6 Water quality

6.1 Introduction

6.1.1 The construction and operation of the Project would generate water quality impact upon nearby and downstream water bodies. This section presents the baseline conditions at the Project Site, relevant legislation, regulations and standards, sources and extent of impact due to the Project, and the mitigation measures recommended to alleviate the impact.

6.1.2 This water quality impact assessment was conducted and presented according to the requirements of Annexes 6 and 14 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) and the EIA Study Brief No. ESB-299/2017 issued in August 2017.

6.2 Environmental Legislation and Criteria

6.2.1 The following relevant legislation and associated guidelines are applicable to the evaluation of water quality impacts associated with the Project:

l Water Pollution Control Ordinance (WPCO) (Cap. 358);

l Environmental Impact Assessment Ordinance (EIAO) and EIAO-TM;

l ETWB Technical Circular (Works) No. 5/2005 Protection of natural streams/ rivers from adverse impacts arising from construction works;

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

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

l Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN5/93) Drainage Plans subject to Comment by the Environmental Protection Department.

Water Pollution Control Ordinance

6.2.2 The Water Pollution Control Ordinance 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 (WQO). The Project Site is located within the watershed of Tuen Mun River and is part of the North Western WCZ. Table 6.1 below summarises the water quality objectives for the North Western WCZ.

Table 6.1 - Water Quality Objectives for the North Western WCZ

	Water Quality Objective	Applicable Area
A	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 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
B	BACTERIA
a)	The level of Escherichia coli should not exceed 610 per 100 milligrams per litre, calculated as the geometric mean of all samples collected in one calendar year.	Secondary Contact Recreation Subzone
b)	The level of Escherichia coli should be less than 1 per 100 mL, calculated as the running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days.	Tuen Mun (A) and Tuen Mun (B) Subzones and Water Gathering Ground Subzones
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.	Tuen Mun (C) 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. Samples should be taken at least 3 times in one calendar month at intervals of between 3 and 14 days.	Bathing Beach Subzones
C	COLOUR
a)	Waste discharges shall not cause the colour of water to exceed 30 Hazen units.	Tuen Mun (A) and Tuen Mun (B) Subzones and Water Gathering Ground Subzones
b)	Waste discharges shall not cause the colour of water to exceed 50 Hazen units.	Tuen Mun (C) Subzone and other inland waters
D	DISSOLVED OXYGEN
a)	Waste discharges shall not cause the level of dissolved oxygen to fall below 4 mg per litre for 90% of the sampling occasions during the whole year; values should be calculated as water column average (arithmetic mean of at least 3 measurements at 1 m below surface, mid-depth and 1 m above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 mg per litre within 2 m of the seabed for 90% of the sampling occasions during the whole year.	Marine waters
b)	Waste discharges shall not cause the level of dissolved oxygen to be less than 4 mg per litre.	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones, Water Gathering Ground Subzones and other inland waters
E	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 unit.	Marine waters except Bathing Beach Subzones
b)	Waste discharges shall not cause the pH of the water to exceed the range of 6.5-8.5 units.	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones 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
d)	The pH of the water should be within the range of 6.0-9.0 units for 95% of samples collected during the whole year. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.5 unit.	Bathing Beach Subzones
F	TEMPERATURE
a)	Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 °C.	Whole zone
G	SALINITY
b)	Waste discharges shall not cause the natural ambient salinity level to change by more than 10%.	Whole zone
H	SUSPENDED SOLIDS
a)	Waste discharges shall neither cause the natural ambient level to be raised by more than 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.	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
c)	Waste discharges shall not cause the annual median of suspended solids to exceed 25 milligrams per litre.	Other inland waters
I	AMMONIA
a)a)	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
J	NUTRIENTS
a)	Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants.	Marine Waters
b)	Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.3 mg per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1 m below surface, mid-depth and 1 m above seabed).	Castle Peak Bay Subzone
c)	Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.5 mg per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1 m below surface, mid-depth and 1 m above seabed).	Marine waters excepting Castle Peak Bay Subzone
K	5-DAY BIOCHEMICAL OXYGEN DEMAND
a)	Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 3 milligrams per litre.	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
b)	Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 mg per litre.	Other inland waters
L	CHEMICAL OXYGEN DEMAND
a)	Waste discharges shall not cause the chemical oxygen demand to exceed 15 mg per litre.	Tuen Mun (A), Tuen Mun (B) and Tuen Mun (C) Subzones and Water Gathering Ground Subzones
b)	Waste discharges shall not cause the chemical oxygen demand to exceed 30 mg per litre.	Other inland waters
M	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 use of the aquatic environment.	Whole zone
N	PHENOL
a)	Phenols shall not be present in such quantities as to produce a specific odour, or in concentration greater than 0.05 mg per litre as C6H5OH.	Bathing Beach Subzones
O	TURBIDITY
b)	Waste discharges shall not reduce light transmission substantially from the normal level.	Bathing Beach Subzones
Note: Reference: Statement of Water Quality Objectives (North Western Water Control Zone)

EIAO-TM

6.2.3 The EIAO-TM specifies the assessment methods and criteria for impact assessment. This Study follows the EIAO-TM to assess the potential water quality impact that may arise during both the construction and operation phases of the Project. Sections in the EIAO-TM relevant to the water quality impact assessment are:

· Annex 6 – Criteria for Evaluating Water Pollution; and

· Annex 14 – Guidelines for Assessment of Water Pollution.

ETWB Technical Circular (Works) No. 5/2005 Protection of natural streams/ rivers from adverse impacts arising from construction works

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

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

6.2.5 All discharges during both the construction and operation phases of the proposed Project are required to comply with the Technical Memorandum Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS) issued under Section 21 of the WPCO.

6.2.6 The TM-DSS defines acceptable discharge limits to different types of receiving waters. Under the TM-DSS, effluents discharged into the drainage and sewerage systems, inshore 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 license conditions for any new discharge within a WCZ.

Practice Note for Professional Persons, Construction Site Drainage

6.2.7 Apart from the above statutory requirements, the Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN1/94), issued by EPD in 1994, also provide useful guidelines on control, handling and disposal of effluent discharges from construction activities. The ProPECC PN1/94 addresses the following discharges from the construction sites:

l Surface runoff;

l Groundwater;

l Boring and drilling water;

l Wastewater from concrete batching and precast concrete casting;

l Wheel washing water;

l Bentonite slurries;

l Water for testing sterilization of water retaining structures and water pipes;

l Wastewater from building construction and site facilities; and

l Acid cleaning, etching and pickling wastewater.

Practice Note for Professional Persons, Drainage Plans subject to Comment by the Environmental Protection Department

6.2.8 The Practice Note for Professional Persons, Drainage Plans subject to Comment by the Environmental Protection Department (ProPECC PN5/93) issued by EPD in 1993 also provides useful guidelines on the environmental design and pollution control in drainage plan submissions under Building (Standards of Sanitary Fitments, Plumbing, Drainage Works and Latrines) Regulations 40(1), 40(2), 41(1) and 90.

6.3 Water Sensitive Receivers

6.3.1 A desktop study including a review of the Approved Tuen Mun OZP (No. S/TM/35) and the Approved Lam Tei and Yick Yuen OZP (No. S/TM-LTYY/10), topographic maps and aerial photos and site visits were conducted to identify the water sensitive receivers (WSRs). There are few watercourses that run through the Project Site. These watercourses are identified as WSRs which may experience water quality impacts during project construction and operation stages. The downstream sections of these watercourses, i.e. Tuen Mun River, are also considered as water sensitive receivers. Some farmlands with potential water abstraction for irrigation are also identified as WSRs. No planned WSRs have been identified within the assessment area.

6.3.2 A number of sensitive uses are identified beyond the Tuen Mun River and they are summarized below:

l Tuen Mun Typhoon Shelter located at the mouth of Tuen Mun River and downstream of the potential housing developments;

l Gazetted bathing beaches near the mouth of the Tuen Mun River, including Castle Peak Beach, Kadoorie Beach, Cafeteria Old Beach and Cafeteria New Beach; and

l WSD Seawater Intake at Castle Peak Bay.

6.3.3 The sensitive uses mentioned in Section 6.3.2 are more than 3.5 km away from the Project Site. They are considered too far from the Project Site to be impacted by the Project, provided that appropriate mitigation measures are implemented at the upstream. Thus, they are not considered as WSRs for assessment under this Project.

6.3.4 The identified WSRs in the vicinity of the Project Site are presented in Table 6.2 and Figure 6.1 and Figure 6.2.

Table 6.2 - Water Sensitive Receivers

ID	Description	Within Project Site (Yes / No)	Type	Directly impacted by the Project	Remarks
Watercourse
S01	Chung Shan Creek	Yes (partially)	Semi-natural stream, channelized near Tsz Tin Tsuen with section near Lam Tei Interchange converted to box culvert	P	Sections within San Hing Road Site and Hong Po Road Site to be diverted. Two aquatic species of conservation interest (Crypotopotamon anacoluthon and Somanniathelphusa zanklon) were found in this watercourse.
S01a	An upstream tributary of Chung Shan Creek	No	Natural stream	O	A freshwater main is proposed to be laid along the footbridge above the stream without direct impact to the stream.
S01b	An upstream tributary of Chung Shan Creek	No	Natural stream	O	-
S01c	An upstream tributary of Chung Shan Creek	No	Natural stream	O	-
S02	A longer downstream tributary of Chung Shan Creek	Yes (Partially)	Natural stream and semi-natural stream upstream, channalized near Villa Pinada and downstream	P	Sections within San Hing Road Site Extension and San Hing Road Site to be diverted
S03	A short downstream tributary of Chung Shan Creek	Yes	Channelized	P	To be diverted
S04	At Chung Shan	No	Natural stream	O	-
S05	At Chung Shan	No	Natural stream	O	Ends up at a storm water chamber (SSH1003061) at Hong Po Road
S06	Near Po Tong Ha	Yes (Partially)	Natural stream	P	A section of the proposed Road L7 will cross the stream. Water flow from stream will be intercepted and diverted to the road drain.
S07	Between Po Tong Ha and Siu Hang Tsuen	No	Channelized	O	Ends up at Tsz Tin Road
S08	West of Po Tong Ha	Yes (Partially)	Natural stream	P	A section of the proposed Road L7 will cross the stream near Site 1 &1A of Tuen Mun Area 54. Water flow from the stream will be intercepted and diverted to the road drain.
S09	North-west of Po Tin Estate	No	Natural stream	O	Connects with a box culvert (SBP1002660) at Hing Fu Street
S10	West of Po Tin Estate	No	Natural stream	O	Connects with a box culvert (SBP1002662) at Po Tin Estate
S11	A nullah to the north of Fu Tai Estate	No	Channelized	O	Connects with Tuen Mun River near MTR Siu Hong Station
S12	Tuen Mun River	No	Channelized	O	Discharges at Tuen Mun Typhoon Shelter
S14	North of Leung King Estate	No	Semi-natural stream and channelized	O	-
S15	At Por Lo Shan	No	Natural stream	O	-
S16	At cut slope west of Shan King Estate	No	Semi-natural stream	O	-
S17	At cut slope west of Shan King Estate	No	Channelized	O	-
S23	Near Unicorn Garden	No	Channelized	O	-
Pond
S13	A pond to the south of Fortress Garden	No	Two active farm ponds for irrigation and one abandoned fishpond	O	-
S24	A pond at Unicorn Garden	No	Fung shui pond	O	-
Potential Water Abstraction for Irrigation
S18	Farmland with potential water abstraction area at Hong Po Road Site	Yes	Farmland	P	Land to be resumed by this Project
S19	Farmland with potential water abstraction area at San Hing Road Site Extension,	Yes	Farmland	P	Land to be resumed by this Project
S20	Farmland with potential water abstraction area between San Hing Road Site and San Hing Road Site Extension	Only a small part of within the Project Site	Farmland	P (for area found within Project Site) O (for area found outside Project Site)	-
S21	Farmland with potential water abstraction area near Tsz Tin Tsuen and Siu Hang Tsuen	No	Farmland	O	-
S22	Farmland with potential water abstraction area south of Fortress Garden	No	Farmland	O	-
Aquatic Species of Conservation Interest
SC1	Crypotopotamon anacoluthon	1 individual within Project Site and 4 individuals adjacent to the Project Site	Freshwater crab	P(for individual found within Project Site) O (for individuals found outside Project Site)	Found in watercourse S01 adjacent and within Hong Po Road Site
SC2	Somanniathelphusa zanklon	12 individuals located upstream or adjacent to the Project Site	Freshwater crab	O	Found in watercourse S01 within upstream and adjacent to Hong Po Road Site and watercourse S02 upstream of San Hing Road Site Extension

6.4 Baseline Conditions

Description of the Environment

6.4.1 The Project involves site formation and infrastructural works, which would include the necessary slope works, road works, sewerage works, drainage works, waterworks, utility works etc. within or outside the housing sites for serving the proposed housing development. In view of the nature of the works involved, water quality issues would be the construction site runoff and drainage, general construction activities, and sewage from the construction workforce on site during construction phase. The proposed development comprises the housing and school sites and associated infrastructure. The water quality issues during operation of the Development would be generation of sewage and urban runoff.

6.4.2 Watercourses identified within the assessment area are presented in Figure 6.1 and Figure 6.2. There are a couple of watercourses passing through the Project Site. Most of these watercourses are modified, channelized or culverted. A major watercourse is identified running from the northwest to the southeast of the Project Site. There are also a number of minor streams running down from the Yuen Tau Shan (northwest of the Project Site) to the Project area. Some of these minor streams are intermittent or ephemeral streams. All the identified watercourses within the vicinity of the Project Site run either into the Tuen Mun River or the drainage system in the urban area.

Baseline Water Quality Conditions

Tuen Mun River

6.4.3 EPD’s River Water Quality in Hong Kong in 2018 indicates that Tuen Mun River showed significant improvement in the last three decades. Its WQO compliance rate rose steadily from 32% in 1988 to 88% in 2018. This improvement was attributed to pollution control efforts as well as implementation of the mitigation measures recommended under the Tuen Mun Sewage Master Plan. In 2018, the upstream station (TN1) was graded “Bad” mainly due to discharge from the unsewered areas. To prevent the pollutants from affecting Tuen Mun River, a Dry Weather Flow Interceptor has been installed at West Rail Siu Hong Station to divert the flow at TN1 to the foul sewers and sewage treatment works for treatment. All other five stations (TN2, TN3, TN4, TN5 and TN6) were graded “Good”. The water quality data in 2018 from EPD monitoring stations along Tuen Mun River are presented in Table 6.3 and the monitoring locations are shown in Figure 6.3 and Figure 6.4.

Table 6.3 - Summary of Water Quality Statistics for Tuen Mun River in 2018

Parameter	unit	TN1	TN2	TN3	TN4	TN5	TN6
Dissolved oxygen	mg/L	4.8	7.7	4.9	5.4	5.4	5.4
		(2.9 - 6.1)	(5.6 - 9.0)	(3.1 - 7.1)	(3.1 - 10.3)	(3.1 - 7.1)	(3.3 - 6.8)
pH		7.4	7.2	7.5	7.6	7.5	7.4
		(7.1 - 7.9)	(6.6 - 7.9)	(7.2 - 7.9)	(7.2 - 8.1)	(7.2 - 7.9)	(7.2 - 7.7)
Suspended solids	mg/L	10.5	8.4	6.5	6.6	6.4	5.6
		(3.0 - 22.0)	(2.5 - 26.0)	(3.2 - 23.0)	(3.2 - 150.0)	(2.6 - 230.0)	(2.4 - 14.0)
5-day Biochemical Oxygen Demand	mg/L	22.0	3.5	2.6	1.7	2.0	2.5
		(10.0 - 45.0)	(0.7 - 28.0)	(1.0 - 13.0)	(0.9 - 6.6)	(1.2 - 28.0)	(1.2 - 4.4)
Chemical Oxygen Demand	mg/L	26	10	15	12	14	12
		(15 - 54)	(3 - 33)	(7 - 23)	(6 - 24)	(7 - 46)	(6 - 21)
Oil & grease	mg/L	0.8	<0.5	<0.5	<0.5	<0.5	<0.5
		(<0.5 - 2.1)	(<0.5 - 0.8)	(<0.5 - <0.5)	(<0.5 - <0.5)	(<0.5 - <0.5)	(<0.5 - <0.5)
E. coli	counts/ 100mL	130 000	53 000	11 000	6 700	8 100	11 000
		(34 000 - 260 000)	(12 000 - 200 000)	(610 - 100 000)	(600 - 71 000)	(500 - 86 000)	(1 400 - 160 000)
Faecal coliforms	counts/ 100mL	460 000	89 000	86 000	46 000	47 000	62 000
		(120 000 - 1 000 000)	(26 000 - 380 000)	(3 800 - 720 000)	(6 100 - 340 000)	(4 400 - 770 000)	(6 900 - 950 000)
Ammonia- nitrogen	mg/L	6.650	1.700	0.420	0.580	0.460	0.480
		(3.800 - 11.000)	(0.510 - 5.700)	(0.270 - 0.820)	(0.190 - 0.990)	(0.230 - 0.800)	(0.200 - 1.100)
Nitrate-nitrogen	mg/L	0.350	2.000	0.520	0.490	0.540	0.525
		(<0.002 - 2.200)	(0.700 - 3.500)	(0.220 - 0.860)	(0.240 - 1.000)	(<0.002 - 0.920)	(0.150 - 0.870)
Total Kjeldahl nitrogen	mg/L	9.35	2.20	0.80	0.73	0.92	0.79
		(5.50 - 13.00)	(0.78 - 8.00)	(0.46 - 1.50)	(0.44 - 1.80)	(0.50 - 2.00)	(0.50 - 1.80)
Orthophosphate Phosphorus	mg/L	0.525	0.180	0.037	0.042	0.039	0.048
		(0.310 - 0.940)	(0.007 - 0.600)	(0.002 - 0.084)	(0.002 - 0.098)	(0.003 - 0.086)	(0.012 - 0.090)
Total phosphorus	mg/L	0.87	0.28	0.07	0.08	0.09	0.08
		(0.54 - 1.20)	(0.06 - 0.87)	(0.04 - 0.16)	(0.04 - 0.18)	(0.04 - 0.34)	(0.04 - 0.16)
Sulphide	mg/L	0.04	<0.02	<0.02	<0.02	<0.02	<0.02
		(<0.02 - 0.08)	(<0.02 - 0.05)	(<0.02 - 0.03)	(<0.02 - <0.02)	(<0.02 - 0.39)	(<0.02 - <0.02)
Aluminium	µg/L	<50	122	54	74	66	53
		(<50 - 113)	(<50 - 614)	(<50 - 128)	(<50 - 193)	(<50 - 152)	(<50 - 149)
Cadmium	µg/L	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
		(<0.1 - <0.1)	(<0.1 - <0.1)	(<0.1 - <0.1)	(<0.1 - <0.1)	(<0.1 - <0.1)	(<0.1 - 0.1)
Chromium	µg/L	<1	<1	2	2	3	2
		(<1 - <1)	(<1 - 2)	(1 - 5)	(1 - 4)	(1 - 4)	(1 - 4)
Copper	µg/L	2	1	4	4	4	4
		(1 - 5)	(<1 - 4)	(1 - 7)	(1 - 13)	(1 - 8)	(3 - 13)
Lead	µg/L	<1	<1	<1	<1	<1	<1
		(<1 - <1)	(<1 - 3)	(<1 - <1)	(<1 - <1)	(<1 - <1)	(<1 - <1)
Zinc	µg/L	13	<10	10	11	11	12
		(<10 - 38)	(<10 - 36)	(<10 - 28)	(<10 - 21)	(<10 - 27)	(<10 - 33)
Flow	L/s	0.154	0.030	NM	NM	NM	NM
		(0.075 - 0.240)	(0.010 - 0.216)

Source: River Water Quality in Hong Kong in 2018 (EPD, 2019).

Notes: 1. Data presented are in annual medians of monthly samples; except those for faecal coliforms and E. coli which are in annual geometric means.

2. Figures in brackets are annual ranges.

3. NM indicates no measurement taken.

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

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

6.4.4 As EPD’s monitoring stations along Tuen Mun River are located downstream and at a distant away from the Project Site, water quality monitoring at the watercourses in the Project vicinity were conducted to establish the baseline water quality in Project area. The locations of project specific water quality monitoring stations and EPD regular monitoring stations are shown in Figure 6.3 and Figure 6.4 respectively. The project specific water quality monitoring was conducted on 21 September 2018, 3 October 2018, 12 December 2018 and 9 January 2019. The results are summarised in Table 6.4 below.

Table 6.4 – Results of Water Quality Monitoring within/near the Project Site

Parameter	unit	WQ1	WQ2	WQ3	WQ4	WQ5
Dissolved oxygen	mg/L	7.99	7.92	5.50	8.45	7.92
		(7.55 - 8.50)	(7.34 - 8.84)	(4.97 – 6.13)	(7.78 – 9.16)	(7.07 - 8.58)
Dissolved Oxygen	% saturation	93.7	93.1	66.0	101.5	93.5
		(86.3 – 99.9)	(86.6 – 99.2)	(57.4 – 72.6)	(89.2 – 117.1)	(91.0 – 98.8)
Conductivity	μS/cm	79.5	109.1	146.5	156.6	127.7
		(55.0 – 117.6)	(82.2 – 149.2)	(90.6 – 177.1)	(136.7 – 195.3)	(108.7 – 160.2)
Salinity	ppt	0.04	0.05	0.07	0.08	0.06
		(0.02 – 0.06)	(0.04 – 0.07)	(0.04 – 0.08)	(0.06 – 0.10)	(0.05 – 0.08)
pH		7.1	7.2	6.6	7.6	7.0
		(6.6 – 7.3)	(7.0 – 7.4)	(6.2 – 7.0)	(7.3 – 8.0)	(6.8 – 7.4)
Turbidity	NTU	6.34	3.67	13.68	5.18	4.13
		(4.20 – 9.69)	(1.69 – 8.97)	(6.33 – 19.30)	(3.91 – 8.48)	(1.70 – 7.06)
Water Depth (at sampling location)	m	<0.5	<0.5	<0.5	<0.5	<0.5
		<0.5	<0.5	<0.5	<0.5	<0.5
Total Suspended Solids	mg/L	3.5	4.3	16.3	6.1	2.4
		(<2 – 6)	(<2 – 6)	(8 – 27)	(3 – 8)	(<2 – 4)
Ammonia- nitrogen	mg/L	0.19	0.22	0.46	0.85	0.21
		(0.03 – 0.48)	(0.04 – 0.45)	(0.28 – 0.77)	(0.33 – 1.64)	(0.03 – 0.52)
Nitrate-nitrogen	mg/L	1.03	0.77	1.61	1.41	1.49
		(0.62 – 1.70)	(0.60 – 0.96)	(1.52 – 1.70)	(1.07 – 1.87)	(0.95 – 1.97)
Total Kjeldahl nitrogen	mg/L	0.5	0.5	0.7	1.4	0.2
		(0.1 – 1.1)	(<0.1 – 1.0)	(0.3 – 1.6)	(0.5 – 2.6)	(0.1 – 0.3)
Total phosphorus	mg/L	0.13	0.11	0.22	0.49	0.11
		(<0.1 – 0.20)	(<0.1 – 0.15)	(0.14 – 0.29)	(0.23 – 1.14)	(<0.1 – 0.13)
Orthophosphate	mg/L	0.09	0.07	0.10	0.35	0.07
		(0.03 – 0.18)	(0.04 – 0.13)	(0.06 – 0.13)	(0.18 – 0.76)	(0.03 – 0.12)
Sulphide	mg/L	<0.1	<0.1	<0.1	<0.1	<0.1
		(<0.1 - <0.1)	(<0.1 - <0.1)	(<0.1 - <0.1)	(<0.1 - <0.1)	(<0.1 - <0.1)
Oil & Grease	mg/L	<5	<5	<5	<5	<5
		(<5 - <5)	(<5 - <5)	(<5 - <5)	(<5 - <5)	(<5 - <5)
Chemical Oxygen Demand	mg/L	6	9	8	15	6
		(<5 – 10)	(<5 – 18)	(<5 – 12)	(<5 – 40)	(<5 – 8)
Biological Oxygen Demand	mg/L	3	3	2	6	2
		(<2 – 6)	(<2 – 5)	(<2 – 2)	(<2 – 18)	(<2 – 4)
Aluminium	µg/L	197.5	72.5	352.5	101.3	127.5
		(100 – 360)	(40 – 110)	(150 – 660)	(70 – 140)	(40 – 250)
Cadmium	µg/L	<0.2	<0.2	<0.2	<0.2	<0.2
		(<0.2 - <0.2)	(<0.2 - <0.2)	(<0.2 - <0.2)	(<0.2 - <0.2)	(<0.2 - <0.2)
Chromium	µg/L	<1	1	<1	<1	<1
		(<1 - <1)	(<1 – 3)	(<1 - <1)	(<1 - <1)	(<1 - <1)
Copper	µg/L	1.5	1.6	2.1	1.8	1.1
		(<1 – 3)	(<1 – 4)	(<1 – 4)	(<1 – 3)	(<1 – 2)
Lead	µg/L	<1	2.5	3	1	<1
		(<1 – 1)	(<1 – 8)	(1 – 5)	(<1 – 1)	(<1 - <1)
Zinc	µg/L	137.5	105.0	26.3	18.8	50.0
		(<10 – 600)	(<10 – 410)	(20 – 40)	(10 – 40)	(10 - 320)
E. coli	CFU/ 100 mL	1653	5896	9438	7810	663
		(110 – 6500)	(310 – 16000)	(340 – 25000)	(680 – 34000)	(50 – 3000)
Faecal Coliforms	CFU/ 100 mL	2093	7323	11500	9525	853
		(160 - 7800)	(680 – 18000)	(680 – 31000)	(800 – 40000)	(57 – 3700)
Flow	L/s	60	20	6	47	12
		(13 – 94)	(7 – 51)	(2 – 7)	(32 – 84)	(3 – 25)

6.4.5 It is noted that high levels of E. coli and faecal coliforms were observed at all monitoring stations, in particular WQ2, WQ3 and WQ4, indicating that the water quality there was likely affected by sewage discharged upstream where scattered houses are not served by public sewers. High levels of ammonia-nitrogen, biological oxygen demand and chemical oxygen demand were also recorded at WQ4 suggesting high organic constituents in the watercourse.

Tuen Mun Typhoon Shelter

6.4.6 The water quality of the Tuen Mun Typhoon Shelter is routinely monitored by EPD. A summary of monitoring data for 2018 at Tuen Mun Typhoon Shelter is provided in Table 6.5 below.

Table 6.5 – Summary of Water Quality Statistics at Tuen Mun Typhoon Shelter in 2018

Parameter	unit	Tuen Mun Typhoon Shelter (NT1)
Temperature	°C	23.9
		(16.4 - 28.5)
Salinity	psu	26.6
		(18.1 - 31.4)
Dissolved oxygen	mg/L	6.0
		(4.3 – 7.2)
Bottom	mg/L	6.0
		(4.3 – 7.2)
Dissolved oxygen saturation	%	83
		(61 – 105)
Bottom	%	83
		(61 – 105)
pH		7.9
		(7.6 - 8.3)
Secchi Disc Depth	m	2.3
		(2.0 - 2.6)
Turbidity	NTU	6.2
		(1.6 - 9.9)
Suspended solids	mg/L	11.3
		(6.1 - 21.5)
5-day Biochemical Oxygen Demand	mg/L	1.1
		(0.4 - 1.9)
Ammonia-nitrogen	mg/L	0.103
		(0.042 - 0.175)
Unionized ammonia	mg/L	0.004
		(0.002 - 0.007)
Nitrite-nitrogen	mg/L	0.072
		0.021 - 0.275
Nitrate-nitrogen	mg/L	0.399
		0.125 - 0.865
Total inorganic nitrogen	mg/L	0.57
		0.20 - 1.19
Total Kjeldahl nitrogen	mg/L	0.34
		(0.19 - 0.63)
Total nitrogen	mg/L	0.81
		(0.45 - 1.33)
Orthophosphate Phosphorus	mg/L	0.016
		(0.008 - 0.032)
Total phosphorus	mg/L	0.04
		(0.03 - 0.06)
Silica (as SiO2)	mg/L	1.90
		(0.54 - 6.50)
Chlorophyll-a	µg/L	8.2
		(1.1 - 32.5)
E. coli	count/ 100 mL	370
		(50 - 4500)
Faecal coliforms	count/ 100 mL	2200
		(270 – 30000)

Source: Marine Water Quality in Hong Kong in 2018 (EPD, 2019).

Notes: 1. Unless otherwise specified, data presented are depth-averaged (A) values calculated by taking the means of three depths: Surface (S), Mid-depth (M), Bottom (B).

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

3. Data in brackets indicate the ranges.

North Western Water Control Zone

6.4.7 The marine water quality of the North Western WCZ is also routinely monitored by EPD. The nearest EPD marine water quality monitoring stations are located near Pearl Island (NM2) and waters off Pillar Point (NM3). The overall WQO compliance rate of the North Western WCZ in 2018 is 89%, improved from 72% in both 2017 and 2016. Compliance with the NH₃-N and TIN objectives in this WCZ was 100% and 67% respectively. The relatively high levels of TIN in 2018 were likely attributed to the higher background level of Pearl River, and some local discharges and surface run-off from the Northwest New Territories as well as North Lantau. A summary of monitoring data for 2018 at NM2 and NM3 are provided in Table 6.6 below.

Table 6.6 - Summary of Water Quality Statistics at NM2 and NM3 in the North Western Water Control Zone in 2018

Parameter	unit	Pearl Island (NM2)	Pillar Point (NM3)
Temperature	°C	23.8	23.9
		(16.7 - 28.2)	(16.9 - 28.6)
Salinity	psu	29.2	29.1
		(24.3 - 32.3)	(24.9 - 32.0)
Dissolved oxygen	mg/L	5.9	5.9
		(4.3 - 7.4)	(4.0 - 7.4)
Bottom	mg/L	5.7	5.6
		(3.1 - 7.5)	(2.9 - 7.7)
Dissolved oxygen saturation	%	82	82
		(64 – 92)	(58 – 98)
Bottom	%	79	78
		(45 – 93)	(42 – 100)
pH		8.0	8.0
		(7.7 - 8.2)	(7.7 - 8.2)
Secchi Disc Depth	m	2.7	2.4
		(2.0 - 3.6)	(1.9 - 3.5)
Turbidity	NTU	4.9	5.7
		(1.7 - 8.0)	(1.9 - 9.2)
Suspended solids	mg/L	9.0	11.1
		(3.0 - 18.7)	(5.3 - 22.0)
5-day Biochemical Oxygen Demand	mg/L	0.7	0.7
		(0.2 - 1.5)	(0.2 - 1.4)
Ammonia-nitrogen	mg/L	0.080	0.078
		(0.006 - 0.133)	(0.016 - 0.137)
Unionized ammonia	mg/L	0.003	0.003
		(<0.001 - 0.005)	(<0.001 - 0.005)
Nitrite-nitrogen	mg/L	0.061	0.066
		(0.013 - 0.130)	(0.013 - 0.157)
Nitrate-nitrogen	mg/L	0.282	0.314
		(0.076 - 0.480)	(0.137 - 0.560)
Total inorganic nitrogen	mg/L	0.42	0.46
		(0.16 - 0.60)	(0.19 - 0.77)
Total Kjeldahl nitrogen	mg/L	0.36	0.32
		(0.18 - 0.81)	(0.18 - 0.69)
Total nitrogen	mg/L	0.70	0.70
		(0.37 - 1.04)	(0.43 - 0.94)
Orthophosphate Phosphorus	mg/L	0.018	0.019
		(0.002 - 0.035)	(0.004 - 0.039)
Total phosphorus	mg/L	0.03	0.04
		(0.03 - 0.04)	(0.03 - 0.05)
Silica (as SiO2)	mg/L	1.31	1.44
		(0.34 - 2.47)	(0.43 - 3.10)
Chlorophyll-a	µg/L	3.5	3.1
		(0.5 - 19.7)	(0.4 - 11.8)
E. coli	cfu/100 mL	54	98
		(3 – 660)	(4 – 3600)
Faecal coliforms	cfu/100 mL	120	230
		(12 – 1100)	(7 – 10000)

Source: Marine Water Quality in Hong Kong in 2018 (EPD, 2019).

Notes: 1. Unless otherwise specified, data presented are depth-averaged (A) values calculated by taking the means of three depths: Surface (S), Mid-depth (M), Bottom (B).

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

3. Data in brackets indicate the ranges.

6.5 Assessment Methodology

6.5.1 In accordance with the EIA Study Brief, the assessment area of the water quality impact includes the area within 500 m from the boundary of the Project Site.

6.5.2 The major issue during construction of the Project is site runoff and sewage from workforce while the issues during operation of the Project relate to sewage from the planned population and non-point sources discharge such as additional surface runoff due to increase in hard paved area. There will be neither dredging, nor reclamation works and all the works will be land-based. Hence, water quality modelling is not proposed.

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

6.6 Identification of Pollution Sources

6.6.1 The Project comprises the following works that would impact water quality of the WSRs within and near the Project Site.

l Site formation, including removal / diversion of watercourses;

l Slope works, including construction of retaining wall;

l Road works and utilities;

l Construction of waterworks, service reservoirs, drainage and sewerage infrastructural works such as box culverts, new sewers and upgrading of existing sewers;

l Construction of superstructures for residential, commercial uses, community / institution, and supporting facilities; and

l Sewage pumping station.

6.6.2 The pollution sources/ impact of the Project during the construction phase, consist of:

l Runoff from construction sites;

l Sewage from construction workforce;

l Accidental spillage of chemicals;

l Contaminated groundwater and wastewater;

l Construction runoff from works at or near watercourses;

l Runoff during demolition / diversion of watercourses; and

l Impact on aquatic species of conservation interest.

6.6.3 The above potential impacts on the WSRs within and outside the Project Site during the construction phase are assessed in Section 6.7.1 to 6.7.12.

6.6.4 Based on the development parameters in Table 3.1, the pollution sources / impact of the proposed development during the operation phase consist of:

l Sewage disposal from the new population;

l Surface runoff from roads, public transport interchanges (PTIs) and paved areas;

l Runoff from landscape areas;

l Temporary discharge (including accidental and regular maintenance discharge) from the proposed San Hing Road Sewage Pumping Station (SHR SPS); and

l Wastewater from commercial activities.

6.7 Prediction and Evaluation of Impacts

Construction Phase

Runoff and Drainage from Construction Sites

6.7.1 Surface runoff from construction sites would be generated after rainfalls. This includes non-point pollution sources of runoff and erosion from exposed site surfaces, interception of drainage channels and watercourses, earth working, excavation and stockpiles that contain increased loads of sediments, and other suspended solids (SS) and contaminants. Potential contaminants include:

l Silt and contaminated runoff from on-site stockpiles;

l Cement, bentonite slurries and other grouting materials for foundation works; and

l Silt and SS within wastewater from dust suppression sprays and wheel washing facilities.

6.7.2 Construction runoff would cause physical, biological and chemical effects. The physical effects include potential blockage of drainage channels and increase in SS levels in watercourses downstream. Runoff that contains significant quantities of concrete and cement-derived material would cause primary chemical effects such as increase in turbidity, discoloration and pH, and accretion of solids. Secondary effects would also result in toxic effects to water biota (due to elevated pH level), and reduced decay rates of faecal micro-organisms and photosynthetic rate due to the decreased light penetration. Nevertheless, water quality impact would become significant only if the runoff and drainage are allowed to discharge directly into the receiving water body without treatment. Thus, provided the surface runoff and drainage are effectively managed and controlled over the site by the mitigation measures recommended in Section 6.8, adverse water quality impacts are not anticipated.

6.7.3 To minimise the risk of flooding and safeguard the public, site staff and contractors should properly maintain the site drainage system and take all necessary precautionary measures as recommended in Section 6.8 to prevent the blockage of the nearby public drains during construction.

Sewage from Construction Workforce

6.7.4 Sewage effluents would arise from the on-site sanitary facilities serving the construction workforce. Sewage is characterised by high levels of biochemical oxygen demand, ammoniacal nitrogen and E. coli. According to Table T-2 of Guidelines for Estimating Sewage Flows for Sewage Infrastructure Planning, the unit flow is 0.23 m³ per day per worker. Assuming that the number of construction workers will be no more than 1000 during the peak construction period, the quantity of sewage generated would be around 230 m³ per day. Since portable chemical toilets and sewage holder tank will be provided, no adverse water quality impact is anticipated.

Accidental Spillage of Chemicals

6.7.5 Chemicals, such as fuel, oil, solvents and lubricants for maintenance of construction machinery and equipment, would be used during construction. If accidentally released and discharged into the water bodies, it would cause water quality impact. To avoid adverse impacts of chemical spillage, best practices of chemical storage such as under covered area, provision of secondary containment (such as bunds), and material safety data sheets are advised. Spill kits are also advised to handle spillage. Staff should be trained for handling spillage and emergency response plan on chemical spillage should be formulated in advance. With the implementation of mitigation measures in Section 6.8, adverse water quality impact to the watercourses is not anticipated.

Groundwater and Wastewater

6.7.6 Ground excavation may be required for site formation, slope works including construction of retaining wall, foundation works and installation of underground utilities. Dewatering will be required for works undertaken below groundwater table. When dewatering takes place in area with groundwater contamination identified, the extracted groundwater requires appropriate handling and disposal. Besides, the extracted groundwater may contain high content of silt. Wastewater and runoff would also be generated from the excavated soil during handling works and rainfalls. As such, extracted groundwater, wastewater and runoff should be collected and treated by on-site wastewater treatment facilities prior to discharge. With proper implementation of mitigation measures recommended in Section 6.8, no significant water quality impact is anticipated.

Construction Runoff from Works at or near Watercourses

6.7.7 Some watercourses are located at or within the Project Site as identified in Section 6.3. Construction works near these watercourses, including ecological enhancement to the retained section of watercourse (S01) adjacent to Hong Po Road Site, may pollute the inland waters due to potential release of construction wastes. Construction wastewater generally contains high levels of SS with elevated pH value. Good housekeeping and mitigation measures should be adopted to reduce generation of construction wastes and potential water pollution. Control of runoff and drainage from construction works adjacent to the inland water should also be implemented to prevent high levels of SS from entering the aquatic environment. Measures recommended in Section 6.8 and ETWB TC (Works) No. 5/2005 should be properly implemented to minimize the potential water quality impact. When designing on-site drainage works, good practices stated under ProPECC PN 5/93 should be referenced.

Runoff during Removal / Diversion of Watercourses

6.7.8 To achieve the target population intake while balancing the constraints from various technical aspects during the formulation of the master layout plan, demolition and diversion of the existing semi-natural streams and drainage channels located within the Project Site is unavoidable. According to the proposed drainage plan of this Study as shown in Figure 6.5, the identified watercourses within the Project Site will be diverted to the proposed u-channel or box culvert and connected to Tuen Mun River downstream. For Chung Shan Creek (S01) and its upstream tributaries (S01c) within HPR Site, the watercourses will be diverted to a box culvert of 2 m (H) × 3 m (W) along the eastern boundary and drainage pipe along the western boundary, respectively. The drainage pipe along the western boundary will only collect water from the upstream S01c and discharge into the downstream S01 near Po Tong Ha. Runoff from Hong Po Road Site will be collected by the proposed U-channel within the site and discharge to the box culvert along the eastern boundary. The watercourse (S02) within SHR Site Extension will be diverted to the proposed box culvert of 2 m (H) × 3 m (W) along the eastern boundary of SHR Site Extension. The watercourse (S03) will be filled. The watercourse (S01) within SHR Site will be diverted to a twin 2 m (H) × 4 m (W) box culvert underneath the realigned Hong Po Road that will connect with the existing box culvert downstream. Watercourses S06 and S08 will be intercepted and diverted to the road drain along the proposed Road L7.

6.7.9 Diversion of watercourses would involve excavation and construction works. Excavated materials, wastewater, chemicals and surplus construction materials may enter the watercourses if they are not properly contained and would give rise to downstream water quality impact. To avoid adverse water quality impact, stream diversion works should be undertaken during dry season (when the flow is low) whenever practicable. Works programme should be duly scheduled to minimize water quality impacts from potentially polluting works such as excavation of sediments and stockpiling of fill materials. A temporary drainage or permanent drainage at most downstream should first be established before the upstream flow is intercepted and diverted. Precaution measures presented in Appendix D of ETWB TC No. 5/2005, such as temporary isolation of rivers using sandbags and silt curtains could be applied to prevent pollutants running into other watercourses nearby. Dewatering of the construction works area shall be completed prior to the construction works. Wastewater treatment facilities should be installed to treat the effluent from dewatering operation prior to discharge. After completion of the construction works, the works area should be cleaned up before receiving any water flow or connecting to any existing watercourses. Section 6.8 presents further discussion on these mitigation measures.

Impacts on Aquatic Species of Conservation Interest

6.7.10 Based on the ecological field survey for this Study, two aquatic species of conservation interest were identified within the assessment area. Freshwater crabs, Somanniathelphusa zanklon and Crypotopotamon anacoluthon were recorded along watercourse S01, S01b and S02, as shown in Figure 6.1. Out of a total of seventeen individuals, sixteen individuals were recorded at the upstream section of the watercourses outside the Project Site or adjacent to but outside the Project Site. As the Project Site will be fenced off with perimeter drainage provided during the construction phase and with the adoption of good site practices recommended in ProPECC PN 1/94, potential impact on these individuals of aquatic species is expected to be minimal. Key measures are recommended in Sections 6.8.11 and 6.8.12.

6.7.11 One individual was recorded at the semi-natural stream within the Project Site (Hong Po Road Site) and will be directly impacted as a result of removal of watercourse. As mentioned in Section 6.7.8, removal and diversion of watercourse within the Site is unavoidable. A precautionary site check and translocation, if necessary, for aquatic fauna of conservation interest have been recommended prior to site formation at the watercourse. Hence, if any individuals are captured, they would be translocated to other watercourse with natural banks and bed outside the Project Site. As such, adverse impact to these species of conservation importance is not expected.

Impacts on Potential Water Abstraction for Irrigation

6.7.12 Patches of farmland with potential water abstraction for irrigation are found within the Project Site. Farmlands S18 and S19 will be resumed for the housing development and the need for water abstraction in the future will no longer be required. Farmlands S21 and S22 and their nearest watercourse are located outside the Project Site and thus not affected. Farmland S20 is located between SHR Site and SHR Site Extension and a small part of it will be resumed for development at SHR Site Extension. As advised by a local resident, water supply for the farmland is from groundwater. As the Project will not affect the groundwater regime, adverse impact on the farmland is not expected. Nevertheless, as farmland S20 is located adjacent to the Project Site, construction runoff shall be controlled by the implementation of mitigation measures. It is provided that surface runoff and drainage are effectively managed and controlled over the site by the mitigation measures recommended in Section 6.8, adverse water quality impacts are not anticipated

Operation Phase

Sewage Disposal from the New Population

6.7.13 During operation, sewage discharge from residential, commercial and community / institutional facilities will be the major source of water pollution. Wastewater would also be generated from the storage, workshop and open storage retained on site.

6.7.14 Local sewerage ranging from 300 mm to 750 mm diameter pipes would be designed to convey the sewage flow generated from the Project Site to the proposed SHR SPS for subsequent conveyance to the downstream Tuen Mun Area 54 Sewage Pumping Station (TM54SPS).

6.7.15 The proposed SHR SPS would collect sewage from SHR Site, SHR Site Extension and HPR Site for conveyance to the downstream sewerage system. The SHR SPS would have a installed capacity of 44,928 m³/d to cater for predicted Peak Flow (Averaged Dry Weather Flow of 14,629 m³/d). A rising mains along Hong Po Road and Tsing Lun Road is proposed to convey sewage from the SHR SPS to the Pillar Point Sewage Treatment Works (PPSTW) via the TM54SPS (proposed to be upgraded by DSD) and the Western Interceptor Sewer Sewage Pumping Station (WIS SPS). Details of the sewerage and sewage treatment implications are provided in Section 7.

6.7.16 With the implementation of the proposed sewerage works and the proposed upgrading of TM54SPS, no adverse sewerage impact is expected.

Surface Runoff from Roads and Paved Areas

6.7.17 The existing Project Area is partially rural in nature. Green coverage would be maximized within the housing sites to minimize the surface run-off. About 7.3 m³/s surface run-off will be generated from the site during operation which has been assessed in the separate Drainage Impact Assessment Report. The indicative site layout and landscape areas, which will be subject to further review and implementation by the Housing Department at a later stage, are shown in Chapter 10 of EIA (Figures 10.24a to 10.24d). The proposed development at HPR Site, SHR Site Extension and SHR Site would potentially have the following hydraulic impact to the downstream stormwater drainage system:

· Change in land use from unpaved to paved surface would reduce the infiltration rate of the catchment and increase the amount of runoff entering into the stormwater drainage system;

· Change in formation level and cross-fall in the Project Area would alter the overland flow pattern and discharge point into the stormwater drainage system; and

· Formation of the proposed development would cut off the flow of the existing watercourses at Chung Shan Creek that would result in ponding in some low-lying area.

6.7.18 A 50-year design return period with a minimum of 500 mm freeboard, as set out in the DSD Stormwater Drainage Manual (SDM), is recommended for the design of urban drainage branch system that should be effective to tackle the potential drainage impact. In addition, a 200-year design return period is assessed for the urban drainage trunk system to ensure adequacy of the stormwater drainage system. The proposed drainage layout shown in Figure 6.5 is considered adequate in conveying the stormwater runoff to the downstream Tuen Mun River. Interior peripheral U-channels would also be installed to properly collect and convey stormwater runoff from the Site to the downstream drainage system.

6.7.19 Surface runoff from roads, PTIs and paved areas would contain pollutants and contaminants that impact the downstream water quality. Substances such as vehicle dust, tyre scraps and oils deposited and accumulated on the road surfaces would be washed into nearby drainage system or watercourses during rainfalls. The worst-case scenario would occur during the first flush under heavy rainfall. Desilting facilities (i.e. silt traps / sedimentation tanks) should be provided at strategic locations at the PTIs and along the Site before discharging into watercourses or permanent drainage systems.

Runoff from Landscape Areas

6.7.20 The indicative landscape areas are shown in Chapter 10 of EIA (Figures 10.24a and 10.24b). Runoff may potentially be contaminated by agrochemicals. Pesticides or fertilizers may be used in the maintenance of the landscaped area, subject to the practice by the future landscape contractor employed by the Housing Authority. Under normal circumstances, any application of pesticides and fertilizers would only be on a need basis based on the health condition of the vegetation and usually at a localized scale. As the scale and nature of landscape area will be similar to those commonly adopted in housing development in Hong Kong, it is expected that the amount of agrochemicals to be used would not cause adverse water quality impact on the runoff. Nevertheless, common good practices would be applied to minimise potential water quality impact from agrochemical application.

Emergency discharge from the SHR SPS

6.7.21 To prevent the occurrence of emergency discharge, a storage tank, twin rising mains, standby pump and treatment facilities and dual electricity supply or backup power supply facilities would be provided to the proposed SHR SPS. Section 6.8 presents further discussion on these mitigation measures. With the above design provision as contingency measures, emergency discharge due to SPS failure is not anticipated.

Discharge to the North Western WCZ

6.7.22 Sewage from the Project Sites will eventually be conveyed to the Pillar Point Sewage Treatment Works (PPSTW) for chemically enhanced primary treatment (CEPT) prior to discharge into Urmston Road. The existing PPSTW has completed the upgrade works in 2014 with a capacity of 241,000 m³ per day. Sewage from the Project Site is estimated to contribute about 5% of the predicted total sewage flow to the PPSTW. The remaining capacity of PPSTW was reduced from 15% to 10% after supporting the proposed development. Discharge from the PPSTW after CEPT and UV disinfection would not cause adverse water quality impact in North Western and Western Buffer waters.

Wastewater from Municipal and Commercial Activities

6.7.23 The wastewater generated from municipal and commercial activities, e.g. refuse collection points, restaurants etc, may contain high levels of pollutants. Each individual commercial tenant should apply for an effluent discharge license under the WPCO and should comply with the standards for effluent discharge into the public sewerage under the TM-DSS. All wastewater will be conveyed to the PPSTW for treatment. No adverse water quality impact is anticipated.

Cumulative Impact with Concurrent Projects

6.7.24 During the construction, a number of development projects at Tuen Mun Area 54, including the public and private housing developments, GIC facilities, and infrastructural works for roads and drains would be constructed and operated simultaneously with the Project. It is anticipated that general construction activities such as site formation and superstructure works would be involved, and potential impact from site run-off, accidental chemical spillage, sewage from construction workforce, etc. would occur.

6.7.25 With proper implementation of the mitigation measures recommended in Section 6.8 and ProPECC PN 1/94, it is anticipated that wastewater generated from the work sites would be properly managed and the potential impact would be contained within the construction site boundaries. Cumulative construction impact of these developments would be kept to minimal.

6.7.26 Sewage flow from the planned SHR SPS will be conveyed to the Tuen Mun Area 54 Sewage Pumping Station (TM54SPS) near Tsz Tin Road via rising mains and gravity sewers. However, the existing TM54SPS was designed to serve the planned public rental housing (PRH), school and Government, Institution or Community (GIC) developments in Tuen Mun Area 54 and would not have enough capacity to handle the extra sewage flow from the planned SHR SPS. Upgrading of the TM54SPS is therefore required to cater for the cumulative sewage from the Project Site as well as sewage from the development in various sites of Tuen Mun Area 54 and other areas within the northern Tuen Mun catchment. The PPSTW has been upgraded with sufficient treatment capacity to cater for the future population growth and development in Tuen Mun. As such, unacceptable cumulative water quality impact associated with sewage flow from other concurrent projects is not anticipated during Project operation.

6.8 Mitigation Measures

Construction Phase

6.8.1 Construction phase mitigation measures, in accordance with the ProPECC PN 1/94, include the use of sediment traps, wheel washing facilities for vehicles leaving the site, adequate maintenance and provision of drainage systems (such as box culvert / pipe) to prevent flooding and overflow due to interception of the existing streams, sewage collection and treatment, and comprehensive waste management (collection, handling, transportation, disposal) procedures. The site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with various types of discharge from a construction site and should be adopted as far as practicable to minimise the potential water quality impacts from various construction activities and construction site run off on WSRs within and outside the Project site as identified in Table 6.2.

General Construction Activities and Site Runoff

6.8.2 The construction of the Project will start with site clearance and formation works, preferably in dry season. During construction, a temporary drainage channel system should be properly designed, maintained or connected to divert any runoff away from the site and eliminate the risk of overflow of surface water from the construction sites during heavy rain. It is important to ensure that the proposed drainage can take its full drainage function prior to the commencement of the earth works and landfilling within the Sites.

6.8.3 In accordance with the ProPECC PN 1/94, best management practices should be implemented as far as practicable as below:

· At the start of the site formation, appropriate site drainage such as perimeter cut-off drains should be constructed to direct off-site water around the site. Internal drainage works and erosion and sedimentation control facilities should be implemented. Channels, earth bunds or sandbag barriers should be provided on site to collect site runoff and prevent untreated runoff from entering nearby watercourses. Silt removal facilities with sufficient capacity should also be adequately designed, installed and properly maintained to treat the collected runoff to appropriate quality (as specified in the effluent discharge license) before discharge. The design of efficient silt removal facilities should be based on guidelines in Appendix A of ProPECC PN 1/94. Detailed design of the sand/ silt traps should be undertaken by the contractor prior to the commencement of the construction.

· Dikes or embankments for flood protection should be implemented around the boundaries of earthwork areas. Temporary ditches should be provided to facilitate the runoff discharge into drains or watercourses, through silt/ sediment traps. Silt/ sediment traps should also be incorporated in the permanent drainage channels to enhance deposition rates.

· The overall slope of the site should be kept to a minimum to reduce the erosive potential of surface water flows. In addition, all the entrances and exits of construction sites should be protected by coarse stone ballast. An additional advantage accruing from the use of crushed stone is the positive traction gained during prolonged periods of inclement weather and the reduction of surface sheet flows.

· Sediment tanks of sufficient capacity, constructed from pre-formed individual cells of approximately 6 to 8 m³ capacity, are recommended as general mitigation measures for settling storm water prior to disposal. The system capacity should be flexible and able to handle multiple inputs from a variety of sources and particularly suited to applications where the influent is pumped.

· All drainage facilities and erosion and sediment control structures should be regularly inspected and maintained to ensure proper and efficient operation at all times and particularly following rainstorms. Deposited silt and grit should be removed regularly, dried and disposed of with other inert C&D material.

· Construction works should be scheduled to minimise surface excavation works during the rainy seasons (April to September). The temporary cut faces, exposes faces of fill slopes or earth stockpiles should be properly protected during rainstorms. All exposed earth areas should be completed and vegetated as soon as possible after earthworks have been completed. If excavation cannot be avoided during the rainy season, or any time of year when rainstorms are likely, exposed slope surfaces should be covered by tarpaulin or other means.

· All open stockpiles of construction materials, such as aggregates, sand and fill materials, should be covered with tarpaulin or similar fabric during rainstorms. Measures should be taken to prevent washing away of construction materials, soil, silt or debris into drainage system.

· Measures should be taken to minimise the ingress of any site drainage into excavations. If excavation of trenches in wet periods is necessary, it should be dug and backfilled in short sections wherever practicable. Water pumped out from trenches or foundation excavations should be discharged into storm drains via silt removal facilities.

· Manholes (including newly constructed ones) should always be adequately covered and temporarily sealed so as to prevent silt, construction materials or debris being washed into the drainage system and storm runoff being directed into foul sewers.

· Precautions to be taken at any time of year when rainstorms are imminent or forecasted, and actions to be taken during or after rainstorms are all summarised in Appendix A2 of ProPECC PN 1/94. Particular attention should be paid to the control of silty surface runoff during storm events, especially for areas located near steep slopes.

· All vehicles and mechanical plant should be cleaned before leaving a construction site to ensure no earth, mud, debris and the like is deposited by them on roads. An adequately designed and located wheel washing bay should be provided at every site exits. Wash-water should have sand and silt settled out and removed at least on a weekly basis to ensure the continued efficiency of the process. The section of access road leading to, and exiting from, the wheel-wash bay to the public road should be paved with sufficient backfall toward the wheel-wash bay to prevent vehicle tracking of soil and silty water to public roads and drains.

· The bentonite, grouting and cement materials should only be delivered to the construction site when they are to be used. They should be stored in a covered warehouse and the excess amount should be removed from the site as soon as the construction is completed.

· Solid waste, building debris and rubbish on site should be collected, handled and disposed of properly to avoid water quality impact. They should be properly held in place when temporary stored on site to avoid blockage of drains and catchpits.

· Oil inceptors should be provided in the drainage system downstream of any potential oil/ fuel pollution sources. They should be emptied and cleaned regularly to prevent the release of oil and grease into the storm water drainage system after accidental spillage. A bypass should be provided for the oil inceptors to prevent flushing during heavy rainfall.

· All fuel tanks and storage areas on site should be provide with locks and placed on sealed areas, within bunds of a capacity equal to 110% of the storage capacity of the largest tank to prevent spilled fuel oils from reaching adjacent water sensitive receivers.

· Expedient discharge of construction site surface water into public foul sewers shall be strictly prohibited. All construction site surface water must be intercepted, desilted, conveyed and discharged into the nearby public stormwater drainage system or watercourse whichever is appropriate.

· Groundwater pumped out of wells, etc, for the lowering of groundwater level in basement or foundation construction should be discharge into storm drains after the removal of silt in silt removal facilities.

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

6.8.4 Regular site inspections should be carried out to prevent any malpractices and ensure that the recommended mitigation measures are properly implemented on site. Notices should be posted at conspicuous locations to remind the workers not discharge any sewage or wastewater into water bodies, marsh and ponds near the construction sites.

Prevention of Accidental Spillage of Chemicals

6.8.5 Chemical used during construction, such as fuel, oil solvents and lubricants, shall be properly stored and contained in designated areas with secondary containment to prevent spillage and contamination of the nearby water bodies. When in use, chemical containers, diesel generator and diesel-powered air compressor should be placed with drip trays underneath to minimise the potential impact of chemicals accidentally spilled. Breaker tips that are not in use and left on ground should be placed with drip trays or tarpaulin underneath to contain the potential impact of lubricants spilled from the tips.

6.8.6 Any maintenance activities and workshops with chemicals use shall be located away from watercourses on hard standings within a bunded area. Sumps and oil interceptors should be provided as appropriate.

6.8.7 The Contractor shall register as a chemical waste producer and employ licensed collector for collection of chemical waste from the construction site. Any chemical waste generated shall be managed in accordance with the Waste Disposal (Chemical Waste) (General) Regulation.

Sewage from Construction Workforce

6.8.8 Portable chemical toilets and sewage holding tanks should be deployed on site for handling the construction sewage generated by the workforce. Location of sewage holding tank should be far away from watercourse nearby. A licensed contractor should be engaged to provide and maintain appropriate and adequate portable toilets to cater for 230 m³ per day and be responsible for collection and disposal of sewage.

6.8.9 Notices should be posted at conspicuous locations to remind the workers not to discharge any sewage or wastewater off-site during construction. Regular environmental site inspection should be conducted by the contractor and environmental team to identify and rectify any malpractices and achieve continual improvement of environmental performance on site. It is anticipated that sewage generation during the construction phase of the Project would not cause water quality impact after undertaking all required measures.

Groundwater and Site Runoff

6.8.10 To avoid the water quality impact due to the pumping and discharge of potentially contaminated water from contaminated area, the following mitigation measures should be adopted.

· Contaminated water, either from groundwater or runoff, should be treated by wastewater treatment facility (WTF) to an acceptable level as indicated in TM-DSS before disposal if the deployment of such WTF is feasible. The need and detail requirement of the WTF cannot be determined at this stage as the presence and type of contaminated groundwater will only be made known after future land contamination site investigation work; and

· Recharging the contaminated groundwater back to the aquifer should be sought if treatment of the contaminated groundwater by WTF is not feasible, subject to the agreement with the EPD.

Construction Works near/ within Watercourses

6.8.11 Besides the general site best management practices, extra care shall be paid for works near watercourses identified in Table 6.2, including ecological enhancement of a section of S01 near Hong Po Road Site, to minimize the potential water quality impacts. The measures described in ETWB TC (Works) No. 5/2005 should be adopted where applicable. The key measures are:

· Temporary shoring or sand bags or water pumping should be installed as appropriate to isolate the flow of the watercourses from the construction works area. The detailed design of the temporary diversion works will be conducted by the contractor during the construction phase to fulfil the requirements in DSD Technical Circular No. 1/2017 “Temporary Flow Diversions and Temporary Works Affecting Capacity in Stormwater Drainage System” for DSD approval so that feasible options of these temporary structures can be formulated.

· Stockpiling of construction materials and dusty materials should be located away from any watercourses, contained in bunded areas and covered with tarpaulin.

· Construction debris and spoil should be covered with tarpaulin during storage. Regular clearance of materials for disposal off-site should be arranged to avoid overwhelming and being washed into the nearby watercourses during rainfalls.

· Water pumps should be used to collect wastewater and construction site surface runoff within the temporary working platform. The collected wastewater shall be properly treated for silt removal prior to discharge.

· Toe-boards and bunds should be provided along the edge of the works area/ temporary platform to prevent wastewater/ debris from falling into the watercourses.

· Any temporary works site inside the watercourses should be temporarily isolated by placing of sandbags or silt curtains with lead edge at bottom and properly supported props to prevent adverse water quality impact.

· Proper shoring may need to be erected to prevent soil/ mud from slipping into the watercourses.

Removal/ Diversion of Watercourses

6.8.12 During removal and diversion of watercourses within the Project Site (i.e. part of S01, part of S02, S03, part of S06 and part of S08), precautionary measures shall be implemented to prevent adverse water quality impact to the surrounding environment. Good site practices as described in ETWB TC (Works) No. 5/2005 and ProPECC PN 1/94 should be adopted where applicable. These include:

· Temporary shoring or sand bags should be installed as appropriate to isolate the water flow from construction works area.

· Dewatering or flow diversion shall be conducted prior to the construction works to prevent water overflow to the surrounding area.

· Watercourse removal and flow diversion should be conducted in dry season as far as practicable when the flow is low.

6.8.13 Water drained along the watercourse shall be diverted to new/ temporary drainage prior to watercourse removal. During removal, water remained at the watercourse should be collected and treated to meet the requirements of TM-DSS prior to discharge. A precautionary site check and translocation, if necessary, for aquatic fauna of conservation interest have been recommended prior to site formation at the watercourse.

Operation Phase

Emergency Discharge from Sewage Pumping Station

6.8.14 To minimize the risk of SPS failure leading to emergency discharge of untreated sewage from the proposed SHR SPS to Tuen Mun River, the design of SPS shall comprise additional provisions, including:

· Twin rising mains in case of one of the duty mains be taken out of operation, the remaining one would still be able to deliver flow;

· Standby pumps and screens in case of unexpected breakdown or maintenance of the pumps and screens such that the standby screens and pumps could take over and allow continuous operation of the SPS;

· Dual electricity supply from different power sub-stations or backup power supply facilities such as diesel generator in case of power failure to sustain the function of pumping facilities;

· A storage tank with 2-hour ADWF capacity (about 1,200m³) to hold temporary discharge for a duration required to restore normal operation of the SPS to cater for breakdown and maintenance of the SPS (actual size to be confirmed in the detailed design stage). The proposed SHR SPS is located in urban area, maintenance team can arrive within short time to investigate the problem and resume the SPS operation as soon as possible. Detail arrangement will be formulated in later design stage;

· Supervisory Control and Data Acquisition (SCADA) system and closed-circuit television (CCTV) will be provided for active monitoring in order to transmit signals showing irregularity or any operational problem of the SPS to the nearby STW or other manned SPS such that immediate actions can be taken in case of emergency; and

· A hand-cleaned bar screen to cover the lower half of the opening of any overflow bypass to prevent the discharge of floating solids to the receiving water bodies.  The clear spacing of the bar screen should normally be 25 mm.

6.8.15 In addition, regular maintenance and plant inspection will be conducted throughout the operation of the SPS to prevent equipment failure. Regular inspection, at least once per week, should cover the essential equipment in the pumping station. With the above measures implemented, abnormality of the equipment should be identified for remedial action to be taken before failure.

6.8.16 In addition, a contingency plan to deal with the unlikely event of emergency discharge during the operation of the SPS should be developed in the detailed design stage and incorporated in the Project contract document. The contingency plan should be prepared by referring to (i) “A Guide on Reporting Sewage Bypass Incidents in Sewage Pumping Stations and Sewers” by the EPD of March 2015 and (ii) "Contingency Plan for Incidents Possibly Encountered in Sewage Treatment Facilities having a Potential of Generating an Environmental Nuisance" by the DSD. The contingency plan should include the following items:

· Locations of waterbodies or WSRs in the vicinity of the emergency discharge;

· A list of relevant government departments (including name, address, email address, phone and fax number of the key persons) to be informed and their respective follow up action in the event of emergency discharge, including key contact persons and telephone numbers;

· A framework of emergency response and reporting procedures required in the event of emergency discharges; and

· Procedures listing the most effective means in rectifying the breakdown of the SPS in order to minimise the discharge duration.

Change in Drainage System and Runoff from Road Surface and PTIs

6.8.17 Following the commissioning of the new roads and PTIs, vehicle dust, tyre scraps and oils might be washed away from the road surface/ open areas to the nearby watercourses by surface runoff or road surface cleaning. A surface water drainage system should be installed to collect and convey the road surface runoff to the existing/ planned drainage system downstream. The road and PTI drainage should be equipped with properly designed silt trap. The design of mitigation measures for the road works during operation phase shall take account of the guidelines as presented in ProPECC PN 5/93 “Drainage Plans subject to Comment by the EPD”.

6.8.18 Best Management Practices (BMPs) should be implemented to control erosion and run-off quantity.

6.8.19 Exposed surface should be avoided within the roads to minimise soil erosion. Thus, all roads shall be hard paved.

6.8.20 The drainage system should be designed to avoid flooding with devices and facilities to control sedimentation, runoff quality, prevent ‘first flush’ pollution, and eliminate pollutant discharge into poor flushing water downstream.

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

6.8.22 Road gullies with standard design, silt traps properly designed and spaced, and oil interceptors should be deployed to remove particles, debris, refuse, fallen leaves, and grease present in stormwater run-off, where appropriate.

6.8.23 Good management measures such as regular cleaning and sweeping of road surface/ open areas should be implemented. The road surface/ open area cleansing should also be carried out prior to rainstorm event.

6.8.24 Manholes, as well as stormwater gullies, ditches provided at the Site should be regularly inspected and cleaned. Additional inspection and cleansing should be carried out before forthcoming heavy rainfall.

6.8.25 By adopting flexible but appropriate management measures for different site conditions, there would be no unacceptable water quality impact of non-point pollution sources upon the receiving water bodies.

Runoff from Landscape Areas

6.8.26 Agrochemicals to be adopted are regulated under the Pesticides Ordinance (Cap.133). Only registered agrochemicals under the Pesticides Ordinance shall be used. Bio-pesticides and pesticides with shorter half-life (i.e. non-persistence in nature) is recommended. The amount of agrochemicals to be applied and application frequency should follow the manufacturer’s instructions. In addition, the application of agrochemicals before heavy rainstorm should be avoided.

Wastewater from Municipal and Commercial Activities

6.8.27 For individual municipal facilities and commercial tenants, effluent discharge license under the WPCO will be required individually for wastewater discharge. The discharge standards specified under the TM-DSS should be observed. Depending on the effluent characteristics, pre-treatment may be required to comply with the standards for discharging wastewater into public sewerage.

6.9 Residual Impacts

6.9.1 With proper implementation of the recommended mitigation measures for drainage and sewerage systems, the Project would not pose residual water quality impacts to the identified WSRs and local environment.

6.10 Conclusions and Recommendations

6.10.1 During the construction phase, with proper implementation of all the recommended mitigation measures, there would be no unacceptable water quality impact from surface runoff of the construction site and sewage generated from construction workforce.

6.10.2 During the operation phase, sewage discharged from the proposed development will be collected by the proposed sewage pipes leading to the SHR SPS and downstream sewerage network and PPSTW for treatment. No adverse water quality impact is expected.

6.10.3 Contingency measures for the proposed SHR SPS such as provision of storage tank, twin rising mains, standby pump, and dual electricity supply or backup power supply facilities should be adopted to prevent emergency situation. For other potential water pollution sources arising from the Project such as runoff from roads, PTIs, paved areas and landscape areas and wastewater from municipal and commercial activities, no unacceptable water quality impact is anticipated provided the recommended mitigation measures are properly implemented.

6.10.4 There would be no residual water quality impact during both construction and operation phases of the Project.

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