Performance quantification of extensive green roof substrate blend : expanded shale and biochar

Abstract

Urban stormwater management practices often involve the redirection of runoff to local waterbodies. As such, the quality of runoff directly affects the condition of these receiving waters. Green roofs offer many benefits to the urban environment including attractive aesthetics, thermal insulation for buildings and stormwater runoff reduction. Unfortunately, in order to promote the spread of vegetation, fertilization is often practiced that can lead to elevated nutrient concentrations in runoff and, ultimately, nearby streams, rivers and bays. Different amounts of biochar, pyrolyzed biomass, were added to model green roof trays to test for the ability of this charcoal-like substance to prevent nitrate and phosphate leaching. Analysis of leachate from natural and simulated rain events showed statistically significant differences of average nitrate concentrations for two out of four rain events, though none for phosphate. Samples from the natural rain event exhibited a clear inverse relationship between nitrate concentration and biochar quantity. The second simulated rain event, however, produced nitrate concentrations that rose and subsequently fell as biochar quantity increased. Further research is needed on the method by which biochar attracts anions though different experimental designs and equipment may more conclusively reveal that biochar can play a role in green roofs as a soil amendment. More noteworthy, though, may be the consistently high nutrient concentrations in leachate originating from the fertilized model trays. The fertilizer application rate of 5 g N/m2 may not be suitable for the substrate and vegetation used in this study but nonetheless it is strongly recommended that controlled-release fertilizer types are used.