Acid Rain and Runoff pH
One of the most consistent runoff water quality influences of a green roof is the ability of a green roof to neutralize acid precipitation. Because acid rain is filtered through the media which is generally buffered to a pH over 7 runoff pH is increased relative to runoff from a non-green roof. This affect is most beneficial in areas with highly acidic precipitation like the Northeastern US. The capacity of a green roof to neutralize acid precipitation is influenced by the parent material of the media. Accelerated aging tests suggest that most of the commercially available media have the potential to neutralize the equivalent of 10-20 years of acid rain. Once the buffer capacity of the media is used up it will be necessary to lime the roof to maintain pH and restore the roofs buffering capacity.
Berghage et al., 2007. Quantifying Evaporation and Transpirational Water Losses from Green Roofs and Green Roof Media Capacity for Neutralizing Acid Rain. NDWRCP 04-DEC-10SG