A new USGS study of pesticides in U.S. rivers and streams reports that, on average, 17 pesticides were detected at least once at the 74 river and stream sites sampled 12 to 24 times per year during 2013–2017. Herbicides were detected much more frequently than insecticides and fungicides.
The number of pesticides detected at a site over the study mirrored the intensity of pesticide use in the region where the site was located. Pesticide use intensity was greatest in the Midwest (49 kg/km2), where 25 pesticides were detected, on average, at each site. Herbicides were heavily used in agricultural settings and were consistently detected in surface waters at concentrations >100 ng/L (nanograms per liter). In contrast, insecticides had lower agricultural-use intensities and surface-water detection frequencies at concentrations >100 ng/L were rare.
An Environmental Protection Agency (EPA) chronic aquatic-life benchmark—estimates of the concentrations below which pesticides are not expected to represent a risk to aquatic life—was exceeded at least once at more than half of the stream sites in every region—Midwest, South, Northeast, West, and Pacific. Such exceedances indicate the potential for harmful effects to aquatic life such as fish, algae, and invertebrates like aquatic insects. However, an EPA human-health benchmark was exceeded only four times (1.1% of samples).
Of the 221 pesticides measured, just 17 were responsible for the aquatic-life benchmark exceedances. Many of these 17 were herbicides, which frequently occurred at relatively high concentrations that exceeded benchmarks for fish, invertebrates, and plants. Others were insecticides, which occurred at lower concentrations, but which are much more toxic to aquatic invertebrates than herbicides.