Table of Contents

5.               WATER QUALITY IMPACT.. 5-1

5.1            Introduction. 5-1

5.2            Environmental Legislation, Standards and Guidelines. 5-1

5.3            Description of the Environment 5-3

5.4            Identification of Water Sensitive Receivers. 5-5

5.5            Assessment Approach and Methodology. 5-5

5.6            Identification of Potential Impacts. 5-5

5.7            Prediction and Evaluation of Potential Impacts. 5-6

5.8            Cumulative Impact 5-7

5.9            Recommended Water Quality Mitigation Measures. 5-7

5.10         Evaluation of Residual Impacts. 5-10

5.11         Environmental Monitoring and Audit Requirements. 5-10

5.12         Conclusion. 5-10





Table 5-1         Summary of WQOs for Victoria Harbour (Phase One) WCZ.. 5-2

Table 5-2         WSD Standards at Flushing Water Intakes. 5-3

Table 5-3         Marine Water Quality for Victoria Harbour (Phase One) WCZ in 2012 at VM12, VM14 and VT8  5-4





Figure 5.1                   Water Control Zones and Water Sensitive Receivers




5.                       WATER QUALITY IMPACT

5.1                   Introduction

5.1.1             This section presents a water quality impact assessment for the construction and operation of the Project, assessing the potential impacts and recommending mitigation measures, where necessary, in order to minimise the identified water quality impacts to an acceptable level.

5.2                   Environmental Legislation, Standards and Guidelines


5.2.1             The EIAO-TM specifies the assessment method and criteria that are needed to be followed in the EIA study.  It also provides details of the assessment criteria and guidelines that are relevant to the water quality impact assessment, including:

·          Annex 6 Criteria for Evaluating Water Pollution; and

·         Annex 14 Guidelines for Assessment of Water Pollution.

5.2.2             Furthermore, the Water Pollution Control Ordinance (Cap. 358) (WPCO) is also applicable to the water quality impact assessment of this Project.

5.2.3             Other relevant guidelines include:

·         Water Supplies Department (WSD) Water Quality Criteria;

·         Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS); and

·         Practice Note for Professional Persons on Construction Site Drainage (ProPECC PN 1/94).

Water Pollution Control Ordinance

5.2.4             The WPCO is the principal legislation to protect and control the water quality in Hong Kong.  Under the ordinance, Hong Kong waters are classified into 10 Water Control Zones (WCZs), in which the corresponding statements of Water Quality Objectives (WQOs) are stipulated.  The Project area is adjacent to the Victoria Harbour (Phase One) WCZ as indicated in Figure 5.1 and the corresponding WQOs are summarised in Table 5-1.

Table 5-1       Summary of WQOs for Victoria Harbour (Phase One) WCZ


Water Quality Objective


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 1,000 per 100mL, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals between 7 and 21 days

Inland waters


Change due to human activity not to exceed 50 Hazen units

Inland waters

Depth-Averaged Dissolved Oxygen (DO)

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

Marine waters

DO within 2m of the Seabed

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

Marine waters


Not less than 4.0mg/L

Inland waters


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

Marine waters

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

Inland waters


Change due to human activity not to exceed 2.0°C

Whole zone



Change due to human activity not to exceed 10%

Whole zone


Suspended Solids (SS)

Not to raise the ambient level by 30% due to caused by human activity and shall not affect aquatic communities

Marine waters


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

Inland waters

Un-ionised Ammonia (UIA)

Annual mean not to exceed 0.021mg/L as un-ionised form

Whole zone



Shall not cause excessive algal growth

Marine waters

Total Inorganic Nitrogen


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

Marine waters

5-day Biochemical

Oxygen Demand (BOD5)

Not to exceed 5mg/L

Inland waters


Chemical Oxygen

Demand (COD)

Not to exceed 30mg/L

Inland waters

Toxic substances

Should not attain such levels as to produce significant toxic 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


Water Supplies Department (WSD) Water Quality Criteria

5.2.5             Besides the WQOs set under the WPCO, the WSD specifies a set of water quality objectives at flushing water intakes as shown in Table 5-2.

Table 5-2       WSD Standards at Flushing Water Intakes

Parameter (in mg/l unless otherwise stated)

WSD Target Limit

Colour (Hazen Unit)

< 20

Turbidity (NTU)

< 10

Threshold Odour Number (odour unit)

< 100

Ammoniacal Nitrogen (NH3-N)

< 1

Suspended Solids

< 10

Dissolved Oxygen

> 2

Biochemical Oxygen Demand

< 10

Synthetic Detergents

< 5

E.coli (no. per 100 ml)

< 20,000

Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS)

5.2.6             Besides setting the WQOs, the WPCO controls effluent discharge into the WCZs through a licensing system.  The TM-DSS provides guidance on permissible effluent discharges based on the type of receiving waters (foul sewers, inland / coastal / inshore / marine waters).  The limits control the physical, chemical and microbial quality of effluents.  Any sewage from the proposed construction and operation activities should comply with the standards for effluents discharged into the inshore waters of the Victoria Harbour (Phase One) WCZ.

ProPECC Notes

5.2.7             The Practice Note for Professional Persons on Construction Site Drainage (ProPECC PN 1/94) was issued by the EPD to provide environmental guidelines for handling and disposal of construction site discharges.  It provides good practice guidelines for dealing with various types of discharge from a construction site.  Practices given in the ProPECC PN 1/94 should be followed during the construction phase in order to minimise the water quality impact due to construction site drainage. 

5.3                   Description of the Environment

5.3.1             According to the “Marine Water Quality in Hong Kong” in 2012, which is the latest available information from EPD at the moment of preparing this Report, after the commissioning of the Stonecutters Island Sewage Treatment Works (SCISTW) under the Harbour Area Treatment Scheme (HATS) Stage 1 in 2002, about 75% of the sewage around Victoria Harbour now receives chemically enhanced primary treatment, resulting in a 70% reduction of the pollution load (in terms of organic pollutants) into the harbour. In the Victoria Harbour WCZ, the 2012 WQOs compliance rate was 77% as compared with 50% in 2011.  Victoria Harbour Water Control Zone had a higher overall compliance rate in which the improvement in 2012 was mainly due to high compliance rates with the Dissolved Oxygen (DO) objective. Compliance with the Total Inorganic Nitrogen (TIN) objective in 2012 remained at 30% as in 2011 and non-compliance are mainly observed at stations in the central and western parts of the Harbour. The overall annual average level of TIN in Victoria Harbour increased could be due to a higher background TIN level under the influence of Pearl River discharge, the year-to-year normal range of fluctuation of the discharge from the Tolo Harbour Effluent Export Scheme and surface run-off, as well as the gradual increase in effluent discharged from the four preliminary treatment plants located in Hong Kong Island during the period.

5.3.2             The representative EPD’s marine water quality monitoring stations within the Victoria Harbour (Phase One) WCZ are in vicinity of the Project area, including VM12 (Rambler Channel (South)), VM14 (Rambler Channel (North)) and VT8 (Rambler Channel for Typhoon Shelter).  The location of these stations is shown in Figure 5.1 and the recent monitoring data are summarised in Table 5-3.  Further to description of marine water environment in Section 5.3.1, the water quality of Victoria Harbour has been on an improving trend during the last decade after implementation of the HATS Stage 1 at the end of 2001, in particular shown by consistent decreasing patterns of ammoniacal-nitrogen and BOD5.


Table 5-3       Marine Water Quality Statistics for the Victoria Harbour (Phase One) WCZ in 2012 at VM12, VM14 and VT8


EPD’s Monitoring Station




Temperature (°C)


(15.9 - 28.3)


(16.0 – 28.5)


(17.4 – 28.6)

Salinity (ppt)


(26.0 – 32.6)


(21.2 – 32.5)


(18.2 – 32.0)

Dissolved Oxygen (mg/L)


(3.8 – 8.4)


(3.6 – 8.1)


(4.9 – 7.4)

Bottom Dissolved Oxygen (mg/L)


(3.5 – 8.4)


(3.3 – 8.1)


(4.2 – 7.3)

BOD5 (mg/L)


(<0.1 – 1.0)


(<0.1 – 1.0)


(0.3 – 1.0)

SS (mg/L)


(2.5 – 16.0)


(2.1 – 13.3)


(3.0 – 16.0)

TIN (mg/L)


(0.34 – 0.85)


(0.32 – 1.08)


(0.31 – 1.15)

NH3-N (mg/L)


(0.113 – 0.247)


(0.075 – 0.273)


(0.047 – 0.270)

Un-ionised Ammonia (mg/L)


(0.002 – 0.007)


(0.001 – 0.006)


(0.001 – 0.005)

Chlorophyll-a (΅g/L)


(0.4 – 3.5)


(0.4 – 4.2)


(0.8 – 9.0)

E.coli (count/100mL)


(75 – 2,100)


(51 – 5,200)


(430 – 890)


   [1] Data presented are depth averaged (except as specified) and annual arithmetic means except for   E. coli (geometric mean).

   [2] Data in brackets indicate ranges.

   [3] Underlined indicates occurrence of non-compliance with that parameter of WQOs.

5.3.3             To further improve the water quality of Victoria Harbour, the construction of HATS Stage 2A has been proceeded with a view to commissioning by the end of 2014.  Upon commissioning, the sewage tunnels will collect the remaining 25% of the sewage currently generated daily from Hong Kong Island region, and convey the sewage to the SCISTW for treatment.  Therefore, it is anticipated that the marine water environment would be further improved.

5.4                   Identification of Water Sensitive Receivers

5.4.1             To evaluate the potential water quality impacts from the Project, areas within 300m from the proposed Project site, and the adjacent water sensitive receivers (WSRs) within the Victoria Harbour (Phase One) WCZ are considered.  No inland watercourse such as river or natural stream located within 300m from the Project site.  The WSRs within the Victoria Harbour (Phase One) WCZ are identified as below and shown in Figure 5.1:-

·      F1 – Kwai Chung Hospital Flushing Water Intake

·      C1 – Kwai Chung Hospital Cooling Water Intake

·      C2 – Tsuen Wan Cooling Water Intake

·      TS1 – Rambler Channel Typhoon Shelter

5.5                   Assessment Approach and Methodology

5.5.1             The criteria and guidelines for assessing water quality impacts as stated in Annexes 6 and 14 of the EIAO-TM have been followed.

5.5.2             The specific construction methods and operational activities of the Project have been reviewed and the potential pollution sources including pollutants from point discharges and non-point sources to surface water run-off, sewage from workforce and polluted discharge generated from the Project have been identified.

5.5.3             The identified pollution sources have been evaluated to determine the significance of impact to the adjacent water system and their representative sensitive receivers.

5.5.4             The potential cumulative impacts due to other related concurrent and planned projects activities or pollution sources within the assessment area have been assessed and mitigation measures proposed where required to control any water quality impacts to acceptable levels.

5.6                   Identification of Potential Impacts

Construction Phase

5.6.1             The Project would involve construction of flyover as described in Section 2. Since no marine construction works are required, potential water pollution sources during the construction phase would be those generated from the land-based works activities including foundation piling, footings, columns, bridge decks, utilities work, general cleaning, wheel washing, dust suppression, etc. 

5.6.2             Major potential sources of water quality impact associated with the land-based construction of the Project have been identified and described as follow:

·      Construction site run-off during foundation piling;

·      Accidental chemical spillage; and

·      Sewage effluent produced by on-site workforce.

Construction Site Run-off

5.6.3             Construction site run-off may contain increased loads of sediments, other suspended solids and contaminants.  Release of uncontrolled site run-off would increase the suspended solid levels and turbidity in the nearby water environment.

5.6.4             Potential pollution sources of site run-off comprise:

·      Run-off and erosion from bare soil and earth, drainage channels, earth working areas and stockpiles;

·      Wastewater from dust suppression sprays and wheel washing facilities at site entrances;

·      Fuel, oil, solvents and lubricants from maintenance of construction machinery and equipment.

5.6.5             With a construction site area of approximately 23,500m2 and return period of 50 years, the estimated peak hour flow would be 0.59m3/s for rainfall duration of 3 minutes. Effluents including any site runoff into the drainage channels, are controlled to comply with the WPCO.

Accidental Chemical Spillage

5.6.6             The on-site general construction activities may cause contamination of the surface soils due to accidental spillage of chemicals used in construction works, which includes spent lubrication oil, diesel and solvents, etc.  The contaminated soil particles may be washed away by construction site run-off or stormwater drainage which in turn causes water pollution.

Sewage Effluent

5.6.7             Potential impacts may arise from wastewater generated from eating areas, temporary sanitary facilities and waste disposal areas provided for the on-site construction workforce.  The characteristics of the wastewater may include high levels of organics (ie. BOD5), ammonia and E. coli. Assuming 150 workers on site with a daily sewage generation of 0.06 m3 per capita in accordance with Sewerage Manual by Drainage Services Department (DSD), the estimated sewage would be 9m3/day.

Operation Phase

5.6.8             The major source of potential impact would be surface runoff containing grit, oil and debris arising from road vehicles.  With additional road area of approximately 4,200m2 and return period of 50 years, the estimated peak surface runoff during storm would be 0.33m3/s for rainfall duration of 3 minutes.

5.7                   Prediction and Evaluation of Potential Impacts

Construction Phase

5.7.1             Potential sources of water quality impacts associated with the construction phase of the Project include:

·      Construction site run-off;

·      General construction activities and accidental spillage of chemicals; and

·      Sewage generated from on-site construction workers.

Construction Site Run-off

5.7.2             Site run-off and drainage from the Site may contain suspended solids and other contaminants.  Potential sources of water pollution from site run-off would comprise:

·      Run-off from exposed bare soil and earth, drainage channels and stockpiles;

·      Release of grouting and cement materials with rain wash;

·      Wash water from dust suppression sprays and vehicle wheel washing; and

·      Fuel, oil and lubricant from maintenance of construction vehicles and mechanical equipment.

5.7.3             Perimeter drains could be applied along the Site boundary to collect site run-off and also intercept run-off from outside.

5.7.4             Mitigation measures of good site practice shall be implemented to control construction site run-off, and to minimise the chance of introducing silt and other pollutants into the storm water drainage system. Silt removal facilities shall be provided and discharge license under WPCO shall be obtained prior to any discharge.

5.7.5             Since all construction works would be land-based in nature, with the implementation of adequate site drainage and provision of silt removal facilities, as well as control under WPCO, no unacceptable water quality impacts to WSRs are anticipated.

General Construction Activities and Accidental Spillage of Chemicals

5.7.6             General construction activities have the potential to cause water pollution as a result of stockpiling, accumulation of debris and rubbish, concrete dust, etc.  Spillage of chemicals, such as oil and diesel from construction plant and equipment and paints, could also result in water quality impacts.

5.7.7             Nevertheless, it is considered that the impact of these activities to WSRs will be minimal provided that the site boundaries are well maintained with the aforesaid perimeter drains.  Good construction and site management practices, such as sediment barriers, site drainage and waste disposal, will also limit the sediment and pollutants to acceptable levels.

Sewage Generated from On-Site Construction Workers

5.7.8             Sewage is characterised by high levels of BOD, ammonia and E.coli.  Water quality impacts from sewage generated by the on-site workforce will be avoided if adequate sewage collection and disposal facilities, such as portable chemical toilets, are properly installed and maintained.

Operation Phase

5.7.9             The road surface runoff will be discharged to decked nullah leading to Rambler Channel via public storm drain. As incorporated in the design, the runoff will be collected by surface water drainage system and discharged to storm drains. The surface runoff can also be controlled by best management practice, etc (e.g. properly designed silt traps with appropriate spacing, sufficient cleaning frequency for silt traps and road gullies, etc). With adequate capacity to cater for treating all surface water and best management practice, it is envisaged that the water quality impact from the land-based operation will be minimal and further mitigation measure would be not be required.

5.8                   Cumulative Impact

5.8.1             There are no major concurrent projects nearby and therefore it is envisaged that the cumulative impact is minimal.

5.9                   Recommended Water Quality Mitigation Measures


5.9.1             General good site practice as described in the following is to be implemented as measures to minimise potential water quality impact during construction phase.

Construction Site Run-off and General Construction Activities

5.9.2             In accordance with the ProPECC PN 1/94, construction phase mitigation measures should include the following:

·      At the establishment of works site, perimeter drains to direct off-site water around the Site should be constructed with internal drainage works and erosion and sedimentation control facilities implemented. Channels (both temporary and permanent drainage pipes and culverts), earth bunds or sand bag barriers should be provided to divert the stormwater to silt removal facilities.  The design of the temporary on-site drainage system will be undertaken by the Contractor prior to the commencement of construction; 

·      Dikes or embankments for flood protection should be implemented around the boundaries of earthwork areas.  Temporary ditches should be provided to facilitate the run-off discharge into an appropriate watercourse, through a silt / sediment trap. Silt / sediment traps should also be incorporated in the permanent drainage channels to enhance deposition rates;

·      The design of efficient silt removal facilities should be based on the guidelines in Appendix A1 of ProPECC PN 1/94, which states that the retention time for silt / sand traps should be 5 minutes under maximum flow conditions.  A sedimentation basin would be required when necessary. The detailed design of the silt / sand traps should be undertaken by the Contractor prior to the commencement of construction;

·      The construction works should be programmed to minimise surface excavation works during rainy seasons (April to September), as possible.  All exposed earth areas should be completed and vegetated as soon as possible after the earthworks have been completed.  If excavation of soil cannot be avoided during the rainy season, or at any time of year when rainstorms are likely, exposed slope surfaces should be covered by tarpaulin or other means;

·      The overall slope of works sites should be kept to a minimum to reduce the erosive potential of surface water flows, and all trafficked areas and access roads should be protected by coarse stone ballast.  An additional advantage accruing from the use of crushed stone is the positive traction gained during the prolonged periods of inclement weather and the reduction of surface sheet flows;

·      All drainage facilities and erosion and sediment control structures should be regularly inspected and maintained to ensure their proper and efficient operation at all times particularly following rainstorms.  Deposited silts and grits should be removed regularly and disposed of by spreading evenly over stable, vegetated areas;

·      Measures should be taken to minimise the ingress of site drainage into excavations.  If the excavation of trenches in wet season is inevitable, they should be dug and backfilled in short sections wherever practicable.  The water pumped out from trenches or foundation excavations should be discharged into storm drains via silt removal facilities;

·      All open stockpiles of construction materials (for example, aggregates, sand and fill material) should be covered with tarpaulin or similar fabric during rainstorms.  Measures should be taken to prevent the washing away of construction materials, soil, silt or debris into any drainage system;

·      Manholes (including newly constructed ones) should always be adequately covered and temporarily sealed so as to prevent silt, construction materials or debris being washed into the drainage system and storm run-off being directed into foul sewers;

·      Precautions to be taken at any time of the year when rainstorms are likely, actions to be taken when a rainstorm is imminent or forecasted and during or after rainstorms, are summarised in Appendix A2 of ProPECC PN 1/94.  Particular attention should be paid to the control of silty surface run-off during rainstorm events;

·      All vehicles and plant should be cleaned before leaving the Site to ensure no earth, mud, debris and the like is deposited by them on roads.  An adequately designed and sited wheel washing facilities / bay should be provided at the exit of the Site where practicable.  Wash-water should have sand and silt settled out and removed at least on a weekly basis to ensure the continued efficiency of the process.  The section of access road leading to, and exiting from, the wheel-washing bay to public roads should be paved with sufficient backfall toward the wheel-washing bay to prevent vehicle tracking of soil and silty water to public roads and drains;

·      Oil interceptors should be provided in the drainage system downstream of any oil / fuel pollution sources.  Oil interceptors should be emptied and cleaned regularly to prevent the release of oil and grease into the storm water drainage system after accidental spillage.  A bypass should be provided for oil interceptors to prevent flushing during heavy rain;

·      The construction solid waste, debris and rubbish on-site should be collected, handled and disposed of properly to avoid causing any water quality impacts.  The requirements for solid waste management are detailed in Section 6 of this EIA report; and

·      All fuel tanks and storage areas should be provided with locks and sited on sealed areas, within bunds with adequate storage capacity to prevent spilled fuel oils.

5.9.3             Site drainage will be implemented according to the requirement and findings from Drainage Impact Assessment which has been submitted to and approved by DSD.

5.9.4             By adopting the above mitigation measures with good management practices, it is anticipated that the impacts of construction site run-off will be reduced to an acceptable level.

Effluent Discharge

5.9.5             There is a need to apply to the 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 run-off and wastewater generated from the works areas should be treated so that it satisfies all the standards listed in the Technical Memorandum.  Minimum distances of 100m should be maintained between the discharge points of construction site effluent and the existing seawater intakes.  In addition, no new effluent discharges in nearby typhoon shelters should be allowed.  The beneficial uses of the treated effluent for other on-site activities such as dust suppression, wheel washing and general cleaning etc., would minimise water consumption and reduce the effluent discharge volume. 

Sewage Generated from On-Site Construction Workers

5.9.6             Portable chemical toilets and sewage holding tanks are recommended for the handling of the construction sewage generated by the workforce.  A licensed contractor should be employed to provide appropriate and adequate portable toilets and be responsible for appropriate disposal and maintenance. 

Accidental Spillage of Chemicals

5.9.7             The 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 (Cap. 354C) should be observed and complied with for control of chemical wastes. 

5.9.8             Any 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. 


5.9.9             The surface runoff will be controlled by best management practice. Surface runoff collection system with silt traps and road gullies should be provided. The road should be designed to direct all surface runoff to the system. The capacity should be properly designed to cater for all surface water. Drainage will be implemented in accordance with the requirement and findings from Drainage Impact Assessment which has been submitted to and approved by DSD.

5.9.10         The system should be properly maintained and cleaned regularly to ensure good service condition in which Highways Department will be responsible for the maintenance of road drains.

5.10               Evaluation of Residual Impacts

5.10.1         Adverse residual impacts during the construction and operation phases of this Project would not be anticipated, provided that the above mitigation measures are implemented.

5.11               Environmental Monitoring and Audit Requirements

5.11.1         With the proper implementation of recommended mitigation measures, the adverse impact during construction and operation of the Project would be insignificant and hence environmental monitoring for water quality is not considered necessary but regular site inspection should be conducted at the construction and work area in order to ensure the mitigation measures are adequately implemented.

5.12               Conclusion

5.12.1         Potential water pollution sources have been identified as construction site run-off, sewage from workforce, and potential risk of chemical spillage.  Adverse residual impacts would not be anticipated with the implementation of adequate mitigation measures including the construction site practices in accordance with the EPD’s ProPECC PN 1/94 Construction Site Drainage, provision and management of portable toilets on-site, and preventive measures to avoid accidental chemical spillages.   Furthermore, implications arising from the surface runoff during the operation phase would be insignificant. Adverse water quality impact would not be anticipated with the implementation of the recommended mitigation measures based on the findings of this EIA study.