Traditionally, the Netherlands' subsurface is mainly used to obtain good quality drinking and industrial waters from the different aquifers. Due to the lack of space on the surface, increasing environmental problems and demand for energy, the subsurface will be used increasingly for other activities, including large underground infrastructural projects, underground storage of waste and greenhousegasses and underground storage capacity for the energy sector.In order to evaluate the effects of the underground activities, detailed knowledge about the subsurface sediments is required. The geochemical composition of the subsurface sediments and the associated mineralogy forms an important part of the information needed to make decisions where and under what restrictions the different activities in the subsurface can best be planned. This thesis is a result of the GEOBON-project, which was started in order to meet this information need. The project studied the chemical composition of subsurface sediments by sampling and analyzing cores that were made by the Geological Survey of the Netherlands as is described concisely in Chapter 2.Chapter 3 present results from two study areas, Brabant and Limburg, which show how sediment source, sorting processes, depositional environment, grain size, weathering and syn- and post-sedimentary diagenesis affects the geochemical composition of Miocene, Pliocene and Lower Pleistocene sediments in the Southern Netherlands:Chapter 3.1 contains the results of a sediment geochemical study that was performed in unconsolidated Upper Cenozoic sediments from the South of the Netherlands. Glauconite-rich sediments (Breda Formation) show anomalously high K contents and low Ba/K ratios. Major shifts in sediment composition as a result of changes in the Rijn system and shifts between Rijn and Schelde provenance as known from heavy-mineral studies are recorded in changes in the grain size dependent variations between Al, Na and K: Pleistocene Rijn sediments (Tegelen Formation) show higher Na ...