Modelling the Wieringermeer effect observed in the Champagne Chalk aquifer

Using the water-table-fluctuation method (WTF) in the Chalk unconfined aquifer of northern Europe leads to low specific yield estimations ( ∼ 2 % ) despite the high porosity of the chalk matrix ( ∼ 40 % ). The reason for this is the Wieringermeer effect, a phenomenon caused by capillarity that has rarely been mentioned in previous studies. In this paper, we highlight the role of the mostly overlooked Wieringermeer effect in explaining the water level records and present a 2D model capable of correctly reproducing the low specific yield in a 40%-porosity matrix. This study focuses on a 10 km 2 experimental site located in the Champagne region of France and is based on six years of water level records. We derived the specific yield and response time using the WTF method and correlograms. The acquired data were then used to construct a distributed numerical model based on physics and the coupling of groundwater flow in unsaturated and saturated zones, using the Metis code. In particular, we tested different retention laws to highlight the effects of capillarity. The piezometric records show that the water table under the hills strongly reacts to precipitation, whereas it reacts only to seasonal signals in the valleys. The specific yields observed are between 0.7 and 5.6%, including matrix and fracture effects. Our model shows that in a 40% porosity matrix, such low specific yields are only possible with high retention, thereby highlighting the Wieringermeer effect within the simulation. • The Chalk is a porous and fractured aquifer with a high retention capacity. • The vadose zone retains a large quantity of capillary water. • This leads to important water-level fluctuations in response to climate forcing. • This corresponds to the Wieringermeer effect.