5 water quality

5.1 Introduction

5.1.1 This section presents an assessment of the potential water quality impacts associated with the construction and operation phases of the proposed traffic improvement works to Tuen Mun Road.  The key environmental issue in respect of water quality impacts associated with the Project would be the proper control of construction site runoff and drainage during site formation and foundation works for the proposed slip road.  Recommendations for mitigation measures have been made, where necessary, to reduce the identified water quality impacts to an acceptable level.  The project area would be located in-land and thus direct impacts on marine water quality would not arise.

5.2 Environmental Legislation and Standards

5.2.1 Legislation and associated guidance relevant to the water quality impacts assessment of the proposed traffic improvement works are described below.

Environmental Impact Assessment Ordinance

5.2.2 This water quality impact assessment will be carried out following the criteria and guidelines as stated in Annexes 6 and 14 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) as required in the EIA Study Brief.

Water Pollution Control Ordinance (WPCO)

5.2.3 The Water Pollution Control Ordinance (Cap. 358), in existence since 1980, is the major legislation relating to the protection and control of water quality in Hong Kong. According to the Ordinance and its subsidiary legislation, Hong Kong waters are divided into ten water control zones (WCZ). The study area for the Project is located within the North Western WCZ.

Technical Memorandum

5.2.4 Besides setting the WQOs, the WPCO controls effluent discharging into the WCZs through a licensing system. A Technical Memorandum (TM) on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters was issued under the WPCO which gives guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, inland waters, marine waters, inshore waters and coastal waters) and the flow rate. The limits control the physical, chemical and microbial quality of effluents. Sewage from the proposed construction activities should comply with the TM standards for effluent discharged into the inshore waters of North Western WCZ (Table 10a of the TM).

Table 5.1 Water Quality Objectives Statement for North Western WCZ

Parameter	Objective	Part(s) of Zone
Aesthetic Appearance	(a) discharge shall not cause objectionable odour or discoluration	Whole Zone
	(b) no tarry residue, floating wood, articles made of grass, plastic, rubber or any other substance	Whole Zone
	(c) Mineral oil not visible on the surface. Surfactants shall not give rise to a lasting foam.	Whole Zone
	(d) no recognizable sewage-derived debris	Whole Zone
	(e) no floating, submerged or semi-submerged subjects likely to interfere with the free movement of, or cause damage to vessels	Whole Zone
	(f) not to contain substances which settle to form objectionable deposits	Whole Zone
E. coli	(a) annual geometric mean not to exceed 610/100 mL	secondary contact recreation subzones
	(b) geometric mean not to exceed 180/100 mL during March to October inclusive in 1 year; sample should be taken at least 3 times in 1 calendar month at intervals of between 3 to 14 days	Bathing beach subzones
	(c) running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days to be less than 1/100 mL	Tuen Mun (A) and Tuen Mun (B) Subzones and Water Gathering Ground Subzones
	(d) running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days not to exceed 1000/100 mL	Tuen Mun (C) and other Inland Waters
Colour	Not to exceed 30 Hazen units	Tuen Mun (A) and Tuen Mun (B) Subzones and Water Gathering Ground Subzones
	Not to exceed 50 Hazen units	Tuen Mun (C) and other Inland Waters
Dissolved Oxygen within 2 m of bottom	not less than 2 mg/L for 90% samples	marine waters
Depth averaged Dissolved Oxygen	not less than 4 mg/L for 90% samples	marine waters
	not less than 4mg/L at any point within the water column	Tuen Mun (A) and Tuen Mun (B) Tuen Mun (C) Subzones, Water Gathering Ground Subzones and other Inland waters
pH value	within the range 6.5 to 8.5; change due to waste discharge not to exceed 0.2	marine waters except bathing beach subzones
	within the range 6.5 - 8.5	Tuen Mun (A) and Tuen Mun (B) Tuen Mun (C) Subzones and water gathering ground subzones
	within the range 6.0 - 9.0	other inland waters
	within the range 6.0 - 9.0 for 95% of samples; change due to waste discharge not to extend by 0.5	Beach Bathing Subzones
Salinity	change due to waste discharge not to exceed 10% of natural ambient level	whole zone
Temperature	change due to waste discharge not to exceed the natural daily temperature by 2^oC	whole zone
Suspended solids	waste discharge not to raise the natural ambient level by 30%, nor cause the accumulation of suspended solids which may adversely affect aquatic communities	marine waters
	annual median not to exceed 20 mg/L	Tuen Mun (A) and Tuen Mun (B) Tuen Mun (C) Subzones and water gathering ground subzones
	annual median not to exceed 25 mg/L	other inland waters
5-Day Biochemical Oxygen Demand	not to exceed 3 mg/L	Tuen Mun (A) and Tuen Mun (B) Tuen Mun (C) Subzones and water gathering ground subzones
	not to exceed 5 mg/L	other inland waters
Chemical Oxygen Demand	not to exceed 15 mg/L	Tuen Mun (A) and Tuen Mun (B) Tuen Mun (C) Subzones and water gathering ground subzones
	not to exceed 30 mg/L	other inland waters
Un-ionized ammonia	annual mean not to exceed 0.021 mg/L	whole zone
Nutrients	not to be present in quantities that cause excessive or nuisance growth of algae or other aquatic plants	marine waters
	annual mean depth average inorganic nitrogen not to exceed 0.3 mg/L	Castle Peak Bay Subzone
	annual mean depth average inorganic nitrogen not to exceed 0.5 mg/L	marine waters except Castle Peak Bay Subzone
Toxins	not to be present at levels producing significant toxic effect, 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
	not to cause a risk to any beneficial use of the aquatic environment	whole zone
Phenol	not to produce a specific odour or concentrations greater than 0.05 mg/L as C6H5 OH	Bathing Beach Subzones
Turbidity	Waste discharges shall not reduce light transmission substantially from the normal level	Bathing Beach Subzones

Practice Notes

5.2.5  A practice note (PN) for professional persons was issued by the EPD to provide environmental guidelines for handling and disposal of construction site discharges.  The ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with various types of discharge from a construction site.  Practices outlined in the PN should be followed as far as possible during construction to minimize the water quality impact due to construction site drainage.

5.3 Description of the Environment

5.3.1 Tuen Mun River is located in the vicinity of the study area and drains to North Western WCZ directly. Six river water quality monitoring stations were set on the river. Locations of these monitoring stations are shown in Figure 5.1. According to EPD’s Annual Report titled “River Water Quality in Hong Kong in 2007”, the Tuen Mun River Channel is reported to have shown an upward water quality trend since the 1980s, including an increase in dissolved oxygen and reductions in organic matters, sewage bacteria, nutrients and metals. The WQO compliance of the Tuen Mun River Channel was reported to be 91% in 2007. One monitoring station namely TN2 achieved grading of ‘excellent’ and four monitoring stations graded as ‘good’. The E.coli levels in the river channel were reported to vary considerably from 3,000 to 130,000 cfu/100mL. The highest E.coli count of 130,000 cfu/100mL was recorded at the upstream monitoring station TN1 (with a grading of ‘bad’) and was reported to be due to pollution from unsewered villages, and the lowest E.coli count of 3,300 cfu/100mL was recorded at the upstream monitoring station TN2 located in another tributary with fewer unsewered premises. E.coli counts of between 4,500 and 9,100 cfu/100mL were recorded at the monitoring stations TN3 to TN6 in the lower part of the river channel.

Table 5.2 Water Quality Index Grading and Compliance with the Water Quality Objectives for Watercourses in 2007

Parameter	Unit	TN1	TN2	TN3	TN4	TN5	TN6
Dissolved oxygen	mg/L	5.2 (3.3 – 6.0)	8.4 (7.5 – 9.7)	5.3 (4.1 – 7.1)	5.5 (3.8 – 11.1)	5.9 (3.9 – 9.3)	4.9 (3.8 – 7.6)
pH		7.9 (7.5 – 8.2)	7.9 (7.5 – 8.1)	7.6 (7.4 – 8.0)	7.6 (7.3 – 8.3)	7.6 (7.5 – 8.2)	7.4 (7.1 – 7.8)
Suspended solids	mg/L	16 (5 – 34)	6 (2 – 67)	6 (2 – 15)	6 (1 – 22)	5 (2 – 24)	4 (1 – 13)
5-day Biochemical Oxygen Demand	mg/L	30 (9 – 69)	1 (1 – 9)	3 (1 – 5)	2 (1 – 4)	3 (2 – 5)	2 (1 – 4)
Chemical Oxygen Demand	mg/L	37 (23 – 89)	6 (2 – 19)	18 (11 – 23)	14 (8 – 26)	15 (7 – 25)	15 (8 – 23)
Oil & grease	mg/L	2.3 (0.5 – 6.6)	0.5 (0.5 – 0.8)	0.5 (0.5 – 0.5)	0.5 (0.5 – 0.5)	0.5 (0.5 – 0.6)	0.5 (0.5 – 0.5)
Faecal coliforms	cfu/100mL	1,200,000 (510,000 – 6,600,000)	8,200 (50 – 240,000)	36,000 (2,300 – 290,000)	54,000 (10,000 – 170,000)	56,000 (8,500 – 57,000)	35,000 (7,100 – 190,000)
E.coli	cfu/100mL	130,000 (44,000 – 310,000)	3,300 (20 – 170,000)	4,500 (110 – 27,000)	9,100 (2,700 – 31,000)	8,000 (3,100 – 100,000)	5,100 (1,300 – 16,000)
Ammonia-nitrogen	mg/L	5.95 (2.50 – 8.30)	0.20 (0.07 – 0.41)	0.50 (0.19 – 0.85)	0.35 (0.14 – 0.99)	0.48 (0.08 – 0.84)	0.46 (0.19 – 0.85)
Nitrate-nitrogen	mg/L	0.48 (0.01 – 1..50)	1.15 (0.69 – 1.40)	0.34 (0.17 – 0.60)	0.38 (0.20 – 0.87)	0.39 (0.26 – 0.57)	0.28 (0.23 – 0.55)
Total Kjeldahl nitrogen, SP	mg/L	8.25 (4.50 – 11.0)	0.48 (0.20 – 1.10)	0.81 (0.45 – 1.30)	0.76 (0.44 – 1.50)	0.81 (0.45 – 1.30)	0.78 (0.40 – 1.10)
Ortho-phosphate	mg/L	0.75 (0.39 – 0.98)	0.04 (0.02 – 0.14)	0.04 (0.02 – 0.08)	0.04 (0.02 – 0.07)	0.04 (0.02 – 0.08)	0.05 (0.02 – 0.08)
Total phosphorus, SP	mg/L	1.15 (0.77 – 1.70)	0.06 (0.03 – 0.17)	0.09 (0.05 – 0.12)	0.08 (0.05 – 0.13)	0.08 (0.06 – 0.14)	0.10 (0.04 – 0.18)
Sulphide, SP	mg/L	0.05 (0.02 – 0.16)	0.02 (0.02 – 0.08)	0.02 (0.02 – 0.03)	0.02 (0.02 – 0.02)	0.02 (0.02 – 0.04)	0.02 (0.02 – 0.02)
Aluminium	µg/L	135 (80 – 480)	60 (50 – 230)	85 (70 – 330)	100 (70 – 950)	90 (70 – 240)	85 (60 – 140)
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 – 18)	1 (1 – 1)	1 (1 – 1)	1 (1 – 1)	1 (1 – 1)	1 (1 – 1)
Copper	µg/L	4 (2 – 25)	2 (1 – 5)	3 (2 – 4)	3 (2 – 4)	3 (2 – 6)	3 (2 – 5)
Lead	µg/L	2 (1 – 6)	1 (1 – 7)	1 (1 – 1)	1 (1 – 1)	1 (1 – 1)	2 (1 – 1)
Zinc	µg/L	30 (20 – 60)	10 (10 – 40)	10 (10 – 20)	10 (10 – 50)	10 (10 – 20)	10 (10 – 60)
Flow	L/s	141 (88 – 304)	100 (88 – 146)	NM	NM	NM	NM

Note:

1. Data presented are in annual medians of monthly samples, except those for faecal coliforms and E.coli which are in annual geometric means. Figures in brackets are annual ranges.

2. SP – soluble and particulate fractions (i.e. total value).

3. NM indicates no measurement taken

5.4 Sensitive Receivers

5.4.1 Potential water quality sensitive receiver likely to be affected by the improvement works construction would be Tuen Mun River.

5.5 Identification of Impacts

Construction Phase

5.5.1 Potential sources of water quality impact associated with the construction of the proposed road improvement works have been identified and include:

Ø construction site runoff and drainage;

Ø general construction activities; and

Ø sewage effluent produced by on-site workforce

Operation Phase

5.5.2 The identified potential sources of impact on water quality during the operation phase would be runoff from the road surfaces and load spillages resulting from accidents. 

5.6 Assessment Methodology

5.6.1 The assessment area for the water quality impact assessment is defined by a distance of 500m from the project boundary. 

5.6.2 The water sensitive receiver that may be affected by the construction works for the road improvement works were identified.  Potential sources of water quality impact that may arise during the construction and operation phases of the Project were described.  This task included identifying pollutants from point discharges and non-point sources to surface run-off.  All the identified sources of potential water quality impact were then evaluated and their impact significance determined.  The need for mitigation measures to reduce any identified adverse impacts on water quality to acceptable levels was determined. 

5.7 Prediction and Evaluation of Impacts

Construction Phase

Construction Runoff and Drainage

5.7.1 Runoff from the construction works areas during site clearance and site formation activities may contain increased loads of sediments, other suspended solids and contaminants.  Potential sources of pollution from site drainage include:

Ø runoff and erosion from exposed soil surfaces, earth working areas and stockpiles;

Ø release of grouting and cement materials with rain wash;

Ø wash water from dust suppression sprays; and

Ø fuel and lubricants from maintenance of construction vehicles and mechanical equipment.

5.7.2 Sediment laden runoff may carry pollutants (adsorbed onto the particle surfaces) into the nearby local storm water drainage system, which may arise include increased suspended solids concentrations in receiving waters and blockage of storm water drains.

5.7.3 Mitigation measures should be implemented to control construction site runoff and drainage from the works areas, and to prevent runoff and drainage water with high levels of suspended solids from entering the nearby local storm water drainage system.  With the implementation of adequate construction site drainage and provision of sediment removal facilities as described in Section 5.8.2, it is anticipated that unacceptable water quality impacts would not be arisen.

General Construction Activities

5.7.4 On-site construction activities may cause water pollution from the following:

Ø discharge of debris and rubbish such as packaging, construction materials and refuse; and

Ø spillages of liquids stored on-site, such as oil, diesel and solvents etc, are likely to result in water quality impacts if they enter into the nearby local storm water drainage system.

5.7.5 Good construction and site management practices should be observed, as detailed in Sections 5.8.3 & 5.8.4, to ensure that litter, fuels and solvents do not enter into the surrounding storm water drains.   

Sewage Effluents

5.7.6 Domestic sewage would be generated from the workforce during the construction phase. However, this temporary sewage can be adequately treated by interim sewage treatment facilities, such as portable chemical toilets, which can be installed within the construction site. It is unlikely that sewage generated from the site would have a significant water quality impact, provided that sewage is not discharged directly to the adjacent watercourse, and chemical toilets are used and properly maintained.

Operation Phase

5.7.7 At the planning and design stages, the highway drainage systems should be properly planned to receive road runoff. The road drainage system will facilitate drainage of runoff during rainstorms directly into the local stormwater drainage system.

5.7.8 Accidental spillage may be a concern during the operation phase of the project. In the event of a spill, it is the responsibility of appropriate authorities to take immediate action to cordon the area of spillage and implement clean-up actions. As there are no water sensitive receivers located in the vicinity of the improved Tuen Mun Road, the impact associated with the discharge of this runoff or accidental spillage is not anticipated to be adverse.

5.8 Mitigation of Impacts

5.8.1 Proposed mitigation measures for containing and minimizing water quality impacts are summarised as below.

Construction Phase

Construction run-off and Drainage

5.8.2 The site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” should be followed as far as practicable in order to minimise surface runoff and the chance of erosion, and also to retain and reduce any suspended solids prior to discharge. These practices include the following items:

Ø Silt removal facilities such as silt traps or sedimentation facilities should be provided to remove silt particles from runoff to meet the requirements of the TM standards under the WPCO. The design of silt removal facilities should be based on the guidelines as stipulated in ProPECC PN 1/94. All drainage facilities and erosion and sediment control structures should be inspected monthly and maintained to ensure proper and efficient operation at all times and particularly during rainstorms.

Ø Careful programming of the works to minimise surface excavations for the road improvement works during the wet season. If excavation of soil cannot be avoided during the wet season, 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.

Ø Exposed soil surfaces should be protected by paving or fill material as soon as possible to reduce the potential of soil erosion.

Ø Open stockpiles of construction materials or construction wastes on-site should be covered with tarpaulin or similar fabric during rainstorms. These materials should not be placed near water courses.

Ø A Drainage Management Plan (DMP) should be prepared by the Contractor and submitted to EPD before the commencement of any construction works to detail the procedures for control of construction site runoff. The DMP should at least cover the construction works areas for slip roads and the local widening of the existing Tuen Mun Road within the project boundary.

General Construction Activities

5.8.3 Debris and refuse generated on-site should be collected, handled and disposed of properly to avoid entering the nearby local stormwater drainage system. Stockpiles of cement and other construction materials should be kept covered when not being used. 

5.8.4 Oils and fuels should only be used and stored in designated areas which have pollution prevention facilities. All fuel tanks and storage areas should be provided with locks and be sited on sealed areas, within bunds of capacity equal to 110% of the storage capacity of the largest tank.

Sewage from Construction Workforce

5.8.5 Temporary sanitary facilities, such as portable chemical toilets, should be employed on-site. A licensed contractor would be responsible for appropriate disposal and maintenance of these facilities.

Operation Phase

5.8.6 A surface water drainage system will be provided to collect road runoff. The following measures are recommended to ensure road runoff will comply with the standards stipulated in the TM for discharges into storm water drains:

Ø The road drainage should be directed through silt traps in the gully inlets to remove silt and grit before entering the public storm water drainage system; and

Ø The silt traps should be regularly cleaned and maintained in good working condition.

5.9 Evaluation of Residual Impact

5.9.1 With the adoption and incorporation of the recommended mitigation measures for both the construction and operation phases, no residual impacts on water quality impact are anticipated to occur due to the works of the proposed project.

5.10 Monitoring and Audit Requirements

5.10.1 No off-site water quality impact would be expected from the construction activities of the Project. All the site effluents and runoff generated from the construction works would be treated and their quality be monitored before discharge under the requirements of the WPCO discharge licence. Water quality monitoring is not considered necessary.  It is recommended that regular site inspections be undertaken to inspect the construction activities and works area in order to ensure the properly implementation of proposed mitigation measures.

5.11 Summary

Construction Phase

5.11.1 The potential water quality impacts arising from construction runoff and drainage, and other land-based construction activities, have been assessed qualitatively.

5.11.2 It is concluded that minimal impacts on the receiving waters are expected provided that mitigation measures are implemented during the construction phase. With the adoption of the recommended mitigation measures, no unacceptable residual impact on water quality is expected.

Operation Phase

5.11.3 The only source of potential impact on water quality during the operation phase will be runoff from the road surfaces. With the implementation of the recommended mitigated measures for the surface water drainage system, it is anticipated that the water quality impacts associated with the operation phase would be minimal and acceptable.