Using medically-derived iodine-131 to track sewage effluent in the Laurentian Great Lakes

Tracking sewage wastewater in a large lake is difficult. Concentrations of pharmaceuticals that can be used as indicator compounds are quickly diluted and not easy to measure. In this study, we examined the potential of using medically-derived iodine-131 (¹³¹I, t_½ = 8.02 d) as a tracer for Milwaukee sewage effluent in Lake Michigan. ¹³¹I activities in sewage effluent from two Milwaukee wastewater treatment plants (WWTPs) were measured in conjunction with ¹³¹I activities in water, sediment and biota in the Milwaukee Outer Harbor and Lake Michigan. ¹³¹I discharge rates from both WWTPS ranged from 34 ± 15 to 1807 ± 24 MBq d^-1, with average and median ¹³¹I discharges of 278 and 129 MBq d^-1. A budget of ¹³¹I in the Milwaukee Outer Harbor - based on measured sediment and water column inventories - showed that ∼11% of the ¹³¹I discharged to the harbor was scavenged to bottom sediments, ∼19% decayed in the harbor water column, and ∼70% was flushed out of the harbor to Lake Michigan. From this budget, we derived a harbor flushing rate of 3.1 days. In Lake Michigan, ¹³¹I activity was found in Cladophora algae (undetected to 91 ± 2 Bq kg^-1) along ∼40 km of shoreline. Benthic trawl samples showed ¹³¹I activity up to 8 km from shore. Calculated ¹³¹I length scales were 30 km alongshore and 3.4 km offshore and corresponded to sewage effluent dispersion rates of ∼2.6 km d^-1 and ∼0.3 km d^-1 in along- and offshore directions. Using ¹³¹I as a tracer of sewage effluent from other coastal municipalities to the Laurentian Great Lakes appears feasible, particularly for larger (>10⁵) population centers.