Concentrations of dissolved As can be related either to natural processes or anthropogenic activities. The aim of this study was to distinguish natural processes from potential anthropogenic effects on As occurrence in landfill areas. The study area is located in Central Sardinia and is characterized by Upper Pleistocene and Holocene sediments (sand and gravel with silt-clay matrix and local occurrence of thin ochreous colour levels; marsh deposits rich in organic matter and vegetal residues). The study area hosts an exhausted landfill where construction and demolition debris were disposed.
The mineralogical composition of solid materials was characterized by quartz, feldspars and clay minerals. Concentrations of As were within the ranges reported in sediments and soils, except one sample that showed 46 mg/kg As.
The waters showed near-neutral pH, variable electric conductivity (<1 to 15 mS/cm) and either predominant chloride or bicarbonate composition. Concentrations above 10 μg/L As were observed in two shallow wells and were associated to relatively high concentrations of bicarbonate, ammonium, iron and manganese, coupled to very low concentrations of sulphate.
The contamination in As was likely due to redox reactions driven by organic matter. The decomposition of organic matter hosted in sediments would cause reducing conditions, favouring in turn the reductive dissolution of solid phases hosting some As, finally resulting in the release of As into solution. Field evidences indicated that such processes were not related to the landfill, and may have occurred when sufficiently reducing conditions were established due to the decomposition of the organic matter buried at a shallow depth. Results showed that mineralogical and geochemical investigations at detailed scale are efficient tools for distinguishing geogenic from anthropogenic sources of contaminants.
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