Atrazine has been in use since the late 1950s, and it has long entered groundwater and surface waters. The USGS has done a lot of work monitoring herbicides in groundwater and surface water, and atrazine is almost always the most commonly detected pesticide in the Midwest (that's their map above). Pesticides are complex organic compounds, and there are many steps needed to break them down to their constitutive molecules (primarily H₂O, CO₂, and various halides such as chloride or bromide). As the “parent” pesticide breaks down, intermediate molecules, or metabolites as they’re sometimes caused, are produced. Some of the intermediates can persist for long times, and there is generally much less known about their toxicities.

Several Illinois towns were part of the class action suit against Syngenta, including Fairfield, Flora, Greenville, Hillsboro, and Mattoon. The U.S. Environmental Protection Agency says that activated carbon filtration is effective in removing atrazine from water, so I guess that’s the technology these towns will use. The drinking water standard is 3 ppb, so we’re talking about very low amounts.