Abstract
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roof (Berndtsson, et al. 2006), but this unpublished thesis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
precipitation runoff.
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
MS thesis text pdf 1.3MB
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