Environ. Sci. Technol. 1995, 29, 848
SIR Murphy takes exception with our use of net volatilization estimates for fluxes of PCBs across the air-water interface of Lake Superior (1). He questions the utility of net fluxes in mass balance models even though this approach has been used successfully numerous times in the freshwater (2-5) literature. Murphy also maintains that our approach performs a great disservice to regulatory agencieswho may misuse such data. We find this statement underestimates the capacity of the U.S. EPA and other federal and state personnel to interpret results such as we present. He correctlypoints out, however, that total (gross) volatilization and deposition is required for estimates of PCB water residence times. We welcome this opportunity to report vapor absorption and dissolvedvapor volatilization fluxes for PCB homologs and CPCB from our data set on Lake Superior. Our model and similar vapor exchange models have been used in a variety of published and submitted mass balance work. The Environmental Protection Agency sponsored the Green Bay Mass Balance Study of 1992 and used an approach similar to ours for the vapor exchange component (2). The EPA study refers to and compares results with our vapor-exchange work in Green Bay (6). Additional mass balance reports on Lake Superior and Lake Michigan have been published or submitted by our group and others (5, 7). The vapor exchange model we have used is clearly useful for whole lake PCB mass balance studies. Under steady-state conditions, water column residence times can be calculated from either the total inputs or the total outputs t=QQII=QIR
(1)
where Q is the total mass in the water column; I and R are the input and output rate, masdtime, respectively (8).In our paper, we reported the net removal, or R - I, due to vapor exchange in Lake Superior. The net removal or volatilization flux was based on the following equation: flux = k,,(C, - C,,,IH)
(2)
where flux (ng m2 day-’) is defined as a net volatilization of a PCB congener, C, is the dissolved PCB congener homolog concentration, C, is the vapor-phase PCB congener concentration, and H i s the dimensionless Henry’s law constant. Here, we report summaries of gross volatilization and gross vapor deposition of PCBs. Gross volatilization and gross vapor deposition are calculated by separating eq 2 and recalculating the homolog fluxes for each 2-week period reported in our original paper (1). Annual vapor volatilization and deposition fluxes for the PCB homologs are reported in Table 1; seasonal vapor volatilization and depositional fluxes for ZPCBs (sum of 80 PCB congeners) are illustrated in Figure 1. More precise estimates of the error and uncertainty in our model results are needed. As we noted in our paper, it is very difficult to estimate the uncertainties in the mass transfer coefficients based on their correlation to wind speed (9). However, using 2-week average wind speeds instead of seasonal or even annual averages dramatically improves the estimates. It should also be noted that the net volatilization flux calculated for Lake Superior in our study agrees with the net CPCB flux estimated by a mass balance
848 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 3, 1995
TABLE 1
Gross Volatilization and Gross Vapor Deposition of PCB Homologs in Lake Superior kglv
volatilization deposition net volatilization
dichlorobiphenyls trichlorobiphenyls tetrachlorobiphenyls pentachlorobiphenyls hexachlorobiphenyls heptachlorobiphenyls octachlorobiphenyls nonachlorobiphenyls
IPCB
77 130 110 98 23 23 0.3 0.0 460
25 51 54 43 20 13 3.0 0.3 210
52 75 57 55 2.8 10 -3.3 -0.3 250
250 200 h
3
150
v
FIGURE 1. Seasonal gross volatilization (kg, white bars) and gross vapor deposition (kg, black bars) in Lake Superior 1992.
approach (5). We do not suggest that our work is the final chapter on vapor exchange in Lake Superior. We hope our paper serves as a reasonable estimate and a guide for future work on a larger scale.
literature Cited (1) Hornbuckle, K. C.; Jeremiason, J. D.; Sweet, C. W.; Eisenreich, S. J. Environ. Sci. Technol. 1994, 28, 1491-1501. (2) Bierman, V. J.; DePinto, J. V.; Young, T. C.; Rodgers, P. W.; Martin, S. C.; Raghunathan, R. USEPA/GLERL, Grosse Ile, MI, Sep 1992. (3) McVeety, B. D.; Hites, R.A.Atmos. Environ. 1988,22,511-536. (4) Swackhamer, D. L.; McVeety, B. M.; Hites, R. A. Environ. Sci. Technol. 1988, 22, 664-672. (5) Jeremiason, J. D.; Hornbuckle, K. C.; Eisenreich, S. 1. Environ. Sci. Technol. 1994, 28, 903-914. (6) Achman, D. A.; Hornbuckle, K. C.; Eisenreich, S. J. Environ. Sci. Technol. 1993, 27, 75-87. (7) Pearson, R. F.; Hornbuckle, K. C.; Golden, K. A,; Eisenreich, S. 1.; Swackharner, D. L. Submitted for publication in Environ. Sci. Technol. (8) Lyman, ‘w. J., Reehl, W. F., Rosenblatt, D. H., Eds. Handbook of Chemical Property Estimation Methods: Environmental behavior of organic compounds; American Chemical Society: Washington, DC, 1990; p 10-1. (9) LVanninkhof, R.; Ledwell, J. R.; Crusius, J. In Air-Water Mass Transfer; Wilhelm, S. C., Gulliver, J , S., Eds.; American Society of Civil Engineers: New York, New York, 1991; pp 441-458.
Keri C. Hornbuckle and Steven J. Eisenreich* Gray Freshwater Biological Institute Department of Civil Engineering University of Minnesota Navarre, Minnesota 55392 ES9409523
0013-936X/95/0929-0848$09.00/0
G 1995 American Chemical Society
