A comprehensive programme of baseline groundwater hydrochemical monitoring has been carried out in connection with the proposed hydraulic fracturing of a 2 to 3 km deep Bowland Shale gas reservoir in borehole KM8 at Kirby Misperton, North Yorkshire, UK. The monitoring infrastructure encompassed: five on-site boreholes with hydraulically open intervals ranging from shallow weathered cover to a c. 200 m deep Corallian limestone aquifer, six off-site wells (hydraulically open in superficial materials and/or Kimmeridge Clay) and four surface water monitoring stations. Groundwater chemistry was high stratified with depth, ranging from slightly acidic, fresh, very hard Ca-HCO3-SO4 waters in shallow weathered cover, to brackish, calcium-depleted, highly alkaline waters in the Corallian aquifer. Dissolved methane was detected in most boreholes, with 10 µg/L being typical of shallow boreholes and around 50 mg/L in the Corallian. Low ethane concentrations and isotopic evidence suggest that the methane was predominantly microbial in origin (carboxylate fermentation at shallow depth, natural methanogenic CO2 reduction at greater depth). Elevated dissolved ethane (20-30 µg/L) was found in one well of intermediate depth, suggesting admixture of a possible thermogenic component, although this could be derived directly from the Kimmeridge Clay penetrated by the well.

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