Stormwater Bioretention: Nitrogen, Phosphorous and Metal Removal by Plants

Stormwater runoff is an environmental concern. It increases the volume and velocity of surface water flow and contains high pollutant concentrations. High flows erode stream channels. Nutrients in the runoff cause eutrophication. Some metals from stormwater runoff is regulated by the Federal National Pollution Discharge Elimination System (NPDES). Bioretention (BR) systems are often used in response to these regulations to retain and treat this stormwater runoff. This study conducted greenhouse and field experiments, and evaluated differences in nutrient and metal removal among plant species that are typically found growing in BR systems but are not typically investigated in this detail. The extent of water quality improvement, the distribution of the nutrients and metals, and plant uptake potential were studied in an effort to better understand how individual plant species interact with, and influence nutrient and metal transport from the stormwater into the soil, plant root, and above ground plant tissue. In the greenhouse experiment six plant species received three levels of water inundation and pollutant loads. Differences in pollutant retention, and plant uptake of nutrients and metals were studied. Biomass production increased with higher water and pollutant loads for all species. Common Reed, Common Field Sedge, and Smallwing Sedge were found to take up significantly more phosphorous and nitrogen into harvestable tissue than Broadleaf Cattail, Soft-stem Bulrush, and Hard-stem Bulrush. More than 89% of cooper, lead and zinc from runoff accumulated in the top 27 cm of the soil. Continued accumulation can result in hazardous levels of metal concentrations. Common Field Sedge, and Smallwing Sedge exhibited higher metal distribution in the plant tissue and exfiltrate water, and lower distribution in the soil media than the other species. It was speculated that these Sedges allow metal uptake through biological and chemical processes. The field BR experiment evaluated citric acid\u27s ability to increase metal uptake potential among three plant species. High citric acid application levels increase the concentrations of metals in the soil pore water, but did not increase plant metal uptake. However, notable differences were found among species, where Carex microptera accumulated more aluminum, chromium, and iron in the above ground tissue than Helianthus meximilani and Typha latifolia