5.                WATER QUALITY

5.1             Introduction

5.1.1       This section presents the assessment of potential water quality impacts that could result from the construction and operation phases of the CWB & IECL.  All construction works for the Project will be land-based and, thus, direct impacts on marine water quality will not arise.

5.2             Environmental Legislation, Policies, Plans, Standards and Criteria

5.2.1       The Water Pollution Control Ordinance (Cap. 358) provides the 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 WCZ based on their beneficial uses.  For this study, the marine waters of Victoria Harbour will be the receiving waters for discharges from the CWB & IECL site.  CWB & IECL site is located in the Victoria Harbour (Phase Three) WCZ.  The corresponding WQOs are listed in Table 5.1.

Table 5.1        Summary of Water Quality Objectives for the Victoria Harbour WCZ




Offensive Odour,  Tints

Not to be present

Whole zone


Not to exceed 50 Hazen units, due to human activity

Inland waters

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 of between 7 and 21 days

Inland waters

Dissolved Oxygen (DO) within 2 m of the seabed

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

Marine waters

Depth-averaged DO

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

Marine waters

Dissolved Oxygen

Not less than 4.0 mg L-1

Inland waters


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


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

Whole zone


Change due to human activity not to exceed 2 oC

Whole zone

Suspended solids


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

Marine waters


Annual median not to exceed 25 mg L-1 due to human activity

Inland waters


Annual mean not to exceed 0.021 mg L-1 as unionised form

Whole zone


Shall not cause excessive algal growth

Marine waters


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

Marine waters


Not to exceed 5 mg L-1

Inland waters

Chemical Oxygen Demand

Not to exceed  30 mg L-1

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).

5.2.2       Besides setting the WQOs, the WPCO controls effluent discharging into the WCZ through a licensing system.  A Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM) was issued under the WPCO which gives guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, storm water drains, inland and coastal waters).  The limits control the physical, chemical and microbial quality of effluents.  Wastewater from the proposed construction activities should comply with the TM standards for effluents discharged into the inshore waters of Victoria Harbour WCZ (Table 9a of the TM), or for effluents discharged into foul sewers leading into Government sewage treatment plants (Table 1 of the TM).

5.2.3       In addition to the WQOs set under the WPCO, the Water Supplies Department has specified a set of seawater quality criteria for flushing water (Table 5.2). The relevant criterion for suspended solids (SS) is less than 10 mg L-1.

Table 5.2        WSD Standards at Seawater Intakes

Parameter (in mg L-1 unless otherwise stated)

WSD Target Limit

Colour (HU)

< 20

Turbidity (NTU)

< 10

Threshold Odour Number (odour unit)

< 100

Ammoniacal Nitrogen

< 1

Suspended Solids

< 10

Dissolved Oxygen

> 2

Biochemical Oxygen Demand

< 10

Synthetic Detergents

< 5

E. coli (number per 100 mL)

< 20,000

5.2.4       A practice note (PN) for professional persons was issued by the EPD to provide guidelines for handling and disposal of construction site discharges. The ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with ten types of discharge from a construction site.  These include surface runoff, groundwater, boring and drilling water, bentonite slurry, water for testing and sterilisation of water retaining structures and water pipes, wastewater from building constructions, acid cleaning, etching and pickling wastewater, and wastewater from site facilities. Practices given in the PN should be followed as far as possible during construction to minimise the water quality impact due to construction site drainage.

5.2.5       Annexes 6 and 14 of the EIAO-TM provide criteria and general guidelines to be used in assessing water quality issues.

5.3             Description of the Environment

Existing Water Quality in the Victoria Harbour

5.3.1       The marine water quality in Victoria Harbour is routinely monitored by the EPD.  The monitoring stations of most relevance to the study area are VM4, VM5 and VM6 located in the central harbour area.  A summary of the monitoring data for these three stations in 1998 is provided in Table 5.3 ([1]).

5.3.2       The water quality monitoring data for 1998 indicates that the dissolved oxygen content in the middle of Victoria Harbour was generally lower than other parts of the harbour.  It was reported that the water quality at stations VM5 and VM6 was subject to the direct impact of sewage discharges from sewage outfalls.  The depth-averaged dissolved oxygen (DO) at stations VM4, VM5 and VM6 in 1998 showed non-compliance with the WQO, whereas the bottom DO showed compliance with the WQO for all three stations.  A reduction in the depth-averaged DO level is observed at Station VM5, in contrast to the results for 1997 that comply with the WQOs. 

5.3.3       The high levels of Total Kjeldahl Nitrogen and Total Nitrogen that were observed in 1997 at stations VM4, VM5 and VM6 also prevailed in 1998.  The level of E. coli remained largely similar to that in 1997, except VM6 where this level was found to be reduced by about 50%.  As for 1997, the levels of Total Inorganic Nitrogen (TIN) at stations VM4, VM5 and VM6 showed non-compliance with the WQO during 1998.  A slight increase in the annual mean of TIN was observed in 1998.  All three stations complied with the WQO for unionised ammonia in 1998 despite a slight elevation of unionised ammonia.

Table 5.3     Summary Statistics of EPD Marine Water Quality data in Victoria Harbour WCZ for 1998



Victoria Harbour (Central)






Number of samples





Temperature (°C)



(17.2 - 32.4)


(17.1 – 27.8)


(17.1 – 27.6)

Salinity (ppt)



(28.1 - 33.5)


(27.3 – 33.2)


(26.8 – 33.1)

Dissolved Oxygen (mg L-1)



(3.1 - 6.5)


(3.0 – 6.5)


(2.9 – 6.8)



(2.7 – 6.3)


(2.8 – 6.4)


(2.0 – 6.0)

Dissolved Oxygen

(% Saturation)



(49 – 93)


(45 – 97)


(46 – 99)



(43 – 80)


(41 – 80)


(29 – 75)




(7.4 – 8.2)


(6.6 – 8.1)


(7.2 – 8.2)

Secchi Disc Depth (m)



(1.8 – 3.5)


(1.7 – 3.0)


(1.0 – 3.0)

Turbidity (NTU)



(2.9 – 6.7)


(3.1- 5.9)


(2.8 – 6.1)

Suspended Solids (mg L-1)



(2.6 – 10.5)


(2.2 – 8.0)


(1.9 – 7.6)

5-day Biochemical Oxygen Demand (mg L-1)



(0.7 – 1.4)


(0.5 – 2.3)


(0.5 – 1.8)

Ammoniacal Nitrogen (mg L-1)



(0.12 – 0.49)


(0.06 – 0.50)


(0.12 – 0.45)

Unionised Ammonia (mg L-1)



(0.003 – 0.022)


(0.003 – 0.020)


(0.004 – 0.019)

Nitrite Nitrogen (mg L-1)



(0.01 – 0.04)


(0.01 – 0.04)


(0.01 – 0.04)

Nitrate Nitrogen (mg L-1)



(0.04 – 0.20)


(0.04 – 0.25)


(0.04 – 0.25)

Total Inorganic Nitrogen (mg L-1)



(0.23 – 0.72)


(0.21 – 0.72)


(0.24 – 0.67)

Total Kjeldahl Nitrogen (mg L-1)



(0.84 – 1.43)


(0.76 – 1.47)


(0.71 – 1.40)

Total Nitrogen (mg L-1)



(0.96 – 1.54)


(0.98 – 1.58)


(0.93 – 1.52)

Ortho-phosphate Phosphorus (mg L-1)



(0.02 – 0.08)


(0.01 – 0.08)


(0.02 – 0.07)

Total Phosphorus (mg L-1)



(0.07 – 0.11)


(0.06 – 0.12)


(0.07 – 0.11)

Silica (as SiO2) (mg L-1)



(0.4 – 1.5)


(0.5 - 1.6)


(0.6 – 1.7)

Chlorophyll-a (mg L-1)



(0.9 – 19.1)


(0.6 – 21.7)


(0.6 – 24.0)

Phaeo-pigment (mg L-1)



(0.2 – 7.1)


(0.2 – 5.6)


(0.2 – 7.5)

E. coli. (cfu per 100 mL)



(910 – 82000)


(570 – 33000)


(1400 – 12000)

Faecal Coliforms (cfu per 100 mL)



(3300 – 120000)


(2000 - 74000)


(3700 – 30000)


1.         Data presented are depth-averaged data, unless specified otherwise.

2.         Data presented are annual arithmetic means except for E. coli. and faecal coliform data which are geometric means.

3.         Data enclosed in brackets indicate the ranges.

Future Water Quality in the Victoria Harbour

5.3.4       The level of pollutant loading to the Victoria Harbour is expected to decrease in the future with the progressive implementation of the various government pollution abatement measures including the Stage I of the Strategic Sewage Disposal Scheme (SSDS) and the Central, Western and Wanchai West Sewerage Scheme.  In addition, with the increased enforcement exerted over industrial effluents to comply with the TM, the pollutant loading is also expected to be reduced in the future.

5.4             Water Sensitive Receivers

5.4.1       In order to evaluate water quality impacts resulting from the CWB & IECL, the proximity of Water Sensitive Receivers (WSRs) to the CWB & IECL site must be considered.  These have been identified below in accordance with Annex 14 of the EIAO-TM:

·        The Victoria Harbour, which will be the receiving water body during the construction and operation of the works.

·        A number of reprovisioned intakes along the waterfront at Central, Wan Chai and Causeway for the WSD salt water pumping stations and cooling water pumping stations.  Direct construction effluent discharge with high SS could impact upon these WSRs, causing problems such as blocked filters and wear on equipment.  Other intakes of WSD salt water pumping stations are too far to be impacted by the proposed Project.

5.4.2       There are no marine biological sensitive receivers such as mariculture zones, commercial fisheries, or shell fisheries within the vicinity of the construction site.

5.5             Assessment Methodology

5.5.1       The assessment of the potential impact of land-based construction activities on water quality has been undertaken in a qualitative manner.  Consideration has been given to controlling potentially harmful impacts from site works and to the use of ‘best’ practice measures to minimise the potential for discharges of pollutants to the nearby waters of the Victoria Harbour.  The assessment of potential water pollution impacts of this Project has involved the following tasks:

·        identifying water sensitive receivers in the vicinity of this Project;

·        defining type and extent of construction and operational activities with the potential to affect water quality;

·        identifying specific locations of those construction activities;

·        assessing the potential impacts arising from the construction activities and operation of the Project; and

·        recommending mitigation measures for minimising any identified adverse impacts on water quality associated with the Project.

5.6             Identification of Environmental Impacts

Construction Phase

5.6.1       In view of present infrastructure construction programmes within the vicinity of the proposed CWB & IECL, all required reclamation will have been completed prior to the commencement of the CWB & IECL, and no reclamation-derived water quality impacts will occur.  In addition, the boxed-tunnel portions of the CWB route will have been constructed as part of the CRIII and WDII reclamation contracts.  Potential sources of water quality impacts arising from the construction of CWB & IECL will therefore be similar to those of other general land-based construction activities and include :

·        construction run-off and drainage;

·        sewage effluent produced by the on-site workforce;

·        general construction activities;

·        diversion of existing watercourses or drainage; and

·        groundwater discharge during sub-surface excavation.

5.6.2       It is expected that the excavation activities during tunnelling will not remove the marine sediment that is left in situ during WDII reclamation.  However, about 730 m3 of marine sediment will be excavated during the construction of the pier foundations for the elevated section of IECL at the Causeway Bay Typhoon Shelter.  Section 6.5.17 provides further discussion on the quality of the excavated marine sediment, and Sections 6.6.11 and 6.7.9 present the appropriate handling and disposal methods of the excavated marine sediment.

Operational Phase

5.6.3       It is considered that impacts resulting from the operation of CWB & IECL, in terms of water quality, will be minimal and similar for both the elevated and tunnel sections of the route.  Surface runoff from slip-roads and elevated structures may be contaminated by oils leaked from passing vehicles, and tunnel seepage would potentially be contaminated to the same extent.  It is considered that impacts upon water quality will be minimal provided that the tunnel and elevated sections of the CWB & IECL are designed with adequate drainage systems and appropriate oil interceptors, as required. 

5.7             Prediction and Evaluation of Environmental Impacts

Construction Phase

Construction Run-off and Drainage

5.7.1       Run-off and drainage from construction sites may contain increased loads of sediments, other SS and contaminants.  Potential sources of pollution from site drainage include:

·        run-off and erosion from site surfaces, drainage channels, earth working and excavated spoil stockpiles;

·        bentonite slurries and other grouting materials;

·        concrete batching plant washout and drainage from dust suppression sprays; and

·        fuel, oil and lubricants from construction vehicles and equipment.

5.7.2       Of concern for the CWB & IECL are the potential physical impacts upon the flushing and cooling water intakes along the waterfront of Central, Wan Chai and Causeway Bay, caused by increased SS concentrations in the Victoria Harbour as a result of any direct discharge of construction runoff.  These impacts may include operational difficulties caused by blocked filters and wear of pump components.  Potential impacts upon culverts and drainage systems in the vicinity of the works are another key concern, as material accumulation within drains and culverts can make areas highly susceptible to flooding.  Any increase in SS from the site could exacerbate the problem, leading to siltation, blocked channels, and associated flooding when heavy rainfall occurs.

General Construction Activities

5.7.3       On-going site construction activities will have the potential to cause water pollution from debris and rubbish, such as packaging and used construction materials, which reduce the aesthetic quality of the water body.  In addition, spillage of liquids such as oil, diesel and solvents are likely to affect water quality if they enter surrounding water bodies.

Sewage Effluent

5.7.4       Sewage effluents arising from the on-site construction workforce also have the potential to cause water pollution.  In general, sewerage effluents should be discharged to the public sewerage system near the works site.  If there are only a relatively small number of workers, appropriate disposal facilities in the form of chemical toilets should be adequate.  TM standards should be applied to any sewage effluent discharges.

Specific Impacts

5.7.5       The extent of water quality impact associated with the works will mainly be dependent upon construction methods and the distance from the sensitive receivers.  The anticipated impacts associated with potential construction activities are outlined below:

Slip-Road Construction

5.7.6       The majority of the CWB will comprise underground tunnel, the construction of which is entrusted to TDD as part of the CRIII and WDII reclamation contracts.  However, some of the CWB route will comprise slip roads linking the tunnel portion to existing infrastructure.  Impacts resulting from construction activities associated with these slip-roads will include generic impacts outlined above, in particular those resulting from runoff, debris entering the Victoria Harbour and sewage effluent disposal.  However, it is considered that these impacts could be effectively controlled through application of appropriate mitigation measures as defined in Section 5.8.  Thus, adverse water quality impact is not predicted.

Elevated Structures

5.7.7       The IECL will be in the form of elevated road.  Most of the construction activities will take place above ground.  Hence, construction runoff would be the primary concern.  Construction activities associated with these elevated structures could therefore lead to those generic water quality impacts outlined above, if runoff were allowed to enter the receiving water body of the Victoria Harbour.  Appropriate mitigation, as detailed in Section 5.8, will therefore be required to prevent water quality impacts associated with the works.


5.7.8       As described in Section 2, a small section of tunnel approximately 100 m long will be constructed at the western portal.  This tunnelling method results in an open construction site that is exposed to the elements including rain for the majority of the construction period.  This could, therefore, lead to construction runoff containing high concentrations of SS, which may result in those physical water quality impacts discussed above.  These could include sediment deposition within the site drainage channels and increased concentrations of SS in the Victoria Harbour, and associated impacts upon cooling water intakes.  However, the area of site will be comparatively small and is, therefore, considered to be readily controllable in terms of appropriate collection and drainage.  These water quality impacts could be effectively controlled through implementation of the mitigation measures defined in Section 5.8.

5.7.9       For the tunnelling construction works entrusted to TDD’s CRIII and WDII contracts, the construction method employed is expected to be diaphragm wall method top-down construction.  This method will also lead to construction runoff containing high concentrations of SS, which may result in those physical water quality impacts discussed above.  However, it is considered that these water quality impacts could be effectively controlled through the implementation of water pollution control measures, which would be addressed in the EIA studies for CRIII and WDII.

Excavated Material Storage and Handling

5.7.10    Construction of the CWB & IECL will generate volumes of excavated material requiring re-handling and disposal.  Runoff and associated water quality impacts could result from storage or stockpiling of this material, which may require mitigation.  However, it is considered that these potential impacts should be adequately reduced through mitigation, as detailed in Section 6.

5.8             Mitigation of Adverse Environmental Impacts

Construction Phase

5.8.1       As detailed above, construction of the CWB & IECL will have the potential to impact upon water sensitive receivers and water quality in the Victoria Harbour, and will therefore require appropriate mitigation.  Proper site management is essential to minimise polluted discharges and site run-off.  In addition, adequate waste disposal facilities (as discussed in Section 6) should be provided which should be used appropriately to ensure that debris and rubbish cannot gain access to the Victoria Harbour and nearby drainage channels.  Additional water quality mitigation measures required during the CWB & IECL construction are detailed below and should be incorporated into the contract specifications.

Site Runoff

5.8.2       All site construction runoff and wastewater produced from construction activities shall be controlled and treated to prevent high levels of contaminants entering surrounding waters and for compliance with the WPCO.  The site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” shall be followed as far as practicable in order to minimise surface runoff and the chance of erosion, and also to retain and reduce any SS prior to discharge.

·        Temporary ditches shall be provided to facilitate runoff discharge into the appropriate watercourses, via sedimentation traps / silt retention ponds.

·        Permanent drainage channels shall also incorporate sediment basins or traps, and baffles to enhance deposition rates.

·        Sand / silt removal facilities such as sand traps, silt traps and sediment basins shall be provided to remove sand / silt particles from run-off.  These facilities shall be regularly cleaned and maintained by the contractor.  The design of silt removal facilities shall be based on the guidelines in Appendix A1 of ProPECC PN 1/94.

·        Collection of spent bentonite / other grouts in a separate slurry collection system for either cleaning and reuse / disposal to landfill shall be implemented.

·        Maintenance and plant areas shall be bunded and constructed on a hard standing with the provision of sediment traps and petrol interceptors.

·        All drainage facilities must be adequate for the controlled release of storm flows.

·        Careful programming of the works to minimise surface excavation works during the rainy season.

·        Exposed soil areas shall be minimised to reduce the potential for increased siltation and contamination of runoff.

·        All fuel tanks and storage areas shall be contained (bunded) such that spills are not allowed to gain access to water bodies.

·        Open drainage channels and culverts adjacent to the site shall be kept safe and free from any debris and excavated materials arising from the works.

·        Open stockpiles of construction materials (for examples, aggregates, sand and fill material) of more than 50 m3 shall be covered with tarpaulin or similar fabric during rainstorms.

5.8.3       Precautions to be taken at any time of year when rainstorms are likely, actions to be taken when a rainstorm is imminent or forecast, and actions to be taken during or after rainstorms are summarised in Appendix A2 of ProPECC PN 1/94.  Any effluent discharged from the site shall be diverted away from any area of embayed water.

Debris and Litter

5.8.4       In order to maintain water quality in acceptable conditions with regard to aesthetic quality, contractors shall be required, under conditions of contract, to ensure that site management is optimised and that disposal of any solid materials, litter or wastes to marine waters does not occur.  Where necessary, entrapment boom shall be provided to avoid loss of floating refuse from the construction site.

Oils and Solvents

5.8.5       All fuel tanks and store areas shall be provided with locks and be sited on sealed areas, within bunds of a capacity equal to 110% of the storage capacity.  All maintenance and plant areas and chemical storage facilities shall be provided with cover as far as practicable to avoid ingress of rainwater.

Sewage Effluent

5.8.6       Temporary sanitary facilities, such as portable chemical toilets, shall be provided on-site by a licensed contractor.  A licensed contractor would be responsible for appropriate disposal and maintenance of these facilities.

Operational Phase

5.8.7       A surface water drainage system would be provided to collect road runoff.  The following operation stage mitigation measures are recommended to ensure road runoff would comply with the TM under the WPCO:

·        The drainage from tunnel sections shall be directed through petrol interceptors to remove oil and grease before being discharged to the nearby foul water manholes.

·        Petrol interceptors shall be regularly cleaned and maintained in good working condition.

·        Oily contents of the petrol interceptors shall be properly handled and disposed of, in compliance with the requirements of the Waste Disposal Ordinance.

·        Sewage arising from ancillary facilities of CWB & IECL (for examples, car park, control room, ventilation and administration buildings and tunnel portals) shall be connected to public sewerage system.  Sufficient capacity in public sewerage shall be made available to the proposed facilities.

·        Road drainage should also be provided with adequately designed silt trap to minimize discharge of silty runoff.

5.8.8       The design of the operational stage mitigation measures shall take into account the guidelines published in ProPECC PN 5/93 “Drainage Plans subject to Comment by the EPD.” All operational discharges from the CWB into drainage or sewerage systems are required to be licensed by EPD under the WPCO.

5.9             Evaluation of Residual Impacts

5.9.1       With the adoption and incorporation of the above recommended mitigation measures for both construction and operation phases, no unacceptable residual impacts on water quality are anticipated.

5.10         Environmental Monitoring and Audit

5.10.1    The potential water quality impacts arising from the proposed land-based construction works for the CWB & IECL have been assessed.  It is predicted that there will not be any insurmountable adverse impacts on marine water quality.  With the implementation of the recommended mitigation measures, it is anticipated that the identified water quality impacts would be kept to within acceptable levels.  Monitoring of marine water quality during the construction phase is therefore not considered necessary for this site.

5.10.2    Regular site audit shall be carried out during construction stage to ensure that no adverse impact on the marine waters and seawater intakes in the vicinity; and to scrutinize timely implementation of the recommended mitigating measures.

5.11         Conclusion

5.11.1    As no reclamation will be undertaken directly for the CWB & IECL, the primary concern with regard to water quality will be the control of runoff during construction.  This could potentially contain elevated concentrations of SS, and could impact upon the flushing and cooling water intakes located along the Victoria Harbour waterfront, identified as potential sensitive receivers.  However, the potential water quality impacts could be controlled to comply with the WPCO standards by implementing the recommended mitigation measures.  No unacceptable residual water quality impact is anticipated during construction.

5.11.2    Mitigation measures with adequate maintenance are also recommended to remove oil and grease from the road runoff during operation.  No unacceptable residual water quality is expected.


([1])   Marine Water Quality in Hong Kong in 1998.  Results for 1998 from the Marine Monitoring Programme of the Environmental Protection Department.