Water reuse is essential to address freshwater scarcity, but improved strategies are needed to ensure water quality. The subsurface has great potential for water reuse through soil passage, allowing infiltration of treated wastewater and surface water. However, these sources contain organic micropollutants (OMPs) that may compromise groundwater quality. Current regulations target only a subset of OMPs and largely overlook their transformation products (TPs), which may also pose risks. This thesis provides new insights into the occurrence and fate of OMPs and their TPs in reused water and the subsurface. With field investigations and laboratory experiments, we assessed OMP occurrence, transport, and transformation, and the environmental factors influencing these processes. Although generally the TPs produced were less toxic, more biodegradable, and more mobile than the respective parent compounds, exceptions were observed. These findings highlight the need to include TPs in monitoring and regulatory frameworks, supporting safer and more sustainable water reuse.