SOME OBSERVATIONS ON ATMOSPHERIC DUST FALLOUT
By Ben R. Urbonas, P.E. and John T. Doerfer
For a full paper on this topic go to http://www.udfcd.org/techpapers.htm.
Dust fallout, as a contributor to the pollutants found on urban surfaces, has
been discussed for years and many studies have been done to quantify it (Sartor
and Boyd, 1972; Pitt and Amy, 1973; Pitt, 1979; Mustard et. al., 1985;
Schroder and Hedley, 1986; Schroder et. al., 1987; NADP, 2003 (full
citations are given in the paper posted on the web). Despite these, there
remains controversy as to how much of the total pollutants that are present on
various urban surfaces come from atmospheric fallout. This paper reports on the
findings of atmospheric fallout observed in 2003 on a roof of single-family
residence and in a winterized swimming pool, both located in Denver, Colorado.
In May of 2003, a roof gutter not cleaned for about 5- to 7-years serving
approximately 700 square feet of a single-family residence was cleaned and the
materials were collected and weighted. These materials consisted of wet leaves,
fine sediment and grit materials typically found on asphalt-composition roofs.
The following were observed:
- Total weight of material removed was between 30 to 40 lbs.
- Approximately 1/3 of the mass was grit particles from the composition
- Approximately 1/3 of the mass was wet leaves and water.
- Approximately 1/3 of the mass was very fine sediments.
From these approximations, it was concluded that the gutter accumulated about
12 lbs of very fine sediment that would be classified as part of the Total
Suspended Solids (TSS) when found in stormwater runoff, or about 2 lbs of TSS
per 100 square feet of roof (870 lbs/acre). This loading rate compares favorably
with a value reported by other studies. What is not known is the quantity of
fine solids that were not trapped in the gutter during these years and were
washed down the gutter onto the lawn or onto the streets and paved alleys that
hydraulically connect to the streams in the Denver area. Clearly, this example
illustrates that roofs in the Denver area are significant sinks of atmospheric
fallout and contributors of TSS to stormwater runoff. Similar to the findings
were reported by Beecham (2001) in Sydney, Australia, namely an average load of
5 kg (11 lbs) of sediment being generated from a typical single-family
residential roof on an annual basis.
A SWIMMING POOL
On Memorial Day weekend the cleanings of a residential swimming pool were
sampled and analyzed. Photograph 1 in the full paper posted on the web page
illustrates the difference in the pool bottom before and after cleanout.
Photograph 2 in the full paper shows a Ziploc(tm) bag filled with fine sediment
that was collected from 200 to 250 square feet of the pool's bottom. This sample
does not contain all of the sediment that was on the cleaning equipment filter
(approximately 15% to 25% of the sediment was not captured and went down the
drain during the washing of the filter). These data indicate:
- The total wet weight of the sample is 3 lbs.
- The weight of the solids removed is around 1.5 lbs, or around 0.9 lbs of
solids per 100 square feet of surface. Extrapolating this to a 12-month
period, we get 1.2 lbs/100 s.f. (700 lbs/acre) a year.
- This material would become part of the TSS load in stormwater
A gradation test was performed to determine the distribution of particle
sizes contained within the atmospheric fallout sample taken from the swimming
pool. The results of the gradation test are shown in Figures 1 and 2 of the
posted paper. One-third of the sample can be classified as fine sand (> 74
microns) and two-thirds of the sample as silt and clay. Little more than
20-percent of the particles are clay sized (<2 microns). Studies have shown
an inverse relationship between particle size and pollutant concentration on
street surfaces (Sartor and Boyd, 1972; Pitt and Amy, 1973; Pitt, 1979) and in
bottom sediments in the South Platte River (Steele and Doerfer, 1983). Because
of their small size, the clay- and silt-sized particles are the most difficult
to remove from runoff by sedimentation processes.
WHAT DOES THIS MEAN?
These observations imply that atmospheric fallout is a significant contributor
of TSS found in stormwater runoff in the Denver area. In a semi-arid climate
wind picks up much dust and fine sediment from many surfaces within and adjacent
to the urban area. Unlike climates with more rainfall and humidity, the
atmosphere in a semi-arid climate does not have many opportunities to cleanse
itself. In addition, native vegetated surfaces comprised of bunch grasses
instead of turf grasses do not protect the soils from scour by wind, nor do they
provide the trapping of dust particles that turf-forming grasses provide after
particles settle to the ground.
It was also found instudies by Sartor and Boyd (1972), Pitt (1979), Mustard
et. al. (1985) that TSS buildup rate on impervious surfaces initially occurs
rapidly and then approaches an asymptotic equilibrium. This phenomenon can be
attributed to wind resuspension and scour of deposited particles so that the
buildup of TSS does not continue at the same rate forever. In a swimming pool,
all solids that fall out of the atmosphere cannot resuspend into the atmosphere.
As a result, a swimming pool, a pond or a lake acts as a perfect sink for these
The findings reported in this paper are not based on accurate scientific
measurements, but do provide a realistic assessment of atmospheric fallout in
the Denver area and how it affects stormwater runoff quality. It is recommended
that these non-scientific initial data be better quantified through the use of
more precise controlled measurements in existing sinks for atmospheric fallout
(e.g., winterized swimming pools that have mesh type winter covers, lined ponds,
This less than formal data collection effort suggests that each 100 square
feet of impervious surface can yield as much a 1.0 to 1.2 lbs (0.45 to 0.55 kg)
of solids on an annual average basis. What fraction of this material actually
makes it into stormwater has yet to be determined. If we assume 100% and an
average of 30% of impervious surfaces in the metropolitan area have a direct
hydraulic connection to the conveyance systems, each square mile of urban
development here can produce about 40 to 50 tons
of TSS in stormwater runoff each year reaching our receiving water systes.
Considering that the Nationwide Urban Runoff Program data collected in the
Denver area at commercial and residential sites by USGS indicates an average TSS
concentration exceed 200 mg/L (EPA, 1983), the estimate using the unscientific
samples collected this year compare well to the annual stormwater TSS loads one
calculates using USGS data.
The observations made using simple atmospheric fallout dust capture techniques
clearly show that:
- Atmospheric fallout in the Denver area is a significant source of TSS in
- The fallout consists mostly of very fine particles that are hard to remove
from the water column.
- It does not matter what form the impervious surface takes, this fallout is
shows up in stormwater runoff.
- The less impervious surfaces that have a direct hydraulic connection to
the conveyance system, the greater the chances for the turf lawns and
landscaping to capture these fine particles before they reach the stormwater
- The BMPs currently recommended in Volume 3 of the District's Urban Storm
Drainage Criteria Manual are well suited for the removal of these fine solid
particles from stormwater.
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