Correspondence/Rebuttal pubs.acs.org/est
Comment on “Polychlorinated Biphenyls in Tree Bark near Former Manufacturing and Incinerator Facilities in Sauget, Illinois, United States”
H
ermanson et al. 1 have recently investigated the concentrations of polychlorinated biphenyls (PCBs) in 27 tree bark samples collected within a few kilometers of an old PCB production facility in Sauget, Illinois. This is a follow-on to a paper by Hermanson and Johnson,2 who measured polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in the same samples. This production facility was the second largest producer of PCBs in the United States (after Anniston, Alabama). Tree bark is an excellent passive sampler of atmospheric persistent organic pollutants (POPs), and in fact, it has been shown that tree bark can be used to locate POPs sources, assuming a simple radial diffusion model.3 Hermanson et al. qualitatively interpreted their data in terms of the sample locations being upwind or downwind from the former PCB production facility.1,2 This interpretation assumednot unreasonablythat the PCB facility was the source of the PCBs and PCDD/F and that these compounds had moved through the atmosphere to the trees that had been sampled. It is possible to be more quantitative in this analysis and to actually determine the location of the source. We have shown previously that a simple radial diffusion model can be used to relate the concentrations of a pollutant (Ci) in a tree bark sample taken at a distance (Di) from a source using Ci = a0Di−a1
(1)
where a0 and a1 are fitted constants. The value of a0 depends on units and is not relevant; the value of a1 is expected to be in the range of 1−2.3 The power of this statistical approach is that one does not need to know a priori where the source is located, but it can be located as part of the statistical process. Using Excel, one can design a spreadsheet using the Solver tool to systematically vary the values of a0 and a1 and the latitude and longitude of the source such that ς=
∑ [(ln(Ci) − ln(a0Di−a ]2
Figure 1. Schematic map of Sauget, Illinois, showing the tree bark sampling sites and concentrations from Hermanson et al.1,2 The brown lines represent major highways. At each site, the nine highest PCB (top) and PCDD/F (bottom) concentrations are indicated by red dots, the nine lowest such concentrations by green dots, and the others by yellow dots. The location of the PCB-laden WGK site is given. The red rectangles represent the locations of the sources as calculated by the radial diffusion model including their Monte Carlo estimated errors. The distance between the y-axes is about 8 km.
1
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causing the tree bark concentrations close to the plant to be lower than those further away from the plant. To help unravel this puzzle, these data were analyzed quantitatively. Figure 2 shows the results of the nonlinear curve fitting done as described above. In both cases, the fit is excellent with r2 > 0.60. The value of a1 for the PCBs is 0.98 ± 0.23, and for the PCDD/F, it is 1.35 ± 0.38. Both of these a1 values are near the expected range. What is more interesting is the location of the sources as determined by this statistical procedure. For the PCBs, this location is 38.6286 ± 0.0014°N, 90.1673 ± 0.0020°W, and for the PCDD/F, this location is 38.6353 ± 0.0033°N, 90.1567 ± 0.0065°W. These errors were obtained by a Monte Carlo analysis in which the
(2)
is minimized.3 Figure 1 shows a map summarizing Hermanson’s PCB and PCDD/F concentrations in tree bark. The color codes for the tree bark concentrations divide the 27 measurements into thirds with the red dots being the highest and the green being the lowest. Figure 1 also shows the location of the former W. G. Krummrich (WGK) PCB production facility, which Hermanson et al.1,2 assumed to be the source of PCBs and PCDD/F to trees in this vicinity. It is interesting to note that some of the lowest PCB and PCDD/F concentrations are located close to the WGK plant; on the other hand, some of the highest PCB and PCDD/F concentrations are located 2−3 km north of the WGK plant. It is difficult to imagine a wind-driven mechanism © XXXX American Chemical Society
A
DOI: 10.1021/acs.est.7b00503 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
Notes
The author declares no competing financial interest.
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REFERENCES
(1) Hermanson, M. H.; Hann, R.; Johnson, G. W. Polychlorinated biphenyls in tree bark near former manufacturing and incineration facilities in Sauget, Illinois, United States. Environ. Sci. Technol. 2016, 50, 6207−6215. (2) Hermanson, M. H.; Johnson, G. W. Chlorinated dibenzo-p-dioxin and dibenzofuran congener and homologue distributions in tree bark from Sauget, Illinois, U.S. Environ. Sci. Technol. 2015, 49, 855−862. (3) Peverly, A. A.; Salamova, A.; Hites, R. A. Locating POPs sources with tree bark. Environ. Sci. Technol. 2015, 49, 13743−13748.
Figure 2. Concentrations in tree bark of total PCBs (top) and total PCDD/F (bottom) as a function of distance (in km) from the calculated sources, which are indicated by the red rectangles in Figure 1. Given the short distances involved, these are Euclidian distances, which assumes the Earth is flat. The two open yellow circles indicate outliers, which were not used in the regressions.
concentration measurements were assigned random errors of ±20%, and the locations of the sources were recalculated. This whole process was repeated 12 times. These coordinates fell into the red rectangles shown on the maps in Figure 1. There are no known sources of PCDD/F or PCBs at either of these sites. There are at least two possibilities to explain these anomalous results. First, the simple radial diffusion model used here may not be appropriate over short distance scales. The second possibility is that a plume from the plant’s incinerator, coming out at a stack height of several tens of meters, does not impact the ground near the plant, thus accounting for the relatively low concentrations near the plant, but the plume does impact the ground at the marked locations shown in Figure 1, thus accounting for the relatively high concentrations further from the plant. In other words, what has been calculated here are the locations where this plume has its major, time-weighted, ground-level impacts. The locations of these two impact sites may be different because of different emission histories of PCBs vs PCDD/F from the WGK site.
Ronald A. Hites*
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School of Public and Environmental Affairs Indiana University Bloomington, Indiana 47405 United States
AUTHOR INFORMATION
Corresponding Author
*Phone 812-855-0193; e-mail:
[email protected]. ORCID
Ronald A. Hites: 0000-0003-0975-5058 B
DOI: 10.1021/acs.est.7b00503 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
