2    WATER QUALITY ASSESSMENT

 

2.1    Introduction

 

This Section describes the impacts on water quality associated with the construction and operation of the proposed East of Sha Chau Facility.  Computer modelling of sediment dispersion has been used to determine the impacts of the proposed development.  Impacts have been assessed with reference to the relevant environmental legislation and standards.  A review of baseline information (Part 1, Section 4) in the Study Area has determined that there are a series of water quality sensitive receivers present as follows:

 

Ecological: Sha Chau and Lung Kwu Chau Marine Park; Seagrass and Horseshoe Crab Habitats; and the critical habitats of the Indo-Pacific Humpback dolphin.

 

Fisheries:  Ma Wan Fish Culture Zone; Artificial Reefs; and Spawning Ground of Commercial Fisheries species.

 

Water Quality:     Beaches at Lung Kwu Tan and around Tuen Mun; Intakes at the Airport, Tuen Mun Area 38; and Castle Peak Power Station.

 

2.2    Water Quality Impact Assessment Methodology

 

A desktop literature review (presented in Part 1, Section 4) was conducted in order to establish the water quality conditions of the area within and surrounding the East of Sha Chau Facility.  Potential impacts due to the construction and operation of the East of Sha Chau Facility have been assessed (following the EIAO-TM Annex 14 guidelines) and the impacts evaluated (based on the criteria in EIAO-TM Annex 6). 

 

2.3    Water Quality Impact Assessment

 

The proposed East of Sha Chau Facility will consist of four purposely dredged seabed pits.  The pits will be dredged sequentially prior to backfilling with contaminated mud and capping with uncontaminated mud.  Impacts associated with the East of Sha Chau Facility are thus divided into those occurring during the dredging of pits and those during backfilling with contaminated mud and capping with uncontaminated mud.  Following this assessment the potential for residual impacts and cumulative impacts associated with concurrent projects, or through the combination of the above works, are discussed.


2.3.1    Backfilling

 

Impacts from the dispersion of sediment in suspension arising from backfilling operations have been assessed using computer modelling. 

 

Suspended Sediment

 

Impacts from suspended sediment may be caused by the transport of sediment plumes to sensitive receivers such as fish culture zones, marine parks etc.  

 

Sediment plumes will cause the ambient suspended sediment concentrations to be elevated and the level of the elevation will determine whether the impact is adverse or not.  The determination of the acceptability of any elevations is based on the criteria defined in Part 1, Section 4.

 

The modelling simulated the release of sediment during backfilling operations in the wet and dry seasons.  The results have been presented as contours of maximum and 90th percentile suspended sediment concentrations above ambient in the surface, middle and bed layers of the water column (Annex A).  Depth averaged contour plots illustrating the maximum and mean values recorded over the 15 day tidal cycle modelling period are presented in Annex A.  In addition, maximum elevations at the sensitive receivers are presented in Tables 2.1a and 2.1b of Annex A.

 

As discussed in Annex A, modelling of backfilling operations has been conducted for trailer disposal (Scenario 1) and through barge disposal (Scenario 3).  Due to the greater loss rates associated with trailer disposal backfilling works, predicted concentrations calculated for these works are discussed below as they thus represent the worst-case scenario.

 

The results of trailer disposal backfilling activities appear to indicate that sediment plumes stay relatively close to the seabed, with no elevations > 15 mg L-1 recorded in the surface layer.  In general, SS increases appear to be confined within the pit boundaries for the surface layer.  Horizontal dispersion is increased in the middle layers, with the maximum dispersion recorded in the bottom layer. Nevertheless, this dispersion stays within relatively close proximity to the pit boundaries with limited horizontal spread following the Urmston Road and around the existing disposal pits in the East of Sha Chau Area.  Wet season contours appear to indicate a similar pattern; however, during this season plumes appear to have less vertical spread throughout the water column, with little or no elevations in SS predicted in the middle and surface layers.  The horizontal spread of SS at the seabed increases, with elevations at the seabed of < 10 mg L-1 recorded on the boundary of the Sha Chau and Lung Kwu Chau Marine Park.  90th percentile concentrations appeared to demonstrate a similar pattern to that described above.  The maximum depth average contour plots for SS indicate that elevations of < 10 mg L-1 cover a relatively small area that is restricted to open waters and does not affect any of the sensitive receivers (Annex A).  The mean depth average plots indicate that the < 10 mg L-1 contour does not extend beyond the boundary of the active pit.

 

The potential impact at each of the water quality sensitive receivers as a result of backfilling operations is discussed below. 

 

Marine Parks:  The maximum depth averaged elevations of SS concentrations at the Marine Park as a result of backfilling operations are predicted to be 2.2 mg L-1 and 1.6 mg L-1 in the dry and wet seasons, respectively.  As such, these elevations are compliant with the WQO.  It is noted that these predicted elevations are similar in range to those predicted in the EIA for CMP IV ([1]).

 

Artificial Reef Deployment Areas:  Predicted elevations of SS concentrations at the ARs within the Marine Park and at the Airport Exclusion Zone as a result of backfilling operations are very low and compliant with the WQO (maximum = 2 mg L-1 (dry season) and 3 mg L-1 (wet season)).  As such, impacts are not expected to occur.

 
Seagrass Beds, Mangroves, Horseshoe Crab Areas:  Sediment dispersion results predict that maximum depth averaged elevations in SS concentrations are expected to be compliant with the WQO at the Tai Ho Bay, San Tau Beach SSSI or at Yam O.

 

Habitat of the Indo-Pacific Humpback Dolphin:  High elevations of SS concentrations appear to only be recorded within close proximity to the boundary of the East of Sha Chau Facility.  Long term monitoring data indicates that disposal of contaminated mud in the East of Sha Chau area does not appear to be having an adverse affect on Sousa chinensis.

 

Fish Culture Zones:  The maximum SS elevation at the FCZ as a result of backfilling operations has been predicted to be < 1 mg L-1.  Impacts to water quality at the Ma Wan FCZ as a result of the backfilling works are thus unlikely to occur as the increases in SS are expected to be negligible.

 

Beaches:  Beaches at Lung Kwu Tan and Tuen Mun are located remotely from the East of Sha Chau Facility (Part 1, Section 4).  As such, impacts from backfilling works were not expected.  This statement has been confirmed by the modelling work that indicates that there are no detectable increases in SS concentrations at each of these sensitive receivers and is therefore acceptable.

 

Intakes:  Modelling results indicate that the maximum elevations at these intakes are negligible (< 1 mg L-1).  As this elevation is within the allowable increase with regard to the WQO, no unacceptable impacts to intakes as a result of backfilling operations are expected to occur.

 

Spawning Area:  Maximum elevations of SS concentrations have been identified in the both the wet and dry seasons to remain close to the seabed, with little or no elevations recorded in the surface later in the wet season.  As most fish larvae, eggs and fry are likely to be found in the surface layer post-spawning, the predicted impacts to water quality will not result in impacts to spawning areas.

 

Sediment Deposition

 

The information presented in the contour plots illustrates that SS concentrations decrease relatively rapidly outside the pit boundary of the East of Sha Chau Facility (Annex A).  This implies that whilst there is a degree of horizontal dispersion of sediment plumes, the majority of suspended sediments settle in close proximity to the works.  The modelling exercise generated contour plots of sediment in the Study Area as a result of backfilling operations (Annex A).  As expected, the majority of sediment settles either within, or within relatively close proximity to, the East of Sha Chau Facility.  Sediment deposition has been predicted within the Marine Park due to backfilling operations, however, maximum deposition has been determined to be no greater than < 25 g m-2 day-1.  The significance of these elevations is discussed in Part 3, Section 3, which has determined that levels such as those predicted are not considered to be a concern.

 

Thus, with the exception of those within the Marine Park that are not considered to be a concern, deposited sediments will not reach water quality sensitive receivers.  As such, adverse impacts to water quality, marine and fisheries sensitive receivers by deposited sediments as a result of backfilling operations at the East of Sha Chau Facility are not expected to occur.

 

Water Quality

 

The loss of sediment through backfilling operations at the East of Sha Chau Facility may impact the quality of the receiving waters.  The modelling approach has simulated the release of nutrients into the water column and examined the subsequent effects on levels of dissolved oxygen, biochemical oxygen demand and nutrients (as unionised ammonia). 

 

The results of the modelling are presented in Annex A and indicate that backfilling operations at the East of Sha Chau Facility are not expected to cause adverse impacts to water quality.  The results indicate that levels of dissolved oxygen, biochemical oxygen demand and nutrients do not change appreciably from background conditions and are compliant with the relevant WQOs.

 

Contaminants

 

The results of modelling suspended sediments released from the disposal of dredged material are presented in Annex B and are discussed above.  Using partitioning coefficients it has been possible to predict the maximum potential release of contaminants (see Methodology in Annex A). 

 

Maximum predicted concentrations of contaminants have been estimated for backfilling operations at the East of Sha Chau Facility.  These predicted concentrations have been used in the bioaccumulation assessment (Annex B) to determine the potential uptake of contaminants into the food chain.  Based on bioconcentration factors determined from the bioaccumulation assessment, the predicted contaminant concentrations in marine water and sediments have been assessed to calculate the risks to humans and marine mammals associated with consuming fish and shellfish collected from the vicinity of the East of Sha Chau Facility.  The results of this assessment are presented in Part 3, Section 5 and in Annex C. 

 

It is also important to investigate the potential for these desorbed contaminants to impact the identified water quality sensitive receivers.  However, for the basis of this assessment, only those water quality sensitive receivers considered to have the potential to be adversely impacted by increases in contaminants in the water column have been assessed ([2]).  These selected water quality sensitive receivers are as follows:

 

·                Airport Exclusion Zone Artificial Reef;

 

·                Sha Chau and Lung Kwu Chau Marine Park;

 

·                San Tau Beach SSSI;

 

·                Tai Ho Bay; and,

 

·                Yam O Bay.

 

Maximum concentrations of contaminants predicted at these sensitive receivers in both the dry and wet seasons are presented in Tables 2.1 and 2.2, respectively and have been evaluated against European Community (EC) Water Quality Standards.  The EC standards which have been used in the absence of quantitative water quality objectives for these contaminants in Hong Kong.

 

Comparison to EC water quality standards, which are presented as dissolved concentrations, requires summation of predicted dissolved concentrations arising from backfilling operations with ambient (soluble) concentrations (see Part 1, Section 4, Table 4.2).  As no EC water quality standards or ambient values are available for PAHs, PCBs and TBT, no comparison between predicted concentrations and these values was possible. 

 

Predicted concentrations of contaminants resulting from a representative operational scenario (Scenario 1 – Trailer disposal) at the East of Sha Chau Facility are extremely low in comparison to EC water quality standards.  As the modelled contaminants represent a range of chemical compounds with varying partitioning coefficients and input values (ie UCELs), the range of results is likely to be representative of other contaminants of concern.  As predicted contaminant concentrations are extremely low (maximum = Chromium, 2.9% of Allowed Levels (wet season)), and modelling results for other operational scenarios are very similar, modelling of contaminants for other operational scenarios at the East of Sha Chau Facility is unlikely to produce detectably different results.  In summary, the predicted contaminant concentrations resulting from operations at the East of Sha Chau Facility are negligible when compared to international water quality standards and thus no unacceptable impacts are anticipated. 


 

Table 2.1    Dissolved Concentrations of Contaminants of Concern through Backfilling Operations at the East of Sha Chau Facility (Dry Season)

COC

Kd

Unit

Max. Sediment

Conc

Unit

Eq. Dissolved Conc.

(mg L-1)

Dissolved Concentration (mg L-1)

Alloweda

(mg L-1)

Minimum Ambient Conc.

(mg L-1)

Maximum Predicted Diss. Conc. as % of Allowed

AR1_3b

MP2(5)b

SG1b

SG2b

SG3b

Metals

 

 

 

 

 

 

 

 

 

 

 

 

 

Ag

200

l/g

2

mg/kg

0.0100

1.1E-03

1.6E-03

1.7E-08

1.3E-06

9.6E-07

-

1

-

As

130

l/ge

42

mg/kg

0.3231

1.5E-02

2.2E-02

9.8E-05

7.6E-03

5.5E-03

-

0.5

-

Cd

100

l/g

4

mg/kg

0.0400

1.1E-03

1.6E-03

3.4E-08

2.7E-06

1.9E-06

2.5

1

0.07%

Cr

290

l/g

160

mg/kg

0.5517

1.3E-01

1.9E-01

8.3E-04

6.5E-02

4.7E-02

15

0.5

1.3%

Cu

122

l/g

110

mg/kg

0.9016

3.6E-02

5.5E-02

2.4E-04

1.9E-02

1.3E-02

5

0.5

1.1%

Hg

700

l/g

1

mg/kg

0.0014

1.9E-03

2.9E-03

8.6E-09

6.7E-07

4.8E-07

0.3

1

1%

Ni

40

l/g

40

mg/kg

1.0000

4.3E-03

6.6E-03

2.9E-05

2.2E-03

1.6E-03

30

0.5

0.02%

Pb

130

l/g

110

mg/kg

0.8462

3.9E-02

5.9E-02

2.6E-04

2.0E-02

1.4E-02

25

0.5

0.2%

Zng

100

l/g

270

mg/kg

2.7000

7.3E-02

1.1E-01

4.8E-04

3.8E-02

2.7E-02

40

5

0.3%

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Organics

 

 

 

 

 

 

 

 

 

 

 

 

 

L PAH

0.075

l/g

3.19

mg/kg

42.1333

6.4E-07

9.7E-07

2.7E-08

2.1E-06

1.5E-06

-

-

-

H PAH

1.14

l/g

9.6

mg/kg

8.4211

3.0E-05

4.5E-05

8.2E-08

6.4E-06

4.6E-06

-

-

-

PCBs

1,585

l/gOC (c)

180

mg/kg

0.0095

9.3E-06

1.4E-05

6.1E-05

4.8E-03

3.4E-03

-

-

-

TBTf

40

l/gOC (c)

0.15

mg/kg

0.0003

1.9E-10

2.9E-10

1.3E-09

1.0E-07

7.2E-08

-

 

-

Notes:      a                 Environmental Quality Standards and Assessment Levels for Surface Water (from HMIP (1994) Environmental and BPEO Assessment Principles for Integrated Pollution Control)

b                 AR1_3 = Airport Exclusion Zone Artificial Reef; MP2(5) = Sha Chau and Lung Kwu Chau Marine Park; SG1 = San Tau Beach SSSI; SG2 = Tai Ho Bay; and SG3 = Yam O Bay

c                 Converted to l/g using the OC content of the sediments

d                 Sediment concentration equal to max. observed value at Kellett Bank

e                 Value is not available, lowest value of other metals has been used, in this case about 10 for Cd

f                  US EPA Aquatic Life Advisory Concentration for Seawater cited in Lau MM (1991) Tributyltin Antifoulings: A Threat to the Hong Kong Marine Environment.  Arch. Environ. Contam. Toxicol. 20: 299-304.

g                 Wen LS, Santschi PH, Paternostro CL, Lehman RD, 1997.  Colloidal and Particulate Silver in River and Estuarine Waters of Texas.  Environ Sci Technol 31: 723-731.


 

Table 2.2    Dissolved Concentrations of Contaminants of Concern through Backfilling Operations at the East of Sha Chau Facility (Wet Season)

COC

Kd

Unit

Max. Sediment

Conc

Unit

Eq. Dissolved Conc

(mg L-1)

Dissolved Concentration (mg L-1)

Alloweda

(mg L-1)

Minimum Ambient Conc.

(mg L-1)

Maximum Predicted Diss. Conc. as % of Allowed

AR1_3b

MP2(5)b

SG1b

SG2b

SG3b

Metals

 

 

 

 

 

 

 

 

 

 

 

 

 

Ag

200

l/g

2

mg/kg

0.0100

8.8E-04

2.1E-03

2.4E-07

9.1E-06

4.8E-07

-

1

-

As

130

l/ge

42

mg/kg

0.3231

1.2E-02

2.9E-02

1.4E-03

5.2E-02

2.7E-03

-

0.5

-

Cd

100

l/g

4

mg/kg

0.0400

8.8E-04

2.1E-03

4.9E-07

1.8E-05

9.6E-07

2.5

1

0.1%

Cr

290

l/g

160

mg/kg

0.5517

1.0E-01

2.4E-01

1.2E-02

4.4E-01

2.3E-02

15

0.5

2.9%

Cu

122

l/g

110

mg/kg

0.9016

3.0E-02

7.1E-02

3.4E-03

1.3E-01

6.7E-03

5

0.5

2.5%

Hg

700

l/g

1

mg/kg

0.0014

1.5E-03

3.7E-03

1.2E-07

4.5E-06

2.4E-07

0.3

1

1.2%

Ni

40

l/g

40

mg/kg

1.0000

3.5E-03

8.4E-03

4.1E-04

1.5E-02

8.0E-04

30

0.5

0.1%

Pb

130

l/g

110

mg/kg

0.8462

3.1E-02

7.5E-02

3.7E-03

1.4E-01

7.2E-03

25

0.5

0.5%

Zng

100

l/g

270

mg/kg

2.7000

5.9E-02

1.4E-01

6.9E-03

2.6E-01

1.4E-02

40

5

0.6%

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Organics

 

 

 

 

 

 

 

 

 

 

 

 

 

L PAH

0.075

l/g

3.19

mg/kg

5.2E-07

1.2E-06

3.9E-07

1.4E-05

7.6E-07

5.2E-07

-

-

-

H PAH

1.14

l/g

9.6

mg/kg

2.4E-05

5.8E-05

1.2E-06

4.4E-05

2.3E-06

2.4E-05

-

-

-

PCBs

1,585

l/gOC (c)

180

mg/kg

7.5E-06

1.8E-05

8.8E-04

3.3E-02

1.7E-03

7.5E-06

-

-

-

TBTf

40

l/gOC (c)

0.15

mg/kg

1.6E-10

3.8E-10

1.8E-08

6.8E-07

3.6E-08

1.6E-10

-

 

-

Notes:      As above

 

 


2.3.2    Dredging

 

Impacts due to the dispersion of sediment in suspension arising from dredging operations have been assessed using computer modelling. 

 

Suspended Sediment

 

The modelling simulated the release of sediment during dredging operations in the wet and dry seasons.  The results have been presented as contours of maximum suspended sediment concentrations above ambient (Annex A).  In addition, tables of elevations at the sensitive receivers are presented in Tables 2.1a and 2.1b of Annex A.

 

As discussed in Annex A, modelling of dredging operations have been conducted for grab dredging (Scenario 3) and through trailer dredging (Scenario 4).  The results appear to indicate that grab dredging results in higher elevations in SS concentrations, they thus represent the worst-case scenario and are discussed below.

 

The results indicate that sediment plumes stay in relatively close proximity to the pit boundaries.  Plumes that extend beyond the boundary of the facility are predicted to remain within the main flow channel of the Urmston Road.  Wet season contours appear to indicate a similar pattern to those predicted for the dry season.  Horizontal spread marginal increases on the boundary of the Sha Chau and Lung Kwu Chau Marine Park. 

 

The potential impact at each of the water quality sensitive receivers as a result of dredging operations is discussed below. 

 

Marine Parks:  The results of the water quality modeling indicate that dredging operations are not predicted to increase SS concentrations within the Marine Park as no detectable concentrations have been identified.

 

Artificial Reef Deployment Areas:  Predicted elevations of SS concentrations at ARs as a result of dredging operations are very low and compliant with the WQO (Marine Park AR: maximum = < 1 mg L-1 (dry season); Airport Exclusion Zone AR: maximum = 3 mg L-1 (wet season)).  The significance of these elevations is discussed in Part 2, Section 4.

 

Seagrass Beds, Mangroves, Horseshoe Crab Areas:  Sediment dispersion results based on dredging operations predict that elevations of SS concentrations are expected to stay relatively close to dredging operations.  As such, elevations at the San Tau Beach SSSI are non-detectable. 

 

Habitat of the Indo-Pacific Humpback Dolphin:  Elevations of SS concentrations appear to only be recorded within close proximity to the boundary of the East of Sha Chau Facility.  Long term monitoring data indicates that operations in the East of Sha Chau area does not appear to be having an adverse affect on Sousa chinensis.


Fish Culture Zones:  Water quality modelling results have shown that the maximum SS elevations at the FCZ as a result of dredging operations is < 1 mg L-1, which is well within the acceptable range and is not expected to cause adverse impacts. 

 

Beaches:  There are no detectable increases in SS concentrations at each of these sensitive receivers due to dredging operations, therefore, no unacceptable impacts are expected to occur.

 

Intakes:  Modelling results indicate that there are no detectable increases at the intakes through dredging operations, therefore, no unacceptable impacts expected to occur.

 

Spawning Area:  Elevations of SS concentrations have been identified to remain close to the seabed.  As most fish larvae, eggs and fry are likely to be found in the surface layer post-spawning, it appears that the predicted impacts to water quality will not result in impacts to spawning areas.

 

Sediment Deposition

 

Predictions of sediment deposition as a result of dredging operations indicate that the majority of sediment settles either within or within relatively close proximity to the East of Sha Chau Facility (Table 2.1, Annex A).  A similar pattern of deposition is predicted for the wet and dry seasons.  In terms of deposition of sediments, the maximum deposition of SS within the Marine

 

Park due to dredging operations has been determined to be no greater than 63 g m-2 day-1.  The significance of these elevations is discussed in Part 3, Section 3, which has determined that levels such as those predicted are not considered to be a concern.

Thus, with the exception of those within the Marine Park that are not considered to be a concern, deposited sediments will not reach water quality sensitive receivers.  As such, adverse impacts to water quality, marine and fisheries sensitive receivers by deposited sediments as a result of dredging operations at the East of Sha Chau Facility are not predicted to occur.

 

2.3.3    Capping

 

Impacts from the dispersion of sediment in suspension arising from capping operations have been assessed using computer modelling. 

 

Suspended Sediment

 

The modelling simulated the release of sediment during capping operations in the wet and dry seasons.  The results have been presented as contours of maximum suspended sediment concentrations above ambient (Annex A).  In addition, tables of elevations at the sensitive receivers are presented in Tables 2.1a and 2.1b of Annex A.


The results of capping operations indicate a similar pattern to barge disposal backfilling operations at the East of Sha Chau Facility in that sediment plumes stay relatively close proximity to the pit boundaries, particularly during the dry season.  Plumes that extend beyond the boundary of the facility appear confined within the main flow channel of the Urmston Road.  In comparison to dredging and backfilling operations, horizontal and vertical spreads of plumes are predicted to be lower.

 

The potential impact at each of the water quality sensitive receivers as a result of capping operations is discussed below. 

 

Marine Parks:  The results of the water quality modeling indicate that capping operations are not predicted to increase SS concentrations within the Marine Park as no detectable concentrations have been identified. 

 

Artificial Reef Deployment Areas:  Predicted elevations of SS concentrations at the ARs within as a result of capping operations are very low and compliant with the WQO (Marine Park AR: maximum = < 1 mg L-1 (dry season); Airport Exclusion Zone AR: maximum = < 1 mg L-1 (wet season)).  No unacceptable impacts are therefore expected to occur.

 

Seagrass Beds, Mangroves, Horseshoe Crab Areas:  Sediment dispersion results based on capping operations predict that elevations at the San Tau Beach SSSI sensitive receiver are non-detectable, as such no exceedance of the WQO would occur. 

 

Habitat of the Indo-Pacific Humpback Dolphin:  Elevations of SS concentrations appear to only be recorded within close proximity to the boundary of the East of Sha Chau Facility.  Long term monitoring data indicates that operations in the East of Sha Chau area do not appear to be having an adverse affect on Sousa chinensis.

 

Fish Culture Zones:  Water quality modelling results have shown that the maximum SS elevations at the FCZ as a result of capping operations is < 1 mg L-1, which is well within the acceptable range and is not expected to cause adverse impacts.

 

Beaches:  There are no detectable increases in SS concentrations at each of these sensitive receivers due to dredging operations, therefore, no unacceptable impacts are expected to occur.

 

Intakes:  Modelling results indicate that there are no detectable increases at the intakes through dredging operations, therefore, no unacceptable impacts expected to occur.

 

Spawning Area:  Elevations of SS concentrations have been identified to remain close to the seabed.  As most fish larvae, eggs and fry are likely to be found in the surface layer post-spawning, it appears that the predicted impacts to water quality will not result in impacts to spawning areas.

 

Sediment Deposition

 

Predictions of sediment deposition as a result of dredging capping operations indicate that the majority of sediment settles either within or within relatively close proximity to the East of Sha Chau Facility (Table 2.1, Annex A).  A similar pattern of deposition is observed between the wet and dry seasons.  In terms of deposition of sediments, the maximum deposition of SS within the Marine Park due to cappingdredging operations has been determined to be no greater than 24 g m-2 day-1.  The significance of these elevations is discussed in Part 3, Section 3, which has determined that levels such as those predicted are not considered to be a concern.

 

Deposited sediments will not reach water quality sensitive receivers.  As such, adverse impacts to water quality, marine and fisheries sensitive receivers by deposited sediments as a result of capping operations at the East of Sha Chau Facility are not predicted to occur.

 

2.4    Water Quality Mitigation Measures

 

The water quality modelling works have indicated that for both the dry and wet seasons, the works can proceed at the recommended working rates without causing unacceptable impacts to water quality sensitive receivers through either elevations of suspended sediment or deposition of sediment.  Changes to other water quality parameters have been demonstrated to be minor, compliant with applicable standards and therefore not of concern.

 

Unacceptable impacts to water quality sensitive receivers have largely been avoided through the adoption of the following measures:

 

·        Siting:  A number of siting options were studied and the preferred location avoids direct impacts to sensitive receivers.

 

·        Reduction in Indirect Impacts:  The East of Sha Chau Facility is located at a sufficient distance from water quality sensitive receivers so that the dispersion of sediments from the construction and operation works does not affect the receivers at levels of concern (as defined by the WQO and tolerance criteria). 

 

·        Adoption of Acceptable Working Rates:  The modelling work has demonstrated that the selected working rates for the dredging and backfilling and capping of the East of Sha Chau Facility will not cause unacceptable impacts to the receiving water quality.

 

Aside from the above pro-active measures that have been instituted for the Project, the following operational constraints should also be applied.  It should be noted that there is no requirement for constraints on timing or sequencing, as all scenarios have been demonstrated to be acceptable with the required mitigation measures in place.


1.      Dredging operations within the East of Sha Chau Facility do not exceed 100,000 m3 week-1.

 

2.      Backfilling operations within the East of Sha Chau Facility do not exceed a disposal rate of 26,700 m3 day-1.

 

3.      Capping operations within the East of Sha Chau Facility do not exceed a disposal rate of 26,700 m3 day-1.

 

4.      No overflow is permitted from the trailer suction hopper dredger but the Lean Mixture Overboard (LMOB) system will be in operation at the beginning and end of the dredging cycle when the drag head is being lowered and raised. 

 

5.      Dredged marine mud shall be disposed of in a gazetted marine disposal area in accordance with the Dumping at Sea Ordinance (DASO) permit conditions.

 

The following good practice measures shall apply at all times:

 

1.      All disposal vessels should be fitted with tight bottom seals in order to prevent leakage of material during transport.

 

2.      All barges should be filled to a level, which ensures that material does not spill over during transport to the disposal site and that adequate freeboard is maintained to ensure that the decks are not washed by wave action.

 

3.      After dredging, any excess materials should be cleaned from decks and exposed fittings before the vessel is moved from the dredging area.

 

4.      The contractor(s) should ensure that the works cause no visible foam, oil, grease, litter or other objectionable matter to be present in the water within and adjacent to the dredging site.

 

5.      If installed, degassing systems should be used to avoid irregular cavitation within the pump.

 

6.      Monitoring and automation systems should be used to improve the crew’s information regarding the various dredging parameters to improve dredging accuracy and efficiency.

 

7.      Control and monitoring systems should be used to alert the crew to leaks or any other potential risks.

 

8.      When the dredged material has been unloaded at the disposal areas, any material that has accumulated on the deck or other exposed parts of the vessel should be removed and placed in the hold or a hopper.  Under no circumstances should decks be washed clean in a way that permits material to be released overboard.

 

9.      All dredgers should maintain adequate clearance between vessels and the seabed at all states of the tide and reduce operations speed to ensure that excessive turbidity is not generated by turbulence from vessel movement or propeller wash.

 

2.5    Residual Environmental Impacts

 

No residual environmental impacts, in terms of exceedances of applicable standards (ie Water Quality Objectives and marine ecology and fisheries tolerance criterion), were predicted to occur as a result of the construction and operation of the East of Sha Chau Facility, provided that the mitigation measures, described in Section 2.4 are implemented.  The mitigation measures were specified in the form of operational constraints and as a series of ‘best practice’ methods.

 

2.6    Cumulative Impacts

 

Cumulative impacts to water quality may arise from concurrent dredging, backfilling or development projects in the area.  In addition, cumulative impacts through the combination of dredging, backfilling and capping operations within the East of Sha Chau Facility have the potential to occur.  A number of planned projects have the potential to result in cumulative impacts with the construction and operation of the proposed East of Sha Chau Facility.  Water quality modelling of the cumulative impacts of these projects has been presented in Annex A.  The findings indicated that no adverse impacts would be expected to water quality sensitive receivers when compared the allowable increases as defined by the WQO.  It should be noted, however, that the assessment has been conducted on maximum operations without the use of operational controls.

 

Unacceptable cumulative impacts as a result of concurrent project construction and operational activities are, therefore, unlikely to occur and hence cumulative impacts to water quality are not anticipated.

 

2.7    Environmental Monitoring & Audit

 

The construction and operation of the proposed East of Sha Chau Facility has been defined at rates that maintain environmental impacts to within acceptable levels.  Actual impacts during the works will be monitored by through a detailed Environmental Monitoring and Audit (EM&A) programme.  Full details of the EM&A programme are presented in the EM&A Manual which has been based on the on-going and previous monitoring programmes conducted at the Contaminated Mud Disposal Facility at East of Sha Chau.  This programme will provide management actions and supplemental mitigation measures to be employed should impacts arise, thereby ensuring the environmental acceptability of the East of Sha Chau Facility.

 

2.8    Conclusions

 

This Section has described the impacts to water quality arising from the construction and operation of the East of Sha Chau Facility.  The purpose of the assessment was to thoroughly evaluate the East of Sha Chau Facility in terms of the acceptability of predicted impacts to water quality from dredging, backfilling and capping of the pits and also concurrent activities.

 

Computer modelling was used to simulate the loss of sediment to suspension during dredging, backfilling and capping operations. 

The assessment concluded that any sediment disturbed by the works would settle rapidly back onto the seabed and the suspended sediment elevations would be of short duration.  This means that there would be little transport of suspended sediment away from the pits and that the sediment would not impact upon sensitive receivers.  The findings of the modelling works are comparable to the elevations predicted during the modelling works for the CMP IV EIA.  The CMP IV EIA predictions have since been verified through the environmental monitoring and audit works.

 

An EM&A programme has been devised to confirm that the works would be environmentally acceptable.


 

 



([1])       ERM-Hong Kong, Ltd (1997).  EIA Study for Disposal of Contaminated Mud in the East Sha Chau Marine Borrow Pit.  For the Civil Engineering Department.

([2])       Sensitive receivers that have been excluded include seawater intakes, bathing beaches stations and fish culture zones as these area either not considered to be sensitive to increases in contaminants or elevated concentrations of SS have been predicted to be negligible at these sites due to backfilling operations.