8.6 Mitigation Measures for Construction Impacts

8.6.1 The main potential impacts arising from construction site activities include an increase in the level of suspended solids (SS), pH value and oil & grease content. The following section describes mitigation measures to minimise impacts on watercourses during specific construction activities. The section is divided into types of impact and makes reference to the activities described in the impacts section.

Mitigation for Excavation Works

8.6.2 Potential run-off from excavation activities at the Sheung Shui end of the alignment and at Kwu Tung to create the station box must be minimised to avoid impacts on adjacent watercourses. Under the Water Pollution Control Ordinance (WPCO), turbid water from construction sites must be treated to minimize the solids content before being discharged into storm drains. The suspended solids load can be reduced by directing the runoff into temporary sand traps or other silt-removal facilities and other good and appropriate site management practices. Advice on the handling and disposal of construction site discharge is provided in the ProPECC Paper (PN 1/94) on Construction Site Drainage.

8.6.3 Five main works areas are proposed at the present time, one at each of Sheung Shui (for eastern Ventilation Building and TBM Launching Shaft), Kwu Tung (for Station Box and western Ventilation Building), Chau Tau (for recovery shaft), Lok Ma Chau Road area (for viaduct and Lok Ma Chau Road widening) and Lok Ma Chau Station.

8.6.4 A drainage system layout should be prepared by the Contractor for each of these Works Areas, detailing the facilities and measures to manage pollution arising from surface runoff from works areas. The drainage layout and an associated management plan to reduce surface runoff sediments and pollutants entering watercourses, should be submitted to the Engineer for approval and to EPD for agreement. The system should be capable of handling stormwater from the site and directing it to sediment removal facilities before discharge. If oil and grease is used on the site or brought to the site, the stormwater should pass through oil interceptors before discharge. The interceptors should have a bypass to prevent washout in heavy storms. The following paragraphs detail measures to be incorporated into this drainage system.

8.6.5 A temporary channel system or earth bunds or sand barriers should be provided in works areas on site to direct stormwater to silt-removal facilities. Stockpiled materials susceptible to erosion of rain or wind should be covered as far as practical especially during the wet season. The presence of flat, exposed areas of permeable soil surface can be formed into pits and used effectively as infiltration areas, into which runoff flows, minimizing the amount of runoff into local watercourses. The success of this measure will depend to a large extent on the permeability of the ground, and the site topography. In Kwu Tung and Chau Tau, the lower water table (relative to the surroundings) will allow implementation of this measure more effectively. Where the ground is insufficiently permeable, sedimentation areas may be used. To allow for the intensive rainstorms in Hong Kong, overflow from these sedimentation areas should pass through silt traps to provide additional pollution removal before discharge.

8.6.6 The largest Works Area will be located alongside the TBM launching shaft at Sheung Shui. A preliminary layout of the proposed treatment plant for dealing with most spoil excavated during tunnelling is shown in Figure 8.3. Details of the equipment proposed for treating the water which is removed from the spoil are given in the section below, Mitigation for Bored Tunnel Operation.

8.6.7 Along parts of the alignment, particularly at the Lok Ma Chau end of the alignment, abandoned fishponds within the works area may act as a sedimentation containment area to receive turbid run-off from the construction areas. Minor modification works such as elevating peripheral earth bunds and maintaining silt removal facilities will contribute to reduction of potentially polluting impacts. The area used for sedimentation should be within the Works Area for the contract. It is proposed that the ponds to the east of the Lok Ma Chau Station can be used initially, while ponds are being filled in the platform area. These ponds cover an area of between 3 to 5 ha, which is considered adequate for setting runoff during pond filling. Selected sections of pond bunds within the overall drainage area should be lowered to ensure a steady flow of runoff in the right direction. The pond water should be tested for turbidity before discharge to make sure the appropriate standard is reached. If discharged is to Shenzhen River, a discharge licence will be required from EPD (LCO).

Mitigation for Diaphragm Walling

8.6.8 Diaphragm walling at each end of the tunnel, Kwu Tung station box construction, viaduct foundation/pier construction and Lok Ma Chau station construction all involve the placing of concrete and have the potential to impact the aquatic environment..

8.6.9 The highly alkaline lime content in cement increases the pH level in water and may endanger aquatic life if it is washed into natural water bodies. Where concrete work is undertaken, concrete washings should be carefully channeled to prevent concrete-contaminated drainage from entering watercourses. Where pH levels are above 8.5, the concrete washings should be channeled to a treatment facility to reduce pH to below 8.5 before discharge. Where ammonia levels are already high under baseline conditions, monitoring of pH levels downstream of concreting work should be carried out to ensure ecotoxic conditions are avoided. The pH levels of surface run-off are particularly important in relation to the active fishponds adjacent to works areas.

Mitigation for Concrete Batching Plant

8.6.10 The use of on-site concrete batching plants should be minimized through casting of viaduct units elsewhere and transporting to the site. If a batching plant is necessary, the drainage system should be carefully designed to minimize the likelihood of concrete washings flowing off-site. Sedimentation or infiltration areas should be established to receive concrete contaminated runoff and works areas covered to minimize runoff from areas of concrete production. In the launching shaft area, the drainage should be designed to enable concrete contaminated wastewaters to be treated to meet the required discharge standard.

8.6.11 Where concreting work is required within a watercourse or fishpond, as in the construction of the viaduct supporting column, the new station and culverting of streams, a dam should be constructed and the water pumped out to an area where solids can be settled out, before sediment removal or concreting works is initiated. Where possible, the concrete washings should be diverted to abandoned fishponds nearby to settle out solids. Adjustment of pH can be achieved by adding an acidic additive or other suitable neutralizing reagents to the waste water prior to discharge. Re-use of the supernatant from sediment pits for washing out concrete lorries, should be practiced wherever possible. Re-instatement work on the sedimentation pond will be required after the construction works are complete. Regular maintenance is required for all drainage systems to enable the pollutant control devices to function properly. Supernatant from settled concrete washings should be tested to determine its contamination status before discharge to a suitable location for treatment.

Mitigation for Bored Tunnel Operation

8.6.12 The bored tunnel operation has been designed to minimise impacts on water quality. Only water and a non-toxic, biodegradable foam will be used at the cutter face to form a wet paste which will be excavated and passed by conveyor back to the launching area where it will be settled in soil basins. The conveyor belt and soil basins should be covered to avoid washout during wet conditions.

8.6.13 The biodegradable foam added to the spoil at the cutter head is a tensoactive and a polymer, in an aqueous solution. It is non-toxic and biodegradable, properties which eliminate any potentially harmful impact on the environment. The material to be used for the Spur Line tunneling will be determined based on the required properties for the material to be excavated. A number of suitable materials are available and details will be submitted by the Contractor for approval before use. As an example of such as agent, the foaming agent used on the West Rail Contract DB320, Product Name CLB F4AD, is a water-soluble alkaline material which fully biodegrades after 15 days and is not considered harmful to the environment (information taken from Material Safety Data Sheet from supplier, Condat). An appropriate and equivalent material for Spur Line tunnel construction will be selected by the Contractor.

8.6.14 The foam is injected with water behind the TBM machine. Approximately 100 litres of tensoactive is added to 5000 litres of water to form a foam solution. Air is pumped into the system as the foam solution and passes down the line to the cutter head, where a foam is formed which is injected at the cutter head. A volume of foam agent (0.1 m³) is required for excavation of approximately 108 m³ ground. Being biodegradable and non-toxic, no adverse impacts are expected during use of the foaming agent.

8.6.15 Excess water from the soil basins will be transferred to the water treatment plant where the addition of flocculants will assist in settlement of solids (Figure 8.3). The treatment plant to be used in the Spur Line project will be similar to that used on Contract DB320 on West Rail (Kwai Tsing to Chau Tau Tunnels). For illustration, a description of this plant is given below. However, it should be emphasized that the detailed design of the treatment plant is not complete and the DB320 treatment system is provided for example only. The treatment plant proposed for treating the wastewater arising from the bored tunnel operation on Contract DB320 comprised a series of units with different functions as described below.

(i) Pre-sedimentation Tank and Oil Trap

Two pre-treatment tanks with a volume of approximately 20 cm³ were used to remove coarse particles and floating matter such as soil.

(ii) Desander

Desanding was carried out in a cyclone and a sand checker screen.

(iii) Flocculant dosing

Flocculant dosing equipment to add flocculant depending on the sediment load.

(iv) Settlement tanks

Two circular settlement tanks with conical bases approximately 6 m diameter and 5.5 m high. Collected sludge can be drawn off from the base and sent to the spoil handling basin or sent for further treatment.

(v) Cake plant

Settled sludge may be passed to the filter press to produce cakes. The extracted water is returned to the beginning of the treatment unit for treatment. The cake is disposed of with the TBM spoil.

8.6.16 Data was obtained from the water treatment system during the EM&A programme for DB320 and an example of the effluent quality is shown in Appendix 8.1. The results indicate that for the majority of the time, the discharge standard of 25 mg/l suspended solids was achieved. The wastewater discharge from the plant at Sheung Shui for Spur Line will also be subject to a discharge licence.

8.6.17 As stated above, the treatment plant at Sheung Shui TBM Launching Shaft is likely to be similar to the DB320 treatment plant but will be specifically designed for the conditions of the area and the nature of wastewater expected. Measures must be incorporated into the monitoring require for the plant to evaluate the performance against discharge standards.

8.6.18 The volume of wastewater produced daily will depend on the volume and type of excavation carried out. In Contract DB 320 for West Rail, the average volume of wastewater produced (which was included in the discharge licence) was around 900 m³. The material being excavated included both rock and soft ground. In Spur Line, the average tunnel drive will excavate around 500 m³ per day. As this is mainly soft material, the water volume of the material may be around 80% water, and the discharge volume will therefore be of the order of 400 m³ per day. Additional water will be required for the foaming agent. At this stage, the exact volumes likely to be produced are uncertain, however, this will be included in the discharge license for the treatment plant. The settleability of the material is also likely to vary and the flocculent addition must be optimised to deal with this.

8.6.19 The addition of flocculants to the wastewater treatment plant should be optimized for the wastewater being treated and regular inspection and monitoring should be undertaken to ensure the discharge meets license standards. The water treatment plant should be regularly maintained, solids removed and dried in a filter press before disposal. The use of bentonite during manned interventions will require careful handling and disposal to landfill following excavation. All discharges from the treatment plant are subject to control under the WPCO.

Hydrological Impacts during Construction

8.6.20 Potential impacts on groundwater levels, particularly in Long Valley, will be avoided through the operation of the TBM in Earth Pressure Balance Mode (EPBM). This mechanism is designed to balance the groundwater pressure at the face with an equal pressure from the TBM machine. In uniform soils, operational problems are unlikely. However, monitoring of groundwater and settlement of the ground above must be conducted during and after a TBM passby.

8.6.21 Contingency plans must be prepared by the Contractor in the case of maloperation of the EPB mode. If operation cannot be carried out in this mode, there is a potential risk of ground blowout or cave in. Anticipation of potential problems will minimise residual impacts on water quality or hydrology from this operation. A detailed assessment of potential impacts and required mitigation is provided in Chapter 3, Hydrology.

Mitigation for Formation of Cross passages

8.6.22 Jet grouting should be carried out under dry conditions as far as possible. Where water addition is required, or the works are carried out in wet conditions, the works area should be bunded to prevent run-off entering adjacent watercourses. In particular, spillage from the grouting production equipment or from other activities, should be contained until the drilling operation is compete. Water should be pumped out of the area and treated (if necessary) before discharge.

8.6.23 Where drilling is carried out near to potentially contaminated land, special care should be taken to avoid run-off contacting contaminated material and entering adjacent watercourses. Settlement areas should be constructed and a local drainage system implemented to contain the water generated. Water should not be discharged from site but pumped out and treated to reduce turbidity and remove contaminants (if present) before discharge.

Mitigation for Works in River Sutlej

8.6.24 Works in the River Sutlej should be conducted in dry season only to avoid washout of the works in the wet season, in compliance with DSD’s requirements. The low flow channel must be suitably relocated and protected to avoid overflow into the excavation or washing of concrete or grout into the channel. Spilt concrete washings should be contained during the works and removed from the channel area as rapidly as possible. A sump or drainage area near the works area should be provided to allow settlement of solids before water is pumped out.

Mitigation for Hydrological Impacts

8.6.25 Mitigation for possible impacts on the underground alluvial phase groundwater flow can be achieved through increasing the permeability of the concrete or grouted mass placed below the River Sutlej before tunnel boring begins. Positioning of suitable sized pipes through the concrete or grout mass and addition of a granular layer either side of the mass, will enable groundwater flow through the structure as before. This measure can be carried out relatively easily where excavation is followed by placement of a lean mix concrete box. In the case of jet grouting, additional excavation may be required to ensure the groundwater flow can be maintained through a pipework system.

Mitigation for Draining works in Fishponds or Rivers

8.6.26 Avoidance of impacts during draining of water from Chau Tau Channel can be achieved through damming of the are in which works is carried out, using sandbags or other means. Measures should be taken to avoid the water coming into contact with water that has been polluted through the works activities. Any water that is impacted and becomes turbid should be confined locally. All work related to Chau Tau channel diversion should be undertaken and completed during the dry season.

8.6.27 Sand filling at Lok Ma Chau fishponds will require displacement of the water from the fishponds. To avoid this water becoming turbid and overflowing to watercourses and ultimately into Shenzhen River, a sedimentation area should be identified within the site and this area used as a stilling basin for water to gradually overflow into. A possible location for this purpose is the area to the east of the station that will be used in future as a reedbed and ecological mitigation area. This area occupies 3 to 5 ha, which should allow sufficient retention time for settlement of the solids in the fishpond water. Once settled, the water can be discharged (through overflow between ponds) into adjacent ponds within the future ecological mitigation area. Water quality should be checked to ensure the water turbidity is similar to the ponds into which water is being discharged. If the water quality is still too turbid, treatment to reduce the turbidity will be required before the water can be discharged. Treatment may comprise a tank into which water can be pumped and flocculant added in a similar way to the treatment for spoil water at the TBM launching shaft. The treatment tank should be sized appropriately for the volume of water to be treated. Water should only be discharged after it meets the required standard.

8.6.28 In all circumstances drained pond mud or river sediment should be removed immediately or stored away from the watercourse. If stockpiled on site, the stockpile should be covered to prevent erosion. The quality of the mud in terms of contaminant concentration should be evaluated prior to removal. Any metal contaminated material, or large quantities of uncontaminated material, requires disposal in designated areas. Stockpiles should be surrounded by drains connected to a sediment trap or containment areas to make sure runoff is adequately treated before discharge. Wherever possible, in situ strengthening of the mud is recommended to minimize fill requirements. Pollutant measurements of sediment in abandoned ponds (Lok Ma Chau Boundary Crossing Expansion EIA, Binnie, 1999) has shown that nutrients are high in pond mud, however, metal levels are low and there is therefore minimal contamination risk.

Mitigation for Bored Piling

8.6.29 The use of bentonite during bore piling requires correct handling and disposal to avoid water impacts. Run-off from bore piling works should be settled in a sedimentation or infiltration pit until the supernatant is clear, after which it can be pumped to a drain, or it may be allowed to infiltrate into the ground.

8.6.30 In some locations along the alignment, abandoned fishponds within the works area may be used as a temporary infiltration or sedimentation pits for settlement of solids, concrete washings or bentonite washings. The pits should be regularly cleared of solids and covered in wet weather, to prevent turbid water from being washed over into storm drains during heavy storms. As described above for the ponds used to settle water around Lok Ma Chau station, the water should be settled to reduce turbidity or treated appropriately before discharge.

Mitigation for the Footbridge between Lok Ma Chau station and Shenzhen

8.6.31 Foundation work for the footbridge will involve piling a cofferdam around the area of the proposed pier for purposes of piling, pile caps and column work. During creation of the piers, care should be taken to ensure no potentially polluting liquid or solid wastes fall into the river. This is essential to avoid impacts downstream in the ecologically sensitive area of Deep Bay and Mai Po marshes. The cofferdam should be constructed to minimize contact of the works area with the surrounding waterbody. In order to avoid water quality impacts after the cofferdam is completed, any waste materials arising should be taken onto the attendant barge and removed for disposal. The pier to be constructed within the Hong Kong boundary is to be located within the alignment of the former riverbank extended for widening of Shenzhen River, and it is therefore unlikely that any of this material will be contaminated. The material can therefore be disposed of as uncontaminated sediment.

8.6.32 Adverse impacts on water quality from deck construction can be minimized by using pre-cast units constructed off site. Where concreting works is carried out on site, it is essential that a suitable mechanism is put in place to avoid concrete washings falling into the river. This may be done by incorporating a drain into the formwork design to catch washings and divert them back to the shore where they should be settled in a sedimentation pit and the supernatant treated before discharge. Any works area alongside the river should have appropriate drainage to remove sediment from surface run-off and incorporate oil and grease traps to capture run-off from the site. Vehicle washing areas should be provided and petrol/oil interceptors incorporated into the drainage. The interceptors should be regularly maintained. Regular maintenance of these pollution control devices is essential to ensure their efficient functioning.

8.6.33 The pier on the Shenzhen side of the river may be located in an area of potentially contaminated sediment. The sediment from the riverbed must first be tested to determine the level and type of contamination, and if necessary, disposed. The disposal site should be agreed with EPD and FMC. If the supernatant after settlement is contaminated, disposal via a suitable route will be required, either to sewer, local sewage treatment works, or, if highly contaminated, to an appropriate treatment location, as agreed with EPD.

8.6.34 These measures are designed to reduce the potential for adverse effects on the water quality and wildlife downstream in Deep Bay. Implemented properly, the potential for pollutants to enter the river will be minimized.

8.6.35 The contractor carrying out the works should submit a detailed Waste Management Plan, which should include a description of works methods and measures incorporated to minimize potential pollution from contaminated material during the construction process. This material may be sediment or contaminated land.

Mitigation for Potential Hydraulic Impacts

8.6.36 The construction of supports for the footbridge across the Shenzhen River has the potential to impact the hydraulic flow of the river. From the results of the DIA carried out for this scenario, the impact is likely to be small. The cross sectional area of the supports is a small proportion of the channel surface area, indicating that the change in the hydraulics of the river flows at this location will be minimal.

Mitigation for Jetties for Materials Transfer

8.6.37 Although impacts from the construction of the jetties are likely to be minimal, care should be taken to avoid waste materials falling into the water during both the construction and dismantling process. Transfer of material onto the barge at the sandfill location should be conducted carefully to allow the displaced water to be discharged slowly, thereby minimize impacts on water quality in the receiving waterbody. Where possible, displaced water should be discharged onto land, into a sedimentation pit where the solids can be allowed to settle or infiltrate into the ground before the clearer supernatant is discharged.

8.6.38 At the construction site, the transfer of materials onto the jetty should also be carried out with care to avoid sand falling into the river from the conveyor belt. The conveyor belt should not be overloaded and covering will assist in containing the sand material. Where stockpiles are set up on shore, they should be covered to prevent run-off entering the river during storm conditions. If water is required for washing purposes during the operation, then is should be pumped into a sedimentation area before clearer water is discharged into the waterbody. The barge should be regularly maintained to minimize the potential for fuel or other contaminants entering the waterbody. All waste generated by the workers onto the barge should be disposed of at allocated sites for waste disposal at the site of collection or delivery. Wastewater collected on the barge should similarly be carefully disposed of at suitable locations to avoid water quality impacts. These working practices and design of the transfer system will provide conditions that minimize impacts downstream in Deep Bay and associated ecologically sensitive mud flats.

8.6.39 Potential impacts from the presence of the jetties on the hydrological regime of the Shenzhen River have been shown to be minimal in the DIA carried out for the Lok Ma Chau station construction works. No mitigation is therefore required.

Mitigation for Site Workshop or Depot

8.6.40 Any contractor generating waste oil or other chemicals as a result of his activities should register as a chemical waste producer and provide a safe storage area for chemicals on site. Hard standing compounds should drain via an oil interceptor. Disposal of the waste oil should be done by a licensed collector. Oil interceptors need to be regularly inspected and cleaned to avoid wash-out of oil during storm conditions. A bypass should be provided to avoid overload of the interceptor's capacity. Good housekeeping practices are required to minimize careless spillage and keep the work space in a tidy and clean condition. Appropriate training including safety codes and relevant manuals should be given to the personnel who regularly handle the chemicals on site.

Mitigation for Additional Population (Workers) on Site

8.6.41 Sewage arising from the additional population of workers on site should be collected in a suitable storage facility, such as an underground septic tank or mobile toilet. Small scale on-site treatment plants should also be considered if the number of workers in one area indicates that this is more appropriate. The collected wastewater from sewage facilities and also from canteens or washing facilities must be disposed of properly, in accordance with the WPCO requirements. At Sheung Shui, connection to foul sewerage in Sheung Shui should be considered. In other areas, wastewater collected should be discharged into foul sewers or collected by licensed collectors and disposed of at government sewage treatment facilities.

8.7 Summary of Mitigation Measures

8.7.1 Table 8.17 shows the range of mitigation measures that should be implemented during Spur Line construction to avoid impacts on waterbodies in the vicinity of the works area.

Table 8.17

Summary of Mitigation Measures for Construction Impacts

Potential impact	Proposed Mitigation
Site Surface Runoff from Excavation Works	· Site management practices in accordance with guidelines in ProPECC PN 1/94. · Creation of suitable drainage system, using channels, sand bags or bunds. · Use of silt trap settlement or infiltration areas for solids settlement.
Diaphragm Walling/Concrete Batching	· Site run-off containing concrete washings should be settled in depressed area and supernatant reused where possible. · Concrete batching plant area should be covered and a suitable drainage system installed to separate concrete washings and clean surface water. Concrete washings to be treated via settlement and appropriate pH adjustment prior to discharge. · Adjustment of pH in concrete contaminated water before discharge.
Bored Tunnel Operation	· Material added to soil at the face should be harmless and biodegradable. · Provide treatment plant to treat wastewater arising from the spoil basin. Regular monitoring and maintenance of water treatment plant. · Monitoring of groundwater levels and settlement during TBM passby. · Preparation of contingency plans in case EPB mode fails.
Formation of Cross passages	· Bunds and drainage system to be installed to contain spillage and potential run-off. · Avoidance of contaminated areas and containment of water that passes over contaminated areas. The contained water should be treated prior to discharge.
Works in River Sutlej	· Works to be carried out in the dry season. · Contain concrete washings and remove from channel. · Relocate and protect low flow channel to avoid contamination of water or overflow into excavation. · Prepare contingency plans for rainfall during works. · Install suitably sized drainage pipes in impermeable box to avoid impacts on groundwater flow in alluvial phase.
Draining works in Fishponds or Rivers	· Undertake and complete diversion work related to Chau Tau channels within the dry season. · Contain any disturbed areas to avoid turbid water entering watercourse. · Minimise mud removal. If necessary, test and dispose of as required. · For fishpond filling, provide settlement areas for the displaced pond water. Use other fishponds as settlement areas if possible.
Bored Piling	· Recycle bentonite wherever possible. · Contain run-off containing bentonite and drain to settlement area.
Footbridge between Lok Ma Chau station and Shenzhen	· Avoid spillage of wastewater and materials into river. · Test sediment prior to dredging, adopt appropriate disposal means for the dredged sediment. · Minimise concrete production on site – use pre-cast units wherever possible. · Minimise stirring up of water, particularly where sediment may be contaminated. · Minimise hydraulic impacts through pier design.
Jetty for materials transfer	· Care should be taken when transferring materials to avoid spillage into river. · Water used for washings should be pumped into settlement area to allow settlement before surface water is discharged.
Site Workshop or Depot	· Chemicals and waste oils should be stored, handled and disposed of to avoid impacts on watercourses. · Oil interceptors should have a bypass installed and be regularly maintained.
Additional Population (Workers) on Site	· Wastewater from workers should be collected and discharged according to discharge license.
Hydrological Impacts during Construction	· Detailed monitoring of groundwater levels during passby of the TBM. · Settlement monitoring and proposed reinstatement to make good any observed decrease in land levels as TBM passes beneath Long Valley.

8.8 Potential Impacts During Operation

8.8.1 The main operational impacts from the Spur Line are:

· Impacts from the presence of an additional pier for the footbridge linking Lok Ma Chau and Huanggang stations across Shenzhen River.

· Hydrological Impacts from the presence of the Tunnel beneath Long Valley

· Stormwater run-off impacts.

· Sewage generation from the station.

8.8.2 Due to the more specific nature of the impacts during operation phase, impacts are presented together with mitigation measures to facilitate continuity of reading in this section.

Potential Impacts from Footbridge Pier

8.8.3 The presence of bridge piers within the river has the potential to create an impact on the hydraulics of the waterway. The significance of the impact can be evaluated by a consideration of the cross-sectional area of the piers compared with the cross-sectional area of the river. The two piers closest the Hong Kong side are small, around 2m diameter and are therefore likely to have minimal impact on the river flow. The main two piers are located beneath the main bridge deck, each 10m at the base, normal to the flow, tapering to 8.1m below the bridge deck. Their width perpendicular to the flow is 2.5m. The main piers will reduce the width of the river, which is 190m at this point, by approximately 1%. From a hydrodynamic point of view, the pattern of water flow will change most significantly around the pier base.

8.8.4 A drainage impact assessment for the material delivery jetty and the Hong Kong side of the bridge has been carried out as part of the detailed design for the Lok Ma Chau station works (KCRC LDD200 DIA, 2001). The results of the hydraulic modeling indicate that the highest water levels will be measured during a 200yr tide and 10yr rainfall event. Under these conditions, the water level in the river will reach 4.149mPD. This is still below the river bank level of 4.5mPD. The greatest change in water level (from the baseline condition) will be experienced during a 200yr rainfall and 10yr tide event. Under these conditions, the water level will increase by 94mm.

8.8.5 Under worst case conditions, the bridge piers may be under construction while the jetties for materials delivery are still present in the river. The jetties will be constructed on piles as described in Section 8.5. The drainage impact assessment shows that the concurrent presence of the jetties and the bridge piers resulted in a top water level of 4.159mPD (under 200yr tide and 10yr rainfall events) and the greatest change in water level (from baseline figures) was 144mm during a 200yr rainfall and 10yr tide event. Based on the current construction programme for the LMC Station, the longest period for the presence of both jetties and bridge piers will be two wet seasons. The chances of such extreme conditions is small, a more likely scenario being the 50yr rainfall event, during which the water level will be raised 81mm to 3.812mPD. The drainage impact assessment concludes that the increased risk during the construction period is considered to be acceptable.

8.8.6 There is also potential for the piers to increase scour on the river bed. Information obtained to date indicates that the bridge piers will be designed to allow minimal headloss in a 100-year storm. In a 2 year storm, the velocity of water is likely to be approximately 0.3m/sec. Assuming that under normal circumstances sediment on the river bed will not be lifted into suspension until the velocity reaches 0.6m/sec, then in the absence of the piers, no sediment suspension would be created. However, around the pier base, the velocity will increase, creating conditions that allow sediment to be brought into suspension. Assuming the sediment is fine and settles at a rate of 0.005m/sec, after it has passed the pier the particles will settle to the river bed again within about 200 to 300 metres. As the distance from Lok Ma Chau to Mai Po and Inner Deep Bay is approximately 2.5km, there will be minimal impact on water quality and downstream ecological resources under these circumstances.

8.8.7 No data is publicly available on sediment quality in Shenzhen River, however, it is expected that the sediment quality is poor due to the volume and type of discharges which enter the river from both sides of the border. Stirring up of the sediment around the pier bases may release pollutants from the sediment. While the sediment will settle relatively quickly in less turbulent flow regimes downstream of the piers, the soluble pollutants may be carried downstream to Mai Po and Inner Deep Bay. The potential impact can be minimized by protecting the river-bed to minimize the effect of additional turbulence around the bridge piers.

8.8.8 In more severe storms, sediment will be lifted into suspension more quickly due to the greater velocities of water travelling down the river. The impact from the piers will be minor compared with the force of the water body lifting the sediment across the entire cross-section of the river. This sediment load will be transported downstream into Deep Bay as a plume of turbid water, not due to the presence of the piers, but due to the water flow in storm events.

8.8.9 Of more concern under these circumstances is the headloss caused by the piers. There is a potential for the piers to hold back the water flow in severe storms. The drainage impact assessment has shown that the presence of the piers will be minimal even in a 200yr rainfall event. In addition, the effect of the footbridge piers would not be greater than the effect of the (much larger) piers at the Lok Ma Chau – Huanggang vehicle bridge further upstream. The shape and number of piers clearly influence the degree of impact, and have been designed to minimize downstream impacts on water quality and hydraulic impacts upstream.

Mitigation for Impacts from Footbridge Pier

8.8.10 Minimising hydraulic impacts have been achieved through several mechanisms. A reduction in the number of piers to two large and two small piers has been incorporated into the design to reduce the proportion of the cross-section occupied by the obstruction. The shape of the large piers has been minimized normal to the flow to improve the streamlining effect of the structures. This design reduces the velocity around the piers. As described above, there will be a local increase in velocity around the piers, resulting in turbulence and lifting of sediment from the riverbed.

8.8.11 In conclusion, each pier in the river will have local impacts on the water velocity. This will lead to stirring up of the sediment and scour around the pier base. In low flow situations, this will cause local turbidity. However, the effect can be minimized by riverbed protection and in any case, the solids lifted are likely to be fine, and downstream of the pier they will quickly settle out. In more severe storms, the sediment on the riverbed will be lifted across the whole river section and the presence of the piers will have minimal additional impact on the water quality downstream. A turbid stream of water will flow into Deep Bay, the impact from the piers being minimal. Headloss from the presence of piers has been considered in the design by minimizing the number of piers. The additional impact from the presence of the footbridge on the ecological resources of Deep Bay and its associated mudflats, will therefore be minimal.

Potential Stormwater Run-off Impacts

Railway track run-off

8.8.12 The subject of urban stormwater run-off is a relatively new study, most of the data on stormwater quality having been collected from roads, residential and commercial areas. There is very little information available on the quality of stormwater run-off from railways, other than reports of impacts such as an increase in run-off due to reduced permeability, potential changes to drainage patterns, increased concentrations of metals from rails and enhancement of vegetation growth due to increased moisture in areas alongside the rail track. Given the lack of data, this section will therefore provide a qualitative description of the types of pollutants that may occur in railway related run-off and hydrological impacts on surrounding land.

8.8.13 The potential impacts from stormwater run-off have been minimised by placing the railway in tunnel. Drainage channels at the entrances will capture most of the surface water before it enters the tunnel. Water that does fall from the train during its passage through the tunnel will drain to the tunnel sumps located at the low points of the tunnel. These sumps must be sized appropriately to ensure no overspill during heavy rainfall events. Pumps will regularly pump the water to the surface where it must be tested before discharge, in accordance with the WPCO licence. Testing will be in accordance with the discharge license which will be prepared according on the discharge location.

8.8.14 Pollutants which may be present in run-off either from the track or from the train and their potential sources and impacts are listed in Table 8.18. The most significant sources of pollution are considered to be the metal grindings washed off after rail grinding and lubricants present on switch points, a proportion of which will be washed off in storm events. The extent of these impacts will depend on the number of trains using the track, the frequency of rail grinding, the train design, the quantity and type of lubricant used and the type of train maintenance carried out.

Table 8.18

Summary of type of pollutants in run-off from railway lines, their potential environmental effects and relative significance in Spur Line operation

Pollutant	Potential source of contamination	Environmental impact	Relative importance on Spur Line operation
Lubricants	Used at the switch points on the track to ensure ease of movement.	Organic pollutant that is not readily biodegradable may have toxic effects on organisms in receiving water bodies. Environmental accumulation may lead to sub-lethal effects, reduction in species diversity.	High impact
Metals	Metal grindings (probably mainly iron) after the train runs on the track and after track grinding	May have toxic effects on some species in receiving waters, may have colour effect, promoting enrichment. Potential accumulative effect in organisms.	From train - low impact Track grinding - high impact
Suspended solids	Suspended solids and dust from the train.	Increase in turbidity of receiving waters, light reduction for primary production, blanketing of habitats, physical damage to some organisms.	Low impact
Oil and grease	Used on the train where parts of the train are exposed.	Reduces surface tension of water surface, may be toxic and cause physical damage to some organisms, change in light penetration.	Low impact
Significant increase or decrease of pH	Lubricants and cleaning materials used on the trains	Can cause ecotoxic conditions in polluted waters, especially where ammonia levels are high. Changes solubility of metals and toxicity of organic species.	Medium impact

8.8.15 At a frequency of approximately twelve trains per hour in each direction, and with half of the alignment in tunnel, the hydraulic impact of stormwater run-off from trains is expected to be small. At grade sections are very short, minimising any change in the hydrology of the area. In the viaduct section, the train is well above the ground, minimizing flooding risk. However, the run-off will be directed to ground level and locally may increase water levels through increased storm flow rates. At Lok Ma Chau station, the presence of an impermeable concrete structure where fishponds were previously located, will increase the stormwater flowrate locally and may cause an increase in the water levels of streams around the area.

8.8.16 The concentration of pollutants in the run-off will depend on the maintenance practices within the depots and the train design. During maintenance, the use of oil & grease and lubricants for moving parts, and cleaning agents for the outer train body, have the potential to impact the environment after the train leaves the depot, by wash-off during storms. The type of chemicals used for cleaning and their removal after cleaning operations will determine the extent of impacts from this source.

8.8.17 Rail grinding is expected to be carried out two or three times annually. During the grinding process, a proportion of the metal dust (probably mainly iron as the tracks is steel construction) will be deposited between the tracks within the plenum unit. While dry, these metals grindings have minimal impact. However, during a storm event, the remaining metal grindings will be washed into the drainage system and, depending on the proximity of watercourses, may immediately enter water bodies. Potential impacts on water quality and aquatic life are described in Table 8.18. The metal loading from this source during each grinding operation may be small relative to impacts from local roads, container storage areas and urban areas. However, the cumulative impact is likely to be significant if mitigation measures are not incorporated into the design and operation of the railway and trains.

8.8.18 The section of the rail alignment where potential impacts are of most significance is within the Lok Ma Chau fishpond area. Run-off from the track will flow into the drainage system of the viaduct that will discharge the run-off through downpipes to ground level. The environmental impact will depend on the location of the discharge. Where run-off flows into nearby fishponds impacts may occur on the ecology and water quality of the ponds. This impact should be minimised through careful design of viaduct drainage.

Station run-off

8.8.19 The station is likely to be completely enclosed and therefore run-off will be limited to wash-off from the outside of the building, polluting activities within the building which release stormwater into the drainage system, and cooling water discharges. The presence of the physical structure of the building will increase the volume of run-off compared with the current environment of fishponds. A drainage channel is included in the design to transport run-off into the adjacent Shenzhen River. In comparison with the extent of the fishpond area around the station and the current flowrate within the Shenzhen River, the proportional increase in run-off flow from the station structure is considered to be small and therefore of low impact. As part of the ecological mitigation design, a small proportion of the run-off from the roof will be diverted from the main station drainage, into the portion of the channel upstream of the station drainage channel. This section of channel forms a storage area during low flows, and in high flows the water will be discharged via the main station drainage channel.

8.8.20 Sources of potentially polluted stormwater that may arise from the station run-off include:

· dust from the roof of the station;

· cleaning agents used for washing outside windows and walls;

· washdown water from car parks (washing of cars within car parks);

· water from trains entering the station during storm events;

· cooling water discharges.

8.8.21 Run-off from the station roof, walls and windows will contain low levels of suspended solids and surfactants from cleaning agents used for washing. With good washing practice, in dry weather and thorough rinsing and collection of rinsate from surfaces, the concentrations of the latter are likely to be low. However, dust build-up will result in turbid run-off, especially during the first rains of the wet season. This will add to the already heavy solids loading in the receiving water bodies.

8.8.22 Run-off from within the station includes washdown water from car washing in car parks. Car washing solutions contain surfactants, which may have high COD values and be toxic to aquatic organisms. Suspended solids levels will be high and will increase turbidity if allowed to drain directly to receiving waters. Pollutants from trains entering the station in storms are expected to be low, as most of the dust and any oil and grease is likely to have been washed off during track running. Minor maintenance that may be required on trains before they leave the station is a small potential source of chemicals and lubricants.

8.8.23 Package air-cooled chillers with condensing in fans will be used in Lok Ma Chau Station. Under normal operation of the chillers, no cooling water will be produced. In emergency conditions, cooling water may be discharged and providing this is directed to the sewage treatment system at the station, no environmental impacts will occur. The type of coolant and volume likely to be released has not yet been decided.

Mitigation measures

8.8.24 Methods of minimising pollution impacts include:

· Avoidance of impact

· Minimization of extent of impact

· Minimization of intensity of impact

Mitigation by washing and maintenance practices

8.8.25 Maintenance practices require the use of oil & grease and lubricants to ensure the safe operation of the train. Good practices should be followed to avoid excessive use of these materials and selection of environmentally friendly chemicals wherever possible.

8.8.26 A number of car parking spaces are allowed for in Lok Ma Chau station. Where car washing practices are carried out, the use of cleaning materials should be minimised and non-toxic materials selected for washing purposes. The washings should be passed through a sediment trap and oil interceptor before passing to the wastewater treatment plant.

8.8.27 Pollution control systems such as oil interceptors and sediment traps require regular maintenance to provide an efficient system for pollutant removal. Oil interceptor contents should be recycled or disposed of to landfill.

Mitigation by design of trains and drainage system

8.8.28 The train body has already been designed to minimize exposure of those areas of the train which will have the greatest potential to release pollutants (suspended solids, oil & grease and metals) during operation. Once the pollutants are released into the stormwater, mitigation measures are required to minimise impacts to the environment.

8.8.29 In the tunnel, the drainage system should incorporate sediment traps to remove solids before the water is pumped to the ground surface. Alternatively, if water is turbid when pumped to the surface, it may be passed through a sediment trap before discharge. An oil interceptor should be installed if oil and grease will be used on the track at any time during maintenance within the tunnel.

8.8.30 Along the viaduct sections of the railway, run-off containing metal grindings, suspended solids, and oil & grease (particularly at the bends at Lok Ma Chau) can be minimised by incorporation of a sediment trap and/or oil interceptors in the viaduct sections at the discharge point of the drainage pipes. This trap should be sufficiently large to function effectively in low flow conditions and include a bypass for storm conditions.

8.8.31 Each of these pollutant control systems should be regularly cleaned and maintained in good working order to ensure their efficiency of function. The contents of sediment traps should be transferred to an appropriate disposal facility.

Mitigation of the extent of the impact

8.8.32 After discharge from the sediment trap, the run-off is likely to enter the wetland areas in Lok Ma Chau. This should have a bypass to prevent washout in heavy rain.

8.8.33 With the recommended facilities included in the drainage system, the level of pollutants in stormwater run-off from the railway is expected to be minimal.

Sewage Generation

8.8.34 The development of the Lok Ma Chau station will alter the landuse in this area considerably, bringing a large number of people into the area that was previously largely undisturbed fishponds. The population working at the station and passing through the station will generate a volume of sewage that will require treatment before discharge to the aquatic environment.

8.8.35 The station will be staffed by approximately 1100 staff members on a daily basis. In addition, the number of passengers expected to cross the border at Lok Ma Chau Station will vary according to the time of day and period of the year as shown below (estimates taken from the Implementation Proposal to Government).

Ultimate scenario (2016)

Weekdays Average number of passengers is 131,000/day

Peak flows during rush hour up to 200,000/day

Weekends Average number of passengers is 196,000/day

Festivals Average number of passengers is 251,000/day

8.8.36 Based on these staff and passenger numbers, the volume of sewage generated has been calculated as shown in Table 8.19.

Table 8.19

Daily volume and concentration of sewage

generated at Lok Ma Chau Station

Criteria	Staff	Passengers	Total
Number of people/day	1100	251000	252000
Sewage flow (l/head/day)	204**	20**
Percentage of passenger/staff using toilet/sink	100**	20**
Total daily sewage flow (m^3)	224	1004	1228
BOD5 load (g/head/day)	55*	14**
Total BOD5 load (kg/d)	60	703	760
Sewage BOD5 concentration (mg/l)			619

EPD Guidelines for design of small sewage treatment plants

** Percentage use follows usage estimated at Lo Wu Station, which was estimated from

British Railway Recommendation WWO Provisional standing order no. 1309

Sewage flow and BOD₅ load for passengers was also taken from British Railway Recommendation.

8.8.37 The peak flow during rush hour periods is estimated to be about 20,000 passengers, which multiplies up to a daily passenger flow of 480,000. The sewage flow during this period should be accommodated by including a balancing tank into the design of the sewage treatment works to hold the peak flows, while the overall design would be based only on maximum daily flows (1228m³/day on festival days).

8.8.38 Sewage is normally disposed of to the public sewerage system and transferred to a sewage treatment works. In the case of Lok Ma Chau Terminus, the closest sewerage system is some distance away, (committed Western Trunk Sewer will reach Pak Shek Au) and it would not be cost-effective to construct a connecting sewer to this point. The only feasible alternative is therefore an on-site sewage treatment facility. In order to reduce the environmental impact of sewage generation from this source, a Rotating Biological Contactor (RBC) or equivalent appropriate sewage treatment process, which will meet the performance requirements, will be incorporated into the design of the station to treat the effluent to the standards required for discharge into the adjacent water body (Shenzhen River or the proposed San Tin Drainage Channel). For the purposes of this assessment, the RBC will be quoted as the potential sewage treatment process. The wastewater treatment system will include a chlorination unit for disinfection of the wastewater to reduce E.coli to acceptable levels. As residual chlorine can also cause toxic effects, a chlorine residual limit of 0.5mg/l is included in the discharge limit.

8.8.39 The discharge standard for the river into which the effluent from the RBC and the polishing reedbed will discharge is defined under the Technical Memorandum for Standards for Effluents discharged into Drainage and Sewerage Systems, Inland and Coastal Waters. The receiving watercourse discharges directly into Shenzhen River and is not used for fish pond culture in the area either now or in the future when the whole area west of the river will be occupied by Wetland Compensation Area for KCRC Spur Line. The most suitable discharge standard is therefore Group B. The standards are summarized in Table 8.20.

Table 8.20

Discharge Standards for Effluents discharged

into Group B Inland Waters

Parameter	Discharge standard based on effluent flowrate of 1208m³/day
pH	6.5 - 8.5
BOD5	20
COD	80
SS	30
E. coli (cfu/100ml)	100
Total residual chlorine (additional standard)	0.5

All values in mg/l unless otherwise stated

8.8.40 In order to achieve these discharge standards, the RBC must have a BOD₅ removal rate of approximately 97%. Assuming this can be achieved, the resulting BOD₅ loading from the effluent discharge would be 24.6kg BOD₅/day and 36.8kg SS/day.

8.8.41 As this pollutant loading is from a new development in the Deep Bay Water Control Zone, mitigation of impacts must be achieved in accordance with EPD’s Zero Discharge Policy (ZDP). The ZDP states that there must be “no net increase in the pollutant load to Deep Bay from new developments”. Mitigation measures are therefore required to off-set the pollutant load that would be discharged into Deep Bay waters from the RBC treatment plant.

Mitigation measures for sewage generation

8.8.42 Two steps are recommended to achieve compliance with the ZDP policy. The first involves a further treatment step to “polish” the effluent discharged from the RBC. The second step is to extract a quantity of water from the San Tin River, for appropriate treatment to remove a quantity of BOD₅ equivalent to the BOD₅ loading discharged in the polished RBC effluent.

Effluent polishing

8.8.43 The effluent discharged from the RBC will contain primarily residual BOD₅, COD, suspended solids and nutrients (N and P in various forms). These pollutants are not highly toxic in the concentrations expected in the discharge and can be reduced further through a “natural” treatment mechanism in the form of reedbeds.

8.8.44 Vegetation has a natural capacity for removing pollutants from water. This function has been studied in wetland vegetation in a number of countries and for a range of effluent types, including domestic wastewater (Reed et al 1985). Mechanisms which remove BOD₅, SS and nutrients from wastewater flowing through the reedbed include:

· quiescent conditions which allow sedimentation of remaining solids;

· bacterial activity on submerged roots;

· adsorption/filtration of dissolved materials by roots and stems; and

· adsorption and ion exchange by sediments.

8.8.45 Suspended matter in water may contain a number of contaminants, such as nutrients, heavy metals, and organic compounds. Physical settling of suspended solids result in efficient removal of the contaminants from the water or wastewater stream. Although wetlands generally provide effective removal of suspended solids, the removal efficiency decreases substantially when the concentration of TSS approaches the natural background level of about 10 mg/L.

8.8.46 There are numerous examples of wetlands being used for wastewater treatment in USA and European countries (Hammer, 1989). Wastewaters that have been treated include domestic sewage, urban stormwater and even mine drainage waters, all of which have a significant organic or inorganic pollution load. Percentage removal efficiencies for these pollutants vary from 50 to 90% for BOD₅, 40 to 94% for SS, 30 to 98% for nitrogen and 20 to 90% for phosphorous (Bastian and Hammer, 1993). Constructed wetlands are often designed on organic loading rates, of which BOD₅ is a good indicator. The lower values reported for BOD₅ removal occur in natural wetlands (not specifically designed for that purpose) or where wastewater concentrations are relatively low, as in the case of Lok Ma Chau station RBC effluent. It is therefore proposed to use the more conservative value of 50% BOD₅ removal for the current assessment, in the absence of proven BOD₅ removal rates in Hong Kong.

8.8.47 Different types of wetland result in different removal efficiencies. The two common designs are surface flow and sub-surface flow reedbeds. The latter usual show higher mass removal rates on a unit area basis because of their capacity to retain pollutant removing bacteria on the rock/gravel fill. However, they are generally more expensive to construct. Table 8.21 shows a comparison of removal rates for surface and sub-surface flow systems. The average values are based on the of North American Wetlands Systems Database (Debusk and Debusk, 2000).

Table 8.21

Comparison of removal efficiency between Surface water flow and

Sub-surface water flow reedbed systems

	FWS				VSB
Parameter	Influent (mg/L)	Effluent (mg/L)	Mass removal rate (kg/ha/d)	Removal (%)	Influent (mg/L)	Effluent (mg/L)	Mass removal rate (kg/ha/d)	Removal (%)
TSS	46	14	7	68	48	10	35	74
BOD5	30.3	8	5.1	71	27.5	8.6	18.4	63
NH4-N	4.9	2.2	0.35	38	6	4.5	0.62	9
NO3 + NO2	5.6	2.2	0.4	51	4.4	1.6	1.89	61
TN	9.03	4.27	1.06	53	18.92	8.41	5.85	56
TP	3.78	1.62	0.17	34	4.41	2.97	1.14	22
Orthophosphate (HPO4^2- + H2PO4^-)	1.75	1.11	0.12	41	NA	NA	NA	NA

8.8.48 In the case of the Lok Ma Chau station reedbed, it will receive effluent from the sewage treatment plant, which has already been treated to the standard required for discharges to Group B inland waters. The pollutant load entering the reedbed is therefore relatively small compared with other wastewater treatment reedbeds. The low organic loading will reduce the amount of vegetation growth to a lower rate relative to other wetlands, which receive high N and P loads, thus reducing the need for harvesting.

8.8.49 There are a number of factors that need to be taken into account in the design of wetlands. An important consideration is the size of the area, which will affect the allowable pollutant loading to the reedbed and the flowrate.

8.8.50 The proposed location for the reedbed lies within an area of approximately 5 ha to the east of the station and north of the railway viaduct, alongside the future San Tin Drainage Channel (Figure 8.4). It is likely that not all of this area will be available for treatment purposes. This assessment investigates the area likely to be required to achieve compliance with the Zero Discharge Policy requirements. This area has been included in the area calculated as being lost or disturbed as a result of the Spur Line. Although it will be highly disturbed due to the close proximity of the station and rail track, discouraging more sensitive birds which require an open area for feeding, establishment of reedbeds will provide not only a wastewater polishing function, but also provide a habitat suitable for fauna which require shelter, such as passerines.

8.8.51 The area of land required for treatment is based on the BOD₅ concentration of the incoming wastewater and the expected performance of the reedbed. A typical upper loading rate is 112kg BOD₅/ha/day (USEPA, 1988). WPCF recommends that minimum areas for effective BOD₅ removal at surface flow and sub-surface flow systems are 3 and 1.2 ha respectively with maximum flow of 1000 m³ per day. A large width / length ratios for sub-surface flow systems can enhance BOD₅ and TSS removal. For significant ammonia conversion, the size of cells needs to be increased to at least 4 ha/1000 m³/d. A high degree of nitrification is due, at least in part, to the low loading of the system.

8.8.52 For the purposes of demonstrating the capability of the reedbed to deal with BOD₅ loads, the USEPA recommended standard be used. Applying a safety factor, and assuming an acceptable loading rate of 80kg BOD₅/ha/day, then approximately 0.25ha of reedbed would be required to reduce 24.6kg BOD₅ in RBC effluent by 50%. The residual pollutant loading discharged into San Tin Channel would be 12.3kg BOD₅. The area available is therefore considerably larger than that required. This allows for the potential for high level treatment and the provision of areas of varied vegetation, providing not only a treatment function but also an ecological function.

8.8.53 In addition to pollutant loading from BOD₅, E. coli levels are also of concern. At a discharge standard of 100cfu/100ml, the health risk is considered to be low (WHO, 1998), however, an order of magnitude lower is considered to be the “safe” level in other countries. Studies carried out on E. coli removal in constructed wetlands (Hammer 1989) have reported 80 to 100% removal. Reduction in E. coli levels is achieved through a variety of mechanisms including die-off from ultraviolet exposure, sedimentation, filtration, adsorption, aggregate formation and bactericidal toxins released by other micro-organisms.

8.8.54 Studies with various treatment wetland configurations have demonstrated a 1-log (90%) to 2-log (99%) reduction for indication bacteria, depending on the system HRT. Studies with pilot-scale sub-surface flow wetlands planted with Phragmites demonstrated 1.1 to 1.9 log removals of E. coli and total coliforms at an HRT as short as 6 hours (Green et al., 1997). These removals increased to as high as 3.1 log after 48-hours detention in the gravel bed. Plant root exudates, or the micro-organisms associated with the plant rhizosphere, may also contribute to pathogen destruction. Gravel filled wetlands containing bulrush (Scirpus) achieved greater total coliform reduction (from 6.7 x 10⁷/100 ml to 5.77 x 10⁵/100 ml) than a non-vegetated gravel bed (Gersberg et al., 1989). While wetlands can be an effective technology for pathogen removal, there remain some concerns for using treatment wetlands, particularly surface flow configurations, in tropical regions. Man human parasites have complex life cycles that rely on intermediate animal hosts. Wetland environments can support many of these avian and mollusk hosts and, therefore, provide an environment for proliferation rather than the destruction of the pathogenic organisms.

8.8.55 Other factors that require consideration in the design of reedbeds for water clean up are listed below.

· As it is unlikely that there will be high concentrations of sediment in the treated effluent flow, no deep water areas are required for settlement. However, it is recommended that a filter bed is located upstream of the reedbed to settle out any suspended solids before the effluent enters the reedbed. This filter bed could be packed with materials such as crushed brick and oyster shells that are know to be effective in removing nitrogen and phosphorous, which is particularly difficult to remove in conventional biological treatment systems.

· Water level variation affects hydraulic residence time, atmospheric oxygen diffusion, plant cover, and all other factors that influence wetland performance. During the summer when water temperatures are elevated and plant productivity is highest, water levels should be lowered to promote better oxygen diffusion to the wetland sediments and plant roots. During colder months, water levels can be raised to provide longer reaction times with starving the plant roots of oxygen. During the dormant plant growth season, plant roots require a smaller fraction of the oxygen needed for metabolism during the summer. After the system has operated for several months, the water level control scheme can be varied in individual cells to develop optimum growth conditions for the specific site.

8.8.56 The reedbed system may be sub-surface flow or surface flow or a mixture of both. Constructed wetlands have been generally grouped into two basic categories. The Water Pollution Control Federation (WPCF) designates surface flow or free-water surface (FWS) systems and sub-surface flow or vegetated submerged bed (VSB) systems under the Natural Systems for Wastewater Treatment design manual (WPCF, 1990). Table 8.22 shows the differences between FWS and VSB systems. FWS systems are commonly found in North America and United States. VSB systems are primarily implemented in Europe for domestic wastewater treatment (Cole, 1998).

· A comparison of surface flow and subsurface flow in terms of reported treatment performance is provided in Table 8.22. Given the irregular shape of the area available, it is recommended that sub-surface flow is used in the wider section, and RBC effluent enters the system in this section (since sub-surface flow has a lower hydraulic capacity than surface flow). This could be followed by a section of deeper water and surface flow reedbed for final polishing. This variety would provide varied habitats, increasing diversity.

Table 8.22

Comparison of requirements of Surface water flow and

Sub-surface water flow reedbed systems

Surface Flow Systems	Sub-surface Flow Systems
Wastewater flow at depth of 1 to 2 inches up to 12 to 18 inches through the basin in a plug flow pattern	Basin is filled with aggregated such as gravel or crushed stone to a depth of 12 to 24 inches.
It can be constructed on both mineral (low organic matter content, high bulk density) and organic (12 to 20% organic matter content, low bulk density, high water holding and cation exchange capacities) soils with simpler hydraulics. Loamy soil is suitable for the roots sand rhizomes penetration and growth.	Subsurface flow wetlands incorporate a rock or gravel matrix that wastewater is passed through in a horizontal, vertical or combined fashion.
Effectiveness depends on the wetland size (volume), location in the watershed, and configuration of inlet and outlet structures. A relatively large spatial requirement.	Less land required.
It offers wildlife habitats, engineering benefits and recreational amenity beyond water treatment. It also uses for treating agricultural and urban runoff to reduce hydraulic runoff peaks from storm events.	It provides no aesthetic or recreational benefits and few, benefits to wildlife.
Water passing through the wetland forming a shallow basin, typically 0.5 to 1.5 m deep with the water column occupied by macrophytes mainly. Depending on depth and nutrient regime, mats of filamentous algae, phytoplankton, or submerged macrophytes will dominate.	No vector problems and odour production. No insect proliferation if surface ponding is avoided. No visible flow, thus, no standing water which limiting the likelihood of human exposure to wastewater pathogens.
Lower installed cost	High cost due to the purchase and transport of the rock or gravel matrix. Concerns over matrix clogging and the potential high cost of renovation also limit the deployment of extremely large area.
Lower temperature tolerance especially during winter.	It provides effective treatment of most wastewater constituents through the winter in temperate climates. The subsurface microbial treatment processes still function, at a reduced rate, even when the surface vegetation has senesced or died, and the matrix surface is covered with snow and ice.

· The shape of the wetland should be designed to maximize the area available for treatment. A possible layout is shown in Figure 8.5. The layout must be capable of meeting the treatment objectives both in terms of pollutant removal and hydraulic capacity.

· The time the treated effluent remains in the wetland is important, to allow opportunity for treatment to occur. Retention times vary greatly, although average values of 5 days are recommended. The retention time within the Lok Ma Chau wetland, for the treated effluent flow alone, and assuming about half of the area would be used for treatment, is about 6 days, which is acceptable. With the addition of water from San Tin River as part of the offsetting technique to comply with the Zero Discharge Policy (ZDP), as described later, the retention time would be reduced. This is still acceptable, considering the low organic loading that will be applied to the reedbed.

· The type of substrate will differ in the sub-surface and surface flow sections. Soils should be low in nutrients to avoid algal blooms in any open water areas. Nutrient removal should be enhanced in the filter bed section, although nitrogen and phosphorous levels on treated sewage are expected to be low.

8.8.57 One of the main problems for surface flow wetlands is short-circuiting and inadequate oxygen transfer. To minimise these impacts, the water should be redistributed by structural means. Both deep channels and earthen berms perpendicular to flow have been used to redistribute flow in wetland. Deep channels, should be approximately 1 m deeper than the rest of the cell to prohibit plant growth. A shallow inlet area can maximise the oxygen transfer capability to enhance nitrification (Kadlec and Knight, 1996).

Zero Discharge Policy Compliance

8.8.58 Under ZDP, discharge of 12.3kg BOD₅ into the Deep Bay Water Control Zone must be offset by treating 12.3kg BOD₅ from a source within the same catchment. Data obtained from water sampling in San Tin River (described in Section 8.3 of this chapter) shows that the BOD₅ concentration can vary from around 30mg BOD₅/l in the wet season to 200mg BOD₅/l in the dry season. The volume of river water to be pumped will therefore vary depending on the time of year. A considerable proportion of the BOD₅ load arises from livestock waste and, with the full implementation of the Livestock Waste Control Scheme (LWCS), the BOD₅ levels are expected to decrease considerably. This assessment of reedbed treatment is based on a changing BOD₅ concentration during the year as shown in Table 8.23.

Table 8.23

Assumed BOD₅ concentrations in San Tin River

for Reedbed Treatment Assessment

Period of year	Estimated River Water quality (mgBOD5/L)	Volume required for off-setting (m^3/day)	HRT in reedbed for RBC effluent + river water (days)*
June to August	30	819	2.9
April, May and September	60	409	3.7
February, March and October	120	205	4.2
November to January	180	136	4.4

Note: * Hydraulic retention times based on assumed area for reedbed of 2ha.

8.8.59 Table 8.23 also shows the volume of river water required to be pumped through either the RBC or the reedbed to offset the 12.3kg BOD₅/day discharged after polishing of the RBC effluent. If the volumes shown are pumped through the RBC, the increase from 760kg BOD₅/day (from passengers and staff) to 772kg BOD₅/day, can be readily accommodated. However, the hydraulic load would increase from 1228m³/day to 2047m³/day in the wet season, requiring a significant increase in the capacity of the RBC. This would also require 100% removal of pollutants from the river water to offset the discharged flow.

8.8.60 If the reedbed is used for treatment, a similar problem arises. Assuming 50% BOD₅ reduction, 24.6kg BOD₅ (equivalent to 1638m³ San Tin River water) would need to be pumped through the reedbed each day. A total loading to the reedbed of 49.2kg BOD₅ (24.6kg BOD₅ from the RBC and 24.6kg BOD₅ from San Tin River) the area of the reedbed required would double. The capacity for increasing the reedbed area is more realistic than increasing the RBC size. This is therefore the recommended option.

8.8.61 The hydraulic loading to the reedbed is determined by the allowable retention time in the system. As described above, a minimum 5 day retention time, at a flowrate of 2866m³/day (1228m³ from the RBC effluent and 1638m³ from the San Tin River) would require an area of approximately 5 ha, assuming a water depth of approximately 0.3 m. As described previously, it is unlikely that 5ha will be available for treatment purposes. To reduce the area requirement, the volume of water to be pumped through the reedbed must be reduced. As an example, Table 8.24 presents one scenario that may be considered at detailed design stage. In this scenario, no pumping would be carried out during the summer months of June, July and August, the off-setting load being dealt with entirely during the remaining 9 months of the year when BOD₅ concentrations are higher and pumped volumes correspondingly lower. This will result in a longer hydraulic retention time (HRT), allowing reduced reedbed area as necessary.

8.8.62 The following assumptions were made:

Total annual BOD volume to be offset = 12.3kgBOD₅/day x 365 days

= 4482kgBOD₅

Table 8.24

A possible pumping regime scenario to

maintain HRT in reduced area reedbed

Estimated River water quality (mgBOD5/L)	Volume pumped (m^3/day)	Number of days of pumping	Organic Loading to reedbed (kgBOD5/d) excluding RBC load	Organic Loading to reedbed (kgBOD5/ha/d) including RBC load	Total Organic Load (kgBOD5)	HRT in reedbed (days) including RBC flow
60	320	120	19	27	2304	3.9
120	320	90	38	39	3456	3.9
180	300	60	54	49	3240	3.9
Total days pumping = 270
Maximum daily volume to be pumped = 320m^3
Total Organic loading (kgBOD5) = 9000
Assuming 50% removal, total BOD5 treated (kgBOD5) = 4500

8.8.63 Table 8.24 shows that the daily volume to be pumped from San Tin Channel can be reduced from 1638m³ to 320m³, allowing a significant increase in the HRT, thus improving the reedbed performance. The reason for this is that pumping is done within the dry season when the pollutant level in the San Tin River is higher, therefore the volume to be pumped for an equivalent pollutant load is smaller. The lowest HRT in this scenario (based on an area of 2 ha and a depth of 0.3m) is 3.9 days. An area of 2.6ha would be required to raise the HRT to 5 days.

8.8.64 Before this proposed pumping system could be implemented, the quality of water in San Tin River during dry and wet seasons needs to be confirmed and agreed with EPD and a programme of pumping finalised. A potential problem is that if the water in San Tin River improves, a larger volume of water will require pumping, reducing the HRT, which may affect reedbed performance. BOD₅ removal can be effectively achieved at an HRT as low as 1 day; however, nutrient removal will not be effective at this retention time. The effectiveness of the RBC to remove nutrients and of the RBC to remove pollutants needs to be confirmed at the detailed design stage.

8.8.65 In achieving BOD₅ removal and off-setting of the pollution load in this way, together with the disinfection facilities of the wastewater treatment plant, the load of E. coli is not expected to have an adverse impact on the environment and will be in compliance with the zero discharge policy requirements.

Key Issues in Construction and Operation of Reedbeds

8.8.66 The following sections describe some of the key factors to be considered in the construction and management of the reedbed.

8.8.67 The area in which the reedbed will be located is currently abandoned fishponds, with areas of deep water and bunds. The ground will need to be reprofiled to form the base of the reedbed. The reedbed areas may vary in width to achieve a sufficient cross section of treatment, particularly in the sub-surface flow section. Bunds wide enough to walk on, should be included in the design for maintenance access.

8.8.68 The base of the channels will be an impermeable material such as clay, which is similar to that material used for fishponds. The outlet from the RBC should be extended to the inlet of the reedbed, where a filter bed should be installed for removal of solids and possibly nutrients, depending on the material used. The RBC effluent can enter the filter bed directly, as solids concentration is likely to be low. River water from the San Tin Channel should be passed through a sediment trap before entering the wetland if high in solids.

Vegetation

8.8.69 Vegetation should be selected from species that establish locally and are effective in removing pollutants. Phragmites is recommended for the Lok Ma Chau wetland as it already grows well in the fishpond area and does not require frequent harvesting, as with some other vegetation such as water hyacinth. Phragmites can establish well in shallow water areas and the main mechanism for pollutant removal is through nutrient uptake through the roots. These plants will therefore function well in sub-surface systems. Surface systems pollutant removal functions primarily through litter accumulation and bacterial action as well as plant uptake.

8.8.70 There will be a permanent supply of water from the Lok Ma Chau station RBC, minimizing the need for a separate supply of water in the dry season. Thus, if the flow of water in the San Tin River decreases in the dry season, and the pumping regime needs to change, the reedbed can be maintained in a wet condition. Monitoring of the water quality during the first dry season will enable adjustments to be made to the pumping regime proposed.

Operation

8.8.71 The nature of operation may also contribute to high level treatment since high loading within a period, no or low loading in the following period or vice versa. Low loading, combined with correspondingly low nutrient application and a relatively shallow gravel depth, could be responsible for the full bed root penetration. This intermittent type of operation may assist the system in staying aerobic so the nitrification reactions can take place (Freeman, 1993).

Management

8.8.72 A management plan will be established for the whole of the ecological compensation area of the Lok Ma Chau station. The reedbed, although not specifically designed for ecology, is likely to have some wildlife value because of its proximity to the fishpond and marshland area being designed for ecological compensation of the Lok Ma Chau station impacts, and the low levels of pollutants which will be present in the system, therefore minimizing the build-up of pollutants in the wildlife using this area.

8.8.73 Wetlands are usually low maintenance systems. Minimal maintenance is required for the soil complex in which the vegetation is established and pollutants are adsorbed. The sediment chamber for sediment settlement and the gravel bed for removal of fine solids, will require more frequent washing and cleaning. The growth of vegetation is expected to be relatively slow, given the low organic load in the wastewater. However, vegetation trimming will be required on an annual basis. The vegetation can be placed on the surface of the surface flow system to improve treatment through litter accumulation.

8.8.74 It is important to ensure that the reedbed is established prior to the discharge of treated RBC effluent into it when the station opens. The reedbed should be established in the year prior to the Spur Line opening, and commissioned through pumping of non-tidal water from the San Tin channel, to monitor the effectiveness of the system in pollutant removal. Monitoring should be carried out as detailed in Chapter 15, EM&A of this report, to determine the performance of the reedbed. Monitoring should continue throughout operation to measure compliance with ZDP.

8.8.75 In the unlikely event that the reedbed system is not operational at the time of opening, contingency arrangements should be made to tanker the treated RBC effluent away.

8.8.76 Implementation of the reedbed will be the responsibility of KCRC during design, construction and management stage. It is important that the management of the reedbed is co-ordinated with the remaining compensation areas for ecological mitigation.

8.8.77 The present assessment is based on a 50% BOD₅ removal rate, which is taken from reported rates in temperate climate reedbeds. From initial results of studies being carried out on reedbed treatment of pig waste septic tank effluent at Kadoorie Farm in Hong Kong, higher rates for removal of BOD₅ and nutrients can be achieved. Data from this study is not available at this stage. Improvement in the rate of BOD₅ removal from the effluent discharged from the RBC would reduce the quantity of BOD₅ required to be off-set which would reduce the volume of river water to be pumped from San Tin River.

8.8.78 In addition, a similar reedbed will be established alongside Lok Ma Chau Boundary Crossing, for the purpose of off-setting pollution loading to Deep Bay, arising from the Boundary Crossing Expansion Project (Binnie 1998). The performance of this system should be carefully evaluated and techniques for improvement of pollution removal incorporated into the design and management of the Spur Line reedbed.

8.9 Concurrent Projects with Potential for Cumulative Impacts

8.9.1 Table 8.25 lists major projects that will be conducted concurrently with the construction of the Spur Line. It also shows the potential impacts from these projects and proposed mitigation measures. The Main Drainage Channels for FanLing, Sheung Shui and Hinterland will be largely complete by the time Spur Line construction starts. However, this project is included because of the potential effect it has on the groundwater levels in Long Valley. This will be taken into account in the assessment of potential operational hydrological impacts from Spur Line.

8.9.2 Each of these projects has the potential to impact water quality in the Study Area and in Deep Bay. The locations of these projects in relation to the Spur Line Alignment are shown on Figure 2.3. Projects of particular significance, due to their proximity to the proposed Lok Ma Chau station, are the construction of the San Tin Eastern Main Drainage Channel, and the expansion of the Lok Ma Chau Boundary Crossing. The drainage from many of the projects listed in Table 8.28 flows into Deep Bay, and therefore the combined impact on Deep Bay water quality from the construction and operational phases of these projects could be significant in the absence of adequate mitigation measures.

8.9.3 Potential impacts and proposed mitigation measures for each project are listed in Table 8.26. The common impacts to all projects are related to site run-off during the construction phase, hydrological impacts and operational impacts. All projects have the potential to impact watercourses during construction, through site run-off of concrete washings, exposed material and potentially contaminated areas. Mitigation measures that are recommended include dry season working where possible, drainage systems to control run-off and treat appropriately before discharge, covering stockpiles, correct handling of excavated material, particularly dredged sediment, and the use of sedimentation tanks for settlement of run-off where appropriate. In the operation phase, sewage collection and treatment is required for all projects, and implementation of stormwater management systems to reduce stormwater pollutant run-off is recommended. Monitoring programmes are also recommended to monitor the effectiveness of the mitigation proposed.

8.9.4 In the case of the Main Drainage Channels for FanLing, Sheung Shui and Hinterland, mitigation recommended for the potential hydrological impacts from the construction of the drainage channel in Long Valley involved maintaining the downstream fabridam in a raised state to prevent a lowering of the groundwater table in Long Valley. The mode operation of the fabridam has not been finalised at this time and the potential for impacts has not therefore been ruled out. The presence of Spur Line has been shown in the current assessment to have minimal impact on the groundwater levels during construction and operation of the tunnel, and a suite of mitigation measures is recommended as contingency to minimise potential impacts.

8.9.5 The recently publicised Kwu Tung North New Development Area (NDA) as part of the Planning and Development Study on NENT covers an area considerably larger than the current Spur Line project. The draft EIA, has not yet been submitted to government. Details on mitigation measures are therefore not yet available.

Table 8.25

Summary of Major Projects Adjacent to the Spur Line

Project description and potential impacts	Proposed Mitigation	Commencement Date	Completion Date
Shenzhen River Training Phase III involves the realignment of the Shenzhen River to improve drainage efficiency and prevent flooding. Impacts include sediment suspension, increased river flow.	a) Construction Method In dredging work, sediment re-suspension should be avoided by reducing transport distance. Clamshell dredger should be used during flood season. During dry season, silt curtain should be used upstream and downstream of dredging spot to reduce impact from the re-suspended sediment. b) Construction Arrangement Wet excavation should be reduced. Due to narrow width and small water flow in original channel, earth excavation should be conducted on each sides of channel sequentially to allow for reduction of river water.	mid 2001	Late 2004
San Tin Eastern Main Drainage Channels -drainage improvement works to alleviate flooding in the San Tin area and provide flood storage ponds for Chau Tau and Pun Uk Tsuen. Impacts of site run-off during construction, increased flow during operation.	A) Construction phase a) Excavation of Stream Channel To minimise leakage and loss of sediments during excavation in rivers, tightly sealed closed grab excavators should be employed. If material is dry and in non-river sections, conventional excavators can be used. b) Works Timing Excavation in existing stream should be undertaken during dry conditions to minimise downstream impacts on downstream sensitive receivers. c) Construction Runoff and Drainage · Provide temporary ditches to facilitate run-off discharge into appropriate watercourses, via a silt retention pond. · Minimize exposed soil areas and cover open material storage stockpiles with tarpaulin or similar fabric. d) Marine Disposal of Excavated Sediment · All off-site vessels (marine dumping disposal barges and floating pontoons) should be sized such that adequate clearance is maintained to ensure turbidity created by turbulence from vessel movement or propeller wash is minimised. · Transport of contaminated marine sediment to the marine disposal grounds should be by split barge (off-site) of not less than 750 m^3 capacity. · Dumping vessels should be stationary throughout the dumping operation. B) Operation phase Ad hoc maintenance dredging during dry season will be adopted to maintain flood capacity.	Undecided	Undecided
Fanling, Sheung Shui & Hinterland main Drainage Channels – drainage improvement works to alleviate flooding in the Fanling, Sheung Shui & Hinterland areas. Impacts of sediment suspension, increased flowrate. Potential impacts on groundwater levels in Long Valley.	A) Construction phase a) Excavation · Excavation should be carried out only in dry conditions. · Excavation of contaminated material for each section of the works must be removed from the site in the dry season and before commencement of the corresponding main excavation works. · In order to keep each sequential works area dry, existing river will be diverted away from construction site if possible. b) Dredging activities · During dredging, watertight dredging equipment should be employed to minimise spillage of contaminated sediment. Water quality monitoring of upstream and downstream should also be conducted. · Silt curtains in the construction area of lower Indus to retain suspended solid are recommended.	1999	2003
	c) Monitoring of water quality · Surface water shall also be monitored to ensure that standards specified in TM are met by measuring the water quality at the outlets of sedimentation tanks. B) Operation phase With the implementation of weirs, sediment traps and fabridams along the rivers, sediment will deposit along the river and would reduce the pollutant transport into Deep Bay. Weirs also have a beneficial effect on water quality by providing oxygenated water downstream and reasonable flow rate.
Lok Ma Chau Control Point Expansion Project – kiosk expansion and works to improve vehicle and passenger throughput and circulation. Impacts of increased run-off, higher pollutant load, sewage generation from staff and passengers.	A) Construction phase a) Silty run-off from site Reduction by diversion into temporary sedimentation or silt removal facilities. Construction of bunds or barriers to prevent runoff to water courses or fishponds. b) Release of nutrients and BOD during excavation Drying of mud and storage in bunded area. c) Elevation of pH in water courses Construction of temporary working jetty underneath concreting work to collect spilt cement and concrete washings. d) Release of oil and grease washings Removal by licensed collectors and appropriate training on handling and storage of chemicals. e) Increase of sewage and other wastewater from additional workers Provision of mobile toilet and proper collection and disposal of sewage. B) Operation phase a) Sewage generation Treatment of sewage using Rotating Biological Contactor (RBC) and further polishing of effluent by passing through reedbed system incorporated into stormwater management area. b) Drainage Stormwater management measures such as oil & grease traps, catchpits, and reedbed for clean up of stormwater before discharge to local watercourses.	mid 1999	2003
Planning & Development Study on Northeast New Territories involves examination of the scope and feasibility of accommodating strategic growth development needs in the NENT.	EIA on-going	Not yet determined	-

8.10 Proposed Monitoring and Audit Programme for Water Receivers

8.10.1 During construction, there are a number of locations where works may impact watercourses and other aquatic sensitive receivers. A regular monitoring programme should be implemented within the vicinity of works to identify and determine the extent of impacts from the works. Areas of particular concern are the works to be carried out close to and within the River Sutlej (launching shaft and River Sutlej works), alongside the River Beas (EAP construction), at Chau Tau where the channel will be diverted, at watercourses in Lok Ma Chau where works will be carried out in fishponds and overspill may enter local watercourses.

8.10.2 At each works area, locations should be identified for monitoring. One upstream and two downstream locations should be identified and sampled during the works period for in situ parameters DO, turbidity, temperature and pH, and samples taken for the analysis of SS against which turbidity measurements can be made.

8.10.3 The potential for impacts to occur to groundwater during the tunnelling operation in Long Valley has been shown in this assessment to be small. However, groundwater levels should be measured throughout the tunnelling period using strategically placed piezometers. Water quality in wells along the alignment should be tested for the above parameters plus nutrients to determine a baseline against which regular measurements can be compared during the tunnelling operation.

8.10.4 During filling of the Lok Ma Chau station platform, the turbidity of settling water should be monitored regularly to ensure that no turbid water overspills into watercourses leading to Shenzhen River. Discharges in this area are of particular concern because of the sensitivity of the downstream area of Deep Bay.

8.10.5 During operation, the performance of the reedbed should be monitored on a regular basis to confirm this mechanism of polishing the treatment plant effluent, and offsetting the pollutant load discharged, is being achieved. Samples of river water should also be tested at regular intervals to confirm the assumptions made in complying with the Zero Discharge Policy.

8.11 Summary of Impacts and Mitigation Measures during Construction and Operation Phases

Construction Phase

8.11.1 The Spur Line alignment will tunnel beneath the areas of Long Valley, Kwu Tung and part of Chau Tau. Above ground impacts from this operation will therefore be minimal. Potential hydrological impacts during tunnel operation are unlikely to be significant, from data obtained from previous similar tunnelling operation. Close monitoring of groundwater levels and settlement as the TBM machine passes by will provide an alarm for any significant changes. The main potential construction activities will be from the construction of structures to be built above ground, i.e. Emergency Access Points, Ventilation Buildings, excavation for Kwu Tung station box and the TBM launching and retrieval shafts, and cut and cover areas at each end of the bored tunnel. These works have the potential to impact several watercourses, wetlands and fishponds that cross or flow close to the proposed alignment. Activities with potential impact include: excavation of soils for cut and cover and for tunnelling, jet grouting from the ground surface, tunnel boring activities, concreting works, piling, works within River Sutlej, draining and filling of fishponds, footbridge construction across Shenzhen River, and excavation of potentially contaminated materials.

8.11.2 Impacts on water quality include generation of turbid run-off, which may contain concrete washings, lubricants, chemicals and other contaminants. The tunnel operation will result in extraction of wet material, the excess water from which must be treated before discharge. Concrete works are of concern due to the potential raise in pH in rivers with high ammonia concentration. Works carried out in River Sutlej and Shenzhen River require particular care in protection of water bodies downstream (Deep Bay) from potential impacts. Impacts can be substantially reduced through the implementation of good site practices, such as installation of a suitable waste handling and treatment system, careful handling of chemicals and proper disposal of wastewater, and incorporation of suitable drainage systems including sedimentation and infiltration pits, and temporary grease trap and septic tank systems. Works in River Sutlej should be carried out in the dry season and with modification to the low flow channel to ensure the water is not contaminated by works run-off. A drainage system designed into the footbridge construction technique (particularly if the deck is cast in-situ) will minimise the risk of impacts from concrete washings entering the Shenzhen River.

8.11.3 Table 8.26 summarizes the mitigation measures for construction impacts. With these mitigation measures in place, the impacts on water quality in watercourses throughout the alignment, and downstream in Deep Bay and the ecologically sensitive areas such as Mai Po marshes, will be minimized.

Table 8.26

Summary of Mitigation Measures for Construction Impacts

Impact from Construction	Works likely to cause impact	Mitigation Measures
Increase of SS and turbidity from turbid site runoff, tunnel spoil discharge and during river related works.	Excavation works at Chau Tau, Kwu Tung, EAPs, Ventilation Shafts, cut and cover tunnels and River Sutlej works. TBM tunnel boring works, major work areas at Sheung Shui, Kwu Tung, Chau Tau and Lok Ma Chau.	· Reduction of suspended matter in site runoff by directing it into temporary sand traps or other sand removal facilities. · Good site management practices according to ProPECC PN 1/94 to minimize soil erosion. · Use of flat and exposed permeable area to encourage infiltration of site runoff. · Works in dry season in River Sutlej and protection of low flow channel. Contingency plans in wet weather. · Careful working practices to minimize sediment disturbance during jetty and footbridge construction.
Release of pollutants from potentially contaminated land	EAP works at Kwu Tung, Kwu Tung Station Box, Chau Tau Launching Shaft, Jet grouting for cross passages	· Isolation of area of potential contamination · Containment of run-off before discharge (through bunding, suitable drainage). Collection, treatment and disposal of contaminated surface water.
Release of nutrients or other contaminants into water courses during dredging activities.	River Sutlej works, footbridge across Shenzhen River, draining and filling of fishponds in Lok Ma Chau and LMC station area.	· Use of silt curtain or sandbag barrier to confine disturbed area. · Quick removal of sediment-laden water away from water courses to an area of infiltration / sedimentation.
Elevation of pH in water courses through discharge of concrete washings into surface runoff.	All areas where concrete will be applied. Cut and cover sections at Sheung Shui and Chau Tau, River Sutlej works, construction of EAP and ventilation buildings, viaducts and LMC station.	· Close monitoring of pH in watercourses. · Construction of cofferdam and pumping out of water where concreting work is carried out within water courses. · Direction of wastewaters to sedimentation pit and treatment (neutralisation) to appropriate standard before disposal to minimise turbidity and potential impacts downstream.
Contamination of groundwater and watercourses with hydrocarbon compounds released from site workshop through spillage or leakages.	Works areas at Sheung Shui, Kwu Tung and Cha Tau. LMC station if works area established there.	· Removal of waste oil by licensed collectors. · Installation and maintenance of oil interceptor of hard standing compound. · Good housekeeping practices to minimize careless spillage. · Appropriate training given to the personnel to handle the chemicals.
Drop in groundwater levels during pass by of TBM, particularly in Long Valley, which is most sensitive to groundwater changes and impacts on wetland function.	Through the tunnel alignment section. Particularity at Long Valley where groundwater level changes are of concern to the ecological function of the wetland.	· Maintain TBM in Earth Pressure Balance mode during soil material cutting. · Monitor groundwater levels and settlement monitoring throughout tunnelling operation, at locations within potential influence zone of the TBM operation.
Increase of sewage and wastewater from additional workers.	All areas where facilities are provided for workers. Likely to be works areas at Sheung Shui, Kwu Tung and Cha Tau.	· Connection to public sewer in the vicinity if possible. Otherwise, provide underground septic tank or mobile toilet to store the sewage. · Discharges from site canteen via grease traps and collected by licensed collector.

Operation Phase

8.11.4 The main operational impacts from the Spur Line railway involve the physical presence of the footbridge piers in Shenzhen River, hydrological impacts from the presence of the tunnel below wetland areas, stormwater run-off from the railway tracks and the trains, and sewage generated by staff and passengers at Lok Ma Chau station. Potential stormwater impacts include both hydraulic and pollution effects. Hydraulic impacts from the railway track will be small. Run-off from the Lok Ma Chau station will marginally change the hydrograph of the area, currently open fishponds.

8.11.5 Hydraulic impacts from culverting of streams are of low significance given their small scale. Potential hydraulic impacts from the supports for the footbridge crossing Shenzhen River can be minimised through riverbed protection and optimising the shape and size of the piers to reduce local turbidity and minimise effects downstream. The ecologically valuable areas such as Deep Bay and its associated mud flats will therefore be protected from adverse impact due to this project.

8.11.6 The most significant potential contaminants in the run-off include lubricants and metal grindings from the above ground track that may have a negative long-term impact on the water quality of receiving watercourses. Other contaminants include oil & grease and cleaning agents from trains and within the station, and suspended solids from dust deposition on the station building. Adverse water quality impacts can be effectively reduced through the implementation of good working practices during cleaning and maintenance, and incorporation of appropriate pollution control measures such as oil interceptors/sediment traps into the drainage system design. The proposed drainage layout should be submitted to EPD when available.

8.11.7 Sewage generated by passengers and staff will be treated in an on site wastewater treatment plant with disinfection to the required standard for discharge to Deep Bay. EPD’s Zero Discharge Policy (ZDP) will be achieved through two mechanisms:

· polishing of effluent in a constructed reedbed (expected to reduce BOD₅ and E.coli); and

· off-setting the pollution load to Deep Bay by treating an equivalent pollution load extracted from the adjacent San Tin Channel. An area of approximately 2.6 ha of reedbed to the east of the station is recommended for reducing BOD₅ and E.coli to an acceptable standard before discharge to San Tin Channel or Shenzhen River.

8.11.8 The reedbed should be established in the year preceding the Spur Line operation and monitoring conducted to evaluate the performance in terms of BOD₅ removal. In the event of inadequate reedbed performance, contingency plans include tankering the effluent off-site or diverting to alternative reedbed sites to meet the ZDP.

8.11.9 Monitoring should be carried out during the construction phase of the project to monitor potential impacts on waterbodies in the Study Area. Groundwater levels should be monitored during the tunnel operation in Long Valley to verify predictions of minimal impact. During operation, the performance of the reedbed should be regularly monitored to achieve compliance with ZDP.

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