The increasing urbanization of land has produced expansive amounts of impervious surfaces, therefore yielding greater quantities of runoff as the water from weather events cannot be absorbed. This runoff may carry pollutants from the surrounding area into the water system and negatively impact the water quality. A possible solution for reducing runoff from impervious surfaces is green roofs, which can absorb and filter water. To determine a green roof’s effects on water quantity and quality, a test site was built in Cleveland, Ohio. This site contains 39 test plots that differ in combinations of plant community type (restoration or horticultural), substrate type (quasi-traditional, conventional, or movable meadow), and the presence of mycorrhizae, alongside empty control plots. The movable meadow plots were uniquely designed to capture the rainwater passing through the soil within an inner system to be reused, whereas the other plots were designed to allow the water to pass through the system and then be released. From October 2017 to October 2018, water samples were collected bimonthly, along with a volumetric measurement of the runoff present. The samples were tested for nitrate, ammonium, and phosphorous using nutrient assays on a microplate reader.
Results/Conclusions
Throughout the course of the experiment, we consistently observed that the movable meadow plots produced little to no runoff. Among the other two substrate types, there was little variation in runoff quantity, though the control plots without plants or soil produced less runoff, likely due to evaporation in the open and contained space. Examining water quality, it was found that plant community type had a significant effect on the total nitrogen present in the runoff (p=0.0015), while the mycorrhizal treatment only had significant impacts as an interaction with plant community type or substrate. Over the duration of the experiment, phosphorous levels were consistently greater than 1.00 mg/L, indicating downstream eutrophication potential which may be due to the recent construction of the experimental plots. Prior research has indicated the possibility of newer green roof substrates to act as a source of phosphorus in the first year of installation, which may explain the unexpectedly high concentrations. Overall, our results indicate the potential for green roofs to be a source rather than a sink of nutrient runoff, which may cause downstream water quality issues and detract from their perceived benefits.