Roxarsone, an organoarsenic compound, is added to poultry feed at 22.7 to 45.5 g per ton, for the purpose of improving weight gain, feed efficiency, and pigmentation. Previous studies have demonstrated that these organoarsenic compounds do not accumulate in poultry tissue or feathers but are rapidly excreted, resulting in elevated concentrations of arsenic (20 to 40 mg/kg) in poultry litter.  Despite the evidence that poultry waste contains significant concentrations of arsenic, there is a surprising lack of information on the fate of organoarsenic compounds in the environment. The central objective of our study is to gain new and fundamental insights into factors that control the fate and transport of arsenic compounds in agricultural watersheds.