The registration of plant protection products in the EU requires a standard risk assessment for aquatic organisms, where toxicity endpoints are compared to PECSW (Predicted Environmental Concentrations in surface waters). The FOCUS model calculations use realistic worst-case scenarios, where pesticides can enter surface waters via spray drift, runoff or drain flow. Simulations result in complex exposure patterns with multiple peaks over long periods. An aquatic risk assessment for mixture toxicity is required for formulations that contain two or more active substances. A common known approach is to evaluate combined effects based on Toxicity/Exposure Ratios for all actives. This approach evaluates the cumulative risk and conservatively assumes a simultaneous occurrence of exposure peaks. However, the FOCUS surface water scenarios often show exposure patterns where occurring maximum concentrations differ in time for active substances, when applied with identical crop/scenario/application timings. A detailed analysis of the ‘realistic worst-case’ exposure profiles for each of the actives is a promising refinement option for critical scenarios. A software tool was developed that allows the user to perform a step-wise evaluation of mixture toxicity in FOCUS surface waters scenarios for formulations containing up to three active substances. The scenarios can be evaluated individually or simultaneously in a first step, resulting in detailed results and visualizations. If scenarios fail the first evaluation step, an in-depth scenario analysis can be performed in a second step, resulting in the identification of relevant key characteristics (e.g. maximum peak duration) for the scenario-compound combinations. Detailed results and professional visualizations provide a profound basis for decision-making in higher-tier risk assessment. An example for a time-dependent additive mixture toxicity assessment is presented introducing the step-wise approach and explaining the results.

MixTox SW – a software tool for mixture-toxicity exposure assessments in FOCUS surface water scenarios

250th National Meeting (2015)

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