Chemical and isotopic composition of CO2-rich magnesium–sodium–bicarbonate–sulphate-type mineral waters from volcanoclastic aquifer in Rogaška Slatina, Slovenia

Abstract

AbstractBottled natural mineral waters from an andesitic aquifer in Slovenia are enriched in magnesium (1.1 g/l), sulphate (2.2 g/l) and dissolved inorganic carbon (204 g/l). We analysed major ions, trace elements, tritium activity, 14C, δ18OH2O, δ2HH2O,δ13CDIC, gas composition and noble gases in six wells. In addition, 87Sr//86Sr, δ34SSO4 and δ11B were analysed here for the first time. Stable isotopes with δ18O = −11.97 to −10.30‰ and δ2H = −77.3 to −63.8 confirm meteoric origin. CO2 degassing is evident at three wells, causing the oxygen shift of about −1.3‰. Tritium activity was detectable only in the shallowest well, where the freshwater component was dated to the 1960s. δ13CDIC in five waters is −1.78 to + 1.33‰, typical of carbonate dissolution. Radiocarbon is low, 1.03–5.16 pMC. Chemical correction with bicarbonate concentration and δ13C correction methods gave best mean residence times, slightly longer than previously published. Sulphate has δ34S 26.6–28.9‰ and δ18O 8.9–11.1‰ due to dissolution of evaporites in carbonate rocks. Boron at concentrations of 1.2–6.1 mg/l has two origins: δ11B = 11.3–16.4‰ from hydrothermal alteration and δ11B = 26.6–31.7‰ from carbonate dissolution. Strontium at concentrations of 0.5–22.0 mg/l has 87Sr//86Sr, indicating three sources: 0.7106 for Miocene clastic rocks, 0.7082 for Triassic carbonates and 0.7070 for Lower Oligocene andesitic rocks. CO2 represents the majority of the dissolved (> 98.84 vol%) and separated gas (> 95.23 vol%). Methane is only found in two wells with a max. of 0.30 vol%. All waters show excess helium and 16–97% of mantle-derived helium. Since all show subsurface degassing, the paleo-infiltration temperature could not be calculated.