Off-site herbicide losses represent risks to human and environmental health, as well as a cost to municipalities for removal. Atrazine, widely used in the US for weed control in crops including corn, is detected in many surface waters. Levels are often significantly higher than the 3 µg/L maximum contaminant level (MCL) permitted in drinking water. The objectives of this study were to determine extent of agricultural drainage water contamination by atrazine, and how watershed scale impacts observed herbicide levels. Outlet water herbicide levels were monitored from nine watersheds from 300 to 19,000 ha in size, representing sub-basin nestings within three main proximal watersheds in northeast Indiana, for four years. Atrazine concentrations peaked during high-flow events, in some instances reaching 20 to 40 times the MCL. Atrazine levels varied more between years than between watershed sizes.