11 Landfill Gas Hazard

11.1 Introduction

11.1.1 This section presents the qualitative risk assessment for landfill gas (LFG) hazards associated with the potential landfill gas migration from Ma Yau Tong West Landfill, Ma Yau Tong Central Landfill and Jordan Valley Landfill to landfill gas sensitive elements of the off-site road improvement works associated with the Anderson Road Quarry (ARQ) Development during construction and operational phases.

11.1.2 The proposed road improvement schemes involve upgrade of several junctions including:

· Junction of (J/O) Lin Tak Road and Sau Mau Ping Road;

· J/O Clear Water Bay Road and On Sau Road and J/O Clear Water Bay Road and New Clear Water Bay Road; and

· J/O New Clear Water Bay Road near Shun Lee Tsuen Road.

11.1.3 There are three closed landfill sites in the vicinity of the ARQ Development including Ma Yau Tong West, Ma Yau Tong Central and Jordan Valley. The proposed works at J/O Lin Tak Road and Sau Mau Ping Road fall within the 250m Consultation Zones of both the Ma Yau Tong West and Ma Yau Tong Central Landfills, whilst the proposed works at J/O New Clear Water Bay Road near Shun Lee Tsuen Road falls within the 250m Consultation Zones of the Jordan Valley Landfill (shown in Figure 11.1).

11.2 Environmental Legislation, Standards & Guidelines

11.2.1 Relevant legislation and associated guidance notes applicable to the assessment of LFG hazards include:

· Section 1.1(f) in Annex 7 of the Technical Memorandum on EIAO (EIAO-TM);

· Section 3.3 in Annex 19 of the EIAO-TM;

· Landfill Gas Hazard Assessment for Development Adjacent to Landfills (ProPECC PN 3/96) ; and

· Landfill Gas Hazard Assessment Guidance Note (1997) (EPD/TR8/97, Guidance Note)

11.2.2 Under Annex 7 of the EIAO-TM, an evaluation of the potential risk posed by landfill gas is required for any development which is proposed within 250m of the edge of waste of a landfill site, known as Landfill Consultation Zone. There are three closed landfill sites in the vicinity of the ARQ Development, namely Ma Yau Tong West (MYTW), Ma Yau Tong Central (MYTC) and Jordan Valley (JV). Since the proposed works at J/O Lin Tak Road and Sau Mau Ping Road falls within the consultation zones of both the MYTW and MYTC Landfills, and the proposed works at New Clear Water Bay Road near Shun Lee Tsuen Road falls within the consultation zone of the JV Landfill (shown in Figure 11.1), a Qualitative Landfill Gas Hazard Assessment (QLFGHA) is required to assess risk associated with potential landfill gas migration from the landfills together with recommended mitigation measures to be implemented for the Project.

11.2.3 ProPECC PN 3/96 and the Guidance Note for assessment of hazards which landfill gas may present to developments close to landfills have been issued by the Environmental Protection Department (EPD). These documents provide an assessment framework to be followed when evaluating risks related to developments described under Section 6.5, Chapter 9 of the Hong Kong Planning Standards and Guidelines. ProPECC PN 3/96 and the Guidance Note apply to all developments proposed within Landfill Consultation Zones.

11.3 Assessment Criteria and Methodologies

Criteria

11.3.1 In accordance with the Landfill Gas Hazard Assessment Guidance Note, risk due to landfill gas may be evaluated based upon the following three criteria:

Source – location, nature and likely quantities / concentrations of landfill gas with potential to affect the development;

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

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

Source

11.3.2 Classification of the landfill gas Sources (i.e. MYTW, MYTC and JV) should be undertaken as follows:

Minor	Landfill sites at which gas controls have been installed and proven to be effective by comprehensive monitoring which has demonstrated that there is no migration of gas beyond the landfill boundary (or any specific control measures) and at which control of gas does 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 locations across the landfill and on at least four occasions over a period of at least 3 months (preferably longer), is less than 5% by volume (v/v).
Medium	Landfill site at which some form of gas control has been installed (e.g. lined site or one where vents or barriers have been retrospectively installed) but where there are only limited monitoring data to demonstrate its efficacy to prevent migration of gas; or Landfill site 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 which is vulnerable to failure.
Major	Recently filled landfill site 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 landfill site at which monitoring has demonstrated that there is significant migration of gas beyond the site boundary.

11.3.3 The “significance” of migration should be assessed by reference to the concentration, frequency and location at which gas is detected. For guidance, it should be assumed that any concentration of methane or carbon dioxide greater than 5% v/v above background levels in any monitoring well outside the landfill's boundary indicates significant migration. Lower concentrations may still be “significant” if they are observed in more than one monitoring well, on several occasions or in monitoring wells located some distance from the site boundary. In general, concentrations of greater than 1% v/v methane or 1.5% v/v carbon dioxide (above background levels in each case) indicate less than adequate control of the gas at source.

11.3.4 In classifying the source term, account needs to be taken of the likelihood and probable effect of a failure of the gas controls. Thus, if it has been demonstrated that there is no migration of gas and there is little danger of the gas controls failing (e.g., if these comprise solely of passive measures such as a liner) it can be assumed that the site represents a "Minor" Source. Where there is no gas migration but this may be as a result of a single, "vulnerable" control measure (e.g., an active extraction system with no warning of failure), the site should be regarded as a "Medium" or even a "Major" Source depending on the other factors (e.g., size of site and age of waste).

11.3.5 Where the effectiveness of the gas controls has not been proven by off-site monitoring or if there is some doubt as to the adequacy of the monitoring, this should be taken into account when considering the impact of the control measures on the Source term. Assessments should always err on the side of caution and, in general, if the effectiveness cannot be demonstrated, the assessment should be undertaken on the same basis as if the controls were not in place.

11.3.6 The reliability of the monitoring, for determining the efficacy of the gas controls, needs to take account of the design, number and location of the monitoring points together with the frequency and duration over which monitoring has been undertaken. Monitoring should have been undertaken under different weather conditions including, in particular, periods of low or falling atmospheric pressure.

Pathway

11.3.7 The broad classification of the Pathway should be undertaken as follows:

Very short / direct	Path length of less than 50m for unsaturated permeable strata and fissured rock or less than 100m for man-made conduits
Moderately short / direct	Path length of 50-100m for unsaturated permeable soil or fissured rock or 100-250m for man-made conduits
Long / indirect	Path length of 100-250m for unsaturated permeable soils and fissured rock

11.3.8 In classifying the pathway, however, adjustment to the above general guidelines will often be required to take account of other factors which will affect the extent of gas migration including the following:

· particular permeability of the soils;

· spacing, tightness and direction of the fissures/joints;

· topography;

· depth and thickness of the medium through which the gas may migrate (which may be affected by groundwater level);

· the nature of the strata over the potential pathway;

· the number of different media involved; and

· depth to groundwater table and flow patterns.

11.3.9 Thus, although there may be permeable soil between the landfill site and a proposed development, say 80m from the edge of the site, if the soil layer is very shallow and thin with its upper surface exposed to the atmosphere, then it will be appropriate to consider this as a long/indirect pathway. This could of course alter if the land between the landfill site and the development was paved over or altered in some other way which reduced the potential for gas release. Similarly, if the land is flat, the surface may be prone to waterlogging which will also effectively seal it at times of heavy rain. In general, a conservative approach should be adopted and it should be assumed that any such permeable surface soils may become less permeable in the future.

11.3.10 If it is known that a conduit (man-made or natural feature such as a fault plane) leads directly from the landfill to the development area, it should be regarded as a "direct/short" pathway even if it is longer than 100m.

Target

11.3.11 Different types of target may be broadly classified as follows:

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 which contain sources of ignition. This would include any developments where there is a possibility of additional structures being erected directly on the ground on an ad hoc basis and thereby without due regard to the potential risks.
Medium sensitivity	Other buildings, structures or service voids where there is access only by authorized, well trained personnel, such as the staff of utility companies, who have been briefed on the potential hazards relating to landfill gas and the specific safety procedures to be followed. Deep excavations.
Low sensitivity	Buildings/structures which are less prone to gas ingress by virtue of their design (such as those with a raised floor slab). Shallow excavations. Developments which involve essentially outdoor activities but where evolution of gas could pose potential problems.

11.3.12 The classification of the above LFG sources, pathway and target are categorized. Having determined into which categories of source, pathway and target the combination of landfill and development fall, a preliminary assessment of the overall risk may be made by reference to Table 11.1. The potential implications associated with the various qualitative risk categories are summarized in Table 11.2.

Table 11.1 Classification of Risk Category

Source	Pathway	Target Sensitivity	Risk Category
		High	Very High
	Very short / direct	Medium	High
		Low	Medium
		High	High
Major	Moderately short / direct	Medium	Medium
		Low	Low
		High	High
	Long / indirect	Medium	Medium
		Low	Low
		High	High
	Very short / direct	Medium	Medium
		Low	Low
		High	High
Medium	Moderately short / direct	Medium	Medium
		Low	Low
		High	Medium
	Long / indirect	Medium	Low
		Low	Very Low
		High	High
	Very short / direct	Medium	Medium
		Low	Low
		High	Medium
Minor	Moderately short / direct	Medium	Low
		Low	Very Low
		High	Medium
	Long / indirect	Medium	Low
		Low	Very Low

Table 11.2 Summary of General Categorization of Risk

Category	Level of Risk	Implication
A	Very high	The type of development being proposed is very undesirable and a less sensitive form of development should be considered. At the very least, extensive engineering measures, alarm systems and emergency action plans are likely to be required.
B	High	Significant engineering measures will be required to protect the planned development.
C	Medium	Engineering measures will be required to protect the proposed development.
D	Low	Some precautionary measures will be required to ensure that the planned development is safe.
E	Very Low (insignificant)	The risk is so low that no precautionary measures are required.

11.3.13 Five generic forms of protection will be used in mitigating the hazards to development. These generic forms corresponding to the five risk levels are set out in Table 11.3. The terms used in Table 11.3 are defined in Table 11.4.

Table 11.3 Generic Protection Measures for Planning Stage Categorization

Category	Generic Protection Measures
A	For the planned development active control of gas, supported by barriers and detection systems. Another, less sensitive form of development should also be considered.
B	Active control of gas, including barriers and detection systems ^(1).
C	Use of “semi active” or enhanced passive controls. Detection systems in some situations.
D	Passive control of gas only.
E	No precautionary measures required.

Note:

(1) The gas protection measures required to allow the safe development of a Category A risk development will need to be more extensive than those for a Category B risk development.

Table 11.4 Definition of Control Terms

Term	Definition
Active Control	Control of gas by mechanical means e.g. ventilation of spaces with air to dilute gas, or extraction of gas from the development site using fans or blowers.
“Semi active” Control	Use of wind driven cowls and other devices which assist in the ventilation of gas but do not rely on electrically powered fans.
Passive Control	Provision of barriers to the movement of gas e.g. membranes in floors or walls, or in trenches, coupled with high permeability vents such as no-fines gravel in trenches or voids/permeable layers below structures.
Detection System	Electronic systems based upon, for example, catalytic oxidation or infra-red measurement principles, which can detect low concentrations of gas in the atmosphere and can be linked to alarms and/or telemetry systems.

11.3.14 The landfill gas hazard assessment aims to provide a qualitative risk assessment of hazard associated with potential landfill gas migration from MYTW Landfill, MYTC Landfill and JV Landfill to landfill gas sensitive elements of the off-site road improvement works associated with the ARQ Development.

11.3.15 The assessment covers the following:

· Review of background information about each landfill that may have potential impacts on the proposed works.

· Assessment of the degree of risks associated with the proposed works.

· Formulation of any precautionary measures considered necessary (commensurate with the degree of risk) for the safe construction and operation of the proposed works.

11.3.16 The Landfill Gas Hazard Assessment Report for “Development of Anderson Road Quarry Site – Design and Construction” is given in Appendix 11.1.

11.4 Description of the Environment

Jordan Valley Landfill

Landfill Site Layout

11.4.1 JV Landfill is located in Ngau Tau Kok next to New Clear Water Bay Road and to the west of Shun Lee Estate. Figure 11.1 shows the location of the landfill.

History of the Landfill

11.4.2 JV Landfill was in operation between 1986 and 1990. The site was closed in 1990 and contains about 1.5 million tonnes of domestic and industrial waste. The total area of the landfill is about 11 ha.

11.4.3 No liner was installed before the deposition of waste. The landfill is underlain by a main spine drain which extends the full length of the landfill and is connected to secondary lateral drains. The leachate is processed at the Leachate Pre-treatment Works, leachate is also received and treated onsite from MYTW Landfill and MYTC Landfill.

Restoration Works

11.4.4 Restoration works were carried out at the landfill between 1997 and 1998. Monitoring of landfill gas and leachate measurements at the periphery of the landfill site is ongoing. The monitoring will continue until the end of the landfill Aftercare Period, which is 20 years from the restoration works completion date.

Landfill Gas Management of Jordan Valley Landfill

11.4.5 The landfill gas management system consists of a LFG control and utilisation system. The system consists of extraction wells, associated piping, condensate removal, and LFG flare. The system is designed to thermally treat 500m³/hr of LFG and is capable of generating 220kW of power.

11.4.6 Landfill gas monitoring data presented in Table B.1 and Table B.4 of Appendix 11.2 respectively indicate that no measurable concentrations of methane or carbon dioxide concentrations < 1.5% were detected at relevant LFG monitoring locations around the periphery of JV Landfill. Based on this monitoring data, gas control at the source is considered to be adequate.

Leachate Management of Jordan Valley Landfill

11.4.7 JV landfill was originally designed/built without lining or a leachate collection system. A capping system was installed on the landfill to prevent infiltration of surface runoff into the landfill and a leachate extraction system was installed to the north 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. Collected leachate is treated onsite.

Ma Yau Tong West Landfill

Landfill Site Layout

11.4.8 MYTW Landfill is located in Kwun Tong next to Tseung Kwan O Road and to the north of Hing Tin Estate. Figure 11.1 shows the location of the landfill.

History of the Landfill

11.4.9 MYTW Landfill was in operation between 1979 and 1981. The site was closed in 1981 and contains about 0.6 million tonnes of domestic and industrial waste. The total area of the landfill is about 6 ha.

11.4.10 No liner was installed before the deposition of waste. The landfill is underlain by a main spine drain which extends the full length of the landfill and is connected to secondary lateral drains. The leachate is collected in a leachate storage tank and tankered to the Leachate Pre-treatment Works at JV Landfill for treatment.

Restoration Works

11.4.11 Restoration works at the landfill commenced in March 1997 and was completed in May 1998. Monitoring of landfill gas and leachate measurements at the periphery of the landfill site is currently ongoing. The monitoring will continue until the end of the landfill Aftercare Period, which is 20 years from the restoration works completion date.

Landfill Gas Management at Ma Yau Tong West Landfill

11.4.12 Landfill gas management consists of passive venting with a standby utility flare integrated into the LFG management system, as a contingency to provide active control of LFG if required.

11.4.13 Landfill gas monitoring data presented in Table B.2 and Table B.5 of Appendix 11.2 respectively indicate no measurable concentrations of methane or carbon dioxide levels greater than 1.5% were detected at any relevant LFG monitoring locations around MYTW Landfill, hence, it is considered gas control at the source is adequate.

Leachate Management at Ma Yau Tong West Landfill

11.4.14 MYTW Landfill was originally designed/built without a lining system or leachate collection system. A capping system constructed to prevent infiltration of surface runoff into the landfill. A leachate extraction system was installed 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 leachate storage tank to the west of the landfill. It is then tankered to the Leachate Pre-treatment Works at JV Landfill for treatment.

Ma Yau Tong Central Landfill

Landfill Site Layout

11.4.15 MYTC Landfill is located in Kwun Tong south of Tseung Kwan O Tunnel and to the East of Lin Tak Road. Figure 11.1 shows the location of the landfill.

History of the Landfill

11.4.16 MYTC Landfill was in operation between 1981 and 1986. The site was closed in 1986 and contains about 1.0 million tonnes of domestic and industrial waste. The total area of the landfill is about 11 ha.

11.4.17 No liner was installed before deposition of waste. The landfill is underlain by a main spine drain which extends the full length of the landfill and is connected to secondary lateral drains. Leachate is collected in a storage tank and tankered to the Leachate Pre-treatment Works at JV Landfill for treatment.

Restoration Works

11.4.18 Restoration works at the landfill commenced in 1997 and were completed in 1998. Monitoring of landfill gas and leachate measurements at the periphery of the landfill site are currently ongoing. The monitoring will continue until the end of the landfill Aftercare Period, which is 20 years from the restoration works completion date.

Landfill Gas Management at Ma Yau Tong Central Landfill

11.4.19 The landfill gas management system consists of active LFG extraction wells and associated piping, blower and flare system, condensate management system, knock out, electrical control and monitoring of LFG both on and off-site. The LFG gas extraction system is integrated with the leachate management system.

11.4.20 LFG is collected from the landfill by active gas extraction and is transferred to the onsite gas management system where the LFG is flared. The system prevents LFG from migrating off-site.

11.4.21 The methane monitoring data is presented in Table B.3 of Appendix 11.2 and revealed that no measurable concentrations of methane were detected at any of the relevant LFG monitoring locations at MYTC Landfill.

11.4.22 Carbon dioxide monitoring data presented in Table B.6 of Appendix 11.2 indicates that carbon dioxide levels were < 1.5% at all the relevant offsite monitoring locations MYTC Landfill. Hence, it is considered that there is adequate control of the gas at the source.

Leachate Management at Ma Yau Tong Central Landfill

11.4.23 MYTC Landfill is originally designed/built without a lining system or leachate collection system underneath. A capping system was installed to prevent infiltration of surface runoff into the landfill. A leachate extraction system was installed to the south 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 north of the landfill. It is then tankered to the Leachate Pre-treatment Works at JV Landfill for treatment.

11.5 Geological Assessment

Geology/Hydrogeology at Jordan Valley Landfill

11.5.1 According to Hong Kong Geological Topography Map (Series: HGM20, 1:20,000 Scale) – Sheet No. 11 (Hong Kong and Kowloon, Edition 1, 1986), the Site is mainly underlain by fine to medium grained granite, with minor intrusions of Quartzphyric rhyolite. The fine to medium grained granite is considered to have low permeability for significant gas migration.

11.5.2 There are three geological faults in the vicinity of JV Landfill. One geological fault trends NW/SE through the centre of the landfill site. This crosses another geological fault trending NE-SW immediately to the south of the landfill. A third geological fault extends to the SE from this fault. The exact locations of the NW-SE faults are not certain and the degree to which the faults are fractured or infilled is also not known, however the NE-SW trending fault has been identified to be lead towards the Project site.

11.5.3 Recent groundwater monitoring data at JV Landfill has been obtained to identify the local groundwater levels. Gauged groundwater levels from March 2013 to September 2014 are summarized in Table B.7 and Table B.8 of Appendix 11.2. The location of the groundwater monitoring wells is shown in Figure 11.2. Groundwater levels ranged from 63.38mPD to 126.73mPD across the landfill. It is conservatively assumed that the unsaturated zone for potential landfill gas could range from ground level to 63.38mPD to 126.73mPD.

Geology/Hydrogeology at Ma Yau Tong West Landfill

11.5.4 According to Hong Kong Geological Topography Map (Series: HGM20, 1:20,000 Scale) – Sheet No. 11 (Hong Kong and Kowloon, Edition 1, 1986), the geology of the site comprises colluvium and granite. The rock (fine to medium grained granite) is considered to have low permeability for significant gas migration.

11.5.5 There are no geological fault lines identified in the vicinity of the landfill.

11.5.6 Recent groundwater monitoring data at MYTW Landfill has been obtained to identify local groundwater levels. Groundwater monitoring data from March 2013 to September 2014 for are summarized in Table B.9 and Table B.10 of Appendix 11.2. The location of the groundwater monitoring wells is shown in Figure 11.4. Groundwater levels ranged from 14.61mPD to 69.13mPD across the landfill. It is conservatively assumed that the unsaturated zone for potential landfill gas could range from ground level to 14.61mPD to 69.13mPD.

Geology/Hydrogeology at Ma Yau Tong Central Landfill

11.5.7 According to Hong Kong Geological Topography Map (Series: HGM20, 1:20,000 Scale) – Sheet No. 11 (Hong Kong and Kowloon, Edition 1, 1986), the Site is mainly underlain by fine grained granite to the west and fine ash to vitric tuff to the east. The rock stratum (fine to medium grained granite) in the direction of the proposed road improvement works is considered to have low permeability for significant gas migration.

11.5.8 There are no geological fault lines identified within the vicinity of the landfill.

11.5.9 Recent groundwater monitoring data at MYTC Landfill has been obtained to identify the groundwater level in the area. Monitoring data from March 2013 to September 2014 for groundwater levels are summarized in Table B.11 and Table B.12 of Appendix 11.2. The location of the groundwater monitoring wells is shown in Figure 11.3. Groundwater levels ranged from 14.61mPD to 69.13mPD across the landfill. It is conservatively assumed that the unsaturated zone for potential landfill gas could range from ground level to 14.61mPD to 69.13mPD.

11.6 Qualitative Landfill Gas Assessment

Landfill Gas Sources

Jordan Valley Landfill

11.6.1 Landfill gas generation is dependent upon a number of factors including temperature, pH, substrate availability, moisture content and oxygen level. At JV Landfill, sufficient LFG is produced for electricity generation which powers the onsite leachate treatment plant. Excess LFG produced is flared onsite to prevent horizontal migration[1].

11.6.2 A post-restoration monitoring programme commenced in 1998 under the restoration contract for JVL. The monitoring is conducted on a regular basis. LFG (including methane and carbon dioxide) and oxygen are monitored at selected monitoring wells installed within and outside the boundary of the landfill. The locations of these monitoring wells are shown on Figure 11.2. Available gas monitoring data provided by EPD from January 2013 to February 2015 are tabulated in Table B.1 and Table B.4 of Appendix 11.2.

11.6.3 As shown in Table B.1 of Appendix 11.2, no detectable concentrations of methane have been measured in monitoring wells outside the landfill boundary in the direction of the proposed development. Background levels of carbon dioxide are unknown; however carbon dioxide levels in monitoring wells ranged from 0.0% to 1.3% at 13 locations. Taking into account the non-detectable level of methane and presence of gas control measures such as vents and barriers to mitigate gas migration, the landfill gas source of JV Landfill is be classified as Medium.

Ma Yau Tong West Landfill

11.6.4 Landfill gas generation is dependent upon a number of factors including temperature, pH, substrate availability, moisture content and oxygen level. At MYTW Landfill extremely low amounts of LFG are produced with low methane content. Due to the low methane content flaring is not required and gas is passively vented to atmosphere to prevent horizontal migration[2].

11.6.5 A post-restoration monitoring programme commenced in 1998 under the restoration contract for MYTW Landfill. The monitoring is conducted on a regular basis. LFG (including carbon dioxide) and oxygen are monitored at selected monitoring wells installed within and outside the boundary of the landfill. The locations of these monitoring wells are shown on Figure 11.4. The available gas monitoring data provided by EPD from January 2013 to February 2015 are tabulated in Table B.2 and Table B.5 of Appendix 11.2.

11.6.6 As shown in Table B.2 of Appendix 11.2, no methane has been detected in monitoring wells installed along the boundary of the landfill in the vicinity of the proposed development. Background levels of carbon dioxide are unknown; however carbon dioxide levels in monitoring wells ranged from 0.0% to 1.2% at 10 of the monitoring locations. Taking into account the non-detectable 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 MYTW Landfill would be considered Medium.

Ma Yau Tong Central Landfill

11.6.7 Landfill gas generation is dependent upon a number of factors including temperature, pH, substrate availability, moisture content and oxygen level. MYTC Landfill produces relatively low amounts of LFG. All LFG produced is flared onsite to prevent horizontal migration[3].

11.6.8 A post-restoration monitoring programme commenced in 1998 under the restoration contract for MYTC 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 11.3. The available gas monitoring data provided by EPD from January 2013 to February 2015 are tabulated in Table B.3 and Table B.6 of Appendix 11.2.

11.6.9 As shown in Table B.3 of Appendix 11.2, no methane has been detected in monitoring wells installed along the boundary of the landfill in the vicinity of the proposed development. Background levels of carbon dioxide are unknown; however carbon dioxide levels in monitoring wells ranged from 0.0% to 1.4% at 13 of the monitoring wells. Taking into account the non-detectable 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 MYTC Landfill would be considered Medium.

Pathway

Natural Pathways

11.6.10 For the natural pathways, the presence of natural cavities, pore paces and fractures is directly related to the prevailing local geography. The rock stratum beneath MYTW and MYTC landfills in the direction of the proposed road improvement works is considered to have low permeability for significant gas migration and there are no geological fault lines directly crossing or in proximity to the proposed road improvement works. However, the distance between proposed road improvement works boundary and MYTW and MYTC landfills will be less than 5m in places, therefore natural pathways are classified as Very Short/Direct for MYTW and MYTC Landfills.

11.6.11 There are three geological faults in the vicinity of the JV Landfill as identified in Section 11.5.2, one of which was identified to run in the direction of the road improvement works. The path length between the JV landfill and the proposed road improvement works boundary is between 150 – 250m; however as the nature of the fault line (i.e. whether it is an open or infilled fault) is unknown, this natural pathway is conservatively classified as Short/Direct.

Man-made Pathways (Utilities)

11.6.12 Based on the available utility layout plans, no existing utilities or services directly link JV, MYTC or MYTCW landfills to the proposed road improvement works. It is therefore considered that there are no man-made pathways between the landfill and the proposed works. In the case of MYTC and MYTW landfills the proposed development is within 5m of the landfill boundary in certain sections. Therefore the man-made pathways for MYTC and MYTW landfills are classified as Very Short/Direct. The man-made pathways for JV Landfill are classified as Long/Indirect.

Target

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

· Excavations for foundations, utilities installation and drainage works during construction phase;

· Slope cutting works during the construction phase;

· Piling works for noise barriers / enclosures and flyover during construction phase; and

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

11.6.14 Excavation for construction of foundation works, earth retaining structures, road works, utility installation etc. during construction phase involve excavation in an open air environment, however, as landfill gas maybe heavier than air, potential exists for landfill gas to accumulate in trenches or excavations. According to Section 3.18 of Landfill Gas Hazard Assessment Guidance Note, deep excavations are categorised as Medium Sensitivity targets.

11.6.15 Slope cutting works to facilitate the road improvement works during construction phase involve excavation in an open air environment. According to Section 3.18 of Landfill Gas Hazard Assessment Guidance Note, shallow excavations are categorised as Low Sensitivity targets.

11.6.16 Piling works for proposed noise barriers / enclosures and flyover during construction phase involve piling in an open air environment. According to Section 3.18 of Landfill Gas Hazard Assessment Guidance Note, creation of a gas pathway could pose potential problems, therefore the target is categorised as Low Sensitivity.

11.6.17 Some manholes, inspection chambers or voids of services/utilities will be present within the project site during the operational phase. Since access to these confined spaces will be restricted to authorized persons who have awareness of potential LFG hazard, these targets are also categorized as Medium Sensitivity.

11.7 Source-Pathway-Target Analysis

11.7.1 Based upon the sources, pathways and targets classified above, the qualitative landfill gas hazard assessment for the proposed development is presented in Table 11.5 to Table 11.7.

Table 11.5 Qualitative Source-Pathway Target Analysis of Proposed Road Improvement Works in the Proximity to Jordan Valley Landfill

Source	Pathway	Target Sensitivity	Risk
Jordan Valley Landfill was closed in 1990. The restoration programme includes LFG management to reduce potential safety and health risks. Gas monitoring is conducted on a regular basis. Methane, carbon dioxide and oxygen concentrations are monitored within monitoring wells installed within and outside the boundary of the landfill. No methane has been detected in monitoring wells installed along the boundary of the landfill in the vicinity of the proposed development. Carbon dioxide levels varied from 0.0% to <1.5% (unknown background level) in 14 of the monitoring wells. Taking into account the zero level of methane and presence of gas control measures such as vents and barriers to mitigate gas migration, JV Landfill is considered a Medium source. (Medium Source)	Natural Pathways A fault line passes through the proposed road (improvement works) alignment and JV Landfill, therefore natural pathways are classified as Direct/Short. (Direct /Short)	Excavation for proposed works during construction phase Excavation for construction of foundation works, earth retaining structures, road works, drainage, sewerage, water works, etc. during construction phase. (Medium Sensitivity Target)	Medium
		Slope works during construction phase  Slope cutting to facilitate road works during construction phase. (Low Sensitivity Target)	Low
		Piling works during construction phase  Piling works for proposed noise barriers / enclosures and flyover during construction phase. (Low Sensitivity Target)	Low
		Maintenance of services / utilities during operational phase  Manholes or inspection chambers of services / utilities during operational phase. (Medium Sensitivity Target )	Medium
	Man-Made Pathways There are no utilities or services directly linking JV Landfill and the proposed road improvement works. Man-made pathways are classified as Long/Indirect. (Long/Indirect)	Excavation for proposed works during construction phase Excavation for construction of foundation works, earth retaining structures, road works, drainage, sewerage, water works, etc. during construction phase. (Medium Sensitivity Target)	Low
		Slope works during construction phase  Slope cutting to facilitate road works during construction phase. (Low Sensitivity Target)	Low
		Piling works during construction phase  Piling works for proposed noise barriers / enclosures and flyover during construction phase. (Low Sensitivity Target)	Low
		Maintenance of services / utilities during operational phase  Manholes or inspection chambers of services / utilities during operational phase. (Medium Sensitivity Target )	Low

Table 11.6 Qualitative Source-Pathway Target Analysis of Proposed Road Improvement Works in the Proximity to Ma Yau Tong West Landfill

Source	Pathway	Target Sensitivity	Risk
Ma Yau Tong West Landfill was closed in 1981. The restoration programme includes LFG management to reduce potential safety and health risks. Gas monitoring is conducted on a regular basis. Methane, carbon dioxide and oxygen concentrations are monitored within monitoring wells installed within and outside the boundary of the landfill. No methane has been detected at the monitoring wells installed along the boundary of the landfill in the vicinity of the proposed development. Carbon dioxide levels varied from 0.0% to <1.5% (unknown background level) in 10 of the monitoring wells. Taking into account the zero level of methane and installation of gas control measures (such as vents and barriers) to prevent gas migration, MYTW Landfill is considered a Medium source. (Medium Source)	Natural Pathways The proposed development is within 5m of the landfill boundary in certain sections. Therefore natural pathways for MYTW Landfill are classified as Very Short/Direct. (Very Short/Direct)	Excavation for proposed works during construction phase Excavation for construction of foundation works, earth retaining structures, road works, drainage, sewerage, water works, etc. during construction phase. (Medium Sensitivity Target)	Medium
		Slope works during construction phase  Slope cutting to facilitate road works during construction phase. (Low Sensitivity Target)	Low
		Piling works during construction phase  Piling works for proposed noise barriers / enclosures and flyover during construction phase. (Low Sensitivity Target)	Low
		Maintenance of services / utilities during operational phase  Manholes or inspection chambers of services / utilities during operational phase. (Medium Sensitivity Target )	Medium
	Man-Made Pathways There are no utilities or services directly linking MYTC Landfill and the proposed road improvement works. The proposed development is within 5m of the landfill boundary in certain sections. Therefore man-made pathways for MYTCW Landfill are classified as Very Short/Direct. (Very Short/Direct)	Excavation for proposed works during construction phase Excavation for construction of foundation works, earth retaining structures, road works, drainage, sewerage, water works, etc. during construction phase. (Medium Sensitivity Target)	Medium
		Slope works during construction phase  Slope cutting to facilitate road works during construction phase. (LowSensitivity Target)	Low
		Piling works during construction phase  Piling works for proposed noise barriers / enclosures and flyover during construction phase. (Low Sensitivity Target)	Low
		Maintenance of services / utilities during operational phase  Manholes or inspection chambers of services / utilities during operational phase. (Medium Sensitivity Target )	Medium

Table 11.7 Qualitative Source-Pathway Target Analysis of Proposed Road Improvement Works in the Proximity to Ma Yau Tong Central Landfill

Source	Pathway	Target Sensitivity	Risk
Ma Yau Tong Central Landfill was closed in 1986. The restoration programme includes LFG management to reduce the potential safety and health risks. Gas monitoring is conducted on a regular basis. Methane, carbon dioxide and oxygen concentrations are monitored within monitoring wells installed within and outside the boundary of the landfill. No 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 <1.5% (with unknown background level) at 11 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; MYTC Landfill is considered a Medium source. (Medium Source)	Natural Pathways The proposed development is within 5m of the landfill boundary in certain sections. Therefore natural pathways for MYTC Landfill are classified as Very Short/Direct. (Very Short/Direct)	Excavation for proposed works during construction phase Excavation for construction of foundation works, earth retaining structures, road works, drainage, sewerage, water works, etc. during construction phase. (Medium Sensitivity Target)	Medium
		Slope works during construction phase  Slope cutting to facilitate road works during construction phase. (Low Sensitivity Target)	Low
		Piling works during construction phase  Piling works for proposed noise barriers / enclosures and flyover during construction phase. (Low Sensitivity Target)	Low
		Maintenance of services / utilities during operational phase  Manholes or inspection chambers of services / utilities during operational phase. (Medium Sensitivity Target )	Medium
	Man-Made Pathways There are no utilities or services directly linking MYTC Landfill and the proposed road improvement works. The proposed development is within 5m of the landfill boundary in certain sections. Therefore man-made pathways for MYTC Landfill are classified as Very Short/Direct. (Very Short/Direct)	Excavation for proposed works during construction phase Excavation for construction of foundation works, earth retaining structures, road works, drainage, sewerage, water works, etc. during construction phase. (Medium Sensitivity Target)	Medium
		Slope works during construction phase  Slope cutting to facilitate road works during construction phase. (Low Sensitivity Target)	Low
		Piling works during construction phase  Piling works for proposed noise barriers / enclosures and flyover during construction phase. (Low Sensitivity Target)	Low
		Maintenance of services / utilities during operational phase  Manholes or inspection chambers of services / utilities during operational phase. (Medium Sensitivity Target )	Medium

11.8 Site Categorization

11.8.1 According to Landfill Gas Hazard Assessment Guidance Note (EPD/TR8/97), for the purpose of categorizing 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.

11.8.2 Referring to Table 11.5 to Table 11.7, the overall hazard level for the proposed project associated with JV, MYTW and MYTC Landfills is Medium for each of the three areas.

11.8.3 In accordance with EPD’s Guidance Note, some precautionary measures are required to protect proposed areas categorized as medium risk. Some recommendations for protection measures for the proposed project (during the construction phase and operational phase) are provided and are presented in the following section.

11.8.4 The general implications and generic precautionary measures for mitigation of the perceived risks as recommended in the Guidance Note are summarized in Table 11.8. Details of the precautionary measures are provided in Section 11.9.

Table 11.8 Summary of the Implication and Generic Protection / Precautionary Measures

Category	Level of Risk	Implication	Generic Protection / Precautionary Measures
C	Medium	Engineering measures will be required to protect the proposed development.	Use of ‘semi active’ or enhanced passive gas controls eg wind driven cowls and other devices which assist in the ventilation of gas but do not rely on electrically powered fans. Detection systems in some situations.
D	Low	Some precautionary measures will be required to ensure that the planned development is safe.	Passive control of gas only such as provision of barriers to the movement of gas eg. membranes in floors or walls, or in trenches, coupled with high permeability vents such as no-fines gravel in trenches or voids/permeable layers below structures

11.9 Recommended Precautionary Measures for Construction Phase and Operational Phase

General

11.9.1 An overall Medium level of risk is derived for the construction stage. General precautionary measures in terms of safety procedures and sensible housekeeping practices are recommended to minimize risk.

11.9.2 Contractors shall note the possible presence of landfill gas in the ground (even if it is unlikely) and shall take this into account in the design, construction of the proposed works.

11.9.3 A Safety Officer or an appropriately qualified person, trained in the use of gas detection equipment, landfill gas related hazards and the appropriate actions to take in the event of adverse circumstances, shall be present on site throughout the works, in particular, when works are undertaken below ground.

11.9.4 The contractor shall take cognizance of the presence of surface water and leachate management system and landfill gas management systems near the proposed works area. The contractor shall take all reasonable care to avoid any damage, loss, injury, interruption or impairment of the integrity of the landfill facilities within the works limits, storage area and across road area. The contractor shall also liaise and seek EPD and their landfill contractor – Hong Kong Landfill Restoration Group Limited (HKLRG) agreement on site arrangement before carrying out the proposed work.

Recommended Safety Measures During Construction

11.9.5 The contractor shall be aware of, and inform all workers accordingly, that methane and carbon dioxide is always likely to be present in the soil voids.

11.9.6 All personnel working on site and all visitors to the site be informed of the nearby landfill site and the possibility of landfill gas in the vicinity of the proposed works area. Safety warning notices shall be posted.

11.9.7 No worker shall be allowed to work alone at any time inside the trenches or joint bays or near to any excavation. At least one other worker shall be available to assist in a rescue in an emergency case.

11.9.8 Smoking and naked flames shall be strictly prohibited within the site or confined space if any. ‘No Smoking’ and ‘No Naked Flame’ notices shall be posted prominently at the site entrance and other conspicuous locations.

11.9.9 All electrical equipment, such as motors and extension cords, shall be intrinsically safe.

11.9.10 Adequate safely equipment shall be available at all times. This includes but is not limited to fire extinguishing equipment, breathing apparatus and personal protective equipment.

11.9.11 In the event of working inside a confined space is required, sufficient approved resuscitation equipment, breathing apparatus and safety torches shall be available. Persons involved in or supervising such work shall be trained and practiced for the use of such equipment. A permit-to-work system for entry into confined space shall be established by an approved qualified person and consistently enforced. All relevant Ordinances, Legislations, Guidelines and Codes of Practice pertaining to work in confined space must be strictly adhered to.

Landfill Gas Monitoring

11.9.12 The works area shall be monitored periodically during construction for the presence of methane, carbon dioxide and oxygen using gas detection equipment. The gas detection equipment shall be an intrinsically safe portable instrument, appropriately calibrated and capable of measuring the following gases in the ranges indicated below:

· Methane 0 – 100% LEL and 0 – 100% v/v;

· Carbon dioxide 0 – 100%; and

· Oxygen 0 – 21%.

11.9.13 During construction, monitoring of excavations shall be undertaken as follows:

For excavation deeper than 1 m, measurements shall be made:

· At the ground surface before excavation commences;

· Immediately before any worker enters an 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.

For excavation between 300 mm and 1 m deep, measurements shall be made:

· Directly after the excavation has been completed; and

· Periodically whilst the excavation remains open.

For excavation less than 300 mm, monitoring may be omitted at the discretion of the Safety Officer or other appropriate qualified person.

11.9.14 The monitoring frequency and area to be monitored shall be set down prior to commencement of ground works either by the Safety Officer or by an appropriately qualified person.

11.9.15 Monitoring should be undertaken by the Safety Officer or by an appropriately qualified person. The monitoring results shall be recorded and kept on site and shall be readily available at all times for inspection by the relevant authority.

Actions in the Event of Gas Being Detected During the Construction Phase

11.9.16 Depending upon the results of measurements, actions will vary. Actions shall be set down by the Safety Officer or other appropriately qualified person prior to commencement of occupancy of the proposed works area. As a minimum these shall encompass those actions specified in Table 11.9.

Table 11.9 Actions in the Event of Gas Being Detected During Construction Phase

Parameter	Measurement	Action
Oxygen (O2)	< 19%	Ÿ Ventilate to restore O2 to >19%
	< 18%	Ÿ Stop works Ÿ Evacuate personnel / prohibit entry Ÿ Increase ventilation to restore O2 to > 19%
Methane (CH4)	> 10% LEL	Ÿ Post ‘No Smoking’ signs Ÿ Prohibit hot works Ÿ Ventilate to restore CH4 to < 10% LEL
	> 20% LEL	Ÿ Stop works Ÿ Evacuate personnel / prohibit entry Ÿ Increase ventilation to restore CH4 to < 10% LEL
Carbon Dioxide (CO2)	> 0.5%	Ÿ Ventilate to restore CO2 to < 0.5%
	> 1.5%	Ÿ Stop works Ÿ Evacuate personnel / prohibit entry Ÿ Increase ventilation to restore CO2 to < 0.5%

Notes:

Sources: Landfill Gas Hazard Assessment Guidance Note
LEL: Lower Explosive Limit

11.10 Safety Measures During Operation

11.10.1 As this project is primarily a road and drainage improvement scheme with no involvement of any building structure, the main precautionary measures during the operational phase relate to inspection or maintenance of buried utilities / services within the 250m LFG Consultation Zone.

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

· The presence of landfill gas should be assumed 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 safe entry procedures;

· Working 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) maintains compliance with the above regulations;

· A strictly regulated “work permit procedure” should be implemented and the relevant safety procedures must be rigidly followed; and

· Adequate communication with maintenance staff should be maintained with respect to landfill gas hazard.

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

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

§ Methane 0 – 100% LEL and 0 – 100% v/v;

§ Carbon dioxide 0 – 100%; and

§ Oxygen 0 – 21%.

· Undertaken for the duration of the site occupancy, or until such time that EPD agrees that surveillance is no longer required.

Actions in the Event of Gas Being Detected During the Operational Phase

11.10.4 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 11.10.

Table 11.10 Actions in the Event of Gas Being Detected During Operational Phase

Parameter	Measurement	Action
Oxygen (O2)	< 19%	Ÿ Ventilate to restore O2 to >19%
	< 18%	Ÿ Stop works Ÿ Evacuate personnel / prohibit entry Ÿ Increase ventilation to restore O2 to > 19%
Methane (CH4)	> 10% LEL	Ÿ Post ‘No Smoking’ signs Ÿ Prohibit hot works Ÿ Ventilate to restore CH4 to < 10% LEL
	> 20% LEL	Ÿ Stop works Ÿ Evacuate personnel / prohibit entry Ÿ Increase ventilation to restore CH4 to < 10% LEL
Carbon Dioxide (CO2)	> 0.5%	Ÿ Ventilate to restore CO2 to < 0.5%
	> 1.5%	Ÿ Stop works Ÿ Evacuate personnel / prohibit entry Ÿ Increase ventilation to restore CO2 to < 0.5%

11.11 EM&A Requirements

11.11.1 For the construction and operation within the Consultation Zones of Jordan Valley Landfill, Ma Yau Tong Central Landfill and Ma Yau Tong West Landfill, the monitoring requirement specified in Sections 11.9.12 to 11.9.16 and Sections 11.10.3 to 11.10.7 shall be followed.

11.12 Conclusions

11.12.1 There are three closed landfill sites in the vicinity of the Project sites including Ma Yau Tong West, Ma Yau Tong Central and Jordan Valley. The proposed works at J/O Lin Tak Road and Sau Mau Ping Road fall within the 250m Consultation Zones of both the Ma Yau Tong West and Ma Yau Tong Central Landfills, whilst the proposed works at J/O New Clear Water Bay Road near Shun Lee Tsuen Road falls within the 250m Consultation Zones of the Jordan Valley Landfill. Thus, a qualitative landfill gas hazard assessment is required to assess risk associated with potential landfill gas migration from the landfills to the Project.

11.12.2 The Source-Pathway-Target analysis of Jordan Valley Landfill and the proposed works indicated a medium source with a very short & direct pathway towards medium to low sensitivity targets resulting in an overall risk category of medium during the construction and operational stages.

11.12.3 The Source-Pathway-Target analysis of Ma Yau Tong West Landfill and the proposed works indicated a medium source with a very short & direct pathway towards medium to low sensitivity targets resulting in an overall risk category of medium during the construction and operational stages.

11.12.4 The Source-Pathway-Target analysis of Ma Yau Tong Central Landfill and the proposed works indicated a medium source with a very short & direct pathway towards medium to low sensitivity targets resulting in an overall risk category of medium during the construction and operational stages.

Construction Phase

11.12.5 Appropriate precautionary measures in terms of safety procedures, sensible housekeeping practices and landfill gas monitoring have been proposed to minimize the landfill gas risk for the Project sites during the construction phase. Provided that all the recommended precautionary measures are implemented properly, the safety of all personnel and general public presence would be safeguarded and there would be no adverse impact anticipated on the Project.

Operational Phase

11.12.6 Appropriate precautionary measures in terms of safety procedures and landfill gas monitoring have been proposed to minimize the landfill gas risk for the Project sites during the operational phase. Provided that all the recommended precautionary measures are implemented properly, the safety of all personnel and general public presence would be safeguarded and there would be no adverse impact anticipated on the Project.

11.12.7 It is anticipated that with the proposed measures in place, potential risk associated with LFG hazard to the respective targets will be negligible.

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[1] Legislative Council of the Hong Kong Special Administration Region of the Peoples Republic Of China (June 2003), Note for public works subcommittee of finance committee, Supplementary information on 45DR – Restoration of Pillar Point Valley Landfill

[2] Legislative Council of the Hong Kong Special Administration Region of the Peoples Republic Of China (June 2003), Note for public works subcommittee of finance committee, Supplementary information on 45DR – Restoration of Pillar Point Valley Landfill

[3] Legislative Council of the Hong Kong Special Administration Region of the Peoples Republic Of China (June 2003), Note for public works subcommittee of finance committee, Supplementary information on 45DR – Restoration of Pillar Point Valley Landfill