Dating groundwater with trifluoromethyl sulfurpentafluoride (SF₅CF₃), sulfur hexafluoride (SF₆), CF₃Cl (CFC‐13), and CF₂Cl₂ (CFC‐12)

<jats:p>A new groundwater dating procedure using the transient atmospheric signal of the environmental tracers SF<jats:sub>5</jats:sub>CF<jats:sub>3</jats:sub>, CFC‐13, SF<jats:sub>6</jats:sub>, and CFC‐12 was developed. The analytical procedure determines concentrations of the four tracers in air and water samples. SF<jats:sub>5</jats:sub>CF<jats:sub>3</jats:sub> and CFC‐13 can be used to date groundwaters in some environments where the CFCs and SF<jats:sub>6</jats:sub> have previously failed because these new tracers have increasing atmospheric input functions, no known terrigenic source, and are believed to be stable under reducing conditions. SF<jats:sub>5</jats:sub>CF<jats:sub>3</jats:sub> has a dating range from 1970 to modern; the mixing ratio (mole fraction) in North American air has increased from the detection limit of 0.005 parts per trillion (ppt) to the 2006 mole fraction of about 0.16 ppt. No evidence was found for degradation of SF<jats:sub>5</jats:sub>CF<jats:sub>3</jats:sub> in laboratory anaerobic systems. The solubility of SF<jats:sub>5</jats:sub>CF<jats:sub>3</jats:sub> was measured in water from 1 to 35°C. Groundwater samples that contained large amounts of terrigenic SF<jats:sub>6</jats:sub> did not contain terrigenic SF<jats:sub>5</jats:sub>CF<jats:sub>3</jats:sub>. CFC‐13 is a trace atmospheric gas with a dating range in groundwater of about 1965 to modern. CFC‐13 has been used primarily in very low‐temperature refrigeration; thus groundwater environments are less likely to be contaminated with nonatmospheric sources as compared to other widely used CFCs. Because of the low solubility of SF<jats:sub>5</jats:sub>CF<jats:sub>3</jats:sub> and CFC‐13 in water, an excess air correction must be applied to the apparent ages. The new dating procedure was tested in water samples from wells and springs from Maryland, Virginia, and West Virginia.</jats:p>