Methane in UK groundwater
Methane (CH4) is an important greenhouse gas and a common trace component of groundwater.
BGS has been studying methane in UK groundwaters since the 1980s to investigate:
- The potential for methane emissions from groundwaters of the UK (Gooddy and Darling, 2005)
- Sources of methane and their hydrogeochemical controls (Darling and Gooddy, 2006)
- baseline concentrations in UK groundwaters
- when methane concentrations reach unsafe levels and become explosive
Groundwater methane contribution to total atmospheric emissions
Using baseline methane concentrations for the main groundwater supply aquifers in the UK, Gooddy and Darling (2005) estimated that water supply groundwater sources contribute only 0.05 per cent to total UK methane emissions (up to 3.3 x 10-4 Tg/year).
In terms of a global budget the groundwater methane input was estimated to be two orders of magnitude smaller.
How does methane get into groundwater?
Methane in groundwater is derived from two main sources:
- biogenic methane is bacterially produced and is particularly associated with anaerobic groundwater environments e.g. peat bogs, wetlands, lake sediments and landfills, although it is detectable in nearly all groundwater
- thermogenic methane is leakage from thermal decomposition of organic matter and is usually associated with coal, oil and gas fields
In the UK most of the methane in groundwater is likely to be of biogenic origin, although thermogenic contributions can be locally important.
Gas phase and carbon stable isotope analysis can be used to identify different methane sources.
Baseline concentrations in UK groundwaters
Understanding the 'baseline' concentrations of methane in UK groundwaters can help us to assess future methane survey or incident data.
Measurements of potable waters from the Cretaceous, Jurassic and Triassic carbonate and sandstone aquifers reveal methane concentrations of up to 500 µg l-1 but a mean value of < 10 µg l-1.
Aquiclude and thermal waters from the Carboniferous and Triassic have concentrations in excess of 1500 µg l-1 (Darling and Gooddy, 2006).
When does methane become explosive and create a potential safety hazard?
Methane becomes an explosive hazard at concentrations of 5–15% by volume in air.
Assuming complete outgassing from water, this requires a minumum dissolved methane concentration of 1600 µg l-1.
This figure is considerably above the general methane baseline (see above), but concentrations of up to ten times higher have been found in tunnels drilled in organic-rich shales.
Contact
Contact George Darling for further information
