Behavior of medically-derived ¹³¹I in the tidal Potomac River

Iodine-131 (t_1/2=8.04d) is administered to patients for treatment of thyroid disorders, excreted by patients and discharged to surface waters via sewage effluent. Radionuclides generally behave like their stable analogs; therefore, medically-derived ¹³¹I is useful as a transport-reaction tracer of anthropogenic inputs and the aquatic biogeochemistry of iodine. Iodine-131 was measured in Potomac River water and sediments in the vicinity of the Blue Plains Water Pollution Control Plant (WPCP), Washington, DC, USA. Concentrations measured in sewage effluent from Blue Plains WPCP and in the Potomac River suggest a relatively continuous source of this radionuclide. The range of ¹³¹I concentrations detected in surface water was 0.076±0.006 to 6.07±0.07BqL^-1. Iodine-131 concentrations in sediments ranged from 1.3±0.8 to 117±2Bqkg^-1 dry weight. Partitioning in the sewage effluent from Blue Plains and in surface waters indicated that ¹³¹I is associated with colloidal and particulate organic material. The behavior of medically-derived ¹³¹I in the Potomac River is consistent with the nutrient-like behavior of natural iodine in aquatic environments. After discharge to the river via sewage effluent, it is incorporated into biogenic particulate material and deposited in sediments. Solid phase sediment profiles of ¹³¹I indicated rapid mixing or sedimentation of particulate debris and diagenetic remineralization and recycling on short time scales.