Abstract
Stable isotopic compositions of δ18O, δ2H and dissolved sulfate δ34S in water from abandoned and flooded coal mines are used to interpret the water's and solutes' origin and interactive history. These isotopic ratios have been determined in mine water from a shallow (<100 m) series of overlapping coal mine workings at the UK Geoenergy Observatories Glasgow Geothermal Energy Research Field Site (UKGEOS GGERFS). Comparison has been made between dissolved sulfate δ34S of water in mined Carboniferous coal-bearing strata, that of water which had interacted with equivalent unmined strata, and δ34S in sulfide-bearing minerals (pyrite) in the host sedimentary rocks and local evaporite (gypsum) of the Lower Carboniferous Ballagan Formation. δ18O and δ2H confirm a meteoric origin for coal mine waters. The δ34S arithmetic mean and standard deviation of the pyrite (+5.0 ± 15.5‰) and water from unmined strata (+0.3 ± 2.1‰) were found to be similar, whereas the mine water exhibited elevated isotopic values (+20.3 ± 1.1‰), plotting closer to that of modern-day seawater (+21.2‰) and Ballagan Formation gypsum (+18.9 ± 0.5‰). Whilst the origin of dissolved sulfate in the mine water remains unclear, it is unlikely to be wholly due to simple pyrite oxidation. Influence of evaporite dissolution, fractionation associated with microbial sulfate reduction, and mixing with saline formation waters of marine, evaporitic or of another origin, cannot be ruled out.
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