Correspondence/Rebuttal pubs.acs.org/est
Comment on “PAH Concentrations in Lake Sediment Decline Following Ban on Coal-Tar-Based Pavement Sealants in Austin, Texas” dated to 2010−2012 for core LBL.2 (r = −0.92, p = 0.009). This trend is also not significant if all 2007−2012 samples are included (r = −0.56, p = 0.09). Excluding the years between 2007 and 2009 or 2010, respectively, when the PAH concentrations were increasing in each of these cores and relying on only decreasing portions of the reported time series introduces uncertainty related to selective inclusion of data for statistical testing, particularly since different groups of years were chosen for each core. Further, the authors state that “core LBL.2 is considered less reliable than other cores, and use of data from this core therefore was limited to the top two intervals, which were deposited in 2012.” Selectively choosing the top six intervals to report a significant decreasing trend for this core is questionable even by the authors’ own characterization. The authors used a modification of the chi-square (χ2) statistic to evaluate similarity between PAH profiles detected in lake sediments and 22 “PAH source materials.” The best matches for the lake sediment profiles were reported to be dust from CT-sealed parking lots in Austin, TX; dust from CTsealed parking lots in six cities; and aged scrapings from CTsealed parking lots in Austin (Authors’ Tables S-5 and S-6).1 While PAHs from CT sealer might be found in these dust profiles, the samples also include PAHs from other parking lot sources (e.g., tire wear, used oil, vehicle emissions, atmospheric deposition). The authors did not attempt to differentiate the contributing sources to this mixture profile. Note that there is a discrepancy between Tables S-2 and S-4 with regard to the anthracene concentrations in LBL.4. Clarification of the differing values would make replication of the authors’ computations easier. We calculated the same modified χ2 test statistics (mean ± s.d.) comparing the same lake sediments to additional PAH profilesaged scrapings from asphalt-sealed parking lots in Austin, TX4 (χ2 = 0.086 ± 0.03), silt and clay sized fraction in street sweepings from New Bedford, Massachusetts5 (χ2 = 0.096 ± 0.03), and dust from unsealed parking lots in Austin, TX4 (χ2 = 0.109 ± 0.05). These values are similar to modified χ2 test statistics for the authors’ best fit PAH sources (with mean values ranging from 0.072 to 0.122) and suggest that the best fit profiles reflect a mixture of multiple PAH sources representative of urban dust rather than strictly CT sealant. As shown in the Authors’ Table S-6, actual CT sealant source profiles (CT-sealant product and week old CT sealant scrapings) exhibit a much poorer fit to lake sediments. This suggests that the CT contribution of PAHs is not the only source driving the match and that the samples reflect most closely general urban dust sources. The timelines from sediment cores reported in the subject paper, other sampling by the COA, and a similar observed
S
ir: In a recent study, Van Metre and Mahler1 reported decreases in polycyclic aromatic hydrocarbon (PAH) levels in sediment from a reservoir lake in Austin, Texas and attributed this to a city-wide ban on coal-tar (CT) based pavement sealer. In a study conducted approximately two years after the ban,2 we found no change in PAH levels in sediment samples collected from occasionally scoured streams in Austin. While lake sediments allow timelines to be developed using dated cores, questions emerge regarding the authors’ interpretations and conclusions based on the following: • Decreases in sediment PAH concentrations are shown to begin in the late 1990s, well before the city ban on CT sealants; • Decreases reported as significant1 were from differing and selected numbers of years by core (2009−2012 in core LBL.1; 2010−2012 in core LBL.2) and are not statistically significant when all postban years (2007− 2012) are included; and • PAH profiles in sediment samples are more consistent with typical parking lot/street dust profiles than CT sealer profiles. The authors report a 44% decrease in the mean sum of 16 priority pollutant PAHs (∑PAH16) when comparing sediment samples dated to 1998−2005 to samples dated to 2006−2014. This is attributed to a 2006 ban on the use of CT sealers instituted in Austin, even though the timelines plotted (Authors’ Figure 2)1 show the decrease beginning in the late 1990s−well before the ban. On this figure, PAHs in only one core (LBL.4) appear to start substantially decreasing after 2006. Notably, when normalized to organic carbon content (Authors’ Figure S-3),1 this core also shows a decreasing trend beginning around 2000. None of the timelines reported by the authors unambiguously support an interpretation attributing the overall decrease observed to one particular source starting around 2007. Researchers from the City of Austin (COA) found no significant difference in ∑PAH16 concentrations in stream sediments collected from 50 of Austin’s largest watersheds when grouped by sample date with date ranges of 1996−1999, 2000−2002, 2003−2005, 2006−2008, and 2009−2010.3 Only when 3-ringed and 4-ringed PAHs were considered separately, were significant decreases reported from the 1996−1999 time frame to more recent time intervals.3 Thus, any apparent decrease was found to have started years before the ban and there has been no significant change in PAH levels since the ban. Van Metre and Mahler1 report a statistically significant downward trend in ∑PAH16 concentrations in LBL.1 core samples dated to 2009−2012 (r = −0.93, p = 0.002), but the trend is not significant if all postban core samples, 2007−2012, are included (r = −0.53, p = 0.12). The authors report a similar statistically significant downward trend relying on six samples © 2014 American Chemical Society
Published: November 19, 2014 14061
dx.doi.org/10.1021/es5046088 | Environ. Sci. Technol. 2014, 48, 14061−14062
Environmental Science & Technology
Correspondence/Rebuttal
pattern of decreasing PAH concentrations beginning in the 1990s at a Texas lake where there was no CT-sealer ban (Fosdic Lake),6 all demonstrate that attributing the entire reported decrease to the ban in Austin is an incomplete explanation of the available data. The similarity of PAH profiles for the lake sediments to non-CT related dust sources also suggests an alternative explanation warranting consideration. The authors have incorporated a degree of conclusiveness to their interpretation of the Austin sealer ban effects that overlooks other available information.
Robert P. DeMott* Thomas D. Gauthier*
■
ENVIRON International Corp., 10150 Highland Manor Drive, Tampa, Florida 33610, United States
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare the following competing financial interest(s): The authors acknowledge financial support from the Pavement Coatings Technology Council.
■
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) DeMott, R. P.; Gauthier, T. D.; Wiersma, J. M.; Crenson, G. Polycyclic aromatic hydrocarbons (PAHs) in Austin sediments after a ban on pavement sealers. Environ. Forensics 2010, 11, 372−382. (3) Richter, A. Monitoring Polycyclic Aromatic Hydrocarbon Concentrations in Austin, TX, After the Coal Tar Sealant Ban; City of Austin, Watershed Protection Department, Environmental Resource Management Division. SR-12−06. March, 2012. (4) Mahler, B. J.; Van Metre, P. C.; Wilson, J. T. Concentrations of Polycyclic Aromatic Hydrocarbons (PAHs) and Major 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. (5) 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 Scientific Investigations Report 2005−5184; U.S. Geological Survey, 2005. (6) 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, 2003.
14062
dx.doi.org/10.1021/es5046088 | Environ. Sci. Technol. 2014, 48, 14061−14062
