If you live in snowy climes, you probably have a generally positive attitude toward the trucks that salt the roads since driving conditions are typically safer afterward. The phrase “salting the Earth,” on the other hand, has a decidedly crueler connotation from antiquity—destroying cropland in a way that ensured food could no longer be grown.

Although no ancient civilization probably ever pulled off the logistical feat of intentionally salting a conquered people’s lands, our modern ones may be doing it unintentionally. Those friendly snowplows (and your sidewalk-shoveling neighbors) are spreading an astounding volume of salt, and it has to go somewhere once it melts.

Road salt became common in the 1940s, and the amount used has increased over time. The US puts down around 18 million tons of salt each year. Roadsides along highways obviously get dosed with more than their fair share of salt, but salt also runs off (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. Although this has sometimes been studied on the local scale, there hasn’t been much big-picture analysis. A new study led by the University of Wisconsin-Madison’s Hilary Dugan works to fill in that gap by estimating how widespread salt contamination is in North America.

The researchers used data from 371 lakes that covered at least four hectares and had chloride measurements going back at least 10 years. That’s not a purely random sampling, but it’s the best way to find trends. Most are in the US Midwest and Northeast, with a handful from southern Canada and some spread over the rest of the US. One-third of the 371 lakes showed a significant upward trend in salt concentration.

Focusing on the 284 lakes in the broad region around the Great Lakes, that number rises to 44 percent. For these lakes, the researchers looked for the characteristics that could predict whether one showed an upward trend. Climate had to be included in this analysis, as wet years could dilute the chloride concentration of a lake, for example.

The dominant factors turned out to be the area covered by roads and concrete within 500 meters of the lake’s edge. Data on road salt application rates was too spotty to be reliable, but there wasn’t much of an obvious relationship there. Every lake with a declining chloride trend had less than one percent concrete cover, though. But even one percent cover was enough to put some lakes in the “increasing chloride” category—and 70 percent of the lakes with more than one percent cover fall in that category.

Looking at almost 150,000 lakes in the continental United States, about 28 percent meet that more-than-one-percent-concrete-cover criterion. If you assume that the small subset the researchers analyzed was representative, that would mean nearly 8,000 lakes in the Great Lakes region would be accumulating chloride salt spread by humans.

Rising salt concentrations mess up ecosystems by shifting chemical balances and driving out sensitive species. In most cases, this isn’t the only pollutant humans are adding to a lake, so those species are probably under multiple stressors. If you extrapolate the trends the researchers found out to the year 2050, 14 of the 284 lakes that were analyzed will exceed the level that the EPA says is toxic for freshwater organisms—which is also about the point that water starts to taste salty. All that may make you look at the salt truck a little differently.

Proceedings of the National Academy of Sciences, 2017. DOI: 10.1073/pnas.1620211114 (About DOIs).