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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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Helium Isotope Analysis and Tritium-Helium Age Dating in the Mirror Lake Basin, Grafton County,New Hampshire

by

S. Drenkard (Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, N.Y. 10964), T. Torgersen (Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, N.Y. 10964), R. Weppernig (Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, N.Y. 10964), K. Farley (Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, N.Y. 10964), P. Schlosser (Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, N.Y. 10964), R.L. Michel (U.S. Geological Survey, National Center, Reston, Va.), A.M. Shapiro (U.S. Geological Survey, National Center, Reston, Va.) and W.W. Wood (U.S. Geological Survey, National Center, Reston, Va.)

Abstract

Ground-water samples from the U. S. Geological Survey's Mirror Lake fractured bedrock site in New Hampshire were analyzed for tritium (3H) and its decay product helium-3 (3He) as well as for helium-4 (4He) and neon (Ne). Parallel measurements of the radioactive mother/daughter pair 3H/3He can be used to determine a ground-water age -- that is, the time elapsed since the water last equilibrated with the atmosphere. Preconditions for precise calculations by this method are separation of the tritiogenic 3He signal from other He components dissolved in water (atmospheric, nucleogenic and mantle 3He) by use of 4He and Ne data. This separation technique is complicated for samples analyzed from the Mirror Lake site because of a large helium excess with an estimated 3He/4He ratio of about 1.65x10-6. Water analysis from shallow wells yield reliable 3H/3He-based ages, but the error in the age calculation increases with depth because of an increase in excess He and the related errors in the associated corrections. Possible origins of the excess He are discussed, and mixing of waters (young, tritium and 3He-enriched with old, 4He-enriched) as an apparent source is ruled out. He isotope measurements of crushed rock samples indicate that the source of excess He is not the local bedrock. On the basis of the observed increase in 3H/3He age and presence of excess He with depth, the potential of the 4He concentration as an additional proxy age-dating tool is explored.

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