Bioaccessibility, mobility, and health risk assessment of arsenic and mercury in soils

Taiwan Environmental Protection Administration has declared more than 2,900 control and remediation sites due to soil and groundwater contamination. People who live on or close to these contaminated sites may have high exposure risk. Moreover, higher-frequency hand-to-mouthing behaviors could lead children under six years old to greater health risk than adults due to direct ingestion. Hence, health risk assessment is necessary for children living in the neighborhood of the contaminated sites. However, the assessment based on total content of contaminants may overestimate the risk; consequently, it is important to determine bioavailable metals in soils. Although in-vivo approaches reflect quasi-physiological conditions, these methods are expensive and time-consuming. Therefore, in-vitro methods are developed and recognized as fast screening tools in assessing bioaccessibility of metals in the soils of contaminated sites. In this study, eight asenic (As)-contaminated and mercury (Hg)-contaminated soils were collected from four different sites in Taiwan (i.e., Guandu, Cianjhen, Anshun and Chishan-Yen sites). One of the objectives is to provide more proper results for exposure assessment via acquisition of As and Hg bioaccessibility by using two in-vitro assays, namely physiologically based extraction test (PBET) and simplified bioaccessibility extraction test (SBET). Moreover, sequential extraction procedure (SEP) was employed to understand the solubilities and motilities of As and Hg in the soils. The experimental results showed that positive significant (P<0.05) relationships were found between As proportions in fraction 1, fraction 2 of SEP and As bioaccessibility; the positive significant (P<0.01) relationships was also established between Hg proportions in fraction 1, fraction 2 of SEP and Hg bioaccessibility. The soil properties (e.g., organic carbon and clay content) had significant (P<0.05) correlation with As and Hg bioaccessibility. As expected, the risk results after adjustment by bioaccessibility were lower than that obtained based on total metal contents. Results obtained in this research provide suggestions to the decision makers on setting new strategies in the risk management of metal-contaminated soil sites.