39Ar (t1/2 = 269 yrs) is a well-established method for dating groundwater on timescales from 50 to 1000 years. Underground production processes with neutrons from natural radioactivity in rock (U and Th decay chains) and primary cosmogenic neutrons were identified as significant contributors to the dissolved activities for many decades (Andrews et al., 1989; Fabryka-Martin, 1988; Loosli et al., 1989; Šrámek et al., 2017). More recently, muon-induced reactions, i.e., slow negative muon capture and reactions with muon-induced neutrons, were documented for 39Ar subsurface production in the deep context of WIMP research (Mei et al., 2010). However, the contribution from these particles was never considered for groundwater dating applications.
Based on updated calculations of the 39Ar depth-dependent production rates, realistic emanation fractions assumptions (Musy et al., 2022), and vertical velocity distributions constrained by numerical modeling tools (DK-Model, Henriksen, et al., 2003), we were able to demonstrate the significance of muon-induced processes for the first time for 39Ar groundwater dating applications. In addition, the potential of 37Ar (t1/2 = 35 days), which is produced by very similar processes to 39Ar, as an indicator of local underground production was also highlighted.
Finally, these conclusions could be extrapolated by explicitly simulating the dissolved 39Ar activities in various recharge conditions with a synthetical 2D numerical model built in HydroGeoSphere (Aquanty Inc., 2022; Delottier et al., 2022). These simulations demonstrated that underground production is negligible when water infiltrates freely in a porous aquifer. In contrast, a confining layer impeding the water recharge at shallow – intermediate depths (< 50 m) leads to over-modern 39Ar activities and, therefore, significant age biases. In the future, special attention should be paid to the recharge patterns when using 39Ar for dating groundwater. In doubt, measuring 37Ar activity in the same sample may indicate signs of underground production.