Use of groundwater lifetime expectancy for the performance assessment of a deep geologic waste repository: 1. Theory, illustrations, and implications

Long-term solutions for the disposal of toxic wastes usually involve isolation of the wastes in a deep subsurface geologic environment. In the case of spent nuclear fuel, if radionuclide leakage occurs from the engineered barrier, the geological medium represents the ultimate barrier that is relied upon to ensure safety. Consequently, an evaluation of radionuclide travel times from a repository to the biosphere is critically important in a performance assessment analysis. In this study, we develop a travel time framework based on the concept of groundwater lifetime expectancy as a safety indicator. Lifetime expectancy characterizes the time that radionuclides will spend in the subsurface after their release from the repository and prior to discharging into the biosphere. The probability density function of lifetime expectancy is computed throughout the host rock by solving the backward-in-time solute transport adjoint equation subject to a properly posed set of boundary conditions. It can then be used to define optimal repository locations. The risk associated with selected sites can be evaluated by simulating an appropriate contaminant release history. The utility of the method is illustrated by means of analytical and numerical examples, which focus on the effect of fracture networks on the uncertainty of evaluated lifetime expectancy.Comment: 11 pages, 8 figures; Water Resources Research, Vol. 44, 200

Modeling groundwater contaminant transport in the presence of large heterogeneity: A case study comparing MT3D and RWhet.

A case study is presented that implements two numerical models for simulating a 30-year PAT operation conducted at a large contaminated site for which high-resolution data sets are available. A Markov chain based stochastic method is used to conditionally generate the realizations with random distribution of heterogeneity for the Tucson International Airport Area (TIAA) federal Superfund site. The fields were conditioned to data collected for 245 boreholes drilled at the site. Both MT3DMS and the advanced random walk particle method (RWhet) were used to simulate the PAT-based mass removal process. The results show that both MT3DMS and RWhet represent the measured data reasonably, with Root Mean Square Error (RMSE) less than 0.03. The use of fine grids and the total-variation-diminishing method (TVD) limited the effects of numerical dispersion for MT3DMS. However, the effects of numerical dispersion were observed when compared to the simulations produced with RWhet using a larger number of particles, which provided more accurate results with RMSE diminishing from 0.027 to 0.024 to 0.020 for simulations with 1, 20, and 50 particles. The computational time increased with more particles used in the model, but was still much less than the time required for MT3DMS, which is an advantage of RWhet. By showing the results using both methods, this study provides guidance for simulating long-term PAT systems. This work will lead to improve understanding of contaminant transport and plume persistence, and in turn will enhance site characterization and site management for contaminated sites with large plumes

Debates-Stochastic subsurface hydrology from theory to practice: A geologic perspective

A geologic perspective on stochastic subsurface hydrology offers insights on representativeness of prominent field experiments and their general relevance to other hydrogeologic settings. Although the gains in understanding afforded by some 30 years of research in stochastic hydrogeology have been important and even essential, adoption of the technologies and insights by practitioners has been limited, due in part to a lack of geologic context in both the field and theoretical studies. In general, unintentional, biased sampling of hydraulic conductivity (K) using mainly hydrologic, well-based methods has resulted in the tacit assumption by many in the community that the subsurface is much less heterogeneous than in reality. Origins of the bias range from perspectives that are limited by scale and the separation of disciplines (geology, soils, aquifer hydrology, groundwater hydraulics, etc.). Consequences include a misfit between stochastic hydrogeology research results and the needs of, for example, practitioners who are dealing with local plume site cleanup that is often severely hampered by very low velocities in the very aquitard facies that are commonly overlooked or missing from low-variance stochastic models or theories. We suggest that answers to many of the problems exposed by stochastic hydrogeology research can be found through greater geologic integration into the analyses, including the recognition of not only the nearly ubiquitously high variances of K but also the strong tendency for the good connectivity of the high-K facies when spatially persistent geologic unconformities are absent. We further suggest that although such integration may appear to make the contaminant transport problem more complex, expensive and intractable, it may in fact lead to greater simplification and more reliable, less expensive site characterizations and models