Water diversion reshapes the multimedia fate of fungicides and intensifies fungicide risks in impounded lakes.

Impounded lakes serve as important sinks for emerging organic pollutants; however, the effects of water diversion on their multimedia fate remain poorly understood, limiting effective ecological risk management in large-scale water-diversion systems. This study investigated the occurrence, ecological risks, sources, and multimedia fate of fungicides under contrasting water-diversion conditions in the largest impounded lake along the world's largest water-diversion project. Results showed higher fungicide concentrations in three media during the water diversion period than during the non-water diversion period, with period-averaged mean total concentrations of 1099.2 vs. 773.6 ng/L in water, 733.7 vs. 262.3 ng/g dw in suspended particulate matter (SPM), and 145.6 vs. 16.2 ng/g dw in sediment. Positive matrix factorization identified agricultural input as the dominant source-related factor (64%), followed by urban-mixed input (26%) and particle redistribution (10%). Level III multimedia fugacity modeling further showed that bidirectional water-SPM exchange dominated active intermedia transfer, while sediment accounted for 64.3-78.4% of the total amounts of the three highest-risk fungicides. Concentrations declined from inflow rivers through the lake to outflow rivers, with greater lake purification efficiency during the water diversion period (38%vs. 25%). Risk quotient assessment identified SPM as the highest-risk medium and carbendazim as the dominant risk driver. Aqueous input concentration was identified as the dominant contributor to uncertainty in model predictions by Sobol analysis. These findings provide a scientific basis for ecological risk management and the control of emerging organic pollutants in large-scale water-diversion systems.