Widespread elevated atmospheric SF6 mixing ratios in the Northeastern United States: Implications for groundwater dating

Publication Type  Journal Article
Year of Publication  2008
Authors  Santella, N.; Ho, D. T.; Schlosser, P.; Stute, M.
Journal Title  Journal of Hydrology
Volume  349
Issue  1-2
Pages  139-146
Journal Date  Jan 30
ISBN Number  0022-1694
Accession Number  ISI:000253002900012
Key Words  groundwater age; urban groundwater; unsaturated zone; northeastern USA; sf6; large urban area; unsaturated zone; sulfur-hexafluoride; transport; chlorofluorocarbons; tracer; cfc-113; trends; water; air

SF6 is a promising transient tracer for groundwater dating, but elevated levels of atmospheric SF6 may limit application of this dating method in urban areas. To determine the magnitude of this limitation within the United States (US), this study derives average atmospheric SF6 mixing ratios from soil air measurements from five major urban areas. All surveyed sites showed SF6 elevated above Northern Hemisphere clean air levels. In many cases, SF6 mixing ratios were highest within large cities, but significantly elevated Levels persisted hundreds of kilometers beyond the urban centers. Estimated SF6 emissions from New York city were anomalously large, resulting in average levels over 200% above remote atmosphere values. Emissions from other urban areas included in this study appear to be typical for large cities in the US and maximum observed elevations of SF6, in these areas, were in the range of 50-100%. These findings imply that caution is needed in choosing an atmospheric input function when using SF6 as a transient tracer within or near urbanized regions. Within the Northeast US, the assumption of remote atmospheric SF6 may lead to significant errors in estimation of groundwater ages, especially in the case of an expected future flattening of the atmospheric mixing ratio curve. (c) 2007 Elsevier B.V. All rights reserved.


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URL  <Go to ISI>://000253002900012
DOI  DOI 10.1016/j.jhydrol.2007.10.031