EGU26-18174, updated on 14 Mar 2026
https://doi.org/10.5194/egusphere-egu26-18174
EGU General Assembly 2026
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
Oral | Thursday, 07 May, 16:55–17:05 (CEST)
 
Room 2.31
Elevated Argon-39 Concentrations in Groundwater of a Sub-Alpine Fractured Mountain Aquifer in the East River Watershed, CO, USA
Christoph West1, Nicholas Thiros3, Franka Neumann4,5,1, Kerstin Urbach6,1, David Wachs1,2, Emmy Hieronimus1,2, Florian Meienburg1,2, Niclas Mandaric2, Alexander Junkermann2, Markus Oberthaler2, Werner Aeschbach1, and W. Payton Gardner3
Christoph West et al.
  • 1Institute for Environmental Physics, Ruprecht-Karls University Heidelberg, Heidelberg, Germany
  • 2Kirchhoff Institute for Physics, Ruprecht-Karls University Heidelberg, Heidelberg, Germany
  • 3Geoscience Department, University of Montana, Missoula, Montana, USA
  • 4Grenoble INP, IGE, University Grenoble Alpes, IRD, CNRS, INRAE, Grenoble, France
  • 5Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia
  • 6Glaciology, Alfred-Wegener-Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany

Mountain groundwater systems face imminent changes due an increasing low-snow winter occurrences related to climate change. Understanding how these systems react to the changing water availability is of great importance, especially the dynamics between recent recharge and older groundwater. Environmental age tracers can be useful to inform on the groundwater mixing dynamics in mountain aquifers. However, these mixing dynamics often remain poorly constrained. The radioactive isotope argon-39 (39Ar, T1/2=268 yr) is an important groundwater age dating tracer, filling the dating gap (ca. 50-1000 years) between the widely used tracers tritium and radiocarbon. With the development of the Atom Trap Trace Analysis (ATTA) technique, 39Ar can now be sampled in smaller sample sizes (~5-10 L), making it accessible for sampling in mountainous regions.

We report on a study sampling groundwater for 39Ar from three bedrock monitoring wells of ~10-70 m depth along a mountain hillslope underlain by fractured shale in the East River Watershed near Crested Butte (Colorado, USA). Measured 39Ar concentrations show a downslope increasing gradient from 1.5 times enrichment, compared to modern concentration, on the upslope to 17 times atmospheric abundance at the bottom of the slope. Modeling results show that the observed elevated 39Ar activities in the groundwater can be reproduced by subsurface production of 39Ar in the rock, predominantly due to muon capture reactions, which has been recently demonstrated in a Danish sand aquifer (Musy et al., 2023). The results and their implications will be discussed in the hydrogeological context of the study area.

 

Musy, S., Hinsby, K., Troldborg, L., Delottier, H., Guillon, S., Brunner, P., and Purtschert, R., 2023. Evaluating the impact of muon-induced cosmogenic 39Ar and 37Ar underground production on groundwater dating with field observations and numerical modeling. Sci. Total Environ., Volume 903, 166588, ISSN 0048-9697

How to cite: West, C., Thiros, N., Neumann, F., Urbach, K., Wachs, D., Hieronimus, E., Meienburg, F., Mandaric, N., Junkermann, A., Oberthaler, M., Aeschbach, W., and Gardner, W. P.: Elevated Argon-39 Concentrations in Groundwater of a Sub-Alpine Fractured Mountain Aquifer in the East River Watershed, CO, USA, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18174, https://doi.org/10.5194/egusphere-egu26-18174, 2026.