Neonicotinoid and s-triazine pesticide transport dynamics in a small karst agricultural watershed

The potential ecological and human health risks posed by agricultural pesticides necessitate a comprehensive understanding of pesticide transport dynamics to guide effective management. Riparian buffers are often implemented to mitigate nutrients and sediment transported in surface runoff. However, the co-benefits they may offer for pesticide mitigation are not well understood, especially in karst landscapes. We monitored nested catchments in a 62 km², karstic, agricultural watershed in the siliciclastic Appalachian Mountain physiographic province of the eastern United States to assess transport dynamics of two surface-applied s-triazine herbicides (atrazine and simazine) and four neonicotinoids commonly coated on seeds (clothianidin, imidacloprid, thiacloprid, and thiamethoxam). In-stream grab samples were collected biweekly from five sites during the 2023 growing season. Simazine, atrazine, and clothianidin were the most frequently detected compounds, found in 93%, 92%, and 75% of samples collected, respectively. Concentration-discharge relationships indicated that clothianidin and atrazine were mobilized in surface runoff and could be retained in buffers. However, simazine appeared to be transported primarily via groundwater from application site to stream through karst features within the fields, making it less likely to be mitigated by riparian buffers. Pesticide fluxes at upstream sub-watersheds were found to be positively correlated to downstream fluxes due to hydrological connectivity typical of head watersheds, agricultural land use distributed across the entire watershed, and the influence of karst features. These patterns suggest the importance of enhancing the common approach of targeting "hot spot" sub-watersheds based on surface hydrology with consideration of subsurface transport pathways that may cross sub-watershed boundaries.