Long-term surface water monitoring of pesticides to evaluate the impact of mitigation measures in an agricultural catchment in Belgium

Once applied, pesticides can enter watercourses through different pathways such as runoff, erosion, drift or point losses and be diffused in the environment. Despite the efforts, many European rivers still show pollution from agricultural sources. Targeted mitigation measures in agriculture could reduce the environmental impact of pesticides and help to reach the desired surface water quality at a particular location. Vegetated buffer strips between water bodies and fields are applied to reduce the export of sediments, pesticides and nutrients from agricultural areas to surface waters. To evaluate and quantify the impact of mitigation measures on runoff and glyphosate loads into surface water, a monitoring study was performed within a typical catchment in SE Flanders (Belgium) for five years (2014-2018) in an area known for its intensive agriculture (>70%) and frequent detection of pesticides in water bodies. A combination of time-based (to establish baseline concentrations and grasp the direct losses) and event-based (to detect the rainfall-runoff related inputs) automatic sampling was used during the agricultural season (April to October). Glyphosate and its major metabolite aminomethylphosphonic acid (AMPA) were selected as monitoring substances for this project since glyphosate has been frequently detected in watercourses in Flanders. Farmers with fields in critical zones were contacted to implement vegetated buffer strips in a voluntary program. Additionally, farmer awareness of risks and good practices related to point losses was raised. The monitoring campaign was conducted for two years before (baseline monitoring) and three years after the implementation of the mitigation measures. By analysing the results during base flow and peak flow conditions, the contribution of different pathways to the pesticide loads and the effectiveness of measures is investigated. Thirty-five rainfall-events were analysed for the five years under different weather conditions during the evaluated seasons. The results of the monitoring campaign, with the evaluation of the effect of several mitigation measures within the catchment on pesticide loads reduction, will be discussed