Human activities are exposing US rivers and streams to a cocktail of salts, with consequences for infrastructure and drinking water supplies. So reports a new study in the Proceedings of the National Academy of Sciences that is the first to assess the combined, long-term changes in freshwater salinity and alkalization across the country.

Using five decades of streamwater data from 232 U.S. Geological Survey monitoring sites, researchers found 37 percent of the drainage area of the contiguous US experienced a significant increase in salinity, with a concurrent increase in alkalization of 90 percent.

Salt ions, damaging in their own right, are driving up the pH of freshwater, making it more alkaline. Both of these variables shape water quality and can influence the stability of pipes and other water delivery infrastructure. For example, when Flint, Michigan switched its primary water source to the Flint River in 2014, the river's high salt load caused lead to leach from water pipes, creating that city's well-documented water crisis.

Co-author Gene E. Likens, president emeritus of the Cary Institute of Ecosystem Studies and a Distinguished Research Professor at the University of Connecticut, Storrs explains, "Long-term monitoring is vital to understanding the pressures facing our nation's freshwaters from increased salt loading, and for guiding strategies that protect drinking water. Road salt, irrigation runoff, and sewage are obvious culprits. But so is acid rain, which can release alkaline salts that compromise the chemical integrity of freshwaters."

Sharp chemical changes were documented in many of the country's major waterways, including the Hudson, Potomac, Neuse, Mississippi, and Chattahoochee Rivers. Many of these rivers supply drinking water for nearby cities and towns, including some of the most densely populated urban centers along the Eastern Seaboard.

Lead author Sujay Kaushal from the University of Maryland, notes, "We created the name 'Freshwater Salinization Syndrome' because we realized it's a suite of effects on water quality, with many different salt ions linked together. We didn't know that before."

Sources of increased salt in waterways vary regionally. In the Northeast, sodium chloride used to maintain roads in winter is a primary culprit. In the Midwest, fertilizers -- particularly those with high potassium content -- are a major contributor. In other regions, mining waste and weathering of concrete, rocks, and soils releases calcium and magnesium salts into nearby waterways.

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Kaushal notes, "Many people assume that when you apply salt to the landscape it just gets washed away and disappears. But salt accumulates in soils and groundwater and takes decades to get flushed out."

The analysis, which has implications for freshwater management and salt regulation strategies, is the first to document a link between increased salinization and alkalization at the continental scale. It is also an important reminder that when different salt compounds combine, their harmful effects can amplify.

"Until now, we didn't fully appreciate the role that different salts play in altering the pH of streams and rivers of our country," Likens said. "Salt content and pH are fundamental aspects of water chemistry, so these are major changes to the properties of freshwater."

"This research demonstrates the value of long-term data in identifying potential threats to valuable freshwater resources," says John Schade, a National Science Foundation Long-Term Ecological Research program director. "Without such long-term efforts, such widespread and significant degradation of water quality by human activities would remain unknown. Now that we know this is happening, we can begin to unravel the causes and develop strategies to mitigate potential effects on public health."

Some strategies for managing road salt pollution already exist, as outlined in the Cary Institute report Road Salt: Moving Toward the Solution. They include pre-wetting salt to allow it to stick to roads, using brine to prevent ice from forming on road surfaces, reducing the salt content of sand, and using pavement sensors and weather information systems to guide salt application.

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"Also, not all salts are created equally in terms of their ability to melt ice at certain temperatures," Kaushal added. "Choosing the right salt compounds for the right conditions can help melt snow and ice more efficiently with less salt input, which would go a long way toward solving the problem."

The team also notes that urban development strategies -- primarily building further from waterways and designing more effective stormwater drainage systems -- can reduce the amount of salt washed away from weathered concrete. They also recommend monitoring and replacing aging drinking water pipes impacted by corrosion, scaling, or the buildup of mineral deposits and microbial films.

Likens explains, "In the US, many rely on a patchwork of aging pipes to bring drinking water into their homes. Lead in pipes, solder, and joints is not uncommon, especially in our older cities. These pipes are vulnerable to saltier, more alkaline water, which can release toxic metals, such as lead, and other contaminants."

"The trends we are seeing in the data all suggest that we need to consider the issue of salt pollution and begin to take it seriously," Kaushal said. "The Environmental Protection Agency does not regulate salts as primary contaminants in drinking water at the federal level, and there is inconsistency in managing salt pollution at the local level. These factors are something communities need to address to provide safe water for future generations."