Reservoir-controlled stable isotope exchange fractionation at pore-scale in the unsaturated zone

• Reservoir-controlled isotope fractionation incorporated into a novel transport model • Model application reveals a significant effect on isotope signals at pore scale. • Multiple impacts on isotope-based estimation methods discussed. • Implementation explains overestimation of residence time and dispersion. Reservoir-controlled stable isotope exchange fractionation forces isotopic signatures of different waters to converge over time and has been incorporated into a one-dimensional numerical pore-space-differentiated isotope transport model. Over two years, the water stable isotope values of δ 18 O and δ 2 H were modeled within a soil profile of 1.80 m depth, with the pore continuum being discretized into fine, medium, and macropores. Equilibrium and kinetic isotope fractionation processes were described using the Craig-Gordon model. Results reveal that accounting for reservoir-controlled stable isotope exchange fractionation significantly affects model outcomes. Above the critical zone, reservoir-specific signature shifts of up to ±5.0 ‰ δ 18 O were observed, while below the critical zone, changes still reached up to ±3.0 ‰ δ 18 O. Interactions among fine, medium, and macropores drive reservoir-controlled isotope exchange, with the medium pores dominating the fractionation process, while temporarily activated macropores strongly influence isotopic signatures of percolating water. Reservoir-controlled exchange fractionation attenuates the variability of isotopic signatures in the unsaturated zone. This study highlights that accounting for reservoir-controlled exchange fractionation can significantly enhance the interpretation of soil water isotope data.