This thesis research was executed in an attempt to understand the salinization process of Garyp well field. This was done with a data analysis and a GMS- MODFLOW- MT3DMS groundwater model.
Salinization of Garyp well field
Garyp drinking water well field is located in the Dutch province of Friesland. It went into operation in 2002. Within the first two years of production, the well screens started to extract groundwater with increasing salinity. The thesis research consisted of an attempt to understand this salinisation through a groundwater model.
The well field consists of six individual wells located inside a glacial gully. This gully was formed during the Mid-Pleistocene. It incises multiple aquifers and aquitards to a depth of around 216 m-msl. The gully is refilled with the Peelo Formation. The well screens are located in the sandy part of the Peelo formation at a depth between 80 to 130 m-msl.
Saline groundwater is expected to be situated on top of several different continuous aquitards due to transgressions of the sea. One of these aquitards is the Peize Balk clay formation which is assumed to be located beneath the reach of the glacial gully at a depth of 216 m-msl. Saline groundwater is also expected inside the Maassluis formation which is located at a depth of around 236 m-msl.
This research is conducted in order to determine the origin of the salinisation process. A data analysis was executed and a GMS- MODFLOW- MT3DMS model is used to simulate the rapid salinization of Garyp well field.
The chloride concentration gradients measured in the monitoring wells and in the extraction wells have been analysed in the data analysis. The data analysis shows that there are large areas in the research area which do not contain any monitoring wells. From this analysis it became clear however that the salinization of Garyp well field is not caused by lateral, but by vertical groundwater flow. Monitoring wells in the Maassluis formation beneath Garyp well field show unexpectedly only fresh groundwater. There is only one monitoring well in a radius of ca. 1200 metres around Garyp well field where brackish/saline groundwater is measured. It is found on top of the Peize Balk clay formation just below the glacial gully.
The base groundwater was developed by Steven Menkveld in 2013. Solute transport is included in the model by the MT3DMS software. Density is not included in the model due to technical errors in the SEAWAT software. Initial salinity patterns, a flow line analysis, plausibility studies and several scenarios have been calculated by the GMS- MODFLOW- MT3DMS model. The accuracy of the calculated initial salinity patterns is highly uncertain, due to the lack of input data in combination with the possibility of extreme differences between fresh and saline ground water on a local scale, as found near Ritskebos well field.
Taking these uncertainties in to account, the flow line analysis as well as the plausibility studies showed that the salinization is most likely to originate from an area on top of, and/or inside the Peize Balk clay formation at a depth of around 216 m-msl. Several scenarios based on reducing the depth of the well screens and / or reducing the extraction rate show no definite solution to the salinization. The scenarios showed that the salinization process can only be delayed.