Subsurface cobalt-60 migration from a low-level waste disposal site

After 28 years of continuous use, a low-level liquid radioactive waste infiltration pit at the Chalk River Nuclear Laboratories will soon be decommissioned. In a study undertaken to predict future releases of 60Co, ratios of sorbed to dissolved 60Co from 0.5 to greater than 130 mL x g-1 were found in the underlying aquifer. This ruled out the application of a simple K/sub d/ model for solution/sediment interactions. In subsequent, more detailed, studies a variety of techniques were used to characterize dissolved 60Co as predominantly (80%) hydrophilic weakly anionic organic complexes of intermediate molecular size. Both naturally occurring and contaminant organics appear to act as complexing ligands. Results of sequential desorption of sediment-bound radiocobalt show that at least three sorption mechanisms (ion exchange, sorption by iron oxide coatings, and ''fixation'') must be involved. On average, more than 80% of the sorbed 60Co is ''fixed''. Two possible scenarios of future release of sediment-bound 60Co are discussed. 20 references