Improved component-based methods for mixture risk assessment are key to characterize complex chemical pollution in surface waters

The present monitoring and assessment of water quality problems fails to characterize the likelihood that complex mixtures of chemicals affect water quality. The European collaborative project SOLUTIONS suggests that this likelihood can be estimated, amongst other methods, with improved component-based methods (CBMs). The use of CBMs is a well-established practice in the WFD, as one of the lines of evidence to evaluate chemical pollution on a per-chemical basis. However, this is currently limited to a pre-selection of 45 and approximately 300 monitored substances (priority substances and river basin-specific pollutants, respectively), of which only a few actually co-occur in relevant concentrations in real-world mixtures. Advanced CBM practices are therefore needed that consider a broader, realistic spectrum of chemicals and thereby improve the assessment of mixture impacts, diagnose the causes of observed impacts and provide more useful water management information. Various CBMs are described and illustrated, often representing improvements of well-established methods. Given the goals of the WFD and expanding on current guidance for risk assessment, these improved CBMs can be applied to predicted or monitored concentrations of chemical pollutants to provide information for management planning. As shown in various examples, the outcomes of the improved CBMs allow for the evaluation of the current likelihood of impacts, of alternative abatement scenarios as well as the expected consequences of future pollution scenarios. The outputs of the improved CBMs are useful to underpin programmes of measures to protect and improve water quality. The combination of CBMs with effect-based methods (EBMs) might be especially powerful to identify as yet underinvestigated emerging pollutants and their importance in a mixture toxicity context. The present paper has been designed as one in a series of policy briefs to support decisions on water quality protection, monitoring, assessment and management under the European Water Framework Directive (WFD).