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TM97-04S Collection, processing, and analysis of ground-water samples for tritium/helium-3 dating;

Detailed Description

The National Water Quality Laboratory (NWQL) of the U.S. Geological Survey (USGS) has contracted with Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, for the analysis of low-level tritium (3H), total dissolved helium (He), total dissolved neon (Ne), and the helium-3/helium-4 isotope ration (3He/4He) of dissolved helium for purposes of groundwater dating based on the tritium/helium-3 (3He/4He) method. The precisions of the measurements are, for tritium, ±4 percent (2 sigma) or ±0.01 Tritium Units (TU)1, for the helium isotope ration, 3He/4He, ±4 percent to ±3 percent (2 sigma), and for the He and Ne concentrations, ±1.0 percent (2 sigma). 

3He/4He dating of ground water applies to the time elapsed since recharge and isolation from the atmosphere. The method is based on the radioactive decay of 3H to 3He.  As these substances are virtually inert in ground water, unaffected by ground-water chemistry and unaffected by contamination from most anthropogenic sources, 3H/3He dating can be applied to a wide range of hydrologic investigations. 3H/3He dating complements existing capabilities within the U.S. Geological Survey for dating of young ground water, such as, uses of chlorofluorocarbons (see joint Office of Water Quality and Office of Ground Water Technical Memorandum 95.03 and attachments, dated December 29, 1994) and can be applied to dating waters recharged within the past approximately 30 years. The purpose of this Memorandum is to define procedures for submittal of 3H/3He samples, describe collection procedures, compare and contrast 3H/3He dating with other methods for dating young ground water (0–50-year time scale), and provide some technical specifics on the 3H/3He dating method and interpretation of ages from the analytical results. The following table summarizes the available analyses and lab codes.


1One TU is equal to 1 3H atom in 1018 atoms of H, or 3.24 picocuries per liter, pCi/L. One liter of water with a concentration of 1 TU produces 7.2 disintegrations per minute (dpm) or 0.12 becquerel (Bq); one Bq corresponds to 1 disintegration per second (dps); one curie (Ci) is equal to 3.7x1010 Bq.


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