6.1
Stage 1 of HATS, comprising
the Stonecutters Island Sewage Treatment Works (SCISTW) and the deep tunnels,
was commissioned in late 2001 to bring early improvement to the harbour water
quality. The deep tunnels collect
sewage from Kwai Chung, Tsing Yi, Tseung Kwan O, parts of eastern
6.2 This Project involves the construction and operation of Stage 2A. Construction of the Project (i.e. Stage 2A) is tentatively scheduled to commence in 2009 for commissioning the Stage 2A scheme by 2014.The scope of Stage 2A includes the following:
·
Upgrading of existing preliminary treatment works (PTW) at
North Point, Wan Chai East, Central, Sandy Bay, Cyberport, Wah Fu, Aberdeen and
Ap Lei Chau on Hong Kong Island;
·
Extension of the deep tunnel network to collect and transfer
sewage from the above mentioned PTWs to SCISTW for treatment and disposal;
·
Expansion of the existing CEPT capacity at SCISTW to
meet the demands of both existing and future developments; and
·
Provision of disinfection to all HATS effluent before
discharging into the harbour.
6.3 Based on the technical review of disinfection technologies and option evaluation as well as the technical environmental assessment conducted under the EIA study for “Provision of Disinfection Facilities at Stonecutters Island STW – Investigation (ADF)”, the purchase of sodium hypochlorite solution for chlorination and sodium bisulphite for dechlorination was recommended as the disinfection technology for SCISTW. Based on the current programme, construction of the chlorination plant would commence in April 2008 for commissioning the advance disinfection facilities (ADF) in October 2009. As for the dechlorination plant, construction would commence in September 2008 for completion in September 2009.
6.4
During the ADF stage, use of
the existing effluent culvert as the chlorine contact tank is proposed. At
Stage
6.5 This Section evaluates the potential water quality impacts that are likely to be generated during the construction and operation of Stage 2A. The water quality impacts of Stage 1, Stage 2A and Stage 2B of the HATS have also been taken into consideration in this water quality impact assessment. Appropriate mitigation measures were identified, where necessary, to mitigate the potential water quality impacts.
6.6
To evaluate the potential water
quality impacts from the Project, water sensitive receivers within the North Western,
Western Buffer,
·
Cooling Water Intakes;
·
WSD
·
Fish Culture Zones (FCZ);
·
Beaches;
·
Sites of Special Scientific
Interest (SSSI);
·
·
Seagrass Beds;
·
Artificial Reefs;
·
Corals;
·
Chinese White Dolphins; and
·
Green Turtle Nesting Grounds
6.7 Figure 6.1 shows the locations of the ecological resources and water sensitive receivers.
6.8 During the operational phase, the potential water quality impacts will be mainly related to the treated effluent discharge from the HATS Stage 2A. Key concerns are:
·
Effects on marine water quality
and sediment quality due to the discharge of disinfected CEPT effluent from
SCISTW at different time horizons under normal plant operation.
·
Water quality effects of occasional
overflow of screened (or untreated) effluent at individual PTW during extreme
storm event under normal plant operation.
·
Water quality impacts of emergency
sewage discharges due to the failure of equipment or power supply or as a
result of treatment process failure.
6.9
The water quality parameters
considered for the assessment of water quality impact from the HATS Stage 2A
effluent include pH, temperature, dissolved oxygen (DO), salinity, suspended
solids (SS), biochemical oxygen demand (BOD), nutrients, chlorophyll, E.coli, sedimentation rates,
chlorination by-products (CBP) and toxic chemicals such as metals, total
residual chlorine (TRC) and unionized ammonia etc.
6.10
A comprehensive review and identification of toxic
contaminants of concern (COC) in the HATS effluent and their respective
assessment criteria and detailed assessment of the toxic effects of the
identified COC were conducted under the human health and marine ecological risk
assessment in Section 7 and Section 8.
6.11
No dredging and filling
activity would be anticipated for the tunnel construction. The general construction activities that
will be undertaken for the upgrading works will be primarily land-based. Key water quality issues associated with
land-based construction would include the impacts from site run-off, sewage
from workforce, accidental spillage and discharges of wastewater from various
construction activities.
6.12
Based on the preliminary
engineering design
6.13 Temporary bypass of sewage effluent via seawall or submarine outfalls of SCISTW and individual PTW would be required during the construction stage. The temporary sewage bypass would cause transient increase of pollution level in the receiving marine water.
6.14 Key water quality impacts related to the provision of disinfection facilities at the SCISTW include:
·
The
reduction of faecal bacteria in the effluent after disinfection.
·
The potential generation of
low-level total residual chlorine (TRC) and chlorination by-products (CBP) in
the effluent due to chlorination of the sewage effluent.
·
The potential impact of TRC in the
event of dechlorination plant failure.
·
The potential impact of faecal
pollution in the event of chlorination plant failure.
·
The potential minor oxygen
depletion impact due to addition of dechlorination chemical.
6.15
Total residual chlorine
(TRC) and chlorination by-products (CBP) are major concerns of chlorination.
TRC includes free chlorine residuals such as hypochlorous acid (HOCl) and
dissolved hypochlorite ion (OCl-) after chlorine is added to water, plus
combined chlorine residuals such as chloramines formed by the reaction of free
residuals with ammonia present in the sewage. CBP refer to chlorinated organic
compounds (or total organic halogen) formed by the reaction of chlorine (mainly
free chlorine residuals) with some specific organic compounds such as humic
substances, which generally are not present in any large quantity in CEPT
effluent. CBP consist of a whole range of halogenated organic compounds, and
are generally considered of concern to human health. Examples of CBP formed
during chlorination include trihalomethanes (THM) and haloacetic acids (HAA).
THM are suspected as being carcinogens and are strictly monitored in drinking
water. CBP concentrations may vary
in orders of magnitude during different chlorination processes. Typical concentrations
of THM and HAA in chlorinated drinking water are usually in the range 1-100 mg/l ([1]). Range of concentrations in chlorinated
sewage effluent for specific CBP compounds has been identified under the EIA
study for ADF and is also presented in this EIA report for completeness.
6.16
Water
quality impacts in relation to the chlorination and
dechlorination of the HATS effluent have been quantitatively assessed
by mathematical modelling under the EIA study for the ADF. Details of the
assessment results are presented in the separate EIA report for ADF. Under the ADF study, water quality model
simulations were performed for 30 days (excluding the spin-up time) each under
the typical wet and dry seasons for normal operation scenarios and 15 days
(excluding the spin-up time) each under the typical wet and dry seasons for
emergency situations (due to temporary failure of chlorination or
dechlorination plant) in accordance with the EIA Study Brief for ADF.
6.17
The
EIA Study Brief for this Project (i.e. Stage 2A) requires that the model
simulations shall be performed for at least one complete calendar year under
normal operation scenarios. As such, the water quality impacts in
relation to the normal operation of the disinfection facilities for HATS have
been re-examined under this EIA based on a series of 1-year model simulations
incorporating monthly variations in Pearl River discharges, solar radiation,
water temperature and wind velocity to confirm the findings of the ADF study.
6.18
According
to the EIA Study Brief for this Project, assessment of temporary or emergency
discharges as well as the short-term construction phase impacts may be assessed
by simulating typical spring-neap cycles (at least 15 days) in the dry and wet
seasons. Furthermore, the HATS flow rates adopted in the EIA study for ADF were
more conservative as compared to the latest flow projections adopted in this
EIA. It is considered that the water quality impacts in relation to the
temporary failure of chlorination or dechlorination plant have been fully
quantified and assessed under the ADF study.
6.19 The Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) is issued by the EPD under Section 16 of the EIAO. It specifies the assessment method and criteria that need to be followed in the EIA. Reference sections in the EIAO-TM provide the details of the assessment criteria and guidelines that are relevant to the water quality impact assessment, including:
·
Annex 6 Criteria for Evaluating
Water Pollution
·
Annex 14 Guidelines for Assessment
of Water Pollution
6.20
The Water Pollution Control
Ordinance (WPCO) provides the major statutory framework for the protection and
control of water quality in
Table 6.1 Summary of Water
Quality Objectives for North
Parameters |
Objectives |
Sub-Zone |
Offensive odour, tints |
Not to be present |
Whole zone |
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
Dissolved oxygen (DO) within 2 m of the seabed |
Not less than 2.0 mg/l for 90% of samples |
Marine waters |
Depth-averaged DO |
Not less than 4.0 mg/l |
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones,
water gathering ground subzones and other inland waters |
Not less than 4.0 mg/l for 90 % sample |
Marine waters |
|
pH |
To be in the range of 6.5 - 8.5, change due to human activity
not to exceed 0.2 |
Marine waters excepting bathing beach subzones |
To be in the range of 6.5 – 8.5 |
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones and
water gathering ground subzones |
|
To be in the range of 6.0 –9.0 |
Other inland waters |
|
To be in the range of 6.0 –9.0 for 95% samples |
Bathing beach subzones |
|
Salinity |
Change due to human activity not to exceed 10% of ambient |
Whole zone |
Temperature |
Change due to human activity not to exceed 2 oC |
Whole zone |
Suspended solids (SS) |
Not to raise the ambient level by 30% caused by human
activity |
Marine waters |
Change due to waste discharges not to exceed 20 mg/l of
annual median |
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones and water
gathering ground subzones |
|
Change due to waste discharges not to exceed 25 mg/l of
annual median |
Inland waters |
|
Unionized ammonia (UIA) |
Annual mean not to exceed 0.021 mg(N)/l as unionized form |
Whole zone |
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
Total inorganic nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen not to
exceed 0.3 mg(N)/l |
|
Annual mean depth-averaged inorganic nitrogen not to exceed
0.5 mg(N)/l |
Marine waters excepting castle peak bay subzone |
|
E.coli |
Not exceed 610 per 100 ml, calculated as the geometric
mean of all samples collected in one calendar year |
Secondary contact recreation subzones |
Should be less than 1 per 100 ml, calculated as the geometric mean of the most recent 5
consecutive samples taken between 7 and 21 days. |
Tuen Mun (A) and Tuen Mun (B) subzones and water gathering
ground subzones |
|
Not exceed 1000 per 100 ml, calculated as the geometric mean of the most recent 5
consecutive samples taken between 7 and 21 days |
Tuen Mun (C) subzone and other inland waters |
|
Not exceed 180 per 100 ml, calculated as the geometric
mean of all samples collected from March to October inclusive. |
Bathing beach subzones |
|
Colour |
Change due to waste discharges not to exceed 30 Hazen
units |
Tuen Mun (A) and Tuen Mun (B) subzones and water gathering
ground subzones |
Change due to waste discharges not to exceed 50 Hazen
units |
Tuen Mun (C) subzone and other inland waters |
|
5-Day biochemical oxygen demand (BOD5) |
Change due to waste discharges not to exceed 3 mg/l |
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones and
water gathering ground subzones |
Change due to waste discharges not to exceed 5 mg/l |
Inland waters |
|
Chemical oxygen demand (COD) |
Change due to waste discharges not to exceed 15 mg/l |
Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) subzones and
water gathering ground subzones |
Change due to waste discharges not to exceed 30 mg/l |
Inland waters |
|
Toxins |
Should not cause a risk to any beneficial uses of the
aquatic environment |
Whole zone |
Waste discharge shall not cause the toxins in water significant
to produce toxic carcinogenic, mutagenic or teratogenic effects in humans,
fish or any other aquatic organisms. |
Whole zone |
|
Phenol |
Quantities shall not sufficient to produce a specific
odour or more than 0.05 mg/l as C6 H5OH |
Bathing beach subzones |
Turbidity |
Shall not reduce light transmission substantially from the
normal level |
Bathing beach subzones |
Source: Statement of Water Quality
Objectives (North Western Water Control Zone).
Table 6.2 Summary of Water Quality Objectives for Western Buffer WCZ
Parameters |
Objectives |
Sub-Zone |
Offensive odour, tints |
Not to be present |
Whole zone |
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
Dissolved oxygen (DO) within 2 m of the seabed |
Not less than 2.0 mg/l for 90% of samples |
Marine waters |
Depth-averaged DO |
Not less than 4.0 mg/l for 90% of samples |
Marine waters excepting fish culture subzones |
Not less than 5.0 mg/l for 90% of samples |
Fish culture subzones |
|
Not less than 4.0 mg/l |
Water gathering ground subzone and other Inland waters |
|
5-Day biochemical oxygen demand (BOD5) |
Change due to waste discharges not to exceed 3 mg/l |
Water gathering ground subzones |
Change due to waste discharges not to exceed 5 mg/l |
Inland waters |
|
Chemical oxygen demand (COD) |
Change due to waste discharges not to exceed 15 mg/l |
Water gathering ground subzones |
Change due to waste discharges not to exceed 30 mg/l |
Inland waters |
|
pH |
To be in the range of 6.5 – 8.5, change due to waste
discharges not to exceed 0.2 |
Marine waters |
To be in the range of 6.5 – 8.5 |
Water gathering ground subzones |
|
To be in the range of 6.0 – 9.0 |
Inland waters |
|
Salinity |
Change due to waste discharges not to exceed 10% of
ambient |
Whole zone |
Temperature |
Change due to waste discharges not to exceed 2 oC |
Whole zone |
Suspended solids (SS) |
Not to raise the ambient level by 30% caused by waste
discharges and shall not affect aquatic communities |
Marine waters |
Change due to waste discharges not to exceed 20 mg/l of
annual median |
Water gathering ground subzones |
|
Change due to waste discharges not to exceed 25 mg/l of
annual median |
Inland waters |
|
Unionized ammonia (UIA) |
Annual mean not to exceed 0.021 mg(N)/l as unionized form |
Whole zone |
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
Total inorganic nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen not to
exceed 0.4 mg(N)/l |
Marine waters |
Toxic substances |
Should not attain such levels as to produce significant
toxic effects in humans, fish or any other aquatic organisms |
Whole zone |
Waste discharges should not cause a risk to any beneficial
use of the aquatic environment |
Whole zone |
|
E.coli |
Not exceed 610 per 100 ml, calculated as the geometric
mean of all samples collected in one calendar year |
Secondary contact recreation subzones and fish culture
subzones |
Not exceed 180 per 100 ml, calculated as the geometric
mean of all samples collected from March to October inclusive in 1 calendar
year. Samples should be taken at least 3 times in 1 calendar month at
intervals of between 3 and 14 days |
Bathing beach subzones |
|
Less than 1 per 100 ml, calculated as the geometric mean
of the most recent 5 consecutive samples taken at intervals of between 7 and
21 days |
Water gathering ground subzones |
|
Not 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 |
|
Colour |
Change due to waste discharges not to exceed 30 Hazen
units |
Water gathering round |
Change due to waste discharges not to exceed 50 Hazen
units |
Inland waters |
|
Turbidity |
Shall not reduce light transmission substantially from the
normal level |
Bathing beach subzones |
Source: Statement of Water Quality
Objectives (Western Buffer Water Control Zone).
Table 6.3 Summary
of Water Quality Objectives for
Parameters |
Objectives |
Sub-Zone |
Offensive odour, tints |
Not to be present |
Whole zone |
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
E coli |
Not to exceed 1000 per 100 mL, calculated as the geometric
mean of the most recent 5 consecutive samples taken at intervals between 7
and 21 days |
Inland waters |
Dissolved oxygen (DO) within 2 m of the seabed |
Not less than 2.0 mg/l for 90% of samples |
Marine waters |
Depth-averaged DO |
Not less than 4.0 mg/l for 90% of samples |
Marine waters |
DO |
Not less than 4.0 mg/l |
Inland waters |
pH |
To be in the range of 6.5 - 8.5, change due to human
activity not to exceed 0.2 |
Marine waters |
Not to exceed the range of 6.0 - 9.0 due to human activity |
Inland waters |
|
Salinity |
Change due to human activity not to exceed 10% of ambient |
Whole zone |
Temperature |
Change due to human activity not to exceed 2 oC |
Whole zone |
Suspended solids (SS) |
Not to raise the ambient level by 30% caused by human
activity |
Marine waters |
Annual median not to exceed 25 mg/l due to human activity |
Inland waters |
|
Unionized ammonia (UIA) |
Annual mean not to exceed 0.021 mg(N)/l as unionized form |
Whole zone |
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
Total inorganic nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen not to
exceed 0.4 mg(N)/l |
Marine waters |
5-Day biochemical oxygen demand (BOD5) |
Not to exceed 5 mg/l |
Inland waters |
Chemical Oxygen Demand (COD) |
Not to exceed 30 mg/l |
Inland waters |
Toxic substances |
Should not attain such levels as to produce significant
toxic, carcinogenic, mutagenic or teratogenic effects in humans, fish or any
other aquatic organisms. |
Whole zone |
Human activity should not cause a risk to any beneficial
use of the aquatic environment. |
Whole zone |
Source: Statement of Water Quality
Objectives (
Table 6.4 Summary of Water Quality Objectives for Eastern Buffer WCZ
Parameters |
Objectives |
Sub-Zone |
Offensive odour, tints |
Not to be present |
Whole zone |
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
Dissolved oxygen (DO) within 2 m of the seabed |
Not less than 2.0 mg/l for 90% of samples |
Marine waters |
Depth-averaged DO |
Not less than 4.0 mg/l for 90% of samples |
Marine waters excepting fish culture subzones |
Not less than 5.0 mg/l for 90% of samples |
Fish culture subzones |
|
Not less than 4.0 mg/l |
Water gathering ground subzone and other Inland waters |
|
5-Bay biochemical oxygen demand (BOD5) |
Change due to waste discharges not to exceed 3 mg/l |
Water gathering ground subzones |
Change due to waste discharges not to exceed 5 mg/l |
Inland waters |
|
Chemical oxygen demand (COD) |
Change due to waste discharges not to exceed 15 mg/l |
Water gathering ground subzones |
Change due to waste discharges not to exceed 30 mg/l |
Inland waters |
|
pH |
To be in the range of 6.5 – 8.5, change due to waste
discharges not to exceed 0.2 |
Marine waters |
To be in the range of 6.5 – 8.5 |
Water gathering ground subzones |
|
To be in the range of 6.0 – 9.0 |
Inland waters |
|
Salinity |
Change due to waste discharges not to exceed 10% of
ambient |
Whole zone |
Temperature |
Change due to waste discharges not to exceed 2 oC |
Whole zone |
Suspended solids (SS) |
Not to raise the ambient level by 30% caused by waste
discharges and shall not affect aquatic communities |
Marine waters |
Change due to waste discharges not to exceed 20 mg/l of
annual median |
Water gathering ground subzones |
|
Change due to waste discharges not to exceed 25 mg/l of
annual median |
Inland waters |
|
Unionized ammonia (UIA) |
Annual mean not to exceed 0.021 mg(N)/l as unionized form |
Whole zone |
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
Total inorganic nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen not to
exceed 0.4 mg(N)/l |
Marine waters |
Dangerous substances |
Should not attain such levels as to produce significant
toxic effects in humans, fish or any other aquatic organisms |
Whole zone |
Waste discharges should not cause a risk to any beneficial
use of the aquatic environment |
Whole zone |
|
E.coli |
Not exceed 610 per 100 ml, calculated as the geometric
mean of all samples collected in one calendar year |
Fish culture subzones |
Less than 1 per 100 ml, calculated as the geometric mean of
the most recent 5 consecutive samples taken at intervals of between 7 and 21
days |
Water gathering ground subzones |
|
Not 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 |
|
Colour |
Change due to waste discharges not to exceed 30 Hazen
units |
Water gathering ground |
Change due to waste discharges not to exceed 50 Hazen
units |
Inland waters |
Source: Statement of Water Quality
Objectives (Eastern Buffer Water Control Zone).
Table 6.5 Summary of Water Quality Objectives for Junk Bay WCZ
Parameters |
Objectives |
Sub-Zone |
Offensive odour, tints |
Not to be present |
Whole zone |
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
Dissolved oxygen (DO) within 2 m of the seabed |
Not less than 2.0 mg/l for 90% of samples |
Marine waters |
Depth-averaged DO |
Not less than 4.0 mg/l for 90% of samples |
Marine waters excepting fish culture subzones |
Not less than 5.0 mg/l for 90% of samples |
Fish culture subzones |
|
Not less than 4.0 mg/l |
Inland waters |
|
5-Bay biochemical oxygen demand (BOD5) |
Change due to waste discharges not to exceed 5 mg/l |
Inland waters |
Chemical oxygen demand (COD) |
Change due to waste discharges not to exceed 30 mg/l |
Inland waters |
pH |
To be in the range of 6.5 - 8.5, change due to waste
discharges not to exceed 0.2 |
Marine waters |
To be in the range of 6.0 –9.0 |
Inland waters |
|
Salinity |
Change due to waste discharges not to exceed 10% of
ambient |
Whole zone |
Temperature |
Change due to waste discharges not to exceed 2 oC |
Whole zone |
Suspended solids (SS) |
Not to raise the ambient level by 30% caused by waste
discharges and shall not affect aquatic communities |
Marine waters |
Change due to waste discharges not to exceed 25 mg/l of
annual median |
Inland waters |
|
Unionized ammonia (UIA) |
Annual mean not to exceed 0.021 mg(N)/l as unionized form |
Whole zone |
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
Total inorganic nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen not to
exceed 0.3 mg(N)/l |
Marine waters |
Dangerous substances |
Should not attain such levels as to produce significant
toxic effects in humans, fish or any other aquatic organisms |
Whole zone |
Waste discharges should not cause a risk to any beneficial
use of the aquatic environment |
Whole zone |
|
E.coli |
Not exceed 610 per 100 ml, calculated as the geometric
mean of all samples collected in one calendar year |
Secondary contact recreation subzones and fish culture
subzones |
Not exceed 1000 per 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 |
|
Colour |
Change due to waste discharges not to exceed 50 Hazen
units |
Inland waters |
Source: Statement of Water Quality
Objectives (Junk Bay Water Control Zone).
Table
6.6 Summary
of Water Quality Objectives for
Parameters |
Objectives |
Sub-Zone |
Offensive odour, tints |
Not to be present |
Whole zone |
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
Dissolved oxygen (DO) within 2 m of the seabed |
Not less than 2.0 mg/l for 90% of samples |
Marine waters |
Depth-averaged DO |
Not less than 4.0 mg/l for 90 % sample |
Marine waters excepting fish culture subzones |
Not less than 5.0 mg/l for 90% of samples |
Fish culture subzones |
|
Not less than 4.0 mg/l |
Inland waters |
|
pH |
To be in the range of 6.5 - 8.5, change due to human
activity not to exceed 0.2 |
Marine waters excepting bathing beach subzones; Mui Wo
(A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo (F) subzones |
To be in the range of 6.0 – 9.0 |
Mui Wo (D) sub-zone and other inland waters. |
|
To be in the range of 6.0 –9.0 for 95% of samples, change
due to human activity not to exceed 0.5 |
Bathing beach subzones |
|
Salinity |
Change due to human activity not to exceed 10% of ambient |
Whole zone |
Temperature |
Change due to human activity not to exceed 2 oC |
Whole zone |
Suspended solids (SS) |
Not to raise the ambient level by 30% caused by human
activity |
Marine waters |
Change due to waste discharges not to exceed 20 mg/l of
annual median |
Mui Wo (A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo
(F) subzones |
|
Change due to waste discharges not to exceed 25 mg/l of
annual median |
Mui Wo (D) subzone and other inland waters |
|
Unionized ammonia (UIA) |
Annual mean not to exceed 0.021 mg(N)/l as unionized form |
Whole zone |
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
Total inorganic nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen not to
exceed 0.1 mg(N)/l |
Marine waters |
E.coli |
Not exceed 610 per 100 ml, calculated as the geometric mean
of all samples collected in one calendar year |
Secondary contact recreation subzones and fish culture subzones |
Not exceed 180 per 100 ml, calculated as the geometric
mean of all samples collected from March to October inclusive in 1 calendar
year. Samples should be taken at least 3 times in 1 calendar month at
intervals of between 3 and 14 days. |
Bathing beach subzones |
|
5-Day biochemical oxygen demand (BOD5) |
Change due to waste discharges not to exceed 5 mg/l |
Inland waters |
Chemical oxygen demand (COD) |
Change due to waste discharges not to exceed 30 mg/l |
Inland waters |
Dangerous substances |
Should not attain such levels as to produce significant
toxic effects in humans, fish or any other aquatic organisms |
Whole zone |
Waste discharges should not cause a risk to any beneficial
use of the aquatic environment |
Whole zone |
Source: Statement
of Water Quality Objectives (Southern Water Control Zone).
6.21 Besides the WQO set under the WPCO, the WSD has specified a set of objectives for water quality at flushing water intakes as listed in Table 6.7.
Table 6.7 WSD
Standards at
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 |
< 1 |
Suspended Solids |
< 10 |
Dissolved Oxygen |
> 2 |
Biochemical Oxygen Demand |
< 10 |
Synthetic Detergents |
< 5 |
E.coli (no. per 100 ml) |
< 20000 |
6.22 A practice note for professional persons has been 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 construction, acid cleaning, etching and pickling wastewater, and wastewater from site facilities. Practices given in the ProPECC PN 1/94 should be followed as far as possible during construction to minimize the water quality impact due to construction site drainage.
6.23 Discharges of effluents are subject to control under the WPCO. The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS) gives guidance on 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 effluent. Any effluent discharge from the proposed construction activities must comply with the standards for effluent discharged into the foul sewers, inshore waters and marine waters of the Victoria Harbour WCZ and Western Buffer WCZ, as given in the TM-DSS.
6.24 Potential impacts on benthic organisms, including corals, may arise through excessive sediment deposition. The magnitude of impacts on marine ecological sensitive receivers was assessed based on the predicted sedimentation rate. According to Pastorok and Bilyard ([2]) and Hawker and Connell ([3]), a sedimentation rate higher than 0.1 kg/m2/day would introduce moderate to severe impact upon corals. This criterion has been adopted for protecting the corals in Hong Kong under other approved EIAs such as Tai Po Sewage Treatment Works Stage 5 EIA ([4]), Further Development of Tseung Kwan O Feasibility Study EIA, Wan Chai Reclamation Phase II EIA, Eastern Waters MBA Study ([5]), West Po Toi MBA Study ([6]) and Tai Po Gas Pipeline Study ([7]). This sedimentation rate was used as the assessment criterion to guard against unacceptable impacts on marine ecological sensitive receivers.
6.25
The
potential impacts on corals were also assessed with reference to the WQO of 30%
from the ambient levels. However, in Eastern Buffer,
6.26 In addition to the statutory WQO stipulated under the WPCO, a set of water quality criteria (WQC) established under the “Environmental and Engineering Feasibility Assessment Studies in relation to the Way Forward of the HATS (EEFS)” was used as reference guidelines to assess the water quality impacts as per the requirement in Section 3.4.3.5 (iv) of the EIA Study Brief.
6.27
The HATS is an overall sewage
collection and treatment scheme for areas on both sides of
6.28 In 2004, the Government of the Hong Kong SAR completed trials and studies on environmental impacts and engineering feasibility to assist in deciding the best way forward for the remaining stages of HATS. Detailed marine water quality, ecological and fisheries assessments were performed as part of the EEFS with the objective of assessing the potential impacts of different treatment and disposal schemes proposed for the remaining stages of HATS and the associated construction activities. The recommended option for HATS Stage 2 is to convey all sewage from the harbour area to SCISTW for centralized treatment. The EEFS recommended that biological treatment plus disinfection should be provided for the HATS on a long-term basis. The marine water quality, ecological and fisheries assessment conducted as part of the EEFS followed the guidelines set forth in the EIAO-TM. The potential water quality impacts were quantitatively assessed using various WQC outlined for the EEFS with a further qualitative assessment based on the collective professional opinion of a team of local and international experts in marine sciences and water quality management.
6.29 Setting of WQC has been recognized from the start of the EEFS as a key to assessing the acceptability and performance of different HATS options. In support of criteria setting, an extensive public consultation exercise on the proposed WQC for HATS was conducted in 2002 as part of the EEFS. These WQC were based on the statutory WQO stipulated under the WPCO, originally developed in the late 1980’s, and further refined by the consultant of the EEFS using the results of the Environmental Impact Assessment of the Strategic Sewage Disposal Scheme (SSDS EIA) and other recently completed studies. The set of HATS specific WQC, developed as a part of the EEFS, has integrated the concerns of various interested stakeholders and the general public through a presentation and briefing with the Advisory Council on the Environment and the Monitoring Group for HATS, public view-sharing workshops and receipt of public comments.
6.30 The findings of this consultation exercise and a full set of the proposed final WQC are documented in the “Report on Community Consultation for the Proposed Water Quality Criteria (October 2002)” prepared under the EEFS.
6.31 The WQC as shown in Table 6.8 and Table 6.9 were used as reference guidelines for far field and near field impact assessment respectively. The far field water quality impact assessment and far field modelling performed under this EIA covered the North Western, Western Buffer, Victoria Harbour, Eastern Buffer, Junk Bay and Southern WCZ as well as the adjacent outer water to take into account all the major pollution sources (including the Pearl River) that may have a bearing on the environmental acceptability of the Project as required by the EIA Study Brief. The near field impact assessment was conducted by mathematical modelling to simulate the characteristics of the sewage plume in the vicinity of the submarine outfall to determine the zone of initial dilution (ZID), plume dimensions, rise height, merging and trapping in various flow and ambient conditions.
6.32
Relevant WQC were derived under
the EEFS for Western Buffer, Eastern Buffer,
Table
6.8 Reference
Marine Water Quality Criteria for Far Field Water Quality Assessment
Parameter |
Value |
Type
/ Period |
Applicable
Zones / Uses (1) |
E.coli |
≤
180/100ml |
Geometric mean for
bathing season (March to October) |
Bathing waters |
≤
610/100ml |
Annual
geometric mean |
Secondary
contact recreation zones and mariculture zones |
|
≤
20,000/100ml |
≥ 90%
of occasions |
Sea
water intakes for flushing and industrial use |
|
Dissolved
oxygen (DO) |
≥ 4 mg/l (water column average) |
≥ 90%
of occasions |
Western
Buffer, Eastern Buffer, |
≥
2 mg/l |
at
all times |
All
WCZ (except mariculture zones) |
|
≥
5 mg/l (water column average) |
Monthly
average |
|
|
≥
5 mg/l (water column average) |
≥ 90%
of occasions |
Mariculture
zones only |
|
≥
2 mg/l (bottom DO within 2 m from the seabed) |
≥ 90%
of occasions |
Mariculture
zones only |
|
Depth-averaged
total inorganic nitrogen (TIN) |
≤
0.2 mg(N)/l |
Annual
mean |
|
≤
0.4 mg(N)/l |
Annual
mean |
Western
Buffer, Eastern Buffer and Victoria Harbour WCZ (except fish spawning ground)
(Figure 6.2) |
|
≤
0.3 mg(N)/l |
Annual
mean |
Junk
Bay WCZ (Figure 6.2) |
|
≤
0.1 mg(N)/l |
Annual
mean |
Semi-enclosed
bays (Figure 6.3) |
|
Depth-averaged
Unionized Ammonia (UIA) |
≤
0.021 mg(N)/l |
Annual
mean |
All
WCZ |
Depth-averaged
total inorganic phosphorus (PO4) |
≤
0.02 mg(P)/l |
Annual
mean |
|
≤
0.04 mg(P)/l |
Annual
mean |
Western
Buffer, Eastern Buffer and Victoria Harbour WCZ (except fish spawning ground)
(Figure 6.2) |
|
≤
0.03 mg(P)/l |
Annual
mean |
Junk
Bay WCZ (Figure 6.2) |
|
≤
0.01 mg(P)/l |
Annual
mean |
Semi-enclosed
bays (Figure 6.3) |
|
Total
residual chlorine (TRC) |
≤
0.008 mg/l |
Daily maximum |
All
WCZ |
Chronic
toxicity |
≤
one chronic toxicity unit (TUc), (derived from NOEC values based on whole effluent
toxicity tests) (2) |
4-day
average chronic toxicity exposure |
All
WCZ |
(Source: EEFS
Report on Community Consultation for the Proposed Water Quality Criteria)
(1) EEFS did not derive WQC for the North Western
WCZ.
(2) USEPA
Technical Support Document for Water Quality-Based Toxics Control (March 1991),
from which one chronic Toxicity Unit (TUc) is defined TUc = 100/NOEC, where
NOEC = % of effluent which gives no observed effect on the most sensitive of
the range of species tested.
Table 6.9 Reference
Marine Water Quality Criteria for Near
Field Water Quality Assessment
Parameter |
Value |
Type / Period |
Applicable Zones / Uses |
Unionized ammonia (UIA) |
≤ 0.021 mg/l (as N) (2) |
Annual average |
At edge of ZID (1) |
≤ 0.035 mg/l (as NH3)
(3) |
4-day average |
At edge of ZID (1) |
|
≤ 0.233 mg/l (as NH3)
(3) |
1-hour average |
At edge of ZID (1) |
|
pH |
6.5 – 8.5, and change ≤ 0.2 |
≥ 90% of occasions |
At edge of ZID (1) |
Temperature |
change ≤ 2 oC |
≥ 90% of occasions |
At edge of ZID (1) |
Sulphide |
≤
0.02 mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Cyanide |
≤
0.005 mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Total residual chlorine
(TRC) |
≤ 0.013 mg/l |
Daily Maximum |
At edge of ZID (1) |
Surfactants |
≤
0.03 mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Copper |
≤
0.005 mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Nickel |
≤
0.005 mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Total chromium |
≤
0.05 mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Zinc |
≤
0.02 mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Mercury |
≤
0.00021 mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Arsenic |
≤
0.02 mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Phenol |
≤ 0.005
mg/l |
≥ 90% of occasions |
At edge of ZID (1) |
Acute Toxicity |
0.3 acute toxicity units (TUa) (derived from
LC50 values based on whole effluent toxicity tests) (4) |
One hour average condition not to exceed this
value |
At edge of ZID (1) |
(Source: EEFS
Report on Community Consultation for the Proposed Water Quality Criteria)
(1) For a surface plume, initial dilution is
defined as the dilution obtained at the centre line of the plume when the
sewage reaches the surface. For a trapped
plume, initial dilution is defined as the dilution obtained at the center line
of the plume where the plume reaches the maximum rise height when the vertical
momentum / buoyancy of the plume becomes zero.
(2)
The WQC for annual averaged UIA was derived with reference to the WQO under
WPCO which is expressed as N.
(3)
The WQC for 1-hour and 4-day averaged UIA was derived with reference to the
USEPA which is expressed as NH3.
(4) USEPA
Technical Support Document for Water Quality-Based Toxics Control (March 1991),
from which one acute Toxicity Unit (TUa) is defined as TUa = 100/LC50, where
LC50 = % of effluent which gives 50% survival of the most sensitive of the
range of species tested.
6.33 The chronic and acute toxicity criteria as shown in Table 6.8 and Table 6.9 respectively were used to assess the toxicity of the sewage effluent with reference to the results of the whole effluent toxicity test (WETT). Details of the WETT results are given in Table 6.33 to Table 6.36.
6.34 Table 6.10 compares the WQC derived under the EEFS (in Table 6.8 and Table 6.9) with the statutory requirements stipulated under the WPCO (in Table 6.1 to Table 6.6) as well as the water quality criteria specified by WSD (in Table 6.7). It should be noted that some parameters considered under the EEFS for far field water quality assessment as listed in Table 6.8, including PO4, TRC and chronic toxicity, are not controlled under the WPCO. In addition, most of the parameters considered under the EEFS for near field water quality assessment (in Table 6.9) are not controlled under the WPCO except for pH and temperature.
Table 6.10 Comparison
of Marine Water Quality Criteria for Water
Quality Assessment
Applicable zones/uses (1) |
Parameters |
EEFS WQC |
WPCO WQO |
WSD Criteria (2) |
||||
Value |
Type/ Period |
Value |
Type/ Period |
Value |
Type/ Period |
|||
Far Field Water Quality Criteria |
||||||||
Bathing waters |
E.coli |
≤ 180/ 100ml |
Geometric mean for the period from March to October |
≤ 180/ 100ml |
Geometric mean for the period from March to October |
Not applicable |
Not applicable |
|
Secondary
contact zones |
E.coli |
≤ 610/
100ml |
Annual
Geometric mean |
≤ 610/
100ml |
Annual
geometric mean |
Not
applicable |
Not
applicable |
|
WSD
flushing water intakes |
E.coli |
≤ 20,000/ 100ml |
≥ 90% of
occasions |
Not
specified |
Not
specified |
≤
20,000/ 100ml |
at all
times |
|
Seawater
intakes for industrial use |
E.coli |
≤
20,000/ 100ml |
≥ 90% of
occasions |
Not specified |
Not specified |
Not
applicable |
Not
applicable |
|
Mariculture
zones |
E.coli |
≤ 610/
100ml |
Geometric
mean |
≤ 610/
100ml |
Annual
geometric mean |
Not
applicable |
Not
applicable |
|
DO |
Depth- averaged |
≥ 5 mg/l |
≥90% of
occasions |
≥ 5 mg/l |
≥ 90% of
occasions |
Not
applicable |
Not
applicable |
|
within
2m from the seabed |
≥ 2 mg/l |
≥ 90% of
occasions |
≥ 2 mg/l |
≥ 90% of
occasions |
Not
applicable |
Not applicable |
||
Semi-enclosed
bays |
TIN |
≤0.1
mg/l (as N) |
Annual
mean |
Not specified |
Not specified |
Not
applicable |
Not
applicable |
|
PO4 |
≤ 0.01
mg/l (as P) |
Annual
mean |
Not specified |
Not specified |
Not
applicable |
Not
applicable |
||
Western
Buffer, Eastern Buffer, and Victoria Harbour WCZ (except fish spawning
ground) |
DO |
Depth- averaged |
≥ 4 mg/l |
≥ 90% of
occasions |
≥ 4 mg/l |
≥ 90% of
occasions |
Not
applicable |
Not
applicable |
within
2m from the seabed |
Not specified |
Not specified |
≥ 2 mg/l |
≥ 90% of
occasions |
Not
applicable |
Not
applicable |
||
TIN |
≤ 0.4
mg/l (as N) |
Annual
mean |
≤ 0.4
mg/l |
Annual
mean |
Not
applicable |
Not
applicable |
||
PO4 |
≤ 0.04
mg/l (as P) |
Annual
mean |
Not specified |
Not specified |
Not
applicable |
Not
applicable |
||
|
DO |
Depth- averaged |
≥ 5 mg/l |
Monthly
average |
≥ 4 mg/l |
≥ 90% of
occasions |
Not
applicable |
Not
applicable |
within
2m from the seabed |
Not specified |
Not specified |
≥ 2 mg/l |
≥ 90% of
occasions |
Not
applicable |
Not
applicable |
||
TIN |
≤ 0.2
mg/l (as N) |
Annual
mean |
≤ 0.1
mg/l |
Annual
mean |
Not
applicable |
Not
applicable |
||
PO4 |
≤ 0.02
mg/l (as P) |
Annual
mean |
Not specified |
Not specified |
Not
applicable |
Not
applicable |
||
Junk Bay
WCZ |
DO |
Depth-averaged |
≥ 4 mg/l |
≥ 90% of
occasions |
≥ 4 mg/l |
≥ 90% of
occasions |
Not
applicable |
Not
applicable |
within
2m from the seabed |
Not specified |
Not specified |
≥ 2 mg/l |
≥ 90% of
occasions |
Not
applicable |
Not
applicable |
||
TIN |
≤ 0.3
mg/l (as N) |
Annual
mean |
≤ 0.3
mg/l |
Annual
mean |
Not
applicable |
Not
applicable |
||
PO4 |
≤ 0.03
mg/l (as P) |
Annual
mean |
Not specified |
Not specified |
Not
applicable |
Not
applicable |
||
All WCZ |
DO at
any depth |
≥ 2 mg/l |
at all
times |
Not specified |
Not specified |
Not
applicable |
Not
applicable |
|
All WCZ |
UIA |
≤
0.021 mg/l (as N) |
Annual
mean |
≤
0.021 mg/l |
Annual
mean |
Not
applicable |
Not
applicable |
|
All WCZ |
TRC |
≤
0.008 mg/l |
Daily
maximum |
Not specified |
Not specified |
Not
applicable |
Not
applicable |
|
Near
Field Water Quality Criteria |
||||||||
At edge
of Initial dilution zone |
pH (3) |
6.5 –
8.5, and
change ≤ 0.2 |
≥ 90% of
occasions |
6.5 –
8.5, and
change ≤ 0.2 |
at all
times |
Not
applicable |
Not
applicable |
|
Temperature
(3) |
change ≤
2oC |
≥ 90% of
occasions |
change ≤
2oC |
at all
times |
Not
applicable |
Not
applicable |
(1) EEFS
did not derive WQC for the North Western WCZ.
(2) The
WSD criteria are applicable to the WSD flushing water intakes only.
(3) The
rest of the parameters listed in Table
6.9 are not presented for comparison as they are not controlled under the
WPCO and the WSD criteria.
6.35 The key differences between the WQC and the statutory requirements for far field water quality assessment are listed below:
·
The TIN criterion derived under
the EEFS for
·
The DO criterion derived under
the EEFS for
· A new DO requirement was derived under the EEFS that the minimum DO level at any location of the water control zone should not be less than 2 mg/l at all the times which is more stringent than the WPCO requirement that only the bottom DO (within 2 m from the seabed) should meet the value of 2 mg/l for 90% of occasions.
· New requirements were derived under the EEFS for TRC and PO4.
6.36 Moreover, additional requirements for UIA, metals, sulphide, TRC, surfactants and phenol etc. were derived under the EEFS for near field water quality assessment (Table 6.9). In particular, the UIA level should not be more than 0.035 mg/l and 0.233 mg/l for 4-day average and 1-hour average respectively based on the WQC. The acceptability of the water quality impacts have also been assessed based on both the WQO stipulated under the WPCO and the WQC established under the EEFS.
6.37 Chlorination is proposed as the disinfection technology for the HATS which would potentially lead to the formation of total residual chlorine (TRC) and chlorination by-products (CBP) in the effluent. Dechlorination will applied to minimize the discharge of TRC into the marine environment.
6.38
The proposed assessment
criteria for TRC are covered by the WQC derived under the EEFS. Based on a CBP selection exercise
conducted under the ADF study, 34 CBP compounds (as listed in Table 6.11 below) were identified for
impact assessment. The CBP
selection process is described in Appendix
7.1 of Section 7 under the human health risk assessment. Water quality
criteria/standards of
·
Rule 1: Criteria from
· Rule 2: Criteria/standards for protection of marine water/saltwater biota were preferred to that of freshwater or that without clear specification (e.g. protection of aquatic environment)
· Rule 3: Chronic criteria/standards specified with averaging time period were preferred and adopted whenever possible.
· Rule 4: National criteria/standards were preferred to local criteria/standards
· Rule 5: If more than one criteria/standards for the same chemical of concern (COC) satisfied the above rules, then the most stringent criterion/standard was adopted to provide conservatism
6.39 The relevant CBP criteria/standards reviewed are provided in Appendix 8.4 of Section 8 under the ecological risk assessment. A summary of the adopted values are given in Table 6.11.
Table 6.11 Marine Water Quality Criteria for CBP
CBP |
Water Quality Criteria (µg/l) |
Bromodichloromethane |
|
Bromoform |
|
Chloroform |
12b (marine water, annual
avg.) |
Dibromochloromethane |
|
Bromoacetic acid |
See Note 1 |
Chloroacetic acid |
See Note 1 |
Dibromoacetic acid |
See Note 1 |
Dichloroacetic acid |
See Note 1 |
Trichloroacetic acid |
See Note 1 |
Methylene chloride |
|
Carbon tetrachloride |
12 b (marine, annual avg.) |
Chlorobenzene |
|
1,1-dichloroethane |
See Note 1 |
1,2-dichloroethane |
10 b (marine, annual avg.) |
1,1-dichloroethylene |
|
1,2-dichloropropane |
See Note 1 |
Tetrachloroethylene |
|
1,1,1-trichloroethane |
100 b (marine annual avg.) |
1,1,2-trichloroethane |
100 b (marine annual avg.) |
Trichloroethylene |
10 b (marine annual avg.) |
2-chlorophenol |
50 b (marine annual avg.) |
2,4-dichlorophenol |
20 b (marine annual avg.) |
p-chloro-m-cresol |
40 b (marine annual avg.) |
Pentachlorophenol |
2 b (marine annual avg.) |
2,4,6-trichlorophenol |
See Note 1 |
Bis(2-chloroethoxy)methane |
See Note 1 |
1,4-dichlorobenzene |
See Note 1 |
Hexachlorobenzene |
0.03 b (marine, annual avg.) |
Hexachlorocyclopentadiene |
See Note 1 |
Hexachloroethane |
See Note 1 |
1,2,4-trichlorobenzene |
|
Alpha-BHC |
See Note 1 |
Beta-BHC |
|
Gamma-BHC |
|
Notes:
1 No criteria/standard was found from literature
review.
a USEPA. www.epa.gov/waterscience/standards/states
b Cole
S.,
c The Canadian
Council of Ministers of the Environment (2005).
d USEPA (2004).
6.40 Environment, Transport and Works Bureau (ETWB) Technical Circular Works (TCW) No. 34/2002 “Management of dredged/excavated sediment” sets out the procedure for seeking approval to dredge / excavate sediment and the management framework for marine disposal of dredged / excavated sediment. This Technical Circular outlines the requirements to be followed in assessing and classifying the sediment. Sediments are categorized with reference to the Lower Chemical Exceedance Level (LCEL) and Upper Chemical Exceedance Level (UCEL), as follows:
Category L Sediment with all contaminant levels not
exceeding the LCEL. The material
must be dredged, transported and disposed of in a manner that minimizes the
loss of contaminants either into solution or by suspension.
Category M Sediment with any one or more contaminant levels
exceeding the LCEL and none exceeding the UCEL. The material must be dredged and
transported with care, and must be effectively isolated from the environment
upon final disposal unless appropriate biological tests demonstrate that the
material will not adversely affect the marine environment.
Category H Sediment with any one or more contaminant
levels exceeding the UCEL. The
material must be dredged and transported with great care, and must be
effectively isolated from the environment upon final disposal.
6.41
The
sediment quality criteria for the classification of sediment are presented in Table 6.12.
Table 6.12 Sediment Quality Criteria for the Classification of Sediment
Contaminants |
LCEL |
UCEL |
Heavy
Metal (mg/kg dry weight) |
||
Cadmium
(Cd) |
1.5 |
4 |
Chromium
(Cr) |
80 |
160 |
Copper
(Cu) |
65 |
110 |
Mercury
(Hg) |
0.5 |
1 |
Nickel
(Ni) |
40 |
40 |
Lead
(Pb) |
75 |
110 |
Silver
(Ag) |
1 |
2 |
Zinc
(Zn) |
200 |
270 |
Metalloid
(mg/kg dry weight) |
||
Arsenic |
12 |
42 |
Organic-PAHs
(µg/kg dry weight) |
||
PAHs
(Low Molecular Weight) |
550 |
3160 |
PAHs
(High Molecular Weight) |
1700 |
9600 |
Organic-non-PAHs
(µg/kg dry weight) |
||
Total
PCBs |
23 |
180 |
Source: Appendix A of ETWB TCW No. 34/2002 Management of Dredged
/ Excavated Sediment
Note: LCEL – Lower Chemical
Exceedance Level
UCEL
– Upper Chemical Exceedance Level
6.42
The
marine water quality monitoring data
routinely collected by EPD were used to establish the baseline
condition. Marine
water quality monitoring is conducted by EPD on a monthly basis. Water samples
are taken at three water depths, namely,
6.43
As the HATS Stage I was
commissioned in late 2001, the data shown in Table 6.13 to Table 6.17
represent the situation after the commissioning of HATS Stage I. The relevant WQO and WQC are included in
Table 6.13 to Table 6.17 for comparison.
It should be noted that WQO for E.coli
is only applicable to stations SM10 and SM11 because only these two selected
stations are located in secondary contact recreation subzones. Descriptions of
the baseline conditions for individual WCZ provided in the subsequent sections
are extracted from the EPD’s report “Marine Water Quality Monitoring in Hong Kong
2006” issued in 2007 which contains the latest information published by EPD on
marine water quality at the moment of preparing this EIA report.
6.44
Due to the effect of the
6.45
Over the years that the EPD has
monitored this WCZ, it has recorded long-term increases in ammonia nitrogen and
TIN at its stations along the
6.46
The levels of E.coli at stations NM1,
NM2, NM3 and NM5 located nearer the local effluent discharges (from SCISTW,
Pillar Point and San Wai Sewage Treatment Works) were generally higher compared
with other stations. Except for the
two western-most stations (nearest to the
6.47
The Central Waters refers to
the
6.48
Commissioning of HATS Stage 1
in late 2001 has brought large and sustained improvements to the water quality
of the Central Waters, especially that of the eastern
6.49 However, water quality improvements were less noticeable in the western harbour area which was still subject to the sewage discharges from local PTW (Central, Wan Chai West and Wan Chai East). Because the Stage 1 effluent has not been disinfected, E.coli levels rose in the vicinity of the SCISTW outfall. As a result, the western harbour (Western Buffer WCZ and northern part of the Southern WCZ), including the water around Tsing Yi and the Tsuen Wan beaches, has experienced increased E.coli counts after commissioning of HATS Stage 1.
6.50
In 2006, the marked water
quality improvements in eastern
6.51
The water quality in Western Buffer WCZ was
largely stable in 2006 as compared to that in 2005 except that there were some
decreases of DO at the western-most station (WM4) causing a non-compliance with
the WQO for DO at this station.
This was due to the lower DO level at the western-most station closest
to the
6.52
The
water quality at
6.53
The Southern Waters consist of
one large WCZ, the Southern WCZ, which covers an area located to the south of
6.54
In terms of WQO compliance, as
the TIN levels at most stations in the WCZ were relatively high, especially in
the summer, they exceeded the WQO for this parameter. In the 1990s, the WQO
compliance rate for TIN was consistently below 20%, and even in 2006 the only
two stations (namely SM1 and SM19 as shown in Figure
6.1) that complied with the WQO were situated in the far eastern side
of the WCZ. When compared to the
less stringent WQC for TIN, about half of the stations still failed to comply
with the TIN standard. Similar TIN levels have been recorded since 1991. In
2006, full compliance with WQO (for DO and UIA) and WQC (for DO, UIA and PO4)
was achieved in the
6.55
E.coli levels were generally low and stable across the WCZ. However, in
some places the bacteriological water quality was affected by coastal sewage
discharges. For example, long-term increases in E.coli levels were recorded at stations (SM7 and SM9) near the
discharge from the SCISTW, in connection with the implementation of HATS Stage
1. Only four stations (SM7, SM9, SM10 and SM11) closest
to the SCISTW outfall were selected for presentation in Table 6.17.
Table 6.13 Baseline
Water Quality Condition for North
Parameter |
Lantau Island (North) |
|
Pillar Point |
|
Chek Lap Kok |
WPCO WQO (in marine waters) |
EEFS WQC (in marine waters) |
||
NM1 |
NM2 |
NM3 |
NM5 |
NM6 |
NM8 |
||||
Temperature (oC) |
23.7 |
23.8 |
23.7 |
24.0 |
24.0 |
23.8 |
Not more than 2 oC in
daily temperature range |
Not available |
|
Salinity |
29.6 (22.2 – 33.1) |
28.6 |
29.4 |
27.2 |
26.0 |
27.6 |
Not to cause more than 10% change |
Not available |
|
Dissolved Oxygen
(DO) (mg/l) |
Depth average |
6.3 |
6.5 |
6.3 |
6.3 |
6.7 |
6.8 |
Not less than 4 mg/l for 90% of the samples |
Not available |
Bottom |
5.9 |
6.3 |
6.1 |
5.9 |
6.6 |
6.7 |
Not less than 2 mg/l for 90% of the samples |
Not available |
|
Dissolved Oxygen (DO) (%
Saturation) |
Depth average |
87 |
90 |
88 |
87 |
92 |
94 |
Not available |
Not available |
Bottom |
82 |
88 |
85 |
83 |
92 |
94 (63 – 118) |
Not available |
Not available |
|
PH |
7.9 |
7.9 |
7.9 |
7.9 |
7.9 |
7.9 |
6.5 - 8.5 (± 0.2 from natural range) |
Not available |
|
Secchi disc Depth (m) |
1.6 (0.9 – 2.5) |
1.7 |
1.5 |
1.4 |
1.2 |
1.3 |
Not available |
Not available |
|
Turbidity (NTU) |
20.3 |
18.0 (5.8 – 75.3) |
18.7 (8.1 – 55.8) |
25.8 |
22.9 |
23.8 |
Not available |
Not available |
|
Suspended Solids (SS)
(mg/l) |
7.4 |
6.4 |
8.1 |
15.7 |
12.6 |
15.8 |
Not more than 30% increase
|
Not available |
|
5-day Biochemical Oxygen
Demand (BOD5) (mg/l) |
0.6 |
0.6 (0.2 – 1.0) |
0.7 |
0.7 |
0.7 |
0.7 |
Not available |
Not available |
|
Ammonia Nitrogen (NH3-N)
(mgN/l) |
0.15 |
0.15 |
0.15 |
0.22 |
0.17 |
0.10 (0.01 – 0.35) |
Not available |
Not available |
|
Unionised Ammonia (UIA) (mgN/l) |
0.005 |
0.005 |
0.005 |
0.008 |
0.006 |
0.004 |
Not more than 0.021 mg/l
for annual mean |
Not available |
|
Nitrite Nitrogen (NO2-N)
(mgN/l) |
0.056 |
0.064 |
0.064 |
0.091 |
0.093 |
0.066 (0.009 – 0.150) |
Not available |
Not available |
|
Nitrate Nitrogen (NO3-N)
(mgN/l) |
0.23 |
0.28 |
0.28 |
0.37 |
0.39 |
0.28 |
Not available |
Not available |
|
Total Inorganic
Nitrogen (TIN) (mgN/l) |
0.43 |
0.49 |
0.50 |
0.67 |
0.66 |
0.44 |
Not more than 0.5 mg/l
for annual mean |
Not available |
|
Total Nitrogen (TN)
(mgN/l) |
0.60 |
0.65 |
0.66 |
0.86 |
0.84 |
0.62 |
Not available |
Not available |
|
Orthophosphate
Phosphorus (PO4) (mgP/l) |
0.03 |
0.03 |
0.03 |
0.03 |
0.02 |
0.02 |
Not available |
Not available |
|
Total Phosphorus (TP)
(mgP/l) |
0.05 |
0.04 |
0.05 |
0.07 |
0.05 |
0.04 |
Not available |
Not available |
|
Chlorophyll-a (µg/L) |
3.6 |
2.8 |
3.3 |
4.2 |
3.9 |
3.5 (1.3 – 14.7) |
Not available |
Not available |
|
E.coli (cfu/100 ml) |
1100 |
470 |
500 |
900 |
64 |
5 |
Not available |
Not available |
|