Program Brief

Statement of Problem: For groundwater models that include large areas in geologically complex regions, it has become increasingly recognized that inclusion of geologic features such as layer dip and fault offset is an important part of flow model development.

Approach and Accomplishments: CNWRA^® maintains state-of-the-art computational facilities for construction and visualization of geographic information system (GIS) databases. CNWRA has also developed a two-phase, mass and energy transport model, METRA, which was recently upgraded to allow use of an unstructured integrated finite-volume model grid system. The unstructured grid system permits the modeler to incorporate dip and offset of hydrostratigraphic layers into a model—similar to a finite-element grid system—while maintaining the computational simplicity of the finite-difference algorithm. Interprocessing programs were written to create unstructured finite difference grids from spatial data obtained from GIS models of site geology. Figure 1 illustrates a model area extracted from an EarthVision™ GIS model. Figure 2 shows the bottom boundary of the groundwater flow model extracted from the GIS model. Note the model boundary incorporates layer dip and fault offset. This inclusion of structural features resulted in improved model calibration to observed hydraulic heads. Perhaps more importantly, use of the nonisothermal flow modeling capability of the METRA code, in combination with the inclusion of structural features, indicated that groundwater convection cells can form in areas where low permeability layers have steep offset. These convection cells resulted in modeled temperature anomalies at the water table consistent with observations at the site. It had been suggested previously such temperature anomalies were indicative of recharge from an underlying aquifer system along fault networks.

Client Benefits: Incorporation of GIS-based geologic data into a nonisothermal groundwater flow model resulted in improved calibration to observed hydraulic heads and the discovery that water temperature anomalies along faults can be explained without the need to assume recharge from below. This focused modeling effort provided the U.S.Nuclear Regulatory Commission with a sound technical basis for evaluating several key assumptions used in a larger-scale model by the U.S. Department of Energy for characterization of a potential high-level waste disposal site.