Dating young groundwater with sulfur hexafluoride: Natural and anthropogenic sources of sulfur hexafluoride

Sulfur hexafluoride (SF6) is primarily of anthropogenic origin but also occ urs naturally. The troposphere concentration of SF6 has increased from a st eady state value of 0.054 +/- 0.009 to more than 4 parts per trillion volum e during the past 40 years. An analytical procedure was developed for measu ring concentrations of SF6 to less than 0.01 fmol/L in water. Groundwater c an be dated with SF6 if it is in equilibrium with atmospheric SF6 at the ti me of recharge and does not contain significant SF6 from other (s)ources. T he dating range of SF6 is currently 0 to 30 years. The tracer was successfu lly used to date shallow groundwater of the Atlantic Coastal Plain sand aqu ifers of the United States and springs issuing near the top of the Blue Rid ge Mountains of Virginia. Significant concentrations of naturally occurring SF6 were found in some igneous, volcanic, and sedimentary rocks and in som e hydrothermal fluids.