Chemical and isotopic composition of CO(2)-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, (14)C, δ(18)O(H2O), δ(2)H(H2O,) δ(13)C(DIC), gas composition and noble gases in six wells. In addition, (87)Sr/(/86)Sr, δ(34)S(SO4) and δ(11)B were analysed here for the first time. Stable isotopes with δ(18)O = −11.97 to −10.30‰ and δ(2)H = −77.3 to −63.8 confirm meteoric origin. CO(2) 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. δ(13)C(DIC) 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 δ(13)C correction methods gave best mean residence times, slightly longer than previously published. Sulphate has δ(34)S 26.6–28.9‰ and δ(18)O 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: δ(11)B = 11.3–16.4‰ from hydrothermal alteration and δ(11)B = 26.6–31.7‰ from carbonate dissolution. Strontium at concentrations of 0.5–22.0 mg/l has (87)Sr/(/86)Sr, indicating three sources: 0.7106 for Miocene clastic rocks, 0.7082 for Triassic carbonates and 0.7070 for Lower Oligocene andesitic rocks. CO(2) 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.