Evaluation of Cesium, Strontium, and Lead Sorption, Desorption, and Diffusion in Cores from Western Pahute Mesa, Nevada Test Site, based on Macroscopic and Spectroscopic Investigations

Abstract

EXECUTIVE SUMMARYThe interaction of radionuclides and other contaminants with minerals and other aquifer materials controls the rate of migration of these contaminants in groundwater. The stronger these interactions, the more a radionuclide will be retarded. Processes such as sorption and diffusion often control the migration of inorganic compounds in aquifers. These processes are often controlled by the nature of the ions of interest, the nature of the aquifer materials, and the specific geochemical conditions. Parameters describing sorption and diffusion of radionuclides and other inorganic ions on aquifer materials are used in transport codes to predict the potential for migration of these contaminants into the accessible environment. Sorption and diffusion studies can reduce the uncertainty of radionuclide transport modeling on the Nevada Test Site (NTS) and other nuclear testing areas.Equilibrium sorption, desorption, and core diffusion experiments were conducted with five different core samples from Western Pahute Mesa (WPM) on the NTS. These cores included samples PM-1 4823, PM-2 4177, UE18R 2228, UE20c 2855, and UE20c 2908. For three of the samples, PM-1 4823, UE18R 2228, and UE20c 2855, a distinction was made between matrix and fracture sub-samples. The samples were characterized based on mineralogy, specific surface area, morphology, density, and porosity. Sample UE18R 2228, a densely welded tuff, was the only sample that had substantially lower porosity and specific surface area compared to the rest. All samples were mainly composed of feldspars and quartz.Based on sorption data with different solid and metal concentrations, linear sorption isotherms were derived for sorption at pH 8.3, representing a common NTS groundwater pH. Following sorption, desorption experiments were conducted to determine the reversibility of the sorption reactions. Lead was the most reactive metal ion and exhibited strong pH-dependent behavior. Typical K d s for Pb sorption were 1x10 -3 to 5x10 -3 m 3 /g. The sorption of Cs was much less pH-dependent and was overall substantially reduced compared to Pb. Typical K d s for Cs were approximately one order of magnitude less than Pb K d s (1 to 3x10 -4 m 3 /g). Strontium sorption was almost negligible for all solids, especially at lower concentrations. The minimal Sr sorption was caused, at least partly, by the substantial amount of Sr already present in the cores. Under these conditions, therefore, Sr would be very mobile. In all cases, K d s obtained from desorption experiments were higher than the K d s obtained from sorption experiments, typically by a factor of 2 to 10.Diffusion experiments with the entire core were conducted with a nonreactive tracer, bromide, before experiments with the reactive tracers, Pb, Cs, and Sr. Based on the bromide diffusion experiments, effective diffusivities and tortuosities were estimated. The effective diffusivities appeared to be directly correlated to sample porosity. The lowest tortuosity, 8.94, was observed for sample PM-1 4823 Matrix, wh...