Beneath the Surface: Soil Particle Size Analysis of Green Stormwater Infrastructure in Washington, D.C.

• Emily Berger, Environmental Engineering, University of Delaware

• Carolyn Voter, Civil, Construction, and Environmental Engineering, University of Delaware

Green stormwater infrastructure (GSI) is a nature-based solution to manage stormwater. Urban areas have been adopting various GSI technologies for over a decade; however, the long-term functionality of GSI has not been well studied. An important factor in the functionality of GSI is the soil composition. The standard methods for soil particle size analysis, such as the sedimentation method, have been extensively studied. But few of these studies were conducted on rain garden soils, which have high levels of organic matter. 

To evaluate the long-term functionality and soil composition of GSI, we collected soil samples at the inlets and low points from rain gardens installed by RiverSmart Homes in Washington, D.C. To overcome the obstacles presented by high organic matter content to particle size analysis, the organic matter was burned off in the furnace at 550 degrees Celsius before testing the samples using the sieve hydrometer method. These findings indicated that the rain gardens have highly variable organic matter levels. Due to the large number of soil samples and the time-consuming nature of the loss-on-ignition and sieve-hydrometer procedures, a faster method was needed to analyze all 310 samples. Using a combination of wet sieving to obtain the sand fraction and laser diffraction to obtain the fines fraction, it is possible to determine the soil composition. Preliminary results from this method indicate that the soils contain a high fraction of sand particles, with about 70 to 80 percent sand.