Tracking the origin of SO4 2- enriched karst groundwater at a regional scale (S Spain)

The oxygen and sulfur stable isotopes (δ18O_H2O, δ34S_SO4, and δ18O_SO4) are commonly used in hydrogeological research to decipher the source/s of dissolved SO4 2- in groundwater flows (i.e. application of fertilizers, sulfide oxidation, gypsum rock dissolution, etc.), both of anthropogenic and/or geogenic origin, which is relevant to establish groundwater protection strategies. This study aims to track the origin of karst waters flowing through selected aquifers in S Spain, by contrasting their chemical composition with their isotopic signature (δ18O_H2O, δ34S_SO4, and δ18O_SO4). Additionally, a compilation of regional to international cases studies is reported to refer the results to the isotopic fingerprinting of enriched-SO4 2- karst groundwater worldwide. A field survey was performed for groundwater sampling in carbonate and evaporitic aquifers located in the Málaga province and its neighbor provinces. The analytical results show SO4 2- concentrations ranging from 39 mg/l to 7197 mg/l (oversaturated in gypsum) and variable δ34S_SO4 and δ18O_SO4 composition, from +7.2 to +15.4‰ and from +9.3 to +15.2 ‰, respectively. Most of the studied karst groundwater (>80 mg/L of dissolved SO4 2-) are within the reference field of Triassic evaporite rocks of the Betic orogen, which determine the water-rock interactions in the deeper compartments of the carbonate aquifers (Triassic to Jurassic age) in contact with the gypsum-bearing basement rocks or within the evaporitic (Triassic) karst aquifers. In all cases, their isotopic relationship δ18O_SO4 vs δ18O_H2O exclude the sulfide oxidation as the precursor of the SO4 2- contents. On contrast, spring waters impoverished in SO4 2- (<80 mg/L) from carbonate lithologies clearly show anthropogenic influence, based on the mixing with fertilizers and wastewater leachates. In this study, the reported values of oxygen and sulfur stable isotopes (δ18O_H2O, δ34S_SO4, and δ18O_SO4) not only have permitted to discriminate the origin of dissolved SO4 2- of karst groundwater but also to refine the hydrogeological knowledge/functioning of some of the studied aquifer systems