Some plants help clear roadsides of salt and pollutants, a research has found. But their contribution is not enough. De-icing applications need to be reduced
by Matteo Cavallito
Some invasive plant species that grow along roadsides and other wet and degraded environments may help clean up soil pollutants. This is supported by a group of researchers from the College of Agriculture, Health, and Natural Resources at the University of Connecticut and Loyola University Chicago. The study, published in the journal Ecological Engineering, focuses on specimens of Typha – a plant genus that includes dozens of species – and swamp reed (Phragmites australi).
Salt promotes diffusion of metals in soil
Particularly tolerant to salinity, these plants grow at the edge of roads where salt is applied as a de-icing agent, researchers recall. And this is no coincidence. Since the second half of the last century, points out Beth Lawrence, a professor in the Department of Natural Resources and Environment at the University of Connecticut in a statement released by the same university, “the quantity of road salt application has grown exponentially, and there are growing concerns about their impact on the environment.”
Roadside ecosystems, in particular, “are increasingly saline and contain high loads of heavy metals like zinc and lead from cars and their emissions.”
The problem, he adds, is that the chemistry of the salt ions mobilizes metals in the soil, making it easier for them to move into the environment and causing problems. The removal of salt from the environment is typically complex. Hence the interest in the ability of plants to absorb it and store it in their tissues thus cleaning up the soil.
“Plant-based bioremediation has been proposed as a strategy to mitigate the growing risks associated with freshwater salinization,” the study says. “But large-scale experimental field tests of how harvesting plants can reduce deicing salts and heavy metals common in roadside environments are lacking.” The authors identified ten wetlands by collecting and analyzing plant biomass for two growing seasons. They also evaluated soil chemistry and plant tissue chemistry.
Ultimately, confirmed the study, “Harvesting both Typha and Phragmites, ubiquitous plants that co-occur in wetlands around the globe, has the potential to remove both salts (Na, Cl, Ca, Mg) and heavy metals (Zn, Cu, Fe, Mn).”
Typha plants have proven most effective in absorbing salts. Swamp straws, in contrast, tend to accumulate metals better, thus coming to contain higher levels of zinc and copper. “Although repeated harvests reduced aboveground biomass and associated salt and metal stocks relative to unmanipulated controls, rotational harvest of common macrophytes growing in road-adjacent wetlands could maximize the reduction of roadside pollutants through harvesting,” the study further explains.
We need to reduce salt application
The percentage of salts removed, the scientists point out, depends on the rate of application, the degree of plant growth, and the amount of time the water is in place. However, they point out, “our data suggest that harvesting invasive macrophytes from urban roadside detention basins is not a silver bullet to remedy freshwater salinization, as it mitigates only a small fraction of added road salt loads.”
In detail, a complete removal of all above-ground biomass within a one-acre basin would remove less than 0.5 percent of the added salt. Therefore, “We really need to focus on reducing the amount of salt that we’re applying to this already very salty system,” Lawrence concludes.