground subzone and other Inland waters
5-Day biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 3 mg/L	Water gathering ground subzones
5-Day biochemical oxygen demand (BOD₅)	Change due to waste discharges not to exceed 5 mg/L	Inland waters
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 15 mg/L	Water gathering ground subzones
Chemical oxygen demand (COD)	Change due to waste discharges not to exceed 30 mg/L	Inland waters
pH	To be in the range of 6.5 – 8.5, change due to waste discharges not to exceed 0.2	Marine waters
	To be in the range of 6.5 – 8.5	Water gathering ground subzones
	To be in the range of 6.0 – 9.0	Inland waters
Salinity	Change due to waste discharges not to exceed 10% of ambient	Whole zone
Temperature	Change due to waste discharges not to exceed 2^oC	Whole zone
Suspended solids (SS)	Not to raise the ambient level by 30% caused by waste discharges and shall not affect aquatic communities	Marine waters
	Change due to waste discharges not to exceed 20 mg/L of annual median	Water gathering ground subzones
	Change due to waste discharges not to exceed 25 mg/L of annual median	Inland waters
Unionized ammonia (UIA)	Annual mean not to exceed 0.021 mg/L as unionized 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 effects in humans, fish or any other aquatic organisms	Whole zone
Toxic substances	Waste discharges should not cause a risk to any beneficial use of the aquatic environment	Whole zone
Bacteria	Not exceed 610 per 100ml, calculated as the geometric mean of all samples collected in one calendar year	Secondary contact recreation subzones and fish culture subzones
	Not exceed 180 per 100 mL, calculated as the geometric mean of all samples collected from March to October inclusive in 1 calendar year. Samples should be taken at least 3 times in 1 calendar month at intervals of between 3 and 14 days.	Recreation subzones
	Less than 1 per 100ml, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days	Water gathering ground subzones
	Not exceed 1000 per 100ml, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days	Inland waters
Colour	Change due to waste discharges not to exceed 30 Hazen units	Water gathering round
Colour	Change due to waste discharges not to exceed 50 Hazen units	Inland waters
Turbidity	Shall not reduce light transmission substantially from the normal level	Bathing beach subzones

Source: Statement of Water Quality Objectives (Western Buffer Water Control Zone).

Technical Memorandum

9.3 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, stormwater 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 and operational stages should comply with the standards for effluents discharged into the inshore waters or marine waters of the Western Buffer and Southern WCZ, as shown in Table 10a and Table 10b, respectively, of the TM.

Practice Notes

9.4 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 Practice Note (PN) for Professional Persons on Construction Site Drainage (ProPECC PN 1/94) issued by EPD 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.

Description of the Environment and Baseline Conditions

9.5 The EPD water quality monitoring stations VM7 and VM8 in the Victoria Harbour WCZ and station WM2 in the Western Buffer WCZ are located in the vicinity of the Project area (Figure 9.1). A summary of the most recently published monitoring data (in 2006) for these stations are presented in Table 9.3. Stations VM7, VM8 and WM2 are considered to be representative of the water quality in the marine waters of the assessment area.

Table 9.3 Summary Statistics of Marine Water Quality in the Victoria Harbour WCZ and Western Buffer WCZ in 2006

Parameter		Victoria Harbour (West)		Hong Kong Island (West)	WPCO WQO (in marine waters)
Parameter		VM7	VM8	WM2	WPCO WQO (in marine waters)
Temperature (^oC)		23.8 (16.9-27.9)	23.8 (16.9-27.9)	23.8 (16.9-27.4)	Not more than 2 ^oC in daily temperature range
Salinity		30.8 (24.2-33.0)	30.7 (23.3-33.0)	30.9 (23.9-33.2)	Not to cause more than 10% change
Dissolved Oxygen (DO) (% Saturation)	Depth average	80 (60-107)	85 (61-105)	86 (59-113)	Not available
Dissolved Oxygen (DO) (% Saturation)	Bottom	77 (48-98)	82 (49-109)	84 (52-114)	Not available
Dissolved Oxygen (DO) (mg/L)	Depth average	5.7 (4.0-7.3)	6.0 (4.1-7.7)	6.1 (4.0-8.1)	Not less than 4 mg/L for 90% of the samples
Dissolved Oxygen (DO) (mg/L)	Bottom	5.5 (3.3-7.0)	5.8 (3.3-7.8)	6.0 (3.5-8.2)	Not less than 2 mg/L for 90% of the samples
pH		8.0 (7.7-8.3)	8.0 (7.8-8.3)	8.0 (7.7-8.3)	6.5 - 8.5 (± 0.2 from natural range)
Secchi disc Depth (m)		2.0 (1.2-2.5)	1.9 (1.0-2.5)	2.0 (1.3-2.5)	Not available
Turbidity (NTU)		11.5 (5.8-20.5)	11.9 (5.3-19.0)	12.0 (6.2-20.3)	Not available
Suspended Solids (SS) (mg/L)		5.5 (1.8-11.6)	5.9 (2.1-11.0)	4.8 (1.4-10.3)	Not more than 30% increase
5-day Biochemical Oxygen Demand (BOD₅) (mg/L)		0.8 (0.1-1.2)	0.7 (0.2-1.1)	0.6 (0.1-1.2)	Not available
Nitrite Nitrogen (NO₂-N) (mg/L)		0.033 (0.014-0.053)	0.035 (0.009-0.060)	0.034 (0.006-0.057)	Not available
Nitrate Nitrogen (NO₃-N) (mg/L)		0.16 (0.06-0.38)	0.16 (0.04-0.40)	0.16 (0.03-0.43)	Not available
Ammonia Nitrogen (NH₃-N) (mg/L)		0.21 (0.07-0.52)	0.17 (0.13-0.26)	0.10 (0.05-0.21)	Not available
Unionised Ammonia (UIA) (mg/L)		0.008 (0.003-0.018)	0.007 (0.004-0.016)	0.004 (0.001-0.009)	Not more than 0.021 mg/L for annual mean
Total Inorganic Nitrogen (TIN) (mg/L)		0.40 (0.24-0.67)	0.37 (0.20-0.64)	0.29 (0.12-0.55)	Not more than 0.4 mg/L for annual mean
Total Nitrogen (TN) (mg/L)		0.58 (0.45-0.75)	0.53 (0.38-0.86)	0.43 (0.23-0.79)	Not available
Orthophosphate Phosphorus (OrthoP) (mg/L)		0.03 (0.01-0.05)	0.03 (0.02-0.04)	0.02 (0.01-0.03)	Not available
Total Phosphorus (TP) (mg/L)		0.05 (0.03-0.08)	0.04 (0.03-0.06)	0.04 (0.02-0.06)	Not available
Chlorophyll-a (µg/L)		2.6 (0.6-9.0)	2.7 (1.0-11.3)	2.8 (0.9-10.9)	Not available
E coli (cfu/100 mL)		9400 (2600-57000)	6100 (350-21000)	910 (16-5400)	Not available
Faecal Coliforms (cfu/100 mL)		23000 (5400-180000)	15000 (930-55000)	2000 (42-11000)	Not available

Note:

1. Data source: Marine Water Quality In Hong Kong in 2006.

2. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, mid-depth, bottom.

3. Data presented are annual arithmetic means of depth-averaged results except for E. coli and faecal coliforms that are annual geometric means.

4. Data in brackets indicate the ranges.

9.6 According to the EPD’s publication “Marine Water Quality in Hong Kong 2006”, the water quality in the Western Buffer WCZ was largely stable with an increase of E coli at the southern part of the WCZ. This may be related to the increased discharges from the sewage treatment works in the western part of Hong Kong Island. In 2006, at the two southern most stations (WM1 and WM2), while the mean dissolved oxygen (DO) level slightly increased by 2%, the bottom DO slighted decreased by 2%. Full compliance with the key WQOs of DO, total inorganic nitrogen (TIN) and unionized ammonia (NH₃-N) was achieved in 2006.

9.7 Based on EPD’s publication “Marine Water Quality in Hong Kong 2006”, the depth-averaged and bottom DO in the Victoria Harbour WCZ were 4.0-7.7 mg/L and 3.3-7.8 mg/L respectively. No hypoxia (i.e. DO<2 mg/L) was observed in the bottom layer. The E. coli counts in the WCZ in 2006 were largely comparable to those in the previous two years, lower at the eastern end and higher in the middle and the western parts of the harbour. E. coli counts at stations VM7 and VM8 in the middle part of the harbour ranged from 350 to 57000 cfu/100ml. A reduction of nitrate nitrogen (NO₃-N) was observed in the Victoria Harbour WCZ, resulting in a slight decrease of TIN (by 11%). The compliance with the key WQOs was 90%, higher than that in 2005 (83%). While the compliance with the WQO for both unionized ammonia and DO were 100%, that for TIN was at 80%.

Identification of Water Sensitive Receivers

9.8 There are no natural streams or rivers within or in close proximity to the proposed works areas. Existing small watercourses within 500m from the Project boundary comprise streams on the hillslopes of Mount Davis and Lung Fu Shan. These streams drain into stormwater drainage culverts on reaching the built-up urban areas. There are no open drainage culverts in close proximity to the proposed works areas.

9.9 Water sensitive receivers (WSR) identified along the harbour front within the assessment area include WSD flushing water intakes at Kennedy Town, Sheung Wan and Central Water Front. The location of these identified WSR is shown on Figure 9.1.

Identification of Potential Impacts

Construction Phase

9.10 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

• wastewater discharge from tunnelling and open cut excavation.

9.11 Specific to the tunnel sections, an identified potential source of water quality impact would be the discharge of tunnelling wastewater from groundwater inflows, drilling and wash-down. Wastewater would also arise from groundwater pumping inside the open cut excavation for Kennedy Town Station.

9.12 Tree walls at Forbes Street located adjacent to the proposed KET Station could be impacted by the potential groundwater drawdown induced by tunnelling.

9.13 No marine-based construction work would be required for the construction of the proposed barging points.

Operation Phase

9.14 The assessment of operational stage water quality impacts of the Project has addressed the following identified potential areas of concern:

• runoff from rail track and operational tunnel drainage

• discharge from any fresh water cooling facilities

• sewage and wastewater effluents from stations

9.15 Seawater would not be used in the cooling system. Both air-cooled and water-cooled chiller schemes are considered to be technically feasible for the WIL project.

Assessment Approach and Methodology

9.16 The Assessment Area for the water quality impact assessment covers the Western Buffer WCZ and the Victoria Harbour WCZ and all areas within 500m from the Project boundary.

9.17 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 and operation phases of the Project are as described in the above section. 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.

Prediction and Evaluation of Impacts

Construction Phase

Construction Site Runoff and Drainage

9.18 Runoff from the surface construction works areas may contain increased loads of sediments, other suspended solids (SS) and contaminants. Potential sources of pollution from site drainage include:

• runoff from and erosion from site surfaces, drainage channels, earth working areas and stockpiles

• runoff from the proposed barging facilities

• wash water from dust suppression sprays and wheel washing facilities

• fuel, oil, solvents and lubricants from maintenance of construction vehicles and mechanical equipment.

9.19 Sediment laden runoff particularly from works areas subjected to excavation or earth works, if uncontrolled, may carry pollutants (adsorbed onto the particle surfaces) into any nearby stormwater drains. For the construction of the cut and cover station at Kennedy Town, bentonite and chemical grouting may be required for diaphragm walling works and as a result may pollute surface runoff.

9.20 As a good site practice, mitigation measures should be implemented to control construction site runoff and drainage from the works areas, and to prevent runoff and drainage water with high levels of SS from entering any nearby stormwater drains. With the implementation of adequate construction site drainage and provision of sediment removal facilities as described in Section 9.29, 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 discharging to the stormwater drain. The contractor would be required to obtain a license from EPD for discharge to the public drainage system.

9.21 No adverse water quality impacts would be expected at the WSD flushing water intakes at Kennedy Town, Sheung Wan and Central Waterfront, given that there would not be any marine based works during the construction activities for the Project.

General Construction Activities

9.22 On-site construction activities may cause water pollution from the following:

• uncontrolled discharge of debris and rubbish such as packaging, construction materials and refuse

• spillages of liquids stored on-site, such as oil, diesel and solvents etc.

9.23 Good construction and site management practices should be observed, as detailed in Section 9.30 - 9.31, to ensure that litter, fuels and solvents do not enter the public drainage system

Sewage Effluent from Construction Workforce

9.24 Domestic sewage would be generated from the workforce during the construction phase. However, portable chemical toilets can be installed within the construction site. 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 water quality impacts would not be expected.

Wastewater Discharge from Tunnelling and Open Cut Excavation

9.25 During tunnelling works, groundwater ingress water pumped out from the tunnel would have a high content of SS and on-site treatment would be required prior to off-site discharge. The water pumped out from the tunnel may be contaminated by bentonite and grouting materials that would be required for the construction of the bored tunnels (for tunnel boring and groundwater treatment) and diaphragm walls for cut-and-cover tunnel sections. Wastewater would also be generated from groundwater pumping inside the open cut excavation for Kennedy Town Station. The quantity of wastewater produced daily would depend on the volume and type of excavation carried out. The maximum wastewater quantities during construction for treatment before discharge into stormwater drains have been estimated by the Preliminary Design Consultant (PDC) and are shown in Table 9.4 for the various works sites.

Table 9.4 Maximum Waste Water Quantities during Construction Phase

Works Site	Wastewater Quantity (L/s)
KET: turnback and station open cut	14.0
Kennedy Town Praya: KET - UNI UNI Construction adit	20.0
Hill Road Vent adit / access UNI - SYP SYP	10.2
Excavations to adit entrances	1.0
Deep ventilation / entrance shafts	5.0
SYP - SHW tunnels (Sai Woo Lane shaft)	6.0

Potential Impact on Tree Walls at Forbes Street due to Groundwater Drawdown

9.26 Potential groundwater drawdown resulted from tunnelling works would have impact on the tree walls at Forbes Street. Such impact would be controlled through regular monitoring of groundwater table together with provisions of recharge well installed at locations where drawdown would be expected to exceed 1m. Monitoring would be conducted on weekly basis during the construction phase.

Operation Phase

Runoff from Rail Track and operational tunnel drainage

9.27 All tracks would be underground and hence there would not be any rainwater runoff. The tunnel wall would be equipped with water-tight liner and designed for no seepage. The amount of groundwater seepage into the tunnel would be insignificant. Any tunnel runoff could be contaminated with limited amount of grease from passing trains or from maintenance activities. 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. The waste would then be disposed of as general refuse and industrial waste as described in Section 7.42. No adverse impacts on marine waters would be expected.

Discharge from Freshwater Cooling System

9.28 According to the preliminary design, both air-cooled and water-cooled chiller scheme would be technically feasible for the WIL project. If water-cooled chiller scheme is to be adopted, the bleed off water from the chiller is recommended to be recycled for flushing use, if possible. The operation of the fresh water cooling system would not be expected to result in unacceptable impacts on water quality since there would not be any spent cooling effluent discharged into marine waters.

Sewage and Wastewater Effluents from Stations

9.29 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. Runoff from cleaning activities at the stations which would enter floor drains would also be connected to the foul sewer. Hence, it is expected that no adverse water quality impact would arise from sewage and wastewater effluents generated during the operation of the stations.

Recommended Water Quality Mitigation Measures

9.30 Proposed mitigation measures for containing and minimizing water quality impacts are summarised below.

Construction Phase

Construction Site Run-off and Drainage

9.31 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, if allowable.

• 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 flow rate 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 summarized 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

9.32 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 7 of this report.

9.33 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

9.34 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

9.35 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 licence 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

9.36 Impact on tree walls at Forbes Street due to the potential for groundwater drawdown induced by tunnelling would be controlled, as a precautionary measure, through monitoring of groundwater table and provision of recharge wells. 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.

Operation Phase

9.37 Mitigation measures are only required to mitigate runoff from track during the operational phase. With the implementation of the following mitigation measures, no residual impact during operational phase is anticipated.

• 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.

• The silt traps and oil interceptors should be cleaned and maintained regularly.

• Oily contents of the oil interceptors should be transferred to an appropriate disposal facility, or to be collected for reuse, if possible.

Evaluation of Residual Impacts

9.38 With the full implementation of the recommended mitigation measures for the construction and operation phases of the proposed Project, no residual impacts on water quality are anticipated.

Environmental Monitoring and Audit Requirements

9.39 Given that there would not be any marine based works for the construction of the Project, water quality monitoring is not considered necessary during the construction phase. However, it is recommended that regular audit of the implementation of the recommended mitigation measures at the work areas be undertaken during the construction phase in order to ensure the recommended mitigation measures are properly implemented.

9.40 As a precautionary measure against potential impact on tree walls at Forbes Street due to groundwater drawdown, monitoring of groundwater level will be conducted on a weekly basis during construction phase of the Project.

Conclusion

Construction Phase

9.41 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 surface works areas, open cut excavation and tunnelling 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 tunnelling wastewater 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 public drainage system and coastal waters. Sewage effluent arising from the construction workforce would also be handled through provision of portable toilets.

9.42 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 construction activities and works areas in order to ensure the recommended mitigation measures are properly implemented.

Operation Phase

9.43 The operational water quality impact for track run-off and tunnel seepage would have no adverse water quality impact provided that mitigation measures are incorporated in the design. The fresh water cooling system for the WIL would not be expected to result in unacceptable impacts on water quality. Sewage and wastewater arisings from the operation of the stations would be discharged to the foul sewer and would not cause adverse water quality impact.