DATING GROUND WATER WITH TRIFLUOROMETHYL SULFURPENTAFLUORIDE (SF₅CF₃), SULFUR HEXAFLUORIDE (SF₆), CF₃CL (CFC-13) AND CF₂CL₂ (CFC-12)

Dating ground waters with CFC-11, CFC-12, CFC-113 and SF₆ on the 0-60 year timescale is sometimes impossible because of excesses of CFCs from anthropogenic sources, excesses of SF₆ from terrigenic sources, degradation of CFCs in some anoxic systems, and/or unresolved CFC ages from the past 10-15 years caused by leveling of their atmospheric concentrations. A new procedure was developed that simultaneously determines concentrations of SF₅CF₃, CFC-13, SF₆ and CFC-12 in air and ground water samples. SF₅CF₃ and CFC-13 can be used to date ground waters in some environments where CFCs and SF₆ have failed previously because SF₅CF₃ and CFC-13 have increasing atmospheric input functions, no known terrigenic source, and are believed to be stable under reducing conditions. The SF₅CF₃ atmospheric mixing ratios in North American air have increased from the detection limit of 0.005 parts per trillion (pptv) to the present atmospheric mixing ratio of about 0.16 pptv. SF₅CF₃ has a dating range from 1975 to modern. No evidence was 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 used primarily 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. Preliminary studies comparing dating applications of SF₅CF₃, CFC-13, SF₆, and CFC-12 have yielded promising results.