EFFECTS OF SOIL GEOCHEMICAL AND PHYSICAL PROPERTIES ON THE BIOACCESSIBILITY OF CHROMIUM IN SOIL

There are numerous Department of Defense sites that are contaminated with toxic metals such as chromium. Ingestion of contaminated soil by children is the usual risk driver motivating remediation. Site assessments are based solely on total soil-metal concentration and do not consider the potential for decreased bioaccessibility due to metal sequestration. This study investigated the effect of soil properties on the bioaccessibility of Cr(III) and Cr(VI) as a function of metal concentration and aging, and sought to develop statistical models based on common soil properties to estimate the bioaccessibility of Cr in soil. The A- and upper B- horizons of 36 DOD relevant soils from 7 major soil orders were treated with Cr(III) and Cr(VI) and allowed to age. The bioaccessibility of the contaminated soils was measured using a physiologically based extraction test (PBET) designed to simulate the digestive process of the stomach. Overall, bioaccessibility of Cr decreased as the duration of exposure to the soil increased. Cr bioaccessibility varied widely as a function of soil type with most soils limiting bioaccessibility to < 30% after 100 d aging. Statistical analysis showed that the bioaccessibility of Cr(III) on soil was correlated with the clay content and total inorganic carbon (TIC) or total organic carbon (TOC) content of the soil, with bioaccessibility decreasing as the soil TIC content increased and as the clay content decreased. The Cr(VI) statistical model indicated that the total organic carbon (TOC) content, cation exchange capacity (CEC), and the soil pH correlated with Cr(VI) bioaccessibility with higher values of TOC, CEC and soil pH resulting in lower Cr bioaccessibility. The models yielded equations based on common soil properties that could be used to predict the Cr bioaccessibility in soils with a reasonable level of confidence. Thus, the models should be useful for assessing Cr(III) and Cr(VI) bioaccessibility at a contaminated site so that the appropriate remediation criteria can be determined.