Urban areas are challenged with developing management solutions to mitigate the effects of urban stormwater. Municipalities across the US are adopting green stormwater infrastructure (GSI) to manage urban runoff due to its potential environmental and economic advantages over conventional “gray” infrastructure. GSI is designed to emulate natural hydrologic and ecological functions by infiltrating, filtering, storing, detaining, and evaporating local runoff and retaining and transforming contaminants through biogeochemical processes. However, GSI ecosystems are unlike their natural analogs in many ways: they are isolated in the environment, experience more frequent and intense flooding events, and receive supplies of organic and inorganic pollutants in urban runoff. Additionally, GSI facilities across climatic regions are designed with similar criteria that may or may not be suited to the region. Little is known about the ecological processes in GSI that are intended to provide environmental benefits. This study investigates the influence of climate and design on pollutant removal, specifically potential nitrogen removal through denitrification [as denitrification enzyme activity (DEA)] in bioswale-type GSI in Baltimore, MD and Portland, OR.
Preliminary results show seasonal differences in DEA response between cities. DEA was higher in Portland during the winter (6.3 mg-N m-2 hr-2) than summer (4.8 mg-N m-2 hr-2) but was higher in Baltimore during the summer (3.2 mg-N m-2 hr-2) than winter (2.3 mg-N m-2 hr-2). Additionally, results show differing relationships between soil infiltration rates and DEA across cities. Slow-infiltrating soils showed higher rates of DEA than fast-infiltrating soils in Portland whereas Baltimore soils showed greater variability in DEA and no clear relationship between infiltration rates and DEA. Future analyses will explore the extent to which design and seasonal factors influence potential nitrogen removal in GSI in these two cities.