Environ. Sci. Technol. 2003, 37, 5837
Comment on “Locating and Quantifying PCB Sources in Chicago: Receptor Modeling and Field Sampling” The paper by Hsu et al. (1) presents the results of a study using PSCF modeling and ambient air data from 1991 and 1994 to locate potential sources of PCBs in the Chicago area. In addition, several suspected emission sites were also investigated. The study presented by the authors is useful, but it requires some clarifications on the results and extrapolations presented in the paper. It is claimed that sludge drying beds are among the potential sources of PCBs to the air in the Chicago area based on upwind and downwind air samples and a laboratory sludge drying test. Also, a K-theory model is used to determine PCB emission rates from the drying beds using the upwind and downwind sample information. The concentrations of total PCB in ambient air samples taken during the years 1991 (LMUATS-IIT) and 1994 (AEOLOS-IIT and Overlake) are used by Hsu et al. (1) in PSCF receptor modeling to locate potential PCB sources. Taking into consideration the absence of grids in the produced graphs, it is highly speculative to pinpoint a specific sludge drying area that is about 0.1 square miles in area superimposed on a 10 000 square mile map of the greater Chicago area. In addition, the PSCF results using LMUATS data collected at the Kankakee, IL, site did not indicate any PCB sources to the northeast, which would be the location of the drying beds. It should be noted that in the PCB analysis of the collected samples, Hsu et al. (1) combined the extracts from a nonsizesegregated glass fiber prefilter and the PUF for a single PCB analysis. Thus, the results reported are total PCBs and not volatilized or gaseous PCBs alone. This is important since the probabilistic models used by the authors assume that the particles move in concert with the air parcel. However, PCBs associated with large solids will deposit quickly and are not expected to be transported substantial distances. Therefore, model estimation of the trajectory might be unrealistically exaggerated. Most PCB congeners are prone to evaporation under intensive heating and forced aeration. Furthermore, PCBs are nonpolar, lipophilic compounds with high affinity to the colloidal soil particles and strongly bind to highly organic soils. A study by Di et al. (2) compares the vapor pressures of several PCB congeners and shows the extremely low volatility of PCB congeners in ambient conditions. The postcentrifuged sludge in the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) contains a high concentration of organic matter and a substantial amount
10.1021/es0304493 CCC: $25.00 Published on Web 11/11/2003
2003 American Chemical Society
of colloidal particles that provide a strong physical binding of any nonpolar lipophilic compounds to it. The laboratory evaporation tests with 60 g of a thin layer of sludge in a fume hood, used by the authors to determine the moisture and PCB losses, took less than 3 d to reach a moisture content of less than 20%. The actual drying process on the paved cells takes about 3-8 weeks to reduce the moisture content to 35%. The authors have also correlated the PCB loss to the moisture loss from the sludge matrix in the laboratory setting. This correlation has serious flaws because the data are only at the extreme ends of the curve and do not include the moisture content between 20 and 60% (3). The authors extrapolated the data to estimate the PCB content of the sludge at 35% moisture. On the basis of this extrapolation, another extrapolation was made using the total annual tonnage of sludge produced. Extrapolating the PCB losses from 60 g of sludge under laboratory conditions to 170 685 ton under field conditions is not scientifically sound. Since the tonnage is a large value, a small change in either the initial PCB concentration of the 60 g of sludge or the resulting PCB losses from the estimated value would markedly affect the resulting estimate of the yearly emission to the atmosphere from the 170 685 ton processed. Emission rates of PCB from the drying beds were also calculated using a dispersion model based on K-theory, using four air samples (operated for a period of 4 h) upwind and downwind of sludge drying cells as input to the model. A 10-fold difference in predicting the emission rate using K-theory and the EPA virtual source model is not explained by the authors (Table 3, ref 1). We agree with the authors that more intensive field studies are needed to quantify and evaluate the significance of potential sources of PCBs in the Chicago area. We believe that the authors have been premature in designating specific small areas of Chicago as the major PCB sources in the metropolitan area.
Literature Cited (1) Hsu, Y.-K.; Holsen, T. M.; Hopke, P. K. Environ Sci. Technol. 2003, 37, 681-689. (2) Di, P.; Chang, D. P. Y. J. Air Waste Manage. Assoc. 2001, 51, 482-488. (3) Hsu, Y.-K. Ph.D. Dissertation, Clarkson University, 2001.
Ali K. Oskouie, David T. Lordi, Bernard Sawyer, and Richard Lanyon* Metropolitan Water Reclamation District of Greater Chicago 100 East Erie Street Chicago, Illinois 60611-3154 ES0304493
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