Gas Detection 11
data had been collected from perimeter boreholes over several years and highly variable gas concentrations and flows had been recorded but the reason could not be explained, see figure 7. As high concentrations and high flows were recorded gas generation was thought to be occurring and a there was a high perceived risk to the adjacent properties, consequently an expensive ongoing intense spot sampling programme was required by the regulator.
To improve the understanding of the processes occurring on the site, continuous monitoring in boreholes 10 and 11 was conducted. This also revealed variable gas concentrations however, a correlation with pressure is clear, see figure 8. When atmospheric pressure decreases borehole pressure lags behind it resulting in periods where the subsurface is positively pressurised allowing gas to migrate. As the pressure regime is identical in both boreholes even though gas regimes are different it indicates that changing atmospheric pressure not gas generation is responsible for the borehole pressure. Now instead of inferring high gas generation from spot sampling, continuous data reveals a pocket of gas migrating posses a much lower risk.
Improved Data Collection
With the ability to collect continuous data it is possible to purge a borehole and collect information on how the concentration recovers. This information is important because it allows a more reasonable inter- borehole comparison to be made and the rate at which the concentration recovers is directly related to the migration/generation potential. In figure 8 the recovery profiles of two different boreholes are compared, both boreholes recover to an absolute value of approximately 10% but one recovers in hours where the other recovers over days, indicating a very different risk.
Figure 5: The expected relationship between atmospheric pressure and gas concentration is clear. Current guidance states a measurement should be taken at 1000 mbar and falling to observe worse case but this can produce significantly different results e.g. 2 or 5%
Figure 6: a The inverse relationship with pressure as expected, b atmospheric pressure has no affect on gas concentration however, when water level increase it triggers a change in concentration.
Figure 7: Schematic of landfill with gas migration issues. Spot sampling in BH 10 and 11 revealed highly variable concentrations and flows indicating gas generation.
Figure 9: A purge test was conducted on two boreholes that spot sampling
revealed a concentration of 10 %CH4. The absolute concentration is the same but one recovers much faster indicating a very different risk. The recovery profile is related to the gas flux providing another vital line of evidence for risk assessment
Conclusions
By collect time series data of gas concentrations and environmental parameters that affect them it is possible to understand the dominant processes controlling gas migration/generation and gain a better understanding of flux. This information can then be feed into the conceptual site model and in turn used to design ongoing monitoring programmes. Following this iterative process a highly site-specific conceptual model can be generated that has a greater understanding of the processes at the site and thereby reduce uncertainty and produce more appropriate risk assessments. This will have to two fold effect of reducing the time and effort of monitoring programmes but also allow appropriate, cost effective, rather than over engineered solutions to migration problems.
References
1. Wilson, S., Oliver, S., Mallett, H., Hutchings, H. and Card, G, 2006. CIRIA Report 665 Assessing risks posed by hazardous ground gases in buildings. CIRIA
2. Environment Agency 2004. Guidance on the assessment of risk from landfill sites. External Consultation, Version 1.0
AUTHOR DETAILS Dr Peter Morris Product Manager Ion Science The Way, Fowlmere, Cambs, SG8 7UJ, UK Tel: +44 (0) 1763 207 235 Mon: +44 (0) 7749 337 105 Fax: +44 (0) 1763 208 814 Email:
Peter.morris@
ionscience.com Web:
www.ionscience.com
Figure 8: Highly variable gas concentrations are observed in BH15 and BH14 but the pressure regimes are identical. The semi-permeable geology means there is a lag between changing barometric pressure and the sub surface equilibrating, resulting in periods where flow would have been recorded and gas generation inferred
New Pump Communicates with Gas Detector
The new G400-MP2 pump from GfG (Germany) communicates with the gas detectors G450/G460. This interaction allows fault indication via the detector display. Low battery, blocked sampling line or interrupted power fault modes are shown on the display and activate different visual and audible warnings.
Gas samples can be taken from distances of up to 100 m with considerable low-pressure performance – the safe solution for confined space measurement.
The G400-MP2 is the only attachable pump available which may remain fixed to the detector. When the pump is turned on, the diffusion inlets are covered and the measurement results are not affected by air flows.
An additional filter system in the sampling line protects pump and sensors from dust and moisture. When the pump is turned off, the gas detector can be operated as usual. The diffusion inlets now allow all gases to enter the sensor chamber.
The G400-MP2 is operated on its own power supply, independently from the gas detector, i.e. the pump does not affect the operational time of the G450/G460. The pump power supply allows at least 10 hours of continuous operation.
Reader Reply Card no 27 November/December 2010 IET
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