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

Bottled 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.