Behavior of 60Co and 134Cs in a Canadian Shield lake over 5 years.

Abstract Radionuclides were added to the anoxic hypolimnion of a Canadian Shield lake to simulate the nuclear fuel waste disposal scenario where radionuclides might enter the bottom waters of a lake. The radionuclides remained in the hypolimnion until lake mixing at autumn turnover after which 60 Co was rapidly lost and 134 Cs was slowly lost from the water. Only 0.4% of the 60 Co and 0.6% of the 134 Cs remained in the water at year 5. Highest concentrations occurred in periphyton and filter feeders, Holopedium gibberum and clams ( Anodonata grandis grandis ). From maximum annual concentrations in clam tissues, it was estimated that the availability of 60 Co for uptake had a half-time ( t 1/2 ) of 835 days in the lake, whereas that for 134 Cs was 780 days. Loss rate coefficients, k , for the radionuclides from taxa ranged from 0.0008 to 0.0043 day −1 ( t 1/2 =161–866 days) for 60 Co and from 0.0009 to 0.005 day −1 ( t 1/2 =139–770 days) for 134 Cs. Cobalt-60 concentrations in forage fish were low, whereas 134 Cs concentrations increased over the first year or two, then slowly declined. On the basis of k values measured for forage fish, the biological half-time of 134 Cs in forage fish ranged from 428 to 630 days. Maximum 134 Cs concentrations in forage fish were higher following hypolimnetic addition than epilimnetic addition. Relatively high 134 Cs concentrations in periphyton at year 5 point to the importance of benthic pathways in the recycling of contaminants to higher trophic levels. The presence of 134 Cs in biota 5 years after the addition, long after concentrations were no longer detectable in surface waters, is evidence of the persistence of Cs in aquatic systems. The k values (or t 1/2 values) for the loss of 60 Co and 134 Cs from water and their uptake and loss from biota can be used to establish parameter values for assessment models. The results demonstrate that assessment models should account for the release of radionuclides from sediment and their subsequent recycling in the food chain when modeling over the long term after the end of a radionuclide release to the environment.