Bridging the ³⁹ Ar– ¹⁴ C Groundwater Dating Gap: A Dual‐Permeability Transport Perspective Based on Numerical Modeling and Field Data

Abstract Groundwater dating studies rely on environmental tracers to estimate residence times, but most available reliable tracers cover either short (days to decades; e.g., 222 Rn, 3 H/ 3 He, 85 Kr) or extended timescales (millennia to millions of years; e.g., 4 He, 36 Cl, 81 Kr). This leaves a critical gap in age information for intermediate residence times (50–30,000 years), which are essential for groundwater resources management. Argon‐39 ( t 1/2 = 269 years) and Carbon‐14 ( t 1/2 = 5,730 years) could fill this gap, yet apparent groundwater ages estimated with these tracers often show systematic discrepancies, with 39 Ar‐ages appearing younger than 14 C‐ages. While mixing and geochemical reactions have been suggested as possible explanations, these mechanisms alone do not fully resolve the observed differences. Despite numerous studies using 39 Ar– 14 C dating, no approach has fully reconciled these inconsistencies, particularly in dual‐permeability systems. This study addresses this gap by explicitly modeling tracer transport and production processes, integrating both numerical simulations and field observations to improve groundwater age interpretations. We combined explicit numerical simulations of reactive tracer transport with multi‐tracer field data from Denmark to systematically evaluate the physical and chemical processes affecting 39 Ar and 14 C activities. Our results demonstrate the systematic biases introduced by depth‐dependent underground production of 39 Ar, mixing processes, and diffusive exchange between mobile and immobile groundwater zones in dual‐permeability media. Thus, this study provides a quantitative framework to address transport biases in 14 C and 39 Ar groundwater dating, allowing for more accurate groundwater residence time estimation and better‐informed decision‐making in water management in both semi‐arid and humid regions.