Assessing pesticide transport and export dynamics from watersheds to Laizhou Bay, China

Pesticide residues from watersheds are increasingly exported into seawater, negatively impacting marine ecosystem health. However, existing watershed and flux models for pesticides are limited and insufficient. This study developed a Pesticides Export from Watersheds to Sea (PEWS) model to assess the transport processes of four common pesticides-atrazine, nicosulfuron, chlorpyrifos, and imidacloprid-in Laizhou Bay, China. The model was calibrated and analyzed using field surveys and geographic information system (GIS) data to evaluate the spatiotemporal distribution of pesticide fluxes within the watershed. The results indicated a decreasing trend in pesticide flux into Laizhou Bay from 2001 to 2023, with annual fluxes in 2023 recorded as 11.5 × 10³, 0.1 × 10³, 0.01 × 10³, and 0.6 × 10³ kg/yr for atrazine, nicosulfuron, chlorpyrifos, and imidacloprid in 2023, respectively. The seasonal pattern followed an inverted U-shape, with fluxes peaking in summer and declining in winter. Spatially, pesticide fluxes were highest in agricultural areas, with atrazine and nicosulfuron fluxes peaking in the downstream regions of the Xiaoqing River watershed, imidacloprid fluxes highest in the Mi and Bailang Rivers, and chlorpyrifos fluxes more prominent in the upstream regions of the Weihe River. The study further revealed that less than 3 % of the total pesticide load was exported into the sea, while 1 %-4 % was absorbed by crops, and over 90 % remained trapped in soil and river systems. The PEWS model couples Global NEWS and SWAT frameworks to simulate watershed-scale pesticide export. By quantifying pesticide export fluxes from watersheds to the sea, this study contributes to the development of effective strategies for controlling pesticide pollution.