Novel Modeling Approach for Sites with Complex, Well‐Characterized Geology

Monday, December 4, 2017
Jessica Barros, PE , TRC Solutions, Concord, CA

A groundwater model was generated to inform the design of a groundwater remedy involving a slurry wall and extensive groundwater extraction well network. The site is underlain by a gently sloping alluvial plain, where hydrogeologic zones consist of mostly fine‐grained low permeability units with localized high permeability regions that form a heterogeneous and interbedded aquifer system.

C Tech’s Mining Visualization System (MVS) software was used to develop a comprehensive geologic model from 233 highly‐detailed boring logs. Initially, a 3D finite difference grid was created in MVS and imported into Modflow USG. This version consisted of 56 layers, each representing a zone of uniform hydraulic conductivity. An attempt was made to simulate the complex geology using the MVS pinchout feature; however, this resulted in many discontinuous lenses and associated model computation/converging challenges. Ultimately, it was determined that this approach was not the best fit for modeling the site’s well‐characterized, heterogeneous geology.

The final version of the model was constructed using a 3D rectilinear grid, generated in MVS, which consists of approximately 120,000 active cells. MVS was used to assign one of four hydrologic conductivity zones (sand with fines, clay, sand, or gravel) to each cell in the model based on imported boring log data. Exporting the hydraulic conductivity zone data from MVS and importing this information into Groundwater Vistas presented particular challenges. Additionally, a boundary condition data gap was identified through model use and resulted in additional field investigation and model refinement.

Jessica Barros, PE, TRC Solutions, Concord, CA
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