Green roofs can affect runoff quality in a number of ways. Because any runoff is filtered through the media and the plants there is potential for both cleansing and contamination. The media and plants act as a particle trap for dust and airborne particulates removing them from the runoff when it rains. The media also acts as an cation exchange filter for charged ions (nutrients and metals) in the rain water. What this means is that if there is a high concentration of an ion in the rain the roof can retain a portion of the ion lowering the concentration in runoff. On the other hand, if the concentration of an ion in the incoming rain is substantially lower than the concentration in the soil (media) solution and on the media cation exchange, then some of the ion will be removed from the soil (media) exchange and the runoff will have a higher concentration of the ion than the rain. This is further complicated by plant uptake and fertilization practices which remove or add nutrients respectively. Runoff concentration of a nutrient is also affected by the parent materials of the media, the use of compost in the media, and the age of the media. It is also important to remember that the amount of a nutrient in the runoff is not only a function of the concentration in the runoff but is also a function of the total runoff. This means that even if the concentration of a nutrient ion in the runoff is the same or is greater from a green than from a non-green roof, the total amount of that nutrient released to the environment may be substantially less from a green roof because the total quantity of the runoff is reduced. The relative quality of the runoff from a green roof also depends on what you compare it with. Runoff from a metal roof will have much higher metal concentrations than runoff from a green roof, which is higher than runoff from a rubber roof.
Robert Berghage, David Beattie, Albert Jarrett, and Thomas O’Conner 2007. Green roof Runoff Water Quality. In Proc. Green Roofs for Healthy Cities, Minneapolis, Mn