The hydrogeochemistry of methane: evidence from English groundwaters by W. G. Darling and D. C. Gooddy, 2006, British Geological Survey
The presence of methane (CH4) in groundwater is usually only noticed when it rises to high concentrations; to date rather little is known about its production or natural ‘baseline’ conditions. Evidence from a range of non-polluted groundwater environments in England, including water-supply aquifers, aquicludes and thermal waters, reveals that CH4 is almost always detectable, even in aerobic conditions. Measurements of potable waters from Cretaceous, Jurassic and Triassic carbonate and sandstone aquifers reveal CH4 concentrations of up to 500 μg/l, but a mean value of <10 μg/l. However, aquiclude and thermal waters from the Carboniferous and Triassic typically contain in excess of 1500 μg/l.
The present paper is not concerned with such incidents but instead focuses on ‘baseline’ conditions by looking at regional distributions of dissolved CH4 and the differences between aquifer types. … The detection limit of the method used was better than 1 part per million by volume (ppmv), corresponding to a dissolved concentration at 10°C of approximately 0.05 μg/l.
Methane was present in nearly all groundwaters above the detection limit of 0.05 μg/l, but potable waters from the Chalk, Lower Greensand, Lincolnshire Limestone and Sherwood Sandstone possessed (with one exception) only trace concentrations, below 50 μg/l and with an average of <10 μg/l.
Concentrations in excess of 1500 μg/l were only found in non-potable waters, the highest values (up to 16,000 μg/l) being found in tunnel inflows from Namurian shales.