Natural-Analog Studies for Partial Validation of Conceptual Models of Radionuclide Retardation at the Wipp

Transport by groundwater within the Culebra Dolomite, an aquifer above the Waste Isolation Pilot Plant (WIPP), is the most probable mechanism for long-term release of radionuclides to the accessible environment. Radionuclides could be retarded by sorption if the groundwater is exposed to sufficient amounts of fracture-lining clays. In this natural-analog study, distributions of U and trace metals have been examined to constrain the strength of clay/solute interactions within the Culebra. p]Uranium solid/liquid distribution ratios, calculated from U concentrations of groundwaters and consanguineous fracture-filling clays, range from ~80 to 800 ml/g and imply retardation factors of 60 to 500 using a fracture-flow model. Retardation factors inferred from uranium-series disequilibria and 14C ages in Culebran groundwaters alone are much lower (~10), implying that clays may contain a significant unreactive component of U. Such a possibility is corroborated by Rb/Sr ages; these imply long-term stability of the clays, with resetting occurring more than 250 Ma ago. Factor analysis and mass-balance calculations suggest, however, that Mg-rich clays are dissolving in Pleistocene-age groundwaters and/or are converting to Na-rich smectites, and that B and Li are taken up from the water by the clays. Apparently, the solution chemistry reflects gradual equilibration of clays with groundwater, but thus far the bulk of the clays remain structurally intact. Measurements of the distribution of U in the Culebra will be more meaningful if the inert and exchangeable components of the U content of the clays can be quantified.