Towards a comprehensive methodology for ecotoxicological assessment: Prioritizing plant protection products for mixture testing in edge-of-field surface waterbodies

Pesticide applications in agriculture result in complex mixtures of Plant Protection Products (PPPs) in the environment. The ecotoxicological effects of these mixtures can occur at concentrations considered safe for individual chemicals, indicating potential risks underestimated by current regulatory assessments focused on individual active ingredients. To address this challenge, our study introduces a methodology for identifying priority PPPs for formulating mixtures, enabling further ecotoxicological testing in water and sediment compartments of edge-of-field surface water bodies, targeting pelagic and benthic organisms. This methodology was primarily based on the actual quantification of PPPs present in these compartments from selected case study sites (CSSs) in Europe and Argentina (11 and 4 for water and sediments, respectively). A conceptual framework was developed that discriminates and selects concerning PPPs based on their individual risk quotient and frequency of occurrence in each CSS, drawing upon two EU regulatory risk assessment approaches, i.e., the general approach under REACH for any environmental contaminant of concern - the European Chemicals Agency (ECHA) approach; and that specifically focusing on PPPs - the European Food Safety Authority (EFSA) approach. Irrespective of whether the focus is on water or sediments, the study revealed disparities in PPP rankings depending on the approach used to identify PPPs of concern, with the ECHA approach being more conservative than the EFSA approach. Despite this, the EFSA approach follows a more standardized assessment factor definition strategy, potentially allowing avoidance of risk overestimation, as well as resulting in a more balanced representation of different PPP classes for subsequent mixtures testing. Overall, the methodological development reported herein, along with the inconsistencies found when comparing different regulatory approaches to assess the risk of environmental contaminants, highlight the need for further discussion on the most appropriate directions towards the standardization of the regulatory risk assessment of PPP mixtures.