BEHAVIOR OF I-125 ADDED TO LIMNOCORRALS IN 2 CANADIAN SHIELD LAKES OFDIFFERING TROPHIC STATES

The main objectives of our investigation were to determine the loss ra te of iodine from water to sediment and to gain a better understanding of the behaviour of iodine in Shield lakes. Iodine-125 and tritium (( HHO)-H-3) 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 I-125/H-3 ratio was used to m easure the loss of I-125 from the epilimnion. Loss rate coefficients, (k), ranged from -0.0017 to -0.0074 day(-1). The I-125 was found prima rily in the <1 nm (<10 000 molecular weight) size fraction of suspende d particles. Partition coefficients (K-d) had geometric means (X/divid ed by geometric standard deviations) of 2526 X/divided by 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 X/divided by 6 l . kg(-1) dw f or bottom sediment. Concentrations in fish ranged from similar to 8 to 184 Bq . g(-1) dw, whereas concentration factors from water to fish r anged from 20 to 390 l . kg(-1) dw. Iodine behaves as a conservative e lement in Shield lakes, although it is available for uptake by biota. The persistence of I-125 in water and its accumulation by fish emphasi zes the potential importance of these pathways in the radiological dos e to humans.