CONSIDERATION OF ARSENIC BIOAVAILABILITY IN HUMAN HEALTH RISK ASSESSMENT: PRELIMINARY GREENHOUSE RESULTS

Human health risk assessments from exposure to metal-contaminated soils are often based on total soil metal concentrations. This practice does not take into account the geochemical fate of metals in soils and its impact on the ultimate gastrointestinal absorption of metals. A critical parameter for realistic human health risk assessment in metal-contaminated soils is an estimate of “bioavailable” metals. Bioavailability is the extent of absorption of a chemical into bloodstream from the gastrointestinal tract, lungs, or skin. Bioavailability of metals in soils is typically an inverse function of soil metal retention. Factors that generally influence arsenic (As) retention in soils include soil pH, amorphous Fe/Al oxide contents, soil organic matter, concentrations of P, Ca, and Mg, cation exchange capacity, and clay content. A greenhouse study was initiated to properly understand the relationship between As speciation and bioavailability in a dynamic interaction between soils, water and plants. The primary goal of this study is to adequately address the issue of soil variability on human bioavailability of As, and its impact on human health risk assessment. Two soils with contrasting chemical properties were selected: Immokalee, an acid sand with low extractable Fe/Al, having minimal As retention capacity, and Millhopper, an acid sandy loam with high extractable Fe/Al oxides and hence, high As retention potential. Soils were amended with sodium arsenate at two rates representing concentrations typically found in Superfund sites: 675 and 1500 mg/kg. Rice (Oryza sativa) was used as the test crop. Soils were sampled three times: 0-time, after 3 mo and 6 mo of soil equilibration. A sequential extraction scheme was employed to identify the geochemical forms of As in soils at time 0, 3-months and after 6-months of soil equilibration. Concentrations of these operationally defined soil As forms were correlated with in-vitro gastrointestinal As fractions to identify the pool of As that is most likely to be bioavailable in a given condition. A Pathway-3 risk assessment was performed based on bioavailable As values instead of total As concentrations, which is the standard practice. Results demonstrated a significant reduction in calculated risk values, which lowered the risk-based goals for potential site cleanup.