7. Landfill gas hazard

7.1 Introduction

7.1.1 Under Annexes 7 and 19 of the Technical Memorandum of the Environmental Impact Assessment Ordinance (EIAO-TM), landfill gas (LFG) hazard assessment is required for any development or re-development within the Consultation Zone, i.e. the area surrounding the landfill boundary as defined by a line running parallel to and 250m away from the edge of the waste. The Project falls within the Consultation Zone (CZ) of Gin Drinker Bay Landfill (GDBL). Therefore, LFG hazard assessment is required in this EIA Study.

7.1.2 EPD has issued the Landfill Gas Hazard Assessment Guidance Note (EPD/TR8/97) (the Guidance Note), which provides a risk assessment framework for developments proposed close to landfill sites. Generally, a qualitative landfill risk assessment is required to ensure that appropriate levels of safety design features are incorporated within the development.

7.2 Assessment Scope

7.2.1 This assessment has been undertaken with close reference to the Guidance Note and in line with Annexes 7 and 19 of the EIAO-TM. The following tasks have been undertaken:

· review of background information relating to the GDBL;

· identification of the sources, nature and likely quantities/concentrations of LFG emissions with the potential to affect the TWR Upgrading works;

· identification of likely viable pathways through the ground, underground cavities, utilities or groundwater via which LFG may reach the TWR Upgrading works;

· the identification of TWR Upgrading works that would be sensitive to the effects of the LFG emissions;

· a qualitative assessment of the degree of risk that the LFG emissions may pose to the TWR Upgrading works for the identified source-pathway-target combination; and

· the proposal of appropriate measures to minimise landfill gas hazard during construction and operation.

7.2.2 The location of TWR relative to GDBL is indicated in Figure 7-1.

7.3 Potential Hazards Associated with Landfill Gas

7.3.1 LFG is a flammable, toxic and asphyxiating mixture of methane and carbon dioxide, often with a trace of volatile compounds. It is a product of the anaerobic decomposition of solid wastes.

7.3.2 Methane (CH₃) is one of the major components of LFG. When methane is mixed with air within the lower and upper explosive limits (LEL and UEL, 5 to 15% by volume) in confined spaces and given a source of ignition such as an electrical spark, an explosion can result. Methane is also asphyxiant, odourless and colourless and its presence (or absence) can only be confirmed by using appropriately calibrated detectors.

7.3.3 Carbon dioxide (CO₂) is the other major component of LFG. It is asphyxiating and causes adverse health effects at relatively low concentrations. The long-term Occupational Exposure Limit (OEL) is 0.5% by volume. Like methane, in the pure form, it is odourless and colourless and its presence (or absence) can only be confirmed by using appropriately calibrated detectors.

7.3.4 The LFG mixture has a density similar to air, although this varies according to its exact composition. Upward movement of LFG is usually a result of excess pressure over the ambient rather than buoyancy. Also, bulk gas movements may be caused by the pumping effect of a rising water table, whereas subsurface lateral diffusion through semi-porous strata and cracks may occur due to a concentration gradient.

7.3.5 LFG is capable of migrating away from its source along permeable pathways such as cracks and fissures in the surrounding rock and other preferential pathways such as utility routes. It has been known to travel along these pathways more than 200m away from the source.

7.4 Assessment Methodology

7.4.1 The methodology set out in the Guidance Note was adopted in this assessment. For ease of reference, key points of the Guidance Note are reproduced in the following sections.

Landfill Gas Assessment Criteria

7.4.2 The risk due to LFG may be evaluated based upon the following three components:

· Source – the location, nature and likely quantities/concentrations of LFG generated by GDBL;

· Pathway - the nature and length of potential pathways through which LFG could migrate (such as geological features, utility services and leachate flow); and

· Target - the level of vulnerability of the development.

7.4.3 These components and their evaluation criteria are discussed in the following paragraphs.

Source

7.4.4 The source of LFG (i.e. GDBL) should be evaluated with respect to the classifications as follows:

· Minor – Landfill sites at which gas controls have been installed and proven to be effective by comprehensive monitoring that has demonstrated that no migration of gas beyond the landfill boundary (or any specific control measures). The landfill must have measures to control gas that do not rely solely on an active gas extraction system or any other single control measure which is vulnerable to failure; or old landfill sites where the maximum concentration of methane within the waste, as measured at several location across the landfill and on at least four occasions over a period of at least six months, is less than 5% by volume (v/v).

· Medium – Landfill sites at which some forms of gas control have been installed (e.g. a lined site or one where vents or barriers have been retrospectively installed), but where there are only limited monitoring data to demonstrate prevention of off-site gas; or landfill sites where comprehensive monitoring has demonstrated that there is no migration of gas beyond the landfill boundary but where the control of gas relies solely on an active gas extraction system or any other single control system that is vulnerable to failure.

· Major – Recently filled landfill sites at which there is little or no control to prevent migration of gas or at which the efficacy of the gas control measures has not been assessed; or any landfills site at which monitoring has demonstrated that there is significant migration of gas beyond the site boundary.

Pathway

7.4.5 The broad classification of the pathway is as follows:

· Very Short/Direct – Path length of less than 50m for unsaturated permeable strata and fissured rock or less than 100 m for anthropogenic conduits.

· Moderately Short/Direct – Path length of 50 to 100m for unsaturated permeable soil or fissured rock or 100 to 250m for anthropogenic conduits.

· Long/Indirect – Path length of 100 to 250m for unsaturated permeable soils and fissured rock.

Target

7.4.6 Target may be classified according to the following evaluation criteria:

· High Sensitivity – Buildings and structures with ground level or below ground rooms/voids or into which services enter directly from the ground and to which members of the general public have unrestricted access or that contain sources of ignition. This would include developments where there is a possibility of additional structures being erected directly on the ground on an ad hoc basis and without regard to the potential risks.

· Medium Sensitivity – Other buildings, structures or service voids where there is access only by authorised, well trained personnel, such as the staff of utility companies, who have been briefed on the potential hazards relating to LFG and the specific safety procedures to be followed; and deep excavations.

· Low Sensitivity – Buildings/structures that are less prone to gas ingress by virtue of their design (such as those with a raised floor slab), shallow excavations or developments that involve essentially outdoor activities but where evolution of gas could pose potential problems.

Qualitative Assessment of Risk

7.4.7 Once the status of each of the source, pathway and target have been evaluated against the above criteria, a qualitative assessment of the overall risk may be made with reference to Table 7-1.

Table 7‑1 Qualitative Risk Assessment Category

Source	Pathway	Target Sensitivity	Risk Category
Major	Very Short / Direct	High	Very High
		Medium	High
		Low	Medium
	Moderately Short / Direct	High	High
		Medium	Medium
		Low	Low
	Long / Indirect	High	High
		Medium	Medium
		Low	Low
Medium	Very Short / Direct	High	High
		Medium	Medium
		Low	Low
	Moderately Short / Direct	High	High
		Medium	Medium
		Low	Low
	Long / Indirect	High	Medium
		Medium	Low
		Low	Very Low
Minor	Very Short / Direct	High	High
		Medium	Medium
		Low	Low
	Moderately Short / Direct	High	Medium
		Medium	Low
		Low	Very Low
	Long / Indirect	High	Medium
		Medium	Low
		Low	Very Low

7.5 Description and History of Gin Drinkers Bay Landfill

Landfill History

7.5.1 Landfilling commenced in 1960 with waste being tipped into the open waters of Gin Drinkers Bay from the northern shoreline (now the north-east corner of GDBL) and the site became a “controlled tip” in 1973 before closing in February 1979. The 29ha landfill received approximately 11.4Mm³ of waste. The thickness of the waste increases from the perimeter towards the centre platform at about +45mPD. Under the platform, the base of the waste is approximately -10mPD, giving a maximum waste depth of approximately 55m.

7.5.2 GDBL was capped using 1 to 2 m completely decomposed granite (CDG). Passive LFG vents were installed around the perimeter but there were no provisions for the collection of leachate, which seeped out of the base of the landfill and into the adjacent Rambler Channel. When leachate levels were elevated, leachate often overflowed onto low-lying parts of the roads and footpaths surrounding the site.

7.5.3 Following completion of the filling and placement of the cover soils, several thousand trees were planted on the site and preliminary works for development of the site as a community park (Kwai Chung Park) were undertaken by Regional Services Department. These works were suspended in the early 1990s as plans were finalised for the construction of a MTRC viaduct that cut through part of the site.

7.5.4 In common with the other closed landfill sites in Hong Kong, restoration works were deemed necessary to reduce the potential health and environmental risks associated with LFG and landfill leachate, and to allow beneficial afteruse of the site. The restoration works for GDBL were carried out as part of a Design-Build-Operate (DBO) contract led by Government, under the management of EPD. Restoration works commenced in February 1999 and were completed in September 2000. The Restoration Contractor installed an active LFG collection system, including gas headers, gas extraction wells, a flare and an electricity generator. A new leachate collection system was installed, as controlling leachate levels was considered to be of paramount importance because of its detrimental effects on the MTRC viaduct. A Leachate Treatment Plant (LTP) was constructed on a platform below the MTRC viaduct to treat leachate from the landfill. A new geosynthetic capping system was installed on top of the platforms, with the heavily vegetated side slopes left uncapped. Prior to landscaping, a number of groundwater, LFG and leachate monitoring wells were installed. The landfill site is developed as a green zone currently.

Geology and Hydrogeology

7.5.5 The deposited waste is underlain by marine deposits and alluvial / colluvial sediments, that are in turn underlain by varying thicknesses of moderately or completely decomposed granite bedrock.

Description of Tsuen Wan Road Upgrading within the CZ

7.5.6 The works to be carried out as part of the TWR Upgrading within the CZ are as follows:

· construction of retaining walls alongside TWR;

· demolition and reprovision of footbridge from the east of TWR leading to GDBL; and

· paving works alongside TWR, adjacent to GDBL.

7.6 Landfill Gas Risk Assessment

Source

7.6.1 GDBL contains a substantial volume of waste and is a major source of gas. When initially constructed the landfill site contained few measures to control either LFG or leachate, although some measures were retrospectively installed.

7.6.2 Restoration works were carried out between Year 1999 and Year 2000 with the aim of minimizing the risks associated with off-site migration of LFG and leachate. As part of the Aftercare Provision of the Restoration Contract, monitoring of LFG is being carried out since completion of the restoration works. The monitoring results are summarized in Tables 7-2 and 7-3 and the locations of monitoring points are shown in Figure 7-2.

Table 7‑2 Methane Concentrations at Gin Drinkers Bay Landfill

	Methane Concentrations at Different Monitoring Points at GDBL (% by volume)
	GDB5	GDB6	GDB7	GDB10	GDBGW7	G1	SV23B	SV24B
Compliance Limit	1	1	1	1	1	1	1	1
Oct-01	<0.1	0.8	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Nov-01	<0.1	0.9	<0.1	0.3	<0.1	<0.1	<0.1	---
Dec-01	<0.1	<0.1	0.4	<0.1	<0.1	<0.1	<0.1	<0.1
Jan-02	<0.1	0.4	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Feb-02	<0.1	0.5	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Mar-02	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Apr-02	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
May-02	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jun-02	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jul-02	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Aug-02	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Sep-02	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Oct-02	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Nov-02	<0.1	<0.1	<0.1	0.2	<0.1	<0.1	<0.1	<0.1
Dec-02	<0.1	0.8	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jan-03	<0.1	0.6	<0.1	0.2	<0.1	<0.1	<0.1	<0.1
Feb-03	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Mar-03	<0.1	<0.1	<0.1	0.6	<0.1	<0.1	<0.1	<0.1
Apr-03	<0.1	0.9	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
May-03	<0.1	3.2	<0.1	<0.1	<0.1	0.2	<0.1	<0.1
Jun-03	<0.1	1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jul-03	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Aug-03	<0.1	0.2	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Sept-03	---	<0.1	<0.1	<0.1	---	<0.1	---	---
Oct-03	---	<0.1	<0.1	<0.1	---	<0.1	---	---
Nov-03	---	<0.1	<0.1	<0.1	---	<0.1	---	---
Dec-03	---	0.3	<0.1	<0.1	---	<0.1	---	---
Jan-04	---	<0.1	<0.1	<0.1	---	<0.1	---	---
Feb-04	---	1.5	<0.1	<0.1	---	<0.1	---	---
Mar-04	---	0.7	0.5	0.2	---	<0.1	---	---
Apr-04	---	<0.1	0.3	0.2	---	<0.1	---	---
May-04	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jun-04	<0.1	0.5	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jul-04	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Aug-04	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Sept-04	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Oct-04	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Nov-04	<0.1	<0.1	<0.1	0.2	<0.1	<0.1	<0.1	<0.1
Dec-04	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jan-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Feb-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Mar-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Apr-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
May-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jun-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jul-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Aug-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Sept-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Oct-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Nov-05	<0.1	1.4	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Dec-05	<0.1	0.6	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jan-06	<0.1	0.3	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Feb-06	<0.1	0.3	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Mar-06	<0.1	5.9	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Apr-06	<0.1	7.6	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
May-06	<0.1	0.7	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jun-06	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jul-06	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Aug-06	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Sept-06	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Oct-06	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Nov-06	<0.1	1.2	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Dec-06	<0.1	3.7	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jan-07	<0.1	4.6	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Feb-07	<0.1	9.7	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Mar-07	<0.1	8.6	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Apr-07	<0.1	9.4	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Notes: ---: Data Not Available Bold: Value exceeds compliance limit Source: Special Waste and Landfill Restoration Group, Environmental Protection Department

Table 7‑3 Carbon Dioxide Concentrations at Gin Drinkers Bay Landfill

	Carbon Dioxide Concentrations at Different Monitoring Points at GDBL (% by volume)
	GDB5	GDB6	GDB7	GDB10	GDBGW7	G1	SV23B	SV24B
Compliance Limit	18.1	21.7	50.5	54.9	12.5	18.7	2	8.7
Oct-01	<0.1	0.9	16.2	18.5	<0.1	7.2	0.1	<0.1
Nov-01	<0.1	3.5	0.3	19.4	<0.1	8.8	<0.1	---
Dec-01	<0.1	<0.1	19.8	19.6	<0.1	8.5	<0.1	<0.1
Jan-02	<0.1	0.6	<0.1	0.1	0.8	8.1	<0.1	1.8
Feb-02	<0.1	1.1	<0.1	<0.1	<0.1	7.9	<0.1	<0.1
Mar-02	<0.1	0.1	<0.1	<0.1	1	8.9	<0.1	<0.1
Apr-02	<0.1	<0.1	13.4	2.4	2.2	7.7	<0.1	<0.1
May-02	<0.1	<0.1	<0.1	<0.1	1.2	9.7	<0.1	<0.1
Jun-02	<0.1	4.2	12.5	1.2	2.4	9.6	<0.1	<0.1
Jul-02	0.3	6.1	<0.1	<0.1	3.7	8.1	<0.1	<0.1
Aug-02	<0.1	7.1	2.3	7.1	4.6	5.5	<0.1	<0.1
Sep-02	<0.1	8.6	7	<0.1	5.4	8	<0.1	<0.1
Oct-02	<0.1	7.4	<0.1	<0.1	0.5	8.9	<0.1	<0.1
Nov-02	<0.1	5.8	21.6	15.7	3.3	10.4	<0.1	<0.1
Dec-02	<0.1	4.2	14.9	17.6	<0.1	10.5	<0.1	<0.1
Jan-03	<0.1	3.4	15.3	4.8	7.1	10.3	<0.1	<0.1
Feb-03	<0.1	2.6	19.8	1.6	3	9.6	<0.1	<0.1
Mar-03	<0.1	3.7	5.6	14.8	0.3	10.4	<0.1	<0.1
Apr-03	<0.1	6.6	7.7	6.2	12.5	11	<0.1	<0.1
May-03	<0.1	6.1	0.1	0.3	4.4	9.3	<0.1	<0.1
Jun-03	<0.1	0.1	7.2	8.0	0.2	<0.1	<0.1	<0.1
Jul-03	<0.1	5.9	12.5	0.3	1.7	10.2	<0.1	<0.1
Aug-03	<0.1	8.2	4.2	1.7	3	11.8	<0.1	<0.1
Sept-03	---	0.6	3.2	12.8	---	8.7	---	---
Oct-03	---	5.1	0.4	<0.1	---	12.6	---	---
Nov-03	---	<0.1	1.1	4.6	---	13.5	---	---
Dec-03	---	4.8	6.5	2.8	---	11.5	---	---
Jan-04	---	<0.1	14.5	12.9	---	11.7	---	---
Feb-04	---	1.2	1.2	14.1	---	10.2	---	---
Mar-04	---	0.7	15.2	10.3	---	11.6	---	---
Apr-04	---	0.2	7.9	5.9	---	10.3	---	---
May-04	<0.1	<0.1	<0.1	<0.1	<0.1	2.3	<0.1	<0.1
Jun-04	<0.1	9.6	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Jul-04	<0.1	0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Aug-04	<0.1	8.4	<0.1	<0.1	<0.1	0.2	<0.1	<0.1
Sept-04	<0.1	8.7	<0.1	<0.1	<0.1	0.3	<0.1	<0.1
Oct-04	<0.1	3.4	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Nov-04	<0.1	8.1	<0.1	<0.1	<0.1	<0.1	<0.1	0.8
Dec-04	<0.1	6.4	<0.1	<0.1	<0.1	0.9	<0.1	<0.1
Jan-05	<0.1	6.7	<0.1	<0.1	<0.1	0.2	<0.1	0.2
Feb-05	<0.1	7	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Mar-05	<0.1	0.5	<0.1	0.1	<0.1	<0.1	<0.1	<0.1
Apr-05	<0.1	2.9	<0.1	3.9	<0.1	0.6	<0.1	<0.1
May-05	<0.1	0.3	<0.1	0.1	<0.1	<0.1	<0.1	<0.1
Jun-05	<0.1	<0.1	<0.1	4.7	<0.1	<0.1	<0.1	<0.1
Jul-05	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Aug-05	<0.1	3.3	<0.1	6.9	<0.1	0.1	<0.1	<0.1
Sept-05	<0.1	7	<0.1	7.1	<0.1	<0.1	<0.1	0.2
Oct-05	<0.1	3.8	<0.1	13.8	<0.1	5.3	<0.1	<0.1
Nov-05	<0.1	5.9	<0.1	13.8	<0.1	14.1	<0.1	<0.1
Dec-05	<0.1	5.4	<0.1	0.1	<0.1	<0.1	<0.1	<0.1
Jan-06	<0.1	4.8	<0.1	7.4	<0.1	<0.1	<0.1	<0.1
Feb-06	<0.1	<0.1	<0.1	<0.1	<0.1	2.3	<0.1	<0.1
Mar-06	<0.1	9.6	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Apr-06	<0.1	0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
May-06	<0.1	8.4	<0.1	<0.1	<0.1	0.2	<0.1	<0.1
Jun-06	<0.1	8.7	<0.1	<0.1	<0.1	0.3	<0.1	<0.1
Jul-06	<0.1	3.4	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Aug-06	<0.1	8.1	<0.1	<0.1	<0.1	<0.1	<0.1	0.8
Sept-06	<0.1	6.4	<0.1	<0.1	<0.1	0.9	<0.1	<0.1
Oct-06	<0.1	6.7	<0.1	<0.1	<0.1	0.2	<0.1	0.2
Nov-06	<0.1	7	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Dec-06	<0.1	0.5	<0.1	0.1	<0.1	<0.1	<0.1	<0.1
Jan-07	<0.1	2.9	<0.1	3.9	<0.1	0.6	<0.1	<0.1
Feb-07	<0.1	0.3	<0.1	0.1	<0.1	<0.1	<0.1	<0.1
Mar-07	<0.1	<0.1	<0.1	4.7	<0.1	<0.1	<0.1	<0.1
Apr-07	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Notes: ---: Data Not Available Bold: Value exceeds compliance limit Source: Special Waste and Landfill Restoration Group, Environmental Protection Department

7.6.3 As part of the restoration works, an active gas extraction system was put in place along the north-eastern boundary of the GDBL site (the western side of TWR) to collect any LFG along this perimeter for venting via the stack at the treatment plant at Kwai Hei Street. This is in addition to a number of passive gas vents along the side of GDBL adjacent to TWR. Landfill gas control is therefore provided by a mixture of active and passive systems.

7.6.4 The results indicate that there have been 11 exceedances of the compliance criteria for methane, with concentrations ranging from 1.2% to 9.7% by volume at monitoring point GDB6 in May 2003, February 2004, November 2005, March to April 2006, and November 2006 to April 2007.

7.6.5 Carbon dioxide concentrations have ranged from below detection limits to 21.6% (in November 2002 at monitoring point GDB7) by volume. There has been a single reading which reached the compliance limit (in April 2003 at monitoring point GDBGW7). Other than this, the compliance limits for carbon dioxide set out in the GDBL restoration contract have not been exceeded. Background concentrations of carbon dioxide, before restoration works commenced, are reported to be in the range of 0 to 49%.

7.6.6 Although elevated methane concentrations have been recorded at GDB6 (up to 9.7%), the methane concentrations from the perimeter wells (GDB7, GDB10 and GDB5, around the perimeter of GDBL) are in all cases less than 1% by volume. The source of the elevated methane concentrations in GDB6 is believed to be due to localized pockets of waste outside the boundary of GDBL which were deposited during the operation of the landfill, and is unlikely to be a result of migration from GDBL, as indicated by the low concentrations in the perimeter wells, since any such migration would also be evident in the perimeter monitoring points. It is considered necessary to mention explicitly in roadwork contract that waste materials may be encountered during the excavation work and proper handling and disposal of the waste will be needed.

7.6.7 Relatively elevated concentrations of carbon dioxide in certain off-site monitoring points have been recorded on certain occasions. However, these concentrations do not exceed the compliance targets for GDBL and are lower than those measured before the restoration works. Carbon dioxide may be present in soil gas as a result of the oxidation of many different types of organic matter (including leakage of sewage pipes, organic sediments, fuel spillage, other possible waste deposits outside the landfill boundary etc.), and its presence is not necessarily indicative of the migration of landfill gas from GDBL. The potential hazards associated with carbon dioxide include the risk of asphyxiation to workers, especially those working in excavations or semi-confined spaces.

7.6.8 In the TWR Upgrading works, only minor excavation at the slope toe outside the boundary fence of the GDBL site will be carried out. It is considered that no stored waste and landfill restoration facilities of GDBL will be affected by this minor excavation. Furthermore, the slope toe will be designed in accordance with the relevant standards in order to ensure the stability of the landfill slope. Since the Project will encroach into a piece of land allocated to EPD under Temporary Government Land Allocation No. TKT 1704, EPD considers that the narrow strip of land within the site boundary of the Project should be reallocated from EPD to HyD. In this respect, CEDD will circulate a land requirement plan to relevant parties, including HyD, in order to address and solve the land allocation issue in the coming stage.

7.6.9 GDBL is a restored landfill with comprehensive LFG control measures consisting both active and passive LFG management systems and there is no evidence indicating that there is significant migration of LFG beyond GDBL. Also, no excavation will be conducted within the boundary fence of the landfill site and it is considered that the stored waste as well as landfill restoration facilities will not be affected. In view of these, the overall Source term is classified as Medium.

Pathway

7.6.10 Since the TWR Upgrading works are adjacent to GDBL and, in the case of the footbridge, within the GDBL site boundary, and because of the existence of localized pockets of waste outside the GDBL site boundary as mentioned in Section 7.6.6, the potential pathway for migration of gas from GDBL to the works is classified as Very Short/ Direct.

Target

Construction Phase

7.6.11 The existing footbridge will be demolished and reconstructed, a new section of pavement will be installed adjacent to the landfill, and a new retaining wall will be constructed. Only minor excavation at the slope toe outside the boundary fence of the landfill site will be carried and it is considered that stored waste as well as the LFG management facilities will not be affected. Detailed design of the foundations for these structures has not been carried out, and hence the depth of excavations cannot be confirmed. The Target is therefore classified as being of Medium Sensitivity during construction.

Operational Phase

7.6.12 The operational phase will encompass users of the road and footbridge. There are no below-ground voids or enclosed spaces where LFG may accumulate, and the development can be classed as “essentially outdoor activities”. The Target is therefore classified as being of Low Sensitivity during operation.

Assessment of Risk

7.6.13 A qualitative assessment of LFG risk posed by GDBL to the TWR Upgrading works is set out in Table 7-4.

Table 7‑4 Qualitative Risk Assessment

Source

Pathway

Target

Qualitative Risk

Medium

Adjacent to GDBL

(Very Short/Direct)

Construction Phase

(Medium Sensitivity)

Medium

Operational Phase

(Low Sensitivity)

Low

7.6.14 The Qualitative LFG Risk Assessment has indicated the risk associated with LFG during TWR Upgrading (construction phase) is Medium and following completion of the works (operation phase) is Low.

7.7 Proposed Protection and Precautionary Measures

Construction Phase

7.7.1 The level of risk from LFG is determined to be Medium during construction. Due to the close proximity to the GDBL site, it is considered prudent to implement precautionary measures during the construction phase, to further minimize any risk to the site workforce. Particular precautions will be required with respect to any trenching or excavation, and any creation of confined spaces at, near to or below ground level.

7.7.2 Construction contractors should be made aware that methane and carbon dioxide are always likely to be present in the soil voids. They should also be aware of the potential hazards and other properties of LFG. In addition, as mentioned in Section 7.6.6, there exists outside the boundary of GDBL localized pocketed of waste which were deposited during the operation of the landfill. Therefore it is necessary to state in the roadwork contract that waste materials may be encountered during the excavation work, and proper handling as well as disposal of the waste may be needed.

Outline of Safety Requirements

7.7.3 In all construction work adjacent to GDBL, safety precautions should be implemented to minimize the risks of:

· Fires and explosions;

· Asphyxiation of workers; and

· Toxicity effects.

7.7.4 Precautions should be clearly laid down and rigidly adhered to with respect to:

· Trenching and excavation; and

· Creation of confined spaces at, near to or below ground level.

7.7.5 In addition to normal site safety procedures, gas detection equipment and appropriate breathing apparatus should be available and used when entering confined spaces or trenches deeper than 1m.

Appointment of a Safety Officer

7.7.6 A Safety Officer, trained in the use of gas detection equipment and LFG related hazards should be present on site throughout the groundwork phase. The Safety Officer should be provided with an intrinsically safe portable instrument (or instruments), appropriately calibrated and capable of measuring the following gases in the ranges indicated:

· Methane 0 to 100% LEL and 0 to 100% by volume

· Carbon dioxide 0 to 100%; and

· Oxygen 0 to 21%

Safety Measures

7.7.7 The following safety measures should be implemented during the construction phase of the Project:

a) All personnel who work on site and all visitors to the site should be made aware of the possibility of ignition of gas in the vicinity of the excavations. Safety notices should be posted warning of the potential hazards.

b) Those staff who work in, or have responsibility for “at risk” areas, including all excavation workers, supervisors and engineers working within the CZ, should receive appropriate training on working in areas susceptible to LFG, fire and explosion hazards.

c) An excavation procedure or code of practice to minimize LFG related risk should be devised and carried out by the contractor.

d) No worker should be allowed to work alone at any time in or near to any excavation. At least one other worker should be available to assist with a rescue if needed.

e) Smoking, naked flames and all other sources of ignition should be prohibited within 15m of any excavation or ground-level confined space. “No Smoking” and No Naked Flame” notices should be posted prominently on the construction site and, if necessary, special areas designated for smoking.

f) Welding, flame-cutting or other hot works should be confined to open areas at least 15m from any trench or excavation.

g) Welding, flame cutting or other hot works may only be carried out in trenches or confined spaces when controlled by a “permit to work” procedure, properly authorised by the Safety Officer or other appropriately qualified person.

h) The permit to work procedure should set down clearly the requirements for continuous monitoring for methane, carbon dioxide and oxygen throughout the period for which the hot works are in progress. The procedure should also require the presence of an appropriately qualified person in attendance outside the “confined area” who shall be responsible for reviewing the gas measurements as they are made, and who shall have executive responsibility for suspending the work in the event of unacceptable or hazardous conditions. Only those workers who are appropriately trained and fully aware of the potentially hazardous conditions which may arise should be permitted to carry out hot works in confined areas.

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

j) Any electrical equipment, such as motors and extension cords, should be intrinsically safe.

k) During piping assembly or construction, 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/conducting should be capped at the end of each working day.

l) Mobile offices, equipment stores, mess rooms etc should be located on an areas which has been proven to be gas free (by survey with portable gas detectors) and ongoing monitoring should be carried out to ensure that these areas remain gas free. The use of permanent gas detectors may be appropriate in some circumstances where there is a relatively high risk but for many developments it will be sufficient to have regular monitoring undertaken manually by the safety officer. The particular arrangements to be adopted at a specific site will need to be determined during the risk assessment/design of protection measures.

m) Alternatively, such buildings should be raised clear of the ground. If buildings are raised clear of the ground, a minimum clear separation distance (as measured from the highest point on the ground surface to the underside of the lowest floor joist) should be 500mm.

n) During construction, adequate fire extinguishing equipment, fire-resistant clothing and breathing apparatus (BA) sets should be made available on site.

o) The Contractor should formulate a health and safety policy, standards and instructions for site personnel to follow.

Monitoring

7.7.8 The following should be noted for LFG monitoring:

a) Periodically during groundwork construction, the works area should be monitored for methane, carbon dioxide and oxygen using appropriately calibrated portable gas detection equipment.

b) The monitoring frequency and areas to be monitored should be set down prior to commencement of groundworks by either the Safety Officer or by an appropriately qualified person.

c) Routine monitoring should be carried out in all excavations, manholes and chambers and any other confined spaces that may have been created by, for example, the temporary storage of building materials on the site surface.

d) All measurements in excavations should be made with the monitoring tube located not more than 10mm from the exposed ground surface,

e) Monitoring of excavations should be undertaken as follows:

i) For excavations deeper than 1m, measurements should be made:

· At the ground surface before excavation commences;

· Immediately before any workers enter the excavation;

· At the beginning of each working day for the entire period the excavation remains open; and

· Periodically through the working day whilst workers are in the excavation.

ii) For excavations between 300mm and 1m deep, measurements should be made:

· Directly after the excavation has been completed; and

· Periodically whilst the excavations remains open.

iii) For excavations less than 300mm deep, monitoring may be omitted, at the discretion of the Safety Officer or other appropriately qualified person.

Actions in the Event of Gas Being Detected

7.7.9 Depending on the results of the measurements, actions required will vary and should be set down by the Safety Officer or other appropriately qualified person. As a minimum these actions should encompass those actions specified in Table 7-5.

Table 7‑5 Summary of Required Actions in the Event of Gas Detected

Parameter	Measurement	Required Action
O₂	< 19%	Increase ventilation to restore O₂ to >19%
O₂	< 18%	Stop work Evacuate Personnel Increase ventilation to restore O₂ to >19%
CH₄	> 10% LEL	Prohibit hot works Increase ventilation to restore CH₄ to < 10% LEL
CH₄	> 20% LEL	Stop work Evacuate Personnel Increase ventilation to restore CH₄ to < 10% LEL
CO₂	> 0.5%	Increase ventilation to restore CO₂ to < 0.5%
CO₂	> 1.5%	Stop work Evacuate Personnel Increase ventilation to restore CO₂ to < 0.5%

Specific Advice Relating to Drilling of Boreholes

7.7.10 If drilling of boreholes is carried out within the CZ, then the advice given in Sections 8.29 to 8.49 of the Guidance Note should be followed, and in particular the drilling contractor should provide a Method Statement detailing the procedures to be followed, which should include:

a) provision of an intrinsically safe portable methane gas detector;

b) no smoking to be allowed within 15m of a borehole;

c) capping of casing at the end of each working day; and

d) exhaust and air-intake stacks to be at least 1.5m above ground level.

Operational Phase

7.7.11 During the operation phase, no specific protection measures are required with respect to the users of the road itself, since road users would not have access to any enclosed spaces in which LFG could accumulate.

7.7.12 Realignment of or new utilities routes that are formed as part of the road-widening works are identified within the CZ (see Figure 7-3). They should be designated as “special routes” and the utility companies notified accordingly, such that the necessary precautions can be implemented during maintenance or extension, in accordance with the requirements of the Guidance Note. Any new utility routes passing through the boundary of the CZ should have a protective impermeable barrier installed (as illustrated in Figures 7.8 and 7.9 of the Guidance Note) at the boundary of the CZ (refer to Figure 7-4).

7.8 Environmental Monitoring & Audit

7.8.1 To ensure the safety of the Contractor’s personnel, landfill gas monitoring for methane, carbon dioxide and oxygen shall be carried out to identify any migration between the landfill and the site.

7.8.2 Monitoring shall be carried out and reported in a similar manner to that for the Restoration Contract to provide comparable data. The presentation format for landfill gas monitoring shall be based on this format provided in Appendix C3 in EM&A Manual and agreed with EPD.

7.9 Conclusions

7.9.1 The LFG Hazard Assessment has been undertaken with respect to the current proposals for the TWR Upgrading works. The assessment has been undertaken in accordance with the EIAO-TM and the Guidance Note issued by EPD.

7.9.2 The risks associated with LFG during the construction phase have been classified as Medium. Since construction works are to be carried out in close proximity to GDBL and within the CZ, certain mitigation measures are recommended for implementation during the construction phase. The LFG mitigation measures stated in Section 7.7 shall be incorporated into the contract documents and the project proponent shall ensure that all the measures will be fully implemented during the construction stage of the Project.

7.9.3 The risks associated with LFG during the operation phase have been classified as Low, and as such any underground utilities formed as part of the road-widening works should be designated as “special routes”, and the necessary precautions outlined in the Guidance Note should be adopted for all maintenance or extension works. Requirements will be incorporated into the contract documents if appropriate.