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Evaluation of the origin of sulphate at the groundwater source Ključ, Serbia

The dual-isotope method of measuring both the δ34S and δ18O values of dissolved sulphate to assess the origin and fate of groundwater sulphate at the Ključ groundwater source, Serbia is applied. A sampling campaign was conducted in September 2007 during low-flow conditions, obtaining river water and groundwater from observation wells completed in a shallow aquifer formed in alluvial sandy–gravelly sediments. In the shallow groundwater, sulphate concentrations ranged from 56.2 to 165.0 mg l−1. The δ34S values of sulphate varied from−5.5 to+3.0 ‰ and values from+1.4 to+4.2 ‰. Oxidation of pedospheric (organic soil S) and lithogenic sulphur sources (e.g. pyrite) were identified as the main causes for the increasing sulphate concentrations in the analysed groundwater. This study shows that combining hydrological, chemical, and isotopic techniques is a powerful approach to identifying sources and processes that control sulphate in water resources.