Hydrogeochemical interpretation of groundwater at Palmottu

The understanding of hydrogeochemical evolution is essential in order to assess the migration of radionuclides at Palmottu. The objective of this study is to evaluate chemical interaction and mixing processes of groundwater evolution, and also to evaluate the residence time over which such processes have persisted. The following interpretation is based on hydrochemical, isotopic, petrographic and hydrological data. Statistical analyses and geochemical modelling are used in order to determine groundwater types, and the relation between conservative mixing and water-rock interaction. Four general groundwater types characterise the Palmottu site. Recently recharged, shallow Ca-HCO 3 type groundwater evolves in the upper part of the bedrock into a Na-HCO 3 type groundwater at increasing depth within a few decades, partly due to Ca to Na ion-exchange processes. Brackish Na-SO 4 and Na-Cl type groundwaters prevail below the HCO 3 groundwater bodies; Na-SO 4 is present only around the mineralised zone whilst Na-Cl is found at greater depths also in the surrounding area. The latter two groundwater types show distinct glacial isotopic signatures. Mixing of water types explains largely the changes of SO 4 , Cl and Na concentrations. Water-rock interaction is specifically connected to carbon cycling. Biogenically produced CO 2 promotes silicate weathering and dissolution of calcite at shallow depths. The heavy 8 7 Sr/ 8 6 Sr ratios measured from the HCO 3 waters are consistent with the dissolution of Rb-rich minerals such as K-feldspar and biotite. Ca to Na ion-exchange maintains calcite dissolution and calcite is saturated in Na-HCO 3 type water. Minor calcite precipitation and silicate dissolution is likely in the brackish groundwater types, thus buffering pH to around 8.5. Anaerobic oxidation of organic matter in the lower part of the Na-HCO 3 layer and CH 4 oxidation in the brackish groundwater layer using SO 4 as an electron acceptor are considered probable processes based on C and S isotope data.