Introduction

The Columbia Plateau Regional Aquifer System (CPRAS) covers about 44,000 mi² within the Columbia River basin in southeastern Washington, northeastern Oregon, and western Idaho (pl. 1). The population of the region is more than 1.3 million people (2000 census) and important agricultural industry has developed despite its arid to semi‑arid climate and limited access to surface-water resources. Groundwater availability in the aquifers of the area is therefore 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 basalts of the Columbia River Basalt Group and the overburden, consisting of overlying basin-fill sediments. Groundwater availability issues in the basin 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 stream 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 characterizing the hydrologic status of the system, identifying trends in groundwater storage and use, and quantifying groundwater availability (U.S. Geological Survey, 2008). The study approach includes documenting changes in the status of the system, quantifying the hydrologic budget for the system, updating the regional hydrogeologic framework, and developing a groundwater-flow simulation model for the system. The simulation model will be used to evaluate and test the conceptual model of the system and later to evaluate groundwater availability under alternative development and climate scenarios.

Purpose and Scope

The scope and objectives of this report are to characterize current (2009) groundwater conditions within each of the primary basalt aquifers and to evaluate changes in groundwater levels since the last regional synoptic measurement in 1984 (Whiteman, 1986; Vaccaro, 1999) through the presentation of maps of groundwater elevation and water-level change for the hydrogeologic units in the CPRAS. Regional assessment of water-level data from the CPRAS is necessary to document regional water-level conditions and to develop a broad view of the effects of groundwater development and management in the sustainability and availability of water supply in the region. This information is useful to delineate areas of water-level changes due to withdrawals, irrigation recharge, or climate and to identify areas with substantial data gaps that may preclude effective management of groundwater resources.