Accumulation of 60Co and 134Cs in lake whitefish in a Canadian Shield lake

Radionuclides were added to the anaerobic hypolimnion of a Canadian Shield lake during the summer of 1989 to simulate nuclear fuel waste leakage into the bottom waters of a lake with deep groundwater flow. Lake whitefish (Coregonus clupeaformis) were sampled in mid-October each year from 1989 to 1994, except for 1992, to determine the concentration of Co-60 and Cs-134 in their tissues. The radionuclides remained in the hypolimnion until lake mixing in the autumn. Subsequently, there was more loss of Co-60 than Cs-134 from the water. After one year, 4% of the Co-60 and 15% of the Cs-134 remained in the water. This decreased to about 0.4% of the Co-60 and 0.6% of the Cs-134 in the water after five years. Little Co-60 and Cs-134 were present in lake whitefish the first autumn following the radionuclide addition. In fish, the highest concentrations were recorded in October, 1990, after which concentrations gradually decreased. Cobalt-60 concentrations were lower than Cs-134 concentrations, probably because Co-60 has a high affinity for organic particles and was not available for uptake. The relatively high Cs-134 concentrations in fish reflects its tendency to remain in the water and its continued availability to biota. Both Co-60 and Cs-134 concentrations were high in the soft tissues. Co-60 was mainly distributed in the flesh, gut, gill, kidney, and liver, whereas Cs-134 was mainly distributed in the flesh. The occurrence of relatively high Cs-134 concentrations in lake whitefish five years after their addition to the lake, long after Cs-134 concentrations are no longer detectable in lake water, attests to the continued persistence of this contaminant in aquatic biota.