Stable water isotopologues tend to fractionate from ordinary water during evaporation processes resulting in an enrichment of the isotopic species in the soil. The fractionation process can be split into equilibrium fractionation and kinetic fractionation. Due to the complex coupled processes involved in simulating soil-water evaporation accurately, defining the kinetic fractionation correctly remains an open research area. In this work, we present a multi-phase multi-component transport model that resolves flow through both the near surface atmosphere and the soil, and models transport and fractionation of the stable water isotopologues using the numerical simulation environment DuMuX. Using this high resolution coupled model, we simulate transport and fractionation processes of stable water isotopologues in soils and the atmosphere without further parameterization of the kinetic fractionation process as is commonly done. In a series of examples, the transport and distribution of stable-water isotopologues are evaluated numerically with varied conditions and assumptions. First, an unsaturated porous medium connected to constant laminar flow conditions is introduced. The expected vertical isotope profiles in the soil as described in literature are reproduced. Further, by examining the spatial and temporal distribution of the isotopic composition, is determined the enrichment of the isotopologues in soil is linked with the different stages of the evaporation process. Building on these results, the robustness of the isotopic fractionation in our model is analysed by isolating single fractionation parameters. The effect of wind velocity and turbulent atmospheric conditions is investigated, leading to different kinetic fractionation scenarios and varied isotopic compositions in the soil.

Employing kinetic interface sensitive (KIS) tracers, we investigate three different types of glass-bead materials and two natural porous media systems to quantitatively characterize the influence of the porous-medium grain-, pore-size, and texture on the “mobile” interfacial area between an organic liquid and water. By interpreting the breakthrough curves (BTCs) of the reaction product of the KIS tracer hydrolysis we obtain a relationship for the specific interfacial area (IFA) and wetting saturation. The immiscible displacement process coupled with the reactive tracer transport across the fluid-fluid interface is simulated with a Darcy-scale numerical model. The results show that the current reactive transport model is not always capable to reproduce the breakthrough curves of tracer experiments and that a new theoretical framework is required. Total solid surface area of the grains, i.e., grain surface roughness, is shown to have an important influence on the capillary-associated IFA by comparing results obtained from experiments with spherical glass beads having very small or even no surface roughness and those obtained from experiments with the natural sand. Furthermore, a linear relationship between the mobile capillary associated IFA and the inverse mean grain diameter can be established. The results are compared with the data collected from literature measured with high-resolution microtomography and partitioning tracer methods. The capillary associated IFA values are consistently smaller because KIS tracers measure the mobile part of the interface. Through this study, the applicability range of the KIS tracers is considerably expanded and the confidence in the robustness of the method is improved.