United States Department of Transportation
i
Evaluating the Particle Size Distribution and Gross Solids Contribution of Stormwater Runoff From Ohio’s Roads
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2019-03-31
[PDF-3.65 MB]
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Edition:Final Report
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Abstract:In order to properly size stormwater treatment systems, or best management practice (BMPs), the sediment particle size distribution (PSD) is needed to quantify the required hydraulic retention time for particle settling or to understand what other treatment processes, such as filtration, might be needed to meet a target sediment removal goal. Gross solids, or particulate matter greater than one-quarter inch in diameter, is transported by stormwater to surface waters. Gross solids include vegetation and anthropogenic sources of litter. A field monitoring study was undertaken across the state of Ohio to determine the PSD of sediment in and to quantify the mass and volume of gross solids conveyed by stormwater runoff. At the monitoring sites, rainfall was continuously measured and weirs combined with bubbler flow meters were installed in existing catch basins to collect runoff hydrology data. These data were utilized to trigger runoff volume-proportional, composite water quality samples collected by automated samplers during rain events. A total of 176 storm event runoff samples were sent to the laboratory for analysis of TSS and PSD. Gross solids monitoring was undertaken by installing a metal mesh basket beneath the existing grate on a second catch basin near the PSD monitoring location. Gross solids samples were collected every 11 days on average and analyzed in the lab for total mass and volume. Additionally, the mass and volume of nine categories of gross solids were measured: vegetation, plastic, glass, metal, paper, cigarettes, gravel, wood, and fabric. Observed TSS event mean concentrations at the 12 monitoring sites were on the low end of those observed in the literature, with an overall mean of 35 mg/L. Annual loading of TSS varied from 87 lb/ac/yr to 463 lb/ac/yr, with a mean value of 242 lb/ac/yr across the 11 sites. At the twelve PSD monitoring sites, a median d50 of 52.5 ??m was observed, which was similar to the 44 ??m median d50 from previous research studies using similar sampling techniques. The NJDEP particle size distribution, which is frequently utilized for laboratory testing of TSS removal for proprietary devices, was very similar to the mean PSD measured herein. Gross solids volume and mass were predominated by natural vegetation (80.3% of volume and 79.7% of mass) and were seasonal in nature with significantly more gross solids contributed in the fall. During the 190-235 day monitoring periods, a total of 7.4 to 58.2 gallons of gross solids were collected by site, with a mean volumetric loading rate of 0.94 gal/ac/day. Mean gross solids weight per collection event across the eleven sites varied from 0.10 to 7.86 lb. The mean observed sample weight was 1.16 lb, resulting in a mean mass loading rate 0.41 lb/ac/day. Similar to gross solids volume, gross solids mass was significantly higher at urban sites than at suburban or rural locations. Grass clippings from mowing of the roadside shoulder and leaf-fall during the autumn season were the principle contributors of vegetation to roadside.
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