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

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.

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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.