CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Response to Comment on “PAHs Underfoot: Contaminated Dust from Coal-Tar Sealcoated Pavement is Widespread in the U.S.”
W
e welcome the opportunity to respond to O’Reilly et al.’s comment regarding our publications on coal-tar-based pavement sealants (CT-sealants) and polycyclic aromatic hydrocarbons (PAHs).1-5 In these publications, we identified CTsealant as a major source of PAHs in urban settings on the basis of multiple lines of evidence: PAH diagnostic ratios in urban stream1 and lake sediments2 were similar to those in CT-sealed pavement dust; PAH profiles for lake sediment were highly correlated (r = 0.95) to those in CT-sealed pavement dust;3 source-receptor modeling attributed 50% of PAHs and upward trends in U.S. urban lakes to CT-sealant;3 and organic petrography identified particles of coal-tar pitch from CT-sealant in pavement and street dust, soils, and stream and lake sediments.5 In response to O’Reilly et al.’s suggestion that we “apply and compare the results of multiple source characterization techniques,” we direct them to these publications. Additionally, we direct them to work by Watts et al.6 that recently identified CTsealant as a major contributor of PAHs to stormwater on the basis of direct monitoring of loads and diagnostic ratios and principal components analysis (PCA). O’Reilly et al.’s principal criticism is a supposed lack of a relation between the CT-sealant contributions estimated using the contaminant mass balance model (CMB)3 and PAH ratios for those samples,2 yet we find a consistent and logical relation. Progressing by quartiles of proportional CT-sealant contribution from low to high for the 120 samples from 40 lakes used in ref 3, 16, 44, 70, and 94% have fluoranthene:pyrene ratios >1.15; 16, 41, 60, and 53% of samples have benzo[a]pyrene:benzo[e]pyrene ratios >0.87; these ratio thresholds used in ref 2 were not arbitrary, but rather bounded the samples of sealed parking lot dust from central and eastern U.S. cities where CT-sealant use is common. O’Reilly et al.’s concerns regarding model requirements of “source sufficiency and stability” are too vaguely stated for us to address. We do not expect our environmental data to plot on the smooth sigmoid curve described by Ahrens and Depree (2010; in O’Reilly et al.) because the sigmoid relation shown is for a hypothetical 2-end-member mixing curve, not environmental data, which are affected by multiple PAH sources3 and natural and analytical variability. Regarding the contention that in ref 3 we did not adequately resolve problems concerning forensic analysis of PAHs raised by Galarneau (2008, in O’Reilly et al.), we note that Galarneau discusses forensic analysis of PAHs in the atmosphere, not aquatic sediment. As discussed in ref 3, little change has been reported in measured PAH profiles between atmospheric particles and lake sediments. Regarding the choice of forensic approaches, we point out that PCA, positive matrix factorization (PMF), and CMB are all receptor models, and all rely on the assumption that all sources have been identified and have a fingerprint. If CT-sealant is not considered as a source, none of these models will identify it. Had Stout and Graan (2010; in O’Reilly et al.) considered CT-sealant in their forensic analysis, they might have found that much of the 20 mg/kg of their “urban This article not subject to U.S. Copyright. Published 2011 by the American Chemical Society
background” PAH was attributable to CT-sealants. Our research3 indicates that those urban lakes with the smallest CT-sealant contributions (50%, n = 20) have concentrations similar to Stout and Graan’s urban background concentration (mean of 25 mg/kg). O’Reilly et al. contend that we “have not eliminated atmospheric deposition as a major source”. In fact, in ref 3 we considered many sources of PAHs with atmospheric pathways, including vehicle emissions, coke-oven emissions, and coal, oil, and wood combustion. Vehicle and coal-combustion sources combined accounted for, on average, 41% of PAHs in the 40 lakes, slightly less than the contribution from CT-sealants of 50%.3 The importance of the contributing source was a function of PAH concentration: PAH concentrations were strongly correlated (r2 = 0.94) to the mass loading of PAH from CTsealants estimated by CMB (ref 3 Figure 6). The importance of CT-sealants as a PAH source is further demonstrated by greatly elevated PAH concentrations in parking lot dust and runoff from CT-sealed lots relative to unsealed pavement;1,2,5-7 in soils near CT-sealed pavement;2,8 in stream sediment downstream from outfalls draining CT-sealed lots;9 and in house dust in residences adjacent to CT-sealed lots.4 The multiple approaches used in our research and that of others lead to a consistent conclusion, that CT-sealant is a major contributor of PAHs to the urban environment. Our research was funded by the U.S. Geological Survey1-5 and by the City of Austin, Texas.1 Peter Van Metre and Barbara Mahler U.S. Geological Survey, 1505 Ferguson Lane Austin, Texas 78754, United States
’ 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 PAHs. 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, 20–25. (3) Van Metre, P. C.; Mahler, B. J. Contribution of PAHs from coaltar pavement sealcoat and other sources to 40 U.S. lakes. Sci. Total Environ. 2010, 409, 334–344. (4) Mahler, B. J.; Van Metre, P. C.; Wilson, J. T.; Musgrove, M.; Burbank, T. L.; Ennis, T. E.; Bashara, T. J. Coal-tar-based parking lot sealcoat: An unrecognized source of PAH to settled house dust. Environ. Sci. Technol. 2010, 44, 894–900. (5) Yang, Y.; Van Metre, P. C.; Mahler, B. J.; Wilson, J. T.; Ligouis, B.; Razzaque, M.; Schaeffer, C.; Werth, C. The influence of coal-tar sealcoat and other carbonaceous materials on polycyclic aromatic
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Environmental Science & Technology
CORRESPONDENCE/REBUTTAL
hydrocarbon loading in an urban watershed. Environ. Sci. Technol. 2010, 44, 1217–1233. (6) Watts, A.; Ballestero, T. P.; Roseen, R. M.; Houle, J. P. Polycyclic aromatic hydrocarbons in stormwater runoff from sealcoated pavements. Environ. Sci. Technol. 2010, 44, 8849–8854. (7) Selbig, W. Concentrations of Polycyclic Aromatic Hydrocarbons (PAHs) in Urban Stormwater, Madison, Wisconsin, 2005-08, OFR 2009-1077; U.S. Geological Survey: Denver, CO, 2009; 46 p. (8) Polycyclic Aromatic Hydrocarbons Released from Sealcoated Parking Lots— a Controlled Field Experiment to Determine if Sealcoat Is a Significant Source of PAHs in the Environment; Final Report to USEPA, University of New Hampshire Stormwater Center: Durham, NH, December 2010; p 51. (9) Scoggins, M.; McClintock, N.; Gosselink, L.; Bryer, P. Occurrence of polycyclic aromatic hydrocarbons below coal-tar-sealed parking lots and effects on stream benthic macroinvertebrate communities. J. N. Am. Benthol. Soc. 2007, 26 (4), 694–707.
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