More than 180 pesticides and their by-products were detected in small streams throughout 11 Midwestern states, some at concentrations likely to harm aquatic insects, according to a new study by the U.S. Geological Survey.

The mixtures of pesticides are more complex than previously reported by the USGS—94 pesticides and 89 pesticide byproducts were detected. On average, 52 pesticide compounds were identified in each stream.

At least one pesticide in at least half of the 100 streams sampled exceeded a threshold predicted to cause harm to aquatic insects and other stream organisms, ranging from acute effects—like death after a short-term exposure—to chronic effects—like longer-term impairments to reproduction and development. Two to four pesticides exceeded that threshold in more than a quarter of the streams. Aquatic insects, like mayfly and stonefly larvae, are critical to stream health because they are an essential link in the aquatic food web. Pesticides were not measured at levels predicted to be toxic to fish in 95 of the 100 streams tested. Potential impacts on human health were not assessed because the small streams sampled in agricultural and urban areas are unlikely to be used as sources of drinking water.

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“About 150 million pounds of pesticides are applied annually in the Midwestern U.S.,” said Lisa Nowell, USGS research chemist and lead scientist for the study. “Understanding which pesticides are occurring at levels potentially toxic to aquatic life, and where they occur, is crucial to informing management decisions.”

While numerous pesticides were detected at low levels, only a few—atrazine, acetochlor, metolachlor, imidacloprid, fipronil, and organophosphate insecticides—were predicted to be major contributors to toxicity. The first three are widely used agricultural herbicides, and the latter three are insecticides used in both residential and agricultural settings.

This is one of the most extensive assessments of pesticides in streams to date: 1,200 samples were collected at 100 Midwest streams over a 12-week period during the 2013 growing season and analyzed for 228 pesticide compounds. Numbers of streams tested in each state include: Illinois (19 sites), Indiana (15), Iowa (17), Kansas (3), Kentucky (3), Minnesota (7), Missouri (14), Nebraska (8), Ohio (7), South Dakota (1), and Wisconsin (6).

This study is the first in a series of five regional stream quality assessments (RSQA) that also includes the Southeast, Pacific Northwest, Northeast, and California. To learn more about the comprehensive Midwest streams assessment and the other regional assessments visit the USGS RSQA website.

The study, “Complex mixtures of dissolved pesticides show potential aquatic toxicity in a synoptic study of Midwestern U.S. streams,” is published in the journal Science of the Total Environment.