This is the time of year when it's not uncommon to see big trucks barreling down highways and streets spreading road salt.
Steve Corsi, a hydrologist with the U.S. Geological Survey, says that translates into high levels of chloride concentrations for rivers like the Milwaukee in Wisconsin or 18 other streams near urban areas in Illinois, Ohio, Colorado and several other states.
"At many of the streams, concentrations have now exceeded those that are harmful to aquatic life," he says.
Corsi says that's especially true during the winter. He and other scientists analyzed chloride levels dating back to 1960 but primarily from the 1990s to 2011. The number of times they found toxic levels of chloride doubled over the two decades.
He says there's plenty of reasons for the increase. "We have lots of businesses that have parking lots and sidewalks and such, we have residents who have driveways and sidewalks and a lot of people use road salt."