Interactions of Phosphorus with Anthropogenic and Engineered Particulate Matter as a Function of Mass, Number and Surface Area
Particulate matter (PM) is ubiquitous in modern urban environments, generated by most anthropogenic activities. Traffic activities, for example, generate significant loads of PM, as do land-disturbing and construction activities. There are also significant sources of biogenic particulate loads in the modern urban environment, such as vegetation. While the ubiquity of PM is generally recognized, the sources, characteristics, transport, treatment and impact of particulate matter in urban runoff (rainfall or snow) continue to be the subject of lively debate. Such knowledge is fundamentally important for modeling urban runoff particulates, particularly regarding the transport and fate of chemicals such as phosphorus and metals. As modeling capabilities begin to couple hydrology, chemistry and PM, improved models such as the USEPA stormwater management model (SWMM) require defensible data and algorithms. Three common topics of debate and new experimental results are presented.
The first concerns particle size distributions (PSDs) in urban runoff. Despite advances in knowledge of PSDs, the sampling and examination of runoff has not significantly changed in practice. Such knowledge should significantly impact model development, the design and performance of best management practices (BMPs) and municipal separate storm sewer systems (MS4). Suspended and colloidal particulates are ecologically and environmentally important as well as acutely bio-available. Sediment and debris size particles represent both a significant load for MS4 and maintenance liability for BMPs that is not accounted for by traditional total suspended solids (TSS). Our measurements, models, treatment and regulations would be well-served by accounting for a wide spectrum of PSDs, despite the additional complexity. This will provide a foundation for BMP numerical effluent limits, monitoring and inventory requirements in MS4s, as well as accurate and quantifiable total maximum daily loads (TMDLs).
The second topic is that of surface area (SA) and pollutant distribution (the focus herein being phosphorus) as a function of granulometry (for example, PSDs) and how SA (units of area L2) relates to the transport and fate of phosphorus. In large part, this debate can be resolved with knowledge of the actual PSD on a gravimetric basis (units of mass M) at a particular location in the watershed or BMP, and knowledge of a normalized quantity that is known as specific surface area (SSA) with units of L2/M (area/mass). By definition, SSA increases with decreasing particle size; yet depending on the gravimetric PSD, total SA can be predominately in the sediment fraction of source area runoff.
The third topic is a quantitative example of the wide disparity between engineered particulate media surface behavior with respect to the fate of phosphorus. Common media, such as perlite, used in stormwater filtration have very little ability to sorb phosphorus in stormwater. This can largely be shown to be a function of available SA, surface charge and species competition. Equilibria, kinetics, breakthrough and desorption are quantifiable phenomena.
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