Green Roofs Can Detain and Remove Pollutants from Stormwater Runoff
Penn State University,
Department of Horticulture, The Pennsylvania State University
Basin Program Funds:
As our country becomes increasingly urbanized, large areas of land are being covered with impervious surfaces: buildings, roads, and parking lots. When storms occur, acidified rain or snowmelt runoff from these surfaces containing nitrogen and particulate pollutants can impose a significant threat to the watersheds that drain into the Great Lakes, degrading them and creating new runoff management problems for state and local officials. Germany has developed a Green Roof technology that, if applied in the US, can reduce runoff and watershed damage from rain deposited nitrate- and ammonium-N well as particulate deposition.
Green Roof technology is not new. After decades of practice, space-cramped and environmentally-conscious Germans have honed the practice to a fine art. Industry figures suggest that 10%, or nearly 600 million square feet, of German roofs are greened. Between 1989 and 1999, German roofing companies installed nearly 350 million square feet of green roofs, and the rate of use is increasing. Although general information about green roofs is available, installation specifics, especially as they relate to roof hydrology and runoff, are mostly patented or proprietary, and performance data related to the cleansing action of Green Roofs are largely anecdotal. In either case, there is little actual quality information available to architects, developers, and builders in this country. Much of the popular semi-technical literature is written only in German, which effectively slows its adoption by Americans. However, there Green Roofs have been installed in the US in places such as Chicago City Hall, the Philadelphia Fencing Academy, and more are planned, like the Ford Motor Company’s River Rouge renovation in Detroit. Adoption of European information and techniques are further limited because our roof conditions are substantially different from those in Germany. So, while principles can be applied, Americans need information about our specific conditions. Pennsylvanians also need to see demonstration roofs so they can be confident about installing them on their own commercial and farm buildings, or homes.
In July, 2000, 6 small (2x2.66M) buildings were erected and placed at the Penn State University Horticulture Research Center in Rock Springs, PA. Three buildings had asphalt roofs and 3 were fitted with extensive green roofs. The slope of all roofs was 8.33% (1:12), and the buildings were oriented in a north-south direction. All buildings had gutters that were enclosed and connected to runoff barrels (capacity 55gal) fitted with pressure transducers ((Omega PX26Series, ±0.2% of 6.89kPa(1psi)) to measure runoff. Water quality was determined by analyzing a composite sample for several selected rain events from each building from October, 2000 to June, 2003. Samples were analyzed for several environmental parameters including pH, turbidity, and nitrates
Green roofs effectively retain (keep on the roof) and detain (slow down) rain. Of the wet deposited rain, sleet, snow, and ice, mature roofs can detain nearly 50% (during 2003) of the year-round wet deposition. Storm water that does leave the roof is delayed, depending on the level of roof medium saturation, anywhere between .10 minutes and 6 hours. When runoff did occur from a green roof, peak runoff for all events never exceeded was always less than 50% that of the non-green roof.
Storm water quality also improved. The pH rose significantly which, when discharged into streams and lakes, would have a beneficial effect on fish and other aquatic life. The seven states that contribute most to acid rainfall are located in the Ohio River Valley, including Pennsylvania and include sulfates and nitrates. While we did not monitor sulfates - their concentration over the last few years seems to have been reduced, nitrates have remained the same or increased. Nitrates also have a significant effect of stream and lake eutrification. Our results show significant nitrate reduction overall as well as for each monitored rain event. Turbidity, responsible for a number of other pollutants was also significantly reduced. In addition to improving the quality of the runoff, the total quantity of runoff is also reduced further reducing the concentration.
Contact: Dr. David Beattie, 814-863-2263