American rivers are under a-salt.

A 50-year analysis of 232 freshwater sites in the US indicates that rivers and streams are getting saltier, according to a study published in the Proceedings of the National Academy of Sciences.  

Coining the term “Freshwater Salinization Syndrome”, researchers found that 37 percent of US drainage areas have increased in salinity. Alkalization, on the other hand, increased by 90 percent. The study is the first to document a link between the two.

The change is attributed to a “suite of effects", and its cause varies from region to region.

In the snowy mid-Atlantic and New England, road salt applied in winter runs off into groundwater systems and streams. Meanwhile, in the agriculturally-rich Midwest, the primary culprit is fertilizer leaching. Other areas lay blame on mining waste.

"Many people assume that when you apply salt to roads and other surfaces it just gets washed away and disappears. But salt accumulates in soils and groundwater and takes decades to get flushed out," said lead author Sujay Kaushal, a biogeochemist at the University of Maryland, in a statement.

The study looked at many of the country’s major rivers including the Mississippi, Hudson, and Potomac – all of which supply drinking water to nearby cities.

Salt and pH are fundamental aspects of water chemistry. Salty, alkaline waters can create big problems for natural ecosystems, drinking water supplies, and urban infrastructure.

The 2014 water crisis in Flint, Michigan, occurred when the city switched its water source to the Flint River, which has a high salt load. Paired with chemical treatments, the water corroded pipe infrastructure and leached lead into the city’s drinking source.  

It’s not just rivers that are getting saltier. A 2017 study of 371 freshwater lakes found that thousands of lakes in the Midwest and Northeast are also at risk of getting saltier.

When fresh water becomes too salty it cannot support most freshwater fish and aquatic species, as is the case with the naturally formed Salt Lake in Utah. This could mean big problems in water systems that support biologically diverse ecosystems.

Most of the research focused on sodium chloride, or table salt, which is the most dominant chemical in road deicers. Scientists say combining certain salts with common minerals like sodium, calcium, magnesium, and potassium could create "toxic cocktails" that create more damage than any one salt alone.

Researchers say this could have implications for freshwater management and salt regulation across North America. Salt regulations have already been passed in the Southwest. Here, researchers say changes in land and water use paired with government efforts have reduced salt inputs, decreasing salinity in a region with historically high levels of salt. 

According to the study, better regulation of road salt, fertilizers, and other salty compounds is essential.