What is tritium ?

Tritium (3H) is the radioactive isotope of hydrogen and, as such, occurs in many forms, as part of the water molecule (HTO), in its elemental form as hydrogen gas (HT), as organically bound tritium in foodstuffs and other organic matter or in methane gas (CH3T).

Natural Environmental Levels

The natural production of tritium is very low, in the units in which it is generally considered, probably less than 3 TU (1 TU = 0.12 Bq/ l water) but, as with carbon-14, a major increase in this value occurred in the 1950s and 1960s with additional input from the atmospheric nuclear tests. In 1964, the time of the Test Ban Treaty, the level of tritium measured in rain water at Wantage, UK, rose to approximately 4000 TU and immediately provided a valuable tool in water resources work, e.g. in the London Basin studies, using enhanced tritium as a 'natural' tracer and later for landfill water tracing. Since 1964 the level of tritium has declined through uptake in the oceans and radioactive decay (half-life = 12.43 yrs) and in the UK rain now varies between 10 and 30 TU.

Tritium and Leachates

Recently the current potential of using tritium as a tool for tracing leachate from land fill sites has been examined in Chapter 6 of 'The Technical Aspects of Controlled Waste Management' (A Review of the Composition of Leachates from Domestic Wastes in Landfill Sites, DOE Report, Project PS072/95). The main conclusions of this section of the report are that high levels of tritium in landfill compared with the surrounding groundwater or surface drainage can provide a powerful technique for establishing whether there is local contamination arising from the landfill.

Tritium Measurement Techniques

Measurements of tritium can be made directly from pure water using the technique of liquid scintillation counting for levels above 25 TU. Impure (dirty or coloured) waters would, however, require a preceding distillation stage. Below 25 TU an electrolytic enrichment procedure enables measurements of current natural rainwater levels or groundwater.

Measurement techniques offered by the RCD laboratory for tritium are as follows:

  1. Direct counting of water sample (5 to 10 ml), with or without distillation depending upon the sample purity as supplied, in an optimised low level liquid scintillation counter (e.g., Wallac Quantulus, LKB SM 1219). Least detection limit 25 TU (3.0 Bq/l) with a counting time of approximately 500 minutes.

  2. 20 times enrichment of the water sample (175 ml) by electrolysis, followed by liquid scintillation counting. Least detection limit 1 TU (0.1 Bq/l) with a counting time of approximately 500 minutes.

Least detection levels are calculated on the basis of being able to detect a sample which produces a count rate greater than 3 times the uncertainty (standard deviation) in the measurement result. The measurement uncertainty expresses the full replicate sample reproducibility and not simply errors estimated on the basis of the counting times employed. Counting uncertainties (assessed on the basis of σ, standard deviation = 1/√n) included in the full assessment are optimally minimised to achieve the overall precision required, but kept in balance with uncertainties from other sources. The counting times given to achieve the Least Detection Limits (LDLs) above are values calculated on times to achieve equal 'counting error' and 'other (random) error' components.