Limits on Conventional Tritium and Tritium/ 3 He Dating of Groundwaters.
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Transcript of Limits on Conventional Tritium and Tritium/ 3 He Dating of Groundwaters.
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 + Q = 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
0.1
1
10
100
1000
Jan-50 Jan-55 Jan-60 Jan-65 Jan-70 Jan-75 Jan-80 Jan-85 Jan-90 Jan-95 Jan-00 Jan-05
pre
cip
wt
Tri
tiu
m T
U
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.