TABLE OF CONTENTS

5............. Water Quality Impact. 5-1

5.1.......... Introduction. 5-1

5.2.......... Environmental Legislation, Standards and Criteria. 5-1

5.3.......... Description of Environment 5-12

5.4.......... Identification of Water Sensitive Receivers. 5-18

5.5.......... Assessment Methodology. 5-20

5.6.......... Identification, Prediction and Evaluation of Environmental Impacts. 5-26

5.7.......... Mitigation of Adverse Environmental Impacts. 5-34

5.8.......... Evaluation of Residual Impacts. 5-38

5.9.......... Cumulative Impacts. 5-41

5.10........ Environmental Monitoring and Audit 5-42

5.11........ Conclusion. 5-42

List of Tables

Table 5.1 Summary of Water Quality Objectives for Deep Bay WCZ. 5-1

Table 5.2 Summary of Water Quality Objectives for North Western WCZ. 5-4

Table 5.3 Standards for effluents discharged into Group B Inland Waters. 5-8

Table 5.4 Standards for effluents discharged into Group C Inland Waters. 5-8

Table 5.5 Standards for effluents discharged into Group D Inland Waters. 5-9

Table 5.6 Standards for effluents discharged into Foul Sewers leading into Government Sewage Treatment Plants. 5-10

Table 5.7 Standards for effluents discharged into Foul Sewers leading into Government Sewage Treatment Plants with Microbial Treatment 5-11

Table 5.8 Reclaimed Water Quality Standards for Non-Potable Uses. 5-12

Table 5.9 Summary EPD’s Routine Marine Water Quality Data for Deep Bay WCZ in Year 2020 5-12

Table 5.10 Summary EPD’s Routine Marine Water Quality Data for North Western WCZ in Year 2020 5-14

Table 5.11 Summary Statistic of 2019 River Water Quality of Yuen Long Creek. 5-16

Table 5.12 Water Quality Survey Results under this Study. 5-18

Table 5.13 Water Sensitive Receivers. 5-19

Table 5.14 Marine Water Sensitive Receivers. 5-19

Table 5.15 Pollution Loading from Livestock Farm and Unsewered Population under “Without YLSEPP” Scenario 5-21

Table 5.16 Assumed Effluent Flow and Qualities of YLSEPP under Normal Operation Scenario. 5-22

Table 5.17 Assumed Effluent Flow and Qualities of YLSEPP under Emergency Discharge Scenario 5-22

Table 5.18 Projects Incorporated in Modelling. 5-24

Table 5.19 Pollution Loads within Deep Bay from Concurrent EIA Projects. 5-26

Table 5.20 Pollution Load to Deep Bay under Scenario 1 and 2. 5-28

Table 5.21 Maximum Percentage Change due to Emergency Discharge. 5-32

Table 5.22 Potential Concurrent Projects. 5-41

List of Figures
Figure 5.1	Locations of Water Sensitive Receivers

List of Appendices
Appendix 5.1	Indicative Locations of Marine Water Sensitive Receivers and Water Quality Monitoring Stations [extracted from EIA Report for “Yuen Long Effluent Polishing Plant” (EIAO Register No.: AEIAR – 220/2019)]
Appendix 5.2	Model Grid Layout, Properties and Model Verifications
Appendix 5.3	Spin-up Test Results
Appendix 5.4	Contour Plots of Water Quality Modelling Results
Appendix 5.5	Predicted Water Quality at Key Water Sensitive Receivers
Appendix 5.6	Time Series Plots of Water Quality Modelling Results - Dry Season
Appendix 5.7	Time Series Plots of Water Quality Modelling Results - Wet Season
Appendix 5.8	Deep Bay Wetland Conservation and Buffer Areas Boundaries [extracted from “Application for Developments within Deep Bay Area under Section 16 of the Town Planning Ordinance” (TPB PG-No. 12C)]

5 Water Quality Impact

5.1 Introduction

5.1.1.1 This section presents an assessment of potential water quality impacts arising from construction and operation of the Project, which has been conducted in accordance with the criteria and guidelines for evaluating and assessing water pollution as stated in Annex 6 and Annex 14 of the “Technical Memorandum on Environmental Impact Assessment Process” (EIAO-TM) as well as the requirements given in Clause 3.4.6 and Appendix D of the EIA Study Brief (No. ESB-313/2019) (hereinafter “the Study Brief”)

5.2 Environmental Legislation, Standards and Criteria

5.2.1 Environmental Impact Assessment Ordinance (EIAO)

5.2.1.1 The EIAO-TM 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

5.2.2 Water Pollution Control Ordinance (WPCO)

5.2.2.1 The Water Pollution Control Ordinance (WPCO, Cap. 358) 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 waters are divided into ten Water Control Zones (WCZs). Corresponding statements of WQOs are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in the WCZ based on their beneficial uses. The Project site is located within Deep Bay WCZs and may have potential impacts to the North Western WCZ. The corresponding WQOs are summarized in Table 5.1 and Table 5.2 below.

Table 5.1 Summary of Water Quality Objectives for Deep Bay WCZ

Parameters	Criteria	Subzone
Aesthetic appearance	Waste discharges shall cause no objectionable odours or discolouration of the water.	Whole Zone
	Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent.
	Mineral oil should not be visible on the surface. Surfactants should not give rise to a lasting foam.
	There should be no recognisable sewage-derived debris.
	Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent.
	Waste discharges shall not cause the water to contain substances which settle to form objectionable deposits.
Bacteria	Should not exceed 610 per 100 mL, calculated as the geometric mean of all samples collected in one calendar year.	Secondary Contact Recreation Subzone and Mariculture Subzone (L.N. 455 of 1991)
	Should be zero 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 (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	Should not 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
	Should not exceed 180 per 100 mL, calculated as the geometric mean of all samples collected from March to October inclusive in one calendar year. Samples should be taken at least 3 times in a calendar month at intervals of between 3 and 14 days.	Yung Long Bathing Beach Subzone (L.N. 455 of 1991)
Dissolve Oxygen (DO) within 2 m of the seabed	Not less than 2 mg/L for 90% of the sample	Outer Marine Subzone excepting Mariculture Subzone
Dissolved Oxygen (DO)	Not less than 4 mg/L for 90% of the sample, taken at 1 metre below surface	Inner Marine Subzone excepting Mariculture Subzone
	Not less than 4 mg/L for 90% of the sampling, calculated as water column average	Outer Marine Subzone excepting Mariculture Subzone
	Not less than 5 mg/L for 90% of the sample, taken at 1 metre below surface	Mariculture Subzone
	Not less than 4 mg/L	Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone, Indus Subzone, Ganges Subzone, Water Gathering Ground Subzones and other inland waters of the Zone
Colour	Human activity should not cause the colour of water to exceed 30 Hazen units.	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	Human activity should not cause the colour of water to exceed 50 Hazen units.	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Temperature	Waste discharges shall not cause the natural daily temperature range to change by more than 2℃	Whole Zone
Salinity	Waste discharges shall not cause the natural ambient salinity level to change by more than 10%	Whole Zone
pH	To be in the range of 6.5-8.5, change due to human activity not to exceed 0.2 units	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% of samples, change due to human activity not to exceed 0.5 units	Yung Long Bathing Beach Subzone
Suspended Solids (SS)	Human activity shall neither cause the natural ambient SS level to be raised by 30% nor give rise to accumulation of SS which may adversely affect aquatic communities	Marine waters
	Human activity shall not cause the annual median of SS 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
Un-ionized ammonia (UIA)	The un-ionized ammoniacal nitrogen level should not be more than 0.021 mg/L, calculated as the annual average (arithmetic mean).	Whole Zone
Nutrients	(a) Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants.	Inner and Outer Marine Subzones
	(b) Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.7 mg/L, expressed as annual mean.	Inner Marine Subzones
	(c) Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.5 mg/L, expressed as annual water column average (arithmetic mean of at least 2 measurements at 1 m below surface and 1 m above seabed).	Outer Marine Subzones
5-Day Biochemical Oxygen Demand (BOD5)	Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 3 mg/L	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 mg/L	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Chemical Oxygen Demand (COD)	Waste discharges shall not cause the chemical oxygen demand to exceed 15 mg/L	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	Waste discharges shall not cause the chemical oxygen demand to exceed 30 mg/L	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Toxins	Waste discharges shall not cause the toxins in water to attain such levels as to produce significant toxic carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to toxicant interactions with each other.	Whole Zone
	Waste discharges shall not cause a risk to any beneficial uses of the aquatic environment.	Whole Zone
Phenol	Phenols shall not be present in such quantities as to produce a specific odour, or in concentration greater than 0.05 mg/L as C6H5OH.	Yung Long Bathing Beach Subzone
Turbidity	Waste discharges shall not reduce light transmission substantially from the normal level.	Yung Long Bathing Beach Subzone

Table 5.2 Summary of Water Quality Objectives for North Western WCZ

Parameters	Criteria	Subzone
Aesthetic appearance	Waste discharges shall cause no objectionable odours or discolouration of the water.	Whole Zone
	Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent.
	Mineral oil should not be visible on the surface. Surfactants should not give rise to a lasting foam.
	There should be no recognisable sewage-derived debris.
	Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent.
	Waste discharges shall not cause the water to contain substances which settle to form objectionable deposits.
Bacteria	Should 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 running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days.	Tuen Mun (A) and Tuen Mun (B) Subzones and Water Gathering Ground Subzones
	Should not 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.	Tuen Mun (C) Subzone and other inland waters
	Should not exceed 180 per 100 mL, calculated as the geometric mean of all samples collected from March to October inclusive in one calendar year. Samples should be taken at least 3 times in a calendar month at intervals of between 3 and 14 days.	Bathing Beach Subzones
Dissolve Oxygen (DO) within 2 m of the seabed	Not less than 2 mg/L for 90% of the sample	Marine waters
Dissolved Oxygen (DO)	Not less than 4 mg/L for 90% of the sampling, calculated as water column average	Marine waters
	Not less than 4 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones, Water Gathering Ground Subzones and other inland waters
pH	To be in the range of 6.5-8.5, change due to human activity not to exceed 0.2 units	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% of samples, change due to human activity not to exceed 0.5 units	Bathing Beach Subzones
Colour	Human activity should not cause the colour of water to exceed 30 Hazen units.	Tuen Mun (A) and Tuen Mun (B) Subzones and Water Gathering Ground Subzones
	Human activity should not cause the colour of water to exceed 50 Hazen units.	Tuen Mun (C) Subzone and other inland waters
Temperature	Waste discharges shall not cause the natural daily temperature range to change by more than 2℃	Whole Zone
Salinity	Waste discharges shall not cause the natural ambient salinity level to change by more than 10%	Whole Zone
Suspended Solids (SS)	Human activity shall neither cause the natural ambient SS level to be raised by 30% nor give rise to accumulation of SS which may adversely affect aquatic communities	Marine waters
	Human activity shall not cause the annual median of SS to exceed 20 mg/L.	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	Human activity shall not cause the annual median of SS to exceed 25 mg/L.	Other inland waters
Un-ionized ammonia (UIA)	The un-ionized ammoniacal nitrogen level should not be more than 0.021 mg/L, calculated as the annual average (arithmetic mean).	Whole Zone
Nutrients	(a) Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants.	Marine waters
	(b) Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.3 mg/L, expressed as annual water column average.	Castle Peak Bay Subzone
	(c) Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.5 mg/L, expressed as annual water column average.	Marine waters excepting Castle Peak Bay Subzone
5-Day Biochemical Oxygen Demand (BOD5)	Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 3 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 mg/L	Other inland waters
Chemical Oxygen Demand (COD)	Waste discharges shall not cause the chemical oxygen demand to exceed 15 mg/L	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
	Waste discharges shall not cause the chemical oxygen demand to exceed 30 mg/L	Other inland waters
Toxins	Waste discharges shall not cause the toxins in water to attain such levels as to produce significant toxic carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to toxicant interactions with each other.	Whole Zone
	Waste discharges shall not cause a risk to any beneficial uses of the aquatic environment.	Whole Zone
Phenol	Phenols shall not be present in such quantities as to produce a specific odour, or in concentration greater than 0.05 mg/L as C6H5OH.	Bathing Beach Subzones
Turbidity	Waste discharges shall not reduce light transmission substantially from the normal level.	Bathing Beach Subzones

5.2.2.2 For mangroves, there is considerable evidence that mangroves are generally adapted to muddy or sandy substrate and used to turbid water, and are survived in dramatic changes in salinity environment due to the nature of the inter-tidal habitat in which they grow (i.e. salinity levels can fluctuate from freshwater to sea water (0-34 ppt). No suspended solids and salinity criteria are recommended for this WSR.

5.2.3 Technical Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters

5.2.3.1 Discharge of effluents is 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) was issued under Section 21 of the WPCO. It sets the discharge limits vary with the effluent flow rates and the effluent should comply with the standards for effluent discharged into different type of receiving waters (foul sewers, storm water drains, inland and coastal waters). The standards control the physical, chemical and microbial quality of effluents.

5.2.3.2 The criteria for discharges to inland waters depend upon the beneficial uses of the waters. The majority of the inland water bodies potentially affected by the proposed developments are used for the agricultural purposes, which are classified as Group B inland waters, for pond fish culture which are classified as Group C inland waters, and for general amenity and secondary contact recreation, which are classified as Group D inland waters. The standards for effluents discharged into Group B, Group C and Group D inland waters are provided in Table 5.3, Table 5.4 and Table 5.5 respectively.

Table 5.3 Standards for effluents discharged into Group B Inland Waters

	Flow rate (m^3/day)
Determinand	≤200	>200 and ≤400	>400 and ≤600	>600 and ≤800	>800 and ≤1000	>1000 and ≤1500	>1500 and ≤2000	>2000 and ≤3000
pH (pH units)	6.5-8.5	6.5-8.5	6.5-8.5	6.5-8.5	6.5-8.5	6.5-8.5	6.5-8.5	6.5-8.5
Temperature (℃)	35	30	30	30	30	30	30	30
Colour (lovibond units) (25mm cell length)	1	1	1	1	1	1	1	1
Suspended Solids	30	30	30	30	30	30	30	30
BOD	20	20	20	20	20	20	20	20
COD	80	80	80	80	80	80	80	80
Oil & grease	10	10	10	10	10	10	10	10
Iron	10	8	7	5	4	3	2	1
Boron	5	4	3	2.5	2	1.5	1	0.5
Barium	5	4	3	2.5	2	1.5	1	0.5
Mercury	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Cadmium	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Selenium	0.2	0.2	0.2	0.2	0.2	0.1	0.1	0.1
Other toxic metals individually	0.5	0.5	0.2	0.2	0.2	0.1	0.1	0.1
Total toxic metals	2	1.5	1	0.5	0.5	0.2	0.2	0.2
Cyanide	0.1	0.1	0.1	0.08	0.08	0.05	0.05	0.03
Phenols	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
Sulphide	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.2
Fluoride	10	10	8	8	8	5	5	3
Sulphate	800	800	600	600	600	400	400	400
Chloride	1000	1000	800	800	800	600	600	400
Total phosphorus	10	10	10	8	8	8	5	5
Ammonia nitrogen	5	5	5	5	5	5	5	5
Nitrate + nitrite nitrogen	30	30	30	20	20	20	10	10
Surfactants (total)	5	5	5	5	5	5	5	5
E. coli (count/100mL)	100	100	100	100	100	100	100	100

Note: All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated.

Table 5.4 Standards for effluents discharged into Group C Inland Waters

	Flow rate (m^3/day)
Determinand	≤100	>100 and ≤500	>500 and ≤1000	>1000 and ≤2000
pH (pH units)	6-9	6-9	6-9	6-9
Temperature (℃)	30	30	30	30
Colour (lovibond units) (25mm cell length)	1	1	1	1
Suspended Solids	20	10	10	5
BOD	20	15	10	5
COD	80	60	40	20
Oil & grease	1	1	1	1
Boron	10	5	4	2
Barium	1	1	1	0.5
Iron	0.5	0.4	0.3	0.2
Mercury	0.001	0.001	0.001	0.001
Cadmium	0.001	0.001	0.001	0.001
Silver	0.1	0.1	0.1	0.1
Copper	0.1	0.1	0.05	0.05
Selenium	0.1	0.1	0.05	0.05
Lead	0.2	0.2	0.2	0.1
Nickel	0.2	0.2	0.2	0.1
Other toxic metals individually	0.5	0.4	0.3	0.2
Total toxic metals	0.5	0.4	0.3	0.2
Cyanide	0.05	0.05	0.05	0.01
Phenols	0.1	0.1	0.1	0.1
Sulphide	0.2	0.2	0.2	0.1
Fluoride	10	7	5	4
Sulphate	800	600	400	200
Chloride	1000	1000	1000	1000
Total phosphorus	10	10	8	8
Ammonia nitrogen	2	2	2	1
Nitrate + nitrite nitrogen	30	30	20	20
Surfactants (total)	2	2	2	1
E. coli (count/100mL)	1000	1000	1000	1000

Note: All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated.

Table 5.5 Standards for effluents discharged into Group D Inland Waters

	Flow rate (m^3/day)
Determinand	≤200	>200 and ≤400	>400 and ≤600	>600 and ≤800	>800 and ≤1000	>1000 and ≤1500	>1500 and ≤2000	>2000 and ≤3000
pH (pH units)	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10
Temperature (℃)	30	30	30	30	30	30	30	30
Colour (lovibond units) (25mm cell length)	1	1	1	1	1	1	1	1
Suspended Solids	30	30	30	30	30	30	30	30
BOD	20	20	20	20	20	20	20	20
COD	80	80	80	80	80	80	80	80
Oil & grease	10	10	10	10	10	10	10	10
Iron	10	8	7	5	4	2.7	2	1.3
Boron	5	4	3.5	2.5	2	1.5	1	0.7
Barium	5	4	3.5	2.5	2	1.5	1	0.7
Mercury	0.1	0.05	0.001	0.001	0.001	0.001	0.001	0.001
Cadmium	0.1	0.05	0.001	0.001	0.001	0.001	0.001	0.001
Other toxic metals individually	1	1	0.8	0.8	0.5	0.5	0.2	0.2
Total toxic metals	2	2	1.6	1.6	1	1	0.5	0.4
Cyanide	0.4	0.4	0.3	0.3	0.2	0.1	0.1	0.05
Phenols	0.4	0.3	0.2	0.1	0.1	0.1	0.1	0.1
Sulphide	1	1	1	1	1	1	1	1
Sulphate	800	600	600	600	600	400	400	400
Chloride	1000	800	800	800	600	600	400	400
Fluoride	10	8	8	8	5	5	3	3
Total phosphorus	10	10	10	8	8	8	5	5
Ammonia nitrogen	20	20	20	20	20	20	20	10
Nitrate + nitrite nitrogen	50	50	50	30	30	30	30	20
Surfactants (total)	15	15	15	15	15	15	15	15
E. coli (count/100mL)	1000	1000	1000	1000	1000	1000	1000	1000

Note: All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated.

5.2.3.3 The TM-DSS also specifies the discharge standards into foul sewers leading into the Government’s sewage treatment plants as shown in Table 5.6 and Table 5.7 below. Subject to the flow rate of the effluents, corresponding standards for the effluent discharge into the Government’s foul sewers should be followed.

Table 5.6 Standards for effluents discharged into Foul Sewers leading into Government Sewage Treatment Plants

	Flow rate (m^3/day)
Determinand	≤10	>10 and ≤100	>100 and ≤200	>200 and ≤400	>400 and ≤600	>600 and ≤800	>800 and ≤1000	>1000 and ≤1500	>1500 and ≤2000	>2000 and ≤3000	>3000 and ≤4000	>4000 and ≤5000	>5000 and ≤6000
pH (pH units)	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10
Temperature (℃)	43	43	43	43	43	43	43	43	43	43	43	43	43
Suspended Solids	1200	1000	900	800	800	800	800	800	800	800	800	800	800
Settleable Solids	100	100	100	100	100	100	100	100	100	100	100	100	100
BOD	1200	1000	900	800	800	800	800	800	800	800	800	800	800
COD	3000	2500	2200	2000	2000	2000	2000	2000	2000	2000	2000	2000	2000
Oil & grease	100	100	50	50	50	40	30	20	20	20	20	20	20
Iron	30	25	25	25	15	12.5	10	7.5	5	3.5	2.5	2	1.5
Boron	8	7	6	5	4	3	2.4	1.6	1.2	0.8	0.6	0.5	0.4
Barium	8	7	6	5	4	3	2.4	1.6	1.2	0.8	0.6	0.5	0.4
Mercury	0.2	0.15	0.1	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Cadmium	0.2	0.15	0.1	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Copper	4	4	4	3	1.5	1.5	1	1	1	1	1	1	1
Nickel	4	3	3	2	1.5	1.5	1	0.8	0.7	0.7	0.6	0.6	0.6
Chromium	2	2	2	2	1	0.7	0.6	0.4	0.3	0.2	0.1	0.1	0.1
Zinc	5	5	4	3	1.5	1.5	1	0.8	0.7	0.7	0.6	0.6	0.6
Silver	4	3	3	2	1.5	1.5	1	0.8	0.7	0.7	0.6	0.6	0.6
Other toxic metals individually	2.5	2.2	2	1.5	1	0.7	0.6	0.4	0.3	0.2	0.15	0.12	0.1
Total toxic metals	10	10	8	7	3	2	2	1.6	1.4	1.2	1.2	1.2	1
Cyanide	2	2	2	1	0.7	0.5	0.4	0.27	0.2	0.13	0.1	0.08	0.06
Phenols	1	1	1	1	0.7	0.5	0.4	0.27	0.2	0.13	0.1	0.1	0.1
Sulphide	10	10	10	10	5	5	4	2	2	2	1	1	1
Sulphate	1000	1000	1000	1000	1000	1000	1000	900	800	600	600	600	600
Total nitrogen	200	200	200	200	200	200	200	100	100	100	100	100	100
Total phosphorus	50	50	50	50	50	50	50	25	25	25	25	25	25
Surfactants (total)	200	150	50	40	30	25	25	25	25	25	25	25	25

Note: All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated.

Table 5.7 Standards for effluents discharged into Foul Sewers leading into Government Sewage Treatment Plants with Microbial Treatment

	Flow rate (m^3/day)
Determinand	≤10	>10 and ≤100	>100 and ≤200	>200 and ≤400	>400 and ≤600	>600 and ≤800	>800 and ≤1000	>1000 and ≤1500	>1500 and ≤2000	>2000 and ≤3000	>3000 and ≤4000	>4000 and ≤5000	>5000 and ≤6000
Copper	1.5	1	1	1	0.8	0.6	0.5	0.4	0.3	0.2	0.15	0.1	0.05

Note: All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated.

Standards in this table apply in place of those in Table 5.6 for the corresponding determinand.

5.2.4 Practice Notes

5.2.4.1 A “Professional Persons Environmental Consultative Committee Practice Note” (ProPECC PN) was issued by the EPD to provide guidelines for handling and disposal of construction site discharges. The ProPECC PN 1/94 "Construction Site Drainage" provides good practice guidelines for dealing with 10 types of discharge from construction sites. These include surface run-off, groundwater, boring and drilling water, bentonite slurry, water for testing and sterilisation of water retaining structures and water pipes, wastewater from building constructions, acid cleaning, etching and pickling wastewater, and wastewater from site facilities. Practices given in the ProPECC PN 1/94 should be followed as far as possible during construction to minimise the water quality impact due to construction site drainage.

5.2.4.2 The “Drainage Plans Subject to Comment by the EPD” (ProPECC PN 5/93) provides guidelines and practices for handling, treatment and disposal of various effluent discharges to stormwater drains and foul sewers during the operation phase. The design of site drainage and disposal of various site effluents generated within the new development area should follow the relevant guidelines and practices as given in the ProPECC PN 5/93.

5.2.5 Technical Circular

5.2.5.1 Environment, Transport and Works Bureau Technical Circular (ETWB TC) (Works) No. 5/2005 provides an administrative framework to better protect all natural streams/rivers from the impacts of construction works. The procedures promulgated under this Circular aim to clarify and strengthen existing measures for protection of natural streams/rivers from government projects and private developments. The guidelines and precautionary mitigation measures given in the ETWB TC (Works) No. 5/2005 should be followed as far as possible to protect the inland watercourses at or near the Project area during the construction phase.

5.2.6 Hong Kong Planning Standards and Guidelines (HKPSG)

5.2.6.1 Chapter 9 of the HKPSG outlines environmental requirements that need to be considered in land use planning. The recommended guidelines, standards and guidance cover the selection of suitable locations for the developments and sensitive uses, provision of environmental facilities, and design, layout, phasing and operational controls to minimise adverse environmental impacts. It also lists out environmental factors that influence land use planning and recommends buffer distances for land uses.

5.2.7 Water Quality Standards for Reclaimed Water

5.2.7.1 The water quality standards for reclaimed water to be adopted by this Project are based on the water quality standards endorsed by the “Working Group on the Implementation of Reclaimed Water Supply in Sheung Shui and Fanling” at its meeting on 13 July 2012 for non-potable uses. Details of the standards are summarized in Table 5.8 below.

Table 5.8 Reclaimed Water Quality Standards for Non-Potable Uses

Parameters	Unit	Recommended Water Quality Standards
E. coli	cfu/100mL	Non detectable
Total Residual Chlorine	mg/L	≥ 1 exiting treatment system; ≥ 0.2 at user end
Dissolved Oxygen	mg/L	≥ 2
Suspended Solids (SS)	mg/L	≤ 5
Colour	Hazen unit	≤ 20
Turbidity	NTU	≤ 5
pH	-	6 – 9
Threshold Odour Number (TON)	-	≤ 100
5-day Biochemical Oxygen Demand (BOD5)	mg/L	≤ 10
Ammoniacal Nitrogen	mg/L as N	≤ 1
Synthetic Detergents	mg/L	≤ 5

Source: Final Environmental Impact Assessment Report under Agreement No. CE 61/2007(CE) - "North East New Territories New Development Areas Planning and Engineering Study – Investigation", Table 6.18.

Notes: 1. Apart from total residual chlorine which has been specified, the water quality standards for all parameters shall be applied at the point-of-use of the system.

2. Where reclaimed water is treated for immediate usage, the level of total residual chlorine may be lower than the one specified in this table.

3. Immediate usage means the collected grey water / rainwater is drawn into the treatment process immediate before a particular round of usage and the treated water will be depleted after that round of usage is completed.

5.3 Description of Environment

5.3.1 Study Area

5.3.1.1 According to Clause 3.4.6.2 of the Study Brief, the Study Area for this water quality impact assessment include areas within 500 metres from the boundary of the Project and shall cover Deep Bay, North Western and other affected WCZs as designated under the WPCO and water sensitive receivers, such as the natural streams and nullah in the vicinity of the Project. The baseline condition of water bodies in the Study Area have been established with reference to routine river and marine water quality monitoring data collected by EPD. Descriptions of the baseline conditions provided in the subsequent sections are extracted from the EPD's reports “Marine Water Quality in Hong Kong in 2020” and “River Water Quality in Hong Kong in 2020” which contains the latest information published by EPD on river and marine water quality.

5.3.2 Marine Water

5.3.2.1 The baseline water quality condition of marine water was established from the marine water quality monitoring data routinely collected by EPD in the Deep Bay and North Western WCZs. A summary of EPD monitoring data collected in 2020 for Deep Bay WCZ and North Western WCZ were presented in Table 5.9 and Table 5.10 respectively.

Table 5.9 Summary EPD’s Routine Marine Water Quality Data for Deep Bay WCZ in Year 2020

		Inner Deep Bay			Outer Deep Bay		WPCO WQO
Parameters		DM1	DM2	DM3	DM4	DM5	(in inland waters)
Temperature (°C)		26.5 (20.8 - 31.2)	26.6 (21.1 - 30.9)	25.4 (20.2 - 30.5)	26.1 (21.1 - 29.8)	25.7 (20.9 - 28.9)	Not more than 2℃ in daily temperature range
Salinity		14.6 (1.6 - 22.6)	16.6 (3.0 - 25.5)	21.5 (4.4 - 29.3)	20.9 (3.8 - 30.7)	23.7 (8.9 - 31.6)	Not to cause more than 10% change
Dissolved Oxygen (mg/L)	Depth Average	5.9 (4.4 - 7.8)	6.1 (4.8 - 7.3)	6.0 (4.3 - 7.1)	5.8 (5.2 - 6.6)	5.8 (4.4 - 6.9)	Marine Subzone excepting Mariculture Subzone: Not less than 4 mg/L for 90% of samples Mariculture Subzone: Not less than 5 mg/L for 90% of samples
	Bottom	N/A	N/A	N/A	5.4 (4.6 - 6.6)	5.5 (2.6 - 7.1)	Outer Marine Subzone excepting Mariculture Subzone: Not less than 2 mg/L for 90% of samples
Dissolved Oxygen (% Saturation)	Depth Average	79 (62 - 105)	83 (69 - 94)	83 (56 - 97)	80 (70 - 94)	81 (59 - 98)	Not available
	Bottom	N/A	N/A	N/A	75 (61 - 94)	77 (36 - 100)	Not available
pH		7.4 (7.1 - 7.8)	7.6 (7.2 - 8.0)	7.8 (7.4 - 8.2)	7.7 (7.3 - 8.1)	7.8 (7.5 - 8.1)	Marine Waters excepting Yung Long Bathing Beach Subzone: 6.5 - 8.5 (±0.2 from natural range) Yung Long Bathing Beach Subzone: 6.0-9.0 for 95% of samples (±0.5 from natural range)
Secchi Disc Depth (m)		1.1 (0.8 - 1.2)	1.1 (0.8 - 1.3)	1.4 (1.1 - 1.9)	1.5 (1.0 - 2.0)	1.6 (1.0 - 2.1)	Not available
Turbidity (NTU)		21.7 (13.3 - 41.4)	28.3 (13.3 - 42.7)	9.5 (5.9 - 14.2)	9.0 (5.1 - 17.4)	9.1 (4.5 - 21.0)	Yung Long Bathing Beach Subzone: Not reduce light transmission substantially from the normal level.
Suspended Solids (SS) (mg/L)		37.0 (19.0 - 59.0)	47.4 (24.0 - 80.0)	15.3 (6.5 - 30.0)	14.5 (6.3 - 21.5)	14.7 (6.8 - 27.3)	Not more than 30% increase
5-day Biochemical Oxygen Demand (BOD5) (mg/L)		1.6 (0.3 - 3.1)	1.8 (0.7 - 4.9)	1.0 (0.3 - 4.3)	0.6 (0.2 - 0.8)	0.6 (0.3 - 1.0)	Not available
Ammonia Nitrogen (NH3-N) (mg/L)		0.455 (0.190 - 0.970)	0.331 (0.075 - 0.930)	0.134 (0.029 - 0.320)	0.102 (0.018 - 0.180)	0.089 (0.011 - 0.170)	Not available
Unionised Ammonia (mg/L)		0.007 (0.002 - 0.014)	0.006 (0.002 - 0.019)	0.004 (<0.001 - 0.009)	0.003 (<0.001 - 0.007)	0.003 (<0.001 - 0.007)	Not more than annual average of 0.021mg/L
Nitrite Nitrogen (NO2-N) (mg/L)		0.131 (0.026 - 0.310)	0.105 (0.040 - 0.310)	0.061 (0.014 - 0.170)	0.067 (0.026 - 0.165)	0.052 (0.024 - 0.130)	Not available
Nitrate Nitrogen (NO3-N) (mg/L)		1.300 (0.900 - 1.600)	1.080 (0.850 - 1.500)	0.753 (0.370 - 1.500)	0.711 (0.265 - 1.400)	0.605 (0.137 - 1.330)	Not available
Total Inorganic Nitrogen (TIN) (mg/L)		1.89 (1.38 - 2.32)	1.52 (1.02 - 2.34)	0.95 (0.43 - 1.78)	0.88 (0.37 - 1.52)	0.75 (0.23 - 1.41)	Inner Marine Subzone: Not more than annual mean of 0.7 mg/L Outer Marine Subzone: Not more than annual water column average of 0.5 mg/L
Total Kjeldahl Nitrogen (TKN) (mg/L)		0.90 (0.58 - 1.70)	0.79 (0.48 - 1.60)	0.39 (0.22 - 0.67)	0.34 (0.14 - 0.49)	0.43 (0.09 - 0.97)	Not available
Total Nitrogen (TN) (mg/L)		2.33 (1.81 - 2.91)	1.98 (1.52 - 3.01)	1.21 (0.77 - 2.11)	1.12 (0.76 - 1.81)	1.09 (0.73 - 1.72)	Not available
Orthophosphate Phosphorus (PO4) (mg/L)		0.151 (0.120 - 0.220)	0.123 (0.086 - 0.160)	0.061 (0.010 - 0.092)	0.035 (0.017 - 0.053)	0.025 (0.011 - 0.037)	Not available
Total Phosphorus (TP) (mg/L)		0.24 (0.19 - 0.34)	0.21 (0.15 - 0.27)	0.10 (0.05 - 0.14)	0.07 (0.05 - 0.10)	0.06 (0.04 - 0.08)	Not available
Silica (as SiO2) (mg/L)		5.88 (3.10 - 10.00)	5.15 (1.90 - 10.00)	3.70 (0.47 - 8.50)	3.89 (0.38 - 9.00)	3.50 (0.34 - 8.97)	Not available
Chlorophyll-a (μg/L)		6.3 (2.5 - 8.9)	8.4 (2.6 - 15.0)	2.8 (1.0 - 11.0)	1.9 (0.5 - 4.0)	1.9 (0.6 - 3.6)	Not available
E. coli (count/100mL)		200 (12 - 1600)	160 (17 - 6100)	22 (<1 - 1100)	63 (4 - 890)	75 (9 - 1500)	Secondary Contact Recreation Subzones and Mariculture Subzone (L.N. 455 of 1991): Not exceed 610 per 100mL Yung Long Bathing Beach Subzone (L.N.455 of 1991): Not exceed 180 per 100mL
Faecal Coliforms (count/100mL)		530 (55 - 2800)	340 (24 - 9600)	66 (3 - 3900)	170 (11 - 3000)	190 (26 - 4600)	Not available

Notes:

1. Data source: EPD Marine Water Quality in Hong Kong in 2020

2. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, Mid-depth, Bottom.

3. Data presented are annual arithmetic means of depth-averaged results except for E. coli and faecal coliforms that are annual geometric means.

4. Data in brackets indicate the ranges.

5. NA (Not Applicable) indicates the measurement was not made due to shallow water.

Table 5.10 Summary EPD’s Routine Marine Water Quality Data for North Western WCZ in Year 2020

		Lantau Island (North)	Pearl Island	Pillar Point	Urmston Road	Chep Lap Kok		WPCO WQO (in inland waters)
						(North)	(West)
Parameters		NM1	NM2	NM3	NM5	NM6	NM8
Temperature (°C)		25.4 (20.5 - 29.3)	25.9 (20.5 - 29.4)	24.8 (18.9 - 29.4)	25.7 (20.8 - 29.4)	26.2 (20.7 - 29.4)	25.9 (20.5 - 29.4)	Not more than 2℃ in daily temperature range
Salinity		28.9 (25.6 - 32.2)	26.6 (19.8 - 31.6)	27.3 (20.2 - 31.9)	26.0 (20.2 - 30.7)	24.0 (14.8 - 30.5)	26.5 (14.3 - 31.0)	Not to cause more than 10% change
Dissolved Oxygen (mg/L)	Depth Average	5.4 (4.4 - 6.4)	5.8 (4.7 - 7.2)	5.7 (4.3 - 6.7)	5.6 (4.3 - 6.7)	5.9 (4.7 - 7.2)	6.0 (4.7 - 7.0)	Not less than 4 mg/L for 90% of samples
	Bottom	5.1 (3.9 - 6.4)	5.6 (4.0 - 7.5)	5.3 (3.5 - 6.9)	5.0 (3.7 - 6.6)	5.9 (4.2 - 7.2)	6.1 (4.1 - 7.2)	Not less than 2 mg/L for 90% of samples
Dissolved Oxygen (% Saturation)	Depth Average	77 (64 - 89)	82 (68 - 104)	81 (64 - 89)	79 (62 - 89)	83 (68 - 102)	85 (67 - 98)	Not available
	Bottom	73 (56 - 91)	79 (58 - 108)	75 (57 - 92)	72 (53 - 93)	83 (61 - 103)	86 (60 - 98)	Not available
pH		7.9 (7.7 - 8.2)	7.9 (7.7 - 8.1)	7.9 (7.7 - 8.2)	7.8 (7.5 - 8.1)	7.9 (7.7 - 8.1)	7.9 (7.7 - 8.2)	6.5 - 8.5 (±0.2 from natural range)
Secchi Disc Depth (m)		2.0 (1.7 - 2.3)	2.1 (1.7 - 2.6)	2.1 (1.7 - 2.7)	1.9 (1.6 - 2.6)	1.9 (1.6 - 2.5)	1.9 (1.5 - 2.8)	Not available
Turbidity (NTU)		5.5 (2.9 - 10.9)	4.3 (1.9 - 7.8)	5.6 (3.4 - 8.9)	8.1 (4.0 - 11.9)	6.7 (4.2 - 11.0)	10.3 (5.0 - 23.3)	Not available
Suspended Solids (SS) (mg/L)		9.0 (2.3 - 15.0)	7.1 (2.4 - 12.7)	8.4 (3.6 - 15.3)	10.1 (3.9 - 20.3)	9.1 (4.5 - 16.7)	13.0 (2.2 - 29.3)	Not more than 30% increase
5-day Biochemical Oxygen Demand (BOD5) (mg/L)		0.6 (<0.1 - 1.1)	0.6 (0.3 - 1.0)	0.5 (0.3 - 0.8)	0.5 (0.2 - 0.8)	0.8 (0.3 - 1.7)	0.6 (0.3 - 1.2)	Not available
Ammonia Nitrogen (NH3-N) (mg/L)		0.086 (0.039 - 0.180)	0.076 (0.031 - 0.177)	0.096 (0.023 - 0.167)	0.076 (0.021 - 0.150)	0.064 (0.016 - 0.157)	0.036 (0.016 - 0.089)	Not available
Unionised Ammonia (mg/L)		0.003 (0.002 - 0.008)	0.003 (0.001 - 0.007)	0.003 (<0.001 - 0.007)	0.003 (<0.001 - 0.006)	0.002 (<0.001 - 0.007)	0.002 (<0.001 - 0.004)	Not more than annual average of 0.021mg/L
Nitrite Nitrogen (NO2-N) (mg/L)		0.068 (0.021 - 0.133)	0.077 (0.023 - 0.150)	0.065 (0.011 - 0.157)	0.086 (0.028 - 0.170)	0.096 (0.028 - 0.203)	0.076 (0.027 - 0.153)	Not available
Nitrate Nitrogen (NO3-N) (mg/L)		0.350 (0.140 - 0.640)	0.469 (0.157 - 0.757)	0.436 (0.160 - 0.850)	0.565 (0.233 - 0.930)	0.591 (0.263 - 0.913)	0.472 (0.167 - 0.967)	Not available
Total Inorganic Nitrogen (TIN) (mg/L)		0.50 (0.28 - 0.81)	0.62 (0.28 - 0.90)	0.60 (0.29 - 0.97)	0.73 (0.37 - 1.05)	0.75 (0.40 - 1.02)	0.58 (0.24 - 1.07)	Not more than annual average of 0.5mg/L
Total Kjeldahl Nitrogen (TKN) (mg/L)		0.35 (0.10 - 0.60)	0.32 (0.07 - 0.62)	0.40 (0.08 - 0.93)	0.32 (0.05 - 0.52)	0.28 (0.05 - 0.54)	0.29 (0.08 - 0.45)	Not available
Total Nitrogen (TN) (mg/L)		0.76 (0.45 - 0.97)	0.87 (0.57 - 1.08)	0.90 (0.49 - 1.15)	0.98 (0.55 - 1.43)	0.96 (0.57 - 1.24)	0.83 (0.52 - 1.48)	Not available
Orthophosphate Phosphorus (PO4) (mg/L)		0.019 (0.011 - 0.031)	0.022 (0.015 - 0.033)	0.021 (0.013 - 0.033)	0.026 (0.018 - 0.041)	0.025 (0.013 - 0.042)	0.019 (0.013 - 0.027)	Not available
Total Phosphorus (TP) (mg/L)		0.06 (0.03 - 0.15)	0.08 (0.03 - 0.24)	0.06 (0.03 - 0.11)	0.07 (0.03 - 0.17)	0.06 (0.03 - 0.12)	0.07 (0.03 - 0.17)	Not available
Silica (as SiO2) (mg/L)		2.11 (0.44 - 3.83)	2.59 (0.54 - 4.27)	2.38 (0.56 - 4.67)	2.98 (0.76 - 4.87)	3.18 (0.91 - 4.90)	2.75 (1.13 - 5.40)	Not available
Chlorophyll-a (μg/L)		3.3 (0.6 - 16.3)	3.8 (0.5 - 22.3)	2.2 (0.4 - 13.0)	1.4 (0.7 - 3.1)	3.3 (0.8 - 16.0)	3.3 (1.1 - 13.0)	Not available
E. coli (count/100mL)		88 (17 - 600)	36 (13 - 140)	66 (16 - 470)	150 (11 - 840)	32 (6 - 600)	3 (<1 - 12)	Not available
Faecal Coliforms (count/100mL)		190 (45 - 1500)	87 (24 - 590)	150 (30 - 1500)	390 (50 - 2500)	71 (8 - 1000)	6 (1 - 25)	Not available

Notes:

1. Data source: EPD Marine Water Quality in Hong Kong in 2020

2. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, Mid-depth, Bottom.

3. Data presented are annual arithmetic means of depth-averaged results except for E. coli and faecal coliforms that are annual geometric means.

4. Data in brackets indicate the ranges.

5.3.2.2 According to the EPD’s publication Marine Water Quality in Hong Kong 2020, the overall WQO compliance rate of the Deep Bay was 67% in 2020, as compared with a ten-year average of 47% in 2009-2018. The DO and NH₃-N WQOs were fully compiled with. Although Deep Bay, as compared with other WCZs, had higher nutrient levels with annual depth-averaged TIN levels exceeding the respective TIN WQOs, a noticeable long-term decrease in TIN levels since mid-2000s has been seen. Also, there were only few reported cases of red tides in Deep Bay, likely ascribed to the presence of considerable areas of unique wetland habitats and high background turbidity which could become a key factor limiting the photosynthesis and growth of phytoplankton in the bay despite ample local nutrients supply.

5.3.2.3 Overall speaking, there have been significant water quality improvements in Deep Bay since mid-2000s as a result of the joint efforts (e.g. provision and upgrading of sewage collection and treatment facilities) of the Hong Kong SAR and Shenzhen Governments in reducing its pollution loads.

5.3.2.4 In 2020, the North Western WCZ attained an overall WQO compliance rate of 67%, with the DO and NH₃-N WQOs fully met. The TIN level, however, could not meet the WQO under the influence of high background level in the Pearl River Estuary. Notwithstanding, the red tides occurrence in these waters remained very low even in the presence of rich nutrients, likely owing to strong tidal flushing hindering the proliferation of phytoplankton community in the area.

5.3.3 Inland Water

5.3.3.1 The water quality monitoring results at stations in Yuen Long Creek, namely YL1, YL2, YL3 and YL4, are shown in Table 5.11. According to the EPD’s publication “River Water Quality in Hong Kong in 2020”, the overall compliance rate of Yuen Long Creek in 2020 was 35%. In terms of Water Quality Index (WQI) grading, the two upstream monitoring stations (YL1 and YL2) were graded as “Bad” and “Fair” respectively in 2020, while the downstream stations (YL3 and YL4) were rated as "Very Bad" in 2020. In terms of E. coli, the four river monitoring stations of Yuen Long Creek recorded “High” or “Very High” E. coli levels (annual geometric mean >10,000 counts/100mL), mainly due to discharges from livestock farms, surface runoffs from unsewered villages as well as expedient connections in old districts.

Table 5.11 Summary Statistic of 2019 River Water Quality of Yuen Long Creek

	Yuen Long Creek				WPCO WQO
Parameters	YL1	YL2	YL3	YL4	(in inland waters)
Dissolved Oxygen (mg/L)	2.9 (1.8 - 5.5)	6.9 (4.9 - 9.5)	2.3 (1.7 - 6.2)	2.0 (1.1 - 3.4)	Waste discharges shall not cause the level of dissolved oxygen to be less than 4 mg/L
pH	7.3 (5.8 - 7.7)	7.8 (7.3 - 8.0)	7.4 (6.8 - 7.9)	7.4 (6.4 - 7.7)	The pH of the water should be within the range of 6.5-8.5
Suspended solids (mg/L)	27.5 (8.0 - 49.0)	7.7 (4.0 - 110.0)	24.0 (2.7 - 67.0)	50.0 (27.0 - 330.0)	Waste discharges shall not cause the annual median of suspended solids to exceed 20 mg/L
5-day Biochemical Oxygen Demand (mg/L)	16.5 (8.8 - 33.0)	7.6 (4.9 - 12.0)	51.0 (19.0 - 120.0)	120.0 (99.0 - 240.0)	Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed: 3 mg/L – YL1 & YL2; 5 mg/L – YL3 & YL4.
Chemical Oxygen Demand (mg/L)	32 (16 - 75)	14 (11 - 30)	47 (20 - 110)	99 (47 - 260)	Waste discharges shall not cause the chemical oxygen demand to exceed: 15 mg/L – YL1 & YL2; 30 mg/L – YL3 & YL4.
Oil & Grease (mg/L)	<0.5 (<0.5 - 1.4)	<0.5 (<0.5 - 0.6)	1.3 (<0.5 - 4.6)	6.7 (0.7 - 16.0)	Not available
E. coli (cfu/100mL)	170000 (31000 - 440000)	87000 (28000 - 390000)	680000 (460000 - 1200000)	2200000 (810000 - 7900000)	Should be zero per 100 mL for YL1 & YL2; Not exceed 1000 per 100 mL for YL3 & YL4, calculated as the running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days
Faecal Coliforms (cfu/100mL)	620000 (130000 - 5200000)	250000 (96000 - 1800000)	1600000 (1000000 - 3200000)	7500000 (1500000 - 30000000)	Not available
Ammonia-nitrogen (mg/L)	5.250 (2.600 - 18.000)	5.300 (2.900 - 8.100)	6.800 (2.100 - 16.000)	7.050 (3.700 - 13.000)	Not available
Nitrate-nitrogen (mg/L)	0.305 (0.013 - 0.910)	0.450 (0.390 - 1.500)	0.005 (<0.002 - 1.000)	0.004 (<0.002 - 0.027)	Not available
Total Kjeldahl Nitrogen (mg/L)	6.60 (3.10 - 49.00)	6.30 (3.50 - 10.00)	9.50 (3.00 - 21.00)	12.00 (5.10 - 24.00)	Not available
Ortho-phosphate Phosphorus (mg/L)	0.615 (0.330 - 7.200)	0.325 (0.190 - 0.450)	0.520 (0.049 - 2.100)	0.455 (0.032 - 1.300)	Not available
Total Phosphorus (mg/L)	1.30 (0.57 - 7.80)	0.56 (0.37 - 0.76)	1.30 (0.52 - 2.80)	1.25 (0.64 - 3.50)	Not available
Sulphide (mg/L)	0.04 (<0.02 - 0.08)	<0.02 (<0.02 - 0.03)	0.07 (<0.02 - 0.17)	0.20 (0.04 - 0.50)	Not available
Aluminium (µg/L)	<50 (<50 - 112)	<50 (<50 - 173)	<50 (<50 - 163)	<50 (<50 - 514)	Not available
Cadmium (µg/L)	<0.1 (<0.1 - <0.1)	<0.1 (<0.1 - <0.1)	<0.1 (<0.1 - <0.1)	<0.1 (<0.1 - <0.1)	Not available
Chromium (µg/L)	<1 (<1 - <1)	<1 (<1 - 2)	<1 (<1 - <1)	<1 (<1 - <1)	Not available
Copper (µg/L)	3 (2 - 7)	2 (<1 - 3)	3 (2 - 6)	3 (1 - 4)	Not available
Lead (µg/L)	<1 (<1 - <1)	<1 (<1 - <1)	<1 (<1 - 1)	<1 (<1 - 2)	Not available
Zinc (µg/L)	11 (<10 - 16)	<10 (<10 - 24)	10 (<10 - 28)	12 (<10 - 17)	Not available
Flow (m^3/s)	0.359 (0.259 - 0.575)	0.015 (0.010 - 0.044)	0.754 (0.483 - 1.758)	0.218 (0.147 - 0.421)	Not available

Notes:

1. Data source: EPD River Water Quality in Hong Kong in 2020

2. Data presented are in annual medians of monthly samples; except those for faecal coliforms and E. coli which are in annual geometric means

3. Figures in brackets are annual ranges

4. cfu – colony forming unit

5. Values at or below laboratory reporting limits are presented as laboratory reporting limits

6. Equal values for annual median (or geometric means) and ranges indicate that all data are the same as or below laboratory reporting limits

5.3.3.2 As part of this assessment, water quality surveys were conducted near the YLSEPP to supplement the baseline water quality information. The surveys were conducted as per a water quality survey plan submitted to EPD. The location of survey point (M1) is shown in Figure 5.1. Water quality surveys were conducted in 3 consecutive non-rainy days. On each sampling day, grab samples for laboratory analysis shall be collected at three specific times, including (i) 10:00am, (ii) 1:00pm and (iii) 4:00pm. All samples should be retrieved at the mid-depth of the water column. Water quality survey results are tabulated in Table 5.12 below and summarized in the following paragraph.

5.3.3.3 Water quality survey were conducted in 2 July 2021 to 4 July 2021 respectively to supplement the water quality data near the YLSEPP. Based on the water quality survey results, the watercourse contained high levels of organic pollutants (BOD₅ ranged from 10-580 mg/L and COD ranged from 10-1100 mg/L), nutrients (Ammonia ranged from 5.1-8.4 mg NH₃-N/L, TKN ranged from 6.4-94 mg N/L and TP ranged from 2.15-33.58 mg PO₄^3--P /L), E. coli (23000-530000 cfu/100mL) and low DO levels (2.63-5.44 mg/L) with limited flow velocity (0.1-0.6 m/s). High pollutants and E. coli levels indicating the watercourse was receiving untreated sewage and wastewater from the nearby existing scattered villages and brownfield sites. To conclude, it can be shown that the watercourse near the YLSEPP is generally subject to disturbance by human activities.

Table 5.12 Water Quality Survey Results under this Study

Parameters	M1
In-situ measurement
Temperature (°C)	29.3 (28.4 - 30.8)
Dissolved Oxygen (mg/L)	4.46 (2.63 - 5.44)
Dissolved Oxygen (% Saturation)	59.5 (34.8 - 70.8)
pH	7.4 (6.8 - 7.6)
Salinity	0.14 (0.10 - 0.56)
Flow Velocity (m/s)	0.2 (0.1 - 0.6)
Laboratory Results
Total Suspended Solids (mg/L)	27 (17 - 360)
5-day Biochemical Oxygen Demand (BOD5) (mg/L)	14 (10 - 580)
Chemical Oxygen Demand (COD) (mg/L)	71 (10 - 1100)
E. coli (cfu/100mL)	180000 (23000 - 530000)
Ammonia (mg NH3-N/L)	14 (5.1 - 84)
Nitrate (mg NO3^--N/L)	0.030 (<0.002 - 0.160)
Nitrite (mg NO2^--N/L)	0.148 (<0.002 - 1.90)
Total Kjeldahl Nitrogen (TKN) (mg N/L)	15 (6.4 - 94)
Total Phosphorus (TP) (mg PO4^3--P/L)	4.07 (2.15 - 33.58)

Notes:

1. Data presented are in medians of samples; except for E. coli which is in geometric means.

2. Figures in brackets are ranges

3. cfu – colony forming unit

4. Values at or below laboratory reporting limits are presented as laboratory reporting limits

5.4 Identification of Water Sensitive Receivers

5.4.1.1 Water Sensitive Receivers (WSRs) within 500m from the boundary of the Project were identified with reference to Annex 14 of the EIAO-TM. These WSRs are presented in Table 5.13 and shown in Figure 5.1.

Table 5.13 Water Sensitive Receivers

ID	Description	Type	Status
S1	Yuen Long Nullah	Watercourse	· Channelised
S2	Immediate upstream of the southern end of Yuen Long Nullah	Watercourse	· Largely natural · Fragmented
S3	Yeung Ka Tsuen Ecologically Important Stream (EIS)	EIS	· Semi-natural
S4	Near YLSEPP	Watercourse	· Semi-natural
S5	Near the southern end of Lam Tai East Road	Watercourse	· Largely natural · Fragmented
S6	Near One Hyde Park	Watercourse	· Heavily modified · Fragmented
S7	Near One Hyde Park	Watercourse	· Heavily modified · Fragmented
S8	Modified watercourse between Pak Sha Shan Road and Wong Nai Tun Tsuen Road	Watercourse	· Channelised
S9	Modified watercourse along the fence of the existing brownfield operation in the northern side of the Project site	Watercourse	· Semi-natural
S10	Catchwater which lead to Wong Nai Tun Irrigation Reservoir	Watercourse	· Channelised
P1	~300m near the southern end of Kung Um Road	Pond	· Overgrown with vegetation
P2	~300m near the southern end of Kung Um Road	Pond	· Overgrown with vegetation
P3	Near Yeung Ka Tsuen	Pond	· Overgrown with vegetation
P4	Near Wong Nai Tun Tsuen	Pond	· Overgrown with vegetation
W1	Within Tai Lam Country Park	Water Gathering Ground	· Rainwater is collected via catchwaters and is then stored in Wong Nai Tun Irrigation Reservoir.
W2	Tai Lam Country Park	Country Park	-
W3	Conservation Area	Conservation Area	-

5.4.1.2 Key marine WSRs in Deep Bay were identified and their indicative locations are shown in Appendix 5.1. These marine WSRs include:

Table 5.14 Marine Water Sensitive Receivers

ID (refer to Appendix 5.1)	Description
E1	Mai Po Marshes SSSI
E2	Mai Po and Inner Deep Bay Ramsar Site / Inner Deep Bay SSSI
E3	Oyster Culture Area
E4	Mangroves
E5	Mangroves along Shan Pui River

5.5 Assessment Methodology

5.5.1.1 According to Clause 3.4.6.2 of the Study Brief, the Study Area for this water quality impact assessment include areas within 500m from the boundary of the Project and would be extended to include WSRs, such as the natural streams and nullah in the vicinity of the Project that may be impacted. The potential water quality impact at the identified WSRs would be assessed with reference to the WQOs or criteria covered in Annex 6 of the EIAO-TM.

5.5.1.2 Potential water quality impacts during construction and operation of the Project are related to wastewater generated from land-based construction works and discharge/reuse of treated sewage effluent from the YLSEPP. There will be neither dredging, nor reclamation works, and all the construction works will be land-based. The methodology employed to assess potential water quality impacts associated with the construction and operation of the Project followed the detailed technical requirements given in Appendix D of the Study Brief and was based on the information presented in Section 2.

5.5.1.3 The assessment approach is referred to Annex 6 – Criteria for Evaluating Water Pollution and Annex 14 – Guidelines for Assessment of Water Pollution under the EIAO-TM.

5.5.2 Construction Phase

5.5.2.1 All the identified sources of potential water quality impacts from the land-based construction works were evaluated and their impact significance determined. Practical water pollution control measures were recommended to mitigate identified water quality impacts.

5.5.3 Operation Phase

Modelling Tools

5.5.3.1 The hydrodynamic and water quality modelling platforms were developed by Delft Hydraulics, namely the Delft3D-FLOW and Delft3D-WAQ respectively.

5.5.3.2 Delft3D-FLOW is a 3-dimensional hydrodynamic simulation programme which calculates non-steady flow and transport phenomena that result from tidal and meteorological forcing on a curvilinear, boundary fitted grid.

5.5.3.3 Delft3D-WAQ is a water quality model framework for numerical simulation of various physical, biological and chemical processes in 3 dimensions. It solves the advection-diffusion-reaction equation for a predefined computational grid and for a wide range of model substances.

5.5.3.4 It should be noted that the effluent of YLSEPP will be discharged to the nearby Yuen Long Nullah. It is expected that the effluent will be fully mixed with the waters in Yuen Long Nullah, flowing through the inland watercourses within the YLS NDA and finally discharge to Deep Bay. The characteristic of the effluent discharge would be considered as a river discharge and initial dilution modelling is considered not applicable.

Modelling Scenarios

5.5.3.5 The following three assessment scenarios have been evaluated in this Study:

· Scenario 1: Without YLSEPP;

· Scenario 2: Normal operation of YLSEPP (ADWF = 65,000 m³/day); and

· Scenario 3: Emergency Discharge from YLSEPP – 2-hr emergency discharge of raw sewage from YLSEPP under power / plant failure (total emergency discharge = 15,000m³).

Scenario 1: Without YLSEPP

5.5.3.6 Scenario 1 represents the “without Project” condition in Inner Deep Bay. According to the approved EIA report “Housing Sites in Yuen Long South” (AEIAR-215/2017), the likely future pollution loadings from the livestock waste and unsewered population which will be removed from the development of the YLS NDA had been calculated. The calculated pollution loadings are extracted and summarized in Table 5.15 below. These pollution loadings will be adopted in this scenario to address the “without YLSEPP” condition.

Table 5.15 Pollution Loading from Livestock Farm and Unsewered Population under “Without YLSEPP” Scenario

		Pollution Loadings
Parameters	Unit	Livestock Farm ^(1)	Unsewered Population ^(2)	Total
5-day Biochemical Oxygen Demand (BOD5)	kg/d	5.62	266	271.62
Suspended Solids (SS)	kg/d	5.62	100	105.62
Ammonia Nitrogen (NH3-N)	kg/d	99	83	182
Organic Nitrogen (Org-N)	kg/d	91.9	-	91.9
Total Inorganic Nitrogen (TIN)	kg/d	99	83	182
Total Nitrogen (TN)	kg/d	190.9	83	273.9
Total Phosphorus (TP)	kg/d	84.8	9.0	93.8
E. coli	count/d	4.11E+15	2.04E+11	4.11E+15

Notes: 1. Data extracted from Table 6.18 of the approved EIA report “Housing Sites in Yuen Long South” (AEIAR-215/2017).

2. Data extracted from Table 6.16 of the approved EIA report “Housing Sites in Yuen Long South” (AEIAR-215/2017).

Scenario 2: Normal Operation of YLSEPP

5.5.3.7 Under normal operation of the YLSEPP, tertiary treatment would be provided to reduce residual pollution loading of the treated effluent. It is expected that a new tertiary effluent polishing plant would be constructed with a capacity to treat ADWF up to 65,000 m³/day. Most tertiary treated effluent will be reused to supply YLS DA and other surrounding areas’ flushing water demand, which will cut down the freshwater demand in the region and will save precious freshwater resources. In normal operation, the treated effluent will be taken up by the Water Reclamation Facilities (WRF). The WRF is planned under CEDD’s D&C consultancy for YLS Development – Yuen Long South (YLS) Development Stage 2 works. Although there is no tentative commissioning year for WRP, it is planned that the WRP shall be available for the first year of population intake for YLS DA. To address the uncertainties on the implementation programme of the WRF, it was assumed as a worst case scenario that the maximum amount of tertiary effluent (65,000 m³/day) from the YLSEPP will be discharged to Yuen Long Nullah under this scenario for conservative assessment. The effluent flow and qualities of the YLSEPP assumed in the water quality modelling are tabulated in Table 5.16.

Table 5.16 Assumed Effluent Flow and Qualities of YLSEPP under Normal Operation Scenario

Parameters ^(4)	Unit	YLSEPP
Flow	m^3/day	65,000
5-day Biochemical Oxygen Demand (BOD5) ^(1)	mg/L	10
Suspended Solids (SS) ^(1)	mg/L	10
Ammonia Nitrogen (NH3-N) ^(2)	mg/L	2
Total Nitrogen (TN) ^(2)	mg/L	10
Total Phosphorus (TP) ^(2)	mg/L	1
E. coli ^(3)	count/100mL	100

Notes: 1. Data are 95th percentile of effluent quality of YLSEPP.

2. Data are annual average of effluent quality of YLSEPP.

3. Data are monthly geometric mean of effluent quality of YLSEPP.

4. The parameters salinity, Org-N and Total Oxidized Nitrogen (TON) are assumed to be <0.1 ppt, 0 mg/L and 8 mg/L respectively as conservative approach.

Scenario 3: Emergency Discharge from YLSEPP

5.5.3.8 Water quality modelling was carried out to address the impact from the discharge of untreated effluent under temporary failure of power supply as well as other incidents such as pump or equipment failure.

5.5.3.9 In the event of emergency situations, untreated effluent would be discharged directly into Deep Bay. The most common reason for system failure events refers to unstable power supply. Given that a number of contingency measures will be provided to the YLSEPP, such as provision of standby unit for all major equipment and back-up power for dual power supply, it is unlikely to have power outage for YLSEPP. In case if unstable power and system hanged, the standby units would serve the process and the system restarting time will be less than 2 hours according to DSD’s normal practice. Interim bypass after the Primary Sedimentation Tank (PST) will also be provided and raw sewage could be treated by primary solid/liquid separation in case there is failure in downstream treatment units to avoid raw sewage discharge.

5.5.3.10 For the purpose of illustrating the possible water quality effect under this emergency situation, it is assumed that the emergency discharge would occur for a period of 2 hours with a total discharge volume of 15,000 m³. This emergency discharge scenario was simulated for both dry and wet seasons. The emergency effluent would be discharged via the YLSEPP outfall to Yuen Long Nullah. The effluent flow and qualities of YLSEPP assumed in the water quality modelling under emergency situation are tabulated in Table 5.17.

Table 5.17 Assumed Effluent Flow and Qualities of YLSEPP under Emergency Discharge Scenario

Parameters ^(3)	Unit	Design Load for YLSEPP (2-hr Emergency)
Flow	m^3	15,000
5-day Biochemical Oxygen Demand (BOD5)	mg/L	210
Suspended Solids (SS)	mg/L	320
Ammonia Nitrogen (NH3-N)	mg/L	30
Total Kjeldahl Nitrogen (TKN)	mg/L	50
Total Phosphorus (TP)	mg/L	7
E. coli	count/100mL	4.0 x10^7

Notes: 1. Peak flow (Peak Factor: 2.74) is assumed during emergency discharge (i.e. 65,000 m³/day x 2/24 x 2.74 = 14,842 m³). The calculated value is rounded up to 15,000 m³ for conservative design assumption.

2. Design loads shown in above table are based on the influent characteristics of San Wai Sewage Treatment Facility (with additional safety margin added). The calculated value is rounded up to achieve a conservative design assumption.

3. The parameters salinity and Total Oxidized Nitrogen (TON) are assumed to be <0.1 ppt and 0 mg/L respectively as conservative approach.

Model Grid Layout and Properties

5.5.3.11 The Delft3D Yuen Long (YL) Model adopted under the EIA study for "Yuen Long Effluent Polishing Plant – Investigation, Design and Construction" (AEIAR-220/2019, hereafter YLEPP EIA Study) was adopted for this Study. Appendix 5.2 shows the grid layout and properties of the YL Model at the study area. As shown in Appendix 5.2, the grids were refined by means of a domain decomposition technique to achieve fine grid sizes near the Project. The YL Model covers the Hong Kong western waters including the North Western, North Western Supplementary, Western Buffer and Deep Bay Water Control Zones (WCZs) and the adjacent Mainland waters including the Pearl River Estuary. The YL Model consists of 38,590 grid cells. Grid size at the open waters is less than 400m in general. The grid cells near Mai Po Nature Reserve are about 70m. The grid quality of the detailed model is generally good except in some areas at or close to the land boundary. In view of the small flow velocity at the land boundary, numerical errors associated with the change of orthogonality should be small. Therefore, the closed grid cells at the coastlines have been adjusted to form a grid line that is parallel to land boundary (rather than keeping these closed grid cells orthogonal). Orthogonality at open grid cells has been checked to be adequate. The grid properties of detailed model grid including orthogonality, N-smoothness and M-smoothness are shown in Appendix 5.2.

5.5.3.12 The YL Model is linked to the Update Model, which was constructed, calibrated and verified under the project “CE42/97 Update on Cumulative Water Quality and Hydrological Effect of Coastal Development and Upgrading of Assessment Tool (Cumulative Study)”. Computations were first carried out using the Update Model to provide open boundary conditions to the YL Model. The Update Model covers the whole HKSAR and the adjacent Mainland waters including the discharges from Pearl River. The influence on hydrodynamics and water quality in these outer regions would be fully incorporated into the YL Model.

5.5.3.13 The performance of the YL Model has already been checked against with the Western Harbour (WH) Model which adopted under the EIA Study for “Cumulative Environmental Impact Assessment Study for the Three Potential Nearshore Reclamation Sites in the Western Waters of Hong Kong - Investigation” (CEIA Study). The results of the actual simulation periods (with sufficient spin-up periods) for water level, depth averaged flow speed, depth averaged flow directions, salinity predicted by the two models have been compared at two indicator points within the modelled area. The results of momentary flows and accumulated flows have been compared at the selected cross section to check for the consistency. Locations of the selected indicator points and cross section are shown in Appendix 5.2-04. Momentary flow represents the instantaneous flow rate at a specific time in m³/s whereas accumulated flow represents the total flow accumulated at a specific time in m³. The comparison plots are given in Appendix 5.2-05 to 5.2-16. The comparison plots indicated that the model results predicted by both models were in general consistent with each other which implied that the model settings of the YL Model as well as the nesting procedures were carried out correctly. There would have minor difference between the two model results which was caused by the difference in grid resolutions between the two models. As the YL Model has relatively finer grid resolution, it should have a more accurate representation of the bathymetry and coastline in Deep Bay waters as compared to the WH Model and hence the deviations are considered to be reasonable.

Model Bathymetry

5.5.3.14 The bathymetry schematization of this model has been updated based on the depth data from marine charts (Charts for Local Vessels 2018) produced by the Hydrographic Office of Marine Department, with incorporation of the projects affecting bathymetry (tabulated in Table 5.18). The hydrodynamic effect of the Contaminated Mid Pit (CMP) at East Sha Chau and The Brothers has also been incorporated and the final level at the CMP after capping was assumed in the modelling scenarios.

Simulation Periods

5.5.3.15 For each modeling scenario, the hydrodynamic simulations will be performed for both dry and wet seasons, and the simulation period covered a 15-day full spring-neap cycle (excluding the spin-up period) for each of the dry and wet seasons. The hydrodynamic results of 15 days will then be used repeatedly to drive the water quality simulations for at least a 15-day full spring-neap cycle (excluding the spin-up period) for each dry and wet season respectively. A spin-up period of 7 days and 45 days will be provided for hydrodynamic simulation and water quality simulation respectively. Hence, the hydrodynamic model simulation period will consist of a spin-up period of 7 days plus an actual simulation period of 15 days (total 22 days). For emergency discharge scenario, the simulation period will consist of a spin-up period of 45 days plus an actual simulation period of at least 15 days spring-neap cycle with long enough for recovery of the receiving water. In order to determine whether sufficient spin-up period is provided for both hydrodynamic and water quality the simulations, a Spin-up Test will be conducted by repeating the model run for one more simulation period to check the spin-up period is sufficient. The Spin-up Test results are presented in Appendix 5.3. It was found that the results of the two successive model runs were consistent with each other, which indicated that the spin-up period was sufficient.

Other Model Settings and Model Parameters

5.5.3.16 The general settings of the model such as the approach to the setup of boundary and initial conditions as well as the model coefficients and parameters followed those adopted under the YLEPP EIA Study.

Assessment Year and Coastline Configurations

5.5.3.17 Major factors that would affect the water quality stimulated would be (i) the change in pollution loading discharged to marine waters; and (ii) the change in coastline configurations in Year 2038.

5.5.3.18 The maximum Project design capacity of 65,000 m³/day was adopted for worst case assessment as it represents the worst case in terms of the amount of Project flow under operation phase.

5.5.3.19 The planned reclamations as listed in Table 5.18 would be completed and therefore would represent a worst case in terms of the tidal flushing and assimilation capacity of the marine water. Table 5.18 shows the coastal development projects incorporated in the coastline configurations for modelling.

Table 5.18 Projects Incorporated in Modelling

Projects	Source of Information on Project Layout
Hung Shui Kiu Effluent Polishing Plant	EIA Study Brief for “Hung Shiu Kiu Effluent Polishing Plant” (EIA Study Brief No.: ESB-312/2019)
Yuen Long Effluent Polishing Plant	EIA Report for “Yuen Long Effluent Polishing Plant” (EIAO Register No.: AEIAR – 220/2019)
Development of Integrated Waste Management Facilities (IWMF) Phase 1	EIA Report for “Development of IWMF Phase 1” (EIAO Register No.: AEIAR – 163/2012)
Harbour Area Treatment Scheme (HATS) Stage 2A	EIA Report for “HATS Stage 2A” (EIAO Register No.: AEIAR – 121/2008)
Hong Kong – Zhuhai – Macao Bridge (HZMB) Hong Kong Boundary Crossing Facilities (BCF)	EIA Report for “HZMB Hong Kong BCF” (EIAO Register No.: AEIAR – 145/2009)
Hong Kong Link Road (HKLR)	EIA Report for “HZMB – Hong Kong Link Road” (EIAO Register No.: AEIAR – 144/2009)
New Contaminated Mud Marine Disposal Facility (MDF) at Airport East / East Sha Chau Area	EIA Report for “New Contaminated Mud MDF at Airport East / East Sha Chau Area” (EIAO Register No.: AEIAR – 089/2005)
Expansion of Hong Kong International Airport into a Three-Runway System (3RS)	EIA Report for “3RS” (EIAO Register No.: AEIAR – 185/2014)
Sha Tin to Central Link (SCL)	EIA Report for “SCL Protection Works at Causeway Bay Typhoon Shelter: (EIAO Register No.: AEIAR – 159/2011), EIA Report for “SCL – Hung Hom to Admiralty Section” (EIAO Register No.: AEIAR – 166/2012) and EIA Report for “SCL – Tai Wai to Hung Hom Section” (EIAO Register No.: AEIAR – 167/2012)
Kai Tak Cruise Terminal	EIA Report for “Dredging, Works for Proposed Cruise Terminal at Kai Tak” (EIAO Register No.: AEIAR – 115/2007)
Tuen Mun – Chek Lap Kok Link (TM-CLKL)	EIA Report for “TM-CLKL” (EIAO Register No.: AEIAR – 146/2009)
Tung Chung New Town Extension (TCNTE)	EIA Report for “TCNTE” (EIAO Register No.: AEIAR – 196/2016)
Contaminated Mid Pit (CMP) at South Brothers	EIA Report for “New Contaminated Mud Marine Disposal Facility at Airport East / East Sha Chau Area” (EIAO Register No.: AEIAR-082/2004) (Remark: The hydrodynamic effect of the capped CMP will be incorporated into the hydrodynamic model. The final level after capping of the CMP is assumed in the model under all modelling scenarios)
CMP at East Sha Chau
Sunny Bay Reclamation	PWP Item No. 751CL - Planning and Engineering Study on Sunny bay Reclamation

Background Pollution Loading

5.5.3.20 The pollution loading of the proposed YLSEPP used for modelling was compiled with reference to the design flow and loads. The pollution loading of other background discharges to the marine water was based on the pollution loads complied for the YL Model under the YLEPP EIA study for cumulative assessment.

5.5.3.21 In addition to the background pollution loading extracted from the YLEPP EIA study, the following pollution loads from concurrent EIA projects were also considered to assess the cumulative impacts.

Table 5.19 Pollution Loads within Deep Bay from Concurrent EIA Projects

Projects	Source of Information on Project Layout
Hung Shui Kiu Effluent Polishing Plant (EIA Study Brief No.: ESB-312/2019)	A new Hung Shui Kiu Effluent Polishing Plant with a design secondary plus treatment capacity of 90,000 m^3/d
Yuen Long Effluent Polishing Plant (EIAO Register No.: AEIAR – 220/2019)	The existing Yuen Long STW will be upgraded to Yuen Long Effluent Polishing Plant with a design tertiary treatment capacity of 180,000 m^3/d.
North East New Territories New Development Areas (EIAO Register No.: AEIAR – 175/2013)	Increase population of around 180,000 with sewage treated at the expanded Shek Wu Hui Effluent Polishing Plant with a design tertiary treatment capacity of 190,000 m^3/d.
Development of Lok Ma Chau Loop (EIAO Register No.: AEIAR – 176/2013)	Development of Lok Ma Chau Loop with sewage treated at the proposed Lok Ma Chau Sewage Treatment Works with a design tertiary treatment capacity of 18,000 m^3/d

5.6 Identification, Prediction and Evaluation of Environmental Impacts

5.6.1 Construction Phase

5.6.1.1 General construction works for the Project would be land-based only. The potential sources of water quality impact associated with the land-based works would include:

· Wastewater from general construction activities;

· Construction site run-off;

· Construction works near watercourses;

· Accidental spillage and potential contamination of surface water and groundwater; and

· Sewage effluent from construction workforce.

5.6.1.2 It should be noted that the site formation works of the YLSEPP site (including the diversion or river works for S2, S4 and S9 as identified in Figure 5.1) are not covered in the scope of this Project and the environmental impact of such works should be duly reviewed in the separate EIA Report: AEIAR-215/2017- Housing Sites in Yuen Long South.

General Construction Activities

5.6.1.3 Wastewater generated from construction activities, including general cleaning and polishing, wheel washing, dust suppression and utility installation may contain high SS concentrations. It may also contain a certain amount of grease and oil. Potential water quality impacts due to the wastewater discharge can be minimised if construction and site management practices are implemented to ensure that litter, fuels, and solvents do not enter public drainage systems. It is expected that if the good site practice suggested in Section 5.7.1 are followed as far as practicable, the potential water quality impacts associated with construction activities would be minimal.

Construction Site Runoff

5.6.1.4 Surface runoff generated from the construction site may contained increased loads of SS and contaminants. Potential pollution sources of site run-off may include:

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

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

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

5.6.1.5 During rainstorms, site run-off would wash away the soil particles on unpaved lands and areas with topsoil exposed, if any. The run-off is generally characterized by high concentrations of SS. Release of uncontrolled site run-off would increase the SS levels and turbidity in the nearby streams. Site run-off may also wash away contaminated soil particles and therefore cause water pollution.

5.6.1.6 Windblown dust would be generated from exposed soil surfaces in the works areas. It is possible that windblown dust would fall directly onto the nearby water bodies when a strong wind occurs. Dispersion of dust within the works areas may increase the SS levels in surface runoff causing a potential impact to the nearby sensitive receivers.

5.6.1.7 It is important that proper site practice and good site management be followed to prevent run-off with high level of SS from entering the surrounding waters. Best Management Practices (BMPs) in controlling construction site discharges are recommended for this Project. With the implementation of BMPs to control run-off and drainage from the construction site, disturbance of water bodies would be avoided and deterioration in water quality would be minimal. Suggested measures to control construction site run-off and drainage are described in Section 5.7.1.

Construction Works near Watercourses

5.6.1.8 Construction activities near the inland watercourses may pollute the stormwater or inland waters due to potential release of construction wastes. Construction wastes are generally characterised by high concentration of SS and elevated pH. Adoption of good housekeeping and mitigation measures would reduce the generation of construction wastes and potential water pollution. The implementation of measures to control run-off and drainage water will be important for the construction works adjacent to the streams in order to prevent run-off and drainage water with high levels of SS from entering the water environment. With the implementation of BMPs and provision of mitigation measures as specified in ProPECC Note PN 1/94 "Construction Site Drainage" and ETWB TC (Works) No. 5/2005 "Protection of natural streams / rivers from adverse impacts arising from construction works" as detailed in Section 5.7.1, it is anticipated that water quality impacts would be minimal.

Accidental Spillage and Potential Contamination of Surface Water and Groundwater

5.6.1.9 The use of chemicals such as engine oil and lubricants, and their storage as waste materials has the potential to impact water quality if spillage occurs and enters adjacent streams. Waste oil may infiltrate into the surface soil layer, or runoff into the nearby streams, increasing hydrocarbon levels. Groundwater pollution may also arise from the improper use and storage of chemicals and petroleum products within the site area where groundwater infiltrates into the area. Infiltration of groundwater may occur at area where there are faults and / or fissures in the rock mass. The potential impacts could however be mitigated by practical mitigation measures and good site practices as described in Section 5.7.1.

Sewage Effluent from Construction Workforce

5.6.1.10 During the construction of the Project, the workforce on site will generate sewage effluent, which is characterized by high levels of BOD, ammonia and E. coli counts. According to Section 5.6.10 of the Construction Industry Council (CIC)’s publication “Reference Materials – Construction Site Welfare, Health and Safety Measures”, the number of toilet facilities should be provided at a ratio of not less than one for every 25 workers. Potential water quality impacts upon the local drainage and fresh water system may arise from these sewage effluents, if uncontrolled.

5.6.1.11 Temporary sewage generation can be adequately treated by interim sewage treatment facilities, such as portable chemical toilets. The number of the chemical toilets required for the construction sites should be subject to later detailed design, the capacity of the chemical toilets, and contractor's site practices. A licensed contractor should be employed to provide appropriate and adequate portable toilets and be responsible for appropriate disposal and maintenance.

5.6.1.12 Provided that sewage is not discharged directly into storm drains or inland waters adjacent to the construction site, temporary sanitary facilities are used and properly maintained, and mitigation measures as recommended in Section 5.7.1 are adopted as far as practicable, it is unlikely that sewage generated from the site would have a significant water quality impact.

5.6.2 Operation Phase

5.6.2.1 Potential water quality impacts associated with the operation phase would include:

· Project effluent discharge;

· Emergency effluent discharge;

· Treated effluent reuse;

· Transportation of organic waste;

· Wastewater from Sludge Treatment;

· Non-point source surface run-off from new impervious areas; and

· Chemical spillage from storage facilities.

Project Effluent Discharge

5.6.2.2 During the operation phase, sewage discharge will be the major water pollution source. As mentioned in the approved EIA report for YLS NDA, the pollution credit would be gained from removal of the existing pollution loads from livestock farms and unsewered population. Table 5.20 summarized the pollution loadings to Deep Bay under Scenario 1 and 2.

Table 5.20 Pollution Load to Deep Bay under Scenario 1 and 2

		Scenario 1	Scenario 2
Parameters	Unit	Pollution credit gained from removal of livestock farms and unsewered population	Pollution Loads discharge to Yuen Long Nullah from YLSEPP
BOD5	kg/d	271.62	650
SS	kg/d	105.62	650
NH3-N	kg/d	182	130
Org-N	kg/d	91.9	-
TIN	kg/d	182	650
TN	kg/d	273.9	650
TP	kg/d	93.8	65
E. coli	count/d	4.11E+15	6.50E+10

5.6.2.3 There would be an increase in pollution loads for BOD₅, SS and TIN/TN due to the proposed effluent discharge from YLSEPP (65,000 m³/day) under Scenario 2 which is the worst situation without implementation of the WRF. The water quality impact will be alleviated once the WRF is implemented.

5.6.2.4 The water quality simulation results of Scenario 1 and 2 are presented in Appendix 5.4 as contour plots for salinity, DO, BOD₅, TIN, UIA, TN, TP, E. coli, SS and sedimentation rates for dry and wet seasons respectively. All contour plots in Appendix 5.4 are presented as arithmetic averages expect for the E. coli levels which are geometric mean values and the DO levels which are 10th percentile values. The model results at different WSR points and EPD monitoring stations in inner subzone of Deep Bay WCZ (i.e. DM1, DM2 and DM3) are summarized in Appendix 5.5. The WSR points include Mai Po Marshes SSSI, Mai Po Inner Deep Bay Ramsar Site, oyster culture area and mangroves. The model results (in Appendix 5.5) are presented as arithmetic depth-averaged values except for the E. coli levels (which are geometric mean depth-averaged values) and the DO levels (which are presented as minimum depth-averaged values at E5 and 10th percentile depth-averaged values for the remaining WSRs and EPD stations) for comparison with the assessment criteria. Locations of WSRs and EPD stations are shown in Appendix 5.1.

Overall Water Quality in Deep Bay due to Project Effluent Discharge

Dissolved Oxygen

5.6.2.5 Under Scenario 1 (without Project), the DO levels predicted at all selected WSRs and EPD stations would meet the WQO except for the Mai Po Marshes SSSI (E1) and mangroves along Shan Pui River (E5) where the annual 10th percentile DO (depth average) was only 3.9 mg/L at E1, and the annual minimum DO was 2.8 mg/L at E5 respectively under Scenario 1. Operation of the YLSEPP would improve the 10th percentile DO (depth average) at E1 to 4.5 mg/L. The annual minimum DO at E5 would be improved to 3.3 mg/L. The low DO levels at these WSRs were mainly contributed from other background pollution sources and commissioning of the Project would improve the overall DO levels in Deep Bay.

Biochemical Oxygen Demand

5.6.2.6 The WQO stated that the BOD levels for Yuen Long & Kam Tin (Lower) subzone [i.e. Mangroves along Shan Pui River (E5)] should not exceed 5 mg/L. As shown in the modelling results, the predicted annual BOD levels at E5 would be 10.8 mg/L under Scenario 1 as compared to the WQO of no more than 5 mg/L. Operation of the YLSEPP would further increase the BOD levels at E5 to 11.2 mg/L [maximum increase in BOD level of 0.4 mg/L (3.7%)]. The high BOD levels at E5 was mainly contributed from other background pollution sources and non-compliances for BOD were predicted under both Scenarios 1 (without Project) and 2 (normal operation of YLSEPP).

5.6.2.7 There is no WQO for BOD available for Deep Bay marine waters. Under Scenario 1 (without Project), the BOD levels at all selected WSRs and EPD stations were predicted to be 3.3-11.0 mg/L. In comparison with the BOD levels under Scenario 1, the BOD levels predicted at the WSRs during the operation of YLSEPP (Scenario 2) would be increased to 3.3-11.4 mg/L (maximum increase in BOD level of 0.4 mg/L). As mentioned in Section 5.6.2.3, the proposed effluent discharge of 65,000 m³/day from YLSEPP under Scenario 2 is the worst situation without implementation of the WRF. The BOD impact will be alleviated once the WRF is implemented.

Salinity

5.6.2.8 The model predicted that the Project would only slightly affect the salinity levels at relevant WSRs and EPD stations in Deep Bay due to the effluent discharge of YLSEPP. The maximum decrease in salinity levels was found at E5 (Mangroves along Shan Pui River). The predicted salinity levels at E5 under Scenario 1 would be 11.2 ppt. The predicted salinity levels at E5 under Scenario 2 would be 10.4 ppt. The percentage decrease in salinity levels at E5 was calculated as 6.6%, which comply well with the WQO of no more than 10% change from the background levels. No adverse water quality impact for salinity would therefore be expected.

Unionized Ammonia / Total Inorganic Nitrogen

5.6.2.9 Without commissioning of the YLSEPP (Scenario 1), the predicted annual TIN and UIA levels at all selected WSRs and EPD Stations were predicted to be 1.81-6.73 mg/L and 0.026-0.126 mg/L respectively, which exceeded the WQO for TIN (0.7 mg/L) and UIA (0.021 mg/L). The operation of YLSEPP would reduce the UIA levels but increase the TIN levels at all selected WSRs in Deep Bay. In comparison with the TIN levels under Scenario 1, the TIN levels predicted at the WSRs during the operation of YLSEPP would be increased to 1.81-6.89 mg/L (maximum increase in TIN level of 0.16 mg/L). In comparison with the UIA levels under Scenario 1, the UIA levels predicted at the WSRs during the operation of YLSEPP would be reduced to 0.026-0.122 mg/L (maximum reduction in UIA level of 0.004 mg/L). The predicted non-compliance for TIN and UIA were mainly contributed by the background pollution sources, which carries a high loading of nitrogen nutrients (as compared to the WQO for TIN and UIA).

Total Nitrogen

5.6.2.10 No WQO for total nitrogen (TN) available for Deep Bay waters. The model predicted that the Project would slightly increase the TN levels (maximum increase in TN level of 0.1 mg/L) at all selected WSRs and EPD stations in Deep Bay. The TN levels in Deep Bay were mainly contributed from other background pollution sources.

Total Phosphorus

5.6.2.11 No WQO for total phosphorus (TP) available for Deep Bay waters. The model predicted that the Project would reduce the TP levels at all selected WSRs and EPD stations in Deep Bay. No adverse water quality impact for TP would therefore be expected.

Suspended Solids

5.6.2.12 The WQO stated that the SS levels should be no more than 30% from the ambient levels in general (except for E5). The model predicted that the Project would slightly increase the SS levels (maximum increase in ambient SS level of <1%) at all selected WSRs and EPD stations in Deep Bay and hence comply with the WQO for SS for no more than 30% increase from the ambient levels. No adverse water quality impact for SS would therefore be expected.

5.6.2.13 The WQO stated that the SS levels for Yuen Long & Kam Tin (Lower) subzone [i.e. Mangroves along Shan Pui River (E5)] should not exceed 20 mg/L. Under Scenario 1 (without Project), the SS level at all E5 was predicted to be 35.0 mg/L, which has already exceeded the WQO for SS (<20 mg/L). In comparison with the SS level under Scenario 1, the SS level predicted at E5 during the operation of YLSEPP (Scenario 2) would be increased to 35.4 mg/L [maximum increase in SS level of 0.4 mg/L (about 1%)]. The predicted non-compliance for SS was mainly contributed by the background pollution sources, which carries a high SS levels (as compared to the WQO).

E. coli

5.6.2.14 The WQO stated that the E. coli level for mariculture subzone [i.e. Oyster Culture Area (E3)] should not exceed 610 per 100 mL (geometric mean in one calendar year). The model predicted that the E. coli level at E3 would comply with the WQO (43 no./100mL, < 610) under all normal operation scenarios. For the mangrove at E5, without commissioning of the YLSEPP (Scenario 1), the E. coli levels was predicted to be 100 no./100mL. The operation of YLSEPP would reduce the E. coli level to 87 no./100mL as compared with the WQO of 1,000 no./100mL for Yuen Long & Kam Tin (Lower) subzone. For E1, E2, and E4, the model predicted that the Project would slightly reduce the E. coli levels (ranged from 21-28 no./100mL under Scenario 1 to 18-27 no./100mL under Scenario 2). Commissioning of the Project would improve the overall E. coli levels in Deep Bay and no adverse water quality impact would therefore be expected

5.6.2.15 It should be noted that the project effluent would be discharged to Yuen Long Nullah and finally discharge to Deep Bay. The identified WSRs in Table 5.13 (except Yuen Long Nullah) are located outside the downstream flow path and hence not affected by the project effluent. As compared to the existing Yuen Long Nullah as presented in Table 5.12, the tertiary treated effluent from YLSEPP (as presented in Table 5.16) would provide better water quality (in terms of BOD₅, SS, Ammonia, TN and TP) than the existing Yuen Long Nullah water quality which provide an opportunity to revitalize the Yuen Long Nullah.

5.6.2.16 In summary, the proposed YLSEPP would reduce the UIA, TP and E. coli levels in Deep Bay. There would be an increase in BOD, TIN, TN and SS levels in Deep Bay due to the effluent discharge from the proposed YLSEPP. The high levels of these parameters were mainly contributed from other background pollution sources which non-compliance for BOD, TIN and UIA were predicted even under the "without Project" (Scenario 1). The overall water quality impact however would be mitigated once the WRF is implemented. The overall DO levels were predicted to be improved in inner Deep Bay. The predicted change in salinity levels would comply well within the WQO of no more than 10% change from the background levels (as compared to Scenario 1). The residual water quality impact due to the project effluent discharge (under Scenario 2) would be further discussed in Section 5.8.

Emergency Discharge

5.6.2.17 During emergency situations, such as loss of power supply at the YLSEPP, or mechanical faults / equipment failures, untreated effluent may overflow and cause potential impacts at downstream WSRs. Emergency discharge due to emergency situations (e.g. power / equipment failure) may occur at the proposed YLSEPP. Scenario 3 assumed that an emergency discharge from the YLSEPP would occur for a period of 2 hours in case of power or plant failure. The model results at different WSR points and EPD monitoring stations in inner subzone of Deep Bay WCZ (i.e. DM1, DM2 and DM3) under Scenario 3 are summarized in Appendix 5.5 for DO, BOD, TIN, UIA, TN, TP, E. coli and SS. It should be noted that the water quality modelling results presented in Appendix 5.5 is for reference only which should not be used for assessing the impact of the short-term emergency discharge under Scenario 3.

Dissolved Oxygen

5.6.2.18 The impact of short-term emergency discharge under Scenario 3 on the WSRs as presented in Appendix 5.6 and 5.7 was insignificant in general. Biggest DO impact were predicted at Mai Po Marshes SSSI (E1) and mangrove along Shan Pui River (E5). According to Appendix 5.5, the predicted DO levels at E1 and E5 were predicted to be 4.4 mg/L and 3.2 mg/L during emergency discharge scenario, which were better than the predicted DO level of 3.9 mg/L and 2.8 mg/L under “without Project” scenario (Scenario 1). Full compliance of DO was predicted at all the remaining WSRs under emergency discharge scenario.

Unionized Ammonia / Total Inorganic Nitrogen

5.6.2.19 The biggest TIN and UIA impacts due to the short-term emergency discharge under Scenario 3 was observed at Mai Po Marshes SSSI (E1). The predicted TIN and UIA levels at E1 were 6.92 mg/L and 0.125 mg/L respectively. As mentioned previously, the Study Area are subject to high nitrogen nutrient loads due to the background discharge from Shenzhen River, with non-compliances for TIN and UIA recorded in Deep Bay WCZ even under both Scenarios 1 (without Project) and 2 (normal operation of YLSEPP).

Suspended Solids

5.6.2.20 The biggest SS impact due to the short-term emergency discharge under Scenario 3 was observed at Mai Po Marshes SSSI (E1) and mangrove along Shan Pui River (E5). The predicted SS levels at E1 and E5 were 40.4 mg/L and 35.5 mg/L, which was less than 30% ambient SS levels as predicted under Scenario 1. Full compliance with the SS objective would be achieved at all selected WSRs under emergency discharge scenario.

Biochemical Oxygen Demand

5.6.2.21 The biggest BOD impact due to the short-term emergency discharge under Scenario 3 was observed at mangrove along Shan Pui River (E5). The predicted BOD level at E5 was predicted to be 11.2 mg/L, which was higher than the predicted BOD level under Scenario 1 of 10.8 mg/L.

E. coli

5.6.2.22 The biggest E. coli impact due to the short-term emergency discharge under Scenario 3 was observed at mangrove along Shan Pui River (E5). The predicted E. coli level at E5 was predicted to be 89 no./100mL, which was better than the predicted E. coli level under Scenario 1 of 100 no./100mL.

Time Series Results at Selected WSRs

5.6.2.23 The time series plots under the emergency discharge scenarios at selected WSRs are presented in Appendices 5.6 and 5.7. The time series plots for E1, E2, E3, E4 and E5 are presented for key parameters of concern including DO, BOD, TIN, UIA, TN, TP, E. coli and SS to illustrate the time changes of pollution elevations at WSRs both close to and further away from the emergency discharge point.

5.6.2.24 Elevation on selected water quality parameters was observed at E1, E4 and E5. The elevated levels at these WSRs were observed right after the emergency discharge. The elevated levels at these WSRs would be recovered to the normal operation of YLSEPP (Scenario 1) in a relatively short time of about 1 day after termination of the emergency discharge. The more distant WSRs, including Oyster Culture Area (E3) was found not to be elevated during or after the emergency discharge event. Table 5.21 presented the maximum percentage change due to the emergency discharge from YLSEPP.

Table 5.21 Maximum Percentage Change due to Emergency Discharge

	Parameters (Depth Averaged)
WSRs	DO	BOD	TIN	UIA	TN	TP	E. coli	SS
Dry Season
E1	-4.1%	3.1%	1.3%	4.2%	1.4%	2.0%	446%	1.8%
E2	-0.9%	0.3%	0.4%	0.8%	0.4%	0.8%	3%	0.4%
E3	-0.1%	0.0%	0.1%	0.2%	0.1%	0.2%	0%	0.0%
E4	-2.3%	1.7%	1.0%	2.6%	1.0%	1.6%	456%	1.3%
E5	-5.1%	4.3%	1.5%	5.3%	1.7%	2.3%	137%	2.6%
Wet Season
E1	-12.1%	8.0%	2.3%	0.2%	2.6%	0.9%	7%	18.2%
E2	-2.4%	0.2%	0.7%	0.0%	0.5%	0.1%	0%	1.1%
E3	-0.1%	0.0%	0.0%	0.0%	0.0%	0.0%	0%	0.0%
E4	-9.1%	1.2%	1.2%	0.1%	1.1%	0.3%	0%	5.1%
E5	-9.3%	9.0%	2.4%	0.2%	3.1%	0.9%	29%	19.8%

Notes: The values in the above table refers to the percentage change between emergency discharge (Scenario 3) and normal operation of YLSEPP (Scenario 2) with maximum elevation for each water quality parameters. Hence,

Maximum Percentage Change = max. of [(concentration under Scenario 3) – (concentration under Scenario 2)] / (concentration under Scenario 2)

5.6.2.25 To minimise the risk of untreated effluent discharge due to emergency events, a number of contingency measures will be provided to the YLSEPP, such as provision of standby unit for all major equipment and back-up power for dual power supply. Details of the mitigation measures are discussed in Section 5.7.2. With the above design provision as contingency measures, the risk of failure of YLSEPP is considered to be negligible.

Treated Effluent Reuse

5.6.2.26 As stated in Section 2.4.1.5 and Table 2.2, reuse of treated effluent inside the YLSEPP would be limited to non-potable uses within YLSEPP only for polymer preparations within the treatment process and via distribution systems without any human contact. The quantity of internal non-potable reuse is about 5,000 m³/day. All the treated effluent reuse will be adopted within the proposed sewage treatment stream via automatic system and well serving as part of sewage treatment process. There would not be any human contacts to the reuse effluent during these processes and thus health impact consideration is not anticipated. While the purpose of effluent reuse will be limited to internal non-potable within the sewage treatment stream, the water quality from final effluent could be well adequate for direct use.

5.6.2.27 With treated effluent would be limited to internal non-potable uses, no adverse water quality impacts are anticipated from reuse of treated effluent inside the YLSEPP.

Transportation of Organic Waste

5.6.2.28 As stated in Section 2, food or organic wastes will be collected and pre-treated in the pre-treatment facilities outside YLSEPP and transported to YLSEPP for co-digestion with sludge generated from sewage treatment. The incoming organic waste will be transported to YLSEPP via fully enclosed trucks and unloaded into the food waste bunker. Automatic cleaning facilities would be provided to clean the exterior of the organic waste delivery trucks and organic waste bins immediately after unloading. Surplus wastewater generated from these cleaning activities should be collected and transferred to the wastewater treatment system in YLSEPP. Leachate collection equipment should be provided to the food waste bunker to convey leachate to the wastewater treatment system in YLSEPP. The organic waste reception facility would be enclosed, spills (if any) would be contained and will not contaminate the surface runoff. There would be no direct discharge of leachate and hence no adverse water quality impact is expected.

Wastewater from Sludge Treatment

5.6.2.29 The YLSEPP would have advanced sludge treatment capacity to cater its sludge and additional organic waste as stated in Section 5.6.2.28. The sludge treatment process of the YLSEPP would involve dewatering process and wastewater would be generated from the sludge treatment process which may have potential to cause water pollution. Notwithstanding the above, wastewater generated from the dewatering process will be fed back into the YLSEPP for treatment before final disposal. There would be no discharge of wastewater from the dewatering process and hence no adverse water quality impact is expected.

Surface Runoff

5.6.2.30 Potential water quality impact may also arise from surface runoff discharge of YLSEPP during operation phase. The surface runoff may contain small amount of suspended solids that may cause water quality impacts to the nearby receiving waters. However, impacts upon water quality would be minimal provided that a proper drainage system would be provided to receive surface runoff to the drainage system at the planning and design stages.

5.6.2.31 According to the DSD "Stormwater Drainage Manual", annual rainfall in Hong Kong is around 2,200 mm. However, the Update Study suggested that only rainfall events of sufficient intensity and volume would give rise to runoff and that runoff percentage is about 44% and 82% for dry and wet season, respectively. Therefore, only 1,386 mm of 2,200 mm annual rainfall would be considered as effective rainfall that would generate runoff (i.e. 1,386 mm = 2,200 mm × (82%+44%)/2).

5.6.2.32 The footprint of the YLSEPP is about 4.6 hectares. With development of the Project, there would be an increase in the total paved area. The additional paved area is about 7,270 m². All the treatment units in YLSEPP will be covered or enclosed to minimize the inflow of surface run-off from entering the treatment processes. Assuming 0.9 as the runoff coefficient, the non-point source pollution from surface run-off is estimated to be 25 m³/day (= 0.9 × 1,386 mm/year × 7,270 m²). It is anticipated that with proper implementation of best management practices as recommended in Section 5.7.2, no adverse water quality impact from non-point source surface run-off is expected.

Chemical Spillage

5.6.2.33 A number of chemicals, including ferric chloride and polymers, would be stored onsite and be used for wastewater treatment process such as sludge conditioning / dewatering. Adverse water quality impacts can be minimised by appropriate storage management and drainage system design as recommended in Section 5.7.2.

5.7 Mitigation of Adverse Environmental Impacts

5.7.1 Construction Phase

5.7.1.1 Measures as listed below are recommended to mitigate the potential water quality impacts from the land-based construction works.

General Construction Activities and Construction Site Runoff

Construction Site Runoff

5.7.1.2 The site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” should be followed as far as practicable to minimise surface run-off and the chance of erosion. The following measures are recommended to protect water quality, and when properly implemented should be sufficient to adequately control site discharges so as to avoid water quality impact.

5.7.1.3 Surface run-off from construction sites should be discharged into storm drains via adequately designed sand / silt removal facilities such as sand traps, silt traps and sedimentation basins. Channels, earth bunds or sand bag barriers should be provided on site to properly direct stormwater to such silt removal facilities. Perimeter channels at site boundaries should be provided as necessary to intercept storm run-off from outside the site so that it will not wash across the site. Catchpits and perimeter channels should be constructed in advance of site formation works and earthworks.

5.7.1.4 Silt removal facilities, channels and manholes should be maintained and the deposited silt and grit should be removed regularly (as well as at the onset of and after each rainstorm) to prevent overflows and localised flooding. Before disposal at the public fill reception facilities, the deposited silt and grit should be solicited in such a way that it can be contained and delivered by dump truck instead of tanker truck. Any practical options for the diversion and realignment of drainage should comply with both engineering and environmental requirements in order to provide adequate hydraulic capacity of all drains.

5.7.1.5 Construction works should be programmed to minimise soil excavation in the wet season (i.e. April to September). If soil excavation cannot be avoided in these months or at any time of year when rainstorms are likely, temporarily exposed slope surfaces should be covered e.g. by tarpaulin, and temporary access roads should be protected by crushed stone or gravel, as excavation proceeds. Intercepting channels should be provided (e.g. along the crest / edge of excavation) to prevent storm run-off from washing across exposed soil surfaces. Arrangements should always be in place in such a way that adequate surface protection measures can be safely carried out well before the arrival of rainstorm.

5.7.1.6 Earthworks final surfaces should be well compacted and the subsequent permanent work or surface protection should be carried out immediately after the final surfaces are formed to prevent erosion caused by rainstorms. Appropriate drainage like intercepting channels should be provided where necessary.

5.7.1.7 Measures should be taken to minimise the ingress of rainwater into trenches. If excavation of trenches in the wet season is necessary, they should be dug and backfilled in short sections. Rainwater pumped out from trenches or foundation excavations should be discharged into storm drains via silt removal facilities

5.7.1.8 Construction materials (e.g. aggregates, sand and fill material) on sites should be covered with tarpaulin or similar fabric during rainstorms

5.7.1.9 Manholes (including newly constructed ones) should always be adequately covered and temporarily sealed so as to prevent silt, construction materials or debris from getting into the drainage system, and to prevent storm run-off from getting into foul sewers. Discharge of surface run-off into foul sewers must always be prevented in order not to unduly overload the foul sewerage system.

Boring and Drilling Water

5.7.1.10 Water used in ground boring and drilling for site investigation or rock / soil anchoring should as far as practicable be re-circulated after sedimentation. When there is a need for final disposal, the wastewater should be discharged into storm drains via silt removal facilities

Wheel Washing Water

5.7.1.11 All vehicles and plants should be cleaned before they leave a construction site to minimise the deposition of earth, mud and debris on roads. A wheel washing bay should be provided at every site exit if practicable and washwater should have sand and silt settled out or removed before discharging into storm drains. The section of construction road between the wheel washing bay and the public road should be paved with backfill to reduce vehicle tracking of soil and to prevent site run-off from entering public road drains.

Rubbish and Litter

5.7.1.12 Good site practices should be adopted to remove rubbish and litter from construction sites so as to prevent the rubbish and litter from spreading from the site area. It is recommended to clean the construction sites on a regular basis.

Effluent Discharge

5.7.1.13 There is a need to apply to EPD for a discharge licence for discharge of effluent from the construction site under the WPCO. The discharge quality must meet the requirements specified in the discharge licence. All the runoff and wastewater generated from the works areas should be treated so that it satisfies all the standards listed in the TM-DSS. The beneficial uses of the treated effluent for other on-site activities such as dust suppression, wheel washing and general cleaning etc., can minimise water consumption and reduce the effluent discharge volume. If monitoring of the treated effluent quality from the works areas is required during the construction phase of the Project, the monitoring should be carried out in accordance with the relevant WPCO licence.

Construction Works near Watercourses

5.7.1.14 The practices outlined in ETWB TC (Works) No. 5/2005 “Protection of natural streams / rivers from adverse impacts arising from construction works” should also be adopted where applicable to minimise the water quality impacts on natural streams or surface water systems. Relevant mitigation measures from the ETWB TC (Works) No. 5/2005 are listed below:

· Construction works close to the inland waters should be carried out in the dry season as far as practicable where the flow in the surface channel or stream is low.

· The use of less or smaller construction plants may be specified in areas close to the water courses to reduce the disturbance to the surface water.

· Temporary storage of materials (e.g. equipment, chemicals and fuel) and temporary stockpile of construction materials should be located well away from any water courses when carrying out of the construction works.

· Stockpiling of construction materials and dusty materials should be covered and located away from any water courses.

· Construction debris and spoil should be covered up and / or disposed of as soon as possible to avoid being washed into the nearby water receivers.

· Proper shoring may need to be erected in order to prevent soil or mud from slipping into the watercourses

Accidental Spillage and Potential Contamination of Surface Water and Groundwater

5.7.1.15 Contractor must register as a chemical waste producer if chemical wastes would be produced from the construction activities. The Waste Disposal Ordinance (Cap 354) and its subsidiary regulations in particular the Waste Disposal (Chemical Waste) (General) Regulation should be observed and complied with for control of chemical wastes.

5.7.1.16 Any service shop and maintenance facilities should be located on hard standings within a bunded area, and sumps and oil interceptors should be provided. Maintenance of vehicles and equipment involving activities with potential leakage and spillage should only be undertaken within the areas appropriately equipped to control these discharges.

5.7.1.17 Disposal of chemical wastes should be carried out in compliance with the Waste Disposal Ordinance. The “Code of Practice on the Packaging, Labelling and Storage of Chemical Wastes” published under the Waste Disposal Ordinance details the requirements to deal with chemical wastes. General requirements are given as follows:

· Suitable containers should be used to hold the chemical wastes to avoid leakage or spillage during storage, handling and transport

· Chemical waste containers should be suitably labelled, to notify and warn the personnel who are handling the wastes to avoid accidents

· Storage area should be selected at a safe location on site and adequate space should be allocated to the storage area.

Sewage Effluent from Construction Workforce

5.7.1.18 The construction workforce on site will generate sewage. Sufficient chemical toilets should be provided in the works area, with a licensed waste collector employed to clean the chemical toilets on a regular basis.

5.7.1.19 Notices should be posted at conspicuous locations to remind the workers not to discharge any sewage or wastewater into the surrounding environment. Regular environmental audit of the construction site will provide an effective control of any malpractices and can encourage continual improvement of environmental performance on site. It is anticipated that sewage generation during the construction phase of the project would not cause water pollution problem after undertaking all required measures.

5.7.2 Operation Phase

Emergency Discharge

5.7.2.1 Given the sensitivity of inner Deep Bay in term of water quality and ecology, extensive effort will be expedited to avoid the occurrence for emergency discharge. In order to achieve this, the design of YLSEPP will be cautiously reviewed to include additional provisions including as follows:

· Applied peaking factors for all major treatment units and electrical and mechanical equipment to avoid equipment failure;

· By-pass mechanism would be provided for both coarse screens and fine screens in the inlet to avoid/minimize failure in coarse/fine screens; Interim by-pass would be provided after the PST to avoid raw sewage by-pass as much as possible;

· Standby unit for all major equipment would be provided in case of unexpected breakdown of pumping and treatment facilities such that the standby pumps and treatment facilities could take over and function to replace the broken pumps; and

· Back-up power for dual power supply would be provided in case of power failure to sustain the function of pumping and treatment facilities.

5.7.2.2 To provide a mechanism to minimise the impact of emergency discharges and facilitate subsequent management of any emergency, an Emergency Response Plan will be formulated prior to commissioning of YLSEPP to set out the emergency response procedures and actions to be followed in case of equipment or sewage treatment failure. The plant operators of YLSEPP should carry out necessary follow-up actions according to the procedures of the contingency plan to minimise any impacts on the identified WSRs due to emergency bypass. Regular maintenances and inspections to all treatment units, penstocks and plant facilities are necessary to maintain a good operation condition. A follow-up water quality monitoring exercise shall be conducted after each emergency discharge event to monitor the recovery of water quality in the vicinity.

Best Stormwater Management Practices

5.7.2.3 Best Management Practices (BMPs) for stormwater discharge are recommended to reduce stormwater pollution arising from the Project.

Design Measures

5.7.2.4 Exposed surface shall be avoided within the Project site to minimise soil erosion. The site shall be either hard paved or covered by landscaping area and plantation where appropriate.

5.7.2.5 Green areas / tree / shrub planting etc. should be introduced within the site as far as possible including open space and along roadside amenity strips, which can help to reduce soil erosion.

5.7.2.6 The existing watercourses in adjacent to the Project site will be retained to maintain the original flow path. The drainage system will be designed to avoid any case of flooding based on the 1 in 50 year return period.

Devices / Facilities to Control Pollution

5.7.2.7 Screening facilities such as standard gully grating and trash grille, with spacing which is capable of screening off large substances such as fallen leaves and rubbish should be provided at the inlet of drainage system.

5.7.2.8 Road gullies with standard design and silt traps and oil interceptors should be incorporated during the detailed design to remove particles present in storm water runoff.

Administrative Measures

5.7.2.9 Good management measures such as regular cleaning and sweeping of road surface / open areas is suggested. The road surface / open area cleaning should also be carried out prior to occurrence of rainstorm.

5.7.2.10 Manholes, as well as storm water gullies, ditches provided among the development areas should be regularly inspected and cleaned (e.g. monthly). Additional inspection and cleansing should be carried out before forecast heavy rainfall.

Chemical Spillage

5.7.2.11 Chemical should be stored on site at bunded area and separate drainage system as appropriate should be provided to avoid any spilled chemicals from entering the storm drain in case of accidental spillage. Also, adequate tools for cleanup of spilled chemicals should be stored on site and appropriate training shall be provided to staffs to further prevent potential adverse water quality impacts from happening.

5.8 Evaluation of Residual Impacts

5.8.1.1 With implementation of the recommended mitigation measures, no adverse residual water quality impact is expected in construction phase.

5.8.1.2 During operation phase, the residual water quality impacts resulting from Project operation are expected to be very minor in scale. The residual impact assessment takes into consideration the following factors:

· Under the worst case scenario, the largest changes in water quality would result in positive impacts to the environment. At Mai Po Marshes SSSI (E1) DO is expected to increase by 15.4%, and E. coli to decrease by 14.3%.

· All other changes in water quality are predicted to be very small variations slightly above or below ambient conditions. At Mai Po Marshes SSSI (E1), UIA and TP levels are expected to decrease by 3.2% and 2.7% respectively. Predicted changes in BOD, TIN, TN and SS would only amount to 3.6%, 2.4%, 1.2% and 0.7% increases above baseline scenario respectively. Given the existing highly eutrophic status of Deep Bay, these changes are very minor in scale.

· Under the worst case scenario, impacts are expected to be relatively localized, with predicted water quality changes at Mai Po Inner Deep Bay Ramsar Site/Inner Deep Bay SSSI (E2) even smaller than those predicted at Mai Po Marshes SSSI (E1).

· Although nutrient levels in Deep Bay WCZ are very high and the water is static, it is anticipated that red tide occurrence in Deep Bay WCZ is limited by the presence of high SS level in the water column, which reduces light penetration and limits the energy source for excessive algal growth. In contrast, the eastern and southern waters, with low background nutrient and low SS levels, have high frequency of red tide occurrences. Hence, nutrient (TIN, TN and TP) concentrations are not a critical factor for algal bloom in the Deep Bay waters, based on the fact that most red tides in Hong Kong occurred in eastern & southern waters with comparatively low nutrient levels. Further effect on the red tide occurrence due to the slight increase in TN (and TIN) level from the Project is not expected given that the existing Deep Bay waters has already contained abundant TN which is considered high enough to trigger red tide but the red tide occurrence in Deep Bay waters was limited by other environmental factors.

5.8.1.3 The residual impact assessment also needs to consider the likelihood of the impacts occurring. As described in Section 5.5.3.7, the worst case scenario assumes full discharge of 65,000m³/day treated YLSEPP effluent to Deep Bay. This scenario is unlikely to occur over an extended time-period for the following reasons:

· The Water Reclamation Facility (WRF) is a planned and committed project under CEDD's D&C consultancy for YLS Development - YLS Development Stage 2 works, that will receive all YLSEPP effluent in the future. Once the WRF is implemented, the treated effluent from YLSEPP will be further polished into reclaimed water which will cut down the freshwater demand in the region, save the precious freshwater resources in the region and reduce the pollution loading discharge to the Deep Bay waters. The residual impact on the water quality would decrease as comparing with the worst-case scenario of YLSEPP (i.e. 65,000m³/day discharge to Deep Bay).

· The YLSEPP will be completed and start operating by 2032, yet full build out of YLS development is not expected until 2038. It is therefore expected that the YLSEPP will not be operating in its full capacity for the first six years of operation. In this case, effluent discharge will also be lower, and potential water quality changes within Deep Bay will be even smaller in scale than that predicted under the worst case scenario.

5.8.1.4 With reference to the factors described in Section 5.8.1.2, the following factors have been considered with reference to Section 4.4.3 of the EIAO-TM regarding the predicted residual water quality impacts:

(i) effects on public health and health of biota or risk to life

5.8.1.5 Decreases in E. coli concentrations would potentially benefit public health and the health of biota. Due to the small scale of changes and existing high levels of eutrophication, other changes to water quality parameters in Deep Bay resulting from the project are not expected to have major adverse impacts to rare and/or endangered species.

(ii) the magnitude of the adverse environmental impacts

5.8.1.6 The magnitude of impacts for most water quality parameters is very small. The only substantial changes would be positive impacts to DO and E. coli.

(iii) the geographic extent of the adverse environmental impacts

5.8.1.7 The small changes to water quality will be most pronounced in the south of the Mai Po Inner Deep Bay Ramsar Site, where the Kam Tin River discharges into Deep Bay. Changes to water quality resulting from the project will attenuate with distance from the discharge point.

(iv) the duration and frequency of the adverse environmental impacts

5.8.1.8 As stated in Section 5.5.3.7 and 5.8.1.3, the WRF is a planned and committed project under CEDD's D&C consultancy for YLS Development - YLS Development Stage 2 works (from Year 2022 to Year 2033 based on the latest information) and the full build up of YLS Development is not expected until 2038. The duration and frequency of worst case scenario impacts are not expected to be long-term, given that the treated effluent will be further polished by the WRF once implemented.

(v) the likely size of the community or the environment that may be affected by the adverse impacts

5.8.1.9 The impact area is restricted to the Inner Deep Bay Area. Changes to water quality will be most pronounced in the south of the Mai Po Inner Deep Bay Ramsar Site, where the Kam Tin River discharges into Deep Bay. Changes to water quality resulting from the project will attenuate with distance from the discharge point.

(vi) the degree to which the adverse environmental impacts are reversible or irreversible

5.8.1.10 The small changes in water quality resulting from the project would be reversible, given that the treated effluent will be further polished by the WRF once implemented.

(vii) the ecological context

5.8.1.11 The intertidal wetlands habitats of the Mai Po Inner Deep Bay Ramsar Site are of high ecological value. Under the worst-case scenario, the largest changes in water quality at Mai Po SSSI would be at site E1, where DO is expected to increase by 15.4%, and E. coli to decrease by 14.3%. Any impact to ecological resources resulting from these water quality changes is expected to be positive. Other water quality parameters are expected increase or decrease by a very small amount compared to the baseline scenario. Given the existing highly eutrophic status of Deep Bay, these changes are considered very minor in scale, and unlikely to have any substantive impact on ecological resources.

(viii) the degree of disruption to sites of cultural heritage

5.8.1.12 The predicted exceedance would not involve any cultural heritage context.

(ix) international and regional importance

5.8.1.13 The intertidal wetlands habitats of the Mai Po Inner Deep Bay Ramsar Site are of high ecological value and recognized international importance. Under the worst-case scenario, the largest changes in water quality at Mai Po SSSI would be at site E1, where DO is expected to increase by 15.4%, and E. coli to decrease by 14.3%. Any impact to ecological resources resulting from these water quality changes is expected to be positive. Other water quality parameters are expected increase or decrease by a very small amount compared to the baseline scenario. Given the existing highly eutrophic status of Deep Bay, these changes are considered very minor in scale, and unlikely to have any substantive impact on ecological resources.

(x) both the likelihood and degree of uncertainty of adverse environmental impacts

5.8.1.14 Residual water quality impacts, which are expected to be minor in scale, are based on the worst-case scenario. As mentioned in Section 5.8.1.3, this worst-case scenario is unlikely to occur over an extended time-period due to the phasing of the YLS development and also commissioning of the planned and committed WRF.

5.9 Cumulative Impacts

5.9.1 Construction Phase

5.9.1.1 Based on the current construction programme, the Project would likely interact with some other projects, which may have cumulative environmental impacts. Table 5.22 summarise the concurrent projects located within 500m from the Project boundary that would contribute to the cumulative environmental impacts during construction phase.

Table 5.22 Potential Concurrent Projects

Potential Concurrent Projects	Tentative Construction Commencement Year	Tentative Commissioning Year
Water Reclamation Facilities	Under YLS DA Stage 2 works. The detailed construction programme is yet to be confirmed
YLS DA Stage 2 works	2022	2033 (target intake year)
YLS DA Stage 3 works	2031	2038 (target intake year)
YLS DA Stage 4 works	2031	2038 (target intake year)

Remark:

- The projects “YLS DA Stage 1 works”, “Hung Shui Kiu/Ha Tsuen New Development Area” and “Yuen Long Barrage Scheme – Investigation, Design and Construction” as mentioned in Table 2.4 are located more than 1.5km away from the YLSEPP. No construction phase cumulative impact would be anticipated.

5.9.1.2 According to the approved YLS DA EIA Study, YLS DA Stage 2 works will tentatively be commenced in 2022 and the intake year will be targeted in 2033; whilst YLS DA Stage 3 and Stage 4 works will tentatively be commenced in 2031 and 2032, while the intake year will be targeted in 2038.

5.9.1.3 The construction works for the concurrent projects as summarized in Table 5.22 would involve land-based construction works only and the potential water quality impact would include construction site runoff and drainage from works areas, wastewater from general construction activities, sewage generated by construction workforce, accidental spillage of chemicals, contaminated groundwater and wastewater, and construction works near watercourses. With proper adoption of mitigation measures and good site practices such as guidelines as given in ProPECC PN 1/94, potential water quality impact would be minimized. No adverse water quality impacts were anticipated.

5.9.1.4 As no significant water quality impact was expected from the Project and the identified concurrent projects during construction phase, no adverse cumulative water quality impacts were hence anticipated.

5.9.2 Operation Phase

5.9.2.1 As stated in Section 5.5.3, cumulative impacts from other projects have already been incorporated in the YL Model in terms of coastline configuration, background pollution loadings, etc. and hence the cumulative water quality impacts have already been assessed in Sections 5.6.2 and 5.7.2. The project effluent would reduce the UIA, TP and E. coli levels but increase in BOD, TIN, TN and SS levels in Deep Bay under the worst case scenario. The overall water quality impact however would be mitigated once the WRF is implemented. No adverse cumulative water quality impacts were anticipated during operation phase.

5.10 Environmental Monitoring and Audit

5.10.1 Construction Phase

5.10.1.1 The potential water quality impact from the land-based construction works can be controlled by the recommended mitigation measures. Nonetheless, in view of that the Project site is surrounded by ecological sensitive areas, water quality monitoring and regular site inspections should be undertaken during the construction to ensure that the recommended mitigation measures are properly implemented. A WPCO license should be obtained if there has construction drainage discharge. Self-monitoring and reporting should be carried out for monitoring the construction drainage discharge in accordance with the WPCO license.

5.10.2 Operation Phase

5.10.2.1 Under normal operation of YLSEPP, monitoring of the treated effluent quality will be governed by the WPCO license to ensure that the effluent quality would comply with the design standards, which is under the ambit of RO of EPD.

5.10.2.2 Water quality monitoring is recommended for the first year of normal operation and emergency discharge of YLSEPP. Detailed environmental monitoring procedures are provided in the standalone EM&A manual.

5.11 Conclusion

5.11.1 Construction Phase

5.11.1.1 Minor water quality impacts would be associated with land-based construction works. Water quality impacts may result from wastewater generated from the general construction activities, construction site runoff, construction works near inland watercourses, sewage effluent from workforce and accidental chemical spillage. The impacts could be mitigated and controlled by implementing the recommended mitigation measures. No adverse water quality impact from construction works for the YLSEPP is anticipated. No adverse residual water quality impact is expected.

5.11.2 Operation Phase

5.11.2.1 Mathematical modelling was conducted under this EIA to study the water quality impacts caused by the effluent discharge from the proposed YLSEPP. The model results indicated that the proposed YLSEPP would reduce the UIA, TP and E. coli levels in Deep Bay. There would be an increase in BOD, TIN, TN and SS levels in Deep Bay due to the effluent discharge from the proposed YLSEPP. The high levels of these parameters were mainly contributed from other background pollution sources which non-compliance for BOD, TIN and UIA were predicted even under the "without Project" scenario. The water quality impact will be alleviated once the Water Reclamation Facilities is implemented. The overall DO levels were predicted to be improved in inner Deep Bay. The predicted change in salinity levels would comply well within the WQO of no more than 10% change from the background levels.

5.11.2.2 It should be noted that the proposed YLSEPP has already adopted tertiary treatment which is the highest treatment standard and the best effluent quality that could be achieved in HK’s sewage treatment. Although the nutrient level in Deep Bay WCZ was very high and the water was static, it is anticipated that red tide occurrence in Deep Bay WCZ was limited by the presence of high SS level in the water column which could reduce the light penetration and limit the energy source for excessive algal growth. Hence, nutrient (TIN, TN and TP) was not a critical factor for algal bloom in the Deep Bay waters. Further effect on the red tide occurrence due to the slightly increase in TN (and TIN) level from the Project is not expected given that the existing Deep Bay waters has already contained abundant TN which is considered high enough to trigger red tide but the red tide occurrence in Deep Bay waters was limited by other environmental factors, e.g. high SS level. Furthermore, there will be a future water reclamation facility to further polish part of the treated effluent from YLSEPP into reclaimed water which will cut down the freshwater demand in the region, save the precious freshwater resources in the region and reduce the pollution loading discharge to the Deep Bay waters. The residual impact on the water quality would be decrease as comparing with worst-case scenario of YLSEPP (i.e. 65,000m³ discharge to Deep Bay).

5.11.2.3 For emergency discharge, the model results indicated that elevated levels of key water quality parameters would be recovered within 1 day after termination of the emergency discharge for WSRs within Inner Deep Bay. The more distant WSR i.e. Oyster Culture Area was found not to be affected by emergency discharge event. The occurrence of emergency discharge from the proposed YLSEPP can be minimised by the implementation of appropriate contingency measures, such as standby unit for all major equipment and back-up power for dual power supply would be provided for the proposed YLSEPP to prevent emergency situation as far as practicable. An Emergency Response Plan will be formulated prior to commissioning of YLSEPP to minimize the impact of emergency discharges and facilitate subsequent management of the emergency.

5.11.2.4 Other water quality impacts associated with the operation phase are identified as surface runoff from paved areas and accidental spillage. It is expected that these potential impacts can be prevented by adopting recommended mitigation measures.