5.         WATER QUALITY IMPACT. 5-1

Introduction. 5-1

Environmental Legislation, Standards and Guidelines. 5-1

Baseline Conditions. 5-4

Water Sensitive Receivers. 5-6

Assessment Methodology & Criteria. 5-6

Identification of Environmental Impacts. 5-9

Evaluation of Impacts. 5-11

Mitigation Measures. 5-15

Residual Environmental Impact 5-18

Environmental Monitoring and Audit Requirements. 5-18

Conclusion. 5-18

 

 

 

Table 5.1        Summary of Water Quality Objectives for the North Western WCZ. 5-2

Table 5.2       Summary Statistics of Marine Water Quality in the North Western WCZ at Station NM1. 5-5

Table 5.3        Ambient and Tolerance Values for Suspended Sediment Concentrations in the Vicinity of Water Sensitive Receivers. 5-10

Table 5.4       Calculated Suspended Sediment (SS) Concentrations from Dredging. 5-12

 

 

Figure 5.1          Locations of EPD Marine Water Quality Monitoring Station

Figure 5.2          Locations of Water Quality Sensitive Receivers

Figure 5.3          Proposed Locations of Stormwater Discharges

Figure 5.4          Typical Plan Configuration of Slit Curtain Arrangement

Figure 5.5          Typical Sectional Configuration of Silt Curtain Arrangement

 

5.                   WATER QUALITY IMPACT

Introduction

5.1               This chapter presents an assessment of the potential impacts on water quality associated with the construction and operation phases of the proposed Road P1 Advance Works at Sunny Bay.  Key environmental issues in respect of water quality include the potential impacts due to dredging works for the reclamation construction, construction site runoff and wastewater from the general site activities, and stormwater discharges from the Project area. 

5.2               The environmental acceptability of these potential water quality impacts is assessed, with a view to identifying appropriate mitigation measures to reduce any identified adverse impacts to acceptable levels. 

Environmental Legislation, Standards and Guidelines

5.3               The criteria for evaluating water quality impacts in this EIA Study include:

·         Technical Memorandum on Environmental Impact Assessment Process (Environmental Impact Assessment Ordinance) (EIAO TM);

·         Water Pollution Control Ordinance (WPCO);

·         Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS);

·         Hong Kong Planning Standards and Guidelines (HKPSG); and

·         Practice Note for Professional Persons (ProPECC), Construction Site Drainage (PN1/94).

Environmental Impact Assessment Ordinance (EIAO), Cap. 499, S.16

5.4               This Project is a Designated Project under Schedule 2 of the EIAO.  The EIAO TM was issued by the EPD under Section 16 of the EIAO.  It specified the assessment method and criteria that was followed in this Study.  Reference sections in the EIAO TM provide the details of assessment criteria and guidelines that are relevant to the water quality assessment, including:

·         Annex 6 – Criteria for Evaluating Water Pollution; and

·         Annex 14 – Guidelines for Assessment of Water Pollution.

Water Quality Objectives

5.5               The Water Pollution Control Ordinance (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, Hong Kong waters are divided into ten Water Control Zones (WCZ).  Corresponding statements of Water Quality Objectives (WQO) are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in the WCZ based on their beneficial uses.  The project area is located within the North Western WCZ and the corresponding WQO are listed in Table 5.1.

Table 5.1          Summary of Water Quality Objectives for the North Western WCZ

 

Parameter	Objective	Part(s) of Zone
Aesthetic Appearance	Discharge shall not cause objectionable odour or discolouration   No tarry residue, floating wood, articles made of grass, plastic, rubber or any other substance   Mineral oil not visible on the surface. Surfactants shall not give rise to a lasting foam   No recognizable sewage-derived debris   No floating, submerged or semi-submerged subjects likely to interfere with the free movement or damage of material   Not to contain substances which settle to form objectionable deposits	Whole Zone     Whole Zone       Whole Zone       Whole Zone   Whole Zone       Whole Zone
E. coli	Annual geometric mean not to exceed 610/100 mL   Geometric mean not to exceed 180/100 mL during March to October inclusive in 1 year; sample should be taken at least 3 times in 1 calendar month at intervals of between 3 to 14 days   Geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days not to be less than 1/100 mL   Geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days not to exceed 1000/100 mL	Secondary contact recreation subzones   Bathing beach subzones           Tuen Mun (A) and Tuen Mun (B) Subzones and Water gathering ground subzones     Tuen Mun (C) Subzone and other inland waters
Colour	Not to exceed 30 Hazen units     Not to exceed 50 Hazen units	Tuen Mun (A) and Tuen Mun (B) Subzones and Water gathering ground subzones Tuen Mun (C) Subzone and other inland waters
Dissolved Oxygen within 2 m of seabed	Not less than 2 mg/L for 90% samples	Marine waters
Depth averaged Dissolved Oxygen	Not less than 4 mg/L for 90% samples   Not less than 4 mg/L	Marine waters   Tuen Mun (A), Tuen Mun (B) and Tuen Mun (c) Subzones, Water gathering ground subzones and other inland waters
pH value	Within the range 6.5 to 8.5 units; change due to human activity not to exceed 0.2 unit   Within the range 6.5 – 8.5 units     Within the range 6.0 - 9.0 units   Within the range 6.0 to 9.0 units for 95% of samples collected during the year; change due to waste discharge not to exceed 0.5 unit	Marine waters excepting Bathing Beach Subzones     Tuen Mun (A), Tuen Mun (B) and Tuen Mun (c) Subzones, Water gathering ground subzones Other inland waters   Bathing Beach Subzones
Temperature	Change due to waste discharge not to exceed 2oC	Whole zone
Salinity	Change due to waste discharge not to exceed 10% of natural ambient level	Whole zone
Suspended solids	Waste discharge not to raise the natural ambient level by more than 30%, nor cause the accumulation of suspended solids which may adversely affect aquatic communities   Annual median not to exceed 20 mg/L     Annual median not to exceed 25 mg/L	Marine waters         Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones, Water gathering ground subzones   Other Inland waters
Un-ionized ammonia	Annual mean not to exceed 0.021 mg/L	Whole zone
Nutrients	Not to be present in quantities that cause excessive growth of algae or other aquatic plants   Annual mean depth-average inorganic nitrogen not to exceed 0.3 mg/L   Annual mean depth-average inorganic nitrogen not to exceed 0.5 mg/L	Marine waters       Castle Peak Bay Subzone     Marine waters excepting Castle Peak Bay Subzone
5-Day Biochemical Oxygen Demand	Not to exceed 3 mg/L     Not to exceed 5 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones, Water gathering ground subzones Other Inland waters
Chemical Oxygen Demand	Not to exceed 15 mg/L       Not to exceed 30 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones, Water gathering ground subzones   Other Inland waters
Toxic Substances	Not to be present at levels producing significant toxic effect, carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to interactions of toxic substances with each other   Not to cause a risk to any beneficial use of the aquatic environment	Whole zone                 Whole zone
Phenol	Not to be present in such quantities as to produce a specific odour, nor to exceed  0.05 mg/L as C6H5OH	Bathing beach subzones
Turbidity	Not reduce light transmission substantially from the normal level	Bathing Beach Subzones

 

Hong Kong Planning Standards and Guidelines (HKPSG)

5.6               The Hong Kong Planning Standards and Guidelines (HKPSG), Chapter 9 (Environment), provides additional information on regulatory guidelines against water pollution for sensitive uses such as aquaculture and fisheries zones, bathing waters and other contact recreational waters.

Technical Memorandum

5.7               Besides setting the WQO, the WPCO controls effluent discharging into the WCZ through a licensing system. A Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS) was issued under Section 21 of the WPCO that gives guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, storm water drains, inland and coastal waters). The limits control the physical, chemical and microbial quality of effluents. Any effluent discharges from the proposed construction activities should comply with the standards for effluents discharged into the inshore waters or marine waters of the North Western WCZ, as shown in Table 10a and Table 10b, respectively, of the TM-DSS.

Practice Note

5.8               A practice note for professional persons was issued by the EPD to provide guidelines for handling and disposal of construction site discharges. The ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with ten types of discharge from a construction site.  These include surface runoff, groundwater, boring and drilling water, bentonite slurry, water for testing and sterilisation of water retaining structures and water pipes, wastewater from building constructions, acid cleaning, etching and pickling wastewater, and wastewater from site facilities.  Practices given in the ProPECC PN 1/94 should be followed as far as possible during construction to minimise the water quality impact due to construction site drainage.

Sediment Quality

5.9               The ETWB TCW No. 34/2002 sets out the management framework for dredged/excavated sediment.  This Technical Circular outlines the procedures to be followed in assessing and classifying sediment and explains the marine disposal arrangement for the classified material.  Dumping permits from EPD are required for marine disposal of dredged/excavated materials.

Baseline Conditions

Water Quality Monitored by EPD

5.10            The EPD water quality monitoring station NM1 in the North Western WCZ is located in the vicinity of the Project area (Figure 5.1).  A summary of the most recently published monitoring data (in 2003) for this station is presented in Table 5.2.

Table 5.2          Summary Statistics of Marine Water Quality in the North Western WCZ at Station NM1

 

Parameter		EPD Monitoring Station (NM1)	WPCO WQOs (in marine waters)
Temperature (oC)		23.2 (17.1 - 28.3)	Natural daily level ± 2 oC
Salinity (ppt)		30.3 (22.9 – 33.5)	natural ambient level ± 10 %
Dissolved Oxygen (DO) (% saturation)		76 (43 – 106)	-
	Bottom	72 (34 – 107)	-
DO (mg L-1)		5.5 (3.0 – 8.4)	³ 4 mg L-1
	Bottom	5.2 (2.4 – 8.5)	³ 2 mg L-1
pH value		8.1 (7.9 – 8.3)	6.5 - 8.5 (± 0.2 from natural range)
Secchi disc (m)		1.8 (0.5 – 3.1)	-
Turbidity (NTU)		12.3 (5.6 – 28.0)	-
SS (mg L-1)		9.7 (2.2 – 32.7)	£ natural ambient level + 30%
Silica (as SiO2) (mg L-1)		1.3 (0.2 – 3.3)	-
BOD5 (mg L-1)		0.9 (0.4 – 1.6)	not applicable to marine waters
Nitrite Nitrogen (mg L-1)		0.05 (<0.01 – 0.16)	-
Nitrate Nitrogen (mg L-1)		0.20 (0.01 – 0.58)	-
Ammonia Nitrogen (mg L-1)		0.09 (0.02 – 0.23)	-
Unionised Ammonia (mg L-1)		0.004 (0.001 – 0.008)	£ 0.021 mg L-1
Total Inorganic Nitrogen (mg L-1)		0.35 (0.07 – 0.75)	£  0.5 mg L-1
Total Nitrogen (mg L-1)		0.48 (0.23 – 0.92)	-
Orthophosphate Phosphorus(mg L-1)		0.023 (0.01 – 0.04)	-
Total Phosphorus (mg L-1)		0.04 (0.02 – 0.06)	-
Chlorophyll-a (µg L-1)		2.2 (0.5 – 5.2)	-
E. coli (cfu per 100 mL)		810 (180 – 3600)	< 610 cfu per 100 mL for keeping live seafood
Faecal Coliform (cfu per 100 mL)		1900 (460 – 7900)	-

Notes:

1.             Data source: EPD (2003). Marine Water Quality In Hong Kong in 2003.

2.             Except as specified, data presented are depth-averaged data.

3.             Data presented are arithmetic means except for E. coli and faecal coliforms that are geometric means.

4.             Data enclosed in brackets indicate ranges.

 

5.11            According to the “Marine Water Quality in Hong Kong 2003”, water quality monitoring data for station NM1 met the WQOs for dissolved oxygen (bottom), unionised ammonia and total inorganic nitrogen.    The depth-averaged dissolved oxygen level failed to comply with the WQO.  Station NM1 was reported to have experienced periods of water stratification in the summer when high salinity and low DO prevailed in the middle and bottom layers, and may have contributed to lower compliance of the DO objective in 2003.

5.12            The geometric mean of E. coli in 2003 failed to comply with the WQO.  According to the EPD report, the increasing trend of E. coli at NM1 noted in 2002 was again observed in 2003.  This increase may be related to the increased volume of effluent discharge from the Stonecutters Island sewage outfall.  The annual mean of depth-averaged suspended solids concentration was similar to the previous year.  The annual means of total inorganic nitrogen, ammonia nitrogen, total phosphorus, chlorophyll-a and silica were also similar to the previous year.  A slight decrease was reported in the depth-averaged dissolved oxygen level at station NM1 in 2003 (a decrease of 0.4 mg/L), and a slight increase was reported in the biochemical oxygen demand (an increase of 0.2 mg/L). 

Water Sensitive Receivers

5.13            In order to evaluate the water quality impacts during the construction and operation phases, the proximity of Water Sensitive Receivers (WSR) to the Project area must be considered.

5.14             The study area for the water quality impact assessment covers a distance of 4km from the Project site boundary.  Water sensitive receivers identified within the study area include Dragon Beach (non-gazetted) on the Tsuen Wan coastline and the fish culture zone at Ma Wan.  The nearest gazetted bathing beach is Anglers’ Beach on the Tsuen Wan coastline. The locations of these sensitive receivers are shown on Figure 5.2.  There is no salt water intake or cooling water intake within the study area.  An area of seagrass bed was identified at the Sunny Bay mudflat during ecological surveys for the Project in 2002/03, although no seagrass was observed growing on the Sunny Bay mudflat during additional surveys conducted in April 2005 (refer to S7.114 - 7.115 for details).  The ecological assessment considered it is entirely possible that seagrasses will re-establish in Sunny Bay at some point in the future (S7.116 refers).  The assessment of potential indirect impacts on this ecological sensitive receiver, that may be present before or during the construction phase of the Project, from water quality impacts during the reclamation works is presented in S7.151 – 7.157.

Assessment Methodology & Criteria

Construction Phase

5.15            The assessment of impacts to water quality during the construction phase has been divided into two aspects, formation of the reclamation and land-based construction activities.

Reclamation Formation

5.16            Impacts to water quality associated with the construction of a reclamation area at Sunny Bay were assessed in the Theme Park EIA Study.[1]  This reclamation at Sunny Bay is a small 10 ha reclamation to provide land for the construction of the public transport interchange (PTI) and was completed in July 2004.  The proposed reclamation area at Sunny Bay for the advance road works of Road P1 would be located immediately adjacent to the Sunny Bay Reclamation for the PTI and would be of smaller size, with an approximate area of 3 ha.  It is therefore appropriate to make reference to the assessment methodology adopted in the Theme Park EIA Study since the assessment addressed water quality impacts from suspended sediment plumes formed during reclamation construction, which is the concern for this Project. 

5.17            In the previously approved EIA Study, a near field model of sediment dispersion^[2] was used to assess the impacts from suspended sediment plumes formed during construction of the Sunny Bay Reclamation.  In this model the depth averaged suspended sediment concentrations were calculated at varying distances from the source of suspended sediments.  The model was used to predict suspended sediment concentrations with distance from the works and hence determine at what distance compliance with the WQOs would be achieved.  Quantitative uncertainties in the assessment of impacts from suspended sediment plumes should be considered when drawing conclusions from the assessment.  In carrying out the assessment, realistic worst case assumptions have been made in order to provide a conservative assessment of environmental impacts including: the assessment is based on the maximum dredging rate; and the calculation of loss rate of sediment to suspension is based on conservative estimates for the types of plant and methods of working.

5.18            The construction phase water quality assessment of dredging works carried out in the Northshore Lantau Development Feasibility Study EIA (NLDFS-EIA) included quantitative assessment of the effects of increases in suspended sediment concentrations from the sediment plume on dissolved oxygen and nutrients levels in the marine waters.  The NLDFS-EIA concluded that the dredging works for the Northshore Lantau Development reclamations would not cause adverse impacts to water quality in terms of reductions in dissolved oxygen concentration and increases in total inorganic nitrogen and unionised ammonia concentrations.  As there has been no change in the surrounding landuse of the proposed reclamation site at Sunny Bay since the completion of the NLDFS-EIA and no new effluent discharges or pollutant sources have been identified in the vicinity of the site, it is considered that the water quality assessment findings of the NLDFS-EIA Report with regard to potential impacts on dissolved oxygen and nutrient concentrations during dredging works remain valid.  Furthermore, the proposed area of dredging for the Project is very small in scale on comparison to the overall size of the proposed reclamations for the Northshore Lantau Development, which was the subject of assessment in the NLDFS-EIA.

5.19            For the preparation of the Sediment Quality Report for this Project in accordance with ETWB TCW No. 34/2002, laboratory testing of sediment sampled from the proposed area of dredging has been undertaken to determine the sediment quality.  A full description of the laboratory testing and classification of the sediment quality is contained in Section 6 (see S6.24 – 6.26 and S6.29).  As requested by EPD, elutriate tests would be carried out if contaminants in the sediment sampled from the proposed dredging area were found to exceed their respective Lower Chemical Exceedance Level (LCEL).  This is because the release of contaminants from sediments containing concentrations of contaminants below the LCEL would not be of concern. 

Land-based Construction Activities

5.20            The assessment of the potential impact of land-based construction activities on water quality has been undertaken in a qualitative manner.  The type and scale of construction activities on land was reviewed to assess the impact of these activities upon the nearby coastal waters.  Consideration has been given to controlling potentially adverse impacts from works sites and to the use of ‘best’ practice measures to minimize the potential for discharges of pollutants to the coastal waters of Sunny Bay through site practices to control site runoff and drainage.

Operation Phase

Hydrodynamics

5.21            The impacts to hydrodynamics from the reclamation formations for the Northshore Lantau Development were assessed in the NLDFS-EIA through computational modelling using the Delft 3D-FLOW hydrodynamic model.  The hydrodynamic model was based on the Upgrade Model, which was set-up, calibrated and validated for the Hong Kong SAR Government^[3].  The Upgrade Model was updated using an extensive new field data set^[4], which resulted in an improved representation of the hydraulics of the Hong Kong SAR waters.   

5.22            The grid of the updated model was refined in the vicinity of the reclamations to provide an improved representation of the features of the reclamations.  The hydrodynamic model was used to simulate the three scenarios defined below.  Each of the scenarios was simulated for 15 days spring-neap tidal cycles in the wet and dry seasons:

·         Baseline, corresponding to all planned reclamations in 2012.

·         Completed Northshore Lantau Development (including the Theme Park in Penny’s Bay).

·         Completed Northshore Lantau Development and Container Terminals.

5.23            The proposed Project forms part of the Road P1 between Sunny Bay and Siu Ho Wan which was studied in the NLDFS-EIA.  The relevant findings of the hydrodynamic modelling conducted for the Northshore Lantau Development with respect to the proposed northshore reclamation at Sunny Bay will be reviewed and presented in this report.

5.24            Quantitative uncertainties in the hydrodynamic modelling were considered in the NLDFS-EIA when making an evaluation of the modelling predictions.  For the hydrodynamic modelling these were considered to be negligible for the following reasons.

·         The computational grid is sufficiently refined to provide representative simulations results;

·         The model has been calibrated and validated to provide reliable predictions of hydrodynamics in the areas affected by the proposed reclamations; and

·         The simulations comprise a sufficient spin up period of 8 days so that the initial conditions do not affect the results.

Stormwater Discharges

5.25            There is an existing stormwater outfall (N10) within the Project area which collects runoff from a natural hillside stream.  The effects of the discharge of stormwater runoff on water quality will be examined in relation to the location of the reprovisioned stormwater discharge point at Sunny Bay, and the increase in additional catchment area generated by the Project. 

5.26            To determine the impacts on water quality from the increased discharges of stormwater from the Northshore Lantau Development reclamations, detailed water quality modelling using the Delft3D-WAQ water quality model was carried out in the NLDFS-EIA.  The water quality model was used to simulate the same three scenarios as the hydrodynamic modelling.  The NLDFS-EIA stated that stormwater from the urban and managed areas in drainage catchments N10 and N11 would be diverted by a tunnel to the west of the Luk Keng headland, while the stormwater from the rural areas would be discharged into Sunny Bay.

Identification of Environmental Impacts

Construction Phase

5.27            The assessment of impacts to water quality during the construction phase has been divided into two aspects, formation of the reclamation and land-based construction activities for the Slip Roads 5 and 6.

5.28            Dredging of marine sediment would be undertaken under the seawall area for construction of the seawall foundation only, and not under the reclaimed area.  The total volume of dredged sediment for the Project was estimated to be approximately 700,000 m³. 

5.29            Key water quality concerns during the construction phase are identified as follows:

·         dredging works that would disturb the marine bottom sediment, causing an increase in suspended solids concentrations in the water column and forming sediment plumes;

·         release of bound organic and inorganic constituents such as heavy metals, PAHs and PCBs into the water column, either via suspension or by disturbance as a result of dredging activities;

·         release of the same contaminants due to leakage and spillage as a result of poor handling and overflow from barges during dredging and transport;

·         construction runoff and drainage from land-based construction activities may enter nearby coastal waters if uncontrolled, with discharges potentially contaminated with silt, oil and grease;

·         sewage effluents from the construction workforce if discharged directly into coastal waters without any appropriate treatment.

5.30            All of the above would result in deterioration of the receiving marine water quality and could have adverse effects on WSR, depending on the level of contamination of the sediment to be dredged and the nature and locations of WSR.

5.31            During dredging, fine sediment would be suspended into the water column.  The extent of fine sediment lost to suspension would depend primarily on dredging rates and working methods.  Impacts from suspended sediment may be caused by sediment plumes being transported to sensitive areas, such as fish culture zones and bathing beaches.  Filling for the reclamation construction would be undertaken behind seawalls which have been constructed to above the water surface and at least 100m in advance of the filling point.  The reclamation filling works would not commence until construction of the seawall reaches the high water level.  Any losses of fine sediment to suspension during filling would thus be contained by the solid seawall structure.  Cumulative impacts from concurrent dredging and reclamation filling works would therefore not result.  The seawall foundation trenches would be backfilled with rockfill and sandfill.  The potential impacts to water quality from this operation are discussed qualitatively and the measures devised in the NLDFS-EIA to minimize the loss of fine sediment to suspension during filling works have been incorporated in this EIA Study (refer to Section 5.40 and 5.62).  Hence, only the losses of fine sediment to suspension from dredging activities are considered to have potential impact on WSR. 

5.32            Suspended sediment plumes passing over a WSR will cause the ambient suspended sediment concentrations to be elevated.  The extent of elevation of ambient suspended sediment concentrations would determine whether or not the impact is adverse.  The determination of the acceptability of any elevation is based on the WQO.  The WQO for suspended sediments for the North Western WCZ is defined as being an allowable elevation of 30% above the background.  As agreed in the NLDFS-EIA and Theme Park EIA, the ambient value may be represented by the 90th percentile of reported concentrations.  EPD routine monitoring data has been used as the source of the reported concentrations, with the monitoring station nearest to the identified WSR being defined as representative of that location.  Monitoring data collected during the past 5 years has been used to characterize the baseline water quality.  EPD monitoring data and the allowable elevations in suspended sediment concentrations are summarised in Table 5.3 below. 

Table 5.3       Ambient and Tolerance Values for Suspended Sediment Concentrations in the Vicinity of Water Sensitive Receivers

 

Sensitive Receiver	Dry Season		Wet Season
(Relevant EPD Monitoring Station)	90th Percentile	30% Tolerance	90th Percentile	30% Tolerance
Ma Wan Fish Culture Zone (WM4)	24.8	7.4	19.9	6.0
Dragon Beach (NM1) Anglers’ Beach (NM1)	23.6	7.1	19.0	5.7
Notes: All values are depth-averaged. Data source: EPD marine water quality monitoring programme for the period January 2000 to December 2004.

 

5.33            The allowable elevation in suspended sediment concentration as defined by the WQO for a particular site corresponds to the 30% tolerance level.  The calculated maximum suspended sediment concentrations from dredging have been compared with the 30% tolerance values in the above table to determine the acceptability of the impacts.

Cumulative Impacts

5.34            If the suspended sediment plumes arising from dredging activities of the proposed Project were to interface with those from other concurrent projects, there would be the potential of cumulative impacts on WSRs. 

5.35            The dredging works for the Sunny Bay Reclamation for the Public Transport Interchange (PTI) – CED Contract No. CV/2002/02 PTI were completed in October 2002 and the filling works are scheduled for completion in mid 2005.  Construction activities for the Disneyland Resort Line (DRL) – MTRC Contracts 581 and 580 are all land-based and are scheduled to be complete by mid 2005.  Hence, there would not be any marine works from other projects within the assessment area concurrent with this Project.

5.36            The concrete units required for the road/bridge structures would be pre-fabricated outside Hong Kong.  Therefore, impacts due to construction of concrete units for the proposed Project would not arise.  Insurmountable off-site impacts on water quality would also not be expected given the small scale of the project and that the quantity of pre-cast concrete units necessary for the construction would not be substantial.

Operation Phase

5.37            During the operation phase of the Project, the potential water quality impact on coastal waters would arise primarily from the discharge of stormwater at the reprovisioned outfall N10.  There would be no effluent discharge associated with the operation of the Road P1 Advance Works.

5.38            The operation of the Northshore Lantau Development would have the potential to affect the water quality of the surrounding marine waters in two ways, by changing the hydrodynamics and through the discharges of sewage effluents and stormwater from the developments on the reclaimed land of the Northshore Lantau Development.  These two aspects were assessed separately in the NLDFS-EIA using computational modelling.  The relevant findings of the hydrodynamic and water quality modelling conducted for the Northshore Lantau Development with respect to the proposed northshore reclamations at Sunny Bay will be reviewed and presented in this report.

Evaluation of Impacts

Construction Phase

Reclamation Formation

5.39            Dredging would be undertaken using two closed grab dredgers, with a maximum total production rate of 8,000 m³/day (assuming 24–hour working per day).  A sediment loss rate from dredging operations of 17kg/m³ was adopted in the NLDFS-EIA Study and has therefore been taken for this Project since the study area is the same.  Based on the proposed daily rate of dredging for the Project, the loss rate was calculated to be 1.57 kg/s.  To minimize the water quality impact during dredging operations, a silt curtain system would be provided at the reclamation site.  Dredging operations would be undertaken within a frame-type silt curtain.  The implementation of frame type silt curtains to fully enclose the grab dredgers would reduce the release of sediment by at least 80%.  The mitigation efficiency of greater than 80% for suspended solids removal has been demonstrated in the silt curtain pilot tests for the Sunny Bay Reclamation under CED Contract No. CV/2000/09.  The silt curtain system comprised two independent silt curtains.  The results of statistical analysis of measurement data taken within the frame type silt curtain and before the second floating type silt curtain gave suspended solids removal efficiencies in the range of 88 – 97% at different depths and tidal conditions.  A pilot test would be conducted to verify the suspended solids removal efficiency of the frame type silt curtain before the start of dredging works for the Project.  With the adoption of this recommended mitigation measure for the construction phase dredging, the mitigated loss rate was calculated to be 0.31 kg/s.  To provide additional protection from sediment loss to outside the Sunny Bay area and to any seagrass bed that may be present to the west of reclamation area, an additional layer of silt curtain would be provided at the eastern and western ends of the reclamation.  The arrangement of this silt curtain should be carefully arranged such that it does not affect the normal operation of the log ponds.

5.40            Following dredging of the seawall foundation, the trench would be backfilled with suitable material such as sandfill and rockfill at a maximum rate of 14,000m³ per day.  This filling rate is based on the current filling rate as adopted in the Sunny Bay Reclamation and considered sufficiently adequate for the reclamation works in this Project.  The loss of fines from the laying of rock bedding in the foundation trench would be expected to be insignificant taking into account the coarse nature of the rockfill.  Sandfill material is likely to contain a small percentage of fine sediment, which could be lost to suspension during placement.  The loss of sediment to suspension could be controlled by placing the sandfill by pumping down the arm of a trailing suction hopper dredger, which is positioned within the trench below the level of the surrounding seabed.  This would mean that the losses of sediment to suspension would be below the surrounding seabed in the trench, where the tidal currents would be lower and as such would settle out into the trench.  This would ensure that the fine sediment would not be transported beyond the immediate vicinity of the works area and hence adverse impacts would not result.

5.41            As discussed in Section 5.17, the same model as was used in the Theme Park EIA for the assessment of the Sunny Bay Reclamation has been adopted to calculate the depth averaged suspended sediment concentrations along the centreline of a plume by solving the advection-diffusion equation for a continuous line source. ^[5]  The formula used is as follows:

 

C(x) = q/(D*x*w*Öp)

 

where        C(x) = concentration of suspended sediment at distance x from the source

      q = sediment loss rate = 0.31 kg/s

      D = water depth = 15 m

      x = distance from source

      w = diffusion velocity = 0.01 m/s

5.42            The value for diffusion velocity is the same as that which was used in the Theme Park EIA for the assessment of sediment plumes from the construction of the Sunny Bay Reclamation and is considered appropriate for use in this Study given that the reclamation for the Road P1 Advance Works would be located immediately adjacent to the Sunny Bay Reclamation.  The diffusion velocity represents reductions in the centre-line concentrations due to lateral spreading. 

5.43            Based on the above equation, the suspended sediment concentrations from dredging have been predicted and are presented in Table 5.4.

 

Table 5.4          Calculated Suspended Sediment (SS) Concentrations from Dredging

 

Distance from Source (m)	SS Concentration (mg/L)
100	11.7
150	7.8
200	5.8
250	4.7
300	3.9
350	3.3
400	2.9
450	2.6
500	2.3
600	1.9
700	1.7
800	1.5

 

5.44            As described in Section 5.14, the closest WSR to the proposed reclamation at Sunny Bay for the Road P1 Advance Works is the Ma Wan Fish Culture Zone, which is located far away from the works area of the Project (at a distance of approximately 3.9 km).  The allowable increases in SS concentration at this sensitive receiver are 7.4 mg/L in the dry season and 6.0 mg/L in the wet season (Table 5.3).  As shown in the above table, the SS concentration at a distance of more than 200 m from the dredging works was predicted to be below 6.0 mg/L.  The predicted SS concentrations at the Ma Wan Fish Culture Zone would be below the WQO.

5.45            The allowable increases in SS concentrations according to the WQO in the vicinity of the dredging works are 5.7 mg/L in the wet season and 7.1 mg/L in the dry season, derived from data at Station NM1.  The data in the above table predicted that at a distance of slightly more than 200 m from the dredging works compliance with the WQO would be achieved (the predicted SS concentration at a distance of 210 m was 5.6 mg/L).  This area does not contain any WSR and as such the area of exceedance of the WQO would be classed as a ‘mixing zone’ and the predicted impacts would be deemed to be acceptable.  No adverse water quality impacts would be expected at Dragon Beach and Anglers’ Beach, which are located a distance of approximately 4 km and 5 km, respectively, from the project site.

Cumulative Impacts

5.46            As discussed in Section 5.35, there would not be any marine works from other projects concurrent with this Project.  Hence, there would not be any cumulative impacts on water quality during the construction works of the proposed Project.

Contaminant Release during Dredging

5.47            The sediment testing results show that the marine sediments to be dredged for the Project are uncontaminated, with no exceedance of the Lower Chemical Exceedance Level (LCEL) recorded, apart from one sediment sample at vibrocore V2 (refer to Table 6.2).  The sediment sample at sampling depth 18.0m to 19.9m at V2 was identified as Category M due to marginal exceedance of the LCEL for arsenic of 12mg/L.  (An arsenic concentration of 13mg/L was reported in sediment sample V2 (18.0m to 19.9m)).   

5.48            An indication of the likelihood of release of arsenic from the sediment during dredging was given by the results of the elutriation test.  As there is no existing legislative standard or guideline for individual heavy metal contents in marine waters, the European Union Water Quality Standards for Coastal Surface Water^[6] was adopted as the assessment criteria.  The European Union Water Quality Standards define a value of 25 ug/L as being the allowable concentration in coastal waters for arsenic. 

5.49            The elutriate test result indicated that the concentration of arsenic did not exceed the water quality standard.  Hence, the potential release of arsenic into the water column would not be anticipated to result in adverse impacts on water quality during the dredging works.  Provided that the dredged materials are properly disposed of at the designated marine dumping grounds and with the adoption of best management practices comprising general working methods for dredging and filling works, no significant impact on water quality would be anticipated to be associated with the dumping activities.

Construction Runoff and Drainage

5.50            During construction works for the road foundations and formation, runoff and erosion from exposed soil surfaces, earth working areas and stockpiles may contain increased loads of sediments, other suspended solids and contaminants.  Other potential sources of pollution from site drainage include release of cement materials with rain wash; wash water from dust suppression sprays and wheel washing facilities; fuel and lubricants from maintenance of construction vehicles and mechanical equipment; and release of any bentonite slurries and other grouting materials with construction run-off or groundwater dewatering process.  Sediment laden runoff may carry pollutants (adsorbed onto the particle surfaces) into the nearby coastal waters.

5.51            Mitigation measures should be implemented to control construction site runoff and drainage from the works areas, and to prevent runoff and drainage water with high levels of suspended sediments from entering the nearby coastal waters or causing blockage of storm water drains.  With the implementation of adequate construction site drainage and provision of sediment removal facilities as described in Section 5.62, it is anticipated that unacceptable water quality impacts would not arise.

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

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

·         spillages of liquids stored on-site, such as oil, diesel and solvents etc, are likely to result in water quality impacts if they enter the adjacent coastal waters or storm water drains.

5.53            Good construction and site management practices should be observed, as detailed in Section 5.65 & 5.66, to ensure that litter, fuels and solvents do not enter the nearby coastal waters or storm water drains.   

Sewage Effluent

5.54            Domestic sewage would be generated from the workforce during the construction phase.  It is unlikely that sewage generated from the site would have a significant water quality impact, provided that sewage is not discharged directly into stormwater drains or coastal waters adjacent to the site and temporary sanitary facilities, such as portable chemical toilets, are used on-site and properly maintained.

Operation Phase

Hydrodynamics and Water Quality

5.55            The overall conclusion in the NLDFS-EIA from the modelling data presented on tidal discharges was that the Northshore Lantau Development generally would not result in large changes in discharge patterns, with the exception of the residual flows through Victoria Harbour in the wet season which were not considered to be detrimental.  The northshore reclamation was predicted to have little effect on current speeds and directions, with only a small deviation in offshore directions in the currents in the immediate vicinity of the reclamation.  Current speeds in the small embayed area, the remainder of Sunny Bay, at the western end of the northshore reclamation were predicted to be significantly reduced due to formation of the embayment and retention times within this area were expected to be long.  In the vicinity of Sunny Bay only small changes in salinity were predicted, which were mainly shown to be increases in the vicinity of the small embayed area at the western end of the northshore reclamation.

5.56            The NLDFS-EIA identified that there is the possibility of changes to water quality due to the predicted changes in discharge patterns, which was investigated through detailed water quality modelling, the results of which are presented below with respect to the proposed reclamations on the northern shore of Lantau Island and Sunny Bay.

5.57            The water quality modelling predicted that there would be no exceedances of the WQO at sensitive receivers caused by the operation of the Northshore Lantau Development.  In general the operation of the Northshore Lantau Development was found to have little effect on water quality.  The water quality modelling predicted water quality conditions in the majority of the areas surrounding the Northshore Lantau Development due to the refined grid resolution used in the modelling.  However, the Sunny Bay following completion of the Northshore Lantau Development was too narrow to be represented by the model and as such it was not possible to quantitatively determine the water quality conditions within the bay.  The modelling was able to determine that the water quality conditions at the mouth of the bay, through the positioning of a sensitive receiver output point at this location, would be acceptable, which gave an indication of likely conditions in the bay.  However, due to the expected long retention times within the remainder of Sunny Bay, the NLDFS-EIA identified that the water quality conditions would be expected to be lower than that at the mouth. 

5.58            To mitigate the potential for poor water quality within the embayed area in Sunny Bay formed as a result of the Northshore Lantau Development reclamation, the NLDFS-EIA recommended that the stormwater system for the Northshore Lantau Development be designed to relocate polluted stormwater discharges away from the bay through the diversion of polluted stormwater to the west of the Luk Keng headland.  Clean run-off from the surrounding hillsides would still be discharged into the bay to maintain a salinity gradient, which may serve to provide suitable conditions for continued mangrove growth on the mudflats at the head of the bay.  The NLDFS-EIA stated that it was not possible at this stage to quantitatively determine whether additional measures would be required.  On a precautionary basis it was recommended that consideration be given to reducing the size of the northshore reclamation to increase the width of the mouth of Sunny Bay and encourage flushing.  The need for and extent of these mitigation measures would be determined during further, more detailed studies for the Northshore Lantau Development.  It was recommended that the detailed studies include the use of a local area water quality model with sufficient resolution to represent the Sunny Bay.  The proposed reclamation for the Road P1 Advance Works represents an interim phase and would be followed in the future by the proposed reclamations at the mouth of Sunny Bay for the Tourism Area Gateway (to be implemented by 2016) and within Sunny Bay for the Future Road P1 (to be implemented by 2014).

5.59            The proposed reclamation for the Project is small in size (3ha) and would take a linear form along the North Lantau shoreline at Sunny Bay.  The reclamation would not reduce the size of the mouth of Sunny Bay and hence would not reduce flushing of the bay.  The mitigation recommendations described above are to mitigate the potential for poor water quality within the small embayed area in Sunny Bay formed as a result of the western end of the Northshore Lantau Development reclamation.  Hence, adverse impacts to water quality conditions in Sunny Bay would not be expected following completion of the small reclamation for the Road P1 Advance Works. 

Temporary Stormwater Discharges

5.60            In view of the recommendations of the NLDFS-EIA that polluted stormwater be discharged away from Sunny Bay, polluted stormwater runoff from the road surfaces and other urban/developed areas of the Project would be collected and discharged outside the bay (Figure 5.3).   The existing outfall N10 which collects runoff from a hillside stream would be relocated to the western end of the reclamation for the Road P1 Advance Works.  This arrangement is in accordance with the NLDFS-EIA that clean run-off from the surrounding hillsides may continue to be discharged into the bay. 

Mitigation Measures

Construction Phase

5.61            The description of mitigation measures to prevent adverse impacts to water quality during the construction phase has been divided into two aspects, formation of the reclamation and land-based construction activities.

Reclamation Formation

5.62            In order to minimize potential impacts on marine water quality due to dredging and filling works for the reclamation formation, the implementation of the following measures is recommended:

·         Dredging should be undertaken using two grab dredgers with a maximum total production rate of 8,000 m³/day.

·         Deployment of frame type silt curtain to fully enclose the grab while dredging works are in progress.  The frame type silt curtain should be designed to enclose local pollution caused by the grab dredger and hung from a frame mounted on the dredger.  This frame type silt curtain shall cover the entire water column from water surface down to the seabed, with ballast at the bottom.  Mid-ballast may be added as necessary.  The frame type silt curtain should be capable of reducing sediment loss to outside by at least 80%.  An illustration of a typical plan configuration of the frame type silt curtain is given in Figure 5.4 and a sectional configuration is given in Figure 5.5. 

·         To provide additional protection from sediment loss to outside of Sunny Bay area and to any seagrass bed that may be present to the west of reclamation area, an additional floated type silt curtain would be deployed at the eastern and western ends of the reclamation area.  This layer of silt curtain should be formed from tough, abrasion-resistant permeable membranes, supported on floated booms in such a way as to ensure that egress of turbid waters from the enclosed dredging area shall be restricted.  The design and location of the floated type silt curtains should not affect the normal operation of the log pond in Sunny Bay.  Recommended arrangement of this additional silt curtain is given in Figure 5.4 and 5.5.

·         The Contractor should submit detailed proposal of the design and arrangement of the frame type and floated type silt curtains prior to installation for approval from the Engineer.  To demonstrate the capability of reducing sediment loss to outside by 80% by the frame type silt curtain, a pilot test shall be conducted.  Prior to commencing the pilot test, the details of it shall be submitted to the Director of Environmental Protection Department for agreement.  After consent is given, the Contractor should install the silt curtains prior to dredging and remove them upon completion of dredging.  Should a suspended solids removal efficiency less than 80% be demonstrated in the pilot test, an additional floated type silt curtain would be deployed near the eastern end of the bay and adjoining Cheung Sok.  The recommended arrangement of this provisional silt curtain is shown in Figure 5.4.

·         All filling activities for the reclamation should be carried out behind seawalls which have been constructed to above the high water level and at least 100m in advance of the filling point.  In the event that the 100 metres lead of seawall construction is not practicable, other suitable barriers shall be implemented to provide an effective lead of 100 metres.

·         If the seawall trenches are to be backfilled with sandfill, then the fill should be placed by pumping down the arm of a trailing suction hopper dredger, which is positioned within the trench below the level of the surrounding seabed.

5.63            The following general working methods should be adopted to supplement the measures described above for dredging and filling works to further minimize potential impacts on water quality:

·         tight-closing grabs should be used to minimize the loss of sediment to suspension during dredging works;

·         all vessels should be sized such that adequate clearance is maintained between vessels and the seabed in all tide conditions, to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash;

·         all pipe leakages shall be repaired promptly and plant shall not be operated with leaking pipes;

·         the decks of all vessels should be kept tidy and free of oil or other substances that might be accidentally or otherwise washed overboard;

·         adequate freeboard shall be maintained on barges to ensure that decks are not washed by wave action;

·         all barges used for the transport of dredged materials should be fitted with tight bottom seals to prevent leakage of material during loading and transport;

·         construction activities should not cause foam, oil, grease, scum, litter or other objectionable matter to be present in the water within and adjacent to the reclamation site;

·         loading of barges should be controlled to prevent splashing of material into the surrounding waters.  Barges should not be filled to a level that will cause the overflow of materials or sediment laden water during loading or transportation;

·         the speed of vessels should be controlled within the works area to prevent propeller wash from stirring up the seabed sediments; and

·         “rainbowing” sand fill will not be permitted.

Land-based Construction Activities

Construction Run-off and Drainage

5.64            The site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” should be followed as far as practicable in order to minimise surface runoff and the chance of erosion, and also to retain and reduce any suspended solids prior to discharge.  These practices include the following items :

·         Before commencing any site formation work, all sewer and drainage connections should be sealed to prevent debris, soil, sand etc. from entering public sewers/drains.

·         Provision of perimeter channels to intercept storm-runoff from outside the site. These should be constructed in advance of site formation works and earthworks.

·         Sand/silt removal facilities such as sand traps, silt traps and sediment basins should be provided to remove sand/silt particles from runoff to meet the requirements of the Technical Memorandum standard under the Water Pollution Control Ordinance.  The design of silt removal facilities should be based on the guidelines provided in ProPECC PN 1/94.  All drainage facilities and erosion and sediment control structures should be inspected monthly and maintained to ensure proper and efficient operation at all times and particularly during rainstorms. 

·         Water pumped out from foundation excavations should be discharged into silt removal facilities.

·         Careful programming of the works to minimise surface excavation works during the wet season.  If excavation of soil cannot be avoided during the wet season, exposed slope surfaces should be covered by a tarpaulin or other means.  Other measures that need to be implemented before, during, and after rainstorms are summarized in ProPECC PN 1/94.

·         Exposed soil surfaces should be protected by paving or fill material as soon as possible to reduce the potential of soil erosion.

·         Open stockpiles of construction materials or construction wastes on-site of more than 50m³ should be covered with tarpaulin or similar fabric during rainstorms.

General Construction Activities

5.65            Debris and refuse generated on-site should be collected, handled and disposed of properly to avoid entering the nearby coastal waters and storm water drains.  Stockpiles of cement and other construction materials should be kept covered when not being used. 

5.66            Oils and fuels should only be used and stored in designated areas which have pollution prevention facilities.  All fuel tanks and storage areas should be provided with locks and be sited on sealed areas, within bunds of a capacity equal to 110% of the storage capacity of the largest tank.  The bund should be drained of rainwater after a rain event.

Sewage Effluent from Construction Workforce

5.67            Temporary sanitary facilities, such as portable chemical toilets, should be employed on-site.  A licensed contractor would be responsible for appropriate disposal and maintenance of these facilities.

Operation Phase

5.68            The NLDFS-EIA recommended on a precautionary principle that silt traps be installed on all storm drains serving developed and landscaped/managed areas.  This would serve to ensure that the pollutants in the stormwater discharges are minimised as far as is practicable.

Residual Environmental Impact

5.69            With the full and strict implementation of the recommended mitigation measures for dredging and filling works for the reclamation formation and land-based construction activities, no unacceptable residual impacts on marine water quality are anticipated to arise. 

Environmental Monitoring and Audit Requirements

5.70            Environmental monitoring and auditing (EM&A) of marine water quality is recommended during the dredging and filling works.  An EM&A programme including pre- and post-dredging/filling monitoring for marine water quality would be required to ensure that the dredging and filling activities do not result in unacceptable impacts.  No monitoring of marine water quality would be required during land-based construction activities on the reclaimed land.  It is recommended, however, that audit of the land-based construction activities be carried out to confirm the implementation of the recommended mitigation measures for construction runoff and drainage and general construction activities.  Details of the EM&A procedures are presented in a separate EM&A Manual. 

Conclusion

5.71            The dredging works for the reclamation formation for the Project were identified to be the key water quality issue for the Project.  The potential water quality impacts associated with the dredging works were assessed by predicting the increases in suspended sediment concentrations due to fine sediment lost to suspension.

5.72            The nearest sensitive receivers identified within the study area include the Ma Wan Fish Culture Zone and Dragon Beach, which are located at approximately 4km from the proposed works area of the Project.  The assessment results show that with the use of two closed grab dredgers operated at a maximum dredging rate of 8,000m³ per day and with frame type silt curtains enclosing the grabs, the elevations in suspended sediment (SS) concentration generated from the dredging activities would be expected to be well below the allowable elevation under the WQO at these sensitive receivers.  Other mitigation measures for the construction of the reclamation were specified in the NLDFS-EIA and comprised operational constraints (i.e. limiting the rates of working, the sequence of construction) and general working methods to prevent adverse impacts on water quality.

5.73            The potential water quality impact arising from the release of contaminants from sediment disturbed during the dredging works was also assessed.  Sediment quality data obtained as part of this Project for the proposed dredging area indicated the level of contamination to be low and therefore adverse impacts on water quality were not predicted.  The potential water quality impacts arising from construction runoff and drainage and other land-based construction activities were assessed qualitatively and no adverse impacts were identified.

5.74            Overall, no unacceptable residual impacts would be expected during the construction phase with the full implementation of the recommended mitigation measures for the reclamation formation and land-based construction activities of the Project.

5.75            In accordance with the recommendations of the NLDFS-EIA, silt traps should be installed on storm drains serving developed and landscaped/managed areas of the Project, and polluted stormwater discharges should be relocated away from Sunny Bay.  With the implementation of the recommended mitigation measures, it is anticipated that the water quality impacts associated with the operation phase of the Road P1 Advance Works would be minimal and acceptable.