The 2000 European Union Water Framework Directive (WFD) states that ‘Member States shall ensure the necessary protection for the bodies of water identified with the aim of avoiding deterioration in their quality in order to reduce the level of purification treatment required in the production of drinking water’. However, it does not specify how to evaluate or quantify this level of purification treatment. The scientific literature contains several different Water Quality Indices (WQIs), but none are suited for this purpose. Therefore, we propose a novel WQI that we specifically designed to quantify the level of purification required to prepare drinking water from source water. It is based on the WQI of the Canadian Council of Ministers of the Environment (CCME WQI), which was chosen because it is widely accepted, can be used with any number of input parameters, does not require expert judgement and has been applied to assess source water quality before. We compare measured contaminant concentrations in source water to drinking water guidelines and additionally incorporate the resilience of contaminants to treatment processes in the index (which is not possible in the CCME WQI). Furthermore, we accommodate for varying sampling frequencies that are characteristic of the ongoing monitoring programme. These changes make our index more robust and sensitive to relevant changes in source water quality. We calculated index scores for source water from the Rhine and the Meuse rivers to monitor the effect of implementation of the WFD on the effort required to produce of drinking water.