12.1.1
The Western Coast Road (WCR) will
require a limited amount of reclamation to accommodate the toll plaza and
associated slip roads. The initial
reclamation area will be a dredged reclamation and so there would be no concern
on any potential generation of biogas underneath this part of the
reclamation. Later phases of the
reclamation would be over sediment left in place, as described in the Working
Paper on the WCR Reclamation submitted in June 2004. Biogas may be generated from decay of organic matter in marine
sediments left in place underneath this reclamation area.
12.1.2
The planned land use on the proposed
WCR reclamation - highway and toll plaza – is not sensitive to potential biogas
emission. Nevertheless, EPD have determined
that should any building be planned on the non-dredged drained reclamation
area, assessment of biogas impact on such land use should be undertaken. The administration building associated with
the toll plaza would be built in this area.
12.1.3
The proposed administration building
would not have a basement and therefore potential biogas emissions are
considered to pose a very limited, if any, risk. For this reason, sampling
and testing of representative samples, as required under S3.4.10 of the EIA Study brief,
was not carried out due to no sensitive land uses being planned on the
proposed WCR reclamation (At the time of preparation of the EIA Study
Brief, the proposed land use was not known). Nevertheless,
adopting a precautionary approach, an assessment has been done using very
conservative (maximising) assumptions.
The results of the assessment, and the reasoning underlying the
conservative nature of the assumptions made, are discussed in the following
sections.
12.2.1
Data on TOC and SOD levels
in sediment samples in Junk Bay is available from a previous study completed in 2001. In that study, sediment
samples collected from three vibrocores had a mean TOC level of 0.36 (% dry wt) and have been used in the
calculation of potential methane flux from the WCR reclamation. In fact, the reclamation for the proposed
toll plaza is situated in a relatively open/exposed position at the outer
portion of TKO Bay, and is located further away from the historical pollution
sources such as village areas and the former Rennie’s Mill at Tiu Keng
Leng. Since the location is not close
to pollution sources and is exposed to wave action, the marine sediments at the
reclamation site are expected to have a lower organic matter content. In fact, diver inspection has confirmed that
the near-shore shallow-water sediment in the area of the proposed WCR
reclamation is sandy, not muddy. Logs
for four vibrocores sampled under the marine ground investigation for this
Project also indicated the near-shore sediment to be sandy and gravel, and not
clay. For these reasons, the TOC values
used in this calculation are considered to be conservative.
12.2.2
Only a fraction of the TOC can be
considered to be biodegradable. With
reference to the biogas risk assessment undertaken in the Comprehensive
Feasibility Study for the Revised Scheme of South East Kowloon Development
(‘CFS’), three grab samples were analysed for carbon compound fractionation
to estimate the percentage of biodegradable TOC. The analytical results indicated that 75% of carbon compounds had
30 or less carbon atoms (it was considered that material with molecules
containing less than 30 carbon atoms is more easily degraded) and this
percentage of 75% has been adopted in this assessment to represent the
worst-case scenario. This value is
considered to be conservative, since data from athe previous
study1 using samples collected in
Junk Bay indicated the biodegradable portion of the TOC to be 12.62%.
12.2.3
Previous studies assumed decay
half-lives for the biodegradable TOC of between 2 years and 5 years. A shorter half-life implies more rapid decay
and faster generation of methane, but the TOC would be depleted more
rapidly. A half-life of 2 years has
been adopted in this assessment for the calculation of potential methane gas
emission to represent a worst-case scenario (as a half-life of 2 years will
result in a higher flux rate after reclamation than that resulting from a
half-life of 5 years).
12.2.4
The hydraulic and water quality
studies undertaken for this project indicated that the siltation
rate at the western coast of Junk Bay was extremely low; around 1.2 mm per
year. The siltation rate was calculated
from the sedimentation flux (g/m2/day) generated by the water
quality model. This extremely low siltation
rate implies that any degradable organic matter in the deeper layers of
sediment would have been anaerobically degraded and would not contribute to gas
formation.
12.2.5
Table 12.1 shows the calculations of
the daily methane flux from the proposed WCR reclamation. The methane concentrations of the boundary
layer at the surface of the WCR reclamation was also estimated for comparison
with the guideline value (1% v/v) as stipulated in EPD’s Landfill Gas Hazard
Guidance Note. The boundary layer is
assumed to be 1 m to represent the worst-case scenario.
Table 12.1 Calculation
of Methane Flux from the WCR reclamation
|
|
Marine Sediment
|
Methane Hazard Assessment Criteria
|
|
Volume (m3)
|
381,000
|
|
|
Density (kg m-3)
|
1750
|
|
|
Dry matter (% w/w)
|
49
|
|
|
Dry matter (kg m-3)
|
857.5
|
|
|
TOC (%)
|
0.36
|
|
|
TOC (kg m-3)
|
3.09
|
|
|
CH4 potential (kg m-3)
|
2.07
|
|
|
Peak annual CH4 potential (kg)
|
273,106
|
|
|
Total area (m2)
|
127,000
|
|
|
Total potential CH4 flux (kg m-2 yr-1)
|
2.15
|
|
|
Total potential CH4 flux (mol m-2 yr-1)
|
134.4
|
|
|
Total potential CH4 flux (l m-2 yr-1)
|
3010.62
|
|
|
Total potential CH4 flux (l m-2 dy-1)
(assuming 75% of TOC biodegradable)
|
6.19
|
18a – 432b
10d
|
|
Potential CH4 concentration (% v/v) at the surface
boundary layer (assuming 75% of TOC biodegradable)
|
0.62
|
1c
|
Notes
a UK Landfill Completion Criterion from Department of the Environment (1993) Landfill Completion. Waste Management Paper No. 26A. London: HMSO.
b Carpenter’s guidance level from Carpenter, R J (1988) Building development on disused landfill sites -
overcoming the landfill gas problem.
In: Proc. 5th International Solid Wastes Conference, Copenhagen,
Denmark, Vol., pp 153-160. London: Academic Press.
c Guideline value from Landfill
Gas Hazard Guidance Note, EPD, HK.
d “Safe” rate of landfill gas emissions from Department of the Environment (1993)
Landfill Completion. Waste Management
Paper No. 26A. London: HMSO.
Significance of Potential Methane Emissions with
Reference to the UK Guidance Values
12.3.1
Taking the UK landfill completion
criterion (i.e. 18 l m-2d-1) as the standard, the
predicted methane emission from the TKO reclamation based on the assumption of
75% TOC biodegradable has been considered as a conservative estimate in this
case. The predicted methane emission
(6.19 l m-2d-1) was found to be approximately 34% of the
UK landfill completion criterion.
12.3.2
The calculations show that the
predicted methane emission, assuming 75% of TOC to be biodegradable, is only
about 1.4% of Carpenter’s guidance level (i.e. 432 l m-2d-1),
providing a safety factor of approximately 70.
It is noted that the predicted methane emission is well below the upper
UK guide value, which is the level at which development would be restricted
according to Carpenter’s guidelines.
12.3.3
Taking the UK “safe” rate of landfill
gas emissions (i.e. 10 l m-2d-1) as the standard, the
predicted methane emission from the TKO reclamation, assuming 75% of TOC
biodegradable, which is a worst-case scenario, is approximately 62% of the UK
“safe” emission rate, which provides a satisfactory level of safety.
Significance of Potential Methane Emissions with
Reference to EPD’s Landfill Gas Hazard Guidance Note
12.3.4
The methane concentration at the
surface boundary layer of the WCR reclamation is estimated to be 0.62 % (v/v),
which is about 1.6 times less than the EPD’s guide value.
12.4.1
The calculations show that, assuming
75% biodegradable TOC (which is a worst-case scenario) the estimated methane
emission from the WCR reclamation is estimated to be well below all relevant
assessment criteria. It is therefore
concluded that the methane gas generation potential is not expected to pose a
development constraint to the proposed WCR reclamation.
12.4.2
Furthermore, the planned land use of
highway and toll plaza on the proposed WCR reclamation is not sensitive to
potential biogas emission. The proposed
administration building of the toll plaza would not have a basement and
therefore potential biogas emissions would be considered to pose a very
limited, if any, risk.