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New Method Tracks Pollutants in Water at Low Cost


Scientists at Columbia University's Lamont-Doherty Earth Observatory have developed a new and inexpensive tool to track the flow of water and the related transport of chemicals, spilled oil, sewage or other pollutants.

The new method, reported in the April issue of Environmental Science & Technology, uses minute amounts of a non-reactive, synthetic gas called sulfur hexafluoride (SF6), which scientists can inject into water in a controlled way and then trace its spreading and mixing patterns. Costing about 1,000 times less than fluorescent dyes, the tracers most widely used today, SF6 greatly expands scientists' ability to visualize and predict the dynamics of moving water.

Using SF6, scientists can track how fast, how extensively and in what directions soluble materials would flow and mix in water. The new environmental tool could be used to aid oil-spill clean-up efforts, to site sewer outfall pipes to reduce pollution risks, or to track the fate of contaminants released into surface water or groundwater, they said.

The scientists say SF6 works as well as current tracers methods using fluorescent dyes, but has significant advantages:

The scientists reported a successful experiment in 1994 using SF6 in New York's Hudson River. The research was conducted by Jordan F. Clark, Peter Schlosser, Martin Stute and H. James Simpson, all geochemists at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y. (Clark has since joined the faculty of the University of California, Santa Barbara.)

Sulfur hexafluoride is manufactured primarily for use as an electrical insulator. Laboratory and field experiments demonstrate that it is non-reactive in natural waters and that it does not react with particles in water or with sediments. Its concentrations in natural waters are extremely low and minute amounts can be measured in water samples.

In their field experiment, the Lamont-Doherty scientists used a "diffusing stone" suspended behind a small boat to bubble a few liters of SF6 about 30 feet deep into the Hudson River just south of Albany, N.Y.

The boat crossed the river channel several times, effectively creating a 1/4-mile-wide line, or "tracer patch," across the river. The total cost of the gas material in 1994 was $16.

For two weeks afterward, the scientists collected daily samples of water downstream of the injection site over various depths and breadths. Using a gas chromatograph, a device that separates components of a chemical mixture, the scientists measured SF6 concentrations and got results right on board.

Over two weeks, the scientists followed the movement downriver of the original tracer patch, which travelled about 5 miles per day at first, then slowed down to less than 2 miles per day. By the end of the experiment, the 1/4-mile-wide tracer patch had dispersed over nearly 25 miles.

The scientists also successfully measured even smaller quantities of another non-reactive gas, helium-3, which was injected into the river along with the SF6. Helium escapes from solution at a faster rate than SF6, so by measuring the ratio of both gases, the scientists can determine how quickly a volatile substance escapes from the water. Helium was measured with a mass spectrometer at Lamont-Doherty.

The Lamont-Doherty scientists say SF6 has a wide range of uses for applied environmental research as well as for basic research to study the dynamics of water bodies. It can be used to study ponds, groundwater and large bodies such as Long Island Sound. It can even be used in the open ocean, as demonstrated by scientist James Ledwell's research carried out at Lamont in the 1980's and more recently at the Woods Hole Oceanographic Institution.

The research was supported by the Hudson River Foundation, and W.M. Keck Foundation and Columbia University's Strategic Research Initiative Program.

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