Average chloride concentrations often exceed toxic levels in many northern United States streams due to the use of salt to deice winter pavement, and the frequency of these occurrences nearly doubled in two decades. 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. The report was published today in the journal Science of the Total Environment.