Wednesday, December 14, 2011

How Old Is Water? By Matthew Van Dusen

               The Nubian Aquifer and ages of water. Courtesy Argonne National Laboratory

How old is the water in your drinking glass? What about the ice cubes floating in it? Any answer is bound to make reference to the water cycle (evaporate, rain, repeat). Still, for most practical purposes, water is both eternal and constantly replenished.

But when water flows underground or freezes into glacial ice, a clock starts ticking from the moment it loses contact with the air. In order to read the time, scientists must trap and count almost infinitesimally small quantities of the radioactive isotope krypton 81. Physicist Zheng-Tian Lu and his team at the U.S. Department of Energy’s Argonne National Laboratory outside of Chicago have honed the technique, called Atom Trap Trace Analysis (ATTA), over more than a decade and successfully used it to map the flow of water in the Nubian Aquifer, two miles beneath the Sahara Desert.

If scientists can determine how long water has been in an underground aquifer, they’ll know how fast it travels and how fast it can be replenished in certain areas – critical information in desert climates where the population depends on groundwater. Krypton 81 can help tell water’s story. Dating glacial ice, a new frontier for the technology, can tell scientists about the atmosphere hundreds of thousands of years ago.

As the technology improves, radio-krypton dating is set to become part of the scientist’s toolkit, going far beyond the range of carbon 14 dating to pinpoint the age of samples 150,000 – 1.5 million years old. But the journey to this discovery has been slow.

“In 1999, when we first demonstrated the principle of ATTA, it would take millions of liters of water for a krypton 81 analysis,” Lu said. “Basically it was impossible.”

These days, sample sizes are 100 liters but Lu hopes to get it down to 1 liter within his career.

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