Dynamic Characteristics of Particle Size Distribution in Highway Runoff: Implications for Settling Tank Design

To understand the characteristics of particle size distribution (PSD) in highway runoff and to facilitate designing best management practices, PSD (2-1,000 (mu)m) was monitored in seven rainfall events in the 2002-2003 rainy season at three highway sites in west Los Angeles. Most of the particles in number were less than 30 (mu)m in diameter and more than 90% were less than 10 (mu)m. Particle number concentration decreased rapidly to 6 mm of accumulated rainfall and then declined more slowly throughout the storm. Particle number concentration was correlated with total suspended solids (TSS) and turbidity. Grab sample particle median diameter increased with increasing TSS. A two-compartment settling tank was evaluated using the measured PSD and was effective in removing both small and large particles. Capturing and retaining the first 20% of the runoff volume on seasonal average can remove 40% of the total particulate load based on calculated particle mass. Using literature data for metal concentrations sorbed to particles, this size holding compartment coupled with a similar size continuous flow clarifier can achieve 65-90% removal for the metals investigated.