3. water quality impact assessment · 2010-07-27 · 3.2.5 water quality criteria for water-cooled...

Consultancy Agreement No. CE 63/2008 Providing Sufficient Water Depth for Kwai Tsing Container Basin and its Approach Channel Environmental Impact Assessment Report

259053/TNI/ENL/23/E July 2010 P:\Hong Kong\MRT\259053 KTCB\01 Project Management\71 Deliverables\07 Environmental Impact Assessment Report\FINAL_PE\EIA_Rpt_Jul 10_Ch 3_Water.doc

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3.1 Introduction

Water quality impacts arising from the construction and operational phases of the Project were assessed in

accordance with Condition 3.4.3 of the EIA Study Brief No. ESB-198/2008 and the specific sub-conditions

of the Study Brief, which are referred to in the following sub-sections where appropriate.

This section presents the findings of the assessment of potential water quality impacts of the proposed

Project in the vicinity of sensitive receivers, primarily in terms of the effects of dredging as well as other

related works necessary to provide sufficient water depth for the KTCB and its Approach Channel, and is

structured as follows:

Section 3.2: Provides discussions on existing environmental legislation, standards, guidelines and criteria;

Section 3.3: Provides a description on the existing environmental conditions;

Section 3.4: Identifies the water sensitive receivers for this Project;

Section 3.5: Describes the assessment methodology adopted for this EIA;

Section 3.6: Provides discussions on the identification of associated environmental impacts of this Project;

Section 3.7: Describes the prediction and evaluation of environmental impacts due to the Project;

Section 3.8: Proposes potential mitigation measures to address the identified impacts;

Section 3.9: Evaluates the residual impacts;

Section 3.10: Proposes appropriate environmental monitoring and auditing programme; and

Section 3.11: Summarises the key findings of this section.

Assessments have been conducted in accordance with the requirements of the Study Brief and Annexes 6

and 14 of the Technical Memorandum on the Environmental Impact Assessment Process. Water quality

impacts in terms of Suspended Solids (SS), Dissolved Oxygen (DO), Heavy Metals, TBT, PCB, PAH,

Chlorinated Pesticides, Ammoniacal Nitrogen (NH3-N), and Unionised Ammonia (UIA) have been assessed

and their impacts due to the project works predicted. Based on the assessment, the parameters NH3-N

and UIA were found to be of concern in certain parts of the Project Area, details of which are further

elaborated in the following subsections.

Appropriate mitigation measures have been recommended to minimise the potential adverse impacts and

to ensure the acceptability of any residual impact, i.e., after mitigation. For the concerned areas with high

NH3-N and UIA values, further confirmatory trials have been proposed to ensure works within these areas

do not contribute to adverse impacts to the receiving environment. In addition, cumulative impacts of

concurrent projects have also been considered in the assessment in order to assess the overall impacts of

the works on the surrounding environment.

3. Water Quality Impact Assessment



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3.2 Environmental Legislation, Standards, Guidelines and Criteria

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

� Technical Memorandum on Environmental Impact Assessment Process (Environmental Impact

Assessment Ordinance) (EIAO-TM);

� Water Pollution Control Ordinance (WPCO);

� Water Supplies Department (WSD) Water Quality Criteria;

� Electrical and Mechanical Services Department (EMSD) Code of Practice for Water-cooled Air

Conditioning Systems Part 2: Operation and Maintenance of Cooling Towers; and

� Other miscellaneous criteria (as described below)

3.2.1 Environmental Impact Assessment Ordinance (EIAO)

The EIAO-TM was issued by the EPD under Section 16 of the EIAO. It specifies the assessment method

and criteria that have been followed in this EIA Study. Reference sections in the EIAO-TM providing details

of the assessment criteria and guidelines that are relevant to the water quality impact assessment include

the following:

� Annex 6: Criteria for Evaluating Water Pollution; and

� Annex 14: Guidelines for Assessment of Water Pollution.

3.2.2 Water Quality Objectives (WQOs)

The Water Pollution Control Ordinance (Cap. 358) provides the statutory framework for the protection and

control of water quality in Hong Kong. According to the WPCO and its subsidiary legislation, Hong Kong

waters are divided into ten Water Control Zones (WCZs). Corresponding statements of WQOs are

stipulated for different water regimes (including marine waters, inland waters, gazetted beaches subzones,

secondary contact recreation subzones, and fish culture subzones) in the WCZs based on their beneficial

uses. The WQOs for the Victoria Harbour, Western Buffer and Southern WCZs are listed in Table 3.1.

Table 3.1: Summary of Water Quality Objectives

Parameters Victoria Harbour WCZ Western Buffer WCZ Southern WCZ

Dissolved Oxygen (DO) (bottom)

Not less than 2.0 mg L-1 for 90% of samples



Depth-averaged DO Not less than 4.0 mg L-1 for 90% of samples


(Not less than 5.0 mgL-1 for 90% of samples for fish culture subzones)


(Not less than 5.0 mgL-1 for 90% of samples for fish culture subzones)

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

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

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

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

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

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

Unionised ammonia Annual mean not to exceed 0.021 mg L-1

Annual mean not to exceed 0.021 mg L-1

Annual mean not to exceed 0.021 mg L-1

Temperature Change due to waste discharge not to exceed 2oC

Change due to waste discharge not to exceed 2oC

Change due to waste discharge not to exceed 2oC



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Parameters Victoria Harbour WCZ Western Buffer WCZ Southern WCZ

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

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

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

Nutrients Annual mean depth-averaged total inorganic nitrogen not to exceed 0.4 mg L-1

Annual mean depth-averaged total inorganic nitrogen not to exceed 0.4 mg L-1

Annual mean depth-averaged total inorganic nitrogen not to exceed 0.1 mg L-1

Toxicants Not to be present at the levels producing significant toxic effect

Not to be present at the levels producing significant toxic effect

Not to be present at the levels producing significant toxic effect

Source: Statement of Water Quality Objectives.

3.2.3 Water Supplies Department (WSD) Water Quality Criteria

Besides the WQOs set under the WPCO, WSD has specified a set of objectives for water quality at flushing

water intakes as shown in Table 3.2.

Table 3.2: WSD’s Water Quality Criteria for Flushing Water at Sea Water Intakes

Parameter (in mg L-1 unless otherwise stated) Target Limit

Colour (HU) < 20

Turbidity (NTU) < 10

Threshold Odour Number (odour unit) < 100

Ammonia Nitrogen (NH3-N) < 1

Suspended Solids (SS) < 10

Dissolved Oxygen (DO) > 2

5-day Biochemical Oxygen Demand (BOD5) < 10

Synthetic Detergents < 5

E. coli (no. per 100 mL) < 20,000

Note: (1) This criteria has been taken as reference when assessing water quality at flushing water intakes. Note that not all

parameters were taken for the assessment. (2) The WSD criteria has been used as it more stringent than EMSD Code of Practice for Water-cooled Air Conditioning

System in terms of suspended solids (3) Insignificant impact due to the presence of E.coli is anticipated due to the fact that E.coli is not detected in sediment

samples taken for the Project.

3.2.4 Water Quality Criteria for Cooling Water Intakes

The Mass Transit Railway Corporation (MTRC) stipulates a limit on SS at its cooling water intakes. For the

MTRC South intake the SS limit is 40 mg L-1

. According to the recently approved EIA for the Hong Kong

Convention and Exhibition Centre Atrium Link Extension (EIA 120/2006) there are no other criteria for

cooling water intakes within the area of influence of this project, however reference should be made to

Section 3.2.5 in connection with the Kwai Chung Hospital intakes.

3.2.5 Water Quality Criteria for Water-Cooled Air Conditioning Systems

EMSD’s Code of Practice for Water-Cooled Air Conditioning Systems “Part 2: Operational and

Maintenance of Cooling Tower”, sets a series of standards associated with intake water. The pertinent

standard is SS for which the criterion is <180 mg L-1

. This criterion could be applied to the Kwai Chung

Hospital which is a water sensitive receiver (WSR) for this Project. However, as the intakes at the Kwai



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Chung Hospital also include a salt water flushing water intake, the WSD’s criterion for SS has been used at

this WSR as it is more stringent than the aforementioned Code of Practice.

3.2.6 Suspended Solids Criterion for Benthic Organisms

It is acknowledged that benthic organisms, including corals, may suffer damage to their respiratory function

as a result of sediment deposition blocking the respiratory and feeding organs of these organisms.

According to Hawker and Connell1, a sedimentation rate higher than 0.1 kg m

-2 per day would introduce

moderate to severe impact upon corals. As such, this limit was adopted as the assessment criterion for

protecting the marine ecological sensitive receivers in this Study. While there are no established legislative

criteria for water quality for corals, an elevation criterion of SS levels less than 30% of ambient baseline

conditions1 has been adopted as the critical value, above which impacts to the habitat may occur. This

criterion is based on that given in a previously approved EIA2 for assessing SS impacts on corals.

3.2.7 Suspended Solids Criterion for Fish Culture Zones

In the fish culture zones the WQO for SS applies, i.e., elevation of SS should be less than 30% of ambient

baseline conditions. In addition, it is noted that the criterion for maximum SS concentration as

recommended by AFCD at the fish culture zones, based on international marine water quality guidelines for

the protection of ecosystems (based on half of the “no observable effect concentrations”), is 50 mg L-1

(CityU’s CCPC 2001).

3.2.8 Unionised Ammonia and Ammonia-nitrogen Criteria for Fisheries and Marine

Ecology in Open Sea and Estuarine Regions

The unionised portion of total ammonia is an important environmental parameter that may cause toxicity to

fish and marine biota and this relates to pH and temperature. The guidelines for protection of marine

ecology follow the Water Quality Objective (WQO) under the Water Pollution Control Ordinance (Cap. 358).

The WQO for unionised ammonia (UIA) is 0.021 mg L-1

and this is the prevailing requirement to be met. In

addition to the WQO for unionised ammonia, the lethal concentrations of ammonia in the form of

ammoniacal-nitrogen (NH3-N) on fisheries and marine ecology may also be derived based on the Hong

Kong toxicity data with reference to international guidelines. The recommended maximum concentration to

protect 80% marine biota according to the water quality assessment guidelines (WQAG) for NH3-N within

Hong Kong marine waters corrected to pH 8.0 is 1.2 mg L-1

(CityU’s CCPC 2001). This figure should be

adjusted for the pH at specific site according to Table 4.10 of the Consultancy Study on Fisheries and

Marine Ecological Criteria for Impact Assessment (City U’s CCPC 2001). For the recommended continuous

concentration to protect 90% of the marine biota a value of 0.7 mg L-1

corrected to pH 8.0 has been

adopted in this EIA using the aforementioned references.

_________________________

1 Hawker, D. W. and Connell, D. W. (1992). “Standards and Criteria for Pollution Control in Coral Reef Areas” in Connell, D. W and

Hawker, D. W. (eds.), Pollution in Tropical Aquatic Systems, CRC Press, Inc.

2 Maunsell Consultants Asia Ltd. (2001), Environmental Impact Assessment for Tai Po Sewage Treatment Works – Stage V, Final EIA

Report, For Drainage Services Department, Hong Kong SAR Government.



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3.2.9 Sediment Quality

Marine disposal of dredged materials is controlled under the Dumping at Sea Ordinance (DASO). The

requirements for marine disposal of sediment are specified in ETWB TCW No. 34/2002: Management of

Dredged/Excavated Sediment. Dredged sediment is classified according to a set of regulatory guidelines

with sediment quality criteria, which include organic pollutants and other toxic substances. Details on

marine dredged sediment quality are presented in Chapter 4.

3.3 Description of the Environment

3.3.1 Marine Water Quality Monitored by EPD

Marine water quality monitoring data routinely collected by EPD was used to establish the baseline

conditions for this Project. A summary of water quality data for the selected and relevant EPD monitoring

stations is presented in Table 3.3 and Table 3.4 for the Victoria Harbour (VM6, 8, 12, 14 and 15), Western

Buffer (WM1 and 4) and Southern (SM3, 5 and 9) WCZs. Locations of these monitoring stations are shown

in Figure 3.1.

Table 3.3: Marine Water Quality in Victoria Harbour Water Control Zone at Selected Stations in 2008

Parameter Rambler Channel Victoria Harbour (West)

Victoria Harbour (Central)

Stonecutters Island

VM12 VM14 VM8 VM6 VM15

Temperature

(°C)

22.7

(14.9 – 27.7)

23.0

(14.8 - 28.5)

22.6

(14.7 - 27.6)

23.5

(16.3 -27.2)

23.6

(16.5 - 27.2)

Salinity 30.8

(26.8 – 32.9)

29.6

(22.5 – 32.7)

31.1

(27.1 - 33.2)

30.9

(27.1 – 32.7)

30.6

(24.0 – 32.8)

Depth average

5.4 (3.2 – 8.0) 5.6 (3.9 – 7.9) 5.8 (3.9 – 7.7) 5.1 (2.8 -7.1) 5.2 (2.9 – 6.3) Dissolved Oxygen (DO)

(mg L-1) Bottom 5.4 (2.8 – 8.0) 5.6 (3.4 – 8.0) 5.7 (3.5-8.1) 4.8 (2.5 – 6.6) 4.9 (2.3 – 6.2)

Suspended Solids (SS)

(mg L-1)

10.2

(6.2 – 16.5)

5.7

(3.0 – 8.2)

5.0

(2.7 – 8.6)

5.3

(2.7 – 7.3)

6.1

(2.8 – 9.2)

Unionised Ammonia (UIA)

(mg L-1)

0.007

(0.003 - 0.009)

0.006

(0.002 - 0.009)

0.007

(0.001 - 0.013)

0.007

(0.002 - 0.011)

0.008

(0.003 - 0.014)

Total Inorganic Nitrogen (TIN) (mg L-1)

0.40

(0.27 - 0.53)

0.46

(0.30 - 0.80)

0.35

(0.22 – 0.51)

0.38

(0.21-0.62)

0.42

(0.25 - 0.87)

Note: 1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: surface,

mid-depth, bottom. 2. Data enclosed in brackets indicate the ranges. (Source: Adopted from EPD Marine Water Quality Hong Kong in 2008)



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Table 3.4: Marine Water Quality in Southern and Western Buffer Water Control Zones at Selected Stations in 2008

Southern WCZ Western Buffer WCZ

Parameter East Lamma Channel

SM3

West Lamma Channel

SM5

West Lamma Channel

SM9

Hong Kong Island West

WM1

Tsing Yi West WM4

Temperature

(°C)

22.5

(14.2 – 26.8)

23.4

(14.2 – 28.9)

22.8

(14.2 – 27.2)

22.3

(14.7 -27.1)

22.6

(14.8 – 27.3)

Salinity 31.9

(28.2 – 33.7)

30.3

(20.8 – 33.7)

29.8

(19.5-32.9)

32.0

(29.9 – 33.6)

31.2

(29.3 – 33.0)

Depth average

6.2

(3.7 – 9.4)

6.9

(5.2 – 9.8)

6.3

(4.3 – 8.8)

6.1

(2.7 – 9.1)

5.7

(3.4 – 8.2) Dissolved

Oxygen (DO)

(mg L-1) Bottom 5.9

(2.8 – 9.9)

6.2

(3.2 – 9.7)

6.1

(3.7 – 8.7)

5.9

(2.1 -9.3)

5.6

(2.7 -8.3)

Suspended Solids (SS)

(mg L-1)

4.8

(2.3 – 11.0)

6.2

(1.9 – 17.0)

7.6

(2.6 – 16.7)

5.5

(1.7 – 13.1)

6.4

(3.0 – 12.1)

Unionised Ammonia (UIA)

(mg L-1)

0.002

(<0.001 – 0.005)

0.002

(<0.001 – 0.006)

0.006

(0.001 – 0.020)

0.002

(<0.001 – 0.004)

0.004

(0.002 – 0.008)

Total Inorganic Nitrogen (TIN) (mg L-1)

0.18

(0.03 – 0.42)

0.24

(0.04 – 0.94)

0.37

(0.11 – 1.15)

0.18

(0.06 – 0.29)

0.32

(0.17 – 0.47)

Note:

1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: surface,

mid-depth, bottom.

2. Data enclosed in brackets indicate the ranges.

(Source: Adopted from EPD Marine Water Quality Hong Kong in 2008)

As reported in the EPD’s “Marine Water Quality in Hong Kong 2008”, the Victoria Harbour WCZ achieved

an 80% compliance with the WQOs in 2008, which was lower than that in 2007. The decrease in

compliance was mainly a reflection of an increase of TIN levels in the harbour.

In contrast, the Western Buffer WCZ achieved 92% compliance with the WQOs in 2008, higher than that in

2007 (83%), whilst the Southern WCZ attained an overall 65% compliance, lower than that in 2007 (73%).

The TIN levels at stations such as SM9 have increased by 48% from late 1980s to 2008.

3.3.2 Sediment Quality

Results and discussions on the sediment quality analysis from the marine site investigation (SI) works for

the Project area are detailed in Chapter 4. A summary of sediment quality data for the selected EPD

monitoring stations in the vicinity of the Rambler Channel (VS9), Tsing Yi (South) (WS1) and Hong Kong

Island (West) (WS2), as extracted from EPD’s “Marine Water Quality in Hong Kong 2008”, is presented in

Table 3.5.

During a recent sampling exercise, sediment samples were collected and analysed for E.coli (Appendix

11.1). The sampling locations were near the Tsing Yi Submarine Sewage Outfall (Figure 3.6), in the

Northern Fairway and Western Fairway. The results indicated that the E.coli concentrations are below

detection limit (< 3 MPN/g) for all locations. Therefore, the dredging activities of the Project are not

expected to induce any adverse impact of E.coli on the water bodies or at the gazetted beaches within the

Project’s area of influence.



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Table 3.5: Bottom Sediment Quality at Selected Stations in 2004 – 2008

Sediment Quality Criteria Contaminants

Rambler Channel

VS9

Tsing Yi (South)

WS1

Hong Kong Island (West)

WS2 LCEL UCEL

Cadmium (Cd) 0.2

(<0.1 – 0.7)

0.1

(<0.1 – 0.2)

<0.1

(<0.1 – <0.1) 1.5 4

Chromium (Cr) 37

(27 – 76)

33

(17 - 41)

36

(32 – 40) 80 160

Copper (Cu) 67

(8 – 230)

47

(13 - 68)

26

(22 - 29) 65 110

Mercury (Hg) 0.11

(<0.05 – 0.23)

0.13

(0.06 – 0.160)

0.09

(<0.05 – 0.14) 0.5 1

Nickel (Ni) 24

(19 – 43)

19

(9 – 24)

24

(21 – 26) 40 40

Lead (Pb) 35

(21 – 81)

39

(24 - 53)

38

(32 – 43) 75 110

Silver (Ag) 1.1

(<0.2 – 6.3)

1.0

(0.2 – 1.7)

0.4

(0.3 – 0.5) 1 2

Zinc (Zn) 100

(66 – 210)

110

(53 – 130)

110

(95 – 120) 200 270

Metalloid (mg/kg dry weight)

Arsenic (As) 5

(3.0 – 8.5)

7.3

(3.8 – 9.4)

8.7

(6.9 – 10.0) 12 42

Organic-PAHs (µg/kg dry weight)

PAHs (Low Molecular Weight)

91

(90 – 100)

94

(90 – 100)

91

(90 – 94) 550 3160

PAHs (High Molecular Weight)

58

(18 – 180)

160

(68 – 420)

59

(35 – 120) 1700 9600

Organic-non-PAHs (µg/kg dry weight)

Total PCBs 19

(18 – 23)

18

(18 – 18)

18

(18 – 18) 23 180

Based on EPD’s monitoring data in 2004 - 2008, the sediment collected at the Rambler Channel VS9 are

contaminated with heavy metals which exceeded the criteria for copper, nickel, lead, silver and zinc.

However, the sediment quality was relatively better for Station WS2 located to the west of Hong Kong

Island.

3.4 Water Sensitive Receivers

In order to evaluate the potential water quality impacts from the Project, water quality sensitive receivers

(WSRs) near the Project area were identified, which include:

� Gazetted Beaches;

� Corals;

� Fish Culture Zones;

� WSD Flushing Water Intakes; and

� Cooling Water Intakes.



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Water quality and ecological sensitive receivers identified near the Project area are shown in Figure 3.1.

These WSRs have been included in the water quality impact assessment as they may be potentially

affected by the proposed dredging works (Condition 3.4.3.2 of the Study Brief).

3.5 Assessment Methodology

The assessment methodology was undertaken in accordance with the requirement of Condition 3.4.3.4 of

the Study Brief.

3.5.1 Hydrodynamic and Water Quality Models

3.5.1.1 Set-up of Hydrodynamic Model

Computer modelling was employed to predict the potential impact on water quality for different tidal

conditions. The hydrodynamic and water quality models were developed by Delft Hydraulics, namely

Delft3D-FLOW and Delft3D-WAQ respectively.

In this study, the basis for modelling of the harbour waters was the existing, validated Western Harbour

Model, with computational grid layout presented in Figure 3.2. This model covers the relevant parts of the

Hong Kong waters, including the Pearl Estuary and the Dangan (Lema) Channel. The grid mesh was

further refined locally to generate higher resolution in the vicinity of the proposed Project area (Figure

3.3a).

3.5.1.2 Coastlines and Bathymetry

To ensure that an accurate coastline profile was used in the modelling, the existing model was updated

with the most recently available information of proposed development from sources such as designated

projects under the EIAO (Figure 3.3b). The bathymetry set up was also updated based on the latest

Admiralty Chart.

3.5.1.3 Simulation Periods

The simulated periods covered a complete spring-neap tidal cycle and the actual simulation period was

preceded by a spin-up period. The simulation periods are specified below:

Dry season: 9 February 12:00 – 24 February 12:00 Wet season: 26 July 04:00 – 10 August 04:00

In order to determine if the spin-up period was sufficient for the simulation, the model was run for one more

spring-neap cycle. It was determined that the results of the two successive model runs were similar, and as

such, a spin-up period of 15 days was adopted for this Project.

3.5.1.4 Initial and Boundary Conditions for Water Quality Models

A spin-up period of 15 days for the sediment plume modelling described below was included. This spin-up

period is considered sufficient since comparable results between two consecutive spring-neap cycle model



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runs were obtained. The boundary conditions were set to zero as the excess suspended solids

concentrations were modelled.

3.5.1.5 Current flow

From the results of hydrodynamic model, the current flow patterns for dry season and wet season are

obtained and are shown in Figure 3.4a and Figure 3.4b respectively.

3.5.1.6 The Mixing Zone Criteria

Section 1.3.1 of the Annex 6 of the Technical Memorandum (TM) under the EIAO provides guidelines on

the determination of mixing zones. According to the TM it is not always necessary to meet all water quality

criteria in all areas and the Authority under the WPCO may allow for the receiving water quality not to meet

water quality criteria in particular circumstances.

The mixing zone defined in this project is the region of marine water where initial dilution of a pollution input

takes place and where water quality criteria will be exceeded. These areas are subject to greater impacts

from the project activity. The mixing zones for this Project are discussed in Section 3.7.1.6.

3.5.2 Sediment Plume Modelling

3.5.2.1 General

Water quality impacts arising from dredging activities associated with the proposed Project that would

disturb the marine bottom sediment include the release of SS into the water column. The impact of

sediment plume dispersion during the marine works was simulated using a three-dimensional Delft3D-

WAQ Model. The WAQ model simulated suspended solids (SS, in mg L-1

), optionally subdivided over

different fractions representing different sediment sources. The simulated SS represented the Project

related discharges only. The calculated concentrations were interpreted as excess concentrations on top of

the background concentrations.

The Delft3D-WAQ model takes into account the sedimentation process by means of a settling velocity,

while erosion of bed sediment, causing resuspension of sediment, is governed by a function of the bed

shear stress. The parameters adopted in the present study are summarised in Table 3.6.

Table 3.6: Summary of Parameters for Sediment Plume Model (Delft3D-WAQ)

Sediment Plume Model Parameters3

Settling velocity 0.5mm/s

Critical shear stress for deposition 0.2N/m2

Critical shear stress for erosion 0.3N/m2

Minimum depth where deposition allowed 0.1m

Resuspension rate 30g/m2/d

_________________________ 3 Mott Connell (2007), Laying of Western Cross Harbour Main and Associated Land Mains from West Kowloon to Sai Ying Pun, Final

EIA Report



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For this Project, the impacts in terms of dissolved oxygen (DO) depletion and total inorganic nitrogen (TIN)

were not explicitly modelled, but estimated on the basis of the calculated sediment concentrations. This

leads to an estimated increase relative to the background of the concentrations of this parameter,

dependent on the quality of the released sediment.

For NH3-N (a component of TIN) and UIA, it has been found from elutriate tests of collected sediment

samples for the purpose of this Study (as discussed in Section 3.7) that some high values of ammoniacal-

nitrogen (NH3-N) were found at some of the sampling stations. As these measured values implied that high

UIA values (also dependent on pH, temperature and salinity of seawater) would also be expected, a more

direct approach for predicting the potential UIA values resulting from the dredging works been used to

assess the potential UIA impacts as discussed later in this section. However, for the purposes of indicating

the background levels of TIN and UIA of the waters within the Study Area, the established methodology for

estimating these ambient conditions has been adopted.

For TIN, it was assumed that the Kjedahl-N (KN) content of the sediment is transformed to TIN in water.

For DO, it was assumed that the entire sediment oxygen demand (SOD) content of the sediment is

transformed to give a decrease in DO concentration. This can be expressed as follows:

( , , , ) ( , , , ) ( )TIN x y z t SS x y z t C C∆ = ∆ × +, 4 ,

( , , , ) ( , , , ) ( )SS NH SS Kj N

TIN x y z t SS x y z t C C−

∆ = ∆ × +

,( , , , ) ( , , , )

SS CODDO x y z t SS x y z t C∆ = −∆ ×

where

TIN concentration of total inorganic nitrogen (mgN/L)

SS concentration of suspended solids (mg L-1

)

CSS,Kj-N concentration of Kjedahl-N in suspended matter (gN/gSS)

DO concentration of dissolved oxygen (mg L-1

)

CSS,SOD concentration of SOD in suspended matter (gO/gSS)

This approach adopts a set of the worst case assumptions. Any removal of pollutants from the water phase

along with the sedimentation of SS and any replenishment of DO from the atmosphere is neglected.

The values used for the DO assessment, as well as for estimating background TIN values, are based on

EPD’s routine marine sediment quality monitoring data recorded at VS9 in 2004 - 2008 near the dredging

area and are summarised in Table 3.7. The DO depletion calculation was performed using the highest level

of SOD measured in the sediment samples collected during the marine SI for conservative predictions. The

highest SOD level was 3620 mgO2/kg.

Table 3.7: Sediment Quality near the Dredging Area

Parameters Dry season Wet season

CSS,Kj-N 296E-6 314E-6

For the estimation of potential UIA values for this EIA,a more direct approach was used to calculate UIA

from elutriate test results, where the ammoniacal nitrogen values were first analyzed, and then the

SOD



3-11

following equations4 used to calculate UIA taking into account of the in-situ temperature, salinity and pH of

the seawater:

For salinity > 18 ppt UIA = NH3-N x F F = 1 / (10

pKas – pH + 1)

pKas = pKaf + I x (0.1552 – 0.000314 x T) pKaf = 0.09018 + 2729.92 / (T + 273.15) I = 19.9273 x S / (1000 – 1.005109 x S) Where,

S – Salinity (ppt) T – Temperature (°C) pH – Power of Hydrogen F – Fraction of Unionized Ammonia (UIA) NH3-N – Ammoniacal Nitrogen UIA – Unionized Ammonia

The above approach is deemed more appropriate for this Project as the prediction for the potential UIA

impacts would then tend to be more representative of the actual conditions of the project site.

3.5.2.2 Concurrent Projects

Assessment of cumulative impacts due to other concurrent projects was undertaken according to the

requirements of Condition 3.4.3.5 (x) of the Study Brief.

In order to assess the cumulative impacts of this Project on the receiving environment, the impacts of other

dredging projects which may be implemented concurrently were also considered in the assessment. To

establish the modelling scenarios it was therefore important to consider the overlap of the construction

programmes for this Project and the concurrent projects.

The dredging operations for this Project are tentatively scheduled to take place between mid 2011 and

2013, and details of the concurrent projects have been identified in Section 2.8.

3.5.2.3 Modelling Scenarios

Scenarios representing the likely “worst case scenarios” for dredging operations were developed. A total of

seven scenarios were derived taking into consideration all the WSRs and the potential impacts of dredging

on them, and noting the large areal extent of the Project areas shown in Figure 3.5a. Scenarios 1 to 5

represent those using grab dredgers working in different parts of the Project area, Scenario 6 represents

the use of two grab dredgers and one cutter suction dredger, and Scenario 7 represents the scenario for

_________________________ 4 Bower C.E. and Bidwell J.P. (1978), Ionization of ammonia in seawater: Effect of temperature, pH and salinity. J. Fish. Res. Board

Can. Vol.35, pp.1012-1016;

K., Russo R.C. & et. al. (1975), Aqueous ammonia equilibrium calculations: effect of pH and temperature. J. Fish. Res. Board Can. Vol.32, pp.2379-2383;

K.H. Khoo, C.H. Culberson, and R.G. Bates, Thermodynamics of the dissociation of ammonium ion in seawater from 5 to 40°C, Journal of Solution Chemistry, 6:281-290, 1977.



3-12

cumulative impacts, i.e., looking at the combined impacts of this Project and the concurrent projects that

were previously identified. The above scenarios are described below:

Scenario 1 – Dredging at Source ID ‘A’, ‘C’ and ‘D’

Scenario 1 assumes three grab dredgers operating simultaneously within the Project area from 2011 to

2013, with the locations of the dredgers as shown in Figure 3.5b.

Scenario 2 – Dredging at Source ID ‘A’, ‘C’ and ‘E’


2013, with the locations of the dredgers as shown in Figure 3.5c.

Scenario 3 – Dredging at Source ID ‘C’, ‘D’ and ‘E’


2013, with the locations of the dredgers as shown in Figure 3.5d. Scenario 3 was developed to assess the

potential impacts on gazetted beaches along Tsuen Wan and Castle Peak Road, as well as the Ma Wan

Fish Culture Zone.

Scenario 4 – Dredging at Source ID ‘A’, ‘B’ and ‘D’


2013, with the locations of the dredgers as shown in Figure 3.5e.

Scenario 5 – Dredging at Source ID ‘A’, ‘B’ and ‘C’


2013, with the locations of the dredgers as shown in Figure 3.5f. Scenario 5 represents the worst case for

the WSRs located at Tsing Yi and Tsuen Wan.

Scenario 6 – Dredging with Cutter Suction Dredger at Source ID ‘A’

Under this scenario, one cutter suction dredger (CSD) will be used for the removal of some hard material

(approximately 92000 m3) in the Rambler Channel near Container Terminal No. 5, located northeast of the

Project area, along with two grab dredgers elsewhere within the Project area.

Scenario 6 assumes the three dredgers to be operating simultaneously from 2011 to 2013, with the CSD

located at Source ID ‘A’, and the two grab dredgers at Source ID ‘C’ and ‘E’, respectively, as shown in

Figure 3.5g. The location of the other two dredgers in this scenario was decided based on the suspended

solids results obtained from Scenarios 1 to 5, which suggested that grab dredgers located at Source ID ‘C’

and ‘E’ tend to produce the highest levels of suspended solid at the WSRs near Tsing Yi and Tsuen Wan.

Scenario 7 – Dredging at Concurrent Project Areas

Scenario 7 simulates the impact from concurrent projects operating in the selected year of 2012 and

Scenario 3. Although Table 2.5 indicated concurrent activities between mid 2011 to mid 2013, year 2012

was selected for this scenario because 2012 was considered to represent the worst year, in terms of the

year with the highest sediment loss rates for the majority of the concurrent projects. In addition, the SS



3-13

impacts predicted under Scenario 3 on the gazetted beaches along Tsuen Wan and Castle Peak Road,

and Ma Wan Fish Culture Zone were also the highest.

The locations of the sediment sources arising from the concurrent projects are shown in Figure 3.5h. The

following marine works have been incorporated:

(i) Dredging works in Wan Chai North and Causeway Bay under the ‘Wan Chai Development

Phase II and Central – Wan Chai Bypass’ project (Source ID: W1, W2, W3, W4)

(ii) Installation of Submarine Gas Pipelines and Associated Facilities from To Kwa Wan to North

Point for Former Kai Tak Airport Development (Source ID: TO1)

(iii) Shatin to Central Link (Source ID: SC1, SC2)

(iv) Marine works north of Chek Lap Kok Airport as part of the project ‘Existing Dredging/ Filling of

the Contaminated Mud Pits in East Sha Chau’ (Source ID: S1, S2)

(v) Marine works of the project ‘Proposed Dredging Works of the Contaminated Mud Pits in South of

Brothers’ (Source ID: SB1, SB2)

(vi) Dredging works near Tuen Mun and Chep Lap Kok Airport under the ‘Tuen Mun – Chek Lap Kok

Link (TM-CLKL)’ project (Source ID: T1, T2, T3, T4, T5)

(vii) Dredging works east of Chek Lap Kok Airport as part of the ‘HZMB Hong Kong Boundary

Crossing Facilities (HKBCF)’ project (Source ID: B1, B2, B3, B4, B5, B6)

(viii) Dredging works east of Chek Lap Kok Airport as part of the ‘HZMB Link Road (HKLR)’ project

(Source ID: L1, L2, L3)

The sediment source locations (Source ID: W1, W2, W3, W4) were based on the spill locations used in the

Wan Chai Development Phase II and Central – Wan Chai Bypass EIA as part of their construction phase

modelling.

The sediment source locations (Source ID: T1, T2, T3, T4, T5, B1, B2, B3, B4, B5, B6, L1, L2, L3, S1, S2,

SB1, SB2) were based on the marine plant locations used in the EIA for HZMB as part of their concurrent

projects assessment.

Alternative Scenario

The “Alternative Scenario” is a hypothetical scenario that assumes five grab dredgers operating

simultaneously at Source ID: A, B, C, D and E (shown in Figure A3.2.1 in Appendix 3.2). This scenario

has been assessed for comparison purposes only, as the scenario is actually considered not practical due

to the busy marine traffic within the Project area. Details of this scenario are provided in Appendix 3.2.

The seven main modelling scenarios for dredging operations are summarised in Table 3.8 and Table 3.9

below.



3-14

Table 3.8: Summary of Assumptions for Scenarios 1 to 6

Scenario Locations Nos. of Dredgers

Dredging Rate m3/day/dredger

Sediment Loss Rate kg/s

Dredging Period

1 Source ID A, C, D 3 grab dredgers 4000*1 1.44 2011 to 2013

2 Source ID A, C, E 3 grab dredgers 4000*1 1.44 2011 to 2013

3 Source ID C, D, E 3 grab dredgers 4000*1 1.44 2011 to 2013

4 Source ID A, B, D 3 grab dredgers 4000*1 1.44 2011 to 2013

5 Source ID A, B, C 3 grab dredgers 4000*1 1.44 2011 to 2013

6 Source ID A, C, E 2 grab dredgers and one CSD

4000*1 for grab dredger, 700 m3/30 mins for CSD (8400

m3/day) *2

1.44 for grab dredger, 2.72 for

CSD

2011 to 2013

* Detailed description of sediment release rates in Section 3.5.2.4 refers. Closed grab dredgers were assumed.

*1 in-situ volume

*2 the CSD operates for 30 minutes over a 1-hr period based on 12 hours (7:00 – 19:00) operation.

Table 3.9: Summary of Assumptions for Scenario 7

Project Source ID

Activity (Equipment)

Number of

Dredgers

Daily Working Hours

Working Rate1

Cycle Time Duration of Each

Operation (minutes)

Sediment Loss

Rate kg/s

The current project

(Scenario 3 in Table 3.8)

C, D, E Dredging (Grab Dredger)

3 24 4000 continuous - 1.44

Hong Kong Boundary Crossing Facility

B1 Filling (Dump Barge)

4 16 769 120 minutes2

5 60.9

B2 Filling (Pelican Barge)

7 16 769 continuous2 - 1.2

B3 Dredging (Grab Dredger)

2 16 6000 continuous - 4.2

B4 Filling (Dump Barge)

6 16 769 60 minutes2 5 60.9

B5 Filling (Pelican Barge)

14 16 769 continuous - 1.6

B6 Dredging (Grab Dredger)

4 16 6000 continuous - 8.3

L1 Filling (Dump Barge)

5 16 769 90 minutes2 5 12.2

L2 Filling (Pelican Barge)

9 16 769 continuous2 - 0.2

HZMB Hong Kong Link

Road

L3 Dredging (Grab Dredger)

3 16 6000 continuous - 6.3

SB1 Filling (Barge)4,5 33.3 24 800 40 minutes 5 60 South of Brothers3

SB2 Dredging (Grab Dredger)

2 24 7143 continuous - 2.8

S1 Dredging (Grab Dredger)

2 24 7143 continuous - 2.8 East of Sha Chau CMP3

S2 Capping (Barge)5 33.3 24 800 40 minutes 5 60



3-15

Project Source ID

Activity (Equipment)

Number of

Dredgers

Daily Working Hours

Working Rate1

Cycle Time Duration of Each

Operation (minutes)

Sediment Loss

Rate kg/s

T1 Dredging (Grab Dredger)

1 16 6000 continuous - 2.1

T2 Filling (Dump Barge)

6 16 769 60 minutes2 5 33.5


1 16 769 8 hours 5 33.5

T4 Filling (Pelican Barge)

2 16 769 4 hours 45 0.7

Tuen Mun Chek Lap Kok Link


3 16 769 3 hours 5 60.9

W1 Dredging (grab) 1 16 6000 continuous - 0.526



Wan Chai Development

Phase II


SC1 Dredging (grab) 2 16 3000 continuous - 2.08 Shatin-Central Link

SC2 Dredging (grab) 1 12 500 continuous - 0.23

Towngas Underwater

Pipeline

TO1 Dredging (grab) 1 16 4000 continuous - 1.39

Notes:

1. The working rate is per grab/TSHD (m3/day) or per barge/event (m

3).

2. Higher frequencies have been assumed compared to the HZMB approved EIA as a conservative approach.

3. Activities in South of Brothers and East of Sha Chau Contaminated Mud Pits were according to CEDD-MFC information

4. No backfilling activities by TSHD are anticipated according to CEDD-MFC information.

5. Figures in New Contaminated Mud Marine Disposal Facility at Airport East / East Sha Chau Area (EIA - 106/2005) refers.

6. Based on values presented in Table 5.10 of Dredging Works for Proposed Cruise Terminal at Kai Tak approved EIA report

In view of the busy marine traffic in the Rambler Channel and Northern Fairway, it is not practical to

assume more than three dredgers working simultaneously within the combined areas. Similarly, a

maximum of two dredgers has been assumed to be operating within the Western Fairway at any one time,

as heavy marine traffic would make it impractical to operate more than two dredgers simultaneously within

the Western Fairway area. The sediment loss rates of the dredgers for the dredging scenarios 1 to 6 within

the Project area as shown in Table 3.8 are further discussed in Section 3.5.2.4.

3.5.2.4 Sediment Loss Rates

The loss rates have been provided according to the requirement of Condition 3.4.3.5 (vi) of the Study Brief.

In the modelling scenarios it was assumed that grab dredger (GD) with a closed grab of 8 m3 would be

used on a continuous basis (i.e. 7 days per week, 24 hours per day), on the assumption of a maximum

daily production rate of 4000 m3 (in-situ volume) per dredger. With respect to the rate of sediment loss

during dredging, the Contaminated Spoil Management Study5 concluded that losses from closed GDs were

_________________________ 5 Mott MacDonald for EPD (1991) Contaminated Spoil Management Study, Final Report.



3-16

estimated to be in the order of 11 to 20 kg m-3

of mud removed. Taking the upper value of 20 kg m-3

to be

conservative, the loss rate is then calculated to be 0.93 kg s-1

per dredger. It is noted that, however, there

are areas within the Rambler Channel where debris or large boulders could hinder proper closing of the

grab, which could result in higher loss rates. In view of this, and to be conservative in the assessment, a

higher loss rate of 1.44 kg s-1

was used for the Project. This loss rate has also been applied in a previously

approved EIA report 6 under similar circumstances. The sediment losses were assumed to be evenly

distributed over the entire water column.

The CSD will only be used for the removal of hard materials under the seabed near Container Terminal

Number 5 as identified earlier. Based on the findings of the 2000 Contaminated Spoil Management Study

as well as input from the Engineering Design Team for this Study, a maximum dredging rate of 700 m3 (in-

situ volume) in 30 minutes over 1 hr period from 7:00 to 19:00 (8400 m3/day) and a loss rate of 7 kg/ m

3 (or

2.72 kg s-1

) for the CSD operation have been assumed. It should be noted that a CSD could operate at a

much higher dredging rate. The value of 700 m3 in 30 minutes over 1 hr period adopted here is specified for

the subject Project only. The sediment losses during the operation of the CSD (via suction of sediment by

pump action during dredging) are assumed to take place in the lowest layer of the model in view of the hard

material located below the existing seabed.

3.5.2.5 Contaminant Release during Dredging

The loss of sediment to suspension during dredging may have chemical effects on the receiving waters,

due to the possibility of the sediment containing hitherto bound organic and chemical pollutants. As part of

the marine site investigation works for this Project, laboratory testing of sediment samples were

undertaken, in accordance with the requirement of Condition 3.4.3.5 (viii) of the Study Brief. A full

description of the sediment quality testing and the classification of the sediment according to the levels of

contaminants found i.e. category L, M and H are detailed in Chapter 4, with recommendations of

appropriate mitigation measures and disposal options provided according to the requirement of Condition

3.4.3.5 (xii) of the Study Brief.

Elutriate tests were performed on sediment samples to simulate and quantify the degree of mobilization of

various contaminants, such as metals and trace organic contaminants, into the water column during

dredging. An indication of the likelihood of release of heavy metals and organics from the sediment during

dredging can be deduced from the results of these tests. If the contaminant levels are higher in the

elutriates compared to the blanks (i.e. marine water from the same site), it may be surmised that the

contaminants could be released into the marine waters during dredging activities. As there is no existing

legislative standard or guideline for individual heavy metal content in marine waters, the UK’s Water Quality

Standards for Coastal Surface Water, the Australian and New Zealand Guidelines for Fresh and Marine

Waters, Environmental Economic and Best Practical Environmental Option (BPEO) Assessment Principals

for Integrated Pollution Control, and United States Environmental Protection Agency (USEPA’s) Salt Water

Criterion have been adopted as the assessment criteria for this aspect.

Where there was an exceedance of the results from the elutriate tests compared to the assessment criteria,

conservative tracers were then introduced into the model to estimate the dilution that could be generated

by the tidal flows and the size of the mixing zone of the contaminants. In such cases the release rate of

tracers followed the sediment release rate discussed in Section 3.5.2.4 at the source (discharge location)

_________________________ 6 Tai O Sheltered Boat Anchorage – EIA (EIA 042/2000)



3-17

and a concentration of 0 g/m3 was defined at all the boundaries. Since there is no decay of the tracer, the

changes in concentration of the tracers at different grid cells would be due to the advection and dispersion

of tidal flows. Comparing the concentration at the grid cell of the source (C0) and the concentration at a

selected grid cell located at the WSR (C1), the dilution rate can be obtained (C0:C1). Results of the

modelling which was undertaken using this approach are presented in Appendix 3.3 and discussed in

Section 3.7.1.6.

3.5.3 Outfall Dilution Modelling

Minor modification works at the Tsing Yi Submarine Sewage Outfall form part of the scope for this Project.

The modification works may cause a change in the performance of the Tsing Yi Submarine Sewage Outfall,

and thus, a quantitative assessment using the USEPA’s Visual Plume (UM3) model has been undertaken

to predict the effluent dilution before and after the modification works. Details of the model inputs are

provided in Appendix 3.6, including outfall specification, effluent characteristics, and ambient conditions in

wet and dry seasons, among others.

For a surface effluent plume, the minimum initial dilution is defined as the dilution obtained when the

effluent reaches the water surface. For a trapped plume, the minimum initial dilution is defined as the

dilution obtained when the plume reaches the maximum rise height.

3.6 Identification of Environmental Impact

3.6.1 Construction Phase

3.6.1.1 Impact of Suspended Sediment

As a result of dredging activities during the construction phase, fine sediment (less than 63 µm) will be lost

to suspension. The suspended sediment will be transported by currents to form sediment plumes, which

will gradually resettle. The impact from sediment plumes is to increase the suspended sediment

concentrations in the receiving water body, which could cause non-compliance with the relevant WQOs

and other criteria for particular sensitive receivers.

The extent of elevation of ambient suspended sediment concentrations would determine if the impact is

adverse, whilst the acceptability of any elevation would be dependent on compliance with the WQOs. The

WQO for SS is defined as being an allowable elevation of 30% above the background. Taking reference

from a previous study on the environmental impacts of released SS7, the ambient value is represented by

the 90th percentile of reported concentrations.

The depth-averaged and surface SS levels in 90 percentiles during dry and wet seasons are summarised

in Table 3.10. These values were derived from EPD’s marine water quality monitoring results at the routine

monitoring stations. The SS levels recorded from 2007 to 2008 were used in this Study. Since seawater

intakes are generally located near the water surface, the ambient surface SS levels were added to the

predicted SS elevations at these sensitive receivers for comparison against the relevant water quality

_________________________ 7 ERM-Hong Kong Ltd (1997). Environmental Impact Assessment for the Disposal of Contaminated Mud in the East Sha Chau

Marine Borrow Pit. Final EIA Report, For Civil Engineering Department, Hong Kong SAR Government.



3-18

criteria. It should be noted that the background level SS concentrations recorded at some stations do not

comply with the WSD’s SS criterion of 10 mgL-1

.

Table 3.10: Background Levels of Depth-averaged and Surface SS

Stations Dry Season Wet Season

Depth-

averaged Surface

Depth-averaged

Surface

VM6, VM8, VM15

Average SS (mg L-1) 5.4 4.4 5.5 4.1

90 percentile (ambient level) 8.5 7.2 9.2 6.8

30% increase above the ambient level 2.6 2.2 2.8 2.0

WM1

Average SS (mg L-1) 5.0 3.7 6.8 3.6



SM3

Average SS (mg L-1) 4.5 3.7 3.9 2.8



SM5

Average SS (mg L-1) 6.6 5.3 3.5 2.8



SM9

Average SS (mg L-1) 5.6 4.7 8.0 5.3



VM14,VM12

Average SS (mg L-1) 8.3 7.2 8.0 5.6



WM4

Average SS (mg L-1) 6.5 5.4 6.3 3.3



Note: Values in Bold indicate exceedance of WSD’s SS criterion. The corresponding values for Stations VM6, VM8, VM12, VM14 and

VM15 are presented in Appendix 3.4. Dry season is from November to March and wet season is from May to September.

3.6.1.2 Impact on Dissolved Oxygen, Total Inorganic Nitrogen and Unionised Ammonia

The extent of depletion of ambient DO concentration and elevation of ambient TIN and UIA will also

indicate if the impact is adverse or not. The determination of the acceptability of any depletion or elevation

is based on the level of compliance with the WQOs. The WQO of DO, DO bottom, TIN and UIA are defined

as being larger than or equal to 4 mgL-1

(larger than or equal to 5 mgL-1

for Fish Culture Subzones), larger

than or equal to 2 mgL-1

, less than or equal to 0.4 mgL-1

(0.1 mgL-1

in Southern WCZ) and less than or

equal to 0.021 mgL-1

respectively.



3-19

An assessment of DO depletion and nutrient release (for the purposes of estimating background levels

only) during dredging was made in relation to the results of the sediment plume modelling of dredging

activities and the sediment quality data of the Study Area. The predicted maximum elevations in tidal and

depth-averaged SS concentrations at the construction site were used to estimate the effects of increased

SS concentrations on DO and TIN. It was assumed that all SOD was exerted and that all TIN in the

sediment were released to the water.

To determine compliance with the water quality criteria, background water quality data were required. The

depth-averaged DO, TIN and UIA and bottom layer DO background levels during dry and wet seasons are

summarised in Table 3.11. The average DO, TIN and UIA values derived from the EPD’s routine marine

water quality monitoring data were used in the assessment. It is noted that the background depth-averaged

TIN concentration recorded at some stations during dry and wet seasons do not comply with the WQO for

TIN, that is less than or equal to 0.1 mg L-1

in Southern WCZ and less than or equal to 0.4 mg L-1

in the rest

of the waters.

Table 3.11: Background Levels of DO, TIN and UIA

Stations (2007 & 2008) Dry Season Wet Season

Depth-

averaged Bottom

Depth-averaged

Bottom

VM6, VM8, VM15

Dissolved Oxygen (mg L-1) 5.0 5.0 3.6 2.6

Total Inorganic Nitrogen (mg L-1) 0.32 - 0.44 -

Unionised Ammonia (mg L-1) 0.005 - 0.009 -

WM1




SM3




SM5




SM9




VM14,VM12




WM4



3-20

Stations (2007 & 2008) Dry Season Wet Season

Depth-

averaged Bottom

Depth-averaged

Bottom




Note: Values in Bold indicate exceedance of relevant criteria. The corresponding values for Stations VM6, VM8, VM12, VM14 and

VM15 are presented in Appendix 3.4. Dry season is from November to March and wet season is from May to September.

3.6.1.3 Impact on Disturbance of Seabed Sediment due to Marine Traffic

Propeller wash from vessels during construction phase could lead to potential disturbance of seabed

sediment, and could generate an elevation in SS concentrations. An assessment has been undertaken in

Section 3.7.1.7 to determine whether there is potential for cumulative impacts to be generated from this

activity and from the propeller wash from the dredgers themselves.

3.6.2 Operation Phase

As noted in Section 2.7 maintenance dredging cannot be ruled out. Even though these activities may be

infrequent, and involving relatively smaller volumes of material to be dredged, this type of dredging still

requires assessment in terms of its impacts on the receiving water quality.

The Project also involves modification works to the Tsing Yi Submarine Sewage Outfall and removal of the

Kwai Tsing Submarine Sewage Outfall (Figure 3.6). Removal of the outfall at Kwai Tsing is not expected to

induce any adverse water quality impact. The impact of the performance of the Tsing Yi Submarine

Sewage Outfall is addressed in Section 3.7.2.2. The potential impact of the deepening of seabed level in

the Rambler Channel and Northern Fairway due to the Project on the effluent dispersion of HATS outfall of

Stonecutters Island was also assessed. There are no other water quality impacts arising during the

operation phase.

3.7 Prediction and Evaluation of Environmental Impacts


3.7.1.1 Impact of Suspended Solids

The models used to simulate the dredging activities and to consider the worst-case scenarios were

described in the foregoing sub-sections. These model simulations covered typical spring and neap tidal

cycles during both dry and wet seasons in Hong Kong. The results are presented as absolute maximum

surface SS concentrations for the complete simulation period at the seawater and cooling water intakes,

taking into account the ambient SS concentrations, for all scenarios.

The predicted SS elevations and concentrations for Scenarios 1 to 6 in the dry and wet seasons at the

WSRs are detailed in Tables 3.12 to Table 3.23. The predicted SS elevations and concentrations at the

WSRs for Scenario 7 are provided in Appendix 3.8.



3-21

Table 3.12: Predicted Suspended Solids Elevations at Gazetted Beaches, Marine Ecology, and Fisheries Sensitive

Receivers for Scenario 1

Assessment Point Maximum SS Elevation (Dry Season)

Maximum SS Elevation (Wet Season)

Depth averaged Depth averaged

Sensitive Receivers

SS Elevation (mgL-1)

SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.7 3.2 0.2 2.5

Approach B2 1.0 3.2 0.5 2.5

Ting Kau B3 0.4 3.2 0.2 2.5

Lido B4 0.3 3.2 0.8 2.5

Casam B5 0.8 3.2 0.4 2.5

Hoi Mei Wan B6 0.8 3.2 0.4 2.5

Gemini B7 0.8 3.2 0.4 2.5

Angler’s B8 0.8 3.2 0.3 2.5

Lo So Shing B9 <0.1 2.8 <0.1 1.9

Hung Shing Yeh B10 <0.1 2.8 <0.1 1.9

Corals

Pak Kok CR1 0.6 2.2 0.2 3.0

Shek Kok Tsui CR2 0.6 2.2 0.2 3.0

Luk Chau CR3 0.3 1.8 <0.1 1.6

Wong Chuk Kok CR4 0.2 1.8 <0.1 1.6

Ap Lei Chau CR5 0.3 1.8 0.1 1.6

Sandy Bay CR6 0.3 2.2 0.2 3.0

Green Island CR7 0.7 2.2 0.2 3.0

Kau Yi Chau CR8 0.5 2.6 1.0 4.4

Kau Yi Chau CR9 0.5 2.6 2.3 4.4

Kau Yi Chau CR10 0.4 2.6 1.3 4.4

Siu Kau Yi Chau CR11 0.5 2.6 1.7 4.4

Siu Kau Yi Chau CR12 0.5 2.6 1.6 4.4

Siu Kau Yi Chau CR13 0.4 2.6 1.1 4.4

Peng Chau CR14 0.3 2.6 0.6 4.4

Peng Chau CR15 <0.1 2.6 0.1 4.4

Peng Chau CR16 0.3 2.6 0.5 4.4

Peng Chau CR17 0.3 2.6 0.8 4.4

Peng Chau CR18 <0.1 2.6 0.2 4.4

Fish Culture Zone

Ma Wan F1 0.6 3.2 0.4 2.5

Lo Tik Wan F2 0.1 1.8 0.1 1.6

Sok Kwu Wan F3 <0.1 1.8 <0.1 1.6

Cheung Sha Wan F4 <0.1 2.6 <0.1 4.4

Values in Bold indicate exceedance of relevant criteria



3-22

Table 3.13: Predicted Suspended Solids Concentrations at Cooling and Sea Water Intakes for Scenario 1

Sensitive Receivers Maximum (1) SS concentration in surface layer (mgL-1)

Assessment Point

SS Criterion (mgL-1)

Dry Season Wet Season

Cooling Water Intakes

Tsuen Wan C1 - 12.0 10.7

MTRC Tsing Yi Station C2 - 12.7 11.3

MTRC Kowloon Station C3 - 7.2 6.8

China H.K. City C4 - 7.2 6.8

Sha Wan Drive C5 - 5.7 5.6

Queen Mary Hospital C6 - 5.6 5.6

Wah Fu Estate C7 - 5.7 5.6

Kwai Chung Hospital *EMSD1 ,<180 11.4 11.1

WSD Flushing Water Intake

Tsing Yi WSD1 <10 13.5 11.4

Kennedy Town WSD2 <10 7.5 6.9

Sheung Wan WSD3 <10 7.2 6.9

Central Water Front WSD4 <10 7.2 6.8

Ap Lei Chau WSD5 <10 5.0 3.9

Kowloon South WSD6 <10 7.2 6.8

Cheung Sha Wan WSD7 <10 7.2 6.8

Tsuen Wan WSD8 <10 12.1 10.8

Near Hong Kong Garden WSD9 <10 10.6 4.5

Lamma Power Station WSD10 <10 8.1 4.1

Kwai Chung Hospital *EMSD1 <10 11.4 11.1

Sensitive Receivers Maximum (1) depth averaged SS concentration (mgL-1)

Assessment Point




Tsuen Wan C1 - 13.2 16.9









Tsing Yi WSD1 <10 15.2 20.3








3-23


Assessment Point




Tsuen Wan WSD8 <10 13.3 16.8




Values in Bold indicate exceedance of relevant criteria.

(1) Absolute value of SS includes the ambient SS level presented in Table 3.10 plus the SS elevations predicted.

* Note that the EMSD Code of Practice requires <180 mg L-1 at cooling water intakes. As Kwai Chung Hospital is a WSR for

both cooling water and salt water flushing intakes, the more conservative criterion has been applied, i.e. <10 mg L-1, as

required for WSD’s salt water flushing water intakes.



Sensitive Receivers





SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.8 3.2 0.2 2.5

Approach B2 1.0 3.2 0.5 2.5

Ting Kau B3 0.4 3.2 0.2 2.5

Lido B4 0.3 3.2 0.8 2.5

Casam B5 0.8 3.2 0.5 2.5

Hoi Mei Wan B6 0.8 3.2 0.4 2.5

Gemini B7 0.8 3.2 0.4 2.5

Angler’s B8 0.7 3.2 0.3 2.5

Lo So Shing B9 <0.1 2.8 <0.1 1.9

Hung Shing Yeh B10 <0.1 2.8 <0.1 1.9

Corals

Pak Kok CR1 0.5 2.2 0.2 3.0

Shek Kok Tsui CR2 0.4 2.2 0.2 3.0

Luk Chau CR3 0.3 1.8 0.1 1.6

Wong Chuk Kok CR4 0.4 1.8 <0.1 1.6

Ap Lei Chau CR5 0.4 1.8 0.1 1.6

Sandy Bay CR6 0.5 2.2 0.3 3.0

Green Island CR7 0.8 2.2 0.3 3

Kau Yi Chau CR8 0.4 2.6 0.9 4.4

Kau Yi Chau CR9 0.4 2.6 2.2 4.4

Kau Yi Chau CR10 0.4 2.6 1.2 4.4

Siu Kau Yi Chau CR11 0.4 2.6 1.7 4.4

Siu Kau Yi Chau CR12 0.4 2.6 1.6 4.4



3-24

Sensitive Receivers





SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Siu Kau Yi Chau CR13 0.3 2.6 1.1 4.4

Peng Chau CR14 0.3 2.6 0.6 4.4

Peng Chau CR15 <0.1 2.6 0.1 4.4

Peng Chau CR16 0.2 2.6 0.5 4.4

Peng Chau CR17 0.2 2.6 0.7 4.4

Peng Chau CR18 <0.1 2.6 0.2 4.4

Fish Culture Zone

Ma Wan F1 0.6 3.2 0.4 2.5

Lo Tik Wan F2 0.1 1.8 0.1 1.6

Sok Kwu Wan F3 <0.1 1.8 <0.1 1.6

Cheung Sha Wan F4 <0.1 2.6 <0.1 4.4




Assessment Point




Tsuen Wan C1 - 12.0 10.7









Tsing Yi WSD1 <10 13.5 11.4







Tsuen Wan WSD8 <10 12.1 10.8






3-25


Assessment Point




Tsuen Wan C1 - 13.2 16.9









Tsing Yi WSD1 <10 15.2 20.3







Tsuen Wan WSD8 <10 13.3 16.8











Sensitive Receivers





SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.8 3.2 0.2 2.5

Approach B2 0.8 3.2 0.5 2.5

Ting Kau B3 0.3 3.2 0.2 2.5

Lido B4 0.3 3.2 0.9 2.5

Casam B5 1.0 3.2 0.5 2.5

Hoi Mei Wan B6 1.1 3.2 0.4 2.5



3-26

Sensitive Receivers





SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Gemini B7 1.1 3.2 0.4 2.5

Angler’s B8 1.0 3.2 0.4 2.5

Lo So Shing B9 <0.1 2.8 <0.1 1.9

Hung Shing Yeh B10 <0.1 2.8 <0.1 1.9

Corals

Pak Kok CR1 0.9 2.2 0.3 3

Shek Kok Tsui CR2 0.8 2.2 0.3 3

Luk Chau CR3 0.5 1.8 0.1 1.6

Wong Chuk Kok CR4 0.5 1.8 <0.1 1.6

Ap Lei Chau CR5 0.5 1.8 0.1 1.6

Sandy Bay CR6 0.6 2.2 0.3 3


Kau Yi Chau CR8 0.7 2.6 1.3 4.4

Kau Yi Chau CR9 0.7 2.6 2.9 4.4

Kau Yi Chau CR10 0.5 2.6 1.4 4.4

Siu Kau Yi Chau CR11 0.7 2.6 2.2 4.4

Siu Kau Yi Chau CR12 0.6 2.6 2.0 4.4

Siu Kau Yi Chau CR13 0.5 2.6 1.4 4.4

Peng Chau CR14 0.4 2.6 0.6 4.4

Peng Chau CR15 <0.1 2.6 0.1 4.4

Peng Chau CR16 0.3 2.6 0.7 4.4

Peng Chau CR17 0.3 2.6 1.1 4.4

Peng Chau CR18 0.1 2.6 0.2 4.4

Fish Culture Zone

Ma Wan F1 0.8 3.2 0.5 2.5

Lo Tik Wan F2 0.2 1.8 0.1 1.6

Sok Kwu Wan F3 <0.1 1.8 <0.1 1.6

Cheung Sha Wan F4 <0.1 2.6 <0.1 4.4




Assessment Point




Tsuen Wan C1 - 11.5 10.6







3-27


Assessment Point







Tsing Yi WSD1 <10 11.8 10.7







Tsuen Wan WSD8 <10 11.5 10.5





Assessment Point




Tsuen Wan C1 - 12.5 15.9









Tsing Yi WSD1 <10 12.7 15.8







Tsuen Wan WSD8 <10 12.4 16.0








3-28






Sensitive Receivers





SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.8 3.2 0.1 2.5

Approach B2 1.0 3.2 0.4 2.5

Ting Kau B3 0.4 3.2 0.2 2.5

Lido B4 0.2 3.2 0.7 2.5

Casam B5 0.5 3.2 0.4 2.5

Hoi Mei Wan B6 0.5 3.2 0.4 2.5

Gemini B7 0.5 3.2 0.3 2.5

Angler’s B8 0.5 3.2 0.3 2.5

Lo So Shing B9 <0.1 2.8 <0.1 1.9

Hung Shing Yeh B10 <0.1 2.8 <0.1 1.9

Corals

Pak Kok CR1 0.5 2.2 0.2 3.0

Shek Kok Tsui CR2 0.5 2.2 0.2 3.0

Luk Chau CR3 0.3 1.8 <0.1 1.6

Wong Chuk Kok CR4 0.2 1.8 <0.1 1.6

Ap Lei Chau CR5 0.1 1.8 <0.1 1.6

Sandy Bay CR6 0.2 2.2 0.1 3.0


Kau Yi Chau CR8 0.4 2.6 0.8 4.4

Kau Yi Chau CR9 0.4 2.6 1.9 4.4

Kau Yi Chau CR10 0.3 2.6 1.1 4.4

Siu Kau Yi Chau CR11 0.4 2.6 1.3 4.4

Siu Kau Yi Chau CR12 0.4 2.6 1.1 4.4

Siu Kau Yi Chau CR13 0.3 2.6 0.9 4.4

Peng Chau CR14 0.2 2.6 0.5 4.4

Peng Chau CR15 <0.1 2.6 0.1 4.4

Peng Chau CR16 0.2 2.6 0.5 4.4

Peng Chau CR17 0.2 2.6 0.5 4.4

Peng Chau CR18 <0.1 2.6 0.2 4.4

Fish Culture Zone

Ma Wan F1 0.4 3.2 0.3 2.5



3-29

Sensitive Receivers





SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Lo Tik Wan F2 0.1 1.8 0.1 1.6

Sok Kwu Wan F3 <0.1 1.8 <0.1 1.6

Cheung Sha Wan F4 <0.1 2.6 <0.1 4.4




Assessment Point




Tsuen Wan C1 - 12.0 10.7









Tsing Yi WSD1 <10 13.5 11.4







Tsuen Wan WSD8 <10 12.2 10.8





Assessment Point




Tsuen Wan C1 - 13.2 17.0







3-30


Assessment Point







Tsing Yi WSD1 <10 15.2 20.4







Tsuen Wan WSD8 <10 13.3 16.9











Sensitive Receivers





SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.8 3.2 0.1 2.5

Approach B2 1.0 3.2 0.4 2.5

Ting Kau B3 0.4 3.2 0.2 2.5

Lido B4 0.2 3.2 0.6 2.5

Casam B5 0.5 3.2 0.4 2.5

Hoi Mei Wan B6 0.5 3.2 0.4 2.5

Gemini B7 0.5 3.2 0.3 2.5

Angler’s B8 0.5 3.2 0.3 2.5

Lo So Shing B9 <0.1 2.8 <0.1 1.9

Hung Shing Yeh B10 <0.1 2.8 <0.1 1.9

Corals

Pak Kok CR1 0.2 2.2 0.1 3.0

Shek Kok Tsui CR2 0.2 2.2 0.1 3.0



3-31

Sensitive Receivers





SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Luk Chau CR3 0.1 1.8 <0.1 1.6

Wong Chuk Kok CR4 0.1 1.8 <0.1 1.6

Ap Lei Chau CR5 0.2 1.8 <0.1 1.6

Sandy Bay CR6 0.2 2.2 0.2 3.0


Kau Yi Chau CR8 0.2 2.6 0.5 4.4

Kau Yi Chau CR9 0.2 2.6 1.6 4.4

Kau Yi Chau CR10 0.2 2.6 1.0 4.4

Siu Kau Yi Chau CR11 0.2 2.6 1.0 4.4

Siu Kau Yi Chau CR12 0.3 2.6 0.8 4.4

Siu Kau Yi Chau CR13 0.2 2.6 0.8 4.4

Peng Chau CR14 0.1 1.8 0.4 1.6

Peng Chau CR15 <0.1 2.6 <0.1 4.4

Peng Chau CR16 0.1 2.6 0.4 4.4

Peng Chau CR17 0.1 2.6 0.4 4.4

Peng Chau CR18 <0.1 2.6 0.1 4.4

Fish Culture Zone

Ma Wan F1 0.3 3.2 0.3 2.5

Lo Tik Wan F2 <0.1 1.8 <0.1 1.6

Sok Kwu Wan F3 <0.1 1.8 <0.1 1.6

Cheung Sha Wan F4 <0.1 2.6 <0.1 4.4




Assessment Point




Tsuen Wan C1 - 12.0 10.7









Tsing Yi WSD1 <10 13.5 11.4





3-32


Assessment Point







Tsuen Wan WSD8 <10 12.2 10.8





Assessment Point




Tsuen Wan C1 - 13.2 17.0









Tsing Yi WSD1 <10 15.2 20.4







Tsuen Wan WSD8 <10 13.3 16.9











3-33



Sensitive Receivers





SS Criteria

(mgL-1)


SS Criteria

(mgL-1)

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.5 3.2 0.1 2.5

Approach B2 0.8 3.2 0.4 2.5

Ting Kau B3 0.3 3.2 0.2 2.5

Lido B4 0.3 3.2 0.8 2.5

Casam B5 0.8 3.2 0.4 2.5

Hoi Mei Wan B6 0.8 3.2 0.4 2.5

Gemini B7 0.8 3.2 0.4 2.5

Angler’s B8 0.7 3.2 0.3 2.5

Lo So Shing B9 <0.1 2.8 <0.1 1.9

Hung Shing Yeh B10 <0.1 2.8 <0.1 1.9

Corals

Pak Kok CR1 0.4 2.2 0.2 3

Shek Kok Tsui CR2 0.4 2.2 0.2 3

Luk Chau CR3 0.3 1.8 0.1 1.6

Wong Chuk Kok CR4 0.4 1.8 <0.1 1.6

Ap Lei Chau CR5 0.4 1.8 0.1 1.6

Sandy Bay CR6 0.5 2.2 0.2 3


Kau Yi Chau CR8 0.4 2.6 0.9 4.4

Kau Yi Chau CR9 0.4 2.6 2.1 4.4

Kau Yi Chau CR10 0.3 2.6 1.2 4.4

Siu Kau Yi Chau CR11 0.4 2.6 1.6 4.4

Siu Kau Yi Chau CR12 0.4 2.6 1.6 4.4

Siu Kau Yi Chau CR13 0.3 2.6 1.1 4.4

Peng Chau CR14 0.3 2.6 0.6 4.4

Peng Chau CR15 <0.1 2.6 0.1 4.4

Peng Chau CR16 0.2 2.6 0.5 4.4

Peng Chau CR17 0.2 2.6 0.7 4.4

Peng Chau CR18 <0.1 2.6 0.2 4.4

Fish Culture Zone

Ma Wan F1 0.6 3.2 0.4 2.5

Lo Tik Wan F2 0.1 1.8 0.1 1.6

Sok Kwu Wan F3 <0.1 1.8 <0.1 1.6

Cheung Sha Wan F4 <0.1 2.6 <0.1 4.4




3-34



Assessment Point




Tsuen Wan C1 - 11.8 10.6









Tsing Yi WSD1 <10 12.5 11.4







Tsuen Wan WSD8 <10 11.9 10.7





Assessment Point




Tsuen Wan C1 - 12.9 16.6









Tsing Yi WSD1 <10 14.4 20.4








3-35


Assessment Point




Tsuen Wan WSD8 <10 13.0 16.5









A summary of the impact arising from Scenarios 1 to 7 is presented below:

Scenario 1

Gazetted Beaches – model results indicated the maximum SS elevations to be 1.0 mg L-1

during the dry

season (worst affected location was B2) and 0.8 mg L-1

during the wet season (worst affected location was

B4). No exceedance of the SS criteria occurred.

Corals – model results indicated the maximum SS elevations to be 0.7 mg L-1

during the dry season (worst

affected location was CR7) and 2.3 mg L-1

during the wet season (worst affected location was CR9). No

exceedance of the SS criteria occurred.

Fish Culture Zone – model results indicated the maximum SS elevations to be 0.6 mg L-1

during the dry

season (worst affected location was F1) and 0.4 mg L-1


F1). No exceedance of the SS criteria occurred.

Cooling Water Intakes – model results indicated the maximum SS concentrations to be 12.7 mg L-1

during

the dry season and 11.3 mg L-1

during the wet season at C2 based on the surface level result.

WSD Flushing Water Intakes – the SS criterion (<10 mg L-1

) was exceeded at four locations (WSD1,

WSD8, WSD9, EMSD1) during the dry season, and at three locations (WSD1, WSD8, EMSD1) during the

wet season. Model results indicated the maximum SS concentrations to be 13.5 mg L-1

during dry season

and 11.4 mg L-1

during wet season at WSD1 based on the surface level result.

Scenario 2


during the dry











3-36


during the dry





during







during dry season

and 11.4 mg L-1


Scenario 3


during the dry

season (worst affected locations were B6 and B7) and 0.9 mg L-1

during the wet season (worst affected

location was B4). No exceedance of the SS criteria occurred.



affected locations were CR1 and CR7) and 2.9 mg L-1


CR9). No exceedance of the SS criteria occurred.


during the dry





during


during the wet season at C1 and C2 based on the surface level result.





during dry season

and 10.7 during wet season at WSD1 based on the surface level result.

Scenario 4


during the dry






affected locations were CR1 and CR2) and 1.9 mg L-1


CR9). No exceedance of the SS criteria occurred.


during the dry






3-37


during







during dry season

and 11.4 during wet season at WSD1 based on the surface level result.

Scenario 5


during the dry










during the dry





during







during dry season

and 11.4 mg L-1


Scenario 6


during the dry

season (worst affected locations were B2, B5, B6 and B7) and 0.8 mg L-1

during the wet season (worst

affected location was B4). No exceedance of the SS criteria occurred.







during the dry





during


during the wet season C2 based on the surface level result.






3-38


during dry season

and 11.4 mg L-1


Scenario 7


during the dry

season (worst affected location was B7 & B8) and 1.6 mg L-1

during the wet season (worst affected location

was B4). No exceedance of the SS criteria occurred.



affected location was CR1 & CR7) and 3.7 mg L-1

during the wet season (worst affected location was CR9).

No exceedance of the SS criteria occurred.


during the dry





during


during the wet season at C2 based on the level surface result.





during dry season

and 11.0 mg L-1


A summary of the results and compliance is given in Table 3.24 below.

Table 3.24: Summary of SS Results for Scenarios 1 - 7

Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5 Scenario 6 Scenario 7

Gazetted Beaches

Maximum SS elevation in dry season (mg L-1) / (Location)

1.0 (B2) 1.0 (B2) 1.1 (B6 & B7)

1.0 (B2) 1.0 (B2) 0.8 (B2, B5-B7)

1.8 (B7 & B8)

Maximum SS elevation in wet season (mg L-1) / (Location)

0.8 (B4) 0.8 (B4) 0.9 (B4) 0.7 (B4) 0.6 (B4) 0.8 (B4) 1.6 (B4)

Exceedance? No No No No No No No

Corals


0.7 (CR7) 0.8 (CR7) 0.9 (CR1 & CR7)

0.5 (CR1 & CR2)

0.6 (CR7) 0.8 (CR7) 1.0 (CR1 & CR7)


2.3 (CR9) 2.2 (CR9) 2.9 (CR9) 1.9 (CR9) 1.6 (CR9) 2.1 (CR9) 3.7 (CR9)


Fish Culture Zone


0.6 (F1) 0.6 (F1) 0.8 (F1) 0.4 (F1) 0.3 (F1) 0.6 (F1) 2.6 (F1)



3-39

Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5 Scenario 6 Scenario 7


0.4 (F1) 0.4 (F1) 0.5 (F1) 0.3 (F1) 0.3 (F1) 0.4 (F1) 2.0 (F1)



Maximum SS level in dry season (mg L-1) / (Location)

12.7 (C2) 12.7 (C2) 11.7 (C2) 12.7(C2) 12.7 (C2) 12.3 (C2) 12.0 (C2)

Maximum SS level in wet season (mg L-1) / (Location)

11.3 (C2) 11.3 (C2) 10.6 (C1 & C2)

11.3(C2) 11.3 (C2) 11.3 (C2) 10.9 (C2)


Flushing Water Intakes

Maximum SS level in dry season (mg L-1) / (Location)

13.5 (WSD1)

13.5 (WSD1)

11.8 (WSD1)

13.5 (WSD1)

13.5 (WSD1)

12.5 (WSD1)

12.0 (WSD1)

Maximum SS level in wet season (mg L-1) / (Location)

11.4 (WSD1)

11.4 (WSD1)

10.7 (WSD1)

11.4 (WSD1)

11.4 (WSD1)

11.4 (WSD1)

11.0

(WSD1)

Exceedance? Yes Yes Yes Yes Yes Yes Yes

Depth-averaged SS elevations were reported for gazetted beaches, corals, fish culture zones while surface SS levels were reported

for water intakes

Exceedance of WSD’s SS criterion of 10 mg L-1

was predicted at the seawater intakes WSD1, WSD8,

WSD9 and EMSD1 due to the exceedance of ambient SS levels at the Rambler Channel and waters near

Tsuen Wan (Table 3.10). Mitigation measures are proposed to protect these WSRs and details are

provided in Section 3.8.

The predicted SS elevations and total concentrations for Scenario 6 during the dry and wet seasons at

WSRs are presented in Tables 3.22 and 3.23. The effects of using a CSD instead of a GD in the Northeast

of the Rambler Channel have been assessed with reference to the predicted results for Scenarios 2 (GD)

and 6 (CSD). The results indicated that there is no significant difference between the predicted results for

these two scenarios. The WSD flushing water intake WSD1 is the closest WSR to the working area of the

CSD. The difference in SS elevation at WSD1 is less than 0.2 mg L-1

.

No non-compliance of the SS criteria was found at any gazetted beach, coral communities, fish culture

zone or cooling water intake. Exceedances of the SS criterion at the WSD flushing water intake are similar

to those described in Scenario 2. As such, it may be inferred that the CSD operation would not induce

significant additional impact compared with those induced by the GD assumed in Scenario 2. It was also

concluded that with either one GD or one CSD operating in the Rambler Channel under Scenarios 1, 2, 4

and 5, there would be no significant change in the predicted SS impacts at any of the WSRs.

As noted in Section 3.5.2.3, an alternative modelling scenario which included 5 dredgers operating

concurrently has also been undertaken and is described in Appendix 3.2 which also included the impact

assessment at the identified WSRs. SS elevations have been presented as colour contours and time

series plots. The extent of maximum surface, bottom and depth-averaged SS elevations over the complete

simulation period during dry and wet seasons, respectively are also presented in Appendix 3.2. As



3-40

illustrated in the contour plots in Appendix 3.2 the SS impact would appear to be confined close to the

dredging areas in the Rambler Channel, Northern Fairway and Western Fairway. Details of the temporal

variations of surface, mid-depth, bottom and depth-averaged SS elevations at various WSRs in close

proximity to the dredging areas during dry and wet seasons were also provided in Appendix 3.2. It must be

stressed that this scenario is only included as an extreme example, because at no time will 5 dredgers

operate together under this Project.

3.7.1.2 Contribution of the Suspended Solids from Potentially Concurrent Projects

The potential concurrent projects included in the cumulative impact assessments were described in

Sections 2.8 and 3.5.2.2. Scenario 7 was specifically developed to examine the potential cumulative

impacts. In the first instance reference was made to the HZMB approved EIAs which included the SS

impact assessment of the concurrent projects of TMCLKL, HKBCF and HKLR projects and other

concurrent projects including the Dredging and Backfilling of East of Sha Chau Contaminated Mud-Pits (V).

Based on the contour plots presented in these EIAs, the predicted maximum SS levels during mitigated

case in year 2012 (spring tide, lowest low) occurred near the proposed BCF and the Brothers. The SS

levels reduced gradually from < 6 mg L-1

near Yam O Wan to < 3 mg L-1

near the Ma Wan Fish Culture

Zone. The SS levels in the plume further reduced to background levels (< 2 mg L-1

) in the Ma Wan Channel

between Tsing Yi and Ma Wan. No noticeable plume with SS levels was observed on the Tsuen Wan coast

or in the Rambler Channel.

The predicted SS elevations and total SS concentrations at the identified WSRs due to the concurrent

projects and Scenario 3 for this Project are provided in Appendix 3.8. Comparison of the results of

Scenario 7 (concurrent projects plus Scenario 3) and Scenario 3 only indicated that insignificant SS

elevations (< 1 mg L-1

) were predicted at all the WSRs, except at the Ma Wan Fish Culture Zone (WSR F1).

The highest SS elevation (1.8/1.5 mg L-1

in dry/wet seasons), due to the contribution of concurrent projects,

was predicted at the Ma Wan Fish Culture Zone (WSR F1). The SS elevation due to the concurrent

projects is also noticeable at the beaches along Tsuen Wan Coast and Castle Peak Road (i.e. WSRs B5 –

B8), with the maximum elevation of 0.8/0.9 mg L-1

in dry/wet seasons. These findings were qualitatively

consistent with those for the HZMB approved EIAs. Further South of Tsing Yi Island, the predicted SS

impacts due to the concurrent projects were reduced.

From the assessment of the predicted data, it may be concluded that the SS impacts of the concurrent

projects on the WSRs are not significant. According to the results given in Appendix 3.8, no SS

exceedance was found at any gazetted beach, coral communities or fish culture zone for this scenario.

The SS exceedance at WSD1, WSD8, WSD9 and EMSD1 in Scenario 7 are due to the high SS

background level as mentioned in previous sections. The maximum contribution from the concurrent

projects to these WSRs is 0.5/0.8 mg L-1

, which is not significant and could be further reduced by the

mitigation measures proposed in Section 3.8.

The maximum predicted sedimentation rate at all the coral communities for Scenario 7 was less than 23 g

m-2

per day (22.2 g m-2

per day at WSR CR9). A maximum of 43 g m-2

per day was obtained when this

value was added to the maximum predicted sedimentation rate given in Section 3.7.1.3 below, which is

well within the criterion of 100g m-2

per day.

In the event the Container Terminal Operators need to undertake maintenance dredging when capital

works for this Project are underway, one of the three dredgers located within container basin (Zone 1 to 6

(including subzones) of Appendix 3.13) for this Project (either the GD or CSD) will be temporarily

suspended during the period of the CTO’s maintenance works. The replacement of one dredger in the



3-41

KTCB for this Project by one dredger for the CTO maintenance works will, by interpolation, result in similar

predicted SS levels at the WSRs as to the previous scenarios assumed for the capital works dredging. In

other words, it can be assumed that if the CTO undertake maintenance dredging concurrently with this

Project, the SS levels will be similar to those predicted for 3 dredgers operating under the capital works

dredging programme.

3.7.1.3 Sedimentation Rate

The sedimentation rates at those locations where coral communities are identified, namely, Peng Chau,

Kau Yi Chau, Western Fairway, East Lamma Channel and north of Lamma Island, were predicted to equal

to or lower than 20 g m-2

per day for the Alternative Scenario (Table A3.4 in Appendix 3.2). In other

words, the scenario which simulates an extreme case of 5 dredgers working at any given time predicted

impacts at all identified WSR’s to be less than 20 g m-2

per day. By inference, the predicted results will be

lower for all other Scenarios (Scenario 1 – 6), since these scenarios assume a lower number of dredgers.

Given that the criterion for corals is 100g m-2

per day, even in the worst case (and unrealistic) scenario,

compliance with the WQO is anticipated. The contours of predicted daily average sedimentation per square

meter during dry and wet seasons are presented in Appendix 3.2.

3.7.1.4 Impacts on the Gazetted Beaches

During the consultations for this Project, some concerns were expressed in relation to the effects of the

dredging works on the Ting Kau gazetted beaches. The modelling results indicated that no significant SS

impact due to the Project was predicted at these beaches. In addition, the sediment analysis also indicated

that no E.coli was identified in the samples collected from nearby the sewage outfalls and as such it may

be surmised that the dredging works will not affect water quality or the health of the gazetted beaches.

3.7.1.5 Dissolved Oxygen and Total Inorganic Nitrogen

The minimum depth-averaged and bottom layer DO for all scenarios in dry and wet seasons at beaches,

fish culture zones, marine ecology sensitive receivers and the cooling and seawater intakes were predicted

based on the methodology in Section 3.5.2. The predicted DO and TIN at individual WSRs are presented

in Appendix 3.9 and summarised in Table 3.25.

No exceedance of WQO of DO in the bottom layer WSD1 – WSD4, WSD6 – WSD8, WSD10, EMSD1, C1

– C4, B9, B10, CR1 – CR4, CR8 – CR18, and F2 – F4, especially for those in the Southern WCZ. TIN

exceedance was recorded at WSRs WSD1 – WSD4, WSD6 – WSD8, WSD10, EMSD1, C1 – C4, B9, B10,

CR1 – CR4, CR8 – CR18, and F2 – F4, especially for those in the Southern WCZ. The elevations are

considered to be due to the non-compliance of the ambient TIN levels with the WQO recorded at the EPD

monitoring stations (Table 3.11). Similarly, the DO non-compliance at WSRs WSD1 – WSD9, EMSD1, C1

– C7, B1 – B8, CR1 – CR7, and F1 – F4 in wet seasons is due to low ambient DO levels with WQO

recorded at the EPD monitoring stations. The results of DO and TIN for the “Alternative Scenario” (i.e., 5

dredgers operating simultaneously) are presented in Table A3.5 in Appendix 3.2 for reference purpose

only.

A summary of the results for Scenarios 1 – 7 is presented below.

Scenario 1



3-42

As presented in Table A3.9a and Table A3.9b of Appendix 3.9, the maximum decrease in depth-

averaged DO for Scenario 1 is predicted to be 0.017 mgL-1

at WSR WSD1. The maximum decrease in

depth-averaged DO at WSR F1 for this scenario is 0.002 mgL-1

and there is insignificant DO depletion at

WSR F2 and F3. The maximum increase in TIN for this scenario is predicted to be 0.0015 mgL-1

at WSR

WSD1.

Scenario 2

As presented in Table A3.9c and Table A3.9d of Appendix 3.9, the maximum decrease in depth-






at WSR

WSD1.

Scenario 3

As presented in Table A3.9e and Table A3.9f of Appendix 3.9, the maximum decrease in depth-averaged

DO for Scenario 3 is predicted to be 0.011 mgL-1

at WSR CR9. The maximum decrease in depth-averaged

DO at WSR F1 for this scenario is 0.003 mgL-1

and there is insignificant DO depletion at WSR F2 and F3.

The maximum increase in TIN for this scenario is predicted to be 0.0009 mgL-1

at WSR CR9.

Scenario 4

As presented in Table A3.9g and Table A3.9h of Appendix 3.9, the maximum decrease in depth-






at WSR

WSD1.

Scenario 5

As presented in Table A3.9i and Table A3.9j of Appendix 3.9, the maximum decrease in depth-averaged


at WSR WSD1. The maximum decrease in depth-

averaged DO at WSR F1 for this scenario is 0.001 mgL-1

and there is insignificant DO depletion at WSR F2

and F3. The maximum increase in TIN for this scenario is predicted to be 0.0015 mgL-1

at WSR WSD1.

Scenario 6

As presented in Table A3.9k and Table A3.9l of Appendix 3.9, the maximum decrease in depth-averaged


at WSR WSD1. The maximum decrease in depth-




at WSR WSD1.

Scenario 7

As presented in Table A3.9m and Table A3.9n of Appendix 3.9, the maximum decrease in depth-


at WSR CR9. The maximum decrease in depth-




at WSR CR9.



3-43

Summary of impacts on DO and TIN

Although there will be decreases in DO levels, the impacts on depth-averaged DO can be surmised as

being very minor which is considered to be a minimal impact on the water quality. The increase in TIN

concentrations at WSRs for all scenarios was predicted to be less than 0.002 mgL-1

, which is considered to

be a minimal impact on the water quality. The existing water quality in some areas has already breached

the WQO for depth-averaged DO and TIN.

It may be surmised that there is no significant impact relating to dissolved oxygen and total inorganic

nitrogen for all dredging scenarios assessed. The non-compliance of minimum depth-averaged DO levels

at the beaches, corals, water intakes and fish culture zones and maximum TIN at WSRs within Southern

WCZ is due to background, and the relative contribution from the dredging activities is minimal.

Table 3.25: Summary of Dissolved Oxygen for Scenario 1-7

Minimum Depth-averaged DO level (mgL-1)

Minimum DO level at bottom layer (mgL-1)

Maximum TIN concentration (mgL-1)#

Scenario 1

Gazetted Beaches

Dry Season 5.50 5.70 0.2903

Wet Season 3.60 2.70 0.3702

Non-compliance? Yes† No No^

Corals

Dry Season 5.90 6.00 0.2502

Wet Season 3.70 2.50 0.5007

Non-compliance? Yes† No Yes*

Fish Culture Zone

Dry Season 5.50 5.70 0.2902

Wet Season 3.60 2.70 0.5000



Dry Season 5.00 5.00 0.3507

Wet Season 3.60 2.50 0.5311



Dry Season 5.00 5.00 0.3509

Wet Season 3.60 2.60 0.5315


Scenario 2

Gazetted Beaches

Dry Season 5.50 5.70 0.2903

Wet Season 3.60 2.70 0.3702




3-44




Corals

Dry Season 5.90 6.00 0.2501

Wet Season 3.70 2.50 0.5007


Fish Culture Zone

Dry Season 5.50 5.70 0.2902

Wet Season 3.60 2.70 0.5000



Dry Season 5.00 5.00 0.3507

Wet Season 3.60 2.50 0.5311



Dry Season 5.00 5.00 0.3509

Wet Season 3.60 2.60 0.5315


Scenario 3

Gazetted Beaches

Dry Season 5.50 5.69 0.2903

Wet Season 3.60 2.69 0.3703


Corals

Dry Season 5.90 6.00 0.2502

Wet Season 3.70 2.50 0.5009


Fish Culture Zone

Dry Season 5.50 5.70 0.2903

Wet Season 3.60 2.70 0.5000



Dry Season 5.00 5.00 0.3502

Wet Season 3.60 2.50 0.5301



Dry Season 5.00 5.00 0.3502

Wet Season 3.60 2.60 0.5302


Scenario 4

Gazetted Beaches

Dry Season 5.50 5.70 0.2903



3-45




Wet Season 3.60 2.70 0.3702


Corals

Dry Season 5.90 6.00 0.2501

Wet Season 3.70 2.50 0.5006


Fish Culture Zone

Dry Season 5.50 5.70 0.2901

Wet Season 3.60 2.70 0.5000



Dry Season 5.00 5.00 0.3507

Wet Season 3.60 2.50 0.5311



Dry Season 5.00 5.00 0.3509

Wet Season 3.60 2.60 0.5315


Scenario 5

Gazetted Beaches

Dry Season 5.50 5.70 0.2903

Wet Season 3.60 2.70 0.3702


Corals

Dry Season 5.90 6.00 0.2501

Wet Season 3.70 2.50 0.5005


Fish Culture Zone

Dry Season 5.50 5.70 0.2901

Wet Season 3.60 2.70 0.5000



Dry Season 5.00 5.00 0.3507

Wet Season 3.60 2.50 0.5311



Dry Season 5.00 5.00 0.3510

Wet Season 3.60 2.60 0.5315


Scenario 6



3-46




Gazetted Beaches

Dry Season 5.50 5.70 0.2902

Wet Season 3.60 2.70 0.3702


Corals

Dry Season 5.90 6.00 0.2501

Wet Season 3.70 2.50 0.5007


Fish Culture Zone

Dry Season 5.50 5.70 0.2902

Wet Season 3.60 2.70 0.5000



Dry Season 5.00 5.00 0.3505

Wet Season 3.60 2.50 0.5310



Dry Season 5.00 5.00 0.3507

Wet Season 3.60 2.60 0.5315


Scenario 7

Gazetted Beaches

Dry Season 5.49 5.69 0.2906

Wet Season 3.59 2.69 0.3705


Corals

Dry Season 5.90 6.00 0.2502

Wet Season 3.70 2.50 0.5011


Fish Culture Zone

Dry Season 5.49 5.69 0.2908

Wet Season 3.59 2.69 0.5000



Dry Season 5.00 5.00 0.3503

Wet Season 3.60 2.50 0.5302



Dry Season 4.99 4.99 0.3503

Wet Season 3.60 2.59 0.5303




3-47

# The WQO for TIN is 0.4 mgL

-1 except at Southern WCZ where the criterion is 0.1 mgL

-1.

^ Except for B9 and B10, where non-compliance of TIN levels are predicted due to the non-compliance of the background TIN levels

with the WQO. TIN levels at B9 and B10 are not the maximum predicted TIN concentration among the gazetted beaches,

* Due to the non-compliance of the background TIN levels with the WQO. † Due to the non-compliance of the background depth-averaged DO levels with the WQO in Wet Season.

3.7.1.6 Potential Contaminant Release During Dredging

An indication of the potential release of contaminants from the marine sediments during dredging was

determined using the results of the elutriate tests from the laboratory testing conducted under the marine

Site Investigation (SI). The locations of sediment sampling are shown in Figure 4.1 and Figure 4.2. The

description of the marine SI is contained within Chapter 4.

Heavy Metals, TBT, PCB’s Total PAHs and Chlorinated Pesticides

Details of the elutriate test results for heavy metals, TBT, PCBs, total PAHs and chlorinated pesticides are

included in Appendix 3.1, with the results summarized in Table 3.26. The measured levels of all the PCBs,

PAHs and chlorinated pesticides were below the detection limits. The highest TBT concentration was 36

ngL-1

measured at D374. Similarly, the measured heavy metal concentrations at most sampling locations

were below the detection limits. Non-compliance of the water quality criteria (Table 3.26) for heavy metals

and organics was only observed in the arsenic concentration measured at sampling location S3, S6, S11,

S12, S15, S32, D174, D196 and D234 in Rambler Channel and Northern Fairway. However, the non-

compliance was considered to be minor with the maximum arsenic concentration of 33 g L-1

(at location S6)

compared with the water quality criterion of 25 g L-1

. Based on the maximum arsenic concentration, the

dilution required to meet the relevant water quality criterion was calculated to be 1.3.

With reference to Figure 3.1 and Figure 4.1, the nearest WSRs to the sampling location S6, S11 and S12

which demonstrated exceedance of arsenic concentration were WSD1, C2 and EMSD1, which are located

at least 600m away from the sampling locations. All of the WSRs are flushing water and cooling water

intakes. There are no criteria for heavy metals or organics in connection with the WSD seawater intakes,

thus reference has been made to the UK Water Quality Standard.

It should be stressed that any release of heavy metals during dredging will be rapidly diluted by the large

volume of marine water within the dredging site and the small exceedance of arsenic at a few points is not

expected to adversely affect WSD intake water quality. According to the elutriate test results, it was

concluded that the potential impacts of release of heavy metals and organics due to seabed disturbance on

all WSRs (in particular the nearby beaches, fish culture zones, ecological sensitive receivers) are

negligible.



3-48

Table 3.26: Elutriate Test Results of Sediment Samples

Metals (µg/l) Total PCBs

(µg/l) Total PAHs

(µg/l) TBT (ng/l)

Vibrocore Sampling Depth As Cd Cr Cu Pb Hg Ni Ag Zn

Water Quality Standards 25(1) 2.5(1) 15(1) 5(1) 25(1) 0.3(1) 30(1) 1.9(2) 40(1) 0.03(2) 3.0(3) 100(4)

S1-2 0-0.9M 14 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <10

S1-2 0.9-1.9M 16 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

S1-2 Elutriate Blank <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <8

S2 0-0.9M <10 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <11

S2 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <9

S2 1.9-2.9M <10 <0.2 <10 1 <1 <0.5 2 <1 <10 <0.05 <0.2 <10

S2 Elutriate Blank <10 <0.2 <10 3 <1 <0.5 2 <1 <10 <0.05 <0.2 <11

S3 0-0.9M 21 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S3 0.9-1.9M 27 <0.2 <10 1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S3 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S3 Elutriate Blank <10 <0.2 <10 2 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S4 0-0.9M 16 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <8

S4 0.9-1.9M 22 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

S4 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S4 Elutriate Blank <10 0.4 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <11

S5-2 0-0.9M 17 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10

S5-2 0.9-1.9M 21 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

S5-2 Elutriate Blank <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

S6 0-0.9M 23 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

S6 0.9-1.9M 33 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

S6 1.9-2.9M 30 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 21

S6 Elutriate Blank <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10

S7 0-0.9M <10 <0.2 <10 <1 <1 <0.5 2 <1 <10 <0.05 <0.2 <15

S7 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15

S7 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15



3-49


(µg/l) Total PAHs

(µg/l) TBT (ng/l)



S8 0-0.9M 12 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15

S8 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15

S8 Elutriate Blank <10 <0.2 <10 2 <1 <0.5 2 <1 <10 <0.05 <0.2 <15

S9 0-0.9M 14 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

S9 0.9-1.9M 18 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9


S10 0-0.9M 21 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

S10 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

S10 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

S10 Elutriate Blank <10 0.2 <10 3 <1 <0.5 1 <1 <10 <0.05 <0.2 <9

S11 0-0.9M 28 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10

S11 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9


S12 0-0.9M 31 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

S12 0.9-1.9M 32 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <8

S12 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <8

S12 Elutriate Blank <10 0.3 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10

S13 Grab Sample <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15


S14 0-0.9M 13 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15


S15 0-0.9M 31 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 63

S15 0.9-1.9M 11 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

S15 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10

S15 Elutriate Blank <10 0.3 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <10

S17 0-0.9M 11 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S17 0.9-1.9M 23 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15



3-50


(µg/l) Total PAHs

(µg/l) TBT (ng/l)



S18 Grab Sample 14 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S18 Elutriate Blank 10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S19 0-0.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

S19 0.9-1.9M 14 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

S19 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10


S21 0-0.9M 16 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S21 0.9-1.9M 15 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S21 1.9-2.9M 18 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15


D174 0-0.9M 31 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

D174 0.9-1.9M 18 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

D174 1.9-2.9M 10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

D174 Elutriate Blank <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

D196 0-0.9M 31 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <8

D196 0.9-1.9M 24 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 30

D196 1.9-2.9M 16 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10


D202 0-0.9M 14 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

D202 0.9-1.9M 12 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

D202 1.9-2.9M 13 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11


D214 0-0.9M 15 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

D214 0.9-1.9M 16 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <14

D214 1.9-2.9M 22 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 12


D221 0-0.9M 17 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10



3-51


(µg/l) Total PAHs

(µg/l) TBT (ng/l)


D221 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10


D234 0-0.9M 18 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

D234 0.9-1.9M 24 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

D234 1.9-2.9M 32 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11


D238 0-0.9M 13 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

D238 0.9-1.9M 14 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

D238 1.9-2.9M 10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15


S29 Grab Sample <10 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15


S30 0-0.9M 16 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S30 0.9-1.9M 17 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15

S30 1.9-2.9M 23 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15


D272 Grab Sample <10 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15


S32 0-0.9M 32 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S32 0.9-1.9M 16 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S32 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15


D298 0-0.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

D298 0.9-1.9M 19 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

D298 1.9-2.9M 21 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11


S34 0-0.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <13

S34 0.9-1.9M 10 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <12



3-52


(µg/l) Total PAHs

(µg/l) TBT (ng/l)


S34 1.9-2.9M 12 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12


S35 0-0.9M 16 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

S35 0.9-1.9M 16 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

S35 1.9-2.9M 18 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12


D320 0-0.9M 11 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

D320 0.9-1.9M 13 <0.2 <10 2 <1 <0.5 <1 <1 <10 <0.05 <0.2 <9

D320 1.9-2.9M 10 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <11


D330 0.9-1.9M 15 <0.2 <10 <1 <1 <0.5 2 <1 <10 <0.05 <0.2 <15

D330 1.9-2.9M 12 <0.2 <10 <1 <1 <0.5 2 <1 <10 <0.05 <0.2 <15


D337 0-0.9M 14 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10

D337 0.9-1.9M 16 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

D337 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10


S40 0-0.9M 18 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

S40 0.9-1.9M 15 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12


D355 0-0.9M 12 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15

D355 0.9-1.9M 13 <0.2 <10 <1 <1 <0.5 1 <1 <10 <0.05 <0.2 <15


D362 0-0.9M 10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10

D362 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <13


S44 0-0.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

S44 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11



3-53


(µg/l) Total PAHs

(µg/l) TBT (ng/l)



D374 0-0.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 36

D374 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <10


S47 0-0.9M 17 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

S47 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15


D378 0-0.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15

D378 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <15


D381 0-0.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

D381 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

D381 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11


S50 0-0.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

S50 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <11

S50 1.9-2.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12


D386 0-0.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

D386 0.9-1.9M <10 <0.2 <10 <1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

D386 Elutriate Blank <10 <0.2 <10 1 <1 <0.5 <1 <1 <10 <0.05 <0.2 <12

(1) Value in Bold indicate exceedance of relevant criteria

(2) UK Water Quality Standard

(3) USEPA, Criteria Maximum Concentration (CMC) of the USEPA Water Quality Criteria (Saltwater)

(4) Australian and New Zealand Guidelines for Fresh and Marine Waters

(5) Michael H. Salazar and Sandra M. Salazar (1996). “Mussels as Bioindicators: Effects of TBT on Survival, Bioaccumulation, and Growth under Natural Conditions” in Organotin, edited by M.

A. Champ and P. F. Seligman. Chapman & Hall, London

(6) WSD Water Quality Objectives of Sea Water for Flushing Supply (at intake point)



3-54

Ammoniacal Nitrogen (NH3-N) and Unionised Ammonia (UIA)

Background: The depth-averaged background TIN (which includes NH3-N and UIA) for dry and wet

seasons are summarized in Table 3.11, as derived from EPD’s routine marine water quality monitoring

data using the methodology described in Section 3.5.2. It is noted that the background depth-averaged

TIN concentration recorded at some stations during dry and wet seasons do not comply with the WQO for

TIN, that is less than or equal to 0.1 mg L-1

in Southern WCZ and less than or equal to 0.4 mg L-1

in the rest

of the waters, while the depth-averaged UIA values show compliance with the WQO criteria of 0.021 mg/l

annual average.

Elutriate Test Results: The first batch of sediment samples results were provided in February 2010, which

included a mixture of grab and vibrocore samples. Based on a review of these test results, a small number

of samples were observed to be suspect (for example, samples with lower than expected pH values), which

gave concerns that these samples may have been contaminated during the sampling process. After a

detailed analysis, further sampling at selected stations as shown in Figure 4.1 was undertaken in May

2010. Based on the two sets of field sampling data, all those data which exhibit consistent parameters

(e.g., with pH greater than 7.8) were subsequently used for the analysis of ammoniacal-nitrogen and

unionised ammonia, as shown in Table 3.27. In this table, the ammoniacal-nitrogen values were measured

directly from the elutriate tests on the collected sediment samples collected, whilst the UIA values were

derived using the formulae as detailed in Section 3.5.2.

Table 3.27: Elutriate Test Results

Site Sampling Depth

NH3-N concentration

(mg L-1)

(NH3-N – blank) concentration

(mg L-1)

UIA concentration

(mg L-1)

(UIA – blank) concentration

(mg L-1)

S1-2 GRAB 2.23 2.08 0.135 0.126

S1-2 ELUTRIATE BLANK 0.15 0.009

S2 GRAB 13.20 13.06 0.969 0.959

S2 ELUTRIATE BLANK 0.14 0.010

S2 0-0.9m 20.06 19.88 1.472 1.459


S2 0.9-1.9m 20.84 20.66 1.530 1.516


S2 1.9-2.9m 3.21 3.03 0.236 0.222


S3 GRAB 3.03 2.90 0.230 0.220


S4 GRAB 4.85 4.74 0.358 0.350


S4 0-0.9m 6.93 6.79 0.512 0.502


S4 0.9-1.9m 2.24 2.11 0.166 0.156


S4 1.9-2.9m 2.06 1.93 0.152 0.143


S5-2 GRAB 2.96 2.81 0.222 0.211



3-55

Site Sampling Depth

NH3-N concentration

(mg L-1)


(mg L-1)

UIA concentration

(mg L-1)


(mg L-1)

S5-2 ELUTRIATE BLANK 0.15 0.011

S6 GRAB 8.24 8.13 0.610 0.601


S7 GRAB 4.02 3.92 0.308 0.301


S8 GRAB 3.67 3.56 0.283 0.274


S9 0-0.9M 13.43 13.14 0.070 0.068

S9 0.9-1.9M 0.70 0.41 0.004 0.002


S10 GRAB 1.22 1.10 0.089 0.081


S10 0-0.9m 2.14 2.03 0.157 0.149


S10 0.9-1.9m 2.63 2.52 0.193 0.185


S10 1.9-2.9m 2.77 2.65 0.203 0.194


S11 0-0.9M 1.11 1.06 0.056 0.053

S11 0.9-1.9M 1.42 1.37 0.071 0.069


S12 0-0.9M 7.82 7.60 0.054 0.052

S12 0.9-1.9M 4.88 4.66 0.034 0.032

S12 1.9-2.9M 5.02 4.80 0.035 0.033


S13 GRAB 3.24 2.62 0.092 0.074


S14 GRAB 2.54 2.49 0.060 0.059


S15 0-0.9M 9.00 8.78 0.058 0.057

S15 0.9-1.9M 4.89 4.67 0.032 0.030

S15 1.9-2.9M 4.50 4.28 0.029 0.028


S17 0-0.9M 5.07 5.02 0.029 0.028

S17 0.9-1.9M 9.09 9.04 0.051 0.051


S18 -- 6.07 6.02 0.146 0.144


S19 GRAB 4.31 4.17 0.305 0.295


S21 GRAB 1.21 1.03 0.087 0.074




3-56

Site Sampling Depth

NH3-N concentration

(mg L-1)


(mg L-1)

UIA concentration

(mg L-1)


(mg L-1)

D174 GRAB 1.63 1.39 0.117 0.100

D174 ELUTRIATE BLANK 0.24 0.017

D196 GRAB 3.23 3.00 0.219 0.203


D202 GRAB 2.01 1.87 0.149 0.138


S29 1.22 1.17 0.035 0.033


D214 GRAB 4.34 4.14 0.311 0.296


D221 0-0.9M 1.24 0.99 0.077 0.062

D221 0.9-1.9M 1.66 1.41 0.103 0.088


D234 GRAB 1.04 0.81 0.071 0.056


D238 GRAB 3.02 2.74 0.210 0.190


D238 0-0.9m 9.23 9.03 0.640 0.627


D238 0.9-1.9m 12.06 11.83 0.837 0.821


D238 1.9-2.9m 13.61 13.40 0.944 0.930


S30 GRAB 3.43 3.19 0.243 0.226


S30 0-0.9m 7.40 7.29 0.525 0.517


S30 0.9-1.9m 8.22 8.10 0.583 0.575


S30 1.9-2.9m 8.44 8.31 0.599 0.590


D272 GRAB 1.19 1.14 0.030 0.029


S32 GRAB 3.39 3.19 0.240 0.226


D298 GRAB 0.25 0.13 0.019 0.010


S34 GRAB 3.00 2.90 0.235 0.228


S35 GRAB 0.56 0.47 0.044 0.037


S35 0-0.9m 1.48 1.32 0.116 0.103



3-57

Site Sampling Depth

NH3-N concentration

(mg L-1)


(mg L-1)

UIA concentration

(mg L-1)


(mg L-1)


S35 0.9-1.9m 0.78 0.62 0.061 0.049


S35 1.9-2.9m 1.49 1.32 0.117 0.103


D320 GRAB 2.59 2.48 0.210 0.201


D330 0-0.9M 0.38 0.33 0.009 0.008

D330 0.9-1.9M 0.11 0.06 0.003 0.001


D337 0-0.9M 0.35 0.30 0.008 0.007

D337 0.9-1.9M 0.44 0.39 0.011 0.009

D337 1.9-2.9M 0.67 0.62 0.016 0.015


S40 0-0.9M 0.44 0.39 0.009 0.008

S40 0.9-1.9M 1.45 1.40 0.029 0.028


D355 0-0.9M 1.01 0.96 0.020 0.019

D355 0.9-1.9M 0.49 0.44 0.010 0.009


D362 0-0.9M 2.66 2.61 0.058 0.057

D362 0.9-1.9M 8.66 8.61 0.188 0.187


S44 GRAB 1.37 1.30 0.115 0.109


D374 GRAB 0.62 0.55 0.051 0.045


S47 GRAB 0.66 0.61 0.053 0.049


D378 GRAB 0.82 0.71 0.070 0.060


D381 GRAB 2.19 2.14 0.182 0.178


S50 GRAB 1.84 1.80 0.157 0.153


D386 GRAB 1.85 1.81 0.157 0.153


Based on the above results, it was observed that the majority of the measured ammoniacal nitrogen values

were less than 5 mg L-1

, with some data in the range of 5 mg L-1

to 13.6 mg L-1

. It was also seen that within

the Project area, a hot spot was found at location S2, where its ammoniacal nitrogen value was recorded at

more than 20 mg L-1

. Upon examination it was surmised that the high reading could be due to historically



3-58

deposited contaminants from activities in the immediate area. Based on the above table, a worst case

scenario was presented for the assessment of ammoniacal nitrogen and UIA.

In addition to the worst case scenario mentioned above, a two-phase approach has also been adopted:

firstly, to assess impacts of the above elutriate test results by “surgically” removing S2 from the overall

data, and secondly, to assess the impacts contributed by the S2 sediment data on its own. The rationale

for adopting such an approach is to allow an assessment of the impacts of the “less contaminated” area i.e.

almost 99% of the dredging area to determine if compliance of the WQO’s and standards can be met

during the main portion of capital works dredging. If compliance cannot be met then mitigation measures

would need to be proposed and residual impacts assessed. The S2 portion of dredging works area has

thus been treated as a separate part of the overall dredging plan, and a highly specific approach to address

the impacts of S2 independently and thus determine a discrete package of mitigation measures for this

particular part of the overall dredging has been proposed.

Ammoniacal Nitrogen (worst case scenario i.e. with all samples including S2 considered,

unmitigated scenario): To assess the impact of ammoniacal nitrogen on the sensitive receivers, the

results from the sampling locations as indicated in Table 3.27 were grouped based on the different dredger

locations (or “source”) as shown in Figure 3.5a and summarised in Table 3.29 making reference to the

overall zonings for the dredging works as discussed in the Marine Traffic Impact Assessment prepared as

part of this Assignment. Table 3.28 below shows the averaged NH3-N values used in the assessment for

each source location.

Table 3.28: Ammoniacal Nitrogen Concentrations at Sediment Sources (All sampling locations included)

Sediment Source Sampling Locations Average NH3-N

concentration (mg L-1)

A S1-2, S2, S3, S4, S5-2, S6, S7, S8, S10, S11 4.47

B S13, S14, S18, S19, S21, D174, D196, D202 2.82

C D272, S32, D298, S34, S35, D320, S29, S30, D234, D238, D221, D214 2.84

D D330, D337, S40, D355, D362, S44 1.52

E D374, S47, D378, D381, S50, D386 1.27

Table 3.29: Combination of Scenarios

Scenario Grouping of Sediment Source

1 A + C + D

2 A + C + E

3 C + D + E

4 A + B + D

5 A + B + C

Note: Please refer to Figure 3.5a for various source locations

Based on the above table as well as dilution factors from the tracer model, the assessment of NH3-N has

been undertaken and full details of the analyses are contained in Appendix 3.12. From the predicted

results it is noted that full compliance is achieved at the corals, fish culture zones and beaches. Some non-

compliance has been identified at two of the WSD intakes based on the guideline of 1 mg L-1

NH3-N. The

following analysis therefore focuses on the seawater intakes and the results in Table 3.30 below show the

predicted concentrations at the various WSD seawater intakes for the worst case scenario.



3-59

Table 3.30: Predicted Elutriate Ammoniacal Nitrogen Concentrations at WSD Flushing Water Intakes (all sampling locations included)

Scenario 1

Elutriate NH3-N concentration (mg L-1)

Scenario 2


Scenario 3


Scenario 4


Scenario 5


Sensitive Receivers

Assess-ment Point

Dry Season Wet Season Dry Season Wet Season Dry Season Wet Season Dry Season Wet Season Dry Season Wet Season

WSD Flushing Water Intakes

Tsing Yi WSD1 1.24 1.14 1.23 1.12 0.09 0.10 1.48 1.28 1.52 1.32

Kennedy Town WSD2 0.10 0.17 0.10 0.16 0.07 0.13 0.10 0.14 0.14 0.19

Sheung Wan WSD3 0.06 0.20 0.06 0.18 0.02 0.11 0.10 0.23 0.12 0.26

Central Water Front WSD4 0.04 0.18 0.04 0.16 0.01 0.10 0.09 0.19 0.10 0.21

Ap Lei Chau WSD5 0.04 0.09 0.04 0.08 0.03 0.07 0.03 0.07 0.04 0.09

Kowloon South WSD6 0.08 0.19 0.08 0.16 0.01 0.10 0.24 0.24 0.24 0.25

Cheung Sha Wan WSD7 0.05 0.16 0.05 0.13 0.01 0.09 0.14 0.18 0.14 0.19

Tsuen Wan WSD8 0.93 0.35 0.92 0.33 0.10 0.08 1.08 0.39 1.13 0.41

Near Hong Kong Garden

WSD9 0.18 0.12 0.17 0.10 0.10 0.08 0.17 0.10 0.22 0.12

Lamma Power Station

WSD10 0.01 0.06 0.01 0.05 0.01 0.05 0.01 0.05 0.01 0.06

Kwai Chung Hospital

EMSD1 0.47 0.56 0.47 0.54 0.06 0.09 0.77 0.72 0.80 0.74

- Value in Bold indicates exceedance of relevant WSD’s Water Quality Criteria.



3-60

The guideline for NH3-N for WSD seawater water intakes is 1 mg L-1

. From the results presented above it

is noted that NH3-N levels exceed the guideline with predicted results of 1.52 mg L-1

and 1.32 mg L-1

at

WSD1 in the dry and wet seasons respectively. WSD8 is also predicted to have an exceedance of the NH3-

N value of 1.13 mg L-1

in dry season for most scenarios except scenario 3. Full compliance is however

predicted for the wet season at WSD8.

The concerns relating to exceedance of NH3-N at WSD’s seawater intakes are due to the possible use of

chemical treatment or physical reductions of the potential contaminant release. Addition of chemicals

would require modifications to current practices at the seawater pumping stations and thus alternatives

such as a physical reduction in the release of contaminants was preferred to address this issue. There is

evidence from work undertaken by the USEPA to support the view that by reducing the release rate of

materials (i.e. SS reduction through control of dredging rate) the contaminant releases can be

commensurately reduced. This mitigation measure is favoured over the addition of chemicals to seawater

at the intakes as it treats the problem at source without creating an additional concern of the use of

chemicals and appropriate (and possibly variable) dosing rates.

In this case the mitigation measures required to reduce the impacts on the WSD1 seawater intake would

however be significant if S2 were retained in the overall dredging plan. This would adversely impact the

dredging programme and the duration of dredging and could also prolong the impacts if dredging rates

were dramatically reduced. To address these exceedances in a more pragmatic manner, the two-phase

approach mentioned earlier has been developed.

Ammoniacal Nitrogen (with excision of hotspot S2, unmitigated): With this approach, Table 3.31

below indicates the averaged NH3-N values to be used in the assessment for each source location.

Table 3.31: Ammoniacal Nitrogen Concentrations at Sediment Sources (with excision of hotspot S2)

Source Sampling Locations Average NH3-N, mg L-1

A S1-2, S3, S4, S5-2, S6, S7, S8, S10, S11 3.40

B S13, S14, S18, S19, S21, D174, D196, D202 2.82

C D272, S32, D298, S34, S35, D320, S29, S30, D234, D238, D221, D214 2.84

D D330, D337, S40, D355, D362, S44 1.52

E D374, S47, D378, D381, S50, D386 1.27

Based on the above table as well as dilution factors from the tracer model, the assessment of NH3-N at the

various WSD seawater intakes for this scenario is shown in Table 3.32.



3-61

Table 3.32: Predicted Elutriate Ammoniacal Nitrogen Concentrations at WSD Flushing Water Intakes (with the excision of hotspot S2)

Scenario 1


Scenario 2


Scenario 3


Scenario 4


Scenario 5


Sensitive Receivers

Assess-ment Point

Dry Season Wet

Season Dry Season

Wet Season

Dry Season Wet

Season Dry Season

Wet Season

Dry Season Wet

Season


Tsing Yi WSD1 0.96 0.88 0.96 0.87 0.09 0.10 1.20 1.03 1.24 1.06

Kennedy Town WSD2 0.09 0.16 0.09 0.14 0.07 0.13 0.09 0.13 0.13 0.17

Sheung Wan WSD3 0.05 0.18 0.05 0.15 0.02 0.11 0.10 0.20 0.11 0.24

Central Water Front

WSD4 0.03 0.16 0.03 0.14 0.01 0.10 0.08 0.17 0.09 0.19

Ap Lei Chau WSD5 0.03 0.08 0.04 0.07 0.03 0.07 0.03 0.07 0.04 0.08

Kowloon South WSD6 0.07 0.17 0.07 0.14 0.01 0.10 0.22 0.21 0.22 0.23

Cheung Sha Wan WSD7 0.04 0.14 0.04 0.11 0.01 0.09 0.13 0.16 0.13 0.17

Tsuen Wan WSD8 0.73 0.28 0.72 0.27 0.10 0.08 0.87 0.32 0.92 0.34


WSD9 0.16 0.11 0.15 0.09 0.10 0.08 0.15 0.09 0.20 0.11

Lamma Power Station

WSD10 0.01 0.06 0.01 0.04 0.01 0.05 0.01 0.05 0.01 0.05

Kwai Chung Hospital

EMSD1 0.37 0.45 0.37 0.43 0.06 0.09 0.67 0.60 0.70 0.63




3-62

Based on the above results, it is noted that the maximum NH3-N concentrations predicted at WSD1 are

1.24 mg L-1

(dry season) and 1.06 mg L-1

(wet season) and at WSD8 are 0.92 mg L-1

(dry season) and 0.34

(wet season) respectively (Appendix 3.12). This means that full compliance with the WSD guideline is

achieved at WSD8, and marginal compliance achieved at WSD1 in the wet season. Mitigation measures

are thus only required for the dry season. However, to be prudent, mitigation measures backed up by

monitoring are recommended for both seasons as described in Section 3.8.

Apart from the WSD flushing water intake guideline for NH3-N, the only other guideline for this parameter in

the water control zones is the published guidelines for the protection of marine biota at 0.7 mg L-1

. The

impacts of the predicted NH3-N based on this guideline are discussed in Chapters 5 and 6.

Ammoniacal Nitrogen (Hotspot S2 only, unmitigated): Using the same approach, i.e. with S2 sediment

being the only source of dredging (source A), the averaged NH3-N values were used in the assessment for

predicting impacts of dredging S2 in isolation.

Based on the results obtained and contained in Appendix 3.12 as well as dilution factors from the tracer

model, the assessment of NH3-N at the various sensitive receivers also indicates non-compliance at some

WSD seawater intakes for this scenario as shown in Table 3.33.

Table 3.33: Predicted Elutriate Ammoniacal Nitrogen Concentrations at WSD Flushing Water Intakes (for only hotspot

S2 case)

Elutriate NH3-N concentration (mg L-1) Sensitive Receivers Assessment Point



Tsing Yi WSD1 3.66 3.33

Kennedy Town WSD2 0.13 0.20

Sheung Wan WSD3 0.12 0.31

Central Water Front WSD4 0.10 0.27

Ap Lei Chau WSD5 0.04 0.09

Kowloon South WSD6 0.24 0.31

Cheung Sha Wan WSD7 0.14 0.23

Tsuen Wan WSD8 2.67 0.87

Near Hong Kong Garden WSD9 0.27 0.14

Lamma Power Station WSD10 0.01 0.06

Kwai Chung Hospital EMSD1 1.34 1.51


Not unexpectedly given the proximity of source A (i.e. S2) to WSD1 seawater intake, the results indicate

that WSD1 has the highest predicted NH3-N level of 3.66 mg L-1

and 3.33 mg L-1

in both the dry and wet

season respectively. A level of 2.67 mg L-1

is predicted at WSD8 for the dry season with compliance

predicted during the wet season. A value of 1.34 mg L-1

and 1.51 mg L-1

of NH3-N is anticipated at EMSD1.

In order to comply with the WSD seawater intake criteria, mitigation measures would thus be required and

are described in Section 3.8.



3-63

Unionized Ammonia (worst case scenario i.e. with all samples including S2 considered,

unmitigated): To assess the impact of UIA on the sensitive receivers, the results from the sampling

locations as indicated in Table 3.27 were similarly grouped based on the dredger locations as shown in

Figure 3.5a. As mentioned in Section 3.5.2, the UIA values were derived based on the measured NH3-N

values; as such, the average UIA value is 0.327 mg/L for Source A, 0.136 mg/L for Source B, 0.195 mg/L

for Source C, 0.046 mg/L for Source D and 0.107 mg/L for Source E.

Based on the dilution factors from the tracer model, the assessment of UIA for the various sensitive

receivers for the worst case scenario is shown in Table 3.34.



3-64

Table 3.34: Predicted Elutriate UIA Concentrations at Gazetted Beaches, Marine Ecology, and Fisheries Sensitive Receivers (all sampling locations included)

Scenario 1

Elutriate UIA concentration (mg L-1)

Scenario 2


Scenario 3


Scenario 4


Scenario 5


Sensitive Receivers

Assessment Point

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.011 0.010 0.011 0.011 0.010 0.011 0.007 0.007 0.007 0.010 0.008 0.009 0.013 0.011 0.012

Approach B2 0.065 0.011 0.038 0.066 0.011 0.039 0.007 0.005 0.006 0.068 0.010 0.039 0.073 0.012 0.043

Ting Kau B3 0.051 0.010 0.031 0.051 0.010 0.031 0.006 0.005 0.006 0.052 0.009 0.031 0.056 0.011 0.034

Lido B4 0.033 0.008 0.021 0.033 0.008 0.021 0.007 0.005 0.006 0.032 0.007 0.019 0.037 0.009 0.023

Casam B5 0.016 0.009 0.013 0.017 0.009 0.013 0.007 0.006 0.006 0.015 0.007 0.011 0.019 0.010 0.015

Hoi Mei Wan B6 0.015 0.009 0.012 0.015 0.009 0.012 0.007 0.006 0.006 0.013 0.007 0.010 0.018 0.010 0.014

Gemini B7 0.014 0.009 0.011 0.015 0.009 0.012 0.007 0.006 0.006 0.012 0.007 0.010 0.017 0.010 0.013

Angler’s B8 0.014 0.009 0.011 0.014 0.009 0.011 0.007 0.006 0.006 0.012 0.007 0.009 0.016 0.010 0.013

Lo So Shing B9 0.001 0.004 0.002 0.001 0.004 0.002 0.001 0.003 0.002 0.000 0.003 0.002 0.001 0.004 0.002

Hung Shing Yeh

B10 0.001 0.004 0.002 0.001 0.004 0.002 0.001 0.003 0.002 0.000 0.003 0.002 0.001 0.004 0.002

Corals

Pak Kok CR1 0.007 0.007 0.007 0.007 0.007 0.007 0.005 0.006 0.005 0.005 0.005 0.005 0.007 0.007 0.007

Shek Kok Tsui

CR2 0.005 0.007 0.006 0.005 0.007 0.006 0.004 0.005 0.005 0.004 0.005 0.005 0.006 0.007 0.006

Luk Chau CR3 0.003 0.003 0.003 0.004 0.004 0.004 0.003 0.003 0.003 0.003 0.002 0.003 0.004 0.004 0.004

Wong Chuk Kok

CR4 0.003 0.002 0.002 0.003 0.002 0.003 0.002 0.002 0.002 0.002 0.002 0.002 0.003 0.002 0.003

Ap Lei Chau CR5 0.003 0.004 0.004 0.004 0.004 0.004 0.003 0.003 0.003 0.002 0.003 0.003 0.004 0.004 0.004

Sandy Bay CR6 0.004 0.008 0.006 0.005 0.010 0.008 0.003 0.008 0.006 0.003 0.006 0.005 0.005 0.009 0.007

Green Island CR7 0.010 0.008 0.009 0.010 0.008 0.009 0.007 0.006 0.007 0.006 0.006 0.006 0.012 0.009 0.010

Kau Yi Chau CR8 0.009 0.010 0.009 0.009 0.010 0.009 0.006 0.007 0.006 0.007 0.008 0.008 0.010 0.011 0.011

Kau Yi Chau CR9 0.009 0.010 0.009 0.009 0.010 0.009 0.006 0.006 0.006 0.007 0.008 0.007 0.010 0.011 0.011

Kau Yi Chau CR10 0.008 0.008 0.008 0.009 0.008 0.009 0.006 0.006 0.006 0.007 0.006 0.007 0.010 0.009 0.010



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Scenario 1


Scenario 2


Scenario 3


Scenario 4


Scenario 5


Sensitive Receivers

Assessment Point

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Siu Kau Yi Chau

CR11 0.009 0.010 0.010 0.009 0.010 0.010 0.006 0.007 0.006 0.007 0.007 0.007 0.011 0.011 0.011

Siu Kau Yi Chau

CR12 0.009 0.010 0.009 0.009 0.010 0.010 0.006 0.006 0.006 0.007 0.007 0.007 0.011 0.011 0.011

Siu Kau Yi Chau

CR13 0.009 0.008 0.009 0.009 0.008 0.009 0.006 0.006 0.006 0.007 0.006 0.007 0.010 0.009 0.010

Peng Chau CR14 0.009 0.009 0.009 0.009 0.009 0.009 0.006 0.006 0.006 0.007 0.007 0.007 0.010 0.010 0.010

Peng Chau CR15 0.008 0.008 0.008 0.008 0.008 0.008 0.005 0.006 0.005 0.006 0.006 0.006 0.009 0.009 0.009

Peng Chau CR16 0.009 0.009 0.009 0.009 0.009 0.009 0.006 0.006 0.006 0.007 0.006 0.007 0.010 0.010 0.010

Peng Chau CR17 0.008 0.009 0.008 0.009 0.009 0.009 0.005 0.006 0.006 0.007 0.006 0.007 0.010 0.010 0.010

Peng Chau CR18 0.008 0.007 0.008 0.008 0.007 0.008 0.005 0.005 0.005 0.006 0.005 0.006 0.009 0.008 0.009

Fish Culture Zones

Ma Wan F1 0.010 0.007 0.009 0.010 0.007 0.009 0.006 0.005 0.006 0.008 0.006 0.007 0.012 0.008 0.010

Lo Tik Wan F2 0.002 0.006 0.004 0.003 0.006 0.004 0.002 0.005 0.003 0.002 0.004 0.003 0.003 0.006 0.004

Sok Kwu Wan

F3 0.002 0.004 0.003 0.002 0.004 0.003 0.002 0.003 0.002 0.001 0.003 0.002 0.002 0.004 0.003

Cheung Sha Wan

F4 0.005 0.006 0.006 0.005 0.006 0.006 0.003 0.005 0.004 0.004 0.005 0.004 0.006 0.007 0.006

- Value in Bold indicates exceedance of relevant annual mean criteria.



3-66

Based on the above results, it is observed that there are no predicted adverse effects at any fish culture

zones or coral sites. This is also true at the gazetted beaches, with Scenario 3 indicating full compliance of

the WQO criteria (for all dry season, wet season and annual average values), whilst all other scenarios

comply for the wet season. Exceedance of the WQO’s UIA (annual mean) criterion of 0.021 mg L-1

is

predicted for the dry season at the Ting Kau, Approach and Lido beaches, with maximum annual average

of 0.043 mg L-1

at Approach (B2), 0.034 mg L-1

at Ting Kau (B3), and 0.023 mg L-1

at Lido (B4).

In the dry season as the worst case, the predicted maximum UIA values are 0.073 mg L-1

at Approach

(B2), 0.056 mg L-1

at Ting Kau (B3) and 0.037 mg L-1

at Lido (B4).

Using a similar approach as for the ammoniacal nitrogen assessment, a two-phase approach for UIA was

also adopted, as elaborated further below.

Unionized Ammonia (with excision of hotspot S2, unmitigated): With this approach, the average UIA

values at various sources are 0.248 mg/L for Source A, 0.136 mg/L for Source B, 0.195 mg/L for Source C,

0.046 mg/L for Source D and 0.107 mg/L for Source E. Based on the dilution factors, the assessment of

UIA for this scenario is shown in Table 3.35.

Consultancy Agreement No. CE 63/2008 Providing Sufficient Water Depth for Kwai Tsing Container Basin and its Approach Channel

Environmental Impact Assessment Report


3-67

Table 3.35: Predicted Elutriate UIA Concentrations at Gazetted Beaches, Marine Ecology, and Fisheries Sensitive Receivers (with the excision of hotspot S2)

Scenario 1


Scenario 2


Scenario 3


Scenario 4


Scenario 5


Sensitive Receivers

Assessment Point

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.010 0.009 0.009 0.010 0.009 0.010 0.007 0.007 0.007 0.008 0.007 0.007 0.012 0.010 0.011

Approach B2 0.051 0.009 0.030 0.051 0.010 0.030 0.007 0.005 0.006 0.054 0.008 0.031 0.058 0.011 0.034

Ting Kau B3 0.040 0.009 0.024 0.040 0.009 0.025 0.006 0.005 0.006 0.041 0.007 0.024 0.045 0.010 0.028

Lido B4 0.026 0.007 0.017 0.027 0.007 0.017 0.007 0.005 0.006 0.026 0.006 0.016 0.030 0.008 0.019

Casam B5 0.014 0.008 0.011 0.014 0.008 0.011 0.007 0.006 0.006 0.012 0.006 0.009 0.017 0.009 0.013

Hoi Mei Wan B6 0.013 0.008 0.010 0.013 0.008 0.011 0.007 0.006 0.006 0.011 0.006 0.008 0.016 0.009 0.012

Gemini B7 0.012 0.008 0.010 0.013 0.008 0.010 0.007 0.006 0.006 0.010 0.006 0.008 0.015 0.009 0.012

Angler’s B8 0.012 0.008 0.010 0.012 0.008 0.010 0.007 0.006 0.006 0.010 0.006 0.008 0.014 0.009 0.012

Lo So Shing B9 0.001 0.003 0.002 0.001 0.003 0.002 0.001 0.003 0.002 0.000 0.002 0.001 0.001 0.004 0.002

Hung Shing Yeh

B10 0.001 0.003 0.002 0.001 0.003 0.002 0.001 0.003 0.002 0.000 0.002 0.001 0.001 0.004 0.002

Corals

Pak Kok CR1 0.006 0.006 0.006 0.006 0.006 0.006 0.005 0.006 0.005 0.004 0.005 0.005 0.007 0.007 0.007

Shek Kok Tsui

CR2 0.005 0.006 0.005 0.005 0.006 0.006 0.004 0.005 0.005 0.004 0.005 0.004 0.005 0.006 0.006

Luk Chau CR3 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.002 0.002 0.002 0.003 0.003 0.003

Wong Chuk Kok

CR4 0.002 0.002 0.002 0.003 0.002 0.003 0.002 0.002 0.002 0.002 0.001 0.002 0.003 0.002 0.002

Ap Lei Chau CR5 0.003 0.004 0.003 0.004 0.004 0.004 0.003 0.003 0.003 0.002 0.003 0.002 0.004 0.004 0.004

Sandy Bay CR6 0.004 0.008 0.006 0.004 0.010 0.007 0.003 0.008 0.006 0.003 0.006 0.004 0.005 0.008 0.006

Green Island CR7 0.009 0.007 0.008 0.009 0.008 0.009 0.007 0.006 0.007 0.006 0.005 0.005 0.011 0.008 0.010

Kau Yi Chau CR8 0.008 0.009 0.008 0.008 0.009 0.008 0.006 0.007 0.006 0.006 0.007 0.007 0.009 0.010 0.010

Kau Yi Chau CR9 0.008 0.009 0.008 0.008 0.009 0.008 0.006 0.006 0.006 0.006 0.007 0.006 0.009 0.010 0.010

Kau Yi Chau CR10 0.007 0.008 0.008 0.008 0.008 0.008 0.006 0.006 0.006 0.006 0.005 0.006 0.009 0.008 0.009




3-68

Scenario 1


Scenario 2


Scenario 3


Scenario 4


Scenario 5


Sensitive Receivers

Assessment Point

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Siu Kau Yi Chau

CR11 0.008 0.009 0.009 0.008 0.009 0.009 0.006 0.007 0.006 0.006 0.006 0.006 0.010 0.010 0.010

Siu Kau Yi Chau

CR12 0.008 0.009 0.008 0.008 0.009 0.008 0.006 0.006 0.006 0.006 0.006 0.006 0.010 0.010 0.010

Siu Kau Yi Chau

CR13 0.008 0.008 0.008 0.008 0.008 0.008 0.006 0.006 0.006 0.006 0.005 0.006 0.009 0.008 0.009

Peng Chau CR14 0.008 0.008 0.008 0.008 0.008 0.008 0.006 0.006 0.006 0.006 0.006 0.006 0.009 0.009 0.009

Peng Chau CR15 0.007 0.007 0.007 0.007 0.007 0.007 0.005 0.006 0.005 0.006 0.005 0.005 0.008 0.008 0.008

Peng Chau CR16 0.008 0.008 0.008 0.008 0.008 0.008 0.006 0.006 0.006 0.006 0.006 0.006 0.009 0.009 0.009

Peng Chau CR17 0.007 0.008 0.008 0.008 0.008 0.008 0.005 0.006 0.006 0.006 0.006 0.006 0.009 0.009 0.009

Peng Chau CR18 0.007 0.006 0.007 0.007 0.006 0.007 0.005 0.005 0.005 0.006 0.005 0.005 0.008 0.007 0.008

Fish Culture Zones

Ma Wan F1 0.009 0.007 0.008 0.009 0.007 0.008 0.006 0.005 0.006 0.007 0.005 0.006 0.011 0.007 0.009

Lo Tik Wan F2 0.002 0.005 0.004 0.003 0.006 0.004 0.002 0.005 0.003 0.002 0.004 0.003 0.002 0.006 0.004

Sok Kwu Wan

F3 0.002 0.003 0.002 0.002 0.004 0.003 0.002 0.003 0.002 0.001 0.003 0.002 0.002 0.004 0.003

Cheung Sha Wan

F4 0.004 0.006 0.005 0.004 0.006 0.005 0.003 0.005 0.004 0.003 0.004 0.004 0.005 0.006 0.006




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The WQO stipulates a (annual mean) criterion of 0.021 mg L-1

for the UIA. Based on the prediction, there

is no adverse impact to Fish Culture Zones (FCZ) and Coral Sites. In addition, full compliance of the WQO

criteria is also achieved for Scenario 3 for all WSR’s.

Regarding compliance with WQO UIA annual average criteria of 0.021 mg/L, marginal compliance is

achieved based on the annual average results for these other scenarios, with 0.034 mg L-1

at B2 (Approach

Beach) and 0.028 mg L-1

at B3 (Ting Kau). For the wet season, the results have shown no UIA value higher

than 0.021 mg/L at all beaches and WSRs with maximum predictions of 0.058 mg L-1

at B2 (Approach

Beach), 0.045 mg L-1

at B3 (Ting Kau) and 0.030 mg L-1

at B4 (Lido) during the dry season.

As noted previously, although compliance is based on annual results, the analysis indicates that it would be

prudent to provide mitigation measure for the dredging operations in the dry season, whilst mitigation is not

anticipated during the wet season, which is not unexpected as the dilution effects are greater in the wet

season compared to the dry season. Notwithstanding the above, a robust monitoring regime will be

required to be provided for both dry and wet season working for these areas, as discussed in Section 3.8.

Unionized Ammonia (hotspot S2 only, unmitigated): With this approach, the value of S2 in Table 3.27

was used in conjunction with the dilution factors to assess the UIA at the sensitive receivers. The results

for this scenario are shown in Table 3.36.

Table 3.36: Predicted Elutriate Unionized Ammonia Concentrations at Water Sensitive Receivers (for only hotspot S2

case)

Elutriate UIA concentration (mg L-1) Sensitive Receivers Assessment Point

Dry Season Wet Season Annual average

Gazetted Beaches

Tung Wan, Ma Wan B1 0.018 0.014 0.016

Approach B2 0.190 0.024 0.107

Ting Kau B3 0.145 0.020 0.083

Lido B4 0.087 0.013 0.050

Casam B5 0.033 0.013 0.023

Hoi Mei Wan B6 0.029 0.013 0.021

Gemini B7 0.026 0.012 0.019

Angler’s B8 0.025 0.012 0.018

Lo So Shing B9 0.001 0.005 0.003

Hung Shing Yeh B10 0.001 0.005 0.003

Corals

Pak Kok CR1 0.009 0.009 0.009

Shek Kok Tsui CR2 0.007 0.008 0.007

Luk Chau CR3 0.004 0.004 0.004

Wong Chuk Kok CR4 0.003 0.003 0.003

Ap Lei Chau CR5 0.004 0.005 0.004

Sandy Bay CR6 0.005 0.011 0.008

Green Island CR7 0.010 0.010 0.010

Kau Yi Chau CR8 0.012 0.015 0.013

Kau Yi Chau CR9 0.013 0.014 0.013



3-70

Elutriate UIA concentration (mg L-1) Sensitive Receivers Assessment Point

Dry Season Wet Season Annual average

Kau Yi Chau CR10 0.012 0.011 0.012

Siu Kau Yi Chau CR11 0.014 0.014 0.014

Siu Kau Yi Chau CR12 0.014 0.014 0.014

Siu Kau Yi Chau CR13 0.013 0.011 0.012

Peng Chau CR14 0.013 0.012 0.012

Peng Chau CR15 0.012 0.011 0.011

Peng Chau CR16 0.013 0.012 0.012

Peng Chau CR17 0.012 0.011 0.012

Peng Chau CR18 0.012 0.009 0.010

Fish Culture Zones

Ma Wan F1 0.015 0.010 0.013

Lo Tik Wan F2 0.003 0.008 0.005

Sok Kwu Wan F3 0.002 0.005 0.003

Cheung Sha Wan F4 0.007 0.008 0.008


Tsuen Wan C1 0.195 0.067 0.131

MTRC Tsing Yi Station C2 0.227 0.216 0.221

MTRC Kowloon Station C3 0.008 0.019 0.013

China H.K. City C4 0.007 0.021 0.014

Sha Wan Drive C5 0.005 0.012 0.009

Queen Mary Hospital C6 0.005 0.010 0.008

Wah Fu Estate C7 0.003 0.006 0.005

Kwai Chung Hospital EMSD1 0.098 0.111 0.105


Tsing Yi WSD1 0.269 0.244 0.257

Kennedy Town WSD2 0.009 0.014 0.012

Sheung Wan WSD3 0.008 0.023 0.016

Central Water Front WSD4 0.007 0.019 0.013

Ap Lei Chau WSD5 0.003 0.007 0.005

Kowloon South WSD6 0.017 0.023 0.020

Cheung Sha Wan WSD7 0.011 0.017 0.014

Tsuen Wan WSD8 0.196 0.064 0.130

Near Hong Kong Garden WSD9 0.020 0.010 0.015

Lamma Power Station WSD10 0.001 0.005 0.003

Kwai Chung Hospital EMSD1 0.098 0.111 0.105


It is observed from the assessment of S2 alone in terms of UIA that annual average UIA values are

exceeded for the beaches at Approach, Ting Kau, Lido and Casam compared with WQO UIA annual mean

criteria. Only the Approach Beach has UIA value higher than 0.021 mg/L in the wet season, whilst all the

gazetted beaches except for Tung Wan, Ma Wan, Lo So Shing and Hung Shing Yeh are affected by UIA in

the dry season (i.e. > 0.021 mg/L) with values ranging from 0.025 mg L-1

to 0.190 mg L-1

.



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A comparatively smaller number of the other sensitive receivers such as cooling water and flushing water

intakes were also predicted to exceed the UIA criterion as shown in Table 3.36. As mentioned previously,

the S2 location is considered a hotspot which needs to be dealt with separately, i.e. isolated from the rest

of the Project area in order not to adversely affect the overall programme of the navigational dredging

(almost 99% of the total dredging area). As such, careful planning of dredging such as carry out field trials,

formulation of dredging sub-zones with the control of dredging rate together with the implementation of

mitigation measures are recommended for this area and are described in Section 3.8.

Mixing Zone

Tracer simulations have been carried out for the 5 source points (i.e. Source A to E) and under the five

combinations of dredging in different zones, referred to as scenarios, in order to determine the maximum

dimensions of mixing zones for the ammoniacal nitrogen (NH3-N) and unionized ammonia (UIA). In each

scenario there are 3 source points according to the combinations shown in Figure 3.5b to Figure 3.5f.

Both dry and wet seasons are covered in the simulations.

In practical terms the mixing zone of the contaminants released from the dredging operation will move as

the dredging proceeds. However in order to simulate the mixing zone the model scenarios have been

derived to demonstrate the worst case situation and the maximum values of pollutant levels in the mixing

zone. The area shown in the mixing zone plots do not therefore represent the actual boundary of

exceedance, rather they may be considered to be the envelope of the extent of the mixing zone over the

entire simulation period. The mixing zone plots of contaminants (NH3-N and UIA) for the dry and wet

seasons of Scenario 1 to 5 are shown in Appendix 3.11.

From the plots of the mixing zones, it can be seen that the areas of exceedance of NH3-N and UIA are

confined to the dredging areas for most of the source points in each of the five scenarios. This is because

contaminants released during dredging would be quickly diluted by the large volume of marine water within

the dredging zone and the high energy i.e. dispersion forces in the Project areas. The area of exceedance

is predicted to be highly localized and confined within 200m for most of the dredging locations. Impact on

the fisheries resources from the potential contaminant release would be limited since the boundaries of

mixing zones are remote from Fisheries Sensitive Receivers.

Cumulative Impact

The cumulative impacts (Scenario 7) associated with the release of TIN and UIA, of both this Project and

other concurrent projects were also assessed on the basis of the mixing zone. In accordance with the

approved EIAs of the Hong Kong Boundary Crossing Facility, the HZMB Hong Kong Link Road and Tuen

Mun Chek Lap Kok Link, the maximum extent of the mixing zones of both TIN and UIA was predicted to be

confined within 560m from the pollution source. With reference to the approved EIA of the Wan Chai

Development Phase II, the predicted mixing zones were well within that project site. This indicates that the

mixing zones of TIN and UIA generated by the concurrent projects are likely to be in the close proximity of

the project areas or the pollution sources and unlikely to intercept the mixing zones of this Project, in view

of the long distance between the concurrent projects and this Project. It is hence anticipated that the

cumulative impacts of the concurrent projects are insignificant.

3.7.1.7 Impact of Disturbance of Seabed Sediment due to Marine Traffic

Aerial photographs of the Project area indicate that in some cases, the large container vessels appear to

generate sediment plumes which could be due to propeller wash. In order to assess whether these plumes



3-72

could combine with SS released during the dredging operations, an assessment was carried out which

included reference to the findings of the Marine Traffic Impact Assessment undertaken for this Project.

Dredging

Under worst case conditions, two dredgers would work within the KTCB area. Each dredger setup will

consist of one tugboat, one hopper barge/derrick barge and one dredger. A typical tugboat registered in

Hong Kong (such as “Cheung Chau”) has a draft of 3.81m while the draft of a Cutter Suction Dredger

suitable for the project use (“Hercules”, Van Oord) is 3.65m. Considering the depth of water in KTCB is

more than 15m, it is expected that the impact due to propeller wash from these vessels is minimal since

only tugboats are self-powered among these vessels. In addition, the Project area is one of the busiest

navigation channels for ocean-going container vessels of which their drafts can be up to 12m. The small

number of construction vessels (propelled tugboat; un-propelled dredger and hopper barge/derrick barge)

used for this Project is expected to have minimal disturbance of seabed sediment comparatively.

Nevertheless, Section 3.8 provides best practice measures to further minimize the impact from propeller

wash.

Other marine traffic

Under the current situation, all large-sized vessels (especially ocean-going container vessels) power off

their propellers and are pulled by tugboats to the berths when they navigate the KTCB area. As mentioned

above, the tugboats will not have a significant impact on the seabed sediment due to their shallow drafts.

However, the deep drafts of the container vessels have been shown to disturb the seabed sediment,

resulting in sediment plumes. However, the increase in suspended solids in the marine water due to these

vessels has been reflected in the background water quality monitoring data and is beyond the control of

this Project.

The issue relating to this Project is the potential disturbance of the area being dredged due to passing

vessels and the potential combination of sediment plumes from the dredging works and passing vessels.

From the MTIA conducted under this Project it is noted that the master of a vessel would attempt to remain

a certain distance away from other vessels, fixed objects and other potential hazards to maintain navigation

safety; therefore, it is expected that the passing vessels will be at a reasonable distance from the dredging

area. A 50m buffer also has been allowed for a vessel’s typical manoeuvring requirements and the

escorting tugboats in the assessment of the marine traffic impact.

It is further noted that the disturbance to the seabed due to the dredging activities is confined to the

dredging area which will be contained within a silt curtain. This mitigation measure as described in Section

3.8 will not only serve to prevent the dispersion of sediment from the dredging area but also to screen off

any disturbance from other marine vessels. Furthermore, non-project-related marine vessels are not

expected to be navigating close to the dredgers for safety reasons. Therefore no impacts due to the non-

project marine vessels are expected.

3.7.2 Operational Phase

3.7.2.1 Impact of Suspended Solids due to Maintenance Dredging

The need for maintenance dredging has been described in Section 2.7 along with the associated details

and assumptions. For the water quality assessment of this particular type of dredging, a similar approach



3-73

has been adopted as for the capital dredging works, i.e., by assessing the volume of dredged material and

considering the number of dredgers involved in the works and duration of works.

From Section 2.7 it has been identified that maintenance dredging of the KTCB and its Approach Channel

would be undertaken by Port Works Department (PWD) and would not exceed 30,000 m3 per annum.

Clearly this maintenance dredging activity would not be spread out over the entire year but rather to a

programme to suit the needs and requirements of the works. For the purposes of this water quality

assessment it was assumed that one dredger and barge configuration would be involved in the

maintenance works on a daily basis. This would be reasonable and in accordance with previous

maintenance dredging operations. Moreover the work would not be conducted over a 24 hour period as for

the capital dredging works, but more likely over a 12-16 hour period. Even if the same rate of dredging

could be achieved for the maintenance dredging as for the capital works, this infers that only 4,000m3

(maximum and in-situ volume) would be dredged on a daily basis. In the event that a rate of 4,000m3 could

be achieved, this would equate to approximately 8 days of maintenance dredging compared to the 24

months for capital dredging. The duration is therefore much shorter than for the capital works programme.

In terms of the release of suspended solids to the receiving waters, the model predictions for the capital

works programme indicate compliance with the WQOs except at WSD’s flushing water intakes. Therefore,

it may be surmised that similar compliance would be achieved during periods of maintenance dredging for

this Project except at the WSD flushing water intakes which are influenced by the ambient levels of SS.

Indeed, the SS concentrations predicted at the WSRs could be surmised to be almost one third of those for

the capital works programme given the reduction in dredging rate on a daily basis.

Additionally, as mentioned in Section 2.7 the Container Terminal Operators (CTO) undertakes

maintenance dredging within the berth boxes. The Container Terminal Operators have provided their

records for maintenance dredging since year 2000 and these are extracted from Section 2.7 and copied

into Table 3.37.

Table 3.37: Volume of Maintenance Dredging undertaken by Container Terminal Operators; 2000 - 2009

Approximate Maintenance Dredge Volumes (m3) from Year 2000 to 2009 Terminal No.

Operator 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

ANNUAL

MAX

1 MTL 15,667 -- 53,0001 -- -- -- -- 12,333 -- 12,667 26,5001

2 MTL 15,667 12,500 12,500 -- -- -- -- 12,333 -- 12,667 15,667

3 DPW -- -- 9,000 11,000 1,600 -- -- -- 17,000 -- 17,000

4 HIT -- 50,0003 -- -- -- -- 65,0002 -- -- -- 50,0003

5 MTL 15,667 12,500 12,500 -- -- -- -- 12,333 -- 12,667 15,667

6 HIT 50,0003 50,0003 10,000 -- 35,000 -- 65,0002 -- 19,800 -- 50,0003

7 HIT 50,0003 -- 10,000 -- 35,000 -- -- -- 19,800 13,000 50,0003

8 East4 COSCO/HIT -- -- -- -- -- -- -- -- -- -- 0

8 West4 ACT -- -- -- -- -- -- -- -- -- -- 0

9 HIT -- -- -- -- -- -- -- -- 4,300 20,000 20,000

Totals 147,000 125,000 107,000 11,000 71,600 0 130,000 37,000 60,900 71,000

Notes:

1. Figure for CT1 in 2002 covers two years, so annual maximum is half of this value.

2. These figures include lowering of the seabed and are therefore not taken as maintenance dredging maxima.



3-74

3. HIT has advised that it will not undertake more than 100,000m3 maintenance dredging annually.

4. COSCO/HIT and ACT have both advised they have no plans to undertake maintenance dredging.

It has been estimated that around 225,000m3 of maintenance dredging could be undertaken in the KTCP

(that is including the 30,000m3 of maintenance dredging assumed for this Project) (refer to Section 2.7) on

an annual basis compared to 4.4Mm3 capital works dredging for this Project over 24 months. Given that

historically the CTO’s schedule their maintenance dredging activities to minimise disturbance to overall

operations of the KTCP it is reasonable to surmise that there will be no greater dredging activity than has

been predicted for the capital works programme. Thus, it may be concluded that concurrent maintenance

dredging programmes will not generate increased impacts compared to those predicted for capital dredging

works.

Given the discussions relating to maintenance dredging for the KTCB and its Approach Channel and the

acceptability of the impacts, it may similarly be surmised that maintenance dredging works undertaken by

the CTO would generate similar or lower levels of SS at the WSRs. Selected results from the water quality

monitoring programme for dredging at Kwai Tsing Container Terminal 1-4 are included in Table 3.41 for

reference.

Sensitivity test on a Lower Dredging rate of 1,500 m3/day

A sensitivity test has been carried out to determine the potential difference if the dredging rate is lowered

from 12,000 m3/day (with 3 dredgers) to 1,500 m

3/day (only 1 dredger) and the results are shown in

Appendix 3.10. Please note that Scenario 3 was chosen as the scenario for the cumulative impact

assessment as described in Table 3.9 with other concurrent projects. As for maintenance dredging, there

will only be one grab dredger deployed for the operation instead of 3 grab dredgers, Source C of scenario

was selected as the source for this sensitive test, i.e. cumulative impact during maintenance dredging.

Comparison of the results for the two cases shows that the impacts of SS elevation, DO depletion, and TIN

elevation will be reduced at most of the WSRs; however, the effects vary for each WSRs due to influences

from concurrent projects and background pollutant concentrations.

Predicted Suspended Solids Elevations at Gazetted Beaches, Marine Ecology, and Fisheries

Sensitive Receivers for Dredging Rate of 1,500 m3/day compared to 12,000 m

3/day

The predicted SS elevations for Gazetted Beaches differ the most at B2 in dry season (SS dropped from 1

to 0.8 mg L-1

) and B4 in wet season (SS dropped from 1.6 to 1.2 mg L-1

) (refer to Table 3.38).

The predicted SS elevations for Marine Ecology Sensitive Receivers differ the most at CR1 in dry season

(SS dropped from 1 to 0.3 mg L-1

) and CR9 in wet season (SS dropped from 3.7 to 1.8 mg L-1

).

However, the predicted SS elevations SS elevations for Fisheries Sensitive Receivers are insignificantly

changed. This is because the majority of the SS elevations at these Sensitive Receivers are due to

concurrent projects.

The WQO requires that the SS elevations due to project works should not exceed 30% of the ambient SS

levels. Referring to the predicted values in Scenario 7, there is no predicted exceedance of SS levels at

Gazetted Beaches, Marine Ecology, and Fisheries Sensitive Receivers. Therefore, under the maximum

dredging rate of 1,500 m3/day case, it may be surmised that there will be no exceedance of water quality

objectives or guidelines.



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Table 3.38: Maximum Reduction in predicted SS elevations at Gazetted Beaches, Marine Ecology and Fisheries

Sensitive Receivers for the Scenario of 1,500 m3/day at Source C Compared to 12,000 m

3/day

Dry season Wet season

Reduction in predicted SS elevations (mg L-1)

Corresponding WSR

Ref. SS criteria (mg L-1)


Corresponding WSR

Ref. SS criteria (mg L-1)

Gazetted Beaches

0.2 B2 2.8 - 3.2 0.4 B4 1.9 – 2.5

Marine Ecology 0.7 CR1 1.8 - 2.6 1.9 CR9 1.6 - 4.4

Fisheries Sensitive Receivers

0 - 1.8 - 3.2 0 - 1.6 - 4.4

Predicted Suspended Solids Elevations at Cooling and Sea Water Intakes for Dredging Rate of

1,500 m3/day compared to 12,000 m

3/day

In the case of Cooling and Sea Water Intakes, the predicted surface layer Suspended Solids elevations

were lowered by a maximum of 0.3 mg L-1

at WSD2 for dry season and 0.1 mg L-1

at C6 and WSD2 for wet

season. The predicted depth averaged Suspended Solids elevations were lowered by a maximum of 0.4

mg L-1

at WSD9 for dry season and 0.5 mg L-1

at C5 for wet season (refer to Table 3.39).

To determine the compliance of the predicted Suspended Solids with the relevant criteria, the values of

suspended solids elevations in addition to the background SS levels were assessed. There are still

exceedances predicted at WSD1, WSD8, WSD9 and EMSD1 since the background SS levels have already

breached the SS criterion for WSD flushing water intake.

Table 3.39: Maximum Reduction in predicted SS elevations at Cooling and Flushing Water Intakes for the Scenario of

1,500 m3/day at Source C Compared to 12,000 m

3/day

Dry season Wet season


Corresponding WSR

Ref. SS criteria (Total SS in mg L-1)


Corresponding WSR

Ref. SS criteria (Total SS in mg L-1)

Cooling Water Intakes (surface layer)

0.2 C5 - 0.1 C6 -

0.3 WSD2 <10 0.1 WSD2 <10

0.2 WSD1 (for reference)

<10 0.0 WSD1 (for reference)

<10

WSD Flushing Water Intakes (surface layer)



<10

Cooling Water Intakes (depth averaged)

0.3 C5 <180 for EMSD1

0.5 C5 <180 for EMSD1

0.4 WSD9 <10 0.1 WSD2 <10



<10

WSD Flushing Water Intakes (depth averaged)



<10



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Reduction in DO depletions for Dredging Rate of 1,500 m3/day compared to 12,000 m

3/day

From the comparison, the difference in DO depth average value of the two cases (1,500 m3/day and

12,000 m3/day) is very small. The difference in DO depletions, ranged from 0.001 to 0.007 mg L

-1 (refer to

Table 3.40)., is insignificant compared to background DO levels 2.6 to 6.2 mg L-1

. Although there are

predicted exceedances at various WSRs in wet seasons, the exceedances are due to low ambient DO

level in the marine water. It is therefore predicted that the change in production rate of dredging has not

affected the DO concentration in the marine water.

Table 3.40: Maximum Reduction in predicted DO depletions at the WSRs for the Scenario of 1,500 m3/day at Source

C Compared to 12,000 m3/day

Reduction in predicted DO depletions (depth averaged) (mg L-1)

Reduction in predicted DO depletions at bottom layer (mg L-1)

Dry season Wet season Ref. DO criteria (mg L-1)

Dry season Wet season Ref. DO criteria (mg L-1)

Gazetted Beaches

0.001 0.001 4.0 0.001 0.003 2.0

Marine Ecology 0.003 0.007 4.0 0.007 0.012 2.0

Fisheries Sensitive Receivers

<0.001 <0.001 5.0 <0.001 <0.001 2.0

WSD Cooling and Flushing Water Intakes

0.001 0.001 4.0 0.001 0.001 2.0

Table 3.41: Water Quality Monitoring Programme for dredging works in Kwai Tsing Container Terminal 1-4.

Station SS concentration (mgL-1) Exceedance?

Aug 2009 Sep 2009 Oct 2009 Nov 2009

Surface

Station 1 3.5 - 22.0 3.0 - 15.0 7.0 - 16.0 6.0 - 11.5 No

Station 2 2.5 - 14.0 3.5 - 15.0 4.0 - 17.0 6.0 - 9.0 No

Station 3 2.5 - 14.0 3.0 - 15.0 4.0 - 17.0 5.0 No

Depth-averaged

Station 1 4.6 - 17.7 3.3 - 13.2 6.3 - 15.3 5.5 - 7.5 No

Station 2 2.8 - 11.7 3.5 - 12.3 5.2 - 13.0 4.7 - 6.5 No

Station 3 3.3 - 12.5 3.5 - 12.7 4.8 - 14.2 6.0 - 6.7 No

* No exceedance in monitoring parameter was reported under the EM&A programme for dredging works at Kwai Tsing Container

Terminal Basin near CT1 to CT4.

Although the monitoring data provided in Table 3.41 indicates that SS releases during previous

maintenance dredging works are compliant with the water quality standards, the elevated background SS

concentrations suggest that there should still be mitigation measures needed for the protection of water

quality at some WSD and EMSD intakes when maintenance dredging is undertaken. Details of the

mitigation measures proposed are provided in Section 3.8.

Regarding the impact relating to NH3-N and UIA, if maintenance dredging rate is reduced to 12.5% of the

capital dredging quantity, the sediment release due to seabed disturbance will commensurately reduce by



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12.5%. Since NH3-N and UIA release are closely related to sediment release due to dredging, by reducing

the dredging rate, it is anticipated the release of NH3-N and UIA will also be reduced. As such, the effect of

NH3-N and UIA during maintenance dredging is expected to be insignificant.

3.7.2.2 Impact on the Performance of Tsing Yi Submarine Sewage Outfall

The existing Tsing Yi Submarine Sewage Outfall was retained under the HATS Stage 1 Scheme (in 2001)

to act as an emergency outfall in the event of failure of HATS Stage1. Due to the proposed dredging of

KTCB down to -17.5mCD, the Tsing Yi Submarine Sewage Outfall will be affected since the diffuser ports

of the outfall needs to be modified to match the new seabed level. The performance (i.e. the dispersion

characteristics) of the modified outfall has been assessed. It should be noted that the emergency function

of this outfall has not been used since 2001.

Figure 3.6 shows the layout plan of the existing Tsing Yi Submarine Sewage Outfall. With the existing

finished level of rubble at approximately -16.5mCD for the Tsing Yi Submarine Sewage Outfall, the seabed

is required to be lowered by about 1m and the existing outfall is required to be modified. A typical section of

the outfall in Drawing No. 259053/CIV/2002 in Appendix 3.5a shows the difference between the existing

and future configurations of diffuser ports. The existing steel pipe riser will be cut down and the non-return

valve and concrete pipe will be removed for replacement. Since the horizontal outfall is still under the

seabed level, there will be no change to the horizontal pipe and thus no environmental impact associated

with that modification.

During the modification works, the existing rubble layer will firstly be removed to a level below -18.9mCD

(i.e. -19.05mPD) and the existing riser pipes will then be cut down. Tailor-made collars will be adopted to

connect the existing riser pipes to the new non-return valves. The tailor-made collar will be combined by

two semi-circular pipes and bolted together to form a circular shape surrounding the existing riser pipe. The

tail of the collar will be welded to the existing riser pipe to prevent leakage of treated sewage. The new non-

return valves, which function in the same way as the existing non-return valves, will be connected to the

existing riser pipes through the collars by bolting. Similar to the existing conditions, the riser pipes protected

by concrete pipes will be extruded from the proposed dredging level to avoid blockage by siltation. The

surrounding rock fill and armour will be replaced around the modified diffusers to afford the same level of

protection as existed prior to dredging.

Upon the completion of this Project, the diffusers ports will be in the same locations as the existing ones

but will be lowered by a depth of approximately 2m. Details of the levels are provided in Appendix 3.5b.

Impact on the Performance of the outfall

As part of the EIA, an additional assessment has been carried out using a Visual Plume (UM3) model, and

outputs of the simulation are contained in Appendix 3.6. It is noted that the model predicted effluent

dilution from single or multiple ports of a single diffuser. The results of comparison of outfall performance

before and after the modification works undertaken were for effluent discharge from one diffuser, and not

from 17 diffusers of the Tsing Yi outfall. The effects such as plume merging due to the interaction of the

plumes from multiple diffusers have not been considered; however, given that the location of the outfall

does not change except in terms of depth, it may be surmised that the approach adopted for a single point

discharge remains valid for the current assessment.

From the results obtained from the modelling (contained in Appendix 3.5b) it may noted that the predicted

changes in ambient current flows before and after the Project are insignificant (Appendix 3.5b). The



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results show that the outfall dilutions from one diffuser after the modification works are enhanced by

between by 4% and 10%, compared to those before the modification works, under different current speed

conditions. The results indicate a slight improvement of the outfall performance, in terms of effluent dilution

for a single diffuser, after the modification works. Since the conclusion for the case of using a single diffuser

for assessment should be similar to the case for multiple diffusers, it may be surmised that the modification

of the Tsing Yi Submarine Sewage Outfall is unlikely to cause any adverse impact on the outfall

performance but rather provide a slight enhancement in the outfall’s performance.

Notwithstanding the foregoing, in the “Final Report Relating to Drainage/Sewerage Matters and Submarine

Outfalls”, March 2010 under this Project, it was proposed that hydraulic performance measurements be

conducted before, during and after the modification works in order to ensure that the hydraulic performance

of the submarine outfall will not be adversely affected due to the proposed dredging and outfall modification

works. As the pipe size, pipe thickness, pipe material, protection material and specification for non-return

valve will be the same as the existing submarine outfalls, the hydraulic performance of the modified

submarine outfall is anticipated to be similar to the original outfall, and dilution and dispersion

characteristics are unlikely to change appreciably.

3.7.2.3 Impact on the HATS Effluent Dispersion

The deepening of the seabed level due to the completion of the Project has been considered in connection

with the potential impacts on the HATS outfall and the potential changes to its effluent dispersion

characteristics. The results of the modelling of the current speed and direction at surface and near-bottom

levels in the proximity of the HATS outfall before (results in red) and after (results in green) the Project are

contained in Appendix 3.7. No significant changes in the current speed or direction were predicted for the

whole spring-neap cycle in wet and dry seasons. As such, it is surmised that there will be negligible effect

on the HATS outfall effluent dispersion due to the Project.

3.8 Mitigation of Adverse Environmental Impact

Recommendation of appropriate mitigation measures is provided according to the requirement of Condition

3.4.3.5 (xii) of the Study Brief. As discussed in Section 3.7, administrative control measures were

suggested based on the assessment results in NH3-N and UIA including dredging rate control, seasonal

dredging, etc. In the following section, more specific measures which are considered effective in controlling

the impact to water sensitive receivers are described.


3.8.1.1 General Dredging Practices

The non-compliance of WSD’s SS criterion at the flushing water intakes WSD1, WSD8, WSD9 and EMSD1

is due to the non-compliance of ambient SS levels at the Rambler Channel and waters near Tsuen Wan.

Deployment of silt screen at the flushing water intakes WSRs WSD1, WSD8, WSD9 and EMSD1 was

recommended to minimise the predicted elevation in SS levels at the four WSRs.



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The implementation of silt screens at the flushing water intakes would reduce the SS level by 60%. The SS

reduction factor has been adopted in the previous approved EIA studies4,8

. The surface SS levels at the

seawater intakes WSD1, WSD8, WSD9 and EMSD1 after the implementation of silt screen were presented

in Table 3.42. The SS concentrations at these WSRs comply with the WSD’s SS criterion.

Table 3.42: Maximum SS Levels (mg L-1

) with Silt Screen

WSR Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5 Scenario 6 Scenario 7

Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet

Surface

WSD1 5.4 4.6 5.4 4.6 4.7 4.3 5.4 4.6 5.4 4.6 5.0 4.6 4.8 4.4

WSD8 4.8 4.3 4.8 4.3 4.6 4.2 4.9 4.3 4.9 4.3 4.8 4.3 4.7 4.3

WSD9 4.2 1.8 4.2 1.8 4.3 1.8 4.2 1.8 4.1 1.8 4.2 1.8 4.5 2.1

EMSD1 4.6 4.4 4.6 4.4 4.5 4.2 4.6 4.5 4.6 4.5 4.6 4.4 4.5 4.2

Depth-averaged

WSD1 6.1 8.1 6.1 8.1 5.1 6.3 6.1 8.2 6.1 8.2 5.8 8.2 5.2 6.5

WSD8 5.3 6.7 5.3 6.7 5.0 6.4 5.3 6.8 5.3 6.8 5.2 6.6 5.1 6.5

WSD9 4.7 3.4 4.7 3.4 4.8 3.4 4.6 3.4 4.5 3.4 4.7 3.4 5.0 3.8

EMSD1 5.0 6.5 5.0 6.5 4.9 6.2 5.1 6.5 5.1 6.5 4.9 6.4 4.9 6.3

Occasionally, the near-surface current speed near the flushing water intake WSD1 could be greater than

0.5 ms-1

especially during spring tides, such that the effectiveness of the silt screen would be reduced.

However, the deployment of silt screen is still recommended at WSD1 since high SS impact occurs due to

poor tidal flushing when current speed is low.

Other mitigation measures that should be undertaken during dredging include:

� maximum dredging rate shall be 4000 m3

(in-situ volume) per day per grab dredger and 700 m3

in 30

minutes in any given hour (max. 8400 m3/day, based on a 12-hour operation per day) per cutter suction

dredger;

� only two types of dredgers will be allowed for this Project: (a) grab dredger with closed grab, and (b)

cutter suction dredger;;

� the allowed maximum number of grab dredgers or cutter suction dredger operating simultaneously

within the Project area shall follow the requirement listed in Table 3.43 below;

� to minimize the potential SS impact from dredging, deployment of silt curtains around the grab dredgers

is recommended (please refer to Figure 3.7 for the schematic design);

� either one cutter suction dredger or one grab dredger shall be working in Zone 2 (including subzones) of

the Container Basin (Appendix 3.13) at any time;

� CSD is only to be deployed for the removal of harder material during daytime only (07:00 to 19:00) in

Zone 2 (including subzones) of the Container Basin (Appendix 3.13);

� Project dredging works within Zone 1-6 (including subzones) of the Container Basin (Appendix 3.13)

shall not be carried out at the same time with Terminal Operator’s maintenance dredging activities;

� if further mitigation measures are required, as demonstrated by the water quality monitoring data under

the EM&A programme, then consideration will be given to reducing the dredging rate, or dredging only

on the state of the tide which would avoid migration of SS towards the WSD and EMSD intakes;

_________________________ 8 Maunsell Consultants Asia Ltd (2001), Wan Chai Development Phase II Environmental Impact Assessment Study (EIA 141/2007)



3-80

� the dredging pump of cutter suction dredger shall be operated during cutting to reduce the sediment

loss to water body;

� no overflow of dredged mud will be allowed. Barges or hopper should not be filled to a level that will

cause the overflow of materials or polluted water during loading or transportation;

� all construction vessels should be sized so that 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;

� the speed of all construction vessels will be controlled within the works area to prevent propeller wash

from stirring up the seabed sediments;

� all barges / dredgers used should be fitted with tight fitting seals to their bottom openings to prevent

leakage of material;

� construction activities should not cause foam, oil, grease, scum, litter or other objectionable matter to be

present on the water within the site or dumping grounds;

� before commencement of dredging works, the holder of the Environmental Permit should submit

detailed proposal of the design and arrangement of the frame type silt curtain to EPD for approval; and;

� the speed of any construction vessels shall not exceed 10 knots when passing through the Project Site

Boundary as shown in Figure 2.1.

Table 3.43: Allowed Maximum Number of Grab Dredgers or Cutter Suction Dredger Operating Simultaneously

Locations

Scenario

Rambler Channel (A)*

Stonecutter Island (B)*

Northern Fairway (C)*

Western Fairway (D)*

Western Fairway (E)*

1 One GD One GD One GD





6 One CSD One GD One GD

* denote reference dredging locations and working zones shown in Figure 3.5b to 3.5j.

3.8.1.2 Specific Dredging Activities

Sediment from S2 has been identified, through testing sediments and elutriates, to be highly contaminated

with ammonia. As such, the impact assessment has identified that it would be prudent to isolate S2 such

that the majority (approximately 99%) of the navigational (i.e. capital work) dredging can proceed without

affecting the overall construction programme.

As discussed in Section 3.7.1.6, the removal of the contaminated sediment identified at S2 from the main

dredging programme can effectively reduce the predicted level of ammoniacal nitrogen and UIA at

seawater intakes and gazetted beaches. This is the first level of mitigation proposed. However, since

there is marginal exceedance of WSD guidelines or WQO, a reduction in dredging rate is still

recommended as the administrative control strategy to control the release of contaminants into the water

column. Tables 3.44, 3.45 and 3.46 below indicate the proposed reduction in dredging rates which would

be needed to achieve compliance with various criteria.

An outline dredging plan is also included in Appendix 3.13, which indicates the sub-zone Z2B where S2 is

located. This sub-zone Z2B is shown to be isolated and with dredging works to be carried out towards the

end of the construction programme. The dredging plan provides a mitigation strategy for the overall

dredging works i.e. the main navigational works and the specific programme for Z2B in which the control of



3-81

dredging rate is employed as the key strategy to control the release of contaminants during dredging. The

detailed plan will be subject to further construction programming, and in particular will be refined to reflect

the specifics of the proposed field trial and monitoring confirmation together with individual subzones

evaluations. It is recommended that the dredging plan and the field trial programme are submitted for

agreement before the construction phase commences.

The field trials will also determine the operation requirements of the CSD (Scenario 6) which is confined to

Zone 2 (Appendix 3.13).

Table 3.44: Mitigation Proposal for Ammoniacal Nitrogen (after excision of S2)

Source Dry Season Wet Season

Scenario A B C D E % Reduction Dredging Rate % Reduction Dredging Rate

1 * * * No reduction is

needed 4,000

No reduction is needed

4,000


needed 4,000


4,000


needed 4,000


4,000

4 * * * 16 3,341 3 3,877

5 * * * 20 3,216 6 3,759

Note: Dredging rate is indicated as volume (cubic meters) for the grab dredger at Source A per day.

To support this proposed mitigation measure a detailed literature survey has been undertaken and results

indicate that releases of contaminants of concerned in dissolved phase to water and releases of volatile

contaminants to air are directly related to sediment resuspension9. A study of contaminated sediments in

New York Harbor10

found that the toxicity found in the toxicity testing was due to ammonia.

Other references indicate that there is evidence that the actual toxicity of NH3-N may be less than

anticipated based on the oxidation of ammonia to nitrate and as a reaction to changing pH associated with

liberation of dredged material at sea.

It is important to note that the actual area affected by this hot spot needs to be confirmed through detailed

sampling and testing; however, initial interpretation of the data indicates that a precautionary principle has

been applied to define the area affected for the S2 effects (sub-zone Z2B), as shown in Appendix 3.13.

Table 3.45: Mitigation Proposal for UIA (after excision of S2)

Source Dry Season Wet Season Annual Average

Scenario A B C D E %

Reduction Dredging

Rate %

Reduction Dredging

Rate %

Reduction Dredging

Rate

1 * * * 59 1,647 No reduction

is needed 4,000 31 2,777

2 * * * 59 1,637 No reduction 4,000 31 2,759

_________________________ 9 Source 2: U.S. Army Engineer Research and Development Center, Environmental Laboratory, “Technical Guidelines for

Environmental Dredging of Contaminated Sediments” Michael R. Palermo, Paul R. Schroeder, Trudy J. Estes, and Norman R. Francingues

10 Invited contribution published as, Jones-Lee, A., and Lee, G.F., “Water Quality Aspects of Dredged Sediment Management, “Water Encyclopedia: Water Quality and Resource Development, Wiley, Hoboken, NJ pp 122-127 (2005)



3-82

Source Dry Season Wet Season Annual Average

Scenario A B C D E %

Reduction Dredging

Rate %

Reduction Dredging

Rate %

Reduction Dredging

Rate

is needed

3 * * * No reduction

is needed 4,000


4,000 No reduction

is needed 4,000

4 * * * 61 1,561 No reduction

is needed 4,000 32 2,700

5 * * * 64 1,443 No reduction

is needed 4,000 39 2,439


Only under Scenario 3 (dredging in Zones C, D, E) can dredging be carried out at a rate of 4,000 m3

per

grab dredger per day and achieve compliance with the criteria for NH3-N and UIA. From the proposed

reduction in dredging rates, the most stringent measures apply, and thus the proposed overall mitigation

‘package’ for dredging to allow compliance with UIA and NH3-N is as follows;

Table 3.46: Mitigation Proposal for Dry and Wet Season for both Ammoniacal Nitrogen and UIA

Dry Season Wet Season Annual Average

Scenario % Reduction Dredging

Rate % Reduction

Dredging Rate

% Reduction Dredging

Rate

1 59 1,647 No reduction is

needed 4,000 31 2,777

2 59 1,637 No reduction is

needed 4,000 31 2,759

3 No reduction is

needed 4,000


4,000 No reduction is

needed 4,000

4 61 1,561 3 3,877 32 2,700

5 64 1,443 6 3,759 39 2,439


Though there is a more mitigation in the dry season, it should be noted that seawater temperature is lower

in dry season and thus the anticipated UIA release could be expected to be lower compared to the wet

season as a result of the slower release rate of UIA at lower temperatures. Also data suggests that release

of contaminants are less during the use of closed grab dredging method compared to those highly

aggressive methods of dredging (e.g. TSHD) and as such release rates may also be lower than predicted

from the elutriates test results.

As such it would be prudent to consider environmental monitoring programmes including monitoring of

Suspended Solids and NH3-N and UIA at seawater intakes and other monitoring locations to assess

release rates against predicted rates. In case of any anomalies, investigations should then be carried out to

assess the source, degree and extent of impact. If it is found to be works related, follow up action shall be

carried out including further reduction in dredging rate, or to dredge in certain tidal windows, etc. If the

release rates are less than predicted then consideration could be given to reinstating the dredging rate to

reduce the duration of the works, while still ensuring compliance with the prevailing standards.

As discussed in earlier in this section, it is proposed to isolate S2 and deal with this small part of the Project

area. Since this location potentially has very high level of NH3-N release, the relationship between dredging

and release of ammoniacal nitrogen was studied and is shown in the following table.



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Table 3.47: Prediction of Unmitigated and Mitigated Ammoniacal Nitrogen Concentrations at Sensitive Receivers

based on hotspot S2 case only

Sensitive Receivers

Assessment Point

0% reduction (Dredging Rate = 4,000)


80% reduction (Dredging Rate = 800)




Dry Season

Wet Season

Dry Season

Wet Season

Dry Season

Wet Season

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.25 0.19 0.12 0.10 0.05 0.04

Approach B2 2.59 0.32 1.29 0.16 0.52 0.06

Ting Kau B3 1.97 0.28 0.99 0.14 0.39 0.06

Lido B4 1.18 0.18 0.59 0.09 0.24 0.04

Casam B5 0.45 0.18 0.23 0.09 0.09 0.03

Hoi Mei Wan B6 0.39 0.17 0.20 0.09 0.08 0.03

Gemini B7 0.36 0.17 0.18 0.09 0.07 0.03

Angler’s B8 0.36 0.17 0.17 0.08 0.07 0.03

Lo So Shing B9 0.01 0.06 0.01 0.03 0.00 0.01

Hung Shing Yeh B10 0.01 0.06 0.01 0.03 0.00 0.01

Corals

Pak Kok CR1 0.12 0.12 0.06 0.06 0.02 0.02

Shek Kok Tsui CR2 0.09 0.11 0.04 0.06 0.02 0.02

Luk Chau CR3 0.06 0.06 0.03 0.03 0.01 0.01

Wong Chuk Kok CR4 0.04 0.04 0.02 0.02 0.01 0.01

Ap Lei Chau CR5 0.05 0.07 0.03 0.03 0.01 0.01

Sandy Bay CR6 0.07 0.15 0.04 0.07 0.01 0.03

Green Island CR7 0.14 0.14 0.07 0.07 0.03 0.03

Kau Yi Chau CR8 0.17 0.20 0.08 0.10 0.03 0.04

Kau Yi Chau CR9 0.17 0.19 0.09 0.09 0.03 0.04

Kau Yi Chau CR10 0.16 0.15 0.08 0.08 0.03 0.03

Siu Kau Yi Chau CR11 0.19 0.19 0.09 0.09 0.04 0.04

Siu Kau Yi Chau CR12 0.18 0.19 0.09 0.09 0.04 0.04

Siu Kau Yi Chau CR13 0.17 0.15 0.09 0.08 0.03 0.03

Peng Chau CR14 0.18 0.16 0.09 0.08 0.04 0.03

Peng Chau CR15 0.16 0.15 0.08 0.07 0.03 0.03

Peng Chau CR16 0.17 0.16 0.09 0.08 0.03 0.03

Peng Chau CR17 0.17 0.16 0.08 0.08 0.03 0.03

Peng Chau CR18 0.16 0.13 0.08 0.06 0.03 0.03

Fish Culture Zones

Ma Wan F1 0.21 0.14 0.10 0.07 0.04 0.03

Lo Tik Wan F2 0.04 0.10 0.02 0.05 0.01 0.02

Sok Kwu Wan F3 0.03 0.07 0.01 0.03 0.01 0.01

Cheung Sha Wan F4 0.09 0.12 0.05 0.06 0.02 0.02




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Sensitive Receivers

Assessment Point







Dry Season

Wet Season

Dry Season

Wet Season

Dry Season

Wet Season

Tsuen Wan C1 2.65 0.91 1.33 0.45 0.53 0.18

MTRC Tsing Yi Station

C2 3.09 2.94 1.54 1.47 0.62 0.59

MTRC Kowloon Station

C3 0.10 0.25 0.05 0.13 0.02 0.05

China H.K. City C4 0.10 0.28 0.05 0.14 0.02 0.06

Sha Wan Drive C5 0.07 0.17 0.04 0.08 0.01 0.03

Queen Mary Hospital

C6 0.07 0.14 0.03 0.07 0.01 0.03

Wah Fu Estate C7 0.05 0.09 0.02 0.04 0.01 0.02

Kwai Chung Hospital

EMSD1 1.34 1.51 0.67 0.75 0.27 0.30


Tsing Yi WSD1 3.66 3.33 1.83 1.66 0.73 0.67

Kennedy Town WSD2 0.13 0.20 0.06 0.10 0.03 0.04

Sheung Wan WSD3 0.12 0.31 0.06 0.16 0.02 0.06

Central Water Front

WSD4 0.10 0.27 0.05 0.13 0.02 0.05

Ap Lei Chau WSD5 0.04 0.09 0.02 0.05 0.01 0.02

Kowloon South WSD6 0.24 0.31 0.12 0.15 0.05 0.06

Cheung Sha Wan WSD7 0.14 0.23 0.07 0.12 0.03 0.05

Tsuen Wan WSD8 2.67 0.87 1.33 0.44 0.53 0.17


WSD9 0.27 0.14 0.13 0.07 0.05 0.03

Lamma Power Station

WSD10 0.01 0.06 0.01 0.03 0.00 0.01

Kwai Chung Hospital

EMSD1 1.34 1.51 0.67 0.75 0.27 0.30

- Value in Bold indicates exceedance of relevant criteria.

- Dredging Rate is indicated as volume (cubic meters) per grab dredger per day.




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Table 3.48: Prediction of Unmitigated and Mitigated UIA Concentrations at Sensitive Receivers based on hotspot S2 case only









Sensitive Receivers

Assessment Point

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Gazetted Beaches

Tung Wan, Ma Wan

B1 0.018 0.014 0.016 0.009 0.007 0.008 0.004 0.003 0.003 0.002 0.001 0.002

Approach B2 0.190 0.024 0.107 0.095 0.012 0.053 0.038 0.005 0.021 0.019 0.002 0.011

Ting Kau B3 0.145 0.020 0.083 0.072 0.010 0.041 0.029 0.004 0.017 0.014 0.002 0.008

Lido B4 0.087 0.013 0.050 0.043 0.007 0.025 0.017 0.003 0.010 0.009 0.001 0.005

Casam B5 0.033 0.013 0.023 0.017 0.006 0.012 0.007 0.003 0.005 0.003 0.001 0.002

Hoi Mei Wan B6 0.029 0.013 0.021 0.015 0.006 0.010 0.006 0.003 0.004 0.003 0.001 0.002

Gemini B7 0.026 0.012 0.019 0.013 0.006 0.010 0.005 0.002 0.004 0.003 0.001 0.002

Angler’s B8 0.025 0.012 0.018 0.012 0.006 0.009 0.005 0.002 0.004 0.002 0.001 0.002

Lo So Shing B9 0.001 0.005 0.003 0.000 0.002 0.001 0.000 0.001 0.001 0.000 0.000 0.000

Hung Shing Yeh B10 0.001 0.005 0.003 0.000 0.002 0.001 0.000 0.001 0.001 0.000 0.000 0.000

Corals

Pak Kok CR1 0.009 0.009 0.009 0.004 0.004 0.004 0.002 0.002 0.002 0.001 0.001 0.001

Shek Kok Tsui CR2 0.007 0.008 0.007 0.003 0.004 0.004 0.001 0.002 0.001 0.001 0.001 0.001

Luk Chau CR3 0.004 0.004 0.004 0.002 0.002 0.002 0.001 0.001 0.001 0.000 0.000 0.000

Wong Chuk Kok CR4 0.003 0.003 0.003 0.001 0.001 0.001 0.001 0.001 0.001 0.000 0.000 0.000

Ap Lei Chau CR5 0.004 0.005 0.004 0.002 0.002 0.002 0.001 0.001 0.001 0.000 0.000 0.000

Sandy Bay CR6 0.005 0.011 0.008 0.003 0.005 0.004 0.001 0.002 0.002 0.001 0.001 0.001

Green Island CR7 0.010 0.010 0.010 0.005 0.005 0.005 0.002 0.002 0.002 0.001 0.001 0.001

Kau Yi Chau CR8 0.012 0.015 0.013 0.006 0.007 0.007 0.002 0.003 0.003 0.001 0.001 0.001

Kau Yi Chau CR9 0.013 0.014 0.013 0.006 0.007 0.007 0.003 0.003 0.003 0.001 0.001 0.001

Kau Yi Chau CR10 0.012 0.011 0.012 0.006 0.006 0.006 0.002 0.002 0.002 0.001 0.001 0.001

Siu Kau Yi Chau CR11 0.014 0.014 0.014 0.007 0.007 0.007 0.003 0.003 0.003 0.001 0.001 0.001




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Sensitive Receivers

Assessment Point

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Siu Kau Yi Chau CR12 0.014 0.014 0.014 0.007 0.007 0.007 0.003 0.003 0.003 0.001 0.001 0.001

Siu Kau Yi Chau CR13 0.013 0.011 0.012 0.006 0.006 0.006 0.003 0.002 0.002 0.001 0.001 0.001

Peng Chau CR14 0.013 0.012 0.012 0.007 0.006 0.006 0.003 0.002 0.002 0.001 0.001 0.001

Peng Chau CR15 0.012 0.011 0.011 0.006 0.005 0.006 0.002 0.002 0.002 0.001 0.001 0.001

Peng Chau CR16 0.013 0.012 0.012 0.006 0.006 0.006 0.003 0.002 0.002 0.001 0.001 0.001

Peng Chau CR17 0.012 0.011 0.012 0.006 0.006 0.006 0.002 0.002 0.002 0.001 0.001 0.001

Peng Chau CR18 0.012 0.009 0.010 0.006 0.005 0.005 0.002 0.002 0.002 0.001 0.001 0.001

Fish Culture Zone

Ma Wan F1 0.015 0.010 0.013 0.008 0.005 0.006 0.003 0.002 0.003 0.002 0.001 0.001

Lo Tik Wan F2 0.003 0.008 0.005 0.001 0.004 0.003 0.001 0.002 0.001 0.000 0.001 0.001

Sok Kwu Wan F3 0.002 0.005 0.003 0.001 0.002 0.002 0.000 0.001 0.001 0.000 0.000 0.000

Cheung Sha Wan F4 0.007 0.008 0.008 0.003 0.004 0.004 0.001 0.002 0.002 0.001 0.001 0.001


Tsuen Wan C1 0.195 0.067 0.131 0.097 0.033 0.065 0.039 0.013 0.026 0.019 0.007 0.013

MTRC Tsing Yi Station

C2 0.227 0.216 0.221 0.113 0.108 0.111 0.045 0.043 0.044 0.023 0.022 0.022

MTRC Kowloon Station

C3 0.008 0.019 0.013 0.004 0.009 0.007 0.002 0.004 0.003 0.001 0.002 0.001

China H.K. City C4 0.007 0.021 0.014 0.004 0.010 0.007 0.001 0.004 0.003 0.001 0.002 0.001

Sha Wan Drive C5 0.005 0.012 0.009 0.003 0.006 0.004 0.001 0.002 0.002 0.001 0.001 0.001

Queen Mary Hospital

C6 0.005 0.010 0.008 0.003 0.005 0.004 0.001 0.002 0.002 0.001 0.001 0.001

Wah Fu Estate C7 0.003 0.006 0.005 0.002 0.003 0.002 0.001 0.001 0.001 0.000 0.001 0.000

Kwai Chung Hospital

EMSD1 0.098 0.111 0.105 0.049 0.055 0.052 0.020 0.022 0.021 0.010 0.011 0.010


Tsing Yi WSD1 0.269 0.244 0.257 0.135 0.122 0.128 0.054 0.049 0.051 0.027 0.024 0.026




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Sensitive Receivers

Assessment Point

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Dry Season

Wet Season

Annual Average

Kennedy Town WSD2 0.009 0.014 0.012 0.005 0.007 0.006 0.002 0.003 0.002 0.001 0.001 0.001

Sheung Wan WSD3 0.008 0.023 0.016 0.004 0.011 0.008 0.002 0.005 0.003 0.001 0.002 0.002

Central Water Front

WSD4 0.007 0.019 0.013 0.004 0.010 0.007 0.001 0.004 0.003 0.001 0.002 0.001

Ap Lei Chau WSD5 0.003 0.007 0.005 0.001 0.003 0.002 0.001 0.001 0.001 0.000 0.001 0.000

Kowloon South WSD6 0.017 0.023 0.020 0.009 0.011 0.010 0.003 0.005 0.004 0.002 0.002 0.002

Cheung Sha Wan WSD7 0.011 0.017 0.014 0.005 0.009 0.007 0.002 0.003 0.003 0.001 0.002 0.001

Tsuen Wan WSD8 0.196 0.064 0.130 0.098 0.032 0.065 0.039 0.013 0.026 0.020 0.006 0.013


WSD9 0.020 0.010 0.015 0.010 0.005 0.007 0.004 0.002 0.003 0.002 0.001 0.001

Lamma Power Station

WSD10 0.001 0.005 0.003 0.001 0.002 0.001 0.000 0.001 0.001 0.000 0.000 0.000

Kwai Chung Hospital

EMSD1 0.098 0.111 0.105 0.049 0.055 0.052 0.020 0.022 0.021 0.010 0.011 0.010

- Value in Bold indicates exceedance of relevant criteria.

- Dredging Rate is indicated as volume (cubic meters) per grab dredger per day.



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From Tables 3.47 and 3.48, it can been seen that the dredging rate has to be reduced to about 80% of the

original rate to reduce the UIA level to allow compliance with WQO at gazetted beaches as well as

compliance of NH3-N level at WSD’s seawater intake (WSD1). As the level of contamination is high in the

area of S2, it is therefore recommended that to clearly define the area affected and thereafter to propose a

field trial or alternative confirmatory methods to be carried out before dredging works at S2 commence.

The field trials may include but are not limited to consideration of methods or options such as hydraulically

closed grab dredgers, specially designed silt curtains, bioremediation or similar methods for treating the

specific cause of the contamination prior to removal of sediments.

The objective of the field trial is to propose the most effective dredging process and rate to control the

release of ammoniacal nitrogen and UIA into the water column and achieve compliance at the WSD1

seawater intake (NH3-N) and at the beaches for UIA. Capital dredging works in the vicinity of S2 should not

therefore be carried out until the proposed method and rate are confirmed. Having taken note of the other

projects in the vicinity, it would be appropriate to time the dredging of S2 to avoid other capital works

projects as far as possible and thus the Q3/Q4 of 2013 would currently be favoured.

From Table 3.35, it is shown that marginal exceedance of annual mean WQO UIA criteria of 0.021 mg L-1

at Approach, Tung Kau and Lido and there is no restriction in carrying out dredging activities during wet

season except the level of UIA during dry season is above 0.021 mg L-1

(WQO criteria as an annual mean).

As the dredging activities has to take into account of the need of the Project and construction programme

as well as not to prolong any environmental effect as a result of the Project. Based on these, it is

considered dredging during dry season is necessary and therefore mitigation measures need to be

implemented to reduce the dredging effects. As such, control of dredging rate is considered the most

effective way in reducing the release of sediments and in turn to control the release of NH3-N and UIA.

3.8.2 Operational Phase

3.8.2.1 Maintenance Dredging

The isopachytes show that there was no significant level of siltation within most of the Study Area, other

than for some isolated high spots adjacent to the berth boxes. Maintenance dredging will therefore only be

considered when the removal of local high spots is required to maintain the navigation depth. The scale of

the maintenance dredging works is far smaller than that of the capital dredging. The reduction in dredging

plant (one dredger compared to three) to be deployed and the reduced volume of sediment to be removed

(30,000m3 per annum maximum for this Project with an estimated 225,000m

3 for all maintenance dredging

associated with KTCP) during maintenance dredging compared to the capital dredging works programme

further suggest a reduction of impacts on receiving water quality.

The SS impacts on the WSRs due to the maintenance dredging may be assumed to be reduced compared

to the impacts arising from the capital dredging due to the reduction in daily production rate (maximum daily

rate of 4,000m3 in-situ volume for maintenance compared to 12,000m

3 in-situ volume for capital dredging

using 3 grab dredgers). To minimise the potential SS impact from the maintenance dredging, PWD has no

objection to deploy silt screens at the flushing water intakes, i.e. at WSD1 and EMSD1. In addition, silt

curtain around the grab dredgers will be installed during maintenance dredging. Subject to the review of

environmental performance of the dredging operations, the deployment of silt curtains may be suspended if

supporting evidence is obtained through the operational phase water quality monitoring and audit

programme.



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In view of the above discussion, the following mitigation measures are proposed when undertaking

maintenance dredging:

� only grab dredger with closed grab will be allowed with a maximum rate of 4000m3/day (in-situ volume);

� no overflow of dredged mud will be allowed. Barges should not be filled to a level that will cause the

overflow of materials or polluted water during loading or transportation;

� all barges used should be fitted with tight fitting seals to their bottom openings to prevent leakage of

material;

� construction activities should not cause foam, oil, grease, scum, litter or other objectionable matter to be

present on the water within the site or dumping grounds,

� in the event that there is an exceedance of SS at the WSD intakes other than WSD1 then further

mitigation measures shall be considered including the use of silt curtains or a reduction in the dredging

rate; and

� no maintenance dredging will be carried out concurrently with other maintenance works by container

terminal operators.

3.9 Evaluation of Residual Impacts

During the construction phase, the key water quality impact associated with dredging activities is the

elevation of SS within the marine water column. Provided that the recommended mitigation measures are

implemented, no unacceptable residual water quality impact due to dredging within the Project areas is

expected.

No residual water quality impact due the operational phase of the Project is anticipated.

3.10 Environmental Monitoring and Audit

Appropriate mitigation measures have been recommended to minimize potential water quality impacts

during the construction phase. Water quality monitoring and audit would be required to obtain a robust,

defensible database of baseline information of water quality before construction, and thereafter, to monitor

any variation of water quality from the baseline conditions and exceedances of WQOs at sensitive

receivers (including fish culture zones) during construction, and to ensure that the recommended mitigation

measures are implemented properly. Details of the water quality monitoring and audit programme and the

Event and Action Plan have been provided in the stand-alone EM&A Manual.

In addition to water quality monitoring, in order to ensure that the hydraulic performance of the Tsing Yi

Submarine Sewage Outfall will not be adversely affected due to the proposed dredging and outfall

modification works, hydraulic performance measurement is therefore recommended to be conducted

before, during and after the proposed modification works. The stand-alone EM&A Manual contains the

details for the hydraulic performance requirements.

3.11 Summary

The water quality impacts arising from the construction and operational phase of the Project have been

assessed. The impacts of the proposed dredging works have been quantitatively assessed using the

Delft3D Model. Suspended solids (SS) were identified as one of the critical water quality parameters

during the dredging operations. The worst-case scenarios for the dredging works have been assessed, in

terms of SS and adverse water quality impact has been predicted on any of the identified WSRs provided

that appropriate mitigation measures are implemented.



3-90

Although high levels of ammoniacal nitrogen and UIA were found within the Project Area detected through

sediment sampling for this Project, assessment showed that for the majority of the area, some 99% of the

project area, navigational dredging will not cause an adverse impacts to the receiving environment through

mitigation measures such as reducing the dredging rates backed up by monitoring. Due to only the

marginal exceedance of the NH3-N and UIA at the sensitive receivers in the dry season, the release of

suspended solids from sediment and hence the associated pollutants (e.g. NH3-N) into water column can

be controlled via the reduced dredging rates. With the implementation of the effect dredging rate control, it

is not considered likely there will be any residual impacts associated with the navigational dredging for this

Project.

A hot spot has been found at the northeastern corner of the Project Area (namely, at sampling station S2),

which abnormally high levels of NH3-N, and hence UIA. To address this issue, it is recommended that

before dredging works at S2 commences, a field trial or alternative confirmatory method is to be carried out

to clearly define the area affected, and thereafter, to propose (through the field trials) an acceptable method

or option such as hydraulically closed grab dredgers, specially designed silt curtain, bioremediation or

similar methods to work at this area. The objective of the field trial is to propose the most effective

dredging process and rate to control the release of ammoniacal nitrogen and UIA into the water column

and achieve compliance at the WSD1 seawater intake (NH3-N) and at the beaches for UIA. Capital

dredging works in the vicinity of S2 should not therefore be carried out until the proposed method and rate

are confirmed. Having taken note of the other projects in the vicinity, it would be appropriate to time the

dredging of S2 to avoid other capital works projects as far as possible and thus the Q3/Q4 of 2013 would

currently be favoured.

An environmental monitoring and audit programme has been proposed to ensure that all the recommended

mitigation measures would be implemented properly.

3. water quality impact assessment · 2010-07-27 · 3.2.5 water quality criteria for water-cooled...

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