Green Roof Media Selection for the Minimization of Pollutant Loadings in Roof Runoff

Stormwater is a leading cause of water quality impairment in estuaries like the Chesapeake Bay. Green roofs are a potential best management practice for urban areas where land for traditional stormwater practices is unavailable. They are aesthetically pleasing, with excellent water retention ability. However, it is unknown how effective they are at treating stormwater. Typical media installations are shallow-depth (2 to 6 inches), and the media have a low organic and a high mineral content. Optimum media must meet the following objectives: lightweight, capable of supporting plants, and able to retain pollutants. The two primary research objectives are the following: 1. To develop an effective media for green roofs that will improve roof runoff quality while maintaining the known water retention benefits of green roofs, and 2. To demonstrate that green roofing will generate lesser pollutant loadings into urban runoff than traditional roofing materials. In Phase I (currently ongoing), several green roof media (formulated from commonlyused expanded minerals, stormwater filter media, and organic matter) are being evaluated for their abilities to retain the pollutants from a synthetic acid rain. The samples are being analyzed for metals, nutrients, pH, and conductivity. The hypothesis is that one media will be “better” at pollutant removal and permanent retention. In Phase II, once the optimized media has been selected, it will be field-tested on a green roof. The water quality of the runoff from the green roof will be compared with the runoff quality from a traditional roof. Field testing will address two objectives: 1. The media is capable of supporting the green roof plants, and 2. The anticipated removals (including water retention) are actually seen in the field. Phase I results to date on the mineral portion of the future media mix demonstrates all media were able to neutralize the acid rain. Comparing the media for pollutant removal and retention, the expanded shale was best able to retain phosphorus, ammonia, and metals from the synthetic acid rain.