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As part of a U.S. Geological Survey Groundwater Resources Program study, a three-dimensional geologic model was constructed for approximately 53,000 square miles of the Columbia Plateau in Idaho, Oregon, and Washington. This model was constructed to define the general aquifer system geometry for use in a regional numerical groundwater flow model. Simplifications and assumptions consistent with this ultimate goal and with the uncertainty in the available data were made. The model units consist of Miocene-age Columbia River Basalt Group strata and younger sedimentary overburden covering approximately 44,000 square miles. Data were compiled from numerous databases and detailed studies that were completed during the past three decades. These data include stratigraphic interpretations of more than 13,000 wells and a contiguous compilation of surficial geology and structural features in the study area. These data were simplified and used to construct piecewise-smooth trend surfaces that represent upper and lower subsurface model 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 interpretations, errors in mapped contact locations, and uncertainty in the shape of the paleosurface upon which flood basalts were emplaced. Surfaces were recombined using a rule-based algorithm to construct a fully three-dimensional model with a 500-foot grid resolution that is consistent with the data and for which error estimates may be made. The modeling process yielded improved estimates of unit volumes, refinement of location of large structural features, and identification of features that may be important for ongoing groundwater studies. Introduction The Columbia Plateau regional aquifer system (CPRAS) covers approximately 44,000 mi2 of Idaho, Oregon, and Washington (fig. 1). The area supports a 6 billion dollar per year agricultural industry, leading the Nation in production of apples and nine other commodities (State of Washington, 2009; U.S. Department of Agriculture, 2009). Groundwater availability in the aquifers of the area is a critical waterresource management issue because the water demand for agriculture, economic development, and ecological needs is high. The primary aquifers of the CPRAS are basaltic lava flows of the Columbia River Basalt Group (CRBG) and overlying basin-fill sediments. Water-resources issues that have implications for current and future groundwater availability in the region include (1) widespread waterlevel declines associated with development of groundwater resources for irrigation and other uses, (2) reduction in base flow to rivers and associated effects on temperature and water quality, and (3) current and anticipated effects of global climate change on recharge, base flow, and, ultimately, groundwater availability. The U.S. Geological Survey (USGS) Groundwater Resources program began a study of the CPRAS in 2007 with the broad goals of (1) characterizing the hydrologic status of the system, (2) identifying trends in groundwater storage and use, and (3) quantifying groundwater availability. The study approach includes documenting changes in the hydrologic condition of the system, quantifying the hydrologic budget for the system, updating the regional geologic and hydrogeologic frameworks, and developing a groundwater-flow simulation model for the system. The groundwater flow model will be used to evaluate and test the conceptual model of the system and then will be used to evaluate groundwater availability under alternative development and climate scenarios. Three-Dimensional Model of the Geologic Framework for the Columbia Plateau Regional Aquifer System, Idaho, Oregon, and Washington By Erick R. Burns, David S. Morgan, Rachael S. Peavler, and Sue C. Kahle 2 Geologic Framework for the Columbia Plateau Regional Aquifer System Figure 1. Study area and data used for construction of the geologic model of the Columbia Plateau regional aquifer system, Idaho, Oregon, and Washington. 0 20 40 60 80 100 Miles 0 20 40 60 80 100 Kilometers Ellensburg

Three-dimensional model of the geologic framework for the Columbia Plateau Regional Aquifer System, Idaho, Oregon, and Washington