Here you can find the NRDC report on atrazine in American drinking water. Atrazine is a weed killer widely used in agricultural areas of the U.S. It has been banned in the European Union because of health hazards.
Unlike the various federal agencies that have been fishing around this issue, NRDC pulled together various sources of data on atrazine in drinking water, "Approximately 75 percent of stream water and about 40 percent of all groundwater samples from agricultural areas tested in an extensive U.S. Geological Survey study contained atrazine. NRDC found that the U.S. EPA's inadequate monitoring systems and weak regulations have compounded the problem, allowing levels of atrazine in watersheds and drinking water to peak at extremely high concentrations."
EPA limits on atrazine depend on annual average concentrations. As any farmer or gardener knows you use large amounts of a pesticide or weed killer occasionally so the concentrations in soil or ground water are going to be subject to intense variation depending on when the chemicals are applied. Nobody sprays for weeds when there's snow on the ground.
More from NRDC, "In watersheds the presence of high one-time peak concentrations of atrazine is just as important as the concentration averaged over a few months or a year. Under the Safe Drinking Water Act (SDWA), the EPA has determined that no more than 3 parts per billion (ppb) of atrazine (as a running annual average) may be present in drinking water. Because it is an average, high maximum concentrations of atrazine contamination during spring runoff may not result in a violation of the standard if the remainder of the year has low or no atrazine contamination.
"The toxicity associated with atrazine has been documented extensively. The adverse reproductive effects of atrazine have been seen in amphibians, mammals, and humans-even at low levels of exposure."
NRDC indicates that "treated" water may still contain fluctuating concentrations of atrazine. When I first blogged about the atrazine pollution of drinking water I suggested that digital tech and proper monitoring could easily give water drinkers and water companies real-time data on atrazkine concentrations. That system would require at least as much political will as installing traffic signals.