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Ecological Engineering 37 (2011) 1709–1717

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Heavy metals in leachate from simulated green roof systems
Sarah E. Alsup a,1 , Stephen D. Ebbs a,∗ , Loretta L. Battaglia a , William A. Retzlaff b a b

Department of Plant Biology, Southern Illinois University Carbondale, 420 Life Science II, 1125 Lincoln Drive, Carbondale, IL 62901-6509, United States
Department of Biological Sciences, Box 1651, Southern Illinois University Edwardsville, Edwardsville, IL 62026-1651, United States

a r t i c l e

i n f o

Article history:
Received 28 February 2011
Received in revised form 21 June 2011
Accepted 29 June 2011
Available online 12 August 2011
Green roofs
Heavy metals
Urban environment
Vegetated roofs
Water quality

a b s t r a c t
The contribution of green roofs to urban water quality, either as sinks or sources of pollutants, is an open question. This study examined leaching of Cd, Fe, Ni, Pb, and Zn from simulated green roof systems that had been deployed under field conditions and naturally leached for 22 months. The objectives were to determine if Arkalyte (an expanded clay), when mixed with pine bark as a substrate, leached metals and if so, whether leaching was influenced by the depth of substrate, structural components of the green roof system, or wet/dry deposition. Leachate was collected from each system after wet deposition events in
June 2007, October 2007, February 2008, and April 2008 and analyzed. The concentration of four elements routinely exceeded USEPA water quality criteria for chronic and/or acute toxicity and were therefore of possible relevance to water quality, particularly for Pb. The frequency and intensity of local wet deposition influenced the volume of leachate recovered from the systems and in some instances the corresponding metal concentration in the leachate. There were no consistent trends with respect to depth and metal concentration in the leachate, due perhaps to the confounding effects caused by leaching of metals from materials used to construct the built-in-place systems and from inputs from deposition. Further evaluation of this substrate and the structural materials is needed to determine if their use in green roof systems will improve or degrade urban water quality.
© 2011 Elsevier B.V. All rights reserved.

1. Introduction
Urbanization and industrialization have significant negative impacts on the urban environment, including the loss of green space, increased impermeable roof surface area, increased storm water runoff, increased burden on water treatment facilities, and decreased water quality. Runoff from rooftops contributes to altering the quality of storm water runoff in the urban environment thus being included among non-point pollutants sources (Chang et al.,
2004). The roofing materials themselves (e.g., roofing membranes, tars, adhesives, drainpipes, guttering) are potential sources of pollutants. With respect to heavy metals, for example, runoff from metallic roofing materials (e.g., aluminum, copper, galvanized iron, zinc) can have Zn concentrations from >1000 to >16,000 ␮g L−1 and
Cu concentrations from >150 to >2500 ␮g L−1 . Plastic, polyester, shingle, and tile roofing materials have also in some instances produced runoff with similar concentrations of Pb and Zn as well as elevated concentrations of Cd, Fe, and Mn (Chang et al., 2004;
Förster, 1996; Gnecco et al., 2005; Mason et al., 1999; Schriewer

∗ Corresponding author. Tel.: +1 618 453 3220; fax: +1 618 453 3441.
E-mail address: (S.D. Ebbs).
Current address: 4304 Springdale Ave., St. Louis, MO 63134, United States.
0925-8574/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2011.06.045 et al., 2008). Industrial activities, coal-burning power plants, incinerators, vehicular traffic, and other sources introduce pollutants onto roofs at comparable