Complementarity of road salt and heavy metal pollutant removal through invasive Typha and Phragmites harvest in urban wetland detention basins

Abstract

Plant-based bioremediation has been proposed as a strategy to mitigate the growing risks associated with freshwater salinization, but large-scale experimental field tests of how harvesting plants can reduce deicing salts and heavy metals common in roadside environments are lacking. We implemented a three-year, large-scale field experiment (across 180 ha) in ten urban wetland detention basins in the Chicago region (Illinois, USA) to investigate the potential of plant biomass harvest to remove salts and heavy metals from roadside environments. 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), as we observed complementarity in the pollutants concentrated in the aboveground tissues of these two species. We observed greater salt stocks in Typha biomass while heavy metals (especially Zn) were more abundant in Phragmites. Salts associated with deicing salts (Na, Cl) were particularly concentrated in live green Typha tissue, while metals tended to be more abundant (>50% of total stocks) in senescent litter, suggesting that plant phenology is important to consider when mitigating target pollutants via harvesting. 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. However, 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.