Elevated concentration of arsenic (As) in ground water associated with basin-fill aquifers has attracted research attention to semi-arid Southwest US. To date, however, few studies have been performed to determine the source and solubilization mechanism of geologic As in these basin-fill aquifers. The shallow aquifer throughout the Cache Valley Basin, Utah, contains As concentrations that exceed EPA’s drinking water limit. Two continuous cores, from the soil surface to 5 m below the water table, were collected from the center of the basin in order to study the redox induced geochemistry that affect As solubility in stratigraphic column. General soil properties, pore water chemistry, and solid phase characterization of As, using sequential extractions, have been determined. Mineral phase As was present throughout the two profiles and As was released into the pore water. The mineral association of As changed with depth, with As accumulating in the redox transition zone. The heterogeneity in the distribution of speciation and mineralogies of As 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.