Tracing contamination in mining areas through sulfur and oxygen isotopes in groundwater sulfates: a case study from the Apuan Alps (Italy).

Groundwater contamination from legacy mining activities is a significant environmental concern, particularly in karst regions with vulnerable aquifers. This study investigates the isotopic composition of groundwater sulfates in a former mining area (Apuan Alps, Italy) to identify contamination sources and assess aquifer vulnerability. Sulfur and oxygen isotopes (δS and δO) of dissolved sulfates, combined with hydrochemical data, distinguish sulfide-derived sulfate, originating from acid mine drainage and sulfide oxidation, from sulfate released through evaporitic dissolution. The results show that some groundwater springs near mining sites are influenced by the oxidation of sulfide minerals and the dissolution of secondary Fe-Al-K sulfates, as evidenced by depleted δS and δO values and the presence of trace metals, including thallium. In contrast, other springs exhibit isotopically enriched δS and δO signatures, characteristic of the dissolution of Triassic evaporites, indicating deeper aquifers protected from mining-related contamination. Seasonal isotopic variations reveal the mixing of shallow and deep groundwater flow components, with dilution effects observed during recharge periods. These findings underscore the effectiveness of isotopic tools in tracing sulfate origin and highlight the risks of contamination in shallow karst systems.