Methane is about 23 times less soluble in water than CO2. If the water is acidic the CO2 will be even more soluble. What you are seeing is methane coming out of solution as the water level rises (less pressure). This is very typical of what happens at sites with elevated ground water. Are you sure the hydrogeology is such that water is continuous with depth..is there any perching. If there is this might cause an additional issue with gas migrating under pressure, with gas trapping under the perched layer.
To answer your question, the gas results are representative of a situation where a headspace is generated and where groundwater is lowered. Beware trenches and buried services. Remember 1.4mg/litre dissolved methane can produce an explosive atmosphere in a pipe....
Keiron Finney MSc MRSC MCIWM CChem CSci CEnv CWMan, Grad IOSH
Midlands Technical Specialist (Landfill Gas)
Environment Agency
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-----Original Message-----
From: Contaminated Land Management Discussion List [mailto:[log in to unmask]] On Behalf Of Paul Burns
Sent: 16 September 2011 11:08
To: [log in to unmask]
Subject: Gas monitoring in saturated ground
I have recently had a site investigation report submitted in support of a planning application. It is for a small site where 1 house is to be built on a plot of land adjoining a former landfill site. High levels of methane are present in the landfill although gas generation is low. Groundwater flow is towards the development site and leachate has been found to contain dissolved methane. The published geology shows alluvial deposits, associated with a former stream, to be present overlying mudstone. The SI on the development site has found several metres of made ground that includes bands of 'organic sands'. The groundwater is very shallow and the whole of the slotted section of the standpipes are within the saturated zone. When the wells are purged of water they recharge within minutes. Gas monitoring has been undertaken immediately following the purging of the wells. CH4 and CO2 levels are up to 4% and 10% respectively. Flow rates up to about 10 l/hr have been measured although this is attributed to the rapid inflow of water into the wells. The gas regime is suggested to be Amber 1. My question is how likely are the gas monitoring results reported to be representative of the gas regime at the site?
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