Radionuclide Migration Studies Associated with the WIPP Site in Southern New Mexico

Abstract

Migration of radionuclides in ground water has been identified as the most likely pathway from a deep geologic nuclear waste repository to the biosphere. An important mechanism in retarding nuclide movement in ground water is sorption, a term which is used herein to encompass all mechanisms pertinent to interactions (which include ion exchange, absorption, and precipitation) between nuclides and the geomedia. Water exiting a repository at the WIPP site would encounter halite in the repository horizon, polyhalite and anhydrite beds, and finally sandstone and dolomite formations which are the upper bound of the repository. To a rough approximation, the concentration of a given nuclide per unit mass, C(solid), sorbed on a solid mineral phase can be related to the concentration per unit volume, C(liquid), in the liquid phase by the relation:

$$C\left( {solid} \right) = Kd \cdot C\left( {liquid} \right)$$

where Kd is generally referred to as the distribution coefficient, with units of ml/g. The use of the term Kd implies an equilibrium state for a given reaction and is used, in that sense, in the mathematical modeling of long term radionuclide migration (1). In the interaction of a nuclide with a complex mineral assemblage, equilibrium conditions may not apply, and the symbol Kd is used as an empirical value applying only to the particular set of conditions used.

This work supported by the United States Department of Energy (DOE), under Contract AT(29-l)-789.

A DOE Facility.