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.

I. Background
The incidence of widespread water pollution began with the onset of the Industrial Age in North America. In the 1970’s, the United States Government reacted to this problem with a series of laws and the creation of the Environmental Protection Agency. One of these laws was the Clean Water Act (CWA) of 1972. Although the Act initially focused on point sources of pollution such as industrial and municipal wastewater discharges, other sources of pollution such as stormwater runoff were later recognized as significant sources of pollution (U.S. EPA, 1996). Point source wastewater discharges and stormwater discharges are now regulated under the CWA and require National Pollutant Discharge Elimination System (NPDES) permits, which limit the amount of pollutants that can enter a body of water.

Stormwater is increasingly becoming recognized as one of the leaders in water quality impairment. Reports from the EPA state that urban runoff was the largest source of water quality impairment in estuaries, and urban runoff was the third leading source of pollution in streams, rivers, and lakes (U.S. EPA, 1994). Also, the 2000 Water Quality Inventory reported that the main sources of pollution affecting the United States shoreline included urban runoff, storm sewers, and non-point source runoff (U.S. EPA, 1994). Development trends of people moving to shorelines will cause increased strain on estuaries like the Chesapeake Bay unless storm management programs are implemented. In 2000, nearly thirteen years after the CWA act addressed stormwater regulations, it was reported that 50% of estuaries that were surveyed in the US were not clean enough to support such uses as fishing and swimming (U.S. EPA, 2002). This shows that there is significant need for improvement in the way urban runoff is managed. One method selected for improving the health of waterbodies is the development and implementation of Total Maximum Daily Loads (TMDLs) for a waterbody based on both the desired use of the river/stream/lake and the stressors that are impairing its use. The implementation of TMDLs may force a change in many NPDES permits as nonpoint source dischargers receive more stringent permit requirements. For many permittees, the new requirements likely will force a survey of pollutant sources ‘upstream’ of the outfall and may result in requiring a site owner to treat the runoff prior to discharge.

Over the past two decades, many stormwater best management practices (BMPs) have been engineered and installed to reduce the amount and improve quality of stormwater runoff. Typical BMPs include wet and dry detention ponds, sand filters, bio-retention areas, constructed wetlands, and, relatively recently introduced to the U.S., green roofs (Moran, 2004). Currently, there is a strong drive for research on the effectiveness of stormwater BMPs as governments
attempt to improve water quality. The focus of this research is green roofing as a stormwater BMP. Green roofs have gained acceptance because of the aesthetics, but there is limited documentation on their effectiveness to improve urban stormwater runoff quality.

Green roofs are vegetated roof covers containing plants, growth media, drainage layers, and waterproof membranes. These roofs take the place of traditional roofing materials and offer additional benefits. The two types of green roofs are intensive and extensive. Intensive green roofs have deep growth media (10 inches to 15 feet) with a high organic content in the media.

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