A Laboratory Comparison of Green-Roof Runoff Water Quality

Increasingly, building owners and city planners are turning to vegetated roofing systems, commonly known as green-roofs, for economic, environmental, and aesthetic benefits. Many studies have shown the potential for green-roofs to mitigate volumetric magnitude and nutrient loadings of rainwater runoff surges immediately following large storms, but further research is needed to help solidify these relationships. Although the insulative capacity of a green-roof has been studied by some researchers, it has yet to be thoroughly tested under a controlled, steady-state environment. Furthermore, a green-roof performance comparison of stormwater runoff has yet to be conducted with controlled rain events and quantifiable antecedent soil-moisture. In this research, rainfall detention and stormwater runoff peak flow attenuation of ten different small-scale green-roof schemes were observed and analyzed in a laboratory environment. Sedum Angelina, Sedum Hispanicum, White Clover, Vinca Major, Ryegrass, and a wildflower blend were grown in separate 2' x 2' trays at separate soil depths of 2" and 6" atop a 1/4" drainage layer to observe how soil depth and root zone development affects stormwater flow-through for each plant type. A constant media mix (porosity=70%) was used consisting of composted paper fiber, leaf compost, pumice, and sand. Antecedent gravimetric soil moisture for each green-roof tray was measured by using initial tray weights and comparing to the residual soil moisture. Water quality levels of temperature, conductivity, and pH were measured on site at Portland State University while nitrate, phosphorus, Total Suspended Solids (TSS), and heavy metals were measured by The Bureau of Environmental Services, Portland, Oregon. This green-roof testing has shown that plant type and antecedent soil moisture have the strongest affect on runoff water quality parameters of pH, nitrate, and selected heavy metal concentrations, while substrate depth primarily dictates runoff conductivity, TSS, and phosphorus concentrations.