Contents:  Water Quality Impact Assessment                                     

4.1     Introduction                                                                                      

4.2     Objectives                                                                                        

4.3     Legislation, Standards and Guidelines                                                

4.4     Assessment Approach                                                                      

4.5     Baseline Conditions                                                                          

4.6     Evaluation Criteria                                                                             

4.7     Construction Phase Impact Assessment                                            

4.8     Operational Phase Impact Assessment                                              

4.9     Impact Mitigation                                                                              

4.10   Residual Impact Assessment                                                            

4.11   Environmental Monitoring & Audit Requirements                                  

4.12   Conclusions & Recommendations                                                      

4.13   References                                                                                      

 

4                    Water Quality Impact Assessment

    4.1                     Introduction

    4.1.1.1            This section presents the Water Quality Impact Assessment (WQIA) for the construction and operational phases of the Project.

    4.1.1.2            The aim of the WQIA is to assess and evaluate impacts of the proposed Project upon water sensitive receivers within the Study Area and to identify measures to avoid or otherwise reduce predicted impacts to within acceptable levels.

    4.2                     Objectives

    4.2.1.1            This section has been compiled in accordance with the evaluation criteria and assessment guidelines as presented in Annexes 6 and 14 respectively of the EIA-TM, and with reference to the requirements of Clause 3.4.1 of the EIA Study Brief.

    4.2.1.2            Key objectives of the water quality impact assessment include the following:

·       To collect and review background information on the existing and planned water system(s) and their respective sensitive receivers;

·       To characterise water and sediment quality and water sensitive receivers based on existing information or appropriate site survey and tests;

·       To identify and analyse physical, chemical and biological disruptions of marine water system(s) arising from the project construction and operation;

·       To predict, quantify and assess any water quality impacts arising from the Project on the water system(s) and the sensitive receivers by appropriate mathematical modelling techniques;

·       To identify and evaluate the best practicable dredging methods to minimize dredging and dumping requirements;

·       To evaluate the potential of and associated water quality impacts arising from accidental vessel collisions within the Project area during construction and maintenance of the wind farm;

·       To identify and quantify all dredging, fill extraction, filling, mud/sediment transportation and disposal activities and requirements; and

·       To devise mitigation measures to avoid or minimize potential impacts, in particular suitable dredging and disposal methods to mitigate any adverse impacts.

    4.3                     Legislation, Standards and Guidelines

    4.3.1.1            Reference has been made to the following local legislation governing water quality:

    4.3.2                  Water Pollution Control Ordinance (WPCO) (Cap. 358)

    4.3.2.1            Defines the boundaries of the ten local Water Control Zones (WCZs) and specifies the requirements Water Quality Objectives (WQOs).  The WQOs set limits for different parameters to be achieved for maintaining the water quality within the WCZs.  In accordance with the Study Brief, the Study Area of the project should cover the Mirs Bay, Port Shelter, Eastern Buffer and Junk Bay WCZs.  Table 4.1 summarises the WQOs for these WCZs. 

Table 4.1       Summary of WQOs for Mirs Bay, Junk Bay, Eastern Buffer and Port Shelter Water Control Zones

Parameter

WQOs

WCZ /  Part (s) of zone /Subzone to which the WQO applies

Dissolved Oxygen (DO)
(bottom)

Not less than 2 mg/L for 90% samples;

Marine waters of Mirs Bay, Junk Bay, Eastern Buffer and Port Shelter WCZs, and Fish Culture Subzones

DO
(Depth-averaged)

Not less than 4 mg/L for 90% samples

Marine waters of Mirs Bay, Junk Bay, Eastern Buffer and Port Shelter WCZs

 

Not less than 5 mg/L

Fish Culture Subzones

Nutrients

Annual mean depth-aver aged inorganic nitrogen not to exceed 0.1 mg/L

Port Shelter WCZ

Annual mean depth-aver aged inorganic nitrogen not to exceed 0.3 mg/L

Marine waters of Mirs Bay and Junk Bay WCZs

Annual mean depth-aver aged inorganic nitrogen not to exceed 0.4 mg/L

Marine waters of Eastern Buffer WCZ

Unionised ammonia

Annual mean not to exceed 0.021 mg/L

Marine waters (all zones) of Mirs Bay, Junk Bay, Eastern Buffer and Port Shelter WCZs

E. coli

Annual geometric mean not to exceed 610cfu/100mL

Secondary contact recreation subzones Port Shelter and Mirs Bay WCZs

Annual geometric mean not to exceed 610cfu/100mL

Fish culture subzones in Port Shelter, Junk Bay, Mirs Bay and Eastern Buffer WCZs

pH

To be in the range 6.5 - 8.5, change due to waste discharge not to exceed 0.2

Marine waters of Mirs Bay, Junk Bay, Eastern Buffer and Port Shelter WCZs

Change due to waste discharge not to exceed 10% of natural ambient level

Whole Zone of Mirs Bay, Junk Bay, Eastern Buffer and Port Shelter WCZs

Temperature

Change due to waste discharge not to exceed 2°C

Whole Zone of Mirs Bay, Junk Bay, Eastern Buffer and Port Shelter WCZs

Suspended Solids (SS)

Waste discharge not to raise the natural ambient level by 30% nor cause the accumulation of SS which may adversely affect aquatic communities

Marine waters of Mirs Bay, Junk Bay, Eastern Buffer and Port Shelter WCZs

Toxicants

Not to be present at levels producing significant toxic effect

Whole Zone of Mirs Bay, Junk Bay, Eastern Buffer and Port Shelter WCZs

Source:  EPD: the Marine Water Quality in Hong Kong 2006

    4.3.3                  Environmental Impact Assessment Ordinance (Cap. 499. S.16)

    4.3.3.1            Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM), Annexes 6 and 14 specifies the assessment method and criteria for water quality impact assessment.  This section follows the details of the assessment criteria and guidelines for evaluating water pollution.

    4.3.4                  Water Supplies Department Water Quality Objectives

    4.3.4.1            Stipulate a set of water quality objectives for water quality at seawater intakes.  Table 4.2 presents the relevant criteria.  The suspended solids and dissolved oxygen requirements are most relevant to this EIA study.

Table 4.2       Water Supplies Department standards at Seawater Intakes

Parameter

WSD Target Limit

Colour

< 20 HU

Turbidity

< 10 NTU

Threshold Odour Number

< 100 Odour Unit

Ammoniacal Nitrogen

< 1 mg/L

Suspended Solids

< 10 mg/L

Dissolved Oxygen

> 2 mg/L

Biochemical Oxygen Demand

< 10 mg/L

Synthetic Detergents

< 5 mg/L

E. coli

< 20,000 no./100mL

 

 

    4.3.5                  Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems Inland and Coastal Water

    4.3.5.1            Provides guidance on the permissible effluent discharges for foul sewers, storm water drains, inland and coastal waters.  Should any effluent be generated from this Project, the effluent quality should comply with the standards for effluents discharged into the inshore waters or marine waters of Junk Bay WCZ, Eastern Buffer WCZ and Mirs Bay WCZ.

    4.3.6                  Environment, Transport and Works Bureau Technical Circular (Works) no. 34/2002 Management of Dredged/Excavated Sediment

    4.3.6.1            Sets out the procedure for seeking approval to dredge/ excavate sediment and the management framework for marine disposal of dredged/ excavated sediment. The Technical Circular also specifies the requirements for determination of sediment quality, classification of sediment and disposal arrangement for the sediment.

    4.4                     Assessment Approach

    4.4.1                  Marine Activities

    4.4.1.1            Various foundation options and construction methods have been evaluated in order to minimize the potential environmental impacts of the proposed Project.  Details of alternative site and construction options are presented in Section 2 of this EIA Report.  Table 4.3 summarizes the preferred foundation and substructure options for Project development.

Table 4.3       Summary of Preferred Foundation and Substructure Options

Item

Preferred Type

Foundation

Suction Caisson Foundations

Substructure

3 or 4 legged jacket substructure

    4.4.1.2            Foundation installation requires the removal of water from inside of the suction caissons to the ambient water through pumping.  The pumped out water may contain a certain amount of sediment.  Transmission power cables and collection power cables will be installed by jetting, with the exception of the section located within Junk Bay.  This section, approximately 3-km long, requires anchor protection which will require the deployment of a dredger.  Both jetting and dredging would cause release of sediment into the water body. 

    4.4.2                  Modelling Tool

    4.4.2.1            A fine grid model has been developed using the Delft3D suite of models for prediction of the impacts due to sediment dispersion in the construction phase and the changes in hydrodynamic regime within the Study Area after the completion the Project.  Details of the model setup and calibration are presented in the Report on Wind Farm Model Calibration (Appendix 4A refers).

    4.4.2.2            The Delft3D-PART module using a particle tracking method has also been used to simulate the concentration distribution of suspended solids (SS).  Depletion of dissolved oxygen (DO) is calculated based on the modelled SS concentrations at WSRs. The concentrations of the other pollutants at the WSRs are estimated based on the results predicted by the model.

    4.4.3                  Pollution Loads

    4.4.3.1            The proposed works will lead to the release of sediment and contaminants into the ambient water, resulting in potential water quality impacts.  Tidal currents are the controlling factor for the dispersion of sediment disturbed by foundation installation and cabling works.  Sediment release rates for different activities are estimated below based on the characteristics of the preferred construction methods and equipment:

Dredging

    4.4.3.2            Rock amour protection involving the dredging of a trapezoidal trench is proposed for the cable in Junk Bay.  The approximate volume of dredged sediment is 135,000m3, based on an approximately 3km long trapezoidal trench of nominal 3m depth (shown in Figure 3.2). The sediment would be extracted by closed-grab dredging followed by backfilling with rock.   The maximum dredging rate for the grab dredger is not expected to exceed 6,300m3 per day.  Sediment loss rates depend on the size of the grab dredger, small dredgers generally have higher sediment loss rates.  The approximate range of the sediment loss rates for large and small dredgers is between 12 kg/m3 and 25 kg/m3 (John et al., 2000).  In this study, a conservative value of the sediment loss rate for the grab dredger of 25 kg/m3 is assumed.

    4.4.3.3            A portion of dredging work in Junk Bay is carried out near to the shore and may not be allowed between 19:00 and 07:00 hours on normal weekdays.  It is assumed that the work would be carried out over 12 hours per day with 6 working days per week for the transmission cable section in Junk Bay.  The worst-case scenario for dredging includes two grab dredgers operating at the same time with a minimum separation of 100 m.  Estimation of the sediment release rate for grab dredging is given below:

Grab size = 11 m3  

Working hours = 12 hr/day

No. of dredgers = 2 dredgers

Daily dredging rate by two dredgers = 6,300m3

Sediment loss rate (S-factor) = 25 kg/m3

Sediment release rate = =

    4.4.3.4            The sediment release due to grab dredging is assumed to be continuous and the sediment load is allocated in the whole water column to represent the sediment loss during the lift motion of the grab.

Jetting

    4.4.3.5            The maximum depth of cable embedment by the jetting machine is 5 m and the width of the trench is approximately 0.4 m.  The maximum jetting speed of the jetting machine is 150 m/hour or 0.0417 m/s.  Therefore, the jetting rate (rate of disturbance) is 0.0417 m/s ´ 0.4 m wide ´  5 m deep trench = 0.0834 m3/s.

    4.4.3.6            The calculation of sediment release rate for jetting is based on the following relationship:

Sediment release rate (kg/s) = jetting rate (m3/s) ´ dry density of the sediment (kg/m3) ´ percentage of loss rate (%)

    4.4.3.7            It is assumed that the percentage of loss rate (% of the disturbed sediment becomes suspension) is 20%[1].  Based on the sediment analysis for this Project, the dry density of the sediment is about 1,105 kg/m3.  The sediment release rate for jetting is therefore:

Sediment release rate (kg/s) =  =

    4.4.3.8            Release of sediment is concentrated at the bottom layer of the water column for jetting and is assumed as a continuous moving source at a speed of 150 m/hr along the offshore transmission power cable sections and at the foundation site.  A 16 working hours per day with 6 working days per week is assumed for the jetting operation in this area. 

Water Pumping Out from Suction Caissons

    4.4.3.9            During the suction caisson installation, water inside the suction caisson would be pumped out and discharged into the surrounding water.  The total amount of water to be pumped out of each foundation is not expected to exceed 8,500 m3.  The pumping rate would not exceed 300 m3 / hour per pump, or 1,200 m3 / hour per foundation.

 4.4.3.10            It is assumed that at the beginning of the operation the water pumped from the suction caissons would be free of suspended solids as the upper water layer within the foundation would be extracted.  As the pumping progresses it may be expected that the lowest water layer within the foundation would contain a certain amount of suspended solids from the seawater / sediment interface.

 4.4.3.11            To verify these assumptions a field trial was conducted in May 2008 to measure turbidity and / or SS concentrations at the discharge location and a various points downstream from the discharge location. Field measurements revealed that the increase in SS above ambient levels was negligible throughout the trial installation, with no sediment plume was observed using underwater video or visible at the water surface.

 4.4.3.12            To take a conservative approach for estimating the sediment release rate used in this water quality impact assessment, it is assumed that the water pumped out from the suction caissons contains an average of 15% sediment, which has been verified to be much higher than the result of the field water quality monitoring of a field trial presented in Section 4.7.2.  The dry density of the sediment, as determined through fieldwork, is 1,105 kg/m3.   The sediment concentration in the water pumped out from the suction caissons is thus 165.8 kg/m3.

 4.4.3.13            With reference to the Liquefied Natural Gas Receiving Terminal and Associated Facilities EIA, 80% of the sediment would fall from the water column to the seabed within a 70 m radius.  The percentage of the disturbed sediment in suspension is assumed to be 20%.  The sediment release rate for each foundation site has therefore been calculated as:

=

 4.4.3.14            The suction pumps are installed at the top of the suction caissons.  Discharge of the water would be conservatively assumed to be highest at 10 m above the seabed as the whole suction caisson will penetrate into the seabed in time.  It is also conservatively assumed that the duration of the discharge is 8 hours for each foundation.  

Modelling Scenarios

 4.4.3.15            As shown in Figure 4.1, the proposed cable route has been divided into three sections in order to derive the worst-case scenarios.  Section 1 represents the transmission cable section in Junk Bay that requires anchor protection to be put in place within a dredged trench.  Two sediment release points (P1 and P2) have been nominated within this section for different scenarios.

 4.4.3.16            No more than two grab dredgers would be deployed and operate at the same time with a minimum separation of 100 m at each proposed sediment release point.  Sediment release point P1 is located near the Seawater Intakes for WSD Pumping Station at Tseung Kwan O and the coral communities at Chiu Keng Wan.  Sediment release point P2 is selected, so that it is located near the Coral Communities at Fat Tong Chau West.

 4.4.3.17            Sections 2 and 3 represent the remaining offshore transmission power cable sections.  Installation of the cables will be by jetting only.   Sediment release points P3 and P4 are the sources representing the movement of the jetting machine within Section 2 and Section 3 respectively.  Only one jetting machine would be deployed for cable laying.  Therefore, jetting can only take place at one location in the entire Project area at any one time.  The jetting operation for this Project takes only one pass per cable installation to fluidize the sediment and lay the cable.

 4.4.3.18            The distance of each of the two sections is approximately 11 km.  Considering the maximum jetting speed of the jetting machine of 150 m/hr, the jetting operation can be expected to take 6 - 9 days depending on the actual length of the working day.  As the period required to complete a single pass is less than the model simulation period of 15 days, it is conservatively assumed that the jetting machine continuously moves along the section throughout the entire simulation period.  This conservative approach covers different tidal stages during the release of sediment from the jetting machine.

 4.4.3.19            At the wind farm foundation site, there would be a maximum of three foundations installed concurrently.  Three sediment release points (P5, P6 and P7) which are the closest to the dredging site in Junk Bay and jetting operation of the transmission power cable sections are allocated on the south-eastern boundary of the foundation site to take into account the worst situation of cumulative impact from the construction activities of the Project at the western side of the Study Area.  These sediment release point locations are also near the coral communities at Tuen Chau Tsai East and at One Foot Rock to represent the worst situation.

 4.4.3.20            In the case where foundation installations are carried out near the Victor Rock, which is one of the identified WSRs, three sediment release points (P8, P9 and P10) allocated at the north-eastern boundary of the foundation site in the closest proximity to this WSR are selected.  Jetting for the array cable laying is also considered to be conducted adjacent to these points to represent the worst situation that may adversely affect the coral communities at Victor Rock.  A moving source at sediment release point (P11) is used to represent the operation of the jetting machine.

 4.4.3.21            There are in total five worst-case scenarios for water quality impact assessment developed from a combination of different sediment release points that represent different construction activities for the entire project area.  Table 4.4 presents all the worst-case scenarios.   

Table 4.4       Worst-case Scenarios

Scenario

Sediment Release Activities from the Wind Farm Project

Concurrent Project

Scenario 1

Section 1 - Dredging in Junk Bay at P1

Section 2 - Jetting at P3

Foundation Site - Water pumping at P5-P7

§                     East Tung Lung Chau mud disposal area

§                     Tseung Kwan O Development

Scenario 2

Section 1 - Dredging in Junk Bay at P1

Section 3 - Jetting at P4

Foundation Site - Water pumping at P5-P7

§                     East Ninepins mud disposal area

§                     Tseung Kwan O Development

Scenario 3

Section 1 - Dredging in Junk Bay at P2

Section 2 - Jetting at P3

Foundation Site - Water pumping at P5-P7

§                     East Tung Lung Chau mud disposal area

§                     Tseung Kwan O Development

Scenario 4

Section 1 - Dredging in Junk Bay at P2

Section 3 - Jetting at P4

Foundation Site - Water pumping at P5-P7

§                     East Ninepins mud disposal area

§                     Tseung Kwan O Development

Scenario 5

Section 1 - Dredging in Junk Bay at P2

Foundation Site - Water pumping at P8-P10 & Jetting at P11

§                     East Ninepins mud disposal area

§                     Tseung Kwan O Development

 

Scenario 1

 4.4.3.22            Scenario 1 is to simulate the impacts due to dredging at the nearest point to the seawater intakes for the WSD pumping station at Tseung Kwan O and coral communities at Chiu Keng Wan in Junk Bay.  Dredging takes place at sediment release point P1 in Section 1.  In order to take into account the potential impacts from the other activities of the same Project, jetting in Section 2 at source P3 and installation of three foundations at the foundation site (as represented by sediment release points P5 to P7) are also included to form this worst-case scenario.

Scenario 2

 4.4.3.23            Scenario 2 is similar to Scenario 1 but jetting takes place in Section 3 of the transmission power cable section as represented by a moving source P4 for.  Dredging also occurs at P1 in Section 1 of the transmission power cable section and water pumping operation takes place at P5 to P7 at the foundation site. 

Scenario 3

 4.4.3.24            Scenario 3 is to simulate the situation where dredging takes place nearest to the coral communities at Fat Tong Chau West in Junk Bay.  Release of sediment due to dredging operation is at P2 in Section 1 and concurrent Jetting is assumed in Section 2 at the moving source P3.  Three sediment release points (P5, P6 and P7) representing the water pumping operation are located at the foundation site.

Scenario 4

 4.4.3.25            Scenario 4 is similar to Scenario 3 but jetting takes place in Section 3 of the transmission power cable section as represented by a moving source P4.  Dredging is also assumed to carry out at P2 in Section 1, which is located nearest to the coral communities at Fat Tong Chau West.  Water pumping operation takes place at P5 to P7 within the foundation site.

Scenario 5

 4.4.3.26            Scenario 5 is to simulate the situation where jetting and water pumping operation for installation of three foundations are located nearest to Victor Rock.  The jetting operation is represented by a moving source P11 for release of sediment and water pumping operation is represented by sediment release points P8, P9 and P10 at the north-eastern boundary of the foundation site.  Dredging is assumed to carry out at sediment release point P2 in section 1 of the transmission power cable section.

 4.4.3.27            The projects or activities that would be carried out concurrently with this Project and are located near the Works include Tseung Kwan O Development and East Tung Lung Chau and East Ninepins mud disposal area.  The assessment of cumulative impacts takes into account the sediment release from these projects in the five worst-case scenarios. EIA’s for the Cruise Terminal at Kai Tak project and the Wan Chai Development Phase II project have suggested that coral colonies be translocated from their current locations to small sites in Junk Bay. Figure 4.2 shows that these potential coral translocation sites are approximately 1.6 kilometers away from the cable corridor. As a result, no adverse impacts are anticipated at the potential coral translocation sites..

 4.4.3.28            The reclamation activity of Tsueng Kwan O Development together with the dredging operation of this Project may further increase the SS elevation in Junk Bay, and is included in all the worst-case scenarios for cumulative impact assessment.  The operation of the East Tung Lung Chau mud disposal area would not overlap with the disposal activity at East Ninepins mud disposal area.  Sediment release from the disposal activity at East Tung Lung Chau mud disposal area is only included in Scenarios S1 and S3 where jetting operation takes place near this disposal area.  Jetting operation as considered in Scenarios 2 and 4 would be carried out near the East Ninepins mud disposal area.  Therefore, sediment release from the disposal activity at East Ninepins mud disposal area is included in Scenarios 2 and 4 for cumulative impact assessment.  The disposal activity at East Ninepins, which is also located near the dredging operation in Junk Bay, is included in Scenario 5 together with the jetting operation at the foundation site to form the worst-case scenario for cumulative impact assessment.

 4.4.3.29            The approach of this study is to first examine the worst-case scenarios without any mitigation measures for reducing sediment release from jetting, dredging and water pumping operations.  Mitigated scenarios are, however, also included in the assessment to achieve compliance with the WQOs.  Therefore, the water quality impacts during the construction stage of the Project examine both the unmitigated and mitigated scenarios.

 4.4.3.30            During the operational stage, the model runs also include drogue tracking for oil spill to assess the areas that are potentially affected by any potential oil spill events.

    4.4.4                  Frictional Effects due to Wind Turbine Sub-structures

    4.4.4.1            The sub-structures of the wind turbines that are submerged in the sea cause friction on tidal flow.  The following method is used to account for the hydrodynamic impact due to the submerged sub-structures.

    4.4.4.2            As the hydrodynamic model grid size is larger than the wind turbine (~30m diameter[2]), it is not practicable to correspondingly refine the model grid size as the computational time would be significantly increased.  The frictional effects due to submerged bridge piers or vertical structures were modelled and assessed in other EIA studies[3].  A similar approach is therefore adopted in this study to model and assess frictional effects caused by the sub-structures of the wind turbines.  With this approach, additional quadratic friction terms are added to the momentum equations to represent the frictional effects of wind turbine columns on the hydrodynamics.  The mathematical expressions for calculating the loss coefficients for accounting the frictional effects are given as follows:

                               (4-1)

Where,

 is the density of water;

,  and  are sizes of the grid cell in x, y and z directions;

  is the velocity,  is the magnitude of the velocity, U and V are velocity components in x and y directions;

 and  are the loss coefficients in x and y directions; and 

Fx and Fy are drag forces induced by the sub-structure of the wind turbine in a grid cell, which are calculated as:

                                                                              (4-2)

 

Where,

n is the number of the turbine columns in the grid cell

 is the drag coefficient;

D is the diameter of the turbine column;

 is the effective approach velocity;

 is the magnitude of the effective velocity,  and  are the effective velocity components in x and y directions;

 is the total cross-section area;

 is the effective cross-section area which is the difference between the total cross-section area and the area blocked by the turbine columns; and

 is the ratio of the total cross-section area to the effective cross-section area.

 

Combining Equations (4-1) and (4-2), the loss coefficient used in the hydrodynamic model is expressed as:

                                                                                    (4-3)

 

    4.4.4.3            It is conservatively assumed that the diameter of the sub-structure is the same as that of the base footprint width of the foundation, i.e. 30 m.  The estimated loss coefficient for the sub-structure in the wind farm location is about 0.2.  

    4.5                     Baseline Conditions

    4.5.1                  Description of the Environment

    4.5.1.1            To assess the existing water quality conditions in the study area covering the Mirs Bay, Port Shelter, Eastern Buffer and Junk Bay WCZs, the most recently published monitoring data collected at the EPD marine water monitoring stations near the proposed wind farm and transmission power cable route have been reviewed.  The data can be used to represent the baseline water quality conditions at representative water sensitive receivers. 

    4.5.1.2            The selected EPD marine water monitoring stations include MM8, MM9 and MM14 in the Mirs Bay WCZ; PM1, PM4, PM6, PM7, PM8, PM9 and PM11 in the Port Shelter WCZ; EM1, EM2 and EM3 in the Eastern Buffer WCZ; and JM3 and JM4 in the Junk Bay WCZ.  A summary of EPD monitoring data collected in between 2002 and 2006 is presented in Table 4.5 to Table 4.8.

Table 4.5       Summary Statistics of Marine Water Quality in Junk Bay WCZ between 2002 and 2006

Parameter

EPD Monitoring Station

 

JM3

JM4

Temperature (ºC)

 

23.3

23.1

(15.9 - 29)

(15.8 - 28.7)

Salinity (ppt)

 

32.5

32.7

(20.9 - 34.9)

(22.2 - 35)

Dissolved Oxygen (mg/L)

 

6.2

6.1

(3.2 - 9.8)

(3.2 - 9.9)

Bottom

6.2

6.1

(3.2 - 9.8)

(3.2 - 9.9)

Dissolved Oxygen (DO) (% saturation)

 

87.0

85.9

(45 - 145)

(46 - 146)

Bottom

87.0

85.9

(45 - 145)

(46 - 146)

pH value

 

8.1

8.1

(7.7 - 8.7)

(7.7 - 8.6)

Secchi Disc Depth (m)

 

2.5

2.5

(1 - 4.1)

(0.5 - 4.5)

Turbidity (NTU)

 

9.1

9.8

(1.7 - 17.9)

(2.6 - 37.4)

Suspended Solids (SS) (mg/L)

 

3.3

4.9

(0.6 - 10)

(0.5 - 110)

5-day Biochemical Oxygen Demand (BOD5) (mg/L)

 

0.9

0.8

(0.2 - 5.9)

(0.1 - 5.8)

Ammonia Nitrogen (NH3-N) (mg/l)

 

0.1

0.1

(0.007 - 0.25)

(0.009 - 0.24)

Unionised Ammonia (mg/L)

 

<0.1

<0.1

(0 - 0.014)

(0 - 0.022)

Nitrite Nitrogen (mg/L)

 

<0.1

<0.1

(0.002 - 0.1)

(0.002 - 0.1)

Nitrate Nitrogen (mg/L)

 

0.1

0.1

(0.003 - 0.38)

(0.008 - 0.36)

Total Inorganic Nitrogen (TIN) (mg/L)

 

0.2

0.1

(0.02 - 0.59)

(0.03 - 0.63)

Total Kjeldahl Nitrogen (mg/L)

 

0.2

0.2

(0.05 - 0.49)

(0.05 - 0.46)

Total Nitrogen (mg/L)

 

0.3

0.3

(0.1 - 0.81)

(0.08 - 0.8)

Orthophosphate Phosphorus (mg/L)

 

<0.1

<0.1

(0.002 - 0.038)

(0.002 - 0.054)

Total Phosphorus (mg/L)

 

<0.1

<0.1

(0.02 - 0.07)

(0.02 - 0.07)

Silica (as SiO2) (mg/L)

 

0.6

0.6

(0.08 - 1.9)

(0.05 - 2)

Chlorophyll-a

 

3.4

2.8

(μg/L)

(0.4 - 30)

(0.4 - 33)

E. coli

 

277.2

225.8

(count/100 mL)

(1 - 7300)

(1 - 3400)

Faecal

 

511.5

541.4

Coliforms

(2 - 11000)

(2 - 8400)

(count/100 mL)

 

 

 

Table 4.6       Summary Statistics of Marine Water Quality in Mirs Bay WCZ between 2002 and 2006

Parameter

EPD Monitoring Station

MM8

MM13

MM14

Temperature (ºC)

 

22.9

23.1

22.9

(15.4 - 29.7)

(15.1 - 30.1)

(15 - 29.8)

Salinity (ppt)

 

33.1

33.2

33.2

(21.2 - 35.1)

(22.1 - 35.2)

(23.1 - 35.2)

Dissolved Oxygen (mg/L)

 

6.5

6.6

6.6

(2.5 - 9.2)

(2.4 - 9.9)

(2.8 - 9.1)

Bottom

6.5

6.6

6.6

(2.5 - 9.2)

(2.4 - 9.9)

(2.8 - 9.1)

Dissolved Oxygen (DO) (% saturation)

 

91.5

92.7

92.4

(35 - 134)

(34 - 149)

(40 - 134)

Bottom

91.4

92.7

92.5

(35 - 134)

(34 - 149)

(40 - 134)

pH value

 

8.2

8.2

8.2

(7.9 - 8.7)

(7.9 - 8.6)

(7.8 - 8.7)

Secchi Disc Depth (m)

 

3.8

4.6

4.1

(1 - 10)

(1.3 - 13)

(1.5 - 10)

Turbidity (NTU)

 

10.9

12.2

9.7

(0.8 - 98.7)

(0.7 - 149.6)

(0.9 - 23.3)

Suspended Solids (SS) (mg/L)

 

4.2

5.6

4.0

(0.5 - 26)

(0.5 - 210)

(0.5 - 24)

5-day Biochemical Oxygen Demand (BOD5) (mg/L)

 

0.6

0.5

0.6

(0.1 - 3.2)

(0.1 - 3.5)

(0.1 - 3.6)

Ammonia Nitrogen (NH3-N) (mg/l)

 

<0.1

<0.1

<0.1

(0.005 - 0.067)

(0.005 - 0.051)

(0.005 - 0.06)

Unionised Ammonia (mg/L)

 

<0.1

<0.1

<0.1

(0 - 0.006)

(0 - 0.005)

(0 - 0.006)

Nitrite Nitrogen (mg/L)

 

<0.1

<0.1

<0.1

(0.002 - 0.053)

(0.002 - 0.045)

(0.002 - 0.045)

Nitrate Nitrogen (mg/L)

 

<0.1

<0.1

<0.1

(0.002 - 0.35)

(0.002 - 0.57)

(0.002 - 0.25)

Total Inorganic Nitrogen (TIN) (mg/L)

 

<0.1

<0.1

<0.1

(0.01 - 0.4)

(0.01 - 0.62)

(0.01 - 0.29)

Total Kjeldahl Nitrogen (mg/L)

 

<0.1

<0.1

<0.1

(0.05 - 0.26)

(0.05 - 0.38)

(0.05 - 0.28)

Total Nitrogen (mg/L)

 

0.2

0.1

0.1

(0.05 - 0.63)

(0.05 - 0.8)

(0.05 - 0.48)

Orthophosphate Phosphorus (mg/L)

 

<0.1

<0.1

<0.1

(0.002 - 0.019)

(0.002 - 0.018)

(0.003 - 0.019)

Total Phosphorus (mg/L)

 

<0.1

<0.1

<0.1

(0.02 - 0.24)

(0.02 - 0.13)

(0.02 - 0.04)

Silica (as SiO2) (mg/L)

 

0.5

0.5

0.5

(0.06 - 1.5)

(0.06 - 3.2)

(0.06 - 1.7)

Chlorophyll-a

 

2.0

1.8

1.8

(μg/L)

(0.3 - 19)

(0.4 - 20)

(0.2 - 19)

E. coli

 

5

2

3

(count/100 mL)

(1 - 25)

(1 - 4)

(1 - 13)

Faecal

 

2

2

2

Coliforms

(1 - 8)

(1 - 6)

(1 - 9)

(count/100 mL)

 

 

 

Notes:     

1.       Date presented are depth-averaged, expect as specified.

2.       Data presented are arithmetic means of the depth-averaged results except for E. coli and faecal coliforms, which are annual geometric means.

3.       Data in brackets indicate the ranges.

 


Table 4.7       Summary Statistics of Marine Water Quality in Port Shelter WCZ between 2002 and 2006

Parameter

EPD Monitoring Station

PM1

PM4

PM6

PM7

PM8

PM9

PM11

Temperature (ºC)

 

23.88

23.77

23.59

23.29

23.07

23.15

23.01

(15.1 - 31.8)

(14.8 - 32)

(15.2 - 30.9)

(15.3 - 31.1)

(15.1 - 30.9)

(14.9 - 31.1)

(15 - 31.3)

Salinity (ppt)

 

32.48

32.55

32.62

32.97

33.09

33.01

33.13

(25.6 - 35.2)

(27 - 36.4)

(26.3 - 35.3)

(25.1 - 35.5)

(21.9 - 35.9)

(23.6 - 35.2)

(21.9 - 35.5)

Dissolved Oxygen (mg/L)

 

6.61

6.44

6.42

6.58

6.56

6.54

6.52

(3.3 - 9.2)

(3.4 - 9.2)

(3.1 - 9.2)

(2.6 - 10.7)

(3.2 - 9.2)

(3.3 - 9.5)

(2.9 - 10.5)

  Bottom

6.61

6.44

6.41

6.56

6.54

6.53

6.51

(3.3 - 9.2)

(3.4 - 10.5)

(3.1 - 9.2)

(2.6 - 10.7)

(3.2 - 9.2)

(3.3 - 9.5)

(2.9 - 10.5)

Dissolved Oxygen (DO) (% saturation)

 

94.20

91.64

91.02

92.88

92.27

92.18

91.67

(47 - 135)

(51 - 141)

(44 - 137)

(36 - 165)

(44 - 141)

(46 - 146)

(41 - 162)

  Bottom

94.20

91.34

91.02

92.80

92.19

92.15

91.63

(47 - 135)

(41 - 141)

(44 - 137)

(36 - 165)

(44 - 141)

(46 - 146)

(7.6 - 8.5)

pH value

 

8.18

8.15

8.11

8.14

8.15

8.16

8.15

(7.8 - 8.6)

(7.7 - 8.6)

(7.3 - 8.6)

(7.4 - 8.6)

(7.5 - 8.6)

(7.7 - 8.5)

(7.6 - 8.5)

Secchi Disc Depth (m)

 

2.95

2.96

3.30

4.44

4.99

4.01

4.63

(1.5 - 5.3)

(1.5 - 6)

(1.5 - 7)

(1.5 - 9)

(1.5 - 11)

(1.4 - 10)

(1 - 11)

Turbidity (NTU)

 

7.49

7.77

7.58

7.68

7.99

7.65

7.93

(1.5 - 18.8)

(1 - 24.2)

(1.3 - 26.9)

(1.5 - 25.9)

(1.2 - 24.4)

(1.6 - 19.4)

(1 - 21.6)

Suspended Solids (SS) (mg/L)

 

2.18

3.10

2.28

2.52

2.54

2.98

2.35

(0.6 - 22)

(0.7 - 60)

(0.5 - 15)

(0.5 - 41)

(0.5 - 15)

(0.5 - 130)

(0.5 - 12)

5-day Biochemical Oxygen Demand (BOD5) (mg/L)

 

0.85

0.80

0.78

0.71

0.62

0.66

0.66

(0.1 - 5.4)

(0.2 - 2.6)

(0.1 - 2)

(0.2 - 2.7)

(0.1 - 2.2)

(0.1 - 1.9)

(0.1 - 4.1)

Ammonia Nitrogen (NH3-N) (mg/l)

 

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

(0.005 - 0.094)

(0.006 - 0.098)

(0.005 - 0.1)

(0.005 - 0.066)

(0.005 - 0.072)

(0.005 - 0.063)

(0.005 - 0.046)

Unionised Ammonia (mg/L)

 

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

(0 - 0.007)

(0 - 0.009)

(0 - 0.009)

(0 - 0.008)

(0 - 0.006)

(0 - 0.007)

(0 - 0.005)

Nitrite Nitrogen (mg/L)

 

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

(0.002 - 0.047)

(0.002 - 0.048)

(0.002 - 0.068)

(0.002 - 0.068)

(0.002 - 0.082)

(0.002 - 0.06)

(0.002 - 0.058)

Nitrate Nitrogen (mg/L)

 

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

(0.002 - 0.11)

(0.002 - 0.088)

(0.002 - 0.13)

(0.002 - 0.15)

(0.002 - 0.25)

(0.002 - 0.22)

(0.002 - 0.25)

Total Inorganic Nitrogen (TIN) (mg/L)

 

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

(0.01 - 0.17)

(0.01 - 0.14)

(0.01 - 0.18)

(0.01 - 0.18)

(0.01 - 0.28)

(0.01 - 0.25)

(0.01 - 0.28)

Total Kjeldahl Nitrogen (mg/L)

 

0.13

0.13

0.13

0.12

0.11

0.12

0.11

(0.06 - 0.35)

(0.07 - 0.37)

(0.05 - 0.32)

(0.06 - 0.25)

(0.06 - 0.23)

(0.05 - 0.45)

(0.05 - 0.31)

Total Nitrogen (mg/L)

 

0.14

0.14

0.15

0.14

0.13

0.14

0.14

(0.05 - 0.36)

(0.06 - 0.37)

(0.05 - 0.34)

(0.05 - 0.35)

(0.05 - 0.46)

(0.05 - 0.48)

(0.05 - 0.47)

Orthophosphate Phosphorus (mg/L)

 

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

(0.002 - 0.033)

(0.002 - 0.02)

(0.002 - 0.028)

(0.002 - 0.029)

(0.002 - 0.021)

(0.002 - 0.022)

(0.002 - 0.019)

Total Phosphorus (mg/L)

 

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

<0.1

(0.02 - 0.06)

(0.02 - 0.06)

(0.02 - 0.04)

(0.02 - 0.05)

(0.02 - 0.04)

(0.02 - 0.13)

(0.02 - 0.04)

Silica (as SiO2) (mg/L)

 

0.53

0.57

0.59

0.57

0.57

0.55

0.56

(0.06 - 1.2)

(0.08 - 1.5)

(0.09 - 2)

(0.06 - 2.5)

(0.07 - 2.1)

(0.07 - 2.1)

(0.08 - 1.9)

Chlorophyll-a

 

2.43

2.37

2.47

1.77

1.63

1.82

1.76

(μg/L)

(0.7 - 31)

(0.4 - 39)

(0.2 - 15)

(0.3 - 25)

(0.3 - 18)

(0.3 - 10)

(0.4 - 19)

E. coli

 

6

3

5

7

4

3

2

(count/100 mL)

(1 - 39)

(1 - 23)

(1 - 90)

(1 - 50)

(1 - 37)

(1 - 9)

(1 - 3)

Faecal

 

14

9

20

7

9

6

30

Coliforms

(1 - 250)

(1 - 130)

(1 - 450)

(1 - 160)

(1 - 280)

(1 - 78)

(1 - 690)

(count/100 mL)

 

 

 

 

 

 

 

Notes:     

1.       Date presented are depth-averaged, expect as specified.

2.       Data presented are arithmetic means of the depth-averaged results except for E. coli and faecal coliforms, which are annual geometric means.

3.       Data in brackets indicate the ranges.


Table 4.8       Summary Statistics of Marine Water Quality in Eastern Buffer WCZ between 2002 and 2006

Parameter

EPD Monitoring Station

 

 

EM1

EM2

EM3

Temperature (ºC)

 

23.1

23.1

23.0

(15.8 - 28.4)

(15.7 - 28.5)

(15.5 - 29.6)

Salinity (ppt)

 

32.7

32.7

33.0

(23.6 - 35)

(22.9 - 35.1)

(22.6 - 35.1)

Dissolved Oxygen (mg/L)

 

6.0

6.2

6.3

(3.2 - 10.5)

(3 - 8.6)

(2.7 - 9.7)

Bottom

5.9

6.1

6.1

(3.2 - 10.5)

(3 - 8.6)

(2.7 - 9.7)

Dissolved Oxygen (DO) (% saturation)

 

83.8

86.9

88.6

(45 - 154)

(43 - 117)

(39 - 133)

 

Bottom

82

84.5

84.8

(45 - 154)

(43 - 117)

(39 - 133)

pH value

 

8.1

8.1

8.1

(7.8 - 8.4)

(7.8 - 8.5)

(7.7 - 8.6)

Secchi Disc Depth (m)

 

2.4

2.5

2.9

(1.3 - 4.5)

(1.3 - 5.3)

(1.2 - 5.8)

Turbidity (NTU)

 

9.9

9.5

10.7

(3 - 43.6)

(2.6 - 26.6)

(2.4 - 96.1)

Suspended Solids (SS) (mg/L)

 

4.0

4.1

4.0

(0.8 - 20)

(0.6 - 64)

(0.6 - 52)

5-day Biochemical Oxygen Demand (BOD5) (mg/L)

 

0.7

0.7

0.6

(0.1 - 3.3)

(0.1 - 5.3)

(0.1 - 3.7)

Ammonia Nitrogen (NH3-N) (mg/l)

 

0.1

0.1

<0.1

(0.006 - 0.23)

(0.007 - 0.2)

(0.006 - 0.2)

Unionised Ammonia (mg/L)

 

<0.1

<0.1

<0.1

(0 - 0.015)

(0 - 0.018)

(0 - 0.012)

Nitrite Nitrogen (mg/L)

 

<0.1

<0.1

<0.1

(0.002 - 0.3)

(0.002 - 0.12)

(0.002 - 0.1)

Nitrate Nitrogen (mg/L)

 

0.1

0.1

<0.1

(0.005 - 0.56)

(0.003 - 0.55)

(0.002 - 0.4)

Total Inorganic Nitrogen (TIN) (mg/L)

 

0.2

0.1

0.1

(0.01 - 0.71)

(0.02 - 0.75)

(0.01 - 0.7)

Total Kjeldahl Nitrogen (mg/L)

 

0.2

0.2

0.1

(0.05 - 0.42)

(0.06 - 0.52)

(0.05 - 0.41)

Total Nitrogen (mg/L)

 

0.3

0.2

0.2

(0.07 - 0.94)

(0.06 - 0.93)

(0.05 - 0.91)

Orthophosphate Phosphorus (mg/L)

 

<0.1

<0.1

<0.1

(0.002 - 0.039)

(0.002 - 0.036)

(0.002 - 0.03)

Total Phosphorus (mg/L)

 

<0.1

<0.1

<0.1

(0.02 - 0.06)

(0.02 - 0.13)

(0.02 - 0.06)

Silica (as SiO2) (mg/L)

 

0.6

0.6

0.6

(0.05 - 1.9)

(0.09 - 2.4)

(0.05 - 2.2)

Chlorophyll-a

 

2.5

2.4

2.3

(μg/L)

(0.2 - 23)

(0.3 - 31)

(0.3 - 27)

E. coli

 

441

279

61

(count/100 mL)

(1 - 7500)

(1 - 5900)

(1 - 2000)

Faecal

 

998

642

137

Coliforms

(1 - 12000)

(1 - 14000)

(1 - 4100)

(count/100 mL)

 

 

 

Notes:     

1.       Date presented are depth-averaged, expect as specified.

2.       Data presented are arithmetic means of the depth-averaged results except for E. coli and faecal coliforms, which are annual geometric means.

3.       Data in brackets indicate the ranges.

 

    4.5.1.3            Between 2002 and 2006, the water quality conditions in the Junk Bay, Mirs Bay, Port Shelter and Eastern Buffer WCZs were satisfactory and had high compliance with WQOs.  The water quality at Eastern Buffer and Junk Bay WCZs had 100% full compliance with the key WQOs.  The water quality in these WCZs has improved when compared to the WQO compliance in 2001 after the implementation of the Harbour Area Treatment Scheme (HATS) Stage 1.  

    4.5.1.4            As shown in Table 4.5 and Table 4.8, the water quality conditions of Junk Bay WCZ at Stations JM3 and JM4, and Eastern Buffer WCZ at Stations EM1, EM2 and EM3 fully complied with the key WQOs including DO, SS and E. coli. 

    4.5.1.5            According to EPD data from 1986 to 2001, the Mirs Bay and Port Shelter WCZs were subjected to effluent discharge from the Sha Tin and Tai Po Sewage Treatment Works and Red tides and fish kills frequently occurred in the 1990s.

    4.5.1.6            After implementation of the Tolo Harbour Action Plan, the water quality in Tolo Harbour has sharply improved.  In 2005, both Mirs Bay WCZ at Stations MM8, MM9 and MM10 and Port Shelter at Stations PM1, PM4, PM 6, PM7, PM8, PM9 and PM11 fully complied with the WQOs for DO, SS, chlorophyll-a and E. coli as shown in Table 4.6 and Table 4.7.

    4.5.2                  Identification of Water Sensitive Receivers

    4.5.2.1            Water sensitive receivers (WSRs) located within the WCZs that could potentially be affected by this Project are listed below and their locations are shown in Figure 4.3, Figure 4.3a and Figure 4.3b. 

·         Seawater Intakes at Chai Wan, Shau Kei Wan and Yau Tong

·         Fish Culture Zones at Port Shelters and mariculture farm at North Tung Lung Chau

·         Potential Marine Park

·         Gazetted / Non-gazetted Beaches at Clear Water Bay

·         Coral Communities

·         Fish Protection Areas

·         Artificial Reef Areas

·         Marine Mammals, in particular finless porpoises (Neophocaena phocaeniodes)

·         Marine benthic communities, in particular amphioxus

 

    4.5.2.2            In order to systematically present the findings of the water quality impact assessment, every key WSR is assigned with a reference identifier.  A list of all the WSRs that have been agreed with EPD and AFCD is presented in Appendix 4B.  The shortest distances of the marine works to the identified WSRs are also shown in this Appendix.

    4.6                     Evaluation Criteria

    4.6.1.1            The key water quality parameters for assessment of the sediment dispersion arising from foundation installation, dredging and jetting activities are suspended solids (SS) and dissolved oxygen (DO).  The pollutants adhered on the marine sediment including metals, metalloid and micro-organic pollutants are also adopted as evaluation parameters for the assessment.

    4.6.1.2            Table 4.9 presents the proposed assessment criteria for SS and DO at the WSRs.  The WQO for SS specifies that human activity or waste discharges shall not raise the ambient SS level by 30% and shall not affect aquatic communities.  Appendix 4C summarises the allowable SS elevations for different categories of WSRs.  The ambient SS level at each of the WSRs was calculated based on the field data from 2002-2006 collected at the EPD’s marine water monitoring stations that are the nearest to the WSRs. 

    4.6.1.3            There is no existing legislative standard or guideline in Hong Kong for individual heavy metals and micro-organic pollutants (PCBs, PAHs and TBT) in marine waters.  In the past EIA studies, various international standards were adopted as the most applicable assessment criteria.  For the present study, comparisons were made amongst standards of EU, Japan, USA, UK, Australia and Singapore.  A conservative selection was carried out using the lowest limiting values from different international standards as the assessment criteria.  Table 4.10 presents the criteria for the evaluation of impacts due to heavy metals and organic compounds at WSRs.

Table 4.9       Proposed Water Quality Assessment Criteria

WSRs

Proposed Water Quality Criteria

Reference

WCZ

WQO for SS

TIN  0.1 mg/L

Unionised Ammonia  0.021 mg/L

 

Seawater Intakes for WSD Pumping Stations

 

WQO for SS

 

SS < 10 mg/L

DO > 2 mg/L

WSD Water Quality Standards at Sea Water Intakes

Other Seawater Intakes

WQO for SS

 

Fish Culture Zones

WQO for SS

 

SS < 50 mg/L

CityU (2001)

Potential Marine Park

WQO for SS

 

Gazetted / Non-gazetted Beaches

WQO for SS

 

Coral Communities

WQO for SS

 

SS deposition rate < 100 g/m2/day

CAPCO Ltd. (2006)

SS < 10 mg/L above ambient level

Pastorok & Bilyard (1985)

Fish Protection Areas

WQO for SS

 

Artificial Reef Area

WQO for SS

 

Marine Mammals

WQO for SS

 

Note 1: WQO for SS refer to the Water Quality Objective for suspended solids for various WCZs stipulated under WPCO.  The WQO specifies that human activity or waste discharges shall not raise the ambient SS level by 30% and shall not affect aquatic communities. Details of the allowable SS elevations for WSRs are summarised in Appenxdix 4C.

 


Table 4.10     Proposed Assessment Criteria for Heavy Metals/Trace Organics

Metal / Contaminant

Proposed Criteria (μg/L)

Remarks

Arsenic

10

Note (1)

Cadmium

2.5*

Note (2)

Chromium

15*

Note (2)

Copper

5*

Note (2)

Lead

8.1*

Note (3)

Mercury

0.16

Note (5)

Nickel

8.2*

Note (3)

Silver

1.9*

Note (4)

Zinc

40

Note (2)

Total PAHs

3.0

Note (6)

PCBs

0.03

Note (3)

TBT

0.01

Note (3)

Notes:

*        Figures expressed in dissolved fraction

(1)      Environment Agency, Government of Japan

(2)      EC Dangerous Substances Directive (76/464/EEC), Environmental Quality Standards for List 1 and List 2 dangerous substances

(3)      USEPA National Recommended Water Quality Criteria, Criterion Continuous Concentration

(4)      USEPA National Recommended Water Quality Criteria, Criterion Maximum Concentration

(5)      United Nations Economic and Social Commission for Asia and the Pacific, ASEAN Marine Water

(6)      Australian Water Quality Guidelines for Fresh and Marine Waters

 


    4.7                     Construction Phase Impact Assessment

    4.7.1                  Identification of Impacts

    4.7.1.1            Land based construction activities are not included in this Study.  The construction principle activities that may cause water quality impact during the construction stages of the Project will be carried out in the sea and are listed below:

·         Dredging and anchor protection for the installation of transmission power cable in Junk Bay using dredgers;

·         Jetting for the installation of transmission power cable connecting to the section in Junk Bay to the offshore foundation site;

·         Jetting for the installation of collection power cable within the foundation site;

·         Installation of foundation involving pumping out seawater from inside of the suction caissons to the surrounding ambient water;

·         Sewage generation due to workforce; and

·         Accidental spillage of chemicals.

 

    4.7.1.2            Project development requires marine works to install turbine foundations and cables.  To prevent damage from anchors and other potential objects, the transmission cable within Junk Bay of approximately 3 km long shall be buried at about 5 m below the seabed, and shall be overlain with rock-amour to a level contiguous with the surrounding seabed.  The remaining transmission cable alignment of approximately 21 km and the array cables within the wind farm footprint shall be buried at about 3 to 5 meters below seabed. Figure 5.2 shows that the transmission cable alignment does not pass through any key coral communities.

    4.7.1.3            There are two transmission power cables for power transmission from the offshore transformer station to a substation facility on land. The location of the transformer station is illustrated in Figure 4.1. These two transmission cables will be buried approximately 50m apart; whilst in Junk Bay where cables will be buried in the same trench and covered with rock amour protection. Each jetting operation is a distinct event with impacts of a similar magnitude to the scenarios modeled. It is identified in Section 2.8.6 that the jetting of the cables will take approximately 2 months to complete for each run.  Therefore any impacts of the operations adjacent to the site will be significantly separated by many tidal cycles and cumulative impacts of suspended sediments are not anticipated

    4.7.1.4            Two closed grab dredgers will be deployed for removing marine sediment along the transmission power cable section in Junk Bay, and the two cables shall be laid at the bottom of the trench side by side.  The fine content of the rock materials is generally low and the rock materials used for backfilling would be inert.  As such, the water quality impact associated with rock fill will be low.

    4.7.1.5            The major concern is sediment release from dredging activity that may cause elevated SS levels in ambient waters leading to reduced sunlight penetration, mobilization of contaminants, and possible direct or induced effects on water sensitive receivers.

    4.7.1.6            The remaining transmission power cable and the collection power cables within the wind farm site will be installed by jetting which uses a strong water jet to fluidise the seabed generating a mixture of water and sediment close to the seabed.  Dispersion of the sediment plumes may affect water sensitive receivers located on or near the seabed such as coral communities.  Since the sediment plumes are generated at the bottom layer of the water column where the flow velocity is low due to the bottom friction from the seabed, the SS would normally settle back onto the seabed quickly.

    4.7.1.7            Installation of wind turbine foundation initially makes use of gravity where the suction caissons are driven down into the seabed by the weight of the foundation structure and suction.  When gravity is balanced out by the frictional force, seawater inside the suction caissons will be mechanically pumped out to reduce the water pressure inside the suction caissons, thereby generating a net downward pressure to ease the foundation further into the seabed with minimal disturbance.

    4.7.1.8            The seawater pumped from the suction caisson foundation may contain a small amount of sediment. It is anticipated that at the beginning of the pumping process, the SS content in the pumped out water should be very low and would gradually increase when the suction caissons are almost completely penetrated into the seabed.  Similar to the dredging and jetting activities, dispersion of the SS may impact nearby water sensitive receivers.  Sediment dispersion modelling has been conducted to predict and assess the potential impact due to the dredging and jetting for cable installation and pumping of seawater from the suction caisson foundations.

    4.7.1.9            The proposed wind farm also comprises of a transformer station. Its foundation will also be installed using the same type of suction caisson foundation technique. Thus, the potential water quality impacts as a result from the foundation installation works will be the same as those predicted for wind turbine foundation.   

 4.7.1.10            The construction activities may involve the use of chemicals such as paint, chemical solvents, mineral oils and fuel oil.  Accidental spillage of these chemicals into the seawater could be harmful to the aquatic life.  The risk of accidental spillage of chemicals can be reduced by implementation of good management practice.  Practicable and effective EM&A requirements are presented in the EM&A Manual of this Study.  Considering that the amount of chemicals to be used in the construction activities would be small, the potential impact of water pollution due to accidental spillage of chemicals is low.

    4.7.2                  Field Measurement & Sampling for Water Quality Impact of Suction Caisson Foundation Installation

    4.7.2.1            The proposed foundation works represent a new construction technique in the marine environment of the HKSAR.  Accordingly, turbidity and suspended solids data were obtained from the field measurements and sampling conducted in May 2008 during the site trial for suction caisson installation to verify that installation would not result in adverse water quality impacts and that the assumptions used in the impact assessment were suitably conservative, or at least would not lead to an under-representation of impacts upon the water sensitive receivers.

    4.7.2.2            The physical parameters adopted for the site trial were as follows:

·         Caisson dimension = 3.5 m (diameter) x 12 m (height);

·         Pumping rate 200 m3 / hour; and

·         Installation duration = 75 minutes (between 15:45 and 17:00). 

    4.7.2.3            Three sampling distances were selected to provide water quality data:

·         S1 – the immediate vicinity of the source;

·         S2 – 70m downstream from the source where 80% reduction in the suspended sediment level was assumed; and

·         S3 – 120m downstream from the source or 50m from S2.

    4.7.2.4            Figure 4.4 illustrates the schematic arrangement of these sampling stations.

    4.7.2.5            Additionally, two sampling depths, 10m and 5m above seabed were adopted for Stations S2 and S3 to represent the upper boundary and the centre of the trajectory of sediment discharge from the foundation as predicted by the mathematical model. Grab samples and in-situ measurements were taken sequentially from the locations every 15 minutes. 

    4.7.2.6            Turbidity and suspended solid baselines were established through reference sample collection conducted prior to the installation works.

    4.7.2.7            Figure 4.5 presents the results of in-situ turbidity data measured at S1, S2 and S3 during and after installation.  The result indicates that the overall turbidity at all stations was low and mostly below baseline level. 

    4.7.2.8            Although a short-lasting spike in turbidity was recorded at S1 at the beginning of the installation, such increase decayed rapidly and was returned back to below the baseline level within 10 minutes as the installation progressed.   The turbidity levels recorded at S1 during the remaining course of installation was steadily low, which reflects no apparent increase in suspended solids in ambient water resulting from discharge of water pumped out from the suction can contained very low level of suspended solids.

    4.7.2.9            Moreover, it is noted that this sudden increase in turbidity at S1 was not detected at either of the downstream stations S2 or S3.  The turbidity data recorded at these two stations during and after the installation was consistently steady and below the baseline levels.

 4.7.2.10            Likewise, the suspended solid levels recorded at S2 and S3 were steadily low at both of the sampling depths of these stations, as illustrated in Figure 4.6, which are consistent with the turbidity results.  The measured results are either at or below baseline levels indicate no significant increase in suspended solid levels resulting from the installation.   

 4.7.2.11            The field monitoring demonstrates that the results predicted by water quality modeling is significantly more conservative than the actual field installation, thus no or insignificant water quality impact arising from the installation of caisson foundation is anticipated. 

 

    4.7.3                  Scenario Impact Assessment

Dispersion of Sediment

    4.7.3.1            The potential water quality impacts in the construction stage of the Project are mainly due to the sediment dispersion and release of pollutants, which are originally adhered on the sediment, from the foundation installation and cabling works.  Disturbance to the marine sediment in the seabed causes suspension of the sediment in the water column.

    4.7.3.2            The foundation installation and cabling works, however, would not introduce additional sources of pollutant into the water column. Suspended solids (SS) and dissolved oxygen (DO) are the key water quality parameters that need to be assessed and compared against relevant criteria.  The Delft3D fine grid model was used to model the proposed worst-case scenarios and to simulate the sediment dispersion in the water environment. The following presents the predicted results of SS and DO without implementation of any mitigation measures:

Scenario 1

    4.7.3.3            Appendix 4D includes the predicted increases in SS at all the WSRs for Scenario 1.  The majority of the WSRs did not show detectable increases in SS, i.e. increase in SS is zero.  In order to show clearly which WSRs would be affected by the construction activities of the Project, the WSRs with detectable increases in SS, i.e. > 0.01 mg/L, in either the dry season or the wet season are presented in Table 4.11.  The other WSRs with no detectable increases in SS are not presented in the table but can still be found in Appendix 4D.

    4.7.3.4            The coral communities at Junk Bay (CC26), Junk Island (CC27), Fat Tong Chau West (CC11) and seawater intake at Tseung Kwan O (SW13) in Junk Bay would be affected by the dredging and jetting operations.  The increases in maximum SS in the wet season at the coral communities at Junk Bay (3.03 mg/L), Junk Island (4.79 mg/L) and Fat Tong Chau West (2.97 mg/L) were higher than the allowable limit (2.03 mg/L).  The time series plots for increases in SS at CC11, CC26 and CC27 presented in Figure 4.7 show the high peaks of SS above the allowable SS elevations for this scenario.  Mitigation measures should be implemented to reduce the SS elevations at these WSRs to a level below the allowable limit.

    4.7.3.5            The increases in mean SS during the dry season (0.00 – 0.03 mg/L) and the wet season (0.00 – 0.36) at these WSRs were below the allowable limits.  The average mean values of the increases in SS were also well below the allowable limits.  

    4.7.3.6            It is likely that both the jetting operation (represented by sediment release point P3) and the dredging operation in Junk Bay (represented by sediment release point P1) contribute to the high peak SS levels at these locations.  The combined effects would be reduced when the jetting machine moves away from Junk Bay.  Hence, elevation of SS would be reduced. 

    4.7.3.7            There would be slight increases in SS levels at the site with amphioxus occurrence (AO8), which is located to the southeast of Tung Lung Chau.  The increases were small, i.e. increases in mean SS were 0.03 mg/L in the dry season and 0.02 in the wet season.  The increases in maximum SS in the dry season (2.18 mg/L) and in the wet season (1.25 mg/L) were below the allowable limits. 

Table 4.11     Predicted Increases in SS (in mg/L) – Scenario 1 (Unmitigated Scenario)

WSD ID

Name

 

 

Allowable SS Elevation

Predicted SS Elevation

Dry

Wet

Average of Dry and Wet

Dry

Wet

Average of Dry and Wet (Mean)

Max

Mean

Max

Mean

CC26

Coral Communities at Junk Bay

2.24

2.03

2.14

0.50

0.02

3.03

0.36

0.19

CC11

Coral Communities at Fat Tong Chau West

2.24

2.03

2.14

0.50

0.00

2.97

0.02

0.01

AO8

Amphioxus Occurrence (Yr 2006 record of summer survey)

2.24

1.87

2.06

2.18

0.03

1.25

0.02

0.03

CC27

Coral Communities at Junk Island

2.24

2.03

2.14

0.23

0.00

4.79

0.20

0.10

SW13

Seawater Intakes for WSD Pumping Station at Tseung Kwan O

1.83

1.38

1.61

0.14

0.00

0.13

0.00

0.00

Remarks: 1. Values of the increases in SS are depth-averaged SS concentrations. 2.The figure in bold    represents that the predicted SS concentration is higher than the allowable SS elevation.

 

Scenario 2

    4.7.3.8            The predicted SS elevations at all the WSRs are included in Appendix 4D.  There were no SS elevations at most of the WSRs.  Table 4.12 shows the predicted SS elevations at the WSRs with detectable increases in SS for Scenario 2.  The WSRs with detectable increases in SS were the coral communities at Junk Bay (CC26), the site with amphioxus occurrence (AO9) and sighting points of marine mammal (MM8 and MM11).  The maximum increase in SS (3.03 mg/L) at coral communities at Junk Bay (CC26) in the wet season was higher than the allowable limit (2.03 mg/L).  Figure 4.8 shows the time series plot of the predicted SS with exceedance in allowable limit during the occurrence of high peaks of SS.

    4.7.3.9            All the seasonal and average mean SS increases were however below the allowable limits.  The transient high peaks of SS at CC26 would be mainly due to dredging.

Table 4.12     Predicted Increases in SS (mg/L) – Scenario 2 (Unmitigated Scenario)

WSD ID

Name

Allowable SS Elevation

Predicted SS Elevation

Dry

Wet

Average of Dry and Wet

Dry

Wet

Average of Dry and Wet (Mean)

Max

Mean

Max

Mean

CC26

Coral Communities at Junk Bay

2.24

 2.03

 2.14

0.50

0.02

3.03

0.36

0.19

AO9

Amphioxus Occurrence (Yr 2006 record of summer survey)

2.24

 1.87

 2.06

0.46

0.01

0.36

0.00

0.005

MM11

Sighting Point of Marine Mammal

2.24

 1.87

 2.06

0.09

0.00

0.00

0.00

0.00

MM8

Sighting Point of Marine Mammal

 2.24

 1.87

 2.06

0.00

0.00

0.32

0.00

0.00

Remarks:  1  Values of the increases in SS are depth-averaged SS concentrations. 2.The figure in bold    represents that the predicted SS concentration is higher than the allowable SS elevation.

 

Scenario 3

 4.7.3.10            The predicted SS elevations at all the WSRs are included in Appendix 4D.  There were no SS elevations at most of the WSRs.  The predicted SS elevations at the WSRs with detectable increases in SS for Scenario 3 are presented in Table 4.13.  Increases in SS were only detected at coral communities at Fat Tong Chau West (CC11) and at the site with amphioxus occurrence (AO8).

 4.7.3.11            There was no exceedance of the increases in seasonal and average mean SS of the dry and wet seasons.  However, the increases in maximum SS in the dry season (5.44 mg/L) and in the wet season (10.26 mg/L) at CC11 exceeded the corresponding allowable limits (2.24 mg/L for the dry season and 2.03 mg/L for the wet season).  The time series plots for increases in SS at CC11 during both the dry and wet seasons are shown in Figure 4.9.  Exceedances during the high peaks of SS are clearly shown in the time series plots.  It is worth noting that no mitigation measures are considered in this scenario.

Table 4.13     Predicted Increases in SS (mg/L) – Scenario 3         (Unmitigated Scenario)

WSD ID

Name

Allowable SS Elevation

Predicted SS Elevation

Dry

Wet

Average of Dry and Wet

Dry

Wet

Average of Dry and Wet (Mean)

Max

Mean

Max

Mean

CC11

Coral Communities at Fat Tong Chau West

 2.24

2.03

2.14

5.44

1.22

10.26

1.18

1.20

AO8

Amphioxus Occurrence (Yr 2006 record of summer survey)

 2.24

1.87

2.06

2.18

0.03

1.25

0.02

0.03

Remarks:  1. Values of the increases in SS are depth-averaged SS concentrations. 2. The figure in bold represents that the predicted SS concentration is higher than the allowable SS elevation.

Scenario 4

 4.7.3.12            The predicted SS elevations with detectable increases in SS for Scenario 4 are presented in Table 4.14.  A complete list of the predicted SS elevations at all the WSRs are included in Appendix 4D.  Increases in SS were recorded at coral communities at Fat Tong Chau West (CC11), the site with amphioxus occurrence (AO9), and sighting points of marine mammal (MM8 and MM11).

 4.7.3.13            Based on the model predictions for this unmitigated scenario, there was no exceedance of the increases in seasonal mean and average mean SS of the dry and wet seasons at these WSRs.  However, the increases in maximum SS at CC11 in the dry season (4.93 mg/L) and in the wet season (7.29 mg/L) exceeded the corresponding allowable limits.

 4.7.3.14            The time series plots for increases in SS at CC11 during the dry and wet seasons in Figure 4.10 show the SS exceedances at different time intervals.  The exceedances would be related to the dredging operation in Junk Bay. 

Table 4.14     Predicted Increases in SS (in mg/L) – Scenario 4 (Unmitigated Scenario)  

WSD ID

Name

Allowable SS Elevation

Predicted SS Elevation

Dry

Wet

Average of Dry and Wet

Dry

Wet

Average of Dry and Wet (Mean)

Max

Mean

Max

Mean

CC11

Coral Communities at Fat Tong Chau West

 2.24

2.03

2.14

4.93

1.22

7.29

1.16

1.19

AO9

Amphioxus Occurrence (Yr 2006 record of summer survey)

2.24

1.87

2.06

0.46

0.01

0.36

0.00

0.005

MM8

Sighting Point of Marine Mammal

 2.24

1.87

2.06

0.00

0.00

0.32

0.00

0.00

MM11

Sighting Point of Marine Mammal

 2.24

1.87

2.06

0.09

0.00

0.00

0.00

0.00

Remarks:  1. Values of the increases in SS are depth-averaged SS concentrations. 2. The figure in bold    represents that the predicted SS concentration is higher than the allowable SS elevation.

 

Scenario 5

 4.7.3.15            The predicted SS elevations at all the WSRs are included in Appendix 4D.  Except at CC11, there were no SS elevations at the other WSRs.  Table 4.15 presents the increases in SS at CC11 for Scenario 5.  The jetting operation was allocated at the wind farm.  The only detectable increases in SS were at coral communities at Fat Tong Chau West (CC11).  The increases in maximum SS in the dry season (4.93 mg/L) and in the wet season (7.29 mg/L) were higher than the corresponding allowable limits (2.24 mg/L in the dry season and 2.03 in the wet season).  The time series plot for the increases in SS at CC11 during the dry and wet seasons are also shown in Figure 4.9.  The results were the same as those of the Scenario 4.  WQO exceedances for SS frequently occurred over the simulation period.  Since the location of CC11 is far away from the foundation site, the model predicted almost no influence from the jetting and water pumping activities at the foundation site. 

Table 4.15     Predicted Increases in SS (in mg/L) – Scenario 5 (Unmitigated Scenario)

WSD ID

Name

Allowable SS Elevation

Predicted SS Elevation

Dry

Wet

Average of Dry and Wet

Dry

Wet

Average of Dry and Wet (Mean)

Max

Mean

Max

Mean

CC11