Department Water Resources and Drinking Water

Tritium and helium are important tracers in hydrology, you can find actual examples in the projects section.

The history of tritium (³H) and helium as tracers in hydrology began in the 1950s and early 1960s, when large amounts of tritium were released at the tests of thermonuclear bombs in the atmosphere. Soon it was discovered, that the radioactive superheavy hydrogen isotope 3H is an ideal tracer for hydrological processes of all kind, because it is readily incorporated in the water molecule to form HTO, and then takes part in the global water cycle (e.g. Begemann and Libby, 1957; Suess, 1969). The International Atomic Energy Agency (IAEA) set up a worldwide network of stations to observe the tritium concentrations in precipitation. These data form the basis for tracer applications of tritium.

A logical complement to tritium is its stable decay product ³He, which is formed in a - decay with a half-life of = 12.4 yr. The ratio of ³He/³H yields a timescale which is independent of the details of the tritium input curve. As long as a water parcel is in contact with the atmosphere, the tritiogenic ³He (³He formed by tritium decay) is exchanged with the atmosphere. If this exchange is cut off, the tritiogenic ³He accumulates; the ³H-³He-clock is running. A water parcel is cut off from the atmosphere for instance when it infiltrates into the groundwater or when it sinks into the deep water of of oceans or lakes.

β τ½
Hence, the ³H-³He age, also called water mass age, measures the duration of the isolation of a water parcel from the atmosphere and is defined as:
with the tritium decay constant = 0.05599 yr^-1.

τ = 1/λ ln(1 + [³HE_tri]/[³H]) λ =ln(2)/τ½

Concentrations of both ³H and ³He in water are extremely low and therefore difficult to measure. In modern surface waters, both isotopes are present at levels on the order of 1 Mio. atoms per gramm of water. Only about 1 out of 10¹⁷ water molecules is tagged with the radioactive hydrogen isotope. Or, even in a large lake such as Lake Geneva, with a volume of 2*10¹¹ m³, there is only about 1 gramm of ³He and ³H each. Measurement of tritium concentrations by - decay counting became feasible in the 1960, when tritium levels were 2 to 3 orders of magnitude higher than natural. Measurement of ³He was made possible at the same time by progress in noble gas mass spectrometry. In 1976, Clarke et al. unified the experimental methods by introducing the measurement of ³H via mass spectrometric determination of its decay product ³He. This paved the way for widespread use of ³H-³He dating, in particular for physical oceanography. Yet, precise analysis of ³H and ³He has remained a difficult experimental task, which is mastered only by a small number of noble gas mass spectrometry labs worldwide.

Pioneering work for the application of the ³H-³He method in lakes was done by Torgersen et al. (1977, 1979). These authors showed that physical-limnological processes such as gas exchange, vertical turbulent diffusion, oxygen depletion rate and mass flux rates in general can be quantified by the use of this tracer pair. Although the method proved promising, its use in physical limnology remained scarce. Since 1989 our group at Eawag and ETHZ has successfully applied the method to a large and still growing number of lakes worldwide.

The use of the ³H/³He tracer pair to study groundwater flow and recharge was first proposed by Tolstikhin and Kamenskiy (1969). However, it took nearly two decades until actual groundwater studies based on this method were published (Takaoka and Mizutani, 1987; Poreda et al., 1988; Schlosser et al., 1988; 1989). These studies showed that the tritiogenic ³He can be detected and separated from other contributions to the total ³He dissolved in groundwater. Schlosser et al. (1989) estimated the effect of dispersion on the tritiogenic ³He peak and the degree of ³He confinement on the basis of model simulations.