CEA presented 1 platform and 9 posters at SETAC Seville in May 2024. We will be showcasing each of these presentations in a series of ‘SETAC Spotlight’ articles. This week it is:

SETAC Spotlight: Comparison of Herbicide Mixtures at Two Catchments in Belgium

Authors: : Hanna Schuster(1), Nadine Taylor(1), Arnd Weyers(2), Robin Sur(4)

¹Cambridge Environmental Assessments, RSK ADAS, Boxworth, UK

A large-scale monitoring study (2010 to 2013) was conducted at two Belgium catchments, Grote Kemmelbeek (GKB) and Kleine Aa (KAA), in an effort to better understand herbicide mixtures in agricultural landscapes. Both sites included a measuring point at the catchment outlet and a 2nd upstream. The concentrations of 12 herbicides and 1 metabolite were assessed on a sub-daily or daily frequency. A recent publication analyses the data from the upstream GKB site using a concentration addition approach to determine measurements that exceeded the Regulatory Accepted Concentration (RAC; exceedance events) resulting in potential effects of the mixture on algae and macrophytes. These exceedance events coincided with seasonal influences, and low rainfall during spring/summer 2011 correlated with a highly reduced number of these events. However, the most significant impact on the exceedance events was found to be a stewardship program that was implemented following an initial 8-month baseline measuring period. The stewardship directed farmers to use more advanced farming techniques, avoid spillages and other point sources. Exceedance events were reduced by more than half for algae and by approx. 10 times for macrophytes from the year the stewardship was established onwards. These findings show how local measures reducing point sources are highly effective and potentially more so than desk-based approaches focusing on diffuse sources. 

Data were further analysed in comparison to the measuring point at the outlet of the GKB catchment, and to the data from both measuring points of the KAA catchment. These catchments differed in their size, land use, erosion likelihood and farms in the surrounding area. The KAA catchment is approx. 10 times larger than the GKB catchment (10K and 950 ha, respectively). Correspondingly, 950 farmers cultivate the land at KAA (120 farmers at GKB) and the KAA %farmland was found to be lower. Due to the comparably large size of the KAA catchment, no stewardship was conducted as part of the monitoring program. 

Initial findings show that the quantity of exceedance events differs significantly between sites, with exceedance events being more prevalent at the GKB catchment compared to the KAA catchment. Although less prevalent, the events at the KAA site were found to be more often driven by true mixtures. The poster will investigate the drivers for mixtures beyond the RAC and discuss concepts and ideas to reduce these.