GREENHOUSE STUDY ON ARSENIC SPECIATION AND BIOAVAILABILITY IN TWO PESTICIDE CONTAMINATED SOILS OF FLORIDA: PRELIMINARY RESULTS

Health risk assessment from human exposure to arsenic (As)–rich soils requires an estimate of bioavailable arsenic. Many baseline risk assessment As-contaminated sites assume that all arsenic present in the soil is bioavailable, thereby potentially overestimating the actual health risk. The risk from As exposure is associated only with those forms of As that are potentially extractable by the human gastrointestinal juices. It has been demonstrated that As may exist in several geochemical forms depending on soil chemical properties, which may or may not be bioavailable. The objective of the reported study was to develop a comprehensive understanding of the relationship between As speciation and bioavailability as a function of soil physico-chemical properties in a greenhouse setting involving dynamic interactions between soil, water and plants. Two different types of soils were chosen based on their potential differences with respect to As reactivity: Immokalee, an acid sand with low extractable Fe/Al, having minimal arsenic retention capacity, and Millhopper, an acid sandy loam with high extractable Fe/Al oxides and hence, higher As retention potential. The soils were amended with sodium arsenate at two rates representing the high end of As contamination, generally representative of Superfund site conditions: 675 and 1500 mg/kg. Rice (Oryza sativa) was used as the test crop. A sequential extraction scheme was employed to identify the geochemical forms of As in soils (soluble, exchangeable, organic, Fe/Al bound, Ca/Mg–bound, residual) immediately after spiking and after 4 mo of equilibration. Concentrations of these As forms were correlated with the in-vitro bioavailable fractions of As to identify those As species that are most likely to be bioavailable. Following the greenhouse study, selective in-vivo bioavailability studies using As-contaminated soils will be conducted on male and female mice to correlate in-vitro results with in-vivo data. The results from this study will help elucidate the effect of soil properties on As biogeochemistry in chemically-variant pesticide-contaminated soils and identify those properties with the greatest influence on availability of As in the human gastrointestinal system.