Evaluation of metal leachability from green roof systems and components.

摘要

Green roof technology is becoming increasingly more popular in the U.S. as it provides several economical, sociological, and ecological benefits to urban environments. One specific advantage a vegetated rooftop is known to provide is its ability to retain water and release it at reduced rates. However, with increased interest in these systems, concerns regarding their influence on water quality have been raised. Green roofs can be complex systems and the degree to which water quality might be affected may be related to green roof construction and components within. To answer questions regarding green roof influence on metal contaminants in storm water runoff, a field study and a related laboratory study were designed to investigate potential impacts these systems may have on water quality with respect to heavy metals and some micronutrients.;Overall, results obtained for Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn content observed in leachate collected from the constructs and media treatments from a green roof field study do not indicate that green roofs are a significant source of heavy metal and micronutrient contaminants in urban stormwater runoff and that planted Green Roof Blocks™ release less or the same metal content as planted built-in-place models. Elemental concentrations determined in effluent collected from treatments in the field study were collected for four rain events (June 28, 2007, October 18, 2007, February 4, 2008, and April 11, 2008) and compared to IEPA effluent standards to assess whether or not there were any negative impacts on water quality. Treatments for this portion of the investigation were empty built-in-place models (controls), planted and non-planted Arkalyte media at depths of 5 cm, 10 cm, 15 cm, and 20 cm in built-in place models, planted and non-planted commercial and potential green roof media in Green Roof Blocks™. With the exception of excess Zn levels in collected effluent from the planted 10 cm treatments in October and the planted 5 cm treatments in February as well as the elevated Fe content in leachate obtained from planted glass media in October, metal concentration in effluent acquired from planted treatments were below effluent standards. Also determined to be above IEPA standards in October were Cu, Fe, and Pb measured in effluent obtained from non-planted glass media.;The laboratory portion of this investigation evaluated several commercially-available substrates, bottom ash, lava rock, and composted pine bark to determine the total acid extractable and plant exchangeable content of eight elements (Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn). In addition to this, effluent collected from planted and non-planted treatments using these substrates incorporated with 20% composted pine bark were examined over a course of three leaching events. While all elements examined were observed to be within levels normally found in soils, results from the total acid extractable experiments demonstrate large variation in metal concentration with each substrate and composted pine bark. With the exception of total extractable Cd, there were differences in concentrations for the remaining total acid extractable metals between the substrates and amendment tested.;An artificial leaching study was conducted in the phytotron at Southern Illinois University Carbondale and consisted of eight repetitions of planted in addition to eight repetitions of non-planted pots containing known volumes of 20 % composted pine bark blended with Arkalyte, Haydite, Lassenite, Axis, bottom ash, Axis + bottom ash, and lava which were subsequently leached over three separate leaching events. Collected effluent was evaluated for Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn content as well as volume of water displaced as a function of planting treatment, number of times leached, and substrate type. Metal content in the leachates were typically an order of magnitude lower than values obtained from the batch studies, and the planting, substrate, and leaching interactions varied for each metal with Pb being the most complex. The volume of water displaced upon leachate collection increased across all substrates as a function of leaching event.