ARSENIC CYCLING IN AGRICULTURAL WATERSHEDS: THE ROLE OF PARTICLES

Poultry are fed organoarsenicals to prevent bacterial infections and improve weight gain. Most of the arsenic is not retained by the animal and is excreted in the poultry litter. Studies have shown that organoarsenicals in the litter are highly soluble in water, and are also rapidly biotransformed to inorganic arsenic in topsoils, raising concern that the more toxic form of arsenic may leach into subsurface water supplies. Recent data collected from a field site in the poultry-dominated Muddy Creek watershed in Rockingham Co., Virginia suggest that inorganic arsenic litter is leached from poultry litter and is transported through the unsaturated zone, despite laboratory evidence that demonstrates that inorganic arsenic adsorbs strongly to surface and near surface soils. Filtration experiments conducted on soil water indicate that arsenic and phosphate, also derived from poultry litter, are associated with Fe oxide particles. Water sampling in streambeds draining the watershed reveals that particles may be collecting within the hyporheic zone, and slowly releasing arsenic through reductive dissolution of Fe oxides or from particle flushing from the hyporheic zone to streamwater. Results from this study suggest that particle-associated transport may be an important element of arsenic cycling in agricultural watersheds.