14 Landfill Gas Hazard

14.1 Introduction

14.1.1 According to the EIA Study Brief, the landfill gas hazards assessment should include a qualitative risk assessment and landfill gas precautionary/protection design. The assessment should include the following tasks:

· review of background information and studies related to the nearest landfills involved;

· identification of the nature and extent of the sources, the possible pathways through the ground, underground cavities, utilities or groundwater and the potential targets associated with the proposed facilities;

· qualitative assessment on the degree of risk which the hazardous emissions may pose to the target for each of the source-pathway-target combinations;

· design of suitable precautionary measures and the types of protection measures necessary for the construction, operation and decommissioning of the proposed developments and associated works; and

· identification of monitoring requirement for assessing the adequacy and performance of the implemented protection measures.

14.2 Government Legislation and Standards

14.2.1 The criteria for assessing landfill gas hazard are outlined in Annex 7 and Annex 19 of the Technical Memorandum on Environmental Impact Assessment Ordinance (EIAO-TM), respectively.

14.2.2 In additional, Environmental Protection Department (EPD) has issued two guidance notes regarding landfill gas hazard assessment, namely ProPECC PN 3/96 Landfill Gas Hazard Assessment for Development Adjacent to Landfill and EPD/TR8/97 Landfill Gas Hazard Assessment - Guidance Note. These two guidance notes set out the conditions under which a landfill gas hazard assessment should be carried out, and provide guidance on undertaking landfill gas hazard assessment. The guidance notes recommend that in general, assessment of landfill gas hazard is required for proposed developments that lie within the 250m Consultation Zone around a landfill.

14.2.3 As the proposed development falls within the 250m Consultation Zone of TKO Stage I, II & III Landfill and Sai Tso Wan Landfill, a landfill gas hazard assessment is required to be undertaken.

14.2.4 The guidance notes recommend that at the planning stage of a development (when no detailed landfill gas site investigation at the development site has been carried out), the landfill gas hazard assessment should be of qualitative nature.

14.3 Description of the Environment

Proposed Development

14.3.1 The TKO further development Study Area includes existing and planned developments in the whole of TKO and East Kowloon including Lei Yue Mun, Yau Tong, Lam Tin and Cha Kwo Ling areas, and Junk Bay. The key elements of the TKO further development comprise the Western Coast Road (WCR), the Cross Bay Link (CBL), a new development at Town Centre South (TCS) and new development at Pak Shing Kok (PSK). The site areas comprise:

· WCR – the western shoreline of Junk Bay, Devil’s Peak, Lei Yue Mun Road, Eastern Harbour Crossing ex-casting basin, Kaolin Mine Site at Cha Kwo Ling and Cha Kwo Ling Road;

· CBL – Junk Bay, land area south of TKO Area 86 and Wan Po Road;

· TCS – the newly reclaimed land south of Po Yap Road, Eastern Drainage Channel and the toe along TKO Stage I Landfill;

· TKL – the newly reclaimed land to the east of Ocean Shores (south of Chui Ling Road); and

· PSK – the three large terraces to the east of TKO Stage I Landfill and Wan Po Road.

Tseung Kwan O Stage I Landfill

History of TKO Stage I Landfill

14.3.2 TKO Stage I Landfill constitutes a marine reclamation covering an area of about 68 ha. Landfilling commenced in 1978 directly onto marine sediments. Northern and southern sections of seawall were then constructed from quarry rock, without prior dredging. The remaining connecting section of seawall was constructed some years later, and was intended to be more permanent in nature. Separating the seawall from the landfill is a 17.5 m wide soil margin.

14.3.3 Between 1978 and 1990, the landfill received approximately 11.9 million tonnes of waste material. The landfill was designated as a co-disposal site and has received, in addition to municipal waste, a range of waste types including construction, industrial and chemical waste. When full, the site was covered over on its northern and western sections with a 2.6 m thick restoration cap. The cap is composed of topsoil, completely decomposed volcanic (CDV) material, and granular drainage material. However, the southern and eastern sections of the site were covered only by a thin layer of CDV material in anticipation of further waste deposition. Both planted and self‑seeded vegetation was established across the site. The vegetation showed no signs of stress from landfill gas (LFG) before a large part of it was removed for additional waste input.

14.3.4 In 1992, the then Civil Engineering Department (CED) let a contract for further on‑site development at TKO Stage I. The works comprised the stripping of the previous capping material for re‑use, and further reclamation to the south of the site. An embankment was constructed along the southern boundary to allow further waste disposal. A permanent seawall was also constructed approximately 100 m from the previous seawall providing capacity for a further 3.2 million m³ of waste. Areas of this new section were lined in an attempt to prevent direct discharges of leachate to the sea. Following the completion of these works, the landfill was capped with 1m of inert cover and hydro‑seeded.

Restoration Works of TKO Stage I Landfill

14.3.5 The restoration works for TKO Stage I commenced in July 1997 and were completed in January 1999. The works included installation of an engineered capping layer, a landfill gas collection system with flaring and electricity generation, a leachate collection system, surface and sub‑surface drainage systems, and works to increase geotechnical stability. The aftercare period commenced from February 1999 onwards.

Capping of TKO Stage I Landfill

14.3.6 The components of the landfill restoration capping system include the following (from top to bottom):

· A 850 mm thick soil layer comprising completely decomposed volcanic (CDV) material or completely decomposed granite (CDG); an additional 650 mm CDV is also provided in the location where trees or shrubs are provided;

· A subsoil drainage layer comprising a synthetic drainage medium, surrounded by suitable geotextile filters.

· An impermeable layer (anchored in CDV at the perimeter) comprising a 1mm thick linear low density polyethylene (LLDPE) geomembrane; and

· The final intermediate layer of well compacted 500 mm thick soil, free from stones or other sharp particles, above the waste.

Landfill Gas Management System of TKO Stage I Landfill

14.3.7 The landfill gas management system consists of active extraction wells, electricity generation from LFG, flaring system for LFG, passive vent trenches/ pipes, and monitoring of LFG both on and off‑site. The LFG gas extraction system is integrated with the leachate management system.

14.3.8 LFG is collected from the landfill by active gas extraction and is transferred to the on site gas utilization system for electricity generation. Surplus LFG is flared at the gas flaring plant at the southeast of the landfill. The system prevents LFG from migrating off-site.

Leachate Management System of TKO Stage I Landfill

14.3.9 TKO Stage I Landfill is an old landfill originally designed/built without a lining system or leachate collection system underneath. A capping system was installed on the landfill to prevent any infiltration of surface runoff into the landfill. A leachate extraction system was installed to the west of the landfill to control the level of leachate on the landfill slope. The leachate extraction / management system is integrated with the LFG management system. The collected leachate is stored temporarily at the gas/ leachate compound at the south of the landfill. It is then transferred to a leachate treatment facility built at the TKO Stage II/III Landfill for treatment before discharge to the sewage treatment plant at Area 85.

Tseung Kwan O Stage II/III Landfill

History of TKO Stage II/III Landfill

14.3.10 TKO Stage II/III is a valley landfill sited in a coastal location south-east of TKO Stage I Landfill. It occupies an area of about 42 hectares. Engineering preparation works were carried out prior to the start of landfilling in 1988. A permanent seawall on a dredged foundation was constructed to the seaward boundary. There is a 15m wide margin of CDV material behind the seawall, and between this and the waste deposit there is a 3m wide trench constructed in coarse aggregate with a continuous length of perforated pipe. The trench forms a leachate interception and collection zone, together with a vent trench for LFG. Collected leachate flows to TKOSTW to the north of the site.

14.3.11 Inert materials were used to raise the formation of the landfill base above sea level. The site was not totally lined, although discrete areas of low permeability membrane were laid, which drain leachate into the leachate collection system. TKO Stage II/III Landfill actual operation to receive waste began in 1988 and ended in 1994. Deposited waste at this site included municipal, construction, industrial and chemical waste. It is estimated that the landfill has received 17 million tonnes of wastes and the density of the deposited waste is approximately 1.3 tonne/m³. The site was temporarily restored by the end of 1995 with an interim cap of 1m of inert cover, hydroseeded, with surface and sub-surface drains installed.

14.3.12 The restoration works of TKO Stage II/III Landfill commenced in July 1997 and completed in January 1999. The aftercare period commenced from February 1999 onwards.

Restoration Works of TKO Stage II/III Landfill

14.3.13 The restoration works generally included installation of an engineered capping layer, a landfill gas collection system with flaring and electricity generation, a leachate collection and treatment system, surface and sub‑surface drainage systems, and works to increase geotechnical stability.

Capping of TKO Stage II/III Landfill

14.3.14 The components of the landfill restoration capping system at TKO Stage II/III Landfill are generally similar to TKO Stage I Landfill as described in section 14.3.6.

Landfill Gas Management System of TKO Stage II/III Landfill

14.3.15 The landfill gas management system at TKO Stage II/III Landfill is generally similar to the TKO Stage I Landfill as mentioned in section 14.3.7 & 14.3.8.

Leachate Management System of TKO Stage II/III Landfill

14.3.16 Leachate management system comprises a leachate collection system and a leachate treatment works. Leachate generated at TKO Stage II/III Landfill is intercepted by the leachate collection system, which then transfers the collected leachate to the on site leachate treatment plant. Leahcate is treated at the treatment plant to meet the discharge standards before it is discharged to the public sewer.

Sai Tso Wan Landfill

History of Sai Tso Wan Landfill

14.3.17 Sai Tso Wan Landfill was operated between 1978 and 1981. It covers about 14 ha and is estimated to have received about 1.6 million tones of domestic and industrial waste. Waste is believed to be up to 50m deep. Prior to waste deposition, the site was underlain by French drains for the collection of groundwater and leachate. The drains were constructed above and separated by a PVC membrane. Side slopes were sealed with bitumenised chunam (Urban Landfills Restoration Contract).

14.3.18 On completion of waste deposition, the landfill was capped with a final cover layer of silty sand containing a trace of clay. The final cover comprises depths averaging 7.5m on the +80mPD afteruse platform and 3.5m on the side slopes.

Restoration Works and Monitoring of Sai Tso Wan Landfill

14.3.19 The restoration works under the Urban Landfills Restoration Contract on the Sai Tso Wan Landfill commenced in April 1997 and were completed in May 1998. Environmental monitoring (e.g. landfill gas, groundwater, leachate, surface water, etc.) and audit was conducted during the Works Period. The Defect Correction Period commenced in May 1998 and ended in April 1999. The Aftercare Period started from May 1998 for 20 years. Sai Tso Wan Landfill was created as a recreation ground and was opened for public use in April 2004.

14.3.20 The restoration works for the landfill include landfill gas management, groundwater management, surface water management, restoration infrastructure, restoration capping system, landscape restoration and slope stability study.

14.3.21 Landfill Gas Monitoring is undertaken during the Aftercare & Afteruse Period, as follows:

· Monthly monitoring of surface emission of landfill gas;

· Monthly monitoring of boreholes and piezometers within and beyond the site boundary;

· Monitoring of buildings and confined spaces;

· Monthly monitoring of controlled landfill gas emissions;

· Monthly monitoring of condensate; and

· Quarterly monitoring of landfill flare.

Landfill Gas Management System of Sai Tso Wan Landfill

14.3.22 An active landfill gas management system was constructed by the Electrical and Mechanical Services Department (EMSD). The system comprises a network of extraction pipes and wells, which deliver the gas to an on-site landfill gas flare facility.

14.3.23 During the restoration works, modification was carried out on the existing active landfill gas extraction system comprising 29 active gas interception and collection wells and a blower/flare facility. Two additional gas extraction wells in the platform area of the site, an additional 3 underground control valves and 1 condensate dripleg were installed for the enhancement of the system capabilities to reduce surface emissions of landfill gas. 4 existing active gas extraction wellhead were relocated outside the barrier trench and 9 new gas monitoring wells were installed at the perimeter of the site in order to supplement the existing 12 gas wells located on and adjacent to the site.

Leachate Management System of Sai Tso Wan Landfill

14.3.24 A leachate collection drain above the liner and a groundwater collection drain below the liner were also installed prior to landfill restoration works. Leachate collected at the site tank and delivered to the Jordan Valley Leachate Pre-treatment Works for treatment before discharge to the foul sewer leading to the Victoria Harbour.

Capping of Sai Tso Wan Landfill

14.3.25 The existing capping system within Sai Tso Wan Landfill was retained and no further works were proposed to the existing capping system under the Urban Landfills Restoration Contract. However, in order to achieve the desirable cross-falls of 1% for the Multi-purpose Pitches, minor reprofiling was performed on the main platform during the Afteruse Period.

Geological Assessment

Geology/ Hydrogeology at TKO Stage I Landfill

14.3.26 TKO Stage I Landfill is a marine reclamation landfill occupying an area of about 68 hectares. According to Hong Kong Geological Survey Map[1], the bedrock of the eastern side of TKO Stage I Landfill comprises fine ash tuffs and eutaxite of the Upper Jurassic age. The landfill was extensively reclaimed from the sea, with natural earth and waste deposited above marine mud.

14.3.27 Groundwater flows westward and south‑westward from the ridge of Clear Water Bay Country Park. Groundwater levels fall significantly from the ridge to the landfill, and finally to the west of the landfill (Eastern Drainage Channel) and the southwest of the landfill (i.e. Junk Bay).

14.3.28 The greatest depth of leachate is in the middle of the landfill where the deposits of waste material are thickest. The hydraulic gradient within the landfill slopes downward towards the sea.

Geology/ Hydrogeology at TKO Stage II/III Landfill

14.3.29 According to Hong Kong Geological Survey Map¹, the geological formation at the west of the landfill is mainly superficial deposits of fill (natural earth and waste) of Holocene Age; at the north and south of the landfill is mainly undivided tuffaceous mud stone, siltstone and breccias of Upper Jurassic Age; at the east of the landfill is mainly trachydacite lava of Upper Jurassic Age; and at the central is mainly alluvium (clay/silt, sand, and gravel) of Holocene Age.

14.3.30 There are a limited number of drillholes in rock strata and boreholes in waste for groundwater and leachate monitoring. These indicate a relatively consistent hydraulic gradient of groundwater/leachate through the wastes to sea level.

Geology/ Hydrogeology at Sai Tso Wan Landfill

14.3.31 According to Hong Kong Geological Survey Map[2], the geological formation beneath Sai Tso Wan Landfill comprises natural earth and waste. On the other hand, the geological formation beneath the area outside the boundary of Sai Tso Wan Landfill comprises fine to medium grained granite. A fault line cuts across the Sai Tso Wan Landfill and links to the offsite area.

14.3.32 Groundwater boreholes are located at the boundary of the landfill site and leachate boreholes are located in the middle of the landfill where the deposits of waste material are thickest. The hydraulic gradient within the landfill slopes downward towards the sea.

Geology at Proposed Development

14.3.33 According to Hong Kong Geological Survey Map^1&2, the geological formation beneath the proposed development in the proximity of Sai Tso Wan Landfill comprises fine to medium grained granite of Jurassic-Cretaceous age. The area adjacent to TKO Stage I & II/III Landfill is reclamation.

14.4 Assessment Methodology

14.4.1 This qualitative landfill gas hazard assessment was undertaken in accordance with the Landfill Gas Hazard Assessment - Guidance Note issued by EPD. The assessment methodology is based on the “Source – Pathway – Target” model. The meaning of these three components are as follows:

· Source – location, nature and likely quantities/ concentrations of landfill gas which has the potential to affect the development.

· Pathway – the ground and groundwater conditions, through which landfill gas must pass in order to reach the development.

· Target – elements of the development that are sensitive to the effects of landfill gas.

14.4.2 The landfill gas source, identified pathway(s), and identified target(s) are then categorised in order to facilitate the assessment process.

14.5 Qualitative Source-Pathway-Target Analysis

Landfill Gas Source

TKO Stage I Landfill

14.5.1 Landfill gas generation is dependent upon a number of factors including temperature, pH, substrate availability, moisture content and oxygen level. Calculations by others[3] indicated that the annual landfill gas production rate by 2004 was predicted at less than 20 million m³yr^-1 assuming moisture content in the waste material of 25%.

14.5.2 A post-restoration monitoring programme commenced in late 1997 under the restoration contract for TKO Stage I landfill. The monitoring is conducted on a regular basis. LFG (including carbon dioxide) and oxygen are monitored at some monitoring wells installed within and outside the boundary of the landfill. The locations of these monitoring wells are shown on Figure 14.1. The available gas monitoring data provided by EPD from April 2003 to March 2004 are tabulated in Appendix 14.1.

14.5.3 As shown in Appendix 14.1, zero level of methane has been detected at the monitoring wells installed along the boundary of the landfill in the vicinity of the proposed development. The carbon dioxide levels varied from 0.0% to 8.6% (with unknown background level) at 9 of the monitoring wells. Taking into account the zero level of methane and that gas control (such as vents and barriers) to prevent gas migration has been installed at the landfill, the landfill gas source of TKO Stage I would be considered Medium.

TKO Stage II/III Landfill

14.5.4 Using the same reference³ with TKO Stage I, the LFG generation rate peaked in 1993, producing a collectable volume of nearly 200 million m³yr^-1. Assuming a moisture content in the waste material of 25%, the annual production rate by 2004 was predicted to be about 22 million m³yr^-1.

14.5.5 A post-restoration monitoring programme commenced in July 1997 under the restoration contract for TKO Stage II/III landfill. LFG (including carbon dioxide) and oxygen are monitored at some monitoring wells installed within and outside the boundary of the landfill. The locations of these monitoring wells are shown on Figure 14.2. The available gas monitoring data provided by EPD from April 2003 to March 2004 are tabulated in Appendix 14.2.

14.5.6 As shown in Appendix 14.2, zero level of methane has been detected at the monitoring wells installed along the boundary of the landfill in the vicinity of the proposed development. The carbon dioxide levels varied from 0.0% to 8.9% (with unknown background level) at 16 of the monitoring wells. Taking into account the zero level of methane and the fact that the landfill site has gas control (such as vents and barriers) to prevent migration, the landfill gas source of TKO Stage II/III would also be considered Medium.

Sai Tso Wan Landfill

14.5.7 According to Urban Landfills – Monthly Report of March 2004 for Landfill Gas Management System[4], the average landfill gas flow rate at the flare station was 320 m³/hr.

14.5.8 As part of the Restoration Contract for Urban Landfills, the restoration monitoring programme commenced in April 1997. LFG (including carbon dioxide) and oxygen have been monitored monthly at monitoring probes installed at the landfill (within and outside the site boundary of the landfill). The locations of the gas monitoring probes located at Sai Tso Wan Landfill are shown in Figure 14.3. The corresponding monitoring data from April 2003 to March 2004 are also provided in Appendix 14.3.

14.5.9 It is shown in Appendix 14.3 that the concentration of methane at the gas probes located at the Sai Tso Wan Landfill ranged from 0.0% to 0.8% with maximum concentration recorded at GW1 during January 2004 and GW7 during March 2004 whilst the concentration of carbon dioxide at the gas probes located at the landfill ranged from 0.0% to 7.0% (background level = 3.1%) with maximum concentration recorded at G2C during March 2004. With reference to Section 3.10 of Landfill Gas Hazard Assessment Guidance Note (EPD/TR8/97), the source of LFG at Sai Tso Wan Landfill is categorised as Medium.

Migration Pathways

Natural Pathways

14.5.10 For the natural pathways, the presence of natural cavities is directly related to the prevailing local geology. The geological formation beneath TKO Stage I & Stage II/III Landfill and beneath the proposed project site mainly comprises earth and waste. There are no fault lines or cracks existing either the TKO Stage I or the Stage II/III Landfill and the project site. The path length between TKO Stage I Landfill and the project site will be about 50-100m and the path length between TKO Stage II/III Landfill and the project site will be about 100-250m. Therefore, the natural pathways are categorised as Moderately Short/Direct for TKO Stage I Landfill and Long/Indirect for Stage II/III Landfill. Geology map for this case is illustrated in Figure 14.4a-b. A cross section between TKO Stage I Landfill and the project site is illustrated in Figure 14.5a-b and a cross section between Tseung Kwan O Stage II/III Landfill and the project site is illustrated in Figure 14.6a-b.

14.5.11 On the other hand, the geological formation beneath the proposed project site near Sai Tso Wan Landfill mainly comprises fine to medium-grained granite of the Jurassic Cretaceous age. Although the path length between Sai Tso Wan Landfill and the project site will be about 100-250m, there is a fault running between Sai Tso Wan Landfill and the project site. Therefore, the natural pathways are categorised as Short/direct. Geology map for this case is illustrated in Figure 14.7a-b. A cross section between Sai Tso Wan Landfill and the project site is illustrated in Figure 14.8a-b.

Man-made Pathways (Utilities)

14.5.12 Based on the available utility layout plan such as CLP Facility Records Map and Town Gas utility layout plan, no existing utilities or services directly linking either the TKO Stage I & II/III Landfill or the Sai Tso Wan Landfill and the proposed development was identified. As such, the man-made pathways are considered to be Long/Indirect for the three areas.

Targets

14.5.13 In general, potential targets associated with a proposed development include:

· Excavations for utilities installation during construction phase.

· Road works and drainage works during construction phase.

· Manholes, inspection chambers or voids of services/utilities during operational phase.

14.5.14 It is anticipated that deep excavations for the utilities would be involved during the construction phase of the proposed project. According to Section 3.18 of Landfill Gas Hazard Assessment Guidance Note, the level of risk for deep excavations is categorised as Medium.

14.5.15 Some manholes, inspection chambers or voids of services/utilities would be present within the project site during the operational phase of the proposed project. Since access to these confined spaces will be restricted to authoriszed persons who have awareness of LFG hazard, the risk level for these targets is also categorised as Medium.

Summary of Qualitative Source-Pathway-Target Analysis

14.5.16 Based upon the sources, pathways and targets classified above, the qualitative landfill gas hazard assessment for the proposed development is presented in Table 14.1-Table 14.3.

Table 14.1 Qualitative Source-Pathway-Target Analysis of proposed development in proximity to Tseung Kwan O Stage I Landfill

Source	Pathway	Target	Risk
The annual landfill gas production rate by 2004 was predicted to be less than 20 million m^3yr^-1. A post-restoration monitoring programme commenced in late 1997 under the restoration contract for TKO Stage I landfill. The monitoring is conducted on a regular basis. LFG (including carbon dioxide) and oxygen are monitored at some monitoring wells installed within and outside the boundary of the landfill. Zero level of methane has been detected at the monitoring wells installed along the boundary of the landfill in the vicinity of the proposed development. The carbon dioxide levels varied from 0.0% to 8.6% (with unknown background level) at 9 of the monitoring wells. Taking into account the zero level of methane and that gas control (such as vents and barriers) to prevent gas migration has been installed at the landfill, the landfill gas source of TKO Stage I would be considered Medium. (Medium Source)	Natural pathways The geological formation beneath Tseung Kwan O Stage I Landfill and the proposed project site mainly comprises natural earth and waste. There are no fault lines or cracks existing between Tseung Kwan O Stage I Landfill and the project site. The path length between TKO Stage I Landfill and the project site will be about 50-100m. (Moderately Short /Direct Pathway)	Deep excavations during construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target)	Medium
		Maintenance of services during operational phase Manholes or inspection chambers of utilities or services. (Medium Sensitivity Target)	Medium
	Man-made pathways There are no utilities or services directly linking between TKO Stage I Landfill and the proposed road. As such, the man-made pathways are considered to be Long/Indirect. (Long/ Indirect Pathway)	Deep excavations during construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target)	Low
		Maintenance of services during operational phase Manholes or inspection chambers of utilities or services. (Medium Sensitivity Target)	Low

Table 14.2 Qualitative Source-Pathway-Target Analysis of proposed development in proximity to Tseung Kwan O Stage II/III Landfill

Source	Pathway	Target	Risk
Assuming moisture content in the waste material of 25%, the annual landfill gas production rate by 2004 was predicted at about 22 million m^3yr^-1. A post-restoration monitoring programme commenced in July 1997 under the restoration contract for TKO Stage II/III landfill. LFG (including carbon dioxide) and oxygen are monitored at some monitoring wells installed within and outside the boundary of the landfill. Zero level of methane has been detected at the monitoring wells installed along the boundary of the landfill in the vicinity of the proposed development. The carbon dioxide levels varied from 0.0% to 8.9% (with unknown background level) at 16 of the monitoring wells. Taking into account the zero level of methane and the fact that the landfill site has gas control (such as vents and barriers) to prevent gas migration, the landfill gas source of TKO Stage II/III would also be considered Medium. (Medium Source)	Natural pathways The geological formation beneath Tseung Kwan O Stage II/III Landfill and the proposed project site mainly comprises natural earth and waste). There are no fault lines or cracks persisting between Tseung Kwan O Stage II/III Landfill and the project site. The path length between TKO Stage II/III Landfill and the project site will be about 100-250m. (Long /Indirect Pathway)	Deep excavations during construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target)	Low
		Maintenance of services during operational phase Manholes or inspection chambers of utilities or services. (Medium Sensitivity Target)	Low
	Man-made pathways There are no utilities or services directly linking between TKO Stage II/III Landfill and the proposed road. As such, the man-made pathways are considered to be Long/Indirect. (Long/ Indirect Pathway)	Deep excavations during construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target)	Low
		Maintenance of services during operational phase Manholes or inspection chambers of utilities or services. (Medium Sensitivity Target)	Low

Table 14.3 Qualitative Source-Pathway-Target Analysis of proposed development in proximity to Sai Tso Wan Landfill

Source	Pathway	Target	Risk
According to Urban Landfills – Monthly Report for Landfill Gas Management System, March 2004 (prepared by the Hong Kong Landfill Restoration Group), the average landfill gas flow rate at the flare station was 320 m^3/hr. As part of the Restoration Contract for Urban Landfills, the restoration monitoring programme commenced in April 1997. LFG (including carbon dioxide) and oxygen have been monitored monthly at monitoring probes installed at the landfill (within and outside the site boundary of the landfill). The concentration of methane at the gas probes located at the Sai Tso Wan Landfill ranged from 0.0% to 0.8% with maximum concentration recorded at GW1 during January 2004 and GW7 during March 2004 whilst the concentration of carbon dioxide at the gas probes located at the landfill ranged from 0.0% to 7.0% (background level = 3.1%) with maximum concentration recorded at G2C during March 2004. With reference to Section 3.10 of Landfill Gas Hazard Assessment Guidance Note (EPD/TR8/97), the source of LFG at Sai Tso Wan Landfill is categorised as Medium. (Medium Source)	Natural pathways The geological formation beneath the proposed project site near Sai Tso Wan Landfill mainly comprises fine to medium-grained granite of the Jurassic Cretaceous age. Although, the path length between Sai Tso Wan Landfill and the project site will be about 100-250m, there is a fault running between Sai Tso Wan Landfill and the project site. Therefore, the natural pathways are categorised as Short/direct. (Short /Direct Pathway)	Deep excavations during construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target)	Medium
		Maintenance of services during operational phase Manholes or inspection chambers of utilities or services. (Medium Sensitivity Target)	Medium
	Man-made pathways There are no utilities or services directly linking between Sai Tso Wan Landfill and the proposed road. As such, the man-made pathways are considered to be Long/Indirect. (Long/ Indirect Pathway)	Deep excavations during construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target)	Low
		Maintenance of services during operational phase Manholes or inspection chambers of utilities or services. (Medium Sensitivity Target)	Low

14.6 Site Categorisation

14.6.1 According to Landfill Gas Hazard Assessment Guidance Note (EPD/TR8/97), for the purpose of categorising a subject site, the category is based on the highest level of risk nominated for any of the potential impacts identified. For example, a subject site with four low risks and one medium risk will fall into the category of medium level of risk; and a site with four low risks, two medium risks, and one high risk will fall into the category of high level of risk. However, if four or more different impacts arise in a particular risk category, then the overall risk classification may be considered to be one category higher. For instance, a site with three low risks and four medium risks will fall into the category of high level of risk.

14.6.2 Referring to Table 14.1-14.3, the overall hazard level for the proposed project associated with TKO Stage I & II/III and Sai Tso Wan Landfill is Medium for each of the three areas.

14.6.3 In accordance with the EPD’s guidance note, some precautionary measures are required to protect a proposed area of medium or high level of risk. Some recommendations for protection measures for the proposed project (during the construction phase and operation phase) are provided and are presented in the following section.

14.7 Recommended Precautionary Measures for Construction Phase and Operational Phase

Construction Phase

14.7.1 During construction phase, the following precautionary measures should be implemented:

a. The Contractor should take special precautions regarding the possible presence of landfill gas. The Contractor should ensure that the site workers are aware of potential hazards of working in confined spaces such as manholes and service chambers, and that appropriate monitoring procedures as per the Code of Practice on Safety and Health at Work in Confined Spaces (Labour Department, Hong Kong) are in place to prevent hazards due to asphyxiating atmospheres in confined spaces;

b. All relevant construction workers should undergo training about the risks and indications of landfill gas and should be thoroughly skilled in first aid and emergency and evacuation techniques;

c. A no smoking policy should be strictly adopted. The possibility of methane rich air being taken into diesel engine plant should not be overlooked, although this is unlikely to occur;

d. Welding, flame cutting or other hot works should only be carried out in trenches or confined area when controlled by a ‘permit to work’ procedure, properly authorised by the Safety Officer;

e. Ground level construction plant should be fitted with vertical exhausts at least 0.6m above ground level and with spark arrestors;

f. During piping assembly or conduiting construction at underground or at ground level, all valves/seals should be closed immediately after installation. As construction progresses, all valves/seals should be closed as installed to prevent the migration of gases through the pipeline/conduit. All piping/conduiting should be capped at the end of each working day;

g. Mobile offices, equipment stores, mess rooms etc. should be located on an area which has been proven to be gas free and on-going monitoring should be carried out to ensure that these areas remain gas free. Alternatively, such buildings should be raised clear of ground with a minimum clear separation distance (as measured from the highest point on the ground surface to the underside of the lowest floor joist) of 500mm. However, in this case, it is highly recommended that all the site offices, equipment stores and mess rooms should be located outside the 250m LFG consultation zone;

h. As a minimum, no work should be undertaken in the absence of fire extinguishers;

i. Adequate fire extinguishing equipment, fire-resistant clothing and breathing apparatus sets should be made available on site;

j. Monitoring of methane, carbon dioxide and oxygen should be undertaken using appropriately calibrated portable gas detection equipment. Monitoring equipment should have, as a minimum, the indication ranges set out below:

Methane	0-100% Lower Explosive Limit (LEL) and 0-100% v/v
Carbon Dioxide	0-100%
Oxygen	0-21%

k. The monitoring frequency and areas to be monitored should be specified prior to commencement of ground-works, either by the Safety Officer, or by an appropriately qualified person. The minimum monitoring frequency would be twice a day (i.e. the morning before site works commence and after the lunch break) during all excavation works below ground level and should always be undertaken by appropriately qualified personnel. All measurements should be recorded and included in the site diary;

l. For excavations deeper than 1 m, measurements should be carried out:

- at the ground surface before excavation commences;

- immediately before any worker enters the excavation;

- at the beginning of each half working day (i.e. morning and afternoon) for the entire period the excavation remains open; and

- periodically through the working day whilst workers are in the excavation.

m. For excavations between 300 mm and 1 m deep, measurements should be carried out:

- directly after the excavation has been completed; and

- periodically whilst the excavation remains open.

n. Depending on the results of the measurements, actions required will vary and should be set down by the Safety Officer of the Contractor or other appropriately qualified person. As a minimum these should encompass those actions specified in Table 14.4.

Table 14.4 Actions in the Event of Gas Being Detected in Excavations

Parameter	Measurement	Action
Oxygen	< 19 %	· Ventilate to restore oxygen to > 19 %
	< 18 %	· Stop works · Evacuate personnel/prohibit entry · Increase ventilation to restore oxygen to > 19 %
Methane	> 10 % LEL (i.e. > 0.5 % by volume)	· Prohibit hot works · Ventilate to restore methane to < 10% LEL
	> 20 % LEL (i.e. > 1 % by volume)	· Stop works · Evacuate personnel/prohibit entry · Increase ventilation to restore methane to < 10 % LEL
Carbon Dioxide	> 0.5 %	· Ventilate to restore carbon dioxide to < 0.5%
	> 1.5 %	· Stop works · Evacuate personnel/prohibit entry · Increase ventilation to restore carbon dioxide to <0.5%

o. Utility companies should be advised of the proposed development being in the vicinity of the TKO Stage I & II/III Landfill and Sai Tso Wan Landfill for their attention to observe the relevant regulations and requirements in such regards when inspection or works are being undertaken at the nearby area.

Operational Phase

14.7.2 Precautionary measures as listed below are recommended for implementation during operational phase:

a. As this project only includes road works or drainage works with no involvement of any building structure, the main precautionary measures during the operational phase will be mainly during the inspection or maintenance of the utilities services at the area within the 250m LFG Consultation Zone.

b. An assumed presence of landfill gas should be adopted at all times by maintenance workers. All maintenance workers inspecting any manhole should be fully trained in the issue of landfill gas hazard. Any manhole which is large enough to permit to access to personnel should be subject to entry safety procedure. Such work in confined spaces is controlled by the Factories and Industrial Undertakings (Confined Spaces) Regulations of the Factories and Industrial Undertakings Ordinance. Following the Code of Practice on Safety and Health at Work in Confined Spaces (Labour Department, Hong Kong) ensures compliance with the above regulations. A strictly regulated “work permit procedure” should be implemented and the relevant safety procedures must be rigidly followed. Adequate communication with maintenance staff should be maintained with respect to landfill gas hazard;

c. The utility companies should undertake a landfill gas surveillance exercise at the utility manholes/inspection chambers. The surveillance exercise shall be:

(i) Undertaken using an intrinsically safe portable instrument, appropriately calibrated and capable of measuring the following gases in the ranges indicated:

Methane	0-100% Lower Explosive Limit (LEL) and 0-100% v/v
Carbon Dioxide	0-100%
Oxygen	0-21%

(ii) Undertaken for the duration of the site occupancy, or until such time that EPD agree that surveillance is no longer required;

(iii) Depending on the results of the measurements, actions required will vary and should be set down by appropriately qualified person. As a minimum these shall encompass those actions specified in Table 14.5.

Table 14.5 Actions in the Event of Gas Being Detected during Operational Phase

Parameter	Measurement	Action
Oxygen	< 19 %	· Ventilate to restore oxygen to > 19 %
	< 18 %	· Stop works · Evacuate personnel/prohibit entry · Increase ventilation to restore oxygen to > 19 %
Methane	> 10 % LEL (i.e. > 0.5 % by volume)	· Prohibit hot works · Ventilate to restore methane to < 10% LEL
	> 20 % LEL (i.e. > 1 % by volume)	· Stop works · Evacuate personnel/prohibit entry · Increase ventilation to restore methane to < 10 % LEL
Carbon Dioxide	> 0.5 %	· Ventilate to restore carbon dioxide to < 0.5%
	> 1.5 %	· Stop works · Evacuate personnel/prohibit entry · Increase ventilation to restore carbon dioxide to <0.5%

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[1] Hong Kong Geological Survey map, Sheet 12, Series HGM20, Edition 1 ‑ 1989 (scale 1:20,000) (published by Geotechnical Control Office in 1989)

[2] Hong Kong Geological Survey map, Sheet 11, Series HGM20 Edition 1- 1986 (scale 1:20,000) (published by Geotechnical Control Office in 1986)

[3] Scott Wilson Kirkpatrick (in association with Aspinwall & Company) (Oct 1994) Agreement CE 11/92 Restoration of Tseung Kwan O Landfills – Final Report.

[4] Urban Landfills – Monthly Report for Landfill Gas Management System, March 2004 (prepared by the Hong Kong Landfill Restoration Group)