The hydrogeology workgroup mainly studies physical flow and transport processes in the subsurface by means of mathematical modeling and field investigations. The spatial variability and uncertainty of hydraulic aquifer parameters pose a major challenge to site characterization, assessment of aquifers, and design of water-resources management schemes. We tackle this problem by stochastic analysis, uncertainty assessment, inverse modeling, and improved site investigation techniques. We analyze the effects of variability and uncertainty on reactive transport, performance of remediation and assessment schemes, and costs. We study interactions between aquifers and other compartments of the water cycle (rivers, unsaturated zone, land surface) by integrated modeling and field investigations. At larger scales, we evaluate the complete hydrosystem response (water fluxes, residence times, biogeochemical status) to changes in climatic forcing and land use within a fully coupled modeling framework supported by high-performance computing environments (parallelization).