6              Water Quality Impact

6.1          Introduction

6.1.1       This section presents an assessment of the potential water quality impacts associated with construction and operation of the Project. Recommendations for mitigation measures have been provided to minimise the identified water quality impacts.

6.2          Environmental Legislation, Plans, Standards and Guidelines

Environmental Impact Assessment Ordinance (EIAO)

6.2.1       The Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) was issued by EPD under Section 16 of the EIAO. It specifies the assessment method and criteria that are to be followed in an EIA Study. Reference sections in the EIAO-TM provide the details of assessment criteria and guidelines that are relevant to the water quality impact assessment, including:

Ÿ   Annex 6 – Criteria for Evaluating Water Pollution

Ÿ   Annex 14 – Guidelines for Assessment of Water Pollution

Water Quality Objectives

6.2.2       The Water Pollution Control Ordinance (WPCO) provides major statutory framework for the protection and control of water quality in Hong Kong. According to the Ordinance and its subsidiary legislation, Hong Kong waters are divided into ten Water Control Zones (WCZs). Corresponding statements of Water Quality Objectives (WQOs) are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in the WCZs based on their beneficial uses. The study area for this water quality impact assessment covers the Victoria Harbour (Phase one) WCZ and Junk Bay WCZ. The corresponding WQOs are listed in Table 6.1 and Table 6.2.

Table 6.1               Summary of Water Quality Objectives for Victoria Harbour WCZ

Parameters

Objectives

Sub-Zone

Offensive odour, tints

Not to be present

Whole zone

Visible foam, oil scum, litter

Not to be present

Whole zone

E coli

Not to exceed 1000 per 100 mL, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals between 7 and 21 days

Inland waters

Dissolved oxygen (DO) within 2 m of the seabed

Not less than 2.0 mg/l for 90% of samples

Marine waters

Depth-averaged DO

Not less than 4.0 mg/l for 90% of samples

Marine waters

DO

Not less than 4.0 mg/l

Inland waters

pH

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

Marine waters

Not to exceed the range of 6.0 - 9.0 due to human activity

Inland waters

Salinity

Change due to human activity not to exceed 10% of ambient

Whole zone

Temperature

Change due to human activity not to exceed 2°C

Whole zone

Suspended solids (SS)

Not to raise the ambient level by 30% caused by human activity

Marine waters

Annual median not to exceed 25 mg/l due to human activity

Inland waters

Unionized ammonia (UIA)

Annual mean not to exceed 0.021 mg(N)/l as unionized form

Whole zone

Nutrients

Shall not cause excessive algal growth

Marine waters

Total inorganic nitrogen (TIN)

Annual mean depth-averaged inorganic nitrogen not to exceed 0.4 mg(N)/l

Marine waters

5-Day biochemical oxygen demand (BOD5)

Not to exceed 5 mg/l

Inland waters

Chemical Oxygen Demand (COD)

Not to exceed 30 mg/l

Inland waters

Toxic substances

Should not attain such levels as to produce significant toxic, carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms.

Whole zone

Human activity should not cause a risk to any beneficial use of the aquatic environment.

Whole zone

Source: Statement of Water Quality Objectives (Victoria Harbour (Phases One, Two and Three) Water Control Zone).

Table 6.2               Summary of Water Quality Objectives for Junk Bay WCZ

Parameters

Objectives

Sub-Zone

Offensive Odour, Tints

Not to be present

Whole zone

Visible foam, oil scum, litter

Not to be present

Whole zone

Dissolved Oxygen (DO) within 2 m of the seabed

Not less than 2.0 mg/L for 90% of samples

Marine waters

Depth-averaged DO

Not less than 4.0 mg/L for 90% of samples

Marine waters excepting fish culture subzones

Not less than 5.0 mg/L for 90% of samples

Fish culture subzones

Not less than 4.0 mg/L

Inland waters

5-Bay Biochemical Oxygen Demand (BOD5)

Change due to waste discharges not to exceed 5 mg/L

Inland waters

Chemical Oxygen Demand (COD)

Change due to waste discharges not to exceed 30 mg/L

Inland waters

pH

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

Marine waters

To be in the range of 6.0 - 9.0

Inland waters

Salinity

Change due to waste discharges not to exceed 10% of ambient

Whole zone

Temperature

Change due to waste discharges not to exceed 2 °C

Whole zone

Suspended solids (SS)

Not to raise the ambient level by 30% caused by waste discharges and shall not affect aquatic communities

Marine waters

Change due to waste discharges not to exceed 25 mg/L of annual median

Inland waters

Unionised Ammonia (UIA)

Annual mean not to exceed 0.021 mg/L as unionised form

Whole zone

Nutrients

Shall not cause excessive algal growth

Marine waters

Total Inorganic Nitrogen (TIN)

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

Marine waters

Dangerous substances

Should not attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms

Whole zone

Waste discharges should not cause a risk to any beneficial use of the aquatic environment

Whole zone

Bacteria

Not exceed 610 per 100ml, calculated as the geometric mean of all samples collected in one calendar year

Secondary contact recreation subzones and fish culture subzones

Not exceed 1000 per 100ml, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days

Inland waters

Colour

Change due to waste discharges not to exceed 50 Hazen units

Inland waters

Source: Statement of Water Quality Objectives (Junk Bay Water Control Zone).

Technical Memorandum on Effluent Discharge Standard

6.2.3       Besides setting the WQOs, the WPCO controls effluent discharging into the WCZs through a licensing system. Guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, stormwater drains, inland and coastal waters) is provided in the Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS). The limits given in the TM cover the physical, chemical and microbial quality of effluents. Any effluent discharge during the construction and operational stages should comply with the relevant standards as stipulated in the TM-DSS.

Practice Notes

6.2.4       The Practice Note (PN) for Professional Persons on Construction Site Drainage (ProPECC PN 1/94) issued by EPD provides good practice guidelines for dealing with various types of discharge from a construction site. Practices outlined in the PN should be followed as far as possible during construction to minimize the water quality impact due to construction site drainage.

6.3          Water Sensitive Receivers

6.3.1       Any discharges from the Project works during construction and operational phase would potentially affect the inland waters within the Junk Bay and Victoria Harbour (Phase One) WCZs.

6.3.2       Major inland water bodies within 500m from the rock cavern site boundary include Tseng Lan Shue Stream as shown in Figure 6.1.

6.3.3       The rock cavern development site is located at about 100m away from the upstream section of Tseng Lan Shue Stream. Other sections of Tseng Lan Shue Stream are further away from the rock cavern development site. The rock cavern development site is located at about +200 mPD, whilst the section of Tseng Lan Shue Stream nearest to the rock cavern development site is also located at a similar vertical level.  The section of Tseng Lan Shue Stream nearest to the rock cavern development is a natural stream flowing towards the northeast (see Figure 6.1).  The stream then merges with the main Tseng Lan Shue Stream, which eventually discharging the water into Junk Bay.

6.4          Description of the Environment

6.4.1       The baseline conditions of the Tseng Lan Shue Stream have been established with reference to the EPD’s report “River Water Quality Monitoring in Hong Kong 2013” which contains the latest information published by EPD on river water quality at the moment of preparing this Report. The water quality monitoring results at stations in Tseng Lan Shue Stream, namely JR3, JR6 and JR11 are shown in Table 6.3 below. In general, the compliance rate for Tseng Lan Shue Stream improved from 79% in 1997 to 90% in 2013. The water quality at JR11 and JR6 were graded as “Excellent” and “Good” and the water quality at downstream (JR3) improved to “Fair” category due to the continued enforcement of the pollution control legislation, the implementation of Sewerage Master Plans and the extended village sewerage in the catchments.

Table 6.3               Summary of Water Quality Monitoring Data for Tseng Lan Shue Stream in 2013

Parameter

Unit

Tseng Lan Shue Stream

JR3

JR6

JR11

Dissolved Oxygen (DO)

mg/L

6.7

(4.1-7.8)

7.9

(7.3-8.4)

9.3

(8.1-10.2)

pH

-

7.4

(6.6-8.3)

7.6

(7.2-7.8)

7.8

(7.3-8.1)

Suspended Solids (SS)

mg/L

7

(2 - 29)

8

(4 - 19)

2

(1 - 4)

5-day Biochemical Oxygen Demand (BOD5)

mg/L

9

(2 - 25)

9

(3 - 29)

<1

(<1 - 25)

Chemical Oxygen Demand (COD)

mg/L

16

(6 - 28)

14

(7 - 27)

5

(3 - 36)

Oil & grease

mg/L

<0.5

(<0.5 – 1.2)

0.6

(<0.5 - 1.2)

<0.5

(<0.5 – 0.8)

Faecal coliforms

cfu/

100ml

85,000

(27,000 - 170,000)

130,000

(35,000 - 580,000)

3,000

(510 - 13,000)

E. coli

cfu/

100ml

51,000

(11,000 - 140,000)

45,000

(11,000 - 170,000)

1,000

(240 – 7,100)

Ammonia-nitrogen

mg/L

3.45

(1.50 - 13.00)

0.35

(0.06 – 1.50)

0.09

(0.03 – 0.28)

Nitrate-nitrogen

mg/L

1.35

(0.74 – 2.10)

2.15

(1.80 – 2.80)

2.80

(1.60 – 6.80)

Total Kjeldahl nitrogen

mg/L

4.90

(1.90 - 14.00)

1.50

(0.30 – 3.30)

0.42

(0.06 – 2.00)

Ortho-phosphate

mg/L

0.57

(0.18 – 1.10)

0.57

(0.30 – 0.91)

0.34

(0.14 – 0.77)

Total phosphorus, SP

mg/L

0.66

(0.24 - 1.70)

0.77

(0.41 - 1.30)

0.38

(0.15 – 0.96)

Total sulphide

mg/L

<0.02

(<0.02 - 0.04)

<0.02

(<0.02 - <0.02)

<0.02

(<0.02 - <0.02)

Aluminium

μg/L

165

(<50 - 279)

140

(70 - 240)

71

(<50 - 200)

Cadmium

μg/L

<0.1

(<0.1- 1.0)

<0.1

(<0.1- 1.0)

<0.1

(<0.1- 1.0)

Chromium

μg/L

<1

(<1- <1)

<1

(<1 - <1)

<1

(<1 - <1)

Copper

μg/L

3

(2 - 6)

3

(2 - 7)

2

(<1 - 3)

Lead

μg/L

1

(<1 - 3)

1

(<1 - 3)

<1

(<1 - <1)

Zinc

μg/L

30

(11 - 48)

48

(20 - 72)

20

(<10 - 43)

Flow

L/s

NM

NM

104

(20 - 204)

Notes:   

(1)    NM indicates no measurement taken.

(2)    Figures in brackets are annual ranges.

6.5          Assessment Methodologies

6.5.1       The study area for the water quality impact assessment covers all areas within 500m from the Project boundary in Victoria Harbour (Phase One) Water Control Zone (WCZ) and Junk Bay WCZ designated under the Water Pollution Control Ordinance (WPCO) and other areas that may have a bearing on the environmental acceptability of the Project.

6.5.2       The Water Sensitive Receivers (WSRs) that may be affected by the Project have been identified.  Potential sources of water quality impact that may arise during the construction and operational stage of the Project were described.  This task included identifying pollutants from point discharges and non-point sources that could affect the quality of surface water bodies.  All the identified sources of potential water quality impact were then evaluated and their impact significance determined.  The need for mitigation measures to reduce any identified adverse impacts on water quality to acceptable levels was determined.

6.6          Identification of Environmental Impacts

Construction Phase

6.6.1       The proposed construction works would not alter the streams and water courses identified in the study area. Potential sources of water quality impact associated with the land-based construction of the Project have been identified and include:

Ÿ   General construction activities

Ÿ   Construction site run-off;

Ÿ   Accidental spillage and potential contamination of surface water and groundwater;

Ÿ   Sewage effluent from construction workforce;

Ÿ   Construction works in close proximity of inland water; and

Ÿ   Infiltration of groundwater.

6.6.2       Based on the findings of land contamination assessment conducted for this EIA, no groundwater contamination issue was identified within the Project works area. 

General Construction Activities

6.6.3       The land-based construction works could have the potential to cause water pollution.  Various types of construction activities may generate wastewater. These include general cleaning and polishing, wheel washing, dust suppression and utility installation. These types of wastewater would contain high concentrations of suspended solids (SS). Various construction works may also generate debris and rubbish such as packaging, construction materials and refuse.    Uncontrolled discharge of site effluents, rubbish and refuse generated from the construction works would lead to deterioration in water quality.

Construction Site Run-off

6.6.4       Construction site run-off would cause potential water quality impacts. The construction works area for the Project would be approximately 1,600 m2.  Assuming the works area is 100% active, the total peak runoff generated from the construction site would be in the order of about 300 m3 per hour under a 10-year-return-period rainstorm and design duration of 5 minutes according to the Stormwater Drainage Manual of the Drainage Services Department (DSD). Potential pollution sources of site run-off may include:

Ÿ   Run-off and erosion of exposed bare soil and earth, drainage channel, earth working area and stockpiles;

Ÿ   Release of any bentonite slurries, concrete washings and other grouting materials with construction run-off or storm water;

Ÿ   Wash water from dust suppression sprays and wheel washing facilities; and

Ÿ   Fuel, oil and lubricants from maintenance of construction vehicles and equipment.

6.6.5       During rainstorms, site run-off would wash away the soil particles on unpaved lands and areas with the topsoil exposed.  The run-off is generally characterized by high concentrations of SS.  Release of uncontrolled site run-off would increase the SS levels and turbidity in the nearby water environment.  Site run-off may also wash away contaminated soil particles and therefore cause water pollution.

6.6.6       Wind blown dust would be generated from exposed soil surfaces in the works areas.  It is possible that wind blown dust would fall directly onto the nearby water bodies when a strong wind occurs.  Dispersion of dust within the works areas may increase the SS levels in surface run-off causing a potential impact to the nearby sensitive receivers.

Accidental Spillage and Potential Contamination of Surface Water and Groundwater

6.6.7       The use of engine oil and lubricants, and their storage as waste materials has the potential to create impacts on the water quality of adjacent inland water bodies or storm drains if spillage occurs. Waste oil may infiltrate into the surface soil layer, or run-off into local water courses, increasing hydrocarbon levels. Groundwater pollution may also arise from the improper use of chemicals and petroleum products within the cavern where groundwater infiltrates into the area. Infiltration of groundwater may occur at area where there are faults and/or fissures in the rock mass. The spillage of petroleum products and chemicals shall be handled properly to avoid any potential surface water or groundwater contamination.

Sewage Effluent from Construction Workforce

6.6.8       During the construction of the Project, the workforce on site will generate sewage effluents, which are characterized by high levels of BOD, ammonia and E. coli counts.     Based on the DSD Sewerage Manual, the sewage production rate for construction workers is estimated at 0.35 m3 per worker per day. For every 100 construction workers working simultaneously at the construction site, about 35 m3 of sewage would be generated per day.  Potential water quality impacts upon the local drainage and fresh water system may arise from these sewage effluents, if uncontrolled. 

Construction Works in Close Proximity of Inland Water

6.6.9       Construction activities in close vicinity to the inland water courses may pollute the inland water bodies due to the potential release of construction wastes. Construction wastes are generally characterized by high concentration of SS and elevated pH. Mitigation measures should be implemented to control the release of construction waste and site effluent into the nearby inland water bodies.

Infiltration of Groundwater

6.6.10    Construction of the rock cavern may result in infiltration of groundwater. The proposed cavern development is located on the hillside of the ARQ Site, which is currently bare rock slope surface, and therefore land contamination issue is not an issue of concern for the development. Thus, infiltration of contaminated groundwater is not anticipated.  The major issue of the potential groundwater infiltration would be the increase in site runoff (and the associated potential drawdown in any soil and aquifer layers). Suspended solids would be the key parameter of concern for the infiltrated water. Groundwater infiltration would affect the construction works and infiltrated water could carry away silt from site into the site drainage. Considerations should be taken in cavern design to minimize the infiltration of groundwater and the potential impacts from the change in groundwater level.

Operational Phase

6.6.11    Domestic and commercial effluent from the future development within the rock cavern would be the key potential sources of water pollution. With reference to the assessment of sewerage and sewage treatment implications for the Project provided in Section 7, the average sewage flow for the cavern development is estimated to be about 8.4m3 per day.  Adequate sewerage and sewage treatment facilities will be provided for the Project development to avoid direct discharge of sewage and wastewater to the nearby drainage system and inland water courses. Assessment on the sewerage and sewage treatment implications for the Project is provided in Section 7.

6.7          Evaluation of Environmental Impacts

Construction Phase

General Construction Activities

6.7.1       Effluent discharged from temporary site facilities should be controlled to prevent direct discharge to the neighbouring inland waters and storm drains.  Such effluent may include wastewater resulting from wheel washing of site vehicles at site entrances.  Debris and rubbish such as packaging, construction materials and refuse generated from the construction activities should also be properly managed and controlled to avoid accidental release to the local storm system and inland waters.  Adoption of the guidelines and good site practices for handling and disposal of construction discharges as specified in Section 6.9 would minimize the potential impacts.

Construction Site Run-off

6.7.2       Construction site run-off and drainage may cause local water quality impacts.  Increase in SS arising from the construction site could block the drainage channels.  High concentrations of suspended degradable organic material in marine water could lead to reduction in DO levels in the water column.

6.7.3       It is important that proper site practice and good site management (as specified in the ProPECC PN 1/94 “Construction Site Drainage”) be followed to prevent run-off with high level of SS from entering the surrounding waters.  With the implementation of appropriate measures to control run-off and drainage from the construction site, disturbance of water bodies would be avoided and deterioration in water quality would be minimal.  Thus, unacceptable impacts on the water quality are not expected, provided that the relevant mitigation measures as specified in the ProPECC PN 1/94 “Construction Site Drainage” are properly implemented.

Accidental Spillage and Potential Contamination of Surface Water and Groundwater

6.7.4       A large variety of chemicals may be used during construction activities.  These chemicals may include petroleum products, surplus adhesives, spent lubrication oil, grease and mineral oil, spent acid and alkaline solutions/solvent and other chemicals.  Accidental spillage of chemicals in the works areas may contaminate the surface water or groundwater nearby. The potential impacts could however be mitigated by practical mitigation measures and good site practices (as given in Section 6.9).

Sewage Effluent from Construction Workforce

6.7.5       Domestic sewage would be generated from the workforce during the construction phase.  However, this temporary sewage can be adequately treated by interim sewage treatment facilities, such as portable chemical toilets. Provided that sewage is not discharged directly into storm drains or inland waters adjacent to the construction site, and temporary sanitary facilities are used and properly maintained, it is unlikely that sewage generated from the site would have a significant water quality impact. 

Construction Works in Close Proximity of Inland Water

6.7.6       Construction activities in close vicinity to the inland water courses may pollute the inland water bodies due to the potential release of construction wastes. Construction wastes are generally characterized by high concentration of SS and elevated pH. The implementation of measures to control runoff and drainage will be important for the construction works adjacent to the inland water in order to prevent runoff and drainage water with high levels of SS from entering the water environment. With the implementation of adequate construction site drainage as specified in the ProPECC PN 1/94 “Construction Site Drainage” and the provision of mitigation measures as described in the ETWB TC (Works) No. 5/2005 “Protection of natural streams/rivers from adverse impacts arising from construction works”, it is anticipated that unacceptable water quality impacts would not arise.

Infiltration of Groundwater

6.7.7       Construction of rock cavern and tunnel may result in infiltration of groundwater. The major concern from these construction activities would be the increase in site runoff (and the associated potential drawdown in any soil and aquifer layers). Practical groundwater control measures are given in Section 6.9 to minimize the potential impacts.

Operational Phase

6.7.8       The identified potential source of impact on water quality during the operational phase of the Project would be the domestic and commercial effluent to be generated from the future development inside the rock cavern. However, with provision of adequate sewerage and sewage treatment facilities for the Project development, adverse impact associated with these operational phase discharges is not anticipated.

6.7.9       As the future development will be fully enclosed inside the rock cavern, this Project will not generate additional storm pollution.  Hence, no adverse impact of non-point source pollution would be induced by this Project.

6.8          Cumulative Impacts from Concurrent Project

6.8.1       The construction of the Project would tentatively commence in 2018 for completion in 2020, which would potentially overlap with the construction period of other nearby concurrent projects as identified in Section 2.3.  However, as all the project works would be land-based and provided that proper mitigation measures will be implemented by the projects, the water quality impact generated from the projects would be localized and no adverse cumulative water quality impacts would be expected.

6.9          Mitigation of Adverse Environmental Impacts

Construction Phase

Construction Site Run-off and General Construction Activities

Boring and Drilling Water

6.9.1       Water used in ground boring and drilling for site investigation or rock / soil anchoring should as far as practicable be re-circulated after sedimentation. When there is a need for final disposal, the wastewater should be discharged into storm drains via silt removal facilities.

Wheel Washing Water

6.9.2       All vehicles and plant should be cleaned before they leave a construction site to minimize the deposition of earth, mud, debris on roads. A wheel washing bay should be provided at every site exit if practicable and wash-water should have sand and silt settled out or removed before discharging into storm drains. The section of construction road between the wheel washing bay and the public road should be paved with backfill to reduce vehicle tracking of soil and to prevent site run-off from entering public road drains.

Rubbish and Litter

6.9.3       Good site practices should be adopted to remove rubbish and litter from construction sites so as to prevent the rubbish and litter from spreading from the site area. It is recommended to clean the construction sites on a regular basis.

Construction Site Run-off

6.9.4       The site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” should be followed as far as practicable to minimise surface run-off and the chance of erosion. The following measures are recommended to protect water quality and sensitive uses of the coastal area, and when properly implemented should be sufficient to adequately control site discharges so as to avoid water quality impact:

6.9.5       Surface run-off from construction sites should be discharged into storm drains via adequately designed sand/silt removal facilities such as sand traps, silt traps and sedimentation basins. Channels or earth bunds or sand bag barriers should be provided on site to properly direct stormwater to such silt removal facilities. Perimeter channels at site boundaries should be provided on site boundaries where necessary to intercept storm run-off from outside the site so that it will not wash across the site. Catchpits and perimeter channels should be constructed in advance of site formation works and earthworks.

6.9.6       Silt removal facilities, channels and manholes should be maintained and the deposited silt and grit should be removed regularly, at the onset of and after each rainstorm to prevent local flooding. Any practical options for the diversion and re-alignment of drainage should comply with both engineering and environmental requirements in order to provide adequate hydraulic capacity of all drains. Minimum distance of 100m should be maintained between the discharge points of construction site run-off and the existing saltwater intakes. No effluent will be discharged into typhoon shelter.

6.9.7       Construction works should be programmed to minimize soil excavation works in rainy seasons (April to September). If excavation in soil cannot be avoided in these months or at any time of year when rainstorms are likely, for the purpose of preventing soil erosion, temporary exposed slope surfaces should be covered e.g. by tarpaulin, and temporary access roads should be protected by crushed stone or gravel, as excavation proceeds. Intercepting channels should be provided (e.g. along the crest / edge of excavation) to prevent storm runoff from washing across exposed soil surfaces. Arrangements should always be in place in such a way that adequate surface protection measures can be safely carried out well before the arrival of a rainstorm.

6.9.8       Earthworks final surfaces should be well compacted and the subsequent permanent work or surface protection should be carried out immediately after the final surfaces are formed to prevent erosion caused by rainstorms. Appropriate drainage like intercepting channels should be provided where necessary.

6.9.9       Measures should be taken to minimize the ingress of rainwater into trenches. If excavation of trenches in wet seasons is necessary, they should be dug and backfilled in short sections. Rainwater pumped out from trenches or foundation excavations should be discharged into storm drains via silt removal facilities.

6.9.10    Construction materials (e.g. aggregates, sand and fill material) on sites should be covered with tarpaulin or similar fabric during rainstorms.

6.9.11    Manholes (including newly constructed ones) should always be adequately covered and temporarily sealed so as to prevent silt, construction materials or debris from getting into the drainage system, and to prevent storm run-off from getting into foul sewers. Discharge of surface run-off into foul sewers must always be prevented in order not to unduly overload the foul sewerage system.

6.9.12    Good site practices should be adopted to remove rubbish and litter from construction sites so as to prevent the rubbish and litter from spreading from the site area. It is recommended to clean the construction sites on a regular basis.

Site Effluent

6.9.13    There is a need to apply to EPD for a discharge licence for discharge of effluent from the construction site under the WPCO.  The discharge quality must meet the requirements specified in the discharge licence.  All the runoff and wastewater generated from the works areas should be treated so that it satisfies all the standards listed in the TM-DSS.  The beneficial uses of the treated effluent for other on-site activities such as dust suppression, wheel washing and general cleaning etc., can minimise water consumption and reduce the effluent discharge volume.  If monitoring of the treated effluent quality from the works areas is required during the construction phase of the Project, the monitoring should be carried out in accordance with the relevant WPCO licence which is under the ambit of regional office (RO) of EPD.  

Accidental Spillage and Potential Contamination of Surface Water and Groundwater

6.9.14    Contractor must register as a chemical waste producer if chemical wastes would be produced from the construction activities.  The Waste Disposal Ordinance (Cap 354) and its subsidiary regulations in particular the Waste Disposal (Chemical Waste) (General) Regulation, should be observed and complied with for control of chemical wastes.

6.9.15    Any service shop and maintenance facilities should be located on hard standings within a bunded area, and sumps and oil interceptors should be provided.  Maintenance of vehicles and equipment involving activities with potential for leakage and spillage should only be undertaken within the areas appropriately equipped to control these discharges.

6.9.16    Disposal of chemical wastes should be carried out in compliance with the Waste Disposal Ordinance.  The Code of Practice on the Packaging, Labelling and Storage of Chemical Wastes published under the Waste Disposal Ordinance details the requirements to deal with chemical wastes.  General requirements are given as follows:

Ÿ   Suitable containers should be used to hold the chemical wastes to avoid leakage or spillage during storage, handling and transport;

Ÿ   Chemical waste containers should be suitably labelled, to notify and warn the personnel who are handling the wastes, to avoid accidents; and

Ÿ   Storage area should be selected at a safe location on site and adequate space should be allocated to the storage area.

Sewage Effluent from Construction Workforce

6.9.17    The construction workforce on site will generate sewage.  It is recommended to provide sufficient chemical toilets in the works areas.  A licensed waste collector should be deployed to clean the chemical toilets on a regular basis.

6.9.18    Notices should be posted at conspicuous locations to remind the workers not to discharge any sewage or wastewater into the surrounding environment.  Regular environmental audit of the construction site will provide an effective control of any malpractices and can encourage continual improvement of environmental performance on site.  It is anticipated that sewage generation during the construction phase of the project would not cause water pollution problem after undertaking all required measures.

 

Construction Works in Close Proximity of  Inland Waters

6.9.19    The practices outlined in ETWB TC (Works) No. 5/2005 “Protection of natural streams/rivers from adverse impacts arising from construction works” should also be adopted where applicable to minimize the water quality impacts upon any natural streams or surface water systems.  Relevant mitigation measures from the ETWB TC (Works) No. 5/2005 are listed below:

Ÿ   Construction works close to the inland waters should be carried out in dry season as far as practicable where the flow in the surface channel or stream is low.

Ÿ   The use of less or smaller construction plants may be specified in areas close to the water courses to reduce the disturbance to the surface water.

Ÿ   Temporary storage of materials (e.g. equipment, chemicals and fuel) and temporary stockpile of construction materials should be located well away from any water courses during carrying out of the construction works.

Ÿ   Stockpiling of construction materials and dusty materials should be covered and located away from any water courses.

Ÿ   Construction debris and spoil should be covered up and/or disposed of as soon as possible to avoid being washed into the nearby water receivers.

Ÿ   Proper shoring may need to be erected in order to prevent soil or mud from slipping into the watercourses.

Infiltration of Groundwater

6.9.20    Appropriate measures during the cavern construction should be implemented to minimize the groundwater infiltration. The water control strategies include:

Ÿ   Probing Ahead: As normal practice, the Contractor will undertake rigorous probing of the ground ahead of excavation works to identify zones of significant water inflow. The probe drilling results will be evaluated to determine specific grouting requirements in line with the tunnel / cavern advance. In such zones of significant water inflow that could occur as a result of discrete, permeable features, the intent would be to reduce overall inflow by means of cut-off grouting executed ahead of the tunnel / cavern advance.

Ÿ   Pre-grouting: Where water inflow quantities are excessive, pre-grouting will be required to reduce the water inflow into the tunnel / cavern. The pre-grouting will be achieved via a systematic and carefully specified protocol of grouting.

Ÿ   In principle, the grout pre-treatment would be designed on the basis of probe hole drilling ahead of the tunnel / cavern face.

Ÿ   The installation of waterproof lining would also be adopted after the formation of the tunnels and caverns.

6.9.21  In the event of excessive infiltration being observed as a result of the tunnelling or excavation works even after incorporation of the water control strategies, post-grouting should be applied as far as practicable as described below:

Ÿ   Post-grouting: Groundwater drawdown will be most likely due to inflows of water into the tunnel / cavern that have not been sufficiently controlled by the pre-grouting measures. Where this occurs post grouting will be undertaken before the lining is cast. Whilst unlikely to be required in significant measure, such a contingency should be allowed for reduction in permeability of the tunnel / cavern surround (by grouting) to limit inflow to acceptable levels.

6.9.22  The practical groundwater control measures stated above are proven technologies and have been extensively applied in other past projects. These measures or other similar methods, as approved by the Engineer to suit the works condition shall be applied to minimize the groundwater infiltration.

6.9.23  In case seepage of groundwater occurs, groundwater should be pumped out from works areas and discharged to the storm system via silt trap.  Uncontaminated groundwater from dewatering process should also be discharged to the storm system via silt removal facilities. 

Operational Phase

6.9.24    All the sewage and wastewater generated from the future development should be properly collected and diverted to public sewers for proper treatment and disposal. Discharge of any commercial effluent to the public sewers will be subject to control under the WPCO and the relevant dischargers shall apply to EPD for a discharge licence for discharge of commercial effluent and the discharge quality must satisfy all the standards listed in the TM-DSS and meet the requirements specified in the discharge licence.  Assessment of the sewage and sewerage implications for this Project is presented in Section 7.

6.9.25    The practices outlined in ProPECC PN 5/93 for handling, treatment and disposal of operational stage effluent should also be adopted where applicable.

6.10        Evaluation of Residual Environmental Impacts

6.10.1    With the full implementation of the recommended mitigation measures for the construction and operational phases of the proposed Project, no residual impact on water quality are anticipated.

6.11        EM&A Requirements

6.11.1    Water quality monitoring is recommended to be carried out at the Tseng Lan Shue Stream during the site clearance and slope excavation works. Details of the recommended water quality monitoring requirements are provided in the stand-alone EM&A Manual for the Project. It is recommended that regular site inspections during the construction phase should be undertaken to inspect the construction activities and works areas in order to ensure the recommended mitigation measures are properly implemented.

6.12        Conclusions

Construction Phase

6.12.1    The key issue from the land-based construction activities would be the potential for release of wastewater from surface works areas, open cut excavation and groundwater infiltration during the formation of rock cavern. Minimisation of water quality deterioration could be achieved through implementing adequate mitigation measures. Regular site inspections should be undertaken routinely to inspect the construction activities and works areas in order to ensure the recommended mitigation measures are properly implemented.

Operational Phase

6.12.2    The key source of potential impact on water quality during the operational phase will be the sewage and wastewater generated from the rock cavern development. However, no adverse water quality impact associated with the operational phase would be anticipated, provided that adequate sewerage and sewage treatment facilities are properly implemented to accommodate all the sewage effluents.