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CORVALLIS - Researchers at Oregon State University have identified a particular horizon in the thick sediments that cover most of Oregon's Willamette Valley at which nitrate in the groundwater is being broken down, protecting the underlying gravel aquifer from nitrate contamination.

Conditions at this reduction-oxidation, or redox boundary form a line of defense against the nitrate, commonly produced by crop fertilizers and livestock waste, experts say. Knowing its location may be of significant value in predicting where the aquifer faces increasing risk of pollution.

However, the redox boundary may also be moving downwards in the sediment towards the aquifer under the pressure of continued nitrate loading up above, and continued research is essential to explore this issue. If the redox boundary is moving, it could place at risk the water quality of the Willamette Aquifer that thousands of people depend upon for drinking water.

This research was presented to today at the annual meeting of the Cordilleran Section of the Geological Society of America, being held in Corvallis.

"A key question is whether this redox boundary is moving downwards through the sediment, and if so, how fast," said Justin Iverson, an OSU masters student who did research on this topic with Roy Haggerty, an OSU assistant professor of geosciences.

According to Iverson and Haggerty, the thick sediments at the surface of the Willamette Valley are actually a relic of its distant geological past, dating back to 12,700-15,000 years before the present when massive floods backed up into the region repeatedly, leaving sediment behind. These Missoula Floods were triggered by the glacial damming of the Clark Fork river in Montana, formation of a large lake and repeated collapse of the ice dam multiple times over 2,300 years.

Each time the lake formed and then collapsed, it resulted in a torrent of water pouring through the Columbia River gorge and ponding up in the Willamette Valley, depositing sediment layers that are thickest at the north end of the valley and thin to less than 10 feet near Eugene. The sediments are about 60 feet thick near Mount Angel where some of the recent studies were done, and throughout most of the valley they are underlain by the porous gravel of the Willamette Aquifer, a vast source of clean, high quality water for thousands of people and multiple other uses.

In their recent study of the hydrologic, chemical and physical qualities of this silt deposit, one thing that became clear to OSU researchers was the very slow rate at which water moves through it. It can take about 25 years for water to physically move through a sediment layer 60 feet thick.

Of particular interest, however, is a horizon within the sediment layer where a process of denitrification appears to be taking place - the breaking down of water-soluble nitrates into other compounds or elements.

"Oxidation has taken place down to the redox boundary, located about 25 feet beneath the ground surface at the field site near Mt. Angel," Iverson said. "At the boundary, reddish-brown, oxidized sediment changes abruptly to a bluish-gray sediment with very little nitrate in it."

The redox boundary, the scientists say, appears to be a place where the unique ingredients for denitrification are available - the necessary bacteria, nitrates, no oxygen and an energy source.

Since ranch animals or nitrate fertilizers had been in agricultural use on that research site for about 60 years, in theory the nitrates should have leached much lower in the sediment column, presumably all the way to the aquifer along with the water in which they were dissolved. But they were apparently stopped and broken down at the redox boundary, protecting the aquifer from contamination.

In points where this redox boundary can be measured to be at or near the aquifer level, the scientists said, it's now clear the underlying aquifer will be at increased risk from nitrates. This compound can cause methemoglobinemia, or "blue baby syndrome," and the EPA has set a maximum allowable level for its presence in drinking water of 10 parts per million.

If further research shows the redox boundary is moving steadily downward in the sediment column, the scientists said, this issue will be of considerably greater concern and could affect agricultural and aquifer water management plans.