PREDICTING ARSENIC BIOAVAILABILITY IN CONTAMINATED SOILS BY USING IN VITRO GASTROINTESTINAL BIOACCESSIBILITY AND SOLID PHASE SPECIATION FOR SITE-SPECIFIC RISK ASSESSMENT (Invited Presentation)

Arsenic is one of the most common contaminants of concern exceeding risk criteria because soil ingestion is the primary human health risk driver. Use of contaminant total content instead of bioavailability is often overly conservative and can result in costly and unnecessary soil remedial action. The study objective was to link the binding mechanism of As in soil (i.e., solid phase speciation) with in vitro and in vivo methods used to predict bioavailability of soil As to humans. Twenty-seven As contaminated soils that represent a wide variety of properties and As sources from DoD installations, industrial sites (mining, smelting, and glass production), residential and agricultural sites were studied. Solid phase speciation were determined using As and Fe XAS EXAFS and Mössbauer spectroscopy. Relative bioavailability (RBA) of As was determined by the adult mouse and the juvenile swine bioassays. In vitro bioaccessible (IVBA) As was determined by the OSU, UBM, PBET, CAB and USEPA Method 9200 IVBA methods. Total soil As ranged from 162 to 12,500 mg/kg with a median value of 464 mg/kg. The most abundant As species was As(V) adsorbed to Fe oxides. Other As species included As (-III) metal sulfides, As (III) sulfides, As (III) oxides, As (III) adsorbed to Fe/ Al oxides, lead arsenates, and hydrous ferric arsenates (HFA). RBA As ranged from 6.37 to 81.2%. IVBA As ranged from <1% to 100%. In vivo-in vitro correlation (IVIVC) analysis showed all of the IVBA methods were predictive of RBA for both the mice and swine bioassays. Despite As(V) adsorbed to mineral surfaces being a major component of most soils (>50%), these soils ranged from ~20-80% in IVBA As and widely ranged in RBA As. Arsenic speciation alone is not predictive of IVBA or RBA As. However, As speciation is very important to provide information on IVBA or RBA As results and/or determine a priori if a bioavailability-based risk assessment is justified by site managers and risk assessors.