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Dating [H.sub.2]O Can Trace Pollution.

Whether it is the birthday of a movie star or the freshness date on a bottle of beer, American culture is obsessed with age. Yet, few give a second thought to the age of water, the mainstay of life.

Robert Criss, professor of earth and planetary sciences, Washington University in St. Louis (Mo.), has developed a new, nonradioactive method to date water. It involves a sophisticated formula that relies heavily on the ratio between oxygen-16, which comprises 99.8% of all oxygen in water, and oxygen-18, a stable isotope of oxygen. This formula gives a distinctive "fingerprint" for the water.

Using the formula, Criss is able to get an average age of water from any system sampled. The method will be essential to future water quality and climate change studies, and eventually will serve as a way to track both the time and severity of pollutant emissions in streams, he indicates.

Isotopes are different variations of the same element. Of the three oxygen isotopes (oxygen-16, -17, and -18), all three behave chemically as oxygen, differing only in mass, or weight. Approximately one oxygen atom in 500 is oxygen-18, and just one in about 2,500 is oxygen-17. "Most methods that date water rely on radioactive isotopes, such as carbon-14, which are usually tied to some trace organic chemical dissolved in water," Criss points out. "But with these methods, one has to ask: Are you really dating the water or looking at when that chemical got in there? You can have an old water sample, put in a tiny amount of some trace chemical, and then what are you going to do two years from now, say that water is two years old? All you know is when that tiny amount of trace material got in there. The oxygen isotope method is intrinsically tied to the bulk water volume itself. Thus, we're dating water, and not the tracer."

In studying the nation's big rivers --the Missouri and the Mississippi--Criss and his collaborators will use oxygen-18 as their linchpin to learn how they operate, where the water comes from, and how old the water is. This will help them learn more about flooding in the watersheds and how the different systems respond to various precipitation events. "We hope with this new method, we'll be able to separate sources of contamination geographically. We're confident that ... we'll be able to identify where different chemicals and contaminants are introduced into the river system. For example, we know now that virtually all of the nitrate in the Missouri River is introduced south, of Bismarck, N.D. Virtually all of the sulfate and sodium in the river are introduced west and northwest of St. Joseph, Mo. We'll be able to narrow down parameters like these even more."

Finally, Criss and his collaborators will use the method to investigate the effects humans have had on the rivers over time. They will examine how the rivers behave now compared with how they would have in the past. "Oxygen-18 is a fingerprint," Criss explains. "How you use it depends on how clever you are as a detective."
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Title Annotation:researcher develops new nonradioactive way to date water
Publication:USA Today (Magazine)
Article Type:Brief Article
Geographic Code:1USA
Date:Jun 1, 2001
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