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

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.