Life cycle assessment of a bioswale amendments to improve nutrient removal from urban storm runoff

Abstract:

Urban storm runoff contributes pesticides, nutrients, metals, suspended solids, and hydrocarbons to surface waters, deteriorating aquatic health. Green infrastructure can restore natural hydrologic functions of watersheds by reducing runoff, removing pollutants, and increasing stormwater runoff infiltration into local aquifers. Bioswales are a type of green infrastructure consisting of a vegetated drainage basin to catch sediments, treat contaminants, and increase infiltration. The Butte College Bioswale treats storm runoff from a 2-ha parking lot in northern California. The current bioswale is performing well overall, except for nutrient removal which could be improved by integrating amendments. We evaluated the environmental impacts of the bioswale by performing a life cycle assessment (LCA) over 20 years and compared the current system to hypothetical bioswales containing amendments meant to aid water treatment, including (1) zerovalent iron, (2) walnut shell biochar, (3) logging residual biochar, (4) rice hull biochar, and (5) water treatment residuals. LCA is a quantitative tool used to measure the environmental impacts of infrastructure, including eutrophication, water acidification, ozone depletion, global warming potential, and smog contribution. To establish the LCA, we measured nutrient and organic carbon loads in inflows and outflows of the bioswale and estimated contributions from the bioswale construction. When comparing the current system to the hypothetical amended systems, water treatment residuals and zerovalent iron displayed the potential to decrease eutrophication by 14%. However, zerovalent iron increased the environmental footprint of the bioswale, while water treatment residuals contributed to an overall decrease in environmental footprint. The three biochars did not have a significant impact on the environment footprint of the bioswale. These LCA results suggest that repurposed amendments such as water treatment residuals and biochars can improve water treatment of storm runoff by the bioswale, while decreasing or not altering its environmental impacts.