10:45 AM - 12:15 PM
[AHW20-P02] Biases of 39Ar groundwater ages at low infiltration rates assessed by numerical modeling and 37Ar/39Ar ratios
Keywords:Groundwater dating, Isotopes hydrology, Argon-39 Argon-37, Numerical Modeling, Underground production
These findings are supported by field data from the Denmark hydrogeological system, where over-modern 39Ar activities (i.e. activity concentration higher than 100 % modern) were observed at relatively shallow depths (30 – 35 m) in non-radiogenic rocks. The Danish national hydrological model was used to calculate the groundwater residence time distributions and steady-state 39Ar and 37Ar profiles in the underground. The results emphasize the importance of muon-induced processes for radioargon subsurface production, which is supported by measured characteristic 37Ar/39Ar ratios for muon-capture reactions.
The significance of cosmogenic underground production for realistic recharge scenarios was assessed using a set of synthetic numerical models explicitly simulating the flow and transport of radioargon isotopes. These simulations reveal that the presence of a confining layer, leading to long water residence times at shallow depths, induces a build-up of 39Ar activities. If this process is not properly accounted for, groundwater ages concluded from 39Ar activities are biased towards young values. Such situations exist e.g. in central and eastern parts of Denmark. In phreatic aquifers with unrestricted infiltration, and thus, short residence times in the shallow domain, subsurface production during the recharge process is negligible.