5. Sulfur Hexafluoride Dating Method
Sulfur hexafluoride ([latex]{SF}_{6}[/latex]) is a relatively stable gas whose concentration in the atmosphere has been steadily increasing since the mid‑1970s. Its low solubility in water combined with parts per trillion (ppt) level concentration in the atmosphere result in low, but measurable, concentrations in young groundwater. Its monotonic increase in the atmosphere makes it suitable as a transient age‑tracer for waters that recharged after about
1975.
Although local sources of [latex]{SF}_{6}[/latex] do not appear to be as common as for CFCs, naturally produced [latex]{SF}_{6}[/latex] can limit the dating range of this tracer in some hydrogeologic environments. Sample collection is relatively simple and the measurement of [latex]{SF}_{6}[/latex] is less complex than [latex]_{}^{3}H[/latex] and noble gases. A significant theoretical advantage of the [latex]{SF}_{6}[/latex] dating method is that since about 1980 the increase in atmospheric concentration has been nearly linear with time. As a result, the computed tracer age represents the flow‑weighted average age of the mixture (i.e., there is minimal bias toward one end member).
In addition to groundwater dating, [latex]{SF}_{6}[/latex] has been used as an injected tracer in surface and groundwater systems. The very low detection limit of [latex]{SF}_{6}[/latex] combined with its inertness, availability, and low cost make it an attractive injected tracer. It has been used in streams to evaluate gas exchange in support of using radon as a tracer of groundwater seepage (Cook, Lamontage, et al., 2006; Wanninkhof et al., 1990). Because the injected concentrations typically exceed concentrations derived from the atmosphere by many orders of magnitude, groundwater dating in the vicinity of an injected [latex]{SF}_{6}[/latex] tracer test is usually problematic. An excellent summary of [latex]{SF}_{6}[/latex] dating including sampling and analytical methods and natural sources of [latex]{SF}_{6}[/latex] is provided by Busenberg and Plummer (2000).