Groundwater levels in eastern Washington’s Columbia Plateau Regional Aquifer System are falling at rates that threaten long-term water availability for farms, communities and ecosystems, according to new research from Washington State University (WSU).
The WSU study, published in Groundwater for Sustainable Development, offers the most detailed analysis yet of water-level trends and vulnerabilities in the Columbia Plateau Regional Aquifer System (CPRAS), which spans parts of Washington, Oregon and Idaho. The layered basalt aquifer system underlies about 44,000 square miles, supporting both agricultural production and municipal water supplies.
The CPRAS underlies the Columbia River Basin, supplying 25-35% of the region’s irrigation water for its multi-billion-dollar agricultural industry and serving as the primary drinking water source for many communities. Crops such as apples, wheat, potatoes, grapes, hops and corn depend heavily on this groundwater.
The study is the first to evaluate groundwater vulnerability across the CPRAS based on “available drawdown” (ADD)—the portion of groundwater that can be reached with existing well infrastructure—rather than total saturated thickness. By focusing on ADD, the research provides a more realistic measure of how close wells are to running dry.
“If you have groundwater 15,000 feet deep, that doesn’t help anybody,” said Sasha McLarty, an assistant professor in WSU’s Department of Civil and Environmental Engineering and the study’s corresponding author.
McLarty likens the aquifer system to a layer cake, with three layers of fractured basalt interspersed with sediment “frosting” that allows water to move more easily. In reality, the layers vary significantly in thickness and arrangement from place to place.
“From location to location, that layering looks different,” McLarty said. “Our paper is the first to really quantify that variability, based on observations, in both trends in water level and vulnerability across the entire aquifer.”
Drawing on data from nearly 3,000 wells provided by the Washington State Department of Ecology, the study analyzed four primary aquifer layers: the Overburden, Saddle Mountains, Wanapum and Grande Ronde layers. The deepest, thickest basalt layer, Grande Ronde, showed the steepest average declines at 1.86 feet per year, with some wells losing up to 7 feet annually. The Wanapum layer followed at a rate of 1.61 feet per year. Declines in the Saddle Mountains and Overburden layers averaged 0.56 and 0.22 feet per year, respectively.
The spatial analysis in the study covered 15 geographic subareas in Washington. Of the 15 subareas examined across all aquifer layers, only a few showed positive average groundwater level trends. These included the Spokane, Klickitat and Lower Snake subareas in the Grande Ronde layer; the Northern CPRAS and Lower Snake subareas in the Wanapum layer; and the Rock Glade and Selah subareas in the Saddle Mountains layer. All remaining subareas recorded negative average groundwater trends in each respective aquifer layer.
McLarty noted the Odessa subarea emerged as a top concern, projected to lose 10% of its available drawdown by 2040 and 50% within 70 years if current trends continue. The Yakima Basin and Extended Toppenish subareas mirror Odessa’s troubling trajectory. While most subareas show declining trends, the Spokane Aquifer is gaining water, thanks in part to strong management and its designation as an Aquifer Protection Area.
Other regions display a complex mix, with some layers declining and others increasing within the same geographic area. In the Rock Glade Water Resources Inventory Area, for example, gains and losses occur side-by-side. This complexity underscores McLarty’s point that groundwater cannot be managed as if it were “a single bucket.”
The study revealed that, overall, 73% of the wells analyzed exhibited declining trends between 2000 and 2020. Of those with statistically significant changes, 87.2% were declines. The most severe drops—2-3 feet or more per year—are concentrated in the Odessa and Yakima areas.
While some areas of the CPRAS remain stable or are improving, the study suggests the overall trend signals growing strain on a resource vital to the region’s agricultural economy and rural communities. The research emphasizes that without significant changes—such as improved irrigation efficiency, expansion of surface water supplies and active recharge efforts—many areas risk exhausting accessible groundwater within decades.
McLarty hopes her findings will guide investment and policy decisions. “What I care about most is will people and ecosystems have groundwater in the future to the extent that they need it?” she asked. “I hope these data can be used to help prioritize investments in improving water security, by showing where that effort is needed.” — Charles Wallace, WLJ contributing editor
