Particle size distribution in highway runoff: Measurement, characteristics, and management implications

Stormwater has become the major pollution source to the receiving water bodies in many urban areas due to increased development and improved control of point source pollution. Since most of the stormwater treatment facilities target solids removal, understanding particle characteristics has become the most important step to choose suitable treatment facilities (e.g. Best Management Practices).; PSD in highway runoff was measured in seven storm events in 2002--2003 rainy season. An experimental protocol was developed to measure PSD carried by highway runoff in order to achieve repeatable and reliable results. Sample contamination, sample representativeness, sample storage time and temperature, and PSD reproducibility were evaluated. PSD changed with time and temperature and particle size analysis must be performed within six hour of sample collection to minimize the change of PSD. Composite samples collected by auto samplers were not suitable for particle analysis purpose due to the change of PSD.; A total of 172 grab samples were analyzed from the three monitoring sites. Particle concentration decreased rapidly to 6 mm of accumulated rainfall, and then declined more slowly throughout the storm. Particle concentration was correlated with total suspended solids (TSS) and turbidity. A two-compartment settling tank was proposed as a BMP and was effective in removing both small and large particles when simulated using the measured PSD. Particle first flush was observed and the associated pollutants also demonstrated a first flush.; Optimization of the two-compartment settling tank design was evaluated by fixing the total volume of two compartments and changing the volume ratio between them. When the design storm for the total volume of the tank is only a few millimeters, no storage compartment produced the highest particle removal efficiency. When the design storm for the total volume of the tank is more than 10 mm, a volume ratio 3:1 of the storage compartment to continuous flow compartment produced the highest particle removal efficiency. Maximum metal and toxicity removal efficiencies increased rapidly with the increasing total design storm size up to 13 mm design storm size, and decreased slowly thereafter.