The U.S. Geological Survey has studied the chloride concentrations of urban streams since 1960. Levels have become increasingly toxic. a trend attributed to public and private winter deicing operations. According to a recent report from the U.S. Geological Survey,
Chloride levels increased substantially in 84 percent of urban streams analyzed, according to a U.S. Geological Survey study that began as early as 1960 at some sites and ended as late as 2011. Levels were highest during the winter, but increased during all seasons over time at the northern sites, including near Milwaukee, Wisconsin; Chicago, Illinois; Denver, Colorado; and other metropolitan areas. ... Road deicing by cities, counties and state agencies accounts for a significant portion of salt applications, but salt is also used by many public and private organizations and individuals to deice parking lots, walkways and driveways. All of these sources are likely to contribute to these increasing chloride trends.