Road Ecology Center

Suspended particulate matter transported from roadway surfaces represents one of the most substantial sources of non-point source pollution in highway runoff. In addition to increasing turbidity and depositional loading, suspended sediment can retain and transport other pollutants to receiving water bodies. Roadway suspended solid loads may be reduced by using Best Management Practices (BMPs) for stormwater. Non-structural BMPs involve the removal of particulate matter from roadway surfaces prior to runoff transport (e.g., street sweeping), whereas various structural BMPs involve end-of-pipe devices that remove suspended solids prior to discharge to receiving waters. Two types of structural BMPs, a deep-sumped hooded catch basin and three 1500-gallon offline water quality inlets (WQIs), were investigated to assess their effectiveness in reducing highway contaminant concentrations along the Southeast Expressway in Boston, Massachusetts. Automatic monitoring techniques were used to characterize the temporal and spatial variability in suspended sediment transport through each structural BMP. The effectiveness of each BMP in reducing suspended sediment loads was assessed using a mass balance approach. The suspended sediment removal efficiency for the catch basin and two 1500-gallon WQIs was 39, 35, and 28 percent, respectively. The particle size distribution of the suspended sediment entering the structural BMPs indicated more than half of the suspended sediment in highway runoff was material less than .062 millimeters (mm) in diameter (sand/silt break). However, particle-size analysis of retained bottom sediment from three WQIs at the conclusion of the study indicated that an average of 74 percent of retained sediment particles were greater than .062 mm in diameter. About 92 percent of the material found in the deep-sumped hooded catch basins was comprised of these larger particles. The primary factor controlling the suspended sediment removal efficiency of each structural BMP was residence time. For example, the average removal efficiency of the WQIs during storms with rainfall depths less than 0.2 inches was 43 percent. This increased efficiency was a function of residence time rather than from “active” stormwater treatment. Flows from small storms simply displaced previously collected stormwater in which the suspended sediments had time to settle during the static antecedent period. The capture efficiency of suspended sediment was further reduced by re-suspension of fine-grained sediments within the WQIs, as well as from high flows bypassing the WQIs. Re-suspension also occurred within the catch basin. Moreover, collection of floatable debris at the outlet of a WQI suggests that floatable debris also was not indefinitely retained by either structural BMP. Due to the exorbitant costs of more frequent clean-up efforts (e.g., street sweeping, and clean-out of catch basins and WQIs) or more WQIs along the Southeast Expressway, source control measures are the most practicable means of reducing Suspended Sediment Concentration (SSC) loading from the highway. Future research includes developing a model that estimates contaminant loading from highway runoff, and evaluating conventional as well as innovative WQIs with real-time, flowweighted sampling to assess whether their performance justifies their widespread use, especially along roadways.