5.                  WATER QUALITY IMPACT ASSESSMENT

 

5.1              Introduction

 

5.1.1        This Chapter details the current condition of the water bodies within the Study Area, and describes the potential impacts on water quality during the construction and operation phases of the Project. Mitigation measures are proposed and the residual impacts after implementation of these measures are evaluated.

 

5.2              Environmental Legislation, Policies, Standards and Criteria

 

5.2.1        The Water Pollution Control Ordinance (WPCO) (Cap.358) enacted in 1980 is the principal legislation controlling water quality in Hong Kong along with Annexes 6 and 14 of the Technical Memorandum on EIA Process (EIAO-TM). Under the WPCO, Hong Kong waters are classified into Water Control Zones (WCZs) and statutory Water Quality Objectives (WQOs) are specified for each WCZ. This Project falls within the catchment of Deep Bay WCZ.

 

5.2.2        The other relevant guideline is the Practice Note for Professional Persons (ProPECC Note) PN 1/94 – “Construction Site Drainage” which provide guidelines for the handling and disposal of construction discharges. This ProPECC Note is generally applicable for control of site runoff and wastewater generated during the construction phase of the Project.

 

5.2.3        The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters under the WPCO defines acceptable discharge limits to different types of receiving water in Hong Kong. Site effluent discharge during the construction phase will be required to comply with the terms and conditions of a discharge licence issued by EPD.

 

5.3              Study Area

 

5.3.1        According to the EIA Study Brief, the Study Area for the water quality impact assessment should include all areas within 300 m from the Project site boundary, including but not limited to Hang Hau Tsuen stream catchment area and Inner Deep Bay. It should be noted that Inner Deep Bay is well over 3 km to the north of the discharge point of the Project.

 

5.4              Baseline Water Quality Conditions of the Study Area

 

Desktop Review and Field Observations

 

5.4.1        Site visits took place over a number of days between December 2007, January, May and September 2008. General descriptions and field observations of the water bodies in the Study Area (Figure 5.2) are provided below and in Table 5.1.

 

San Hing Tsuen Channel

 

5.4.2        San Hing Tsuen Channel is a rectangular concrete channel originating from the under road box culvert at Lau Fau Shan Road which collect drainage flow from San Hing Tsuen and nearby villages. It traverses across open storage sites and container yards, then flows parallel along Deep Bay Road joining Fong Kong Tsuen Channel before draining into Hang Hau Tsuen stream. In addition to village drainage, the channel also collect domestic wastewater from nearby unsewered villages, runoff from surrounding open storage sites / container yards as well as some wastewater discharged from restaurants in Lau Fau Shan.

 

Fung Kong Tsuen Channel

 

5.4.3        Fung Kong Tsuen Channel is a rectangular channel originating near Fong Kong Tsuen which collects drainage flow from Fong Kong Tsuen and nearby villages. This drainage channel traverses across a number of open storage sites / container yards and collects effluents from these industrial premises in additional to village runoff and domestic wastewater from nearby unsewered villages. It drains into Hang Hau Tsuen stream after joining with San Hing Tsuen Channel.

 

Hang Hau Tsuen Stream

 

5.4.4        Hang Hau Tsuen stream is a semi-natural tidal stream of about 435 m in length. It collects drainage flow from the San Hing Tsuen Channel and Fung Kong Tsuen Channel and discharges into Deep Bay. It also receives surface runoff and domestic wastewater from Hang Hau Tsuen as well as various other discharges collected by the two upstream drainage channels.

 

5.4.5        Prior to 1997, a large number of squatter houses were built on the stream which affected water flow. The squatter houses were subsequently demolished in 1997.

 

5.4.6        The stream is covered with grasses along the section above tidal influence and partly lined with mangroves below the high tide level.

 

Fishponds

 

5.4.7        A total of four fishponds can be found within the Study Area. All appeared to be abandoned. Grassy and weedy vegetation were recorded in the pond bunds. The conditions of the water inside the ponds are fairly turbid. Rubbish and construction waste can be found at the edge of the ponds.

 

Deep Bay

 

5.4.8        Hang Hau Tsuen stream directly discharges into Deep Bay. Deep Bay is a large shallow bay fed by outflow from the Shenzhen River and run-off from the surrounding land. Water from Deep Bay flows into the main Pearl River estuary. Extensive mudflats can be found along the Deep Bay coast from Tsim Bei Tsui down to Ha Pak Nai. Culture of oyster is practiced along the intertidal mudflat of Deep Bay.

 

Inner Deep Bay

 

5.4.9        Inner Deep Bay is situated on the north western part of the New Territories and is bordered by Shenzhen to the north. It directly receives discharges from Shenzhen River. Inner Deep Bay comprises a range of habitats from shallow water to inter-tidal and estuarine mudflats and contains the largest and most important dwarf mangrove communities in Hong Kong. It also provide feeding and resting ground for large numbers of bird species including visitors or passage migrants some of which are considered as rare or endangered. It has been designated as a Site of Scientific Interest (SSSI) and as a Ramsar Site.

 

Table 5.1

Field Observations of the Inland Water Bodies in the Study Area

 

Stream / Channel

Description

Water Quality Observations

San Hing Tsuen Channel

Concrete rectangular drainage channel flowing across open storage sites and container yards. Water is almost stagnant during dry season.

Water is grossly polluted with visible floating scum, oil stains and rubbish. Water is black in colour and very turbid. The lower reaches of the channel bed along Deep Bay Road is strewn with rubbish and noticeable odour is observed.

Fung Kong Tsuen Channel

Concrete rectangular drainage channel flowing across open storage sites and container yards. Low water flow during dry season.

Water is grossly polluted with visible oil stains and floating rubbish. Water is black in colour and very turbid.

Hang Hau Tsuen Stream (upstream section)

The upstream portion is a natural stream with rocky bottom and is overgrown with grasses. It receives flows from San Hing Tsuen Channel and Fung Kong Tsuen Channel via a box culvert underneath Deep Bay Road. Water is shallow given the wideness of the stream at this section.

Blackish turbid water, with rubbish trapped within the grasses along the stream bed.

Hang Hau Tsuen Stream (mid-stream section)

Natural stream with dark colour muddy bottom and is partly overgrown with grasses and mangroves. The stream is narrower in the upstream portion with faster water flow. Extensive grasses occupy the stream bank. The stream widens in the middle portion where mangroves and shrubs can be found. Stream bank is partly occupied by footpaths and village houses. A sub-standard wooden bridge provides access across the stream.

Blackish turbid water, presence of rubbish along the stream bed.

Water quality appears to slightly improve during the start of the wet season of 2008, probably due to the continuous heavy rain during the period.

Hang Hau Tsuen Stream (downstream section)

Natural stream section affected by tidal action with a slightly lighter colour muddy bottom. Smaller patches of mangroves. Stream bank is occupied by footpaths and village houses. Presence of broken stilt structures. A sub-standard wooden bridge provides access across the stream.

Blackish turbid water.

Water quality appears to slightly improve due to the continuous heavy rain during the start of 2008 wet season, but still turbid probably due to the tidal action of Deep Bay.

Hang Hau Tsuen Stream (outfall section & towards Deep Bay)

Natural stream section affected by tidal action with extensive intertidal mudflats along the coast. Stream is slightly narrower at the outfall due to presence of large concrete platform. Oyster shell hash can be found at various locations. A small concrete / metal footbridge with landing step cases provide access across the stream. No mangroves exist to the east of the footbridge due to the platform. To the west of the footbridge, there is a large patch of mangroves at one side and mooring areas with concrete landing places for small boats on the other side.

Blackish turbid water. Presence of mudskipper, intertidal crabs and other invertebrates on the mudflats.

Water quality appears to slightly improve due to the continuous heavy rain during the start of 2008 wet season, but still turbid probably due to the tidal action of Deep Bay.

 

Baseline Conditions

 

Stream Water Quality

 

5.4.10    There is no EPD river water quality monitoring station near the Study Area. Water quality sampling was conducted in January 2008 to characterise the water quality of Hang Hau Tsuen stream.

 

5.4.11    The sampling locations are shown in Figure 5.1. The results are summarized in Table 5.2 and presented in Appendix 5. The results verified the poor water quality observed from field survey. The major pollution sources are mainly attributed to wastewater discharges from unsewered villages (Hang Hau Tsuen and other villages upstream) and industrial effluent from upstream sources.

 

5.4.12    Levels of E. coli were relatively high compared to levels found in Deep Bay and well above WQOs levels. This reflects the presence of sewage which is being discharged to the stream untreated.

 

Table 5.2

Results of Water Quality Sampling at Hang Hau Tsuen Stream

 

Parameters

Unit

Sample Location

W1 (upstream)

W2 (downstream)

Temperature

oC

15.1

15.7

pH

-

7.32

7.02

Salinity

ppt

0.2

2.2

Turbidity

NTU

134

56.2

Dissolved Oxygen

mg/L

5.48

3.52

Dissolved Oxygen (DO)

%

54.7

36

Suspended Solids (SS)

mg/L

70

34

Ammonia Nitrogen (NH3-N)

mg/L

2.97

1.38

Reactive Phosphorus (PO4)

mg/L

0.34

0.03

Total Kjeldahl Nitrogen (TKN)

mg/L

5.1

3.6

Total Phosphorus (P)

mg/L

0.8

0.4

Nitrate Nitrogen (NO3-N)

mg/L

1.46

0.97

Nitrite Nitrogen (NO2-N)

mg/L

0.2

0.16

Arsenic (As)

µg/L

<10

<10

Cadmium (Cd)

µg/L

0.5

0.3

Chromium (Cr)

µg/L

5

3

Copper (Cu)

µg/L

96

47

Lead (Pb)

µg/L

235

115

Mercury (Hg)

µg/L

<0.5

<0.5

Nickel (Ni)

µg/L

7

8

Silver (Ag)

µg/L

<1

<1

Zinc (Zn)

µg/L

308

217

E. coli

cfu/100mL

66,000

26,000

Biochemical Oxygen Demand (BOD)

mg/L

10

8

Chemical Oxygen Demand (COD)

mg/L

122

122

Oil & Grease

mg/L

8

<5

Total Organic Carbon (TOC)

mg/L

20

16

 

Notes:

The sampling locations are shown in Figure 5.1.

Marine Water Quality

 

5.4.13    Water and sediment quality in Deep Bay is monitored regularly as part of EPD’s marine water quality monitoring programme. The EPD’s monitoring locations near the Study Area is shown in Figure 5.1.

 

5.4.14    A summary of EPD water quality data from 2005 to 2007 for the monitoring station closest to the Project (DM3) is shown in Table 5.3.

 

5.4.15    The water quality of Deep Bay (including Inner Deep Bay) was the poorest in the territory, characterised by high concentrations of organic and inorganic pollutants and low levels of DO. In 2007, the WQO compliance in the Deep Bay WCZ was low at 27% (EPD 2008).

 

Table 5.3

Summary of EPD Marine Water Quality Data for Deep Bay (DM3) from 2005 to 2007

 

Parameters

Unit

DM3

2005

2006

2007

DO

mg/L

5.3
(4.1 – 7.2)

5.8
(3.8 – 6.8)

6.4
(2.7 – 10.4)

DO

%

72
(56 – 89)

80
(55 – 95)

86
(39 – 127)

Turbidity

NTU

38.0
(12.9 – 87.6)

21.2
(13.1 – 39.2)

19.3
(10.6 – 35.6)

SS

mg/L

40.7
(7.8 – 93.0)

16.9
(3.8 – 38.0)

13.4
(4.3 – 36.0)

NH3-N

mg/L

0.75
(0.27 – 1.40)

0.69
(0.20 – 1.40)

0.84
(0.26 – 1.90)

NO2-N

mg/L

0.25
(0.16 – 0.37)

0.19
(0.12 – 0.25)

0.21
(0.11 – 0.46)

NO3-N

mg/L

0.73
(0.27 – 2.00)

0.63
(0.35 – 1.10)

0.54
(0.24 – 1.20)

TKN

mg/L

1.19
(0.47 – 2.00)

0.95
(0.66 – 1.50)

1.28
(0.60 – 2.60)

PO4

mg/L

0.13
(0.07 – 0.19)

0.11
(0.02 – 0.21)

0.14
(0.04 – 0.31)

TP

mg/L

0.21
(0.10 – 0.31)

0.16
(0.08 – 0.34)

0.20
(0.09 – 0.39)

E. coli

cfu/100mL

150
(2 – 3800)

120
(12 – 1800)

38
(7 – 1300)

BOD

mg/L

1.7
(0.4 – 3.9)

1.6
(0.5 – 3.7)

2.3
(0.4 – 6.2)

 

Sources:

EPD (2006) Marine Water Quality Monitoring in Hong Kong in 2005

EPD (2007) Marine Water Quality Monitoring in Hong Kong in 2006

EPD (2008) Marine Water Quality Monitoring in Hong Kong in 2007

 

5.4.16    An updated baseline water quality should be established before the commencement of the construction phase of the Project to allow appropriate action and limit levels to be determined for impact monitoring and audit.

 

Sediment Quality

 

5.4.17    A Sediment Quality Report (SQR) has been undertaken to ascertain the volume and degree of contamination of the stream sediment that needs to be excavated such that appropriate marine disposal sites can be allocated by the relevant authorities. Details are presented in Chapter 6 – Waste Management.

 

5.5              Water Sensitive Receivers

 

5.5.1        The Water Sensitive Receivers (WSRs) likely to be impacted by the Project include the Hang Hau Tsuen stream itself, the nearby abandoned fishponds, the mudflats along the Deep Bay coast, the mangroves near the mid and downstream section of the stream, the oyster bed and the waters of Deep Bay into which the proposed channel discharges. Inner Deep Bay SSSI is located over 3 km to the north of the channel discharge point. Adverse impact from the Project on Inner Deep Bay is not expected. The locations of these WSRs are presented in Figure 5.2.

 

5.6              Potential Impacts During Construction Phase

 

Site Preparation / Clearance

 

5.6.1        Preparation of the land for construction of the drainage channel will involve removal of surface vegetation and excavations, removal and import of construction materials. Construction of some temporary infrastructure may also be necessary such as drainage, bunds and access roads.

 

5.6.2        The permeability characteristics of the site may change, reducing infiltration into surface layer. This may lead to increased site surface runoff with high suspended solids loadings during the wet season. Erosion of soil enriched in organic matter may also release nutrients into the nearby water bodies.

 

De-Watering / Excavation of Streams / Pond and Removal of Sediment

 

5.6.3        During excavation, the water in the stream will be temporarily diverted to provide a dry working stream bed prior to the channel works. This may lead to disturbance of organic rich bottom sediments which will release nutrients and suspended solids into the water.

 

5.6.4        Sediment characterisation surveys were conducted in January 2008 and November 2008 to determine whether there are any contaminants in the sediments. The results indicated that some of the sediments are “uncontaminated” (Category L) and some are “contaminated” (Category M and H). Details of the sediment testing and its recommended disposal route can be found in Chapter 6 – Waste Management.

 

5.6.5        Excavation of the stream sediments will be required for channel formation at the upstream portion of the stream. The stream sediment at the downstream section will largely be left in-situ. Excavated sediments, whether clean or contaminated, may have to be stored on-site before being sorted, reused or transported to disposal sites. If stored as open air stockpiles and left uncovered during rainfall, there is a potential risk of the excavated sediments being washed away, back into the stream water or the nearby waterbodies including the abandoned fishponds, thereby causing sudden increase of oxygen demand, turbidity and heavy metals (from contaminated sediments) during rainy condition.

           

Concreting Work

 

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

 

5.6.7        With the use of natural substrates such as mud and rip-rap as well as retention of the existing natural stream bed in parts of the proposed channel, the use of concrete will be substantially reduced.

 

            Site Workshop or Depot

 

5.6.8        Works areas and site offices will be provided for site staff and contractors, which may include workshop or depot to provide maintenance and repair services for the equipment on site. The workshop may store engine oil, lubricants, chemicals and waste oil/materials which may create impacts if accidental spillage occurs. Waste oil may infiltrate the surface topsoil and contaminate the ground. Stormwater runoff from the site may wash the oil spillage into adjacent water bodies.

 

General Construction Works

 

5.6.9        The channel construction works are scheduled to start in end 2009 for completion in end 2012. General construction works will be continuous throughout both wet and dry seasons. During the wet season, silty runoff from the sites will be generated from areas cleared of vegetation. Spillage, hydraulic leakage and runoff from the surface of standby construction equipment during rainy conditions may release oil and lubricants to the surrounding environment. If the site runoff is not adequately managed, it may enter the water bodies and increase the suspended solids, turbidity and oil & grease levels of these water bodies.

 

            Presence of Additional Population (Workers)

 

5.6.10     The presence of site staff and construction workers will generate sewage and domestic refuse. Impacts include waste and wastewater generated from eating areas, toilets and waste disposal areas. Sewage arising from on-site construction workforce would have the potential to cause water pollution if it is discharged directly into adjacent water bodies without any appropriate treatment.

 

5.7              Mitigation Measures for Construction Phase

 

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

 

            General

 

5.7.2        The Contractor shall observe and comply with the Water Pollution Control Ordinance (WPCO) and its subsidiary regulations. The Contractor shall carry out the works in such a manner as to minimise adverse impacts on the water quality during execution of the works. In particular the Contractor shall arrange his method of working to minimise the effects on the water quality within and outside the site and on the transport routes.

 

5.7.3        The Contractor shall follow the practices, and be responsible for the design, construction, operation and maintenance of all the mitigation measures below and as specified in ProPECC PN 1/94 – “Construction Site Drainage”. The design of the mitigation measures shall be submitted by the Contractor to the Engineer for approval.

 

            Site Preparation / Clearance

 

5.7.4        Proper construction site drainage management measures should be implemented to control site runoff and drainage, and thereby prevent high sediment loadings from reaching Deep Bay or the nearby abandoned fishponds. Site runoff and wastewater should not be discharged into the nearby fishponds irrespective of the status of the fishponds.

 

5.7.5        Turbid water from construction sites must be treated to minimise the solids content before being discharged. Advice on the handling and disposal of site discharge is given in the ProPECC Note PN 1/94 – “Construction Site Drainage”.

 

5.7.6        In general, surface run-off from construction sites should be discharged into water bodies via adequately designed silt removal facilities such as sand traps, silt traps and sediment basins. Channels or earth bunds or sand bag barriers should be provided on site to properly direct stormwater to such silt removal facilities. Perimeter channels at site boundaries should be provided to intercept storm run-off from outside the site so that it will not wash across the site (or into the proposed channel works area). Catchpits and perimeter channels should be constructed in advance of earthworks.

 

5.7.7        Silt removal facilities and diversion channels should be maintained and the deposited silt and grit should be removed regularly, especially at the onset of and after each rainstorm to ensure proper functioning of these facilities at all times.

 

5.7.8        Measures should be taken to prevent the washing away of construction materials, soil, silt or debris into the nearby water bodies. Open stockpiles susceptible to erosion should be covered with tarpaulin or similar fabric and provided with containment such as bunds, sand bag barriers or equivalent measures, especially during the wet season (April – September) or when heavy rainstorm is predicted. Runoff to watercourses should be reduced by minimising flat exposed areas of permeable soil, and by forming pits or diversion channels into which runoff can flow to suitable treatment facilities before discharge.

 

De-watering / Excavation of Stream / Pond and Removal of Sediment

 

5.7.9        Excavation works within the existing stream section and pond should be programmed to be carried out during dry season from 1st October to 31st March as far as practicable to minimise impacts on downstream water quality and nearby sensitive receivers.

 

5.7.10    The use of containment structure such as sheet pile barriers, earth bunds, sand bag barriers wrapped with geotextile fabric or similar material, diversion channels or other similar techniques should be installed surrounding the excavation area to facilitate a dry or at least confined excavation within the stream. Schematic diagram of typical drainage measures during excavation of the stream is shown in Figure 5.3. The Contractor should submit details of the temporary drainage measures along with the proposed measures to ameliorate the potential water quality impacts to the Environmental Team (ET) for verification and to the Engineer for approval before commencement of the construction works.

 

5.7.11    The excavation area should be limited to section of half width of the stream in order to maintain continuous water flow within the stream during the construction phase.

 

5.7.12    After dewatering of the stream and pond, the sediments should be allowed to dry before excavation (yet still maintain a moist state to avoid dust nuisance). This will facilitate excavation of the sediments and also minimise the risk of drained water flowing back into watercourses as the sediment is handled. Where time or weather constraints require handling of wet sediment, care should be taken in the removal of sediment and the storage area should be bunded to prevent silty runoff entering water bodies.

 

5.7.13    Tightly sealed closed grab excavators should be employed to minimize leakage and loss of sediments during excavation works within the stream.

 

5.7.14    Excavated sediment material from stream should be stored in covered impermeable skips and disposed within 2 days, to avoid inadvertent release of silty runoff and contaminants to nearby water bodies. If sediment material is identified to be suitable for reuse as stream bed material, it should be properly stockpiled, adequately covered and provided with containment to prevent runoff during wet season.

 

5.7.15    Regular monitoring of suspended solids and turbidity should be conducted during excavation works. Any exceedance of water quality in the nearby water bodies caused by inadvertent release of site runoff should be rectified in accordance with EM&A programme for this Project.

 

            Concreting Work

 

5.7.16    Runoff should be carefully channelled to prevent concrete-contaminated water from entering watercourses. Adjustment of pH can be achieved by adding a suitable neutralising reagent to wastewater prior to discharge. Reuse of the supernatant from the sediment pits for washing out of concrete lorries should be practised.

 

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

 

            Site Workshop or Depot

            General Construction Works

 

5.7.18    Any Contractor generating waste oil or other chemicals as a result of his activities should register as a chemical waste producer and provide a safe designated storage area for chemicals on site. The storage site should be located away from existing water courses.

 

5.7.19    All compounds in works areas should be located on areas of hard standing surface with provision of diversion channels and settlement ponds where necessary to allow interception and controlled release of settled / treated water. Hard standing compounds should drain via an oil interceptor. The oil interceptor should be regularly inspected and cleaned to avoid wash-out of oil during storm conditions. A bypass should be provided to avoid overload of the interceptor's capacity. To prevent spillage of fuels or other chemicals to water courses, all fuel tanks and storage areas should be sited on sealed areas within a bund of a capacity equal to 110% of the storage capacity of the largest tank. Where temporary storage of chemicals or fuel drums outside the storage area is necessary, drip tray should be provided. Disposal of the waste oil should be carried out by a licensed collector. Good housekeeping practices should be implemented to minimise careless spillage and to keep the storage and the work space in a tidy and clean condition. Appropriate training including safety codes and relevant manuals should be given to the personnel who regularly handle the chemicals on site.

 

Emergency Contingency Plan

 

5.7.20    The Contractor should prepare an emergency contingency plan (spill response plan) for the Project to contain and remove accidental spillage of chemicals and all hazardous materials on-site including fuels at short notice and to prevent or to minimize the quantities of contaminants from entering the stream water or nearby water bodies and affecting the sensitive habitats. The Contractor should submit the emergency contingency plan to the ET for review & comment and the Engineer for approval. The Plan should include, but not limited to, the following:

 

(i)                  potential emergency situations

(ii)                 chemicals or hazardous materials used on-site (and their location)

(iii)               emergency response team

(iv)               emergency action plans and procedures

(v)                list of emergency telephone hotlines

(vi)               locations and types of emergency response equipment

(vii)             training plan and emergency drill

(viii)            schedules for review and audit.

 

General Guidance for Handling of Spillage / Leakage

 

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

 

·                     Contact person in charge or nominated person immediately and initiate action plans based on the emergency contingency plan.

 

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

 

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

 

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

 

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

 

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

 

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

 

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

 

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

 

           


Presence of Additional Population (Workers)

 

5.7.22    Sewage arising from construction workers on site should be collected in a suitable storage facility, such as portable chemical toilets. An adequate number of portable toilets should be provided for the construction workforce. The portable toilets should be maintained in a state that will not deter the workers from using them. Wastewater collected should be discharged into foul sewers and collected by licensed collectors.

 

5.7.23    The collected wastewater from sewage facilities and also from eating areas or washing facilities of site offices should be disposed to foul sewer. If there is no foul sewer in the vicinity, a septic tank and soakaway system or for larger flow, a sewage treatment plant should be provided. All domestic sewage discharges (except into foul sewer) are controlled under the WPCO. The Contractor must apply for a discharge licence from EPD and must comply with the terms and conditions of a valid WPCO licence.

 

5.8              Potential Impacts During Operational  Phase

 

Introduction

 

5.8.1        Potential water quality impacts during operational phase of the Project include:

 

•                     pollution loads entering the proposed channel (from polluted upstream water, runoff from adjacent land / villages and flood waters);

•                     pattern of sediment deposition due to change in flow regime;

•                     increase in suspended solids during maintenance of the proposed channel; and

•                     road surface runoff from the proposed access road, viewing point and carpark.

 

5.8.2        It should be noted that the operation of the proposed channel does not in itself generate any new pollution loading to Deep Bay.

 

Pollution Loads entering the Channel

 

5.8.3        Long term reduction of pollution load into the channel is required to improve the water quality in the channel and the receiving waters of Deep Bay. The first step would be the continuous enforcement of Livestock Waste Control Scheme by the relevant authority and the provision of sewerage system to the unsewered villages in the area. The stream currently receives domestic wastewater from unsewered villages and industrial effluent which will continue even after the Project is complete. The level of impact will be similar to the current situation and the long term solution will be to provide sewer connections from village households to sewage treatment works. Provision of public sewers to the unsewered areas / villages in Northwest New Territories is being investigated but outside the scope of this Project.

 

5.8.4        Vegetation removes dissolved and particulate pollution through absorptive, filtration and biological mechanisms. As concrete drainage environment will not provide such functions, parts of the existing natural stream bed will be retained. In addition, some sections of the channel will be lined with original stream bed materials. Vegetations will be allowed to re-establish along the channel naturally.

 

5.8.5        Standard drainage design of catchpits with sand traps can effectively reduce the amount of sands, grits and refuse contained in surface runoff of Hang Hau Tsuen from entering the proposed channel.

 

Pattern of Sediment Deposition due to Change in Flow Regime

 

5.8.6        The proposed channel will need to effectively collect and convey storm water to alleviate the risk of flooding in Hang Hau Tsuen. The increase in flowrate in the channel as a result of better conveyance will likely increase the amount and rate of sediment discharge from the channel which will affect the pattern of sediment deposition especially at the outfall of the channel. This would be exacerbated by the use of concrete lining in the channel. Such changes in sediment deposition would have an adverse effect in particular to the established mangrove stands and intertidal fauna at the mudflats located near the channel outfall.

 

5.8.7        With the proposed design of using rip-rap and original stream bed materials lining which mimics the current stream condition, sediment will be allowed to accumulate on the channel bed which will reduce sudden discharge of sediment downstream during storm flow. By raising the bank level and widening of the stream, water flowrate will not increase significantly. Furthermore, the retained mangroves and stream bed at the downstream section would have similar effect in retaining the sediment as in the current condition. The proposed channel will therefore have negligible effect on sediment deposition pattern. The sediment deposition pattern will remain the same as in current situation which is predominantly controlled by the tidal action of Deep Bay. As such, the Project will not affect the shoreline or bathymetry of Deep Bay as well as the mangroves and intertidal mudflats of Deep Bay.

 

Maintenance of the Channel

 

5.8.8        Maintenance would be necessary for the proposed channel to remove excessive sediment, vegetation, rubbish, debris and obstructions in order to maintain its hydraulic performance and structural integrity. Siltation will generally be allowed to accumulate and removal of excess silt would be carried out at locations where it would impede water flow. Little or no maintenance will be necessary for the natural stream bed section of the channel. A maintenance access will be provided along the southern downstream portion of the channel to allow access by maintenance personnel to access and maintain the channel bank and box culvert (footbridge). The alignment of the maintenance access has been selected to minimize disturbance to the existing mangroves lying along the north bank. The potential temporary impacts include an increase of suspended solids during removal of the accumulated sediments and disturbance to the aquatic communities from workers and equipment. The impacts associated with the maintenance works are not expected to create adverse water quality impacts.

 

5.8.9        There would be no chemicals or hazardous materials stored / used on-site during operation, as such the likelihood of accidental spillage is not expected. Emergency contingency plan for operation phase is therefore considered not necessary.

 

Road Surface Runoff from the Proposed Access Road, Viewing Point and Carpark

 

5.8.10    Contaminants from exhaust emissions, tyre wear and leakage of oil & grease from vehicles deposited on the road surface and carpark have the potential to cause water quality impacts to the receiving waters of Deep Bay during rainfall events. Such road surface runoff would also contain a small amount of silts, grits and litter. The materials deposited on the surface will be removed from the road surface during rain events and enters the road drainage system. The initial runoff which occurs during the first rain event (also referred to as the “first flush”) will contain most of the particulate matters and associated contaminants and can impact the receiving waters of Deep Bay.

 

5.8.11    Given the low numbers of vehicles (and visitors) expected to use the access road and carpark, the amount of pollutants is expected to be very minimal. Moreover, the standard highway drainage design consisting of gullies and silt / grit traps can effectively trap such pollutants. Provided that the road drainage systems are properly maintained, the proposed access road and viewing point and carpark are not expected to create adverse water quality impacts.

5.9              Mitigation Measures for Operational  Phase

 

Measures to Reduce Pollution Loadings entering the Channel

 

5.9.1        Appropriate location along the toe zone of the channel will be filled with about 200 mm thick original stream bed materials. The upstream dry weather flow channel will also be filled with a layer of about 100 mm thick original stream bed materials on top of the rip-rap lining. The most important feature of such measure is the prospect of natural re-colonization of benthic communities and re-establishment of vegetation along the toe zone of the channel replicating riparian vegetation. The vegetation is not expected to be detrimental in any way to the channel structure or to the conveyance of flood flow. Specific planting is considered not necessary as vegetation will recolonized and established itself naturally similar to current condition. Non-woody species (such as sedges) is preferred for ease of maintenance and pruning as they are easier to be pruned and will impede the flow of water to lesser extent during large flow as the vegetation will just collapse and spring back. However, seasonal cutting and clearance of vegetation, particularly in advance of the wet season may be required. This mitigation measure has additional benefits of aesthetic and ecological value.

 

5.9.2        In addition, the use of rock fill base or original stream bed materials for the channel bed has the benefit of providing uneven surfaces and cavities for sediment to accumulate. Ultimately a sediment layer will build up on the channel bed, forming a natural layer for development of the benthic community. Removal of the upper layer of this sediment will only be necessary once the layer thickness has built up to around 300 mm thick, and sediment is likely to be washed downstream in heavy storms. A minimum of 100 mm thick sediment should be allowed to accumulate at the channel bed to permit recolonizing of benthic communities. Growth of vegetation will inhibit washout of sediment and sediment removal can be carried out at the same time as vegetation harvesting during the dry season when flows are minimal.

 

5.9.3        Catchpits with sand traps will be provided in the drainage system to trap sands, grits and rubbish in the Hang Hau Tsuen surface runoff prior to discharge to Deep Bay. The catchpit should be cleaned and maintained especially before the onset of the wet season to ensure its performance.

 

Environmental Considerations for Maintenance of the Proposed Channel

 

5.9.4        Maintenance may be necessary for the proposed channel at regular intervals to remove excessive silts, vegetation, rubbish, debris and obstruction. Little or no maintenance will be necessary for the natural stream bed section of the channel. Likewise, the retained and compensated mangroves within the mangrove zone (Figure 2.7) will not require any long term maintenance. Good practice guides for the planning and execution of desilting and maintenance works are recommended in the following sections.

 

5.9.5        The following considerations should be included in planning for the maintenance works of the proposed channel:

 

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

 

(b)               The management of woody / emergent vegetation should be limited to manual cutting, to be carried out during dry season and only when unchecked growth of such vegetation is very likely to impede channel flow.

 

(c)                Mangroves within the mangrove zone should be retained if the hydraulic capacity of the channel is adequate. Mangroves found outside the mangrove zone but within the proposed channel should be remove as they will affect the hydraulic capacity of the channel. Rip-rap that are used to delineate the mangrove zone should be replaced if found damaged.

 

(d)               A minimum of 100 mm thick sediment should be allowed to accumulate on the channel bed to permit recolonization of benthic communities.

 

(e)                Phasing of the works should be considered to better control and minimize any impacts caused, and to provide refuges for aquatic organisms. Where possible, works should be carried out along half width of the channel in short sections. A free passage along the channel is necessary to avoid forming stagnant water in any phase of the works and to maintain the integrity of aquatic communities.

 

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

 

(g)                Where no maintenance access is available for the channel, temporary access to the works site should be carefully planned and located to minimize disturbance caused to the channel, adjacent vegetation (especially mangroves) and nearby sensitive receivers by construction plants.

 

(h)                The use of lesser or smaller construction plants should be considered to reduce disturbance to the channel bed. Quiet construction plants should be used.

 

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

 

Mitigation Measures for the Proposed Access Road, Viewing Point and Carpark

 

5.9.6        The following measures should be implemented to ensure no adverse water quality impacts during the operation of the access road, viewing point and carpark.

 

5.9.7        Highways Department (HyD) standard road drainage system should be provided along the proposed access road and viewing point and carpark to collect the road runoff. The road drainage design should incorporate gullies and silt / grit traps to trap any pollutants in the road surface runoff prior to discharge into Deep Bay.

 

5.9.8        Regular cleansing of the access road and viewing point and carpark following normal established practices should be carried out to remove any accumulated silts, grits and litters. The gullies and silt / grit traps should also be regularly cleaned and maintained in good working condition.

 

5.10          Residual and Cumulative Impacts

 

5.10.1    With the effective implementation of mitigation measures as described, residual impacts on water quality of the receiving water bodies due to the implementation of this Project are expected to be negligible.

 

5.10.2    There are no other concurrent projects in the vicinity of the Project that will give rise to cumulative impacts.

 

5.11          Monitoring and Audit Requirement

 

5.11.1    Potential water quality impacts during the construction of the proposed channel can be mitigated by the implementation of the recommended site practices and other measures to control runoff and to minimise water pollution.

 

5.11.2    A water quality monitoring and audit programme is recommended to check for compliance with relevant water quality standards, to ensure the proper implementation of the mitigation measures and to verify the effectiveness of the recommended mitigation measures.

 

5.11.3    Monitoring of pH value, dissolved oxygen, suspended solids and turbidity will need to be carried out to ensure that any deteriorating water quality could be readily detected and timely action taken to rectify the situation. Monitoring should be provided at both downstream (as impact station) and upstream (as control station) as shown in Table 5.4.

 

5.11.4    No monitoring is required during operational phase.

 

5.11.5    The monitoring requirement is broadly presented in Chapter 10 of this Report. Details of the monitoring and audit of water quality will be presented in the separate EM&A Manual.

 

Table 5.4

Water Quality Monitoring Stations during Construction Stage

 

Water Quality Monitoring Station

Description

Purpose of Placing Sampling Station

W1

Downstream of the works immediately at the discharge point to Deep Bay

Monitor and audit potential impacts from the works, check water quality discharging to Deep Bay, impact station

W2

Near the oyster bed in Deep Bay

Monitor and audit potential impacts from the works, check water quality discharging to Deep Bay, impact station

W3

Near the oyster bed in Deep Bay

Monitor and audit potential impacts from the works, check water quality discharging to Deep Bay, impact station

W4

Immediate downstream of any specific works within Hang Hau Tsuen channel

Temporary / mobile station to monitor and audit potential impacts from any specific works, check water quality discharging to Deep Bay, impact station

W5

Upstream of the works at the confluence of San Hing Tsuen Channel and Fung Kong Tsuen Channel

Establish background water quality levels entering Hang Hau Tsuen stream, control station

W6

Waters of Deep Bay

Establish background water quality levels in Deep Bay, control station

W7

Waters of Deep Bay

Establish background water quality levels in Deep Bay, control station

 

Notes:

Subject to the location of the construction works and to be proposed by the ET Leader.

5.12          Summary

 

5.12.1    The current water quality of Hang Hau Tsuen stream and Deep Bay in the Study Area is relatively poor as these water bodies receive a variety of polluting inputs including domestic wastewater from unsewered villages and effluent from open storage sites and container yards.

 

5.12.2    The construction of the channel could adversely impact the water bodies through silt-laden site runoff, disturbance of stream sediment during excavation, concreting works, runoff from workshops & depot and increased sewage and wastewater resulting from the additional workforce on site. These impacts can be readily mitigated through the construction of a suitable drainage system with silt traps, good site management practices, careful working practices when excavating sediments and proper sewage collection and disposal system.

 

5.12.3    The Project will not generate any new pollution loading to Deep Bay. Provision of catchpits with sand traps will remove some of the pollutants from the surface runoff at Hang Hau Tsuen before being discharged into Deep Bay. The impact can be further mitigated through the incorporation of natural stream bed section into the drainage channel design to allow recolonization of benthic communities and vegetation. The re-established vegetation can acts as a natural pollutant trap and filter. The proposed channel will require regular maintenance to ensure effective functioning, to prevent the unacceptable build up of sediments and to remove obstructions. The routine maintenance of the channel may pose potential impacts if it is not properly executed. By following the recommended good practice guides during maintenance works, no adverse impacts are expected.

 

5.12.4    The provision of the access road and viewing point and carpark would have minimal impact during operation with the incorporation of gullies and silt / grit traps in the road drainage design.

 

5.12.5    With the implementation of the recommended mitigation measures, no adverse residual water quality impact is expected during construction and operation phases of the Project.

 

5.13          References

 

EPD (2006) Marine Water Quality Monitoring in Hong Kong in 2005

EPD (2007) Marine Water Quality Monitoring in Hong Kong in 2006

EPD (2008) Marine Water Quality Monitoring in Hong Kong in 2007