Reviewing 39 Ar and 37 Ar underground production in shallow depths with implications for groundwater dating

• Published in: The Science of the total environment 2023-08, Vol.884, p.163868
• Main Authors: Musy, Stephanie, Purtschert, Roland
• Format: Article
• Language: English
• ISSN: 1879-1026
• DOI: 10.1016/j.scitotenv.2023.163868

Argon-37 ( Ar) and Argon-39 ( Ar) are used for groundwater dating on timescales from weeks to centuries. For both isotopes, the quantification of underground sources is essential to accurately infer water residence times from sampled dissolved activities. Subsurface production resulting from interactions with neutrons from the natural radioactivity in rocks and with primary cosmogenic neutrons has been known for a long time. More recently, the capture of slow negative muons and reactions with muon-induced neutrons were documented for Ar subsurface production in the context of underground particle detectors (e.g. for Dark Matter research). However, the contribution from these particles was never considered for groundwater dating applications. Here, we reevaluate the importance of all potential depth-related production channels at depth ranges relevant for Ar groundwater dating [0 - 200 meters below the surface (m.b.s)]. The production of radioargon by muon-induced processes is considered in this depth range for the first time. The uncertainty on the total depth-dependent production rate is estimated with Monte Carlo simulations assuming a uniform distribution of the parameter uncertainties. This work aims to provide a comprehensive framework for interpreting Ar activities in terms of groundwater residence times and for exposure age dating of rocks. The production of Ar is also addressed since this isotope is relevant as a proxy for Ar production, for the timing of river-groundwater exchanges, and in the context of on-site inspections (OSI) within the verification framework of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). In this perspective, we provide an interactive web-based application for the calculation of Ar and Ar production rates in rocks.