| MadSci Network: Earth Sciences |
Geoff: There are actually several ways to estimate the age of ground water. For very old ground water (thousands of years up to about 70,000 years), carbon-14 dating works well. For water that was last on the surface during the industrialized era, anthropogenic contaminants can be used. For instance, nuclear testing began in about 1952, and this released tritium into the atmosphere. High levels of tritium in ground water indicate water younger than 50 years. Tritium can be used to distinguish "new" water from "old" water that entered the ground before nuclear testing, because the natural level of tritium in ground water is 5 to 10 times less than the level in new ground water that formed during the period of nuclear testing. Because people are no longer injecting large quantities of tritium into the atmosphere, the tritium levels of new ground water are getting closer and closer to natural levels. At the same time, the tritium underground is decaying. Tritium is an unstable isotope of hydrogen and has a half-life of 12.3 years. This limits its residence time in ground-water, because ground water that is isolated from the atmosphere does not acquire new tritium. The result of the cessation of nuclear testing and the short half-life of tritium is that in the not too distant future it will no longer be possible to use tritium to date ground water (or at least, not in the relatively crude way just described). Other chemicals, such as chlorofluorocarbons, can be used in similar ways. Chlorofluorocarbons have also been in the atmosphere for about 50 years. The concentrations of certain chlorofluorocarbons in the ground water correspond to the amounts that were put into the atmosphere in different years, so the method can be used to determine a rather specific age estimate. The same kind of thing can be done with other contaminants. Dating ground water can be tricky, and it is not something I have ever done. I suspect that the analyses are fairly expensive. If you want to know more, I recommend you get in touch with the U.S. Geological Survey (water.usgs.gov); a number of experienced researchers work for that agency and date ground water. I do not know if anyone in New Zealand has been dating ground water, but there should be a government agency in New Zealand equivalent to the Water Division of the U.S. Geological Survey. Good luck, David Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861 www.gsa.state.al.us REFERENCES http://www.generaloceanics.com/genocean/goesgrw.htm Fetter, C. W., 1994, Applied Hydrology, 3rd ed., New York, Macmillan College Publ. Co., 691 p.
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