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September 16, 2020

A mix of multiple pesticides is common in many U.S. streams, reports a new study by the U.S. Geological Survey National Water Quality Program.

In the study of 72 rivers and streams across the contiguous U.S., five or more pesticides were detected in 88% of the more than 5,000 samples collected during 2013–17. Land use in the watersheds investigated spanned a range, including agricultural, urban, and mixed.

The potential for toxicity of the pesticide mixtures to fish was low, but about 12% of samples were predicted to have potential acute (rapid) or chronic (longer-term) toxicity to aquatic invertebrates. Potential toxicity was evaluated using the Pesticide Toxicity Index (PTI), a screening approach that uses the concentrations of all the pesticides in a water sample to estimate potential toxicity. Aquatic invertebrates—a critical part of the food chain—include cladocerans, a large group of miniature freshwater crustacean species, and benthic invertebrates, such as damselfly larvae.

In a given sample, a single pesticide compound generally was responsible for most of the potential toxicity of the pesticide mixture, based on the PTI score, but that pesticide varied among samples and aquatic groups. For example, in a sample with a PTI indicating potential chronic toxicity to fish, the greatest contributor to that toxicity was likely to be the herbicide acetochlor, the fungicide degradate carbendazim, or the synergist piperonylbutoxide. Identification of primary contributors to toxicity could aid efforts to improve the quality of rivers and streams to support aquatic life.


Citation: Pesticide mixtures show potential toxicity to aquatic life in U.S. streams, 2013–2017. Covert SA, Shoda ME, Stackpoole SM, Stone, WW. 2020. Science of the Total Environment,

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