Despite national regulations to eliminate pollutant discharge, untreated stormwater runoff continues to detrimentally affect downstream water bodies. Planted bioretention (BR) systems have been proven to remove significantly more pollutants from storm runoff than unplanted systems and are increasingly used in response to United States Environmental Protection Agency (EPA) National Pollutant Discharge Elimination System (NPDES) regulations. A portion of the pollutants removed from stormwater runoff is taken up into the aerial portions of plants. This biomass can be harvested and disposed of off-site, decreasing potentially hazardous buildup of pollutants in BR systems. It is well understood that certain species are more capable at surviving the stressful conditions of a stormwater BR system and some are better accumulators of pollutants. However, little work has been done to investigate the fate of pollutants within a BR system (water, plant, soil matrices), nor to evaluate the differences that might exist among plant species. Given the increase in BR use for stormwater pollutant reduction, and the lack of comprehensive information on individual species’ removal efficiencies, there is a critical need to determine the extent of pollutant uptake as a function of individual plant species, and to develop inexpensive and environmentally responsible systems for maximum stormwater pollutant removal. The goal of this study was to quantify differences in constituent (total phosphorous, nitrogen, copper, zinc and lead) distribution, and evaluate the differences in plant uptake of these constituents among species typically found within stormwater BR systems. This was accomplished by determining the fate of stormwater pollutants within each of the phases of a BR system (soil, plant roots, plant shoots, water) undergoing three hydraulic and pollutant loading regimes using a mass-balance approach. Six plant species typically found in stormwater basins were used in this study. They were: Phragmites communis (Common Reed), Typha latifoliaI (Broadleaf Cattail), Scirpus validus (Soft-stem Bulrush), Scirpus acutus (Hard-stem Bulrush), Carex microptera (Clustered Field Sedge), and Carex praegracilis (Smallwing Sedge). Study results showed that there is a significant difference in biomass production and constituent distribution as function of species and as a function of hydraulic and pollutant loading regime. For example, the highest loading rate consistently allowed higher fractions of metals to pass through the vegetated system and be discharged in the soil filtrate. Also, BR systems populated with the sedge species consistently accumulated lower fractions of metals in the soil than unplanted controls, and produced higher metals mass fluxes from the system in the soil filtrate. This suggests that the sedge species produce a root exudate to solubilize metals, decrease adsorption to the soil matrices, and increase metal mobility in the soil-pore water. The results of this study will enable municipalities, institutions, and state governments to improve plant recommendations for BR systems to optimize management and harvest procedures for reducing stormwater impacts to surface and groundwater.
Available at: http://works.bepress.com/r-dupont/14/