The logic of the problem is deceptively simple. In coastal regions, fresh groundwater recharged by rainfall floats like a lens on top of denser saline groundwater, with a brackish mixing zone sandwiched in between. Pump fresh water too aggressively and the underlying brackish water rises toward the well screen in a process called upconing, eventually salinizing the well. Once a well turns salty, recovery can take years, and operators often abandon it altogether. The scavenger well concept counters this by placing a second, deeper extraction well beneath the freshwater well, intercepting the upconing brackish water before it can reach the freshwater screens. But the approach carries a hidden cost: the deeper well inevitably captures some fresh water as well, and disturbing the delicate transition zone can trigger chemical changes that alter water quality in ways that simple mixing models do not predict.

To quantify these trade-offs, the research team, led by Teun van Dooren of KWR Water Research Institute and Delft University of Technology, installed a pilot well field with two extraction wells surrounded by six monitoring wells, all fitted with multilevel piezometers and electrode cables for continuous geoelectrical resistivity measurements. The aquifer system beneath the site consists of unconsolidated Pleistocene and Holocene sediments, with a phreatic aquifer in the top 15 meters, a second freshwater-bearing aquifer between 25 and 40 meters below sea level, and a third aquifer from roughly 50 to 105 meters whose lower portion hosts a brackish transition zone between about 83 and 102 meters. Drinking water utilities have recharged pretreated river water through infiltration ponds in these dunes since the 1950s, sustaining a thick freshwater lens, but temporary interruptions in recharge can still push deeper wells toward salinization.

Even the act of drilling turned out to matter. The team used uncased rotary circulation drilling with fresh drinking water as drilling fluid, and they monitored the fluid’s electrical conductivity as it recirculated through aboveground settling containers. The measurements revealed that the relatively fresh drilling fluid intruded into the deeper, more saline parts of the subsurface, freshening them substantially. Well development largely restored the original sharp salinity transition at 83 meters depth at the screened intervals, but residual drilling fluid persisted for years in clay seals and hydraulically isolated layers, slowly back-diffusing into the surrounding groundwater. Counterintuitively, the researchers argue that using fresh drilling fluid is the right choice: salinization-induced chemical disturbances last longer than freshening ones, and the modest volumes lost during drilling, between 470 and 690 cubic meters per extraction well, are trivial compared with the gains the scavenger well delivered.

The first extraction experiment ran for a year and a half, during which the team pumped 586,000 cubic meters of brackish groundwater through three well screens positioned across the transition zone. The fresh-brackish transition followed the flow-averaged extraction depth, dropping from 83 meters to about 95 meters at the central wells, and the freshwater lens locally grew by an estimated 36,000 cubic meters within a 100-meter radius. Yet the mixing analysis delivered a sobering verdict: up to 58 percent of the extracted brackish water was actually admixed fresh groundwater, meaning roughly 338,000 cubic meters of fresh water was lost, about nine times the volumetric growth of the lens. Pumping brackish water alone, in other words, is an inefficient way to expand a freshwater reserve.

The second experiment told a far more encouraging story. When the team extracted fresh water alone from well screen 1.1 for four months, upconing salinized the well within about seven weeks, after 48,000 cubic meters had been pumped, and chloride concentrations exceeded the Dutch drinking water standard of 0.15 grams per liter. But when brackish extraction was run concurrently, using progressively shallower screens to intercept the rising saline water, the same freshwater well yielded at least 142,000 cubic meters over the same period without salinization. The freshwater intercepted and lost in the brackish well amounted to at most 19,000 cubic meters, only about 20 percent of the gain. The scavenger well, operated directly beneath the freshwater well, proved dramatically more effective at increasing net freshwater extractability than any strategy aimed at enlarging the lens itself.

The chemistry beneath the numbers proved equally revealing. As fresh water was pumped and saline groundwater crept upward, the higher ionic strength of the inflowing water triggered cation exchange on the aquifer’s sediment surfaces: ammonium, barium, calcium, iron, and manganese, all relatively abundant in fresh water, were mobilized into solution, while sodium, magnesium, and potassium were adsorbed. Barium concentrations in the extracted fresh water climbed to five times the level expected from conservative mixing alone. This mobilization appeared only after chloride exceeded the drinking water threshold, but the researchers caution that utilities applying more lenient salinity limits could see other constituents breach their own standards before the water tastes salty, meaning scavenging rates may need to be higher than salinity-based designs suggest.

Concurrent brackish extraction stabilized the quality of the fresh water, suppressing these ion-exchange excursions, but it shifted the chemical burden to the brackish stream itself, which showed salinization-induced mobilization of barium and other freshwater cations. The team also observed a striking hysteresis: groundwater compositions during salinization did not retrace the path taken during freshening, and the extent of mobilization depended on how completely the groundwater had been flushed with fresh water beforehand. Trace metals such as arsenic, cobalt, and nickel were unaffected by the exchange reactions, though their concentrations were temporarily elevated after drilling, when oxygen-rich drilling fluid oxidized sulfide minerals in the aquifer. Elevated turbidity at the freshening front, likely caused by swelling and mobilization of clay particles, never reached the extraction wells and posed no clogging risk.

The study’s practical guidance is clear. A scavenger well works best when placed directly below the freshwater well, where it most effectively prevents upconing saline water from bypassing the brackish screens, and its extraction rate should be tuned so the flow-averaged extraction depth stays beneath the bottom of the freshwater screens. The high-resolution geoelectrical monitoring proved capable of predicting the salinity of water that would be extracted when pumping resumed, giving operators a real-time tool for managing the transition zone. And because the extracted brackish water can be desalinated by reverse osmosis, with lower feedwater salinity yielding higher freshwater recovery, even the water lost to mixing may be partially reclaimed, while mixing brackish streams of different salinities reduces scaling risk in treatment facilities.

For coastal cities from the Netherlands to small islands where freshwater lenses are the only local supply, the message is one of cautious optimism. Managed carefully, scavenger wells can squeeze substantially more drinking water from limited coastal aquifers without sacrificing them to the sea. But the Dutch trial also shows that success depends on respecting the geochemistry as much as the hydraulics: the transition zone is not a passive boundary but an active chemical reactor, and every design choice, from the salinity of the drilling fluid to the placement of clay seals and the timing of pumping, echoes through the quality of the water that finally reaches the tap.

Subject of Research: Field study of scavenger well extraction of fresh and brackish groundwater from the fresh-saline transition zone beneath the Dutch coastal dunes and its implications for coastal freshwater extractability

Article Title: 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

Article References: van Dooren, T., Hartog, N., Zwolsman, G., Raat, K., & Bakker, M. (2026). 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. Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03129-9

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03129-9

Keywords: coastal aquifers, freshwater lens, scavenger wells, brackish groundwater, salinization, cation exchange, hydrogeology, geoelectrical monitoring, managed aquifer recharge, saltwater upconing, drinking water, Dutch coastal dunes