Monitoring of Representative Particle Size Distributions and Gross Solids from Roadways and Application to SCM Design

Road runoff treatment is a challenge due to lack of space in the right of way. Therefore, it is imperative to understand the quality of runoff from roadways so that efficient stormwater control measures (SCMs) may be designed. In terms of TSS control, the particle size distribution of sediment in the runoff controls the amount of sediment (based on Stokes' Law) that may be removed through settling in a typical SCM. For best treatment of TSS, the flow depth in vegetated SCMs during the design storm should be limited to the height of the vegetation. Typical roadside stormwater controls (vegetated filter strips and swales) could be designed for water quality by calculating a hydraulic retention time for a particular removal of sediment. To do so, an understanding of average particle size distributions (PSDs) was needed for roads across North Carolina (NC). A field-monitoring study was conducted during May-November 2012 to obtain PSDs from roads in the three ecoregions of NC: mountains, piedmont, and coastal plain. Two sites in the mountains, six in the piedmont, and four in the coastal plain were monitored for TSS concentrations and PSDs for a minimum of six storms apiece. Roadway types were distributed across Interstate highways (six sites), four lane divided highways (one site), four lane primary roads (three sites), and secondary roads (two sites) to determine the effects that annual average daily traffic have on runoff PSDs. At the Interstate highway sites, two monitoring locations had a permeable friction course (PFC) overlay and one site had a NovaChip overlay, which may modify the expected PSD from a highway. Because nutrients are a concern in NC, nutrient concentrations (TN and TP) were monitored at four sites both in the stormwater and sorbed to the sediment. This will allow for modelling of nutrient removal performance for typical roadway SCMs by estimation of TSS removal from Stokes' Law. Gross solids (trash, debris, and particles greater than 5 mm in diameter) are often overlooked in loading of nutrients to waterways. Automated stormwater samplers are not able to capture most gross solids, and total maximum daily loads (TMDLs) for these pollutants have been established for some watersheds. Monitoring for gross solids was undertaken at four sites: one in the mountains, two in the piedmont, and one in the coastal plain. Three sites were Interstate highways while one was a divided four lane highway. Dry mass of gross solids and nitrogen and phosphorus content were determined in the laboratory. It was determined that gross solids are a substantial portion of the nutrient load, and that they cannot be disregarded in stormwater sampling for nutrient load estimation.