Measurements of nutrients and mercury in green roof and gravel roof runoff

Green roofs are a popular stormwater management technique because they are effective in reducing runoff volume from buildings. However, this does not necessarily result in a reduced pollutant load. Runoff from experimental gravel and green roof plots and two sets of real gravel and green roofs, were analyzed for mercury, nitrogen, phosphorus, and metals. The effects of roof type (green versus gravel) and underlayments (drainage layer, water-retention layer) on runoff volume, pollutant concentration, and pollutant load were evaluated. Although mercury concentrations in runoff were often higher from green roofs than gravel roofs, the reduction in runoff volume from green roofs typically resulted in no significant difference in runoff load. The green roofs also leached significantly higher concentrations of the nutrients phosphorus and nitrogen than the gravel roofs, but reduction in runoff volume did not similarly mitigate the nutrient load. Underlayment type had no significant effect on runoff volume, mercury, or phosphorus concentrations. Plots with a water-retention layer embedded with fertilizer initially leached higher nitrogen, but were similar to other green roof treatments two months following installation. Alum and Ultra-Phos Filter (UltraTech International, Inc.) were also tested for their potential to reduce the phosphorus in runoff. The average reduction in phosphorus in runoff directed through alum-filled pouches was 22%. Based on the elevated nitrogen and phosphorus load running off of the green roofs, caution is recommended when considering the application of fertilizer during installation or maintenance of green roofs. When choosing green building technology, materials and methods must be carefully chosen to match the most critical needs of the local environment.