4. Tritium/Helium-3 Dating Method
The tritium‑helium age-dating method relies on the measurement of tritium ([latex]_{}^{3}H[/latex], as discussed in Section 3) and noble gases in groundwater samples. Because the method relies on measurement of a parent (tritium: [latex]_{}^{3}H[/latex]) and child product (tritiogenic helium: [latex]_{}^{3}{He}_{trit}[/latex]) of radioactive decay, the [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] age dating is not reliant on a historical record of atmospheric concentrations or activities. Measured noble gases—[latex]He[/latex] and [latex]Ne[/latex], at a minimum, but preferably [latex]Ne[/latex], [latex]He[/latex], [latex]Ar[/latex], [latex]Kr[/latex], and [latex]Xe[/latex]—are used to determine the amount of [latex]_{}^{3}{He}[/latex] produced from decay of [latex]_{}^{3}H[/latex] during transport in the aquifer: tritiogenic helium, [latex]_{}^{3}{He}_{trit}[/latex]. Given [latex]_{}^{3}H[/latex] and [latex]_{}^{3}{He}_{trit}[/latex] for a groundwater sample, it is possible to use the known half‑life of [latex]_{}^{3}H[/latex] (12.32 years; Lucas & Unterweger, 2000) to calculate groundwater tracer age.
Because groundwater age involves both time (recharge year, or age) and space (the location of sampling relative to the water table), the concept is well suited for many applications related to aquifer resilience and groundwater quality issues. For example, recharge rates (dimensions of LT-1) based on [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] ages were first reported by Solomon and Sudicky (1991), and this technique has been used broadly (e.g., Table 1 in McMahon et al., 2011, which includes studies based on [latex]{SF}_{6}[/latex] and CFC tracers).
Similarly, estimation of recharge year for groundwater samples is useful for investigating historical input of contaminants to aquifers, including non‑point sources (e.g., Böhlke, 2002; Puckett et al., 2011) and point sources (Shapiro et al., 1999). Knowledge of historical contaminant inputs to aquifers can then be useful for predicting future discharge of contaminants from aquifers, including discharge from wells—for interactive examples, see Böhlke and others (2014) and related web interface—and groundwater discharge to streams (Gilmore, Genereux, Solomon, Farrell, et al., 2016; Gilmore, Genereux, Solomon, & Solder, 2016).
[latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] age dating has been used to constrain and/or calibrate groundwater models. One example is McMahon and others (2010), where six [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] age estimates—out of 2,574 total field observations including other parameters—had a major influence on refining the numerical model. Similarly, in a groundwater quality study, Wells and others (2021) used existing [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] groundwater age estimates (Böhlke et al., 2007; Wells et al., 2018) in a machine‑learning framework to constrain the range of groundwater and vadose zone transport rates. Additional examples of [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] applications can be found in Gilmore and others (2021).