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Limits on Conventional Tritium and Tritium/ 3 He Dating of Groundwaters. by E. Calvin Alexander, Jr. Geology & Geophysics Department University of Minnesota. Age or Residence Time of Groundwater.
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Limits on Conventional Tritium and Tritium/3He Dating ofGroundwaters. by E. Calvin Alexander, Jr. Geology & Geophysics Department University of Minnesota
Age or Residence Time of Groundwater • The length of time between a when a drop of water falls out of the sky as rain or snow and when it comes out of your facet. • This concept readily and correctly grasped by the general public and therefore a powerful management tool. • 103 to 104 water samples analyzed each year by commercial companies in North America.
Simple Tritium Dating • 3H → 3He + b Qb = 18.6 kev t½ = 12.33 yr • Pre-nuclear age cosmogenic 3H level in the atmosphere is poorly known but is conventionally taken to be about 3 TU. • Tritium Units = 1 atom of 3H per 1018 atoms of H. = 1.19 Bq/kg of water. = 32.2 pCi/kg of water. • Atmospheric testing of nuclear weapons between ~ 1953 and 1964 introduces over 2000 TU of 3H to the northern hemisphere precipitation
Ottawa Canada Tritium Record 12 month-running, precipitation-weighted average decay corrected to 2002
Simple Tritium Dating • Current precipitation in Minnesota contains about 10 TU of tritium (which appears to be supported by the tritium releases of the nuclear industry. • The detection limits of conventional commercial (~ $50/sample) tritium analyses is about 5-8 TU. • Enriched tritium analyses lower the detection limit to about 0.2 – 0.8 TU and cost about ($150 - $300/sample)
Simple Tritium DatingAnalytical Techniques • Liquid scintillation counting of 10 to 20 cm3 vials with paired photomultipliers in a coincidence mode. • The same counting systems are used for both standard and enriched samples. • Background in such systems is about 1.6 to 1.8 counts/min – and is the detection limiting factor.
Tritium/3He Dating • Calculates age from the 3H to 3He ratio in the water sample today. • Involves both at 3H (counting) and 3He (noble gas MS). • Assumes pure piston flow – which is a priori violated by any real flow system.
14C dating of Groundwater • First technique – counting several grams of C as CO2 in a Geiger Counter – anti-coincidence counting. • Second technique – counting several grams of C as benzene in scintillation counters – coincidence counting – same systems as used for tritium counting. • Third technique – measuring the 14C/12C ratio directly in an accelerator mass spectrometer – uses a few mg of C.