Occurrence and environmental risk of pesticides in a hill-to-plain transitional river: Impacts of land use, seasonal dynamics, and sediment bacterial community.

Pesticides are emerging pollutants that have attracted global concern due to their environmental risks. However, the presence of pesticides and their ecological impacts remain unclear, especially in rivers within hill-to-plain transitional zones. This study examined seasonal changes and environmental risks of pesticides, along with their relationships with land use and sediment bacterial communities in a river at the upper Lake Taihu basin. Among the 119 current-use pesticides, 22 were found in overlying water and 54 in sediment. The mean detection rates (DFs) of fungicides, insecticides, and herbicides were, respectively, 74 %, 70 %, and 82 % in overlying water sampled in three (wet, normal and dry) seasons. The average total pesticide concentrations in water peaked in the wet season (11, 598 ng/L), followed by the normal (342.26 ng/L), and dry seasons (130.55 ng/L). Pearson correlation analysis and partial least squares structural equation modeling show that land use directly affected water quality and pesticide levels. Positive matrix factorization analysis indicated that insecticides, bactericides, and herbicides contributed 45 %, 32 %, and 23 % of the pesticide residual in water, respectively. Notably, in surface water, isoprothiolane, isocarbophos, carbendazim, and metolachlor posed moderate risks both in the wet and dry seasons. Co-occurrence network analysis revealed decreasing bacterial interaction strength along the upstream-to-plain gradient, with Actinobacteria and Firmicutes members likely aiding pesticide degradation in sediments. These findings offer a comprehensive understanding of how land use influences pesticide behavior and microbial communities, providing valuable insights for pesticide management in small watersheds of the Lake Taihu basin.