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 water-resource 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 water-level 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.


The geologic model described in this report will be the foundation for the hydrogeologic framework of the groundwater flow model. The hydrogeologic framework will be constructed by identifying geologic features with similar or distinct hydraulic characteristics.


Purpose and Scope


The specific objective of this work was to update the geologic framework for the CPRAS by developing a digital, three-dimensional geologic model using available geologic mapping and well information. The model is intended to be used as the geologic foundation for a hydrogeologic model that will be used as input to a groundwater flow model for the CPRAS that will be developed as part of the groundwater availability assessment. This report describes the process used to generate the digital geologic model, as well as summarizes the resulting datasets.


Previous Investigations


Numerous reports, some regional and some site-specific, document investigations regarding the geologic and hydrogeologic framework of the CPRAS. Each investigation has improved the understanding of the system within the limitations of the scope of the investigation or investigative techniques available at the time the study was conducted. The hydrogeologic units defined for most of these studies correspond to the geologic model units defined for this study.


In the early 1980s, the USGS began a study of the CPRAS as part of its Regional Aquifer-System Analysis (RASA) program. The geologic and hydrogeologic frameworks of the Columbia Plateau aquifer system were described in several reports including Drost and Whiteman (1986), Drost and others (1990), Gonthier (1990), and Whiteman and others (1994). The RASA study delineated seven hydrogeologic units—the overburden aquifer, the Saddle Mountains Basalt unit, the Wanapum Basalt unit, the Grande Ronde Basalt unit, two interbed units corresponding to the Mabton and Vantage interbeds, and a basement confining unit (Whiteman and others, 1994). The geologic and structure mapping of Swanson and others (1979a, 1979b, 1981) was the basis for the RASA framework in addition to analysis of geophysical logs, geologists logs, and chemical analysis of core or drill cuttings (Drost and others, 1990). Using data from the RASA study, two USGS studies covering subareas of the RASA study near the Horse Heaven Hills (fig. 1A) were completed in the 1980s (Davies-Smith and others, 1988; Packard and others, 1996). Both studies represented the CRBG as three basalt aquifers—the Saddle Mountains Basalt, Wanapum Basalt, and Grande Ronde Basalt units. 


In 2000, the USGS began a hydrogeologic investigation of the Yakima River basin that resulted in two principal framework reports describing an area that covers about 6,900 mi². This area generally covers the Yakima River and all of its tributary drainage basins within the footprint of the CRBG and upstream of the Columbia River (fig. 1A). The first report described the geologic and hydrogeologic framework of sedimentary deposits within the Yakima River basin (Jones and others, 2006); the second report described the extent and depth to top of basalt and interbed hydrogeologic units within the basin (Jones and Vaccaro, 2008). As in the earlier RASA and Horse Heaven Hills work, the major units recognized in the Yakima River basin study include, from youngest to oldest, the Saddle Mountains, Wanapum, and Grande Ronde Basalt units. In addition, the Mabton and Vantage interbed units that consist of sediments between individual basalt layers were included. Maps showing the extents and depth to top of these units were constructed on the basis of geologic mapping, previously constructed structure contour maps, and unit interpretations from about 3,000 well-log records. Well-log records included lithologic, geochemical, and geophysical descriptions.


In 2001, the Columbia Basin Groundwater Management Area (GWMA) Plan was adopted by the State of Washington to guide efforts to reduce nitrate concentrations in groundwater in Franklin, Grant, and Adams Counties. In 2005, Lincoln County was added to the GWMA resulting in a four-county area that covers about 8,300 mi² in the central part of the study area. Detailed geologic and hydrogeologic framework investigations have been conducted for the GWMA and are summarized by Lindsey and others (2007) and Tolan and others (2007). A key component of the work was to refine the description of the geologic framework of the CRBG and overlying sediments within the GWMA. Information used to map the units included geologic maps, measured sections at outcrops, interpreted lithologic contacts from water well logs and specialty drill holes, petrographic and geochemical analyses of drill cuttings, and selected hydrogeologic reports.


Within the GWMA, the sediment overlying the CRBG was divided into five major units: (1) Quaternary sand dunes, (2) Quaternary alluvium, (3) Quaternary cataclysmic flood deposits, (4) Quaternary loess, and (5) Miocene-Pliocene Ringold Formation (Tolan and others, 2007). The upper two CRBG units (Saddle Mountains Basalt and Wanapum Basalt) were subdivided and the members were mapped by geochemistry, magnetic polarity, and relative position. The Saddle Mountains Basalt was subdivided into seven members and the Wanapum Basalt into three members (Tolan and others, 2007). Additionally, seven sediment interbeds that occur between basalt members were mapped. The lowest CRBG unit, the Grande Ronde Basalt, was not mapped at member level because of the great thickness of its uppermost member and relative lack of deep drilling information.


In 2005, the 700 mi² Palouse Basin basalt aquifer system was characterized on the basis of stratigraphic, lithologic, chemical, and magnetic data, and the information was used to develop a hydrogeologic geographic information system (GIS) database (Leek, 2006). The CRBG was divided into four units, Saddle Mountains, Wanapum, Grande Ronde, and Imnaha (Leek, 2006). Sedimentary interbeds were recognized as the Latah Formation, which is contemporaneous with the Vantage interbeds. Structure maps were constructed for the tops of the two dominant basalt aquifers, the Wanapum and Grande Ronde.


In 2006, the USGS created a website to present information about the Columbia River Basalt stratigraphy (U.S. Geological Survey, 2009a). In addition to background information about the CRBG and an extensive bibliography, the website contains detailed information for 151 wells in Oregon and 32 wells in Washington. Geologic information for water wells was on the basis of visual and geochemical analysis of carefully sampled borehole cuttings and interpretation of the driller’s water well reports. Geochemical analysis of selected basalt cuttings provided oxide and trace element information to assist in classifying basalt into formation, members, and flow units.


First posted February 25, 2011