Quantifying Dead‐End Impacts on Radon Signals in Fractured Rocks

Abstract Understanding radon transport in fractured rock is fundamental for interpreting geochemical signals associated with subsurface deformation and fluid transport. The role of dead‐end fractures (DEFs), one key geometric feature of fractured rocks, remains poorly understood. Using conceptual fracture‐system models consisting of a through‐going main fracture and distributed DEFs, and explicitly coupling α ‐recoil release, diffusion, advection, and decay loss, we quantify a two‐stage radon breakthrough behavior: an early advection‐dominated stage along the main fracture followed by diffusive contributions from DEFs. Across cases with different DEF characteristics, the equilibrium radon activity displays a logistic constitutive relation with the flushing index Zh (the ratio of advective removal rate to radon decay rate), whose constitutive parameters are quantitatively characterized by DEF number and length. Furthermore, the maximum effective tracing length for DEFs, l max , was identified and quantitatively characterized. This study advances our mechanistic understanding of radon nuclide transport in fractured rocks.