Effectiveness of Three Best Management Practices for Highway-Runoff Quality along the Southeast Expressway, Boston, Massachusetts
Water-Resources Investigations Report 02-4059
Best management practices (BMPs) near highways are designed to reduce the amount of suspended sediment and associated constituents, including debris and litter, discharged from the roadway surface. The effectiveness of a deep-sumped hooded catch basin, three 2-chambered 1,500-gallon oil-grit separators, and mechanized street sweeping in reducing sediment and associated constituents was examined along the Southeast Expressway (Interstate Route 93) in Boston, Massachusetts. Repeated observations of the volume and distribution of bottom material in the oil-grit separators, including data on particle-size distributions, were compared to data from bottom material deposited during the initial 3 years of operation. The performance of catch-basin hoods and the oil-grit separators in reducing floating debris was assessed by examining the quantity of material retained by each structural BMP compared to the quantity of material retained by and discharged from the oil-grit separators, which received flow from the catch basins. The ability of each structural BMP to reduce suspended-sediment loads was assessed by examining (a) the difference in the concentrations of suspended sediment in samples collected simultaneously from the inlet and outlet of each BMP, and (b) the difference between inlet loads and outlet loads during a 14-month monitoring period for the catch basin and one separator, and a 10-month monitoring period for the second separator. The third separator was not monitored continuously; instead, samples were collected from it during three visits separated in time by several months. Suspended-sediment loads for the entire study area were estimated on the basis of the long-term average annual precipitation and the estimated inlet and outlet loads of two of the separators. The effects of mechanized street sweeping were assessed by evaluating the differences between suspended-sediment loads before and after street sweeping, relative to storm precipitation totals, and by comparing the particle-size distributions of sediment samples collected from the sweepers to bottom-material samples collected from the structural BMPs. A mass-balance calculation was used to quantify the accuracy of the estimated sediment-removal efficiency for each structural BMP. The ability of each structural BMP to reduce concentrations of inorganic and organic constituents was assessed by determining the differences in concentrations between the inlets and outlets of the BMPs for four storms. The inlet flows of the separators were sampled during five storms for analysis of fecal-indicator bacteria.
The particle-size distribution of bottom material found in the first and second chambers of the separators was similar for all three separators. Consistent collection of floatable debris at the outlet of one separator during 12 storms suggests that floatable debris were not indefinitely retained.
Concentrations of suspended sediment in discrete samples of runoff collected from the inlets of the two separators ranged from 8.5 to 7,110 mg/L. Concentrations of suspended sediment in discrete samples of runoff collected from the outlets of the separators ranged from 5 to 2,170 mg/L. The 14-month sediment-removal efficiency was 35 percent for one separator, and 28 percent for the second separator. In the combined-treatment system in this study, where catch basins provided primary suspended-sediment treatment, the separators reduced the mass of the suspended sediment from the pavement by about an additional 18 percent. The concentrations of suspended sediment in discrete samples of runoff collected from the inlet of the catch basin ranged from 32 to 13,600 mg/L. Concentrations of suspended sediment in discrete samples of runoff collected from the outlet of the catch basin ranged from 25.7 to 7,030 mg/L. The sediment-removal efficiency for individual storms during the 14-month monitoring period for the deep-sumped hooded catch basin was 39 percent.
The concentrations of 29 inorganic constituents in bottom sediments were typically higher for the size fraction less than 0.062 mm in diameter. Concentrations of total organic carbon (TOC) were similar for the size fraction less than 0.062 mm in diameter and the fraction greater than 2.00 mm in diameter. Concentrations of total polyaromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) were larger in the fraction greater than 2.00 mm in diameter than in the size fractions less than 2.00 mm in diameter. Since PAHs and PCBs are commonly associated with TOC (including organically coated sediment), it is believed that the PCB and PAH concentrations in the solid fraction less than 0.062 mm in diameter were similar to the concentrations in the solid fraction greater than 2.00 mm in diameter, if not greater because of the effects of the surface area.
The estimated annual suspended-sediment load for the entire study area was about 29,000 kg. Approximately 24,000 kg discharged directly to Malibu Beach and Tenean Beach embayments, and the remaining 5,000 kg discharged to the land surface. These loads do not include an estimated 2,000 kg of suspended sediment retained by the five oil-grit separators in the area.
Mechanized street sweepers were used on the pavement three times during the study. Samples of sweepings were collected each time for the analysis of particle size. The first mechanized street sweeping had no observable effect on subsequent storm loads of suspended sediment. Following the second sweeping, a net increase of the suspended-sediment load was observed at one station and a net decrease of the suspended-sediment load was observed at the second station. These effects, however, were only temporary. The third time the highway was swept was after continuous monitoring was terminated. The particle-size distribution in sweeper samples for the size fraction less than 4 mm in diameter was similar to the particle-size distribution in bottom sediment in the catch basin. The concentration of particles greater than 0.5 mm in diameter was higher in sweeper samples than in samples from the separators, so the sweepers were successful in removing the larger particles. Because the highway lacks curbing that would provide a physical boundary to trap debris and sediment, and the equipment was inefficient in trapping particles less than 0.062 mm in diameter, pavement sweeping provided few water-quality benefits for the Southeast Expressway.
The primary factor controlling the efficiency of each structural BMP in removing suspended sediment was retention time. Examination of constituent-sediment relations suggests that the retention time for many highway-related constituents was short; these constituents either were dissolved and not subject to treatment by simple gravity separation, or were associated with particles less than 0.062 mm in diameter, which commonly passed through the BMPs. Thus, the potential effectiveness of the separators and the catch basins to reduce loads of inorganic and organic constituents was much less than their ability to reduce loads of suspended sediment.
The average relative percent difference (RPD) between concentrations of trace metals in stormwater samples from the inlets and the outlets of the separators ranged from 15 to 30 percent. The average RPD for concentrations of organic constituents was commonly less than about 10 percent and negative in several cases. The separators did not affect the concentrations of dissolved solids as they passed through their chambers. The average RPD between the event mean concentrations for trace metals in samples collected from the inlet and outlet of the catch basin during storms was about 25 percent. The average RPD for concentrations of organic constituents was typically less than 20 percent and even negative in several cases, except for oil and grease, which was near 30 percent. The observed ranges of the RPDs for both types of BMPs were probably larger than the actual ranges because the particle-size and concentration of suspended sediment sampled in the BMP inflows during the four storms introduced a bias in favor of higher concentrations.
Concentrations of fecal and Enterococci bacteria were found throughout the storms at the inlets of the two continuously monitored separators; this result indicated that the pavement washoff process was inefficient or that there was a continuous source of bacteria in the drainage area. The efficiency of the structural BMPs tested in this study in reducing fecal-indicator bacteria concentrations was not quantified; each BMP chamber is likely to retain a quantity of fecal-indicator bacteria proportional to its storage volume after a storm. Removal of bacteria from the BMP is dependent on how well the bacteria survive until the next storm and the potential for bacterial export during the next storm.
Description of Study Area and Highway-Drainage Systems
Methods for Continuous Monitoring of Highway Drainage
Design of Highway-Drainage Monitoring Stations
Measurement of Discharge
Measurements of Turbidity, Specific Conductance, and Water Temperature
Collection and Analysis of Samples
Automatic Sample Collection
Miscellaneous Debris Samples
Measurement of Volume and Mass of Bottom Material in the Oil-Grit Separators and the Catch Basin
Collection of Bottom-Material Samples from the Oil-Grit Separators
Sediment Samples from Mechanical Street Sweepers
Quality Assurance/Quality Control
Analysis of Data
Efficiencies of Structural BMPs
Annual Suspended-Sediment Loads
Predictors for Unsampled Suspended-Sediment Loads
Assessment of Best Management Practices
Temporal Variability of Retained Bottom Material
Particle-Size Distribution and Contents of Retained Bottom Material
Reduction of Debris and Litter
Variability in Suspended-Sediment Concentration Samples
Effectiveness of Oil-Grit Separators in Reducing Suspended-Sediment Concentrations
Effectiveness of Deep-Sumped Hooded Catch Basins in Reducing Suspended-Sediment Concentrations
Estimated Loads of Suspended Sediment in the Study Area
Mechanized Street Sweeping
Variability of Concentrations of Chemical Constituents in Sediment
Reduction of Chemical Constituent Loads Discharged in Storm Flows
Reduction of Fecal-Indicator Bacteria
Appendix 1. Summary Statistics and Analytical Results from the Evaluations of Effectiveness for Three Best Management Practices for Highway-Runoff Quality along the Southeast Expressway, Boston, Massachusetts
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Outside Cover (186 KB) --1
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Table of Contents (80 KB) --6 pages
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Appendix 1B through 1G (939 KB) --12 pages
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Whole report (11 MB) --170 pages
The citation for this report, in USGS format, is as follows:
Smith, K.P., 2002, Effectiveness of Three Best Management Practices for Highway-Runoff Quality along the Southeast Expressway, Boston, Massachusetts: U.S. Geological Survey Water-Resources Investigations Report, 62 p., + 1 CD-ROM.
Send questions or comments about this report to the author, Kirk Smith .
For more information about USGS activities in Massachusetts-Rhode Island District, visit the USGS Massachusetts-Rhode Island Home Page.