14                          Landfill Gas Hazard

14.1                   Legislation and Standards

14.1.1             Relevant legislation and guidelines applicable to the landfill gas (LFG) hazards include:

·        Section 1.1(f) in Annex 7 of the EIA Technical Memorandum (TM);

·        Section 3.3 in Annex 19 of the EIA TM;

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

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

14.1.2             These legislation and guidelines recommend that, in general, a qualitative assessment of the risk posed by LFG will be required for a development within the 250m Consultation Zone of a landfill site to ensure appropriate precautionary measures would be designed and implemented to safeguard the development.  For particular sensitive developments and/or where the development is particularly close to the landfill site, it may be necessary to undertake a quantitative risk assessment (QRA).  The requirement for a QRA is usually identified during the qualitative assessment.

14.1.3             Two landfills, namely Tseung Kwan O Stage I Landfill (TKOL-I) and Tseung Kwan O Stage II/III Landfill (TKOL-II/III), are located in the proximity of the eastern end of the proposed CBL development.  The CBL Wan Po Road junction, which aligns with Wan O Road and links up Wan Po Road, encroaches into the 250m Consultation Zone of TKOL-II/III.  A plan showing the relative locations of the landfill and the proposed development is shown on Drawing No. 209506/EIA/LFG/001.  A qualitative risk assessment of LFG hazard on the potential sensitive receivers is carried out for the safety of the development, mainly focusing on the TKOL-II/III. 

14.1.4             As the proposed CBL is at least 500m away from TKOL-I, a landfill gas hazard assessment is not required under the Study Brief.

14.2                   Scope of the Assessment

14.2.1             In accordance with the procedures recommended in Landfill Gas Hazard Assessment Guidance Note and the requirement listed in the Study Brief, the following tasks have been undertaken to allow a full consideration of the potential risk of LFG from the TKOL-II/III:

·        Review of background information (including landfill gas monitoring data) and studies related to the TKOL-II/III;

·        Identification of the nature and extent of the sources, including the likely concentrations and/or amounts of hazardous emissions which might have the potential impacts on the Project and impacts from the Project to the potential receivers;

·        Identification of possible pathways through the ground, underground cavities, utilities or groundwater, and the nature of these pathways through which hazardous emissions must traverse if they were to reach the Project;

·        Identification of the potential receivers associated with the Project which are sensitive to the impacts of the hazardous emissions;

·        Qualitative assessment on the degree of risk which the hazardous emissions may impose on the receivers for each of the source-pathway-receiver combinations; and

·        Design of suitable level of precautionary measures and contingency plan for the Project and the potential receivers, if needed. 

14.3                   Description of Existing Environment

 

Desktop Study

14.3.1             Previous studies have been undertaken at, or involving, the TKOL-II/III.  The information and documents which have been used as background materials for the preparation of this assessment include the following:

·        Restoration of Tseung Kwan O Landfills monthly reports, Tseung Kwan O Landfill Restoration Contract, Swire SITA Waste Services Ltd. (provided by EPD)

·        Landfill gas monitoring data for the TKOL-II/III  and background information regarding the restoration works and landfill gas control measures provided by EPD;

·        Agreement No.  CE 87/2001 (CE).  Further Development of Tseung Kwan O Feasibility Study: Environmental Impact Assessment Report, Maunsell Consultants Asia Ltd, July 2005;

·        Agreement No.  CE 10/2005 (EP) South East New Territories (SENT) Landfill Extension – Feasibility Study: Environmental Impact Assessment Report, ERM-Hong Kong, Limited, December 2007;

·        Geology Map, Hong Kong Geological Survey, GEO / Planning Division.

 

Background Information

History of Tseung Kwan O Stage II/III Landfill

14.3.2             Based on the information provided by the approved EIA report of Agreement No. CE 87/2001 (CE) – Further Development of Tseung Kwan O Feasibility Study, a review of the existing conditions at and around the TKOL-II/III is given in this section.

14.3.3             TKOL-II/III is located at TKO Development Area 105 on the eastern shoreline of Junk Bay.  It is a valley landfill sited in a coastal location approximately 1km south-east of TKOL-I.  The site covers an area of about 42 hectares.  To the east of the site lies the Clear Water Bay Country Park; to the west lies reclaimed land which contains the comprehensive development area (e.g.  Lohas Park and MTR Depot) and the TKO Industrial Estate.

14.3.4             Engineering preparation works were carried out prior to the start of landfilling in 1988.  A permanent seawall on a dredged foundation was constructed to the seaward boundary.  There is a 15m wide margin of completely decomposed volcanic (CDV) material behind the seawall, and between this and the waste deposit there is a 3m wide trench constructed in coarse aggregate with a continuous length of perforated pipe.  The trench forms a leachate interception and collection zone, together with a vent trench for LFG.  Collected leachate flows to TKO Sewage Treatment Works at the northwest of the landfill site.

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

 

Restoration Works and Aftercare

14.3.6             The restoration works of TKOL-II/III commenced in July 1997 and completed in January 1999.  The restoration works generally included installation of an engineered capping layer, a landfill gas collection system with flaring and electricity generation, a leachate collection and treatment system, surface and sub‑surface drainage systems, and works to improve geotechnical stability and landscaping of the site.  A site plan on completion of final cap and the cross sections of TKOL-II/III is illustrated in Appendix 14.1.

14.3.7             The engineered capping layer (with low permeability) and surface water drainage system are installed to reduce infiltration of rain water into the waste mass thereby reducing the amount of leachate to be treated.  Typical details of a restoration capping system are shown in Detail 2 and Detail 5 of Appendix 14.1.  The components of the landfill restoration capping system include the following (from top to bottom):

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

·        Filtration Geotextile-Geonet Composites: A subsoil drainage layer comprising a synthetic drainage medium, surrounded by suitable geotextile filters.

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

·        Final intermediate Cover: A well compacted 500 mm thick soil, free from stones or other sharp particles, above the waste.

14.3.8             The landfill gas management system consists of active extraction wells, electricity generation from LFG, flaring system for LFG, passive vent trenches/ pipes, and monitoring of LFG both on and off‑site.  The gas extraction system is integrated with the leachate management system.  LFG is collected from the landfill by active gas extraction.  It is transferred to the on-site gas utilization plants for electricity generation and used for heating in the leachate treatment process.  Surplus LFG is flared at the gas flaring plant at the southeast of the landfill for complete destruction.  The system aims to control LFG from migrating off-site in sub-surface layer.

14.3.9             Leachate management system comprises a leachate collection system and a leachate treatment works.  Leachate generated at TKOL-II/III is intercepted by the leachate collection system, which then transfers the collected leachate to the on site leachate treatment works.  Leachate is treated at the treatment plant to meet the discharge standards prior to discharge at the public sewer.

14.3.10        The aftercare period commenced from February 1999 onwards.  Environmental monitoring work for the landfill may continue for more than two decades or up to 30 years. The methane content in the landfill gas remained fairly constant at 44%-48% between 1999 and 2003[1]. Such landfill gas quantity and methane content levels still require monitoring as the landfills could only be considered as fully restored from the perspective of landfill gas safety when the methane content is reduced to 1% or below.  The site has been an open space / green zone as its tentative afteruse. The Hong Kong Air Cadet Corps has also been using the top platform at TKOL-II/III as a model aeroplane training field on weekends and public holidays.

 

Geology/ Hydrogeology at Tseung Kwan O Stage II/III Landfill and Proposed Development

14.3.11        According to Hong Kong Geological Survey Map (Appendix 14.2), the geological formation at the west of the landfill is mainly superficial deposits of fill (natural earth and waste) of Holocene Age, on top of which is the extension of reclaimed land towards Junk Bay; at the north and south of the landfill is mainly undivided tuffaceous mud stone, siltstone and breccias of Upper Jurassic Age; at the east of the landfill is mainly trachydacite lava of Upper Jurassic Age; and at the central is mainly alluvium (clay/silt, sand, and gravel) of Holocene Age.

14.3.12        There are a few numbers of drillholes in rock strata and boreholes in waste for groundwater and leachate monitoring.  These have indicated a relatively consistent hydraulic gradient of groundwater/leachate through the wastes to sea level.

14.4                   Potential Hazards and Properties of Landfill Gas

14.4.1             Methane is odourless and colourless, and typically associated with numerous highly odoriferous compounds in LFG which will give some warning of its presence.  However, the absence of odour should not be taken to mean that there is no methane – this can only be confirmed by using appropriately calibrated methane detectors.  Methane is a flammable gas and will burn when mixed with air between approximately 5% and 15% by volume, the Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL) respectively.  A mixture of methane and air with a composition between the LEL and UEL ignited in a confined space could lead to an explosion.  Methane is also an asphyxiant.

14.4.2             Carbon Dioxide, which is another major component of landfill gas, could induce asphyxia and adverse health effects.  The long-term eight hour Occupational Exposure Limit (OEL) is 0.5% by volume.  Similar to methane, it is also odourless and colourless and can only be detected using appropriately calibrated detectors.

14.4.3             Gas Buoyancy: Methane is lighter than air whereas carbon dioxide is heavier than air.  Typical mixtures of LFG are likely to have a density close to or equal to that of air.  However, site conditions may result in a ratio of methane to carbon dioxide which may make the gas mixture lighter than air or heavier than air.  As a result, LFG may be concentrated in the bottoms of trenches or excavations, or may rise up and accumulate beneath structures and foundations.

 

14.5                   Assessment Methodology

 

Assessment Criteria

14.5.1             In accordance with the Guidance Note, the risk due to LFG may be evaluated based upon the following three criteria:

·        Source – the rate and concentration of LFG generated by the landfill;

·        Pathway – the nature and length of potential pathways through which LFG can migrate and leachate flow, such as geological strata, utility services; and

·        Target (Receiver sensitivity) – the level of vulnerability of various elements of the development to LFG.

14.5.2             Each of these criteria is further described below.

 

Source

The classification of the Source (i.e. the landfill) is determined as follows:

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.

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.

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 6 months, is less than 5% (v/v).

14.5.3             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.

 

Pathway

14.5.4             Generally, three types of pathway are considered for the transmission of LFG.  They are:

·        Man-made pathways e.g. utility connections, stormwater channels, etc.;

·        Natural pathways such as rock jointing planes, fissures and other naturally occurring phenomena which may promote or give rise to the transmission of gas over distances; and

·        A combination of the previous categories.  An example of the latter may be, for instance, where a specific geological feature promotes gas transmission but which stops short of directly linking the landfill and target.  A man made connection, however my also co-exist near the edge of the geological feature, which in combination with the former, may act to link the two sites.  In this instance, careful assessment of the likelihood of the mechanism acting to link the two pathways needs to be undertaken before assigning an appropriate pathway classification.

14.5.5             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 100m for man-made conduits.

Moderately short / direct:  Path length of 50 to 100m for unsaturated permeable soil or fissured rock or 100 to 250m for man-made conduits.

Long / indirect:                   Path length of 100 to 250m for unsaturated permeable soil and fissured rock.

14.5.6             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:

·        permeability of the soil;

·        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 groundwater flow patterns.

 

Target

14.5.7             Different levels of vulnerability or sensitivity of potential Targets for LFG have been 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 for 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 LFG and the specific safety procedures to be followed.

                                             Deep excavations.

Low Sensitivity:                  Buildings and 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.

14.5.8             The above examples of different categories within each criteria are to be used as a general guide only and specific aspects of a development may render it more or less sensitive than indicated.  Account needs to be taken of any particular circumstances when assigning a target to one of the three indicated categories.

 

Assessment Methodology

14.5.9             Following the determination of the categories of source, pathway and target in which the combination of landfill, pathway and development fall, a qualitative assessment of the overall risk is undertaken with reference to Table 14.1, which is extracted from EPD’s Guidance Note.  The potential implications associated with the various qualitative risk categories are summarized in Table 14.2.  It should be noted that the different levels of risk determine the likely extent of the protection measures required to ensure the safety of a development.

 

Table 14.1     Classification of Risk 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

 

Table 14.2    Summary of General Categorizations of Risk

Category

Level of Risk

Implication

A

Very high (undesirable)

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.

14.6                   Qualitative Risk Assessment

The Source

14.6.1             Reference has been made to the EIA Report of Agreement No.  CE 87/2001 (CE) Further Development of Tseung Kwan O Feasibility Study to evaluate the potential risk at the TKOL-II/III.  Available gas monitoring results of the TKOL-II/III from February 1998 to May 2009 provided by EPD are tabulated in Appendix 14.3.

14.6.2             LFG generation is dependent upon a number of factors including temperature, pH, substrate availability, moisture content and oxygen level.  The LFG generation rate peaked in 1993, producing a collectable volume of nearly 200 Mm3 yr-1.  By the time of 2007, the quantity of gas collected has reduced to 11 Mm3 yr-1 (1271 m3 hr-1; EPD’s website[2]).  TKOL-II/III is one of the closed landfill that is able to generate LFG with more than 35% methane content (utilization value) and with high enough production rate. The LFG collected by active extraction system is utilized to provide thermal energy to treat leachate and generate electricity to operate the on-site thermal destructor, leachate treatment works and site office.

 

Restoration Stage

14.6.3             A post-restoration monitoring programme commenced in July 1997 under the restoration contract for TKOL-II/III.  LFG management system was installed to control gas emission and prevent off-site gas migration.  LFG (mainly methane and carbon dioxide) and oxygen were monitored at some monitoring wells deployed within and at the perimeter boreholes around the boundary of the landfill.  Drawing No. 209506/EIA/LFG/002 shows the locations of the off-site monitoring wells and their identification codes are given as follows:

·        Down gradient monitoring wells:      2DG1, 2DG2, 2DG3, 2DG4, 2DG5, 2DG6, 2DG7

·        Up gradient monitoring wells:           2UG1, 2UG2, 2UG3, 2UG4, 2UG5, 2UG6, 2UG7, 2UG8, 2UG9

14.6.4             Summary of the available gas monitoring data from the 15 out of 16 off-site drillholes provided by EPD during the restoration stage (March 1998 to January 1999) are tabulated in Table 14.3.

Table 14.3       Landfill Gas Monitoring Results at Tseung Kwan O Stage II/III Landfill (March 1998 – January 1999)1

Location

Methane (%)

Carbon Dioxide (%)

Oxygen (%)

Range

Average

Range

Average

Range

Average

2DG1

0.0-0.0

0.0

0.3-7.1

3.5

10.2-19.3

15.5

2DG2

0.0-0.0

0.0

9.6-12.8

11.7

1.7-7.4

4.1

2DG3

0.0-12.0

7.2

0.1-8.5

5.5

0.1-10

2.0

2DG4

0.0-0.4

0.0

0.0-10.0

5.2

5.5-18.9

5.2

2DG5

0.0-0.6

0.1

0.0-0.8

0.1

16.9-20.5

19.5

2DG6

0.0-0.8

0.1

0.0-0.2

0.1

18.9-20.4

19.8

2DG7

0.0-0.0

0.0

0.0-3.2

1.2

14.3-20.4

18.2

2UG1

0.0-0.0

0.0

0.0-0.0

0.0

20.0-20.3

20.2

2UG2

-

-

-

-

-

-

2UG3

0.0-0.0

0.0

0.0-0.1

0.1

19.8-20.4

20.3

2UG4

0.0-0.0

0.0

0.0-0.2

0.1

19.2-20.4

20.0

2UG5

0.0-0.0

0.0

0.0-0.1

0.0

20.1-20.4

20.3

2UG6

0.0-0.0

0.0

0.0-0.1

0.0

20.1-20.4

20.3

2UG7

0.0-0.1

0.0

0.0-0.1

0.1

20.1-20.4

20.3

2UG8

0.0-0.0

0.0

0.0-0.3

0.1

19.8-20.4

20.2

2UG9

0.0-2.1

0.5

0.0-2.9

0.7

18.1-20.4

19.4

Note: All information is provided by EPD.

(1) The background level of methane is taken to be 0.0% v/v and the background level of carbon dioxide is below the instrument detection limit of 0.1% v/v.

(2) Standard compliance level of methane is taken to be 1% v/v and that of carbon dioxide is 1.5% v/v above natural background level. As stated in EPD’s Guidance Note, it should be assumed that 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. In addition, 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.

14.6.5             Low concentrations of methane have been observed at all off-site monitoring drillholes, except from 2DG3 and 2UG9, where significant methane concentrations (>1% v/v) have been detected. 2DG3 is located immediately adjacent to the existing leachate treatment plant at the south-western landfill boundary while 2UG9 is located at the northern boundary. The maximum methane concentrations of 2DG3 and 2UG9 are 12.0% v/v and 2.1% v/v, recorded on 10-Sept-98 and 12-Jan-99 respectively. It is worth noting that, according to the information provided by EPD, an illegal petrol filling station was sited near 2DG3 before 2001, which resulted in the contamination of this monitoring well by gasoline and diesel. Therefore, constant high levels of methane, as well as carbon dioxide, were recorded at 2DG3.

14.6.6             Carbon dioxide concentrations above 5% v/v were observed at 2DG1, 2DG2, 2DG3 and 2DG4. Maximum gas concentrations from 2DG1, 2DG2, 2DG3 and 2DG4 were 7.1, 12.8, 8.5 and 10.0% v/v, which were detected on 31-Aug-98, 10-Sep-98, 11-Nov-98 and 14-Dec-98 respectively. The average carbon dioxide concentrations at other monitoring wells were well below 5% v/v.

 

Aftercare Period

14.6.7             The aftercare period commenced in February 1999. A summary of the 10-year monitoring data is given in Table 14.4.  

Table 14.4    Landfill Gas Monitoring Results at Tseung Kwan O Stage II/III Landfill (February 1999 – May 2009)1

Location

Methane (%)

Carbon Dioxide (%)

Oxygen (%)

Range

Average

Range

Average

Range

Average

2DG1

0.0-0.5

0.0

0.0-7.2

1.8

4.8-21.2

17.5

2DG2

0.0-0.3

0.0

0.0-13.8

5.7

1.9-20.8

12.7

2DG3

0.0-12.4

0.8

0.0-14.9

4.0

0.0-20.9

4.2

2DG4

0.0-0.0

0.0

0.0-7.3

2.6

11.0-21.4

17.4

2DG5

0.0-0.1

0.0

0.0-3.7

0.4

16.9-21.7

19.7

2DG6

0.0-0.8

0.0

0.0-3.1

0.6

0.7-26.6

18.9

2DG7

0.0-2.0

0.0

0.0-3.6

1.0

13.4-22.4

18.7

2UG1

0.0-0.1

0.0

0.0-7.2

0.6

14.6-21.4

19.2

2UG2

0.0-0.2

0.0

0.0-2.1

0.4

10.5-22.3

19.6

2UG3

0.0-0.2

0.0

0.0-1.6

0.4

15.8-21.9

19.7

2UG4

0.0-0.2

0.0

0.0-2.2

0.4

0.9-21.1

19.3

2UG5

0.0-0.2

0.0

0.0-3.2

0.3

13.6-22.6

19.7

2UG6

0.0-0.1

0.0

0.0-1.7

0.3

15.7-22.7

19.6

2UG7

0.0-0.2

0.0

0.0-0.7

0.2

16.5-22.5

20.0

2UG8

0.0-0.1

0.0

0.0-2.4

0.4

15.4-22.4

19.7

2UG9

0.0-3.5

0.1

0.0-3.6

0.6

10.2-21.6

19.3

Note:

(1)        All information is provided by EPD.

(2)        The background level of methane is taken to be 0.0% v/v and the background level of carbon dioxide is below the instrument detection limit of 0.1% v/v.

(3)        Standard compliance level of methane is taken to be 1% v/v and that of carbon dioxide is 1.5% v/v above natural background level. As stated in EPD’s Guidance Note, it should be assumed that 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. In addition, 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.

14.6.8             As shown in the above table, an average zero level of methane has been detected at most of the monitoring wells installed along the boundary of the landfill.  However, 2DG3, 2DG7 and 2UG9 have been recorded with elevation of methane levels of above 1% v/v. All these exceedances occurred before 2002 and the recent recorded methane level is significantly low. As mentioned above, an illegal petrol filling station was sited near 2DG3 before 2001, which resulted in the contamination of this monitoring well by gasoline and diesel. The contractor spent a year to decontaminate the site after the petrol station was shut down. Therefore, constant high levels of methane, as well as carbon dioxide, were recorded at 2DG3 before 2002 and the methane levels reduce to below 1% v/v thereafter, indicating that there is no sub-surface off-site migration of methane in recent years. It has also been concluded that the methane content detected at 2DG3 was not caused by gas migration from the landfill.

14.6.9             The carbon dioxide levels varied from 0.0% to 14.9% (with unknown background level) at the 16 monitoring wells, averagely well below 5% v/v except for 2DG2.  Elevated carbon dioxide concentration (>5% v/v) was recorded in 2DG1, 2DG2, 2DG3, 2DG4 and 2UG1, with maximum concentrations of 7.2, 13.8, 14.9, 7.3 and 7.2 detected on 6-Dec-05, 18-Jan-06, 24-Jan-02, 6-Oct-06 and 22-Apr-99 respectively.  According to the Guidance Note, a carbon dioxide concentration greater than 5% v/v above background levels in any monitoring well outside the landfill’s boundary indicates significance migration.  Hence, the potential of off-site migration of LFG cannot be eliminated.

Classification of Source

14.6.10        In view of the insignificant evidence of off-site methane migration (though potential gas migration may still occur at some locations, particularly 2DG1, 2DG2, 2DG3 and 2DG4 due to occasional >5% v/v carbon dioxide concentrations measured), and that the landfill site has multiple LFG control measures (such as vents and barriers; Section 14.3), the LFG source of TKOL-II/III would be classified as Medium.

The Pathways

Natural Pathways

14.6.11        For the natural pathways, the presence of natural cavities is directly related to the prevailing local geology.  The geological formation beneath TKOL-II/III and beneath the proposed CBL mainly comprises natural earth and waste.  A cross section between TKOL-II/III and the Project site is illustrated in Appendix 14.4.  There are no fault lines or cracks existing in the Landfill and the Project site and the area in between.  The landfill and the future CBL site should however be cautiously considered as conducive to LFG migration.  Taking into consideration of the precautionary assumption of the strata’s permeability and the path length between the landfill and the proposed CBL site, which is about 100-250m, it is considered that the natural pathway should be categorised as Long/Indirect. 

Man-made Pathways

14.6.12        Based on the available utility layout plan (Appendix 14.5) such as that of stormwater drainage and sewerage, communications services cable and CLP power cables, it is known that underground utilities exist within the landfill Consultation Zone. Possible transmission of LFG to the proposed CBL junction may occur through these linkages. The path length is between 100-250m. However, there should be no direct connection between the existing utilities or services and the landfill with the proposed works area. As such, man-made pathways are considered to be Moderately Short/Direct.

The Targets

14.6.13        The layout of the CBL work boundary is illustrated in Drawing No. 209506/EIA/LFG/001. The construction works which fall within the 250m Consultation Zone include modification or improvement of the existing Wan O Road and Wan Po Road around CBL / Wan Po Road junction.

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

·        Excavations for utilities installation during construction phase,

·        Road works and drainage works during construction phase, and

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

14.6.15        It is expected that excavations for utilities (<2m depth) would be involved during the construction phase of the proposed project.  Site worker in areas of confined space and trenches are prone to higher risk of exposure to LFG. Therefore, the level of risk for excavations is categorised as Medium.

14.6.16        Based on the preliminary design, some manholes, inspection chambers and voids of services/utilities will be present within the Project site during operational phase.  Since access to these confined spaces will be restricted to authorised personnel who have been briefed on the potential LFG hazards and specific safety procedures, the risk level for these targets is also categorised as Medium.

Source – Pathway – Target Analysis

14.6.17        On the basis of the source, pathways and targets identified above, a source – pathway – target analysis has been undertaken and is presented in Table 14.5 according to EPD’s assessment framework. The combination of a medium source term and moderately short to long distances between the landfill site and the proposed site results in an overall risks of “Low to Medium” depending on the sensitivity of the particular targets.

Table 14.5              Qualitative Assessment of LFG Hazard to the Proposed Junction of CBL

Source

Pathway

Target

Hazard

TKOL-II/III is one of the closed landfill that generates LFG with more than 35% methane content (utilization value) and with high enough production rate. The LFG collected by active extraction system is utilized to provide thermal energy to treat leachate and generate electricity to operate the on-site leachate treatment plant.

A post-restoration monitoring programme commenced in July 1997 under the restoration contract for TKOL-II/III.  LFG management system has been installed to control gas emission and prevent off-site gas migration.  LFG (mainly methane and carbon dioxide) and oxygen are monitored at some monitoring wells deployed within and at the perimeter boreholes around the boundary of the landfill. 

Average zero level of methane has been detected at each of the monitoring well installed along the boundary of the landfill during recent years, indicating that there is no sub-surface off-site migration of methane. 

The carbon dioxide levels varied from 0.0% to 14.9% (with background level <0.1% v/v) at 16 of the monitoring wells.  Elevated carbon dioxide concentration (>5% v/v) was recorded in 2DG1, 2DG2, 2DG3, 2DG4 and 2UG1 only.  According to the Guidance Note, a carbon dioxide concentration greater than 5% v/v above background levels in any monitoring well indicates significance migration.  Hence, the potential of off-site migration of LFG cannot be eliminated.

In view of the insignificant evidence of off-site methane migration and that the landfill site has multiple LFG control measures (such as vents and barriers; Section 14.3), the LFG source of TKOL-II/III would be classified as Medium.

Natural Pathways

The geological formation beneath TKOL-II/III and beneath the proposed CBL mainly comprises natural earth and waste. There are no fault lines or cracks existing in the Landfill and the Project site and the area in between.  The landfill and the future CBL site should however be cautiously considered as conducive to LFG migration.  Taking into consideration of the precautionary assumption of the strata’s permeability and the path length between the landfill and the proposed CBL site, which is about 100-250m, it is considered that the natural pathway should be categorised as Long/Indirect.

 

Construction Phase

It is expected that excavations for utilities would be involved during the construction phase of the proposed project.  Site workers in areas of confined space and trenches are especially prone to higher risk of exposure to LFG. Therefore, the level of risk for excavations is categorised as having Medium Sensitivity.

 

Operational Phase

Although detailed design information is not available at this stage, some manholes, inspection chambers or voids of services/utilities would be present within the project site during operational phase.  Since access to these confined spaces will be restricted to authorised personnel who have been briefed on the potential LFG hazards and specific safety procedures, the risk level for these targets is also categorised as having Medium Sensitivity.

 

 

 

Low

 

 

 

 

 

 

Low

 

 

 

 

 

 

 

 

 

 

 

Medium

 

 

 

 

 

 

Medium

 

Man-made Pathways

Based on the available utility layout plan such as that of stormwater drainage and sewerage, communications services cable and CLP power cables, it is known that underground utilities exist within the landfill Consultation Zone. Possible transmission of LFG to the proposed CBL junction may occur through these linkages. The path length is between 100-250m. However, there should be no direct connection between the existing utilities or services and the landfill with the proposed works area.  As such, man-made pathways are considered to be Moderately short/Direct.

 

 

Construction Phase

It is expected that excavations for utilities would be involved during the construction phase of the proposed project.  Site workers in areas of confined space and trenches are especially prone to higher risk of exposure to LFG. Therefore, the level of risk for excavations is categorised as having Medium Sensitivity.

 

Operational Phase

Although detailed design information is not available at this stage, some manholes, inspection chambers or voids of services/utilities would be present within the project site during operational phase.  Since access to these confined spaces will be restricted to authorised personnel who have been briefed on the potential LFG hazards and specific safety procedures, the risk level for these targets is also categorised as having Medium Sensitivity.

 

Site Categorisation

14.6.18        According to the Guidance Note, for the purposes of categorizing the site, the category will be based upon the highest level of risk nominated for any of the potential impacts identified. For example, a site with three low risks, and one medium risk will fall into Category C, and a site with three low risks, three medium risks and one high risk will fall into Category B. Table 14.2 summarizes the general implications for developments which fall into the different overall risk categories. If four or more different impacts arise in a particular risk category, however, then the overall risk classification may be considered to be one category higher. For example, a site with two low risks and five medium risks will fall into Category B.

14.6.19        Table 14.5 depicts that the overall hazard level for the proposed CBL associated with TKOL-II/III should be falling into the risk category of Medium level (Category C). Engineering measures are required to protect a proposed area of medium or higher level of risk. Recommendations for protection measures are provided for the proposed project (during construction phase and operational phase) in the following section.

14.7                   Precautionary and Protective Measures

14.7.1             The qualitative hazard assessment undertaken in Section 14.6 has concluded that the potential for landfill gas to affect the development during the construction and operational phases is ‘low’ to ‘medium’ depending upon the location and nature of the target being considered.

14.7.2             This section provides general advice and recommendations for the avoidance of environmental impacts related to LFG during the construction and operational phases. Where applicable, specific measures for handling the hazards identified during the construction and operational phases should be addressed to further reduce the likelihood of incidents and increase the level of safety to the workers and the public. These measures should be reviewed taking into account of the findings of the detailed Qualitative LFG Hazards Assessment (QLFGHA) to be undertaken by the project proponent (Refer to Section 14.7.12).

14.7.3             In general, the measures being taken for the restoration of the landfill site and the control of LFG should not be relied upon to ensure the safety of adjoining developments. However, it must also be acknowledged that the works being undertaken would have the effect of lowering the potential for an incident to occur off-site when compared to the historical situation.

 

Precautionary Measures

14.7.4             All contractors participating in the works should be aware that methane and carbon dioxide are always likely to be present in the soil and rock voids and all works should be undertaken on the basis of an “assumed presence of LFG”.

14.7.5             Risks in the construction works mainly result from construction workers’ contact with LFG. Whilst the risks are not expected to be significant, owing to the use of powered mechanical equipment to undertake excavation works, there may still be instances where human exposure may be inevitable when personnel may have to enter confined spaces. Precautionary measures to be adopted by the contractors (for both site formation and infrastructure development) for the period of construction of infrastructure within the 250m Consultation Zone are outlined in Paragraphs 8.3 to 8.49 of EPD’s Landfill Gas Hazard Assessment Guidance Note. The following guidance has been extracted from this Guidance Note to ensure a robust and comprehensive set of measures to protect workers are provided.

·        During all works, safety procedures should be implemented to minimize the risks of fires and explosions, asphyxiation of workers (especially in confined space) and toxicity effects resulting from contact with contaminated soils and groundwater.

·        Safety officers who are specifically trained with regard to LFG and leachate related hazards and the appropriate actions to take in adverse circumstances should be present on all worksites throughout the works.

·        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 works, the possible presence of contaminated water and the need to avoid physical contact with it.

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

·        Enhanced personal hygiene practices including washing thoroughly after working and eating only in “clean” areas should be adopted where contact may have been made with any groundwater which is thought to be contaminated with leachate.

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

·        During piping assembly or conduiting 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 / conduiting should be capped at the end of each working day.

·        Mobile offices, equipment stores, mess rooms etc. should be located on an area 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. 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. However, in this case, it is highly recommended that all the site offices, equipment stores and mess rooms should be located outside the 250m Consultation Zone.

·        Smoking and naked flames should be prohibited within confined spaces. “No Smoking” and “No Naked Flame” notices in Chinese and English should be posted prominently around the construction site. Safety notices should be posted warning of the potential hazards.

·        Welding, flame-cutting or other hot works may only be carried out in confined spaces when controlled by a “permit to work” procedure, properly authorized by the Safety Office. The permit to work procedure should set down clearly the requirements for continuous monitoring of methane, carbon dioxide and oxygen throughout the period during which the hot works are in progress. The procedure should also require the presence of an appropriately qualified person 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.

·        During the construction works, adequate fire extinguishers and breathing apparatus sets should be made available on site and appropriate training given in their use.

 

Monitoring

14.7.6             Monitoring should be undertaken when construction works are carried out in confined space within the 250m Consultation Zone. The monitoring requirements and procedures specified in Paragraphs 8.23 to 8.28 of EPD’s Guidance Note are highlighted as follows:

·        The monitoring equipment used should be capable of measuring methane, carbon dioxide and oxygen concentrations. The equipment should be intrinsically safe and calibrated according to the manufacturers instructions.

·        When portable monitoring equipment is to be used, the frequency and areas to be monitored should be set down prior to commencement of the works either by the Safety Officer or by an appropriately qualified person.

·        All measurements should be made with the monitoring tube located not more than 10mm from the surface.

·        A standard form, detailing the location, time of monitoring and equipment used together with the gas concentrations measured, should be used when undertaking manual monitoring to ensure that all relevant data are recorded.

·        If methane (flammable gas) or carbon dioxide concentrations are in excess of the trigger levels or that of oxygen is below the level specified in the Emergency Management in the following section, then evacuation should be initiated.

 

Actions in the Event of Abnormal Gas being Detected

14.7.7             Depending on the results of the measurements, actions required will vary and should be set down by the Safety Officer or another appropriately qualified person. As a minimum these should encompass those actions specified in Table 14.6

 

Table 14.6    Actions in the Event of LFG Being Detected in Excavations

Parameter

Monitoring Results

Actions

O2

<19% v/v

Increase underground ventilation to restore O2 to >19% v/v

<18% v/v

Stop works; evacuate all personnel, prohibit entry; and increase ventilation to restore O2 level to >19%

CH4

>10% LEL

Prohibit hot works; increase ventilation to restore CH4 to <10% LEL

>20% LEL

Stop works; evacuate all personnel; increase ventilation further to restore CH4 to <10% LEL

CO2

>0.5% v/v

Increase ventilation to restore CO2 to <0.5% v/v

>1.5% v/v

Stop works; evacuate all personnel; increase ventilation further to restore CO2 to <0.5%

 

Emergency Management

14.7.8             In order to ensure that evacuation procedures are implemented in the event of the trigger levels specified in Table 14.6 above being exceeded, it is recommended that a person, such as the Safety Officer, is nominated, with deputies, to be responsible for dealing with any emergency which may occur due to LFG.

14.7.9             In an emergency situation the nominated person, or his deputies, shall have the necessary authority and shall ensure that the confined space is evacuated and the necessary works implemented for reducing the concentrations of gas. The following organizations should also be contacted as appropriate:

·        Hong Kong Police Force;

·        Fire Services Department;

·        Environmental Protection Department.

 

General Recommended Precautionary and Protection Measures – Design Phase

14.7.10        At the preliminary assessment stage, it is not practicable to determine detailed protection, but a provisional classification of the site into one of five categories will allow the Professional Person a means of understanding the generic types of protection which would be appropriate. The potential implications associated with the various qualitative risk categories are summarized in Table 14.2.

14.7.11        According to the source-path-target analysis in Section 14.6, the risk category at the proposed CBL development ranged from low to medium. Therefore, “some precautionary measures” to “engineering measures required” should be applied by the project proponent to protect the proposed development. To avoid engineering measures, underground rooms or void should be avoided in the CBL development as far as practicable.

14.7.12        The need and practicality of incorporating measures for higher risk category should be reviewed in the detailed QLFGHA during the detailed design stage.

14.7.13        The CBL project proponent should make the utility companies aware of the location and features of the site within the 250m Consultation Zone during the respective detailed design stage as part of the QLFGHA. The utilities companies should have a responsibility to train and ensure their staff to take appropriate precautions at all times when entering enclosed spaces or plant rooms.

 

General Recommended Precautionary and Protection Measures – Operational Phase

14.7.14        Precautionary measures below are recommended for implementation during operational phase.

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

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

14.7.17        The utility companies should undertake a LFG 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% Lower Explosive Limit (LEL) and 0-100% v/v

Carbon Dioxide

0-100%

Oxygen

0-21%

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

·         Depending on the results of the measurements, actions required will vary and should be set down by appropriately qualified person. As a minimum, these shall encompass those actions specified in Table 14.1.

 

Monitoring Requirement

14.7.18        The existing monitoring wells will continue to be monitored by EPD’s Landfill Restoration Contractors as part of the restoration contract for TKOL-II/III.

14.7.19        To protect the site workers and future owners within the 250m Consultation Zone, it is recommended that monitoring of any LFG which may be migrated to the site should be undertaken during the construction of CBL infrastructure when the works involve confined spaces. The monitoring requirement has been discussed in Section 14.7.6. The requirements of operational monitoring by future site developers should be determined in the detailed QLFGHA during the detailed design stage when the risk potential and mitigation measures, if required, are confirmed.

14.8                   Conclusion

·        This chapter provides a qualitative risk assessment for LFG hazards associated with the proposed CBL development. The TKOL-II/III is considered as a “Medium” source of gas migration due to occasional high carbon dioxide levels of more than 5% v/v above natural background level detected in the gas monitoring wells though LFG control measures such as an active gas extraction system have been installed. The source-pathway-target analysis indicates that the overall risks to the receivers within the 250m Consultation Zone during both construction and operational phases would be categorised as “Low to Medium”.  Appropriate precautionary measures have been recommended to reduce such risk to comply with the requirements of the TM-EIAO.  It is expected that with the proposed measures in place, the potential risk of LFG migration to the respective targets will be minimal and has complied with Section 1.1(f) in Annex 7 and Section 3.3 in Annex 19 of the TM-EIAO. Moreover, a detailed assessment will be carried during the detailed design of the project.

 


[1]  Environment, Transport and Works Bureau, October 2004. Legislative Council Panel on Environmental Affairs: PWP Item 5166DR – Restoration of Tseung Kwan O Landfills – post-completion environmental monitoring work

[2] EPD’s website. Waste: Problems and Solutions. Landfill Gas to Energy at Closed Landfills [http://www.epd.gov.hk/epd/english/environmentinhk/waste/prob_solutions/msw_lgu.html]