CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Comment on “PAHs Underfoot: Contaminated Dust from Coal-Tar Sealcoated Pavement is Widespread in the U.S.”
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n a series published in Environmental Science and Technology1-3, Mahler, Van Metre, and their colleagues have investigated the role of coal tar-based pavement sealants (CT-sealants) as a source of polycyclic aromatic hydrocarbons (PAHs) in urban sediments. Data from single sediment samples from 10 urban lakes were used to support the claim that CT-sealants provided a “substantial contribution” to many urban watersheds.2 These authors have recently provided sediment data from 40 lakes.4 We applied these data4 to the forensics analysis methods used in ref 2. A lack of consistency in the results of the methods used in these two papers calls into question whether this source characterization supports the hypothesis concerning the relative role of CT-sealants.5,6 The authors1,2 relied upon PAH double ratio plots of fluoranthene/pyrene (Fl/Py) and benzo[a]pyrene (BaP)/benzo[e]pyrene (BeP) as an important line of evidence. An arbitrary line in Figure 42 divided sediments supposedly impacted by the CT-sealants from unimpacted samples. In ref 4 the EPA Chemical Mass Balance (CMB) Model was used to estimate the contribution of coal tar to urban sediments. We have concerns about the results because it appears that model requirements of source sufficiency and stability were violated. While discussed in ref 4, many problems with application of such models highlighted in a recent review7 were not resolved. Despite these concerns, we used the results as published. If the results of refs 2 and 4 are consistent, there should be a relationship between the sealant contribution estimated using CMB and the Fl/Py versus BaP/BeP ratios.6 We observe no relationship as samples within each of four classes of % CT-sealant contribution (e25%, 26-50%, 51-75%, g76%) are spread across the range of PAH ratios (Figure 1a). Samples from all classes are both in and outside of the box highlighted in ref 2 as indicating CT-sealant impacts. Looking at each ratio individually (Figure 1b), there is not the expected sigmoid relationship6 with modeled CT-sealant contribution. Methods such as ratio analysis or receptor modeling have been described as “biased by a priori assumptions as to the number and nature of the contributing sources7.” A means of overcoming this bias is to use multiple lines of evidence and demonstrate consistency among the methods. To evaluate PAH contributions to a localized watershed, Stout and Graan (2010)5 used chemical fingerprinting, PAH ratio analysis, principal component analysis (PCA), and positive matrix factorization (PMF). Unlike receptor models, the multivariate methods PCA and PMF do not require pre-identification of potential source chemistries. Consistency was demonstrated by observing the expected differences in GC-FID chromatograms and PAH concentration histograms for samples separated by PCA, and by being able to predict total PAH concentration using the results of PMF.5 Sediment PAH source identification has been the subject of significant research, and atmospheric deposition has been recognized as a primary contributor. While challenging conventional r 2011 American Chemical Society
Figure 1. Consistency between refs 2 and 4 would be demonstrated by a relation between the PAH ratios and the modeled sealant contribution.6.
wisdom is the essence of science, it is critical to have sufficient and consistent technical support. Mahler and Van Metre have not evaluated the data in a way to test their null hypothesis, so have not eliminated atmospheric deposition as a major source of PAHs. In papers published in ES&T, PAH ratios were used to link PAHs associated with CT-sealed parking lots to urban sediments.1,2 Without this evidence, there is little basis for claims regarding the relative role of CT-sealants, and less for the use of such superlatives as the “dominant” source1 or “substantial” contribution.2 While the points raised in this commentary do not eliminate CT-sealants as a potential PAH contributor in some urban systems, we suggest that the authors apply and compare the results of multiple source characterization Published: March 11, 2011 3185
dx.doi.org/10.1021/es200240g | Environ. Sci. Technol. 2011, 45, 3185–3186
Environmental Science & Technology
CORRESPONDENCE/REBUTTAL
techniques including sample chemistry (PAH ratios and concentration histograms), receptor models, and multivariate methods. Kirk O’Reilly,* Jaana Pietari, and Paul Boehm Exponent, 15375 SE 30th Pl, Suite 250, Bellevue, WA, 98007, 425-519-8700
’ ACKNOWLEDGMENT We acknowledge funding from the Pavement Coatings Technology Council. ’ REFERENCES (1) Mahler, B. J.; Van Metre, P. C.; Bashara, T. J.; Wilson, J. T.; Johns, D. A. Parking lot sealcoat: An unrecognized source of urban polycyclic aromatic hydrocarbons. Environ. Sci. Technol. 2005, 39, 5560–5566. (2) Van Metre, P. C.; Mahler, B. J.; Wilson, J. T. PAHs underfoot: Contaminated dust from coal-tar sealcoated pavement is widespread in the United States. Environ. Sci. Technol. 2009, 43 (1), 20–25. (3) Yang, Y.; Van Metre, P. C.; Mahler, B. J.; Wilson, J. T.; Ligouis, B.; Razzaque, MD. M.; Schaeffer, D. J.; Werth, C. J. Influence of coal-tar sealcoat and other carbonaceous materials on polycyclic aromatic hydrocarbon loading in an urban watershed. Environ. Sci. Technol. 2010, 44 (4), 1217–1233. (4) Van Metre, P. C.; Mahler, B. J. Contribution of PAHs from coal-tar pavement sealcoat and other sources to 40 U.S. lakes. Sci. Total Environ. 2010, 409, 334–344. (5) Stout, S.; Graan, T. Quantitative source apportionment of PAHs in sediments of Little Menomonee River, WI: Weathered creosote versus urban background. Environ. Sci. Technol. 2010 44 (8) 29322939. (6) Ahrens, M. J.; Depree, C. J. A source mixing model to apportion PAHs from coal tar and asphalt binders in street pavement and urban aquatic sediments. Chemosphere 2010, 81 (11), 1526–1535. (7) Galarneau, E. Source specificity and atmospheric processing of airborne PAHs: Implications for source apportionment. Atmos. Environ. 2008, 42 (11), 8139–8149.
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