Evaluating the impact of muon-induced cosmogenic 39 Ar and 37 Ar underground production on groundwater dating with field observations and numerical modeling.

Autor: Musy S; Climate and Environmental Physics and Oeschger Center for Climate Change Research, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland. Electronic address: stephanie.musy@unibas.ch., Hinsby K; Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark., Troldborg L; Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark., Delottier H; Centre for Hydrogeology and Geothermics (CHYN), Faculty of Science, University of Neuchâtel, Rue Emile-Argand 11, 2000 Neuchâtel, Switzerland., Guillon S; MINES ParisTech, PSL Research University, Centre de Géosciences, 35 rue Saint Honoré, 77305 Fontainebleau, France., Brunner P; Centre for Hydrogeology and Geothermics (CHYN), Faculty of Science, University of Neuchâtel, Rue Emile-Argand 11, 2000 Neuchâtel, Switzerland., Purtschert R; Climate and Environmental Physics and Oeschger Center for Climate Change Research, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland.
Jazyk: angličtina
Zdroj: The Science of the total environment [Sci Total Environ] 2023 Dec 10; Vol. 903, pp. 166588. Date of Electronic Publication: 2023 Aug 25.
DOI: 10.1016/j.scitotenv.2023.166588
Abstrakt: Groundwater dating by radioactive cosmogenic tracers such as 39 Ar relies on the decay rate from a known initial atmospheric activity (100%modern). Thereby, it is assumed that cosmogenic 39 Ar production in the subsurface is negligible at depths below the water table and that contributions from natural rock radioactivity are minor or missing. Here we present 39 Ar data from aquifers located in quaternary glacial sediments and tertiary limestones in Denmark, which unequivocally demonstrate that cosmogenic production can induce considerable age biases. 39 Ar values larger than 100%modern are observed at relatively shallow groundwater depths in non-radiogenic rocks. These activities are compared to calculations based on previously assessed depth-dependent production rates in rocks and realistic estimates of the emanated fractions to the water phase. The water residence time distribution with depth, which was determined by numerical flow modeling and particle tracking, underpinned the significance of muon-induced 39 Ar production. The short-lived isotope 37 Ar is produced by similar processes as 39 Ar and demonstrated its usefulness as an indicator of local underground production in an aquifer. The significance of cosmogenic underground production in other possible recharge scenarios was then assessed by explicitly simulating the radioargon accumulation and decay in a 2D synthetical numerical model. These simulations demonstrated that underground production is negligible when the water infiltrates freely in a porous aquifer. However, in the presence of a confining layer impeding the infiltration at shallow depths (<30 m), as is the case in our study site in Denmark for instance, over-modern 39 Ar activities (>100%modern) may occur. The age concluded from the dissolved activities is then possibly biased towards young values. Special attention should thus be paid to the recharge rates when using 39 Ar for dating groundwater. 37 Ar activities provide complementary information about the strength and mechanisms of underground production.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)
Databáze: MEDLINE
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