Encroachment of residential development on agricultural lands in the United States where arsenical pesticides were extensively used prior to the 1990s has increased the potential for human exposure to arsenic (As), a group A carcinogen. Soil ingestion by children is a critical issue in assessing health risks from exposure to As-enriched soils. In the absence of a universal “soil model” on As bioavailability, many baseline risk assessment studies use the assumption that all (100%) As present in soil is bioavailable. However, As exists in many geochemical forms as dictated by soil chemical properties. Because As bioavailability is a function of soil speciation, using total soil arsenic values potentially overestimates human health risk, thereby increasing site cleanup expenses. A laboratory incubation study was conducted to estimate in vitro As bioavailability as a function of soil properties in four chemically variant soil types contaminated with sodium arsenite pesticide. Results demonstrate that As speciation in certain soils translates to significant lowering of As bioavailability and hence potential cancer risk.