The hydrochemical composition of groundwater and the isotopic composition of sulphur compounds in sediments (δ³⁴S of sulphide, inorganic sulphate) and groundwater (δ³⁴S, δ¹⁸O of dissolved sulphate) have been investigated to reveal the reasons for elevated concentrations of dissolved groundwater sulphate in the drinking water catchment area of Torgau–Mockritz (Germany).

The three most important anthropogenic sources of sulphate identified in this study are historic atmospheric sulphate deposition, predominantly of anthropogenic origin, inorganic fertilization, and dissolution of gypsum from waste dumps. Owing to their overlapping isotopic range, no clear differentiation between the individual anthropogenic sources is possible. Sulphate from the oxidation of sedimentary sulphides, however, was recognized by its negative sulphur isotope signature (δ³⁴S < − 5‰).

The mobilization of sulphate from different soil sulphur species is closely connected with variable isotopic fractionations, resulting in a wide isotopic variation range for the sulphate entering the saturated zone. Generally, sulphur and oxygen isotope ratios of the dissolved groundwater sulphate range from −19 to +37‰ (Vienna Cañon Diablo troilite) and from −2 to +19‰ (Vienna standard mean ocean water), respectively. Sulphate from the majority of groundwater samples can be assigned to anthropogenic sources. Sulphate from sulphide oxidation is especially present in samples from the upper sampling level and the groundwater surface. Sulphate concentrations above 700 mg l⁻¹ are mostly caused by the oxidation of sulphide. Sulphate that occurs in low and moderate concentrations dominantly originates from anthropogenic sources.

δ³⁴S distribution patterns were used to locate the sources for the elevated sulphate concentrations in the raw water. It was shown that the sulphate is not mobilized in the immediate vicinity of the production wells. Rather, it originates in an area ∼1·5 km west of the intake. The main mobilization mechanism there is the oxidation of reduced sedimentary sulphur species.

Beyond its local significance, this study presents a general example how changes in the sulphur cycle indicate major, anthropogenically induced alterations of the hydrochemical situation in catchment areas.