Managing freshwater lenses in a Dutch coastal setting; increasing freshwater availability by aquifer storage and recovery.
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Farmers in the coastal area of Walcheren, Zeeland are faced with issues of saltwater intrusion and summer drought (Oude Essink et al., 2010). These coastal areas are naturally brackish-saline environments due to Holocene marine transgressions (Stuyfzand and Stuurman, 1994; Post et al., 2003). Farming in these coastal areas relies on the existence of freshwater lenses that float on top of saline groundwater. These freshwater lenses are much smaller (< 10 m) than what is typically found in the dunes and are the product of the balance between infiltrating rainwater and saline seepage (Eeman et al., 2011; De Louw et al., 2011). This research provides an investigation into the hydrology of the dune area between Oostkapelle and Vrouwenpolder and on the freshwater lenses found on the farmlands. This research comprises a physical investigation of the shallow hydrological system using geophysical methods and a density dependent groundwater flow and solute transport model with the numerical transport code MOCDENS3D. Electrical conductivity (EC), temperature-EC probe and EM-31 results pinpoint three likely locations of saline seepage. It is advised that the saline water be routed away from the interest area towards the east so as to ensure several ditches remain fresh for aquifer freshwater storage. Continuous vertical electrical sounding measurement transects reveal freshwater lenses of no average between 6 m and 8 m deep. Two proposed measures to increase the availability of freshwater and decrease saline seepage are investigated with the model. Firstly, phreatic aquifer storage and recovery scenarios on two separate farmland plots are investigated. Surplus freshwater during winter is pumped into the subsurface through a horizontal well for recovery and use during times of need as often occurs in summer. Second, a proposed new ditch drainage level scheme is tested to investigate its effects on saline seepage and the shallow subsurface freshwater system. Results of both aquifer storage and recovery scenarios indicate that an injection of 10000 cubic m of freshwater over six months in winter and an abstraction of 6000 cubic m of water in summer is sustainable over long periods of time and does not cause further salinization of the shallow subsurface. Results of the scenario study for the newly proposed drainage ditch water levels showed that no significant change was observable to the depths of the freshwater lenses on the farmland plots, and there was a marked reduction of seepage and chloride concentrations in the ditches due to the increase in ditch water levels. However it was observed that the size and amount of more diffusive like seepage on the farmland plots increased. Farmers in the coastal areas of the Netherlands will be increasingly faced with the challenges of summer droughts and saline seepage due to climate change and sea level rise. The findings of this research outlines a low cost and effective way to store water for later use such as in dry summer month and has great potential to help coastal farming and irrigation practices in similar coastal environments.