Advances in agrogeophysics: geophysical monitoring of the fresh–saline water interface in an agricultural field in the Belgian polders

Polders are coastal low-lying areas reclaimed on the sea mainly for agriculture. They are constantly drained by pumps, ditches and subsurface drains. In West-Flanders, Belgium they are composed of a shallow freshwater lens on top of denser saline groundwater. This freshwater lens is essential for crop growth but during dry periods, it can disappear and the saline groundwater can enter the root zone by capillary rise and endanger yield. To prevent this, farmers can use controlled drainage to raise the water table in their field during winter months to increase the freshwater lens thickness. To study the effectiveness of the technique, we monitored three fields with piezometers, resistivity sticks alongside field mapping using electromagnetic induction for three years. The resistivity sticks highlighted lithological differences, but also enabled us to monitor the fresh–saline water interface and the water table accurately. Controlled drainage retained additional rainfall during intense summer precipitation but little effect was observed on the fresh–saline water interface. Soil heterogeneity and past land use of the field seem to have a larger effect on the fresh–saline water interface. Despite the limited effect on the freshwater lens, the value of geophysical methods for monitoring its thickness for field-scale study was demonstrated.