Analysis and management of herbicidal mixtures in a high-intensity agricultural landscape in Belgium.

Waterbodies located in anthropogenically influenced environments, such as agricultural landscapes, may be exposed to different chemicals simultaneously, or sequentially. Yet, current environmental risk assessments focus on single active substances for unintended mixtures. For 3.5 years, the present study monitored the mixture of herbicides, within an intensively managed agricultural catchment, accompanied by a stewardship programme. Twelve herbicides and one metabolite were monitored on a daily to sub-daily basis, generating a unique, high temporal resolution dataset, enabling an assessment of cumulative exposure in a worst-case scenario. Analyses focused on the number of events at which the herbicide mixture concentration exceeded the regulatory accepted concentration for algae and macrophytes, based on concentration addition, and the potential factors influencing the frequency of these events are considered. A low number of individual herbicides drove the toxicity and only two of these overlapped for the two organism groups, algae and macrophytes. The observed exceedance events coincided with seasonal influences, and low rainfall during the 2011 season correlated with a highly reduced number of these events. The major influence was found to be the implementation of the stewardship programme, which directed farmers to use more advanced farming techniques, avoid spillages and other point sources. The number of exceedance events was reduced by more than half for algae (9% of daily mean samples in 2010 and 4% in 2013) and by approximately 10 times for macrophytes (36% in 2010 to 3% in 2013). This high-resolution monitoring data set illustrates how knowledge of the influencing factors can help reduce unintended exposure to chemicals and achieve real-world improvements. Overall, a single-substance assessment is protective of mixture effects. Where mixture effects do play a role, local measures to manage point sources are more effective than changes to the desk-based environmental risk assessments that focus on diffuse sources. This article is protected by copyright. All rights reserved. Integr Environ Assess Manag 2022;00:0-0. © 2022 SETAC.