Drug analysis as a tracer of pesticide pollution from wastewater treatment plants in the Western Cape, South Africa

South Africa has one of the most productive and diverse agricultural economies in Africa. Consequentially, it is the leading pesticide user in Sub-Saharan Africa. The Western Cape is a dominant agricultural region in South Africa, making it particularly vulnerable to pesticide pollution. After application, pesticides can be transported to rivers, potentially causing adverse ecological and human health effects. Thus, there is an urgent need to understand the sources and risk of aquatic pesticide pollution.

To achieve this, we deployed passive samplers for two-week intervals every month from February 2022 - March 2023 in three rivers within agricultural catchments (Piketberg, Grabouw, and the Hex River Valley) in the Western Cape. A control sample was deployed in Jonkershoek Nature Reserve. A pesticide monitoring campaign from 2017-2019 in the same agricultural catchments identified year-round detections despite few agricultural applications, making sources and drivers of pesticide pollution unclear. Thus, in addition to 44 pesticides, 20 drugs were analysed using LC-MS/MS as an indicator for wastewater treatment plant effluent. 22 pesticides and seven drugs were detected above the limit of quantification.

While some pesticides showed elevated concentrations and detections during the main pesticide application period which indicates rainfall and application as a contamination driver, some pesticides without year-round agricultural applications (e.g., imidacloprid) had high detection frequencies and concentrations out of the main application season. However, such compounds typically had high Groundwater Ubiquity Scores. This suggests constant leaching of pesticides into groundwater connected to rivers as a possible contamination source.

Piketberg had high cumulative drug concentrations which correlated strongly with cumulative pesticide concentrations, whereas Grabouw and Hex River Valley did not. This holds particularly true for carbendazim and terbuthylazine. This suggests that some pesticides in Piketberg are likely sourced from both wastewater treatment plants and agriculture, whereas the absence of drugs in Grabouw and Hex River Valley suggests that pesticides are more likely sourced from agriculture. Herbicide detections in Jonkershoek Nature Reserve (e.g., atrazine) indicate contamination possibly sourced from atmospheric transport or invasive plant control and trail maintenance.

A risk evaluation using European Environmental Quality Standards revealed that four pesticides were detected at concentrations exceeding their respective threshold levels, namely imidacloprid, chlorpyrifos, terbuthylazine, and spiroxamine. The omnipresence of imidacloprid in all agricultural catchments and monitoring campaigns are cause for concern. This highlights the need for distinct monitoring approaches and the implementation of tailored mitigation measures. Future sampling of groundwater and wastewater influent and effluent in all study catchments is key to improve our understanding of pesticide transport pathways.