William Retzlaff

The exchangeability and leachability of metals from select green roof growth substrates

Batch extraction and leaching studies were conducted with potential green roof substrates (e.g., Axis, Arklayte, coal bottom ash, Haydite, Lassenite, lava rock, and composted pine bark). The results indicated that these materials would not likely be sources of Cr, Cu, Fe, Ni, or Zn and that Lassenite would be considered a source of Mn if the leachate concentrations were compared to USEPA drinking water standards for these elements. Lassenite would not be a source of Mn if the data was compared to a USEPA standard for Mn toxicity to aquatic life. All of the substrates tested leached Cd and/or Pb concentrations that exceeded the USEPA water quality standards at least once during the 6-month leaching study, so these materials may be potential sources of Cd and Pb in green roof storm water runoff. The leaching of Cu, Cd, Fe, Mn, Pb, and Zn was differentially influenced by time and/or the presence of a single Sedum hybridum ‘immergrauch’ plant. The leaching of Cd, Cu, and Pb displayed complex, three-way interactions between main effects (substrate type and the presence or absence of a plant) and between leaching events. For all substrates except Lassenite, the presence of a S. hybridum plant decreased the leaching of Pb over time. The leaching of Cd was generally enhanced by plants for most substrates with time. Collectively the results suggest that changes in the biogeochemical conditions within green roof systems may alter metal solubility, decreasing the leaching of some elements and increasing the leaching of others.

Green Roof Storm-Water Runoff Quantity and Quality

AbstractThe design of a green roof plays a significant role in performance. In a ground-level field study, 28 built-in-place green roof models, four modular units, and four roof decks with standard black membrane were arranged in a completely randomized design. Storm-water runoff quantity and quality were monitored from September 2005 through June 2008. Results indicate that green roof systems significantly reduced storm-water runoff and that system design, growth media depth, and presence of plants impacted storm-water retention. In a second study on a building roof, two modular systems were used to evaluate water loss through evapotranspiration. Water loss of both systems was significant and influenced by system design, growth media type, and presence of plants. Finally, runoff quality analyses from the ground-level field study and from a laboratory pot experiment indicate that nitrate, total suspended solids, and turbidity are generally low in runoff after a first-flush event. Runoff water quality was ...

Suspended Solids in and Turbidity of Runoff from Green Roofs

Green roof technology is used to reduce the quantity of stormwater runoff, but questions remain regarding its impact on quality. This study analyzed the total suspended solids (TSS) in and the turbidity of runoff from green roof growth media mixed with composted pine bark in an indoor pot study. The results showed that there were elevated levels of TSS and turbidity in the runoff that decreased over time for all growth media. Both TSS and turbidity are affected by the type of growth media. Lava and haydite had higher mean TSS and mean turbidity than arkalyte and bottom ash. Vegetation reduced the mean turbidity and mean TSS of the first flush by an average of 53% and 63%, respectively, but generally had no statistically significant effect thereafter. The results indicate that the media, rather than the vegetation, has a greater effect on TSS and turbidity in the runoff. In areas with stringent water quality regulations for stormwater runoff from developed sites, media selection may be an important consideration. It may also be necessary in these regions to ensure that the roof is planted prior to receiving rainfall to minimize the first flush effect and that any irrigation does not result in runoff.

Green Roof Systems as Sources or Sinks Influencing Heavy Metal Concentrations in Runoff

The contribution of green roofs to urban water quality is a subject of increasing interest. This study examined heavy metals in runoff from simulated green roof systems after 22-32 months in the field to assess source or sink behavior. The green roof systems were not sources of metals, except for one instance of elevated Cu in runoff. On some dates, runoff metal concentrations were significantly lower than that from an ethylene propylene diene terpolymer (EPDM) membrane roof simulation, suggesting intermittent sink behavior. This sink behavior did not in most cases decrease runoff metals below the U.S. EPA water quality criteria for chronic Cd or Pb toxicity. If there were metals in the rainwater input from wet deposition, the green roof systems did not seem to sequester those metals, as runoff metal concen- trations were not reduced relative to the EPDM roof simulation. The lack of consistency in source or sink behavior indicates that pollutant concentrations in runoff are influenced by a complex interaction of factors. The contribution to urban water quality provided by these specific substrates if used in green roofs systems, or from modular green roofs, could be equally inconsistent. DOI: 10.1061/(ASCE)EE.1943-7870 .0000601.

Using Turbidity to Determine Total Suspended Solids in Storm-Water Runoff from Green Roofs

AbstractGreen roofs are a technology used to control the quantity of the storm-water runoff and reduce energy consumption. Questions remain about their effect on water quality. Total suspended solids and turbidity are of the main parameters for water quality. This study investigated whether turbidity could produce a satisfactory estimation for total suspended solids (TSS) in the storm-water runoff from green roofs. Measuring turbidity is much faster than measuring TSS; a log-linear model showed strong positive correlation between TSS and turbidity (R2=0.9374) with a regression equation of [ln (TSS)=0.979 ln (Turb.)+0.574]. This equation shows that turbidity is a suitable monitoring parameter for TSS when TSS sampling and testing are impractical or not feasible.

Energy Conservation Analysis of Various Green Roof Systems

The present paper illustrates an experimental analysis on the comparative analysis of air-conditioning energy savings of different green roof applications based on experimental measurements. Tests were carried out on nine different green roof samples, involving three types of growth media (lava, arkalyte, pumice) matched with three sedum types (kamtchaticum, spurium, sexangulare). Temperature readings at the bases of each sample were recorded for 32 months continuously at every 15 minutes. The data was processed and the heat gain of a sample building with each green roof application was calculated theoretically. The energy consumption comparisons showed that the right selection of growth media and vegetation can yield significant energy savings for air-conditioning applications.

Experimental and Theoretical Analysis of Evapotranspiration in Green Roof Systems

This study focuses on the water loss of green roof systems due to evapotranspiration, which is the combination of evaporation and transpiration processes. Water loss through evapotranspiration increases the capacity of a green roof to intercept additional stormwater during the following rain event and keep it out of the municipal stormwater system. Hence it is important that amount of water loss through a green roof, with known plant and growth medium properties and climate conditions, can be quantified. A theoretical model based on energy equation is used and MATLAB is employed in solving the equations due to the complexity of the problem. Relevant heat transfer equations with appropriate thermophysical properties of materials are used. Values obtained from the theoretical analysis are compared with experimental values that were gathered via a field project in which different green roof samples were tested for 2.5 years in terms of their weight under wet and dry seasonal conditions. The results from the theoretical model are found to be in close agreement with the experimental measurements, which is encouraging for predicting water loss of green roof systems in different geographic locations with known climate conditions.

Thermal Evaluation of Green Roof Systems Containing Bottom Ash Growth Media

This paper presents a study on the use of furnace bottom ash (FBA) as a growth medium in green roof applications. Experimental tests were performed on three different green roof samples, involving FBA as the growth media and three sedum types (kamtchaticum, spurium, sexangulare) matched with it. Theoretical analysis was conducted for modeling the heat transfer through the green roof samples. Temperature readings at the surface center, bottom center, and bottom edge of each sample were recorded for 12 weeks with a data sampling frequency of 15 minutes. The results were compared with roofs without vegetation. These reference roofs are covered with black (EPDM), and white (TPO) roofing membranes. The study showed that green roof samples with FBA yielded better thermal performance compared to conventional roof membranes. However plant survival and coverage could be an issue in green roof systems using FBA if necessary amount of plant maintenance is not performed.