We develop and demonstrate the coupling of the variably saturated flow and transport model HYDRUS-1D, with the geochemical reaction model ORCHESTRA, for simulating multicomponent reactive transport in soils and other porous media. We implement a loose coupling of HYDRUS and ORCHESTRA that feeds flow information simulated by HYDRUS into ORCHESTRA, where arbitrary multicomponent reactions, transformations, and kinetics can be implemented. The HYDRUS-ORCHESTRA coupled model is evaluated with four test cases of increasing complexity in a 100-cell, 1-meter deep unsaturated zone, including transient flow conditions and active internal sinks (root water and solute uptake). The aim was to examine the accuracy of the coupled model against HYDRUS, which can simulate simple reactive transport scenarios. The coupled model is accurate, with <0.65% deviation in outflow breakthrough times, and ∼1% deviation in terminal solute mass balances compared to HYDRUS. The coupled model’s agreement with HYDRUS increases with transport distance and grid refinement, because the deviations stem from numerical dispersion. This novel methodology makes it possible to simulate the fate of arbitrary reactive substances, nutrients, energy, and isotopes under transient, variably saturated conditions including plant-soil–water interactions. Such flexibility is particularly crucial for emerging contaminants, which can exhibit diverse novel and complex reactive behaviours. Read more here: https://doi.org/10.1016/j.jhydrol.2026.136010
