Tritium (³H) is the radioactive isotope of hydrogen with a half-life of 12.43 years (IAEA, 1981) and is an excellent indicator of ground water recharged since 1952 (Clark and Fritz, 1997). Production of ³H in the atmosphere naturally occurs by cosmic ray spallation, but the principal source was the atmospheric testing of thermonuclear weapons. The standard unit of measure for ³H is a tritium unit (TU) for which one TU is equivalent to one ³H atom per 10¹⁸ atoms of hydrogen or in terms of radioactivity 3.2 picocuries per liter (Clark and Fritz, 1997). Tritium content in precipitation closely mimics world events during the Cold War years with a maximum concentration occurring in 1963, commonly referred to as the "bomb peak". Atmospheric concentrations have gradually declined since 1963 and present-day ground water typically contains <1 to 10 TU, seldom exceeding 50 TU (Clark and Fritz, 1997).

The tritium/helium-3 (³H/³He) method is based on the radioactive decay of ³H to ³He. Both tritium and helium are relatively inert gases. Multiple sources of ³He are present in the environment such as the Earth's mantle and atmosphere, fluid inclusions within rocks, and excess air entrained in ground water during recharge (Schlosser, 1992). This method separates ³He derived from ³H (tritogenic ³He) from ³He derived from natural sources. Apparent age estimates from the ³H/³He method can be extremely accurate (within months) for ground water containing high ³H concentrations (waters recharged since 1963). Unlike the chlorofluorocarbon dating method, ³H/³He is a valid technique for sites contaminated with organic compounds.

Clark, I.D., and Fritz, Peter, 1997, Environmental isotopes in hydrogeology: New York, Lewis Publishers, 328 p.

International Atomic Energy Agency (IAEA), 1981, Statistical treatment of environmental isotope data in precipitation: Vienna, IAEA, Technical Report Series No. 206.

Michel, R.M., 1989, Tritium deposition in the Continental United States, 1953-1983: U.S. Geological Survey Water-Resources Investigations Report 89-4072, 46p.

Schlosser, Peter, 1992, Tritium/³He dating of waters in natural systems, in Isotopes of noble gases as tracers in environmental studies: Vienna, IAEA, p. 123-145.