Go Easy on the Salt

Did road salt help cause the Flint water crisis?

Photo: Wisconsin Department of Natural ResourcesA giant pile of salt for deicing roads in Milwaukee, Wisconsin

Nothing can excuse the inadequate treatment, insufficient monitoring, and overall incompetence that caused the Flint drinking water crisis. But it’s still worth asking why Flint River water is so corrosive in the first place. What made the water so efficient at stripping lead from service pipes?

Early attention has fallen on the overuse of road salt in Michigan. Nationwide, Americans dump about 22 million tons of salt on our roadways each year to melt snow and ice, and not without justification: De-icers reduce accidents on major highways by 93 percent, according to a motorists’ advocacy group. But road salt has negative consequences as well. Here’s some background on how road salt can affect drinking water and ecosystems.

First, how might road salt have contributed to the Flint crisis?

The cheapest and most common way to de-ice a road is by applying ordinary salt. Road salt is 40 percent sodium ions and 60 percent chloride ions. When the snow and ice melt, the salt applied to roads eventually washes its way into both groundwater and nearby rivers and streams, where the ions dissociate from each other.

Water from the Flint River has an unusually high concentration of chloride ions—eight times higher than the tap water in Detroit, which supplied Flint with drinking water until April 2014, when the city made the switch. According to the Virginia Tech water experts who helped blow the lid off the public health scandal, those high chloride levels are likely responsible for the corrosiveness of Flint River water. Without an aggressive corrosion-inhibiting treatment, like the one Detroit uses, the water readily dissolved lead from the city’s pipes.

Are high chloride levels a problem elsewhere?

Yes, and the problem is getting worse. Average chloride concentrations in northern rivers and streams doubled between 1990 and 2011, according to a 2015 study led by Steven Corsi of the U.S. Geological Service. The increase is not entirely due to road salt—some fertilizers as well as the water softeners used in sewage treatment plants contain chlorides—but road salt is the primary suspect. And the problem extends beyond wintertime. Corsi found elevated chloride levels year-round at most sample sites, suggesting that chloride is accumulating in groundwater and slowly and continuously moving into rivers.

It’s not yet clear whether road salt is causing clinically significant spikes in lead exposure in other cities, but the potential is certainly there. The average concentration of chloride in Flint water was listed as 92 milligrams per liter in a 2014 report. That was high enough to strip the lead out of pipes, and Corsi measured levels that high, or higher, in several other localities.

Photo: Michael Pereckas/FlickrWorkers manually spread salt from a salt truck

“In many urban streams, the average annual concentration, meaning that we smoothed out seasonal peaks and valleys, was approximately 600 milligrams per liter,” says Corsi. In winter, when road salt applications peak, Corsi measured concentrations above 1,000 mg/L, sometimes even above 10,000.

Municipalities like Detroit manage elevated chloride concentrations using corrosion inhibitors like orthophosphates, which create a barrier between lead pipes and corrosive water. But there are limits to their effectiveness at extremely high chloride concentrations.

Is road salt bad for ecosystems, too?

Yes. Some species are more salt tolerant than others, so increased salt concentrations limit biodiversity and alter the makeup of an ecosystem. The loss of native plants can change the diet of animals throughout the food chain, and plants like reed canary grass and narrow-leaved cattail, considered invasive in the United States, tend to thrive in salt-laden landscapes.

Road salt can also have more direct impacts on wildlife: Birds, deer, and other animals like to eat potentially toxic salt crystals. For the sake of wildlife protection, the EPA suggests keeping continuous chloride concentrations below 86 mg/L. The highest levels that Corsi’s team observed are not only well above these limits, they would be immediately toxic to many aquatic species.

So what’s the solution?

Use less road salt. That sounds glib, but as of now, there aren’t better options. Magnesium chloride and calcium chloride, which are popular substitutes for ordinary road salt, still contain chlorides. Organic de-icing salts, such as calcium magnesium acetate, feed bacteria and can cause bacterial films to grow on the surfaces of streams. The bacteria also consume oxygen, dropping dissolved oxygen levels and contributing to aquatic dead zones. Some municipalities are experimenting with early salting to reduce total salt required, or mixing the salt with water, but there is insufficient data at this point to suggest that these approaches will significantly lessen the risks.

Some legislators worry that placing limits on road salt could expose the government or private entities to liability for automobile accidents, and while many legislatures are considering the issue, few states have effective salt standards. Meanwhile, the only practical limitations are the expense of the salt and the trucks that deliver it—and, of course, the amount of salt readily available. When snow has fallen in heavy amounts in the past, some states have gotten creative with cheese brine, pickle and beet juice, and wastewater from oil and gas wells—again, all imperfect substitutes.


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