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Atrazine concentrations in the Big Blue River Basin (BRB) in Nebraska and Kansas periodically exceed the U.S. EPA Maximum Contaminant Level (MCL) of 3 µg L-1. The present study is focused on watershed variables influencing surface runoff of atrazine. The assessment includes the impact of corn and sorghum planting progress (indicating atrazine application), rainfall, antecedent soil water content, and soil restrictive layer on stream-measured weekly atrazine load in independent BRB subwatersheds for 1997 - 2004. Maximum atrazine loading occurred after most of the corn had been planted but during sorghum planting from mid-May to early June, immediately following multiple rainfall events that saturated the soil profile and caused runoff from fields. Analysis of covariance was conducted from day 110 when about 10% of the corn was planted to day 170 when 90% of the sorghum was planted. Results from the independent subwatersheds imply that atrazine load weighted by area is related to cumulative weekly rainfall across all years. Statistical analysis showed rainfall was the most significant factor associated with atrazine loading, but soil water content, corn and sorghum planting progress, and the presence of a restrictive layer at the soil surface were also important.
Adviser: Patrick J. Shea