DEVELOPMENT OF A THREE DIMENSIONAL DIGITAL GEOMODEL FOR THE COLUMBIA PLATEAU REGIONAL AQUIFER SYSTEM, WASHINGTON, OREGON, AND IDAHO

As part of a U.S. Geological Survey Groundwater Resources Program study, a three-dimensional geomodel of generalized hydrogeologic units was constructed for approximately 100,000 km² of the Columbia Plateau in Washington, Oregon, and Idaho. These units are comprised of Miocene age Columbia River Basalt Group rocks and younger overburden. Data were compiled from numerous available databases and detailed studies completed during the past three decades. These data include stratigraphic picks made on more than 13,000 wells and a contiguous compilation of surficial geology and structural features spanning the study area. These data were simplified and used to construct piecewise-smooth trend surfaces that represent subsurface hydrogeologic unit boundaries in this complex folded and faulted terrain. The smoothness of the surfaces implicitly represents uncertainty in prediction of each surface resulting from data gaps, errors in borehole picks, errors in mapped contact locations, and uncertainty in the shape of the paleo-surface upon which flood basalts were emplaced. Using a rule-based algorithm, surfaces were then recombined to construct a fully three-dimensional model that is consistent with the data and for which error estimates may be made. The modeling process has yielded improved estimates of unit volumes, refinement of location of the largest structural features, and identification of features that may be important for ongoing groundwater studies.