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Posted 9-22-99
How far and how fast sediments move through a river can help scientists understand how pollutant and nutrient runoff affects rivers and lakes.
The answer to this will assist conservationists in managing land uses along river channels by knowing whether pollutant problems will exist in the river channels or down river at the receiving waters.
Geologists at Case Western Reserve University employed naturally occurring radionuclides (a type of radioactive atom) from the atmosphere which absorb into fine sediments as a new way of following sediment movement through rivers. They found that fine sediments traveled in steps of up to 60 kilometers (37 miles) along a small Idaho river during a spring runoff from snow melting.
To test this measuring technique on larger rivers, associate professors Gerald Matisoff and Peter Whiting will begin a follow-up study this summer on the Yellowstone River. They have a two-year, $163,000 grant from the National Science Foundation to support the research along 1,000 kilometers (62 miles) of the river.
They will set up 10 test sites on the river, starting in the northeastern part of the national park, continuing through Montana, and ending near the North Dakota border at the confluence of the Missouri River.
"We will have a chance in the Yellowstone study to determine whether fine sediment in that system behaves like it did in the Gold Fork (River)," says Whiting.
The researchers selected the Yellowstone River because it has the longest undammed river reach in the continental United States.
In the new study, Matisoff and Whiting will look at whether longer transit times will allow the sediments to settle or degrade before they reach the receiving waters of a new river, a lake, or other water source. For pollutants that undergo degradation at similar rates to the radionuclides, they can determine the pollutants' toxicity.
Pollutants, including most pesticides and metals, attach themselves to fine sediment as radionuclides do and move from land to water during runoff from snow melting or thunderstorms. These pollutants eventually settle in river beds, wetlands, lakes, estuaries, or oceans.
Whether these particles travel in one giant leap from the headwaters to the receiving waters, or whether they take many small steps, was among reasons for looking at the markers on the sediment particles, says Matisoff.
In a study of Idaho's Gold Fork River, a local consortium of ranchers, environmentalists, state officials, and members of the lumber industry were pointing fingers at who was causing the pollution in the Cascade Reservoir, into which the Gold Fork River feeds, notes Matisoff.
The one-year, $35,000 study, which the Idaho group funded, did not settle any disputes, but it gave geologists a new tool to track sediment in river channels.
The geologists were surprised to find that particles can move great distances through the channel once they leave the land. Much of the fast travel took place at the headwaters, where the drop in altitude through narrow canyon walls caused the faster flow of the water.
Matisoff, Whiting, and Everett Bonniwell, a doctoral candidate in geological sciences, published results from the Idaho study -- "Determining the times and distances of particle transit in a mountain stream using fallout radionuclides" -- in the international journal Geomorphology in February.
The researchers used three atmospheric fallout radionuclides -- berylium-7 (Be-7), lead-210 (Pb-210), and cesium-137 (Cs-137) -- as tracers.
At higher altitudes, the radionuclides remain in the snowpack until the spring thaw. As the snow melts into the ground, the radionuclides attach to fine soil particles and wash into streams by the runoff from the snowmelt.
The geologists were particularly interested in the ratio of Be-7 to the other radionuclides. Be-7 was the primary tracer, according to Whiting, because it has a half-life of 53.4 days, with less than 1 percent remaining at the end of a year. The Be-7 tracer can identify new sediment from the older sediments stirred up from the river bottom.
Cesium, delivered to Earth through the testing of thermonuclear weapons in 1954-68, has a half-life of 30.1 years, while Pb-210 has a half-life of 20.4 years.
Before the snow melted in April 1996, the researchers gathered core snow samples in the headwater region. During the runoff between May and July 1996, they collected sediment samples from filtered water about one meter above the river bed at five testing sites and tested for the Be-7 tracer in comparison to Pb-210 and Cs-137.
They also compared the river sediments with nearby soil samples at the five testing sites, where they found that Be-7 is delivered evenly throughout the landscape.
Matisoff adds that this new tool for determining sediment transit has great potential in land-management practices.
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