Event-based size distributions of particulate matter transported during urban rainfall-runoff events

Source area rainfall-runoff conveyances are capable of mobilizing and transporting a very hetero-disperse size gradation of particulate matter (PM). This PM is most commonly characterized through a single event-based gravimetric index such as total suspended solids (TSS) or suspended sediment concentration (SSC) without an examination of PM fractions or particle size distributions (PSDs). Recognizing the utility of such indices, this study went further to examine PM suspended-settleable-sediment fractions and PSDs to representatively characterize the entire event-based PSD of the PM non-colloidal fractions from 1 μm to larger than 24,500 μm transported by urban source area runoff events. This study examined the hetero-disperse PSD, and provided a mass balance analysis to ensure representative event-based results. On an event basis, fine PM (<75 μm; the suspended and settleable fractions) accounted for from 25% to 80% of the gradation on a mass basis, and gravel-size PM (>2000 μm) ranged from 0.5% to 30%. Measured PSDs were compared to literature PSDs on paved surfaces, and in urban runoff from paved surfaces as well as the associated sampling and analysis methods. Results indicate that published urban street surface PSDs are generally within a similar range, with wider variability for published runoff PSDs. PSD variability is attributed to differences in sampling and analytical methods between studies. Knowledge of PSDs is critical for PM transport and fate, pollutant partitioning and distribution, as well as the non-stationary behavior of unit operations.