3-D GEOSTATISTICAL LITHOFACIES MAPPING OF ICE-FLOODS DEPOSITS IN A PORTION OF THE HANFORD NUCLEAR SITE

The complexity of large-scale Pleistocene catastrophic floods deposits of the Hanford formation in the Pasco Basin present challenges for constructing reliable flow and transport models for predicting contaminant migration. Previous hydrogeologic models of the Hanford formation in the 200 West Area of the Hanford Site relied on traditional sequence stratigraphy deduced from boreholes and outcrops to produce a ‘layer cake' representation of hydrogeologic properties. Those models are limited in that heterogeneity and uncertainty within each layer is not adequately addressed. Indicator geostatistics provides a tool for stochastic simulation of the heterogeneity of the sediments within each stratigraphic sequence and a quantification of the uncertainty in the distribution of lithofacies. This study classified the glacial flood deposits into five lithofacies: silty sand, fine sand, coarse sand, gravelly sand, and sandy gravel using data retrieved from the Hanford Borehole Geologic Information System. Borehole data from the study area provide data on the vertical heterogeneity of the subsurface sediments but only limited information on the lateral heterogeneity. Excavation sites near the study area provided a qualitative assessment of the lateral heterogeneity of the lithofacies. Indicator variogram models were developed to characterize the spatial continuity of each lithofacies. Conditional indicator simulation techniques were applied to produce realizations of the distribution of lithofacies. Analysis of the realizations allowed for the quantitative assessment of uncertainty in the spatial distribution of the lithofacies. The realizations can be used as input for flow and transport modeling choosing the extreme lithofacies distributions and the modal distribution to capture the range of behavior in flow and transport predictions. Hydraulic conductivities were assigned to each lithofacies based on frequency distributions of measured hydraulic conductivity data from each lithofacies. The resulting 3-D geostatistical models of hydraulic conductivity provide an improved understanding of the heterogeneity of Hanford formation sediments and also provide geologically plausible constraints on flow and transport modeling of the study area.