Whether it's 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, Ph.D., a Washington University in St. Louis professor of earth and planetary sciences in Arts & Sciences, 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. Researchers and environmentalists will be able to use the isotopic fingerprint to answer questions "of time and the river."
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.
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.
More accurate picture
"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 explains. "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 says, 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 April 26, 2000, at the Geochemical Perspectives on Environmental Processes (GPEP) 2000 conference, held at Washington University in St. Louis. His work is supported by the National Science Foundation. The method was first published in a paper co-authored with G. C. Frederickson, a recent graduate of Washington University, in a 1999 issue of Chemical Geology and later in Criss' book, "Principles of Stable Isotope Distribution," published by Oxford University Press, 1999.
Time provides the key baseline for researchers to understand how a water system works and how undesirable pollutants move through the system. "The primary characteristic of a ground water system is how long the water is stored," Criss says. "Along with the size of the system and the depth of its basin, these parameters help us understand water systems."
He uses an isotopic mass spectrometer to separate and identify atoms by weight and other characteristics to determine age and other 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, 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.
Examining pollutants
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 the water is. They plan to learn more about flooding in the watersheds and how the different systems respond to various precipitation events. Also, they will examine pollutants.
"We hope with this new method, we'll be able to separate sources of contamination geographically," he says. "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 says. "How you use it depends on how clever you are as a detective."
by Tony Fitzpatrick