A concept for a site evaluation with respect to groundwater protection is explained. In this concept isotope methods are used as a tool to verify and confirm the results. Two examples are described where tritium, stable isotopes, tritium-3He and radiocarbon are used to determine the infiltration rate, turnover time/mean residence time of aquifer systems. These results were compared with those obtained from classical hydrogeologic data. Assuming different scenarios for possible contaminant output from landfills, waste-management facilities, or waste disposal sites possible consequences for the groundwater quality are derived. It can be shown that the reliability and accuracy of the predictions can be increased considerably by the application of isotope methods.