Every winter, the surface of the earth in the northern United States becomes considerably more salty. The reason is, for availability, cost, and effectiveness, nothing beats salt-based deicers for keeping roadways clear of ice. But the effects of road salt on aquatic ecosystems, freshwater drinking supplies, infrastructure, and vehicles is significant. When chlorides get into groundwater, it can be very difficult to get them out. They do not biodegrade over time, and the accumulation in soils can be retained for decades. As few as 50 pounds of salt can contaminate 10,000 gallons of water. The New Hampshire Department of Environmental Services estimates that there are almost 50 chloride-impaired watersheds within the state, and it lists over 100 of the state's drinking water sources as contaminated due to chlorides. Groundwater experts suggest that the chloride problem may be much larger than we know, due to limited testing and the cumulative impact of the chemical.
Therefore, given what we know about the harmful effects of salt, it makes sense to use it sparingly. But as any homeowner who has tossed it on a sidewalk knows, it is hard to estimate the right amount to use and, if anything, we err on the side of caution, resulting in liberal applications. Municipalities have an even tougher time getting it right. A public works department must deploy dozens or hundreds of spreaders, managing them so they do not miss a road and adjusting their management approach to accommodate changing temperatures and the unique weather fluctuations of each winter event. When trucks are moving through complex road systems it can be challenging for operators to know the last time deicing material was applied to a particular surface. When in doubt, operators apply more material.