Salts that de-ice roads, parking lots and sidewalks keep people safe in winter. But new research shows they are contributing to a sharp and widely rising problem across the U.S. At least a third of the rivers and streams in the country have gotten saltier in the past 25 years. And by 2100, more than half of them may contain at least 50 percent more salt than they used to. Increasing salinity will not just affect freshwater plants and animals but human lives as well—notably, by affecting drinking water.
Sujay Kaushal, a biogeochemist at the University of Maryland, College Park, recounts an experience he had when visiting relatives in New Jersey. When getting a drink from the tap, “I saw a white film on the glass.” After trying to scrub it off, he found, “it turned out to be a thin layer of salt crusting the glass.”
When Kaushal, who studies how salt invades freshwater sources, sampled the local water supply he found not just an elevated level of the sodium chloride, widely used in winter to de-ice outdoor surfaces, but plenty of other salts such as sodium bicarbonate and magnesium chloride. He also found similar concentrations of these chemicals in most rivers along the east coast, including the Potomac, which provides drinking water for Washington, D.C. Where did all of it come from?
De-icing salts, Kaushal determined, are part of the problem, slowly corroding our infrastructure. Estimates put the cost of repairs at about $1,000 per ton of de-icing salt imposed on the environment. But he also found a link to acid rain, caused by the air pollution from burning fossil fuels in power plants and cars. “Decades of acid rain have dissolved not just portions of rock and soils but buildings and roads as well—all of which have added various salts to the water,” he says. Although the acidity of the rain is decreasing, it is still present. Meantime the amount of concrete and asphalt in the world have continued to expand.
Salts can free up other pollutants, too. In his own house near Washington, D.C., Kaushal once had black water coming from the tap. “The salts in the water were leaching manganese—a neurotoxin—from the old pipes in the neighborhood,” he says.
A similar issue recently arose in Flint, Mich., where the decision to start drawing drinking water from a saltier local river mobilized lead from pipes into the water supply. Nationwide, salts are crusting the insides of home boilers and the cooling tanks of power plants. They are also coating the land where crops grow. And they are stressing plants and animals in freshwater ecosystems, in some cases until they disappear.
Land Use Is a Primary Culprit
In January 2018 Kaushal and his colleagues published the nationwide study that showed at least a third of U.S. streams and rivers have gotten saltier over 25 years. On December 3 a modeling analysis by freshwater scientist John Olson at California State University, Monterey Bay, confirmed these findings, which indicated the future looks briny, as well.
If salt use continues at the current rate, Olson’s group showed, salinization levels would likely rise at least 50 percent in half of U.S. streams by 2100. This could be a problem for drinking water, and it also would double the number of streams that are too salty for irrigation—from 3 to 6 percent. Effects of climate change were included in the study.
How people use the land is another important factor. “Today, the saltiest streams are in the northern Great Plains,” Olson says. “Salinity is naturally high, and mining and oil and gas extraction are releasing more salt by exposing new rock and pumping out saline groundwater.” In those places, he adds, it is not unusual to find streams that are about half as salty as ocean water.
Other hotspots include the highly urbanized Northeast, where Kaushal works, and the agricultural Midwest, where irrigation flushes salts from the surface into the streams. The largest predicted increases are in the arid Southwest, however. The combination of expanding agriculture and reduced rainfall there would require careful irrigation management, Olson says. In the Colorado River Basin, where several such projects are ongoing, the economic cost of salinization is estimated at $300 million per year, including damage totaling $176 million to crops and $81 million to households. In California salty water is costing the agricultural sector billions of dollars in yield losses annually.
Of course Olson is concerned with salt’s effects on people. But he is also worried about ecosystems. One experiment in which he tracked the survival and growth of a dozen freshwater species in two Nevada streams of different salinity levels taught him some animals are very sensitive to the levels. “Because their body tissues have a higher salt concentration than the water around them, freshwater animals are adapted to pump out the water that keeps rushing in while trying to hold on to the salts,” Olson notes. But when the salt levels suddenly go up, the animals may get an overdose, adds entomologist John Jackson, who was not involved in Olson’s study.
The Stroud Water Research Center in Pennsylvania, where Jackson is based, has a long tradition of testing water quality impacts on mayfly larvae. According to Jackson’s latest research, as the salt level approaches about a tenth that of seawater, which is not unheard of in some streams, at least three of four species tested are likely to die. He thinks some of the insects’ physiological mechanisms fail. The larvae of dozens of mayfly species populate healthy streams all year long and provide food for many animals, including fish. “Beyond that,” Jackson says, “they are the canary in the coal mine, warning us that other insects may also be at risk.”
Insects are not the only life-forms that will suffer, adds ecologist William Hintz of the Darrin Fresh Water Institute in New York State, who has studied salinity effects on rainbow trout. “Adult fish are quite resilient, but the young are vulnerable,” he says. “High salt levels may slow down their growth, which has negative effects on their survival and reproductive capacity.”
This week Hintz published a new paper focusing on the fate of the water flea, a tiny critter that tends to thrive in such large numbers that it has a key influence on ecosystem health. “Under experimental conditions water fleas do seem able to evolve adaptations to moderate salt levels,” Hintz says. But when he exposed the adapted ones to higher but still realistic levels of salinity (less than one tenth of seawater), they all died. The containers in which they were kept were soon overtaken by the algae common in algal blooms often observed in unnaturally salty freshwater in the wild.
“The demise of water fleas does not just affect the clarity of the water, but will likely also impact the small fish that feed on them,” Hintz adds. “They provide food for the larger fish that humans eat.” In the face of acute problems with our drinking water and food production this may seem insignificant. But as Jackson puts it, the natural streams that collect water across the landscape and carry it along to the rivers and lakes we get our drinking water from are like a neural network connecting us to nature. “If they are unhealthy, sooner or later we’ll pay the price.”