Constraints on the hydrogeochemistry and origin of the CO2-rich mineral waters from the Eastern Carpathians - Transylvanian Basin boundary (Romania)

The Eastern Carpathians, along the Neogene to Quaternary volcanic chain, host important natural resources of mineral waters. Most of these are in the form of CO2-rich natural springs of different types, associated with CO2 gas emissions. In order to understand the origin of these waters, the natural processes responsible for the observed physical-chemical features and to check the exposure and vulnerability of the hydrogeological system to potential contamination and environmental changes, 121 mineral water springs and wells were investigated. The investigations performed, offered an insight on the processes affecting the waters after their infiltration that led to a wide range of chemical and isotopic compositions. The diversity in the geological structure of the area, alteration processes and interaction with CO2, all explain the variety of the hydrogeochemical facies of the investigated waters. Three major hydrogeochemical types that are the result of the geological variety of the study area and water-rock interaction were identified: Ca-Na-Mg-HCO3, Ca-Na-Mg/Na-Ca +/- Mg-HCO3-Cl +/- SO4, and Na-Cl waters. Further changes in the geochemical features are attributable to mixing between different water types during their ascent, enhanced by the tectonic setting of the area. Considering the isotopic composition (delta O-18, delta D) of the waters, most of them proved to be of meteoric origin. Extreme isotopic shifts towards enriched delta O-18 and delta D values of + 8.59 and -12.2 parts per thousand (V-SMOW) respectively were observed in the case of 11 springs. These shifts were attributed to the interaction of water with clay minerals and diagenesis. As most of these waters are of meteoric origin, their discharge may be influenced by meteorological factors. Moreover, the mixing of aquifers with very shallow waters most likely facilitates their exposure to external contamination.