Water Quality Impacts of Green Roofs Compared with Other Vegetated Sites: Fid-Bau Portal

Water Quality Impacts of Green Roofs Compared with Other Vegetated Sites

Barr, Catherine M. / Gallagher, Patricia M. / Wadzuk, Bridget M. / Welker, Andrea L.

Green roofs are a convenient stormwater management strategy in highly developed, urbanized areas. Research has demonstrated that green roofs are effective at mitigating stormwater runoff volume, in addition to providing a range of other environmental benefits. Previous studies on the overflow from extensive green roofs have indicated that the overflow from these roofs, particularly those maintained with fertilizer, may contribute nutrients to nearby streams, storm sewers, and adjacent waterways. Whereas many studies have compared green roof nutrient concentrations with that of conventional roofs and urban streams, few studies have compared green roof effluent with other vegetated systems’ effluent. In this study, located in Villanova, Pennsylvania, green roof effluent was evaluated and compared with vegetated land uses (e.g., woods and grass) and other stormwater control measures (e.g., bioinfiltration rain garden and constructed stormwater wetland) typically found in urban watersheds. Effluent samples from all sites were tested and analyzed for concentration and mass loading of nitrogen (nitrate, nitrite, total Kjeldahl nitrogen, and total nitrogen) and phosphorus (orthophosphate and phosphorus). Overall, the green roof effluent concentrations for nitrogen and phosphorus species were statistically different than the other land uses, and often with higher concentrations. The green roof effluent was most statistically similar to the wooded land use. From a mass loading perspective, in terms of unit area of the contributing watershed, the green roof had a higher loading than the other land uses. However, the fertilized green roof exported less than 15 percent of the total input mass of nitrogen and phosphorus, demonstrating that volume reduction aids in managing the effluent.