This project will develop and publish a practical guide for Department of Defense (DoD) contractors to evaluate and incorporate advanced geologic models into more efficient, accurate numerical models of groundwater flow and contaminant fate and transport that are scaled appropriately for the purpose of the models.
Advanced models of subsurface geology are being developed at many DoD sites using advanced stratigraphic analyses such as Environmental Sequence Stratigraphy. Manual construction of numerical groundwater model grids from these new geologic models is tedious and inefficient. In recent years, there has been strong development in software packages used by geologists to create three-dimensional geologic models of the subsurface. Outputs from these geologic modeling software packages now include rectangular grids or meshes that can be directly imported into conventional finite difference and finite element groundwater models. On other projects, especially those with complex geology and a need to accurately simulate contaminant partitioning and transport processes that occur at fine scales (e.g., matrix diffusion), simulations using numerical models with unstructured grids are necessary.
The project team plans to construct and run three numerical models for three sites (a total of nine models), with all three models from each site incorporating the same advanced geologic conceptual site model. The first set of models will be conventional MODFLOW models with rectangular grids. The second set of models will be MODFLOW 6 models constructed using unstructured grids. The third set of models will be similar to the second set with a significantly reduced grid refinement in the low hydraulic conductivity zones where the matrix diffusion process can be modeled through the REMChlor-MD semi-analytic method. The gridded output from the geological modeling software will be optimized to honor geological contacts between geological materials that control groundwater flow and plume development along both horizontal and vertical migration paths.
The output and efficiency of each model will be compared and contrasted. Criteria used in this comparison will include contaminant mass discharge, source mass depletion, local contaminant minima and maxima in time and space, plume attenuation rates, and model efficiency, including time for model construction, calibration efficiency, and model run times.
Completion of this project will have significant benefits for the DoD, resulting in the availability of more accurate modeling tools to design effective remediation systems, better estimation of the duration of site remediation programs, and more accurate evaluation of potential risks to human and ecological receptors. The new models will be better able to simulate matrix diffusion of contaminants in the subsurface. While matrix diffusion can cause “tailing” of contaminants that delays attainment of low concentration based cleanup goals, diffusive sequestration can be an important attenuation mechanism at sites with heterogeneous geology. (Anticipated Project Completion - 2024)