SF6 dating
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Summary
Sulfur hexafluoride is primarily of anthropogenic origin but also occurs naturally in minerals, rocks, and volcanic and igneous fluids. Concentrations of SF6 in air have rapidly increased from a steady state value of about 0.02-0.05 to 4 parts per trillion during the past 35 years, and are expected to continue increasing while atmospheric concentrations of CFCs are steady or beginning to decrease.� An analytical procedure was developed for measuring concentrations of SF6 to less than 0.01 femtomol per liter (fmol/L) in waters.� Several hundred ground-water samples were analyzed from different parts of the U.S.� The results indicate that the SF6 in water is derived from the atmosphere, volcanic and igneous sources, and local anthropogenic point sources. Ground water can be dated with SF6 if it is in equilibrium with atmospheric SF6 at the time of recharge, and does not contain significant SF6 from other sources.� The dating range of SF6 is 0 to 35 years.� SF6 concentrations in water in equilibrium with 1999-air are 0.9-2 fmol/L. The tracer was successfully used to date shallow ground waters in a coastal-plain sand aquifer in eastern Maryland, and water from springs near the top of the Blue Ridge Mountains, Shenandoah National Park, Virginia.� Concentrations of SF6 as high as 200 fmol/L were found in ground water samples collected from volcanic rift zones and igneous areas.� The concentrations of SF6 measured in water from springs at the base of the Blue Ridge Mountains near the crystalline-sedimentary rock fault contact exceeded 3 fmol/L, and decreased in the sedimentary rocks away from this contact to less than 1 fmol/L.� Water samples from the eastern Snake River Plain aquifer in Idaho contained up to 17 fmol/L of SF6 with the highest concentrations measured directly above areas with the highest geothermal gradient.� Concentrations of SF6 of 3 fmol/L were common in ground water in the Albuquerque Basin, New Mexico, and concentration of both He increased with depth, however, there was no correlation between He and SF6 concentrations in the ground water.� The highest SF6 concentrations were found in water recharged at the eastern boundary basin in the Sandia Mountains.� The silicic igneous rocks appear to be the source of the SF6 within the basin.� Rio Grande river water was usually in equilibrium with the atmosphere; however, concentrations of SF6 greater than air-water equilibrium were found in some samples within the city limits of Albuquerque, and in the effluents of a sewage treatment plant.
Investigators:
Eurybiades Busenberg and L. Niel Plummer (U.S. Geological Survey, 432 National Center, Reston, VA 20192)
Publications:
Busenberg, E., and Plummer, L.N., 1997, Use of sulfur hexafluoride as a dating tool and as a tracer of igneous and volcanic fluids in ground water. Geological Society of America, Salt Lake City, 1997, Abstracts and Programs, v. 29(6), p. A-78.
Busenberg E., and Plummer, L.N., 2000, Dating young ground water with sulfur hexafluoride: Natural and anthropogenic sources of sulfur hexafluoride. Water Resources Research, v. 36(10), 3011-3030. View this publication (PDF, 38.2 MB)
Illustrations:
Cross section at locust grove, MD shows SF6 model ages (solid lines) and nitrate concentrations as mg/l nitrogen. (dashed lines) SF6 model ages are in agreement with ages obtained with other environmental tracers)
Concentration of SF6 in U.S. natural waters. Some sample from the rift zones in New Mexico (Middle Rio Grande Basin) and Idaho (Snake River Plain Aquifer) have concentrations of SF6 that exceed modern air-water equilibrium. Excess SF6 is believed to be of igneous origin.
Conclusions
Methodologies were developed for collecting water samples and measuring environmental concentrations of SF6 in ground water, gases, and rocks.� The analytical procedure can measure concentration to less than 0.01 fmol/L in ground water. SF6 concentrations were measured in about 250 North American air samples, in minerals and rocks, and about three thousand natural waters and ground water samples.
1. Two sources of SF6 were identified, a small natural background and a large anthropogenic component.� The natural background concentration constitutes about 1.2 percent of the 1999 total atmospheric partial pressure of more that 4 parts per trillion.� The steady-state natural background SF6 concentration in the atmosphere was calculated from old ground waters and is 0.054 ± 0.009 parts per trillion volume.
2. The atmospheric history of the tracer is now well established [Maiss and Brenninkmeijer, 1998], and a vast majority of the 250 atmospheric measurements reported here are in agreement with the results of Maiss and Brenninkmeijer [1998].
3. A procedure was developed for dating ground water from sedimentary rocks.� The dating range applies to water recharged in 1970 to modern. The SF6 method is particularly useful in dating very young (post-1993) ground water, and recharge in urban environments where CFCs can be elevated due to local anthropogenic sources. When the terrestial flux of SF6 from igneous rocks and mineral grains is high, ground waters cannot be dated by the SF6 method.
4. The SF6 method was used to date spring water issuing from the overburden near the top of the Blue Ridge Mountains, Virginia, ground waters from the unconfined Atlantic Coastal Plain aquifer of the U.S., and water from other U.S. locations.� The results are in good agreements with other dating methods.
5. Small but significant concentrations of SF6 were measured in 16 minerals and rocks of igneous metamorphic, hydrothermal, and sedimentary origin. Concentrations of SF6 were generally highest in silicic igneous rocks and lowest in mafic rocks. Significant concentrations of SF6 may be present in some diagenetic fluids.
6. Concentrations of SF6 significantly higher than equilibrium with modern air-water were measured in ground water from fractured silicic igneous rocks, from some hot springs, and in some ground waters from volcanic areas.� Concentrations of SF6 may be a useful natural tracer of igneous and volcanic fluids.
