Medically derived (131)I (t1/2 = 8.04 d) is discharged from water pollution control plants (WPCPs) in sewage effluent. Iodine's nutrient-like behavior and the source-specificity of (131)I make this radionuclide a potentially valuable tracer in wastewater nitrogen studies. Iodine-131 was measured in Potomac River water and sediments in the vicinity of the Blue Plains WPCP, Washington, DC, USA. Dissolved (131)I showed a strong, positive correlation with δ(15)N values of nitrate (δ(15)NO3(-)) in the river, the latter being a traditional indicator of nutrient inputs and recycling. Surface water δ(15)NO3(-) values ranged from 8.7 to 33.4‰; NO3(-) + NO2(-) concentrations were 0.39-2.79 mg N L(-1) (26-186 μM). Sediment profiles of particulate (131)I and δ(15)N indicate rapid mixing or sedimentation and in many cases remineralization of a heavy nitrogen source consistent with wastewater nitrogen. Values of δ(15)N in sediments ranged from 4.7 to 9.3‰. This work introduces (131)I as a tool to investigate the short-term fate of wastewater nitrogen in the Potomac River and demonstrates the general utility of (131)I in aquatic research.