Relationship between Quaternary history and groundwater in the IJsselmeer area

Abstract

Concentrations of salt ions in groundwater bearing Quaternary sediments in the IJsselmeer area have been changed by different processes such as transport, diffusion and dispersion. and the exchange of dissolved and adsorbed ions. Two case studies at Oostelijk Flevoland, the IJsselmeerpolder which was drained in 1957, illustrate how the predominating process alternated, in relation with the geological and hydrological configuration and with human interference. At Lelystad, in the central part of the IJsselmeer, the variation of chloride content with depth could originally be explained by diffusion. However, recently ion exchange and mixing of different types of groundwater are the main processes which determine groundwater quality, as the drainage of the polder Oostelijk Flevoland has initiated significant groundwater flow. Using the Piper diagram an interpretation is given for the origin of the various groundwater types, found near Lelystad. A second case study (Bremerberg) revealed the importance of a semi-impermeable layer of Eem clay in the underground. The groundwater under this layer initially had low concentrations of ions as it originated from the Veluwe area. As a consequence of the polder construction and the subsequent Iowering of the piezometric head, the seepage direction was inversed. Surface water from the border lake with higher ion contents started to affect the original groundwater. Contrary to this negative development, the initiated downward seepage resulted in a decrease of the relatively high ion content of the groundwater outside the area with Eem clay. The importance of the water quality in the border lake is stressed, in relation to use of the groundwater for the public water supply.