4.1 Background and Historical Development
Tolstikhin and Kamensky (1969) first reported on the concept of using the [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] parent‑child concentrations to determine an age of water. The method was used to investigate circulation of ocean waters (Jenkins & Clarke, 1976) and later applied to lakes (Torgerson et al., 1979). About a decade later, the first use of tritium‑helium age dating for groundwater was reported by Schlosser and others (1988) and Poreda and others (1988).
Helium has two naturally occurring and stable isotopes. Most natural [latex]He[/latex] is [latex]_{}^{4}{He}[/latex] with an abundance that is nearly a million times greater than[latex]_{}^{3}{He}[/latex] in the atmosphere. The [latex]_{}^{3}{He}[/latex] found in groundwater can be derived from the following:
- the atmosphere,
- subsurface production in the Earth’s crust,
- primordial [latex]He[/latex] (e.g., degassing from mantle rocks), or
- the decay of [latex]_{}^{3}H[/latex].
The [latex]He[/latex] isotopic composition of each of these sources is vastly different. The atmosphere contains about 5.24 ppmv (ppm volume) [latex]He[/latex] with a [latex]\frac{_{}^{3}{He}}{_{}^{4}{He}}[/latex] ratio of 1.382 ± 0.005×10-6 (Sano et al., 2013). In the subsurface, [latex]_{}^{4}{He}[/latex] is a radioactive decay product of [latex]U[/latex] and [latex]Th[/latex] and a small amount of [latex]_{}^{3}{He}[/latex] is produced from the fission of [latex]_{}^{6}{Li},[/latex] which produces [latex]_{}^{3}H[/latex] that then decays to [latex]_{}^{3}{He}[/latex].
The [latex]\frac{_{}^{3}{He}}{_{}^{4}{He}}[/latex] ratio produced by the Earth’s crust ranges from about 10‑8 to 10‑7 and depends on many factors including neutron production and the [latex]Li[/latex] content of crustal rocks (Solomon, 2000). Primordial [latex]He[/latex] that degasses from the mantle is relatively rich in [latex]_{}^{3}{He}[/latex] with a [latex]\frac{_{}^{3}{He}}{_{}^{4}{He}}[/latex] ratio that ranges from about 1×10–5 to 2×10–5 . When produced from [latex]_{}^{3}H[/latex] decay, [latex]_{}^{3}{He}[/latex] is called [latex]_{}^{3}{He}_{trit}[/latex] and is used for [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] dating. As is discussed later in this section, separating [latex]_{}^{3}{He}_{trit}[/latex] from other components is an essential task for dating.