Abstract
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.
Highlights
Methane (CH4) is released to the environment both by natural processes and anthropogenic activities, the latter conceivably including the exploitation of unconventional gas sources (UGS) such as shale gas or coal-bed methane
The existing data points available before the present survey began are shown in Fig. 2, together with the areas currently licensed for hydrocarbon exploration by the Oil and Gas Authority (OGA)
Aliquots of the headspace gas are expanded into the evacuated inlet system of the gas chromatograph (GC), from where they are admitted to a 1/8th-inch
Summary
Methane (CH4) is released to the environment both by natural processes and anthropogenic activities, the latter conceivably including the exploitation of unconventional gas sources (UGS) such as shale gas or coal-bed methane. There was no consistent collection of baseline data on CH4 concentrations in groundwater in the USA before shale gas exploitation began, making it sometimes problematic to correctly identify and attribute the reasons for the existence of high-methane groundwaters This has been identified as a key data gap and uncertainty in investigations into the impact of UGS exploitation (USEPA, 2016). If dissolved CH4 concentrations exceed the equilibrium solubility at the ambient partial pressure of ~1850 parts per billion by volume (NOAA, 2016), which is ~ 0.05 μg/l under the open atmosphere, methane will degas from the water until the system re-equilibrates This degassing may create a potential explosion hazard or asphyxiation risk if it occurs in a confined space, and constitutes the main risk of methane to human health. In practice a CH4 risk ‘action level’ of 10,000 μg/l is considered realistic (Eltschlager et al, 2001)
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