In-Vitro Determination of Arsenic Bioavailability in Contaminated Soil and Mineral Beneficiation Waste from Ron Phibun, Southern Thailand: A Basis for Improved Human Risk Assessment

The post-ingestion bioavailability of arsenic (As) in alluvial soil and mineral beneficiation waste from Ron Phibun, Nakhon Si Thammarat Province, Thailand has been investigated using a physiologically-based extraction test (PBET). The method utilises synthetic leaching fluids closely analogous to those of the human stomach and small intestine, upon which the leaching duration and ambient temperature of the experimental procedure are also directly based. Replicate analyses of Ron Phibun alluvium samples holding 1406 and 2123 g As g-1 respectively indicated an average stomach absorption of 11.2% (of total soil As). Gross absorption increased to 18.9% following translocation through a simulated small-intenstine regime. Higher gross absorption (35.7%) was recorded during PBET analysis of a flotation waste sample holding c. 2% total As. Within- and between-site variations of As bioavailability can principally be ascribed to mineralogical factors, notably the relative abundances of sulphide, arsenide, arsenate and oxide hosts within the soil/flotation waste matrix. The PBET provides a potentially valuable mechanism for refining risk assessments of sites subject to natural or anthropogenic As contamination. Through the substitution of total soil As values with input data which relate specifically to bioavailable As, risk calculations derived using established models such as the US-EPA package Risk Assistant may prove more realistic, thus facilitating improved cost-benefit analysis of site remediation options. Assessment of the relative human risks associated with potable water consumption and contaminated soil ingestion at Ron Phibun has signified that the latter could constitute a more significant As exposure pathway than recognised previously. Further evaluation of the precise soil ingestion levels of inhabitants residing on the As-rich alluvium which covers much of the district is, therefore, warranted.