Comparison of measured and predicted herbicide concentrations in surface water catchments in Belgium

In the process of approval of active substances and authorization in the European Union and at the member state level, it must be shown by the applicant that an unacceptable risk for nontarget organisms in the aquatic environment by the active substance can be excluded. To achieve this aim, standardized models, scenarios, and agreed pesticide input parameters have to be used to calculate the exposure as defined by the FOrum for the Co-ordination of pesticide fate models and their Use (FOCUS). During a period of daily surface water sampling lasting 3.5 years, a survey was conducted among farmers in the highly vulnerable catchment of Grote Kemmelbeek in Belgium to collect agronomic data on crops, application dates, and application rates of 12 applied herbicides and one metabolite. Daily surface water concentrations for the same herbicides were measured over 2.5 years for a second, much larger but less vulnerable, catchment of Kleine Aa (KAa). A comparison of realistic worst-case predicted environmental concentrations in surface water (PEC_sw) according to FOCUS with measured concentrations shows that, in the GKb catchment, the PEC_sw was never exceeded for six substances, was exceeded on only 1–2 days for five substances, and was exceeded on 9–27 days for two substances. For the KAa catchment, the PEC_sw was only exceeded on two days for one compound and never for the other 12 compounds. These numbers correspond to a level of protection of the FOCUS PEC_sw between 100% and 97% and are much higher than the regulatory protection goal of 90%. These two case studies demonstrate the protectiveness of the FOCUS surface water approach. Integr Environ Assess Manag 2024;20:1447–1462. © 2024 Bayer AG. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).