Report shows declining ground water levels in region

Ground water levels declined over the past quarter-century in 83 percent of 470 wells measured in the sprawling Columbia Plateau Regional Aquifer system in southeastern Washington, northeastern Oregon and western Idaho, a new report from the U.S. Geological Survey found.

The steepest drops occurred in the deepest basalt aquifer in the 44,000-square-mile plateau. Smaller declines were recorded in shallower aquifers that receive recharge water from irrigation, precipitation or discharge from rivers or wells, according to the survey.

Hydrologists compared readings taken in 1984 with measurements recorded in 2009 for the report, which was released this week.

Declines were reported throughout the Columbia Plateau, but were more widespread in parts of the Yakima River Basin, in western Lincoln, eastern Grant and southwestern Adams counties, and in the Pullman-Moscow area.

Average annual declines over the 25 years in the Moscow-Pullman area, which taps ground water for municipal water supplies, averaged more than 1 foot per year, the survey found.

Irrigation accounted for much of the use elsewhere, the survey said.

The average annual decline in the Grande Ronde basalt unit -- the oldest and deepest level of the aquifer tapped for water use -- averaged 2 feet a year over the period. Thickness of the basalt in the Grande Ronde may be greater than 15,000 feet near the central part of the basin, according to the survey.

Declines also occurred in the Yakima River Basin because of heavy pumping from the Wanapum basalt unit, which averages up to 1,200 feet thick, the survey said.

But the drop was less in the shallower Overburden unit, at less than 0.3 feet per year, the survey said.

Overall, results of the study -- launched to assess ground water levels in the plateau and identify trends in storage and use -- came as no surprise to hydrologists for the U.S. Geological Survey, said Sue Kahle, project manager for the agency.

"Water level declines have been tracked for many years in smaller (areas). This the first time we looked at the Columbia Plateau as a whole," Kahle said.

Historically, ground water levels in the regional aquifer system have dropped since the 1930s.

The study "confirms some of the concerns swirling around for years," said Joye Redfield-Wilder, a spokeswoman for the Washington Department of Ecology in Yakima.

Darryll Olsen of the Columbia-Snake River Irrigators Association, which represents agricultural interests and irrigation districts in Eastern Washington, said the most interesting part of the survey is the decline in water levels in the Grande Ronde unit.

Water levels dropped by more than 100 feet in some of the 136 Grande Ronde unit wells where measurements were taken, with declines greater than 25 feet measured in 60 percent of the wells, the survey said.

"I'd like to know what is going on in the Grande Ronde," Olsen said.

One explanation may be the depth of the formation, said Guy Gregory, hydrogeologist with Ecology's water resources program in Spokane. He supervised a group of scientists involved in the study.

"Recharge has a hard time getting there and there is not a lot of recharge to begin with," Gregory said. "We're using more of (the recharge water) and it's harder to get more to it because of its depth."

The study will help scientists develop a ground water-flow simulation model that water managers hope to use to test ways of managing the region's ground water supplies under different development and climatic conditions, Kahle said.

-- Kevin McCullen: 509-582-1535; kmccullen@tricityherald.com