The influence of a whole-lake addition of stable cesium on the remobilization of aged 137Cs in a contaminated reservoir

To document the short-term dynamics of Cs, 4 kg of 133Cs were introduced into an 11.4-ha, 157 000 m3 reservoir previously contaminated with 137Cs from past reactor operations at the US Department of Energy's Savannah River Site near Aiken, South Carolina, USA. The 133Cs addition resulted in an increase of 6.1 MBq of 137Cs (1.9 μg 137Cs) in the water column over the following 260 days. Possible sources for the increased 137Cs included (1) release from the sediments, (2) release from the approximately 26 000 kg of aquatic macrophytes that occupied 80% of the reservoir, and (3) wash-in from the pond's watershed. Data are presented to indicate that release from the sediments was the principal source of the 137Cs increase. The fraction of 137Cs released from the sediments (0.7%) is consistent with laboratory measurements of 137Cs desorption from neighboring ponds on the Savannah River Site.