Preprints
https://doi.org/10.5194/egusphere-2025-1625
https://doi.org/10.5194/egusphere-2025-1625
06 Jun 2025
 | 06 Jun 2025

Different tracer, different bias: using radon to reveal flow paths beyond the Window of Detection

Mortimer L. Bacher, Julian Klaus, Adam S. Ward, Jasmine Krause, Catalina Segura, and Clarissa Glaser

Abstract. Slug tracer experiments have greatly advanced our understanding of solute transport in streams. Breakthrough curves (BTCs) from these experiments are biased toward faster flow paths, highlighting the need for alternative tracers to cover longer timescales. The radioactive tracer radon (222Rn) is increasingly used to quantify transit times in subsurface transient storage zones, as it traces transit times of up to 21 days. However, it remains unclear whether calibrating transient storage models (TSMs) with radon yields longer subsurface timescales of transit times – and thus greater transient storage areas – than with slug tracers such as sodium chloride (NaCl). To address this, we conducted radon measurements and NaCl slug tracer experiments in Oak Creek (Oregon, USA) and jointly and individually calibrated TSM parameters with both tracers. We applied parameter identifiability analysis and evaluated the information provided by both tracers in constraining model parameters. Our results show that calibrating the TSM with radon and chloride increases information on model parameters compared to calibrating the TSM with each tracer individually. This suggests that incorporating radon in calibration improves solute transport estimates in future studies. However, when calibrating the TSM with only radon measurements, all resulting parameters of the TSM were non-identifiable. This non-identifiability arises from steady state activity of radon in streams and radon's high sensitivity to the amount and location of groundwater inflow, which is not explicitly accounted for in TSMs. As a result, radon measurements are biased toward longer-timescale flow paths, limiting its usefulness for characterizing solute transport in calibrating TSMs without chloride.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
Share
Mortimer L. Bacher, Julian Klaus, Adam S. Ward, Jasmine Krause, Catalina Segura, and Clarissa Glaser

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2025-1625', Anonymous Referee #1, 08 Jul 2025
  • RC2: 'Comment on egusphere-2025-1625', Robert Runkel, 18 Jul 2025
Mortimer L. Bacher, Julian Klaus, Adam S. Ward, Jasmine Krause, Catalina Segura, and Clarissa Glaser
Mortimer L. Bacher, Julian Klaus, Adam S. Ward, Jasmine Krause, Catalina Segura, and Clarissa Glaser

Viewed

Total article views: 304 (including HTML, PDF, and XML)
HTML PDF XML Total Supplement BibTeX EndNote
247 43 14 304 25 7 21
  • HTML: 247
  • PDF: 43
  • XML: 14
  • Total: 304
  • Supplement: 25
  • BibTeX: 7
  • EndNote: 21
Views and downloads (calculated since 06 Jun 2025)
Cumulative views and downloads (calculated since 06 Jun 2025)

Viewed (geographical distribution)

Total article views: 298 (including HTML, PDF, and XML) Thereof 298 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
1
 
 
 
 
Latest update: 30 Aug 2025
Download
Short summary
Slug tracer experiments are biased toward faster flow paths, underscoring the need for tracers that reveal temporally longer timescales. We explore integrating solute tracers with naturally occurring radon to quantify flow paths of different timescales at the reach scale. Joint calibration of a transient storage model with both tracers better constrains model parameters, highlighting that this approach is critical for improving solute transport estimates in future studies.
Share