9 LANDFILL GAS IMPACT ASSESSMENT
9.1 INTRODUCTION
This section of the EIA Study Report presents the landfill gas impact assessment for the proposed Gas Receiving Station (GRS) at the Tai Po Gas Production Plant (GPP) and the proposed twin gas pipelines from the proposed LNG receiving terminal at Cheng Tou Jiao, Shenzhen to the production plant. The assessment has been based on the Consideration of Alternatives (Section 2) and on the Project Description (Section 3).
There are a number of risks associated with any development close to a landfill site relating to the generation and migration of landfill gas and leachate. Since part of the Project works lie within the 250 m landfill Consultation Zone of Shuen Wan landfill (see Figure 9.1a), a qualitative landfill gas hazard assessment is considered necessary. The objectives are to advise on the potential hazards relating to landfill gas and leachate migration, and to recommend appropriate measures to ensure that the Project is able to be constructed and operated without undue risk.
9.2 PROCEDURES AND GUIDLINES
A Guidance Note for the assessment of the hazards which landfill gas may present to developments close to landfills has been published by the Environmental Protection Department (EPD) of the Government of the Hong Kong SAR. This provides an assessment framework to be followed when evaluating the risks related to developments described under Section 6.5, Chapter 9 of the Hong Kong Planning Standards and Guidelines. The Guidance Note applies to all developments proposed within 250 m of the edge of a landfill site, known as the landfill Consultation Zone (CZ). Although the GRS is located at approximately 1.5 km from the Shuen Wan landfill, a short section of the proposed Pipelines and the associated landing point fall within the Shuen Wan landfill Consultation Zone and hence, a Qualitative Landfill Gas Risk Assessment (QLFGHA) must be carried out.
Whilst no specific guidelines concerning the assessment of potential risks from leachate are currently in force in Hong Kong, similar principles to those outlined in the Guidance Note for landfill gas have been applied in this assessment to develop an overview of the potential risk from leachate.
9.3 PREVIOUS STUDIES UNDERTAKEN AT THE SHUEN WAN
LANDFILL
A number of previous studies have been undertaken at, or involving, the Shuen Wan landfill, including the Restoration of Shuen Wan Landfill series of documents and an EIA of a proposed golf course development. The documents which have been used as background material for the preparation of this assessment include the following:
* Restoration of Shuen Wan Landfill, Final Phase I Report, Environmental Assessment for the Landfill Extension, Mouchel Asia Ltd, October 1992;
* Restoration of Shuen Wan Landfill, Final Phase II Report, Landfill Restoration Scheme, Mouchel Asia Ltd, May 1993;
* Hong Kong Geological Survey Sheet 7, Sha Tin; and
* Landfill Gas Hazard Assessment: Guidance Note, EPD 1997.
The EPD has also provided the landfill gas monitoring data covering the period from June 2000 to May 2002 and groundwater and leachate monitoring data for the period of June 2001 to May 2002.
9.4 SCOPE OF THIS STUDY
The following tasks have been undertaken as part of this study:
* review of background information and studies related to the Shuen Wan landfill;
* identification and investigation of the sources of landfill gas and leachate which have the potential to affect the development from both desktop studies and the previous monitoring conducted at the site;
* identification of viable pathways through the ground, underground cavities, utilities or groundwater and the conditions of these pathways through which the landfill gas and leachate must pass if they are to reach the development;
* review of the pipeline and landtake point design and identification of elements which are sensitive to the effects of landfill gas and leachate during construction and subsequent operation;
* qualitative assessment of the degree of risk which landfill gas and leachate pose to the development taking account of each source-pathway-target combination; and
* proposal of suitable types of protection and precautionary measures to minimise any risks due to landfill gas and leachate hazards during the construction and operation of the pipeline.
9.5 LANDFILL GAS ASSESSMENT CRITERIA AND METHODOLOGY
9.5.1 General
In accordance with the Landfill Gas Hazard Assessment Guidance Note, EPD, 1997, the risk due to landfill gas may be evaluated based upon the following three criteria:
* Source - the rate and concentration of gas generation by the landfill;
* Pathway - the nature of and length of potential pathways through which landfill gas can migrate and leachate flow, such as geological strata, utility services; and
* Target - the level of vulnerability of various elements of the development to landfill gas.
Each of these criteria is further described in the subsections below.
9.5.2 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.
9.5.3 Pathway
Generally, three types of pathway are considered for the transmission of landfill gas. 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 two 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 may 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.
The broad classification of a 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 250 m for man-made conduits
Long/indirect
Path length of 100 to 250m for unsaturated permeable soils and fissured rock
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:
* a broad assessment of the specific 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.
9.5.4 Target
Different levels of vulnerability or sensitivity of potential Targets for landfill gas 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 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 authorised, 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.
The above examples of different categories within each criterion 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.
9.5.5 Assessment of Risk Criteria
Following the determination of the categories of source, pathway and target in which the landfill, pathway and development fall, a quantitative assessment of the overall risk may be made by reference to Table 9.1 which is extracted from the EPD Landfill Gas Hazard Assessment Guidance Note. The potential implications associated with the various quantitative risk categories are summarised in Table 9.2. 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, but with the possible exception of the very high risk category, development is not precluded for any of the assessed levels of risk.
Table 9.1 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 9.2 Summary
of General Categorisations of Risk
|
Level of Risk |
Implication |
|
Very high |
At the very least,
extensive engineering measures and alarm systems are likely to be
required. An emergency actions plan
should also be developed so that appropriate actions may be immediately taken
in the event of high gas concentrations being detected within the
development. |
|
High |
Significant engineering measures
will be required to protect the planned development. |
|
Medium |
Engineering measures
required to protect the development. |
|
Low |
Some precautionary measures
will be required to ensure that the planned development is safe. |
|
Very low |
No protection or
precautionary measures are required. |
9.5.6 Leachate/Contaminated Groundwater
The same principles used for the assessment of risk of landfill gas may be applied to the proposed pipelines in terms of leachate/contaminated groundwater concerns. Although guidelines have not been developed for categorising each of the three input terms, the same general philosophy may be applied whereby the source, pathway and target terms are assigned to one of three classifications and the overall risk evaluated based on the same assessment matrix presented in Table 9.1.
9.6 ASSESSMENT OF POTENTIAL RISK
This section reviews the information described in Section 9.3 and evaluates the data presented with reference to the assessment definitions given in the Guidance Note. The qualitative assessments of the potential hazard associated with landfill gas and leachate/contaminated groundwater migration to the proposed Project works are then concluded.
9.6.1 The Source
Landfill History
Waste disposal at the site commenced in 1974 with waste being deposited into a void enclosed by a sea wall. The enclosed void was filled with domestic, commercial and industrial waste, displacing the enclosed water into the harbour. Under successive contracts, further sea walls have been built and the landfill extended to its present 50 hectares. The Shuen Wan landfill subsequently received approximately 15 million m3 of waste prior to its closure in 1995. The key restoration facilities completed include the followings:
* construction of a multi-layer capping system consisting of compacted soil, geomembrane, geonet and geotextile;
* a landfill gas management system comprising about 90 gas extraction wells, a series of headers, and a flaring and utilization plant; and
* a leachate management system with a pre-treatment works, a series of leachate extraction wells and pumping chambers.
A passive venting trench is located along the western boundary of the landfill to reduce the potential for sub-surface off-site migration. A comprehensive landfill gas monitoring programme which acts as the secondary control, has also been implemented to monitor the effectiveness of the restoration facilities and provide an early warning of any off-site migration of landfill gas.
Landfill Gas Monitoring Results
A number of landfill gas monitoring wells were adopted by the landfill Restoration Contractor and used to monitor off-site landfill gas migration under the Restoration Contract. The monitoring wells, which are relevant for this assessment, are shown in Figure 9.6a. Recent monitoring are summarised in Table 9.3. Details of the monitoring results are presented in Annex E.
Table 9.2 Summary of Landfill Gas Monitoring Results
(From June 2000 to May 2002)
|
Location(a) |
Methane (%v/v) (b) |
Carbon Dioxide (% v/v) (b) |
Oxygen (%v/v) |
||||
|
|
Range |
Average |
Range |
Average |
Range |
Average |
|
|
GMP01M |
0.0 – 0.8 |
0.0 |
0.0 – 17.7 |
6.1 |
4.9 – 20.4 |
13.0 |
|
|
GMP01S |
0.0 – 0.3 |
0.0 |
0.0 – 15.4 |
2.5 |
8.9 – 20.3 |
17.7 |
|
|
GMP47D |
0.0 – 0.0 |
0.0 |
0.0 – 2.9 |
0.6 |
11.9 – 20.4 |
19.1 |
|
|
GMP47M |
0.0 – 0.3 |
0.0 |
0.0 – 14.2 |
3.7 |
5.3 – 20.3 |
15.1 |
|
|
GMP47S |
0.0 – 0.0 |
0.0 |
0.0 – 5.0 |
0.4 |
15.5 – 20.5 |
19.6 |
|
|
GMP48D |
0.0 – 0.0 |
0.0 |
0.0 – 4.3 |
1.5 |
13.7 – 20.5 |
18.7 |
|
|
GMP48M |
0.0 – 0.0 |
0.0 |
0.0 – 4.1 |
1.4 |
6.2 – 20.1 |
18.2 |
|
|
GMP48S |
0.0 – 0.0 |
0.0 |
0.0 – 8.5 |
0.5 |
2.8 – 20.5 |
19.3 |
|
|
P60M |
0.0 – 0.3 |
0.0 |
0.0 – 6.1 |
1.9 |
10.0 – 20.3 |
18.4 |
|
|
P60S |
0.0 – 0.3 |
0.0 |
0.0 – 7.9 |
2.0 |
13.9 – 20.3 |
18.7 |
|
|
P61M |
0.0 – 0.4 |
0.1 |
0.0 – 3.0 |
1.2 |
17.5 – 20.1 |
19.8 |
|
|
P61S |
0.0 – 0.5 |
0.1 |
0.0 – 2.1 |
0.8 |
15.1 – 20.2 |
19.1 |
|
|
Notes:
(a) Depth indication: “D” = Deep (3m below ground); “M” = Medium (6m below ground); “S” = Shallow (9m below ground) (b) According to the Guidance Note, 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. 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. |
|||||||
Zero or low concentrations of methane have been observed in all of the monitoring wells (1) at the western boundary of the landfill. Slightly elevated methane concentrations were found in GMP01M and P61S. However, the average methane levels from these wells were well below 1% (v/v). This suggests that off-site landfill gas migration in this area is under effective control.
Positive carbon dioxide readings are not in themselves indicative of a landfill gas presence; however, it is acknowledged that under some circumstances, the methane component of landfill gas may be oxidised leaving reduced concentrations of oxygen and relatively high concentrations of carbon dioxide. Under these circumstances, elevated readings of carbon dioxide and reduced oxygen concentrations are assumed to be associated with a potential landfill gas presence. Carbon dioxide concentrations in monitoring wells are generally expected to be of the order of 1% (v/v), however, under some circumstances, according to geological and hydrogeological conditions, concentrations of 3% or more may reasonably be expected. Oxygen concentrations in monitoring wells are typically 20% (v/v). It is within this context that the results of carbon dioxide and oxygen monitoring are reviewed.
The average carbon dioxide concentrations detected in all the concerned monitoring wells along the western boundary ranged from 0.6% to 6.1% (v/v). Elevated levels of carbon dioxide were detected occasionally in GMP01M, GMP01S, GMP47M, GMP48S, P60M and P60S. However, the average carbon dioxide concentrations detected in these wells were all well below 5 % (v/v), except in GMP01M. Since the maximum gas concentrations ranged from 2.1% to 17.7% (v/v), this indicated an elevated concentration compared to the typical background level.
The oxygen concentrations measured in these wells are all lower than or approximately 20%. The reduced oxygen concentrations correspond well with the increased levels of carbon dioxide.
In summary, although zero or no methane was measured at all the monitoring wells since June 2000, the elevated carbon dioxide and reduced oxygen levels suggest that there is still a potential for off-site migration of landfill gas in these area. However, it is likely the migration, if any, is limited and the area of influence is close to the landfill boundary. Migration of carbon dioxide presents less of a risk to the Pipelines (the nearest section is about 150m from the landfill boundary) and the GRS at approximately 1.5 km away from the landfill.
Groundwater Quality
The locations of groundwater monitoring wells are shown in Figure 9.6a. The results of recent groundwater monitoring (see Annex E) showed some contamination with leachate. However, it is noted that readings of all parameters were not high relative to leachate quality. Since the landfill is in a coastal location it is envisaged that further dilution of the contaminated groundwater is taking place.
Groundwater Level
The average groundwater level is in the range of 1.4 to 4.9 mPD (See Annex E). The lowest groundwater level was recorded in GW07, which is located at a waterfront location and heavily influenced by tidal fluctuations. Compared with a ground level of approximately 5.5 mPD, the unsaturated zone is approximately 0.6 to 3.2 m (excluding the data of GW07).
Classification of Source
The restoration of the landfill has been completed in December 1997. The landfill gas control measures includes the provision of a passive gas venting trench along the western boundary of the landfill, as well as an active gas extraction system for the control of gas from central areas of the landfill. The active gas extraction system is however, susceptible to power failure and under these circumstances, would be ineffective at controlling gas migration. The passive venting trench would provide an appropriate form of backup to areas south of the landfill under these circumstances.
The retroactive mitigation measures installed at the landfill site should not be relied upon to provide an adequate level of safety to adjacent developments but, whilst not comparable to gas containment measures at modern landfill sites, the combination of measures installed would be expected to be reasonably effective at controlling landfill gas migration and therefore, are expected to provide a substantial reduction in potential hazard over the historical situation.
Taking account of the landfill gas control measures installed at the Shuen Wan landfill as part of the Restoration Contract, the gas pressure within the landfill is significantly reduced and hence reduces the driving force for off-site landfill gas migration. These control measures are supplemented by regular monitoring of landfill gas at the off-site monitoring wells. This will ensure that no significant off-site landfill gas migration will occur.
The monitoring results have demonstrated that there has been no or very low migration of methane gas beyond the landfill boundary for the last 24 months and the carbon dioxide levels are generally low. However, it is noted that elevated carbon dioxide levels (above 5% (v/v)) were occasionally detected in some of the monitoring wells.
With respect to leachate control, a leachate management system with a pre-treatment works (2), a series of leachate extraction wells and pumping chambers has been installed in the Shuen Wan landfill to reduce the total volume of leachate within the landfill and hence the potential for off-site impacts.
Groundwater monitoring results indicate the presence of minor contamination, probably due to leachate from the landfill. The possibility of methane generation from the groundwater is considered low.
Taking the landfill gas and leachate control systems installed at the landfill and the recent landfill gas and groundwater monitoring data into account, the Shuen Wan landfill has been considered to constitute a Medium source (3) in this assessment.
9.6.2 The Pathway
General
The potential pathways through which landfill gas may enter the pipeline trench and then subsequently the GRS are threefold; namely, through transmission along natural pathways such as fissures or joints in rock, man-made pathways such as through permeable backfill in utilities trenches or a combination of both. The likely potential for each mode of transmission are clearly dependent on the geological and hydrogeological conditions, which are discussed below.
Natural Pathway
Geology and Hydrogeology: A review of geological conditions has been conducted with reference to previous study reports on restoration of the Shuen Wan landfill and the Hong Kong Geological Survey documents
The bedrock beneath the landfill site comprises block-bearing volcanic tuff and tuffite of the Repulse Bay Group (Upper Jurassic), and granodiorite of the major intrusive Jurassic-Cretaceous period. The tuff is also shown to be thermally altered and may be expected to contain hornfels at the location. A quartzphyric rhyolite dyke is shown at the southern end of Sirmio Headland, and trends in a north-east/south-west direction, which if extended, crosses the landfill site.
Superficial deposits comprise fine and medium marine sands, or fill deposits at the landfill site, and debris flow deposits, alluvium and fill in the immediate surrounds. In the marine areas, the superficial deposits are shown to be marine mud of the Holocene Epoch.
The landfill waste generally comprises wood, plastic bags, steel, cloth, paper or mixed soils. Some more uniform fill materials, such clays or sands, are also present. These probably form cover, base or separating layers placed during previous filling operations. The industrial estate is reclaimed from fill material. Fault lines were not identified at the Project area.
Based on groundwater monitoring data, the groundwater level is approximately 0.6 to 3.2 m below ground level. This indicated that the unsaturated zone is relatively shallow. Since landfill gas is not readily soluble in water, given the shallow unsaturated zone and the separation distance of 150 m between the nearest pipeline trench and the landfill boundary, the natural pathway for potential landfill gas migration is considered long/indirect. In addition, given that a passive venting trench is located at the western boundary of the landfill, it is quite likely that any small amount of gas migration from the landfill will diffuse to the atmosphere and/or any methane will get oxidised to carbon dioxide in the shallow unsaturated zone. A cross-section of the landfill relative to the proposed pipeline trench is shown in Figure 9.6b. Since the GRS is located at approximately 1.5 km from the landfill, the natural pathway for potential landfill gas migration is considered very long.
With respect to potential leachate or contaminated groundwater migration pathway, any migrating leachate /contaminated groundwater from the landfill will have to travel about 150m to the pipeline trench. As the reclamation area is also influenced by tidal movement and groundwater flow from the north, it is considered that any migrating leachate in this area will be significantly diluted. Taking account the leachate management plan of the Restoration Contract and the fact that the groundwater taken at the western boundary of the landfill is only slightly contaminated, the potential for landfill gas generation from the migrating leachate/contaminated groundwater is considered low.
Man-made Pathway
Utility connections to the proposed Pipelines and GRS have also been considered as potential pathways for the migration of landfill gas. It is possible that a preferential route for landfill gas migration may comprise the permeable backfill in which service utilities are installed and following connection to the Pipelines may allow the direct transmission of landfill gas to the proposed pipeline trench and subsequently followed through to the GRS.
The existing utilities in the vicinity of the Pipelines and landfill are shown in Figures 9.6c. For landing route option 1, the Pipelines will be located in a dedicated trench separated from the existing utilities. Hence, any potential landfill gas migration via the existing utilities is not relevant in this case. For landing route option 2, the Pipelines will be running parallel to the existing utilities along Dai King Street and Dai Hei Street. According to the engineering design information, the proposed Pipelines will be running in a separate trench except at locations of cross-over, where sandbags are used to separate the pipelines from the existing utilities (see Figures 9.6d and 9.6e for the proposed design at the cross-over locations). As shown in Figure 9.6c, the existing utilities will run close to the landfill in an east-west direction along Dai Hei Road, which provides a preferential pathway for potential landfill gas migration from the boundary of the landfill westwards to the industrial estate. Although the proposed Pipelines will not be connected to the landfill and will be running in a separate trench, landfill gas can diffuse through the layer of sandbags at the cross-over location to the pipelines trench. The closest cross-over location to the landfill is approximately 150 m from the landfill boundary although any potential landfill gas will have to travel for more than 1 km before reaching the GRS. Hence, it is considered that the man-made pathway for landfill gas migration from the landfill to the Pipelines and subsequently the GRS should be classified as long/indirect.
Combination (Natural Plus Man-made Pathways)
The consideration of this potential pathway allows for a combination of migration through natural strata and subsequently, through man-made sources such as utility connections, as the potential means of landfill gas affecting the proposed Pipelines and subsequently the GRS in the GPP.
This mode of transmission generally requires a series of events to occur in order for gas to potentially affect the developments. The path length for the transmission of gas by a combination of natural and man-made pathways can generally be assumed much longer than the direct distance between the landfill and the particular receptor.
9.6.3 The Targets
A short marine section of the Pipelines falls within the Consultation Zone of Shuen Wan Landfill. However, the pipelines will be laid under water, which will not be associated with explosion and asphyxiation risk, and therefore landfill gas hazard will not be a concern.
For the onshore section of the Pipelines, the laying of pipelines will involve excavation of trenches. For the Option 1 route, the Pipelines would be buried in one single trench approximately 2 m in width and to a minimum depth of 1.7 m. An open cut method of construction would be used. Although the construction method involves working in a trench, the trench will be in open cut and hence any potential landfill gas migration is likely to diffuse to atmosphere and/or any methane will be oxidised to carbon dioxide. Hence, the sensitivity to landfill gas hazards is low.
For the Option 2 route, the Pipelines will be laid separately beneath each carriageway in 1.0m wide trenches. An open cut method will be used for the pipe laying works. The sensitivity to landfill gas hazards is low.
Although the Pipelines are not sensitive to landfill gas hazard, the GRS in the GPP, will be a sensitive receiver. Based on the design information, the GRS will be an open structure with no underground rooms. Hence, its sensitivity to landfill gas hazards is considered low.
9.6.4 Source-Pathway-Target Analysis
On the basis of the source, pathways and targets identified above, a source-pathway-target analysis has been undertaken and is presented in Table 9.4 according to EPD's assessment framework. The combination of a moderate source term, a long distance between the landfill site and the target and low sensitivity at GRS, results in an overall hazard level assessed as Very Low.
Table 9.4 Qualitative Assessment of Landfill Gas Hazard to the Proposed Pipelines and Towngas Complex
|
Source |
Pathway |
Target |
Assessment of Hazard |
|
Shuen Wan Landfill. Actively producing landfill gas,
retrospective gas and leachate control measures installed including active
gas extraction system and a leachate management system as well as a passive
gas venting trench along western boundary. Monitoring has shown
effective control of landfill gas is being achieved. |
Permeable marine
sand/reclamation fill. Any
migrating landfill gas has to pass through the passive venting trench and
travel about 150 m shallow depth unsaturated zone to the pipeline trench and
a further 1 km to the GRS. |
GRS - it is an open structure without any
underground rooms. Pipeline trench – buried underground pipes, no access is required under normal
operation except during maintenance. |
|
|
(Medium source) |
(Long/indirect) |
(Low
sensitivity) |
Very low |
|
|
|
|
|
9.7 RECOMMENDED PRECAUTIONARY MEASURES FOR THE
CONSTRUCTION AND OPERATION OF THE PROJECT
9.7.1 Introduction
In general, the measures taken for the restoration and aftercare of the Shuen Wan Landfill and the control of landfill gas and leachate should not be relied upon to ensure the safety of adjoining developments; however, it must also be acknowledged that the restoration works undertaken will have the effect of lowering the potential for an incident to occur off-site when compared to the historical situation. Allowance for this has been made in the qualitative assessment undertaken.
The qualitative hazard assessment undertaken in Section 9.6 has concluded that the potential for landfill gas and leachate to affect the proposed Project during the construction and operational phase is very low. During the construction period, similar hazards may also arise related to the flammability or the potential asphyxiating properties of landfill gas and/or the potentially toxic nature of leachate. Given the results of the qualitative risk assessment, it should be assumed that the risks due to the landfill gas or leachate during construction and operational phases is very low. No mitigation measures are considered necessary.
This section of the report provides advice and recommendations for the avoidance of environmental impacts related to landfill gas and leachate/contaminated groundwater during the construction and operational phases. The recommendations are made on a precautionary basis.
9.7.2 General Hazards Related to Landfill Gas and
Leachate
Landfill Gas
All contractors or Towngas personnel participating in the construction and operation of the Pipelines and Towngas complex should be aware that potential of methane and carbon dioxide present in the soil and all works should be undertaken on the basis of an "assumed presence of landfill gas". In addition the following properties of landfill gas should be noted.
* Methane is odourless and colourless, although in landfill gas it is typically associated with numerous highly odoriferous compounds, which gives some warning of its presence. However, the absence of odour should not be taken to mean that there is no methane. Methane levels can only be reliably confirmed by using appropriately calibrated portable methane detectors.
* Methane is a flammable gas and will burn when mixed with air between approximately 5 and 15% (v/v). If a mixture of methane and air with a composition between these two values is ignited in a confined space, the resulting combustion may give rise to an explosion. Methane is also an asphyxiant.
* Carbon dioxide, the other major component of landfill gas is an asphyxiating gas and causes adverse health effects at relatively low concentrations. The long-term Occupational Exposure Limit (OEL) is 0.5% (v/v). Like methane, it is odourless and colourless and its presence (or absence) can only be confirmed by using appropriately calibrated portable detectors.
* Gas density. Methane is lighter than air whereas carbon dioxide is heavier than air. Typical mixtures of landfill gas 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 or heavier than air. As a result, landfill gas may accumulate in either the base or top of any voids or confined spaces.
Leachate
The main problem associated with leachate is its potential for the corrosion of steel and concrete structures and pollution of receiving waters. Leachate also presents a potential health risk to anyone who comes into contact with it. In particular, it may cause severe irritation if there is contact with skin or eyes due to its acidic nature. Many of the compounds likely to be present in the leachate are toxic, if present at a sufficiently high concentrations.
9.7.3 General Recommended Precautionary Measures
Notwithstanding the development of other safety and protection measures and procedures, which will be required through the construction contracts or according to site conditions, the following recommendations should also be considered when works are to be carried out within the Consultation Zone of the Shuen Wan Landfill.
* During all works, safety procedures should be implemented to minimise the risks of fires and explosions, asphyxiation of workers and toxicity effects resulting from contact with contaminated soils and groundwaters.
* Safety officers, specifically trained with regard to landfill gas and leachate related hazards and the appropriate actions to take in adverse circumstances, should be present on the site throughout the works, in particular, when works are undertaken below ground.
* All personnel who work inside the pipeline trench 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.
* Monitoring for landfill gas should be undertaken when underground works (i.e. excavation below 2m ground level) are to be undertaken through the use of an intrinsically safe portable instrument, appropriately calibrated and capable of measuring the following concentrations:
¡V methane 0 to 100% (v/v)
¡V carbon dioxide 0 to 100% (v/v); and
¡V oxygen 0 to 21% (v/v)
* 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.
9.8 CONTRACTUAL OBLIGATIONS OF THE SUCCESSFUL TENDERER
The construction of the Pipelines and GRS will be undertaken in accordance with the construction contract, which specifies the relevant statutory and other recommended environmental, health and safety guidelines to be followed. In addition, and specifically, the tenderers will be made aware of hazards associated with landfill gas and other issues relevant to construction in the vicinity of a landfill through the Materials and Workmanship Specification and further, will be provided with copies of this report in order to inform them of the likely significance of these issues and any additional recommended safety and precautionary measures that may need to be implemented during the construction phase.
Prior to works commencing, the Towngas should ensure through review and verification of the Contractors Safety Plan that adequate consideration has been given by the Contractor to the occurrence and management of emergency situations and that emergency plans have been developed to cover contingencies relating to landfill gas and leachate.
The construction contract also calls for Contractors to submit "method statements" or statements of working procedures to the Engineer prior to commencing specific activities on site.
The method-of-working statement should cover, inter alia:
* monitoring proposed;
* emergency procedures, including fire fighting;
* supervisors responsibilities;
* storage and use of safety equipment;
* safety procedures; and
* signs, barriers and guarding.
It is recommended that the Towngas's Environmental and Safety Manager provides a review of all method statements, where there is a potential for landfill gas and leachate impacts to occur, to ensure that adequate measures are incorporated in the procedures to counter any possible adverse effects. The Contractor should not be allowed to commence work prior to review by these persons and where recommendations are made, until appropriate adjustments to the work procedure as required by these persons have been incorporated and agreed.
Monitoring for methane, carbon dioxide and oxygen should be undertaken during the construction phase at work areas within the Consultation Zone of the Shuen Wan landfill where works involve deep excavations or entry into confined spaces (i.e. manhole). The frequency and areas to be monitored should be documented in the Contractors safety plan prior to commencement of the works.
All monitoring should be carried out by trained technicians and equipment calibrated according to the manufacturers instructions. A standard record form, detailing the location, time of monitoring and equipment used, together with the gas concentrations measured, should be used to ensure all relevant data are recorded for future reference.
Depending on the results of gas monitoring and the environment under which it is conducted (ie confined space or atmospheric works), appropriate actions will vary and should be stated as part of the Contractor's safety plan.
In emergencies, the following bodies should be contacted:
* Fire Services Department;
* Hong Kong Landfill Restoration Group Ltd; and
* EPD.
9.8.1 Precautionary Measures for the Operation of the
Project
During the normal operation of the Pipelines and the GRS, the risk associated with landfill gas and leachate/contaminated groundwater is very low and hence mitigation measures are not required. However, under all scenarios where entry to the pipelines trench deeper than 1m within the Consultation Zone of the Shuen Wan landfill is proposed, e.g. for maintenance purpose, Towngas staff should rigidly enforce safety procedures and any works should be carried out in accordance with the Safety Guide to Working in Confined Spaces produced by the Labour Department of the HKSAR Government. No entry should be allowed until monitoring for methane, carbon dioxide and oxygen has been undertaken and the levels of these gases have been determined to be safe by a qualified safety officer.
9.9 SUMMARY
A qualitative hazard assessment of the migration of landfill gas and leachate/contaminated groundwater has been undertaken. It is concluded that the potential for landfill gas and leachate to affect the proposed Project during the construction and operational phase is very low. During construction, similar hazards may arise from the flammability or the potential asphyxiating properties of landfill gas and/or the potentially toxic nature of leachate. Whilst the risks associated with landfill gas and leachate are expected to be very low, precautionary measures have been recommended for the avoidance of environmental impacts during construction and operational phases.
_____________________
(1) Within 250m of the proposed alignment of the Pipelines.
(2) The collected leachate is currently being treated at the Tai Po Sewage Treatment Works.
(3) Mainly respects to the increased carbon dioxide levels.