Origin, mobility, and temporal evolution of arsenic from a low-contamination catchment in Alpine crystalline rocks

Highlights: ► δ34SSO4 vs. [As] and δ18OSO4 vs. AsV/AsIII correlations provide new tools even for low [As]. ► Long-term and high-resolution monitoring shows droughts enhance pyrite dissolution. ► Major effect of 2003 European heatwave on pyrite dissolution and [As] increase. -- Abstract: The reduction to 10 μg/l of the limit for arsenic in drinking water led many resource managers to deal with expensive treatments. In the very common case of arsenic levels close to the recommended maximum concentration, knowing the origin and temporal evolution of As has become of great importance. Here we present a case study from an alpine basin. Arsenic speciation, isotopic compositions of pyrite, sulfate and water, and concentrations of major and trace elements demonstrate a geogenic source for arsenic linked to the dissolution of pyrite. We provide new tools to further study As at low concentrations where many processes may be masked. The observed negative correlation between δ34SSO4 and [As] is interpreted as a Rayleigh-type sulfur-isotope fractionation during increasing pyrite dissolution. The observed positive correlation between δ18OSO4 and AsV/AsIII could help to retrieve initial redox conditions. A 3-year long monitoring at high-resolution demonstrated that drought conditions enhance pyrite dissolution whose degradation products are scavenged by recharge water. An increase in As in groundwater may result from droughts due to enhanced oxygen entry in the unsaturated zone. The 2003 European heatwave had a major effect