Abstract

Rooftops are both a source of and a pathway for contaminated runoff in
urban environments. To investigate the importance of
particle-associated contamination in rooftop runoff, particles washed
from asphalt shingle and galvanized metal roofs at sites 12 and 102 m
from a major expressway were analyzed for major and trace elements and
PAHs. Concentrations and yields from rooftops were compared among
locations and roofing material types and to loads monitored during
runoff events in the receiving urban stream to evaluate rooftop sources
and their potential contribution to stream loading. Concentrations of
zinc, lead, pyrene, and chrysene on a mass per mass basis in a majority
of rooftop samples exceeded established sediment quality guidelines for
probable toxicity of bed sediments to benthic biota. Fallout near the
expressway was greater than farther away, as indicated by larger yields
of all contaminants investigated, although some concentrations were
lower. Metal roofing was a source of cadmium and zinc and asphalt
shingles a source of lead. The contribution of rooftop washoff to
watershed loading was estimated to range from 6 percent for chromium
and arsenic to 55 percent for zinc. Estimated contributions from
roofing material to total watershed load were greatest for zinc and
lead, contributing about 20 and 18 percent, respectively. The
contribution from atmospheric deposition of particles onto rooftops to
total watershed loads in stormwater was estimated to be greatest for
mercury, contributing about 46 percent.

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4.2.2. Roofing material

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The asphalt-shingle roofing material sampled for this study was found
to be a source of lead and possibly mercury. Lead concentrations and
yields were significantly higher in particles washed from asphalt
shingle roofs than in those washed from metal roofs. Asphalt shingle
roofs may also be a source of mercury. Concentrations of mercury from
asphalt shingle roofs were significantly higher than concentrations
from metal roofs, but yields of mercury were not. However, in those
cases where mercury yields from asphalt shingle roofs did exceed those
from metal roofs, the difference was much greater (3-8 times greater)
than when the yields from metal roofs exceeded those from asphalt
shingle (1-2 times greater). Regional atmospheric fallout, generally
thought to be the major source of mercury in the environment ([Swain et
al]), does not explain the elevated concentrations of mercury often
seen in urban sediments, thus the possibility that asphalt shingle
roofs may be a source of mercury to the urban environment is intriguing
and warrants further investigation.

5. Summary and conclusions

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The relative contributions of atmospheric deposition versus roofing
material to yields from rooftop runoff were also evaluated. Lack of a
roofing material source indicated that PAH, arsenic, chromium, and
copper were from atmospheric sources. Nickel was also dominantly from
atmospheric sources, however, about 40 percent of the lead, 37 percent
of the zinc, and 16 percent of the cadmium in rooftop runoff (median
values) were determined to come from roofing materials, with the
remainder coming from atmospheric deposition. The possibility that
asphalt shingle roofs are a significant source of lead to urban
waterbodies has not, to our knowledge, been reported previously. Some
mercury may also be contributed by asphalt shingle roofs.

HTH,

D