3H and 3He

Tritium (3H) is the only radioactive isotope of hydrogen, and has a half-life of 12.43 years (Unterweger et al., 1980). Large quantities of tritium were introduced into the hydrological cycle by atmospheric thermonuclear testing in the 1950s and 1960s, providing a useful environmental tracer for water originating from this period. Tritium decays by beta-emission to 3He, the rare, stable isotope of helium. Under favourable conditions, measurements of both 3H and 3He in groundwater allow the reconstruction of tritium concentrations in precipitation and the determination of water flow paths. Ratios of 3H to 3He can be applied to quantify the extent of radioactive decay, and hence determine subsurface water residence times up to 40 years.