Auxiliary analyses in support of performance assessment of a hypothetical low-level waste facility: Groundwater flow and transport simulation. Volume 3

Stochastic subsurface hydrologic theory is applied to data for a hypothetical low-level radioactive waste site to demonstrate the features of the hydraulic parameter estimation process, as developed by Gelhar and others. Effective values of hydraulic conductivity, macrodispersivity, and macrodispersivity enhancement are estimated from the data in this manner. A two-dimensional saturated flow and transport finite-element computer code is used to model the site. Four different isotope inputs and two types of input configurations contribute to an evaluation of model sensitivities. These sensitivities of the mean concentrations and the uncertainties around the mean are explored using an analytical model as an example. Results indicate that the spatial heterogeneity of isotope sorption, through its contribution to longitudinal dispersivity enhancement, has a large effect on the magnitude of concentration predictions, especially for isotopes with short half-lives in comparison to their retarded mean travel times. This observation emphasizes the need for accurate site data measurements that compliment the parameter estimation process. A comparison of simplified analytical screening models with the numerical model predictions shows that the analytical models tend to underestimate concentration levels at low times, potentially as a result of oversimplification of the flow field. Future models could address aspects that are neglected in this report, such as three-dimensionality or unsaturated flow and transport