The Reston Chlorofluorocarbon Laboratory

Dating with SF6 Background

Sulfur hexafluoride (SF6) is a trace atmospheric gas that is primarily of anthropogenic origin but also occurs naturally in fluid inclusions in some minerals and igneous rocks, and in some volcanic and igneous fluids. SF6 holds considerable promise as a dating tool of post-1990s groundwater because, unlike the chlorofluorocarbons with steady or declining atmospheric mixing ratios, atmospheric concentrations of SF6 are expected to continue increasing (Busenberg and Plummer, 1997). SF6 is also stable in reducing ground-water environments, and because there are relatively few uses of the compound, few environments are contaminated by anthropogenic sources.

Significant production of SF6 began in the 1960s for use in high voltage electrical switches. SF6 is extremely stable, with an estimated atmospheric lifetime of 800 (Morris et al., 1995) to 3200 years (Ravishankara et al., 1993). SF6 can be analyzed by GC-ECD techniques to a precision of 1-3% (Wanninkhof et al., 1991; Law et al., 1994), so, even though atmospheric mixing ratios are small (currently about 7.5 pptv), dating is possible from about 1970 to the present.

The earliest reported measurement of the atmospheric concentration of SF6was 0.03 pptv in 1970 (Lovelock, 1971). SF6 is accumulating rapidly in the atmosphere with a current growth rate of about 6.9% per year (Geller et al., 1997). The historical atmospheric mixing ratio of SF6 is being reconstructed from production records and atmospheric measurements (Ko et al., 1993; Elkins et al., 1996; Maiss et al., 1996; Geller et al., 1997; Levin and Hesshaimer, 1996), and retrieved from concentrations measured in seawater (Law et al., 1994) and in previously-dated groundwater (Busenberg and Plummer, unpub. data). As atmospheric CFC concentrations fall, an even more sensitive dating tool will be the ratio of SF6 to, for example, CFC-12 (Figure 2). However, because of the low solubility of SF6 (Mroczek, 1997), apparent ages can be very sensitive to excess air, so this needs to be accurately determined.

Busenberg and Plummer (1997) used SF6 to date shallow groundwater on the Delmarva Peninsula, Maryland, and water from springs in the Blue Ridge Mountains of Virginia. Concentrations between one and two orders of magnitude higher than that possible for modern air-water equilibrium were measured in groundwater from volcanic rift zones in New Mexico and Idaho (Busenberg and Plummer, 1997). SF6 concentrations about 3 times higher than possible for modern air-water equilibrium were measured in spring water along igneous-sedimentary contacts in parts of Maryland, Pennsylvania and Virginia, and in old (>1000 years) groundwater from the Middle Rio Grande Basin in New Mexico (Busenberg and Plummer, 1997). Therefore, there is likely a natural, igneous source of SF6 that will complicate dating in some environments.

SF6 has also been used as an artificial tracer (Wilson and Mackay, 1996; Schlatter et al., 1997) that is injected into groundwater or surface water. Clark et al. (1995b) found that wastewater discharged to the Hudson River at New York City was contaminated with SF6. Busenberg and Plummer (1997) found minor contamination of SF6 in Rio Grande water near an outfall from a sewage treatment plant. All other surface waters had SF6concentrations near equilibrium with air.