A primer on the role of mass, number and surface area of anthropogenic and engineered particles

Particulate matter is ubiquitous in our modern urban environments. Most anthropogenic activities of our daily routines generate particulate matter. For example traffic activities generate copious loads of particulate matter, as do land-disturbing and construction activities. Yet, there are also significant sources of biogenic particulate loads in the modern urban environment; sources such as vegetation. While the ubiquity of this particulate matter 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 vigorous and lively debate. Despite this debate, such knowledge is of fundamental importance for modeling of urban runoff particulate phenomena, in particular with respect to the transport and fate of chemicals such as metals, nutrients and organics. As our modeling capabilities begin to couple hydrology and chemistry, models such as SWMM will require defensible data and algorithms for such constitutive phenomena. Three common topics of debate are presented herein.

The first is particle size distributions (PSDs) in urban runoff. Despite advances in knowledge of PSDs, how runoff is sampled and examined has not significantly changed in practice, yet such knowledge has a significant impact on model development, the design and performance of BMPs and MS4 inventories as well as acute and chronic impacts of particulate matter in receiving waters. The reality is that suspended and colloidal particulates are ecologically and environmentally important as well as acutely bio-available. Yet sediment and debris size particles are generally more labile than suspended particles and represent both a significant inventory load for MS4s and maintenance liability for BMPs that is not accounted for with traditional TSS. Our measurements, models, treatment and regulations are well-served by accounting for this wide spectrum of PSDs, despite the additional complexity. Such knowledge will also provide a foundation for BMP numerical effluent limits, monitoring and inventory requirements in MS4s, as well as accurate and quantifiable TMDLs.

The second topic is that of surface area as a function of granulometry (for example, PSDs) and how surface area (SA) with units of L2 relates to transport and fate of chemicals. In large part, this debate can be resolved with knowledge of the actual PSD on a gravimetric basis (units of 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. By definition, SSA increases with decreasing particle size; yet depending on the gravimetric PSD, SA can be predominately in the settleable and sediment fractions of source area runoff.

The third topic is a quantitative example of the wide disparity between engineered particulate surface behavior with respect to fate of chemicals. Common media, such as perlite, used in stormwater filtration have very little ability to sorb stormwater chemicals of concern. In large part this can be shown to be a function of available SA, surface charge and species competition. Equilibria, kinetics, breakthrough and desorption are quantifiable phenomena.