H138-0012
Estimating Groundwater Age in the Nebraska Sand Hills from SF6 in Stream Water: An Application of the Reach Mass-Balance Approach to Groundwater Mean Transit Time
Monday, 14 December 2020
Poster
Craig R Jensen1, David P Genereux2, Troy E Gilmore3,4, Douglas Kip Solomon5, Aaron R. Mittelstet3, Eric Humphrey5, Markus R. MacNamara1, Caner Zeyrek6 and Vitaly A Zlotnik7, (1)North Carolina State University, Marine, Earth, and Atmospheric Sciences, Raleigh, NC, United States, (2)North Carolina State University Raleigh, Department of Marine, Earth, and Atmospheric Sciences, Raleigh, NC, United States, (3)University of Nebraska - Lincoln, Biological Systems Engineering Department, Lincoln, NE, United States, (4)University of Nebraska Lincoln, Conservation and Survey Division - School of Natural Resources, Lincoln, NE, United States, (5)University of Utah, Department of Geology and Geophysics, Salt Lake City, UT, United States, (6)University of Nebraska - Lincoln, Conservation and Survey Division - School of Natural Resources, Lincoln, NE, United States, (7)University of Nebraska - Lincoln, Earth and Atmospheric Sciences, Lincoln, NE, United States
Abstract:
Stream reach mass-balance has previously been employed with sulfur hexafluoride (SF
6) as an age-dating tracer to estimate groundwater mean transit time (MTT) in the North Carolina (NC) coastal plain. We further examined this approach by applying it in a drier mid-continent location in Nebraska. The reach mass-balance framework uses stream measurements to solve for the flow-weighted mean concentration of SF
6 in the groundwater discharge to a stream reach (C
gw); the groundwater age associated with C
gw values is implicitly an estimate of groundwater MTT. Work was conducted in the South Branch of the Middle Loup (SBML) River and its major tributary stream in the Nebraska Sand Hills at stream flows of 27 – 1899 L/s spanning high flow conditions in May 2019 and baseflow conditions in August 2019.
Stream water SF6 concentrations were less than would be expected for water in equilibrium with the atmosphere, and were consistent with groundwater ages of at least 2 – 19 yr. Thus, the SF6 signal of groundwater age was at least partly preserved in the stream water and not eliminated by gas exchange. Cgw in two reaches correlated to MTT values of 20 yr and about 0 yr. Four of the calculated Cgw values were negative (not physically possible). Monte Carlo analyses showed that Cgw values were highly uncertain. Low groundwater discharge, relative to stream discharge, was identified as the main source of uncertainty. The SF6 concentration in stream water at the downstream end of a stream reach (Cd) emerged as a second significant contributor to uncertainty, a non-intuitive result of the mathematical sensitivity of Cgw to Cd.
Comparing the results from the present study with those from prior work in the NC coastal plain, it seems that the groundwater recharge rate divided by watershed drainage density (R/D), and distance from the headwaters (L), may be useful screening variables to assess the likelihood of success for reach mass-balance estimation of groundwater MTT. For the NE study site, R/D was 95 m2/yr and L was 11 to 116 km; corresponding values at the NC study site were 231 m2/yr and 15 km. Reach mass-balance age-dating may be more successful in reaches of low L in areas of high R/D.
