Forensic Assessment of Refined Tar-Based Sealers as a Source of Polycyclic Aromatic Hydrocarbons (PAHs) in Urban Sediments

Abstract

Atmospheric deposition of particles and their subsequent transport by stormwater are a major source of polycyclic aromatic hydrocarbons (PAHs) in urban sediments. Recently, the results of forensic analysis have been used to promote a hypothesis that refined tar-based pavement sealers (RT-sealers) are another significant source. To evaluate this hypothesis, a suite of forensic methods was applied to a wider range of PAH data for this study. Sediments PAH profiles are no more similar to RT-sealers than they are to a number of other environmental inputs. While RT-sealers were not eliminated as a potential source in some locations, forensic methods did not differentiate their contribution from other sources of PAHs, indicating RT-sealers are not a unique or readily quantifiable source of PAHs to the urban environment.

In response to O'Reilly et al. (2014)

“…Finally, on a double‐ratio plot, samples of lake sediment and sealed parking lot dust from the central and eastern United States grouped together, and apart from 6 other commonly identified urban PAH sources, whereas samples of lake sediment and parking lot dust from the western US plotted in a group distinct from that of central and eastern samples, and closer to the six urban sources. In an earlier publication, O'Reilly et al (), using a different set of ratios, illustrated the same phenomenon (Fig. 2, O'Reilly et al ); their graphical results, as separate from their interpretation of those results, are consistent with the findings of Van Metre et al ().…”

Parsing pyrogenic polycyclic aromatic hydrocarbons: Forensic chemistry, receptor models, and source control policy

“…In a complex setting with numerous sources that change over time, multiple lines of evidence are required (Boehm ; O'Reilly et al ). Although the focus of this article is receptor models, their use should be part of a broader evaluation that includes considerations of site‐specific history and application of a range of environmental forensic tools (Johnson et al ).…”

“…The authors suggested greater similarity between PAHs in sediment samples and RT‐sealed runoff than was observed in either unsealed or petroleum‐asphalt–based sealer in 2 of the double‐ratio plots. Although this was a local study and it ignored the fact that the ratios were within the ranges expected for urban background (Stout et al ), the abstract claimed that these findings indicated that RT‐sealers “may dominate loading of PAHs to urban water bodies in the United States.” To investigate this claim, we used multiple lines of evidence forensic evaluation to consider a wider range of PAH‐affected particles that act as sources to sediments and found that, because of overlapping ranges, diagnostic ratios cannot differentiate among them (O'Reilly et al ). As presented in Mahler et al (), which specific sediment samples were used in the ratio analysis is not clear.…”

“…Overlapping ratios between the sediments and lot dusts, again, are not unexpected, because both are influenced by heavy pyrogenic PAHs. Publication of the 9‐lakes study highlights the similarities between sediments and RT sealer–associated samples, but fails to note that they are no more similar to each other than they are to environmental samples affected by atmospheric deposition such as soil or roof dust (O'Reilly et al ). More importantly, the use of only 1 forensic method without consideration of the many potential sources of PAHs is insufficient to associate cause and effect.…”

“…Comparing the results of the CMB model with results using a multiple lines of evidence forensic approach, we found the CMB output to be inconsistent with either diagnostic ratio analysis or PCA. The expected relationship between the sealer contribution suggested by CMB (O'Reilly et al ; ) and the ratios used to suggest sealer impacts is not observed (Mahler et al ; Van Metre et al ). PCA was unable to separate weathered RT‐sealer sources from the PAH profiles of other environmental particles, and the ranges overlapped between these and some sediment samples.…”