Atrazine Ecological Monitoring Program: Two decades of generating daily or near-daily monitoring data in highly vulnerable watersheds.

Surface water monitoring of pesticides ensures adherence to environmental and human health regulatory requirements. This study focuses on an unprecedented monitoring program spanning two decades with daily or near-daily sampling across 13 states in the US Midwest and Southern United States, targeting watersheds in the upper 20th percentile of runoff vulnerability based on the United States Geological Survey watershed regressions for pesticides model. The Atrazine Ecological Monitoring Program (AEMP), mandated by the United States Environmental Protection Agency (USEPA), aims to collect extensive high-frequency atrazine exposure data alongside key environmental parameters to better understand the dynamics of atrazine fate, transport, and concentrations in these watersheds. Note, the AEMP is also referred to by the USEPA as the Atrazine Ecological Exposure Monitoring Program, or AEEMP, though the former is more commonly cited. Analysis of the 322 site-years of data revealed that sampling frequency is paramount in accurately estimating seasonal chemical concentrations and runoff loads in flowing waters. The AEMP has advanced with improved sampling techniques and a focus on increasingly vulnerable watersheds (i.e., currently representing 97th-99th centile runoff vulnerability), as evidenced by analysis of variance results indicating higher atrazine concentrations in later years, particularly post-2012. Factors such as soil conditions and precipitation were significant in influencing atrazine levels in surface water. Regression analyses underscored the interaction between agricultural activity and weather patterns as predictors of atrazine concentrations. The AEMP's detailed dataset has notably contributed to environmental risk assessment and the refinement of regulatory models. This study highlights the value of high-resolution data in vulnerable regions, emphasizing that high-frequency monitoring and inclusion of detailed environmental data significantly enhance our understanding of pesticide fate and transport in surface waters and informs stewardship efforts.