As the bee debate raged, scientists studying the country’s waterways started to detect neonicotinoid pollutants. In 2015, the U.S. Geological Survey collected water samples from streams throughout the United States and discovered neonicotinoids in more than half of the samples.

And on Wednesday, a team of chemists and engineers at the USGS and University of Iowa reported that they found neonicotinoids in treated drinking water. It marks the first time that anyone has identified this class of pesticide in tap water, the researchers write in Environmental Science & Technology Letters.

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Gregory LeFevre, a study author and U of Iowa environmental engineer, told The Washington Post that the find was important but not immediate cause for alarm.

“Having these types of compounds present in water does have the potential to be concerning,” he said, “but we don’t really know, at this point, what these levels might be.”

If the dose makes the poison, the doses of insect neurotoxin reported in the new study were quite small. The scientists collected samples last year from taps in Iowa City as well as on the university campus and found neonicotinoid concentrations ranging from 0.24 to 57.3 nanograms per liter — that is, on a scale of parts per trillion. “Parts per trillion is a really, really small concentration,” LeFevre said, roughly equal to a single drop of water plopped into 20 Olympic-size swimming pools.

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The Environmental Protection Agency has not defined safe levels of neonicotinoids in drinking water, in part because the chemicals are relative newcomers to the pesticide pantheon. “There is no EPA standard for drinking water,” LeFevre said.

The pesticides, most of which were released in the 1990s, were designed to be more environmentally friendly than other chemicals on the market. The compounds work their way into plant tissue rather than just coating the leaves and stems, requiring fewer sprays. And though the pesticides wreak havoc on insect nervous systems, neonicotinoids do not easily cross from a mammal’s bloodstream into a mammalian brain.

In 2015, environmental health scientists at George Washington University and the National Institutes of Health published a review of human health risks from neonic pesticide exposure. Acute exposure — to high concentrations over a brief period — resulted in “low rates of adverse health effects.” Reports of chronic, low-level exposure had “suggestive but methodologically weak findings,” with a Japanese study associating neonicotinoids with memory loss.

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Melissa Perry, a public health researcher at George Washington University who was involved in that review, said via email that the new study “provides further evidence that neonicotinoid pesticides are present in our daily environments. From a public health standpoint, this issue clearly needs better attention.”

The Iowa scientists tracked neonicotinoid concentrations in the local drinking supply from May to July, the seven-week span after the region’s farmers planted maize and soy crops. Every sample contained three types of neonicotinoids: clothianidin, imidacloprid and thiamethoxam.

“Everything in the watershed is connected,” LeFevre said. “This is one of many types of trace pollutants that might be present in rivers.” (The USGS released an interactive map of the nation’s water quality on Tuesday, where those inclined can track trends in common pollutants.)

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Most water filtration systems target clay, dirt or other particles, as well as pathogenic contaminants like bacteria. They’re not designed to eliminate chemical pesticides — and the properties of neonicotinoids make these compounds unusually challenging to remove. Other types of pesticides stick to soil particles, which are then filtered out. But neonicotinoids can slip past sand filters because they are polar chemicals. “They dissolve very readily in water,” LeFevre said. He invoked a chemistry aphorism: “Like dissolves like.”

This proved out as the research team looked at how effectively the university’s sand filtration system and Iowa City’s different water treatment technique blocked the three neonicotinoids studied. The university’s sand filter removed 1 percent of the clothianidin, 8 percent of imidacloprid and 44 percent of thiamethoxam. By contrast, the city’s activated carbon filter blocked 100 percent of clothianidin, 94 percent of imidacloprid and 85 percent of thiamethoxam. That finding was “quite a pleasant surprise,” LeFevre said. “It’s definitely not all bad news.”

The activated carbon filters are relatively economical, he said. In fact, after the research was completed, the university installed a similar system on its campus.

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Given the study’s small sample size and geographical span, Perry said more comprehensive assessments of water supplies are needed “to determine how ubiquitous neonics are in water supplies in other parts of the country.” The chance of that happening is unclear. “There is currently no national effort to measure to what extent neonicotinoids are making it into our bodies, be it through water or food,” she noted.