Green Roofs as An Urban Stormwater Best Management Practice for Water Quantity and Quality in Florida and Virginia.

Green roofs are well known as an urban stormwater volume Best Management Practice (BMP) in northern climates, but information regarding water quality benefits/impacts and optimal plant-growing media combinations for green roofs in the sub-tropics is lacking. The objectives of this study were to: 1) determine the optimal plant-growing medium combination for water and nutrient retention and 2) characterize green roofs’ capability to reduce stormwater volume and peak runoff and 3) determine whether green roofs behave similarly (as a sink or source) for nutrients and metals, in Florida and Virginia. The results of the green roof bin study showed that growing medium type affected water retention and nutrient leaching more than plant type. Water retention ranged from a low of 24% for Building Logics (B) medium with no vegetation to a maximum of 83% for UCF (U) growing medium with perennials. Differences among media were attributed to physical characteristics of the media: pore-size distribution and OM content. Plants increased water retention by 7-10% above bare medium, with perennials having the greatest, and succulents having the least effect.  TP and TN loads for the establishment period (initial 6-weeks) ranged from 110 mg P m^-2 (U-perennials) to 1800 mg P m^-2 for (H-succulents or bare medium); and from 190 mg N m^-2 (U-runners) to 1800 mg N m^-2 (H-succulents) . The majority (60-90%) of the nutrient load leached out in the establishment period of the 24-week study period. Green roofs monitored in Virginia and Florida behaved similarly for water retention and peak reduction. In both Florida and Virginia, small rain events (< 0.254 cm), had significantly (p<0.05) higher mean retention (79% and 98%, Florida and Virginia respectively) than large rain events (26% retention and 72%, Florida and Virginia, respectively). Green roofs significantly (p<0.05) reduced the peak runoff in both Florida (94% for small and 60% for large events) and Virginia (100% for small and 79% for large events). Green roofs behaved similarly for nutrients (sources for phosphorus, sinks for N-NO₃, and buffered pH) and differently for metals. Al and Fe levels were significantly higher (p<0.05) in green roof (GR) runoff than conventional roof (CR) runoff in Florida; while Pb was significantly lower (p<0.05) in Virginia GR runoff. In conclusion, this study found that green roofs in the subtropics are better suited as a stormwater volume control BMP, than a nutrient control BMP.