6. Water Quality Impact
6.1. Introduction
6.2. Environmental
Legislation, Policies, Plans, Standards, and Guidelines
6.3. Description of
the Environment
6.4. Water Sensitive
Receivers (WSRs)
6.5. Description of
Assessment Methodologies
6.6. Identification,
Prediction and Evaluation of Environmental Impacts
6.7. Mitigation of
Adverse Environmental Impacts
6.8. Definition and
Evaluation of Residual Environmental Impact
6.9. Identification
and Evaluation of Operational Phase Environmental Impact
6.10. Environmental Monitoring and
Audit
6.11. Conclusion and Recommendation
This chapter presents the assessment of potential water quality impacts which may arise from the Project. Water quality impacts associated with decommissioning, site formation, temporary drainage works and road works of the west portion and southern edge of the Middle Ash Lagoon are assessed.
6.2.1. 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 EIAO (TM) have provided details of 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
6.2.2. The Water Pollution Control Ordinance (WPCO) provides the major statutory framework for the protection and control of water quality in Hong Kong. According to the WPCO and its subsidiary legislation, Hong Kong water is divided into ten Water Control Zones (WCZ). Corresponding statements of Water Quality Objectives (WQO) are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in WCZ based on the beneficial uses. With reference to the EIA Study Brief (No. ESB-243/2012), the Study Area for this water quality assessment covers Deep Bay WCZ (refer to Figure 6.1). The corresponding WQOs are listed in Table 6.1
Table 6.1 Summary of Water
Quality Objectives for Deep Day 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 2m of the seabed |
Not less than 2.0 mg/L for 90% of samples |
Outer Marine Subzone excepting Mariculture
Subzone |
|
Dissolved Oxygen (DO) within 1m below surface |
Not less than 4.0 mg/L for 90% of samples |
Inner Marine Subzone excepting Mariculture
Subzone |
|
Not less than 5.0 mg/L for
90% of samples |
Mariculture Subzone |
|
|
Depth-average DO |
Not less than 4.0 mg/L for 90% of samples |
Outer Marine Subzone excepting Mariculture
Subzone |
|
Not less than 4.0 mg/L |
Yuen Long &Kam Tin
(Upper and Lower) Subzones, Beas Subzone, Indus Subzone, Ganges Subzone,
Water Gathering Ground Subzones and other inland water of the Zone |
|
|
5-Day Biochemical Oxygen Demand (BOD5) |
Not to exceed 3 mg/L |
Yuen Long &Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone,
Ganges Subzone and Water Gathering Ground Subzones |
|
Not to exceed 5 mg/L |
Yuen Long &Kan Tin
(Lower) Subzone and other inland waters |
|
|
Chemical Oxygen Demand (COD) |
Not to exceed 15 mg/L |
Yuen Long &Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone,
Ganges Subzone and Water Gathering Ground |
|
Not to exceed 30 mg/L |
Yuen Long &Kam Tin
(Lower) Subzone and other inland waters |
|
|
Parameters |
Objectives |
Sub-Zone |
|
pH |
To be in the range of 6.5 – 8.5, change due to waste discharges not to
exceed 0.2 |
Marine waters excepting Yung Long Bathing Beach Subzone |
|
To be in the range of 6.5 –
8.5 |
Yuen Long &Kam Tin (Upper
and Lower) Subzones, Beas Subzone, Indus Subzone, Ganges Subzone 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, change due to waste discharge not to exceed 0.5 |
Yuen Long Bathing Beach
Subzone |
|
|
Salinity |
Change due to waste discharge not to exceed 10% of ambient |
Whole zone |
|
Temperature |
Change due to waste discharge not to exceed 2oC |
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 |
|
Not to cause the annual
median to exceed 20 mg/L |
Yuen Long & Kam Tin (Upper
and Lower) Subzones, Beas Subzone, Ganges Subzone, Indus Subzone, Water
Gathering Ground Subzones and other inland waters |
|
|
Unionized Ammonia (UIA) |
Annual mean not to exceed 0.021 mg/L as unionized form |
Whole zone |
|
Parameters |
Objectives |
Sub-Zone |
|
Nutrients |
Shall not cause excessive algal growth |
Marine waters |
|
Total Inorganic Nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen not to exceed 0.7 mg/L |
Inner Marine Subzone |
|
Annual mean depth-averaged inorganic
nitrogen not to exceed 0.5 mg/L |
Outer Marine Subzone |
|
|
Bacteria |
Not to exceed 610 per 100 ml, calculated as the geometric mean of all samples
collected in one calendar year |
Secondary Contact Recreation Subzones and Mariculture
Subzones |
|
Should be zero per 100 ml,
calculated as the running median of the most recent 5 consecutive samples
taken between 7 and 21 days. |
Yuen Long &Kam Tin
(Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water
Gathering Ground Subzones |
|
|
Not to exceed 180 per 100ml,
calculated as the geometric mean of the collected from March to October
inclusive in one calendar year. Samples should be taken at least 3 times in a
calendar month at intervals from 3 to14
days. |
Yuen Long Bathing Beach
Subzone |
|
|
Not to exceed 1000 per 100 ml,
calculated as the running median of the most recent 5 consecutive samples
taken at intervals of between 7 and 21 days. |
Yuen Long &Kam Tin
(Lower) Subzone and other inland waters |
|
|
Parameters |
Objectives |
Sub-Zone |
|
Colour |
Not to exceed 30 Hazen units |
Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone,
Ganges Subzone and Water Gathering Ground Subzone |
|
Not to exceed 50 Hazen units |
Yuen Long & Kam Tin
(Lower) Subzone and other inland waters |
|
|
Turbidity |
Shall not reduce light transmission substantially form the normal
level |
Yuen Long Bathing Beach Subzone |
|
Phenol |
Quantities shall not be sufficient to produce a specific odour or more
than 0.05 mg/L as C6H5OH |
Yuen Long Bathing Beach Subzone |
|
Toxins |
Should not cause a risk to any beneficial uses of the aquatic
environment |
Whole zone |
|
Should not attain such
levels as to produce toxic carcinogenic, mutagenic or teratogenic effects in humans,
fish or any other aquatic organisms. |
Whole zone |
Source:
Statement of Water Quality Objectives (Deep Bay Water Control Zone)
6.2.3. 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 Water (TM-DSS), issued under Section 21 of WPCO, give guidance on permissible effluent discharges based on the types of receiving water (foul sewers, storm water drains, inland and coastal water). The limits control the physical, chemical and microbial quality of effluent. Any effluent from the proposed construction activities must comply with the standards for effluent discharged into the foul sewers, inshore waters and marine waters of the Deep Bay WCZ provided in TM-DSS.
6.2.4. To comply with EPD’s No Net Increase in Pollution Loads Requirement in Deep Bay, the Project aims to provide protection to the inland and marine water quality of the Deep Bay WCZ. The policy requires that developments within Deep Bay catchment area not to result in an increase in pollution loads in the inland and marine waters.
6.2.5. The Practice Note for Professional Persons on Construction Site Drainage (ProPECC PN 1/94) was issued by the EPD to provide environmental guidelines for handling and disposal of construction site discharges. Practices as outlined in the ProPECC PN 1/94 should be followed during the construction phase in order to minimize the water quality impact due to construction site drainage. Moreover, other ProPECC Notes including the ProPECC PN 5/93 Drainage Plans (subject to Comment by the Environmental Protection Department) would also be considered.
6.3.1. The Subject Site is currently located in the west portion of the Middle Ash Lagoon at Tsang Tsui, Tuen Mun. The Site is currently operated by Castle Peak Power Company Limited (CAPCO) for the placement of Pulverised Fuel Ash (PFA), a by-product of burned coal from the adjacent Castle Peak Power Station. The ash lagoon is formed by reclamation. As detailed in Section 2.3.2, the majority of the Subject Site was filled up to about +10.5mPD with PFA by CAPCO which shall be maintained until site handover by CAPCO currently planned in the first quarter of 2015. A man-made water channel of approximately 30m wide is located to the north of the Subject Site, part of water channel falling into the Subject Site. Figure 1.1 depicts the existing water channel condition. As advised by CAPCO, the 30m water channel will be maintained in the north for discharging the surface water to the existing outlet tower.
6.3.2. The decommissioning of the west portion of the Middle Ash Lagoon at Tsang Tsui in Tuen Mun has the potential to affect the inland watercourse within Deep Bay WCZ. There is one EPD routine water quality monitoring station (DB8) along the Tsang Kok Stream (see Figure 6.1). A summary of the published monitoring data (in 2012) for this station is presented in Table 6.2.
Table 6.2 Summary of Water Quality
Monitoring Results for Tsang Kok Stream in 2012
|
Parameter |
Unit |
Tsang
Kok Stream (DB8) |
|
Dissolved
oxygen |
mg/L |
8.0 (6.8 – 10.0) |
|
pH |
|
7.5 (7.2 – 8.3) |
|
Suspended
solids |
mg/L |
5
(2 – 36) |
|
5-day
Biochemical Oxygen Demand |
mg/L |
<1
(<1 – 1) |
|
Chemical
Oxygen Demand |
mg/L |
3
(<2 – 8) |
|
Oil
& grease |
mg/L |
<0.5
(<0.5 – <1.2) |
|
Faecal
coliforms |
mg/L |
1700 (110 – 5400) |
|
E.
coli |
cfu/100
mL |
130
(38 – 1200) |
|
Ammonia-nitrogen |
mg/L |
0.04 (0.02 – 0.08) |
|
Nitrate-nitrogen |
mg/L |
1.19 (0.13 – 8.10) |
|
Total
Kjeldahl Nitrogen |
mg/L |
0.14 (0.05 – 0.46) |
|
Ortho-phosphate |
mg/L |
<0.01
(<0.01 – <0.01) |
|
Total phosphorus |
mg/L |
<0.02
(<0.02 – 0.04) |
|
Total sulphide |
mg/L |
<0.02
(<0.02 – <0.02) |
|
Aluminium |
µg/L |
120 (<50 – 350) |
|
Cadmium |
µg/L |
<0.1
(<0.1 – <0.1) |
|
Chromium |
µg/L |
<1
(<1 – <1) |
|
Copper |
µg/L |
<1
(<1 – 3) |
|
Lead |
µg/L |
1
(<1 – 10) |
|
Zinc |
µg/L |
10 (<10 – <40) |
|
Flow |
L/s |
NM |
Notes:
(i).Data source:
River Water Quality in Hong Kong in 2012
(ii).Data
presented are in annual medians of monthly samples; except those for faecal
coliforms and E. coli which are in annual geometric means.
(iii).Figures
in brackets are annual ranges.
(iv).NM indicates
no measurement taken.
(v).cfu – colony forming unit.
(vi).Values
at or below laboratory reporting limits are presented as laboratory reporting
limits (Appendix B).
(vii).Equal values for annual medians (or geometric
means) and ranges indicate that all data are the same as or below laboratory
reporting limits.
6.3.3. River water quality monitoring data for the year 2012 at station DB8 showed the overall compliance rate of the Tsang Kok Stream with the WQOs was 100%, with full compliance with the WQOs of pH, suspended solids, dissolved oxygen, COD and BOD. The water quality of this minor stream was reported to be excellent and free from point source pollution.
6.3.4. The EPD water quality monitoring stations DM4 and DM5 located at the Outer Deep Bay WCZ are the nearest monitoring stations in the vicinity of the Project area (see Figure 6.1). Monitoring data collected at the Outer Deep Bay in 2012 is extracted from the EPD’s publication 2012 Marine Water Quality in Hong Kong, which is the latest information published on the EPD website at the moment of preparing of this Report. A summary of the monitoring data (in 2012) for these stations are presented in Table 6.3.
Table 6.3 Summary of Marine Water
Quality in Outer Deep Bay in 2012
|
Parameter |
Outer Deep Bay |
WPCO WQOs (in marine waters) |
||
|
DM4 |
DM5 |
|||
|
Temperature
(oC) |
24.7
(15.9 – 30.9) |
24.3 (16.5
– 29.2) |
Change due to waste
discharge not to exceed 2oC |
|
|
Salinity |
20.7
(9.2 – 29.8) |
23.9
(12.9 – 31.1) |
Change due to waste
discharge not to exceed 10% of ambient |
|
|
Dissolved
Oxygen (DO)
(mg/L) |
|
6.6
(4.3 – 8.3) |
6.4
(4.4 – 7.9) |
Not
less than 4.0 mg/L for 90% of samples |
|
Bottom |
6.5 (4.3 – 8.1) |
6.1 (3.3 – 7.6) |
Not less than 2.0 mg/L for 90% of
samples |
|
|
Dissolved
Oxygen (DO) (%
saturation) |
|
88
(64 – 117) |
87
(65 – 111) |
Not available |
|
Bottom |
88 (63 – 114) |
83 (48 – 101) |
Not available |
|
|
pH |
7.5
(6.8 – 7.7) |
7.6
(7.4 – 7.8) |
To be in the range
of 6.5 – 8.5, change due to waste discharges not to exceed 0.2 |
|
|
Secchi disc
Depth (m) |
1.3
(1.0 – 1.5) |
1.4
(1.0 – 1.6) |
Not available |
|
|
Turbidity
(NTU) |
8.6
(2.2 – 15.0) |
9.6
(1.7 – 25.2) |
Not available |
|
|
Suspended
Solids (mg/L) |
11.5
(2.5 – 32.0) |
8.6
(1.8 – 22.0) |
Not to raise the
ambient level by 30% caused by waste discharges and shall not affect aquatic
communities |
|
|
5-day
Biochemical Oxygen Demand (BOD5) (mg/L) |
0.6
(0.2 – 1.3) |
0.5
(0.1 – 1.2) |
Not available |
|
|
Ammonia
Nitrogen (NH3-N) (mg/L) |
0.256
(0.070 – 0.560) |
0.185
(0.065 – 0.437) |
Not available |
|
|
Unionized
Ammonia (UIA) (mg/L) |
0.004
(0.002 – 0.007) |
0.003
(0.001 – 0.006) |
Annual mean not to
exceed 0.021 mg/L as unionized form |
|
|
Nitrite
Nitrogen (NO2-N) (mg/L) |
0.183
(0.043 – 0.355) |
0.129
(0.017 – 0.280) |
Not available |
|
|
Nitrate
Nitrogen (NO3-N) (mg/L) |
1.060
(0.365 – 1.950) |
0.824
(0.260 – 1.570) |
Not available |
|
|
Total
Inorganic Nitrogen (TIN) (mg/L) |
1.50
(0.71 – 2.76) |
1.14
(0.38 – 2.16) |
Annual mean
depth-averaged inorganic nitrogen not to exceed 0.5 mg/L |
|
|
Total
Kjeldahl Nitrogen (TKN) (mg/L) |
0.48
(0.25 – 0.85) |
0.42
(0.25 – 0.74) |
Not available |
|
|
Total
Nitrogen (TN) (mg/L) |
1.72
(0.91 - 3.04) |
1.37
(0.56 – 2.36) |
Not available |
|
|
Orthophosphate
Phosphorus (Ortho P) (mg/L) |
0.050
(0.026 – 0.089) |
0.036
(0.020 – 0.046) |
Not available |
|
|
Total
Phosphorus (TP) (mg/L) |
0.07
(0.05 – 0.12) |
0.07 (0.05 – 0.22) |
Not available |
|
|
Silica
(as SiO2) (mg/L) |
4.14
(1.30 – 8.55) |
3.26
(1.03 – 6.73) |
Not available |
|
|
Chlorophyll-a
(µg/L) |
2.5
(0.4 – 6.7) |
2.1
(0.5 – 6.8) |
Not available |
|
|
E.
coli (cfu/ 100 mL) |
55
(1
– 420) |
290
(120 – 810) |
Not available |
|
|
Faecal
Coliforms (cfu/100 mL) |
120
(6
– 580) |
530
(230 – 1400) |
Not available |
|
Note:
(i).
Data
source: 2012 Marine Water Quality in Hong Kong
(ii).
Unless
otherwise specified, data presented are depth-averaged (A) values calculated by
taking the means of three depths: Surface (S), Mid-depth (M), Bottom (B).
(iii).
Data
presented are annual arithmetic means of depth-averaged results except for E.
coli and faecal coliforms which are annual geometric means.
(iv).
Data
in brackets indicate the ranges.
6.3.5. According to the 2012 Marine Water Quality in Hong Kong, most of the parameters were found in compliance with the WQO. Only non-compliance was recorded with the WQO for total inorganic nitrogen which was reported to be result of a persistent nutrient pollution problem.
6.3.6. The Marine Water Quality Report shows that the water quality trend for the streams and marine water concerned (DB8, DM4, DM5) has been stable since over the past 5 years. The future water quality of the water sensitive receivers is expected to remain stable in the future.
6.3.7. The construction phase of the Project does not involve any alteration of water courses, natural streams, ponds, wetlands, catchment types etc.
6.3.8. There are no planned water system and existing water system within the Subject Site.
6.4.1. The Subject Site is located at the west portion of the Middle Ash Lagoon at Tsang Tsui in Tuen Mun. With reference to Section 3.4.6.2 of the EIA Study Brief (No.ESB-243/2012) for the Project, the assessment area for decommissioning and construction water quality impact assessment includes all areas within 500m from the boundary of the Subject Site. Three water sensitive receivers have been identified within 500m from the site boundary of the Project.
6.4.2. Two moderate sized streams are located at the southern part of ash lagoon and discharged into a tidal channel to the east of the ash lagoon area (refer to Figure 6.1). The lower reaches of stream W1 are routed through a man-made, tidally influenced channel to the south of the ash lagoons. Although the substrate of this channel is natural, the banks have been lined with geo-textile matting. The second stream (stream W2) drains into the tidal channel from the southeast. The section of stream flowing through the existing WENT Landfill site has been wholly channelized with concrete.
6.5.1. The criteria and guidelines for assessing water quality impacts as stated in Annex 6 and 14 of the EIAO (TM) have been followed. The assessment area as specified in the EIA Study Brief (No. ESB-243/2012) covers an area within 500m of the Subject Site boundary, and all the relevant water sensitive receivers, nearby watercourses and the associated water systems in Deep Bay WCZ.
6.5.2. The water quality sensitive receivers which may be affected by the construction activities have been identified, and potential sources of water quality impact which may arise during the construction of the Project are also described. All the identified sources of potential water quality impact were then evaluated followed by determining the impact significance. The need for mitigation measures to reduce any identified adverse impacts on water quality to acceptable levels was determined.
6.5.3. Potential pollution sources including point discharges and non-point sources to surface water run-off, sewage and polluted discharge generated from the Project have been identified. The identified pollution sources have been evaluated to define the significance of impact.
6.6.1. Potential sources of key water quality impact associated with the decommissioning, site formation, slope stabilization works, construction of surface drainage outfall, temporary drainage works and road works of the Project have been identified and include:
· Drainage and construction site runoff during decommissioning;
· Construction of retaining structure for slope stabilization, construction of surface drainage outfall, and general construction site activities;
· Sewage effluent by on-site workforce;
· Accidental Spillage; and
· Release of PFA Leachate from Ash Lagoon
6.6.2. The drainage works in the Project mainly involve the setup of perimeter drainage channels. Runoff from the construction work areas at the west portion of the Tsang Tsui Middle Ash Lagoon may increase sediment loads and contaminants to the nearby water sensitive receivers. The proposed location for water discharge during construction is depicted in Figure 6.1.
6.6.3. Potential sources of pollution from site drainage include:
· Runoff and erosion from exposed PFA/soil surfaces, earth working areas and stockpiles;
· Wash water from dust suppression sprays;
· Fuel and lubricants from construction vehicles and mechanical equipment maintenance area; and
· Release of cement materials during rain wash.
6.6.4. As a good practice, mitigation measures should be implemented to control construction site runoff and drainage from the Project, and to prevent runoff and drainage water with high levels of suspended solids from entering the nearby water bodies. Moreover, as a part of construction works, general fill will be deposited and compacted to about 1m thick above the existing PFA platform to prevent PFA at the Subject Site from being significantly disturbed due to any future development.
6.6.5. Adoption of the relevant guidelines and good site practices during the construction phase would reduce the potential water quality caused by construction runoff and contaminated PFA surface runoff. As such, adverse water quality impacts on the nearby WSRs within the Deep Bay WCZ would not be anticipated. The construction site drainage would be collected by the temporary drainage system installed by the Contractor and then treated on-site before discharge. The Contractor would be required to obtain a license from EPD for discharge to the channel located south of the Subject Site.
6.6.6. As detailed in Section 2.6, the decommissioning works in the Project will mainly involve site formation works. On-site construction activities may cause water pollution from the following:
· Uncontrolled discharge from debris and rubbish such as packaging, construction materials and refuse; and
· Spillages of liquids stored on-site, such as oil, diesel and solvents etc, are likely to result in water quality impacts if they enter water bodies.
6.6.7. The existing access road shall be upgraded to suit the development and would involve the upgrading of pavement through decommissioning. The minor reinstating works will involve removing of the weeds, repairing the cracked surface, filling up holes by concrete and installing gates and fence of STF boundary. Road widening works and slope stabilization works shall be carried out through decommissioning of approximately 0.8 hectares of the Construction Access Road (South). PFA levelling and filling works at the Construction Access Road Area would cause possible surface runoff to the existing stream located at the south of the Subject Site.
6.6.8. Construction of retaining structure for slope stabilization at the embankment may cause possible surface runoff to the man-made water channel located at the north of the Subject Site. Associated effects which may arise include increased suspended solids concentrations and contaminants in the water receiving bodies. As the construction will be carried out during dry season, the adverse impact is considered as limited.
6.6.9. Installation of surface drainage outfall located at the north-west corner of the Subject Site involves the construction of pipe racks in the 30m wide water channel as support to the 1.05m in diameter drainage pipe, which crosses the existing water channel and connected to a sand trap located on the existing sea wall.
6.6.10. Anchors and pipe rack shall be erected on strip of newly filled aggregate cross the man-made water channel. No dredging activities will be required, no increase of turbidity and introduce contaminants into the man-made water channel is anticipated.
6.6.11. The activities mentioned above are likely to result in water quality impacts if they enter the nearby streams or drainage channels near the works area. However, general construction activities are unlikely to have adverse impacts on water quality provided that the site is well maintained and good construction practices are well implemented.
6.6.12. Sewage effluent might be generated from the workforce during the decommissioning and construction phase. All sewage discharge is subject to control and illegal discharge of untreated sewage is prohibited. Providing a suitable sewage collection system and treatment facilities on site will avoid the potential sewage pollution, and it is unlikely that sewage generated from the site would have a significant impact on water quality.
6.6.13. For any wastewater generated within the construction site, a discharge license should be obtained from EPD under the WPCO before making any discharge. Wastewater treatment facilities should be provided on site in order to treat the effluent to meet the required water quality standards before discharge.
6.6.14.
However, this temporary sewage
can be adequately treated by interim sewage treatment facilities, such as
installing portable chemical toilets within the construction site. It is
anticipated that sewage generation during the construction phase of the Project
will not cause a water pollution problem once all required measures have been
implemented.
6.6.15. Site drainage system and facilities will be well maintained and good construction practices shall ensure that oil, fuels and solvents are managed, stored and handled properly and do not enter the nearby streams. With good site practices and proper implementation of mitigation measures, no adverse water quality impacts are expected.
6.6.16. The decommissioning will be located at the west portion of the Middle Ash Lagoon at Tsang Tsui. The Middle Lagoon, along with the East and West Lagoons, were constructed in the 1980s. The lagoons have been used by China Light and Power (CLP) Company Limited for the storage of pulverized fuel ash (PFA), a by-product of the coal burning. PFA is a fine, grey powder formed from rock particles contained within coal, consisting mainly of silica, aluminium and iron oxide.
6.6.17. The Middle Lagoon along with the West Ash Lagoon is still used for storage of PFA. In which, the Middle Lagoon has been used as part of its water collection and conservation system by CLP since 1997. The ash lagoon area is underlain by marine deposits. The approximate top level of the Subject Site is about +10.5mPD. The ash lagoon PFA is underlain by marine deposits thickness varying from 3.5m to 5m with marine deposit (MD) base levels varying from -5mPD to -7mPD. The bottom level of the seawall is in the range of -8mPD to -4.4mPD with alluvium as the founding stratum.
6.6.18. Marine clay comprising the original seabed forms the base of the lagoon. A seawall, composed of layers of woven geotextile material sandwiching an impermeable layer of injected bentonite is located at the north of Middle Lagoon, outside the embankment.
6.6.19. Referring to Section 2.6, construction works associated with the Project mainly involve site formation, excavation and deposition works at the Subject Site and filling and levelling works at the Construction Access Road Area. Thus the seawall liner would not be disturbed during the entire construction phase and PFA leachate is unlikely to be released from ash lagoon into the aquatic environment.
6.6.20. No below ground construction works is anticipated during the decommissioning of site, site formation works and construction of drainage and road works. As such, no groundwater contamination would be expected.
6.6.21. The seawall liner shall not be disturbed during the entire construction phase. The site conditions of Middle Ash Lagoon during decommissioning and construction phases would not be much different from the existing conditions. Foundation activities would not be present during the entire construction and the base of the Lagoon would not be disturbed. Hence, the leakage through the base of the Lagoon would not be significant and PFA leachate in the Middle Ash Lagoon is unlikely to cause unacceptable water quality impact. As the ash would remain in the Middle Ash Lagoon and would not be disposed of, detailed ecotoxicological assessment and additional toxicity tests are considered not necessary.
6.6.22. The potential cumulative impacts due to other related concurrent and planned projects activities or pollution sources within the area around the alignment have been assessed and mitigation measure proposed where required ensuring that any water quality impacts would be controlled to acceptable levels. The following concurrent projects are identified from a water quality perspective:
· The operation of Sewerage Treatment Facility
· The planned WENT Landfill Extension (WLES)
· The existing WENT landfill
6.6.23. The operational phase of the Sludge Treatment Facility will be concurrent with the construction phase of the Project. During the operation phase of the STF, wastewater will be generated from sanitary facilities used by plant personnel, commercial activities and sludge treatment activities in the STF. According to the Sludge Treatment Facilities (STF) EIA Report (EIA-155/2008), all generated wastewater will be discharged to the on-site wastewater treatment plant. The treated effluent from the wastewater treatment plant will be reused in the STF and there would be no wastewater effluent discharged to the coastal waters of Deep Bay. Therefore cumulative water quality impact from other EIA designated projects is not expected.
6.6.24. The leachate generated would be treated by the treatment plant prior discharge to the Lung Kwu Shueng Tan Outfall Chamber and then Urmston Road Submarine Outfall. Therefore cumulative water quality impact from other EIA designated projects is not expected.
6.6.25. The construction phase of the WENT Landfill Extension (WLES) would not be concurrent with the construction phase of the Project. According to the WLES EIA Report (EIA-171/2009), proper implementation of construction site runoff control measures will be implemented during the construction phase of the WLES.As such, adverse water quality impact from WLES during construction phase is not anticipated.
6.6.26. The water quality impacts associated with the drainage and construction site runoff during decommissioning, general site activities, accidental spillage and sewage effluent produced by on-site workforce can be controlled and minimized through good site practices.
6.6.27. As the seawall liner would not be disturbed during the entire construction phase; the PFA leachate in the Middle Ash Lagoon is unlikely to cause unacceptable impact on the aquatic environment. The site condition of the Middle Lagoon during decommissioning and construction phases would not be much different from the existing condition.
6.6.28. Aside from the proposed development, 3 concurrent projects are identified from a water quality perspective. Namely the operation of Sewerage Treatment Facility, the planned WENT Landfill Extension (WLES), and the existing WENT landfill. No adverse water quality impact due to the discharge from concurrent sources is expected. Hence, no adverse existing and future water quality impacts from these sources are anticipated.
6.6.29. Through implementation of a systematic and effective site management plan, identified potential water polluting activities and its associated water quality impacts could be controlled and minimized.
6.7.1. In accordance with the Practical Note for Professional Persons on Construction Site Drainage, Environmental Protection Department, 1994 (ProPECC PN 1/94), and DSD Technical Circular TC 14/2000, construction phase precautionary, handling and disposal of construction site discharges will be adopted where appropriate. Through the adoption of proper mitigation measures, some of the pollution problems currently associated with construction activities can be prevented or minimized, as following:
6.7.2. At the start of the site establishment, perimeter cut-off drains will be constructed around the site to direct off-site water. Internal drainage works and erosion and sedimentation control facilities will be implemented. Channels, earth bunds or sand bag barriers will be provided on site to direct storm water/runoff to silt removal facilities. The design of the temporary on-site drainage system will be undertaken by the Contractor prior to the commencement of construction.
6.7.3. Dikes or embankments for flood protection will be implemented around the boundaries of earthwork areas. Temporary ditches will be provided to facilitate the runoff discharge into an appropriate watercourse, through a silt/sediment trap.
6.7.4. The surface run-off from construction sites should be discharged into storm drain via adequately designed sand/silt removal facilities such as sand traps, silt traps and sediment basins. Reference shall be made to Appendix A1 of ProPECC PN 1/94. The detailed design of the sand/silt traps will be undertaken by the Contractor prior to the commencement of construction.
6.7.5. To prevent possible runoff into the water channel during slope stabilization works at the embankment located to the north of the Subject Site, stabilization works at the embankment shall be carried out during dry season to minimize surface and storm water runoff discharge into the water channel located to the north of the embankment. Silt fences shall be erected along the embankment to prevent contaminated surface runoff entering the water channel.
6.7.6. To minimize adverse water quality impact caused by the construction of surface drainage outfall, works shall be scheduled to commence at the dry season to prevent possible disturbance to breeding activities and adversely increased turbidity due to the combined effects of construction activities and rainfall. Moreover, silt fences shall be erected to prevent contaminated surface runoff from entering the water channel.
6.7.7. Nevertheless, all exposed PFA/earth areas will be covered immediately after the earthworks have been completed. If excavation in PFA/soil could not be avoided in these months or at any time of the year when rainstorms are likely to happen. For the purpose of preventing soil erosion, temporarily exposed slope/soil surfaces should be covered e.g. by tarpaulin, and Construction Access Road Area should be protected by crushed stone or gravel, as excavation proceeds. Earthwork final surfaces should be well compacted and subsequent permanent work or surface protection should be immediately performed.
6.7.8. Precautionary measures will be taken at any time of year when rainstorms are likely to occur. Actions will be made with reference to the actions summarized in Appendix A2 of ProPECC PN 1/94. For this Project, particular attention will be paid to the control of silt surface runoff and the well function of silt removal facilities.
6.7.9. Open stockpiles of construction materials (e.g. aggregates, sand and fill material), excavated piles and exposed PFA surfaces on sites shall be covered with tarpaulin or similar fabric during rainstorms. Measures should be taken to prevent the washing away of construction materials, soil, silt or debris into any drainage system.
6.7.10. All vehicles and plant shall be cleaned before leaving the construction site to ensure that no earth, mud, debris and the like is deposited on public roads. A wheel washing bay will be provided at every site exit if practicable and wash-water will have sand and silt settled out or removed before discharging into storm drains on a weekly basis to ensure the efficiency of work process. The section of construction road between the wheel washing bay and the public road will be paved with backfill to reduce vehicle tracking of soil and prevent site run-off from entering public road drains.
6.7.11. Construction solid waste, debris ad rubbish on site will be collected, handled and disposed of properly to avoid water quality impacts.
6.7.12. Regular environmental audit on the construction site shall be provided to ensure an effective control of any malpractices and to achieve continual improvement of environmental performance on site. Thus, no residual water quality impact during construction phase is anticipated.
6.7.13. Temporary sewage will be adequately treated by interim sewage treatment facilities, such as portable chemical toilets, which can be installed within construction site. It is anticipated that sewage generation during the construction phase of the Project will not cause a water pollution problem. The Contractor shall be responsible to provide appropriate and adequate potable chemical toilets as well as appropriate disposal and maintenance.
6.7.14. Workers will be reminded not to discharge and sewage or wastewater into the nearby environment during the construction phase of the Project. No residual sewage impact on the surrounding water system is anticipated.
6.7.15. Contractor must register as a chemical waste producer of chemical wastes that would be produced from construction activities. The Waste Disposal Ordinance (Cap 354) and its subsidiary regulations in particular the Waste Disposal (Chemical Waste) (General) Regulation should be observed and complied with for control of chemical wastes.
6.7.16.
Service workshops and
maintenance facilities will be located within a bundling area, and sumps and
oil interceptors will be provided. Potential oil or fuel leakage/spillage
activities are mainly associated with the maintenance of equipment. Oils and fuels
should only be used and stored in designated areas with pollution
prevention facilities. All fuel tanks and storage areas should be sited on
sealed areas in order to prevent spillage of fuels and solvents to the nearby
watercourses. All waste oils and fuels should be collected in designated tanks
prior to disposal. The activities shall only be
undertaken within areas appropriately equipped to control these discharges.
Disposal of chemical wastes should be carried out in compliance with the Waste
Disposal Ordinance. Any fuel and waste lubricant oil from the on-site
maintenance of machinery and equipment will be collected by a licensed chemical
waste collector.
6.7.17. By adopting the above precautionary measures with Best Management Practices (BMPs), it is anticipated that the impact of runoff from the construction site will be reduced to satisfactory levels before discharge.
6.7.18. The construction runoff discharged from the Subject Site shall fully comply with the standards stated in Section 6.2, otherwise the discharge shall be collected and conveyed to the on-site leachate treatment prior discharge. By adopting precautionary measures with Best Management Practices (BMPs), it is anticipated that there will be no significant water quality impact during the construction phase.
Residual environmental impact is the net impact remaining with the mitigation measures in place. With incorporation of the Practical Note for Professional Note for Professional Persons on Construction Site Drainage, Environmental Protection Department, 1994 (ProPECC PN 1/94),DSD Technical Circular TC 14/2000 and EM&A programme, adverse residual water impact during the construction phase is not expected.
As defined in Section 2.5, operational phase shall be defined as the period when the decommissioning works as detailed in Section 2 are completed. The operational phase of the Project shall be an idle buildable land to cater any future developments by the government. Since the operational phase does not involve any equipment operation or human activities, adverse water quality impacts is not expected.
Further details of the specific EM&A requirements are detailed in Section 12 of this report and in the EM&A Manual, together with event action plans and procedures for complaints.
6.11.1. The potential sources of water quality impact arising from construction phase of the Project include drainage and construction site-runoff, sewage effluent produced by workforce and wastewater generated from general construction activities.
6.11.2. The temporary sewage can be adequately treated by interim sewage treatment facilities, such as portable chemical toilets, which can be installed within construction site. With proper implementation of construction site mitigation measures and good site practices, adverse water quality impacts during construction phase is not anticipated. Potentially significant effects were not identified. Reference shall be made to ProPECC PN 1/94 to ensure that the proper construction phase precautionary measures are adopted. Residual impacts on water quality are not anticipated.