Reclaimed water from wastewater treatment plants can be a significant contributor of salt to drinking-water reservoirs, according to new research that highlights the complexities of addressing the growing issue of freshwater salinization.

Scientists compared salt levels from two tributaries and a Virginia water-reclamation facility, a type of treatment plant designed to supplement human water supplies, under different weather conditions. Although the watersheds contributed more sodium annually, the reclaimed water was the primary source of sodium during dry periods. The researchers reported the findings April 19 in Nature Sustainability.

"What's really interesting about this particular reservoir is it's kind of a microcosm of what's happening across the United States," said Stanley Grant, a professor of civil and environmental engineering at Virginia Polytechnic Institute and State University and co-author of the study. "In this really complicated system, where do you start to try to control the problem?"

The salt concentration in streams, lakes and other bodies of freshwater is on the rise worldwide. In the United States, freshwater salinization is becoming a problem in densely populated Northeast and Mid-Atlantic states and in parts of the Midwest important for agriculture, Grant and his colleagues noted in the study.

This salt comes from many sources, including roadway de-icing, agricultural runoff, the weathering of concrete, household and industrial waste, water softeners and chemicals added during wastewater treatment. At high-enough concentrations, these salts can pollute drinking water and harm sensitive ecosystems.

"It's not the number one environmental threat that many people think about, but it could be a really serious problem as it unfolds," Grant said.

He and his team focused on the Occoquan Reservoir, in northeast Virginia, which supplies water to nearly 2 million people. The concentration of sodium in the reservoir began rising in the mid-1990s and now often exceeds the U.S. Environmental Protection Agency's recommended thresholds.

Removing the salt through a process such as reverse osmosis is difficult and expensive, Grant said.

For the study, the authors analyzed sodium and streamflow measurements collected regularly between 2010-18 from three sources that drain into the reservoir: a water-reclamation facility and the rapidly urbanizing Occoquan River and Bull Run watersheds. 

"The expectation for the Mid-Atlantic region of the United States ... is that most of the salinization signal is coming from de-icers used on roads, highways and parking lots," Grant said. "What we wanted to look at was whether that's the whole story."

He and his colleagues found that runoff from the two watersheds was the primary source of sodium during bursts of wet weather, while the water-reclamation facility sent more sodium to the reservoir during long stretches of dry weather. 

"Those relatively rare rain events end up dominating the annual signal," Grant said. "At the daily level, you start to see a much richer picture."

Additionally, the reclaimed water usually had higher sodium concentrations than those seen in water flowing from the two watersheds.

Grant and his colleagues estimated that, on average, 46.5% of the sodium in the reclaimed water came from chemicals used in water treatment, industrial waste and human excretion. It wasn't clear where the rest originated, although the researchers suspect that sodium-containing potable water that ran down drains, as well as household products such as detergents, played an important role.

Combating freshwater salinization is complicated by the interconnected nature of the key contributors, he said.

"It's a big ouroboros, in a way," Grant said. "You think, 'Where is the source?' And it really is everywhere, and it's just getting circulated around."

One way that salt enters freshwater is through chemicals used in wastewater treatment plants to dechlorinate water before it is released. However, plants can minimize salt levels by using ultraviolet light rather than chlorine to kill pathogens in the water.  

Low-salt versions of common household products could also help reduce the amount of salt in water that flows into treatment plants and, ultimately, reservoirs, Grant said.  

"The other thing I would say is taking a hard look at discharge permits and having that public debate about the potential economic trade-offs," he said. "Do we allow the businesses or industries to discharge this salt, knowing that there could be downstream consequences both to ecosystems and to human water security?"

Grant and his colleagues examined only one reservoir, so more research is needed to determine whether similar patterns are playing out in other locations. 

"Beginning to unpack what's going on in the sanitary sewer system — where the sources are, how they get delivered to the wastewater treatment plant, how the ways in which wastewater is treated can add additional salt or not — I think that has very general applicability," Grant said.

Indirect potable reuse, the practice of boosting water supplies by adding highly treated wastewater to reservoirs and groundwaters, may become more prevalent in the future under the EPA's Water Reuse Action Plan.

"How should that be done in a way that minimizes the impact on rising salinity?" Grant said. "Hopefully, our work will help inform that discussion, too."

The study, "Addressing the contribution of indirect potable reuse to inland freshwater salinization," published April 19 in Nature Sustainability, was authored by Shantanu V. Bhide, Stanley B. Grant, Emily A. Parker, Megan A. Rippy, Adil N. Godrej, Peter Vikesland, Ayella Maile-Moskowitz, Marc Edwards, Kathryn G. Lopez and Todd Schenk, Virginia Polytechnic Institute and State University; Sujay Kaushal, University of Maryland; Greg Prelewicz and Niffy Saji, Fairfax Water; Shannon Curtis, Public Works and Environmental Services; and Thomas A. Birkland, North Carolina State University.