HR: 0800h
AN: H21F-1093    [Abstracts]
TI: Terrestrial Sources of Perfluorinated Gases: Excess CF$_{4}$ and SF$_{6}$ in Mojave Desert Groundwaters
AU: * Deeds, D A
EM: dan@gaslab.ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 8675 Discovery Way, La Jolla, CA 92037 United States
AU: Vollmer, M K
EM: martin.vollmer@empa.ch
AF: Department of Biogeochemistry, Max Planck Institute for Chemistry, P.O. Box 3060, Mainz, 55020 Germany
AU: Vollmer, M K
EM: martin.vollmer@empa.ch
AF: Air Chemistry Department, Swiss Federal Laboratories for Materials Testing and Research, Ueberlandstrasse 129 , Duebendorf, 8600 Switzerland
AU: Kulongoski, J T
EM: kulongos@usgs.gov
AF: United States Geological Survey, San Diego Field Office, 5735 Kearny Villa Road Suite O, San Diego, CA 92123 United States
AU: Miller, B R
EM: ben@gaslab.ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 8675 Discovery Way, La Jolla, CA 92037 United States
AU: Hilton, D R
EM: drhilton@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 8675 Discovery Way, La Jolla, CA 92037 United States
AU: Izbicki, J A
EM: jaizbicki@usgs.gov
AF: United States Geological Survey, San Diego Field Office, 5735 Kearny Villa Road Suite O, San Diego, CA 92123 United States
AU: Harth, C M
EM: chris@gaslab.ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 8675 Discovery Way, La Jolla, CA 92037 United States
AU: Weiss, R F
EM: rfw@gaslab.ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 8675 Discovery Way, La Jolla, CA 92037 United States
AB: The recent discovery of perfluorinated gases in fluid inclusions of granites and fluorites suggests a geologic source for the estimated 40 parts-per-trillion (ppt) of tetrafluoromethane (CF$_{4}$) and $<$0.006 ppt of sulfur hexafluoride (SF$_{6}$) in the preindustrial atmosphere. The accumulation of these gases in groundwaters with long residence times enables the detection of even small emissions from the surrounding aquifer material. We have measured high concentrations of CF$_{4}$ and SF$_{6}$ in groundwaters from the Mojave Desert, California. Dissolved SF$_{6}$ was extracted by a purge and trap technique and analyzed by gas chromatography with electron capture detection. Dissolved CF$_{4}$ was sampled by headspace extraction, using liquid helium to cryofocus the analytes prior to injection into the Medusa gas chromatograph/quadrupole mass spectrometer analytical system. Current precisions and accuracies for these measurements are on the order of 2% for both gases. Initial measurements of dissolved CF$_{4}$ concentrations range from $\sim$0.05 to $\sim$1.5 pmol kg$^{-1}$, about 5 to 15 times higher than expected for water in equilibrium with the preindustrial atmosphere at the local temperature and altitude of the recharge site. SF$_{6}$ concentrations range from $\sim$0.3 to $\sim$16 fmol kg$^{-1}$, up to several thousand times higher than expected for air-saturated water. Taking into account the large uncertainties in the estimated preindustrial atmospheric concentration of SF$_{6}$, and in the estimated atmospheric lifetimes of both SF$_{6}$ and CF$_{4}$, the ratio of their excess abundances in Mojave Desert groundwaters agrees within an order of magnitude with the estimated ratio of natural fluxes required to sustain their preindustrial atmospheric concentrations. Relationships among dissolved CF$_{4}$ and SF$_{6}$ concentrations and the other geochemical properties of the aquifer, including groundwater residence times (ages), helium abundances and isotopic ratios, and fluoride concentrations will be presented.
DE: 1800 HYDROLOGY
DE: 1886 Weathering (1625)
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0322 Constituent sources and sinks
DE: 0330 Geochemical cycles
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting