PULLMAN, Wash. -- Groundwater is declining across Eastern Washington's complex, interconnected aquifer system, as people draw on it for irrigation, drinking and other uses at a pace that threatens its sustainability, according to a new study by a Washington State University researcher.
In certain "hot spots" – such as the Odessa region and the Yakima Basin – the rates of decline are particularly significant, with groundwater levels dropping two to three feet a year or more.
The data is built upon a new metric of water vulnerability; rather than simply calculating how much groundwater there is, it measures how much is actually accessible with current wells.
"With these numbers we can say, 'Hey, this is a problem now,'" said Sasha McLarty, an assistant professor in the Department of Civil and Environmental Engineering at WSU and the corresponding author of the new study. "It's not a problem in the future, it's a problem now."
The research, published in the journal Groundwater for Sustainable Development , provides a detailed new picture of the Columbia Plateau Regional Aquifer System. Although not all areas in the system showed declines, the study lends urgency to the need for increased water supplies, for example through water conservation, additional use of surface water, and aquifer-recharging projects.
The paper focuses on a little-studied element of the aquifer system – its variability in both geographic location and depth below the surface. The system underlies the Columbia River Basin in Washington, Idaho and Oregon, providing a quarter to a third of irrigation water across the region.
It is comprised of four main geological layers—three basalt layers formed at different periods in history, and a top layer of sedimentary materials.
"Imagine a layer cake, where you have these chunks of actual cake, which is mostly fractured basalt in this case, and then there's frosting in between, the parts where water moves more easily," McLarty said. "That's our aquifer."
Unlike a layer cake, however, the layers don't sit atop each other in a neat, orderly pattern.
"From location to location, that layering looks different," she 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."
Previous groundwater studies have compared the rate of water usage with the volume of water in the aquifer layers, a metric known as saturated thickness. But a lot of that water lies below the depth of current wells. McLarty's paper measures the water accessible to the current well infrastructure, which they define as available drawdown (ADD), and is a more useful metric to assess how vulnerable groundwater users may be to declines.
"If you have groundwater 15,000 feet deep, that doesn't help anybody," McLarty said.
Using state Department of Ecology data collected from nearly 3,000 wells drawing water from the aquifers, researchers calculated trends in the four aquifer layers. The thickest basalt layer—known as the Grand Ronde—had the steepest declines in groundwater, at 1.86 feet per year on average and up to about 7 feet per year. The top layer of sedimentary materials, known as the Overburden layer, had the smallest annual decreases, at 0.22 feet per year.
But the Overburden layer is still vulnerable because it has less available drawdown, the study shows.
Most but not all are in decline
McLarty focused on 15 geographic subareas within the state, which highlighted the points of greatest concern. One of these is the Odessa area in Eastern Washington, which is on a pace to consume 10% of available drawdown by 2040—and half within 70 years.
Wells in the Yakima Basin showed a similar trend to those in Odessa, and most subareas showed a pattern of declining groundwater. However, not every area did. The Spokane Aquifer is gaining water, one of three subareas with a positive trend. McLarty said that this is thanks to very active management and monitoring efforts, including a designated Aquifer Protection Area. Other areas are more complex with some layers declining and some increasing even in the same area, like in the Rock Glade Water Resources Inventory Area. The study shows that you can't just view groundwater as a single bucket.
McLarty hopes that the new research will encourage efforts to improve groundwater sustainability.
"What I care about most is will people and ecosystems have groundwater in the future to the extent that they need it?" she said. "I hope these data can be used to help prioritize investments in improving water security, by showing where that effort is needed."
