Environmental Pollution xxx (2014) 1

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Letter to the Editor

Letter to the Editor on “Article title misstates the role of pavement sealers” Editor: In Kirk O’Reilly’s recent Letter to the Editor (O’Reilly, 2013), he discounted the results of Witter et al.’s (2014) paper which suggested coal tar-based sealant (CT-sealant) dust is an important source of polycyclic aromatic hydrocarbons (PAHs) in sediment collected from urban areas of Conodoguinet Creek, PA. However, O’Reilly made several errors in the modeling work that supported his position. In this letter, I will discuss these errors in the context of my experience using the U.S. EPA’s Chemical Mass Balance (CMB) model for the source apportionment of PAHs in sediment samples (Crane, 2014). The first overarching error O’Reilly made was in his selection of PAH source profiles to use in the CMB model. His assumption that Witter et al. (2014) used the same PAH source profiles as Van Metre and Mahler (2010) is not correct. Witter et al. (2014) averaged several CT-sealant dust profiles, including some newer profiles, whereas Van Metre and Mahler (2010) considered CTsealant dust source profiles separately in their statistical evaluation and CMB model runs. In addition, Van Metre and Mahler (2010) evaluated the use of several other PAH source profiles not considered by Witter et al. (2014). O’Reilly used the same five source profiles used by Van Metre and Mahler (2010) for their model run “A” without determining whether these sources were statistically significant to Witter et al.’s (2014) PAH proportional P data (i.e., individual PAH concentrations divided by PAH12). He also did not consider whether the data should be divided into two categories for urban and rural land uses per the principal components analysis in Fig. 4 of Witter et al.’s (2014) paper. Van Metre and Mahler (2010) ran over 200 model runs of their 40 lake nationwide data set, which did not include any lakes in Pennsylvania. It was inappropriate of O’Reilly to only perform two runs of the CMB model with PAH sources that may not all be relevant to Witter et al.’s (2014) data set. The second overarching error O’Reilly made was in his interpretation of the CMB model performance criteria. He indicated that 26 of the 35 sediment samples met model acceptability criteria, but he did not identify these criteria. He also did not try other combinations of source profiles that could have improved the results. Furthermore, O’Reilly did not provide any source apportionment data for the model runs. He incorrectly used the coefficient of determination (R2) value calculated between P measured and modeled concentrations of PAH12 as a purported measure of model performance with and without using CTsealant dust as a source. Instead, CMB model performance is based

on the following fitting statistics: 1) Chi-square, which is the weighted sum of squares of the differences between the calculated and measured fitting species concentrations for individual PAHs, 2) R2, which is the fraction of the variance in the measured species concentrations that is explained by the variance in the calculated species concentrations as determined by a linear regression of measured versus model-calculated values for the P fitting species (i.e., not just for PAH12), and 3) the percent mass estimated by the model that represent the percent ratio of the sum of the model-calculated source contribution estimates to the measured mass concentration (Coulter, 2004). These performance measures could be used to conduct pairwise statistical tests between model runs as done in Crane (2014), who showed statistically better results were obtained when CT-sealant sources were included in the CMB-modeled source apportionment of PAHs in stormwater pond sediments from the Minneapolis-St. Paul, MN metropolitan area. In addition, O’Reilly could have calculated the relative percent difference (RPD) between measured and P modeled PAH12 concentrations (instead of his Fig. 1 plot), and a pairwise statistical test could have been conducted of the RPDs between model runs that included and excluded CT-sealants. In conclusion, O’Reilly’s major errors compromised his CMB model results, and these results cannot be used to support his position.

References Coulter, C.T., 2004. EPA-CMB 8.2 users manual. EPA 452/R-04e011. U.S. Environmental Protection Agency, Research Triangle Park, NC (accessed 18.02.14.). http://www.epa.gov/scram001/models/receptor/EPA-CMB82Manual.pdf. Crane, J.L., 2014. 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. 66, 176e200. O’Reilly, K., 2013. Letter to the Editor: article title misstates the role of pavement sealers. Environ. Pollut. http://dx.doi.org/10.1016/j.envpol.2013.11.029. Van Metre, P.C., Mahler, B.J., 2010. Contribution of PAHs from coal-tar pavement sealcoat and other sources to 40 U.S. lakes. Sci. Total Environ. 409, 334e344. Witter, A.E., Nguyen, M.H., Baidar, S., Sak, P.B., 2014. Coal-tar-based sealcoated pavement: a major PAH source to urban stream sediments. Environ. Pollut. 185, 59e68.

Judy L. Crane Minnesota Pollution Control Agency, 520 Lafayette Road North, St. Paul, MN 55155-4194, USA E-mail address: [email protected]. Available online xxx

DOI of original article: http://dx.doi.org/10.1016/j.envpol.2013.11.029. http://dx.doi.org/10.1016/j.envpol.2014.03.029 0269-7491/Published by Elsevier Ltd.

Please cite this article in press as: Crane, J.L., Letter to the Editor on “Article title misstates the role of pavement sealers”, Environmental Pollution (2014), http://dx.doi.org/10.1016/j.envpol.2014.03.029

Letter to the Editor on "Article title misstates the role of pavement sealers".

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