|Runoff from street surfaces is a major contributor of pollution in the City of Madison. One way to control roadway runoff is to use street sweeping to remove pollutants before being transported in stormwater runoff. This option may be preferable to structural Best Management Practices (BMPs) since these can be costly and/or take up considerable space.
This study's objective was to determine if the dirt load on residential streets is reduced by various street sweeping scenarios and to what degree. The City already performs street sweeping, but this study may indicate that we need to modify our practices.
Samples were collected from two basins to determine any effects on water quality. The water quality sampling results from these basins and the street dirt load data were used to estimate the benefits that may be achieved using the other street sweeping programs.
The study used a paired basin approach: data was collected from four basins. The control basin had no sweeping except at the start of the equilibration periods. A second basin had sweeping as it is currently done throughout Madison. Another basin was swept with sweepers the city currenlty uses but with parking regulations in place to allow full access to the curb. The final basin was swept with the same parking regulations as the previous basin but using a new generation sweeper.
The USGS collected vacuum samples once a week from the four basins for the duration of the study. Street dirt data was used to determine the pick-up efficiency of the street sweepers and the rate of dirt build-up on the streets.
The schedule had three equilibration periods in it, which were meant to allow the street dirt levels to rise back up to baseline levels after a sweeping period concludes. At the beginning of these periods, the control basin were swept.
The study period ran for 30 months, but Dec. to March were not monitored. This resulted in 22 months of monitoring, or approximately 95 weeks, 48 of which will have street sweeping occurring. This schedule resulted in 95 street dirt collections, for a total of 572 street dirt samples.
The study also resulted in 80 storm samples from the control and test sites for a total of 160 runoff water quality samples. An additional 30 samples were collected for quality assurance. These water samples were sent to the USGS sediment laboratory in Iowa for total sediment concentration and sand-silt split particle size analysis.
Concentrations of the following constituents were measured at the City of Madison Department of Public Health Laboratory:
- NO2+ NO3
- Kjeldahl, Nitrogen
- Total Phosphorus
- Dissolved Phosphorus
- Suspended Solids
- Total Dissolved Solids
- Chloride, Automated
- Hardness, Total Recoverable, Calc. Method
- Calcium, Total Recoverable, ICP
- Cadmium, Dissolved
- Copper, Total Recoverable
- Coper, Dissolved
- Lead, Total Recoverable
- Lead, Dissolved
- Zinc, Total Recoverable
- Zinc, Dissolved
Data analyses was performed by USGS and the City of Madison to summarize the efficiency of the sweeping operation. Accumulation rates of constituent buildup on the street surface and the runoff loads were developed.
Street dirt data was used to determine the pick-up efficiency of the street sweepers and the rate of dirt build-up on and wash off from the streets. The sieved dirt data was used to see if there is a difference in the collection efficiency of the two size fractions between the new generation sweeper and current sweepers. The data on collection efficiency will allow for estimation of street dirt load reductions using the various street sweeping test scenarios.
For more information contact:
Greg Fries, Engineer