Jarrett Okita 1 , Cara Poor , PhD, PE 1 , * , , Jessica M. Kleiss , PhD 2 , Ted Eckmann , PhD 3
Spring 2018
aging and green roofs, long term impact of green roofs, concentrations of zinc, copper, phosphorous and phosphate in green roof stormwater runoff, soil moisture, water quality
Green roofs have become a common method to increase water retention on-site in urban
areas. However, the long-term water quality of runoff from green roofs is poorly understood.
This study evaluated the water quality of stormwater runoff from a regular (non-vegetated)
roof, a green roof installed 6 months previously, and a green roof installed 6 years
ago in Portland, Oregon. Samples of runoff were taken during every rain event for
10 months, and analyzed for total phosphorus (TP), phosphate (
), total nitrogen (TN), nitrate (
), ammonia (NH
3), copper (Cu), and zinc (Zn). Runoff from the green roofs had higher concentrations
of TP and
and lower concentrations of Zn compared to the regular roof. Average TP concentrations
from the 6-year old roof and 6-month old roof were 6.3 and 14.6 times higher, respectively,
than concentrations from the regular roof, and average
concentrations from the 6-year old roof and 6-month old roof were 13.5 and 26.6 times
higher, respectively, compared to the regular roof. Runoff from the 6-month old green
roof had higher concentrations of TP and
than the 6-year old green roof during the wet season, but lower concentrations during
the dry season. The 6-month old green roof installations where receiving waters are
sensitive or impaired may need additional treatment methods to reduce phosphorus levels.
As green roofs age, water retention decreases and phosphorus leaching increases during
the dry season.