Limits on Conventional Tritium and Tritium/3He Dating of Groundwaters. 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 Title

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 Dating Analytical 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.