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An urban stormwater treatment unit was tested as part of an ongoing program
of urban nonpoint-pollution research in Madison, Wis. Flow measurements were
made and water samples were collected at the inlet to, outlet from, and bypass
around the treatment chamber of the device that was installed to collect the
runoff from a city maintenance yard. About 90 percent of the runoff water from
the 4.3-acre basin was treated by the unit. The remaining 10 percent bypassed
the treatment chamber when the flow rate reached approximately 500 gallons per
minute. A 24-percent difference between the estimated amount (405 kilograms)
and the actual amount (536 kilograms) of retained material in the treatment
chamber may be attributed to bedload material that the automatic samplers could
not effectively collect. Assuming this, 8 percent of the total mass in the untreated
runoff water was estimated as the unsampled bedload. On the basis of water-sample
data collected over the course of the study, the suspended solids removal efficiency
of treatment chamber was about 25 percent, and the efficiency of the unit as
a whole was 21 percent. If the unsampled bedload material was accounted for,
the treatment-chamber efficiency was 33 percent. About 19 percent of the total
phosphorus was removed from the water that passed through the treatment chamber
and 17 percent was removed by the unit as a whole. Total polycyclic aromatic
hydrocarbon (PAH) loads were reduced about 39 percent by the treatment chamber
and 34 percent by the unit as a whole; these were some of the most effectively
removed constituents. Total metals were reduced about 20 to 30 percent by both
the treatment chamber and the unit as a whole. In general, dissolved constituents
were unaffected by the unit. The material retained in the treatment chamber
had high concentrations of lead and PAH and may be subject to special disposal
restrictions based on those concentrations and the presence of benzo(a)anthracene.
The chemical makeup of the retained material in other similar stormwater treatment
units will probably vary depending on the land use and activities in the drainage
basin.
U.S. Department of the Interior, U.S. Geological Survey
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