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Health and Environmental Effects Research

Water Isoscape Suggests That Willamette Basin Is Vulnerable to Climate Change Effects

U.S. Environmental Protection Agency scientists in Oregon's Willamette Valley are using the isotopic signals of small tributary streams and rivers within the Willamette Basin to paint a picture of where their water comes from and how water resources might change in the future.

"The isotopic signal is determined by the rainfall," explains Renée Brooks, an investigator in the EPA's Western Ecology Division. "A storm comes in, and the heavier isotopes preferentially collect in the water droplets and fall first. Then, as that storm proceeds across the valley and up the mountains, it gets isotopically lighter and lighter. You get a very strong signal change associated with elevation." Unless the water in the watershed evaporates — and less than 5% of their samples indicated evaporation— the isotopic signature of the water will not change again. Because of its stable isotopic signal, a drop of water in the river can be traced back to the elevation at which it fell to the ground as rain or snow.

The Willamette River Basin supports Portland at its outlet to the Columbia River and Salem, Corvallis, and Eugene upstream; it is the most populous valley in Oregon. But, in addition to urban water needs, the watershed supports heavy agricultural use— for the growth of grass seed, wheat, Christmas trees, and all of the bounty of the area's small food farms. Summers are dry in Oregon when water demand is at its highest and river water is at its lowest flow. The trend toward warmer temperatures raises an important question: In the future, will there be enough water to sustain current levels of use in the Willamette Valley?

To describe the surface waters of the Willamette River Basin, Brooks' team sampled a total of 65 small watersheds feeding six major river tributaries within the Coast Range and the Cascade Mountains, as well as the major rivers, including the Willamette River. After two years of regular seasonal sampling and analysis, the researchers developed a water isoscape of the basin (a map of water isotopes) and determined that the mean elevation of the Willamette River's source water shifts seasonally by approximately 700 meters. In the winter, when it is raining in the valley and snowing high in the mountains, the river's composition is isotopically heavy, meaning that its water is coming from precipitation in the valley and foothills. In the critical low-flow summer months, melting snow pack water— water coming from elevations higher than 1200 meters — constitutes up to 80% of the river.

According to Brooks, the area's reliance on high-elevation water during summer months highlights the vulnerability of the watershed to the influences of a warming climate, where snowpack is expected to decrease in the future and drain off earlier. By recognizing the sources of the water and how they likely will change, water managers will be able to better allocate those scarce resources for agriculture and urban use.

Isoscapes also may offer clues to investigators seeking sources of water pollution. Because the Willamette River is made up of high elevation water, it is more depleted of heavy isotopes than the local precipitation. When levels of pollutants like nitrates in groundwater spike, land and water managers want to know if river water is carrying the pollutants from elsewhere in the watershed, or if local agricultural runoff is to blame. Determining the isotopic signal of the polluted water potentially could help unravel these kinds of mysteries.

"What we did was very specific to the Willamette," Brooks says, "but the approach can be used to address a lot of issues that hinge on where water is coming from." Brooks' study was published in the May 2012 issue of Ecosphere.

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