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Metals

Metal concentration in rain in central PA is very low. In most rain events measured concentrations were below detection limits. As with any soil, the green roof media has metals on the cation exchange and as a result the concentration of metal in runoff from green roofs is higher than the concentration of runoff from a non-green rolled asphalt shingle roof. For example iron was only detectable in runoff from one of 5 measured storms from non-green roofs. In contrast iron was detectable in runoff from 4 of 5 rains from green roofs, although in all cases the concentrations were very low (Figure 1 and 2). Similar results were obtained with other metals including zinc. Zinc was detectable in runoff from 2 of 5 measured rains from non-green roofs and 4 of 5 rains from green roofs (Figure 3 and 4).

Although the data would suggest that the green roof contributes to metals in the runoff (Figure 1-4), it is very much a question of what you compare it with (FigureĀ  5). If you compare the runoff from a green roof with runoff from a new galvanized metal roof the green roof has much lower zinc concentration in the runoff. In fact the runoff from a metal roof exceeds the aquatic toxicity limit fro zinc many fold and the total zinc load is many times greater from a metal roof.