Excessive application of salt to icy roads
Common salt, or sodium chloride, lowers the freezing point of water and so is an effective agent in melting ice and snow. Salt is relatively inexpensive. It is common practice in cold countries to spread salt on highways and walkways to avoid the formation of ice. When excess salt washes off the roads, it can poison roadside vegetation or raise salt concentrations in streams and reserves of underground water. It also accelerates the deterioration of concrete and metal.