Many U.S. waterways contain pesticide levels that pose potential harm to aquatic life, according to a new study from the United States Geological Survey.

Drawing on pesticide concentration data from large rivers in five regions comprising the continental U.S., USGS researchers found that at least half of the testing sites in each region contained at least one pesticide whose concentration exceeded Environmental Protection Agency benchmarks for the safety of human and aquatic life, even though most agricultural pesticide use was in the Midwest. The team detailed the results of its analysis in a study published April 17 in Science of the Total Environment.

"We're saying that, based on the concentrations we found and what the EPA has said can be tolerated by these organisms, we would think that the level is problematic, and you might need to look into this a little bit more," said lead author Sarah Stackpoole, a USGS research ecologist. "We're not saying that it killed all these organisms at this site. It's just that the potential is there."

While Stackpoole and her colleagues did not witness the death of any organisms, recent research shows that overall pesticide use in the U.S. decreased from 1992 to 2016, though pesticide toxicity increased. According to Stackpoole's study, average pesticide use in the U.S. from 2013-17 totaled 353 million kilograms per year across 438 different pesticides. 

This congressionally mandated project is part of a program called the National Water-Quality Assessment Project, or NAWQA, which monitors large rivers and streams to better understand how human activity and natural environmental changes are affecting water quality. All water samples analyzed from this study came from NAWQA sites that have been active for up to 40 years. 

Stackpoole and her colleagues gathered publicly available data based on USGS National Water Quality Laboratory analysis of these samples, which determined their pesticide concentrations through mass spectrometry and direct-aqueous liquid injection chromatography, a technique that can separate and identify the components of a solution. 

The researchers found 564 instances from 2013-17 across 74 USGS monitoring locations where pesticide concentrations exceeded what the EPA considers chronic or acute for humans and aquatic life. Of the 221 pesticides studied, 17 were bypassing benchmarks. 

An acute benchmark exceedance would likely be strong enough to kill an organism, but a chronic benchmark sets the point where a pesticide would inflict sub-lethal damage, such as interfering with a species' ability to reproduce, or inhibiting its ability to function at a high capacity, according to Stackpoole. 

Most of these exceedances were chronic. Only four pertained to human health, and three times they occurred in the same location. All others affected aquatic life, including fish, invertebrates and plants. 

"We're trying to get a sense at the big scale, in these flowing waters, is this happening a lot or not? Is it of a high concern or not? And I think what we would say about the human health benchmark is: It is rare," Stackpoole said in an interview with The Academic Times. "The aquatic life benchmarks are a different story, because the aquatic invertebrates, the aquatic plants and the fish are all actually living within that environment."

Pesticides enter waterways by a number of routes, according to Stackpoole, including surface runoff, groundwater contamination or through the air. 

Overall, imidacloprid, commonly used in fruit, vegetable and soybean production, was the worst offender by far, leaping over the EPA benchmark for the health of aquatic life 245 times at 60 sites. This insecticide so easily exceeds this benchmark, according to Stackpoole, because its chronic benchmark for invertebrates, for instance, is only 0.01 micrograms per liter.

Imidacloprid is a member of the neonicotinoid class of pesticides that has recently been blamed for declining bee populations, and three of the most commonly used ones, including imidacloprid, are currently banned in Europe.

The USGS is continuing to collect samples from its monitoring sites for U.S. rivers. According to Stackpoole, these samples are still being analyzed for concentration levels of pesticides and herbicides, and different departments, including the USGS, will be able to plan and acquire funding for future projects on the resulting data.

The study, "Pesticides in US Rivers: Regional differences in use, occurrence, and environmental toxicity, 2013 to 2017," published April 17 in Science of the Total Environment, was authored by Sarah M. Stackpoole, Megan E. Shoda, Laura L. Medalie and Wesley W. Stone, United States Geological Survey.