Modeling Transient Redox Zonation at a Riverbank Filtration Scheme

Riverbank filtration (RBF) is a low cost water treatment or pre-treatment technology that has been practiced for over a century in Europe and more than half a century in the United States. When wells are placed in alluvial aquifers along river banks and pumped, a large portion of the pumped water is the induced infiltration of surface water. As the water passes through the aquifer, it undergoes a number of physical, chemical and biological processes. In this study the biogeochemical processes that occur during subsurface passage at an existing RBF system, the Flehe Waterworks located along the Rhine River in Düsseldorf Germany, were analyzed with a reactive transport model. Observed transient hydrochemical data, including those collected during a major flooding event were used to constrain the numerical model. In an initial step flow and nonreactive transport of the RBF system were simulated with MODFLOW and MT3DMS using measured chloride and temperature data for model calibration. Based on the calibrated conservative model reactive transport was simulated with PHT3D. Thereby degradation of organic carbon was found to be the primary driver for hydrochemical changes, associated with the temporary depletion of dissolved oxygen and nitrate. The model simulations were able to clearly elucidate the impact of the temporal changes in river water temperature on the reaction kinetics and therefore explain the observed transient redox zonation.