There is reasonable agreement about some hydrologic conditions that preclude consideration of a site as a mined geologic repository for high-level radioactive waste. Defining the hydrologic characteristics that make a site suitable for a repository is much more difficult. Hydrologic characterization of proposed repositories generally attempts to address three questions.

· How quickly will groundwater containing dissolved radionuclides move from the repository to the "accessible environment" where it discharges to the surface or can be pumped from a well?

· What are the expected fluxes of radionuclides over time, and cumulatively, to the accessible environment?

· What are the expected concentrations of radionuclides in water to which humans or other species might be exposed?

If there were a set of hydrologic characteristics that simultaneously maximized travel time while minimizing fluxes and concentrations, it might be possible to develop a simple set of hydrologic criteria for repository siting. However, in many cases the hydrologic conditions that enhance suitability from one perspective may serve to limit suitability with respect to another. For example, a limited flux of water through the repository would limit the cumulative flux of radionuclides but might also lead to high concentrations in a narrow plume. Societal choices of what constitutes an acceptable risk will ultimately dictate the relative importance of travel time, fluxes and concentrations in evaluating the suitability of a site. Estimates of expected groundwater travel times, fluxes and concentrations are subject to uncertainties resulting from incomplete knowledge of aquifer hydraulic properties, uncertainties in future boundary conditions (including those related to human activities), and limited understanding of the governing transport processes. Societal choices also play an important role in determining what levels of uncertainty can be tolerated in repository siting decisions.