Medically-Derived ¹³¹I as a Tool for Investigating the Fate of Wastewater Nitrogen in Aquatic Environments

Medically derived ¹³¹I (t_1/2 = 8.04 d) is discharged from water pollution control plants (WPCPs) in sewage effluent. Iodine's nutrient-like behavior and the source-specificity of ¹³¹I 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 ¹³¹I showed a strong, positive correlation with δ¹⁵N values of nitrate (δ¹⁵NO₃^-) in the river, the latter being a traditional indicator of nutrient inputs and recycling. Surface water δ¹⁵NO₃^- 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 ¹³¹I and δ¹⁵N indicate rapid mixing or sedimentation and in many cases remineralization of a heavy nitrogen source consistent with wastewater nitrogen. Values of δ¹⁵N in sediments ranged from 4.7 to 9.3‰. This work introduces ¹³¹I as a tool to investigate the short-term fate of wastewater nitrogen in the Potomac River and demonstrates the general utility of ¹³¹I in aquatic research. (Figure Presented).