Environ. Sci. Technol. 2006, 40, 3657-3658
Comment on “Parking Lot Sealcoat: An Unrecognized Source of Urban Polycyclic Aromatic Hydrocarbons” In a recent paper, Mahler et al. (1) reached the conclusion that coatings applied to seal and preserve paved parking lots (sealcoat) “may dominate loadings of PAHs to urban water bodies”. This finding is of particular interest because this source, stated to be “previously unrecognized”, is not simply identified as one urban PAH source. The authors go further to suggest that their latest study has identified the dominant source overlooked in their own sequence of previous PAH source investigations as well as in extensive research by other groups that have consistently characterized loading from automobile-related sources and airborne transport of combustion byproducts as the dominant urban PAH sources. We reviewed the specified sources of the data presented in this paper and were unable to reproduce or identify some of the values shown in several figures. In some cases, the data presented in the cited references differed from data presented in the paper (1), and in other cases, the cited sources did not include data needed to support the values plotted in the figures. The authors maintain that their conclusion about the dominance of sealcoat is supported by two lines of reasoning: (i) similarities between particulate PAH loading generated by simulated rainfall events and storm event loads measured in-stream for four urban watersheds and (ii) a comparison of PAH ratios from particles washed off sealed parking lots to ratios from suspended sediments collected from four urban streams. Information used to illustrate these comparisons is presented in Figures 4-6 of the Mahler et al. paper (1). In Figure 4 of ref 1, the authors present three PAH ratios comparing particles washed from sealed parking lots to corresponding values specified to be from suspended sediment samples. The authors state that the particular ratios included represent the best indicators of coal tar sources of PAHs, citing an article by Canton and Grimault (2). One of the ratios presented is benzo[a]pyrene:benzo[e]pyrene. Since the specified citation (2) actually points out the instability of benzo[a]pyrene in the environment, a factor that precludes its use in ratios serving as particularly reliable indicators of PAH source, further clarification of the basis for suggesting benzo[a]pyrene:benzo[e]pyrene to be among the best indicators should be provided. No description of the specific stream sediment results plotted in Figure 4 is provided, and no description of the sample collection and analysis methodologies or information substantiating the comparability of the results from different studies is provided. The text suggests that the sources for the stream sediment results are two previous publications by the same research group (3, 4). However, in ref 3, no concentrations of individual PAHs are presented. In the cited USGS report (4), benzo[e]pyrene, indeno[1,2,3-cd]pyrene, and benzo[ghi]perylene results are excluded from the report and appended data tables. Accordingly, neither of the cited sources contains results that can be used to replicate the computation of the ratios included in Figure 4 and ascribed to stream sediment samples. Unable to replicate the stream sediment values plotted in Figure 4 (1), we attempted to identify other sediment sampling results where the necessary PAH concentrations were available. The City of Austin, TX, where the parking lot 10.1021/es060326t CCC: $33.50 Published on Web 04/29/2006
2006 American Chemical Society
FIGURE 1. Double ratio plot of fluoranthene:pyrene vs indeno[1,2,3cd]pyrene:benzo[ghi]perylene comparing urban stream sediment samples collected by Geismar (5) to particles washed from sealed and unsealed parking lot surfaces (1). runoff experiments were performed, has released such measurements in a report prepared by Geismar (5). Using the results in this report, we calculated fluoranthene:pyrene and indeno[1,2,3-d]pyrene:benzo[ghi]perylene ratios for the Austin samples in which these PAHs were detected. The results are shown in Figure 1 along with those from the particulate washoff samples from sealed and unsealed parking lots presented in the original paper (1) and ratios calculated for National Institute of Standards and Technology (NIST) standard reference materials for diesel particulate matter (SRM 1650a) and PAHs from coal tar (SRM 1597). Using the Austin stream sediment results, we find distinct differences between the PAH ratios calculated for the urban stream sediments and particulates washed off coal-tar sealed parking lots in Austin. Our comparisons indicate that PAH profiles in Austin urban stream sediments more closely resemble diesel exhaust particles than coal tar-sealed parking lot runoff. In contrast, the ratio comparisons selected by Mahler et al. (1) were interpreted as indicating that stream sediments closely resembled the PAH profile from coal tarbased sealcoat. Since this ratio comparison is one of the two lines of information used to support the conclusion that sealcoat can be a dominant source of PAH loading in urban streams, it is of particular importance to consider the differences between the ratios presented in Figure 4 (1) for which we could not identify the corresponding stream samples and those corresponding to the samples reported by the City of Austin and Geismar (5). For their mass balance evaluation, the hypothetical storm event load of ΣPAH from sealed parking lots in a watershed was estimated by multiplying the mean yield of ΣPAH from rainfall simulations on parking lots by the sealed parking lot area of the watershed. The authors note that this estimate assumes that all particles mobilized from the pavement actually enter a stream. This simplification produces an overestimate of parking lot related loading since in actual runoff events engineered sedimentation features, overland flow, and sewer/drainage systems function to reduce PAH transport efficiency. In Figures 5 and 6 of ref 1, PAH event load estimates are identified as corresponding to four watersheds: Lake Como, Echo Lake, and Fosdic Lake in the Fort Worth, TX, area and Williamson Creek in Austin, TX. The sources cited for the event load estimates plotted are again previous publications by the same research team (3, 4). Neither of these sources VOL. 40, NO. 11, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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contains event load data from Williamson Creek, so none of the values purported to illustrate relative source contributions of PAHs to Austin urban streams could be substantiated or replicated. The source of the data for Williamson Creek should be identified and, if it has not been published with a comprehensive description elsewhere, corresponding study design, sampling details, and analysis details should be provided. For the three watersheds where the source information could be identified, the measured in-stream event loads reported in cited references differ significantly from the values presented in Figure 5 of ref 1. Van Meter et al. (4) estimate total PAH loads of 114-1400 g for Echo Lake inflow during four separate storm events. In contrast, the information presented in Figure 5 show data for four separate storm events with only one event exceeding 100 g. PAH loadings measured at the inflow to Lake Como and Fosdic Lake in ref 4 also appear to be higher than the values presented in Figure 5 of ref 1. In the authors’ approach to mass balance analysis, these discrepancies are important to resolve because using lower values for total stream load than those in the cited source serves to exaggerate the apparent contribution related to the parking lot input. For example, based on the previously published PAH event loads for Echo Lake (4), the average value of 616 g would suggest that the estimated contribution from all PAH sources on sealed parking lots in the watershed (33 g) amounted to only 5% of the storm event load to Echo Lake. In contrast, the same parking lot estimate appears to account for more than one-third of PAH loading when compared with the average of less than 100 g of total load to Echo Lake shown in Figure 5 of ref 1. Use of the type of mass balance comparisons presented by the authors to establish the relative importance of PAHs from parking lots and the subfraction that may be related to sealcoat necessitates addressing this apparent range of uncertainty. In conclusion, we were unable to replicate the computations and identify values from cited sources for a number of the data points represented in Figures 4-6 of ref 1. With regard to the PAH ratio analysis, we could not identify the source of the values presented for stream sediment samples,
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and the values that we could identify from the City of Austin appear to contradict the interpretation developed by the authors. With regard to the mass balance analysis, we could not identify the source for values from one watershed, the values presented for the other watersheds do not appear to match those from the cited sources, and the previously published values suggest the relative contribution of PAHs from parking lot sources is substantially less than the “majority” source suggested by the authors. Because these uncertainties relate to the two lines of argument indicated to support a conclusion that parking lot sealcoat could be a dominant source of PAHs in urban streams, clarification is important for understanding the strength of the conclusions of this paper.
Literature Cited (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. (2) Canton, L.; Grimalt, J. O. Gas chromatographic-mass spectrometric characterization of polycyclic aromatic hydrocarbon mixtures in polluted coastal sediments. J. Chromatogr. 1992, 607, 279. (3) Mahler, B. J.; Van Metre, P. C. A simplified approach for monitoring of hydrophobic organic contaminants associated with suspended sediment - methodology and applications. Environ. Sci. Technol. 2003, 44, 288. (4) Van Metre, P. C.; Wilson, J. T.; Harwell, G. R.; Gary, M. O.; Heitmuller, F. T.; Mahler, B. J. Occurrence, Trends, and Sources in Particle-Associated Contaminants in Selected Streams and Lakes in Fort Worth, Texas; U.S. Geological Survey Water Resources Investigations Report 03-4169; U.S. Geological Survey: Denver, CO, 2003. (5) Geismar, E. Identifying sediment contamination sources in watersheds of Austin, Texas. NWQMC National Monitoring Conference, 2000.
Robert P. DeMott* and Thomas D. Gauthier ENVIRON International Corp. 10150 Highland Manor Drive Tampa, Florida 33610 ES060326T
