Mixtures of dissolved pesticides in 100 streams in the Midwestern U.S., 2013

Dissolved pesticides were measured in weekly water samples from 100 freshwater streams across eleven states in the Midwestern U.S. during May-August, 2013. A total of 182 pesticide compounds (94 pesticides and 88 degradates) were detected, with a median of 25 compounds detected per sample and 54 detected per site. Potential aquatic toxicity was evaluated using the Pesticide Toxicity Index and by comparison to U.S. Environmental Protection Agency aquatic-life benchmarks. In a screening-level assessment, about 5% of streams were predicted to have potential effects on fish and 58% potential effects on invertebrates. In 75% of streams, short-term effects on algal growth were predicted, with potential for longer-term effects on algal communities predicted in 9% of streams. A relatively small number of pesticidesthe herbicides atrazine, acetochlor, and metolachlor, the insecticides imidacloprid, fipronil, and organophosphate (OP) insecticides, and to a lesser extent, the fungicide/degradate carbendazimwere predicted to be the largest contributors to potential toxicity of pesticide mixtures. This data release provides pesticide compound concentrations, watershed characteristics, and aquatic-life benchmarks used in the analysis presented in the journal article, Pesticide mixtures and potential aquatic toxicity in a synoptic study of Midwestern U.S. streams, May-August, 2013, by Nowell, L.H., Norman, J.E., Moran, P.W., Schmidt, T., Mahler, B., Nakagaki, N., Van Metre, P.C., Shoda, M.E., Stone, W.W.