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

## TL;DR

The study examines how pesticides move through a karst agricultural watershed, finding that some pesticides can bypass riparian buffers via groundwater, while others are mitigated by surface runoff.

## Contribution

The study provides new insights into pesticide transport dynamics in karst landscapes, highlighting the role of subsurface pathways in mitigating or bypassing riparian buffers.

## Key findings

- Clothianidin and atrazine were transported via surface runoff and could be mitigated by riparian buffers.
- Simazine was primarily transported through groundwater, bypassing buffer treatment.
- Karst features enable upstream pesticide applications to reach downstream areas with minimal dilution.

## Abstract

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.

Stream sampling most often detected surface‐applied s‐triazines and highly water‐soluble clothianidin.Karst hydrology enables upstream applications to transport downstream with minimal dilution.Local buffers may not capture contaminated karst flow that discharges to streams through streambank seeps.Atrazine and clothianidin were transported in surface runoff and could likely be mitigated by riparian buffers.Surface‐applied simazine was largely transported in groundwater, likely bypassing riparian buffer treatment.

Stream sampling most often detected surface‐applied s‐triazines and highly water‐soluble clothianidin.

Karst hydrology enables upstream applications to transport downstream with minimal dilution.

Local buffers may not capture contaminated karst flow that discharges to streams through streambank seeps.

Atrazine and clothianidin were transported in surface runoff and could likely be mitigated by riparian buffers.

Surface‐applied simazine was largely transported in groundwater, likely bypassing riparian buffer treatment.

Vegetative buffers protect streams from contaminants transported by surface runoff. Surface‐applied and seed‐coated pesticides can be transported to streams through surface and groundwater. The extent to which buffers may mitigate pesticides is unclear, particularly in karst watersheds. We sampled five locations in a karst agricultural watershed during the 2023 growing season. Samples were analyzed for two surface‐applied pesticides and four seed‐coated pesticides. How concentrations varied with flowrate was used to identify transport pathways. We found that two pesticides (one surface‐applied and one seed coated) were mobilized in surface runoff, suggesting that buffers could help mitigate them. However, one surface‐applied pesticide moved through groundwater, likely bypassing buffer treatment. The remaining pesticides showed no dominant transport pathways. This study highlights the need to consider transport pathways to mitigate pesticide transport, particularly in karst watersheds.