Researcher studies the power of native plants to combat road salt pollution

Salt pollution in freshwater is a growing global concern.

Excessive salt harms plants, degrades soil, and compromises water quality. In urban areas, road salts used for de-icing during winter often wash into stormwater systems, posing health concerns and challenges for infrastructure.

Specifically, salts can impact the processes like filtration and contaminate retention basins that are used to manage and treat urban stormwater runoff. Megan Rippy, assistant professor in civil and environmental engineering, is on a mission to understand how salt affects plants in stormwater detention basins and assess whether certain plants can mitigate salt pollution through a process called phytoremediation.

“Plants play an important role in green infrastructure performance, but only 1 percent of plants, known as halophytes, can handle highly saline environments,” said Rippy. “This makes it important to characterize the threat salts pose to green infrastructure as well as the potential of salt tolerant species to mitigate that threat.”

Rippy led a yearlong study funded by a National Science Foundation Growing Convergence Research program award. She studied stormwater detention basins in Northern Virginia, exploring the impacts of road salts on plants, soils, and water quality in green infrastructure systems. These basins, designed to manage stormwater runoff and improve water quality, face challenges from road salts used during winter months.

The research published in Science of the Total Environment shows that the amount of salt present in green infrastructure systems does reach levels that threaten plant communities. However, relying on salt-tolerant plants to mitigate the program is unlikely to be effective because they simply don’t assimilate enough salt.

Salt levels and plant resilience

The research found that basins draining roads exhibited the highest salt levels, causing significant stress to plants. Parking lots were next with moderate salt levels, while basins draining grassy areas had little to no salt stress.

Of the 255 plant species identified in the basins, 48 native species showed the ability to tolerate high salt concentrations. Certain plants, particularly cattails, absorbed substantial amounts of salt, significantly higher than other species.

The researchers focused on 14 detention basins across Northern Virginia, measuring salt levels in water, soil, and plant tissues throughout the four seasons. The water samples were analyzed at the Occoquan Watershed Monitoring Laboratory for electrical conductivity and major salt ions. The basins provide drainage for different types of land, including roads, parking lots, and grassy areas.

Can plants solve the salinity problem?

While salt-tolerant plants like cattails showed promise, their impact on overall salt removal was limited. Even in a basin densely planted with cattails, only about 5 to 6 percent of the road salt applied during winter could be removed. This suggests that phytoremediation alone cannot resolve salt pollution but could complement broader salt management strategies that also address winter salt application.

“The amount of salt cattails remove is roughly equivalent to the mass of one to two adults,” said Rippy. “That pales in comparison to the amount we actually apply to the roads and parking lots, suggesting that we shouldn’t expect plants to be a silver bullet solution to our salinization problem.”

Climate change also may alter salt stress dynamics in stormwater systems. As winters in transitional climate zones become milder with more rain and less snow, the amount of salt applied to roads could decrease. This shift might bring salt levels in basins more in line with plants’ ability to absorb and process the salinization.

However, regions with persistent snow cover may experience different challenges, such as delayed deicer wash-off and plant emergence, which could affect salt stress profiles and phytoremediation capacity.

Resilient systems for managing urban salt pollution

This study provides valuable insights into the interplay between plants, salt pollution, and green infrastructure. By understanding how plants tolerate and process salt, Rippy is one step closer to developing sustainable solutions for protecting freshwater ecosystems.

While plants alone cannot solve our salt pollution problem, their role in integrated management strategies is critical. This can offer guidance for urban planners, engineers, and environmental scientists to design more effective stormwater systems to manage runoff, reduce salt pollution, and create greener, more resilient cities.