Het kwantificeren van verticale grondwaterstroming in de Horstermeerpolder met behulp van temperatuurmetingen

Temperature measurements were collected in three deep (230 m) piezometers in the brackish Horstermeerpolder using the stop-and-go method. With future groundwater extraction planned, the aim was to increase our understanding of the local groundwater system. The artesian properties of the piezometers resulted in water spillage and thereby lower quality temperature measurements for one of the three piezometers. At the same location, a convex in the temperature-depth profile was present, caused by intra-well flow of groundwater towards the surface. The temperature-depth profiles inside the polder showed higher temperatures than those at the edge due to vertical flow towards the surface. IAt the edge of the polder an inversion in the temperature-depth profile was identified (inflection point) at a depth of 37 m, suggesting warming of the land surface and dominant lateral flow. The upper aquifer was identified to range until a depth of approximately 150 m below the surface. Below this is the Maassluis Formation, which contains the first aquitard. The vertical flow and hydraulic conductivity of the first aquitard unit of the Maassluis Formation were calculated with the Bredehoeft and Papadopulos method using density corrected hydraulic head measurements. The upward flow rate across this unit was determined to be rather small: 0.0004 m/d. The hydraulic conductivity was calculated to be approximately 0.02 m/d, which is a factor of seven larger than the value given by the national hydrogeological model REGIS II TNO. This difference is attributed to methodological differences and parameter uncertainty. In addition to spatial variation, as the values in REGIS II are calculted for a regional scale, opposed to the local estimates based on our temperature measurements.