Occurrence and ecological risk of typical pesticides in a river–lake system

Pesticides are widely used in agriculture and can enter river–lake systems through surface runoff, adversely affecting non-target organisms and threatening ecological security. This study investigated the occurrence and distribution of 52 pesticides in a typical river–lake system in China and evaluated their ecological risks to aquatic organisms. The average total pesticide concentration in surface water was 203.05 ng/L, with carbendazim being the dominant pollutant, contributing 23.66% to the contamination. In sediments, the average pesticide concentration was 6.34 ng/g, with tebuconazole being the primary contributor at 28.57%. Fungicides were the main pesticide type in both river water and sediments, accounting for 76.86% and 85.10%, respectively. Pesticides predominantly accumulated in lake sediments, with the small lake showing high pesticide concentrations near river outflow areas and the large lake accumulating pesticides near lake inlets. Pesticide concentrations in both water and sediments increased downstream along rivers. Ecological risk assessment revealed high mixed risks to algae, daphnia, and fish, with risk levels rising along with trophic levels of aquatic organisms. The correlation between pesticide concentration and mixed ecological risk was weaker for algae than for daphnia and fish, and certain pesticides posed high risks to algae even at low concentrations, indicating more targeted toxicity for lower trophic organisms. These findings provide reference data for ecological risk assessment and pesticide pollution management in river–lake systems in agricultural regions.