Mineralogy and geochemistry affecting arsenic solubility in sediment profiles from the shallow basin-fill aquifer of Cache Valley Basin, Utah

Elevated arsenic concentrations have been reported in groundwater samples collected from the semi-arid Western U.S., including the Cache Valley Basin, Utah. The volcanic rock in the basin-fill aquifers underlying portions of the West is considered the primary source of arsenic, but there is debate about the mechanisms that control arsenic solubilization in these semi-arid and arid climates. Sediment cores were collected from a shallow basin fill aquifer in the Cache Valley Basin to systematically determine arsenic mineralogy and solubilization mechanisms in relation to non-redox and redox induced soil processes. Soluble arsenic was present throughout the two studied profiles in varying abundance and oxidation state, with the highest concentration of soluble As(V) and As(III) at the depth of the water table. Sequential chemical extractions of arsenic, with oxidation preservation strategies, revealed mineral sources and sinks of arsenic vulnerable to altering redox conditions down the profile. Weathering of primary arsenic-bearing minerals resulted in soluble arsenic in the vadose zone. Once soluble arsenic was leached to the deeper profile, arsenic solubility was controlled by carbonate minerals that concentrate at the water table. In the zone with alternating oxidizing and reducing conditions, iron oxides became the controlling mineral phase. The association between arsenic and sulfides limited arsenic solubility at depths under permanent water saturation. Arsenic solubility was revealed to be controlled by a sequence of processes that prevail under different geochemical regimes down the profile.