Estimated Toxicity to Aquatic Organisms in Midwestern Streams Driven by Relatively Few of the 227 Pesticides Analyzed

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U.S. Geological Survey (USGS) scientists measured and estimated potential acute and chronic toxicity for 227 pesticides in agricultural and urban streams in Midwestern United States. Numerous pesticides were detected at low levels. Atrazine, acetochlor, metolachlor, imidacloprid, fipronil, selected organophosphate insecticides, and carbendazim were determined to be major contributors to toxicity that was estimated in over half of the streams studied.

The Midwestern landscape has been extensively modified, with row crops and pavement replacing grasslands and woodlands. A wide variety of pesticides are used in these modified landscapes to control insects, weeds, and fungi. Exposure to pesticides can result in acute toxicity (caused by a single exposure) or chronic toxicity (caused by long-term exposure) to aquatic plants, aquatic insects, and fish.

With new pesticides continually being approved or discontinued, the exposure and effects of pesticides and pesticide mixtures on aquatic life is uncertain. USGS scientists started an extensive assessment of pesticides in Midwestern streams to understand the complexity of stream organism exposure and to estimate toxicity in 100 streams over a 12-week period during the 2013 growing season. Samples were analyzed for 227 pesticide or pesticide breakdown products using a sensitive, broad-spectrum method.

Pesticide toxicity to aquatic organisms was estimated by comparing the concentrations measured in the streams to U.S. Environmental Protection Agency Office of Pesticide Programs (OPP) aquatic-life benchmarks for individual pesticides and to a Potential Toxicity Index (PTI). The PTI is a model that assesses the potential toxicity of mixtures of pesticides to fish, cladocerans (small crustaceans commonly called water fleas), and benthic invertebrates (bottom-dwelling organisms in water bodies).

Map of Streams Tested for Pesticides in 11 Midwestern States

A map of sampling sites across midwestern states showing estimated toxicity of stream samples to aquatic insects.  At least one pesticide in more than half of the 100 streams sampled in the Midwest exceeded a toxicity threshold predicted to cause harm to aquatic insects and other stream organisms, ranging from acute effects (mortality after short-term exposure) in 12 streams to chronic effects (longer term impairments to reproduction and development) in 41 streams.  Data from Nowell and others, 2017.

Over 180 of the 227 measured pesticides were detected in streams at low levels. However, relatively few of measured pesticides—atrazine, acetochlor, metolachlor, imidacloprid, fipronil, selected organophosphate insecticides, and carbendazim—were measured at levels likely to be major contributors to aquatic toxicity, according to aquatic life benchmarks and the PTI. The first three pesticides are widely used agricultural herbicides, the next three are insecticides used in both urban and agricultural settings, and the last is a fungicide breakdown product. At least one pesticide in more than half of the 100 streams sampled exceeded the concentration estimated to be toxic to aquatic invertebrates.

This study provided a short-term temporal assessment of currently used pesticides and their potential aquatic toxicity in agricultural and urban streams in the Midwestern U.S. The acute and sublethal effects of complex pesticide mixtures detected in this study on aquatic organisms are unknown.

This study is part of the Midwest Stream Quality Assessment, the first in a series of five USGS Regional Stream-Quality Assessments that aim to evaluate the relation between ecological stressors, such as contaminants in water and habitat disturbance, and stream ecology across large regions of the United States.

The USGS National Water-Quality Assessment Project provided the funds for this study.

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