Pesticide concentration patterns in agricultural drainage networks in the lake Erie Basin

Abstract This paper presents information on pesticide concentrations in Lake Erie tributaries draining agricultural watersheds, information distilled from data sets spanning nearly a decade and including up to 750 samples per tributary. Pesticide concentrations are strongly skewed and approximately lognormal. Average concentrations in tributaries are correlated with the amount applied in the basin, but with important secondary effects from chemical properties and modes of application of the pesticides. During runoff of storm events following application, concentrations rise rapidly, peak about the time of peak discharge, and decline slowly thereafter. These patterns do not match those for nutrients, major ions, or sediment, indicating a different pathway from the fields for pesticides. On an annual basis, elevated monthly average concentrations are usually observed from May to August, and low concentrations are present during the rest of the year. Monthly average concentrations of atrazine and alachlor generally exceed maximum contaminant levels (MCLs) in at least one month following application, but those of other herbicides do not. Annual averages are below MCLs for all compounds. No long-term trends are apparent. Comparisons of patterns in large and small tributaries show that small tributaries have higher maximum concentrations, more frequent concentrations below detection limit, and fewer intermediate concentrations. Smaller tributaries have more strongly skewed distributions and much greater temporal variability in concentrations than do larger rivers.