Groundwater is increasingly subjected to excessive over-exploitation and contamination in many parts of the world. It faces the challenge of balancing its multiple functions in a sustainable manner. This thesis explores several specific problems related to contaminant migration, groundwater-surface water interactions and managed aquifer recharge, using a variety of simulation methods and combined simulation-optimization modelling approaches. The validity of these approaches for evaluating groundwater quantity and quality management options is explored. Case studies were performed on field applications in Belgium, Germany and Oman. The results of these studies demonstrate that better insights and improved groundwater resource management can be achieved through a combination of different simulation and optimization methods that take into account data availability and specific site conditions.