Salinization and freshening by extracting fresh and brackish groundwater from transition zones: Insights from a field study in the Dutch coastal dunes and implications for freshwater extractability

Extracting brackish groundwater using scavenger wells beneath freshwater lenses can mitigate salinization of freshwater wells. However, disturbing the fresh–saline transition zone may induce losses from limited freshwater reserves through mixing and groundwater quality changes. A field study was conducted in the Dutch coastal dunes that combines geophysical and hydrogeochemical analyses to examine disturbances from well installation, salinity dynamics during fresh and brackish groundwater extraction, and mixing and ion-exchange processes affecting extracted groundwater quality. Fresh drilling fluid introduced during well installation primarily freshened deeper, more saline groundwater. Residual drilling fluid persisted mainly in clay seals after well development, affecting geophysical measurements. Nevertheless, applying fresh drilling fluid and clay seals improved long-term representativeness of groundwater quality monitoring by limiting ion exchange and vertical mixing within the borehole. Extracting brackish groundwater in addition to freshwater nearly tripled the 4-month freshwater extractability (from 48,000 to 142,000 m 3 ) at the pilot site, with limited losses owing to freshwater mixing in the brackish scavenger well (19,000 m 3 ). Alternatively, when extracting brackish groundwater only, the local expansion of the freshwater lens was limited relative to the estimated losses by mixing in the brackish well. Freshwater extraction triggered salinization-induced mobilization of freshwater cations (ammonium, barium, calcium, iron, and manganese). Concurrent brackish groundwater extraction stabilized extracted freshwater quality, but instead triggered ion exchange in extracted brackish groundwater. Overall, this study identifies freshwater losses and water quality changes as key design constraints for optimizing scavenger well installation and operation, aiming to increase coastal freshwater extractability.