A geochemical and stable isotope investigation of groundwater/surface-water interactions in the Velenje Basin, Slovenia

The geochemical and isotopic composition of surface waters and groundwater in the Velenje Basin, Slovenia, was investigated seasonally to determine the relationship between major aquifers and surface waters, water-rock reactions, relative ages of groundwater, and biogeochemical processes. Groundwater in the Triassic aquifer is dominated by HCO3 (-), Ca2+, Mg2+ and delta C-13(DIC) indicating degradation of soil organic matter and dissolution of carbonate minerals, similar to surface waters. In addition, groundwater in the Triassic aquifer has delta O-18 and delta D values that plot near surface waters on the local and global meteoric water lines, and detectable tritium, likely reflecting recent (< 50 years) recharge. In contrast, groundwater in the Pliocene aquifers is enriched in Mg2+, Na+, Ca2+, K+, and Si, and has high alkalinity and delta C-13(DIC) values, with low SO4 (2-) and NO3 (-) concentrations. These waters have likely been influenced by sulfate reduction and microbial methanogenesis associated with coal seams and dissolution of feldspars and Mg-rich clay minerals. Pliocene aquifer waters are also depleted in O-18 and H-2, and have H-3 concentrations near the detection limit, suggesting these waters are older, had a different recharge source, and have not mixed extensively with groundwater in the Triassic aquifer.