Colloid-facilitated migration of plutonium in fractured rock has been implicated in both field and laboratory studies. Other reactive radionuclides may also experience enhanced mobility due to groundwater colloids. Model prediction of this process is necessary for assessment of contaminant boundaries in systems for which radionuclides are already in the groundwater and for performance assessment of potential repositories for radioactive waste. Therefore, a reactive transport model is developed and parameterized using results from controlled laboratory fracture column experiments. Silica, montmorillonite and clinoptilolite colloids are used in the experiments along with plutonium and Tritium. The goal of the numerical model is to identify and parameterize the physical and chemical processes that affect the colloid-facilitated transport of plutonium in the fractures. The parameters used in this model are similar in form to those that might be used in a field-scale transport model.

Many contaminants in groundwater strongly interact with the immobile porous matrix, which retards their movement relative to groundwater flow. Colloidal particles, which are often present in groundwater, have a relatively small size and large specific surface area which makes it possible for them to also adsorb pollutants. The sorption of tracers to colloids may enhance their mobility in groundwater, relative to the case where colloids are not present. A class of pollutants for which colloid-facilitated transport may be of particular significance are radioactive isotopes. A major reason for why geologic repositories are considered suitable for the disposal of spent nuclear fuel is the strong affinity of many radionuclides to adsorb onto the porous matrix. Therefore, radionuclides accidentally released, would be contained in the geological media by adsorption or filtration until sufficient decay takes place. However, the presence of colloids may enhance radionuclide mobility in the groundwater, and reduce the efficiency of geologic media to act as a natural barrier.

In this report, we review the results of Reimus et al. (2000a; 2000b) regarding matrix diffusion and colloid-facilitated transport in fractured rock and evaluate the implications of these results on modeling fracture flow at the Nevada Test Site (NTS). In particular, we examine these data in the context of the recent Cheshire hydrologic source term (HST) model results (Pawloski et al., 2001). This report is divided into several sections. In the first, we evaluate the effective diffusion coefficient (D{sub e}) data reported in Reimus et al. (2000a) for conservative tracer species (3H, 14C, and 99Tc) and fit a simple effective diffusion model to these data. In the second, we use the fitted effective diffusion model, in conjunction with a surface complexation model, to simulate plutonium-colloid transport and compare model results to data reported in Reimus et al. (2000b). In the third, we evaluate the implications of these data with regards to radionuclide transport through fractures at the field scale and, in particular, with regards to the Cheshire HST model (Pawloski et al., 2001). Finally, we make recommendations regarding future radionuclide transport modeling efforts at the NTS.

This book presents a collection of essays that address various facets of the hydrogeology of Israel. Despite its small geographic size, Israel exhibits a variety of climates and is located between two regional fluctuating base levels. The respective chapters discuss the variety of hydrogeological configurations and hydrological processes produced by these geographical circumstances. In some cases, the interpretation of these aspects is deliberately left open to debate, because the authors were asked to provide, in addition to their own views, also alternative and even conflicting ones. Hydrogeological configurations similar to those in Israel can be found in other countries around the world. Therefore, researchers, scholars and professionals in this interdisciplinary field can benefit from and directly apply the considerable experience and expertise that has been gathered in Israel over the past few decades.