On April 5, a team of chemists and engineers at the U.S. Geological Survey (USGS) and the University of Iowa reported that they had discovered neonicotinoids, a class of pesticides, in treated drinking water. This finding marks the first time anyone has identified the chemicals in tap water. 
Gregory LeFevre, a study author and University of Iowa environmental engineer, said:
“Having these types of compounds present in water does have the potential to be concerning, but we don’t really know, at this point, what these levels might be.” 
Researchers 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’s based on a scale of parts per trillion, so the concentrations were very small – roughly equal to a single drop of water placed into 20 Olympic-size swimming pools, according to LeFevre.
However, the EPA has not defined safe levels of neonicotinoids in drinking water, partly because the pesticide class is relatively new. Most neonicotinoids were released in the 1990s and were designed to be more environmentally friendly than the competing chemicals. But neonicotinoids kill bees, and the EPA finally admitted as much in January 2016.
Even pesticide manufacturers have admitted that neonicotinoids kill bees, although they claim this is only the case when the toxins are used improperly.
Neonicotinoids attack insects’ nervous systems. Once a bee or other insect is exposed, it enters into a brief spurt of hyperactivity, but eventually becomes paralyzed and dies. 
In humans, acute exposure to neonicotinoids resulted in “low rates of adverse health effects,” according to a 2015 report by environmental health scientists at George Washington University and the National Institutes of Health (NIH). Chronic, low-level exposure had “suggestive but methodically weak findings.” One Japanese study linked the pesticides with memory loss.
In 2015, USGS scientists took samples from nine sites in Nebraska and Iowa and found varying levels of neonicotinoids in every single one, and some of the sites registered unhealthy levels of the chemicals.
In the latest study, published in Environmental Science and Technology Letters, neonicotinoids were found to be passing through water treatment plants and entering the water supply. Water treatment plants are not designed or intended to filter out pesticides, but that’s not to say that no pesticides are filtered out. 
Samples from the University of Iowa treatment plant barely removed any of the three main neonicotinoid chemicals: clothianidin, imidacloprid, and thiamethoxam. However, water taken from the Iowa City treatment facility removed 100%, 94%, and 85% of those pesticides, respectively. 
If it turns out that the levels uncovered in the latest research are a problem (is there really any “safe” level of pesticides in our drinking water?), then either water treatment facilities will have to be modified to filter them out, or farmers will have to find a way to fight pests minus the pesticides.
 BBC News