Occurrence and distribution of arsenic in the sediments of a carbonate-rich unsaturated zone

Three combined approaches were used to study the arsenic (As) occurrence in carbonate-rich unsaturated zone from a large plain environment (Pampean plain, Argentina). A sequential extraction procedure (SEP) was used to identify the As-bearing phases and distribution along a 2-m-long unsaturated zone profile, whereas XRD and SEM/EDS were used to determine the mineralogical features of the sediments. In this hydrogeologic environment, groundwater levels tend to be close to the surface (<5 m) and hydrogeochemical processes that develop during infiltration and recharge through the unsaturated zone may release the As hosted in the sediments to shallow aquifers. According to SEP results, the unsaturated zone was divided in an upper and a lower zones. Disregarding the As contained in the silicates, low crystalline Fe(III) oxyhydroxides were the main As-bearing minerals in the upper zone. Its presence was confirmed by SEM/EDS analysis. In the lower zone, carbonates were the main As-bearing minerals, also supported by XRD analysis. The chemical composition from the resulting water-extractable step of the SEP was constrained using geochemical inverse modeling (PHREEQC) to elucidate those potential geochemical processes that may control the As release or retention during, for instance, an infiltration event. They showed that Na/Ca exchange promoted the calcite dissolution which, in turn, raised the pH as a result of proton consumption. High pH and HCO3− contents favored the As desorption from low crystalline Fe(III) oxyhydroxides in the upper zone, where they were present in higher proportion.