Dr. Shirley Clark is one of the hidden gems of the Chesapeake Bay stormwater research community, tirelessly working out in her fine labs at Penn State- Harrisburg. She recently sent me a paper on the quality of roof runoff from various kinds of roofing materials.
The main conclusion is that roof runoff quality is fundamentally determined by the nature of the roof surface, which can be quire diverse. Some kinds of roof surfaces are benign, whereas others can be a major source of metals and other pollutants for a long time. Her paper points to the importance of product substitution to get the cleanest possible runoff, and is a must read for anyone who is serious about designing rainwater harvesting systems.