A new study shows U.S. rivers and streams have developed higher salt concentrations and become more alkaline in recent decades, blamed in part on the application of road salt.

Of 232 U.S. Geological Survey river and stream monitoring sites across the country, 34 percent showed an increasing trend in sodium and 66 percent had become more alkaline, according to the study published this month in the Proceedings of the National Academy of Sciences.

The study links increased salt concentration with alkalinization, which can cause ammonia toxicity and algae blooms.

“The syndrome is caused by salt pollution (e.g., road de-icers, irrigation runoff, sewage, potash), accelerated weathering and soil cation exchange, mining and resource extraction, and the presence of easily weathered minerals used in agriculture (lime) and urbanization (concrete),” according to the study.

Salinization and alkalinization have increased most rapidly in urban parts of the East Coast and Midwest, the study notes.

Iowa’s statewide average for chloride concentration in rivers and streams has fluctuated between 17 milligrams per liter and 25 mg/L since 2001, with 2017 on the low end of the spectrum, data provided by the Iowa Department of Natural Resources show.

Historical data from the Iowa Geological Survey show average chloride concentrations at a handful of sites on Iowa rivers in the 1940s and 1950s were less than 10 mg/L.

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The statewide water quality standard for chloride is 389 mg/L for chronic (long-term or multiple measures) and 629 mg/L for acute (one-time).

“I don’t think there is a major cause for alarm here, but clearly stream chloride levels are far higher than they were 60 to 70 to 100 years ago,” said Chris Jones, a research engineer for the University of Iowa’s IIHR-Hydroscience & Engineering.

As base salt concentration rises, spikes caused by a manure spill, for example, will be closer to thresholds that could kill wildlife, Jones said.

Eastern Iowa streets superintendents are mindful of the environmental effects of rock salt, applied to melt ice on streets and highways.

“I am aware of some of those levels going up because of our use of rock salt,” said Brock Holub, Iowa City streets superintendent.

Iowa City hires an Illinois company to treat salt reserves with an agricultural product that keeps the salt from clumping and reduces the amount of salt needed.

“Our object is not to melt snow,” Holub said. “It’s to break the bond between the pavement and the snow so we can move it (snow) sideways. It’s more cost effective and more environmentally friendly.”

Cedar Rapids usually uses 8,000 to 9,000 tons of salt a year, according to Mike Duffy, the city’s public works streets operations manager.

“Iowa’s winters require that we use a variety of tools at our disposal for public safety,” Duffy wrote in an email.

Cedar Rapids applies up to 16,000 gallons of salt brine and beet juice to streets before a snow event to keep ice and snow from sticking. Duffy and his team work with plow operators to use only as much salt as necessary. Trucks are designed to stop spreading sand or salt when the vehicle stops, he said.

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