GREEN ROOFS AND ECOSYSTEM SERVICES: RUNOFF WATER QUALITY AND QUANTITY

The I-ZEB Project (Intelligent Zero Energy Buildings) was recently approved within the third framework agreement between Lombardy Region and National Research Council with the main aim of providing technological solutions and innovative materials in buildings from an energy, environmental and structural point of view. Among the proposed strategies, vegetated roofs are becoming a widespread technology in our cities in a NZEB (Nearly Zero Energy Buildings) perspective, since they increase thermal insulation, inertia and evapotranspiration of roof and they contribute to the reduction of the building energy demand for heating and cooling. Besides, they provide many ecosystem services in densely populated areas, such as reduction of stormwater runoff, mitigation of urban heat island effect and noise pollution, aesthetic improvement. A less investigated aspect is the contribution to the reduction of urban runoff pollution by absorbing pollutants present in wet and dry atmospheric deposition. Recent researches showed that during periods of intense evapotranspiration, water-soluble substances may be absorbed in the vegetated roof reducing the pollution transfer to the environment. Contaminant concentrations in green roof runoff may be comparable to those found in precipitation (e.g. nitrogen, trace elements), but the strong capability of these roofs to reduce annual runoff may result in a significant reduction of the total pollutant load. Notwithstanding this, if not correctly designed, vegetated roofs may contribute to water pollution by releasing contaminants from soil, fertilizers or drainage systems. For example, significant release of nutrients, DOC and some trace elements were emphasized. The potential of providing ecosystem services in terms of water quality are strongly determined by the green roof type/arrangement and by local climatic conditions. A direct experimental research in Milan will clarify the potential of this technology in a metropolitan area.