Aquifer Recharge and Exchangeable Cations in a Tertiary Clay Layer (Bartonian Clay, Flanders-Belgium)

Abstract

The survey area comprises the northern part of Eastand West-Flanders in Belgium. The recharge of the Tertiary Ledo-Paniselian detrital aquifer is occurring by infiltration through the Bartonian clay in the highest regions in the south of the survey area. Both in the aquifer and in the overlying clay layer, marine conditions were prevailing until the last regression at the end of Tertiary times. The infiltration of fresh water has led to the gradual freshening of the aquifer. The present distribution of the groundwater types shows the increasing marine influence in the direction of groundwater flow to the north (Walraevens, 1990). In the westerly recharge area around Ursel, the hydraulic head above and below the Bartonian clay is accurately known, by the presence of 56 piezometers. The local thickness of the Bartonian being known as well (on the average l0 to 20 m), the hydraulic conductivity of the clay layer could be deduced. In the modeling of groundwater flow, an excellent agreement between observed and calculated heads is obtained with a value of l0 -9 m/s (Walraevens, 1987). Laboratory measurements mostly lead to values that are at least an order of magnitude lower. The discrepancy has been ascribed to the presence of preferential pathways in the clay, through which the flow is preferentially taking place.