Methane (CH4) is only a trace constituent of the atmosphere but an important greenhouse gas. Although groundwater is unlikely to be a major source of atmospheric CH4, its contribution to the CH4 budget of the UK has up to now been poorly characterised. Groundwater CH4 concentrations have been measured on 85 samples from water-supply boreholes and a further eight from other miscellaneous water sources. Concentrations in abstracted groundwaters ranged from <0.05 – 42.9 µg/l for Chalk, <0.05 – 22 µg/l for the Lower Greensand, 0.05 – 21.2 µg/l for the Lincolnshire Limestone and from <0.05 – 465 µg/l for the Permo-Triassic sandstone. Having the largest abstraction volume, the Chalk is likely to be the main UK groundwater contributor to global CH4 emissions. A calculation to estimate the total emissions of CH4 from water supply groundwater sources based on the median and the maximum CH4 concentrations gave a values of 2.2 10-6 Tg/yr and 3.3 10-4 Tg/yr. Estimates show groundwater contributes a maximum of 0.05% of all UK CH4 emissions and a further two orders of magnitude less in terms of the global CH4 budget. Other groundwater sources such as inflows to tunnels may have significantly higher CH4 concentrations, but the volume of water discharged is much lower and the overall amount of CH4 outgassed is likely to be of the same order as the aquifer release. The generally low concentrations of CH4 in groundwater supplies suggest no threat of explosion, although groundwater released by excavations remains a hazard
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