2D and 3D Representations of Outcrop Heterogeneity from Lidar Imagery 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, in part, due to material heterogeneity. In order to characterize heterogeneity at the 2-3m scale, a series of high-resolution (millimeter-scale) terrestrial lidar scans were taken of sediments located in braided stream exposures west of Albuquerque, New Mexico, and of fluvial sediments near the Hanford Site in Washington. For the Hanford site, multiple scans of the same outcrop were projected onto a 2D plane. Mean and standard deviation were calculated from the projected multiple points in the 2D grid. The mean and standard deviation results were processed through a series of filters to enhance textural information and distinguish between sand and gravel. The resulting binary image was converted to a text array of numerical color values, one representing each lithology (e.g. sand, gravel). Each lithology was assigned a reasonable hydraulic conductivity value. In order to evaluate the influence of 3D heterogeneity, successive ~2 cm intervals of the braided channel deposits near Albuquerque were exposed by scraping the outcrop. We are developing a 3D volume by scanning each successive slice. Point clouds from the scans will be interpolated into a regular 3D grid and facies segmented from this grid. 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 the 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.