Aquifer storage and recovery in a fossil creek bed: managing droughts in a brackish environment
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In the Dutch coastal province of Zeeland, there is a lack of freshwater that can be used for irrigation during dry spells in May-August. A promising solution may lie in a geological feature of the region. There are sandy fossil creek beds that, after the surrounding marshland subsided, became elevated in the area. The permeable sand provides good conditions for the growth of freshwater lenses. The goal of the research is to find effective ways to increase the thickness of the lens, such that it can be used to sustainably pump freshwater from this reservoir in times of drought. On a creek ridge north of Serooskerke, Walcheren, electrical conductivity measurements were done using several different geophysical methods. This showed freshwater lenses of around fourteen metre thick. The area was then modelled in a 3-D density dependent groundwater flow model with coupled salt transport, using the code MOCDENS3D. Model results showed that there are currently two main factors limiting freshwater lens growth. Firstly, there is a primary ditch cutting deep through the sandy creek. Fresh groundwater flows from the creek bed into the brackish ditch. Also, if the current conventional drainage on the sandy creek were to be replaced by controlled drainage, unnecessary drainage could be prevented. Whilst eliminating these two points would already thicken the freshwater lens by a few metres, further success could be made by artificially infiltrating extra freshwater into the creek ridge. This can be done through the controlled drainage, during winter months when surplus freshwater is available. Artificially infiltrating extra water could help balance for the water that is pumped out of the creek ridge in summer, to prevent upconing of brackish water.