Arsenic (As) concentrations in groundwater that exceed the Maximum Contaminant Level (MCL) (10 μg/L) for drinking water have been reported throughout the United States, with higher occurrences in the Southwestern basin-fill aquifers. Levels of As above the MCL were measured in wells throughout the Cache Valley Basin, Utah. The As is naturally occurring in geologic material from the soil surface to depths of groundwater. This study reports on the mechanisms of retention and solubilization of As through these zones using geochemical modeling and microcosm studies. Two cores (NP 9 and NP 13) were collected from the soil surface to the depth of groundwater and sectioned based on observed redoximorphic features. Pore water was analyzed for As and iron( Fe) redox species, general water quality parameters and solid phase As, Fe and Mn using sequential extractions. These data were used in PHREEQC and MINTEQ geochemical models to predict mechanisms of As retention. Microcosm studies were performed using sediments from the water table zone. The sediments were exposed to oxidized, reduced, and poisoned conditions over time to evaluate the effect of the seasonal fluctuating water table on As release. Modeling results indicated As(V) was dominantly sorbed to hydrous ferric oxides (HFO) throughout both profiles. Although much less As(V) was sorbed to CaCO3, the percentage associated with calcite was 1.7 to 3.3% and 6 to 59% in the surface and water table zones for NP 9 and NP 13, respectively. As(III) solubility was controlled by the formation of an As-S mineral, orpiment. Microcosm findings, over 113-day incubation, concluded that regardless of treatment condition, As is released. For reduced samples As in solution was primarily As(III), while oxidized and poisoned samples only released As(V). The release of As under every condition, and the lack of reduced As and Fe in the poisoned samples, indicates that As release is abiotically controlled, while reduction is microbially driven. Carbonate minerals were the source of As(V) under treatment conditions as determined using an acetate extraction. Desorption of As(V) from carbonate minerals and the reduction of As(V) to As(III) played a significant role in explaining solution phase As(III) concentrations.