The issue of salt contamination in freshwater is becoming an urgent global challenge.
High levels of salt are detrimental to plants, deplete soil quality, and deteriorate water purity. In cities, salts spread on roads for de-icing during winter often flow into stormwater systems, raising health risks and putting pressure on infrastructure.
Specifically, excessive salts can interfere with vital functions like filtration and jeopardize retention basins designed to manage and purify urban stormwater runoff. Megan Rippy, an assistant professor in civil and environmental engineering, is investigating how salt influences plants in stormwater detention basins and whether some plants can help alleviate salt pollution through a process known as phytoremediation.
“Plants are crucial for the effectiveness of green infrastructure, yet only about 1 percent of existing plants, referred to as halophytes, can thrive in highly saline settings,” noted Rippy. “Therefore, it’s essential to understand the risk that salts pose to green infrastructure, alongside the potential of salt-resistant species to help diminish that risk.”
Rippy spearheaded a year-long study, funded through an award from the National Science Foundation’s Growing Convergence Research program. She examined stormwater detention basins in Northern Virginia, focusing on the effects of road salts on plants, soil conditions, and water quality in such green infrastructure setups. These basins are intended to manage stormwater runoff and enhance water quality but are challenged by the salts applied to roads during the winter months.
The findings, which were published in Science of the Total Environment, indicate that the levels of salt within green infrastructure systems can indeed reach harmful levels for plant communities. Nonetheless, relying on salt-resistant plants to address this issue is likely to be insufficient, as they do not absorb salt in substantial amounts.
Salt concentration and plant resilience
The study revealed that basins draining roads had the highest concentrations of salt, leading to considerable stress on plants. Next were the basins tied to parking lots with moderate salt levels, while those linked to grassy regions exhibited minimal to no salt stress.
Among the 255 plant species mapped in the basins, 48 native varieties displayed some tolerance to elevated salt levels. Certain plants, especially cattails, absorbed notable amounts of salt, far exceeding other species.
The researchers concentrated on 14 detention basins throughout Northern Virginia, monitoring salt concentrations in water, soil, and plant tissue across all four seasons. Water samples underwent analysis at the Occoquan Watershed Monitoring Laboratory to assess electrical conductivity and primary salt ions. The basins serve as drainage systems for various land types, including roads, parking lots, and grassy landscapes.
Can plants tackle the salinity dilemma?
Although salt-tolerant plants like cattails showed potential, their contribution to overall salt reduction proved limited. Even in a basin densely populated with cattails, only about 5 to 6 percent of the road salt applied during the winter could be eliminated. This outcome suggests that phytoremediation on its own cannot entirely resolve salt pollution and should be part of wider salt management approaches, which also address winter salt use.
“The quantity of salt removed by cattails corresponds to the weight of one to two adults,” Rippy explained. “This is trivial compared to the volume we actually distribute on roads and parking spaces, indicating that we shouldn’t view plants as a sole remedy for our salinization issues.”
Climate change may also shift the dynamics of salt stress within stormwater systems. As winters in transitional climate zones shift towards milder conditions with increased rainfall and reduced snowfall, the amount of salt utilized on roads may decline. This transition could align salt levels in basins more closely with plants’ capacity to absorb and manage salinization.
However, areas with consistent snow cover might face different problems, such as prolonged deicer runoff and delayed plant growth, potentially affecting salt stress dynamics and the capacity for phytoremediation.
Building resilient systems to tackle urban salt pollution
This research offers significant insights into the complex interactions between plants, salt contamination, and green infrastructure. By gaining a deeper understanding of how plants endure and process salt, Rippy moves closer to formulating sustainable solutions for safeguarding freshwater ecosystems.
While plants cannot single-handedly solve our salt pollution challenges, their inclusion in integrated management strategies is vital. This can provide direction for urban planners, engineers, and environmental scientists to create more effective stormwater systems that manage runoff, mitigate salt pollution, and foster greener, more resilient cities.
