Hydrogeologic Setting

Overviews of the geology and hydrology of the Columbia Plateau presented in this section summarize detailed descriptions in reports that describe the groundwater flow system, including its boundaries, hydraulic characteristics, and components of recharge and discharge (Vaccaro, 1999); discuss the hydrology based on numerical modeling (Bauer and Hansen, 2000); and discuss the thickness, extent, and lithology of hydrogeologic units within the CPRAS (Kahle and others, 2009).

The Columbia Plateau is an intermontane basin between the Rocky Mountains and the Cascade Range filled mostly with Cenozoic basalt and sediment. The Columbia River Basalt Group consists of a series of basalt flows that erupted during various stages of the Miocene Age, 17 million to 6 million years ago. More than 300 flows have been identified and individual flows range in thickness from 10 ft to more than 300 ft (Tolan and others, 1989; Drost and others, 1990). Soils derived from flows or sediments deposited on the surface of a flow were sometimes preserved, creating sedimentary interbeds between flows. The CPRAS occurs in an area of several structural regions (pl. 1). The geologic properties of these structural regions may influence groundwater flow through compartmentalization (the creation of distinct zones within an aquifer with limited interconnectivity) of the hydrogeologic units. The presence and importance of compartments in the CPRAS has been recognized and discussed in numerous studies (for example Newcomb, 1959; Porcello and others, 2009). Groundwater-level data analyzed for this study, however, were insufficient to evaluate compartmentalization fully.

The largest structural regions in the CPRAS consist of the Yakima Fold Belt and the Palouse Slope, which occupy the Columbia River basin. The Yakima Fold Belt includes the western and central parts of the Columbia River basin and consists of a series of anticlinal ridges and synclinal valleys trending roughly east-west (Reidel and others, 2002). Folds and faults within this region affect groundwater movement by offsetting or altering permeable interflow zones to create effective barriers to flow (Drost and others, 1990; Reidel and others, 2002). The Palouse Slope in the eastern part of the basin is much less deformed and has a gently southwestward dipping slope. The other structural regions within the CPRAS are the Blue Mountains, a composite anticlinal structure that forms the southeastern extent of the Columbia River basin, and the Clearwater Embayment, which marks the eastward extent of the CPRAS along the foothills of the Rocky Mountains.

Generalized hydrogeologic units recognized in this investigation, listed in order of generally increasing age, include Overburden, Saddle Mountains, Wanapum, and Grande Ronde. The hydrogeology was simplified by grouping the Mabton and Vantage Interbeds into the Saddle Mountains and Wanapum units, respectively. The Overburden unit consists predominantly of undivided, unconsolidated to semi‑consolidated sedimentary deposits ranging from Miocene to Holocene in age (Drost and others, 1990). These include many types of deposits of local and (or) regional extent including fluvial, glaciofluvial, eolian, and volcanoclastic deposits that can range in thickness from 0 to 1,300 ft. The Saddle Mountains hydrogeologic unit consists mostly of the Saddle Mountains Basalt and interbed members and is the least extensive and youngest formation of the Columbia River Basalt Group. Most of the unit is in the west-central part of the study area, with less continuous occurrences in the Blue Mountains and eastward into Idaho. Thickness of the Saddle Mountains unit can range from 0 to about 1,000 ft. The Wanapum hydrogeologic unit, composed mostly of basalt and interbed members of the Wanapum Basalt, is in the north-central part of the study area. Much of the unit lies beneath the Overburden and Saddle Mountains units. Thickness of the Wanapum unit ranges from 0 to about 1,200 ft. The Grande Ronde hydrogeologic unit is the oldest and most extensive of the basalt units. This unit underlies most of the study area, except for an area along the southern boundary of the CPRAS in Oregon and along the eastern edge of the aquifer system in Idaho. The Grande Ronde unit contains the basalt and interbed members associated with the Grande Ronde Basalt. Thickness of the unit is largely unknown, but may be greater than 15,000 ft near the central part of the basin (Reidel and others, 2002).

Seasonal fluctuations of groundwater levels, due to recharge from precipitation, irrigation, or canals, or due to discharge to rivers or wells, are typical in the surficial or shallow aquifers of the CPRAS, although this also has been observed in deeper, non-surficial units. Long-term groundwater levels in the CPRAS, however, have been undergoing substantial changes since the 1930s (fig. 1). Groundwater level declines due to pumping have caused many wells to go dry, which required deepening of these wells to remain productive. Rises in groundwater due to recharge from canals and irrigation have created concerns of potential water logging of crops. Continued monitoring is needed to further evaluate human- or climate-induced changes to the groundwater system.