Behaviour of 125I added to limnocorrals in two Canadian Shield lakes of differing trophic states

The main objectives of our investigation were to determine the loss rate of iodine from water to sediment and to gain a better understanding of the behaviour of iodine in Shield lakes. Iodine-125 and tritium (3HHO) were added to the epilimnion of limnocorrals (enclosures) in mesotrophic Lake 226 and eutrophic Lake 227, Experimental Lakes Area, northwestern Ontario. The change in the 125I3H ratio was used to measure the loss of 125I from the epilimnion. Loss rate coefficients, (k), ranged from −0.0017 to −0.0074 day−1. The 125I was found primarily in the < 1 nm (< 10 000 molecular weight) size fraction of suspended particles. Partition coefficients (Kd) had geometric means (×÷geometric standard deviations) of 2526 ×÷2.1 l·kg−1 dry weight (dw) for suspended sediment, 1362 ± 2.9 l·kg−1 dw for particles in sediment traps and 132 ×÷6 l·kg−1 dw for bottom sediment. Concentrations in fish ranged from ∼8 to 184 Bq·g−1 dw, whereas concentration factors from water to fish ranged from 20 to 390 l·kg−1 dw. Iodine behaves as a conservative element in Shield lakes, although it is available for uptake by biota. The persistence of 125I in water and its accumulation by fish emphasizes the potential importance of these pathways in the radiological dose to humans.