Heavy metals in leachate from simulated green roof systems

• Published in: Ecological engineering 2011-11, Vol.37 (11), p.1709-1717
• Main Authors: Alsup, Sarah E., Ebbs, Stephen D., Battaglia, Loretta L., Retzlaff, William A.
• Format: Article
• Language: English
• Subjects: acute toxicity; Bark; Cadmium; clay; Deposition; dry deposition; Green roofs; Heavy metals; iron; Leaching; lead; Lead (metal); nickel; pollutants; Roofs; Simulation; United States Environmental Protection Agency; Urban environment; Vegetated roofs; Water quality; water quality criteria; wet deposition; zinc
• ISSN: 0925-8574; 1872-6992
• DOI: 10.1016/j.ecoleng.2011.06.045

► Deposition can introduce heavy metals into green roof systems. ► Materials and substrates used in green roofs can represent sources of heavy metals. ► An Arkalyte:pine bark substrate tested in field green roof systems leached metals. ► Metal leaching from the substrate was variable with time and deposition and not dependent on depth. ► Metal leaching from this substrate may adversely impact urban water quality. 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.