The Influence of Soil Composition on Stormwater Retention and Runoff in Green Roofs at Portland State

The purpose of this study is to determine what soil composition is best for green roofs at Portland State; we aim to compare the current soil to the original substrate, to measure which composition retains the most water, and which filters out the most pollutants in stormwater runoff. Five different soil compositions were tested — original, current, layered mixed, solid mixed, and potting soils — and 1,000mL of high and low intensity rainfall (in/hr) was simulated for each of the mixtures. Water was allowed to filter through the soils for a predetermined time (10min for a high application rate and 20min for a low application rate), and runoff water was collected in a graduated cylinder. Contaminant levels in the simulated stormwater runoff and water retention was calculated as a percentage. Results suggest that the original soil mixture retains more water than the current soil mixture during low intensity rainfall, but falls short during high intensity events. All soils were outperformed by potting soil in terms of retention regardless of application rate. Inversely, potting soil had the worst environmental impact for pollutant runoff. More studies need to be done to determine the best composition of soil for Portland State’s green roofs