Chemical and isotopic evidences on evaporite dissolution as the origin of high sulfate water in a karst geothermal reservoir

Using multiple isotopes including 87Sr/86Sr in water, delta 34SSO4, delta 18OSO4, delta 18OH2O and delta 2HH2O, delta 13CDIC and 14CDIC in water, we have investigated the origin and evolution of sulfate concentration (1472 mg/L) in the Tangquan Karst geothermal system in Nanjing, China. The sulfate has been found to be mainly derived from dissolution of gypsum and other evaporite minerals occurring in the carbonate reservoir. The dissolution of evaporite minerals is relatively easier than carbonate at elevated temperatures. We confirmed this differentiated dissolution process by reverse hydrogeochemical simulation using the PHREEQC code. The origin of sulfate is supported by 87Sr/86Sr isotope ratio (0.7088-0.709) with a typical marine signature. We further constrained the origin of sulfate with delta 34S in aqueous SO4 (27.6%o-34.2%o) and delta 18O in aqueous SO4 (14.3%o-18.4%o). Such sulfate-rich geothermal water usually occurs in open karst geothermal systems, favorable for Spa therapy in many places of the world.