title: Further developments of radio-noble gas groundwater dating – assessment of 39Ar and 37Ar underground production, and development of a new 85Kr sampling technique
Creators: name: Musy, Stéphanie Lisa
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 530 Physics
subject: 550 Earth sciences &amp; geology

description: Groundwater supplies billions of people with freshwater for domestic, agricultural, energy, and industrial purposes. Understanding its recharge rates, flow paths, and residence times is essential for protecting the resource in terms of quantity and quality and implementing management policies that ensure equitable and sustainable access to clean water. In this context, assessing the age of the water allows for a better understanding of the water cycle at all scales. Dating methods using radioactive noble gas isotopes rely on the accumulation and/or decay from a known atmospheric activity, which dissolves in the recharge. The focus of this thesis is the application of these dating methods for the radioactive isotopes of argon (39Ar and 37Ar) and krypton (85Kr).
Subsurface production of radioactive isotopes is an inherent problem associated with these methods. Taking this phenomenon into account is essential to avoid biasing the ages concluded from the dissolved activities towards young values. For the methods based on argon-39 (39Ar) and argon-37 (37Ar), some processes are now clearly understood and documented. This is the case of the interactions with neutrons originating from cosmic rays or from the naturally present radioactivity in rocks. In contrast, reactions involving muons, which are different cosmogenic particles, are documented for other applications but have never been considered in the context of groundwater dating methods.
In this thesis, the significant impact of muon-induced reactions on 39Ar and 37Ar activities could be demonstrated for the first time. The activities measured in aquifers in Denmark could be compared with those theoretically produced by the documented and/or the newly-assessed muon processes. For this purpose, a review of all nuclear processes involved in the production of radioargon in the underground was required. In addition, the emanation, which is the transfer process for the atoms produced in the rock to the pore space, was assessed by irradiation experiments in a particle accelerator. These results were then combined with information on depth-dependent recharge residence time to compute the activities accumulating during the infiltration of a water particle. Finally, using numerical modeling tools, the significance of subsurface production processes was extrapolated to a broader context. The simulation of various recharge scenarios allowed a better understanding of the situations where groundwater production is susceptible to induce significant age biases.
In parallel, the use of 37Ar as an indicator of surface water-groundwater interactions was investigated in a pumping experiment in an alluvial aquifer in Emmental (CH). The combination of multiple tracers (222Rn and 4He) allowed to distinguish the mixing fraction and the travel time of the freshly infiltrated river water from the regional component.
85Kr is a tracer commonly used to date young (< 50 years) groundwater. Conventionally, its sampling is associated with logistical and practical challenges, such as disruption of the age stratigraphy in the well and preferential drainage of permeable aquifer fractions. As part of this thesis, a new in-situ sampling method was developed for the 85Kr. The latter consists of small quasi-passive samplers placed directly in the well, thus avoiding the need to pump water to the surface. Thereby, the fieldwork is facilitated, and the accessibility of this dating method is improved. This new technique has been tested and validated in a porous aquifer of the Swiss Plateau.