Hydrogeochemistry and Redox Potential Evolution in Managed Aquifer Recharge: Numerical Modelling of Column Experiments

Managed Aquifer Recharge (MAR) is a technology to deal with water stress and water scarcity worldwide under present or uncertain future scenarios. Challenges faced about this technology are related on the one hand to the quantity or availability, but also to the quality of the input water. Depending on the source and the degree of pretreatment, recharging water may have undesirable concentrations of Emerging Organic Compounds (EOC), aside from other potentially hazardous substances and pathogens. Understanding the processes that influence the fate of EOCs within the soil and the aquifer during the infiltration path and the transport to the collection point is therefore a key point for evaluating and predicting contaminant plumes and risk assessment. A good understanding and knowledge of the processes taking place is therefore essential, these including both hydrogeochemical and bacterially induced processes.

An important factor controlling hydrogeochemical bacterially mediated reactions is the redox potential, which governs the reducing or oxidizing capacity of a system. If organic substances are present, the formation of a steep gradient of the redox potential due to the utilization of different electron acceptors is typical for the sediment-water interface. The spatial distribution into different redox zones significantly affects the degradation of EOCs, as different reactions occur in these zones. In the present work, the software PHT3D was used to create a numerical model specific for this purpose. Data from column experiments tracing EOC's fate were used as a basis for modeling. Then, a numerical model was built, successfully reproducing the experimental data on chemical species, redox potential and EOC's concentrations along the columns.