Re: Naturally-occurring Tritium

[Paul Lavely <lavelyp@uclink4.berkeley.edu>]

The following is from the Miami Tritium lab at 
http://www.rsmas.miami.edu/groups/tritium/

Tritium Analysis
     Tritium is the radioactive isotope of hydrogen ( half-life of 
12.4 years, decay rate of 5.6 % per year). Tritium is produced 
naturally in the upper
atmosphere by cosmic radiation. It can replace hydrogen in H2-gas, 
forming HT, and in water, forming HTO. The release of excess tritium 
into the
atmosphere from nuclear weapons tests conducted between 1952 and 1963 
'tagged' rain water, and thereby all surface waters with HTO. This 
tracer
perfectly follows the water in atmospheric, oceanic and hydrological 
transport and mixing processes. Atmospheric tritium concentrations 
peaked between
1962 and 1965 and most of this excess (i.e. bomb produced) tritium 
was precipitated during the same time period and a few years 
afterward. Since then the
deposition rate has tapered off sharply. Thus the presence of excess 
tritium in the water of an aquifer unequivocally proves that recharge 
occurs on a time
scale of years to decades. The actual tritium level, combined with 
approximate local tritium history of precipitation may give more 
specific information about
the make-up of the aquifer.

     The Tritium Laboratory was originally established in 1960 as a 
radiocarbon dating facility for ocean waters and sediments. In 1964, 
it was expanded to
perform low-level measurements of environmental tritium, mainly for 
oceanographic purposes. Due to lessening demand from oceanography, we 
are now
able to offer low-level tritium measurements to other fields such as 
hydrology. Our present yearly capacity is about 2500 tritium 
measurements on water
samples relating to research projects mainly from the hydrological community.

     Tritium assay at the very low levels in nature is often given in 
tritium units (TU), where one TU represents a tritium/hydrogen ratio 
of 10-18. In water of
1 TU, the specific activity is equal to 3.2 picocuries per liter (pCi 
L-1) or 7.1 disintegrations per minute per liter (dpm L-1). We 
measure these very low
activities by internal gas proportional counting of H2-gas made from 
the water sample. The counting equipment consists of nine ultra-low 
background
proportional gas counters of various sizes, operating in 
anticoincidence with a surrounding cosmic-ray detector system. The 
whole system is encased in a
30-ton iron shield. The electronics mainly consist of commercially 
available instruments modified for our purpose. The entire operation 
is computerized and
includes many quality control features, from sample data input and 
machine parameter checks to final statistical tests.

     Following conversion to hydrogen gas, "high-level"  tritium 
samples of > 100 TU (300 pCi L-1) may be admitted to counters 
directly. Low-level
hydrological water samples go through an electrolytic enrichment 
step, in which tritium concentrations are increased about 60-fold 
through volume
reduction. Accuracy of the low-level measurement with enrichment is 
0.10 TU (0.3 pCi L-1 of H2O), or 3.5 %, whichever is greater; that of 
low-level
counting without enrichment is 3 TU (9.6 pCi L-1of H2O) or 3.5 %, 
whichever is greater.

     Procedures are now available in our Laboratory for measurement of 
tritium in samples other than ordinary waters. We routinely do 
extractions of certain
types of H2O-absorbers used in atmospheric tritium sampling. We also 
offer the extraction of water from soils and subsequent measurement 
of tritium in the
extracted water. Tritium concentrations in water found in soils, 
combined with the local precipitation tritium history can be used to 
estimate the travel time of rainwater through the soil.



Paul Lavely
UC Berkeley
lavelyp@uclink4.berkeley.edu

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