Iron (hydr)oxide transformation and release of arsenic from tropical soils during iron and sulfate reduction.
Benjamin Kocar1, Samantha Ying1, Matthew Polizzotto1, Ung Mengieng2, Sopheap Samreth2, Leng Moniphea2, Mickey Sampson2, and Scott Fendorf1. (1) Stanford University, Ges - Braun Hall, Ges - Braun Hall, Stanford, CA 94305-2115, United States of America, (2) Resource Development International-Cambodia, Royal Brick Road, Kean Svay, Kandal Province, Cambodia
Soils and surface sediments are enriched with arsenic bearing iron (hydr)oxides throughout sedimentary basins of southeast Asia– arsenic concentrations typically range from 10 to 40 mg kg -1 (DW). Seasonal monsoons result in drastically fluctuating water levels, which drive wide swings in redox processes within soils and near-surface sediments. Extensive flooding results in anaerobic soil/sediment conditions, with ensuing reduction of iron (hydr)oxides and sulfate. Aqueous concentrations of arsenic vary with redox conditions and processes and when elevated, result in migration through the soil profile to the deeper subsurface. Using ceramic-cup lysimeters and passive samplers (peepers), we measured a suite of reduced aqueous constituents, including arsenic, within soil profiles in the lower Mekong delta, Cambodia. Within a zone of seasonal wetting/drying, we observed increased aqueous concentrations of arsenic in surface soils during periods of flooding. Arsenic release is coincident with the production of Fe(II) and, to a lesser degree, S(-II) in the porewater. These results indicate that arsenic mobilization occurs in near-surface soil environments upon soil wetting, and that iron reduction and sulfidogenesis are likely dominant processes responsible for arsenic release.