Field experiment determinations of distribution coefficients of actinide elements in sulfate lake environments

The predictions of the distance that a given radionuclide may travel over a given period of time from a high-level nuclear waste repository are dependent on a number of factors. These include the numerical value assigned for solubility and sorption-desorption particle affinities (K/sub d/'s). We have already shown that pure phase solubilities and laboratory scale K/sub d/ determinations do not accurately predict radionuclide concentration measured in alkaline brines (Simpson et al., 1983, 1984). Data from other natural systems must be included in order to complete our understanding of radionuclide behaviour in natural aquatic systems. The concentrations of a number of radioisotopes of some elements (Pu, U, Th, Pa, Ac, Ra, Bi, Po, Pb, Cs, Sr, and K) were measured in a group of lakes that are dominated by SO42- ion in their anionic composition. Only Pu and the Th show possible enhancement of solubility at high sulfate concentrations in some Saskatchewan lakes, although never to the extent as observed for alkaline lakes. Surface waters of Green Lake (meromictic) which are at saturation with respect to calcite have approximately the same radionuclide content as seawater. The deepwaters (anoxic) show a marked increase in Th and Ra. This indicates that these elements may be coupled with the redox cycle of Fe and Mn which under oxygenated conditions effectively sequester Pu, Th, and Ra as Fe-Mn oxyhydroxides. Another possibility for the enhanced 226Ra in the deep water is by coprecipitation with CaCO3 in the surface water and subsequent dissolution in the deep water thereby releasing radium into the water. 38 references, 17 figures, 28 tables