4.         WATER QUALITY

 

4.1       Introduction

 

This section outlines the potential impacts associated with the proposed cable laying activities between Chi Ma Wan Peninsula and Cheung Chau via Pui O on receiving water quality. The potential water quality impacts associated with the Project have been considered. These include impacts from cable laying, general construction works and runoff., and tThe environmental acceptability of the impacts on receiving water quality wereas determined and mitigation measures recommended where necessary..

 

 In order to define the nature and quantify the extent of the potential impacts associated with the marine based works, sediment plume modelling was undertaken using an approved and calibrated mathematical model which simulated various likely scenarios in accordance with the requirements of the Study Brief. Other water quality assessments have been undertaken using qualitative assessments and are based on practical experience of similar projects in Hong Kong (such as “Focussed Environmental Impact Assessment Study : Laying a Second 132kV Submarine Cable Transmission link from Lau Fau Shan to Shekou”, ERM 1996)..

 

The potential water quality impacts associated with the Project have been considered. These include impacts from dredging, general construction works and runoff, and the environmental acceptability of the impacts on receiving water quality were determined and mitigation measures recommended.

 

The water quality assessment has followed the guidelines given in Annexes 6 and 14 of the EIA-TM and has focussed on assessing the construction impacts associated with implementing the project and provides details of any mitigation measures and monitoring requirements which may be necessary to ensure residual impacts are acceptable and comply with current standards and guidelines.

 

 

4.2       Relevant Legislation, Regulations, Guidelines and Evaluation Criteria

 

The Water Pollution Control Ordinance (WPCO) (Cap. 358) enacted in 1980 is the principal legislation controlling water quality in Hong Kong along with Annexes 6 and 14 of the Technical Memorandum on Environmental Impact Assessment Process (EIA-TM).  Under the WPCO, Hong Kong waters are classified into 10 Water Control Zones (WCZs) and statutory Water Quality Objectives (WQOs) are specified for each WCZ. 

 

This cable laying project will take place in the Southern Water Control Zone which was gazetted in L.N. 204 of 1988.  The WQOs for this WCZ are the evaluation criteria for assessing the water quality impacts during dredging or jet ploughing activities.  The WQOs also apply to the protection of water quality with respect to the potential off-site discharges from construction sites, and disposal of sewage from the construction work force.

 

WQOs have been established in terms of the physical, chemical and biological water quality in the specific Water Control Zone to achieve the level of protection required for each beneficial use.  For the current situation beneficial uses are:

 

·         human health; and

·         aquatic life

 

Relevant WQOs for this assignment are included in Table 4-1 which indicate the ambient value and the compliance level.

 


Table 4-1         Water Quality Objectives (WQOs) for Southern Water Control Zone

 

Water Quality Objective

Part or Parts of Zone

A.                  AESTHETIC APPEARANCE

(a)     Waste discharges shall cause no objectionable odours or discolouration of the water.

(b)    Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substance should be absent.

(c)     Mineral oil should not be visible on the surface. Surfactants should not give rise to a lasting foam.

(d)    There should be no recognisable sewage-derived debris.

(e)     Floating, submerged and semi-submerged object at a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent.

Waste discharges shall not cause the water to contain substance which settle to form objectionable deposits.

 

Whole Zone

 

Whole Zone

 

Whole Zone

 

Whole Zone

Whole Zone

 

 

Whole Zone

 

B.                   BACTERIA

(a)     The level of Escherichia coli should not exceed 610 per 100 ml, calculated as the geometric mean of all samples collected in one calendar year.

(b)    The level of Escherichia coli should not exceed 180 per 100 ml, calculated as the geometric mean of all samples collected from March to October inclusive in one calendar year. Samples should be taken 3 times in a calendar month at intervals of between 3 and 14 days.

 

Secondary Contact Recreation Subzones and Fish Culture Subzones

 

Bathing Beach Subzones

C.               DISSOLVED OXYGEN

(a)     Waste discharges shall not cause the level of dissolved oxygen to fall below 4 milligrams per litre for 90% of the sampling occasions during the year, values should be calculated as the water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth, and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year.

(b)    The dissolved oxygen level should not be less than 5 milligrams per litre for 90% of the sampling occasions during the year; values should be calculated as water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year.

(c)     Waste discharges shall not cause the level of dissolved oxygen to be less than 4 milligrams per litre.

 

Marine waters excepting Fish Culture Subzones

 

 

 

 

 

 

Fish Culture Subzones

 

 

 

 

 

 

 

Inland waters of the Zone

D.                  PHpH

(a)     The pH of the water should be within the range of 6.5-8.5 units. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units.

 

 

(b)    The pH of the water should be within the range of 6.0-9.0 units.

 

(c)     The pH of the water should be within the range of 6.0-9.0 units for 95% of samples. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.5 units.

 

Marine waters excepting Bathing Beach Subzones; Mui Wo (A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo (F) Subzones.

Mui Wo (D) Subzones and other inland waters.

Bathing Beach Subzones

E.                   TEMPERATURE

(a)   Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 degree Celsius.

 

Whole Zone

F.                   SALINITY

(a)   Waste discharges shall not cause the natural ambient salinity level to change by more than 10%.

 

Whole Zone

G.                   SUSPENDED SOLIDS

(a)     Waste discharges shall neither cause the natural ambient level to be raised by 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities.

(b)    Waste discharges shall not cause the annual median of suspended solids to exceed 20 milligrams per litre.

 

(c)     Waste discharges shall not cause the annual median of suspended solids to exceed 25 milligrams per litre.

 

Marine waters

 

 

Mui Wo (A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo (F) Subzones.

Mui Wo (D) Subzones and other inland waters.

 

H.                  AMMONIA

(a)   The ammonia nitrogen level should not be more than 0.021 milligram per litre, calculated as the annual average (arithmetic mean), as unionised form.

 

Whole Zone

I.                     NUTRIENTS

(a)     Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants.

(b)    Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.1 milligram per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed).

 

Marine waters

J.                    5-DAY BIOCHEMICAL OXYGEN DEMAND

(a)   Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 milligrams per litre.

 

Inland waters of the Zone

K.                  CHEMICAL OXYGEN DEMAND

(a)   Waste discharges shall not cause the chemical oxygen demand to exceed 30 milligrams per litre.

 

Inland waters of the Zone

L.                   DANGEROUS SUBSTANCES

(a)     Waste discharges shall not cause the concentration of dangerous substances in marine water to attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to toxicant interactions with each other.

(b)    Waste discharges of dangerous substances shall not put a risk to any beneficial uses o the aquatic environment.

 

Whole Zone

 

 

 

 

Whole Zone

(Source: Adopted from CAP. 358, section 5, 1988 and L.N. 453 of 91)

 

 

The Technical Memorandum (TM), “Standards for Effluent Discharge into Drainage and Sewerage Systems, Inland and Coastal Waters”, issued under Section 21 of the WPCO defines acceptable effluent discharge limits to different types of receiving waters.  With regard to inland waters, there is no distinction between different zones and the beneficial use of the inland waters is the only factor governing the quality and quantity of the effluent that should be met. Under the TM, inland waters are classified into four groups.  These are given below in Table 4-2.

 

Table 4-2         Different Groups of Inland Water Specified in the TM

 

Inland Water Grouping

Beneficial use

Group A

Abstraction for potable water supply

Group B

Irrigation

Group C

Pond fish culture

Group D

General amenity and secondary contact recreation

(Source: Adopted from Technical Memorandum of Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters)

 

 

For this Project Group D waters prevail for the inland waters the :-.

 

 

The WQOs which prevail for this Project with discharge standards for Group D waters are given in Table 4-3.

 


 

            Table 4-3         Standards for Effluents Discharged into Group D Inland Waters

(All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

 

Flow rate

(m3/day)

Determinand

£200

>200 and £400

>400 and £600

>600 and £800

>800 and £1000

>1000 and £1500

>1500 and £2000

>2000 and £3000

pH (pH units)

6-10

6-10

6-10

6-10

6-10

6-10

6-10

6-10

Temperature (oC) 

30

30

30

30

30

30

30

30

Colour (lovibond units)

(25mm cell length)

1

1

1

1

1

1

1

1

Suspended solids

30

30

30

30

30

30

30

30

BOD

20

20

20

20

20

20

20

20

COD

80

80

80

80

80

80

80

80

Oil & Grease         

10

10

10

10

10

10

10

10

Iron

10

8

7

5

4

2.7

2

1.3

Boron

5

4

3.5

2.5

2

1.5

1

0.7

Barium

5

4

3.5

2.5

2

1.5

1

0.7

Mercury

0.1

0.05

0.001

0.001

0.001

0.001

0.001

0.001

Cadmium

0.1

0.05

0.001

0.001

0.001

0.001

0.001

0.001

Other toxic metals individually

1

1

0.8

0.8

0.5

0.5

0.2

0.2

Total toxic metals

2

2

1.6

1.6

1

1

0.5

0.4

Cyanide

0.4

0.4

0.3

0.3

0.21

0.1

0.1

0.05

Phenols

0.4

0.3

0.2

0.1

0.1

0.1

0.1

0.1

Sulphide

1

1

1

1

1

1

1

1

Sulphate

800

600

600

600

600

400

400

400

Chloride

1000

800

800

800

600

600

400

400

Fluoride

10

8

8

8

5

5

3

3

Total phosphorus

10

10

10

8

8

8

5

5

Ammonia nitrogen

20

20

20

20

20

20

20

10

Nitrate + nitrite nitrogen

50

50

50

30

30

30

30

20

Surfactants (total)

15

15

15

15

15

15

15

15

E. coli (count/100ml)

1000

1000

1000

1000

1000

1000

1000

1000

(Source: Adopted from Technical Memorandum of Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters)

 

 

4.3       Potential Sources of Impact

 

4.3.1     Sources Inventory

 

   The potential sources of water quality impacts are from:As identified in Section 2 there are various activities taking place in the marine environment which could adversely affect water quality and impact on the marine sensitive receivers. These are primarily :

 

The main aspects of the project which have the potential to impact water quality and thus potentially sensitive receivers these are:

·         Cable laying which is described in section 2  detail in section ___ and may take place using dredging or direct burying methods although it is recognised that the method favoured method (by the the Client) is theis  direct burying method as it is shorter, has fewer environmental impacts to address and is generally more in keeping with the approach of minimising at source the environmental impacts of projects (through choice of method or plant);

·         Formation of the temporary workings platform at the tunnel portal at Pui O; and

·         Laying the cable in the typhoon shelter at Cheung Chau.

The methods considered for cable laying have been subjected to detailed modelling using a sediment plume model. For the pther aspects of the work, qualitative assessments have ben adopted. Justifications for this are given in the form of the construction method as described below.

 

The temporary working platforms area at Pui O will be formed by placing a seawall in the location shown in Figure 3.3.  Material excavated from the tunnel will be used to form the works platform and by adopting a strategy of filling behind the forward formed seawall, no fines will be released to the receiving marine waters..  The method of construction which is designed for pollution prevention, will be to place concrete blocks as an outer seawall (within no gaps) and fill behind the seawall with excavated material from the tunnel.  Tunnel spoil will largely be large fraction rock and rubble. Notwithstanding this there will be no opportunity for any small fraction materials (“fines”) to be released to the receiving waters as there will be no gaps permitted in the seawall.  Once the tunnelling works have been completed the fill material will be excavated, disposed of off-site and the seawall dismantled, thus reinstating the beach to its original state.  No detailed modelling was undertaken as the impacts are temporary and very minor. and transient.

 

When laying the cables in the typhoon shelter, an opening will need to be made by dismantling the area shown in Figure 4.21.  The cast concrete sleeve will then be placed in the seawall, which shall be reconstructed and divers deployed to lay” the cables through the sleeve to rest in the cable trough.  The “U” shaped sleeve is 4.9m (long) x 0.75m (high) for each circuit which passes through the breakwater.  Only two circuits are placed through the breakwater according to the current design proposed.

 

It is estimated that the diver would need around 10 days to lay each circuit in the typhoon shelter (by hand). This work will have minimal impact on the sediments in the bottom of the typhoon shelter and no detailed assessment has been carried out.  As a precaution the Contractor should be prepared to provide a silt screen for the diver to work within should this be required.  Reference should be made to section 4.6 which details mitigation measures.

 

Other potential impacts include the off-site runoff during land based construction activities and dewatering of slurry generated during the driving of the tunnel, and .

 

Other issues include the liquid waste (sewage and greywater) generated by the construction workers. which is discussed in Section 9 under the solid and liquid wastes section.

 

For the assessment of off-site spillage the focus was on qualitative assessments and definition of mitigation measures to be included in the Contract Documents.  Guidelines following ProPECC Note PN1/924 “Construction”, “Site Drainage” would be used as a reference guide. Mitigation measures are also described in Section 4.6.

 

For the treatment of wastewater generated by the tunnel construction, it is likely that this would need to focus on removal of sediment.  This could be by sedimentation tank or by the use of a mobile microfiltration plant, should the required discharge standard not be met by conventional sedimentation techniques.  The standards to achieve are set out in the Technical Memorandum on “Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters”. Further discussion of this waste is described in Section 9.

Dredging for and/or laying of cables;

·Reclamation works at Pui O Beach;

·Construction runoff; and

·Sewage generated by the workforce.

 

The water quality assessment has followed the guidelines given in Annexes 6 and 14 of the EIA-TM. The water quality assessment h and has focussed on assessing the construction and operational impacts associated with implementing the project and provides details of any mitigation measures and monitoring requirements which may be necessary to ensure residual impacts are acceptable and comply with current standards and guidelines.

 

It should be noted that as the working platform at Pui O is temporary there will be no potential impediment to tidal flows following completion of the works, therefore no post construction assessment is required.

 

For the remaining assessments, attention has focussed on the cable laying works.

 


In accordance with the requirements of the EIA-TM, the assessments given in the following sections are, as far as practical, quantitative..

 

4.3.2     Baseline Conditions

 

Marine Water Quality

 

The Study Area is located within Southern Waters in a water body where oceanic and estuarine waters interchange. The Southern Water Control Zone which is the second largest water control zone (WCZ) with a marine mud disposal area to the south of Cheung Chau.  The WCZ also covers a statutory marine reserve in Cape D’Aguilar which is of great scientific interest and conservationversation  significance but which is remote from the subject site.  The major land areas in the zone include the southern part of Lantau Island, all outlaying islands in the southern  territorial waters of Hong Kong Island. and the southern part of Hong Kong Island. 

 

There are fFour maricultural zones are located within this WCZ at Cheung Sha Wan, Po Toi Island, Cheung Sha Wan (southeast coast of Lantau Island), Sok Kwu Wan and Lo Tik Wan of Lamma Island.  Apart from Chi Ma WanCheung Sha Wan all other fish culture zones are remote from the influence of the proposed cable layingmarine works. Moreover, a navigation fairway is running through  The Adamasta Channel is a designated navigation fairway which is used by high speed vessels plying to and from Macau, inter-island ferries and fishing vessels .  Consequently water movements are complex with seasonal variations observed with estuarine or oceanic influences dominating at different times of the year.

 

Based on the data obtained from Marine Water Quality in Hong Kong in 1999 published by the Environmental Protection Department (EPD), the closest monitoring stations to the project are at SM11, SM12 and SM13 within the WCZ to the north and the west of Cheung Chau and to the southeast of Lantau Island.  Locations of the indicating representative water sampling stations in the Southern WCZ (in the current context) are shown in Figure 4.12. Water quality statistics for the representative stations are presented in Table 4-4.

 

 

Table 4-4      Marine Water Quality in the Southern Water Control Zone at Selected Stations

 

Determinand

SM11

SM12

SM13

Temperature (oC) 

23.5

(17.5 – 27.2)

23.7

(18.5 – 27.5)

23.8

(18.6 – 27.3)

Salinity (psu)       

30.3

(25.4 – 32.7)

30.5

(25.8 – 33.4)

30.4

(24.7 – 33.4)

Dissolved Oxygen (mg/L)

6.5

(5.0 – 9.3)

6.6

(5.3 – 9.6)

6.3

(4.7 – 8.0)

Dissolved Oxygen Bottom (mg/L)

6.5

(4.9 – 8.9)

6.5

(5.3 – 9.4)

6.4

(5.2 – 8.2)

Dissolved Oxygen (% Saturation)    

91

(70 – 118)

92

(75 – 125)

88

(67 – 104)

Dissolved Oxygen Bottom

(% Saturation)     

91

(70 – 112)

91

(76 – 123)

89

(74 – 107)

PpH

8.0

(7.8 – 8.3)

8.0

(7.8 – 8.4)

8.0

(7.9 – 8.4)

Secchi Disc Depth (m)

2.0

(1.0 – 3.5)

1.8

(1.0 – 3.0)

1.8

(1.0 – 3.5)

Turbidity (NTU)

8.2

(3.7 – 10.7)

9.2

(4.0 – 13.6)

12.7

(3.8 – 22.4)

Suspended Solids (mg/L)

6.4

(2.5 – 11.3)

8.0

(2.8 – 12.7)

9.7

(2.0 – 20.7)

5-day Biochemical Oxygen Demand (mg/L)

0.7

(0.3 – 1.2)

0.8

(0.3 – 1.3)

0.7

(0.3 – 1.2)

Ammonia Nitrogen (mg/L)

0.06

(0.04 – 0.11)

0.04

(0.01   – 0.05)

0.03

(0.01 – 0.05)

Unionized Ammonia (mg/L)

0.003

(0.001 –  0.006)

0.002

(0.001 –  0.003)

0.002

(<0.001 –  0.004)

Nitrite Nitrogen (mg/L)

0.03

(0.01 – 0.05)

0.02

(<0.01 – 0.04)

0.02

(<0.01 – 0.04)

Nitrate Nitrogen (mg/L)

0.14

(0.02 – 0.29)

0.14

(0.01 – 0.30)

0.13

(<0.01 – 0.32)

Total Inorganic Nitrogen (mg/L)

0.23

(0.08 – 0.38)

0.19

(0.04 – 0.35)

0.19

(0.02 – 0.37)

Total Kjeldahl Nitrogen (mg/L)

0.2

(0.12 – 0.36)

0.22

(0.13 – 0.35)

0.21

(0.08 – 0.33)

Total Nitrogen (mg/L)

0.41

(0.28 – 0.51)

0.38

(0.24 – 0.55)

0.36

(0.18 – 0.52)

 

 

 

 

Ortho-phosphate (mg/L)

0.02

(0.01 – 0.04)

0.02

(0.01 – 0.03)

0.02

(0.01 – 0.03)

Total-Phosphorus (mg/L)

0.04

(0.02 – 0.05)

0.03

(0.02 – 0.04)

0.03

(0.02 – 0.04)

Silica (as SiO2) (mg/L)

0.9

(0.1 – 3.0)

1.0

(0.1 – 2.8)

1.1

(0.1 – 3.0)

Chlorophyll-a (mg/L)

3.3

(1.0 – 6.6)

3.2

(1.2 – 6.0)

2.4

(0.6 – 5.4)

Phaeo-pigment (mg/L)

1.0

(0.4 – 1.6)

1.0

(0.5 – 2.0)

0.8

(0.2 – 1.9)

E.coli (cfu/100mL)

4

(1 – 120)

12

(2 – 200)

2

(1 – 15)

Faecal Coliforms (cfu/100mL)

9

(1 – 350)

28

(4 – 510)

5

(1 – 90)

Note:   1. Except as specified, data presented are depth-averaged results.

2. Depth-averaged results at each station are calculated as arithmetic means of measurements at all available depths (i.e. S, M, B) except for E.coli and faecal coliforms which are geometric means.

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

4. Data enclosed in brackets indicate the ranges.

5. Shaded cells indicate non-compliance with the WQOs.

(Source: Adopted from EPD Marine Water Quality for 1999)

 

 

 

The wWater quality at the representative stations was generally good with a low level of sewage bacteria (indicative of faecal organismsations) being present and with .  Aall the parameters, of water quality except for total inorganic nitrogen (TIN), complying with the relevant WQOs.  As these representative monitoring stations are shielded by Lantau Island, the influence of the Pearl River flow is insignificant, which is .  This can be reflected by the relatively small range of in the salinity (25 to 33 psu).

 

With reference to the report entitled  of the project “Focussed Environmental Impact Assessment (EIA) Study : Laying a second 132kV Submarine Cable Transmission Link from Lau Fau Shan to Shekou it was noted that the . The ambient level of suspended solids wais assumed to be the 90th percentile of the reported concentrations. Using this precedent case the ,  and the predicted values of 90th percentiles at SM11, SM12 and SM13 were calculated to be are 9.1, 12.2 and 17.8 mg/L respectively.  The mean ambient value is calculated to be 13.0 mg/L. 

 

As the WQO is defined as 130% of the ambient, the WQO compliance threshold is set (for this assessment) at 16.9 mg/L. Therefore, the maximum allowable elevated level for suspended sediment is due to the proposed works is 3.9 mg/L. It should be stressed that this compliance level acceptability level should be confirmed through the baseline monitoring to be carreid carried out by the Contractor prior to commencing marine works. This baseline monitoring will provide athe more accurate level of suspended solids which can be released into the water column while still complying with the Water Quality Objectives – as it will represent the situation at the time of the works.

 

Analysis of dDissolved oxygen is defined both in terms of concentration (mg/L) and saturation (%).  According to the WQO compliance evaluation, only DO concentration is concerned and the ambient level of dissolved oxygen is assumed to be the 10th percentile of the reported concentrations.  The ranges and mean values at the representative sampling stations are presented in Table 4-4.  The 10th percentiles at SM11, SM12 and SM13 are predicted to be 5.4, 5.4 and 5.1 mg/L respectively.  The mean ambient value is estimated to be 5.3 mg/L.  For the DO saturation, the 10th percentiles at SM11, SM12 and SM13 are predicted to be 78.2%, 77.9% and 74.1% respectively and the mean ambient level is estimated to be 76.7%.  These ambient values indicate a well-oxygenated baseline environment.

 

 

 

Sediment Quality

 

Marine sediment quality data is collected by EPD in the vicinity of the sStudy aArea. has revealed that the sediments are classified as The “Classification of Dredged Sediment for Marine Disposal” under Work Bureau Technical Circular (WBTC) No. 3/2000 is summarised in.  The sediment quality criteria for the classification of sediment is summarised as shown in Table 4-5.  This defines the level of further testing (if any) which needs to be carried out before disposal routes can be defined assuming that disposal of material is required. It should be noted that only if dredging is undertaken will disposal of marine deposits be requiriedd. The direct burying method does not generate waste for off-site disposal which is another reason why it is a favoured construction method..

 

            Table 4-5         Sediment Quality Criteria for the Classification of Sediment

 

Contaminants

Lower Chemical Exceedance Level (LCEL)

Upper Chemical Exceedance Level (UCEL)

Metals (mg/kg dry wt.)

 

 

Cadmium (Cd)

1.5

4

Chromium (Cr)

80

160

Copper (Cu)

65

110

Lead (Pb)

75

100

Mercury (Hg)

0.5

1

Nickel (Ni)*

40

40

Zinc (Zn)

200

270

Metalloid (mg/kg dry wt.)

 

 

Arsenic (As)

12

42

Organic-non-PAHs (mg/kg dry wt.)

 

 

PCBs

23

180

Organic-PAHs (mg/kg dry wt.)

 

 

Low Molecular Weight PAHs

550

3160

High Molecular Weight PAHs

1700

9600

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

 

 

SS5 and SS6 are the two closest sediment monitoring stations to the proposed submarine cable across the Adamasta Channel.  In addition, the proposed submarine cable will pass through Cheung Chau typhoon shelter albeit using divers to lay the cables in this final section rather than direct burying or dredging techniques.  Therefore, the quality of typhoon shelter sediments at SS7 iswas also taken into account. The routine sediment quality monitoring data at SS5, SS6 and SS7 collected by EPD are summarised in Table 4-6.

 

            Table 4-6         Monitoring Results of Sediment Quality between 1995 and 1999

 

Parameter

Average Concentrations

Unit

SS7

SS5

SS6

SS7SS6

Cadmium

0 - 0.9

0 - 0.9

0 - 0.90 - 0.9

(mg/kg dry wt.)0 - 0.9

Chromium

0 – 49

0 - 49

0 - 490 - 49

0 - 49

Copper

0 - 54

0 - 54

³650 - 54

³65

Lead

0 - 64

0 - 64

65 - 740 - 64

65 - 74

Mercury

0 - 0.7

0 - 0.7

0 - 0.70 - 0.7

0 - 0.7

Nickel

0 - 34

0 - 34

0 - 340 - 34

0 - 34

Zinc

0 - 140

0 - 140

150 - 1900 - 140

150 - 190

Arsenic

0 - 14

0 - 14

0 - 140 - 14

0 - 14

PCBs

6 - 10

0 - 5

11 - 200 - 5

(mg/kg dry wt.)11 - 20

PAHs

51 - 100

0 - 50

101 - 200

 

Note:       1.     Data presented are in annual medians of monthly samples

2.     Shaded cells indicate non-compliance with exceedance of the LCEL.

(Source: Adopted from EPD Marine Water Quality in Hong Kong for 1999)

 

  

Based on the above monitoring results, the qualities of the sediment quality is relatively similar for the three locations except for Zn in SS7.samples collected at the marine sediment monitoring locations are similar.  The average concentrations of the heavy metals in the marine sediments at SS5 and SS6 fall within the same range while the typhoon shelter sediments at SS7 hasve higher average concentrations of copper, lead and zinc.  The results show that the contents of Mercury and Arsenic exceed the Lower Chemical Exceedance Level (LCEL) but complies with the Upper Chemical Exceedance Level (UCEL).  Therefore no contamination of sediment is considered.

 

 

The average concentrations of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) are different at the three monitoring stations.  The tendency of increasing concentrations is in the sequence of SS6, SS5 and SS7.  The high PAHs at Cheung Chau typhoon shelter may be related to the combustion use of of petrol eum ion the fishing boats.

 

4.4       Sensitive Receivers

 

Beneficial uses have been defined in accordance with the requirements of the Hong Kong Planning Standards and Guidelines (HKPSG), which have been transposed into the EIA-TM.  As required under the Study Brief all water bodies, water and stream courses, groundwater systems and other sensitive or beneficial uses have been identified within a 6km radius from the project sites.  The study area of the water quality impact assessment is presented in Figure 4.3. In some cases however the effects of the works cannot be discerned so far from the site and thus these sensitive receivers may be discounted.  Potentially sensitive receivers include :include: These include Adamasta Channel, Cheung Chau Typhoon Shelter, Hei Ling Chau Typhoon Shelter, Silver Mine Bay, Cheung Sha Lower Beach, Pui O Beach and other gazetted and non-gazetted beaches within the Study.. y Area.

 


Monitoring locations within the Study Area are illustrated on Figure 4.23. Monitoring locations for beneficial uses have been identified in accordance with the requirements of the EIA-TM. These include:

 

·         Marine life;

·         Mariculture;

·         Beaches and other recreational areas;

·         Fish spawning grounds and fish culture zones; and

·         Areas for navigation/shipping including typhoon shelters, marinas and boat parks.

 

The project essentially requires cable laying across the Adamasta Channel and the potential impacts relate to elevation of suspended solids in the water column. Taking these factors into account only two of the potentially sensitive receivers within the 6km radius have the potential to be adversely affected during the course of the cable laying works. These are the Cheung Sha Wan Fish Culture Zone (shown as station R1 on Figure 4.4) and Tai Kwai Wan beach on Cheung Chau (shown as R13 on Figure 4.4).

 

Other potentially sensitive receivers such as Pui O Wan are too far away from the works area and the effects of tidal currents would disperse any sediment plumes before reaching this area and for this reason Pui O Wan has been discounted from any further assessment relating to sediment plume modelling. Similarly the beach at Tung Wan (R7) has been discounted as it is on the east of Cheung Chau whereas the works will take place on the west of the island.

 

In order to assess the extent of the impacts of sediment release to the water column a series of assessment stations were selected which include the two sensitive receivers as indicated above. Although the results from all other assessment points shown on Figure 4.4 have been included to assist in the interpretation of the data, attention is focussed upon the impacts of the works on the sensitive receivers  R1 and R13.

 

Sensitive receivers Those which could be potentially affected include:

·Adamasta Channel

·Cheung Chau Typhoon Shelter

·Pui O Beach

·Tai Kwai Wan

·Tai Long Wan

·Yi Long Wan

·Sai Wan

·Po Yue Wan

·Pak Tso Wan

·Tung Wan Tsai

 

Those which arwe outsidewith the area of influence of the works are not considered further and include:

·Cheung Sha Wan Fish Culture Zone

·Fish Fry Nursery Area

·Hei Ling Chau Typhoon Shelter

·Silver Mine Bay

·Cheung Sha Lower Beach

·Tung Wan Chai

·Sai Wan Typhoon Shelter

·Kwun Yam Wan

·Mong Tung Wan

Of the foregoing identified sensitive receivers within the six km radius of the works, some have been identified as being outside the area of influence of the work.  Some of the sensitive receivers have not been ascribed specific assessment points.  These include the Cheung Sha Wan Fish Culture Zone is outwith the main tidal flows and sediment plumes are expected to disperse before reaching this area.  Hei Ling Chau and Sai Wan are typhoon shelters both have impediments to flow and ingress of pollutants due to the breakwaters and Pui O beach was not included as the landmass of the Chi Ma Wan Peninsula and the tidal regime in the area were considered to be impediments to the migration of sediments from the main stream of the Adamasta Channel.

 

For the water quality impact assessment, 38 monitoring locations for beneficial uses have been selected as part of the assessment process. 27 monitoring locations represent the water sensitive receivers of beneficial uses and the rest of 11 are used for assess ing purposeThe assessment points indicated on Figur.  Locations of these WSRs and assessment points are presented in Figure 4.42., with aare included with a brief description in Table 4-.7. A brief description of the WSRs is summarised given in Table 4-7 and 4-8 respectively.

 

 


Table 4-7         Description and Coordinates of the Water Quality AssSenstiveessment PointReceivers Monitoring Locations

 

LocationDescription

Beneficial UseType

Assessment Point

Easting

Northing

Cheung Sha Wan Fish Culture Zone

Sensitive ReceiverFish Cultural Zone

R1*

810 687.330

818 743.944

Tai Kwai Wan

Sensitive Receiver (Non-gazetted Beach)

R13*

808 727.202

820 715.976

Tung Wan

Sensitive Receiver

Gazetted Beach

R7*

808 103.527

821 798.480

Nam Tam Wan

Rugged Coast

R8*

806 894.147

821 408.058

Po Yue Wan

Rugged Coast

R24**

806 725.004

819 775.129

Pak Tso Wan

Non-gazetted Beach

R25*

806 385.343

820 077.863

Tai Long Wan

Non-gazetted Beach

R26**

808 794.506

817 867.812

Tai Long Wan

Non-gazetted Beach

R27**

808 789.923

817 679.750

Yi Long Wan

Non-gazetted Beach

R28**

808 374.320

817 346.308

Yi Long Wan

Non-gazetted Beach

R29**

808 478.223

817 170.606

Yi Long Pai

Non-gazetted Beach

R30*

808 262.784

817 069.695

Mong Tung Wan

Non-gazetted Beach

R38*

809 440.312

815 880.115

Cheung Chau Typhoon Shelter

Typhoon Shelter

R18**

808 108.111

820 693.041

 

 

R19**

807 906.423

820 885.690

 

 

R20**

807 584.012

820 871.393

 

 

R21**

807 474.000

820 605.354

 

 

R22**

807 498.292

820 453.979

 

 

R23**

807 194.387

820 265.925

Adamasta Channel

Navigation Fairway

R2*

810 458.139

820 129.182

 

 

R3*

810 052.929

821 064.905

 

 

R4*

809 792.935

821 551.115

 

 

R5*

809 791.652

820 500.719

 

 

R6*

809 559.160

821 055.732

 

 

R9*

809 515.618

820 009.596

 

 

R10*

809 272.676

820 560.022

 

 

R11**

809 067.820

819 542.974

 

 

R12*

808 887.635

820 165.550

 

 

R14**

808 466.217

820 569.500

 

 

R15**

808 648.422

819 138.386

 

 

R16**

808 442.957

819 774.439

 

 

R31**

808 131.778

818 010.005

 

 

R32**

808 035.518

817 505.448

 

 

R33*

807 966.761

817 005.478

 

 

R34*

807 499.947

816 504.704

 

 

R35**

807 687.148

818 339.435

 

 

R36**

807 572.552

817 784.422

 

 

R37*

807 471.708

817 253.170

Note:   *   Monitoring locationsWater sensitive receivers lie outside the area of 100m away from the submarine cables.

   **   Monitoring locationsWater sensitive receivers fall within the area100m in the vicinity of the submarine cables.

 

Table 4.8  Description and Coordinates of the Supplementary Assessment Points

 

Note:       *      Assessment Points lie outside the area of 100m away from the submarine cables.

                **    Assessment Points fall within the area100m in the vicinity of the submarine cables.

 

 

The identified sensitive receivers include a fish culturale zone and a fish fery nursery area, nine non-gazetted and on gazetted beaches and the navigation fairway. The existing water quality at these sensitive receivers can be represented by the closest EPD’s monitoring station, SM12, as discussed in the previous section 4.3.2.

 

Cheung Sha Wan Fish Cultural Zone and the fish fry nursery area located toon the east side of Chi Ma Wan Peninsula and a. They are not directly affected by the proposed installation of submarine cables.

 

Tai Long Wan, Yi Long Wan, Yi Long Pai and Mong Tung Wan are locateds along the south-west coast of Chi Ma Wan Peninsula while Tai Kwai Wan, Po Yue Wan, Pak Tso Wan, Nam Tam Wan and Tung Wan are on Cheung Chau. Only Tung Wan is a gazetted beach.

 

The proposed submarine cables are running extend across the Adamasta Channel. As ships and ferries travel through this fairway regularly and transverse the alignment of the submarine cables, the dispersion of the water pollutants may be complicated. It should be noted that the assessment has not taken account of propeller wash as this was deemed to be minor compared to other natural hydrodynamic forces.

 

4.5       Construction Phase Impacts

 

4.5.1     Impact Sources

 

Construction activities include land-based works for the installation of the underground cable.  Construction activities may cause adverse impacts on the water quality of the receiving waters due to silt laden runoff, and direct contamination of waters during construction works. 

 

Potential impacts associated with the cable laying activities include the physico-chemical changes to the water column due to the release of suspended solids and the hitherto bound contaminant.  s.

 

The nature and extent of the impacts depend on the quality of the sediments as well as:

 

 

·         Dispersion characteristics within the receiving waters

·         Method of cable laying

·         Rate of cable laying

·         Proximity and nature of the sensitive receivers

 

Environmental impacts associated with dredging can be subdivided as follows:

 

·physical effects : resuspension and redeposition of particles during the dredging operations and changes in physical habitat due to smothering of benthos.

 

·chemical effects : oxidation of released sediments, potential proliferation of bacteria which feed on resuspended organic matter.

 

·biological effects : short term alteration in phytoplankton productivity as a result of decreased light penetration or proliferation of some species at the expense of others due to overabundance of nutrients, and adverse impacts of silt and clay particles on the branchiae of fish, and abrasive effects on crustaceans.

 

·social implications : visual impacts of dredging in terms of the recreational use of waters, damage to commercial fishing grounds or high levels of suspended solids at seawater intake points.

 


Key issues which were addressed through this assessment include:

 

(i)         the definition of the extent of the potential sediment contaminationplume; and

 

(ii) determination of the response of the marine environment to the potential release into the water column of trace metals and organic micropollutants from the material being dredged; and

 

(iii)        definition of the mitigation measures which will minimise the impacts of dredging release of sediments to the lowest acceptable level.

 


The geochemical form of trace metals within sediment samples plays a significant role in determining the potential impacts on the marine environment.  Although bulk chemical analyses provides an indication of the total contaminant levels within a sediment sample, the mere presence of a contaminant does not necessarily infer that it will either have an adverse impact on water quality or be available for uptake by aquatic organisms.

 

Metals may be :

 

(i) bound tightly within the crystalline lattice structure of the minerals within sediments and are not thus release except through the weathering process.  Such metals, including aluminium and magnesium, are usually inert and are thus biologically unavailable;

 

(ii) bound through a variety of ionic interactions, involving negatively charged surfaces of minerals or large organic molecules and positively charged cations including trace metals.  Substances in this form, often termed exchangeable cations, are relatively easily mobilised particularly under acidic conditions; or

 

(iii) dissolved in interstitial waters (pores) and although this fraction is generally relatively small these contaminants are comparatively easily mobilised and are often available to biota.  Contaminants in interstitial waters exist as free ions, in various organic and inorganic complexes, and their concentration is independent of the total contaminant level.

 

Metals, nutrients and organic materials which are bound into the sediment interstitial waters or adsorbed to the cation exchange complex, are the most mobile and potentially available contaminants in dredged material.  Anoxic sediments will frequently contain trace elements which are readily mobilised.

 

Contaminants, such as PCBs and chlorinated hydrocarbons are more complicated than organo-metals as they are not bound within mineral lattices, they do not occupy positions in clays or form sulphides or other insoluble compounds, nor are they part of the exchangeable fraction.  Instead, synthetic organics are almost exclusively found in the adsorbed form, usually associated with dissolved and particulate organic pollutants and carbon in sediments, connected to various organic molecules by Van der Waals forces.

 

Only a fraction of the PCBs and chlorinated hydrocarbons which are bound to the sediments are available in the interstitial water of sediments, and therefore it may be concluded that only a very small percentage of the overall contaminant load will be bioavailable.

 

Redox potential (Eh) is one of the most important factors influencing the remobilization of metals from sediments.  Anoxic sediments which are characterised by an Eh of -100 mV or less, while well oxygenated waters will, in contrast, have an Eh of >400mV.  If anoxic sediments are dredged from or disposed of in well oxygenated waters (i.e. dissolved oxygen levels greater than 4 mg/L), the physio-chemical state of metals within the sediments may be affected and some metals will become more mobile.

 

In addition to redox potential, the pH of both the sediments and the receiving waters can also affect the availability of some metals with a consequential increase in the amount of metal released initially to the water column.

 

In addition to the increase in suspended solids to the water column potential impacts also relate to the possible smothering of benthic biota and marine organisms through irritation of the gills or other membranes. A change in the deposition layer may result in the disturbance to the benthic infauna although recent survey data has shown that the rate of reworking of the seabed is much more rapid than previously considered and this should not be such a problem as formerly considered.

 

 

Another aspect which has often been considered in assessments of dredging impacts is the change in dissolved oxygen levels due to the release of suspended solids into the water column (during dredging). This is a particular issue if the sediments are heavily contaminated especially with organic matter. In this regard however cognisance should be given to the actual field survey data collected by CLP on another project which involved monitoring of the dissolved oxygen levels during the dredging works.

 

During construction there is the potential for erosion and sediments to be washed into receiving waters.  The potential sources of these impacts and the potential effects have also been assessed.

 

4.5.2          4.5.2   Dissolved Oxygen Depletion

 

Another aspect which has often been considered in assessments of dredging impacts is the change in dissolved oxygen levels due to the release of suspended solids into the water column (especially during dredging).

 

The effects of the resuspension of sediments in terms of dissolved oxygen depletion has been may be estimated using athe standard relationship relating the sediment oxygen demand and daily oxygen uptake rate with concentration of sediment release during dredging. : However such

 

DOdep = C x SOD x K x 0.001

 

where DOdep = reduction in dissolved oxygen level in mg/l

C = tidal average suspended solids concentration in kg/m3

SOD = sediment oxygen demand in mg/kg sediment

K = daily oxygen uptake rate, 0.23

 

This Such equations provides very conservative results which need to be subsequently verified during onsite monitoringby collecting sediment samples from the dredging location and conducting laboratory analyses for SOD.  It should be stressed that the sediments in the area to be disturbed are not generally dredged would not be expected to be highly polluted and coupled with a relatively low dredging rate, it would be sediment release, it is expected that DO depletion would be negligible and a very transient, situationshort term impact.  This has not in this regard however cognisance should be given to the actual field survey data collected by CLP on another project which involved monitoring of the dissolved oxygen levels during the works and confirmed that the level of dissolved oxygen depletion was negligible.

 

 


4.5.3     Modelling Assumptions

 

            Hydrodynamic Regime

 

As tThe reclamation area on formation of the temporary working platforms area to the east side of Pui O beach is small (18055180 m2) and temporary, no and will be formed behind a seawall with no dredging work will be involved. Thus As noted previously, the water quality impact caused by construction of this temporary working platformreclamation area is expected to be minimal and negligible when comparing with the laying of submarine cables. The temporary working platform was not refore, the construction of this reclamation was not included in the modelsin the hydrodynamic models as this facility will be dismantled following excavation of the tunnel.. The tidal flow models used for this assessment were therefore not modified to take account of any potential impediment to flow associated with this project. The model used was that set up under EPD contract No WP00-84 using the local fine grid model with a coastline representing the coastline .for the year 2000.