Iodine-131: Measurement and Application of a Novel Tracer in Lake Michigan

Iodine-131 is a short-lived (half-life=8.0233 days), gamma emitting, radiopharmaceutical that, when excreted by patients, enters aquatic systems via sewage effluent discharged from water reclamation facilities (WRFs). Here, I report on 131I activities in the nearshore of southwest Lake Michigan in the vicinity of Milwaukee, Wisconsin. This is the first report on 131I activity in any of the Great Lakes of North America. The flux of 131I from Milwaukee’s two WRFs was monitored from July 2013 to December 2014. Mean discharge of 131I from the Jones Island WRF was (0.664 ± 0.012)×108 Bq d-1 (mean effluent 131I activity: ~0.25 Bq L-1; n = 29). Mean discharge of 131I from the South Shore WRF was (2.07 ± 0.68)×108 Bq d-1 (mean effluent 131I activity: ~0.74 Bq L-1; n = 29). The mean combined flux of 131I from both WRFs into Lake Michigan was (2.78 ± 0.72)×108 Bq d-1. Measureable activities of 131I were found in samples of Cladophora algae that were collected along a ~40 km section of Lake Michigan shoreline, from Atwater Park (8 km north of the Jones Island WRF) to Wind Point (13 km south of the South Shore WRF). 131I activity in all Cladophora algae samples (n = 30) ranged from below detection to ~39 Bq g-1 dry weight. Cladophora samples containing 131I were found at the shoreline of most public beaches in the area including Atwater Park, Bradford Beach, South Shore Beach, Bay View Park, Grant Park, and Bender Park. Detection of 131I in algae still attached to bottom substrate is an unequivocal metric of recent (days to weeks) exposure to treated sewage effluent. 131I activity in dreissenid mussels (n = 17) ranged from below detection to ~0.7 Bq g-1 dry weight, and its mean was approximately one order of magnitude less than activity found in Cladophora algae. Bottom trawls of lakebed material were made at 1, 5, and 8 km from shore in the vicinities of both WRFs. Analysis of collected materials revealed most 131I activity in sloughed Cladophora algae, with overall 131I activity decreasing with distance from shore but still measureable at 8 km out. Under steady state conditions, estimates of sloughed Cladophora transport along the lake bottom ranged from 200 to 500 m d-1 in both the south and east (offshore) directions. Estimates of sediment deposition in the Milwaukee outer harbor were made with measurements of 131I and the naturally occurring cosmogenic radionuclide 7Be (half-life = 53.44 days) in bottom sediment (n = 67) and the outer harbor water column (dissolved + particle-bound fractions; n = 21). Concordance between both radionuclide derived particle flux estimates could only be achieved if it was assumed that bottom scavenging of both particle-bound and dissolved nuclide fractions was occurring, with particle transport dominated by vertical convection rather than gravitational settling. Mean non-steady state estimates of sediment deposition in the outer harbor from 29 July to 8 October, 2014 were 5.48 ± 0.93 tonne d-1 as derived by 131I, and 7.6 ± 1.3 tonne d-1 as derived by 7Be. Estimates of sediment loading from the Milwaukee River to the outer harbor, which were derived from USGS gauged discharge and turbidity measurements, had a mean of 9.1 tonne d-1 over the same time interval. I conclude from these results that as much as 60 to 80% of riverine particulate material delivered to the outer harbor is retained within the harbor and not discharged to the lake during periods of relatively low river flow