Tritium

Tritium is the common name for hydrogen-3 (³H), which is a radioactive isotope of hydrogen. Like ordinary hydrogen (¹H or hydrogen-1, called protium) and deuterium (²H or hydrogen-2), tritium has a single proton in its nucleus. Unlike ordinary hydrogen, deuterium and tritium have neutrons in their nucleus. Deuterium has one neutron in its nucleus and is stable, while tritium's nucleus contains two neutrons and is unstable. Tritium decays spontaneously to helium-3 (³He) through ejection of a beta particle (essentially a high-energy electron). The half-life of tritium is about 12.32 years. Since the number of protons determines chemical bonding, tritium behaves like ordinary hydrogen and can replace ordinary hydrogen in water molecules. Thus, tritium readily cycles through the hydrologic and biologic components of the environment. Tritium has three times the mass of ordinary hydrogen due to the two extra neutrons. Because of this extra mass, water containing tritium evaporates at a slightly slower rate than water containing only hydrogen-1.

The unit of measure of tritium in water is the tritium unit (TU). One tritium unit equals 1 tritium atom in 10¹⁸ hydrogen atoms. In SI units, one tritium unit is about 0.118 bequerels per liter (Bq/L), where the bequerel is one decay per second. In picocuries per liter, 1 TU is approximately 3.19 pCi/L. Tritium occurs in very small quantities naturally, being produced in the upper atmosphere by cosmic rays. Natural (pre-nuclear age) levels of tritium in precipitation are on the order of 1 to 5 TU. Nuclear-weapons testing during the 1950s and 1960s created relatively large amounts of tritium in the atmosphere that can be detected in ground water that was recharged during this period. Greatly elevated levels of tritium can be present in ground water contaminated with radioactive wastes.

Tritium concentrations in the atmosphere. Image from USGS Circular 1213

Using Tritium to Date the Age of Ground Water

Tritium can be used to estimate whether ground water has been recharged before or after 1953. Tritium in the atmosphere quickly reacts to form water vapor, which can subsequently condense to form precipitation (rain, hail, sleet, or snow). The amount of tritium in infiltrating precipitation that subsequently becomes ground-water recharge is controlled primarily by the quantity of tritium in the atmosphere when the precipitation formed, plus the length of time it takes water to move from the land surface to the water table. Because tritium decays with a half-life of 12.32 years, ground water that does not contain detectable tritium is inferred to have infiltrated into the subsurface prior to 1953. Conversely, ground water that contains detectable tritium is inferred to contain at least some water that infiltrated after 1953. Because tritium is decaying rapidly, it is becoming more difficult to use bomb tritium to date the age of ground water. Scientists can also use the ratio of tritium to its decay product helium-3 (³He) to date ground water (the time since the water last equilibrated with the atmosphere). The ³H/³He dating method can be used to date ground water recharged within the past 30 years.

More Information

• Desert Plants Reveal Contaminant Transport Pathways
• Using Plants To Detect Tritium Contamination
• Waste Disposal and Contaminant Migration in the Arid Southwest, Amargosa Desert Research Site, Nevada (includes research on the processes affecting transport of tritium in the subsurface)
• Global Environmental Change Research Project, Ice Core Project, Tritium
• Virginia Aquifer Susceptibility Project, Dating Ground Water with Tritium
• Periodic Table--Hydrogen, Resources on Isotopes from the USGS Isotope Tracers Project

References

• Cordy, G.E., Gellenbeck, D.J., Gebler, J.B., Anning, D.W., Coes, A.L., Edmonds, R.J., Rees, J.A.H., and Sanger, H.W., 2000, Water quality in the central Arizona Basins, Arizona, 1995-98: U.S. Geological Survey Circular 1213, 38 p.
• Freeze, R.A. and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, New Jersey, Prentice-Hall, 604 p.
• Hem, J.D., 1985, Study and interpretation of the chemical characteristics of natural water: U.S. Geological Survey Water-Supply Paper 2254, Third Edition, 264 p.
• Lucas, L.L., and Unterweger, M.P., 2000, Comprehensive review and critical evaluation of the half-life of tritium: Journal of Research of the National Institute of Standards and Technology, v. 105, no. 4, p. 541-549.
• Thatcher, L.L., Janzer, V.J., and Edwards, K.W., 1977, Methods for determination of radioactive substances in water and fluvial sediments: U.S. Geological Survey Techniques of Water-Resource Investigations, Book 5, Chapter A5, 95 p.

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