U.S. farms use hundreds of millions of pounds of insect, weed and fungus killers each year and the runoff washes into waterways — especially in the middle of the country, where commodity crops are king.
A new study by the U.S. Geological Survey builds a clearer picture of pesticide pollution in streams. The scope of the study is notable, with scientists checking regularly for 80 chemicals across 81 sites for a decade.
At most of the sites, they found troubling levels of at least one pesticide between 2013 and 2022. The levels were high enough to possibly harm aquatic plants or invertebrates — creatures like mayflies and stoneflies.
And the data only provide a snapshot of the situation.
"This is the largest consistently monitored nationwide network," said Meg Shoda, a hydrologist with USGS based in Cleveland, Ohio, and the study's lead author. And yet, she added, "This sampling at this rate is likely underestimating real conditions."
The USGS doesn't take samples every day or even every week at the sites, so it's easy to miss pollution spikes when rain washes chemicals off of farmland, lawns and golf courses.
They're also sampling just a small portion of the millions of miles of streams in the U.S. for 80 chemicals that are just a portion of the many pesticides currently in use.
Three chemicals stood out
Still, researchers found patterns. Some chemicals turned up again and again, and the study highlights three as the biggest threats to clean water among those in the study: atrazine, metolachlor and imidacloprid.
Atrazine (the second-most common farm herbicide in the country as of 2017) and metolachlor fight weeds — often on corn, soybean, sugar cane or sorghum fields. Imidacloprid scrambles the brains of insects. Farmers spray it on crops or buy seed pretreated with the stuff to protect their plants as they grow.
The results don't sit well with public health groups worried that pesticides are becoming far too common in drinking water supplies. Take atrazine.
"Atrazine has been used for a really, really long time," said Sydney Evans, a senior science analyst at the Environmental Working Group, a nonprofit research organization. "It's a hormone disruptor, which is why we're concerned about it."
The herbicide has become one of the most prevalent chemical contaminants in U.S. surface water and accumulates in groundwater, too, particularly in the Midwest.
Atrazine has been detected in samples from more than 2,000 water utilities serving 40 million people, according to the Environmental Working Group's Tap Water Database.
Some public health studies associate atrazine with human birth defects or other effects for children's development, though the findings aren't conclusive. Other research has found that it can harm frogs, fish and aquatic plants.
The European Union banned the chemical two decades ago.
The five states where atrazine has turned up in the most public water systems are all important agricultural hubs in the middle of the country: Texas, Wisconsin, Missouri, Kansas and Illinois.
The chemical's maker, Syngenta, argues that the weedkiller is vital to U.S. crop yields and benefits the environment by helping farmers lean less on tilling the soil to kill weeds. Tilling leads to its own long list of environmental impacts, including erosion that depletes topsoil and leads to other water quality problems in streams.
Pesticide pollution could hurt plants and animals
For the new study, the Geological Survey used its network of long-term water monitoring sites, collecting water samples 10 to 24 times a year at each location.
The Geological Survey scientists found 19 pesticides had topped benchmarks set by the U.S. Environmental Protection Agency.
The EPA's benchmarks indicate safety for plants, invertebrates or other creatures. If a pesticide's concentration level stays below benchmarks, it's not expected to affect these living things.
Two-thirds of the sites experienced pesticide pollution above at least one acute and one chronic benchmark. (Acute levels reflect the fact that higher pesticide exposure can cause immediate harm. Chronic levels are lower but last longer, also posing risks.)
Atrazine was the single pesticide affecting the most sites at acute levels. Imidacloprid impacted the most sites with chronic exposures.
The good news, Shoda said, is that sometimes scientists didn't find pesticides at all. The flip side is that sites struggling with pesticide mostly weren't getting better over time. The contamination tended to be on the rise.
That was the case at many stream sites in the Midwest and Mississippi River Basin.
Shoda said the findings fit with research that shows agriculture is a key driver of pesticide pollution.
"A lot of agriculture happens in this basin," she said.
For the study's authors, one lingering question is what happens to plants and animals when they're hit by multiple pesticides at once. EPA benchmarks reflect the impact of an individual chemical, but Shoda said in the field, scientists often find a "soup."
"How does this sort of soup of different pesticides affect (them)?" she said. "What's the toxicity of that?"
Aquatic ecologist Debbie Baker said pesticides are formulated to do damage to plants and insects, which carries clear implications for streams.
"They have the same effect in the water," said Baker, an associate scientist at the Kansas Biological Survey & Center for Ecological Research who was not involved in the study. "We know this and yet it's still happening."
Baker said harming one category of living thing in the water — say, aquatic plants — can have cascade effects for others.
"Just like terrestrial plants, plants in the aquatic community provide oxygen," she said, "and the base of the food chain for the other aquatic organisms."
Aquatic insects are key to the food web, too, not just as protein-packed morsels that feed fish and birds, but also as predators. A dragonfly, for example, helps control mosquito populations both while it is a water-dwelling nymph and also once it's a flying adult. Other invertebrates play their own roles, too, as plant grazers, prey or predators.
"It's really important to see the community," Baker said. "Not just an individual mayfly species or an individual plant species."
Baker would like to see more research on invertebrate communities in streams with pesticide pollution versus those without it.
"What's really happening in the long term to the macroinvertebrate community?" she said, underscoring the importance of understanding whether the "mix of the predators and the grazers and the omnivores" is changing.
A few of the central U.S. rivers with this pollution
Atrazine showed up dozens of times in the Kansas River at De Soto, Kansas, and the White River near the Indiana-Illinois border at levels above the EPA's benchmark for potential acute (immediate) harm.
Ditto for Maple Creek near Omaha, Nebraska, and the Little Arkansas River north of Wichita. These rivers also had times when the weedkillers acetochlor and metolachlor and the insecticide imidacloprid showed up in concerning concentrations.
Like atrazine, the European Union has banned acetochlor, as well as a form of metolachlor that raised concerns of poisoning mammals that feed on earthworms contaminated with it. The EU has also banned farmers from using imidacloprid on outdoor crops because it hurts bees and other pollinators.
Imidicloprid popped up in other rivers across the middle of the country above chronic or acute levels, including the Brazos River south of Houston, Texas; the Clinton River on the north side of Detroit, Michigan; Shingle Creek in Minneapolis, Minnesota; the Des Moines river near the Iowa-Missouri border; and the North Canadian River on the east side of Oklahoma City.
The Environmental Working Group says many questions remain unanswered about how specific pesticides affect people and how many people are exposed to them.
The EPA has not set a regulatory limit for the amount of imidacloprid allowed in public water supplies. Also, the federal agency does not consider the neonicotinoid insecticide a carcinogen, though chronic exposure has been shown to cause liver damage in rats.
Evans, with the Environmental Working Group, would like to see testing stepped up for imidacloprid in water supplies.
The EPA gathers data on a limited number of unregulated chemicals to understand their prevalence in drinking water, but this one has yet to make the list.
"That's concerning to me," she said, "because of the studies that are showing it in drinking water — in places where it has no business being."
Celia Llopis-Jepsen is an environment reporter for Harvest Public Media and host of the environmental podcast Up From Dust. You can follow her on Bluesky or email her at celia (at) kcur (dot) org.
Harvest Public Media is a collaboration of public media newsrooms in the Midwest and Great Plains. It reports on food systems, agriculture and rural issues.
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