Sulphur isotopes in deep groundwater reservoirs: Evidence from post-stimulation flowback at the Pohang geothermal facility, Korea

•The 4.2 km deep Pohang PX-1 well was subject to hydraulic stimulation.•Sulphate concentration and δ34S increase during post-stimulation flowback.•Excess SO42− demonstrates mineral dissolution during hydraulic stimulation.•Isotopes and modelling indicate anhydrite dissolution as probable sulphate source.