Water Dating Technique Finds Many Potential Applications
By Calla Cofield
A technique for determining the age of water using three atmospheric radioisotopes is coming into its own. The Atom Trap Trace Analysis method, or ATTA, was first developed by researchers at Argonne National Laboratory in 1999, but it is only in the past 18 months that it has become a practical way for geologists and hydrologists to determine the age of water samples from the field. In the last 12 months the Argonne team has analyzed samples from seven continents, and can determine when those samples became isolated from the atmosphere. Now the team has begun a project with the International Atomic Energy Agency’s water resources program to determine the age of water samples from many of the world’s major underground aquifers.
Zheng-Tian Lu, a senior physicist at Argonne and a part-time professor at the University of Chicago, leads the team that developed ATTA more than a decade ago. Lu spoke about the recent ATTA developments at the 2013 APS April Meeting in Denver, Colorado.
The ATTA method uses lasers to trap and isolate three radioisotopes, krypton-81, krypton-85, and argon-39, that are dissolved in water samples. All three isotopes occur naturally in Earth’s atmosphere and can be used to measure the time since a sample became isolated from the atmosphere. The different isotopes each have a unique half-life and can date samples of different ages. Argon-39 has a half-life of 269 years, and is ideal for dating samples between 100 and 1000 years. This fills a gap between the ideal dating ranges of carbon-14 (half-life 5730 years) and hydrogen 3 (tritium, half-life 12 years).
Hydrologists interested in tapping underground water sources can use the technique to determine how frequently those sources refill or drain to keep them from being exhausted. Finding out how isolated one is from other sources matters especially if, for example, the water table is located beneath a nuclear waste storage facility. Glaciers are largely organized into sequential layers of ice, but sometimes the oldest layers are pushed up and out to the sides, disrupting the chronology. Glacial layers provide information about the history of our planet, and ATTA helps chart that history more precisely.
The ATTA apparatus is a tabletop device, about two meters long, which can be operated by a single person. Liquid or ice samples are vaporized, funnelled into a beam, and then sent through a vacuum chamber and into a magneto-optical trap. A laser tuned to a transition frequency of one of the isotopes excites the atoms, causing them to fluoresce. A CCD camera measures the fluorescence, which can be used to count the number of individual atoms in the trap.
Lu and his team published the first results using the ATTA method in the journal Science in 1999. At the time, the device could only capture only about one in ten million krypton-85 atoms.
Coupled with the rarity of the isotopes, the method required roughly a kiloton of water to gather enough atoms to determine the age of the sample.
The newest version of the instrument, the ATTA-3, is now ten thousand times more efficient. The team requires only about 100 kilograms of water to determine the age of the sample, which is more reasonable for scientists to collect from the field. Lu adds that the team hopes to continue to improve the efficiency.
There are two other methods for dating krypton 85 and argon 39, and Lu says at the moment ATTA’s contribution is a useful alternative, but with its current efficiency, it certainly doesn’t replace these. However, ATTA appears to be the most feasible way to date krypton 81. Its half-life is 229,000 years, so dating methods that rely on observing particle decays take far too long. Dating krypton 81 is possible with Accelerator Mass Spectrometry (AMS), the technique most commonly used to date samples using carbon-14, but this required many tons of water and was largely abandoned.
“As ATTA-3 became operational,” said Lu, “krypton 81 dating, an idea that had been discussed for more than 40 years, finally became available to the earth science community at large.”
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