4.7 Summary
The tritium‑helium‑3 age‑dating method is a robust method for estimating the age of groundwater that was recharged after the mid‑1900s. Because the method relies on measurement of a parent ([latex]_{}^{3}H[/latex]) and child product ([latex]_{}^{3}{He}_{trit}[/latex]) of radioactive decay, [latex]_{}^{3}H[/latex]/[latex]_{}^{3}{He}[/latex] age dating is not reliant on a historical record of atmospheric concentrations or activities.
The process for determining groundwater tracer age (Figure 15) includes determination of [latex]_{}^{3}{He}_{trit}[/latex] by first measuring [latex]_{}^{3}{He}[/latex] in a water sample and then subtracting the [latex]_{}^{3}{He}[/latex] components from atmospheric and terrigenic reservoirs, including [latex]_{}^{3}{He}[/latex] from the solubility equilibrium concentration at recharge temperature and elevation, [latex]_{}^{3}{He}[/latex] from excess air, and [latex]_{}^{3}{He}[/latex] from terrigenic production. The method is especially sensitive to the choice of noble gas model—particularly regarding excess air—and parameters related to different reservoirs of [latex]_{}^{3}{He}[/latex] such as the signature of terrigenic [latex]He[/latex]. Measurement of [latex]_{}^{3}H[/latex] is also required. For young waters without significant amounts of radiogenic [latex]He[/latex] and small amounts of excess air, age uncertainties of ±2 years are typical, but large amounts of excess air (e.g., > 10 ccSTP/kg) and uncertainty in the appropriate excess air model can lead to uncertainties greater than ten years.
Although the basic age equation for the [latex]_{}^{3}H[/latex]/[latex]_{}^{3}{He}[/latex] method is straightforward, its application is sometimes complicated by the need to determine a relatively small value ([latex]_{}^{3}{He}_{trit}[/latex]) by taking the difference of two or more large values ([latex]_{}^{3}{He}_{m}[/latex], [latex]_{}^{3}{He}_{sol}[/latex]). Thus, it is important to choose an appropriate noble gas model for the available data. Furthermore, uncertainty in very young groundwater—age less than a few years—can be large on a percentage basis. The [latex]_{}^{3}H[/latex]/[latex]_{}^{3}{He}[/latex] age-dating method is well suited for determination of groundwater recharge rates greater than about 30 mm yr-1; lower recharge rates may not allow confinement of produced [latex]_{}^{3}{He}_{trit}[/latex] in the groundwater. Groundwater tracer ages from samples containing mixtures of young and old groundwater will be biased (sometimes strongly) toward the [latex]_{}^{3}H[/latex]/[latex]_{}^{3}{He}[/latex] age of the young groundwater.