While salt on sidewalks and roadways helps prevent slips, falls and deadly auto accidents during wintry weather, a new study led by a University of Maryland geologist warns that introducing salt into the environment can hurt us in other ways—releasing toxic chemical cocktails that create a growing global threat to our freshwater supply and human health.
Added salts can interact with soils and infrastructure to release metals, dissolved solids and radioactive particles, according to previous studies by Professor Sujay Kaushal of the Department of Geology and the Earth System Science Interdisciplinary Center and his team. They termed these cascading effects “freshwater salinization syndrome,” which can poison drinking water and harm agriculture, infrastructure, wildlife and the stability of ecosystems.
The new study is the first comprehensive analysis of the interconnected effects of freshwater salinization syndrome and their impact on human health. It suggests that water supplies could face serious threats at local, regional and global levels if a coordinated management and regulation approach is not applied to human sources of salt. The study, which calls on regulators to approach salts with the same level of concern as acid rain, loss of biodiversity and other high-profile environmental problems, was published Monday in the journal Biogeochemistry.
“We used to think about adding salts as not much of a problem,” said Kaushal. “We thought we put it on the roads in winter and it gets washed away, but we realized that it stuck around and accumulated.”
When Kaushal and his team compared data and reviewed studies from freshwater monitoring stations throughout the world, they found a general increase in chloride concentrations on a global scale. Chloride is the common element in many types of salts like sodium chloride (table salt) and calcium chloride (commonly used for road salt). The researchers also uncovered a 30-year trend of increasing salinity in places like the Passaic River in the New York City suburbs of northern New Jersey and a 100-mile-plus stretch of the Potomac River that supplies drinking water to Washington, D.C.
The major human-related salt source in areas including the Northeastern U.S. is road salts, but sewage leaks and discharges, water softeners, agricultural fertilizers and fracking brines enriched with salts also contribute. Indirect sources include the weathering of roads, bridges and buildings, which can release salt from limestone, concrete or gypsum as they break down. Ammonium-based fertilizers can also lead to the release of salts in urban lawns and agricultural fields. In some coastal environments, saltwater intrusion from sea-level rise is another source.
The study points to a growing body of research that shows how chemical cocktails released by all of these salt sources harm both natural and built environments. For example, changes in salt levels can allow invasive, more salt-tolerant species to take over a stream. Chemical cocktails released by salts can change the microbes in soil and water, and because microbes are responsible for decay and replenishment of nutrients in an ecosystem, that shift can lead to even more changes in the release of salts, nutrients and heavy metals into the environment.
In the built environment, salts can degrade roadways and infrastructure. They can also corrode water pipes, causing the release of heavy metals into drinking water supplies as they did in Flint, Michigan, in 2014 after the city switched to a new, saltier water source.
“I am greatly surprised by the increasing scope and intensity of these problems as highlighted from our studies,” said study co-author Gene E. Likens, founding president emeritus of the Cary Institute of Ecosystem Studies and a distinguished research professor at the University of Connecticut. “Increased salinization of surface waters is becoming a major environmental problem in many places in the world.”
The study calls for a more comprehensive, modern approach to water quality monitoring, and suggests that management strategies must evaluate salt contributions from different sources on a watershed-ecosystem level and prioritize regulation accordingly, much the way nutrient loads in watersheds are currently managed.
“Ultimately, we need regulation at the higher levels, and we’re still lacking adequate protection of local jurisdictions and water supplies,” Kaushal said. “We have made dramatic improvements to acid rain and air quality, and we’re trying to address climate change this way. What we need here is a much better understanding of the complicated effects of added salts and regulations based on that. This can allow us to avert a really difficult future for freshwater supplies.”