Technical Abstract: High concentrations of arsenic in the shallow and deep ground waters of the Tulare basin in California, USA, pose a hazard risk to drinking water supplies in the region.. Both natural processes and anthropogenic activities affect arsenic accumulation and movement in this region. Agricultural activities, particularly irrigation and drainage, enhance arsenic mobilization, and high evapoconcentration rates concentrate arsenic in surface and shallow groundwaters. Important biogeochemical processes affecting arsenic solubility and mobility include adsorption/desorption and redox reactions. The later process, which transforms inorganic arsenate [As(V)] to arsenite [As (III)], remains poorly understood as to under what conditions it occurs and to the extent of its influences. The role of nearly impermeable layers, e.g., E-clay, in potentially retarding arsenic movement into deeper aquifers has also not been well evaluated. Future research involving reactive transport models is necessary to better our understanding of arsenic reactivity, accumulation, and mobility in the groundwaters of the Tulare Basin.