Vegetated, or “green” roofs are used in urban architecture and sustainable building to capture stormwater and enhance urban wildlife. Most research to date has focused on their water retention capabilities, but green roofs also have the potential to treat anthropogenic acid precipitation in urban environments. More specifically, green roofs could reduce sulfate, nitrate and hydrogen ion concentrations in runoff. One previous study has demonstrated that nitrate was reduced in water draining from a green roof (Berndtsson, et al. 2006), but this unpublished thesis appears to be the first comprehensive study of the effects of vegetated roof materials on major ions and acidity in runoff. The objective of this current research was to test whether green roofs could significantly modify acidified precipitation. Experiments were conducted using a variety of materials commonly used in modern green roof design.
Twenty different experimental roof surfaces, each with an area of 0.5 m2, were constructed and exposed to ambient rainfall during the 2008 growing season. Twenty of these plots simulated green roof constructions varying by vegetation, drainage material and soil (substrate) depth and composition. Substrate types included one commercially available topsoil and three hydroponic soils; low density, inorganic materials commonly used in green roofing. Runoff samples were collected from the green roof plots and a control, a traditional asphalt-shingled roof plot. Precipitation was collected in a funnel-style bulk precipitation collector. Collections followed ten precipitation events that occurred between June and October, 2008. Water samples were measured for pH and alkalinity and analyzed via ion chromatography for concentrations of 10 major ions.
As a group, the experimental green roofs effectively lowered nitrate ion concentrations in runoff and raised pH to near-neutral values (from 6.8 to 7.8, varying with roof construction), compared to a mean pH in precipitation of 5.3. The weighted average of nitrate concentrations in precipitation was 0.03 meq/L while average concentrations in the three types of hydroponic substrate plots were one-third lower, at 0.02 meq/L for each substrate type. In contrast, roofs with topsoil substrates had the opposite effect; average nitrate concentration was 0.40 meq/L. This increase could be due to nutrients leaching from the topsoil.
Sulfate ion concentrations were increased in water captured from all green roofs. The weighted average sulfate concentration of precipitation was 0.03 meq/L. Topsoil substrate plots had average concentrations between 0.46-0.86 meq/L, while hydroponic plots had lower increases in sulfate concentration with values ranging between 0.04-0.19 meq/L. The effects of vegetation, substrate thickness, and drainage material on ion concentrations in runoff were insignificant.
In addition to these results, this study indicates that the geographic origins and pathways of weather can control sulfate and nitrate ion concentrations and pH of precipitation falling in the northeastern United States. In precipitation from two tropical systems (Tropical Storm Hanna and Hurricane Kyle), sulfate and nitrate concentrations were at their lowest values (≤0.01 meq/L) and pH reached the highest levels measured throughout the experiment (6.9 for Hanna and 6.1 for Kyle) as compared to all other precipitation events. These results are likely due to the absence of sulfur and nitrogen emitters in the path of these two weather systems.
Although sulfate wet deposition in the northeastern United States has declined since the passage of the Clean Air Act in 1970 and amendments in 1990, concentrations of reactive forms of nitrogen remain high, and pH remains low. This study is one of the first to demonstrate that, in addition to intercepting stormwater, many green roof materials can improve stormwater quality, enhancing their environmental value as elements of sustainable urban building design.
Gillen, J., 2009. The effect of vegetated roofs on acid
Unpublished MSc. thesis, State University of New York at Albany. 82 pp., +xi
University at Albany Science Library call number: SCIENCE Oversize (*) QC 869 Z899 2009 G55
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