A baseline survey of dissolved methane in aquifers of Great Britain.

Interest in dissolved methane (CH4 ) concentrations in aquifers in England, Scotland and Wales ('Great Britain' or GB) has grown concurrently with interest in the exploitation of unconventional gas sources (UGS). Experience, mainly from North America, has shown the importance of a pre-production baseline against which changes possibly due to UGS extraction can be compared. The British Geological Survey, aided by water utilities, private users and regulators, has compiled a unique dataset for CH4 in groundwaters of GB. This focuses principally on areas where UGS exploration is considered more likely, as indicated by the underlying geology. All the main water supply aquifers (Principal aquifers) were targeted, plus Secondary aquifers where locally important. The average dissolved CH4 concentration across GB in the aquifers sampled was 45μg/l. Out of a total of 343 sites, 96% showed dissolved CH4 concentrations <100μg/l, 80% <10μg/l, and 43%<1μg/l. No site had a CH4 concentration above the US Department of the Interior suggested risk action level of 10,000μg/l. While most sites were sampled only once, a subset was monitored quarterly to determine the magnitude of seasonal or other variations. Generally these variations were minor, with 84% of sites showing variations within the range 0.5-37μg/l, but some aquifers where the porosity was primarily fracture-related showed larger changes (0.5-264μg/l). This may have been due to the nature of sampling at these sites which, unlike the others, did not have installed pumps. Since the regulatory compliance monitoring attending UGS operations will include the measurement of parameters such as dissolved CH4 , it is essential that sampling methods are tested to ensure that reliable and comparable datasets can be obtained.