HUMAN HEALTH RISK FROM EXPOSURE TO SOIL-ARSENIC: DOES ONE SIZE FIT ALL?

Rapid intrusion of suburban development on former agricultural lands has greatly increased the potential for human exposure to arsenic (As), a group A carcinogen used extensively as pesticides prior to the 1990s. Recent studies have focused on the health risk posed by long-term human exposure to low-level As-contaminated systems, particularly due to soil ingestion from incidental hand-to-mouth activity by children playing in the backyards. In the absence of a “soil model” on As bioavailability, many baseline risk assessments of As-enriched sites assume that all (100%) As present in the soil is bioavailable. This assumption seriously overestimates the actual risk (thereby increasing site clean-up expenses), since various geochemical forms of As are stable and/or insoluble in human digestive juices and are not likely to be bioavailable. A laboratory incubation study was conducted to identify the relationship between geochemical speciation and “in-vitro” bioavailability of As in soils as a function of soil properties. Five different soil types were chosen based on their potential differences with respect to As reactivity: an acid sand with minimal As retention capacity, a sandy loam with relatively high concentration of Fe/Al-oxides (hence, higher As retention capacity), a low-pH clay soil, an organic (muck) soil, and a high pH calcareous soil. The soils were amended with sodium arsenite at three rates: normal (45 mg/kg), above normal (225 mg/kg) and excessive (450 mg/kg). A sequential extraction scheme was developed to identify the various geochemical forms of As in pesticide-applied soils (soluble, exchangeable, organic, Fe/Al-bound, Ca/Mg-bound, residual). Concentrations of these operationally defined soil As forms were correlated with the “in-vitro” bioavailable fractions of As to identify the As species that are most likely to be bioavailable under a variety of soil/pesticide scenarios. Data obtained from arsenic bioavailability studies at 0-time (immediately after spiking the soils with pesticides), 4-month and 1-year incubation period was used to calculate the reduction in potential cancer risk using soil-specific bioavailability data as opposed to total As data. Results demonstrate that As speciation in soils result in significant lowering of As bioavailability, and hence, cancer risk.