Garyp well field (Friesland) was taken into use for drinking water extraction by Vitens in 2002, but salinity levels increased rapidly in the following years. In this thesis research GMS-MODFLOW is used to reconstruct the local geohydrological situation using deep drillings combined with regional data from the MIPWA model, in order to simulate groundwater flow in the vicinity of the well field and determine the cause for the rapid salinization.
The Garyp well field (Friesland)
The Garyp well field (Friesland) was taken into use for drinking water extraction by Vitens in 2002. Salinity levels increased rapidly in the following years. This increase in salinity was not predicted by models. In this thesis research GMS-MODFLOW is used in order to simulate groundwater flow in the vicinity of the well field in more detail than in the large scale MIPWA model, and thus determine the cause for the rapid salinization.
The area is characterized by deep former glacial gullies, formed in the Elsterien age, which incised through the original layering of aquifers and aquitards. The gullies have been refilled with sandy sediments of the Peelo Formation and have a large effect on groundwater flow patterns due to their depth and high hydraulic conductivity. Reconstruction of the gully using deep drilling in the area shows that the gully is deeper than assumed by other models and literature, and that water extraction takes place in this filled gully at a depth of between 80 to 130 m-msl. In order to implement the complex hydrogeological situation a local and a regional model have been created. In the local model deep drillings are used to embed the local hydrogeological situation in detail. This model is afterwards combined with MIPWA-data to create a regional model to simulate regional groundwater flow.
From streamline analysis in this regional model it becomes apparent that Garyp well field attracts groundwater from open water as well as from depths of between 230 and 300 m-msl in the south-east of the model area. This is water is stored in marine sediments of the Maassluis Formation, and is the reason for salinization of Garyp well field. Groundwater flow towards the well field preferentially follows the glacial gully due to its high hydraulic conductivity and lack of aquitards. A possible solution to counter salinization is to replace the well screens to a depth of 50 to 80 m-msl, which reduces the attraction of deep groundwater.