Diffusive–Dispersive and Reactive Fronts in Porous Media: Iron(II) Oxidation at the Unsaturated–Saturated Interface

Diffusive–dispersive mass transfer is important for many groundwater quality problems as it drives the interaction between different reactants, thus influencing a wide variety of biogeochemical processes. In this study, we performed laboratory experiments to quantify O₂ transport in porous media, across the unsaturated–saturated interface, under both conservative and reactive transport conditions. As reactive system we considered the abiotic oxidation of Fe²⁺ in the presence of O₂. We studied the reaction kinetics in batch experiments and its coupling with diffusive and dispersive transport processes by means of one-dimensional columns and two-dimensional flow-through experiments, respectively. A noninvasive optode technique was used to track O₂ transport into the initially anoxic porous medium at highly resolved spatial and temporal scales. The results show significant differences in the propagation of the conservative and reactive O₂ fronts. Under reactive conditions, O₂, continuously provided from the atmosphere, was considerably retarded due to the interaction with dissolved Fe(II), initially present in the anoxic groundwater. The reaction between dissolved O₂ and Fe²⁺ led to the formation of an Fe(III) precipitation zone in the experiments. Reactive transport modeling based on a kinetic PHREEQC module tested in controlled batch experiments allowed a quantitative interpretation of the experimental results in both one- and two-dimensional setups.