7. HYDRODYNAMIC, WATER QUALITY AND SEDIMENT QUALITY IMPACTS

7.1 Introduction

7.1.1 This section presents the assessment of potential water quality impacts that may arise during the construction and operational phases of the SWC project. Figure 7.1 shows the alignment of the SWC bridge. Assessment of the changes in hydrodynamic and water quality conditions due to the proposed SWC bridge in Deep Bay is included. The Deep Bay Model was used as a modelling tool to predict the hydrodynamic and water quality impacts.

7.1.2 The construction phase impacts covered in the assessment include construction site runoff, wastewater generated from construction activities, sewage generation, accidental spillage of chemicals on site, sediment dredging and changes in hydrodynamic conditions during the bridge pier construction. The operational phase impacts include hydrodynamic and water quality changes due to the presence of the SWC bridge piers and reclamation on the Shenzhen side, changes in erosion and sedimentation patterns in Deep Bay, road runoff from the SWC bridge, and accidental spillage of chemicals during accidents.

7.1.3 This section also includes the assessment of sediment quality, classification of sediment and recommendations on sediment disposal.

7.2 Environmental Legislation, Policies, Plans, Standards and Criteria

7.2.1 Relevant legislation and guidelines used for water quality impact assessment of the proposed SWC project are described in this section. The proposed SWC bridge alignment covers a corridor in Deep Bay linking between Ngau Hom Shek on the Hong Kong side and Dongjiaotou on the Shenzhen side. Part of Deep Bay is within the HKSAR and some regions are within the boundary of the Mainland. As the water quality model covers the whole water body in Deep Bay, the changes in water quality conditions within the Hong Kong waters should be assessed using the relevant HKSAR legislation and guidelines. To assess the water quality conditions on the Shenzhen side, it is more appropriate to compare the results with the relevant Mainland legislation and guidelines.

HKSAR

Environmental Impact Assessment Ordinance (EIAO), Cap.499, S16

7.2.2 The proposed SWC is a Designated Project under Schedule 2 of the EIAO. Under Section 16 of the EIAO, Environmental Protection Department (EPD) issued the "Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM)" which specifies the assessment methods and criteria for environmental impact assessment. This Study follows the EIAO-TM to assess the potential water quality impacts that may arise during the construction and operational phases of the Project. Sections in the EIAO-TM relevant to the water quality impact assessment are:

· Annex 6 - Criteria for Evaluating Water Pollution; and
· Annex 14 - Guidelines for Assessment of Water Pollution.

Water Quality Objectives (WQOs)

7.2.3 The Water Pollution Control Ordinance (WPCO) (Cap.358) provides the major statutory framework for the protection and control of water quality in Hong Kong. According to the Ordinance and its subsidiary legislation, the whole Hong Kong waters are divided into ten Water Control Zones (WCZs). Water Quality Objectives (WQOs) were established to protect the beneficial uses of water quality in WCZs. Specific WQOs are applied to each WCZ. The proposed SWC is located within the Deep Bay WCZ and the corresponding WQOs are listed in Table 7.1. The WQOs for the Deep Bay WCZ are used as the basis for assessment of water quality impacts in the present Study.

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

7.2.4 As specified in the EIA Study Brief, the "Technical Report on Environmental Protection of Deep Bay and its Catchment, Appendix T, Hong Kong Guangdong Environmental Protection Liaison Group, December 1992" should also be used as a reference in assessing the water quality impacts due to the SWC project. Table 7.2 summarises the parameters included in "Appendix T" of the technical report. Reference is made to "Appendix T" for the parameters that are not specified in the WQOs for the Deep Bay WCZ.

Table 7.2 Water Quality Objectives for Deep Bay – Appendix T, Technical Report on Environmental Protection of Deep Bay and its Catchment

Technical Memorandum on Effluent Discharge Standards

7.2.5 Discharges of effluents are subject to control under the WPCO. The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM) sets limits for effluent discharges. Specific limits apply for different areas and are different between surface waters and sewers. The limits vary with the rate of effluent flow. Standards for effluent discharged into the waters of Deep Bay WCZ are presented in Table 7.3.

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 9a, Environmental Protection Department.

Practice Note for Professional Persons on Construction Site Drainage

7.2.6 The Practice Note for Professional Persons (ProPECC Note PN1/94) on Construction Site Drainage provides guidelines for the handling and disposal of construction discharges. This note is applicable to this Study for control of site runoff and wastewater generated during the construction phase of the SWC project. The types of discharges from construction sites outlined in the ProPECC Note PN1/94 that are relevant to the present Study include:

· Surface run-off;
· Boring and drilling water;
· Wastewater from concrete batching and precast concrete casting;
· Wheel washing water; and
· Wastewater from construction activities and site facilities.

Sediment Quality

7.2.7 Relevant legislation and guidelines for disposal of contaminated material at marine disposal sites are listed below:
· Dumping at Sea Ordinance (Cap. 466);
· Works Bureau Technical Circular No. 3/2000, (WBTC No. 3/2000) Management of Dredged/Excavated Sediment; and
· Works Bureau Technical Circular No. 12/2000, (WBTC No. 12/2000) Fill Management.

7.2.8 The Dumping at Sea Ordinance is the major statutory legislation to control dumping of sediment at sea. This safeguards the water quality and ecology of the Hong Kong waters. The WBTC No. 3/2000 sets out the management framework for dredged/excavated sediment disposal. The project, which will commence on or after 1 January 2002, should follow the WBTC No. 3/2000. This technical circular is applicable to the SWC project.

7.2.9 The WBTC No. 3/2000 provides guidelines for the classification of sediment based on their contaminant levels with reference to the Chemical Exceedance Levels. Sediment quality criteria for sediment classification include metals (cadmium, chromium, copper, mercury, nickel, lead, silver and zinc); metalloid (arsenic); and organic micro-pollutants (PAHs, PCBs and TBT). Based on the sediment quality criteria, the sediment is defined as Category L material (low contaminant levels), Category M material (medium contaminant levels) or Category H material (high contaminant levels).

7.2.10 The WBTC No. 3/2000 stipulates a three-tier screening for sediment assessment. Tier I screening is a desktop study of available data. The data will be used to determine whether the sediment is Category L material and is suitable for open sea disposal. If decision cannot be made as a result of insufficient information, Tier II screening, which categories the sediment based on testing the chemical contaminant levels, is required. Tier II screening determines the suitability of open sea disposal for the sediment and decides whether further testing is required. Tier III screening should be conducted to identify the most suitable disposal option for Category M material and certain Category H material identified in Tier II screening.

7.2.11 The WBTC No. 12/2000 defines the responsibilities of the Marine Fill Committee (MFC) and the Public Fill Committee (PFC). The circular sets out the terms of reference and membership of the two committees and provides explanation on the management of fill resources, construction and demolition material, and dredged/excavated sediment disposal.

Mainland

Sea Water Quality Standard (GB3097-1997)

7.2.12 The Sea Water Quality Standard GB3097-1997 ((海水水質標準GB 3097-1997) was established under the National Standard of the People's Republic of China UCD 551463 (中華人民共和國國家標準UCD 551463). It specifies water quality objectives for different beneficial uses of marine water in Mainland. There are four categories of receiving water under the regulation. Relevant Mainland water quality objectives are listed in Table 7.4.

Remarks:
1. Category 1 represents marine fisheries zone, marine natural reserve area and critically endangered marine habitat protection area;
2. Category 2 represents marine cultural zone, marine bathing water, secondary contact or marine recreation area, and marine water which is directly related to human consumption;
3. Category 3 represents marine water for general industrial use and marine scenic area;
4. Category 4 represents marine harbour area and marine development area; and
5. All units in mg/L unless otherwise stated.

Source: Sea Water Quality Standard GB3097-1997

Integrated Wastewater Discharge Standard (GB8978-1996)

7.2.13 The Integrated Wastewater Discharge Standard GB8978-1996 (污水綜合排放標準 GB 8978-1996) was stipulated under the National Standard of the People's Republic of China UCD 551463 (中華人民共和國國家標準 UCD 551463). The regulation specifies two categories of pollutants in the effluent discharges. Standards for the pollutants of the first category apply to all discharges regardless of the types of effluents and uses of receiving waters and are listed in Table 7.5.

7.2.14 Standards for the pollutants of the second category set different limits for different uses of receiving water. There are 3 classes of limits for the pollutants of the second category. Class 1 limits apply to the effluent discharged into marine water of Category 2 under the Sea Water Quality Standard GB3097-1997 as described in the above section. Class 2 limits apply to the effluent discharged into marine water of Category 3 under the Sea Water Quality Standard GB3097-1997. Class 3 limits apply to effluent discharged into sewers leading to secondary wastewater treatment plant. The relevant standards for selected parameters applicable to enterprise constructed on or after 1 January 1998 are presented in Table 7.6.

7.3 Description of the Environment

HKSAR

Water Quality

7.3.1 The proposed SWC is located within the Deep Bay WCZ. EPD has been carrying out routine marine water quality monitoring at a number of monitoring stations in this WCZ. Evaluation of the baseline conditions of the water bodies covered by the Deep Bay WCZ for the present study has been based on the monitoring data from EPD's monitoring stations.

7.3.2 There are 5 marine water quality monitoring stations in the Deep Bay WCZ. Stations DM1, DM2 and DM3 are located within the inner sub-zone whereas stations DM4 and DM5 are located in the outer sub-zone. Water quality in the outer sub-zone was better than that in the inner sub-zone based on EPD's monitoring results in 2000. The BOD5, SS and inorganic nutrient levels were comparatively higher in the Inner Deep Bay. There was an increase in DO level (0.4 mg/L) in the bay. At DM1 and DM2, the recorded DO levels were the lowest (3.6 mg/L at DM1 and 3.9 mg/L at DM2). There were remarkable increases in E. coli levels at all marine monitoring stations in Deep Bay. The increases ranged from 40% to 400%. There was an overall 17% decrease in BOD5 level based on the data recorded at the 5 monitoring stations. The nitrogen and phosphorus levels did not have significant variations from the data recorded in 1999.

7.3.3 Exceedances of the WQOs for dissolved oxygen (DO), total inorganic nitrogen (TIN) and unionised ammonia (UIA) were mostly recorded in the inner sub-zone in 1999. There were some improvements of the DO level in the bay and no WQO exceedance for DO was recorded in 2000. However, the TIN levels recorded at all the monitoring stations exceeded the WQO indicating high nutrient levels in the Deep Bay waters. Exceedances of the WQO for UIA were observed at DM1, DM2 and DM3 within the Inner Deep Bay. Table 7.7 summaries the water quality monitoring results in the Deep Bay WCZ in 2000.

7.3.4 The ammonia nitrogen and TIN levels at DM1 to DM4 increased from 1986 to 2000. There were also long-term increases in E. coli level at DM2, DM4 and DM5. The stations in the Outer Deep Bay (DM4 and DM5) showed a long-term decreasing trend in depth-averaged DO.

Notes:
1. Data are depth-averaged data.
2. Data presented are annual arithmetic means except for E.coli and faecal coliforms, which are geometric means.
3. The value in bold indicates that the parameter exceeds the WQO for the Deep Bay WCZ.

Source: Marine Water Quality in Hong Kong in 2000 by EPD.

Sediment Quality

7.3.5 There are four EPD sediment sampling stations (DS1, DS2, DS3 and DS4) in the Deep Bay Water Control Zone (Deep Bay WCZ). Locations of the sediment sampling stations are shown in Figure 7.2. DS1 is located close to the outlet of Shenzhen River. The location of DS2 is near the proposed SWC alignments. DS3 and DS4 are in Outer Deep Bay. EPD adopts the sampling method of taking grab samples of the top 10cm layer of sediment for sediment metal analysis.

7.3.6 Based on the sediment quality data collected by EPD from 1995 to 2000 (see Appendix 7A), the average zinc levels at DS1 were high ranging between 86 mg/kg and 360 mg/kg. It appeared that the data collected in 1999 were comparatively lower than the previous years, but the zinc level significantly increased in 2000. The recorded zinc levels at DS1 in 1997, 1998 and 2000 exceeded the UCEL of 270 mg/kg as specified in WBTC No. 3/2000.

7.3.7 The nickel levels in the sediment collected at DS1 were also recorded high in 1997 - 1998 and subsequently reduced in 1999. The nickel level increased again in 2000. The highest nickel level recorded in August 1998 was above the UCEL of 40 mg/kg. The 6-year records indicated that the cadmium levels at DS1 were low (0.1 - 0.5 mg/kg) and were much below the LCEL of 1.5 mg/kg. High concentrations of copper (84 - 98 mg/kg) and arsenic (14 - 20 mg/kg) were also recorded in 1997, 1998 and 2000. These levels exceeded the LCEL for copper (65 mg/kg) and arsenic (12 mg/kg), but they were below their corresponding UCEL (110 mg/kg for copper and 42 mg/kg for arsenic). The copper and arsenic levels in the sediment decreased in 1999 but increased again in 2000 with the values of 98 mg/kg and 20 mg/kg respectively. There were two records (77 and 87 mg/kg) exceeded the LCEL for lead (75 mg/kg) at DS1 in January 1997 and January 2000. No exceedance of the LCEL for chromium and mercury was found at this station. There were no records for silver from 1995 to 1997. The recorded silver levels at DS1 between 1998 and 2000 were at the LCEL of 1 mg/kg.

7.3.8 For the organic micro-pollutants, the PCB levels (5 - 23 ug/kg) at DS1 were all below the LCEL of 23 mg/kg from 1995 to 1999, but the LCEL was exceeded with the value of 24 mg/kg in 2000. The PAH levels were 46 - 523 mg/kg (1995 - 2000). There was no measurement of TBT.

7.3.9 Based on the available data from EPD, the sediment (surface layer) at DS1 changed from Category L material in 1995 - 1996 to Category H material in 1997 - 1998. The condition appeared to be improved in 1999 as the sediment was Category M material. But the condition deteriorated in 2000 as the sediment was Category H material.

7.3.10 The EPD sediment sampling station DS2 is nearest to the SWC alignments. There was one exceedance of the LCEL for zinc in January 1997 with a value of 220 mg/kg. The rest of the data measured between 1995 and 2000 were below the LCEL for zinc with a range between 69 and 190 mg/kg. There were quite a number of exceedances of the LCEL for arsenic at DS2. In 2000, the measured arsenic level was 17 mg/kg. Except for the copper level (66 mg/kg) recorded in January 1997, most of the copper levels were lower than the LCEL of 65 mg/kg. The concentrations of cadmium (0.1 - 0.4 mg/kg), chromium (21 - 47 mg/kg), nickel (11 - 28 mg/kg), lead (33 - 69 mg/kg) and mercury (0.05 - 0.2 mg/kg) recorded from 1995 and 2000 were below their corresponding LCEL. The silver levels recorded in 1998 and 2000 were 1 mg/kg at the LCEL. The PAH levels (39 - 423 ug/kg) and PCB levels (5 - 20 ug/kg) were recorded low at DS2. Comparing the available EPD data with the WBTC No. 3/2000, the sediment at DS2 was mainly Category M material.

7.3.11 At DS3, the parameters with concentrations below the LCEL include zinc (69 - 150 mg/kg), nickel (14 - 32 mg/kg), lead (30 - 60 mg/kg), mercury (0.05 - 0.18 mg/kg), copper (19 - 53 mg/kg), chromium (23 - 48 mg/kg) and cadmium (0.1 - 0.3 mg/kg). High arsenic levels were, however, recorded at this station with values ranging from 12 to 20 mg/kg. The silver levels recorded in 1998 and 2000 were 1 mg/kg at the LCEL. The PAH levels and PCB levels measured at this station were 40 - 118 ug/kg and 5 - 10 ug/kg respectively. The 6-year records indicated that the sediment at DS3 would likely to be Category M material.

7.3.12 DS4 is located near the mouth of Deep Bay and is distance away from the Shenzhen River discharge. In general, the sediment contaminant levels at this station were low when compared to those at the other stations. Similar to the conditions for DS3, the parameters with concentrations below the LCEL included zinc (36 - 140 mg/kg), nickel (7 - 24 mg/kg), lead (18 - 68 mg/kg), mercury (0.05 - 0.15 mg/kg), copper (6 - 45 mg/kg), chromium (14 - 44 mg/kg) and cadmium (0.1 - 0.2 mg/kg). Most of the arsenic levels (10 - 19 mg/kg) were higher than the LCEL of 12 mg/kg. The silver levels records in 1998 and 2000 were 1 mg/kg at the LCEL. The PAH levels and PCB levels measured at DS4 were 39 - 96 ug/kg and 5 - 40 ug/kg respectively. Based on the WBTC No. 3/2000 for sediment classification, the sediment was mainly Category M material.

Mainland

7.3.13 The wastewater pollution sources in Deep Bay (the name "Shenzhen Bay" is adopted by the Shenzhen authority) are mainly from the land. A significant amount of untreated domestic and industrial/commercial wastewater and cultivation wastewater is discharged into Deep Bay through Shenzhen River. Other major rivers leading to the bay on the Shenzhen side include Dasha River, Futian River and Xinzhou River.

7.3.14 Based on the information from the 深港西部通道(深圳灣公路大橋)環境影響報告書 (Shenzhen Western Corridor (Shenzhen Bay Bridge) Environmental Impact Assessment) dated 1998 (Reference 1), the total wastewater discharged into Deep Bay was approximately 169 million cubic meters a year of which 85 million m3 was industrial wastewater and 52 million m3 was domestic wastewater, and the rest was irrigation wastewater. Major pollutants in wastewater entering the Deep Bay waters were COD, SS, inorganic nitrogen, and inorganic phosphorus.

7.3.15 In the EIA of 深港西部通道口岸場坪填海及地基處理工程環境影響報告書 (Shenzhen Western Corridor Reclamation and Foundation Treatment Engineering) dated 1999 (Reference 2), water quality monitoring survey was conducted in Shenzhen Bay. Six marine water sampling locations were selected and monitored. Figure 7.3 shows the locations of the marine water sampling stations in Shenzhen.

7.3.16 The parameters including pH, As, Hg, and Cr met the Category 3 Standard in both spring tide and neap tide. DO, CODMn, BOD5, and oils measured at the sampling stations met the standards in neap tide but not in spring tide. The DO, SS, CODMn and BOD5 levels measured at the stations during spring and neap tides were in the ranges from 3.14 to 6.98 mg/L, 4.6 to 87.6 mg/L, 0.67 to 4.51 mg/L, and 1.2 to 5.87 mg/L respectively. CODMn and BOD5 exceeded the standards at Station 4 and the measured DO levels exceeded the standard for DO at Stations 2 and 3 in ebb tide. Oils exceeded the standard substantially by 5.8 times in ebb tide at Station 1 and in spring tide at Station 4. Non-ionic nitrogen exceeded the standard in both spring tide and neap tide. The concentrations of total phosphorus were high, exceeding the standard by 0.74-2.57 times, and total nitrogen was also very high.

7.3.17 The pollutant concentrations decreased gradually from inner bay to outer bay indicating that the pollutants were mainly from the land. The rivers discharging into the bay were the main pollution sources. Most of the pollutants entering Deep Bay came from Shenzhen River, which generated high pollution levels in the inner part of the bay. Besides, Xinzhou River and Dasha River entered Deep Bay from the northern shore and caused pollution problem. Figure 7.4 shows the locations of the outlets of major rivers in Shenzhen.

7.3.18 The distance from Shenzhen River estuary to Shenzhen Bay mouth is about 15km. The pollutants would be transported within the bay for a certain period before flowing out of the bay. The pollutants would be diluted during this process, and the pollutant concentrations become lower when arriving at the mouth of the bay.

7.3.19 The pollution patterns during flood and ebb tides were not the same. Basically, the pollutant concentrations in ebb tide were comparatively higher than those in flood tide, This phenomenon indicated that the water quality outside of the bay was better than that inside of the bay. The dilution process in the water might contribute to this difference.

7.4 Water Sensitive Receivers

7.4.1 Indicator points were selected within the Deep Bay WCZ to provide hydrodynamic and water quality outputs for evaluation of water quality impacts. The selected indicator points included water quality sensitive receivers and EPD marine water sampling stations.
7.4.2 The water quality sensitive receivers that are potentially affected by the proposed Project are listed below:

· Mangrove near Ngau Hom Shek
· Cooling water intake for China Light & Power (CLP) Black Point Power Station
· The Marine Park at Sha Chau/Lung Kwu Chau
· Oyster beds near Lau Fau Shan
· Mai Po Nature Reserve in the Inner Deep Bay
· Pak Nai Site of Special Scientific Interest (Pak Nai SSSI)
· Tsim Bei Tsui SSSI
· Mangroves and mudflat at Futian
· Oyster beds at Shekou
· Chinese White Dolphin feeding ground in the Urmston Road Channel
· Seagrass and horseshoe crabs at Ha Pak Nai
· Ramsar site (north and south)

7.4.3 Figure 7.5 shows the locations of these water quality sensitive receivers. The locations of EPD marine water sampling stations (DM1 - DM5) within the Deep Bay WCZ are also shown in the figure.

7.4.4 All the sensitive receivers and EPD marine water sampling stations were defined as water quality monitoring points in the model to output the key water quality parameters for determination of water quality changes as a result of the construction and operational phase activities. The modelling results are presented in form of contour plot, time series plot and table for both the dry and wet seasons in this section.

7.4.5 A list of the indicator points is presented in Table 7.8.

7.5 Assessment Methodology

7.5.1 The Deep Bay Model was calibrated and validated for hydrodynamic and water quality modelling under Agreement No. CE17/95 - Deep Bay Water Quality Regional Control Strategy Study. The model was also used in the Feasibility Study for Additional Cross Border Links Stage 2 under Agreement No. CE 48/97 (Cross Border Links Study, Reference 3) to predict the changes in flushing capacity and water quality due to the bridge construction in Deep Bay. In view of the model being successfully applied in two studies for hydrodynamic and water quality modelling in Deep Bay, the Deep Bay Model is therefore used as an assessment tool in the present Study to perform similar modelling tasks.
Hydrodynamic and Water Quality Model Set-up
Grid Layout and Bathymetry Schematisation

7.5.2 The grid layout and bathymetry schematisation of the Deep Bay Model are shown in Figures 7.6 and 7.7 respectively. The reference level of the model is Principal Datum Hong Kong and the depth data are relative to this datum. The arrangement of the model grid in the Deep Bay Model has been designed to match with the varying seabed conditions in Deep Bay. The small channels in the shallow water region and the areas near the Inner Deep Bay have a finer grid size. The grid sizes are comparatively larger at the open boundaries of the model. The grid sizes vary from 25 m in the small channels to 800 m near the open boundaries. The active grid cells in the Deep Bay Model are approximately 2700 in number.

7.5.3 The bathymetry schematisation of the Deep Bay Model was analysed and constructed using available data and field measurements were also conducted to provide additional bathymetry data of Deep Bay under the Deep Bay Water Quality Regional Control Strategy Study (Reference 4). For the present Study, the same set of bathymetry data was checked to be consistent with the measured data presented in the following charts:

· Admiralty chart for Shekou Gang to Ma Wan Gang based on surveys of 1994 and 1995 and later information has been included (Source: Chinese Government chart of 1999) by United Kingdom Hydrographic Office, edition date: 9 August 2001
· Sea bed data chart for the region from Shekou to Ma Wan based on survey of 1998 by Marine Bureau of the PRC, edition date: January 1999.

7.5.4 The charts listed above and the bathymetry schematisation of the Deep Bay Model were sent to the Mainland authorities on 20 September 2001 for comment and confirmation, the reply from the Mainland authorities on 29 September 2001 accepted the validity of the data. It is considered that the bathymetry data in the present form can be used in the Deep Bay Model for the present modelling exercise.

7.5.5 Site Investigation (SI) was carried out as part of the assignment in this Study. The bathymetry data was also checked against the sounding data obtained from the SI within the study area envelope for the SWC project. The bathymetry data in the model was then updated based on the sounding data.

Simulation Periods

7.5.6 The simulation periods for the dry and wet season water quality modelling are summarised in Table 7.9. The main differences for the dry and wet season modelling were the monthly variations in fresh water inflows and meteorological factors. The first 7 days of the simulation were used for model spin up. The remaining days represented a 15-day spring-neap tidal cycle and were used for analysis of hydrodynamic and water quality impacts. The hydrodynamic outputs from the Deep Bay Model provided inputs for the water quality simulation. Computational time steps of 1 minute and 10 minutes were adopted in the hydrodynamic simulation and water quality simulation respectively. The hydrodynamic forcing including averaged fresh water flows, wind and boundary conditions for the dry season and wet season were applied separately in the corresponding dry and wet season hydrodynamic simulations. Similarly, the dry and wet season pollution loads were applied in the corresponding dry and wet season water quality simulations.

7.5.7 The completion date of the SWC project is planned to be in 2005 - 2006. No reclamation is required on the Hong Kong side for the construction of the SWC bridge alignment and there is no other planned reclamation within Deep Bay. Liaison with the Mainland authorities had indicated that there would be no major planned reclamation on the Shenzhen side except for the reclamation near the landing point at Dongjiaotou.

7.5.8 A site visit to the proposed landing area at Dongjiaotou on 13 September 2001 found that there was no reclamation activity in the area. A reclaimed land protruding from the existing shoreline had been formed. The existing coastline configuration in Deep Bay is shown in Figure 7.8.

7.5.9 The Mainland authorities confirmed that construction of external seawall would first be carried out and would take about 6 months for completion. The subsequent reclamation activities would then be carried out behind the seawall. It is likely that seawall construction would be completed before the commencement of the SWC project in August 2003. Figure 7.9 shows the latest reclamation layout at Dongjiaotou. To take into account the impacts from the potential reclamation sites adjacent to the landing point at Dongjiaotou, the unconfirmed reclamation sites are also considered in this Study. Figure 7.11 shows the coastline configuration with the unconfirmed reclamation sites.

7.5.10 The formation of seawall at the outer boundary of the reclamation area at the early stage of the reclamation would alter the coastline in that area. The coastline configuration in the present modelling exercise had taken into account the reclaimed areas. In the Deep Bay Model, the proposed reclaimed areas were defined as dry points where no calculation of flow dynamics was conducted.

Meteorological Forcing

7.5.11 Based on the Deep Bay Model set-up and some typical coefficients that were used in the hydrodynamic and water quality modelling of the Hong Kong waters, the wind conditions applied in the hydrodynamic simulation were assigned to be 5 m/s NE for the dry season and 5 m/s SW for the wet season. The horizontal eddy viscosity and diffusivity were 1 m2/s. The values for vertical eddy viscosity and diffusivity were computed with the k-e model. A minimum value for the vertical eddy diffusivity was set at 10-7 m2/s. For the vertical eddy viscosity, a minimum value was set at 5 x 10-5 m2/s.

7.5.12 The ambient environmental conditions were closely linked to the processes of water quality changes. Meteorological forcing including solar surface radiation and water temperature was defined in the model for water quality simulation. The model adopted monthly averaged values of solar surface radiation and water temperature. Solar radiation and water temperature were assumed to be constant over the entire domain of the model. Solar radiation is recorded only at King's Park station by Hong Kong Observatory. The monthly averaged solar radiation was calculated based on the hourly data recorded at this station. The average values of solar radiation adopted in the model were 132 W/m2 in the dry season and 237 W/m2 in the wet season.

7.5.13 The ambient water temperature was determined based on the EPD routine monitoring data collected within the Deep Bay WCZs from 1990 to 1999. The average water temperature values used in the water quality model were 17 °C in the dry season and 29 °C in the wet season.

Flow Aggregation

7.5.14 The water column in the vertical direction was divided into 10 layers for hydrodynamic simulation. Aggregation of the hydrodynamics was performed for water quality simulation to reduce the vertical resolution from 10 layers to 5 layers. The vertical distribution of the layers in the model for water quality simulation was 10%, 20%, 20%, 30% and 20% of the hydrodynamic layers from surface to bottom. This optimised the computational time and data storage without a significant influence on the quality of the modelling results. A 2x2 flow aggregation was also applied in the spatial level.

Initial and Open Boundary Conditions

7.5.15 The Deep Bay Model was linked to the Update Model, which was constructed, calibrated and verified under the Update on Cumulative Water Quality and Hydrological Effect of Coastal Development and Upgrading of Assessment Tool. Hydrodynamic computations were first carried out using the Update Model to provide open boundary conditions to the Deep Bay Model. A restart file from previous hydrodynamic computations was then used to provide initial conditions to the Update Model. The initial conditions for the Deep Bay Model were selected to be the same as those for the Update Model. This was done by using a utility program to map the information contained in the restart file of the Update Model to the restart file of the Deep Bay Model.

7.5.16 Open boundary conditions were transferred from the Update Model to the Deep Bay Model through a nesting process. Both the water level and velocity boundary definitions were defined in the Deep Bay Model. As the Update model covered the discharges from the major Pearl River estuaries, which include Humen Jiaomen, Hongqili, Hengmen, Muodaomen and Aimen, the influences on hydrodynamics due to the discharges from Pearl River estuaries were therefore incorporated into the Deep Bay Model.

7.5.17 Similarly, water quality simulation was first conducted using the Update Model to provide boundary conditions to the Deep Bay Model. The concentrations of various modelling parameters were defined at the open boundary of the Deep Bay Model for the dry and wet seasons. In order to start the water quality simulation from a more realistic condition, a spin-up period of two full spring-neap cycles were adopted for the first simulation period. After performing the spin-up, the influence from initial conditions would be subsided and would not affect the concentrations of the simulated parameters. The computed water quality conditions at end of the first simulation period were used as the initial conditions for the actual simulation.

7.5.18 The planned major reclamation projects outside of Deep Bay and within the Hong Kong side were incorporated into the Update Model for hydrodynamic simulation. These major reclamation projects included the Central and Wan Chai Reclamation, Yau Tong Bay Development, South East Kowloon Development, Penny's Bay Stages I and II Reclamation, Tuen Mun Area 38 Reclamation, Container Terminal No. 9 and Tseung Kwan O Reclamation. It is, however, considered that the influence to the open boundary conditions from these projects would not be significant due to the remote distance.

Model Outputs

7.5.19 The model runs covered the baseline and operational scenarios. Comparisons were made between the baseline scenario and the other scenarios to show the degree of influence due to the SWC project and the reclamation adjacent to the SWC landing point at Dongjiaotou.

7.5.20 Statistical analysis of hydrodynamic and water quality changes was conducted at representative indicator points in the study area. Some of the indicator points were located at the same locations as EPD marine water sampling stations (DM1, DM2, DM3, DM4 and DM5) as well as in the vicinity of the proposed SWC bridge alignment. The other indicator points represented the water quality sensitive receivers. The locations of the water quality sensitive receivers and EPD marine water sampling stations are shown in Figure 7.5. Figure 7.12 shows the locations of the indicator points near the proposed SWC bridge alignment.

7.5.21 The key water quality parameters assessed in this section using the Deep Bay Model included salinity, dissolved oxygen (DO), suspended solids (SS), biochemical oxygen demand (BOD), E. coli, unionised ammonia (UIA) and total inorganic nitrogen (TIN). The Deep Bay Model had taken into account the interaction of the modelled parameters. The changes in the erosion and sedimentation patterns in Deep Bay were assessed using the model.

7.5.22 Mean depth-averaged results in the form of table, contour plot and time series plot for salinity, depth-averaged 90%ile DO, bottom 90%ile DO, SS, BOD, E. coli, UIA and TIN are presented in this section. All the key water quality parameters at the water quality sensitive receivers and EPD marine water sampling stations are summarised in tables for comparison with relevant criteria. The annual average results would be determined by averaging the dry and wet season results. The maximum, minimum and mean values of the concerned parameters are also presented. The modelling results would compare with the WQOs for marine waters of Deep Bay WCZ to check for compliance. For parameters that are not specified in the WQOs for the Deep Bay WCZ, the "Technical Report on Environmental Protection of Deep Bay and its Catchment, Appendix T, Hong Kong Guangdong Environmental Protection Liaison Group, December 1992" would be used as a reference in assessing the water quality impacts due to the SWC project.

7.5.23 The contour plot showed the spatial distribution of the concerned parameters and covers all the indicator points selected for the assessment. Time series plots for the wet and dry season results for DO, SS, BOD, E. coli, UIA and TIN at indicator points including oyster beds near Lau Fau Shan, Pak Nai SSSI, EPD marine water sampling station DM4 and oyster beds at Shekou were generated for the different modelling scenarios.

7.5.24 For both the dry and wet season simulations, time-series plots for the computed current speeds and salinity were produced at the indicator points located in the vicinity of the SWC bridge alignment.

7.5.25 In order to address the potential impacts of the SWC project to the Inner Deep Bay Ramsar site, the average of water quality parameters in the areas within Sections 1 and 2 as shown in Figure 7.13 was assessed. The impacts in terms of percent deterioration of water quality for different modelling scenarios were determined to assess the water quality changes.

Pier Friction

7.5.26 The grid sizes of the Deep Bay Model would be larger than the pier sizes of the proposed SWC bridge. The Deep Bay Model was therefore used to assess the global impacts of the bridge piers. The present modelling exercise was based on the approach adopted in the Cross Border Link Study (Reference 3) for determination of the effect of bridge piers. The SWC bridge in the hydrodynamic simulation was represented by an additional quadratic friction term added to the momentum equations. The forces on the flow due to the piers were used to determine the energy loss. This approach was based on the previous work by Delft Hydraulics (References 4 - 8) to generate correct discharge through the bridge section without an explicit adjustment of the cross-sectional area.

7.5.27 The mathematical expressions for representation of pier friction were based on the Cross Border Link Study and the Delft3D-FLOW module developed by Delft Hydraulics. A quadratic friction term added to the momentum equations can be expressed in the form:

Friction loss in the x-direction = [C_{loss, u} U || ] / D x (m/s²)

Friction loss in the y-direction = [C_{loss, v} V || ] / D y (m/s²) (7.1)

 

Where: C_{loss, u} and C_{loss, v} are the loss coefficients in the x and y directions;

is the velocity vector (U, V), and U and V are the velocities in the x and y directions (m/s);

|| is the magnitude of the velocity vector = (m/s); and

D x and D y are the grid distances in the x and y directions (m).

Notes:
1. The loss coefficients were calculated based on bridge configuration shown in Figures 7.14, 7.15, 7.16 and 7.17; and
2. It was assumed that the dimensions of bridge piers for different typical spans (50m, 75m, 100m and 200m) were similar and were based on the configuration of 75m span. The same pier configuration used for different spans was to show the effect due to the changes in span length only.

7.5.35 During the bridge pier construction period, cofferdam for typical bridge pier with a size of 10x10m would be installed at each pier site. The size of cofferdam is larger than the normal size of the bridge pier (6m x 2.5m) and pile cap (8.5m x 8.5m). The loss coefficient for a typical bridge pier with cofferdam was calculated to be 0.2.

Pollution Loading

7.5.36 The pollution loading inventory used in the water quality modelling is provided in Tables 7.11 and 7.12 for dry season and wet season respectively. Full details of the pollution loading inventory were presented in the Revised Report on Water Quality Model Input Data and Model Methodology. Figure 7.4 shows the locations of the discharge points in Deep Bay.

Table 7.12 Pollution Loading for Wet Season

7.5.37 For the discharge points outside the study area, the water quality simulation using the Update Model was based on the pollution loading inventory compiled under Agreement No. CE28/99 Review of North District and Tolo Harbour Sewerage Master Plans to provide the pollution load data at those discharge points.

Modelling Scenarios

7.5.38 The reclamation layout on the Shenzhen side was based on the latest information provided by the Mainland authorities during the meeting on 26 Feb 2002. A circular viaduct section immediately south of the landing point located mainly within the seawater was included in the latest reclamation layout. However, the final layout of the landing point at Dongjiaotou has not been finalised by the Mainland authorities at this stage. The inclusion of circular viaduct represents the possible worst-case scenario. For the case without the circular viaduct section, the potential impacts on hydrodynamics and water quality would be less significant than the worst-case scenario. The water quality modelling scenarios adopted in the water quality impact assessment had taken into account the latest reclamation layout and are listed as follows:

7.5.39 Scenario 1 is to represent the existing baseline conditions in Deep Bay and is used to provide a basis for comparison with the other scenarios. Scenario 2 represents the conditions only with the SWC reclamation at Dongjiaotou and without the SWC bridge. Scenario 3 is the operational scenario representing the conditions with the SWC reclamation and the SWC bridge (including the circular viaduct section). The purpose of including Scenario 4 is to project the future development in Deep Bay by the Shenzhen side. There is no confirmed programme for the estimated reclamation included in Scenario 4 and the reclamation is not associated with the SWC project.

7.5.40 Comparisons of Scenarios 2 to 4 with Scenario 1 (baseline scenario) were made in this section to show changes in hydrodynamic and water quality conditions due to the presence of the SWC bridge and the reclamation sites. The cases with and without the SWC bridge were also compared to show the reduction in accumulated fluxes or flushing capacity through the bridge alignment.

7.5.41 The pollution loading inventory compiled under this Study mainly focuses on the future scenarios after the completion of the SWC bridge. There would be variation in actual pollution loads entering Deep Bay in different years. The pollution loads would increase due to the increases in population and urbanisation in the Deep Bay area. A conservative approach was taken to apply the pollution loads for 2010 to all the modelling scenarios. This approach provided a clear picture of the water quality changes when making comparisons between different scenarios.

Sediment Quality

7.5.42 The guidelines specified in the WBTC No. 3/2000 are adopted for assessment of dredging and disposal of sediment for the SWC project. As specified in the WBTC No. 3/2000, sediments are classified into three categories based on their contaminant levels with reference to the Chemical Exceedance Levels (CEL). The classification is defined as follows:

· Category L - Sediment with all contaminant levels not exceeding the Lower Chemical Exceedance Level (LCEL). The material must be dredged, transported and disposed of in a manner, which minimizes the loss of contaminants either into solution or by resuspension.
· Category M - Sediment with any one or more contaminant levels exceeding the Lower Chemical Exceedance Level (LCEL) and none exceeding the Upper Chemical Exceedance Level (UCEL). The material must be dredged and transported with care, and must be effectively isolated from the environment upon the final disposal unless appropriate biological tests demonstrate that the material will not adversely affect the marine environment.
· Category H - Sediment with any one or more contaminant levels exceeding the Upper Chemical Exceedance Level (UCEL). The material must be dredged and transported with great care, and must be effectively isolated from the environment upon the final disposal.

7.5.43 The sediment quality criteria for the classification of sediment are shown in Table 7.13. Sediment can be classified into Category L, Category M or Category H material after carrying out Tier II screening test. There are three types of disposal options: Types 1, 2 and 3 represent open sea disposal, confined marine disposal and special treatment/disposal respectively. Category L material is suitable for open sea disposal. Tier III screening test is required to determine the disposal option (Type 1 open sea disposal (dedicated sites) or Type 2 confined marine disposal) for Category M material. For Category H material with one or more contaminant levels 10 times higher than the LCEL, Tier III screening test (dilution test) is required to determine whether the sediment is suitable for Type 2 confined marine disposal or Type 3 special treatment/disposal. If contaminant levels are lower than 10 x LCEL, Type 2 confined marine disposal should be adopted.

7.5.44 Depending on the results of the sediment chemical quality, biological tests may need to be conducted to determine the disposal option if Category M or Category H sediments are identified. Biological tests consist of the following items:
· A 10-day burrowing amphipod toxicity test (for Category M sediment)
· A 20-day burrowing polychaete toxicity test (for Category M sediment)
· A 48-96 hour larvae (bivalve or echinoderm) toxicity test (for Category M sediment)
· Dilution test of the above 3 toxicity tests (for Category H sediment with one or more contaminant levels exceeding 10 times Lower Chemical Exceedance Level)

7.5.45 Table 7.14 details the test endpoints and failure criteria of the three toxicity tests. The biological test is deemed to have failed if any one of the three toxicity tests (10-day burrowing amphipod toxicity, 20-day burrowing polychaete toxicity and 48-96 hour larvae toxicity) is failed.

7.5.46 Reference sample should also be collected to use as control sediment in the biological tests.

Note:
1. The contaminant level is considered to have exceeded the UCEL if it is greater than the value shown.

Table 7.14 Test endpoints and decision criteria for Tier III biological screening

Notes:
1. Statistically significant differences should be determined using appropriate two-sample comparisons (e.g., t-tests) at a probability of p£0.05.
2. Dry weight means total dry weight after deducting dead and missing worms.
3. Normality survival integrates the normality and survival end points, and measures survival of only the normal larvae relative to the starting number.

7.5.47 Site Investigation (SI) was conducted to identify the sediment quality in the study area. Vibrocore and grab sediment samples were collected during the SI for analysis of sediment quality. Sediment samples were collected using vibrocoring. Vibrocoring was conducted along both the north alignment (D1 to D6 in total 6 nos.) and the south alignment (D7 and D8 in total 2 nos.), as shown in Figure 7.18. The sampling depth was the depth of the unconsolidated mud layer. The vibrocore penetrated into the base of the marine deposits until the more compact consolidated sand layer is encountered. This was distinguished by the different penetration rates of the two different layers during vibrocoring. The sediment depth was checked by visual observation of the collected vibrocore samples.

7.5.48 The numbers of vibrocore samples collected at each location are detailed as follows:

	Sampling depth less than 5 m – 3 samples (upper, middle and bottom) with 1 m respectively at the top 1-m layer, middle 1-m layer and bottom 1-m layer of each vibrocore sample
	Sampling depth between 5 m and 10 m – 1 sample (middle 1-m layer)
	Sampling depth between 10 m and 20 m – 1 sample (middle 1-m layer)
	Sampling depth between 20 m and 30 m – 1 sample (middle 1-m layer)
	Sampling depth between 30 m and 40 m – 1 sample (middle 1-m layer)

7.5.49 Grab samples of the upper deposits were also collected at locations (A1 to A16 and D1 to D8). Figure 7.19 shows the sampling locations for grab samples. The grab sediment samples collected at A1 to A16 were taken for analysis of the parameters as specified in the WBTC No. 3/2000 including heavy metals, metalloid and organic micro-pollutants. Elutriate tests were also conducted for these sediment samples to estimate the release of contaminants during the dredging activities of the SWC project. The grab sediment samples collected at D1 to D8 were used for the elutriate tests only.

7.5.50 Reference sediment (surface grab) was collected at the same time as sampling for vibrocore samples for biological testing. The location of the reference sediment was the location PS6 in Port Shelter.

7.5.51 The collected vibrocore samples (D1 to D8), surface grab samples (A1 to A16) and reference sediment sample (at location PS6) were analysed for:

· 9 heavy metals and metalloid including cadmium, chromium, copper, mercury, nickel, lead, zinc, silver and arsenic;
· 3 organic micro-pollutants including polychlorinated biphenyls (PCB), polyaromatic hydrocarbons (PAH), and tributyl tin (TBT in interstitial water).

7.5.52 Analytical methods in accordance with the Standards Methods for the Examination of Water and Wastewater by APHA and relevant testing methods (e.g. USEPA) were adopted in analyzing the above listed parameters.

7.5.53 Elutriate tests were also conducted on the surface grab samples (samples collected at A1 to A16 and D1 to D8) to simulate the release of pollutants during dredging operation. The surface grab samples for elutriate tests were analysed for:
· 9 heavy metals and metalloid including cadmium, chromium, copper, mercury, nickel, lead, zinc, silver and arsenic
· 3 organic micro-pollutants including PCB, PAH, and TBT
· TKN, NO3-N, NO2-N, NH4-N, PO4-P, total phosphorus and chlorinated pesticides

7.5.54 After chemical testing, the categories of sediment were determined based on WBTC No. 3/2000. Biological tests were conducted due to some of the sediment samples being classified as Category M material. There was no Category H sediment with contaminant levels exceeding 10 x LCEL. Dilution test was not required. The biological tests were conducted on composite samples. According to the WBTC No. 3/2000, composite samples could be prepared by mixing up to 5 samples of the same category (M or H), which are continuous in vertical or horizontal profile. The method for sediment mixing for the biological testing was discussed with EPD. Composite samples and the reference sediment sample were analysed for:
· A 10-day burrowing amphipod toxicity test (for Category M sediment)
· A 20-day burrowing polychaete toxicity test (for Category M sediment)
· A 48-96 hour larvae (bivalve or echinoderm) toxicity test (for Category M sediment)

Sediment Dredging

7.5.55 Based on the preliminary design, the SWC bridge decks and piers are supported by bored pile foundation. Cofferdams would be installed for pile cap construction. No release of sediment is normally expected according to the proposed construction method. However, a sensitivity test is included to address the worst-case scenario should there be any release during the course of construction.

7.5.56 Delft3D-WAQ module was used to model dispersion of sediment during dredging. The hydrodynamic conditions generated from the Deep Bay Model provided basic hydrodynamic information for modelling of sediment plume dispersion. The processes of settling of sediment particles and exchange of sediment particles between the water column and the seabed govern the sediment transport. Sediment deposition and erosion occur when the bed shear stress is below or above the critical shear stress. The deposition rate and erosion rate can be calculated using the following equations:

(1) Bed Shear Stress (t) < Critical Shear Stress for Deposition (td = 0.05 Pascal)
Deposition rate = Vs Cb (1 - t / td)

Where: Vs = settling velocity (= 1 mm/s); and Cb = bottom layer SS concentration
(2) Bed Shear Stress (t) > Critical Shear Stress for Erosion (te = 0.4 Pascal)
Erosion rate = Re (t / te - 1)

Where: Re = erosion coefficient (= 0.0002 kg/m2/s).

(1) Bridge Pier Construction Within the Hong Kong Waters

7.5.57 The most common grab size in the Hong Kong fleet is 8 m3 in capacity. A 8 m3 closed grab was therefore used to determine the sediment loss rate for each dredger during the dredging operation. It was assumed that dredging would be carried out between 0700 and 1900 each day with a total working period of 12 hours a day. There would be 6 working days per week. Dredging is assumed to be continuous and dredging would take place in the dry and wet seasons. The hourly production rate for sediment dredging using a grab dredger of 8 m3 capacity adopted in the Kellett Bank Study (Reference 9) was 208.3m3/hr. The same hourly production rate was also used in this Study.

7.5.58 A typical value of the dry density of marine mud is 1,340 kg/m3. The sediment loss rate is related to water depth, current speed and sediment characteristics. The Kellett Bank Study also recommended a sediment loss rate of 25 kg/m3 for modelling the grab dredging operations. This value takes into account the occasional failure to close the grab. The calculated sediment loss rate for the hourly production rate of 208.3m3/hr is 1,447 g/s per dredger. Assuming a 5% loss of the dredged sediment during the transfer of the sediment from the dredging point to the barge, the estimate sediment loss rate for each dredger was calculated to be 72 g/s.

7.5.59 The elutriate tests conducted in the SI provided information on the release of contaminants from the marine mud during dredging operation. An inactive tracer was defined in the model at a point corresponding to the dredging location to determine the dilution in the vicinity of the dredging site. The dilution information was then used to determine the decreases in concentrations of the concerned parameters and to evaluate the potential impacts to the aquatic environment.

(2) Reclamation on the Shenzhen Side

7.5.60 Reclamation will be carried out to provide additional land for the landing point at Dongjiaotou in Shenzhen. The potential cumulative water quality impacts from reclamation at Dongjiaotou are included in the assessment. The reclamation area is approximately 153 ha. Assuming an average water depth of 1.5m, the total volume of the reclaimed land is 229.5 x 104 m3 not including the reclamation above the sea level. The duration for the filling activities is about 1 year. In order to take into account the variability of the filling activities, a "peaking factor" of 2 is included in the estimation of filling rate. The factored volume is therefore 459 x 104 m3. It is further assumed that "unprocessed" public fill material would be used for reclamation. In Hong Kong, percentage of fines for "unprocessed" public fill material would be up to 40 %. This percentage is included in the estimation of the sediment loss rate for filling.

7.5.61 The calculation of sediment loss rate for filling is detailed below:

Factored volume of reclamation = 459 x 104 m3
Working rate of reclamation = 459 x 104 m3 / 1 year (52 weeks) = 88,270 m3/week
Working hour = 16 hours a day (7am to 11pm)
Working day = 7 days a week
Bulk density = 2000 kg/m3
Fine portion = 40%
Portion of fine lost during filling = 5%
Release rate of fine portion = 88,270 m3/week x 2000 kg/m3 x 40% = 175 kg/s
Sediment loss rate for filling = 175 kg/s x 5% = 8.75 kg/s

7.5.62 Since the filling activities would be carried out behind seawall, the release of sediment may only occur if there were leakage from seawall boundary. It is therefore considered that the sediment loss rate during the reclamation period would be much lower than the average sediment loss rate of 8.75 kg/s. A conservative approach was made to assume that there would be 15% of the sediment loss through the seawall. The sediment loss rate would be 1,313 g/s. This value was used in the model runs to assess the impacts from reclamation at Dongjiaotou.

7.5.63 The modelling cases to assess the potential water quality impacts arising from dredging and filling include:

· Case D1: Bridge pier construction (dredging) within the Hong Kong waters
· Case D2: Bridge pier construction (dredging) within the Hong Kong waters and reclamation at Dongjiaotou (filling)
· Case D3: Bridge pier construction (dredging) on both the Hong Kong and Mainland sides and reclamation at Dongjiaotou (filling)

7.5.64 To consider the worst-case scenario, there would be 8 pairs of pier sites (16 discharge points) under construction along the SWC alignment within the Hong Kong waters. All the discharge points were located near the shoreline at Ngau Hom Shek such that the impacts to the nearby water sensitive receivers are potentially higher. It was assumed that 5 pairs of pier sites (10 discharge points) of the SWC section within the Mainland boundary would be constructed simultaneously. The sediment loss rate for dredging was assumed to be the same as that for bridge pier construction within the Hong Kong waters, i.e 72 g/s. The impacts on the water sensitive receivers in Hong Kong are a concern. All the discharge points were therefore allocated close to the borderline. For the landing point reclamation, the discharge point was allocated at the southern edge of the reclamation site. Figure 7.20 shows the discharge locations.

7.6 Identification of Environmental Impact

7.6.1 The potential water quality impacts arising from the construction phase of the SWC project would include:

· Construction site runoff and wastewater from general construction activities and bored piling work;
· Sewage from workforce;
· Accidental spillage of chemicals on site;
· Sediment dredging along the SWC alignment and sediment disposal;
· Sediment dredging at Mai Po and sediment disposal; and
· Changes in hydrodynamic conditions during the bridge pier construction period.

7.6.2 Construction projects, which would be carried out concurrently with the SWC project, in the vicinity of the SWC project site may generate cumulative construction impacts. These projects and cumulative impacts are identified and addressed in the next section.

7.6.3 The potential water quality impacts arising from the operational phase of the SWC project include:
· Changes in hydrodynamic conditions;
· Changes in water quality conditions;
· Changes in sedimentation and erosion patterns in Deep Bay;
· Accidental spillage of chemicals during accidents; and
· Road runoff from the SWC bridge.

7.7 Prediction and Evaluation of Environmental Impacts

Construction Phase

Construction Site Runoff and Wastewater from General Construction Activities and Bored Piling Work
Ngau Hom Shek and SWC Alignment

7.7.1 The works area for SWC would be located near the landing point of Ngau Hom Shek. Figure 7.21 shows the boundary of works area at Ngau Hom Shek. This works area is within the DBL project limit. Most of the permanent works for the SWC bridge would be located offshore with certain activities to be carried out on land within the works area and along the access roads. During the construction stage, a temporary access bridge (see Figure 2.7) would be installed in between the bridge piers of the southbound carriageway and the northbound carriageway to provide access to the pier sites.

7.7.2 Preparation of the works area may increase exposed topsoil. During a rainstorm, site runoff would be generated washing away the soil particles. The runoff is generally characterised by high concentrations of suspended solids. Release of uncontrolled site runoff would increase the SS levels and turbidity in the water near Ngau Hom Shek. Tidal currents would disperse the turbid water along the coastline in Deep Bay causing visual nuisance and hazards to the aquatic life.

7.7.3 Wind blown dust would be generated from exposed soil surface in the works area. Due to the close proximity to the Deep Bay waters, it is possible that wind blown dust would fall directly onto the nearshore water when a strong wind occurs. Dispersion of dust within the works area may increase the SS levels in surface runoff causing a potential impact to the nearby water bodies such as the Deep Bay waters and the nearby stream courses.

7.7.4 Various types of construction activities to be involved in the SWC bridge construction may generate wastewater. The activities include bored pile construction, excavation and filling, general cleaning and polishing, wheel washing, dust suppression, utility installation and upgrading of Fung Kong Tsuen Road to provide access to the works area. These types of wastewater contain high concentrations of suspended solids.

7.7.5 Bored pile foundation would be constructed to support the bridge piers and decks. The construction works involve excavation and cleaning of foundation. A casing is first driven into the marine sediment layer before the excavation of marine sediment inside the casing. Sediment dredging and washing of foundation are carried out within the casing. The wastewater generated from the bored piling works contains high concentrations of suspended solids. A submersible pump would be placed inside the casing to pump out the wastewater to a settling tank for removal of suspended solids. The water in the settling tank would then be reused in the foundation cleaning process.

7.7.6 Depending on the locations of the bored piling sites, the piling works would be carried out in a slightly different manner. In this project, the bored piling sites would be located within the works area at Ngau Hom Shek, in the shallow water region and in the deep water region.

7.7.7 The bored pile foundation to be constructed within the works area would be carried out from a land-based operation. Any uncontrolled release of wastewater generated from the bored piling works in this area may enter the small stream course at Ngau Hom Shek. The mangroves and mudflat near the outlet of the stream may be affected by the sediment-laden wastewater if no mitigation measures were to be undertaken.

7.7.8 A temporary bridge of about 1.8 km located between the northbound alignment and the southbound alignment would be constructed in the shallow water region. The temporary bridge would be branched off and extended to the pier locations. Piling equipment is placed on the extended section of the temporary bridge to carry out the piling works. During low tides, part of this shallow water region is exposed to the atmosphere providing feeding ground to birds. Discharges of untreated wastewater in this region would enter the mudflat or the seawater affecting the birds feeding in mudflat and the aquatic organisms.

7.7.9 Beyond the 1.8 km shallow water region, bored piling works would be carried out from a barge-based operation. The barge equipped with piling equipment is moved to the pier location for carrying out the bored pile construction at sea. Discharges from the bored piling sites in this deep water region would directly enter the seawater leading to water pollution. Sediment plume would be generated and dispersed away from the bored piling sites increasing the turbidity and SS levels in the region. The aquatic organisms would be affected.

7.7.10 Good working practices would avoid releasing the wastewater into the surrounding seawater or mudflat. Should any discharges from the bored piling sites be required, a suitable wastewater treatment system needs to be provided to avoid water pollution and to minimize any harmful effect on mudflat. A discharge licence should be applied from EPD for effluent discharge from the sites. Effluent quality should be in compliance with the requirements specified in the discharge licence.

7.7.11 Excavation and filling activities generate stockpiles of excavated soils. Site runoff would carry the soil particles to the nearby stream courses and the nearshore water. Good site practices should be implemented to handle and treat the excavated soils and fill materials on site.

7.7.12 Washing of concrete lorry on site generates wastewater with elevated pH values and the wastewater should be properly treated.

Lung Kwu Sheung Tan

7.7.13 A barging point at Lung Kwu Sheung Tan has been allocated for casting yard and storage. It would also be used as a precasting yard with concrete batching operations for the SWC project. This site would not involve any bored piling activities. A large amount of concrete would be processed in this area. Cement is alkaline in nature. Washing of concrete mixers generates wastewater, which contains waste concrete particles and has a high pH value. Release of the concrete washings with a high pH value into the seawater may increase the level of unionized ammonia, which is highly toxic to aquatic organisms, in the receiving water. This causes ecological impacts to the surrounding environment. Therefore, concrete washings should be properly collected and treated before final discharge.

7.7.14 Site runoff generated from the precasting yard and concrete batching plant may contain waste concrete and would enter drains and the seawater adjacent to the site. Direct discharges of site runoff may cause impacts on water quality and ecology in the receiving water body. Suitable mitigation measures should be implemented on site to minimize the water quality and ecological impacts associated with the precasting and concreting activities at Lung Kwu Sheung Tan.

Sewage from Workforce

7.7.15 The presence of workforce for the construction of SWC bridge generates sewage. Sewage discharge is subject to control and illegal discharge of untreated sewage would not be acceptable and would affect the water quality in Deep Bay. Provision of suitable sewage collection facilities on site could avoid the sewage pollution problem. It is anticipated that sewage from workforce would not cause water pollution to the Deep Bay waters.

7.7.16 The Mainland EIA Report on the SWC Reclamation and Foundation Treatment Engineering indicated that the workforce for construction of the landing point on the Shenzhen side would be about 500 persons and the volume of domestic sewage to be produced was about 100 m3/day. The sewage would be discharged to foul sewer network. In case where this option is not feasible, the sewage would be treated and discharged to Deep Bay. The quality of treated effluent reported in the Mainland EIA was: 130 mg/L ³ COD, 30 mg/L ³ BOD5, 20 mg/L ³ NH3-N and 100 mg/L ³ SS. These pollution loads were included in the water quality model to assess the impacts from the sewage discharge at the reclamation site. The final stage of the bridge pier construction period was considered as a worst situation. It was therefore assumed that all the bridge piers were near completion and were placed in the seawater along the bridge alignment. It was further assumed that the treated effluent would be discharged at the outer edge of the reclamation site.

7.7.17 The predicted annual depth-averaged DO, BOD5, SS, UIA, TIN and E. coli levels at the nearest indicator point "I" (oyster beds at Shekou) were 4.83mg/L, 1.61mg/L, 25.4mg/L, 0.059mg/L, 1.46mg/L and 3286count/100mL respectively. There were no significant variations in water quality conditions when compared to the case without the sewage discharge (DO = 4.85mg/L, BOD5 = 1.61mg/L, SS = 26.4, UIA = 0.06mg/L, TIN = 1.50mg/L and E. coli = 3184 count/100mL). Based on the model predictions, discharge of treated effluent from the reclamation site is not likely to cause unacceptable water quality problems during the construction period.

Accidental Spillage of Chemicals on Site

7.7.18 There would be a large variety of chemicals to be used for carrying out construction activities. These may include surplus adhesives, spent paints, petroleum products, spent lubrication oil, grease and mineral oil, spent acid and alkaline solutions/solvent and other chemicals.

7.7.19 Accidental spillage of chemicals in the works area would contaminate the surface soils. The contaminated soil particles may be washed away by construction site runoff or storm runoff causing water pollution. Accidental spillage of chemicals on the bridge sections may directly affect the aquatic environment in Deep Bay. It is recommended that the Contractor should develop an emergency plan to deal with chemical spillage in case of an accident.

7.7.20 It is required to register as a chemical waste producer if chemical wastes are produced from the construction activities. The Waste Disposal Ordinance (Cap 354) and its subsidiary regulations in particular the Waste Disposal (Chemical Waste) (General) Regulation should be observed and complied with for control of chemical wastes.

Sediment Dredging along the SWC Alignment and Sediment Disposal

7.7.21 Dredging would cause disturbance to the seabed. Release of marine mud and contaminants may affect the water quality in Deep Bay. The key concern of the bridge construction is sediment dredging along the bridge alignment. Marine sediment would be excavated during the construction of piles and pile caps at the pier locations. During the critical construction period, there would be 8 pair of pier sites under construction at the same time. These pier sites would be located in the mudflat, inter-tidal and sub-tidal regions. For the case of unconfined dredging, the potential impacts to the surrounding water may be significant.

7.7.22 Sediment samples were collected and analysed in the SI to determine the sediment quality in the study area. Figures 7.19 and 7.18 show the sampling locations of grab samples and vibrocore samples. Laboratory analysis of the sediment samples included the parameters of cadmium, chromium, copper, mercury, nickel, lead, silver, zinc, arsenic, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and tributyltin (TBT). All the chemical test results are summarized in Table 7.15.

7.7.23 At the early stage of the Project, vibrocore samples (D1 to D8) were collected along both the south alignment (D7 and D8) and the north alignment (D1 to D6). Since the s-curve (north) alignment has been selected as the preferred option, the results for vibrocore samples collected at D1 to D6 are considered more relevant in this assessment. A comparison of the vibrocore samples collected along the south and north alignments is shown in Table 7.16.

7.7.24 The laboratory results for the vibrocore samples collected at D1 to D6 showed that only arsenic levels at D3 were higher than LCEL and the sediment was classified as Category M material according to WBTC No. 3/2000. The maximum arsenic level at D3 was 14 mg/kg and was detected at the lowest sampling depth of 3.90 - 5.10 m. The arsenic levels at this location were also found to be high in both the upper and lower sediment layers. The levels of other heavy metals namely cadmium, chromium, copper, nickel, lead, zinc and silver were all below LCELs. The measured concentrations ranged from < 0.1 to 0.2 mg/kg for cadmium, 14 to 37 mg/kg for chromium, 3.1 to 25 mg/kg for copper, 2.8 - 23 mg/kg for nickel, 14 to 49 mg/kg for lead, 8.9 to 70 mg/kg for zinc, and < 0.1 to 0.1 mg/kg for silver. The levels of mercury, PAHs, PCBs and TBT were lower than the detection limits and below the LCELs. There were no vibrocore samples classified as Category H material. Except the sediment at D3 was classified as Category M material, the other 5 sediment samples at D1, D2, D4 - D6 were classified as Category L material. The contaminant levels of the vibrocore samples collected along the proposed s-curve alignment were low.

7.7.25 For the vibrocore samples collected at D7 and D8, the measured concentrations ranged from < 0.1 to 0.2 mg/kg for cadmium, 1.4 to 22 mg/kg for chromium, 1.4 to 14 mg/kg for copper, 0.8 - 15 mg/kg for nickel, 3.6 to 35 mg/kg for lead, 4.5 to 82 mg/kg for zinc and 1.9 - 12 mg/kg for arsenic. Mercury and silver were below detection limits of 0.05 mg/kg and 0.1 mg/kg respectively. The vibrocore samples collected at D7 and D8 were all classified as Category L material.

7.7.26 The total numbers of vibrocore samples collected along the north alignment (6 nos.) were more than those collected along the south alignment (2 nos.). From the SI results, no Category H material on both the alignments was detected. The sediment quality along the two alignments was generally low in contaminant level. Most of the vibrocore samples collected along the north alignment were Category L material, except that Category M material was found at D3. The measured arsenic levels at D3 (9.5 - 14 mg/kg) slightly exceeded the LCEL (> 12 mg/kg). All the vibrocore samples collected along the south alignment were Category L material but the arsenic levels measured at D8 (7.8 - 12 mg/kg) nearly exceeded the LCEL. The cadmium, mercury, silver, PAH, PCB and TBT levels in the sediment samples collected along the two alignments were rather consistent. The levels of chromium, copper, nickel, lead and arsenic measured along the north alignment were comparatively higher than those measured along the south alignment but the differences were small. Overall, there were no significant differences in sediment quality between the vibrocore samples collected at D1 to D6 along the north alignment and at D7 - D8 along the south alignment.

7.7.27 Different from the vibrocore samples, the grab samples were collected not along the north and south alignments but were collected in the vicinity of the alignments. Most of the grab sediment samples were found to be below the LCELs except for the parameters of zinc, arsenic and copper. There were exceedances of the UCEL for zinc at two sampling stations A14 ad A15 with values of 320 mg/kg and 280 mg/kg respectively. Figure 7.22 shows the contour plot for zinc in the sampling area. The concentrations of zinc were higher towards the central region of Deep Bay with the highest level at A14.

7.7.28 The location of EPD's sediment monitoring station DS2 is near the proposed SWC alignments. Comparing with EPD's data at DS2 collected between 1995 and 2000, the measured zinc levels during the SI showed a higher value (64 - 320 mg/kg) than the EPD's monitoring data (69 - 220 mg/kg).

7.7.29 As the zinc levels exceeded the UCEL at A14 and A15 stations, the sediment at A14 and A15 was classified as Category H material. However, as the zinc levels at these two stations were below 10 times of the LCEL, dilution test was not required to determine the disposal option for the sediment. Confined marine disposal could be adopted for this sediment.

7.7.30 There were also quite a number of exceedances of the LCEL for arsenic from the collected sediment samples, with a maximum value of 18 mg/kg at A8. There were 11 out of 16 grab sampling stations exceeded the LCEL for arsenic. These included the grab sediment samples collected at A1, A3, A5, A6, A7, A8, A9, A11, A13, A14 and A15. Figure 7.23 shows the contour plot for arsenic in the sampling area. The concentrations of arsenic (8.6 - 18 mg/kg) at all the grab sampling locations were consistent with EPD's monitoring results at DS2 (9.8 - 18mg/kg).

7.7.31 Except for the copper level (70 mg/kg) at sampling location A14, the copper levels at all the other locations were lower than the LCEL of 65 mg/kg. The results were also consistent with the EPD's monitoring data at DS2.

7.7.32 The concentrations of cadmium (0.1 - 0.5 mg/kg), chromium (17 - 46 mg/kg), nickel (11 - 34 mg/kg), lead (26 - 64 mg/kg) and mercury (less than 0.05 mg/kg) were below the corresponding LCELs. The silver levels ranged from less than 0.1 mg/kg to 1 mg/kg. These results were comparable with EPD's monitoring results at DS2. Figure 7.24 shows the distribution of lead in the sampling area based on the laboratory results for the grab sediment samples. The lead concentrations at the upper layer of the marine sediment were evenly distributed over the sampling area. All the measured lead concentrations were below the LCEL for lead.

7.7.33 The sediment samples collected at all the grab sampling locations contained PAH levels less than 330 mg/kg, PCB levels less than 2 mg/kg (each individual) and TBT (in interstitial water) levels less than 15 hg/L. The PAH, PCB and TBT (in interstitial water) levels were all below their corresponding LCELs. The measured PAH and PCB levels by EPD were also below LCELs.

7.7.34 There were in total 2 grab sediment samples classified as Category H material (A14 and A15), 9 samples as Category M material (A1, A3, A5-A9, A11 and A13) and 5 samples as Category L material (A2, A4, A10 A12 and A16).

7.7.35 A comparison of the characteristics of the grab sediment samples collected near the north alignment (A1, A2, A5, A6, A9, A10, A13 and A14) and the south alignment (A3, A4, A7, A8, A11, A12, A15 and A16) showed that the sediment in the vicinity of both the alignments contained relatively high arsenic levels. Exceedances of the LCEL for arsenic were found at 6 sampling stations near the north alignment (A1, A5, A6, A9, A13 and A14) and at 5 sampling stations near the south alignment (A3, A7, A8, A11 and A15). The zinc level at A14 (near the north alignment) exceeded the UCEL and the copper level exceeded the LCEL. The zinc level at A15 (near the south alignment) also exceeded the UCEL. The similar results would be due to the close proximity of the two sampling stations. Based on the WBTC No. 3/2000, the 8 grab samples collected near the north alignment had 1 sample identified as Category H material, 5 samples as Category M material and 2 samples as Category L material. The other 8 grab samples collected near the south alignment had 1 sampled identified as Category H material, 4 samples as Category M material and 3 samples as Category L material. Overall, there was no significant variation in sediment quality for both the north alignment and the south alignment when comparing the grab samples collected near these two alignments.

Table 7.15 Sediment Chemical Quality Results