Human health risk from exposure to As-enriched soils is associated only with those forms of soil As that are potentially extractable by the human gastrointestinal juices. Arsenic may exist in several geochemical forms in soils depending on soil chemical properties; some of these forms may not be bioavailable. The soil properties that most likely to influence As retention are listed in Table 1. Soils from the Immokalee, Millhopper, Pahokee Muck, and Orelia series were characterized to determine their pH, salinity, soil organic matter (SOM) content, cation exchange capacity (CEC), and total and available P, Mg, Ca, Fe, and Al. Immokalee soil is a sandy spodosol with low Fe/Al, Ca/Mg, and P contents. Being sandy and lacking positively charged surfaces (e.g., amorphous Fe/Al oxides), the Immokalee soil is likely to have minimal As retention capacity (Pierce and Moore 1980; Oscarson et al. 1981), thereby potentially increasing the bioavailable fraction of As. This soil was used as a control to study the effects of the variant soil chemical properties of the other soils. The Millhopper soil has a pH similar to Immokalee but much higher concentrations of Fe, Al, and P; the Pahokee Muck soil has 85% SOM and much higher concentrations of Fe, Al, Ca, and Mg; and the Orelia soil, in addition to having higher concentrations of Fe, Al, Ca, and Mg, has a much higher pH compared to Immokalee soil. The soils also varied widely in their salinity (measured as electrical conductivity [EC]) and the CEC, both likely to affect As geochemistry. The higher the CEC, the greater the amount of positive charge on the surface and the higher the potential of the As oxyanions to form electrostatic bonds with the positively charged surface sites. According to Chen et al. (1999), the major factors controlling trace metal concentrations in soils are clay content, organic carbon content, pH, CEC, and Fe, Al, Ca, Mg, and P concentrations. Because both As and P occur as oxyanions in environmental systems and have similar chemical properties, high P content of the Millhopper soil could result in desorption of retained As. Reportedly, As is strongly adsorbed onto Fe and Al oxides (Jacobs et al. 1970; Barringer et al. 1998); hence, Millhopper, Pahokee Muck, and Orelia soils are likely to have strong As retention capabilities. Generally, sorption of As decreases with increasing pH (Adriano 2001). This can be attributed to the negative surface charge on the adsorptive surface at higher pH as well as the negative charge of As oxyanions (Wasay et al. 2000). Certain components of SOM (such as fulvic acid) tend to complex As, thereby making it more soluble and hence bioavailable (Gough et al. 1996). On the other hand, adsorption of As by certain other components of SOM, such as humic acids, is high in the pH range of 5 to 7 and when the humic acids have high ash and calcium content (Mok and Wei 1994). Humic acids can contribute more to the retention of As in acidic environments than do clays and some metal oxides, thereby lowering its ultimate bioavailability. The major retention sites on the humic acids at low-pH systems are the amine groups (Thanabalasingam and Pickering 1986). Moreover, As bound to the Ca/Mg fraction in Millhopper, Pahokee Muck, and Orelia soils has the potential solubilize in the highly acidic environment of human stomach, thus becoming bioavailable. All the soils studied had similar native As concentration, ranging between 14 mg/kg in Pahokee Muck and 17 mg/kg in Orelia.