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A new study finds that the freshwater we rely on for drinking water and industry is getting saltier. The study, published in the Proceedings of the National Academy of Sciences, pulls data from 232 US Geological Survey monitoring sites across the country that have been collected over decades. 

The overall trend is that freshwater is increasing in salinity and is more alkaline, especially in the east and Midwest. That means the majority of U.S. waters are saltier and have a higher pH. Study authors call it the “freshwater salinization syndrome,” and say there are multiple reasons behind it.

LISTEN: “America’s Freshwater is Getting Saltier. And That’s Not Good.”

Ryan Utz is an assistant professor of sustainability at Chatham University and one of the study authors. He says agricultural lime used on farm fields eventually dissolves into the water that drains into nearby streams and rivers. And Utz says we’re still seeing the effects of acid rain from the 70s and 80s, which caused minerals to leach out of rocks and soil, changing the chemical composition of water to be more alkaline. Climate change could also be a contributing factor, because warmer water can speed up the dissolution of chemical compounds.

This map shows changes in the salt content of fresh water in rivers and streams across the United States over the past half-century. Warmer colors indicate increasing salinity, while cooler colors indicate decreasing salinity. The black dots represent the 232 U.S. Geological Survey monitoring sites that provided the data for a new study. (Ryan Utz / Chatham University)

And this time of year, another obvious contributor of salinity is the rock salt used to melt icy roads and sidewalks. About 23 million metric tons of rock salt is applied to roads in the United States each year. “Rock salt isn’t going to disappear and vanish into the atmosphere,” Utz explains. “It ends up in our rivers and streams, where there could be ecological consequences.”

Utz says there could be a mix of positive and negative impacts from this trend. One negative is that as more dissolved chemical compounds related to alkalinization get in the water, they can end up becoming a solid again. This can cause deposits to build up in municipal and industrial pipes, becoming a real challenge to the country’s aging water infrastructure. Also power plants that use water as a cooling substance could be affected. “The more stuff that is dissolved in the water from the rivers, the harder it is for that water to be released to steam,” explains Utz. “You need a higher temperature or you need to remove those chemical compounds before you use it for a cooling purpose.”

Increased levels of salinity in the water also have been shown to increase rates of hypertension in humans. A particularly brutal winter in 2014 caused the state of New Jersey to use 80% more road salt than usual. And when the ice began to melt, some of the state’s water companies issued a warning to residents that the water was too salty for people on low-sodium diets.

Utz says researchers are unsure at this stage how aquatic organisms will be affected. “Stream ecosystems are probably one of the most physically dynamic ecosystems on earth because they endure floods and droughts and may do so even within the same year.” Utz says some organisms may really benefit from higher levels of alkalinity, like mussels and clams who build shells out of calcium carbonate. And the elevated levels of salt in freshwater could potentially help the oceans, which are becoming more acidic as a result of increased levels of carbon dioxide in the atmosphere. “If rivers that feed into marine waters like the Chesapeake Bay are becoming more alkaline, that might buffer the effects of the increasing acidity that we’re seeing in marine waters,” Utz says.

Utz says the research is still pretty new, and they’re not really sure how rising rates of salinity and alkalinity will affect specific rivers and streams. That’s for other scientists to pick up. But he says the study might at least prompt a discussion about how much and how often salt is applied to roads and sidewalks during the winter months.