Response to Comment on “PAH Concentrations in Lake Sediment

Nov 19, 2014 - (1) We reported that, following a 2006 ban on coal-tar-based pavement sealant (CT sealant) in Austin, Texas, the sum of the 16 Priority...
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Correspondence/Rebuttal pubs.acs.org/est

Response to Comment on “PAH Concentrations in Lake Sediment Decline Following Ban on Coal-Tar-Based Pavement Sealants in Austin, Texas”

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dust on nearby unsealed pavement, resulting in elevated PAH concentrations.6,9,10 DeMott and Gauthier indicate that declines in ∑PAH appear to begin in the late 1990s, prior to the 2006 ban (Figure 2 in ref 1). An apparent decline in ∑PAH from 1998−2005 in the USGS sediment cores is not statistically significant (p-value = 0.71 using Spearman’s rank correlation) and the variability is within analytical uncertainty of ∑PAH measurements. Although it is not known why the rapid increase in PAH concentrations of the 1960−1990s slowed in about 2000, the source-apportionment modeling of PAHs indicates that CT sealant continued to be the dominant source of PAHs to Lady Bird Lake until at least 2014,1,5 and that changes in the CTsealant contribution have been the major driver of PAH trends in this lake. DeMott and Gauthier’s contention that the trend testing involved “selective inclusion of data” refers to the exclusion of flood deposits from the statistical analysis. As detailed in Supporting Information for ref 1, a major flood in 2007 brought in sediment from upstream reservoirs and reaches of the less developed greater watershed to Lady Bird Lake. Because the flood sediment is not representative of urban runoff from Austin, it was not included in the trend tests. Statistical testing that includes all postflood-affected samples yields significant downward trends in ∑PAH for both cores.1 Additionally, as noted in ref 1, a Kruskal−Wallis test comparing the ∑PAH for samples deposited during 1998−2005 (mean 7.98 mg kg−1) to samples deposited during 2012−2014 (mean 3.32 mg kg−1) indicates that the decrease of 58% realized 6−8 y after the ban is significant (p-value = 0.0004). The conclusion that the significant decrease in PAH concentrations can be primarily attributed to Austin’s 2006 ban on CT sealant is consistent with published academic and government research on the importance of CT sealant as an urban PAH source. Pavlowsky11 concluded that elimination of use of coal-tar sealants in Springfield, MO, would lead to a 80− 90% reduction in PAH concentrations in streams and ponds, but noted that it might take years to decades for that reduction to be realized. Source-apportionment modeling by the Minnesota Pollution Control Agency estimated that 67% of PAHs in Minnesota urban stormwater ponds were from CT sealant.12 Researchers at the University of New Hampshire Stormwater Center reported that approximately 30 times more PAHs were exported on a per area basis from CT-sealed pavement than from an unsealed parking lot.9 Our recent article provides direct evidence that elimination of use of CT sealants resulted in a substantial reduction in PAH contamination in Lady Bird Lake. We thank DeMott and Gauthier for identifying an error in Supporting Information Table S-2.1 A corrected version of

e welcome the opportunity to respond to the comment by DeMott and Gauthier, which was funded by the Pavement Coatings Technology Council, on our recent article.1 We reported that, following a 2006 ban on coal-tar-based pavement sealant (CT sealant) in Austin, Texas, the sum of the 16 Priority Pollutant PAHs (∑PAH) in sediment in Lady Bird Lake, the principal receiving water body for Austin runoff, declined 58%.1 CT sealant, a black liquid that is sprayed or painted on asphalt pavement, typically is 15−35% by weight coal tar or coal-tar pitch, which are known human carcinogens.2 CT sealants have an average ∑PAH concentration of about 66 000 mg kg −1,3 100s to 1000s of times greater than concentrations in other common PAH sources such as used motor oil, tire particles, and weathered asphalt.4 Declining concentrations of PAHs in Lady Bird Lake sediment are consistent with earlier estimates, based on PAH profiles, that about 77% of PAHs in the lake sediment were from CT sealant.5 In addition to the trend testing, the subject publication provides a quantitative evaluation of PAH sources by using both statistical analyses of similarity in PAH profiles and the Contaminant Mass Balance source-apportionment model, and discusses other potential PAH contributors in detail.1 DeMott and Gauthier write that PAH profiles in Lady Bird Lake sediments “are more consistent with typical parking lot/ street dust profiles than CT sealer profiles” and contend that PAHs in dust and scrapings from CT-sealed pavement are from a mixture of various urban sources. This disregards the ordersof-magnitude differences between PAH concentrations in dust from unsealed parking lots and in dust and scrapings from CTsealed lots.6 Samples in the CT-sealant dust profile for six cities, for example, had a mean total PAH (sum of 12 PAHs) of 2200 mg kg−1, 81 times greater than the mean of samples from unsealed lots (27 mg kg−1) in the same cities,6 indicating that about 99% of the PAHs in dust from the CT-sealed lots was from CT sealant, precluding any appreciable contribution to the CT-sealant profile from other urban PAH sources. DeMott and Gauthier question why selected urban dust and scraping samples7,8which they assume are not CT-sealant relatedhave PAH profiles similar to those of Lady Bird Lake sediments. Urban dust is a PAH receptor, not a primary PAH source. The profiles are similar because the three sets of samples selected by DeMott and Gauthier likely contain PAHs from CT sealant. One set of samples, from two lots in Austin, was scrapings of asphalt-based sealant applied over CT sealant (total PAH concentrations of 340 and 2000 mg kg−1 in scrapings8 compared with an average of 50 mg kg−1 in asphalt products3). The other two sample sets were for dust from unsealed parking lots and streets in cities where use of CT sealants is, or was, common, and the PAH profiles therefore likely reflect off-site transport. At least three studies have identified off-site transport of PAHs from CT-sealed lots to This article not subject to U.S. Copyright. Published 2014 by the American Chemical Society

Published: November 19, 2014 14063

dx.doi.org/10.1021/es5053107 | Environ. Sci. Technol. 2014, 48, 14063−14064

Environmental Science & Technology

Correspondence/Rebuttal

Supporting Information for ref 1 has been provided to the journal.

Peter C. Van Metre* Barbara J. Mahler

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U.S. Geological Survey, 1505 Ferguson Lane, Austin, Texas 78754, United States

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Van Metre, P. C.; Mahler, B. J. PAH concentrations in lake sediment decline following ban on coal-tar-based pavement sealants in Austin, Texas. Environ. Sci. Technol. 2014, 48 (13), 7222−7228. (2) IARC. Chemical agents and related occupations: Volume 100F, A review of human carcinogens; International Agency for Research on Cancer Lyon, France, 2012. (3) Mahler, B. J.; Van Metre, P. C.; Crane, J. L.; Watts, A. W.; Scoggins, M.; Williams, E. S. Coal-tar-based pavement sealcoat and PAHs: Implications for the environment, human health, and stormwater management. Environ. Sci. Technol. 2012, 46, 3039−3045. (4) 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 (15), 5560−5566. (5) Van Metre, P. C.; Mahler, B. J. Contribution of PAHs from coaltar pavement sealcoat and other sources to 40 U.S. lakes. Science of the Total Environment 2010, 409, 334−344. (6) Van Metre, P. C.; Mahler, B. J.; Wilson, J. PAHs underfoot: Contaminated dust from sealcoated pavements. Environ. Sci. Technol. 2009, 43 (1), 20−25. (7) Breault, R. F.; Smith, K. P.; Sorenson, J. R. Residential Street-Dirt Accumulation Rates and Chemical Composition, And Removal Efficiencies by Mechanical- And Vacuum-Type Sweepers, New Bedford, Massachusetts, 2003−04; U.S. Geological Survey: Denver, CO, 2005; p 27. (8) Mahler, B. J., Van Metre, P. C., and Wilson, J. T. Concentrations of Polycyclic Aromatic Hydrocarbons (PAHs) and Major and Trace Elements in Simulated Rainfall Runoff From Parking Lots, Austin, Texas, 2003, U.S. Geological Survey Open-File Report 2004−1208; U.S. Geological Survey: 2004; p 25. (9) Watts, A. W.; Ballestero, T. P.; Roseen, R. M.; Houle, J. P. Polycyclic aromatic hydrocarbons in stormwater runoff from sealcoated pavements. Environ. Sci. Technol. 2010, 44 (23), 8849−8854. (10) Yang, Y.; Van Metre, P. C.; Mahler, B. J.; Wilson, J. T.; Ligouis, B.; Razzaque, M. 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, 1217−1223. (11) Pavlowsky, R. T. Coal-tar pavement sealant use and polycyclic aromatic hydrocarbon contamination in urban stream sediments. Phys. Geogr. 2013, 34 (4−5), 392−415. (12) Crane, J. L. Source apportionment and distribution of polycyclic aromatic hydrocarbons, risk considerations,and management implications for urban stormwater pond sediments in Minnesota, USA. Arch. Environ. Contam. Toxicol. 2014, 66 (2), 25.

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dx.doi.org/10.1021/es5053107 | Environ. Sci. Technol. 2014, 48, 14063−14064