Studies of nuclear waste migration in geologic media. Annual report, October 1978-September 1979

Abstract

Experimental results obtained this year confirm the results obtained in previous years - that nuclides migrating by fluid flow in rock often exhibit complex behavior not predicted by simple chromatographic-type models. A phenomenon found previously to lead to complex behavior for leached radionuclides is that the amount of adsorbed nuclide was not proportional to the nuclide concentration in solution (nonlinear adsorption isotherm). For cesium adsorption on limestone and on basalt, nonlinear isotherms were found this year to occur in the range of cesium concentrations in the groundwater of about 10/sup -3/ to 10/sup -9/M. Because cesium concentrations in this range can readily be attained by the leaching of solid waste by groundwater, the effects of nonlinear isotherms are germane to nuclide migration. This dependence of cesium migration on the leached concentration of cesium emphasizes the importance of treating the leaching and migration processes simultaneously such as is done in the leach-migration experiments performed in this work. The existence of nonlinear isotherms precludes the use of a single partition coefficient (K/sub d/) to describe cesium migration at an arbitrary cesium concentration above 10/sup -9/M. Nonetheless, nonlinear isotherms can be studied experimentally (e.g., to give K/sub d/ as a function of concentration) and effects of nonlinear adsorption can be predicted quantitatively. Comparison of results from column and batch tests indicate that, in addition to nonlinear adsorption, kinetic effects need to be considered in predicting nuclide migration from the partition coefficients measured in batch tests. Results of batch experiments of 2 weeks or longer duration pertained to migration expected only at the very lowest (< 50 m/y) groundwater flow rates of interest.