5.7 Summary

[latex]{SF}_{6}[/latex] is a relatively stable gas and the atmospheric concentration has been monotonically increasing since the mid‑1950s. As a transient age‑dating tracer, [latex]{SF}_{6}[/latex] benefits from nearly linear increase in the atmosphere of seven to eight percent per year since about 1980. Its low solubility in water combined with ppt‑level concentration in the atmosphere result in low but measurable concentrations in young groundwater.

Further, measurement of [latex]{SF}_{6}[/latex], while still somewhat specialized, is less complicated than with tracers such as [latex]_{}^{3}H[/latex] and noble gases. The process of calculating a tracer age involves measuring an aqueous concentration, calculating the atmospheric (gas-phase) concentration that would be in equilibrium—after correcting for excess air—with the aqueous concentration, and comparing the atmospheric‑equivalent value with the history of [latex]{SF}_{6}[/latex] in the atmosphere.

The primary limitation to [latex]{SF}_{6}[/latex] dating is uncertainty that results from the amount of excess air in a sample, and from the existence of terrigenic [latex]{SF}_{6}[/latex]. Despite these limitations, excellent agreement between [latex]{SF}_{6}[/latex] ages and CFC ages (e.g., Busenberg & Plummer, 2000) and [latex]_{}^{3}H[/latex]/[latex]_{}^{3}{He}[/latex] ages (e.g., Solomon et al., 2015) have been reported. When samples contain less than about 2 ml/kg of excess air and no terrigenic sources, age uncertainties of ± 5 years are typical for [latex]{SF}_{6}[/latex] dating.

While [latex]{SF}_{6}[/latex] is a potent greenhouse gas and was included in the Kyoto Protocol, there is currently no indication that the rate of atmospheric growth is declining. From a groundwater‑dating point of view, higher atmospheric values are 1) easier to measure, and 2) tend to minimize complications due to terrigenic sources. As such, [latex]{SF}_{6}[/latex] is likely to become an increasingly common dating method in the future.