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.