Redox controlled biogeochemical processes affecting arsenic solubility in a shallow aquifer in semi-arid Cache Valley UT

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The poisoning of millions of people in South and Southeast Asia through the ingestion of As-laden ground water used for drinking water has focused attention on the sources and geochemistry of As in these humid regions. Elevated concentrations of As, however, are not limited to humid environments but are also observed in arid and semi-arid regions worldwide, including the Southwest US. The shallow aquifer throughout the Cache Valley, Utah, contains As concentrations that exceed EPA’s drinking water limit. Two continuous cores, from the soil surface to 1.5 m below the water table, were collected from the center of the valley in order to describe the biogeochemistry that controls the solubility of naturally occurring arsenic. General soil properties, pore water chemistry, and solid phase characterization of arsenic, using sequential extractions, have been determined. Geologic arsenic contents were present throughout the two profiles and arsenic was released into the pore water. The mineral association of arsenic changed with depth, with arsenic accumulating in the redox transition zone. The heterogeneity in the distribution of speciation and mineralogies of arsenic has been confirmed by synchrotron-base X-ray absorption spectroscopy. Reduced As species were identified in the vadose zone as orpiment. In the redox transition zone, As predominately exists as As(V) and co-occurs with Fe oxides. Deeper in the sediment profile, the depletion zone contains only reduced As species presumably realgar. The results also revealed an unanticipated zone of active As redox cycling between the vadose zone and redox transition zone, located within the seasonally oscillating ground water wetting front. The fluctuating redox conditions resulted in mixed oxidation states for As. The pore water from this zone has the highest As concentration within the profile. Understanding the behavior of geologic As in our study area is important because similar processes may also affect other regions that withdraw ground water from basin-fill aquifers including parts of California, Nevada, Arizona, Utah, New Mexico, and Colorado.

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