America’s freshwater is changing. According to an analysis of 232 sites in streams and rivers over several decades, it has become saltier in some places, and in almost all places, it’s becoming less acidic and more alkaline. These two processes may be related, and researchers have dubbed it “freshwater salinization syndrome.”

There are many likely causes: road salt (particularly in the Midwest and Northeast, where increased freshwater salinity is concentrated), sewage, mining, fertilizers used in agriculture. These continental-scale changes will affect wildlife unused to salty, more alkaline water. It will also likely come around to affect humans and the water we drink.

It’s not just that the water will taste a little saltier—though after a snowy winter in 2014, New Jersey did have to warn residents that its water was too salty for people on sodium-restricted diets. It’s that America’s aging water infrastructure is not built to handle saltier water. Just as road salts rust the metal of your car, dissolved salt in water reacts with the metal in water pipes.

For Sujay Kaushal, a geologist at the University of Maryland who led the recent analysis of 232 sites, this is personal concern. “This affected my house directly,” he says. In 2015, the drinking water in parts of Maryland, where he lives, started to turn brown. The water utility explained the color likely came from manganese, which had been carried out of old pipes by water full of dissolved road salt.

A storm drain corroded by road salt (PNAS)

Road salt may have contributed to the lead crisis in Flint, Michigan, too. When the city began drawing water from the Flint River rather than relying on Detroit’s tap water to save money, it also began sending water with eight times as much chloride through its old lead pipes. Chloride can come from road salt (aka sodium chloride), and it corrodes the pipes.

“When we’re throwing down road salt, we might be thinking about the fact that we’re putting salt into the water, but we’re not thinking that it may also mobilize lead,” says Hilary Dugan, a limnologist at the University of Wisconsin at Madison, who was not involved in the study. Dugan has studied lakes in North America, which she also found to be increasing in salinity.

In the wild, many aquatic organisms will have difficulty adjusting to saltier waters. When Charles Hawkins, an ecologist at Utah State University, studied streams in Nevada, he expected to find toxic heavy metals to correlate with losses in biodiversity. Instead, he found increased salinity to be the big factor. “Salinization of stream systems and aquatic systems in general is just starting to emerge as a really big problem that has gone unrecognized for decades,” he says.

Kaushal’s study did not find increased salinity in the West generally, though the U.S. Geological Survey data he relied on was quite limited for Nevada. Still, the overall pattern in the West is somewhat surprising, as water salinity is a problem traditionally associated with dry regions. Extensive water management in the West—with a focus on salt in some cases—may be a mitigating factor. Hawkins suspects the smaller streams he has studied may also be more sensitive to salt sources than larger rivers. This analysis of 232 sites at rivers and streams in the United States is the most extensive study of its type, but there are still many gaps in the data and unanswered questions.

Another big question is why rivers and streams are becoming more alkaline across the whole continent. Kaushal and his team found that alkalization is happening across 90 percent of the drainage area of the continental United States. Some salts—though notably not the sodium chloride in road salt—make water more alkaline when they dissolve. Other salt sources like mining and agriculture may be the driver behind more alkaline waters. Kaushal thinks that acid rain may, paradoxically, make freshwater less acidic. Acid rain dissolves rocks, which can release an alkaline salt called calcium carbonate.

This all adds up to a complicated picture, but one in which humans are major players. When I spoke to Kaushal late last week, cities up and down the East Coast were salting their roads to prepare for the bomb cyclone. “What happens in winter is a very exciting time from the perspective of stream chemistry,” he says. But “it is also a scary time.”