Salt levels in drinking water could be near tipping point

Salt is in the food we eat, on the pavement under our car tires in winter, and in the powdered laundry detergent we use to wash our clothes. And an ever-increasing amount is ending up in local waters — waters that, by definition, should not be salty.

Yet across the world, sodium levels in freshwater rivers, lakes and reservoirs have been trending upward. The causes are difficult to pinpoint and will likely be even harder to reverse.

Recognizing how intractable the problem can be, the National Science Foundation in 2020 awarded a group of Chesapeake Bay area researchers a $3.6 million grant to tackle the issue as one of society’s “grand challenges.” Funded through the foundation’s Growing Convergence Research program, the effort, now in its second of five years, has brought scientists and leaders from a range of backgrounds to the table to solve complex issues.

Freshwater salinization, as it’s called, is certainly one of them. There are many sources of salt in a waterbody, ranging from salt and salt brine spread on wintry roads to home water softener systems. Wastewater treatment plants are designed to reduce levels of nitrogen, phosphorous and other pollutants from the water before discharging it, but rarely do they remove salt.

Although there are scientific methods for removing salt from water, such as reverse osmosis, they are energy-intensive and far too expensive for most water authorities to seriously consider using them in treatment plants.

Keeping salt out of the water in the first place, the experts say, is by far the best approach — though it’s not an easy one. The Virginia Department of Environmental Quality spent years developing a salt management strategy to reduce levels of salt that were polluting Accotink Creek in Fairfax County. The effort aims to balance the safety benefits of road de-icing with the harmful impact of excess salt on living resources. It spells out steps that government, businesses and citizens can take to that end.

But winter road salt isn’t the only source of the problem. Powdered laundry detergents, from those on supermarket shelves to homemade alternatives, often contain salt, which is then flushed with the wastewater to the nearest treatment plant or through a septic system. Industrial cooling systems, like those used at large data centers, include salt as a disinfectant in water. Wastewater treatment doesn’t remove salt, so it eventually makes its way to the nearest creek, river or bay.

Stanley Grant, a civil and environmental engineering professor at Virginia Tech University, is leading the National Science Foundation effort from his post as director of the Occoquan Watershed Monitoring Lab in Prince William County, VA. The lab has been tracking rising sodium levels in the Occoquan Reservoir for decades.

Among the questions Grant hopes to address with the project is not just which methods work to reduce salt, but also which ones policymakers and the broader population will consider both palatable and achievable. And can the various stakeholders agree?

“We are a microcosm,” he said from his office in the lab’s nondescript building in Manassas, VA. “[But] the solutions that we develop here are absolutely translatable to many other water supplies and watersheds around the country and the world.”

Also involved in the project is University of Maryland professor Sujay Kaushal, who first wrote about saltier waters in Maryland in a 2005 paper. He later began referring to the phenomenon, which he saw increasing across the country and the world, as “freshwater salinization syndrome,” identifying a range of symptoms that accompany the condition. More have been discovered since.

“It’s like a systemic illness the watershed is facing as it is fed a high-salt diet,” he said.

Just as too much salt in a human diet can contribute to high blood pressure, heart disease and stroke, too much salt in a waterbody can have similarly damaging effects. Higher salt levels in freshwater can reduce biodiversity, increase the presence of certain salt-loving species and cause infrastructure such as pipes to corrode more quickly, for example.

And in waterbodies that supply public drinking water, too much salt poses risks to human health as well as to the environment. While unnatural levels of salt might gradually impact freshwater systems, the impact on drinking water can be immediate. One day the water might not be too salty to drink, but the next day it will be. Researchers say it can be difficult and incredibly costly to return a waterbody to health once it’s reached such a tipping point. 

In the United States, the U.S. Environmental Protection Agency does not have a regulatory limit on sodium as a pollutant in drinking water. But the agency’s guidance documents recommend that drinking water sodium levels remain less than 20 milligrams per liter for people on low-sodium diets and less than 30-60 milligrams per liter as a threshold for taste.

When water exceeds those thresholds, “you wouldn’t necessarily call it salty, but it just starts to taste bad,” Grant said.

The Occoquan Reservoir is no stranger to such careful monitoring. It was already a source of drinking water when, in the 1960s and ’70s, it became so polluted by development runoff and poor sewage treatment that the state stepped in to address the problems. As a result, several smaller sewage treatment plants in the area were consolidated to create the more advanced Upper Occoquan Service Authority, which could treat the water to a higher degree before discharging it into the waters feeding the reservoir.

“This was one huge experiment,” said Tom Faha, director of the Virginia Department of Environmental Quality’s Northern Regional Office, at a public meeting in June. “We were taking all of our wastewater for the area and treating it and discharging it into one of our primary water supplies.”

To oversee the outcomes of that experiment — which at the time included a suite of new water quality regulations — the state created the Occoquan Watershed Monitoring Lab in 1972, the same year Congress passed the Clean Water Act. The lab has been collecting water quality data ever since, recording the success of the early effort to use wastewater to help recharge a reservoir.

Wastewater treatment and runoff control practices have helped the reservoir maintain water quality over the years. But increased sodium levels have emerged as a threat, steadily rising since the lab’s inception.

In recent years, sodium levels in the reservoir have begun to “routinely exceed” the federal drinking water advisory levels for both low-salt diets and occasionally for taste. The Fairfax County Water Authority, which serves more than 2 million people in the region, gets 30–40% of its drinking water from the reservoir.

The utility’s other source of drinking water is the Potomac River, which is also getting saltier over time, though the impact is diluted for now by greater volumes of water. The Washington Suburban Sanitary Commission has recorded a 230% increase in salt levels in the river over the past 30 years.

“We have an urbanizing watershed that we’re also using as a water supply, and that is inherently a conflicting situation,” Grant said during a presentation this summer to the Prince William County Board of Supervisors. “How close are we to getting to the point where we have one more needle on the camel’s back, and we have a problem?”

Meanwhile, the county board has been on the cusp of approving zoning changes that would allow more intense development on a 2,100-acre swath of the watershed for the Occoquan Reservoir and Bull Run, a tributary of the Potomac. Those changes could pave the way for dozens of warehouse-like data centers where there used to be farmland, forests and widely scattered homes. 

Environmental groups have opposed the projects for several reasons, but chief among them is their potential impact on water quality. The correlation between development and saltier waters is well-established.

The more parking lots, buildings and roads, the higher the amount of sodium chloride in the water — and not just in winter, Kaushal said. As a chemical element that can take many forms, sodium can seep with groundwater through soil over time, interacting with other chemicals along the way.

Some of the more promising solutions for removing salt from the environment are natural ones. One of Kaushal’s students is publishing a paper on how forests naturally decrease sodium levels in the water as it filters through the ecosystem. This might sound similar to the way trees absorb other nutrients, but the process is more complex, with electrical interactions between certain elements. 

Megan Rippy, an assistant professor of civil and environmental engineering at Virginia Tech University and a co-principal investigator on the National Science Foundation salt project, is studying whether certain plants can absorb excess salt in stormwater retention ponds. Based on soil and water samples so far, cattails could be among the best native salt accumulators, she said.

But even environmental functions like these can be negatively impacted if there is too much salt in the system. Just as salty foods make humans thirsty, too much salt can dehydrate some plants, reducing their ability to survive and filter other pollutants. In the water, salt can be detrimental to aquatic organisms that live in systems that are supposed to be either freshwater, saltwater or a mix of both.

Researchers who are still grappling with the issues say there are no simple solutions. The main goal of the Science Foundation project is to learn whether — in the absence of regulations that restrict salt in runoff and wastewater — stakeholders can agree on what constitutes too much and at what point they’re willing to do something about it. That includes all the humans who live in a watershed.

“We’re salty creatures,” Kaushal said. “We require salt when we build things, eat things, dispose of things, so we all play a role in this.”

For tips on reducing your salty contributions to the environment:

• Izaak Walton League
• Northern Virginia Regional Commission