Geochemical side effects of potential Managed Aquifer Recharge during infiltration of monovalent partial desalinated water into different dune sands

Managed Aquifer Recharge (MAR) with desalinated saline water is of increasing importance to mitigate groundwater overexploitation and improving its quality. Besides high energy demand, fully desalinated water needs to be post-treated to increase the total dissolved solids concentration for its further use. As a new approach, the aim of the cooperative project “innovatION” is the development of a monovalent-selective membrane capacitive deionization method to improve the ecological footprint and to deliver a purposeful removal of ions. Nonetheless, the infiltration of a water with different water chemistry than natural pore- or groundwater causes geochemical interactions between water and sediment. 

Here, we present first insights of geochemical water-sediment interactions during infiltration of a monovalent partial desalinated water (mPDW) into three different dune sediments from the barrier island Langeoog, Northern Germany, by conducted column experiments. The island of Langeoog was chosen as one of the demonstration sites of the project. The results of the column experiments show that ongoing processes such as cation exchange and calcite dissolution depend clearly on the sediment characteristics. The more pedogenically developed the infiltrating media is, the more complex the geochemical interactions get. Calcite dissolution takes place during infiltration into beach sediment with a higher carbonate content, whereas infiltration into decalcified brown dune sands shows accumulation/adsorption of Ca²⁺. Grey dune sands appear to be a suitable location for a potential MAR application on Langeoog due to less distinct geochemical reactions. Numerical investigation of the respective experiments is shown in a companion study by Schloo et al. (submitted to EGU2023). Trace element mobilization was shown to not just depend on shifting redox conditions but also on the chemical composition of the infiltrating water potentially linked to colloidal transport. Especially, As and V mobilization were periodically retained during mPDW infiltration. Nevertheless, all reactions are shown to be time limited during the experiments and unlikely to cause major problems, hence MAR with mPDW on Langeoog might be a suitable approach to secure the freshwater lens volume in future in an energy efficient way.