In exploring groundwater quality patterns in sandy phreatic aquifers this thesis follows three fundamental tracks, which are of a methodological, of a conceptual, and of a process based kind. First, in chapter 1 the three principle factors that together determine the groundwater quality distribution, which are flow, input, and geochemical processes, are introduced. Also the two perspectives of relating groundwater flow to groundwater quality are presented. The first perspective is in terms of future and past pathways of point source pollution, the second is of a spatiotemporal type, where the water quality in streams, wells, or wetlands is related to the historical landuse within their recharge areas. These concepts form the framework for the following chapters. In chapter 2 groundwater flow is mapped in terms of the two mentioned perspectives. I show that some powerful 3D aspects of groundwater flow models are underexplored, and that this is due to (1) the lack of adequate visualization methods, and (2) the incompleteness of existing definitions of groundwater flow systems. Both problems are resolved by formalizing groundwater flow systems analysis in three dimensions and by the development of new mapping techniques. This is illustrated for the Hengelo area. Some implications are elaborated upon, both in terms of groundwater flow system and model scale as well as from a conceptual and spatial planning viewpoint. However, such maps are of limited practical value when the uncertainty of groundwater flow is unknown. In chapter 3 it is shown that existing techniques for estimating uncertainty mainly focus on groundwater levels, whereas in relating groundwater quality to groundwater flow other characteristics of groundwater flow are important. A methodology was developed that assesses the uncertainty in groundwater flow system configuration, and is illustrated for the Salland area. The resulting maps of uncertainty due to anisotropy, drainage resistance, recharge, and drainage level are discussed. In chapter 4 the ...