In the winter, snow and ice make the road unsafe to drive, and for this reason, salt is a lifesavor. Salt lowers the freezing point of water and rids the roadways of the dangers of ice and snow. The problem with using salt, however, is that it doesn't just dissapear after the snow melts. "Salt is a natural ingredient, but what is not natural is the concentrations," said Richard Hanneman, President of the Salt Institute, based in Alexandria, Va. And evidence is growing that the salt concentration of streams, lakes and groundwater is steadily increasing. Salt levels in some places are high enough to harm roadside plants and aquatic life. Besides damaging organic matter, salt also effectively corrodes bridges and cars.