One of those few is Robert E. Criss, Ph.D., professor of earth and planetary sciences in Arts & Sciences, who has developed a new, nonradioactive method to date water. The method involves a sophisticated formula that relies heavily on the ratio between oxygen-16, which comprises 99.8 percent 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 he samples.
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. Criss is incorporating it into an ambitious study of water quality in the watersheds of the Mississippi and Missouri rivers, together the largest river system in North America.
Isotopes are different variations of the same element. There are three oxygen isotopes, oxygen-16, -17 and -18. All three behave chemically as oxygen, differing only in their mass, or weight. About one oxygen atom in 500 is oxygen-18, and only 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 the water," Criss explained. "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."
Similarly, Criss said, some researchers put dyes into water to get information about a water system. While this can reveal information about the flow of water, its fastest pathway and other clues about a system, it gives no clear picture of the water's age.
Criss described his method in a paper, "Geochemical Hydrology of the Rivers and Springs of Missouri," delivered this spring at the Geochemical Perspectives on Environmental Processes (GPEP) 2000 conference held here. His work is supported by the National Science Foundation.
Time provides the key baseline for researchers to understand how a water system works and how undesirable pollutants move through the system.
He uses an isotopic mass spectrometer to separate and identify atoms by weight and other characteristics to determine age and additional factors. Older ground water is more pristine, normal in its carbon, oxygen and hydrogen isotopic ratios and lower in nitrates, compared with younger ground water.
Younger ground water can be high in nitrates and higher in carbon-14 and will have a distinctive isotopic signature if a significant fraction of the water has been lost by evaporation. This signature helps Criss trace the water to agricultural or industrial sources.
In studying the nation's big rivers, Criss and his collaborators will use oxygen-18 as their linchpin to learn how the rivers operate, where the water comes from and how old it is. They plan to learn more about flooding in the watersheds and how the different systems respond to various precipitation events. They also will examine pollutants.
"We hope with this new method we'll be able to separate sources of contamination geographically," he said. "We're confident that reach by reach, 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 Bismark, 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 examine 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 said. "How you use it depends on how clever you are as a detective."