Geochemical interactions during infiltration of monovalent-partial desalinated water into different coastal dune soils

• Innovative deionization method produces monovalent-partial desalinated water (mPDW). • First geochemical evaluation of mPDW infiltration in a German dune landscape. • Column experiments reveal cation exchange, carbonate dissolution and Fe 2+ release. • Hydrogeochemical reactions vary by soil type. • Results demonstrate feasibility of MAR with mPDW, grey dunes most suitable location. Coastal freshwater resources, particularly freshwater lenses, are increasingly vulnerable to saltwater intrusion due to overexploitation and climate change. Managed Aquifer Recharge (MAR) using desalinated water offers a promising countermeasure to mitigate seawater intrusion by sustaining freshwater heads and restoring natural hydraulic gradients. However, high costs and nutrient removal restrict its sustainability. A novel approach applies capacitive deionization with monovalent-selective membranes to produce monovalent-partial desalinated water (mPDW), reducing mainly Na + and Cl − while retaining divalent cations. This study evaluates geochemical interactions during infiltration of synthetic mPDW into soils from the barrier island Langeoog (Germany) to assess the applicability of MAR with mPDW. Column experiments with beach sand and soils of the grey and brown dunes monitored major ions, pH and electric conductivity to assess hydrochemical changes linked to soil development. Across all soils, cation exchange dominated with temporary adsorption of Ca 2+ and Mg 2+ and desorption of Na + . Carbonate dissolution occurred in beach sand and grey dune soil as evidenced by elevated Ca 2+ and HCO 3 − (up to 6.5 mmol/L). Conversely, brown dune soils, enriched in organic matter and Fe oxides, showed complete retention of Ca 2+ and Mg 2+ , and reductive dissolution of Fe oxides (Fe 2+ up to 2.2 mmol/L). The infiltration of mPDW slightly increased the effective cation exchange capacity in brown dune soils by 50 %. Overall, the soil type strongly controlled the hydrogeochemical response to mPDW infiltration and the grey dune is the most suitable site. These results highlight the feasibility of MAR with mPDW from geochemical and hydrological perspectives.