H138-0013
Using empirical transit time distributions to forecast stream water tracer concentration
Monday, 14 December 2020
Poster
Eric Humphrey1, Douglas Kip Solomon1, Troy E Gilmore2, Aaron R. Mittelstet2, Vitaly A Zlotnik3, David P Genereux4, Caner Zeyrek5, Markus R. MacNamara4 and Craig R Jensen4, (1)University of Utah, Department of Geology and Geophysics, Salt Lake City, UT, United States, (2)University of Nebraska - Lincoln, Biological Systems Engineering Department, Lincoln, NE, United States, (3)University of Nebraska - Lincoln, Earth and Atmospheric Sciences, Lincoln, NE, United States, (4)North Carolina State University, Marine, Earth, and Atmospheric Sciences, Raleigh, NC, United States, (5)University of Nebraska - Lincoln, Conservation and Survey Division - School of Natural Resources, Lincoln, NE, United States
Abstract:
Groundwater transit time distributions (TTDs) are critical for understanding contaminant flushing from aquifers to streams and can provide insight into other hydrologic characteristics. While models of shallow groundwater systems are commonly assumed to discharge higher fractions of young groundwater into a stream (e.g., an exponential TTD), few field-based observations of the TTD have been reported. In this study, we (a) observed the groundwater TTD within five reaches over 99km of stream length based on flow-weighted tracer data (noble gases,
3H, and
14C) collected at approximately 100 point locations below the streambeds and (b) applied the observed, unmodelled TTDs to forecast the stream concentration of these tracers.
Groundwater samples were collected from piezometers installed 30-50 cm into the streambed of the South Branch of the Middle Loup River in the Sand Hills, Nebraska USA. At each of the sampling locations, the rate of groundwater discharge into the stream was measured using a tube seepage meter, a novel device that can directly measure seepage rates with relatively low uncertainty. Using these seepage rates and the 3H/3He and 14C dating method, we have computed flow-weighted TTDs for each of the five reaches and cumulative TTDs based on the integration of upstream reaches. The flow-weighted mean age in the headwaters is approximately 40 years and approximately several thousand years in the downstream reaches. The TTDs suggest high inputs of old water (>20 yrs) and low inputs of young water (<20yrs) contrary to continental-scale models. An empirical (unmodelled) TTD was created from the large number of flow-weighted age measurements and was used to calculate the expected integrated stream concentration of 3H and 14C. These results were mostly consistent with the measured integrated stream concentration indicating that the directly measured TTD could be a powerful tool for understanding and predicting stream water contamination from groundwater aquifers.
