CONTENTS

5. WATER QUALITY IMPACT ASSESSMENT 5-2

5.1 Introduction 5-2

5.2 Environmental Legislation, Policies, Standards and Criteria 5-2

5.3 Study Area 5-7

5.4 Baseline Water Quality Conditions 5-7

5.5 Water Sensitive Receivers 5-12

5.6 Potential Sources of Impacts 5-12

5.7 Impact Assessment 5-13

5.8 Mitigation Measures 5-17

5.9 Cumulative Impacts 5-22

5.10 Residual Impacts 5-24

5.11 Monitoring and Audit Requirement 5-24

5.12 Conclusion 5-25

5. WATER QUALITY IMPACT ASSESSMENT

5.1 Introduction

5.1.1 This Section presents the water quality impact assessment for the construction and operational phases of the Project. Potential impacts have been identified and their significance on the Water Sensitive Receivers (WSRs) evaluated. Mitigation measures are recommended, where necessary, to reduce the potential water quality impacts in order to control residual impacts to acceptable levels.

5.2 Environmental Legislation, Policies, Standards and Criteria

5.2.1 The following legislation and relevant guidance or non-statutory guidelines are applicable to the evaluation of water quality impacts associated with the construction and operation of the Project:

· Water Pollution Control Ordinance (WPCO);

· Technical Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM- DSS);

· Environmental Impact Assessment Ordinance (Cap. 499. S.16) and the Technical Memorandum on EIA Process (EIAO-TM), Annexes 6 and 14;

· Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN1/94);

· Drainage Services Department Practice Note No. 1/2015: Guidelines on Environmental and Ecological Considerations for River Channel Design; and

· Town Planning Board Guideline No. 12C (TPB PG-No. 12C) Requirement on No Net Increase in Pollution Load to Deep Bay.

Water Pollution Control Ordinance (WPCO)

5.2.2 The Water Pollution Control Ordinance (WPCO) is the primary legislation for the control of water pollution and water quality in Hong Kong. Under the WPCO, Hong Kong waters are divided into 10 Water Control Zones (WCZs). Each WCZ has a designated set of statutory Water Quality Objectives (WQOs). The proposed Project is located within the Deep Bay WCZ. The corresponding WQOs are summarised in Table 5.1 below.

Table 5. 1 - Water Quality Objectives of the Deep Bay Water Control Zones

Parameters	Objectives	Sub-Zone
Aesthetic appearance	(a) Waste discharges shall cause no objectionable odours or discolouration of the water.	Whole zone
	(b) Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent.	Whole zone
	(c) Mineral oil should not be visible on the surface. Surfactants should not give rise to a lasting foam.	Whole zone
	(d) There should be no recognisable sewage-derived debris.	Whole zone
	(e) 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.	Whole zone
	(f) Waste discharges shall not cause the water to contain substances which settle to form objectionable deposits.	Whole zone
Bacteria	(a) The level of Escherichia coli 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)
	(b) The level of Escherichia coli 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
	(c) The level of Escherichia coli 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
	(d) The level of Escherichia coli 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)
Colour	(a) Waste discharges shall 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
	(b) Waste discharges shall not cause the colour of water to exceed 50 Hazen units.	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Dissolved Oxygen	(a) Waste discharges shall not cause the level of dissolved oxygen to fall below 4 milligrams per litre for 90% of the sampling occasions during the year; values should be taken at 1 metre below surface.	Inner Marine Subzone excepting Mariculture Subzone
	(b) Waste discharges shall not cause the level of dissolved oxygen to fall below 4 milligrams per litre for 90% of the sampling occasions during the year; values should be calculated as water column average (arithmetic mean of at least 2 measurements at 1 metre below surface and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year.	Outer Marine Subzone excepting Mariculture Subzone
	(c) The dissolved oxygen level should not be less than 5 milligrams per litre for 90% of the sampling occasions during the year; values should be taken at 1 metre below surface.	Mariculture Subzone
	(d) Waste discharges shall not cause the level of dissolved oxygen to be less than 4 milligrams per litre.	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
pH	(a) The pH of the water should be within the range of 6.5-8.5 units. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units.	Marine waters excepting Yung Long Bathing Beach Subzone
	(b) Waste discharges shall not cause the pH of the water to exceed the range of 6.5-8.5 units.	Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	(c) The pH of the water should be within the range of 6.0-9.0 units.	Other inland waters
Temperature	Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 degrees Celsius.	Whole Zone
Salinity	Waste discharges shall not cause the natural ambient salinity level to change by more than 10%	Whole Zone
Suspended solids	(a) Waste discharges shall neither cause the natural ambient level to be raised by 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities.	Marine waters
	(b) Waste discharges shall not cause the annual median of suspended solids to exceed 20 milligrams per litre.	Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone, Ganges Subzone, Indus Subzone, Water Gathering Ground Subzones and other inland waters
Ammonia	The un-ionized ammoniacal nitrogen level should not be more than 0.021 milligram per litre, 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 milligram per litre, expressed as annual mean.	Inner Marine Subzone
	(c) Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.5 milligram per litre, expressed as annual water column average (arithmetic mean of at least 2 measurements at 1 metre below surface and 1 metre above seabed).	Outer Marine Subzone
5 day biochemical oxygen demand	(a) Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 3 milligrams per litre.	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	(b) Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 milligrams per litre.	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Chemical oxygen demand	(a) Waste discharges shall not cause the chemical oxygen demand to exceed 15 milligrams per litre.	Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones
	(b) Waste discharges shall not cause the chemical oxygen demand to exceed 30 milligrams per litre.	Yuen Long & Kam Tin (Lower) Subzone and other inland waters
Toxins	(a) 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
	(b) 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 milligrams per litre 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

Technical Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM- DSS)

5.2.3 All discharges during both the construction and operation phases of a proposed development are required to comply with the Technical Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS) issued under Section 21 of the WPCO. The TM-ICW defines acceptable discharge limits to different types of receiving waters. Under the TM-ICW, effluents discharged into the drainage and sewerage systems, inland and coastal waters of the WCZs are subject to pollutant concentration standards for specified discharge volumes. These are defined by the Environmental Protection Department (EPD) and are specified in licence conditions for any new discharge within a WCZ.

Technical Memorandum on EIA Process (EIAO-TM)

5.2.4 Annexes 6 and 14 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) provide general guidelines and criteria to be used in assessing water quality issues.

Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN 1/94)

5.2.5 Apart from the above statutory requirements, the Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN 1/94) issued by EPD in 1994, also provide useful non-statutory guidelines on the management of construction site drainage and prevention of water pollution associated with the construction activities.

Drainage Services Department Practice Note No. 1/2015: Guidelines on Environmental and Ecological Considerations for River Channel Design

5.2.6 The Practice Note presents the essential environmental and ecological considerations that should be taken into account in the design of river channels.

Town Planning Board Guideline No. 12C (TPB PG-No. 12C) Requirement on No Net Increase in Pollution Load to Deep Bay

5.2.7 The Project Site will partly fall within the Wetland Buffer Area. According to the TPB PG-No. 12C, the development within the Wetland Buffer Area shall not cause net increase in pollution load to Deep Bay.

5.3 Study Area

5.3.1 In accordance to Section 3.4.8.2 of the EIA Study Brief, the Study Area for this water quality impact assessment include areas within 500 meters from the boundary of the Project Site.

5.4 Baseline Water Quality Conditions

5.4.1 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 “River Water Quality in Hong Kong in 2018” and “Marine Water Quality in Hong Kong in 2018” which contains the latest information published by EPD on river and marine water quality.

EPD’s River Monitoring Stations

5.4.2 The nearest water quality monitoring stations are Yuen Long Creek (YL1 – YL4) and Kam Tin River (KT1 – KT2) (Figure 5.1). Table 5.2 and Table 5.3 summarise the water quality monitoring data for YL1 – YL4 and KT1 – KT2 respectively.

Table 5. 2 - Summary of River Water Quality Monitoring Data collected by EPD River Water Quality Monitoring Programme for Stations in YLTN, MN and SHN (2018)

Parameter	Unit	River WQOs (Yuen Long and Kam Tin Upper Subzone)	River WQOs (Yuen Long and Kam Tin Lower Subzone)	YL1 (MN)	YL2 (SHN)	YL3 (YLTN)	YL4 (YLTN)
Dissolved oxygen (DO)	mg/L	≥ 4	≥ 4	4.3	3.5	2.8	2.9
pH	-	6.5 – 8.5	6.5 – 8.5	7.1	7.3	7.3	6.9
Suspended solids (SS)	mg/L	≤20	≤20	15.5	7.3	25.5	61.0
5-Day Biochemical Oxygen Demand (BOD5)	mg/L	≤3	≤5	18.0	10.1	68	140
Chemical Oxygen Demand (COD)	mg/L	≤15	≤30	31	35	92	165
Escherichia coli (E.coli)	cfu/100mL	0	1,000	120,000	87,000	1,000,000	1,700,000
Ammonia-nitrogen (NH3-N)	mg/L	n.a.	n.a.	6.300	17.000	9.250	6.850
Total Kjeldahl nitrogen (TKN)	mg/L	n.a.	n.a.	7.80	19.50	13.50	11.00
Total phosphorus	mg/L	n.a.	n.a.	0.97	2.65	1.45	1.10
Flow rate	m^3/s	n.a.	n.a.	0.125	0.023	0.400	0.135

Notes:

· Data source: EPD River Water Quality in Hong Kong in 2018

· Data presented are in annual medians of monthly samples; except those for E. coli which are in annual geometric means.

· cfu - colony forming unit.

· n.a. indicated the absence of applicable WQOs

· Underlined figures = non-compliance of WQO

· The WQO for Nitrogen under the WPCO refers to level of Un-ionized Ammoniacal Nitrogen. As such, there is no applicable WQO to Ammonia-Nitrogen.

· MN = Main Nullah; SHN = San Hui Nullah; YLTN = Yuen Long Town Nullah

5.4.3 The 2018 monitoring data indicate that river water quality at the upstream area (i.e. YL1 and YL2) of MN and SHN is generally better than that at the downstream area (i.e. YL3 and YL4) of the YLTN. The levels of Suspended Solids (SS), Chemical Oxygen Demand (COD), 5-day Biochemical Oxygen Demand (BOD₅) and E. coli were higher at the downstream than the upstream stations.

5.4.4 It is also noted that generally high levels of E. coli were recorded at all stations. For Dissolved Oxygen (DO), the values were higher at the upstream stations (MN station YL1 and SHN station YL2). BOD₅, COD and E. coli levels exceeded the WQO at all stations.

5.4.5 In terms of WQO compliance rate, overall compliance rate of the four monitoring stations in 2018 was 38%. In 2018, the Water Quality Index (WQI) was graded as “bad” at MN station YL1 and SHN station YL2 and as “very bad” at YLTN stations YL3 and YL4.

Table 5. 3 - Summary of River Water Quality Monitoring Data collected by EPD River Water Quality Monitoring Programme for Stations in Kam Tin River (2018)

Parameter	Unit	River WQOs (Yuen Long and Kam Tin Upper Subzone)	River WQOs (Yuen Long and Kam Tin Lower Subzone)	Stations in Kam Tin River
				KT1	KT2
Dissolved oxygen (DO)	mg/L	≥ 4	≥ 4	5.1	3.1
pH	-	6.5 – 8.5	6.5 – 8.5	7.1	7.3
Suspended solids (SS)	mg/L	≤20	≤20	8.5	31.5
5-Day Biochemical Oxygen Demand (BOD5)	mg/L	≤3	≤5	8.5	26.5
Chemical Oxygen Demand (COD)	mg/L	≤15	≤30	20	50
Escherichia coli (E.coli)	cfu/100mL	0	1000	71,000	82,000
Ammonia-nitrogen (NH3-N)	mg/L	n.a.	n.a.	4.850	9.000
Total Kjeldahl nitrogen (TKN)	mg/L	n.a.	n.a.	6.60	12.00
Total phosphorus	mg/L	n.a.	n.a.	0.91	2.10
Flow rate	m^3/s	n.a.	n.a.	0.333	0.229

Notes:

· Data source: EPD River Water Quality in Hong Kong in 2018

· Data presented are in annual medians of monthly samples; except those for E. coli which are in annual geometric means.

· cfu - colony forming unit.

· n.a. indicated the absence of applicable WQOs

· Underlined figures = non-compliance of WQO

· The WQO for Nitrogen under the WPCO refers to level of Un-ionized Ammoniacal Nitrogen. As such, there is no applicable WQO to Ammonia-Nitrogen.

5.4.6 In 2018, BOD₅, COD and E. coli levels exceeded the WQO at both stations. The overall compliance rate of the two monitoring stations in 2018 was 47%. The WQI at KT1 and KT2 was graded as “fair” and “bad” respectively in 2018.

EPD’s Marine Monitoring Stations

5.4.7 The existing marine water quality can be referred to EPD’s marine water quality data obtained from routine monitoring carried out at Inner Deep Bay WCZ (i.e. Stations DM1, DM2, DM3) (Figure 5.2). The EPD monitoring data collected in 2018 is summarised in Table 5.4.

5.4.8 Deep Bay is a large shallow and semi-enclosed bay on the eastern side of the Pearl River Estuary with an average depth of about 3m. Deep Bay has the largest and most important mangrove habitat in Hong Kong. The mudflats of inner Deep Bay also have high conservation value as an important feeding ground for a huge number of resident and wintering birds. The water body’s shallowness, presence of muddy habitats as well as the strong riverine inputs from within and outside, have contributed to its naturally high level of suspended solids.

5.4.9 The overall WQO compliance rate of the Deep Bay WCZ was 53% in 2018 as compared with a ten-year average of 46% in 2008-2017. Similar to previous years, the Deep Bay WCZ had relatively high nutrient levels. The Deep Bay WCZ is further divided into two subzones with different TIN objectives. The annual depth-averaged TIN levels in the Deep Bay Inner Marine Subzone and Deep Bay Outer Marine Subzone were higher than their respective Total Inorganic Nitrogen (TIN) WQO. Although the TIN levels are relatively high in this WCZ as compared with other WCZs, there were few reported cases of red tides in Deep Bay because the high background suspended solid level has limited the photosynthesis and growth of phytoplankton in water. In 2018, all five stations had NH3-N WQO full compliance, while two of the stations in Inner Subzone of Deep Bay (DM1 and DM2) did not meet the DO WQO.

5.4.10 Despite that the TIN level in Deep Bay is relatively high, there are noticeable signs of water quality improvement since the mid-2000s as a result of joint efforts by both Hong Kong and Shenzhen Governments in reducing pollution loads, e.g., the voluntary farm surrender schemes for poultry and pig farms implemented in 2005-2008.

Table 5. 4 - Summary of EPD’s Routine Marine Water Quality Data for Inner Deep Bay WCZ in Year 2018

Parameter	Unit	Inner Deep Bay
		DM1	DM2	DM3
Temperature	°C	25.4 (18.8 - 31.6)	25.3 (19.0 - 31.3)	25.5 (18.7 - 31.5)
Salinity	-	14.5 (3.2 - 23.3)	16.7 (4.8 - 25.3)	21.3 (10.0 - 27.9)
Dissolved Oxygen (Depth average)	mg/L	4.5 (1.5 - 8.5)	5.1 (2.9 - 7.5)	5.5 (3.4 - 7.7)
Dissolved Oxygen (Bottom)	mg/L	NA	NA	NA
Dissolved Oxygen (Depth average)	% saturation	59 (20 - 112)	68 (40 - 99)	75 (49 - 98)
Dissolved Oxygen (Bottom)	% saturation	NA	NA	NA
pH	-	7.2 (6.9 - 7.5)	7.3 (6.9 - 7.6)	7.4 (7.0 - 7.7)
Secchi Disc Depth	m	1.2 (1.0 - 1.8)	1.2 (0.9 - 1.6)	1.5 (1.0 - 2.0)
Turbidity	NTU	27.1 (12.8 - 54.1)	26.6 (12.2 - 66.2)	10.6 (3.9 - 21.9)
Suspended Solids	mg/L	39.9 (21.0 - 98.0)	38.7 (18.0 - 92.0)	16.5 (8.6 - 31.0)
5-day Biochemical Oxygen Demand	mg/L	2.4 (0.8 - 4.0)	2.2 (0.9 - 4.2)	1.1 (0.6 - 3.4)
Ammonia Nitrogen	mg/L	0.679 (0.016 - 1.900)	0.475 (0.210 - 0.960)	0.121 (<0.005 - 0.220)
Unionised Ammonia	mg/L	0.007 (<0.001 - 0.026)	0.005 (<0.001 - 0.015)	0.002 (<0.001 - 0.005)
Nitrite Nitrogen	mg/L	0.4 (0.093 - 1.000)	0.347 (0.065 - 1.000)	0.192 (0.035 - 0.480)
Nitrate Nitrogen	mg/L	1.513 (0.570 - 2.400)	1.378 (0.710 - 1.800)	0.965 (0.750 - 1.300)
Total Inorganic Nitrogen	mg/L	2.59 (1.54 - 4.00)	2.2 (1.78 - 3.06)	1.28 (0.94 - 1.78)
Total Kjeldahl Nitrogen	mg/L	1.48 (0.59 - 4.00)	0.91 (0.57 - 1.80)	0.37 (0.19 - 0.74)
Total Nitrogen	mg/L	3.39 (2.33 - 5.52)	2.64 (2.19 - 3.90)	1.53 (1.11 - 2.10)
Orthophosphate Phosphorus	mg/L	0.174 (0.099 - 0.320)	0.144 (0.077 - 0.250)	0.076 (0.031 - 0.160)
Total Phosphorus	mg/L	0.26 (0.18 - 0.36)	0.23 (0.18 - 0.36)	0.11 (0.08 - 0.16)
Silica (as SiO2)	mg/L	8.15 (3.80 - 18.00)	7.12 (2.00 - 17.00)	4.26 (1.00 - 9.80)
Chlorophy ll-a	µg/L	4.5 (1.6 - 9.0)	5.1 (1.6 - 15.0)	2.7 (0.9 - 7.0)
E.coli	cfu/100mL	990 (49 - 200000)	380 (18 - 7900)	11 (1 - 370)
Faecal Coliforms	cfu/100mL	1900 (110 - 600000)	820 (67 - 31000)	30 (2 - 900)

Notes:

1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, mid-depth, bottom.

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

3. Data enclosed in brackets indicate the ranges.

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

5. Data adopted from EPD Marine Water Quality Hong Kong in 2018

Monitoring data from EIA Reports

5.4.11 Water quality sampling were conducted for the Improvement of Yuen Long Town Nullah (Town Centre Section) EIA. The samplings were conducted in four consecutive non-rainy days between 5 August and 8 August 2017 and the result are presented in Table 5.5. The upstream dry weather flow (DWF) coming from Kung Um Road Nullah (Southwest branch at the junction of Kung Um Road and Tai Shu Ha Road East) has better water quality when compared with DWF from San Hui Nullah / Town Centre Section / East Nullah and West Nullah.

Table 5.5 – Water Sampling Result in YLTN EIA

	Kung Um Road Nullah	San Hui Nullah	End of Town Centre Section	YLTN – West Nullah	YLTN – East Nullah
Water Quality Index	9 (Fair)	11 (bad)	13 (bad)	11 (bad)	15 (very bad)
DO (mg/L)	5.46	3.80	1.86	2.38	1.28
Suspended Solid (mg/L)	14.70	7.45	14.60	4.39	31.1
Ammonia Nitrogen (mg/L)	3.55	12.77	3.79	1.79	8.80
Total Kjeldahl Nitrogen (mg/L)	4.41	13.90	5.01	2.94	11.33
Total Phosphorus (mg/L)	0.53	2.59	0.64	0.32	1.15
BOD (mg/L)	8.47	7.15	14.20	6.20	64.4

Notes:

1. Underlined figures = non-compliance of WQO.

2. The presented results are the average measured value.

5.5 Water Sensitive Receivers

5.5.1 Key water sensitive receivers identified within the Study Area of the Project include:

· Yuen Long Town Nullah;

· Shan Pui River;

· Kam Tin River;

· Old Kam Tin River;

· Deep Bay Wetland Buffer Area;

· Deep Bay Wetland Conservation Area;

· Mai Po Inner Deep Bay Ramsar Site;

· Mangroves; and

· Active and inactive fish ponds.

5.5.2 Locations of the identified WSRs are shown in Figure 5.2.

5.6 Potential Sources of Impacts

Construction Phase

5.6.1 The main construction activities associated with the Project that have the potential to cause water quality impacts involve the following:

· Construction activities for the Barrage Scheme including excavation, drainage channel training and diversion works, ground breaking, excavation, backfilling works;

· Construction activities for parapet walls, refinements to the existing intersection of YLN & YLBF, revitalisation works including ground breaking, concreting works, excavation, backfilling works;

· Decommissioning work of existing low flow pumping station /inflatable dam;

· Land-based site runoff and pollutants entering the receiving waters.

Operation Phase

5.6.2 Potential water quality impacts that may arise during the operation phase of the Project are identified as follows:

· Increase in suspended solids during operation and maintenance of the revitalised YLN;

· Changes in hydrodynamic conditions; and

· Change in sediment deposition and erosion pattern.

5.7 Impact Assessment

Construction Phase

Construction Activities for the Barrage Scheme

5.7.1 Discharges and runoff from the site during the construction phase, particularly during site formation, sediment removal, drainage channel training and diversion works, excavation and backfilling works, will contain suspended solids which could be a source of water pollution. For sediment removal, excavation works will not be carried out in open waters. A containment structure as described in Section 5.8.8 will be setup to avoid water quality impact during the sediment removal operation.

Potential adverse water quality impacts may thus arise at the WSRs if construction site runoff is allowed to spill outside the construction site area and drain into the nearby streams, storm drain or natural drainage without treatment. However, it is anticipated that no unacceptable water quality impacts on the identified WSRs would arise from the land-based works if standard site practices and mitigation measures, described in Section 5.8 are in place and properly implemented.

Land-based Construction Activities for Parapet Walls at YLN and KTR

5.7.2 The construction of parapet walls will only involve land-based works. Potential adverse water quality impacts may thus arise at the WSRs if construction site runoff is allowed to spill outside the construction site area and drain into the nearby YLN and KTR. However, it is anticipated that no unacceptable water quality impacts on the identified WSRs would arise from the land-based works if standard site practices and mitigation measures, described in Section 5.8 are in place and properly implemented.

Refinements to the existing intersection of YLN & YLBF

5.7.3 The proposed modification works will be conducted during low flow conditions, and the proposed works involve minor excavation and concreting works at the existing concrete diversion structure. With the implementation of mitigation measures, adverse water quality impact during construction phase is not anticipated. A photo for the diversion structure is shown in Appendix 6.4B (View Point I).

Concreting work

5.7.4 Concreting work will be required for part of the construction of the Barrage Scheme and other ancillary infrastructure. If concrete spillage, washdown or concrete curing water is inadvertently introduced into the water course, the pH level will be raised and may result in contamination of the water or creation of toxic conditions for the aquatic life. Turbidity and suspended solids levels will also increase if contaminated by runoff containing waste concrete. Other possible sources of concrete-enriched waste water include water from wheel wash facilities, and washout from concrete lorries/pumps.

Decommissioning Works of Existing Low-flow Pumping Station / Inflatable Dam

5.7.5 The decommissioning of the existing low-flow pumping station (LFPS) and inflatable dam will be conducted during low flow conditions for safety issues. The proposed works would involve suspending the pumping operations and deflation / removal of the existing inflatable dam. The operation will not involve any excavation and building works, adverse water quality impact during construction phase is therefore not anticipated.

Revitalisation works on the YLN

5.7.6 Ground breaking, excavation and backfilling works for revitalisation works inside the YLN may lead to elevated SS levels and dispersal of SS to the downstream section of the YLN if in contact with river water. River water will be required to be diverted from the works area as far as practicable before works commence (i.e. working in dry condition) which will limit the potential of release of SS into river water and subsequent dispersal to downstream area. Water quality impacts due to release and dispersal of SS from construction activities on the nullah bed are thus not expected to be unacceptable. During revitalisation works, the water level within YLN will be minimized by the operation of barrage so as to ensure no overflow is occurred from YLN to the works area (i.e. the dry zone with the use of containment structures, in conjunction with the provision of a silt curtain where practicable).

5.7.7 Overall, no unacceptable adverse impacts to water quality are anticipated to occur as a result of the revitalisation works on the nullah bed with proper implementation of the mitigation measures as proposed in Section 5.8.

Sewage Discharge from the Construction Workforce

5.7.8 Sewage will be generated from the construction workforce, site office’s sanitary facilities and from portable toilets. If not properly managed, these wastewaters could cause adverse water quality impacts, odour and potential health risks to the workforce by attracting pests and other disease vectors.

5.7.9 According to the latest construction programme and estimate, there will be a maximum of 100 construction workers on-site during the peak construction period of the Project. With an estimated sewage generation rate of 0.15 m³/worker/day, about 15 m³ of sewage will be generated per day. Adequate number of portable toilets will be provided at the Project Site to ensure that sewage from site staff is properly collected and managed. No unacceptable environmental impacts are envisaged provided that the portable toilets are properly maintained by the contractor and the collected sewage is disposed at the designated sewage treatment works.

Accidental Spillage / Uncontrolled discharge from General Construction Activities

5.7.10 The following pathways of uncontrolled discharge and spillage of contaminants from general construction activities may lead to adverse water quality impacts to nearby WSRs:

· Uncontrolled discharge of wastewater generated from concrete and vehicle washing;

· Uncontrolled discharge of debris and rubbish such as packaging, construction waste and refuse etc; and

· Spillages of liquid and chemicals stored on-site, such as oil, diesel and solvents etc.

5.7.11 Wastewater generated from concrete and vehicle washing may contain elevated levels of SS. Wastewater from concrete washing is also noted with high pH value. Debris and rubbish generated by the construction activities, if allowed to enter nearby streams, storm drain or natural drainage, may cause blockages. The spillage of liquids and chemicals stored on-site may also result in water quality impacts by leading to increase in levels of contaminants (e.g. oil and grease) when they enter nearby water bodies.

5.7.12 The effects on water quality from the construction activities are, however, likely to be acceptable, provided that the site is well maintained and that good construction practices and well-designed temporary drainage system and mitigation measures, as described in Section 5.8, are implemented properly.

Operation Phase

5.7.13 The Barrage Scheme will shift the existing tidal flushing conditions of the area between the existing inflatable dam and the proposed location of the barrage. The affected area is about 500m in length. The salinity of the area will be lowered as here is no/little ingress of saltwater entering the barrage while the water quality in the affected area will be improved by the exclusion of polluted water from downstream of Shan Pui River. The barrage will also reduce the tidal flushing at the downstream area. However, considered the long distance (3km) from the estuarine to the proposed barrage, the tidal movement at the barrage will be weak. Therefore, change in hydrology and increase in sedimentation is expected to be minimal at the downstream of the proposed barrage.

5.7.14 Potential water quality impacts during operational phase of the Barrage Scheme and the revitalised YLN include:

· pollution loads entering the channels (from polluted upstream water, runoff from adjacent land and flood water; and

· increase in suspended solids during maintenance of the revitalised YLN

5.7.15 As mentioned in Section 2.9.15, the designed discharge flow rate for the Barrage will be lower than the existing flow rate of the YLN. Practicable designs including energy dissipators or orientation of the pump outlets will be optimised in the detail design stage to dissipate excess energy of flowing water downstream such that the hydraulic performance of the downstream will be similar to the existing condition. Therefore, the potential sediment erosion impact or increases in suspended solids to nearby waters, mangroves, ponds and ultimately Deep Bay, due to the operation of the barrage are not anticipated. The proposed general arrangement of energy dissipators at the pump outlets are shown in Figure 5.4 and will be optimised during detailed design.

5.7.16 Since the Project will not generate or discharge any water quality pollutants, therefore the TPB PG-No. 12C that there will be no net increase in pollution load to deep bay will be complied. Moreover, long term reduction of pollution load into the channels would improve the water quality in the YLN. The stream currently receives some domestic waste discharges and industrial effluent in the upstream. However, during operational stage, those discharges will be reduced due to the implementation of the improvement works from separate Projects including improvement works of YLTN and Yuen Long South development. The level of impact will be less than the current situation.

5.7.17 Failure of tidal barrier and low flow pumps could lead to temporary accumulation of upstream water at the YLN, adverse water quality impact is not expected. Quarterly maintenances and inspections to the tidal barrier and pumping stations are necessary to maintain a good operation condition to minimize the chance of failure. In addition, provision of standby units and dual power supply could also minimise occurrence of failure of tidal barrier and low flow pumps.

5.7.18 Vegetation removes dissolved and particulate pollution through absorptive, filtration and biological mechanisms. As the existing concrete drainage environment will not provide such functions, natural stream bed instead of concrete channel will be proposed in the revitalisation scheme where feasible.

5.7.19 Additional flow would be diverted to the YLBF for the barrage’s operation at the downstream. Based on the Drainage Impact Assessment, YLBF & KTR have been identified to have adequate capacity to receive such additional flow. Therefore, no potential water quality impact has been identified for such operation.

5.7.20 The proposed parapet walls will not generate any water quality pollution impact during operational stage.

5.8 Mitigation Measures

Construction Phase

5.8.1 The main potential impacts from the construction stage of the Project include an increase in suspended solids, pH value, oil & grease and general site effluent entering adjacent water bodies. The following sections discuss the proposed mitigation measures for the potential water quality impacts identified above.

General Construction Site Practice

5.8.2 The Contractor should observe and comply with the Water Pollution Control Ordinance and its subsidiary regulations and obtain a discharge license under the Ordinance. The Contractor should carry out the Project works in such a manner as to minimize adverse impacts on the water quality during execution of the works. In particular, the Contractor should arrange the working method to minimize the effects on the water quality within and outside the Project Site and on the transport routes. In addition, the management of construction site drainage from the Project will follow guidelines provided in ProPECC PN 1/94.

5.8.3 In addition to the above working method arrangements, the Contractor would also be required to make reference to HKO’s weather forecast and regional rainfall patterns, or equivalent, for the arrangement of works and implement the necessary mitigation measures to further minimise water quality impact during adverse weather.

5.8.4 The Contractor would be required to observe HKO’s weather forecast and regional rainfall pattern to minimise water quality impact during adverse weather.

5.8.5 Portable toilets should be located away from watercourses as far as practicable.

Concreting Works

5.8.6 Runoff should be carefully channeled, with the adoption of containment structures as necessary, to prevent concrete-contaminated water from entering watercourses. Adjustment of pH can be achieved by adding a suitable neutralising reagent to wastewater prior to discharge. Re-use of the supernatant from the sediment pits for washing out of concrete lorries should be practised.

5.8.7 Any exceedance of acceptable range of pH levels in the nearby water bodies caused by inadvertent release of site runoff containing concrete should be monitored and rectified under the EM&A programme for this Project.

Construction Site Runoff and Drainage

5.8.8 Proper site management measures should be implemented to control site runoff and drainage, and thereby prevent high sediment loadings from reaching downstream sections of the river/stream. The Contractor should follow the practices, and be responsible for the design, construction, operation and maintenance of all the mitigation measures. For reference, the estimated sewage generation rate of 0.15m³/worker/day is adopted from Table T-2 of EPD’s Report No.: EPD/TP 1/05 Guidelines for Estimating Sewage Flows for Sewage Infrastructure Planning Version 1.0. The design of the mitigation measures should be submitted by the Contractor to the Engineer for approval. These mitigation measures shall include the following practices to minimize site surface runoff and the chance of erosion, and also to retain and reduce any suspended solids prior to discharge:

· Before commencing any work, all sewer and drainage connections should be sealed to prevent debris, soil, sand etc. from entering public sewers/drains.

· Provision of perimeter channels to intercept storm-runoff from outside the site. These should be constructed in advance of the construction works.

· Temporary ditches such as channels, earth bunds or sand bag barriers should be included to facilitate runoff discharge into the stormwater drain, via a sand/silt basin/trap. Where necessary, sedimentation tanks would be implemented in conjunction with bank-up sedimentation tanks to minimise water quality impact.

· Works programme should be designed to minimize works areas at any one time, thus minimizing exposed soil areas and reducing the potential for increased siltation and runoff.

· Sand/silt removal facilities such as sand traps, silt traps and sediment basins should be provided to remove the sand/silt particles from run-off where necessary. These facilities should be properly and regularly cleaned and maintained. These facilities should be carefully planned to ensure that they would be installed at appropriate locations to capture all surface water generated on site.

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

· Temporary access roads (if any) should be protected by crushed gravel and exposed slope surfaces shall be protected when rainstorms are likely; and

· Open stockpiles of construction materials on-site should be covered with tarpaulin or similar fabric during rainstorms to prevent erosion.

Use of Containment Structures and Diversion Channels

5.8.9 The use of containment structures and diversion channels is recommended, in conjunction with the provision of a silt curtain, wherever practicable to facilitate a dry or at least confined excavation within the nullah. For example, nullah water should be contained within the works area before the commencement of excavation by the use of concrete blocks or sandbag barriers. Water within the contained area should be discharged to the nullah before excavation commences to create the dry conditions. Sediment removal works shall not be carried out in open waters. Nullah water should also be diverted from the works area through the use of diversion channel constructed by materials such as concrete blocks. Indicative details of the containment structures and diversion channels are provided in Figure 5.3 and would be provided by the Contractor to the Engineer for approval before commencement of construction works for the Project. By limiting or confining the works areas the extent of disturbance to the surrounding water bodies will be significantly reduced, and thus resulting impacts on water quality from sediment re-suspension will be reduced. These measures will be implemented to ensure compliance with the Water Pollution Control Ordinance and its subsidiary regulations.

Sewage and Wastewater Discharge

5.8.10 All discharges during the construction phase of the Project are required to comply with the Technical Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-ICW) issued under Section 21 of the WPCO. Domestic sewage/wastewater generated by workforce on-site should be collected in a suitable storage facility such as portable chemical toilets. An adequate number of portable toilets will be provided during the construction phase. These toilets should be maintained in a state that will not deter the workers from using them. The collected sewage/wastewater will be discharged into the foul sewer or transferred to the Government sewage treatment works by a licensed collector.

Storage and Handling of Oil, Other Petroleum Products and Chemicals

5.8.11 The following mitigation measures should be implemented for the storage and handling of oil, other petroleum products and chemicals:

· Waste streams classifiable as chemical wastes should be properly stored, collected and treated for compliance with Waste Disposal Ordinance or Waste Disposal (Chemical Waste) (General) Regulation requirements.

· All fuel tanks and chemical storage areas should be provided with locks and be sited on paved areas.

· The storage areas should be surrounded by bunds with a capacity equal to 110% of the storage capacity of the largest tank to prevent spilled oil, fuel and chemicals from reaching the receiving waters.

· Waste oil should be collected and stored for recycling or disposal, in accordance with the Waste Disposal Ordinance.

· Vehicle and plant servicing areas, vehicle wash bays and lubrication bays should, as far as possible, be located within roofed areas. The drainage in these covered areas should be connected to foul sewers via a petrol interceptor.

Handling of Spillage / Leakage

5.8.12 In the event that accidental spillage or leakages of hazardous substances / chemical wastes occur, the response procedures as listed below should be followed. It should be noted that the procedures below are not exhaustive and the Contractor should propose other response procedures in the emergency contingency plan based on the particular types and quantities of chemicals or hazardous substances used, handled and stored on-site.

· Oil leakage or spillage should be contained and cleaned up immediately. Waste oil should be collected and stored for recycling or disposal in accordance with the Waste Disposal Ordinance.

· Instruct untrained personnel to keep at a safe distance well away from the spillage area.

· If the spillage / leakage involve highly toxic, volatile or hazardous waste, initiate emergency evacuation and call the emergency service.

· Only trained persons equipped with suitable protective clothing and equipment should be allowed to enter and clean up the waste spillage / leakage area.

· Where the spillage/ leakage is contained in the enclosed storage area, the waste can be transferred back into suitable containers by suitable handheld equipment, such as hand operated pumps, scoops or shovels. If the spillage / leakage quantity is small, it can be covered and mixed with suitable absorbing materials such as tissue paper, dry soft sand or vermiculite. The resultant slurry should be treated as chemical waste and transferred to suitable containers for disposal.

· For spillage / leakage in other areas, immediate action is required to contain the spillage / leakage. Suitable liquid absorbing materials such as tissue paper, dry soft sand or vermiculite should be used to cover the spill. The resultant slurry should be treated as chemical waste and transferred to suitable containers for disposal.

· Areas that have been contaminated by chemical waste spillage / leakage should be cleaned. While water is a soluble solvent for aqueous chemical wastes and water soluble organic waste, kerosene or turpentine should be used for organic chemical wastes that are not soluble in water. The waste from the cleanup operation should be treated and disposed of as chemical waste.

· In incidents where the spillage / leakage may result in significant contamination of an area or risk of pollution, the EPD should be informed immediately.

Operation Phase

5.8.13 Maintenance may be necessary for the revitalised YLTN at regular intervals to remove excessive silts, vegetation, debris and obstruction.

5.8.14 The following considerations should be included in planning for the maintenance works:

(a) Maintenance of the channels should be restricted to annual silt removal when the accumulated silt will adversely affect the hydraulic capacity of the channel, except during emergency situations where flooding risk is imminent. Desilting should be carried out by hand or light machinery during the dry season (October to March) when water flow is low.

(b) Phasing of the works should be considered to better control and reduce any impacts caused. Where possible, works should be carried out along half width of the drainage channel in short sections. A free passage along the drainage channel is necessary to avoid forming stagnant water in any phase of the works.

(c) Containment structures (such as sand bags barrier) should be provided for the desilting works area to facilitate a dry or at least confined working area within the drainage channel.

(d) The locations for the disposal of the removed materials should be identified and agreement sought with the relevant departments before commencement of the maintenance works. Temporary stockpile of waste materials should be located away from the channel and properly covered. These waste materials should be disposed of in a timely and appropriate manner.

(e) Effective temporary flow diversion scheme should be implemented and the generated wastes should be collected and disposed off-site properly to avoid adversely affecting the water quality of the drainage system.

5.8.15 Practicable designs including energy dissipators or orientation of the pump outlets will be optimised in the detail design stage to dissipate excess energy of flowing water downstream such that the hydraulic performance of the downstream will be similar to the existing condition.

5.9 Cumulative Impacts

5.9.1 According to publicly best available information at time of writing, the following major developments in Yuen Long will be constructed and/or operated concurrently with the construction and operation of the Project.

· Improvement of Yuen Long Town Nullah (YLTN);

· Elevated Pedestrian Corridor in Yuen Long Town Connecting with Long Ping Station (EPC);

· Housing Sites in Yuen Long South (HSYLS); and

· Yuen Long Effluent Polishing Plant (YLEPP).

Construction Phase

5.9.2 The construction period of Improvement of Yuen Long Town Nullah will overlap with that of the Project during 2021 to 2026. According to the Project Profile, construction site runoff and increase suspended solids due to removal of river sediment would cause impact on water quality. With proper adoption of mitigation measures and good site practices, no unacceptable water quality impact is expected.

5.9.3 The construction period of EPC will overlap with that of the Project during 2021 to 2025. According to the EIA Report, the main water pollution sources of EPC is construction activities, construction surface runoff and sewage from site workforce. No unacceptable water quality impact is expected with implementation of the mitigation measures.

5.9.4 The project area of HSYLS may have potential interface with the upstream end of the Project Site. The construction period of HSYLS will overlap with that of the Project during 2021 to 2026. Site runoff during decking works may flow into the nullah. According to the EIA Report, mitigation measures such as providing toe boards or bunds along the works platform over the nullah shall be implemented. No unacceptable water quality impact is expected with implementation of the mitigation measures.

5.9.5 The construction period of YLEPP will overlap with that of the Project during 2021 to 2026. According to the EIA Report, the major water pollution sources of YLEPP is construction activities, sewage from site workforce, accidental spillage and surface run-off. No unacceptable water quality impact is expected with implementation of the mitigation measures.

5.9.6 In consideration of the Project and the concurrent projects will not generate significant water quality impact during construction phase. Adverse cumulative water quality impacts during construction phase are not anticipated.

Operation Phase

5.9.7 The operation of YLTN will occur concurrently with the Project. According to the Project Profile, a dry weather flow interception (DWFI) system will be constructed to intercept all polluted dry weather flow being discharged to the Town Centre Section of the YLTN which will then be conveyed to the YLEPP for treatment. The reduction in pollution load caused by the operation of the YLTN Project will thus lead to beneficial impacts of improvement of water quality and alleviation of odour nuisance within the Town Centre Section of the YLTN. Overall, unacceptable residual water quality impacts to WSRs are not expected to occur during operation.

5.9.8

5.9.9 The operation of EPC will occur concurrently with the Project. According to the EIA Report, surface runoff from the elevated pedestrian corridor would be the source of water pollution from EPC and no adverse water quality impact is anticipated with the provision of the drainage system. There would be no increase of the amount of surface runoff and velocity. Increase of the water level of nullah may occur due to the decrease of cross-section area as a result of the permanent structures of the EPC. The flood risk can be mitigated to acceptable level and adverse water quality impact is not anticipated with the implementation of the mitigation measures recommended in the EIA Report.

5.9.10 During the operation of HSYLS, water quality impact may mainly arise from sewage disposal, surface run-off from the PDA, emergency discharge from sewage treatment works and pumping stations, maintenance flushing for reclaimed water service reservoir and wastewater from industrial and commercial activities. On the other hand, water quality of watercourse nearby (including YLTN) and Deep Bay WCZ would be improved due to the decommissioning of 3 pig farms and 2 chicken farms as stated in approved EIA report AEIAR-215/2017 - Housing Sites in Yuen Long South as well as the construction of new sewerage and sewage treatment works for the unsewered area. No significant water quality impact is anticipated with the implementation of mitigation measures.

5.9.11 During the operation of YLEPP, the overall water quality in the inner Deep Bay would be improved for the upgrading treatment level from secondary treatment to tertiary treatment. Adverse water impact may arise from the emergency discharge. With implementation of appropriate mitigation measures such as dual power supply, the occurrence of emergency discharge from the YLEPP and the subsequent water quality impact will be minimized. According to the tentative schedules, the YLTN Project will be completed one year before the commissioning of Phase 1 of YLEPP. During the gap year, the collected DWF will be conveyed directly or as close as possible to the desilting basin of the existing LFPS and mixed with the DWF of Kung Um Road Nullah and West Nullah before being discharged to Shan Pui River through the existing LFPS. After the commissioning of Phase 1 of YLEPP, up to 18,000 m³/day of DWF from the YLTN Project will be intercepted and released to the YLEPP for treatment. Based on the findings in Section 6.6.13 and the EIA Report of YLEPP, no net increase in pollution loads to the Deep Bay is anticipated for both projects.

5.9.12 In consideration of both the Project and the concurrent projects will not generate significant water quality impact during operation phase. Adverse cumulative water quality impacts during operational phase are not anticipated.

5.10 Residual Impacts

5.10.1 With the proper implementation of the recommended mitigation measures described in Section 5.8 above, no unacceptable residual water quality impacts are envisaged from the construction and operation of the Project.

5.11 Monitoring and Audit Requirement

5.11.1 With proper implementation of the recommended mitigation measures, unacceptable water quality impacts at the identified WSRs are not expected to occur. However, a water quality monitoring programme is recommended.

5.11.2 Detailed approach and methodology of the water quality monitoring programme are presented in the Environmental Monitoring and Audit (EM&A) Manual under a separate cover and are briefly described below.

Construction Phase

5.11.3 Baseline monitoring should be undertaken for three times per week for a period of four weeks before commencement of the construction works to establish baseline water quality conditions of the area. Impact monitoring should be undertaken for three times per week during the construction period to obtain water quality data of the area throughout the construction period for comparison with the baseline water quality data and hence determine any water quality impacts from the construction activities. Post Project monitoring should also be undertaken three times per week for four weeks after the completion of construction works.

5.11.4 The following parameters will be monitored under the water quality monitoring programme:

· pH (in situ measurement);

· Water temperature (^oC) (in situ measurement);

· Salinity (ppt) (in situ measurement);

· Dissolved Oxygen (DO) (% saturation and mg L^-1) (in situ measurement);

· Turbidity (NTU) (in situ measurement); and

· Suspended Solids (SS) (mg L^-1) (laboratory analysis).

5.11.5 Weekly site inspections and audits will be conducted to ensure that the recommended mitigation measures are properly implemented during the construction stage.

Operation Phase

5.11.6 Unacceptable water quality impacts are not expected during the operation of the Project. Therefore, environmental monitoring and audit for water quality is not recommended for the operation phase of the Project.

5.12 Conclusion

5.12.1 The potential sources of water quality impacts associated with the construction and operation of the Project have been identified and the potential impacts were evaluated.

5.12.2 Potential impacts arising from the proposed construction works are predicted to be largely confined to the specific works areas. With proper implementation of the recommended mitigation measures, in particular the establishment of dry condition for excavation works within the existing nullah and adoption of good construction site practices as recommended in relevant regulatory guidelines, unacceptable water quality impacts are not expected at the identified WSRs including the Deep Bay Wetland Conservation Area and Mai Po Inner Deep Bay Ramsar Site.

5.12.3 During the operation phase, the existing tidal condition will be shifted downstream from the existing inflatable dam to the proposed barrage. The water quality at the upstream of the barrage is expected to be improved. Practicable designs including energy dissipators or refinements of the orientations of the pump outlets will be optimised in the detail design stage to dissipate excess energy of flowing water downstream such that the hydraulic performance of the downstream will be as similar to the existing condition as possible. Therefore, the potential sediment erosion impact or increases in suspended solids to nearby waters, mangroves, ponds and ultimately Deep Bay, due to the operation of the barrage are not anticipated. With regular maintenance works to remove excessive sediments, it is anticipated that the Project will not lead to any unacceptable water quality impacts. Unacceptable water quality impacts are also not expected to occur at any identified WSRs due to the small-scale and infrequent maintenance works.

5.12.4 With proper implementation of the recommended mitigation measures, no unacceptable residual water quality impacts are envisaged from the construction and operation of the Project. Nevertheless, a monitoring programme is recommended during the construction phase to verify the predictions of the EIA and ensure compliance with the assessment criteria.

5.12.5 Cumulative water quality impacts associated with concurrent projects within the Study Area have been considered with no unacceptable impact anticipated.