This section gives a brief description of relevant legislation and
guidelines for Landfill Gas (LFG) hazard assessment. Ma Tso Lung Landfill
(MTLL, close to KTN NDA) is located near the proposed KTN NDA; as shown
in Figure 10.1.
MTLL is situated in the vicinity of the KTN NDA. A portion of the
KTN development falls within the 250m Consultation Zone. A qualitative risk
assessment is conducted with landfill gas precautionary/protection design
recommenced.
The landfill gas hazard assessment
has been conducted in accordance with the requirements of Annexes 7 and 19 of
the TM-EIAO as well as the requirements set out under Clause 3.4.11 of the EIA
Study Brief.
10.2.1 Criteria and Guidelines
The relevant legislations,
standards and guidelines applicable to the present study for the assessment
of 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).
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.
The KTN NDA developments will encroach into part of the MTLL 250m
Consultation Zone. Hence, a qualitative risk assessment is needed for the
safety of the developments.
10.2.2 Scope of the Assessment
In accordance with the procedures recommended in Landfill Gas Hazard
Assessment Guidance Note, the following tasks have been undertaken to allow a
full consideration of the potential risk of LFG from the MTLL to KTN NDA:
·
Review of background information (including landfill gas monitoring
data, if any) and studies related to the MTLL;
·
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;
·
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 are to reach the development;
·
Review of design of the proposed developments within the 250m
Consultation Zone and identification of the potential receivers which are
sensitive to the impacts of the hazardous emissions during both construction
and operational phases;
·
Qualitative assessment on the degree of risk which the hazardous
emissions may pose to the developments taking account of each
source-pathway-target combination; and
·
Design of suitable level of precautionary measures and contingency
plan for the Project and the types of protective measures for safe operations,
if needed during both construction and operational phases of the proposed NDA
developments.
10.3.1 Ma Tso Lung Landfill and KTN
A number of
previous studies have been undertaken at, or involving, the MTLL including
Planning and Development Study on North East New Territories; the Feasibility
Study on Restoration of North-west New Territories Landfills; North-west New
Territories Landfills and Gin Drinkers Bay Landfill Restoration – Contract
Arrangements; and the EIA of the Restoration Works under Contract No
EP/SP/30/95. The documents which have been used as background material for the
preparation of this assessment include the following:
·
Landfill gas and surface gas monitoring data for the MTLL and
background information regarding the restoration works and landfill gas control
measures provided by EPD in 2009 and 2013;
·
Agreement No. CE 64/96. Planning and Development Study on North East
New Territories: Technical Paper 13 – Environmental Impact Assessment,
Maunsell Consultants Asia Ltd, June
2003;
·
Agreement No. CE 66/94. North-west New Territories Landfills and
·
Agreement No. CE 10/92. Restoration of North-west New Territories
Landfills: Initial Environmental Impact Assessment Report, Scott Wilson
Kirkpatrick Consulting Engineers, April 1995;
·
Agreement No. CE 10/92. Restoration of North-west New Territories
Landfills: Final Report, Scott Wilson Kirkpatrick Consulting Engineers, August
1995; and
·
Hong Kong Geological Survey Sheet 2, Geology of San Tin.
10.3.1.2 Background Information
The MTLL is located approximately 750m north of Pak Shek Au near the
Closed Border Area. The landfill site is at the head of a small valley,
surrounded on three sides by higher land (except the east side). The whole site
is within the Study Area and is next to the proposed development in KTN NDA.
The landfill site occupies an area of about 2 ha and was operated
between 1976 and 1979. About 180,000 tones of domestic, industrial and
construction wastes were deposited at the site during this period. Restoration
of landfill site has been completed in 2000 and the site is currently for recreational use by Tung Wah
Group of Hospitals. EPD is currently implementing “Aftercare” of the landfill which
includes operating and maintaining the landfill gas management system and
environmental monitoring work. The contract period of the Aftercare Phase lasts
for 30 years, from June 2000 to May 2030.
As shown in Figure 10.2, the development of
KTN NDA will encroach within
the MTLL and its 250m Consultation Zone. The developments
falling within the Consultation Zone include the Standard
Swimming Pool and Sports Centre Zone (E1-5), the Residential Zones (A2-2),
Educational Zones (E1-2, E1-4, A3-1 and
A3-2), Government Zones (E1-3),
Amenity Zones (A2-1), Green Belt Zone (H1-1) and vehicular roads, whilst
the developments falling within the MTLL include the District Open Space Zone
(E1-1) where recreational area is proposed. There will only be minor construction
works required in
site E1-1 and the landfill restoration facilities within
MTLL will remain unaffected.
10.4.1 Potential Hazards and Properties of LFG
LFG presents a number of potential hazards and has particular
properties of note, as follows:
·
Methane is odourless, 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.
·
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.
·
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.
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 developments to LFG.
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% by volume (v/v).
PATHWAY
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 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 50 m for unsaturated permeable strata and
fissured rock or less than 100 m 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 250 m for unsaturated permeable soil 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:
·
Particular permeability of the soils;
·
spacing, tightness and direction of the fissures/joints;
·
topography;
·
depth and thickness of the medium through which the gas may migrate
(which may be affected by groundwater level);
·
the nature of the strata over the potential pathway;
·
the number of different media involved; and
·
depth to groundwater table and groundwater flow patterns.
TARGET
Different levels of vulnerability or sensitivity of potential
Targets for LFG have been classified as follows:
Highly sensitive: 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/ structures which are less prone to gas ingress by virture
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 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.
10.4.3 Classification of Risk Category
Following the determination of the categories of source, pathway and
target in which the landfill, pathway and development fall, a qualitative
assessment of the overall risk is undertaken with reference to Table 10.1,
which is extracted from EPD’s Guidance Note. The potential implications
associated with the various qualitative risk categories are summarized in Table 10.2.
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 10.1 - 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 10.2 - 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. |
10.5.1 Ma Tso Lung Landfill and KTN
10.5.1.1
The Source
LFG Monitoring Results before Restoration
Various site investigations and monitoring programmes have been
undertaken between 1993 and 1999 and provided lots of information on the
activities of the landfill. A total of eleven drillholes are installed within
the waste and around the periphery of the landfill beyond the waste boundary. These
were used for the monitoring of off-site migration of LFG and groundwater
quality. The monitoring drillholes located on- and off-site at the landfill are
shown in Figure 10.3 and as follows:
On-site drillholes: DH302 and DH303 (The locations were not provided).
Off-site drillholes: DH304, DH305, DH307, DH308, A351, A352, A353,
A354 and A356.
High concentrations of methane and carbon dioxide have been observed
in the on-site monitoring drillholes DH302 and DH303. At DH302 the highest
concentrations of methane and carbon dioxide recorded were 72.1% v/v in October
1998 and 38.7% v/v in February 1997 respectively. The highest concentrations
found at DH303 were 74.8% gas methane in September 1998 and 39.5% gas carbon
dioxide in February 1997, respectively. This indicates that the landfill was
still actively generating LFG.
The monitoring undertaken during this period revealed no methane in
the off-site monitoring drillholes DH304, DH305, DH307, DH308, A352, A353, A354
and A356. The only off-site monitoring drillhole where significant methane
concentrations have been detected is A351 located immediately adjacent to the
southern landfill boundary. The maximum methane concentration found at A351
throughout the monitoring period was 29% v/v in September 1998. The maximum
carbon dioxide concentration in drillhole A351 was 5.2% in March 1998.
Elevated concentrations of carbon dioxide were observed in all of
the off-site monitoring drillholes, with the highest concentrations ranging
from 3.44% gas in September 1998 in monitoring drillhole DH308 to 32.4% gas in
February 1998 in monitoring drillhole DH304. There have been no obvious trends
in the gas concentrations recorded in these drillholes.
Restoration Stage
EPD has commissioned Swire SITA Waste Services Limited in 1999 to
undertake a Design, Build and Operate (DBO) Contract (North-west New
Territories Landfills and Gin Drinkers Bay Landfill Restoration (Contract No
EP/SP/30/95)) to restore the Siu Lang Shui, Ma Tso Lung, Ngau Tam Mei and Gin
Drinkers Bay Landfills. The restoration works of the MTLL have been completed
in 2000. The following restoration works with respect to LFG and leachate
control have been undertaken at the MTLL.
·
Installation of a new capping system (comprising an impermeable LLDPE
liner, sub-soil drains and surface water drainage systems) for the whole
landfill site;
·
Installation of a passive LFG venting system;
·
Installation of a leachate
collection and storage system; and
·
Installation of new monitoring wells.
The passive venting system which acts as the primary control will
minimize the LFG pressure within the landfill and hence reduce the potential
for sub-surface off-site migration. A comprehensive LFG monitoring programme
which acts as the secondary control has also been implemented to monitor the
effectiveness of the passive venting system and provide an early warning of any
off-site migration of LFG.
A number of the existing off-site LFG monitoring drillholes
(including DH304, DH305, DH307, DH308, A351, A352, A353, A354 and A356) were
adopted by the Landfill Restoration Contractor and used to monitor off-site LFG
migration under the restoration contract. One off-site drillhole GG1 which is
the downstream of the MTL Landfill was also applied for LFG monitoring (Figure
10.3). LFG monitoring results during restoration (1999-2000) are
summarized in Table
10.3.
Table 10.3 - Summary of MTLL LFG monitoring
results (April 1999 to June 2000) (1)
|
Location |
Methane (% v/v) |
Carbon Dioxide (% v/v) (3) |
|||
|
Range |
Average |
Compliance Level |
Range |
Average |
|
|
DH304 |
<0.01 - 0.19 |
0.03 |
1 |
1.52 - 28.4 |
11.8 |
|
DH305 |
<0.01 |
0.01 |
1 |
1.43 - 17.1 |
8.06 |
|
DH307 |
<0.01 - 0.16 |
0.02 |
1 |
3.3 - 13.5 |
7.07 |
|
DH308 |
<0.01 |
0.01 |
1 |
1.8 - 8.2 |
3.84 |
|
A351 |
0.14 - 3.8 |
0.92 |
15 (2) |
0.71 - 5.95 |
4.00 |
|
A352 |
<0.01 - 0.03 |
0.01 |
1 |
1.92 - 15.8 |
5.07 |
|
A353 |
<0.01 - 0.04 |
0.03 |
1 |
1.85 - 19.5 |
9.14 |
|
A354 |
<0.01 - 0.02 |
0.01 |
1 |
2.26 - 6.64 |
4.99 |
|
A356 |
<0.01 - 0.21 |
0.07 |
1 |
1.89 - 16.2 |
5.60 |
|
GG1 |
<0.01 - 0.24 |
0.10 |
- |
4.86 - 18.3 |
10.71 |
Note:
1. Information was extracted from the Report of
Agreement No. CE 64/96 Planning and Development Study on North East New
Territories: Technical Paper 13 – Environmental Impact Assessment,
Maunsell Consultants Asia Ltd, June 2003
2. Due to high background level
3. There are no standard compliance levels for
carbon dioxide. However, as stated in EPD’s Guidance Note, it should be assumed
that any concentration of carbon dioxide greater than 5% v/v above background
levels in any monitoring well outside the landfill’s boundary indicates
significant migration.
Low concentrations of methane have been observed in all off-site
monitoring drillholes. The only drillhole where significant methane
concentrations have been detected is A351 located immediately adjacent to the
southern landfill boundary. The maximum methane concentration was 3.8% v/v on
June 2000, which is nevertheless significantly lower than that before the
restoration. This suggested that the restoration measures have been effectively
implemented to control gas migration.
The carbon dioxide concentrations detected in off-site drillholes
were similar before and after the restoration work. Maximum gas concentration
of 28.4% v/v in drillhole DH304 was recorded in June 1999. The average carbon
dioxide concentrations in most of the monitoring wells are above 5% v/v.
Recent Monitoring Results (Aftercare Phase)
Table 10.4 -
Summary of MTLL methane monitoring results
(January 2008 to December 2012) (1)
|
Location |
Methane (% v/v) |
Oxygen (% v/v) |
|||
|
Range |
Average |
Compliance Level |
Range |
Average |
|
|
DH304 |
<0.1 |
<0.1 |
1 |
0.1 - 20.0 |
14.6 |
|
DH305 |
<0.1 |
<0.1 |
1 |
5.4 - 21.0 |
17.3 |
|
DH307 |
<0.1 |
<0.1 |
1 |
9.7 - 21.2 |
15.7 |
|
DH308 |
<0.1 |
<0.1 |
1 |
2.1 - 21.0 |
18.4 |
|
A351 |
<0.1 |
<0.1 |
1 |
3.3 - 21.3 |
16.7 |
|
A352 |
<0.1 |
<0.1 |
1 |
5.5 - 20.8 |
13.6 |
|
A353 |
<0.1 |
<0.1 |
1 |
3.1 - 21.0 |
17.4 |
|
A354 |
<0.1 |
<0.1 |
1 |
1.8 - 20.2 |
14.5 |
|
A356 |
<0.1 |
<0.1 |
1 |
5.8 - 19.0 |
15.2 |
|
GG1 |
<0.1 |
<0.1 |
1 |
1.2 - 20.9 |
13.9 |
Notes:
1.
All information is provided by EPD.
All monitoring data in past five years tally with the compliance
level of 1% methane. No exceedance or abnormal data was detected.
Table
10.5 - Summary of MTLL carbon dioxide monitoring results (January 2008 to December 2012) (1)
|
Location |
Carbon Dioxide (% v/v) |
|||
|
Range |
Average |
Background Level |
Compliance Level |
|
|
DH304 |
<0.1 – 19.3 |
5.0 |
32.4 |
33.9 |
|
DH305 |
<0.1 – 8.7 |
2.6 |
20.5 |
22.0 |
|
DH307 |
<0.1 - 10.0 |
3.4 |
13.5 |
15.0 |
|
DH308 |
<0.1 - 5.3 |
1.6 |
8.2 |
9.7 |
|
A351 (2) |
0.1 – 8.6 |
2.4 |
7.3 |
8.8 |
|
A352 |
<0.1 – 13.7 |
6.0 |
18.6 |
20.1 |
|
A353 |
<0.1 – 12.7 |
3.6 |
19.5 |
21.0 |
|
A354 |
0.6 – 12.1 |
5.4 |
15.1 |
16.6 |
|
A356 |
2.1 – 7.1 |
5.1 |
16.2 |
17.7 |
|
GG1 |
<0.1 - 15.9 |
6.1 |
18.3 |
19.8 |
Notes:
1.
All information is provided by EPD.
2.
Only one data was detected in drillhole A351 (May 2012, 8.6%) above background level.
Most of the recorded carbon dioxide concentrations were at
relatively high level (>5% v/v); however, the carbon dioxide background
level for each drillhole is very high too and has corresponding to its high compliance
level. Only one data detected in drillhole A351 (May 2012, 8.6%) was
higher than background level but not exceeded the compliance levels.
As stated in EPD’s Guidance Note, any concentration of carbon
dioxide greater than 5% v/v above background levels in any monitoring well
outside the landfill’s boundary indicated significant migration. The MTLL has
potential hazards due to one location (A351, 8.6% CO2) may still has
migration despite the concentration of carbon dioxide is not greater than the
5% v/v above background level of. 7.3%.
Based on surface gas monitoring results of 17 sampling points (Figure
10.4) from January 2008 to December 2012, no carbon
dioxide was detected and oxygen in all tests were 21.0% (v/v), most methane was
0 ppm, while only few points show slightly elevated results at some time (5-30 ppm).
Classification of Source
A passive landfill gas venting system has been installed at the MTLL
under the North-west New Territories Landfills and Gin Drinkers Bay Landfill
Restoration Contract. Recent LFG monitoring results (Tables 10.4 and 10.5)
in Aftercare Phase indicated methane concentrations were at very low level
(<1% v/v). Although relatively high carbon dioxide concentrations were
recorded, due to the high backgrounds and corresponding high compliance levels,
only one data was detected above the background levels. The results indicated
that there were no serious LFG migration problems; however, migration might
still happen at one location (i.e. A351). According to Paragraph 3.9 and 3.11
of EPD’s Landfill Gas Hazard Assessment Guidance Note, the restored MTLL will
be classified as a “Medium” Source.
It should be noted that the Landfill Restoration Contractor is
required to undertake regular LFG monitoring at the off-site monitoring drillholes.
The Restoration Contract specified a stringent compliance level of 1% methane
(20% LEL) for the off-site monitoring drillholes.
General
The potential pathways through which LFG may enter the KTN NDA site
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 Pathways - Geology and Hydrogeology
The occurrence of fissures along the Ma Tso Lung fault (Figure
10.5), near the south and south-eastern boundary of the landfill may
provide a preferential pathway for LFG migration. The faults are thought to be
relatively deep and below the permanent groundwater level. Therefore it is
expected that it has little effect on the gas migration potential to the
proposed development. In addition, there are no direct connection between the
landfill and the fault line.
Migration Potential Due to Natural Pathways
The previous sections have examined the geological and
hydrogeological settings of the landfill and the area between the landfill and
the proposed development. Taking account of the permeability of the strata
between the landfill and proposed development, depth and path length of the
unsaturated permeable strata, and the location of the Ma Tso Lung fault, it is
considered that this natural pathway for LFG migration from the landfill to the
proposed developments
within the Consultation Zone should be classified as “moderately short to long”, whilst
that to the proposed development within the MTLL should be classified as “very short/ direct”.
Man-made Pathways
Utility connections to the proposed NDA site have also been
considered as potential pathways for the migration of LFG. It is possible that
a preferential route for LFG migration may comprise the permeable backfill in
which service utilities are installed and following connection to the site, may
allow the direct transmission of LFG to the proposed development. However, the
proposed utilities which are connected to the proposed developments within the Consultation Zone are about 50m away from
the MTLL. There is no direct connection between the landfill and these
utilities. An access road will be built to provide access to the existing road
adjacent to the MTLL from Kwu Tung NDA West Road. The access road will be
formed by filling at the junction connecting the existing road. Shallow
utilities and road drainage are proposed along the road.
Therefore it is considered that the man-made pathway for LFG
migration from the landfill to the proposed development should be classified as
“moderately short to long”.
For the proposed development
within the MTLL, since the future utilities to be installed (if any) will pass
directly through the landfill, the man-made pathway for the proposed
development within MTLL would be considered as “very short/ direct”.
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 LFG affecting the
proposed developments at the NDA.
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 too much longer than the direct distance between the
landfill and the particular receptor in the development within the Consultation Zone, but still consider
direct for the development within MTLL.
LFG related impacts may occur in areas at/below ground at the
proposed development in the NDA during both construction and operational
phases.
Construction Phase
The works boundary in KTN NDA is shown in Figure 10.2. The
construction works to be carried out within the Landfill Consultation
Zone include the Standard Swimming Pool and Sports Centre Zone (E1-5), Residential
Zone (A2-2), Educational Zones (E1-2, A3-1, A3-2 and E1-4), Government Zones (E1-3), Amenity
Zones (A2-1), Green Belt Zone (H1-1) and vehicular roads, whilst the construction works to be carried
out within the MTLL will include the District Open Space Zone (E1-1) where
recreational area is proposed.
Excavation is expected (though only minor excavation works is expected in site E1-1) and the areas of confined space and trenches are especially at certain level of risk to exposure of LFG. Construction staff working in indoor
environment (e.g. site office) may also be at risk to
exposure of LFG. In view of the close proximity to the MTLL, construction monitoring
of landfill gas at confined spaces and trenches, and general precautionary
measures are therefore recommended.
Operational Phase
As described above, proposed developments falls in the Consultation
Zone include
the Standard Swimming Pool and Sports Centre Zone (E1-5), the Residential Zones (A2-2), Educational Zones (E1-2, E1-4, A3-1 and A3-2),
Government Zones (E1-3), Amenity Zones (A2-1), Green Belt Zone (H1-1) and vehicular
roads, whilst the
proposed developments falling within the MTLL include the District Open Space
Zone (E1-1) where recreational area is proposed. There
will only be minor construction works required in site E1-1 and the
landfill restoration facilities within MTLL will remain unaffected.
To minimize the potential
landfill gas hazard to the future occupants of the proposed developments, it is recommended that below ground rooms/voids should be avoided as far
as practicable in
the developments within the Consultation Zone and should totally be avoided in the recreational
developments within the MTLL (i.e. Site E1-1). Avoiding below ground rooms/
voids in the developments within the MTLL will also help to prevent
interruption to the landfill restoration facilities.
Although detailed design information is not available at this stage, it is recommended that below ground rooms/voids should be avoided as
far as practicable in the detailed design stage. To facilitate the assessment
of hazards to the various types of uses at, or below ground, they have been
initially grouped into either highly, moderately or low sensitivity categories,
based on guidance give in the Guidance Note. This involves a consideration of
the following elements:
·
Intended use and contents;
·
Provision and reliability of
ventilation;
·
Frequency of use;
·
The ability to restrict access
by the public; and any other features of the design or specifications which may
render the area of more or less sensitivity.
Highly Sensitive
For the developments within MTLL,
the uses that would likely fall into this category may include: utility pits
and structures into which services enters directly from the ground and to which
members of general public have unrestricted access or which contain sources of ignition.
For the developments within the
Consultation Zone, the uses that would likely fall into
this category may include: underground pump room, utility pits and structures
with ground level or below ground rooms/void or into which services enters
directly from the ground and to which members of general public have
unrestricted access or which contain sources of ignition.
Moderate Sensitivity
The uses that would likely fall into this category may include
transformer room, pump room, and telephone / cable rooms at ground level where
access only by authorized, well trained personnel who have been briefed on the
potential hazards relating to LFG and the specific safety procedures to be
followed.
Low Sensitivity
The uses likely to fall into this category may include open
playground, car park and security post.
10.5.1.4 Source – Pathway – Target Analysis
On the basis of the source, pathways and targets identified above, two
source – pathway – target analysis have been undertaken and is presented in Table 10.6a
(i.e. for the development within the Consultation Zone) and Table 10.6b (i.e. for the development
within MTLL) according to EPD’s assessment framework. The combination of a
medium source term, a pathway
of moderately short to long distance between the
landfill site and the development sites within the Consultation Zone, and a pathway of very short/ direct
between the landfill and the development site within MTLL, results in a range of overall risks depending on the sensitivity of
the particular targets.
Table
10.6a indicates the overall risk level of LFG
hazards to various targets (receivers) in Construction and Operational Phases
within the Consultation Zone is categorized as “Low” (Category D) to “High”
(Category B). Similarly, Table 10.6b also indicates the overall risk level
of LFG hazards to various targets (receivers) in Construction and Operational Phases
within MTLL is categorized as “Low” (Category D) to “High” (Category B). According
to Table 10.2,
some precautionary measures will be required for Category D, engineering
measures will be required for Category C, and Significant engineering measures
will be required for Category B.
Table 10.6a - Qualitative assessment of LFG hazard to the proposed developments
within the Consultation Zone
|
Source |
Pathway |
Target |
Hazard |
|
Ma Tso Lung Landfill A passive LFG
venting system has been installed at the MTLL under the Restoration Contract.
A comprehensive LFG monitoring programme is being conducted under the
Landfill Restoration Contract to provide an early warming and carry out
mitigation measures if in case any significant migration of LFG is detected. LFG monitoring
results in past five years in MTLL have indicated methane concentrations were
at very low level and only one data of carbon dioxide concentration was above the
background levels. According to Para 3.10 of EPD’s 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”, as a conservative approach for the
hazard assessment, the restored MTLL will be classified as a “Medium” source. (Medium source) |
Natural Pathway The proposed
development in KTN NDA is close to the south-east of the MTLL. A fault line
is present at the south-eastern boundary of the landfill and about 140 m to
the south of the landfill. However, there are no direct connections between
the fault line and the proposed development within the Consultation Zone. In addition, the fault line is thought to be relatively deep and
below the permanent groundwater level. It is expected it will have little
effect on the gas migration potential to the proposed development within the Consultation Zone. (Moderate short/direct) Man-made
Pathway There is no
extensive existing under ground utility structures within the Consultation
Zone.
A telecommunication cable can be found along the northern boundary of the
existing community sports centre. The proposed underground utilities which
will connect to the proposed development in the NDA within the Consultation Zone includes water mains, power and telecommunication cables. These
utilities are running parallel to Kwu Tung NDA West Road and are about 50m
away from the landfill. An access road will be built to re-provide access to
the existing road adjacent to the MTLL from Kwu Tung NDA West Road. The
access road will be formed by filling at the junction connecting the existing
road. Shallow utilities and road drainage are proposed along the road. (Moderate short/direct) Combination
of Natural and Man-made Pathways The combination
of natural and man-made pathway may allow potential gas migration to the
proposed developments at the NDA. However, this mode of transmission
generally requires a series of events to occur in order for gas to
potentially affect the developments. (Moderate short/direct) |
Construction Phase Medium Sensitivity The
construction works to be carried out within the Landfill Consultation Zone
include the Standard Swimming
Pool and Sports Centre Zone (E1-5), Residential Zone (A2-2), Educational Zones (E1-2, A3-1, A3-2 and E1-4), Government Zones (E1-3), Amenity Zones (A2-1), Green
Belt Zone (H1-1) and vehicular roads. Excavation is
expected and the areas of confined space and trenches are
especially at a higher risk to exposure of LFG. Construction staff working in
indoor environment (e.g. site office) may also at risk to exposure of LFG. Nevertheless, during
the construction phase all of these development sites would only be
accessible by authorised, well trained personnel who would have been briefed
on the potential hazards relating to landfill gas and the specific safety
procedures to be followed. As such, the risk level of the development sites
during the construction phase is considered as Medium Sensitive. Operational
Phase Highly Sensitive The uses that
would likely fall into this category may include: underground pump room,
utility pits and structures with ground level or below ground rooms/void or
into which services enters directly from the ground and to which members of
general public have unrestricted access or which contain sources of ignition. Medium
Sensitivity The uses that
would likely fall into this category may include transformer room, pump room,
and telephone / cable rooms at ground level where access only by authorized,
well trained personnel who have been briefed on the potential hazards
relating to LFG and the specific safety procedures to be followed. Low
Sensitivity The uses likely
to fall into this category may include open playground, car park and security
post. |
Medium High Medium Low |
Table 10.6b - Qualitative assessment of LFG hazard to the proposed
developments within MTLL
|
Source |
Pathway |
Target |
Hazard |
|
Ma
Tso Lung Landfill A
passive LFG venting system has been installed at the MTLL under the
Restoration Contract. A comprehensive LFG monitoring programme is being
conducted under the Landfill Restoration Contract to provide an early warming
and carry out mitigation measures if in case any significant migration of LFG
is detected. LFG
monitoring results in past five years in MTLL have indicated methane
concentrations were at very low level and only one data of carbon dioxide concentration was above the background levels. According to Para 3.10 of EPD’s 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”, as a conservative approach for the hazard
assessment, the restored MTLL will be classified as a “Medium” source. (Medium source) |
Natural
Pathway For the proposed
development within MTLL, since it is located directly above the landfill, a
certain degree of gas migration potential to the proposed development would
be expected. (Very short/ direct) Man-made
Pathway For the proposed
development within the MTLL, the future utilities to be installed (if any)
are likely to be connected directly to the landfill. (Very short/ direct) Combination
of Natural and Man-made Pathways The
combination of natural and man-made pathway may allow potential gas migration
to the proposed developments within MTLL. However, this mode of transmission
generally requires a series of events to occur in order for gas to
potentially affect the developments. (Very short/
direct) |
Construction
Phase Medium Sensitivity The construction works to be carried out
within the MTLL will include the District Open Space Zone (E1-1) where
recreational area is proposed. Minor
excavation is expected and the areas of confined space and trenches are especially at a higher risk to exposure
of LFG. Construction staff working in indoor environment (e.g. site office)
may also at risk to exposure of LFG. Nevertheless,
during the construction phase the development site would only be accessible
by authorised, well trained personnel who would have been briefed on the
potential hazards relating to landfill gas and the specific safety procedures
to be followed. As such, the risk level of the development site during the
construction phase is considered as Medium Sensitive. Operational
Phase Highly Sensitive The
uses that would likely fall into this category may include: utility pits and
structures into which services enters directly from the ground and to which
members of general public have unrestricted access or which contain sources
of ignition. Medium
Sensitivity The
uses that would likely fall into this category may include transformer room,
pump room, and telephone / cable rooms at ground level where access only by
authorized, well trained personnel who have been briefed on the potential
hazards relating to LFG and the specific safety procedures to be followed. Low
Sensitivity The
uses likely to fall into this category may include open playground, car park
and security post. |
Medium High Medium Low |
10.6.1 Introduction
The qualitative landfill gas hazard assessment undertaken in Section 10.5.1 has concluded that the level
of risk for KTN development within Consultation Zone during the
construction phase is “Medium” and during operational phase is ‘Low’ to
‘High’ depending upon the location and nature of the target being considered. Similarly,
the level of risk for the developments within MTLL during the construction
phase is also “Medium” and during operational phase is ‘Low’ to ‘High’ depending upon the
location and nature of the target being considered.
This section provides general advice and recommendations for the
avoidance of environmental impacts related to LFG during the construction and
operational phase. A detailed qualitative LFG hazard assessment (QLFGHA) should
be carried out by individual developer during the detailed design stage (such requirement could be
included in the lease condition by Lands D) in accordance with the Guidance
Notes for Landfill Gas Hazard Assessment. Where
applicable, specific measures for handling the hazards identified during the
operational phases should be addressed to further reduce the likelihood of
incidents and increase the level of safety to the public. These measures should
be reviewed taking into account the findings of the detailed QLFGHA to be
undertaken by the developers during the detailed design stage as mentioned above.
In addition, the design, construction and operation of the proposed
development within the MTLL (i.e. the proposed recreational area in site E1-1)
should be fully compatible with the landfill restoration and aftercare works
and impose no adverse impact to them. Caution should also be exercised to
ensure long term integrity of the capping system and other restoration facilities.
Design and Construction of the proposed development within the MTLL should be
provided to EPD for agreement in the design stage.
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.
10.6.2 General Recommended Precautionary and Protection Measures – Design Phase
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
10.2.
According to the source-path-target analysis in Tables
10.6a and 10.6b,
depending upon the locations and nature of the targets being considered, the
risk category at the proposed developments within the Consultation Zone and
within MTLL are both ‘Low’ to ‘High’ during the operational phase. This implied “some precautionary measures” to “significant
engineering measures required” by the future site developers to protect
proposed development.
For the high risk category, the use of active control of gas,
including barriers and detection systems are recommended. These measures
include the control of gas by mechanical means e.g. ventilation of spaces with
air to dilute gas, or extraction of gas using fans or blowers. For the low risk
category, the provision of barriers to the movement of gas is recommended. Measures
recommended include the use of membranes in floors or walls, or in trenches,
coupled with high permeability vents such as no-fines gravel in trenches or
voids/permeable layers below structures. The need and practicality of
incorporating such measures should be reviewed in the detailed Qualitative LFG
Hazards Assessment (QLFGHA) to be undertaken during the detailed design
stage for developments within the Consultation Zone and within MTLL. Detailed
precautionary and protection measures should be recommended in the QLFGHA.
Nevertheless, to
minimize the risk of landfill gas hazard to the future occupants of the
proposed developments, below ground rooms/ voids should be avoided as far as
practicable in the developments within the Consultation Zone. For the proposed
developments within the MTLL, underground rooms/ voids should totally be
avoided not only to minimize the risk of landfill gas hazard to the future
occupants, but also
to prevent interruption to the landfill restoration facilities. In addition, buildings or structures within the MTLL (i.e.
the proposed recreational area in site E1-1) should be at ground level with raised floor
slabs which are less prone to gas ingress.
In addition, the design and construction method of the proposed
development within MTLL (i.e. the proposed recreational area in site E1-1)
should be provided to EPD for agreement in the design stage to ensure
compatibility with the landfill restoration facilities and
aftercare works
within MTLL, such that these facilities and works will not be affected by the
construction or operation of the proposed development.
10.6.3 General Recommended Precautionary and Protection Measures –
Construction Phase
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”.
Risks in the construction works are mainly resulted 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 most
of the piling and excavation works, there may be 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 landfill Consultation Zone and within the MTLL 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 and appended to this and 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, 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 bore pilling and excavation
works, should receive appropriate training on working in areas susceptible to
LFG.
·
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.
·
Any offices / quarters set up
on site should take precautions against LFG ingress, such as being raised off
the ground. Other storage premises, e.g. shipping containers, where this is not
possible should be well ventilated prior to entry.
·
Adequate precautions to prevent
the accumulation of LFG under site buildings and within storage shed should be
taken by raising buildings off the ground where appropriate and “airing”
storage containers prior to entry by personnel and ensuring adequate
ventilation at all times.
·
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 Officer. 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.
·
Ongoing gas monitoring should
be considered for offices, stores etc set up on site.
10.6.3.1 Monitoring
Monitoring should be undertaken when construction works are carried
out in confined space within the Consultation Zone and within MTLL. Routine
gas monitoring should be undertaken during
groundwork construction and in all excavations. Monthly gas monitoring should
also be conducted for offices, stores etc set up on site. 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 10 mm 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.
10.6.3.2 Actions in the Event of Abnormal Gas being Detected
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 10.7.
Table
10.7 - Actions in the event of LFG being detected
|
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% |
10.6.3.3 Emergency Management
In order to ensure that evacuation procedures are implemented in the
event of the trigger levels specified in Table 10.7
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.
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:
·
·
Fire Services Department;
·
Environmental Protection
Department.
10.6.4 General Recommended Precautionary and Protection Measures –
Operational Phase
10.6.4.1 Utility Companies
The developers should make the utility companies aware of the
location and features of the site within the 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.
Should utility installation be
required in site E1-1, the developers should make the utility companies aware
of the potential constraints imposed by the landfill restoration facilities and
aftercare works to ensure these facilities and works will remain unaffected. Appropriate
precautionary measures against landfill gas should also be taken should utility
installation be required within the MTLL.
10.6.4.2 Building Management
The management committee of the building estate will hold a special
responsibility to ensure that the occupants of the building, its staff and
maintenance workers are protected from LFG and that visitors to the site within the Consultation Zone and
within the MTLL are also made aware as to the dangers
and the precautions required to be taken.
Of primary importance to satisfactorily upholding this
responsibility will be to ensure that strict procedures for maintaining control
over all temporary and /or permanent works proposed at the site are reviewed
with regard to the LFG hazard. This needs to be accompanied by a comprehensive
contingency plan in case of incidents, including liaison with EPD officers,
Fire Services Department, Landfill Restoration Contractors and others, as
necessary.
All construction and maintenance (including utilities) personnel
working at the site should be made aware of the hazards of LFG and its possible
presence on site. This should be achieved through a combination of posting
warning signs in prominent places and also by access to detailed information on
LFG hazards and the designs and procedural means by which these hazards are
being minimized on site. In addition, entry to confined spaces such as
refuse/store rooms, drainage manholes etc. should be preceded by a period of
“airing” the space by opening the door widely allowing fresh air to enter.
Where appropriate, monitoring of gas should also precede entry.
Any proposed modifications or additions to the building structure
should be subject to a further assessment of LFG hazard, particularly in areas
where a gas membrane has been installed. Any penetrations of the membrane must
be repaired as soon as possible after detection or works completion using
similar products.
The building management company should also make arrangement with
Landfill Restoration Contractor so that they are advised of all situations
which may potentially threaten the safety of the building occupants resulting
from any accidents or failures at the landfill site. The building management
company should also have available suitable gas monitoring equipment for any ad
hoc investigations necessary relating to LFG and be in a position to undertake
any future routine monitoring of gas which may be considered necessary soloing
completion of the defects correction period.
Precautionary and protection measures recommended for building
management should generally be taken up by the developer of the sites.
To ensure that all the above protection and precautionary measures
and issues pertaining to LFG are properly and consistently addressed by future
users and owners of the site, it is recommended that a comprehensive LFG hazard
management system be developed by the owner of the building or its property
management agency. The system should be developed as part of the QLFGHA before
the occupation of the building and implemented during its operational phase (such requirement could be
included in the lease condition by Lands D).
To protect the site workers and future owners of the developments within the Consultation Zone and within MTLL, it
is recommended that monitoring of any LFG which may be migrated to the site
should be undertaken during the construction of infrastructure and the
development within the Consultation Zone and within MTLL when
the works involve confined spaces. Routine gas monitoring should be undertaken during
groundwork construction and in all excavations. Monthly gas monitoring should
also be conducted for offices, stores etc set up on site. The monitoring
requirement has been discussed in Section
10.6.3.1.
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.
The
design and construction within the sites E1-2 and E1-3 etc should avoid
interference or disturbance to the off-site landfill gas, surface water and
ground water monitoring wells as part of the landfill restoration facilities of
the MTLL. Should it be technically unavoidable, prior approval should be
obtained from EPD for the required modification or relocation of the monitoring
wells.
This section has provided a preliminary qualitative assessment on
potential hazards associated with LFG migration from MTLL to the proposed
development in KTN NDA and within MTLL.
The MTLL is considered as a “Medium” source of gas migration since LFG
monitoring results in past five years in MTLL have indicated methane
concentrations were at very low level and only one data of carbon dioxide concentration
was above the background levels. The risk categories
associated with the source-pathway-target have been identified. It is concluded
that the potential risk during construction phase for developments within the Consultation
Zone is “Medium” and during operational phase is ‘Low’ to ‘High’
depending upon the location and nature of the target being considered. Similarly,
the level of risk for the development within MTLL during the construction phase
is also “Medium” and during operational phase is ‘Low’ to ‘High’ depending upon the
location and nature of the target being considered. Therefore, this implied “some
precautionary measures” to “significant engineering measures” required by the
future site developers to protect the proposed development.
To minimize the
risk of landfill gas hazard to the future occupants of the proposed developments,
below ground rooms/ voids should be avoided as far as practicable in the developments
within the Consultation Zone. For the proposed developments within the MTLL,
underground rooms/ voids should totally be avoided not only to minimize the
risk of landfill gas hazard to the future occupants, but also to prevent interruption to
the landfill restoration facilities. In addition, buildings or structures within
the MTLL (i.e. the proposed recreational area in site
E1-1) should be at
ground level with raised floor slabs which are less prone to gas ingress. It is also
recommended that further LFG monitoring should be
carried out prior to the commencement of the detailed design of the
developments to provide the latest LFG data for the detailed QLFGHA.
General protection and precautionary measures have been proposed for
consideration during the design, construction and operational
phases of the developments. In addition, the design, construction and operation
of the proposed development within the MTLL (i.e. the proposed recreational
area in site E1-1) should be fully compatible with the landfill restoration and
aftercare works and impose no adverse impact to them. Caution should be
exercised to ensure long term integrity of the capping system and other
restoration facilities.
The design and construction method of the proposed development within MTLL
should also be provided to EPD for agreement during the design stage.
It is expected
that with the proposed precautionary measures in place, the potential risk of
LFG migration from MTLL to KTN development would be minimal. Nevertheless, a detailed QLFGHA
should be undertaken during the detailed design stage of the developments to
review the need and practicality of the protection and precautionary measures proposed
and provide recommendations on the detailed protection and precautionary measures to
be adopted. Such requirement could be imposed on developers by including it in
the lease conditions by Lands D.