The Reston Groundwater Dating Laboratory

SF5CF3 and CFC-13 Background

SF5CF3 and CFC-13 are trace atmospheric gases that can be used to date ground water. The main advantage of using these new tracers is that they may provide useful age information for waters from some environments where the CFCs, SF6, and 3H/3He have previously failed. A new analytical method was developed by the Reston Chlorofluorocarbon Laboratory that determines concentrations of SF5CF3, CFC-13, SF6 and CFC-12 in a sample. These new tracers have increasing atmospheric input functions, no known terrigenic source, and are believed to be stable under reducing conditions. SF5CF3 has a dating range from 1975 to modern; the atmospheric concentration in North American air has increased from the detection limit of 0.005 parts per trillion by volume (pptv) to the 2006 concentration of about 0.16 pptv. No evidence has been found for degradation of SF5CF3 in laboratory anaerobic systems, or of a terrigenic source. Ground-water samples that contained large amounts of terrigenic SF6 did not contain excess SF5CF3. CFC-13 is a trace atmospheric gas with a dating range in ground water of about 1965 to modern. CFC-13 has been primarily used in very low-temperature refrigeration, thus ground-water environments are less likely to contain non-atmospheric sources as compared to other widely used CFCs. Because of the low solubility of SF5CF3 and CFC-13 in water, an excess air correction must be applied to the apparent ages.