The role of inorganic green roof substrates in water quality improvement

Extensive green roof systems are gaining popularity as an alternative method to reduce impervious surfaces in urban areas. A key design component of extensive green roof systems is the light-weight substrates typically made from heat-expanded shale, clay and slate, amongst other materials. The physical properties of extensive green roof substrates, primarily water holding capacity and air-filled porosity are essential for system performance. Crumb rubber, a recycled tire material, is a potential amendment for these manufactured substrates. As other recycled products, its use is appreciated for reducing environmental impacts with respect to extraction and production of new materials. Its low bulk density may reduce substrate loads, resulting in decreased costs in engineering for retrofits, maintenance and renovation, while possibly improving the longevity of the substrate's characteristics. During the vulcanization process of tire fabrication, zinc is added, and crumb rubber tends to leach this metal. A hydroponic study comparing three different zinc levels demonstrated that Sedum kamtschaticum, a commercially representative green roof species, completely collapses at concentrations higher than 80 ppm when grown in glass beads after three months of weekly fertigation. However, the plants exhibited a great tolerance to the same levels when grown in green roof substrate. Apparently, the cation exchange capacity of this substrate prevents available zinc to become toxic to plants and reduces the chances for zinc to become an environmental pollutant when leached from crumb rubber-amended green roofs. This study also provides an estimated comparison of the volume being leached from both substrates. The results also demonstrate how extensive green roof substrates contribute to reduce stormwater runoff. Attendees will gain an understanding of the essential properties of extensive green roof substrates and this topic will generate discussion as to the importance, effectiveness and environmental consequences of using recycled components as amendments to these media.