9.
landfill
gas hazard
9.1
Introduction
9.1.1.1
Section
9 of the EIA Report presents the assessment of potential landfill gas (LFG) hazard
to the Project.
9.2
Environmental Legislation, Standards
and Guidelines
9.2.1.1 Relevant legislation and associated guidance notes applicable to the assessment of the LFG hazards include:
·
Section 1.1(f)
in Annex 7 of the Technical Memorandum on EIAO (EIAO-TM);
·
Section 3.3 in
Annex 19 of the EIAO-TM;
·
Landfill Gas
Hazard Assessment for Development Adjacent to Landfills
(ProPECC PN 3/96);
·
Landfill Gas
Hazard Assessment Guidance Note (1997) (EPD/TR8/97, Guidance Note).
9.2.1.2 ProPECC PN 3/96 and the Guidance Note provide 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. ProPECC PN 3/96 and Guidance Note apply to all developments proposed within a Landfill Consultation Zone, which is the area of land surrounding the landfill boundary as defined by a line running parallel to and 250 m away from the edge of the waste if this can be identified or, if not, the recognized landfill site boundary.
9.2.1.3 In accordance with aforementioned framework, the following tasks have been undertaken to assess potential LFG hazard associated with the closed and restored Ngau Tam Mei landfill (NTML) to potentially sensitive elements of the Project area situated within the NTML Consultation Zone.
·
A review of
background information (including landfill gas monitoring data) and studies
related to the NTML including sections 14 (Landfill Gas Hazard Assessment) and
Section 15 (Impacts on the Restored Ngau Tam Mei Landfill) of approved EIA, AEIAR-143/2009 for the Hong Kong Section of Guangzhou - Shenzhen - Hong Kong
Express Rail Link;
·
Identification
of the nature and extent of the source, including the likely concentrations and
/ or amounts of hazardous emissions with potential to impact the Project;
·
Identification
of possible subsurface pathways and the nature of these pathways through which
hazardous emissions must traverse if they are to reach the development;
·
Identification
of potentially sensitive receivers / elements of the development that maybe
susceptible to landfill gas ingress/accumulation;
·
Qualitative
assessment of the degree of risk which the hazardous emissions may pose to aspects
of the development taking account of each source-pathway-target combination;
and
·
A description of
possible precautionary measures (if needed) during construction and operation
phases assuming development constraints are surmountable, and identification of
any monitoring requirements during construction and operation.
9.3
Assessment
Methodology
9.3.1
Criteria
9.3.1.1 In accordance with the Landfill Gas Hazard Assessment Guidance Note, risk associated with landfill gas may be evaluated by assessment of the following three criteria:
·
Source –
location, nature and likely quantities/ concentrations of landfill gas with
potential to affect the development;
·
Pathway – the
ground and groundwater conditions through which landfill gas must pass in order to reach the development; and
·
Target –
elements of a development that may be sensitive to the effects of landfill
gas.
9.3.2 Source
9.3.2.1 The classification of the Source (i.e., Ngau Tam Mei Landfill) is undertaken as follows:
|
Minor |
Landfill sites at which gas controls have been installed and proven to be effective by comprehensive monitoring which has demonstrated that there is no migration of gas beyond the landfill boundary (or any specific control measures) and at which control of gas does not rely solely on an active gas extraction system or any other single control measure which is vulnerable to failure; or old landfill sites where the maximum concentration of methane within the waste, as measured at several locations across the landfill and on at least four occasions over a period of at least 3 months (preferably longer), is less than 5% by volume (v/v). |
|
Medium |
Landfill site at which some form of gas control has been installed (e.g. lined site or one where vents or barriers have been retrospectively installed) but where there are only limited monitoring data to demonstrate its efficacy to prevent migration of gas; or a landfill site where comprehensive monitoring has demonstrated that there is no migration of gas beyond the landfill boundary but where the control of gas relies solely on an active gas extraction system or any other single control system which is vulnerable to failure. |
|
Major |
Recently filled landfill site at which there is little or no control to prevent migration of gas or at which the efficacy of the gas control measures has not been assessed; or any landfill site at which monitoring has demonstrated that there is significant migration of gas beyond the site boundary. |
9.3.2.2 The “significance” of migration is assessed by reference to the concentration, frequency and location at which gas is detected at routinely monitored locations, especially those peripheral to the area of waste deposition. Any concentration of methane or carbon dioxide > 5% v/v above background levels in any monitoring well outside the landfill boundary may indicate significant migration. Concentrations > 1% v/v methane or 1.5% v/v carbon dioxide (above background) indicate less than adequate control of the gas at source.
9.3.2.3 If monitoring data demonstrates that there is no migration of gas and little danger of gas controls failing (e.g., if these comprise solely of passive measures such as a liner) it can be assumed that the site represents a "Minor" Source. Where there is no gas migration but this may be as a result of a single, "vulnerable" control measure (e.g., an active extraction system with no warning of failure), the site should be regarded as a "Medium" or even a "Major" Source depending on the other factors (e.g., size of site and age of waste). Where the effectiveness of gas controls has not been proven by off-site monitoring, assessment always errs on the side of caution and is undertaken on the same basis as if the controls were not in place.
9.3.2.4 The reliability of the monitoring to assess the efficacy of gas control needs to take account of the design, number and locations of monitoring points and the frequency and duration over which monitoring has been undertaken. Monitoring should have been undertaken under different weather conditions including periods of low or falling atmospheric pressure.
9.3.3 Pathway
9.3.3.1 The broad classification is as follows:
|
Very short / direct |
Path length <50m for unsaturated permeable strata and fissured rock or < 100m for man-made conduits |
|
Moderately short / direct |
Path length of 50-100m for unsaturated permeable soil or fissured rock or 100-250m for man-made conduits |
|
Long / indirect |
Path length of 100-250m for unsaturated permeable soils and fissured rock |
9.3.3.2 Factors affecting the extent of gas migration such as soil permeability or spacing and direction of the fissures/joints, vadose zone thickness and the topography and nature of ground over the potential pathway need to be considered. If a preferential pathway from the landfill to the development area exists the pathway is classified as "direct/short" even if it is longer than 100m.
9.3.4 Target
9.3.4.1 Target sensitivities are classified as follows:
|
High sensitivity |
Buildings or 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 |
Buildings, structures or service voids where there is access only by authorized, well trained personnel, such as the staff of utility companies, who have been briefed on the potential hazards relating to landfill gas and the specific safety procedures to be followed or deep excavations. |
|
Low sensitivity |
Buildings/structures which are less prone to gas ingress by virtue of their foundation design (such as those with a raised floor slab). Shallow excavations and developments which involve essentially outdoor activities but where evolution of gas could pose potential problems. |
9.3.5 Risk Categorization
9.3.5.1 Having determined the categories of source, pathway and target, qualitative assessment of overall risk is made by reference to Table 9.1. The potential implications associated with the various qualitative risk categories are summarized in Table 9.2.
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 Categorization of Risk
|
Category |
Level of Risk |
Implication |
|
A |
Very High |
A less
sensitive form of development should be considered otherwise 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 |
No
precautionary measures are required. |
9.3.5.2 Five generic forms of protection are used for mitigation of hazards to a development. These correspond to the risk levels set out in Table 9.3 with the terms used defined in Table 9.4.
Table 9.3 Generic
Protection Measures for Planning Stage Categorization
|
Category |
Generic Protection Measures |
|
A |
Active
control of gas, barriers and detection systems |
|
B |
Active
control of gas, including barriers and detection systems (1) |
|
C |
Semi active
controls. Detection systems in some
situations |
|
D |
Passive
Control |
|
E |
No precautionary
measures required. |
Note (1): The gas protection measures required to
allow the safe development of a Category A risk development will need to be
more extensive than those for a Category B risk development.
Table 9.4 Definition
of Control Terms
|
Terms |
Definition |
|
Active |
Control of
gas by mechanical means e.g. ventilation to dilute gas, or extraction of gas
from the development site using fans or blowers. |
|
Semi active
|
Use of wind
driven cowls and other devices which assist in the ventilation of gas but do
not rely on electrically powered fans. |
|
Passive |
Provision
of barriers to the movement of gas e.g. membranes in floors or walls, or in
trenches, coupled with high permeability vents such as gravel in trenches or
a clear void/permeable layer below structures. |
|
Detection |
Electronic
systems which can detect low concentrations of gas in the atmosphere and can
be linked to alarms and/or telemetry systems. |
9.4
Description of Environment
9.4.1.1 The closed and restored Ngau Tam Mei Landfill is located to the southwest of The Project (Figure 9.1) with a small portion of the planned development node lying within Ngau Tam Mei Landfill Consultation Zone. In general, if a proposed development is to be located within the Consultation Zone of a landfill, the Project Proponent is required to undertake a landfill gas hazard assessment and submit a report to the Environmental Protection Department (EPD) for vetting.
9.4.1.2 Where a proposed development or elements of a development are of higher sensitivity, Landfill Gas Hazard Assessment (LFGHA) is required to provide preliminary technical input for formulating and evaluating development options by ascertaining the risk acceptability for development within or in close proximity to a landfill site.
9.4.1.3 Whilst development details are not finalized, the Recommended Outline Development Plan indicates the affected area is planned for development of an electricity substation with no public access.
9.4.1.4
A phased approach is adopted to qualitatively
assess landfill gas hazard risk and outline a range of possible mitigation
measures for consideration in the substation building design to afford an
appropriate level of protection dependent upon the calculated risk.
9.5
Identification
of Landfill Gas Generation, Characteristics and Hazards
9.5.1 Landfill Gas Generation
9.5.1.1 Infiltration of water into a landfill causes gases to be generated as decomposition of organic materials occurs. Once biodegradation has started the oxygen is soon exhausted and as no replenishment of the free oxygen is available, the waste mass becomes anaerobic. During anaerobic fermentation methanogens generate methane and carbon dioxide, the primary constituents of landfill gas. A typical composition of LFG is about 60% by volume of methane and 40% by volume of carbon dioxide, although these percentages can vary widely depending on the site conditions. Also present are trace quantities of hydrogen sulphide, nitrogen and gaseous hydrocarbons.
9.5.1.2 Due to the high variability in the settings of biodegradation, waste composition, and individual site characteristics, the rate of degradation and the volume of landfill gas produced per unit of waste can vary greatly. The generation of LFG is dependent on numerous environmental conditions including temperature, pH, substrate availability, moisture content and oxygen content.
9.5.2 Landfill Gas Characteristics
9.5.2.1 Whilst methane has relatively low solubility in water, is colourless and odourless, and generally of little influence in groundwater quality, it occurs in gaseous form in the unsaturated zone. The gas, which is also an asphyxiant, is highly flammable and can be explosive when all the following conditions exist at the same time:
·
Its
concentration in air is between 5% of the Lower Explosive Limit (LEL) and 15%
of the Upper Explosive Limit (UEL);
·
the gas is in a
confined space; and
· a source of ignition exists.
9.5.2.2 The relationship between methane and oxygen where flammable mixtures can occur is shown in Plate 9.1 (from 30 CFR § 57.22003, MSHA Illustration 27).
Plate 9.1 Flammability
Levels of Methane and Oxygen
9.5.2.3 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.
9.5.2.4 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.
9.5.3 Landfill Gas Hazard
9.5.3.1 Given the potentially flammability, asphyxiant properties and gaseous density of LFG, potential hazard arises in the event that LFG is able to migrate from the landfill and accumulate in confined spaces such as building basements, underground car parks, lift shafts, pumping stations, and maintenance chambers etc. For the same reasons, temporary structures such as site huts and any other unventilated enclosures erected during construction stage may also be exposed to landfill gas hazards.
9.5.4 Landfill Gas Migration
9.5.4.1 Methane will migrate along pressure gradients from areas where it is present at higher pressures to areas where it is present at lower pressures. The primary mechanism for significant methane migration in subsurface unsaturated soils is pressure-driven flow. Diffusion also occurs but at rates too low to result in unacceptable indoor air concentrations under reasonably likely scenarios.
9.5.4.2 The ability for landfill gas to migrate beyond the waste boundary varies according to the type of landfill construction details, presence of gas and leachate control measures, restoration details and permeability of the ground through which gas must travel. Factors such as changes in atmospheric pressure can also encourage gas migration.
9.5.4.3 If gas is able to intercept any buried service routes especially where the utility has been laid in an open conduit or the trench excavation has been backfilled around the utility line with coarse gravel; these may also be susceptible to potential hazards and/or they may act as preferential gas migration pathways.
9.6
Quantitative Assessment of Potential
Risk
9.6.1 Source
9.6.1.1 Ngau Tam Mei Landfill occupies approximately 1.7ha and was formed in a natural stream valley generally oriented northeast to southwest. A review of information suggests that waste disposal occurred on an informal basis as early as 1963, with more formal waste placement occurring between 1973 and 1975 resulting in approximately 90,000m3 of waste disposed of prior to landfill closure.
9.6.1.2
The landfill was configured as two platforms; an
upper platform between +32mPD and +36mPD which gently slopes from northeast to
southwest while the lower platform is between elevations +24mPD and +26mPD,
with a slightly steeper slope towards the southwest. The toe of the landfill is approximately
+16mPD.
9.6.1.3 Landfill restoration works in 1999 consisted of placement of a “high integrity” capping system over the two platforms; minor modifications to the existing leachate management system to provide for collection and transport to an off-site treatment facility; installation of a passive landfill gas (LFG) ventilation system; and on-going monitoring of groundwater, leachate levels and landfill gas. The leachate management system consists of a simple piping network installed at the base of the landfill, and a concrete chamber near the toe of the lower slope. The landfill gas management system consists of nine vertical passive vent pipes (VV-1 to VV-9) installed to depths to 3.0-9.0m across the upper platform and horizontal pipes installed in relatively shallow trenches with vertical passive vent risers aligned around the perimeter of the upper platform, along the toe of the upper slope and diagonally across and down the lower slope. The network of passive horizontal trenches and vertical risers around the perimeter of the upper platform likely have a limited depth of waste beneath.
9.6.1.4 The passive venting system acts as the primary control will minimize build-up of LFG pressure within the landfill and hence reduce the potential for sub-surface off-site migration. Under the North-west New Territories Landfills and Gin Drinkers Bay Landfill Restoration Contract No. EP/SP/30/95A LFG a monitoring programme is in place which acts as a secondary control to monitor the effectiveness of the passive venting system and provide an early warning of any off-site migration of LFG. The locations of LFG and groundwater monitoring wells are presented on Figure 9.3.
9.6.1.5 From the landfill gas monitoring data provided for the period July 2019-June 2021, no methane is detected in any of the monitoring wells within the waste boundary or outside the waste boundary. The majority of wells are located to the west and south of the landfill and the nearest gas monitoring wells to the Project boundary are monitoring wells A451 and DH408 located within the northern portion of the landfill. A451 is located at the top of the slope to the east of waste boundary while DH408 is located close to the toe of the slope to the west of waste boundary.
9.6.1.6 Averaged and ranged landfill gas monitoring data for monitoring well A451 and DH408 over the 24 monitoring events between July 2019 and June 2021 is summarized in Table 9.5. Full monitoring data presented in Appendix 9.1.
Table 9.5 Summarized LFG monitoring data for A451
and DH408 July 2019- June 2021
|
Well |
Parameter |
July 2019-June 2021 Average |
July 2019- June 2021
Range |
|
A451 |
Methane |
<0.1% |
- |
|
Carbon
Dioxide |
6.13% |
0.8-9.9% |
|
|
Oxygen |
14.48 % |
8.8-20.6% |
|
|
DH408 |
Methane |
<0.1% |
- |
|
Carbon
Dioxide |
2.25% |
<0.1-6.1% |
|
|
Oxygen |
17.80% |
12.9-20.7% |
9.6.1.7 As shown in the monitoring data, there is no evidence of accumulation of methane in any of the monitoring wells suggesting that methane production within the waste mass is extremely low and/or that pressure heads are insufficient to drive any lateral migration of gas beyond the waste mass. Elevated carbon dioxide concentrations are occasionally recorded an in the absence of background soil gas concentrations for reference, a conservative assumption is that the potential for off-site migration of landfill gas cannot be eliminated.
9.6.1.8 Typical landfill gas production phases are shown in Plate 9.2. The phase duration will vary according to specific landfill conditions such as composition of the waste, the restoration of the landfill, and the provision of landfill gas and leachate management systems (Crawford and Smith 1985).
9.6.1.9 Based upon the timeline of historic operations alone, gas production at Ngau Tam Mei Landfill may be in the latter stages of Phase IV or beyond with end of methane production if the majority of the organic matter has been degraded.
Plate 9.2 Production
phases of typical landfill gas (USEPA 1997)
9.6.2
Classification
of Source
9.6.2.1 NTML can be considered an ‘old landfill’ site where the maximum concentration of methane within the waste, as measured at (ongoing) monthly intervals as demonstrated by EPD data from August 2019 to July 2021 is <0.1% by volume. Whilst detections of carbon dioxide greater than 5% v/v have occasionally been measured in A451 located outside the site’s eastern boundary and in DH408 located close to the toe of the slope to the west of waste boundary but within the site boundary; given the age of the landfill and likely phase of gas production, volumes of gas evolution resulting in a pressure gradient and lateral migration of gas are not anticipated to be significant especially as passive venting creates a preferential vertical gas migration pathway from the landfill.
9.6.2.2 Assessment suggests that there is no serious LFG migration problem therefore the restored NTML can be classified as a “Minor” Source.
9.6.3 Pathways
9.6.3.1 Potential pathways through which landfill gas may migrate include; 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.
9.6.3.2 Recent groundwater monitoring data indicates the thickness of the unsaturated zone of the subsurface through which gas may migrate.
Table 9.6 Depth (m) to groundwater measured from
the wellhead cover
|
Well /Date |
17-8-19 |
6-11-19 |
19-2-20 |
14-5-20 |
14-8-20 |
18-11-20 |
5-2-21 |
18-5-21 |
|
A458 |
4.33 |
6.99 |
8.57 |
9.18 |
4.02 |
8.09 |
8.94 |
9.45 |
|
DH403 |
16.12 |
17.04 |
19.12 |
19.94 |
19.52 |
19.31 |
20.22 |
20.71 |
|
DH404A |
8.4 |
10.53 |
13.04 |
13.85 |
12.25 |
12.85 |
13.98 |
14.54 |
|
DH405 |
6.33 |
7.33 |
8.73 |
9.23 |
7.64 |
8.29 |
8.74 |
9.19 |
|
DH407 |
24.15 |
23.28 |
25.07 |
26.07 |
26.4 |
26.15 |
Dry |
Dry |
|
DH408 |
20.98 |
20.66 |
22.19 |
Dry |
22.76 |
22.66 |
Dry |
Dry |
|
GW1 |
18.13 |
18.47 |
20.5 |
21.28 |
21.62 |
20.47 |
21.05 |
21.92 |