Fate of Co-60 and Cs-134 added to the hypolimnion of a Canadian Shield lake

Tritium (H-3), cobalt-60 (Co-60), and cesium-134 (Cs-134) were added to the anaerobic hypolimnion of a Canadian Shield lake during the summer of 1989 to simulate the nuclear fuel waste disposal scenario in which nuclides are assumed to enter the bottom of a lake with deep groundwater flow. The radionuclides initially remained confined to the hypolimnion and there was little loss from the water. Lake mixing during autumn turnover resulted in the rapid loss of Co-60 (first order rate constant, k, is 0.036 d(-1)) and much slower loss of Cs-134 (k = 0.006 d(-1)) from the water to sediment over the next 41 and 238 d, respectively. One year after the addition, <1% of the Co-60 and Cs-134 and 6% of the tritiated water added to trace water movement had been lost to the outflow. About 6% of the Co-60 and 19% of the Cs-134 remained in the water column. Loss rates to the sediment observed following autumn turnover in the present study were slower than those previously observed from the epilimnion; however, there was little difference in the amount of Co-60 and Cs-134 remaining in the water after 1 year when the input was to the hypolimnion as opposed to the epilimnion.