Transport of 60Co between Water and Sediments in a Small Shield Lake

Abstract

Low levels of 60Co, discharged from a waste management area, have flowed into a small shield lake over a period of 22 yr. Constructing a mass balance using measured waterflows, measurements of 60Co activities in the inflow, in the bottom sediments, and in the lake outlet provides a unique record of the isotope's behaviour. The long-term, annual, and seasonal movements of 60Co between the water, sediments, and the outflow have been examined and compared. An instantaneously mixed, single reservoir model with constant parameters accurately simulated the average annual variations in 60Co activity in the water and in the sediments. The lake sediments were the major sink for 60Co. More than 90% of the 60Co present in the lake system at any one time was found in the sediments. However, less than half of the total 60Co inputs were retained in the sediments over the 22-yr period. The remainder left the lake through the outflow. The fractional rate of 60Co deposition in the sediments was independent of the rate of 60Co inputs and independent of the 60Co activity in the lake sediments. Seasonal changes in 60Co concentrations in the lake water were very large. Spring and summer concentrations were often 50–500% lower than predicted from a model that simulated annual average concentrations. The bias was likely due to rapid removal of 60Co by increased scavenging of the isotope by particulates. During the winter, 60Co concentrations in the lake water were greater than expected, probably due to the exclusion of 60Co from lake ice, incomplete mixing of the inflow with the lake water, or release of 60Co from the sediments. These seasonal variations in the 60Co activity were accurately simulated by a single reservoir model using different removal rates for the summer and winter periods.