4 WATER QUALITY IMPACT

4.1 Legislation, Policies, Plans, Standards and Criteria

4.1.0.1 Annexes 6 and 14 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) issued under section 16 of the Environmental Impact Assessment Ordinance (EIAO) set out the criteria and guidelines for assessment of water quality impacts. General and project-specific criteria should be considered in assessing the potential water quality impacts associated with development projects. Assessment should take into account the assimilative capacity of the receiving water body and water quality conditions.

4.1.0.2 Water Control Zones (WCZs) were declared under the Water Pollution Control Ordinance (WPCO) (Chapter 358), which was enacted in 1980 and amended in 1990 and 1993. Water Quality Objectives (WQOs) were established for each WCZ to protect the beneficial uses of water quality. There are 10 WCZs and 4 supplementary WCZs in Hong Kong. The Deep Bay and North Western WCZs are applicable to the present study. The appointed days for the Deep Bay WCZ and North Western WCZ were 1 December 1990 and 1 April 1992 respectively.

4.1.0.3 The major areas covered by the Deep Bay WCZ include Ping Che, Fanling, Sheung Shui New Town, Kwu Tung, Mai Po, Kam Tin, Shek Kong, Yuen Long, Tin Shui Wai, Lau Fau Shan and San Tin. The North Western WCZ mainly includes Tuen Mun, Castle Peak, Lam Tei, Lung Kwun Sheung Tan, Tap Shek Kok, Pillar Point, Tai Lam Chung, Yam O, Chek Lap Kok, Tung Chung and Tai O. A large portion of the Study Area is located within the Deep Bay WCZ and a relatively small portion of the Study Area falls within the North Western WCZ.

4.1.0.4 The marine WQOs for the Deep Bay and North Western WCZs are presented in Table 4.1.

Source: Marine Water Quality in Hong Kong in 1993 by EPD

4.1.0.5 Discharges of effluents are subject to control under the WPCO. Effluent discharge standards were defined in the Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM) issued under section 21 of the WPCO. Four groups of inland waters were defined in the TM. The beneficial uses of these four groups of inland waters are listed as follows:

Group A : abstraction for potable water supply
Group B : irrigation
Group C : pond fish culture
Group D : general amenity and secondary contact recreation

4.1.0.6 Groups B, C and D inland waters are applicable to this study. The construction site discharges normally contain high concentrations of suspended solids. COD and pH are also of concern. Standards for these parameters for Groups B, C and D are summarised in Table 4.2.

Note:

1. All units for suspended solids in mg/L and all figures are upper limits.

Sources: Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters, Tables 4, 5 and 6, Environmental Protection Department.

4.1.0.7 As Deep Bay is an ecological sensitive area, a "zero discharge policy" for Deep Bay has been implemented in Deep Bay catchment. Effluents discharged into Deep Bay are required to be properly treated prior to final disposal so as not to cause net increase in pollution load to Deep Bay. Suitable mitigation measures should therefore be applied to minimise the identified water quality impacts. Standards for effluents discharged into the coastal waters of the Deep Bay WCZ are shown in Table 4.3.

Table 4.3 Standards for Effluents Discharged into the Coastal Waters of Deep Water Control Zone

Notes: 1. All units in mg/L unless otherwise stated; and

           2. All figures are upper limits unless otherwise indicated.

Source: Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters, Table 8, Environmental Protection Department.

4.1.0.8 Guidelines for the handling and disposal of construction discharges are provided in ProPECC Note PN1/94 on Construction Site Drainage. The types of discharges from construction sites outlined in the guidance note include:

• surface run-off;
• groundwater;
• boring and drilling water;
• wastewater from concrete batching and precast concrete casting;
• wheel washing water;
• bentonite slurries;
• water for testing and sterilization of water retaining structures and water pipes
• wastewater from building construction;
• acid cleaning, etching and pickling wastewater; and
• wastewater from site facilities.

4.1.0.9 Methods to prevent water pollution from the construction activities are recommended in the guidelines and are considered relevant to the present study.

4.2 Description of Environment

4.2.0.1 This section reviews the background conditions in the Study Area. Water quality sensitive receivers, which may be affected by the DBL project, are identified.

4.2.1 Background Conditions

4.2.1.1 EPD performs routine marine water quality monitoring at the monitoring stations in the Deep Bay and North Western WCZs. The Deep Bay WCZ includes three monitoring stations in the inner sub-zone (DM1, DM2 and DM3) and two monitoring stations in the outer sub-zone (DM4 and DM5). According to the monitoring results in 2000, water quality in the outer sub-zone was better than that of the inner sub-zone. Exceedances of the WQOs for dissolved oxygen, total inorganic nitrogen and unionized ammonia were mostly recorded in the inner sub-zone. The past 10-year monitoring results indicated increasing trends for E. coli, total inorganic nitrogen and ammonical nitrogen in most of the areas in Deep Bay. The depth-averaged dissolved oxygen levels at most of the stations showed a decreasing trend.

4.2.1.2 There are six water quality monitoring stations (NM1, NM2, NM3, NM5, NM6 and NM8) in the North Western WCZ. Outfall discharges from the Pillar Point, San Wai and Siu Ho Wan Sewage Treatment Works affected the water quality in terms of E. coli and BOD5 in the regions near the discharge locations. Based on EPD's marine water monitoring results for 2000, the dissolved oxygen (DO) and unionised ammonia (UIA) levels complied with the WQOs at all the stations. Except for station NM5, all other stations met the WQO for total inorganic nitrogen (TIN). The increases in sewage discharge from the NWNT outfall into Urmston Road might cause the rise in E. coli levels. This was reflected in the data measured between 1998 and 2000. Under the influence from the Pearl River flow, there were increasing trends in TIN and orthophosphate phosphorus at NM5 and NM3.

4.2.1.3 EPD also carries out routine monitoring of river quality in Hong Kong. A Water Quality Index (WQI) with grading from excellent to very bad is used to indicate the extent of organic pollution in a river. The WQO compliance rates of the rivers located near the Study Area in 2000 are summarised in Table 4.4.

4.2.1.4 In the Deep Bay WCZ, there was substantial improvement in water quality of Tin Shui Wai Nullah in 2000. The compliance rate improved from 49% in 1999 to 88% in 2000. The WQIs of the upstream and downstream stations in Tin Shui Wai Nullah were improved to "good" and "fair".

4.2.1.5 The WQI of Ngau Hom Sha Stream was graded as "good" with a relatively low WQO compliance rate for BOD5. Full compliance with five WQOs was recorded in Sheung Pak Nai Stream and Ha Pak Nai Stream reflecting excellent water quality in these streams. The compliance rate for pH was 92% for Pai Nai Stream and the other WQOs were in full compliance.

4.2.1.6 In the North Western WCZ, the WQIs for Tuen Mun River Channel ranged from "bad" to "good". The WQO compliance increased slightly in 2000 in particular with DO and chemical oxygen demand (COD). The suspended solids (SS) levels increased at the monitoring stations located near the active construction sites, e.g. West Rail project sites.

4.2.2 Sensitive Receivers

4.2.2.1 Scattered stream courses are located within the proposed road alignment. These local stream courses would be directly affected by the DBL project.

4.2.2.2 Flows from some of the local stream courses run through fishponds and agricultural land. The changes of water quality in the stream courses would have a direct impact on the stream water users. All the flows from the local stream courses enter the water body covered by the Deep Bay and North Western WCZs. These two WCZs are sensitive to the potential water quality changes due to the DBL developments.

4.2.2.3 The main streams located close to the proposed road alignment include Ngau Hom Sha Stream, Sheung Pak Nai Stream, Pak Nai Stream and Ha Pak Nai Stream. These streams are at some distance from the proposed DBL alignment. It is not likely that there would be direct water quality impacts to these streams due to the developments. Discharges from the proposed road alignment would mainly run to Tin Shui Wai Nullah and Tuen Mun River. Figure 4.1 shows the locations of the main streams, nullah and river.

4.2.2.4 Tsim Bei Tsui Site of Special Scientific Interest (SSSI) is located near the outlet of Tin Shui Wai Nullah and Pak Nai SSSI is located to the west of the proposed road alignment. These areas are of scientific interest and are sensitive to the changes in water quality. In addition, mangroves growth along the Deep Bay Road at Ngau Hom Shek are identified as sensitive receivers which are potentially impacted by the proposed developments.

4.2.2.5 The presence of the DBL may affect the existing fresh/salt water supply pipes, foul sewers and storm drains. Re-alignment of some of these facilities would be required, wherever necessary.

4.2.2.6 San Wai Sewage Treatment Works (San Wai STW) is located near the DBL alignment. This STW would be upgraded and expanded to deal with the sewage generated from future developments in the North-west New Territories. As the recommended alignment of the DBL project would not include any boundary control facilities, there would be no sewage discharge from the DBL developments into San Wai STW. San Wai STW would not be affected by the DBL developments in terms of sewage treatment capacity.

4.3 Assessment Methodology

4.3.0.1 The assessment of potential water quality impacts arising from the construction and operational phases of the DBL project is based on the guidelines outlined in the Annexes 6 and 14 of the EIAO-TM. The adopted assessment methodology is specific to the construction sites and activities of this project. The following areas are considered in the assessment:

• Review of relevant environmental legislation, polices, plans, standards and criteria
• Understanding of the background conditions of the Study Area
• Identification of the water quality sensitive receivers
• Identification of impact-causing factors
• Prediction and evaluation of impacts
• Recommendation on impact mitigation measures
• Evaluation of residual impacts

4.3.0.2 Mitigation measures to control discharges from construction sites are based on the guideline outlined in ProPECC Note PN1/94 on Construction Site Drainage. Best Management Practices (BMPs) are outlined in this section to recommend ways to reduce potential water quality impacts.

4.3.0.3 The proposed mitigation measures for the operational phase of the DBL project are targeted to reduce pollutants at source.

4.4 Identification, Prediction and Evaluation of Potential Impacts

4.4.0.1 This section identifies, predicts and evaluates the potential water quality impacts that would arise during the construction and operational phases of the project.

4.4.1 Construction Phase

4.4.1.1 Water quality impacts arising from the construction phase would include:

• Re-alignment of local stream courses;
• Re-alignment of existing fresh/salt water supply pipes, foul sewers and storm drains;
• Excavation and filling of road sections;
• Construction of foundation and road sections; and
• Sewage generated from the construction sites.

Local Stream Courses

4.4.1.2 Several stream courses are located within the proposed road alignment. Figure 4.2 shows the local stream courses that would be affected by the proposed developments.

4.4.1.3 A small stream near Ngau Hom Shek is located in the line of the end section of the DBL alignment. It runs through abandoned agricultural land and discharges into the Deep Bay WCZ. The stream courses located to the west of Ha Tsuen are relatively large. These stream courses run across the proposed road alignment in a north-easterly direction. Another small stream located to the south-west of Tin Sam would be affected by the proposed DBL alignment in the lower steam section. Discharges from local villages and farms have polluted the water quality in this section of the stream. Several stream courses run across the DBL alignment near Lam Tei Interchange. Some of the stream sections are artificially channelled.

4.4.1.4 During the construction phase of the proposed developments, the upper sections of these local stream courses would not be directly affected by the construction activities. At the locations where the DBL alignment is crossed by these stream courses, disruption of stream flows due to construction activities should be avoided. Re-alignment or diversion of the stream courses would be required in order to minimise impacts to the aquatic life and uses of water in the downstream regions. The stream courses, which would be physically affected at the points where the DBL alignment intersects with their existing alignments, are indicated in Figure 4.2. Box culvert or diversion channels are proposed at the same locations to mitigate the impact to the stream courses. The other existing stream courses would not be affected as the DBL is elevated at the intersect points.

4.4.1.5 Release of site runoff into the diverted stream courses during the construction phase of the project has the potential to deteriorate the stream water quality if not appropriately controlled. Site runoff may contain high concentrations of suspended solids leading to the increase in turbidity and suspended solids levels in the affected stream courses. Deposition and accumulation of solid particles in the stream courses would cause damage to habitats and aquatic organisms. Sufficient preventative measures and protective works need to be undertaken to minimise the potential water pollution. It is anticipated that site runoff could be effectively controlled through the implementation of preventative measures and protective works, the impacts to the stream courses and the associated ecological systems would be low.

Pipeworks and Drains

4.4.1.6 Re-alignment of the existing fresh/salt water supply pipes, foul sewers and storm water drains need to be undertaken wherever necessary. In the sections where the proposed road alignment is away from residential areas, the influence on the existing foul sewers and storm water drains is likely to be minimal.

4.4.1.7 Construction of road sections near Lam Tei would have a higher potential to affect the existing pipeworks and drains. The impacts to the existing pipeworks and drains would be covered in the drainage impact assessment. It is anticipated that the impacts would be within acceptable levels after the implementation of suitable re-alignment and diversion works.

Excavation and Filling

4.4.1.8 Excavation of land generates stockpiles of excavated soils, which should be re-used for filling of road sections in suitable locations. The existing vegetation along the road alignment would be removed. Soils would then be exposed to the atmosphere. A rainstorm may induce considerable soil erosion at the exposed soil surfaces.

4.4.1.9 Release of the excavated and filling materials into the local stream courses would increase the suspended solids, turbidity and nutrient levels of the stream water. The potential impacts on the local stream courses may include the reduction in light penetration into water for primary production, and damage to habitats and aquatic organisms. The increase in nutrient levels would deteriorate the water quality in the local stream courses limiting the activities of aquatic organisms. The suspended solids concentrations for effluent discharging into Groups B, C and D inland waters are limited to low levels (5 to 30mg/L for flow rates between £100 and £2000 3/day). Details of the allowable concentrations of suspended solids and other parameters are shown in Table 4.2.

4.4.1.10 Accumulation of sediment in the streams would reduce stream carrying capacity and would possibly increase surcharges. Proper handling of exposed soils should be fully considered as part of the site management practices. Similar practices should be applied to stockpiles of filling materials.

4.4.1.11 Excavation and filling would interrupt the stream flows leading to the changes in flow regimes. The filling of ponds and wetlands would cause the changes in water table and reduce flood storage capacity. The detention time of storm runoff would be reduced causing rapid runoff. There would be potential risk of flooding and soil erosion in the downstream areas of the construction sites.

4.4.1.12 The stream courses with high concentrations of suspended solids discharging into the inshore water near Ngau Hom Shek and Tsim Bei Tsui may affect the growth of mangroves and cause an increase in the turbidity of the Deep Bay waters. The impacts to the estuarine systems in Deep Bay would be more severe if a large-scale discharge of sediment occurs. Mitigation measures should be implemented to control the release of sediment from the excavation and filling activities. Sediment-laden effluent should be treated prior to discharging into the local stream courses.

Construction of Foundation and Road Sections

4.4.1.13 Bored pile foundation would be used for supporting the DBL road structures. Soil excavation and cleaning of foundation would be required. The excavated material has high water content. A large amount of wastewater would be generated from bored piling works. This type of wastewater is characterised by high concentrations of suspended solids. DBL is connected to SWC at Ngau Hom Shek, a pair of piers is located approximately 15m to the northwest of Deep Bay Road. If discharge of wastewater generated from bored piling works is not properly controlled, there would be potential impacts to Deep Bay mudflat at this location during construction. Suitable site arrangement and wastewater treatment facilities need to be provided to avoid water pollution and any harmful effect on mudflat.

4.4.1.14 Construction of diaphragm wall as part of the foundation works would generate a large amount of excavated soils. It is necessary to prevent migration of the excavated soils to the local stream courses through implementation of suitable mitigation measures.

4.4.1.15 The potential water quality impacts arising from construction of roads and associated facilities would involve wastewater generation. Attention should be paid to stockpiles of construction materials and exposed soils. The vehicle wheel washing facilities installed at all the site exits use water to clean the soil particles adhered to the vehicles and eventually produce sediment-laden wastewater. Washing of concrete lorry mixers generates wastewater with elevated pH values and high suspended solids contents. Wastewater would also be generated from the activities of construction equipment polishing and dust suppression.

4.4.1.16 In general, the wastewater generated from construction activities contains high concentrations of suspended solids. When mixing with the wastewater from washing of concrete lorry mixers, the pH value of the wastewater would be raised. Release of the wastewater into the local stream courses may increase or decrease the pH causing ecotoxic conditions in the receiving water. The aquatic organisms would be adversely affected. Provision of wastewater collection and treatment facilities and good planning of the construction works are required to minimise water pollution.

4.4.1.17 Illegal disposal and accidental spillage of chemicals in the construction sites cause soil contamination. This would have direct impacts on the groundwater quality. When the chemicals are transported by site runoff to the local stream courses, the water quality in the streams would be polluted. Organic pollutants may have toxic effects on aquatic organisms in the stream water. Long-term accumulation of toxic substances is also harmful to organisms affecting species diversity. The stream flows carries the pollutants downstream to Tin Shui Wai Nullah. Without proper mitigation measures, this may eventually cause serious impacts to the ecological conditions of Tsim Bei Tsui SSSI. Discharges of polluted stream flows at Ngau Hom Shek would potentially affect the existing wetlands and Pak Nai SSSI.

On Site Sewage Generation

4.4.1.18 Sewage would be generated from the workforce and domestic activities on site. Discharges of untreated sewage into the local stream courses are illegal and are harmful to the aquatic environment. In accordance with the "zero discharge policy" for Deep Bay, the generated sewage should be properly treated and disposed of to achieve the requirements of no net increase in pollution load to Deep Bay. It is necessary to provide temporary storage facilities or in-situ sewage treatment systems in order to comply with the policy.

4.4.1.19 Table 4.5 summarises the construction activities that may cause water quality impacts during the construction phase of the proposed developments. The sensitive receivers that would be potentially affected by the release of runoffs and wastewater from the construction sites are also included in the table.

4.4.1.20 The water quality impacts that may arise during the construction phase of the DBL project would be temporary. To minimise the potential water quality impacts, it is recommended to monitor the quality of the treated effluent discharged from construction sites to ensure that the treated effluent is in compliance with the discharge licence requirements. Details of the recommended water quality monitoring requirements are included in the Environmental Monitoring and Audit (EM&A) Manual. Site environmental audit should also be implemented to ensure that all the recommended mitigation measures to control water pollution are fully implemented on site by the contractors.

4.4.1.21 The DBL project and the concurrent major projects in the vicinity may cause cumulative impacts during the construction period. These major projects include Shenzhen-Hong Kong Western Corridor (2003-2006), Yuen Long Highway Widening (2003-2005), Hung Shui Kiu New Development Area (2004-2010), Route 10 North Lantau to Yuen Long Highway (2004-2007), and San Wai Sewerage Treatment Works (2004-2007).

4.4.1.22 Shenzhen-Hong Kong Western Corridor is a dual-3 carriageway and will be connected to DBL near Ngau Hom Shek. The permanent works for the SWC project would mostly be located offshore. Some construction activities would be carried out on land in the works area near Ngau Hom Shek. The potential water quality impacts that would be generated in Ngau Hom Shek area are mainly the construction site runoff, wastewater and sewage generation, and accidental spillage of chemical substances. The water quality sensitive receivers located near Ngau Hom Shek may be affected by the DBL and SWC projects. It is, however, considered that provision of site drainage systems, sedimentation/wastewater facilities and chemical toilets would be recommended as mitigation measures in the SWC EIA study and these water pollution control facilities should be provided on site during the construction period to minimise the water quality impacts. With the implementation of suitable mitigation measures, good site management practices, and environmental monitoring and audit, the DBL and SWC projects are not likely to cause adverse cumulative water quality impacts in Deep Bay.

4.4.1.23 Yuen Long Highway Widening project involves the widening of a dual two-lane highway to a dual three-lane highway. DBL will be connected to Yuen Long Highway via Lam Tai Interchange. The project of Hung Shui Kiu New Development Area involves residential and commercial developments, and construction of associated facilities and infrastructures. For the Route 10 North Lantau to Yuen Long Highway project, potential cumulative impacts may arise in the areas near the Route 10 tunnel portal and toll plaza in conjunction with DBL. Site runoff and wastewater generated from all types of construction activities from these project sites may cause cumulative impacts with DBL. All these projects would be carried out from a land-based operation. Mitigation measures defined in ProPECC Note PN1/94 - Construction Site Drainage would be implemented in each of the projects to control site runoff and to avoid water pollution. It is anticipated that cumulative water quality impacts due to these developments and highway projects would not be significant.

4.4.1.24 San Wai Sewage Treatment Works project is to expand the existing facilities at San Wan STW and Ha Tsuen Pumping Station. The treated effluent would be discharged via the NWNT effluent tunnel. The EIA study of San Wai Sewage Treatment Works project is still ongoing. All the construction phase impacts and mitigation measures would be addressed and assessed in that EIA study. Site runoff from the project sites may enter Deep Bay via the local stream courses and Tin Shui Wai Drainage Channel. Implementation of mitigation measures to control water pollution from site runoff and other water pollution sources would minimise the cumulative water quality impacts with the DBL project and the other concurrent projects.

4.4.2 Operational Phase

4.4.2.1 Upon completion of the project, the roads and associated facilities will be in full operation. Potential water quality impacts arising from the operational phase would mainly include:

• Storm runoff from road sections; and
• Spillage of chemicals or toxic substances due to accident.

4.4.2.2 Sewage facilities are not likely to be available in the small vehicle holding area near Ha Tsuen Interchange. If necessary, provision of chemical toilets to collect the sewage generated from the personnel who manage the facility would avoid water pollution. There would be no boundary control facilities in the DBL alignment. Sewage generation is unlikely to be a problem during the operational phase of the project.

Storm Runoff from Road Sections

4.4.2.3 The construction of road sections increases the impermeable areas in the catchment areas covered by the DBL alignment and reduces infiltration in the impermeable areas leading to the increase in quantity of road runoff. There would be potential impacts to the streams and channels downstream of the DBL road sections as a result of the increase in runoff.

4.4.2.4 The runoff from road sections near Lam Tei would eventually enter the North Western waters via Tuen Mun River whilst most of the road runoff from the DBL alignment would be directly/indirectly discharged into the Deep Bay waters. Deep Bay is a designated water control zone and there should be no net increase in pollution load to Deep Bay.

4.4.2.5 The amount of pollutants in road runoff is related to the pollutants accumulated on the road surface. The research from the University of New South Wales found that dry weather build up of pollutants reaches equilibrium after 10 days. After 10 days, contaminant deposition rates were similar to removal rates caused by air turbulence. The pollutant equilibrium would be maintained until a cleaning event. The cleaning events were defined as wind events exceeding 21 km/hr or storm events with rainfall exceeding 7 mm/hr. The research also indicated that pollutant concentrations in runoff are negligible after 20 to 30 minutes.

4.4.2.6 The existing coastal roads used by the vehicles travelling between Hong Kong and Shenzhen lie within the Deep Bay catchment. Currently, road runoff from the existing roads is discharged directly to Deep Bay through the normal road drainage systems. No special treatment facilities are installed in the road drainage systems to reduce the contaminant levels of road runoff. The construction of the DBL and Shenzhen Western Corridor (SWC) provides a more direct route for vehicles travelling between Hong Kong and Shenshen. The vehicle-generated pollutants in the road systems, however, still remain within the Deep Bay catchment. As Deep Bay is an ecological sensitive area, it is necessary to ensure that the potential impacts to Deep Bay due to road runoff from DBL are minimal.

4.4.2.7 Studies have been conducted to investigate pollutant concentrations in road runoff. Table 4.6 gives a list of the concentrations of some common pollutants found in road runoff from general roadways and highways.

Table 4.6 Pollutant Concentrations from Road Runoff

4.4.2.8 A study by Clark et al. (2000) investigated chemical characteristics of road sweeping sludge. Metal concentrations in road sweeping sludge were found to be affected by the mode of driving and were directly proportional to traffic density. A summary of chemical concentrations of road sweeping sludge is given in Table 4.7. The surface runoff in the road sections would carry the contaminants into the drainage systems and the local stream courses. The provision of end of pipe treatment is considered as a secondary control and would be less effective. The most effect way to minimise the impact is to reduce the contaminants at source. Regular cleaning of road sweeping sludge would reduce the contamination levels of the runoff, hence minimise the potential impacts to the receiving water body.

Remark:

1. Clark, C. F., Smith, P. G., Neilson, G., and Dinnie, R. M. (2000). “Chemical characterisation and legal classification of sludges from road sweepings.” Journal of CIWEM, 14, 99- 102.

4.4.2.9 Much of the road runoff that would be discharged to Deep Bay is via local stream courses and Tin Shui Wai Nullah. The pollutants contained in road runoff would first be diluted by the water in Tin Shui Wai Nullah and be further diluted by the tidal water when reaching the outlet of the nullah. The dilution of pollutants is due to the mixing of the polluted runoff with a large amount of water in the nullah or in the sea. Tidal flow would carry the pollutants offshore. The potential impacts from the road runoff discharging through Tin Shui Wai Nullah are likely to be low. The key concern is the discharge of road runoff to the mudflat near Ngau Hom Shek.

4.4.2.10 The end section of DBL connects to SWC near Ngau Hom Shek. A viaduct would be constructed over land from the coast and links to DBL. The road runoff from this section would be discharged directly to Deep Bay. During low tides, a large extent of mudflat along Deep Bay Road exposes to atmosphere. Release of polluted road runoff to the mudflat during the low tide period may affect the birds and fauna feeding at the mudflat. The impacts should be minimised through implementation of suitable mitigation measures.

4.4.2.11 The first flush flow generated from rainstorms usually contains most of the pollutants. The data presented in Tables 4.6 and 4.7 provides information on the concentrations of pollutants in road runoff and chemical characteristics of road sludge. The subsequent storm runoff from the road sections is in general less polluted. To effectively minimise the potential impacts to the receiving water environment and the mudflat, the vehicle-generated pollutants are best to be reduced at source. This could be achieved by frequent removal of the road sludge from the road sections. When mixing the pollutants with the storm water, the removal of these pollutants becomes more difficult. As such, provision of special treatment facilities to lower the pollutant concentrations in the road runoff would be less effective and should be considered as an alternative method with a lower priority.

4.4.2.12 Littering of rubbish from the DBL users may obstruct the storm drain systems. When a rainstorm occurs, storm runoff may not be effectively collected and diverted to the suitable discharge points. Illegal or accidental release of oil and grease from vehicles may cause pollution to the local stream courses and the seawater in the two water control zones. A tight control on these types of pollution should be implemented.

Accidental Spillage

4.4.2.13 Accidental spillage of chemicals or toxic substances is extremely dangerous to the water quality sensitive receivers. The worst case would be the accident of chemical transport vehicles. Leakage of fuel oils and/or other toxic substances may spread on road surface and enter the road drainage systems. When discharging into the local stream courses and eventually to Deep Bay and North Western waters, the damages to the fishponds, wetlands, agricultural land, Tsim Bei Tsui SSSI, Pak Nai SSSI and mangroves at Ngau Hom Shek would be severe.

4.4.2.14 The emergency action framework should be established to deal with the water pollution from accidental spillage so as to minimise the potential risk to the environment. In the event of oil pollution at sea, Marine Department (MD) is the designated authority for the clean-up of oil. MD has developed a Maritime Oil Spill Response Plan (MOSRP) to deal with oil spill and their potential hazard to the Hong Kong waters. The Plan is targeted to: 1) co-ordinate the responses from different government departments to control oil pollution; 2) provide a guide to the MD's response to oil pollution; 3) specify the limitation of the Plan; and 4) to identify priority of coastal areas for protection and clean-up. Through the emergency services and relevant works departments, Government has established measures and good practice for dealing with dealing with accidental spillage.

4.5 Mitigation of Adverse Impacts

4.5.0 Construction Phase

Local Stream Courses, Pipeworks and Drains

4.5.0.1 Construction of the DBL highway disrupts the flows of some of the local stream courses as indicated in Figure 4.2. It is recommended that supporting columns and piers for the elevated sections of the DBL should be located away from existing stream courses as far as possible. In the sections where the proposed road alignment intersects with the local stream courses, re-alignment or diversion of the local stream courses would be required. Box culverts and diversion channels should be constructed to divert the stream flows downstream.. The design of the diverted sections of the stream courses should minimise loss of flow section and avoid generating unstable flow conditions. The proposed box culverts and diversion channels are shown in Figure 4.2.

4.5.0.2 To minimise the potential water quality during the construction of box culverts or diversion channels, the construction period for re-alignment or diversion of stream courses should be shortened as far as possible through a better coordination with the other DBL construction activities. The contract documents should clearly specify the responsibilities of the Contractor to ensure that runoff from the works area should pass through silt traps and suitable sedimentation facilities prior to discharging into stream courses. A construction site drainage layout and management plan should be developed by the Contractor to detail the procedures for control of construction site runoff.

4.5.0.3 The existing pipes, which would be affected by the DBL project due to the increased runoff from DBL, should be upsized to increase the pipe capacity.

4.5.0.4 Monitoring of the local stream course water quality during the construction phase of the project is included as part of the Environmental Monitoring and Audit programme. Details are presented in the EM&A manual.

Excavation and Filling

4.5.0.5 The Study Area falls within the Deep Bay and North Western WCZs. Discharges from the construction sites are subject to control under the WPCO. The site management should apply for a discharge licence from EPD for discharge of effluent. The quality of the discharged effluent in terms of pH, suspended solids, chemical oxygen demand and other parameters as required should comply with the licence conditions. Pollution control and mitigation measures recommended in the ProPECC PN1/94 - Construction Site Drainage provide guidelines for preventing and minimising water pollution arising from construction activities. The Contractor should develop a construction site drainage layout and management plan with the consideration of the mitigation measures recommended in the ProPECC PN1/94. The following paragraphs provide recommendations on mitigation measures.

4.5.0.6 In areas where excavation and filling are carried out, it is recommended to build temporary earth bunds or use sand bags to confine the runoff or wastewater generated from the construction activities. Excavation works should be minimised in rainy season.

4.5.0.7 Open stockpiles of construction materials and dusty materials should be covered with tarpaulins during rainstorms. These materials should not be placed near the stream courses. This avoids the release of materials into the stream water.

4.5.0.8 To minimise the release of soil particles into the local stream courses during rainstorms, digging of trenches and holes should be carried out in short sections. After finishing a section of works, trenches and holes should be immediately back-filled to minimise the inflow of rainwater during rainstorms.

4.5.0.9 To prevent storm runoff from washing across exposed soil surface, intercepting channels should be provided. It is recommended to pave haul roads with concrete and protect temporary access roads using crushed stone or gravel. The exposed slope surfaces should be lined or hydroseeded.

4.5.0.10 The changes in water table due to filling of fishponds and wetlands would be localised. The reduction in flood storage capacity and changes in flow regimes should be regulated by provision of flow detention systems and suitable drainage arrangement. The Drainage Impact Assessment of the DBL project has identified that a stream course, which traverses the Hung Shui Kiu New Development Area, would need improvement and the use of a detention tank is the preferred option. The size of the proposed detention system has been determined based on a worse case scenario for a 1 in 10 year and 4-hour duration rainfall event. Figure 4.2 shows the location of the proposed detention system.

4.5.0.11 Drainage systems should be provided on site to collect all the contaminated runoff. Installation of sediment basin/trap and/or oil/grit separator is also recommended to reduce the pollution levels of contaminated runoff.

Construction of Foundation and Road Sections

4.5.0.12 Recommendations on water quality impact mitigation measures outlined in ProPECC PN1/94 - Construction Site Drainage should be adopted. The wastewater generated from bored pile foundation construction and related activities should be collected and recycled. When disposal of the wastewater is required, suitable treatment systems should be provided to reduce the suspended solids levels below the standards specified in the discharge license. The recommended mitigation measures against release of muddy wastewater from bored piling sites located approximately 15m to the northwest of Deep Bay Road into Deep Bay mudflat are shown in Figure 4.3. First, a row of interlocking sheetpiles should be installed to essentially fence off the area from the nearby stream course. Second, a wastewater treatment system comprising of chemical coagulation, sedimentation and pH control processes should be installed on site to treat the wastewater generated from the bored piling works. Chemical coagulation is to add coagulant into the wastewater to enhance the formation of larger sediment particles and to increase the removal of suspended solids. Sedimentation is a physical process of settling of particles in quiescent water. Wastewater generated from bored piling works may have slightly higher pH values, the pH control process is to add acid to neutralise the treated effluent to the levels within the requirements of discharge licence before final discharge. Figure 4.4 shows an example of the performance of a commercially available wastewater treatment system (treatment capacity: 50 m3/hr), which was used to treat the wastewater generated from a bored piling site. A schematic sketch of the wastewater treatment system is also shown in the same figure. The major pollutant in the wastewater generated from bored piling works is SS. As shown in the figure, the influent SS levels were about 10,000 to 30,000 mg/L. After the treatment processes, the SS levels of treated effluent were all below 30 mg/L. In general, the capacity of wastewater treatment system for treating wastewater generated from bored piling sites ranges from 40 m3/hr to 120 m3/hr depending on the scale of the bored piling works. Should the wastewater treatment system be properly selected and operated, the wastewater generated from bored piling works would be treated to acceptable levels before final discharge. A section of the drainage measures during bored pile construction along with the piling arrangement is shown in Figure 4.5 for information.

4.5.0.13 To ensure the Contractor would implement the proposed mitigation measures to properly handle the wastewater generated from bored piling works, it is recommended that the proposed mitigation measures should be included in the contract and reflected in the tender drawings. It is also recommended that the treated effluent could be used for vehicle washing, dust suppression and general cleaning.

4.5.0.14 The Contractor should submit a construction site drainage layout and management plan prior to the commencement of the DBL construction works to detail the procedures for control of construction site runoff. Wastewater treatment systems should also be installed on site to treat the wastewater generated from the construction sites.

4.5.0.15 Bentonite slurries used in bore-pile or diaphragm wall construction should be reconditioned and reused wherever practicable to minimise the volume of used bentonite slurries to be disposed of. A licensed waste collector should be deployed for collection and disposal of the used bentonite slurries.

4.5.0.16 Adequate surface channels should be constructed along the site boundaries to avoid release of surface and storm runoffs out of the sites. The channel system to collect the runoffs in the construction sites should be well designed prior to the commencement of the site formation works. Provision of drains at the lowest points of the sites could effectively collect the runoff. Silt and sand traps, which remove large soil particles in the runoffs, should be provided in the channels. Regular maintenance and cleaning of the channels would ensure that the channel system is in good conditions and is not obstructed by sediments. Provision of oil/grit separator in the channel system is recommended to reduce the pollution levels of the potentially contaminated runoff. It is recommended that the Contractor should submit a construction site drainage layout and management plan prior to the commencement of the DBL construction works. This can ensure a better site management and control of surface runoff on site.

4.5.0.17 Vehicle wheel washing facilities should be provided at all site exits to avoid the escape of soil and dirt from the construction sites. The wastewater generated from the vehicle wheel washing facilities should be recycled wherever practicable. Excess wastewater should be transferred to a suitable wastewater treatment system for removal of suspended solids.

4.5.0.18 The wastewater generated from washing of concrete lorry mixers could be pre-treated by discharging into a sedimentation pit, which provides a quiescent environment for the concrete particles to settle and consolidate. The upper layer water in the sedimentation pit with low concentrations of concrete particles should be further treated to the standards acceptable for final discharge. The concrete wastes deposited on the bottom of the pit should be removed regularly.

4.5.0.19 Covers should be provided to the newly constructed manholes to prevent any kinds of wastewater from entering into the drainage systems during the construction phase. The pipes connected to the manholes need to be temporarily sealed to avoid debris and construction materials get into the drainage systems.

4.5.0.20 The most critical parameter of the site runoffs and the wastewater generated from various construction activities is suspended solids. The wastewater treatment system for treating wastewater generated from bored piling works would be able to handle these types of wastewater.

On Site Sewage Generation

4.5.0.21 Chemical toilets should be provided on site for collection and temporary storage of sewage. Sewage storage tank is also an alternative option. The collected sewage should be tankered away by a licensed waste collector for off-site disposal at sewage treatment plants, i.e. DSD's Sewage Treatment Facilities in North West New Territories. Impacts due to off-site disposal of the collected sewage would be minimal.

4.5.0.22 Mitigation of water quality impacts could be achieved through Best Management Practices (BMPs). A detailed planning and sensible scheduling of construction works would minimise most of the impacts. Elements of BMPs should include a clean site policy, suitable design and well planning, establishment of a site environmental team to work closely with the site management, good housekeeping, and monitoring and audit.

Clean Site Policy

4.5.0.23 A clean site policy should be initiated from the top management and be implemented at all levels of the management and working teams. The policy statement needs to be periodically reviewed and assessed for effectiveness. Information related to the government's environmental protection regulations and guidelines should be provided to the site management for improving the environmental awareness of the site staff. It is recommended that mitigation measures to prevent water quality impacts could be included in the tender documents. Any appointed sub-contractors are aware of their responsibilities in preventing water pollution when they bid for the project and carry out the construction works.

Design and Planning

4.5.0.24 Recognising the potential water quality impacts that may arise during the construction and operational phases of the project, a full consideration of suitable water pollution controls at the design and planning stages is essential. The following items should be focused on:

• Site planning appropriate to site conditions
• Re-alignment of the existing streams
• Minimisation of storm water runoff
• Adequacy of storm drain and grit/sand removal facilities
• Measures to deal with generation of sewage
• Treatment facilities to handle site discharges

Environmental Team and Site Management

4.5.0.25 Formation of a site environmental team would ensure the proper implementation of mitigation measures in order to deal with the potential water pollution associated with the construction activities. The team would be able to response immediately to the water pollution issues happened on site. Any unforeseen issues could be identified and remedied without delay. Control measures outlined in ProPECC Note PN1/94 on Construction Site Drainage should be implemented where appropriate.

4.5.0.26 The team members should carry out routine inspection and provide feedback to the site management forming a framework of continual improvement of the water quality protection work. Regular meetings with all the working parties are required to review the environmental performances of each individual party. Relevant environmental protection regulations and practical guidelines should be delivered and explained to the working parties in the meetings.

Good Housekeeping

4.5.0.27 A wide variety of materials including steel and wooden materials, concrete (sand, aggregate and cement) and chemicals would be delivered to the construction sites. Good housekeeping avoids pollutants originated from these materials from entering storm and site runoffs. This reduces adverse impacts to the downstream watercourses. Generation of wastewater and illegal discharges of chemicals and sewage would be minimised. The following areas need to be implemented to achieve good housekeeping on site:

• Provision of designated areas for storage of raw materials, chemicals and toxic substances
• Control and keeping records of raw materials, chemicals and toxic substances being delivered to and out of the construction sites
• Provision of control areas for equipment maintenance and repair
• Provision of adequate sewage collection facilities at convenient locations
• Registration as a "Chemical Waste Producer" if necessary and provision of proper storage, labelling and disposal of chemical wastes generated from the construction activities
• Collection of site debris and solid wastes on a regular basis
• Notification to all working parties of no illegal disposal of wastewater and chemical wastes
• Minimisation of the volume of wastewater generated from dust suppression activities and unattended water faucets
• Provision of treatment facilities including sediment removal facilities and oil/grit separator to control site discharges to the standards in compliance with the requirements specified in the discharge licence

Monitoring and Audit

4.5.0.28 The site environmental team and site management play an important role for the implementation of environmental monitoring and audit on site. Through the monitoring and audit, the potential water quality impacts would be reduced.

4.5.0.29 Monitoring of the discharged effluent quality at the final discharge points during the construction phase should be undertaken to ensure that the effluent quality is in compliance with the discharge licence requirements. In view of the length of the DBL alignment, several discharge points are proposed at different locations along the alignment. Figure 4.6 shows the discharge points / monitoring locations (WQ1 to WQ11) along the DBL alignment. The proposed discharge points are consistent with those identified in the Drainage Impact Assessment to provide adequate routes to release the runoff or treated effluent from the DBL road sections to the local stream courses.

4.5.0.30 Construction site runoff and any treated wastewater generated in the section near the SWC at Ngau Hom Shek would be discharged directly into the Deep Bay waters at this discharge point. The wetlands at Ngau Hom Shek would be sensitive to the water quality changes due to the discharges from the construction sites. Special attention should be paid to ensure that the discharge quality at this point is in compliance with the standards specified in the discharge licence and would not cause adverse impacts to the wetlands. Under the SWC project, a works area would be demarcated at the immediate north of the interface between DBL and SWC along Deep Bay Road. As the two projects would be carried out at the same time, construction site runoff and wastewater generated in the works area and from the DBL section at Ngau Hom Shek would be collected and treated in the works area prior to discharging into the Deep Bay waters. Effluent quality at the discharge point in the works area would be monitored under the SWC project to ensure the compliance with discharge licence requirements. Water quality monitoring has been proposed under the SWC project as part of the environmental monitoring and audit programme. A number of control and impact monitoring stations have also been proposed in the inter-tidal and sub-tidal areas along the SWC alignment. The SWC monitoring stations (W1, W2, W11 and W12) in the inter-tidal area near Ngau Hom Shek and at the nearby sensitive receivers could serve the purpose of monitoring the potential water quality impacts arising from both the SWC and DBL construction activities in the works area and the DBL section near Ngau Hom Shek. Details of the construction phase water quality monitoring are presented in the EM&A manual.

4.5.0.31 The treated effluent to be discharged at WQ1 to WQ5, WQ10 and WQ11 would enter the local stream courses. The major flow direction of these stream courses is to the north and all these stream courses are linked to the Tin Shui Wai Nullah. The flows would be eventually discharged into the Deep Bay WCZ. The discharges at WQ6, WQ7, WQ8 and WQ9 would enter two separate stream courses. The major flow direction of these two stream courses is to the south. Through the Tuen Mun River, all the flows would be discharged into the North Western WCZ.

4.5.0.32 The parameters to be monitored at the discharge point and monitoring frequency should meet the requirements specified in the discharge licence. In general, suspended solids, pH and chemical oxygen demand are the parameters of concern. Laboratory analysis of these parameters would be required to monitor the discharge quality. Recommendations on the site environmental audit are included in the EM&A Manual.

4.5.0.33 Effluent discharged from the construction sites would enter the Groups B, C and D inland waters, Deep Bay WCZ and North Western WCZ. The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM) issued under section 21 of the WPCO defines the quality of effluents discharged into these waters. The discharge licence issued by EPD would be based on the TM to specify the maximum discharge limits of relevant parameters. For different discharge flow rates, the effluent standards vary slightly. Estimation of discharge flow rate is required when applying for the discharge licence from EPD. With regard to the construction site discharges, the major parameters to be monitored may include suspended solids (SS), pH, chemical oxygen demand (COD) and others as specified in the discharge licence.

4.5.0.34 The ET and site management should be responsible for monitoring the quality of effluent discharged from the construction sites and be response to the any non-compliance of effluent discharge. The pollution sources need to be identified and the effectiveness of mitigation measures should be reviewed to avoid the reoccurrence of the non-compliance.

4.5.0.35 When non-compliance of the effluent quality is detected, the ET should inform the site management and identify the sources of pollutants and causes of non-compliance. The causes might be a result of sudden increases in pollution loads and flows from the construction or operation activities. It might also be related to lack of protective works provided in the project site. Remedial actions are required to ensure that the recommended mitigation measures are fully implemented and the non-compliance is suitably rectified.

4.5.0.36 A sample monitoring record sheet is shown in Table 4.8 for reference.

Table 4.8 Water Quality Monitoring Data Record Sheet

Note:

The laboratory results for the monitored parameters are to be filled up once they are available from the laboratory.

4.5.1 Operational Phase

Storm Runoff from Road Sections

4.5.1.1 At the planning and design stages, the highway drainage systems should be properly planned to receive storm runoff from the DBL highway. The DBL road sections consisting of impervious paving would generate a large quantity of runoff. The potential impacts to the downstream channels and risk of flooding is assessed in the drainage impact assessment.

4.5.1.2 At the northern end DBL near Ngau Hom Shek runoff from DBL would discharge direct to Deep Bay. There are concerns that during low tides contamination could adversely affect the mudflats, which have ecological value. Therefore, adoption of mitigation measures to protect the mudflats is considered.

4.5.1.3 Use of treatment facilities including infiltration basin, reed bed or sedimentation tank requires the provision of a large area of land. The pollutants in road runoff from DBL highway may include heavy metals, oil and grease and inorganic matters. The pollutants in dissolved phase would not be removed by these facilities. The treatment efficiency of these facilities would also be uncertain when a large amount of road runoff is generated during a heavy rainstorm. A drawback of these facilities is the potential for toxic build-up. The nearby vegetation and groundwater may be polluted. This would affect birds and other fauna feeding in the region. Mikkelsen et al (1997) pointed out that stormwater infiltration systems might effectively trap pollutants but the systems might also pose a potential solid waste disposal problem. In fact, disposal of polluted material may generate secondary pollution and reduce the capacity of landfill sites.

4.5.1.4 The University of New South Wales (UNSW) Research Report No. 204 entitled 'Stormwater Quality from Road Surfaces - Monitoring of Hume Highway at South Strathfield' reported that dry weather build-up of contaminants reaches equilibrium after 10 days. After 10 days, contaminant deposition rates are similar to removal rates caused by air turbulence. Also, the report indicated that the contaminant equilibrium would be maintained until a cleansing event. Cleansing events were defined as wind events exceeding 21 km/hour or storm events with rainfall exceeding 7 mm/hr. The contaminant concentrations in runoff are negligible after 20 to 30 minutes.

4.5.1.5 Currently the most effective mitigation measure to prevent the vehicle-generated pollutants from entering the Deep Bay waters and to Deep Bay mudflat is therefore to remove the pollutants from the road surface prior to the occurrence of a rainstorm. When the contaminants are removed from the road surface, the pollution level of the first flush flow would be low. It is recommended that vacuum air sweepers/trucks should be deployed to remove the pollutants deposited on the road surface. This mitigation measure would also be applied in SWC, which is connected to DBL at Ngau Hom Shek. Vacuum air sweepers/trucks are commercially available and are designed to remove debris and road sludge from road surface. The highway would be cleaned twice per week, which is shorter than the time required for the build-up of contaminants to an equilibrium level (10 days). Most of the pollutants deposited on the road surface would therefore be removed through the cleaning operation and would not be washed away to the mudflat and the Deep Bay waters. For the case without road cleaning, the pollutants may be blown away by air turbulence from the road surface to the surrounding environments, i.e. the Deep Bay waters and the mudflat, after reaching equilibrium if there is no occurrence of rainstorm. Implementation of vacuum cleaning at an interval of twice per week in this case would effectively reduce the volume of pollutants to be blown away by air turbulence. A rough estimate of the reduction in pollutants to be eventually released into the seawater would be about 80 to 90%. In the event that a rainstorm occurs just before the vacuum cleaning to take place, the pollutants that would be washed away to Deep Bay are only approximately 30% of the loading for the equilibrium case. Taking into consideration of all the vehicle-generated pollutants within the Deep Bay catchment, the removal of pollutants in DBL offsets the increases in pollution loads as a result of the increases in traffic flow within the catchment to a certain extent. The proposed mitigation measure is consistent with the road cleaning recommended in the SWC EIA Study to remove the contaminants on the road surface along the SWC bridge. Vacuum air sweepers should incorporate side broom to sweep road sludge and debris into the suction nozzle to increase the removal efficiency of vehicle-generated pollutants. As most of the pollutants would concentrate near the road kerb, the cleaning path of the vacuum air sweeper should mainly cover the region about 1 to 2 m from the road kerb. Road washing could also be carried out to remove oil and grease on the road surface. The side broom and the suction nozzle installed in the vacuum air sweeper could remove the cleaning water from road surface. After the removal of the pollutants, the pollution levels in storm runoff would be much reduced. The cleaning operation is more suitable to be carried out in the period with low traffic flow. Since road runoff from the end section of DBL at Ngau Hom Shek as shown in Figure 4.7 would be directly discharged to the mudflat during ebb tides, vacuum cleaning should be carried out on this section.

4.5.1.6 HyD is the management authority of both the DBL and SWC projects. Under the SWC project, the frequency of road cleaning using vacuum air sweeper would be reviewed through an operational phase bridge runoff monitoring programme. Since the DBL is connected to SWC, similar operational phase bridge runoff monitoring programme to cover the end section of DBL at Ngau Hom Shek would be required to determine the effectiveness of the mitigation measure. The cleaning frequency would be reviewed based on the monitoring data. Details of the monitoring requirements are included in EM&A Manual.

 4.5.1.7 The remainder of DBL carriageway would discharge to inland drainage systems similar to the existing highways and urban areas. HyD undertakes routine road inspection, maintenance and cleaning for public roads, expressways, trunk roads, and other roads. With the routine cleaning practices adopted by HyD in place, the potential water quality impacts due to road runoff from the remaining sections of DBL are expected to be low. In addition, road runoff would be discharged into the Deep Bay waters via Tin Shui Wai Nullah and North-western waters via Tuen Mun River. The pollutants would be rapidly diluted by tidal flows near the outlets of the nullah/river minimising the water quality impacts. Special mitigation measures to deal with road runoff from the remaining sections of DBL would not be required. Overall, vacuum air cleaning would be provided only for the section of DBL that drains directly into Deep Bay. The remaining sections of DBL would be cleaned under the routine HyD cleaning programme.

4.5.1.8 Standard HyD road gullies with silt traps would be installed in the road drainage systems of the entire DBL alignment to intercept and enable removal of residual grit, particulate matter and pollutants in road runoff. These facilities serve as a control to minimise the pollutants from discharging to the Deep Bay waters and mudflat. Regular cleaning of rubbish and sediment from the drainage systems following the normal highway maintenance practices is required to maintain the normal operation of the systems at all times.

4.5.1.9 DBL also includes a vehicle recovery and weighing station. In this area vehicles may be parked. As stationary vehicles particularly older and defective vehicles can drip oil and grease. Oil and grease interceptors would be incorporated as part of drainage system. There would be no office associated with the weighing station. Therefore, no domestic sewage would be generated from this area and domestic sewage would not be an issue.

Accidental Spillage

4.5.1.10 Road accidents can cause accidental spillage of fuel or other chemicals along DBL. This can cause adverse environmental impacts and hazard to adjacent areas. The issue for DBL is similar to all highways and roads in Hong Kong. Government through the emergency services and relevant works departments have appropriate procedures for dealing with accidental spillage. These are specific to the nature of the spillage and the impacts and hazards the spillage presents. As far as practical containment and collection of the spillage is preferred for appropriate disposal. The following issues require to be considered and appropriate action taken on a case by case basis:

• measures to prevent the potential pollution to the local stream courses and the downstream water body in case of an accident;
• requirements to inspect the drainage systems to ensure that the channels or drain pipes are in normal function;
• procedures and guidelines to deal with different categories of spillage;
• actions to inspect and monitor the water quality of the nearby receiving water body after an accident;
• provision of pollution control equipment; and
• actions to restore the affected water body.

4.5.1.11 Appendix 4A includes the emergency action framework for DBL.

4.6 Definition and Evaluation of Residual Impacts

4.6.0.1 Residual impacts are the remaining impacts that cannot be totally eliminated after implementation of mitigation measures. In this study, potential water quality impacts would be mainly generated from construction activities during the construction phase. With the implementation of the proposed mitigation measures, residual impacts should comply with legislative requirements and would not cause any deviations of water quality in Deep Bay from the existing conditions. Implementation of BMPs and water quality monitoring during the construction and operational phases of the proposed developments would ensure that the water quality impacts to the local water environment are minimal. Use of vacuum air sweepers to remove vehicle-generated pollutants from the road surface would effectively minimise the potential impacts to the Deep Bay waters and mudflat. The removal of pollutants from the end section of DBL reduces the overall pollution loading generated from vehicles within the Deep Bay catchment. The residual pollutants to be directly released into the Deep Bay waters and mudflat would be low in quantity. The exchanges of tidal flows in Deep Bay during flood and ebb tides minimise the long-term effect due to the residual pollutants. It is anticipated that there would be no insurmountable water quality impacts as a result of the DBL developments.

4.7 Environmental Monitoring and Audit

4.7.0.1 Both construction phase and operational phase EM&A are recommended for water quality. The details of the EM&A program are given in Section 11 of this report and the separate EM&A manual.