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
Groundwater dating with radioactive tracers requires a known concentration input of recharge. It is further assumed that subsurface sources in the water and rocks are negligible. For 39Ar (t1/2=269 yrs) these assumptions are fulfilled in most cases, making this tracer an ideal dating tool. In-situ production induced by U/Th decay chains in rocks can be estimated with neutron flux calculations in the subsurface (Fabryka-Martin, 1988; Šrámek et al., 2017). Cosmogenic production was found to be irrelevant due to the short reach of cosmic rays’ neutrons. Recently, this theory has been challenged by a reevaluation of the significant production channels at depths relevant to groundwater studies (0 – 200 m). Direct muon capture reactions and interactions with muon-induced neutrons have been demonstrated to be the prevailing 39Ar (and 37Ar) production channels between 5 – 150 m, leading to distinct 37Ar/39Ar ratios (Musy and Purtschert, In prep.).
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.
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.