Abstract

Green roofs are contemporary technological solutions that provide multiple benefits to the urban environment. Special attention has recently been attributed to their capacity to retain or absorb air- born pollutants. The current study aimed to evaluate the capacity of extensive green roof systems to retain eleven elements including heavy metals deposited as urban air pollutants. The study was performed at the Agricultural University of Athens. Thirty specialized lysimeters were constructed and placed on Agricultural University of Athens library building rooftop. They were equipped with automated runoff data loggers and sub- sampling collectors. Extensive green roof layering system was constructed within each lysimeter followed by substrate which was placed at two different substrate depths (either 8 cm or 16 cm). The lysimeters were planted with different plant types: a) turfgrass (Festuca arundinacea), b) xerophyte (Origanum onites)and c) succulent CAM plant (Sedum sediforme). Empty lysimeters and lysimeters with bare substrate without plant cover were also included as control simulations of conventional roofs and non- vegetated green roofs, respectively. Samples were collected from the runoff and analyzed for pH, EC, NO3-, Arsenic (As), Cadmium (Cd), Chromium (Cr), Copper (Cu), Iron (Fe), Nickel (Ni), Lead (Pb), Antimony (Sb), Selenium (Se), Vanadium (V) and Zinc (Zn). Results revealed a complex pattern of interactions regarding the response of conventional, vegetated and non- vegetated green roof systems and substrate depths to the parameters measured. The internationally health established limits were not surpassed with only some exceptions, recorded on specific dates and not as a continuous trend. The vegetated and non- vegetated green roof systems exhibited the potential to act as buffer zones when substrate was subjected to acidic rainfall events. Arsenic concentrations in the runoff of the non- vegetated lysimeters were higher compared to the vegetated lysimeters exhibiting a potential capacity of green roofs to improve water runoff quality. Cadmium, Cu, Se, Sb and V concentrations in the runoff of both vegetated and non- vegetated green roof systems were lower compared to the conventional roofs, presenting a potential to mitigate pollution in urban runoff. During the second study year of Cr, Fe, Ni, Pb exhibited increased leaching in the runoff of the vegetated and non- vegetated green roof systems. The "first flush" phenomenon was observed on several occasions (Cd, Cu, Pb, Sb, Se, V, and Zn), though its intensity varied. The substrate depth was found to affect the behavior of some vegetated systems since shallow substrates exhibited lower V concentrations and deep substrate occasionally exhibited elevated Zn runoff. Festuca arundinacea in lysimeters with shallow substrate reduced As and Cr leaching in the runoff. The green roof systems planted with Origanum onites in deep substrate presented increased Zn levels in the runoff and in shallow substrate elevated As and Se levels. Sedum sediforme planted lysimeters with deep substrate, exhibited lower Se and higher runoff concentrations of As, Cd, Cr, and Ni compared to the other vegetated lysimeters. In conclusion, the results of the present study, highlight the potential of green roof systems to act as urban biofilters, and reveal the paramount importance of the multifactor pattern of interactions among the distinct environmental (substrate, climate) and vegetative (plant species, development habit, growth) factors in all green roof systems.