Strategic Sewage Disposal Scheme (Harbour Area Treatment ... CE7395.pdf · disposal of sewage for all of Hong Kong. One main element of the Sewage Strategy was the Strategic Sewage

Mouchel

August 2004

Mouchel

Strategic Sewage Disposal Scheme

(Harbour Area Treatment Scheme)

Stage 1

Agreement No. CE 73/95

Baseline Monitoring &

Performance Verification

Executive Summary

Baseline Monitoring &

Performance Verification

Executive Summary

CE 73/95

Environmental Protection DepartmentThe Government of the Hong Kong

Special Administrative Region

( )

Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary

C O N T E N T S Page 1 PROJECT BACKGROUND 1 SSDS Stage I Interim Outfall, Construction and Commissioning 1 2 THE RECEIVING ENVIRONMENT AND POTENTIAL IMPACTS 2 Water Column, Seabed Sediments 2 Intertidal and Benthic Ecosystems 2 Fisheries 2 Birds and Cetacea 2 Beneficial Uses and Sensitive Receivers 3 3 STUDY OBJECTIVES 3 4 BROAD APPROACH 3 5 POLLUTION SOURCE CHARACTERISATION 4 6 FLOWS AND LOADS OF THE EFFLUENT DISCHARGE 4 7 EFFLUENT TOXICITY 5 8 MOVEMENT AND DISPERSION OF THE EFFLUENT PLUME 5 Computer Model Studies 5 Dye Tracing Studies 6 9 ENVIRONMENTAL IMPACT 6 Marine Water Quality 6 Bathing Beach Water Quality 8 Sediment Quality 8 Benthic Fauna 9 Marine Fisheries 10 Intertidal Ecology 11 10 SUMMARY AND DISCUSSION 11 Overall Impacts on the Marine Environment 11 Zone of Influence of the Interim Outfall Discharge 12 Recommendations for Future Monitoring 13 Tables 1 Interim Outfall Commissioning Sequence 1 2 Effluent Quality 4 3 Monthly Mean Daily Flows 4 4 Marine Water Quality Rested Parameters 7 5 Annual Geometric Mean E. coli Concentrations 8 6 E. coli and Ammoniacal Nitrogen Concentration at Eastern and Western Harbour 12 7 Detailed Survey Programme 14 Drawings 1 Major Effluent Load Diverted from the Eastern Victoria Harbour, Tathong Channel and Rambler

Channel to the Stage I Outfall 2 Study Area and Sensitive Receivers 3 Location of Monitoring Stations 4 Measured Minimum Dilution at Selected Locations

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5 Distribution of NH4-N (mg/L) at High Water (Depth Average)

1

Major Effluent Loads Diverted from the Eastern Victoria Harbour,Tathong Channel and Rambler Channel to the Stage I Outfall

Drawing No.

SSDSStage I OutfallSSDSStage I Outfall

Stage 1Stage 1

Stage 4Stage 4

Stage 3Stage 3

Stage 2Stage 2

Flow Diversion Dates

Stage 1: 7th November 2001 (9.4cum/s)Stage 2: 3rd December 2001 (10.2cum/s)Stage 3: 5th December 2001 (11.8cum/s)Stage 4: 12th December 2001 (14.2 cum/s)

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1 PROJECT BACKGROUND In 1989, the Environmental Protection Department (EPD) of the Hong Kong Government concluded the Sewage Strategy Study (SSS) which provided the basis for an overall strategy for the collection, treatment and disposal of sewage for all of Hong Kong. One main element of the Sewage Strategy was the Strategic Sewage Disposal Scheme (SSDS) in which a series of deep tunnel collection systems would collect and transfer sewage from the central urban area of Hong Kong and Kowloon to treatment facilities on Stonecutter’s Island before discharge through the Stage I Interim Outfall in the Western Harbour (Drawing 1). The SSDS Stage I works, once completed, were intended to provide an early improvement in water quality in Victoria Harbour and Northern Tathong Channel but possibly at the expense of water quality in the vicinity of the Interim Outfall. What was the territory wide Strategic Sewage Disposal Strategy at the commencement of the current Agreement has now been superseded by new studies under what is now known as the Harbour Area Treatment Scheme (HATS). For consistency with previous reports under the current Agreement and to avoid confusion with newer studies under HATS, however, in this summary, reference may still be made to the SSDS rather than the HATS.

In 1994, EPD commissioned a consultancy study to review options for the final sewage treatment arrangements and an International Review Panel recommended that Chemically Enhanced Primary Treatment (CEPT) should be adopted prior to finalisation of the ultimate treatment and disposal options. Based on the assumed CEPT treatment, a full Environmental Impact Assessment (EIA) of the Interim Outfall and proposed treatment level was then carried out.

Construction of the Interim Outfall was initially anticipated to be completed by the end of May 1997 and, in March 1996, Mouchel Asia was commissioned by the EPD to carry out the current Assignment, the Baseline Monitoring and Performance Verification studies.

SSDS Stage I Interim Outfall, Construction and Commissioning

The Interim Outfall consists of two separate diffuser sections, the north and south sections, which are linked with a short section of 1.5 m diameter pipe at diffuser manifold level. Each diffuser section comprises twelve risers at 52 m

centres with each riser having 8 rosette discharge ports giving a total number of 192 (2 x 12 x 8) ports. The total length of the diffuser system is 1,234 m.

Location of the Interim Outfall Following delays in the construction of the feeder tunnels, the Interim Outfall was initially commissioned in late June 1999 using treated effluent from North West Kowloon with seawater augmentation to prevent siltation within the system. The Interim Outfall was then finally commissioned with the previously planned design discharge in four stages in November and December 2001 by diverting effluent from pre-existing outfalls in Victoria Harbour, the Tathong channel and the Rambler Channel. The commissioning sequence is summarised in Table 1 below:

Table 1 Interim Outfall Commissioning Sequence

Flow Commencement

Date

Discharge (m3/s)

Percentage of 1997

Design Flows Initial : June 1999 3.7 26% Stage 1: 7th November 2001 9.4 66%

Stage 2: 3rd December 2001 10.2 72%

Stage 3: 5th December 2001 11.8 83%

Stage 4: 12th December 2001 14.2 100%

The outfalls which were decommissioned during the commissioning process are also shown in Drawing 1.

Stages 1, 2 and 3 were expected to reduce the direct pollution load to the Eastern Victoria

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Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary Harbour and Tathong Channel by up to 90%-95% which was expected to result in a measurable improvement in water quality in those areas. 2 THE RECEIVING ENVIRONMENT AND

POTENTIAL IMPACTS

It must be expected that the treated effluent discharge from the Interim Outfall will have some impact on the local receiving waters and the current Assignment was commissioned to assess the acceptability of the effluent discharge as predicted in the earlier EIA. However, the effluent being discharged has been diverted from other outfalls and, as a result of the CEPT treatment process, the total effluent load to Hong Kong’s Territorial waters has actually reduced.

Water Column, Seabed Sediments The Project Area (Drawing 1) lies within the outer estuarine reaches of the Pearl River Delta where hydraulic conditions vary seasonally with vertically well mixed conditions in the winter dry season but stratified conditions in the summer wet season when a layer of oxygen rich brackish water overlies a denser layer of oceanic water generally depleted in dissolved oxygen.

It was anticipated that the effluent plume could have impacts in several gazetted Water Control Zones (WCZ) including the Victoria Harbour, North Western, Western Buffer, Southern and Eastern Buffer WCZs. In each WCZ, gazetted Water Quality Objectives (WQO) have been defined to protect the Beneficial Uses (BUs) of each WCZ. At the time of the commencement of this study (March 1996) and prior to the commissioning of the Interim Outfall, as detailed in the EPD Annual Reports on Marine Water Quality available at that time, the Western Buffer, and North Western WCZs had a high level of compliance with their respective WQOs while Victoria Harbour failed to meet the WQOs for dissolved oxygen and Total Inorganic Nitrogen (TIN). The Southern WCZ also failed to meet the WQO for TIN although this failure was considered to be due to the high TIN concentrations in the waters to the south of Hong Kong’s Territorial boundaries.

The sea bed in the study area is mainly soft and is made up of a mixture of sand and mud, the exact composition of which varies between locations. Data from EPD’s routine sediment

quality monitoring programme showed that about 75% of the sediment sampled is in the very fine (<63µm) silt fraction which is the most chemically and biologically active fraction and, over the years, the sediments in Victoria Harbour and the Rambler Channel have been enriched by organic mater and contaminated materials derived from the discharge of untreated sewage, urban storm water and industrial effluents.

Intertidal and Benthic Ecosystems

The intertidal fauna and flora of Hong Kong as a whole are considered to be both rich and diverse and both hard and soft shores in less developed parts of the study area are regarded as rich habitats.

The naturally seasonally varying salinities and, among other anthropogenic factors, intensive trawling impose severe stresses on benthic organisms and it has been suggested that these natural and anthropogenic stresses maintain the benthic infaunal communities in an early successional state. The findings of the present study were also consistent with the recent study of marine benthic communities carried out by the City University for AFCD in 2002.

Fisheries

Intensive fishing activity occurs principally from small inshore boats using lines, purse seines or trawls. The majority of fish caught are pelagic, e.g. Sardines (Clupeidae), anchovies (Engraulidae), small jacks (Carangidae), mackerel (Scombridae) which are all commercially important species, but many benthic species are also important.

Birds and Cetacea

Many sea birds feed in the area, principally gulls, reef egrets (Egretta sacra) and red-necked phalarope (Phalaropus lobatus). Reef egrets are resident but are unlikely to feed off the Interim Outfall. Gulls, principally black-headed gulls (Larus ridibundus) over the winter, are present in large numbers where they roost and feed in the area and, like the large number of phalarope which are passage migrants, may feed off the Interim Outfall. One Cetacean species, the Indo-Pacific Humpback Dolphin (Sousa chinensis), is most commonly found in the waters north of Lantau Island and therefore might impinge on the outer edges of the study area.


Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary Beneficial Uses and Sensitive Receivers There are a number of key bathing beaches which could be impacted by the effluent plume from the Interim Outfall in the Tsuen Wan and Tuen Mun districts. Shortly before the commencement of this Assignment, during the 1994 bathing season, beach water quality in the Tsuen Wan District showed effects of sewage discharges. Anglers’ beach in the Tsuen Wan District was classified as very poor, Lido, Casam, Gemini, Hoi Mei Wan, Ting Kau and Approach beaches as poor and Tung Wan as fair. In 2001, just before the final stages of commissioning were completed, Anglers beach improved to poor.

Secondary Contact Recreation Sub-zones are those areas in which water based activities such as windsurfing, water skiing, dinghy sailing and sea canoeing take place. Within the study area, secondary contact recreation zones are located along all coastlines except those of Victoria Harbour, West Kowloon, South Tsing Yi and the Rambler Channel.

There are a number of seawater intakes in the vicinity of the Interim Outfall (Drawing 2) which abstract water for cooling systems and for treatment and distribution by WSD for toilet flushing water. Within the study area, seawater abstraction for keeping live fish has also been observed at several locations and growing marine fish in cages is common in Hong Kong and is carried out in five locations within the study area at Cheung Sha Wan; Ma Wan; Lo Tik Wan; Sok Kwu Wan; and Po Toi. 3 THE STUDY OBJECTIVES The study objectives can be summarised broadly as follows:

(a) To establish the environmental baseline prior to commissioning of the interim treatment facilities and outfall;

(b) To confirm the performance of the Interim Outfall and acceptability of the impact of the effluent discharge on the marine environment; and

(c) To assess the need for any improvements to the effluent treatment levels and the requirements for any future monitoring.

4 BROAD APPROACH

In order to achieve these objectives, bearing in mind the potential environmental impacts which

could follow the commissioning of the Interim Outfall, a number of separate tasks were carried out including: (a) Design of Baseline and Impact Monitoring

Programmes covering all environmental compartments of concern including: • Marine Water Quality (16 Monitoring

Stations); • Marine Sediment Quality (chemistry and

toxicity) (16 Stations); • Benthic community (Initially 16

monitoring Stations reduced to 4 Reference Stations and one Impact station following a programme review);

• Demersal Fishery (6 Monitoring Stations); • Intertidal Ecology (3 Monitoring Stations);

(b) Dye Tracer Studies; (c) Three-dimensional Computer Modelling; (d) Effluent Quality and Toxicity Testing.

The principal task was the design and execution of the environmental monitoring programme and, except for the intertidal ecology studies, for each environmental compartment, monitoring was undertaken at stations grouped in the near-field (impacts expected), mid-field (impacts possible) and far-field/Reference areas (detectable impacts unlikely or not expected) (Drawing 3). It was important to monitor in the far-field, remote from the Interim Outfall, in order to identify any large scale changes in environmental conditions which would not be related to the commissioning of the Stage I Outfall but which could confound the data collected. For the intertidal ecology, all monitoring stations were located within the mid-field area.The monitoring programme had several requirements and was required to:

(1) identify any spatial variations in the parameters being monitored; and

(2) identify and differentiate between temporal variations in the parameters monitored due to seasonal variations, long term large scale variations due to changing background conditions and temporal variations resulting from the commissioning of the Stage I Outfall or other short term anthropogenic activities.

It was considered that some water quality parameters and fisheries characteristics could be subject to tidal and seasonal variations while, sediment quality would be less likely to change over short periods of time. As a result, in order


2

Study Area and Sensitive ReceiversMouchel

Drawing No.

B2

North LantauSand Borrow Area

Stage 1Interim Outfall

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West of Sulphur ChannelMarine Borrow Area

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ContainerTerminal No. 9

Lammer PowerStation Extension

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COOL Cooling Water Intake

COR Coral

FLUSH Flushing Water Intake

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Sand Borrow Area

Major Reclamation Site

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Lung Kuw ChauLung Kuw Chau

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SSDS Stage 1- Baseline Monitoring and Performance Verification

Location of Monitoring Stations

NEW TERRITORIESNEW TERRITORIESNEW TERRITORIESNEW TERRITORIES

LANTAU ISLANDLANTAU ISLANDLANTAU ISLANDLANTAU ISLAND

HONG KONG ISLANDHONG KONG ISLANDHONG KONG ISLANDHONG KONG ISLAND

KOWLOONKOWLOONKOWLOONKOWLOON

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Deep Bay WCZ

North WesternWCZ

North WesternWCZ Western

BufferWCZ

WesternBufferWCZ

Victoria HarbourWCZ

VictoriaHarbour

WCZ

Junk BayWCZ

EasternBufferWCZ

Port ShelterWCZ

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3

Drawing No.

KEY

Marine Water StationsSediment Quality StationsIntertidal Ecology StationsDemersal Trawling Stations

Secondary Contact Recreation Sub-zoneWater Control Zone BoundaryBeach (gazetter & non-gazetted)Existing Outfall


Table 2 Effluent Quality to minimise tidal variations, water quality monitoring was always carried out within a tidal window of 3.5 hours spanning high and low water on large amplitude tides with similar amplitudes (around 2m) and the frequency of monitoring in each environmental compartment was selected to ensure that any expected temporal variations could be resolved while avoiding unnecessary expenditure of resources. After completion of the first annual round of Baseline Monitoring, the programme was reviewed and some modifications to sampling frequencies, parameters determined and numbers of replicate samples collected were recommended in order to maximise the value of the data being collected bearing in mind the variability in the data during the Baseline monitoring and the analysis which could be carried out. Details of the monitoring programme are summarised in Table 7 at the back of this report and discussed in more detail below.

Contaminant Units LoR SSDS1 EIA2

µg/L 10 ND 6 Arsenic Total µg/L 10 ND - µg/L 0.2 ND - Cadmium Total

Dissolved µg/L 0.2 ND - µg/L 2 10 40 Chromium Total

Dissolved µg/L 2 4 - µg/L 1 24 120 Copper Total

Dissolved µg/L 1 3 - µg/L 3 34 140 Nickel Total

Dissolved µg/L 3 29 - µg/L 2 ND - Lead Total

Dissolved µg/L 2 ND - µg/L 5 41 70 Zinc Total

Dissolved µg/L 5 16 - µg/L 0.02 0.04 Mercury Total

Dissolved µg/L 0.02 < 0.02 - Salinity Kg/m3 0.01 10.9 - Suspended Solids mg/L 2 27 72-82 Silica mg/L 0.05 8.9 - Ammonia mg/L 0.1 19.3 - NOx as N mg/L 0.1 <0.1 - TKN as N mg/L 0.1 25 - Total Phosphorus mg/L 0.05 2.4 - BOD mg/L 0.5 59 -

E. coli Cfu/ 100mL 1 1.5 x107 -

Total PCBs µg/L 0.1 ND - Naphthalene µg/L 0.1 0.3 - Acenaphthylene µg/L 0.1 0.2 - Organochlorine Pesticides µg/L 0.1 -

20 ND -

5 POLLUTION SOURCE

CHARACTERISATION

Within Hong Kong’s coastal waters, there are a large number of effluent discharges from sea outfalls and storm drains as well as diffuse pollution sources in the New Territories as a result of sewage contamination of streams and groundwater. In the most recent pollution load inventory developed for Hong Kong waters, over 300 discreet effluent loads were identified. The major component of these effluent loads is sewage and the effluent to be diverted to the Stonecutter’s Island Treatment Works was expected to comprise mainly domestic sewage.

Notes: ND Not Detected 1 Average over all 8 sampling occasions in this study 2 Assumed effluent characteristics in the EIA

A summary of the effluent quality assumed in the EIA is also given in the table above. During the impact monitoring period when the effluent samples were collected, the mean effluent flow over the 15 month period was 15.4m3/s. Table 3 below summarises the monthly mean daily flows during the impact monitoring period:

6 FLOWS AND LOADS OF THE EFFLUENT DISCHARGE Table 3 Monthly Mean Daily Flows

Month Mean Daily Flow (m3/s) January 2002 14.6 February 2002 9.9 March 2002 15.1 April 2002 15.2 May 2002 16.0 June 2002 16.1 July 2002 16.2 August 2002 16.6 September 2002 17.3 October 2002 16.1 November 2002 15.7 December 2002 15.6 January 2003 15.6 February 2003 15.1 March 2003 15.7

Following the full commissioning of the outfall in December 2001, effluent samples were collected from a seal pit downstream of the treatment works before the effluent was discharged through the outfall. Effluent samples were collected approximately bimonthly with 24 samples being collected at hourly intervals over a 24 hour period. In order to obtain a measure of the mean effluent characteristics, equal volumes of each hourly sample were combined to form one composite sample which was then subject to chemical and biological testing. Summary results from the chemical testing are presented in Table 2 below:


Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary It was found that the CEPT effluent was of a higher quality than had been assumed in the EIA. As a result, any impacts from the CEPT effluent discharge as assessed in the EIA would not have been underestimated.

The effluent was tested for a large range of PAHS, PCBs and Organochlorine Pesticides and it was found that only Naphthalene and Acenaphthylene were detected on occasion at or very close to the limits of detection of the test protocol. With respect to the interpretation of the impact of the effluent discharge on the receiving waters, sediment quality and tissue contamination levels, one important finding from the effluent quality testing was that it contained no Arsenic or Cadmium and Lead was only detected on one occasion suggesting it is not a persistent contaminant in the effluent. Any changes in the levels of these contaminants in the receiving waters, therefore, would not be related to the commissioning of the Interim Outfall. 7 EFFLUENT TOXICITY Effluent toxicity tests were carried out using bivalve larvae (Mytilus galloprovincialis), one of the most sensitive of the established international toxicity tests. A range of effluent concentrations (obtained by diluting the neat effluent with seawater) were tested according to the standard protocol ASTM E724-89 and it was found that the EC50, the concentration above which 50% of the test population showed abnormal development, was in the range 16% to 19% effluent, a dilution rate of the order of 5 to 6 times. In practice, the initial dilution determined from the dye tracer studies and from the outfall design studies was in the range 50 to 300 in the immediate vicinity of the outfall. As a result, it was concluded that the toxicity of the effluent would not be of concern in the receiving waters. 8 MOVEMENT AND DISPERSION OF THE

EFFLUENT PLUME

In order to design the monitoring programme effectively, it was important to appreciate how the effluent plume would be transported and dispersed by the tidal currents. In addition, given the contaminant concentrations in the effluent and the rate of dilution of the effluent in the receiving waters with distance from the Interim Outfall, it was also important to assess

the likely detectability of the effluent contaminants at each monitoring station. As a result, three-dimensional computer models were applied before the Interim Outfall was commissioned in order to aid in the design of the monitoring programme and to assist in the interpretation of the water quality data collected. The computer models were vital in order to achieve the study objectives.

Following the outfall commissioning, tracer studies were also carried out to provide farther data on the effluent plume characteristics and dilution rates.

Computer Model Studies Initially, at the commencement of the study, a previously calibrated large area model of tidal flows throughout Hong Kong waters was applied.

Large Area Model Grid This model was used to aid in the design of the monitoring programme by identifying the areas most likely to be impacted by the effluent plume.

Subsequently, a higher resolution three-dimensional model of tidal flows, dynamically linked to an initial dilution model, was set up and validated. This model was used to review the monitoring programme in order to ensure that optimum use was being made of resources available and to determine the likelihood that impacts from the effluent discharge would be detectable at the monitoring stations. By simulating a conservative tracer, the model was also used to assess whether the sampling times within the tidal cycle were likely to allow the maximum effluent concentrations to be detected. Typical model results are illustrated below for dry season conditions:



The principal objectives of the tracer studies were:

Predicted Tracer Concentrations at Station W05

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l Lev

el (m

PD

)

Tracer ConcentrationTracer Concentration During Measurement PeriodWater Level

(1) to determine the initial dilution of the effluent in the vicinity of the Interim Outfall in order to assess the performance of the diffuser system;

(2) to track the effluent plume into the far field and to determine the dilution rates with distance from the outfall over the area impacted by the plume; and

(3) to assist in the interpretation of the water quality data.

Typical results from the wet and dry season tracer studies in terms of dilution rates achieved at distances from the Interim Outfall are presented in Drawing 4.

Dye Tracer Studies Following the final commissioning of the Interim Outfall, Rhodamine WT fluorescent dye was injected into the effluent stream and then tracked in the receiving waters in both the wet season (on 9th and 11th July 2002) and dry season (on 21st and 23rd November 2002) in order to ensure that the expected range of seasonally changing conditions were monitored. The dye was injected on two occasions in each season such that it emerged from the diffuser system at low water on the first survey and, a day later, at high water. In this way, the dye could be tracked for complete flood and ebb tidal cycles and the maximum area over which the dye remained detectable could be determined. It was not intended that the dye should be clearly visible over a large area (the fluorometers can detect concentrations as low as 0.25ppb while concentrations in excess of 1.5ppb are usually visible) but small dye patches, just visible at the sea surface in the vicinity of the diffuser, re illustrated below.

The tracer studies proved valuable in interpreting the results of the marine water quality monitoring and confirmed that:

♦ the initial dilutions being achieved by the outfall were in agreement with the outfall design studies;

♦ the dilution rates being achieved were no less than those assumed in the EIA; and

♦ even at the high dilution rates observed, the potential for E. coli contamination of beaches remote from the outfall remained as discussed farther below.

9 ENVIRONMENTAL IMPACT

As discussed above, all environmental compartments considered to be at risk from the treated effluent discharge from the Interim Outfall were monitored and a large and comprehensive dataset was assembled and analysed in detail. Table 7 provides a summary of the monitoring programme from which the frequency of monitoring with respect to the outfall commissioning sequence can be seen.

a

Rhodamine Dye Visible Above The Diffuser

Marine Water Quality In August 1996, one year before the outfall was planned to be commissioned, the water quality monitoring programme designed under the current Assignment began. Water samples were collected approximately bimonthly (Table 7) on tides of similar amplitude at 16 stations (Drawing 3) and analysed for a range of parameters. The parameters to be tested were selected based on their expected presence in the effluent and which were relevant to the established water


Drawing No.

Mouchel

4

Measured Minimum Dilution at Selected Locations

Measured Minimum Dilution at Selected Locations - Dry Season

* Dilution Rounded to nearest 25 times

250125 100

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B

COR

COOL

Bathing BeachCooling Water Intake

CoralFlushing Water Intake

Measured Minimum Dilution at Selected Locations - Wet Season

Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary quality objectives and are detailed in Table 4 below:

Table 4 Marine Water Quality Tested Parameters

Laboratory Analysis In-situ Measurement

Ammoniacal Nitrogen Cadmium1 Temperature

Total Oxidised Nitrogen

Chromium1 Salinity

Total Kjeldahl Nitrogen

Copper1 pH

Total Phosphorous

Lead1 Dissolved Oxygen

BOD5 Mercury1 Turbidity E. coli Nickel1 Water Velocity Suspended Solids Zinc1 Silica2 Arsenic1 Chlorophyll-a

Notes: 1 The total and dissolved fractions of the Heavy Metals and Arsenic were determined.

2 Following completion of the Baseline monitoring, the analysis of water samples for Silica was abandoned because it was found that the background Silica concentration was heavily dependent on the freshwater flow from the Pearl River Delta and was not present in the effluent to the extent that any impacts could be detected.

Following completion of the Baseline Monitoring, as a result of delays in the construction of the tunnel system, the monitoring programme was suspended in June 1997. In early 1999, it was planned to commission the Interim Outfall in June 1999 using effluent from North West Kowloon (26% of the design flows) and so two rounds of Supplementary Baseline Monitoring were carried out in March and May 1999. A year of impact monitoring at bimonthly intervals was then carried out from July 1999 until May 2000 while the Interim Outfall was discharging 26% of the design flows. The water quality monitoring programme was again suspended in June 2000 until it recommenced in November 2001 immediately before, the commissioning sequence began (Table 1). The monitoring programme was then finally completed in March 2003. In addition to the main water quality monitoring programme, as the final stages of commissioning took place, additional monitoring works were undertaken in order to focus attention on, for example, beneficial impacts on water quality in Victoria Harbour.

Water sampling was carried out using the rosette sampler shown below. The sampler was triggered automatically by the onboard computer data acquisition system. This system records the depth of the instruments, the position of the

survey vessel using dGPS and the in-situ parameters listed above ensuring a consistent dataset. In total, 24 rounds of water quality monitoring were completed at the 16 monitoring stations with measurements and sampling taking place 1m below the sea surface, mid-depth and 1m above the seabed.

Rosette Water Sampler

Both seasonal and spatial variations were identified for many parameters as illustrated in Drawing 5 for Ammoniacal Nitrogen where it can be seen that the data from the inner harbour area reflects the location of the principal effluent sources. A typical data set for Ammoniacal Nitrogen is also presented below for Station W06 close to the Interim Outfall where the Baseline and Impact datasets have been compared with the historic EPD Routine Monitoring data.

Station W06 - Ammoniacal Nitrogen

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Jan

Feb

Mar

Apr

May Jun

Jul

Aug

Sep

Oct

Nov

Dec

Month

Con

cent

ratio

n (m

g/L)

EPD Maximum EPD Minimum Impact (26%) BaselineImpact (100%)


E:\E20 job\HATS data\Executive summary\ES (English)\[Dwg 5.xls]NH4

SSDS Stage I - Baseline Monitoring and Performance Verification

MouchelDistribution of NH4-N (mg/L) at High Water (Depth Average)

Station W01

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0.1

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0.4

1 4 7 10 13 16 19 22

3.2b(ii)Drawing No.

5

Survey Dates 1: 28 Oct 1999 13: 26 Mar2002 2: 28 Oct 1996 14: 28 Mar1999 3: 04 Nov 2001 15: 26 Apr1997 4: 05 Nov 2002 16: 07 May2000 5: 16 Nov 2001 17: 19 May1999 6: 21 Dec 1999 18: 26 May

Station W02

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1 4 7 10 13 16 19 22

Station W03

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1 4 7 10 13 16 19 22

Station W05

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0.4

1 4 7 10 13 16 19 22

Station W04

0.0

0.1

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1 4 7 10 13 16 19 22

Station W07

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1 4 7 10 13 16 19 22

Station W06

0.0

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0.4

1 4 7 10 13 16 19 22

Station W08

0.0

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1 4 7 10 13 16 19 22

Station W10

0.0

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1 4 7 10 13 16 19 22

Station W09

0.0

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1 4 7 10 13 16 19 22

Station W11

0.0

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0.4

1 4 7 10 13 16 19 22

Station W13

0.0

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0.4

1 4 7 10 13 16 19 22

Station W12

0.0

0.1

0.2

0.3

0.4

1 4 7 10 13 16 19 22

Station W15

0.0

0.1

0.2

0.3

0.4

1 4 7 10 13 16 19 22

Station W14

0.0

0.1

0.2

0.3

0.4

1 4 7 10 13 16 19 22

Station W16

0.0

0.1

0.2

0.3

0.4

1 4 7 10 13 16 19 22 Impact-100%

Impact-66%

Impact-26%

Supplementary

Baseline

Baseline

15/09/2004

Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary It can be seen that the data recorded under the Baseline and Impact monitoring programmes generally lay within the range of concentrations recorded by EPD before the Interim Outfall was commissioned and there is no obvious impact from the commissioning of the Interim Outfall.

It is to be expected that the discharge of effluent from the Interim Outfall must have an impact on water quality but the only impacts which could be assigned to the commissioning of the Interim Outfall were increases in E. coli and Ammoniacal Nitrogen concentrations in the Western Harbour and reductions in concentrations of these contaminants in Victoria Harbour where major local effluent loads had been diverted to the Interim Outfall. Increases in Ammoniacal Nitrogen concentrations close to the outfall were of limited extent but increases in E. coli concentrations extended to the beaches in the Tuen Mun district. Baseline and Impact geometric mean E. coli concentrations at each monitoring station are presented in Table 5 below and the changes in concentration following commissioning of the Interim Outfall can be seen. Table 5 Annual Geometric Mean E. coli

Concentrations (No/100mL)

Depth Averaged Surface Layer Station

Baseline Impact1 Baseline Impact1 W01 219 261 91 99 W02 342 982 162 547 W03 195 967 124 865 W04 2,785 4,441 2,695 4,147 W05 846 7,080 1,082 6,787 W06 2,224 7,199 1,933 9,207 W07 393 3,170 344 3,706 W08 4,345 9,456 5,373 9,820 W09 151 646 129 489 W10 6,943 402 6,040 407 W11 82 7 107 10 W12 779 2,410 712 2,367 W13 36 318 30 306 W14 236 241 122 270 W15 4 7 5 9 W16 24 16 20 14

Note: 1 Impact data comprises the annual tide averaged data set following full commissioning of the Interim Outfall

It was concluded that:

♦ there was no indication that the achievement of the Water Quality Objectives in all relevant WCZ would be endangered by the treated effluent; and

♦ if the Mixing Zone for the Interim Outfall is

defined as that area within which the WQO may not be achieved as a result of the effluent discharge, then, based on the water quality data collected under this Assignment, apart from E. coli, the Mixing Zone would be confined to within an area extending no farther than the monitoring stations closest to the outfall, a distance of approximately 500m.

Bathing Beach Water Quality As discussed above, a number of bathing beaches could have been put at risk with respect to E. coli contamination by the diversion of effluent to the Interim Outfall. Those considered to be most at risk were those in the Tsuen Wan district. An analysis of beach water quality data collected by EPD under their beach water quality monitoring programme indicated a reduction in E. coli concentrations at the beaches in the Tuen Mun District after 1997 which may be the result of the ongoing sewage improvements in this district. In the Tsuen Wan District, however, there was a marked increase in E. coli concentrations after 2001 and the full commissioning of the Interim Outfall and this increase in bacterial contamination in beach waters may well be the result of the full commissioning of the Interim Outfall. If this is the case, once disinfection is implemented at the Stonecutter’s Island Sewage Treatment Works, beach water quality in the Tsuen Wan District should improve.

Sediment Quality Sediment samples were collected at a total of 16 stations (Drawing 3) from the commencement of the monitoring programme and a total of 11 rounds of sediment monitoring took place (Table 7) covering the Baseline period, the period during which the effluent discharge was 26% of the design flows and during the final Impact monitoring following the full commissioning of the Interim Outfall. In the Baseline, the sediment samples were analysed for the following physical properties and contaminants:

Sediment Properties and Contaminants % of silt/clay (<63µm) Chromium Total Organic Carbon Copper Acid Volatile Sulphide Lead PCBs Mercury PAHs Nickel Organochlorine Pesticides Zinc Cadmium Arsenic

Following the Baseline surveys, the results of the



Based on all available data, it was concluded that:

effluent quality testing from a pilot CEPT treatment plant became available and it was found that the effluent did not contain significant concentrations of PAH, PCB or Organochlorine Pesticides. As a result, under the Monitoring Programme Review and Refinement carried out as part of this Assignment on completion of the Baseline monitoring, it was recommended that the determination of these compounds in the sediment samples should be discontinued because, if the effluent does not contain these compounds, their presence in marine sediments, if found, can have no bearing on the performance of the outfall. By optimising the monitoring programme in this way, large savings in laboratory costs were achieved.

♦ No statistically significant impact attributable to the effluent discharge from the Interim Outfall was found; and

♦ There was no indication that, for all the contaminants assessed, the treated effluent discharge had had any impact on sediment quality within the study area.

The toxicity of the sediment was also assessed using standard international test protocols based upon Amphipod and Polychaete tests. On 9 occasions during the Baseline and Impact monitoring periods, samples from two near-field stations and 3 mid-field stations were tested and it was found that: Unavoidably, the Impact monitoring took place

during the construction of two large reclamations within the study area (Penny’s Bay Stage I and Container Terminal 9) and the dredging and reclamation works may have resulted in some impacts on the sediment samples during the Impact monitoring at some stations. In addition, when assessing the contaminant levels in the sediment, allowance must be made for the natural variability in sediment contaminant levels and a detailed statistical analysis of the data was carried out based on an analysis of variance in which data from the near-field, mid-field and far-field stations were compared with each other and with respect to any changes between the Baseline and Impact monitoring periods. A typical dataset for sediment Nickel contamination from Station S05 is presented below:

♦ The sediments were not acutely toxic to the marine organisms; and

♦ There was no indication that the effluent discharge from the Interim Outfall had had any impact on sediment toxicity.

Benthic Fauna During the Baseline monitoring, 5 replicate sediment samples from each of the 16 sediment quality monitoring stations were collected and the benthic fauna identified and counted. It was found that, overall, the species diversity and abundance were low which was consistent with previous studies in Hong Kong including the recent (2000-2002) survey of benthic communities in Hong Kong carried out by the City University for the AFCD. Following the analysis of the Baseline data, a review of the monitoring programme was carried out and it was recommended that the sampling programme should be amended with 4 replicates being collected at each of the 4 nearfield stations (S04, S05, S06 & S07, Drawing 3) and pooled to represent a single impact area consisting of 16 replicates and with 16 replicates being collected at each of the 4 far-field Reference stations.

S ediment Nickel ConcentrationS tation S 05

0

20

40

60

Sep-96

Dec-96

Feb-97

May

-97

Mar

-99

Jul-9

9Dec-

99

Jun-00

Nov-01

Aug-02

Mar

-03

Ni m

g/kg

Baseline Impact (26%) Impact (100%)

This adjustment to the monitoring programme, while preserving the same total sample numbers analysed, was designed to improve the robustness of the data collected while preserving the value of the Baseline data set. The data collected was used to calculate, for example, the Shannon-Wiener Diversity Index which is a measure of both abundance and evenness of the species present and the Pielou Evenness Index which is a measure of how evenly species are


Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary distributed within a community. The Pielou Evenness Index varies from 0 (low evenness indicating that that the sample is dominated by a few species) to 1 (high evenness indicating the species are present in equal abundance). The data and results were also subject to a range of univariate and multivariate statistical analysis and a new Biotic Index was calculated at each station. The biotic index was used as a measure of marine soft-bottom habitat quality and is based on biological indicators within the community. The biotic index used in the present study varied from 0 (benthic community in good condition) to 7 (azoic). Typical results for the Pielou Evenness Index at the Impact Station covering the whole Baseline and Impact monitoring programme are presented below:

Impact (S04, S05, S06 & S07)

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0.6

0.8

1.0

Sep-

96

Mar

-97

Sep-

97

Mar

-98

Sep-

98

Mar

-99

Sep-

99

Mar

-00

Sep-

00

Mar

-01

Sep-

01

Mar

-02

Sep-

02

Mar

-03

Piel

ou's

Eve

nnes

s In

dex

While natural variability in the benthic communities is to be expected, it was found that:

♦ The commissioning of the Interim Outfall had not had an identifiable impact on the benthic communities.

Marine Fisheries In order to determine whether the treated effluent discharge from the Interim Outfall had had an impact on the demersal fishery, a monitoring programme was carried out to determine principally whether any significant contamination of commercially important species resulted which could indicate an increased risk to public health and secondly whether any significant impact on catch rates of commercially important species followed the commissioning of the Interim Outfall. Monitoring took place at a total of 6 stations with 2 nearfield stations, 2 midfield stations and 2 far field Reference

stations (Drawing 3). A total of 26 data sets were assembled for the catch analysis and 21 data sets were assembled for the tissue contamination assessment.

Between the Baseline Monitoring, the initial Impact monitoring when the effluent discharge was 26% of the design flows and the final Impact monitoring following the full commissioning of the Interim Outfall, a pattern of general reductions in the total numbers of individuals caught and catch weight were identified. Typical results for the Yield Per Unit Effort (YPUE) for Mantis Shrimp is illustrated below and the decreasing trend can be seen:

Mantis Shrimp YPUE

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Jul-9

6

Jan-

97

Aug-

97

Feb-

98

Sep-

98

Mar

-99

Oct

-99

May

-00

Nov

-00

Jun-

01

Dec

-01

Jul-0

2

Jan-

03

YPU

E

Station F1Station F2Station F3Station F4Station F5Station F6

These reductions were evident at all monitoring locations including those remote from the Stage I Outfall (Stations F5 and F6) indicating that the decrease in catch numbers was a large scale effect and not related to the commissioning of the Stage I Outfall.

The analysis of tissue samples showed that, in general, contamination levels had reduced between the Baseline and the 100%-Impact monitoring periods and no impact from the full commissioning of the Stage I Outfall with respect to increased contamination levels in fish tissue obtained close to the outfall was identified. Tissue contaminant levels were also well below the limit concentrations set in the Hong Kong Food Adulteration (Metallic Contamination) Regulations and there was no indication that the


Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary commissioning of the Interim Outfall had resulted in an increased hazard to human health as a result of the consumption of locally caught fish.

Based on the data obtained, it was concluded that:

♦ There was no indication that the effluent discharge from the Stage I Outfall was having an impact on the capture fisheries; and

♦ Contamination levels had reduced following the Baseline period and no adverse impact from the full commissioning of the Stage I Outfall could be identified.

Intertidal Ecology During the Baseline monitoring, the flora and fauna on both hard and soft intertidal shores were surveyed on Green Island, Hei Ling Chau and Kau Yi Chau but it was found that the soft shores yielded little data. The hard shores, however, were much more productive and large temporal changes in the intertidal flora and fauna species were typical of the 12 data sets collected over the Baseline and Impact monitoring periods. However, similar changes were found on all hard shores surveyed and it was concluded that:

♦ there was no indication that the full commissioning of the Stage I Outfall had impacted on the intertidal ecology.

10 SUMMARY AND DISCUSSION

In anticipation of the Interim Outfall being commissioned in 1997, the EPD commissioned Mouchel Asia to carry out a programme of Baseline Monitoring and Performance Verification studies beginning in March 1996. All environmental compartments of concern were monitored and a number of associated tasks including computer modelling, dye tracer and effluent toxicity studies were carried out.

Three-dimensional computer models were applied to aid in the design of the monitoring programmes, to help in the assessment of the data collected and to allow a detailed review of the monitoring programme to be carried out following the Baseline monitoring. Dye tracer studies were also carried out in order to verify the performance of the outfall with respect to the initial dilution rates being achieved and to assess the rates of dilution of the effluent in the

near-, mid- and far-field regions of the plume. By determining the dilution rates of the dye (and so by implication, the effluent) with distance from the outfall, the potential impacts from the treated effluent discharge could be assessed without interference from natural variations in background water quality.

Overall Impacts on the Marine Environment The principal conclusions from the assessment of the data sets collected were that, with respect to the treated effluent discharge:

♦ There was no detectable impact on: sediment quality; o

o o o o

marine fisheries; fish tissue contamination levels; Intertidal hard shore ecology; and benthic communities;

♦ Ammoniacal Nitrogen concentrations may have been elevated at times at the Impact stations within 500m of the Interim Outfall but not at larger distances from the outfall;

♦ Increases in E. coli concentrations were found at all nearfield stations and at some mid-field and far-field stations including Station W02 to the west of Ma Wan at high water and to the south of the outfall at Station W13 at low water which were considered to be due to the effluent discharge;

♦ Significant improvements in water quality (notably E. coli concentrations) were found in the eastern part of Victoria Harbour from where major effluent loads had been diverted to the Interim Outfall following CEPT treatment at the Stonecutter’s Island Sewage treatment Works; and

♦ The performance of the Interim Outfall with respect to dilution rates being achieved (as determined in the tracer studies) was consistent with the outfall design predictions.

The elevations in E. coli concentrations over a relatively large area extending to the beaches in the Tsuen Wan districts were considered to be due to the effluent discharge. Dye tracer studies also confirmed that, based on the dilution rates achieved, the time taken by the effluent to travel from the outfall and the E. coli mortality rates and concentrations in the effluent, elevations in E. coli concentrations were to be expected at the beaches. The only sensitive receivers to be


Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary adversely impacted by the treated effluent discharge, therefore, were found to be beaches in the New Territories. These adverse impacts, however, could be mitigated by disinfection of the effluent before discharge.

The effluent was subject to both chemical and biological testing and was found to be of a higher quality than had been assumed in the preceding EIA. The dye trace studies also confirmed that the initial dilution rates assumed in the earlier EIA and predicted in the outfall design studies were being achieved by the outfall. As a result, possible impacts assessed in the EIA had not been underestimated.

The demersal fisheries studies indicated that the effluent discharge had not resulted in increased contamination levels in the commercially important species tested and, as a result, there should be no increased risk to public health or to cetaceans from the consumption of locally caught fish. Public health concerns, however, remain with respect to the bacteria levels at the beaches in the Tsuen Wan district. At these beaches, however, regular water quality monitoring is carried out by EPD and the acceptability of the nearshore waters for bathing is assessed directly on a regular basis.

As noted above, the commissioning of the Interim Outfall was achieved by diverting primary treated effluent from outfalls in the Eastern Victoria Harbour, Northern Tathong Channel and Rambler Channel to the Stonecutter’s Island Treatment Works. As a result, the effluent discharge from the Interim Outfall is not a new effluent load to Hong Kong waters but rather a diversion following CEPT treatment of pre-existing effluent loads. The total effluent load to Hong Kong’s coastal waters, therefore, will have reduced as a result of the improved treatment level and no large scale deterioration in water quality was anticipated as a result of commissioning the Interim Outfall.

The monitoring programme identified reductions in water quality in the vicinity of the Interim Outfall with respect to Ammoniacal Nitrogen concentrations and increases in E. coli concentrations over the Western Harbour extending to the Beaches in Tsuen Wan District. However, much larger reductions in E. coli and Ammoniacal Nitrogen concentrations were found in the Eastern Victoria Harbour and Northern Tathong Channel where water quality improved markedly following the diversion of the major

local effluent loads from these areas to the Stonecutter’s Island Treatment Works. Typical data of E. coli and Ammoniacal Nitrogen concentrations at relevant monitoring stations are presented below:

Table 6 E. coli and Ammoniacal Nitrogen Concentration at Eastern and Western Harbour

Annual Geometric Mean E. coli

Concentration, Depth Averaged, (CFU No/100ml)

Annual Mean Ammoniacal Nitrogen Concentration, Depth

Average (mg/L) Stn WZC

Baseline Impact Baseline Impact

W02 North Western 342 982 0.08 0.12

W03 Western Buffer 195 967 0.10 0.11

W10 Victoria Harbour 6942 402 0.20 0.13

W11 Eastern Buffer 82 7 0.04 0.03

While some reduction in water quality in the vicinity of any sea outfall is to be expected, the beneficial impacts of the SSDS Stage I works on water quality in Victoria Harbour and the Tathong Channel should also be noted.

Zone of Influence of the Interim Outfall Discharge

As noted above, the only identified impacts on the marine environment from the treated effluent discharge related to E. coli and Ammoniacal Nitrogen concentrations. As expected, concentrations of these parameters in the coastal waters reduced in the vicinity of the major effluent loads diverted to the Stonecutter’s Island Sewage treatment Works. In the vicinity of the Interim Outfall, occasional increases in Ammoniacal Nitrogen concentrations at the Impact Stations around 500m from the outfall were thought to be due to the treated effluent discharge while relatively large scale increases in E. coli concentrations extending as far as Station W02 to the North West and Station W13 to the south were considered to be due to the treated effluent discharge. If the Zone of Influence (ZoI) of the Interim Outfall is defined as that area within which adverse impacts on the marine environment can be detected, then the ZoI would extend to Station W02 in the North West, Stations W13 and W14 in the South and Station W08 at the western entrance to Victoria Harbour on the East. At present, this ZoI is considered to include the bathing beaches in the Tsuen Wan district but it should be noted that significant


Strategic Sewage Disposal Scheme Stage I Outfall Mouchel Asia Environmental Baseline Monitoring and Performance Verification Executive Summary improvements in water quality and a reduction in the size of the ZoI could be expected if disinfection of the effluent is implemented.

Recommendations for Future Monitoring

The monitoring programme implemented under the current study has proved effective in identifying impacts from the commissioning of the Stage I Interim Outfall with respect to increases in the concentrations of E. coli and Ammonia. Future monitoring may be required to confirm the longer term impact of the effluent discharge at its current discharge rate, to assess improved levels of treatment (e.g. disinfection) or to assess the impact of future increases in the discharge rate.

The current monitoring programme has established a robust set of data which describe environmental conditions before, during and after the staged commissioning of the Stage I Outfall and it would be important that any future monitoring programme is able to use the existing data set to its full. As a result, it is recommended that, as far as possible, the monitoring stations employed in this study are again employed in any future monitoring programme. However, as might be the focus of attention in any future study, additional water quality monitoring stations could be considered in the nearshore waters at the beaches where E. coli concentrations have been identified as being of concern.



Table 7 Detailed Survey Programme Survey Type

Marine Sediment Fisheries Marine Ecology Effluent Date Marine Water Chemistry Eco-

Toxicity Fisheries Tissue Analysis

Benthic Fauna Inter-tidal Chemistry Eco-

Toxicity Aug-96 Sep-96 Oct-96 Nov-96 Dec-96 Jan-97 Feb-97 Mar-97 Apr-97 May-97 Jun-97 Jul-97 Mar-99 May-99 Jul-99 Aug-99 Oct-99 Nov-99 Dec-99 Feb-00 May-00 Jun-00

4-Nov-01 16-Nov-01 30-Dec-01

Mar-02 May-02 Jun-02 Jul-02 Aug-02 Nov-02 Dec-02 Jan-03 Mar-03

Numbers of Surveys Baseline

Baseline 6 4 4 12 6 4 4 0 0 Supplementary 2 1 1 2 2 1 1 0 0

Impact Flow @ 26% 7 4 2 7 7 4 4 0 0 Flow @ 66% 1

Flow @ 100% 8 2 2 5 5 2 3 8 8 TOTAL 24 11 9 26 20 11 12 8 8

Notes

No Shade Baseline Periods (Zero discharge) Light Shade Surveys at 26% 1997 Design Flow

Dark Shade Surveys at 66% 1997 Design Flow White Text Surveys at 100% 1997 Design Flow Note: Refer also to Table 1 for details of the commissioning sequence


Documents

Strategic Sewage Disposal Scheme (Harbour Area Treatment ... CE7395.pdf · disposal of sewage for all of Hong Kong. One main element of the Sewage Strategy was the Strategic Sewage