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.