Stormwater September 2010 : Page 11

abundant in the environment. The relative concentration of individual PAHs in environmental samples and the relative amount of alkylated versus parent PAHs found in environmental samples depends on the source(s) of the PAHs. When high temperatures (i.e., greater than several hundred degrees Celsius) are involved in a process that produces PAHs, these high temperatures tend to eliminate the alkyl side chain, so processes such as fossil fuel combustion (in gasoline or diesel powered vehicles, or in wood burning), the manufacture of steel, and coal gasification result in a predominance of unsubstituted or “parent” PAHs. Specific known sources of such combustion-related or “pyrogenic” PAHs include combustion-linked atmospheric de-position from diesel and automobile vehicle exhausts, smelters, power plants, legacy manufactured gas plants, creosote from wood treatment applications, forest fires, and residential wood burning. The effect of temperature on the formation of PAHs is elegantly explained in Lima et al. (2005). Crude petroleum and refined petroleum products and unprocessed coal are also major potential sources of PAHs. However, unlike pyrogenic PAHs, these fossil fuel or “petrogenic” PAHs are formed at lower temperatures over a longer period of time. As a result, petrogenic PAH sources are char-acterized by a higher relative abundance of alkyl-substituted PAHs (Figure 1). Petrogenic sources to stormwater commonly include motor fuels, lubricants, asphalt, and petroleum-based solvents. Sources of PAHs in Stormwater and Urban Sediments Sources of pollutants to urban waterways and sediments are nu-merous (Boehm et al. 2002) and include municipal and industrial effluents and discharges, stormwater runoff whether direct or as part of combined sewer overflows, direct deposition on water, and spills, among other pathways. With regard to stormwater pathways in particular, atmospheric deposition on roadways and other paved surfaces has been found to be a major source of PAHs in storm-water and sediments (VanMetre et al. 2000, Stein et al. 2006). The mass (Moon et al. 2006) and specific chemistry (Yunker et al. 2002, Boehm 2006) of the PAHs deposition depends on land use, urban density, and industrial processes, among other factors. Combustion products from transportation, power generation, and coking processes are significant manmade sources that result in atmospheric deposition. Other potential sources of pyrogenic PAHs to stormwater are the runoff from historical sites such as manufac-tured gas plants, wood treatment facilities, and smelters. In addition to combustion, releases of petroleum can also be a source of PAHs in stormwater. Petrogenic PAHs can originate from point sources—such as oil spills or petroleum handling facilities—from asphalt pavements, and from more dispersed sources such as leaking oils from individual vehicles (Boehm 2006). Typically, the percent of PAHs in petroleum products ranges from zero to about 20% by weight. While uncombusted gasoline contains almost none, PAHs are present in diesel fuel, home heating oil, motor oil and other lubricants, and crude oils. In addition, as crankcase oil is used and heated, the concentrations of PAHs increase with time (Pruell and Quinn 1988). Because of the role of combustion products in atmospheric deposition, it has been observed that the average PAH flux (g/m2) from urban areas is up to 45 times as great as flux from undeveloped areas (Stein et al. 2006). In addition, mass per unit time of PAHs flow-ing into stormwater systems per storm event increases with the September 2010 11 www.stormh2o.com

Kirchner Block

 

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