Particulate Matter Particle Size Distributions Transported in Urban Runoff

A very wide size range of particulate matter (PM) transported in urban stormwater poses significant challenges to sampling, analysis and control. In addition, mis-application of particle analysis often have the potential to cause misrepresentation of hetero-disperse PSD in urban stormwater by failing to characterize the entire gradation; or are carried out after sample holding time (< 12 hours) has resulted in natural coagulation and flocculation to occur. A methodology is developed which parallels particle size analysis from geotechnical engineering to characterize event-based size distribution of PM transported in urban stormwater by sampling and also analysis of the entire PM size range transported in runoff. This paper presents the non-colloidal PSDs for event-based size distribution of PM ranging in size from 1 μm to larger than 24,500 μm transported during urban runoff events. Through a literature review, the PSD results in this study were compared to those PSDs in urban runoff previously published in the literature with details of sampling and analytical methods employed. In order to demonstrate the validity of stormwater PSD results, mass balance error analysis, requiring a 90% mass recovery, was carried out for each monitored event. While the non-colloidal PSD results in this study demonstrate that urban stormwater transports PM in size ranging from 1 μm to larger than 24,500 μm, PSDs from the literature focus on more limited size ranges. With the common stormwater application of automatic samplers appropriately developed for less dense suspended floes from steady wastewater flows, the coarser fraction of the gradation is truncated by such sampling, in particular for source area runoff. Additionally, sample holding times that allow coagulation and flocculation truncate the suspended fraction of the gradation. Ultimately, urban environs and receiving water are impacted by the entire PM gradation, but through differing mechanisms and time scales; resulting in the need to characterize and control the entire gradation. Because of the hetero-disperse nature of runoff PM, the suspended fine fraction is appropriately characterized and controlled based on particle volume/number and in contrast, on a gravimetric basis for the coarse fraction larger than 75 μm.