Arsenic Speciation and Availability in Orchard Soils Historically Contaminated with Lead Arsenate

Lead arsenate is an environmentally hazardous contaminant that was applied as a pesticide in orchards during the early 1900s. Elevated arsenic (As) concentrations persist in soils where lead arsenate was applied. To assess the risk associated with historic lead arsenate contamination, the retention, bioavailability, and speciation of soil As were evaluated in three historically contaminated orchard soils. Stirred-flow desorption studies and in vitro physiologically based extractions were used to assess the mobility and relative bioavailability of soil As, respectively. Synchrotron-based X-ray absorption spectroscopy was used to determine soil As speciation. Arsenic concentrations in former orchard soils ranged from 11.8 to 59.0 mg kg. Less than 22% of total As was considered bioavailable according to in vitro extractions. Up to 15% of soil As was desorbed in 10 mM KCl, but desorption with phosphate solutions resulted in release of up to 70% of total As dependent on soil type. Desorption data suggest that arsenate is primarily sorbed via inner-sphere complexation, and elevated concentrations of competing ions in soil solution may increase mobility of soil As. Arsenic was primarily present in the As(V) oxidation state, the less mobile form of As. Combined results from X-ray absorption spectroscopy and desorption studies indicate that As primarily exists as sorbed species, likely to ubiquitously present Fe- and Al-oxides in soils. Results demonstrate that soil As from these historically contaminated orchards is mostly in stable, nonhazardous forms, but factors such as Fe- and Al-oxide content and land management practices have a significant effect on As transport and bioavailability.