Characterisation of the stable isotope composition of methane in UK groundwaters

Abstract

The primary aim was to use stable isotope analysis to characterise the origin and alteration of methane (CH4) in UK groundwater. CH4 is produced from a variety of processes, including archaeal metabolism in anaerobic environments, thermal breakdown of organic matter during burial, and abiogenic geological processes. The stable isotope composition depends initially on production pathway, composition of substrates, temperature and energetic conditions. Post-formation processes, such as transport or CH4 oxidation, can continue to alter the stable isotope composition. Methane may enter groundwater from neighbouring formations, as well as be produced microbially in situ; alteration processes may further affect stable isotope composition. CH4 in groundwater is often observed in low concentrations historically difficult to sample. A field sparging method was developed enabling stable isotope analysis with a lower limit of 0.5 - 1 µg L-1 for δ13C(CH4) and 2 - 3 µg L-1 for δ2H(CH4). Site investigation was carried out in three parts: 1. Sites within a syncline of the Lower Greensands aquifer where conditions change from confined to unconfined; 2. Sites within a shale gas development area in Lancashire; and 3. Sites within several aquifers in the UK to explore dominant geological and environmental controls on groundwater δ13C(CH4) and δ2H(CH4) values and the degree of variability. Local geology was observed to be an influencing factor on environmental conditions affecting methane production and oxidation processes. Redox conditions were observed to strongly influence the concentration of microbially produced CH4 within groundwater in the UK. Strong correlations between CH4 and both Eh and DO were recorded. A wide range of d13C(CH4) and d2H(CH4) values were measured ranging from -79 to -29 ‰ and -353 to +248 ‰, respectively. As CH4 concentration decreases, both δ13C(CH4) and δ2H(CH4) values increase. A clear correlation of δ13C(CH4) and δ2H(CH4) values was observed as they increased proportionally to each other. These results indicate that in UK aquifers a kinetic isotope effect caused by post formational bacterial CH4 oxidation is the dominant control on δ13C(CH4) and δ2H(CH4) values. Though CH4 in UK aquifers is predominantly microbial, thermogenic CH4 was also present in areas with known source rocks, such as the Coal Measures and evidence of mixing between sources was observed.

Date of Award	6 Nov 2018
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Edward Hornibrook (Supervisor), Daren Gooddy (Supervisor), W. George Darling (Supervisor) & Ian J Parkinson (Supervisor)