5. water quality impact

5.1 Introduction

5.1.1 This Chapter presents the assessment of the potential impacts of the Project on the aquatic environment. The following sections are included:

· definition of applicable water-related legislation;

· description of the existing water environment along TWR;

· identification and preliminary assessment of potential water quality impacts during construction and operation of the Project;

· recommendations of potential mitigation measures for the amelioration of adverse impacts on the aquatic environment; and

· preliminary identification of monitoring requirements.

5.2 Environmental Legislation, Policies, Plans and Standards

5.2.1 The criteria for evaluating water quality impact are provided in Annexes 6 and 14 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM).

5.2.2 Under the Water Pollution Control Ordinance (Cap. 358) (WPCO), Hong Kong waters are divided into different Water Control Zones (WCZs). Each WCZ has a designated set of statutory Water Quality Objectives (WQOs).

5.2.3 The majority of TWR lies within Victory Harbour (Phase I) WCZ and Western Buffer WCZ, which were declared on 1 November 1994 and 1 June 1993 respectively. WQOs for the Victory Harbour (Phase I) and Western Buffer WCZs are listed in Tables 5-1 and 5-2, respectively. Locations of TWR and the two WCZs are shown in Figure 5-1.

5.2.4 Any discharge to WCZs is required to comply with the standards specified in the Technical Memorandum on Standards for Effluents into Drainage and Sewerage Systems, Inland and Coastal Waters (Cap. 358, S.21) (referred to hereafter as the TM on Effluent Standards). Inland waters are divided into four different groups based on their beneficial uses. It is likely that the inland waters of the Study Area is defined as Group D, which serves general amenity and secondary contact recreation as beneficial uses. Depending on the classification and the selected effluent disposal methods, any discharge from the site to receiving waters will be required to meet the standards given in Tables 5-3 and 5-4.

Table 5‑1 Water Quality Objectives for Victory Harbour (Phase I) Water Control Zone

	Water Quality Objectives	Part of Parts of Zone
A.	AESTHETIC APPEARANCE
	a) There should be no objectionable odours or discolouration of the water.	Whole zone
	b) Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent.	Whole zone
	c) Mineral oil should not be visible on the surface. Surfactants should not give rise to a lasting foam.	Whole zone
	d) There should be no recognisable sewage-derived debris.	Whole zone
	e) Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent.	Whole zone
	f) The water should not contain substances which settle to form objectionable deposits.	Whole zone
B.	BACTERIA
	The level of Escherichia coli should not exceed 1000 per 100 mL, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days.	Inland waters
C.	COLOUR
	Human activity should not cause the colour of water to exceed 50 Hazen units.	Inland waters
D.	DISSOLVED OXYGEN
	a) The level of dissolved oxygen should not fall below 4 mg per litre for 90% of the sampling occasions during the whole year; values should be calculated as the annual water column average (see Note). 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) The level of dissolved oxygen should not be less than 4 mg per litre.	Inland waters
E.	pH
	a) The pH of the water should be within the range of 6.5-8.5 units. In addition, human activity should not cause the natural pH range to be extended by more than 0.2 unit.	Marine waters
	b) Human activity should not cause the pH of the water to exceed the range of 6.0-9.0 units.	Inland waters
F.	TEMPERATURE
	Human activity should not cause the daily temperature range to change by more than 2.0 degrees Celsius.	Whole zone
G.	SALINITY
	Human activity should not cause the salinity level to change by more than 10%.	Whole zone
H.	SUSPENDED SOLIDS
	a) Human activity should neither cause the suspended solids concentration to be raised more than 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities.	Marine waters
	b) Human activity should not cause the annual median of suspended solids to exceed 25 mg per litre.	Inland waters
I.	AMMONIA
	The un-ionized ammoniacal nitrogen level should not be more than 0.021 mg per litre, calculated as the annual average (arithmetic mean).	Whole zone
J.	NUTRIENTS
	a) Nutrients should 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 0.4 mg per litre, expressed as annual water column average (see Note).	Marine waters
K.	5-DAY BIOCHEMICAL OXYGEN DEMAND
	The 5-day biochemical oxygen demand should not exceed 5 mg per litre.	Inland waters
L.	CHEMICAL OXYGEN DEMAND
	The chemical oxygen demand should not exceed 30 mg per litre	Inland waters
M	TOXIC SUBSTANCES
	a) Toxic substances in the water should not 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 interactions of toxic substances with each other.	Whole zone
	b) Human activity should not cause a risk to any beneficial use of the aquatic environment.	Whole zone

Note: Expressed normally as the arithmetic mean of at least 3 measurements at 1 m below surface, mid depth and 1m above the seabed. However in water of a depth of 5m or less the mean shall be that of 2 measurements (1m below surface and 1m above seabed), and in water of less than 3 m the 1 m below surface sample only shall apply.

Table 5‑2 Summary of Water Quality Objectives for Western Buffer Water Control Zone

	Water Quality Objectives	Part of Parts of Zone
A.	AESTHETIC APPEARANCE
	a) There should be no objectionable odours or discolouration of the water	Whole zone
	b) Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent.	Whole zone
	c) Mineral oil should not be visible on the surface. Surfactants should not give rise to a lasting foam.	Whole zone
	d) There should be no recognisable sewage-derived debris.	Whole zone
	e) Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent.	Whole zone
	f) The water should not contain substances which settle to form objectionable deposits.	Whole zone
B.	BACTERIA
	a) The level of Escherichia coli should not exceed 610 per 100 mL, calculated as the geometric mean of all samples collected in a calendar year.	Secondary Contact Bathing Beach Subzones and Fish Culture Subzones
	b) The level of Escherichia coli should not exceed 180 per 100 mL, calculated as the geometric mean of all samples collected from March to October inclusive in 1 calendar year. Samples should be taken at least 3 times in 1 calendar month at intervals of between 3 and 14 days.	Bathing Beach Subzones
	c) The level of Escherichia coli should be less than 1 per 100 mL, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days.	Water Gathering Ground Subzones
	d) The level of Escherichia coli should not exceed 1000 per 100 mL, calculated as the geometric mean of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days.	Other inland waters
C.	COLOUR
	a) Human activity should not cause the colour of water to exceed 30 Hazen units.	Water Gathering Ground Subzones
	b) Human activity should not cause the colour of water to exceed 50 Hazen units.	Other inland waters
D.	DISSOLVED OXYGEN
	a) The level of dissolved oxygen should not 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 excepting Fish Culture Subzones
	b) The level of dissolved oxygen should not be less than 5 mg per litre for 90% of the sampling occasions during the years; 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.	Fish Culture Subzones
	c) The level of dissolved oxygen should not be less than 4 mg per litre.	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, human activity should not cause the natural pH range to be extended by more than 0.2 unit.	Marine waters
	b) Human activity should not cause the pH of the water to exceed the range of 6.5-8.5 units.	Water Gathering Ground Subzones
	c) Human activity should not cause the pH of the water to exceed the range of 6.0-9.0 units.	Other inland waters
F.	TEMPERATURE
	Human activity should not cause the natural daily temperature range to change by more than 2.0 degrees Celsius.	Whole zone
G.	SALINITY
	Human activity should not cause the natural ambient salinity level to change by more than 10%.	Whole zone
H.	SUSPENDED SOLIDS
	a) Human activity should 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) Human activity should not cause the annual median of suspended solids to exceed 20 mg per litre.	Water Gathering Ground Subzones
	c) Human activity should not cause the annual median of suspended solids to exceed 25 mg per litre.	Other inland waters
I.	AMMONIA
	The un-ionized ammoniacal nitrogen level should not be more than 0.021 mg per litre, calculated as the annual average (arithmetic mean).	Whole zone
J.	NUTRIENTS
	a) Nutrients should 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 0.4 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
K.	5-DAY BIOCHEMICAL OXYGEN DEMAND
	a) The 5-day biochemical oxygen demand should not exceed 3 mg per litre.	Water Gathering Ground Subzones
	b) The 5-day biochemical oxygen demand should not exceed 5 mg per litre.	Other inland waters
L.	CHEMICAL OXYGEN DEMAND
	a) The chemical oxygen demand should not exceed 15 mg per litre.	Water Gathering Ground Subzones
	b) The chemical oxygen demand should not exceed 30 mg per litre.	Other inland waters
M	TOXIC SUBSTANCES
	a) Toxic substances in the water should not 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 interactions of toxic substances with each other.	Whole zone
	b) Human activity should not cause a risk to any beneficial use of the aquatic environment.	Whole zone
N.	TURBIDITY
	Waste discharges should not reduce light transmission substantially from the normal level.	Bathing Beach Subzones

Table 5‑3 Standards for Effluents Discharged into the Marine Waters of Victoria Harbour Water Control Zone (All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

Flow rate (m³/day)	≦10	＞10 and ≦200	＞200 and ≦400	＞400 and ≦600	＞600 and ≦800	＞800 and ≦1,000	＞1,000 and ≦1,500	＞1,500 and ≦2,000	＞2,000 and ≦3,000	＞3,000 and ≦4,000	＞4,000 and ≦5,000	＞5,000 and ≦6,000
Determinand	≦10	＞10 and ≦200	＞200 and ≦400	＞400 and ≦600	＞600 and ≦800	＞800 and ≦1,000	＞1,000 and ≦1,500	＞1,500 and ≦2,000	＞2,000 and ≦3,000	＞3,000 and ≦4,000	＞4,000 and ≦5,000	＞5,000 and ≦6,000
pH (pH units)	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10
Temperature (^oC)	45	45	45	45	45	45	45	45	45	45	45	45
Colour (lovibond units) (25mm cell length)	4	1	1	1	1	1	1	1	1	1	1	1
Suspended Solids	700	600	600	500	375	300	200	150	100	75	60	40
BOD	700	600	600	500	375	300	200	150	100	75	60	40
COD	1500	1200	1200	1000	700	600	400	300	200	100	100	85
Oil & Grease	50	50	50	30	25	20	20	20	20	20	20	20
Iron	20	15	13	10	7.5	6	4	3	2	1.5	1.2	1
Boron	6	5	4	3.5	2.5	2	1.5	1	0.7	0.5	0.4	0.3
Barium	6	5	4	3.5	2.5	2	1.5	1	0.7	0.5	0.4	0.3
Mercury	0.1	0.1	0.05	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Cadmium	0.1	0.1	0.05	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Other toxic Metals Individually	2	1.5	1	0.8	0.6	0.5	0.32	0.24	0.16	0.12	0.1	0.1
Total toxic Metals	4	3	2	1.6	1.2	1	0.64	0.48	0.32	0.24	0.2	0.14
Cyanide	1	0.5	0.5	0.5	0.4	0.3	0.2	0.1	0.1	0.08	0.06	0.04
Phenols	0.5	0.5	0.5	0.3	0.3	0.2	0.1	0.1	0.1	0.1	0.1	0.1
Sulphide	5	5	5	5	5	5	2.5	2.5	1.5	1	1	0.5
Total Residual Chlorine	1	1	1	1	1	1	1	1	1	1	1	1
Total Nitrogen	100	100	100	100	100	100	100	100	100	100	100	50
Total Phosphorus	10	10	10	10	10	10	10	10	10	10	10	5
Surfactants (Total)	30	20	20	20	15	15	15	15	15	15	15	15
E. coli (Count/100ml)	5,000	5,000	5,000	5,000	5,000	5,000	5,000	5,000	5,000	5,000	5,000	5,000

Table 5‑4 Standards for Effluents Discharged into the Marine Waters of Western Buffer Water Control Zone (All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

Flow rate (m³/day)	≦10	＞10 and ≦200	＞200 and ≦400	＞400 and ≦600	＞600 and ≦800	＞800 and ≦1,000	＞1,000 and ≦1,500	＞1,500 and ≦2,000	＞2,000 and ≦3,000	＞3,000 and ≦4,000	＞4,000 and ≦5,000	＞5,000 and ≦6,000
Determinand	≦10	＞10 and ≦200	＞200 and ≦400	＞400 and ≦600	＞600 and ≦800	＞800 and ≦1,000	＞1,000 and ≦1,500	＞1,500 and ≦2,000	＞2,000 and ≦3,000	＞3,000 and ≦4,000	＞4,000 and ≦5,000	＞5,000 and ≦6,000
pH (pH units)	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10
Temperature (^oC)	45	45	45	45	45	45	45	45	45	45	45	45
Colour (lovibond units) (25mm cell length)	4	1	1	1	1	1	1	1	1	1	1	1
Suspended Solids	500	500	500	300	200	200	100	100	50	50	40	30
BOD	500	500	500	300	200	200	100	100	50	50	40	30
COD	1,000	1,000	1,000	700	500	400	300	200	150	100	80	80
Oil & Grease	50	50	50	30	25	20	20	20	20	20	20	20
Iron	20	15	13	10	7	6	4	3	2	1.5	1.2	1
Boron	6	5	4	3.5	2.5	2	1.5	1	0.7	0.5	0.4	0.3
Barium	6	5	4	3.5	2.5	2	1.5	1	0.7	0.5	0.4	0.3
Mercury	0.1	0.1	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Cadmium	0.1	0.1	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Other Toxic Metals Individually	2	1.5	1.2	0.8	0.6	0.5	0.32	0.24	0.16	0.12	0.1	0.1
Total Toxic Metals	4	3	2.4	1.6	1.2	1	0.64	0.48	0.32	0.24	0.2	0.14
Cyanide	1	0.5	0.5	0.5	0.4	0.3	0.2	0.15	0.1	0.08	0.06	0.04
Phenols	0.5	0.5	0.5	0.3	0.25	0.2	0.13	0.1	0.1	0.1	0.1	0.1
Sulphide	5	5	5	5	5	5	2.5	2.5	1.5	1	1	0.5
Total Residual Chlorine	1	1	1	1	1	1	1	1	1	1	1	1
Total Nitrogen	100	100	80	80	80	80	50	50	50	50	50	50
Total Phosphorus	10	10	8	8	8	8	5	5	5	5	5	5
Surfactants (Total)	30	20	20	20	15	15	15	15	15	15	15	15
E. Coli (count/100ml)	4,000	4,000	4,000	4,000	4,000	4,000	4,000	4,000	4,000	4,000	4,000	4,000

5.2.5 Sam Dip Tam Stream and its associated catchments at the western end of TWR lie within Victoria Harbour (Phase I) WCZ. Therefore, discharges generated in this part of the highway must comply with the standards specified for the Victoria Harbour WCZ as detailed in the TM on Effluent Standards.

5.3 Sensitive Receivers

5.3.1 The Study Area includes the following water bodies that could potentially be affected by the Project:

· isolated ponds;

· streams and rivers; and

· channels.

Isolated Ponds

5.3.2 There are only a few isolated ponds found in Clague Garden Estate and Tsuen Wan Park. They are not treated at sensitive receivers since water from the Project should not be discharged into these water bodies.

Rivers, Streams and Channels

5.3.3 There are 3 rivers/streams in the vicinity of the Project Area, namely Sam Dip Tam Stream, Kau Wa Keng Stream (both discharging into Victoria Harbour (Phase I) WCZ) and Pai Min Kok (Anglers’) Stream (discharging into Western Buffer WCZ). Since Pai Min Kok Stream and Kau Wa Keng Stream are too far away from the Project Area (more than 5km and 1km, respectively), they are not considered as sensitive receivers.

5.3.4 Since Sam Dip Tam lies in close proximity to the Project Area, it is considered as a sensitive receiver. Sam Dip Tam Stream drains into an underground box culvert.

5.3.5 The surface water collected by the above rivers/streams is directed into Rambler Channel.

5.4 Description of the Environment

5.4.1 The following sections consider the existing stream and marine water quality conditions in the main water sensitive receivers as identified above.

Sam Dip Tam Stream System

5.4.2 The Sam Dip Tam Stream system has an approximate catchment area of 4.5km², the 12km long Sam Dip Tam Stream runs through Tsuen Wan and drains into Rambler Channel through an underground box culvert. Water quality is routinely monitored by EPD at three locations. Water quality data at the three monitoring stations, namely TW1, TW2 and TW3, along Sam Dip Tam Stream are presented in Table 5-5. Locations of the three monitoring stations are shown in Figure 5-1. Table 5-5 illustrates that water quality in Sam Dip Tam Stream is classified as “Excellent”.

Table 5‑5 Water Quality at EPD Monitoring Stations in Sam Dip Tam Stream (mg/L unless Indicated) (EPD, ~~2005~~2006)

Parameter	TW1	TW2	TW3
Dissolved Oxygen	~~7.6~~7.6	~~8.2~~8.4	~~8.2~~8.2
pH	~~7.5~~7.6	~~7.9~~7.9	~~7.6~~7.5
Suspended Solids	23	22	22
BOD₅	11	22	21
COD	84	76	86
Oil & Grease	~~0.5~~0.5	~~0.5~~0.5	~~0.5~~0.5
Faecal Coliforms (cfu/100 mL)	~~250,000~~73,000	~~120,000~~110,000	~~53,000~~30,000
E. Coli (cfu/100 mL)	~~55,000~~10,000	~~37,000~~31,000	~~14,000~~9,000
Ammonia-Nitrogen	~~0.04~~0.05	~~0.13~~0.29	~~0.12~~0.11
Nitrate-Nitrogen	~~0.51~~0.78	~~1.60~~1.20	~~1.90~~1.55
Total Kjeldahl N	~~0.21~~0.24	~~0.30~~0.69	~~0.28~~0.31
Ortho-Phosphate	~~0.05~~0.03	~~0.14~~0.16	~~0.15~~0.13
Total Phosphorus	~~0.07~~0.06	~~0.15~~0.19	~~0.16~~0.14
Sulphide	~~0.02~~0.02	~~0.02~~0.02	~~0.02~~0.02
Aluminium (µg/L)	5050	5050	5050
Cadmium (µg/L)	~~0.1~~0.1	~~0.1~~0.1	~~0.1~~0.1
Chromium (µg/L)	11	11	11
Copper (µg/L)	22	22	22
Lead (µg/L)	21	11	11
Zinc (µg/L)	4020	3020	2010

Note: Data presented is the annual average of monthly samples.

Ponds and Groundwater

5.4.3 There are no water quality data for either the ponds identified in the Study Area or the areas of groundwater resources.

Marine Water

5.4.4 EPD carries out routine water quality monitoring of marine water and the report is published annually. Routine monitoring stations relevant to the Project are VM12 and VM14 which locations are indicated in Figure 5-1. Water quality results of these stations are presented in Table 5-6.

Table 5‑6 Marine Water Quality at EPD Monitoring Stations in Vicinity of this Project (mg/L unless indicated) (EPD, ~~2005~~2006)

Parameter	VM12	VM14
Temperature (^oC)	~~23.1~~23.8	~~23.4~~24.2
Salinity	~~31.0~~30.8	~~29.6~~28.7
Dissolved Oxygen (Surface)	~~5.4~~5.5	~~5.7~~6.0
Dissolved Oxygen (Bottom)	~~5.3~~5.3	~~5.6~~5.9
Dissolved Oxygen, % Saturation (Surface)	7577	8084
Dissolved Oxygen, % Saturation (Bottom)	7475	7983
pH	~~8.0~~8.0	~~8.1~~8.0
Secchi Disc Depth (m)	~~1.7~~1.5	~~1.8~~1.5
Turbidity (NTU)	~~14.4~~15.6	~~11.3~~12.6
Suspended Solids	~~7.2~~11.0	~~4.7~~5.9
BOD₅	~~0.7~~0.7	~~0.8~~0.7
Ammonia Nitrogen	~~0.20~~0.18	~~0.17~~0.16
Unionised Ammonia	~~0.008~~0.006	~~0.008~~0.007
Nitrite Nitrogen	~~0.04~~0.036	~~0.05~~0.053
Nitrate Nitrogen	~~0.20~~0.17	~~0.27~~0.27
Total Inorganic Nitrogen	~~0.44~~0.39	~~0.49~~0.48
Total Kjeldahl Nitrogen	~~0.40~~0.36	~~0.34~~0.32
Total Nitrogen	~~0.63~~0.57	~~0.66~~0.64
Orthophosphate Phosphorus	~~0.03~~0.03	~~0.03~~0.03
Total Phosphorus	~~0.06~~0.05	~~0.05~~0.04
Silica (as SiO₂)	~~1.1~~1.2	~~1.3~~1.6
Chlorohpyll (μg/L)	~~1.8~~2.1	~~2.8~~3.4
E. Coli (cfu/100mL)	~~4,000~~3,400	~~2,100~~1,300
Faecal Coliforms (cfu/100mL)	~~9,700~~8,100	~~4,700~~2,800

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), and Bottom (B).

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

5.5 Identification and Evaluation of Water Quality Impact

Construction Phase

Sources of Impact

5.5.1 During the implementation of the Project, there are a number of activities which have the potential to impact the water environment. These activities are highlighted below:

· spillages of oil/fuel, construction chemicals etc.;

· generation of silt-laden surface run-off from vegetation stripping and reworking of embankments, dust suppression activities, wheel washing facilities, spoil importation, soil/material storage/stockpiling areas; and

· discharge of sewage and wastewater generated by construction workers.

5.5.2 It is considered that the principal concern will be related to the discharge of surface run-off heavily laden with suspended solids from the work sites and stockpiling areas into the drainage system during the raining periods. Potential impacts will include increased sediment accumulation, turbidity, discoloration, BOD and nutrient enrichment. However, as discussed in Chapter 6 - Waste Management Implications, there will be an incentive to the Contractor to reduce the stockpiling areas as far as practicable to minimize “double handling” of materials. The potential water quality impacts due to construction activities are assessed further below.

Chemical Spillage

5.5.3 During road widening, fuel of construction plants may need to be stored on site. Spillage of fuel and oil has the potential to migrate towards the aquatic environment and impacts the aquatic life therein.

Construction Site Run-off

5.5.4 During construction phase, water quality in the vicinity of the Study Area may be affected by the generation of wastewater and construction site run-off. The major sources for the generation of silt-laden run-off during raining events include storage of imported fill, works during vegetation stripping and reworking of road embankments. Such run-off, if discharged directly into the aquatic environment, has the potential to elevate the suspended solids loading in the watercourse and impact any life forms therein. It is noted that no works will be carried out in the identified water sensitive systems.

5.5.5 In order to mitigate impacts related to silt-laden run-off, mitigation measures are required to prevent the generation of run-off, as well as to minimize the potential of such effluents reaching the aquatic environment. Feasible mitigation techniques have been considered in accordance with the Practice Note for Professional Persons on Construction Site Drainage (ProPECC PN 1/94), which provides good practice guidelines for dealing with various discharges from construction sites and should be followed as far as possible during any construction activities in order to minimize water quality impact.

Operational Phase

5.5.6 Water quality impact during the operation of the Project may arise from the following:

· discharges of road surface run-off containing sediment and chemical contaminants into the water system via the road drainage system; and

· spillages of chemicals onto roads and into the water system.

Road Run-off

5.5.7 Currently road run-off is already occurring during raining periods, which leads to discharges to the existing waterway. As such, during operation of the upgraded TWR, there will be no new contamination sources. However, the road-widening scheme will increase the surface area of the road surface and thus increase the volume of road run-off and the total pollutant loading.

5.5.8 Material will accumulate on the widened road during dry periods, both from surface run-off from adjacent areas and from dust generated by vehicles using the road. Most accumulation is expected in slightly depressed and at grade sections where sediment and silt will be carried and deposited. Material deposited on the road will contain a whole array of organic and inorganic chemicals.

5.5.9 Material deposited on the road surface will be removed from the carriageway during raining events. A large proportion of the rainfall landing on road hard surfaces may reach the surface water drainage system. Climatic conditions and intensity of precipitation have a particular significant influence on the characteristics of run‑off. Any further rainfall run‑off is likely to be relatively less polluted. The initial run‑off which containing most of the particulate matter and the associated contaminants is referred to as the "first flush" and can impact the sensitive receiving watercourses. This is especially the case following high intensity storms which tend to scour the road surface and can result in a relatively great run‑off pollutant loading. Pollutant levels tend to be dependent on the local conditions, topography, climate and the degree of urbanization.

5.5.10 A wide range of concentrations has been reported for contaminants in road run-off. Contaminants of greatest concern with respect to water quality are particulates and heavy metals (such as iron, lead and zinc). Particulates may settle out rapidly in any receiving water system and can cause smothering of the bed, while some heavy metals are toxic to some sensitive life forms.

5.6 Identification and Evaluation of Cumulative Water Quality Impact

5.6.1 As illustrated in Chapter 42, referring to the construction programme of the Project (tentatively from June 2011 to May 2015) and other construction projects within the Study Area, it is noted that the construction period of other projects listed below might overlap with the Project:

· Reconstruction and Improvement of Tuen Mun Road (Anticipated Completion Year: 2011~~Anticipated Completion Year: 2011~~): Construction of noise barriers at sections of Tsuen Wan (close to the Panorama, Belvedere Garden, Greenview Court and Yau Kom Tau Village) and Sam Shing Hui;

· Property Development at Tsuen Wan West Station TW5 (Anticipated Completion Year: 2015 – 2016~~Anticipated Completion Year: 2015 – 2016~~): Construction of residential blocks and relevant facilities;

· Property Development at Tsuen Wan West Station TW6 (Anticipated Completion Year: 2011 – 2012~~Anticipated Completion Year: 2011 – 2012~~): Construction of residential blocks and relevant facilities; and

· Property Development at Tsuen Wan West Station TW7 (Anticipated Completion Year: 2012 – 2014~~Anticipated Completion Year: 2012 – 2014~~): Construction of residential blocks and relevant facilities.

5.6.2 Locations of the abovementioned concurrent projects are shown in Figure 4-3.

5.6.3 These projects would be confined and involve typical construction activities. It is anticipated that with the adequate implementation of mitigation measures to control construction site run-off and drainage, potential impacts on water quality would be well controlled within the site boundary.

5.6.4 Therefore, the cumulative impact at the water sensitive receiver is expected to be limited during both construction and operation phases.

5.7 Mitigation Measures for Water Quality Impact

Construction Phase

Control and Mitigation of Silt-laden Run-off

5.7.1 Silt-laden surface run-off should be prevented from directly entering the sensitive receivers during the construction works. The mitigation measures described below for the construction phase are in accordance with ProPECC PN 1/94:

a) works sites and areas used for imported fill stockpiling should, as far as possible, avoid the water sensitive receivers;

b) stripping of existing vegetation should be sequential to avoid exposure of large areas of embankment slopes;

c) special precautions should be taken when working in the vicinity of streams and channels, especially when bridges along TWR are being widened. This may involve the installation of temporary drainage works to ensure that run-off does not enter the water bodies directly; typical example of this type of measure is the provision of suitable temporary drainage system, such as peripheral channels around the site, to intercept all on-site runoff to water quality treatment devices such as sedimentation pond / sand trap. Only treated run-off from these devices will be discharged offsite. Sizes and arrangement details of these drainage works depend on local conditions and will be addressed during the detailed design stage;

d) perimeter cut-off drains to direct off-site water around the works sites should be constructed. Internal drainage works, erosion and sedimentation control facilities should be implemented. Channels, earth bunds or sandbag barriers should be provided on site to direct stormwater to silt removal facilities. The design of efficient silt removal facilities should be based on the guidelines provided in ProPECC PN 1/94;

e) sedimentation tanks of sufficient capacity, constructed from pre-formed individual cells of approximately 6-8 m³ capacity should be adopted as a general mitigation measure which can be used for settling wastewaters prior to disposal. The tanks are readily available and used primarily for recycling water for bored piling operations. The system capacity should be flexible and be able to handle multiple inputs from a variety of sources and particularly suited to applications where the influent is pumped. Various physical enhancement and chemical additives can be added to refine the sedimentation process;

f) construction works should be programmed to minimise surface excavations / cutting during the rainy period (April to September). If excavation of soil cannot be avoided during the rainy period, or at any time of year when rainstorms are likely, exposed slope surfaces should be covered by a tarpaulin or other means. Other measures that need to be implemented before, during and after rainstorms are summarized in ProPECC PN 1/94. Particular attention should be paid to the control of silty surface run-off during storms events, especially for sites located near steep slopes;

g) all exposed earth areas should be completed and re-vegetated promptly after earthworks have been completed, or alternately, within 14 days of the cessation of earthworks.

h) earthworks final surfaces should be well compacted and subsequent permanent work or surface protection should be carried out immediately after final surfaces are formed in order to prevent rainstorm erosion;

i) the overall slope of the site should be kept to a minimum to reduce the erosive potential of surface water flows and all trafficked areas and access roads 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;

j) silt contained water and drilling water collected from any boring operations, dewatering etc. should be removed with properly designed silt removal facilities, such as sedimentation tanks referred to above, such that the TM on Effluent Standards are achieved prior to the discharge of waters;

k) all drainage facilities, erosion and sediment control structures should be inspected regularly and maintained to ensure proper and efficient operation at all times and particularly following rainstorms. Deposited silt and grit should be removed regularly and disposed of by spreading evenly over stable and non-sensitive vegetated areas;

l) measures should be taken to minimise the ingress of site drainage into excavations. If the excavation of trenches in rainy period (April to September) is necessary, they should be dug and backfilled in short sections. Water pumped out from trenches or foundation excavations should be discharged into the silt removal facilities;

m) all open stockpiles of construction materials (e.g. aggregates, sand and fill material) should be covered with a tarpaulin or similar fabric during rainstorms. Measures should be taken to prevent the washing away of construction materials, soil, silt or debris into any drainage system;

n) manholes (including newly constructed ones) should always be covered and temporarily sealed so as to prevent silt, construction materials or debris being washed into the drainage system;

o) all vehicles and plants should be cleaned before leaving the construction site to ensure no earth, mud and debris is deposited on roads. An adequately designed and automatic wheel washing facilities should be provided at every site exit and wash-water should have sand and silt settled out and removed at least on a regular basis to ensure the continued efficiency of the process.

p) the section of access road leading to, and exiting from the wheel-wash bay to the public road should be paved with sufficient backfill toward the wheel-wash bay to prevent vehicle tracking of soil and silty water to public roads and drains;

q) water used for construction purposes on site should, as far as practical, be recycled for use;

r) information detailing storm run-off and wastewater discharge points, and the corresponding maximum (or range of) volumes of discharges expected from the construction sites on a dry day should be provided in the WPCO licence application. In general, assuming adequate information has been provided together with the license application, EPD would need at least 20 days for the processing of a license for a discharge. It is therefore recommended that the Contractor submit the licence application to EPD as early as possible before the commencement of any discharge.

5.7.2 If the good management practices abovementioned are implemented, adverse impacts on the aquatic environment due to surface run-off should be avoided.

Construction Materials

5.7.3 In order to prevent water quality impact associated with construction material, the following mitigation techniques are recommended:

a) stockpiles of cement and other construction material should be kept covered when not being used;

b) stockpiles of cement and other construction material should not be located adjacent to nullahs and streams;

c) entry points into the surface drainage system should be fitted with oil interceptors;

d) waste oil and other chemical waste as defined in the Waste Disposal (Chemical Waste) (General) Regulation require disposal by an appropriate means and require pre-notification to EPD prior to disposal. An appropriate disposal facility should be the Chemical Waste Treatment Centre (CWTC) at Tsing Yi. If chemical wastes are to be generated, the Contractor will need to register with EPD as a chemical waste producer and observe the requirements for chemical waste storage, labelling, transportation and disposal. The requirements for waste storage, transportation and disposal are considered in Chapter 6 - Waste Management Implications;

e) impact associated with spillages should be managed through careful handling procedures. Oil and fuel should only be used and stored in designated areas with pollution prevention facilities. Fuel tanks and drums of fuel oils and other polluting fluids/chemicals should be provided with locks and bounded to a capacity of 110% of the storage capacity of the largest tank. The bund should be drained of rainwater after raining events.

Sewage from Construction Workers

5.7.4 Sewage effluent arising from the on-site construction workforce has the potential to cause water pollution. Therefore, plans for the collection, treatment and disposal of sewage during construction phase should be specified. Sewage generated on site should be disposed of through connection of the sanitation facilities with the existing foul sewerage system. Where this is not possible, temporary portable chemical toilets, septic tanks or package sewage treatment plants may need to be used. Overall it is considered that no water quality impact is expected to arise from on-site generated sewage if such sewage facilities are provided.

Operational Phase

5.7.5 The amount of sediment accumulating on the road surface during operation is not expected to be large, particularly due to the proposals for landscaping of adjacent embankments, which will minimize soil exposure immediately next to the carriageways. The road drainage system will be incorporated as part of the general road improvement scheme and will facilitate drainage of run-off of floodwater directly into the water system.

5.7.6 If the measures highlighted above are adopted, and if the drainage network is maintained appropriately, the impact on the aquatic environment should be minimal.

5.8 Residual Impacts on Water Quality

5.8.1 With the full implementation of the recommended mitigation measures for the construction and operation phases of the Project, no unacceptable residual impacts on water quality are anticipated. It is recommended that regular audit of the implementation of these measures be carried out during the construction phase.

5.9 Environmental Monitoring and Audit

5.9.1 For protection of the water quality during construction of the road widening works, the following environmental monitoring and management measures should be incorporated into the Environmental Protection and Pollution Control Requirements in the Contractor’s specification, as a minimum:

a) discharges should be monitored to ensure that they comply with the TM on Effluent Standards and any applicable discharge licenses; and

b) effluent containing cement-generated material should be checked periodically with respect to pH.

5.10 Conclusions

5.10.1 Potential water quality impact generated by this Project are mainly through the generation and discharge of silt-laden surface run-off from spoil stockpiling areas and during landscape stripping as well as embankment reworking. Specific mitigation measures have been specified to control such impacts to the identified water sensitive receivers.

5.10.2 Road run-off from the upgraded TWR during operation phase may contain sediment and organic/inorganic pollutants. If the measures highlighted in this Chapter are adopted, and if the drainage network is maintained appropriately, possible impact on the water environment should be minimal.

5.10.3 This water quality impact assessment has identified none of particular insurmountable problems associated with either the road widening construction works or the completed road operation. A number of mitigation measures have been recommended, which are generally related to good construction site management. Given the implementation of these measures, potential impact associated with the construction and operation of the highway is not considered significant.