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Particle Size Distribution of Highway Runoff and Modification Through Stormwater Treatment

by Ana Marie Karamalegos, M.S.E., Michael E. Barrett, Ph.D., Desmond F. Lawler, Ph.D., Joseph F. Malina, Jr., Ph.D.

ABSTRACT

Investigating the particle size distribution and density of particles in stormwater runoff is important to reduce the degradation of the quality of the receiving water resulting from nonpoint source pollution. The physical properties of stormwater runoff impact the distance that pollutants travel during a storm event as well as the types of constituents transported. The small particle size range affects transport and characteristics of the constituents since the large particles will settle out easily. Therefore, comparing size distributions, especially in the small size range, of runoff and of the effluent of the Best Management Practice (BMP) in place to treat runoff adds insight into the removal efficiencies of small particles by these treatment systems. The objective of this research was an investigation of the characteristics of stormwater runoff from various sites throughout Austin, TX. The sites included a bridge approach highway, a bridge deck itself, and other sites on a stretch of highway away from bridges. The BMPs included a vegetated filter strip, a full sedimentation sand filter (also known as an Austin sand filter), and an extended detention basin. A protocol was developed to determine the average particle density of the runoff samples, which involved measuring and analyzing the suspended sediment concentration (SSC), particle size distribution function, and volume concentration distribution of the runoff samples. The SSC is a measure similar to the suspended solids concentration, but the entire sample is analyzed rather than a measured aliquot to ensure that the large particles are included in the measurement. The SSC and particle size distributions were determined for untreated runoff samples as well as the associated effluent of the BMP. SSCs for the bridge approach highway samples were higher compared to the bridge deck runoff samples; however, a similar mass range was observed between the 75 μm and 125 μm sieves. The particle size distribution function for all of the bridge approach highway runoff samples suggested that the mid-size particle range (1.8 μm < dp < 25 μm) influenced the SSCs; however, small particles (dp < 1.8 μm) did not have much impact on the SSC. The calculated densities of almost all of the runoff samples collected were below the density of sand (2.65 g/cm3), which commonly is assumed to be the density of the particulate material in runoff. The density of the particles in stormwater runoff affects the particle removal efficiency of treatment processes; therefore, knowing the particle density is important in ensuring the maximum amount of particle removal when designing BMPs. The full sedimentation sand filter at the bridge approach highway site appeared to be the most efficient BMP in removing sediment in the small particle size range since this treatment process reduced the amount of particles with a diameter larger than 1 μm. Full sedimentation sand filters should be considered when deciding which type of BMP should be used for stormwater treatment at a particular site; however, the location and economic conditions must also be considered
before choosing a BMP treatment process. The extended detention basin alone effectively can treat the runoff. However, particles with a diameter between 1 μm and 3 μm (0 < log dp < 0.5) are not removed, and the Austin sand filter removed these particles. Vegetated filter strips did not remove small particles efficiently; however, this BMP could be more appropriate based on financial and location considerations. Future research should examine the influences of storm size on the SSCs and the resulting average particle density of stormwater runoff to continue to improve stormwater treatment.

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