The baseline concentrations of methane in Great British groundwater : the National Methane Baseline Survey

This report describes the BGS research programme evaluating the baseline concentration of methane in groundwater which ran from November 2011 to completion in March 2016. The aim of the survey has been to improve knowledge of conditions in aquifers overlying potential shale gas source rocks present at depth, thus providing baseline knowledge relevant to the management of future exploration of new hydrocarbon sources. The necessity for doing this has been prompted by evidence from elsewhere (notably the USA) which has revealed very high methane concentrations in groundwater in some areas of shale gas extraction. Although this has often been directly attributed to shale gas operations, there have generally been no pre-development data on methane concentrations available to test this. Before the start of the current survey, BGS held some 170 analyses of methane in groundwaters from aquifers across Great Britain, acquired from the 1980s onwards. These data have been combined with new survey data, to give a total of 439 methane data points. In this combined dataset, 96% of samples show methane concentrations of less than 100µg/l and indicate that methane is rarely present at concentrations high enough to be potentially explosive (there are no health limits for methane in groundwaters). In the minority of samples with elevated concentrations, this was generally considered to be due to the proximity of organic rich coal seams or peats. The highest concentrations were found in the Cretaceous aquifers of the Weald Basin, a known area for occurrences of methane gas in the shallow subsurface. The new survey covers the majority of principal aquifers in Great Britain, including the Chalk, Permo-Triassic Sandstones, Carboniferous Limestone, and the Lower Greensand. Samples from over twenty different aquifers have been collected. In general, methane concentrations in carbonate aquifers (Chalk, limestones, Oolites etc.) are low, similar to those seen in the Permo-Triassic sandstone aquifers, although methane is widely present above detection limit (approximately 1µg/L) in all these aquifers. The Coal Measures of South Wales have the highest median value across Great Britain and this aquifer also shows the greatest temporal variability. Methane concentrations in the Carboniferous sediments of Scotland are also elevated, likely due to the impact of mining and the presence of coal seams. While little temporal variability is generally seen in aquifers used for public water supply or otherwise regularly pumped, further work is required to understand the impact of borehole use, pumping regime and aquifer type on the variability of the methane baseline of Great British aquifers. It should be noted that the Survey is not intended to replace any oil and gas operator’s local monitoring as required by the regulators. This is a national scale survey to enable a broad understanding of the distribution of methane in aquifers across relevant areas of Britain and cannot replace an understanding of groundwater quality at a local scale