The EU Water Framework Directive and ist Groundwater Daughter Directive call for a cost-effective protection of all water resources. For groundwater, the assessment of cost-effective protection measures is complicated by the long time lags and the flow dynamics within aquifers. Together with the water supply company OOWV and the Lower Saxony Board of Agriculture, Ecologic has analysed the cost-effectiveness of groundwater protection measures in a drinking water protection area, and thereby developed a practical solution for dealing with the specific problems of cost-effectiveness analyses related to groundwater.