Comment on "Seasonal Variations in Air- Water Exchnge of Polychlorinated Biphyls in Lake Superior" SIR: In their Introduction, Hornbuckle et al. (1)state " ... quantitative estimation of the atmospheric component of the sources and sinks [of PCBs] in the Great Lakes is vital to predicting chemical residence time, ecosystem exposure, and the atmospheric component of whole-lake mass balance". However, the results reported in the paper significantly underestimate the atmospheric sources and sinks of PCBs to Lake Superior; the results reported are not relevant to mass balance calculations or models. If their results were used in such models, they could lead regulatory agencies to make management decisions that would not have the anticipated result. The principal problem is with their model. A mass balance is based on determining the magnitude of each individual source and sink component (2-5). Therefore a whole-lake mass balance model requires estimates of the vapor deposition flux and of the volatilization flux. However, only the net vapor flux is reported and discussed by Hornbuckle et al. ( 1 ) . They state "Whether volatilization or [vapor] deposition occur is dependent on season". This is not correct. As long as vapor phase and dissolved PCBs are present, volatilization and deposition are continually occurring. The vapor deposition flux depends only on the vapor-phase concentration of the particular compound and the appropriate physical parameters (temperature, wind speed, etc.) (6). This flux is not affected by the dissolved concentration of the compound (oxygen continues to invade a body of water that is saturated, and at the same rate as if the water were fully unsaturated). Vapor-phase PCB molecules that dissolve in a body of water are real inputs-they can partition to particles, be taken up by organisms, evaporate, flow out of the lake, etc. Likewise PCBs that enter the lake from other sources (streams, discharge, rain, etc.) may evaporate from the lake. The two vapor fluxes are independent. Just as tributary inflows and outflows are separately included in mass balances, so must vapor inputs and outputs (2-5). The net flux (vapor or other) of a contaminant is an indication only of whether that exchange happens to be at equilibrium. This could be of interest in some situations, but it is not related to mass balances that seek to determine the relative magnitude of each input or loss term. For example, a lake could have vapor inputs and losses of a particular contaminant of 1 kg/yr for a net vapor flux of zero or net vapor inputs and losses of lo6 kg/yr, also with a net flux of zero. Even though the net flux in each of these cases is zero, the effects on the lake and its biota of an input flux of 106kg/yr of a toxic compound will be a very different than if the flux were 1 kg/yr. A mass balance based on individual fluxes would reflect these differences, while one based on net fluxes would not (your lifestyle is related to your annual cash flow-how much money you make and
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 3,1995
TABLE 1
Input Mass Balance Models for Lake Ontario after Mackay et a/. (4) source
Niagara River precipitation dry deposition vapor atmospheric other
individual flux model (kg/yr)
net flux model (kg/yr)
344
344
35 7 65
35 7 0 107 (17%) 175
total
626
42 (7%)
175 561
spend in a year, not on whether your bank balance happensto be higher, lower, or the same at the end of the year than it was at the beginning). An illustration of the errors which result in mass balances by the use of the net flux method is the modeling results by Mackay etal. (4) for PCBs in Lake Ontario. They report annual PCBvapor inputs of 65 kg/yr and volatilization losses of 251 kg/yr. These yield a net vapor flux of PCBs out of the lake of 186 kg/yr. Table 1 shows their input mass balance terms. The middle column shows these data where the vapor deposition flux is used. The last column shows the same data, but with the application of the net flux model of Hornbuckle et al. (1). In this case, the vapor input term is zero, since the net annual vapor flux happens to be out of the lake. The data in Table 1 indicate that while there is a large net annual vapor loss from Lake Ontario (three times the inputs; 186 kglyr), the vapor inputs still comprise 60% (65 kg/yr) of the atmospheric inputs to the lake! Inclusion of these vapor inputs into the mass balance model more than doubles the total atmospheric contribution of PCBs to Lake Ontario from 42 to 107kg/yr and changes the atmospheric contribution from 7% to 17% of the total PCB inputs. Since the inputs to Lake Ontario are dominated by the Niagara River, management decisions based on an atmospheric load of 42 kglyr rather than the correct load of 107 kg/yr probably would not be affected. However, for lakes where the atmospheric contributions are similar to the inputs from other sources, the uses of the net flux method would lead to significant errors. Using the net flux model, Strachan and Eisenreich (7) estimate that the atmosphere contributes ~ 6 0 % of the PCBS entering Lakes Michigan and Huron. Table 2 shows example mass balances (using the net flux and the individual flux methods) for a lake where the atmosphere contributes 60% of the total PCB inputs. The proportion of the inputs follow Mackay's results ( 4 ) that vapor inputs are 150% of the other atmospheric inputs and the precipitation input is five times that of dry deposition. It can be seen that the net flux method makes the nonatmospheric sources appear to be about twice as
0013-936X/95/0929-0846$09.00/0
@ 1995 American Chemical Societv
TABLE 2
Example Lake input sources
precipitation dry deposition vapor atmospheric other total
individual fluxes (ka/yr)
net flux (kglyr) 100 20
100 20
180 300 (79%) 80 (21%)
380
120 (60%) 80 (40%)
200
important compared to when the vapor inputs are included. Inclusion of the vapor inputs makes the nonatmospheric contribution relatively small. Despite the use of individual vapor fluxes in its earlier Green Bay Mass Balance Project (51,the U.S. EPA currently proposes to use only the net flux in its mass balance models for Lake Michigan (8)and the Great Waters Program (2,9). Management decisions based on these models can be expected to overestimate the effectivenessof controls on nonatmospheric sources since the net flux method overestimates the importance of nonatmospheric inputs. Regardless of the model used, the results of Hornbuckle etal. ( I ) should be takenwith some caution: (a)they suggest that their use of an average wind speed rather than a wind speed distribution for their calculations probably leads to their results being low by a factor of 2; (b) the shore-based wind speeds they used for over-lakegas exchange are biased low since they did not correct for the increase in wind speed (factorof 0.75-2.5; mean -1.5) that occurs as the air moves over the water (9); (c) this wind speed correction for the lower surface resistance of the water surface is a strong function of the atmospheric stability (IO),which they do not discuss; (d) while they report the sign and magnitude of the net flux, this result has little significance since it is the difference between two numbers, each with high uncertainty and of similar magnitude. Finally, uncertainties are not shown either for their reported net input result or for the data in the figures. From the discussion in their paper and the problems indicated here, it is clear that these uncertainties are substantial. They need to be shown so that the significance of their results and conclusions can be assessed by potential users of the data. In conclusion,whether the net vapor flux of a compound is into or out of a lake for 1 h, 1 day, 1 season, or 1 year is
irrelevant to a mass balance determination. It is the magnitude of each of the fluxes over the time period of interest that must be used. Hornbuckle etal. (1)have done considerable work to estimte PCB vapor fluxes and much of the work necessary to estimate the terms needed in a mass balance model. Unfortunately,they do not determine these two terms or report data that would permit the required numbers to be calculated. Thus, their results, and those of others who use net vapor fluxes in mass balances (2,7-9,11-14), should not be used to determine the relative magnitude of different input sources or to make decisions regarding the management of sources of toxics to lakes.
Literature Cited (1) Hornbuckle, K. C.; Jeremiason, J. D.; Sweet, C. W.; Eisenreich, S . J. Environ. Sci. Technol. 1994, 28, 1491-501. (2) US.EPA. Deposition ofAirPollutants to the Great Waters; EPA453/R-93-055; U.S. EPA Washington, DC, May 1994. (3) Mackay, D. J. Great Lakes Res. 1989, 15, 283-97. (4) Mackay, D. M.; Sang, S.; Vlahos, P.; Diamond, M.; Gobas, F.; Dolan, D. J. Great Lakes Res. 1994,20, 625-42. (5) Sweet, C. W.; Murphy, T. J.; Bannasch, J. H.; Kelsey, C. A.; Hong, J. J. Great Lakes Res. 1993, 19, 109-28. (6) Chester, R. Marine Geochemistry Chapman and Hill: London, 1990; p 234. (7) Strachan, W. M. J.; Eisenreich, S. J. Mass balancing of toxic chemicals in the Great lakes;The role of atmospheric deposition. Report to the Intemational Joint Commission,Windsor, Ontario, May 1988. (8) Horvatin, P. J. Draft Lake Michigan mass b a l a n c e l m s budget Work Plan; U.S.EPAIGLNPO: Chicago, IL, Oct 1993. (9) Baker, J. E.; Church, T. M.; Eisenreich, S. J.; Fitzgerald, W. F.; Scudlark, J. R. Relative atmospheric loadings of toxic contaminantsand nitrogen to theGreat Waters;US.EPA Washington, DC, 1993. (10) Schwab, D. J.; Morton, J.A. J. GreatLakesRes. 1984,10,68-72. (11) Achman, D. R.; Hornbuckle, K. C.; Eisenreich, S. J. Environ. Sci. Technol. 1993, 27, 75-87. (12) Eisenreich, S. J.; Strachan, W. M. J. Estimating atmospheric deposition of toxic substances to the Great Lakes, an update; National Water Research Institute: Burlington, Ontario, Feb 1992. (13) Baker, J. E.; Eisenreich, S. J. Environ. Sci. Technol. 1990, 24, 342-52. (14) Swackhamer, D. L.; McVeety, B. D.; Hites, R. A. Environ. Sci. Technol. 1988,22, 664-72.
Thomas J. Murphy DePaul University Department of Chemistry 1036 West Belden Avenue Chicago, Illinois 60614-3214 ES940585V
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