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

► 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.