If you live in a snowy climate, you probably take a generally positive view of the trucks that litter the roads, as driving conditions tend to be safer afterwards. The phrase “salt the earth,” on the other hand, has a decidedly crueler antiquity connotation—destroying cropland in a way that left no food to grow.
Although no ancient civilization probably ever achieved the logistical feat of on purpose salting the land of a conquered people, our moderns may be doing it unintentionally. Those friendly snowplows (and your sidewalk shoveling neighbors) are spreading an amazing amount of salt, and it needs to go somewhere once it melts.
Road salt became common in the 1940s and the amount used has increased over time. The US deposits about 18 million tons of salt each year. Road shoulders along highways naturally get more than their fair share of salt, but salt also drains (sometimes via storm drains) into streams and lakes where it can accumulate. That makes road salt a common target in local efforts to protect bodies of water. While this has sometimes been studied on a local scale, there hasn’t been much analysis of the big picture. A new study led by Hilary Dugan of the University of Wisconsin-Madison seeks to fill that gap by estimating how widespread salt pollution is in North America.
The researchers used data from 371 lakes covering at least four hectares with chloride measurements going back at least 10 years. That’s not a purely random sample, but it’s the best way to spot trends. Most are in the Midwestern and Northeastern US, with a handful from southern Canada and some scattered throughout the rest of the US. One third of the 371 lakes showed a significant upward trend in salt concentration.
If we focus on the 284 lakes in the broad Great Lakes region, that number jumps to 44 percent. For these lakes, the researchers looked for the characteristics that could predict whether they showed an upward trend. Climate had to be included in this analysis, because wet years could, for example, dilute the chloride concentration of a lake.
The dominant factors appeared to be the area of roads and concrete within 500 meters of the lake’s edge. Data on the dosage of road salt was too spotty to be reliable, but there was not really a clear connection. Any more with one decreasing However, the chloride trend had less than one percent concrete cover. But even one percent coverage was enough to put some lakes in the “increasing chloride” category — and 70 percent of lakes with more than one percent coverage fall into that category.
Looking at nearly 150,000 lakes in the continental United States, about 28 percent meet that criterion of more than one percent concrete cover. Assuming that the small subset the researchers analyzed was representative, that would mean that nearly 8,000 lakes in the Great Lakes region would collect human-dispersed chloride salts.
Rising salt concentrations disrupt ecosystems by shifting chemical balances and driving out sensitive species. In most cases, this isn’t the only pollutant humans add to a lake, so those species are likely under multiple stressors. If you extrapolate the trends the researchers discovered to the year 2050, 14 of the 284 lakes analyzed will exceed levels the EPA says are toxic to freshwater organisms — which is also about the point at which water starts to taste salty. All of that may make you look at the salt truck a little differently.
Proceedings of the National Academy of Sciences2017. DOI: 10.1073/pnas.1620211114 (About DOIs).