Estimating groundwater mean transit time from SF6 in stream water: field example and planning metrics for a reach mass-balance approach

Craig R. Jensen, David P. Genereux, Troy E. Gilmore, D. Kip Solomon, Aaron R. Mittelstet, C. Eric Humphrey, Markus R. MacNamara, Caner Zeyrek, Vitaly A. Zlotnik

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


Stream reach mass-balance can be used to estimate groundwater age at scales larger than individual wells. Atmospherically derived SF6 was used as an age-dating tracer in reach mass-balance analysis of six stream reaches in Nebraska, USA. The goal was to estimate the flow-weighted mean SF6 concentration in the groundwater discharge to each reach (Cgw) and the groundwater mean transit time (MTT) based on Cgw. The Cgw in one reach (1.48 fmol/kg) was consistent with a MTT of 27 years. High uncertainty in Cgw prevented plausible MTT estimates in the other reaches. Monte Carlo analyses indicated that low rates of groundwater inflow to the stream reaches (Qgw) led to high uncertainty in Cgw. This issue can affect the success of any reach mass-balance analysis, for age dating tracers or other solutes. To improve the likelihood of success in future applications of reach mass-balance, two new planning metrics are proposed for use, separately or together: the ratio of groundwater recharge rate (R) to stream density (D) within a watershed, and the long-term average Qgw/Qd in a stream reach (where Qd is stream discharge at the downstream end of the reach). In comparison to the Nebraska study site (R/D = 107 m2/year), more accurate estimation of Cgw was achieved in a previous SF6 study in North Carolina (USA) with greater R/D (231 m2/year). Larger Qgw/Qd is associated with more accurate outcomes in reach mass-balance, and long-term average Qgw/Qd in stream reaches may be estimated by GIS analysis of watershed areas and/or stream lengths.

Original languageEnglish (US)
Pages (from-to)479-494
Number of pages16
JournalHydrogeology Journal
Issue number2
StatePublished - Mar 2022


  • Age-dating tracers
  • Field techniques
  • Groundwater/surface-water interaction
  • High Plains Aquifer
  • Sulfur hexafluoride
  • USA

ASJC Scopus subject areas

  • Water Science and Technology
  • Earth and Planetary Sciences (miscellaneous)


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