6. WATER QUALITY IMPACT ASSESSMENT

6.1 Introduction

6.1.1 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.

6.2 Environmental Legislation, Policies, Standards and Criteria

6.2.1 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; and

· 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.

· 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)

6.2.2 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 WQOs designated for this zone are thus relevant for assessing the water quality impacts from the construction and operation of the Project (Table 6.1).

Table 6.1 Key Water Quality Objectives of the Deep Bay WCZ for Yuen Long & Kam Tin (Upper) Subzone and Yuen Long & Kam Tin (Lower) Subzone

Water Course within the Study Area	pH Range	Maximum 5-day Biochemical Oxygen Demand (BOD5)	Maximum Chemical Oxygen Demand (COD)	Maximum Annual Median Suspended Solids (SS)	Minimum Dissolved Oxygen (DO)	Maximum Annual Mean Unionised Ammoniacal Nitrogen
Yuen Long & Kam Tin (Upper) Subzone	6.5 – 8.5	3 mg/L	15 mg/L	20 mg/L	4 mg/L	0.021 mg/L
Yuen Long & Kam Tin (Lower) Subzone	6.5 – 8.5	5 mg/L	30 mg/L	20 mg/L	4 mg/L	0.021 mg/L

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

6.2.3 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)

6.2.4 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)

6.2.5 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

6.2.6 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

6.2.7 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.

6.3 Baseline Water Quality Conditions

Hydrology and Hydrodynamics

6.3.1 YLTN has a total length of about 12 km and is made of concrete lining at the bottom and side walls. The nullahs are divided into five sections, namely East, West, Town Centre, Upstream (including Main and San Hui) and Downstream sections, according to their location (Figure 6.1). The proposed Project involves the Town Centre Section which aligns from north to south in the middle of Yuen Long Town area and separates the town into western and eastern parts. The Town Centre Section is approximately 30 m wide and 4 m deep. It is a rectangular concrete channel collecting flows from western and eastern areas of the town centre. The DWF in the Town Centre Section has an estimated average flow rate of 600 m³/day.

6.3.2 YLTN can currently provide flood protection standard of 1 in 50 years return period. Its hydraulic performance has been assessed using the InfoWorks ICM model ⁽¹⁾. The performance assessment was carried out by comparing the peak water-levels with bank levels for open channels to quantify the level of protection at each section of the channel. The results identified some minor flooding blackspots, including Ting Fook Villas, Wang Chau Public School, Tai Kiu Tsuen, Shui Pin Tsuen and Tai Kei Leng, in Yuen Long Town area.

6.3.3 It is anticipated that the flooding problem in YLTN could be worsened subsequent to the increasing population and the development of residential, commercial and industrial buildings.

Baseline River Water Quality

6.3.4 The proposed Project is located between Shap Pat Heung Road and Tung Tau Wai San Tsuen (Figure 1.1). Water courses identified within the Study Area (defined as 500 m from the boundary of the Project Site) include the YLTN, Main Nullah (MN), San Hui Nullah (SHN), East Nullah, West Nullah and Kam Tin River. The Study Area falls within the Deep Bay WCZ and the WQOs designated for the whole zone are thus relevant to this Project and are summarized on Table 6.1 above.

Water Quality of YLTN, MN and SHN

6.3.5 River water quality has been regularly monitored by the EPD since 1986. Measurements and laboratory tests of over 40 physic-chemical and biological parameters, including organics, nutrients, metals and E. coli levels are recorded on a monthly basis. In 2018, a total of 6 stations in the Yuen Long District were monitored, four of them were relevant to the monitoring of YLTN (stations YL3-4), MN (YL1) and SHN (station YL2) (Figure 6.2).

6.3.6 Data of key water quality parameters measured in 2018 at the four monitoring stations as reported in the Annual River Water Quality in Hong Kong ⁽²⁾ are presented in Table 6.2.

Table 6.2 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.

6.3.7 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.

6.3.8 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.

6.3.9 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.

Water Quality of Kam Tin River

6.3.10 In 2018, water quality of Kam Tin River was monitored at two stations (please refer to Figure 6.2 for locations of the monitoring stations). Data of key water quality parameters measured in 2018 at the two monitoring stations as reported in the Annual River Water Quality in Hong Kong are presented in Table 6.3.

Table 6.3 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.

6.3.11 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.

Water Quality of DWF in YLTN

6.3.12 Water sampling and the discrete flow measurement were conducted in four consecutive non-rainy days between 5 August and 8 August 2017. On each of the four sampling days, five grab samples were collected at the sampling locations at five specific times including (1) 9:00am, (2) 11:00am, (3) 6:00pm, (4) 9:00pm and (5) 11:30pm. The grab samples were sent to HOKLAS accredited laboratory for chemical analysis immediately after sampling. According to the results of water sampling of DWF as presented in Table 6.4, the upstream 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 6.4 Water Sampling Results

	Kung Um Road Nullah	San Hui Nullah	End of Town Centre Section	YLTN – West Nullah	YLTN – East Nullah
WQI (Water Quality)	9 (Fair)	11 (bad)	13 (bad)	11 (bad)	15 (very bad)
DO (mg/L)	5.46	3.80	1.86	2.38	1.28
SS (mg/L)	14.70	7.45	14.60	4.39	31.1
Ammonia as N (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
Note: Underlined figures = non-compliance of WQO The presented results are the average measured value

Discharges into the Yuen Long Town Nullah

6.3.13 Discharges of sewage from unsewered villages and livestock farms are the main source of pollution to the YLTN. As reported in the Feasibility Study for the Project, a total of six livestock farms, including three pig farms and three poultry farms, are located within the catchment of YLTN ⁽¹⁾⁽³⁾. As discussed in Section 6.3.9 above, the WQI in 2018 was graded as “very bad” at YL3 and YL4 and “bad” at YL1 and YL2.

6.3.14 An Expedient Connection Survey was carried out in early 2005 under Agreement No. 99/2002 (DS) – Provision of Sewerage to Unsewered Areas/Villages in Northwest New Territories – Feasibility Study. The results of the survey indicated that there were a number of expedient connections along the Yuen Long Nullah (e.g. due to the unsewered areas in Yuen Long). Under the Project, it is estimated that about 60 numbers of stormwater outfalls will be intercepted to the DWFI system.

Existing Water Quality of Treated Effluent from the Yuen Long Sewage Treatment Works

6.3.15 According to the existing sewage treatment process of the YLSTW, treated effluent from the YLSTW has undergone preliminary, primary and secondary treatment before being discharged to the Shan Pui River (refer to the Figure 6.3 for the Sewage Treatment Process Flowchart). In general, the raw sewage will firstly undergo preliminary screening, degritting and subsequently primary sedimentation. The sewage will then be purified by means of a secondary biological treatment process, during which the organic matter in the settled sewage will be decomposed by micro-organisms. Eventually, treated effluent for discharge to Shan Pui River should meet the discharge license standard of not more than 30 mg/L for SS and 20 mg/L for BOD₅.

6.3.16 Monthly mean BOD₅ and SS levels recorded for the treated effluent from YLSTW are shown in Table 6.5. Review of the treated effluent quality from YLSTW in the period of January 2009 to December 2018 revealed that the water quality of the treated effluent was well within the discharge license standard, with a range of monthly means from less than 5 mg/L to less than 10 mg/L (recorded in Dec 2018) for BOD₅ and from less than 5 mg/L to 19 mg/L (recorded in March 2018) for SS, respectively ⁽⁴⁾.

Table 6.5 Monthly Mean BOD₅ and SS Levels recorded for the Treated Effluent from YLSTW from January 2009 to December 2018

BOD5 (mg O2/L)

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

2009

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

2010

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

2011

<5

<6

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

2012

<5

<6

<5

<5

<5

<5

<5

<5

<5

<5

<5

<6

2013

<5

<5

<7

<6

<5

<5

<6

<5

<5

<5

<6

<5

2014

<5

<5

<5

<5

<5

<5

<5

<5

<5

<7

<6

<9

2015

<9

<7

<6

<5

<5

<5

<5

<5

<5

<6

<5

<5

2016

<6

<6

<5

<6

<5

<5

<5

<5

<6

<6

<5

<6

2017

<5

<5

<5

<5

<7

<5

<5

<5

<5

<6

<5

<7

2018

<7

<7

<9

<6

<6

<8

<6

<6

<5

<6

<5

<10

SS (mg/L)

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

2009

<10

11

<12

<10

<7

<7

<7

<8

<6

<7

<6

<7

2010

<8

<5

<8

<7

<7

10

<9

<9

<6

<8

<8

<6

2011

<6

<8

<6

<6

<6

<7

<6

<5

<6

<5

<5

<5

2012

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

2013

<6

<7

<6

<7

<5

<5

<5

<5

<5

<6

<9

<6

2014

<9

<8

<8

<8

<9

<6

<6

<6

<8

13

<8

12

2015

15

<8

<11

<8

<9

<7

<8

<7

<7

<7

<7

8

2016

<9

<8

<9

<10

<7

<7

<8

<9

<7

<8

<6

<8

2017

<6

<6

<9

<8

<7

8

<9

<11

11

<12

<15

14

2018

18

18

19

15

<12

14

16

<11

<9

<8

<8

<18

6.4 Water Sensitive Receivers

6.4.1 The Water Quality Sensitive Receivers (WSRs) that may be affected by changes in water quality arising from the Project are identified in accordance with the EIAO-TM. The identified WSRs include the sections of YLTN, Deep Bay Wetland Buffer Area, Shan Pui River, Deep Bay Wetland Conservation Area, Kam Tin River, mangrove and Mai Po Inner Deep Bay Ramsar Site. The approximate distance from the identified WSRs to the Project Site is detailed in Table 6.6.

Table 6.6 Identified Water Quality Sensitive Receivers (WSRs)

WSR	Minimum Distance away from the Project Site Boundary	Minimum Distance away from the 500m Study Area Boundary
Yuen Long Town Nullah	Within the Project Site	Within the 500m Boundary
Deep Bay Wetland Buffer Area	Within the Project Site	Within the 500m Boundary
Shan Pui River	Within the Project Site	Within the 500m Boundary
Deep Bay Wetland Conservation Area	~80m	Within the 500m Boundary
Kam Tin River	~80 m	Within the 500m Boundary
Mangrove	~100m	Within the 500m Boundary
Mai Po Inner Deep Bay Ramsar Site	~1300m	~800m

6.4.2 Locations of the identified WSRs are shown in Figure 6.2. All the fish ponds within the 500-m study area are currently inactive. Based on our desk-top review, these inactive ponds are enclosed waterbody and there is currently no water intake point where water can be abstracted from Yuen Long Town Nullah/Shan Pui River into the ponds. These ponds are therefore not considered as a potential WSR.

6.5 Potential Sources of Impacts

Construction Phase

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

· Construction activities on the nullah bed, including ground breaking, excavation, pipeline laying, backfilling and reinstatement works associated with construction of DWFI system and the section of rising main inside the YLTN;

· Sewage discharges from the construction work force;

· Construction runoff and drainage; and

· Pollutants entering the receiving waters due to accidental spillage /uncontrolled discharge from the general construction activities.

Operation Phase

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

· Changes in water quality of the YLTN due to interception of YLTN DWF;

· Release of excess DWF;

· Maintenance works for the pipes of the DWFI system to remove excessive silt, vegetation, debris and obstructions within the drainage channel which may lead to disturbance and re-suspension of river sediments and thereby affecting water quality;

· Emergency discharge of the collected DWF in case of pumping station breakdown;

· Changes in hydrodynamic conditions; and

· Change in sediment deposition and erosion pattern.

6.5.3 As the treated effluent from YLEPP will not be reused for maintaining water flow, there will be no biocide discharge upon the latest project scope.

6.6 Impact Assessment

Construction Phase

Construction Activities on the Nullah Bed

6.6.1 Ground breaking, excavation, pipeline laying and backfilling works for construction of DWFI system and rising main section inside the YLTN may lead to elevated SS levels and dispersal of SS to the downstream section of the YLTN 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 (please refer to Section 3.6.8 for details). Water quality impacts due to release and dispersal of SS from construction activities on the nullah bed are thus not expected to be unacceptable.

6.6.2 Overall, no unacceptable adverse impacts to water quality are anticipated to occur as a result of the construction works on the nullah bed with proper implementation of the mitigation measures as proposed in Section 6.7.

Sewage Discharge from the Construction Workforce

6.6.3 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.

6.6.4 According to the latest construction programme and estimate, there will be a maximum of 50 construction workers on-site during the peak construction period of the Project. With an estimated sewage generation rate of 0.15 m³/worker/day ⁽⁵⁾, about 7.5 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.

Construction Runoff and Drainage

6.6.5 During construction of DWFI system, rising main and DWF pumping station, runoff from the construction site, particularly during excavation, pipeline laying and backfilling, will contain high SS level which could be a source of water pollution. Wastewater with high pH value may be generated by in-situ concreting works for reinstatement of the nullah. 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, with proper implementation of general good construction site practices as described in Section 6.7 below, it is anticipated that the land based construction works will not cause unacceptable water quality impact through site runoff and drainage.

Accidental Spillage / Uncontrolled discharge from General Construction Activities

6.6.6 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.

6.6.7 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.

6.6.8 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 6.7, are implemented properly.

Operation Phase

Interception of YLTN DWF

6.6.9 During operation of the Project, the DWFI system will collect the polluted DWF which is originally discharged to the Town Centre Section of YLTN and causing the odour nuisance in the area. The system will then convey the DWF to the future YLEPP (upgraded from the existing YLSTW) for tertiary treatment. Eventually, treated effluent from YLEPP will be discharged to Shan Pui River and it is expected that the treated effluent will meet the proposed discharge license standard of not more than 10 mg/L for SS, 10 mg/L for BOD₅, 10 mg/L for TN, 1 mg/L for TP and 100 counts/100mL for E.coli.

6.6.10 With interception of YLTN DWF and subsequent released of treated effluent at Shan Pui River, it is expected that pollutant loading to Shan Pui River will change as a result of the operation of the Project.

6.6.11 As the residual pollution loading from YLEPP has been accounted for in their approved EIA Report, the pollution loading reduction due to this Project is assumed to be same as the original pollution loading of the 18,000 m³/day DWF before operation. To calculate the pollution loading reduction, average water quality data of stations YL3 and YL4 as recorded from 2014-2018 by EPD are used. Water quality data of YL3 are used to represent pollution loading from the San Hui Nullah and Town Centre Section of the YLTN, whereas water quality data of YL4 are used to represent loading from the East Nullah. Using the above data and flow rates of the nullahs, the pollution loading reduction to Shan Pui River due to interception of YLTN DWF is calculated and presented in Table 6.7.

6.6.12 As shown in the results, it is anticipated that the pollution loads to the Shan Pui River will be reduced by 742 kg/day for SS, 1,906 kg/day BOD₅, 197 kg/day for TN, 22 kg/day for TP and 2.8×10¹⁴ counts/day for E.coli due to operation of the Project. The reduction in pollution load caused by the operation of the Project will thus leading to beneficial impacts of improvement of water quality and alleviation of odour nuisance. The Project would not generate a net increase in pollution loading to the receiving water in the Deep Bay WCZ.

6.6.13 It is anticipated that the quantity of the DWF, particularly from the upstream of YLTN (at the junction of Kiu Hing Road and Tai Shu Ha Road West), would be subsequently reduced after completion of the sewerage works at the area. Eventually, the DWF at upstream (i.e. Kung Um Road Nullah and San Hui Nullah) is supposed to be cleaner and may lead to improvement of water quality in the YLTN. In order to ensure that the Project will not retard recovery of the water body if level of pollution from other sources decrease (e.g. from upstream of YLTN as presented above), water quality of the sources of DWF to the YLTN should be regularly reviewed.

Table 6.7 Anticipated Improvement to Water Quality in YLTN

For SS
Source of DWF	Flow Rate of DWF (m^3/day)	SS conc. in DWF (mg/L)^(1)	Estimated SS conc. in merged DWFs at downstream section of YLTN (mg/L)	Predicted Reduction in Pollution Load for SS during Operation (kg/day)
San Hui Nullah, Town Centre Section of YLTN	1,600	23.1	41.22	742
East Nullah	16,300	43.0
For BOD5
Source of DWF	Flow Rate of DWF (m^3/day)	BOD5 conc. in DWF (mg/L) ^(1)	Estimated BOD5 conc. in merged DWFs at downstream section of YLTN (mg/L)	Predicted Reduction in Pollution Load for BOD5 during Operation (kg/day)
San Hui Nullah, Town Centre Section of YLTN	1,600	43.4	105.87	1,906
East Nullah	16,300	112.0
For TN
Source of DWF	Flow Rate of DWF (m^3/day)	TN conc. in DWF (mg/L)^(1)(2)	Estimated TN conc. in merged DWFs at downstream section of YLTN (mg/L)	Predicted Reduction in Pollution Load for TN during Operation (kg/day)
San Hui Nullah, Town Centre Section of YLTN	1,600	9.26	10.92	197
East Nullah	16,300	11.09
For TP
Source of DWF	Flow Rate of DWF (m^3/day)	TP conc. in DWF (mg/L) ^(1)	Estimated TP conc. in merged DWFs at downstream section of YLTN (mg/L)	Predicted Reduction in Pollution Load for TP during Operation (kg/day)
San Hui Nullah, Town Centre Section of YLTN	1,600	1.14	1.19	22
East Nullah	16,300	1.20
For E.coli
Source of DWF	Flow Rate of DWF (m^3/day)	E.coli conc. in DWF (counts/100mL) ^(1)	Estimated E.coli conc. in merged DWFs at downstream section of YLTN (counts/100mL)	Predicted Reduction in Pollution Load for E.coli during Operation (counts/day)
San Hui Nullah, Town Centre Section of YLTN	1,600	715,165	1,565,686	2.8 × 10^14
East Nullah	16,300	1,649,173

Note (1): Average water quality data of stations YL3 and YL4 as recorded from 2014-2018 by EPD are used.

(2): TN data is not available. The sum of nitrate-nitrogen and TKN concentrations has been used for conservation estimation of TN.

Release of Excess DWF

6.6.14 During operation of the Project, the DWFI system will collect the polluted DWF currently being discharged to the Town Centre Section of YLTN and causing the odour nuisance in the area. The system will then convey the DWF to the YLEPP for tertiary treatment. The DWFI system will convey up to 18,000 m³/day of the intercepted DWF to the YLEPP for treatment. When the above limit is reached, the exceeded DWF will be overflown directly or as close as possible to the desilting basin of the existing low flow pumping station (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. The discharge point of the existing LFPS is indicated in Figure 6.4. It is anticipated that such release of excess DWF will not lead to deterioration of water quality when compared to the existing baseline conditions under which the DWF with the same water quality is not being collected and thus flowing in the YLTN.

Maintenance Works

6.6.15 Maintenance works will be undertaken for the DWFI system. Diversion of the DWF to the nearby sewerage system can be adopted during maintenance of the pipe so that the water quality at YLTN would not be adversely affected. It is expected that maintenance will only be undertaken on an as-needed basis and frequent maintenance (i.e. monthly) will not be required. Such small-scale maintenance would require only light mechanical equipment such as a small loader and/or a small crane truck. Hand-held equipment will be used for vegetation removal. Given the infrequent maintenance requirements and the collected solid wastes will be collected and disposed off-site properly, unacceptable water quality impact is not expected.

6.6.16 Whilst possible changes to water quality may be expected during the removal works, such as increases in SS due to disturbance of nullah bed material and subsequently increased sedimentation onto the nullah bed, it is expected that these changes will be short-term and occur only within the area of maintenance works and for a short distance downstream due to rapid settling out of any disturbed nullah bed material. Changes in SS concentrations and sedimentation rate would be expected to be within the typical ranges experienced under ambient conditions following heavy storms and increased sediment run-off. Therefore, no unacceptable water quality impacts are anticipated to occur as a result of the small-scale maintenance works. Water quality impacts to the ecological sensitive receivers located downstream of the proposed drainage channel (Wetland Conservation Area and Mai Po and Inner Deep Bay Ramsar Site near the Shenzhen River estuary) are also not expected to occur due to the small-scale and temporary nature of the maintenance works. Please refer to Section 8 for more detail on ecological impacts.

Emergency Discharge of the collected DWF

6.6.17 An emergency bypass system will be provided for the DWF pumping station. In case of pumping station breakdown, the collected DWF will be discharged directly or as close as possible to the desilting basin of the existing low flow pumping station (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. As such, unacceptable adverse water quality impacts are not expected during emergency discharge. In addition, the u-channels/pipes of the DWFI system will be designed to accommodate the designed flow rate of the DWF and with adequate structural strength to withstand the loading during normal circumstances and maintenance works. It is thus highly unlikely that the pipes will burst causing adverse water quality impacts to the receiving waters.

As added protection, the following measures will be provided in the DWF pumping station to avoid breakdown of the pumping station:-

· One standby pump will be installed in the pumping station for emergency.

· Dual power supply to the DWF pumping station is agreed with CLP Power.

Changes in Hydrodynamic Conditions and Salinity

6.6.18 The current flood-protection capacity of the YLTN (i.e. 1 in 50 years) will not be altered by the Project. In addition, according to the Drainage Impact Assessment conducted for this Project⁽¹⁾, adequate freeboard could be maintained after the implementation of DWFI system and mitigation works in hydraulic aspect.

6.6.19 A maximum of 18,000m³ of DWF to the YLTN will be intercepted and conveyed to YLEPP for treatment, and the treated effluent will be released to the Shan Pui River. According to the Engineer’s estimation, interception of the DWF to the YLTN would only lead to a reduction of 3-4% of total volume of water to the section of Shan Pui River near the confluence with Kam Tin River, where key ecological habitat is identified (please refer to Section 8 for details on habitats identified). It should, however, be noted that the concerned river section is joined to Deep Bay and is thus expected to be affected by its tidal influence. For instance, the Inner Deep Bay station DM1, which is the closest station to the Study Area, had salinity varies from 0.2 – 29.5 psu when measured over the past years (i.e. 1986-2017 ⁽⁶⁾). This would indicate the tidal influence from Deep Bay within the Study Area. Due to the predominant tidal influence, the reduction in volume due to the DWF interception is not expected to be significant. It should also be noted that the reduction of flow volume in the nullah is due to interception of some of the polluted flow which should not have been drained into the nullah.

6.6.20 On the other hand, the flow rate reduction within the section of YLTN between the Project Site and the YLEPP due to the DWF interception would reduce the amount of freshwater input to and cause the increase of salinity at downstream where ecological receiver (i.e. mangroves) is identified. The associated ecological impact assessment is provided in Section 8.8.

6.6.21 In addition, as presented in Section 6.6.13 above, the DWF interception is expected to lead to reduction in pollution load to Shan Pui River which is considered as a beneficial water quality impact. Considering the above, unacceptable impact to hydrodynamic conditions that would lead to adverse water quality impacts is not expected to occur.

Change Sediment Deposition and Erosion Pattern

6.6.22 As discussed in Section 6.6.13, it is anticipated that pollution loads to Shan Pui River will be reduced by 742 kg/day for SS, 1,906 kg/day BOD₅, 197 kg/day for TN, 22 kg/day for TP and 2.8×10¹⁴ counts/day for E.coli due to operation of the Project which are being considered as beneficial water quality impacts. As such, although reduction in SS loading may change the pattern of sediment deposition and erosion within the drainage channel and along the downstream section of YLTN leading to improvement of water quality, no unacceptable water quality impacts are anticipated to occur due to the alteration.

6.6.23 In addition, regular maintenance works will be undertaken within the proposed drainage channel and at the downstream sections to remove excessive sediments and thus reduce sediment pollution to the Deep Bay (regular maintenance works will be undertaken by DSD at these downstream sections which are not within the scope of the Project). As such, it is not anticipated that the Project will lead to any unacceptable water quality impacts within the Project Site and along the downstream section of YLTN by altering sediment deposition and erosion patterns.

6.7 Mitigation Measures

Construction Phase

6.7.1 Potential impacts on water quality as a result of construction activities of the Project, including ground breaking, excavation, pipeline laying, backfilling and restatement works on the nullah bed associated with construction of DWFI system have been assessed in Section 6.6 above. The following section describes the mitigation measures proposed to alleviate water quality impacts during construction of the Project.

General Construction Site Practice

6.7.2 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.

Construction Site Runoff and Drainage

6.7.3 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. 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.

· 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

6.7.4 The use of containment structures and diversion channels is recommended wherever practicable to facilitate a dry or at least confined excavation within the nullah (please refer to Section 3.6.8 for details). 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. Nullah water should also be diverted from the works area through the use of diversion channel constructed by materials such as concrete blocks (please refer to Section 3.6.8 for details). Details of the containment structures and diversion channels should 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. Furthermore, excavation works in the nullah should be carried out during periods of low flow (dry season from November to March) as far as practicably to reduce impacts on downstream water quality and sensitive receivers. These measures will be implemented to ensure compliance with the Water Pollution Control Ordinance and its subsidiary regulations.

Sewage and Wastewater Discharge

6.7.5 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

6.7.6 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 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

6.7.7 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

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

6.7.9 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.

Emergency Response Plan

6.7.10 An Emergency Response Plan should be developed before the commencement of the Project’s operation in order to provide details on the emergency arrangement in case of breakdown of the DWFI system.

6.7.11 The proposed system includes overflowing pipes with outlets on both sides of the nullah.  When water rises to a certain level, stormwater within the underground system will be released and directly discharged into the nullah.  This prevents further back-up into the upstream system and the side branches.  The discharge of stormwater directly into the nullah is consistent with the existing drainage pattern.

6.8 Cumulative Impacts

6.8.1 According to publicly available information at time of writing, the following major developments in north western part of New Territories in Hong Kong and are downstream of the Project will be constructed and/or operated concurrently with the construction and operation of the Project.

· Yuen Long Barrage Scheme (YLBS);

· 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

6.8.2 The construction period of YLBS 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.

6.8.3 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.

6.8.4 The project area of HSYLS will not encroach but in close vicinity to the Project Site of the YLTN Project. The construction period of HSYLS will overlap with that of the Project during 2021 to 2026. Most of the works are at least 400m away from the YLTN Project except decking over sections of YLTN along Kung Um Road and Kiu Hing Road which is about 100m away from the Project Site. 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.

6.8.5 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.

6.8.6 In consideration of the YLTN 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

6.8.7 The operation of YLBS will occur concurrently with the Project. According to the Project Profile, the proposed barrage conveys stormwater from YLTN to Shan Pui River for discharge, no adverse impact on water quality is expected. Based on the latest preliminary design of YLBS, excess flow from DWF pumping station will enter the closed interception pipe from current discharge point to the future LFPS. The proposed DWF interception arrangement under the YLTN Project will be affected due to the decommissioning of the existing low flow pumping station and inflatable dam under the YLBS. Potential changes in DWF interception arrangement, hydrology and sedimentation rate will be assessed during the EIA study of YLBS.

6.8.8 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.

6.8.9 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 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.

6.8.10 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.

6.8.11 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.

6.9 Residual Impacts

6.9.1 With the proper implementation of the recommended mitigation measures described in Section 6.7 above, no unacceptable residual water quality impacts are envisaged from the construction and operation of the Project.

6.10 Monitoring and Audit Requirement

6.10.1 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 to verify the predictions of the EIA and ensure compliance with the assessment criteria.

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

Construction Phase

6.10.3 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.

6.10.4 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).

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

Operation Phase

6.10.6 Unacceptable water quality impacts are not expected during the operation of the Project. It is anticipated that the pollution loads to the Shan Pui River will be reduced by 742 kg/day for SS, 1,906 kg/day BOD₅, 197 kg/day for TN, 22 kg/day for TP and 2.8×10¹⁴ counts/day for E.coli due to operation of the Project. The reduction in pollution load caused by the operation of the Project will thus leading to beneficial impacts of improvement of water quality and alleviation of odour nuisance.

6.10.7 Therefore, environmental monitoring and audit for water quality is not recommended for the operation phase of the Project. On the other hand, the water quality of the branches of YLTN will be regularly reviewed. When the water quality has reached the prevailing acceptable standards, EPD will be consulted and the DWF interception system would be reviewed and modified, e.g. re-connect the branches with better water quality to the existing DWF channel.

6.11 Conclusion

6.11.1 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.

6.11.2 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.

6.11.3 During the operation phase, the interception of polluted DWF by the Project is expected to lead to improvement of water quality within the YLTN and at Shan Pui River and is thus considered to be beneficial. With regular maintenance works to remove excessive sediments, it is anticipated that the Project will not lead to any unacceptable water quality impacts by altering the sediment deposition and erosion pattern of the Project Site and its downstream area. Unacceptable water quality impacts are also not expected to occur at any identified WSRs due to the small-scale and infrequent maintenance works.

6.11.4 During emergency discharge of the collected DWF to the YLTN (e.g. due to breakdown of the DWF pumping station), water quality of the nullah is predicted to be similar to the existing condition before operation of the Project. In addition, given the low likelihood of the emergency discharge and proper Emergency Response Plan will be in place and implemented, no unacceptable water quality impact is expected to be associated with such discharge.

6.11.5 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.

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

6.12 References

1) B&V (2019) Improvement of Yuen Long Town Nullah (Town Centre Section) – Investigation, Draft Drainage Impact Assessment Report (Issue 2)

2) EPD (2018) River Water Quality Report in Hong Kong 2018

3) http://www.epd.gov.hk/epd/english/laws_regulations/enforcement/lw_map.html - Accessed on 25 March 2019

4) http://www.dsd.gov.hk/EN/Sewerage/Sewage_Treatment_Facilities/Effluent_Quality_of_Major_Sewage_Treatment_Works/index.html

5) Table 2 of the Drainage Services Department's Sewerage Manual

6) Environmental Protection Department Website: Marine Water Quality Data (1986-2017) Available at http://epic.epd.gov.hk/EPICRIVER/marine/history [Accessed March 2019]