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
  
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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