Lidar Imagery Based 2D and 3D Representations of Outcrop Heterogeneity for Groundwater Modeling

Development of models that preserve the influence of heterogeneity at varying scales is vital for modeling solute dispersion in groundwater. Dispersion, the result of varying velocities in a flow field, is partly due to material heterogeneity. To characterize such heterogeneity at the 2-3m scale while preserving realistic facies distributions, a series of high-resolution (millimeter-scale point spacing) terrestrial lidar scans were taken of fluvial sediments near the Hanford Site in Washington, and of sediments located in braided stream exposures west of Albuquerque, New Mexico. Multiple scans of the Hanford site outcrop were projected onto a 2D plane. Mean and standard deviation were calculated from the projected multiple points in the 2D grid. The images were processed through a series of filters to enhance textural information and distinguish between sand and gravel. In order to evaluate the influence of 3D heterogeneity, we scrape back successive ~2 cm intervals of the Albuquerque site outcrop and scan each slice to develop a 3D volume. Point cloud intensity data from the scans will be interpolated on a regular 3D grid and segmented into sand and gravel facies. Each lithology was assigned a realistic hydraulic conductivity value consistent with the grain size distribution of that facies. Groundwater flow and solute transport time were simulated using MODFLOW and RWHet, respectively. 2D simulations show that flow and solute transport are focused by cross-bedding into the coarser-grained units, and we expect the 3D simulations to show this as well. Thus, the majority of flow may be focused into a smaller volume of the material making up an aquifer. These solute transport simulations result in non-Fickian breakthrough due to the variable velocity field caused by heterogeneity.