Environ. Sci. Technol. 1988, 22, 229-230
CORRESPONDENCE Comment on “Estimation of the Atmospheric and Nonatmospheric Contributions and Losses of Polychlorinated Biphenyls for Lake Michigan on the Bask of Sediment Records of Remote Lakes” SIR Swackhamer and Armstrong (1) recently estimated the magnitude and the relative importance of the different sources and sinks of polychlorinated biphenyls (PCBs) in Lake Michigan (LM). To distinguish between atmospheric and nonatmospheric sources of PCBs, they used an atmospheric deposition model calibrated on several small Wisconsin lakes (WLs) whose only known source of PCBs is the atmosphere. Estimates of the total inputs to all of the lakes were based on the accumulations in the sediments. Their conclusions are based on a number of assumptions. I would like to discuss four of their assumptions. (1)The atmospheric deposition model calibrated on the WLs indicates that 75% of the PCBs that enter from the atmosphere evaporate and only 25% accumulate in the sediments. Swackhamer and Armstrong applied the same ratio of evaporation to accumulation to LM. Thus, the 1.9 pg/ (m2-yr)of the PCBs accumulated in the sediments of LM, which they assume came from the atmosphere, implies a loading of 7.5 pg/(m2.yr) from the atmosphere. However, in determiningthe loading of nonatmospherically derived PCBs to LM, they assume with no explanation that none of these PCBs evaporate! Thus the sediment accumulation, 5.7 pg/(m2.yr), equals the lake loading from this source. This results in the atmospheric loading being 5747% of the total loading. However, if one assumes that PCBs from all sources have similar tendencies to evaporate, only 25% now comes from the atmosphere-a big difference. Their assumption that nonatmospherically derived PCBs behave very differently in the lake than atmospherically derived PCBs do has a major effect on their conclusions and needs to be justified. Also, if the atmospheric contribution to the sediment accumulation in LM is closer to 1.08 pg/ (m2.yr),see below, then the atmosphere may contribute only ~ 1 6 % of the total lake loading. (2) There is abundant evidence that PCB inputs to the environment have increased over time, at least through the mid-l970s, since they were first manufactured in the late 1920s. The sediment data presented by Swackhamer and Armstrong support these findings. However, in calculating the sediment fluxes of PCBs, Swackhamer and Armstrong averaged the accumulations found in the sediments in Lake Michigan over 51 years (1930-1980). But the average accumulations found in the WLs were based on an average of only the last 30 years. By averaging the sediment accumulations in LM over an additional 20 years of much lower accumulations,the average flux to the LM sediments is lowered. If their assumption is true that atmospheric inputs to LM have been the same as to the WLs, then the PCB accumulations found in the WLs should also be divided by 51 years. If this is done, an average flux to the 0013-936X/88/0922-0229$01.50/0
WLs sediments of 1.08 f 0.53 pg/(m2.yr) is found, with a range of 0.25-1.75 pg/(m2.yr). This is only ~ 6 0 % of the 1.9 f 1.1pg/(m2.yr) they used for those lakes. Also, the mean total flux of PCBs to the four WLs differs by as much as a factor of 4 (a factor of 8 in the 51-year means). This casts doubt on the assumption of uniformity of atmospheric inputs, or it is a reflection of errors in correcting for sediment focusing in the WLs. (3) In applying the WLs results to LM, Swackhamer and Armstrong assume that the meteorological, climatological, hydrological, and hydrodynamic environment of LM (58000 km2; 5000 km3) is the same as that of the WLs (0.1-3 km2; 0.001-0.03 km3). But there are significant differences that need to be taken into account. In particular, the Great Lakes with their large surface area and very large heat capacity significantly affect the regional climatology and local meteorology (2-4). This should affect atmospheric inputs to them compared to smaller lakes. For instance, the ice cover on LM, averaged over the 4 months of the typical ice season, is only 19% (5). The higher average wind speeds and the unstable atmospheric conditions over LM during this time, caused by the water being warmer than the air, should enhance dry deposition and vapor exchange with the atmosphere. In contrast, the WLs are completely frozen over and isolated from the atmosphere during the winter. Also, to the extent that wind speeds affect the exchange of PCBs between air and water, the PCB exchange should be lower with the more sheltered WI lakes, due to lower wind speeds and shorter fetches, than with LM. (4) Swackhamer and Armstrong assumed that the PCB inputs/m2 from the atmosphere to LM were the same as to the WLs. However, the Milwaukee-Chicago-Gary urban/industrial area is along the southwest shore of LM. Air concentrations of PCBs in Chicago, IL (6),Madison, WI (7), and Minneapolis, MN (8), are all a factor of 5 or more above the concentrations measured over LM (7). This is in agreement with literature reviews that indicate that PCB concentrations are a factor of 10 higher in urban areas than in rural areas (9). Precipitation inputs of PCBs are also higher in Chicago than in the more remote, northern regions of Lakes Michigan and Huron. PCB concentrations determined in event wet-only rain samples in 1979-1980 averaged 75 ng/L in Chicago (41O55’ N) (6). Monthly integrated wet-only samples collected during a similar time period averaged 12 ng/L at Point Betsie, MI (44’41’ N), and 45 ng/L at Waukegan, IL (42O42’ N). In addition, 32 wet-only event rain samples collected during 1977-1978 at Whitestone Point (44’06’ N) on Saginaw Bay along Lake Huron averaged only 13 ng/L (IO, I I ) . The legitimacy of the conclusions in a modeling paper such as this is very dependent on the soundness of the assumptions made. And the important assumptions need to be thoroughly justified. To me, the lack of justification and reasonableness of some of the assumptions made in this paper severely limits the validity of the conclusions reached.
Literature Cited
0 1988 American Chemical Society
(1) Swackhamer, D. L.; Armstrong, D. E. Enuiron. Sci. Technol. 1986,20, 879-83. Environ. Sci. Technol., Vol. 22, No. 2, 1988 229
Environ. Sci. Technoi’. lQ88,22, 230-231
(2) Changnon, S. A.; Jones, D. M. A. Water Resour. Res. 1972,
8, 360-71. (3) Eichenlaub, V. L. Weather and Climate of the Great Lakes Region; University of Notre Dame Press: Notre Dame, IN,
1979. (4) Cole, H. S.; Lyons, W. A. Proceedings of the 14th Conference on Great Lakes Research; International Association for Great Lakes Research Ann Arbor, MI, 1971;pp 436-63. (5) Assel, R. A.; Quinn, F. H.; Leshkevich, G. A.; Bolsenga, S. J. Great Lakes Ice Atlas; Great Lakes Environmental Research Laboratory: Ann Arbor, MI, 1983; Table 2. (6) Murphy, T. J.; Rzeszutko, C. P. J. Great Lakes Res. 1977, 3, 303-12. (7) Doskey, P. V.; Andren, A. W. J. Great Lakes Res. 1981, 7,
15-20. ( 8 ) Hollod, G. J. Ph.D. Thesis, University of Minnesota,
Minneapolis, MN, 1979. (9) Eisenreich,S. J.; Looney, B. B.; Hollod, G. J. Environ. Sci. Technol. 1981, 15, 879-83. (10) Murphy, T. J.; Paolucci, G.; Schinsky, A. W.; Combs, M. L.; Pokojowczyk,J. C. “Inputs of PCBs from the Atmosphere to Lakes Huron and Michigan”;report of U.S. EPA project R-805325; Duluth Environmental Laboratory: Duluth, MN, May 1982. (11) Murphy. T. J. In Toxic Contaminants in the Great Lakes; Simmons, M., Nriagu, J., Eds.; Wiley: New York, 1984.
Thomas J. Murphy Chemistry Department DePaul University Chicago, Illinois 60604
SIR: Murphy questions assumptions made in our estimations of the atmospheric and nonatmospheric contributions of PCBs to Lake Michigan ( I ) . Although we explained our assumptions at some length, we are happy to provide further clarification. Murphy argues that we assumed PCBs from nonatmospheric sources do not evaporate and that this assumption leads to an overestimate of the fraction of the PCB loading to Lake Michigan derived from nonatmospheric sources. However, we did not make this assumption. Clearly, PCBs from nonatmospheric sources are also subject to volatilization. We chose to use the calculated loss to the atmosphere from remote lakes as a conservative estimate of the corresponding loss from Lake Michigan. We did not feel that alternative assumptions, such as Murphy’s suggestion that loss to the atmosphere is directly proportional to loading, were justified. Importantly, our estimate of the nonatmospheric loading to Lake Michigan was presented as the net rather than the total value, representing the amount remaining (in the lake sediments) after losses by volatilization or other processes. As a point of further clarification, we did not obtain the atmospheric loading (7.5 pg m-2 yr-l) on the basis of a ratio of evaporation to sediment accumulation of PCBs, as indicated by Murphy. The atmospheric loading was obtained from a separate investigation (2). We measured sediment accumulation of PCBs in remote lakes and calculated loss to the atmosphere as the difference between atmospheric loading and sediment accumulation of PCBs. Our use of different periods of time for the remote lakes and Lake Michigan in calculating average sediment fluxes of PCBs was also questioned. Fluxes were averaged over the periods for which PCBs were detectable in the sediments. This time interval was longer in Lake Michigan than in the remote lakes. We believe it would be erroneous to use a time period other than the period over which PCBs were actually deposited. In contrast to Murphy’s 230
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conclusion, we believe PCBs could have been delivered to Lake Michigan earlier than to the remote lakes, i.e., from nonatmospheric sources. As discussed previously ( I ) , the range in sediment fluxes of PCBs in the remote lakes probably reflects uncertainties in time resolution (sectioning sediment cores and calculating sedimentation rates), measurement of low PCB concentrations, and sediment focusing. We did not attempt to correct for differences in sed’,nent focusing among remote lakes. Were sediment focusing occurring in the remote lakes, the difference between the remote lakes and Lake Michigan would be even greater, making our estimates conservative. We share Murphy’s concern regarding meteorological and physical differences between Lake Michigan and the remote lakes and acknowledged this limitation (I). In the absence of a pristine lake identical in size and climate with Lake Michigan, the remote lakes served as a useful first approximation. The suitability of the remote lakes is supported by data from Lake Superior. Lake Superior receives approximately 80% of its PCBs through atmospheric input (3, 4 ) . The average sediment PCB flux in Lake Superior is estimated to be 4 hg m-2 yr-l ( 4 ) , which makes the atmospheric contribution similar to the flux for the remote lakes (1). Furthermore, sediment PCB cogener fluxes in Siskiwit Lake on Isle Royale in Lake Superior are similar to those of Lake Superior, even though Siskiwit Lake is ice-covered during winter months (5). Two factors may account for the agreement in fluxes: (1) although possibly enhanced by high wind speeds, dry deposition is not the dominant process involved in transport of atmospheric PCBs to lakes (2), and (2) during winter, vapor exchange from ice-free lakes may be reduced through the influence of low surface water and air temperatures on Henry’s Law constants. In fact, Murphy (6)suggests this temperature effect in a recent publication. Our assumption that atmospheric inputs per unit area are similar for Lake Michigan and the remote lakes is supported by air and rain measurements, including data cited by Murphy. Air concentrations of PCBs over the open waters of Lake Michigan (3, Wisconsin remote sites (8),and Lake Superior ( 4 ) are similar. As we stated previously ( I ) , concentrations in air and rain are higher in urbanized areas in localized regions along the southern shores of Lake Michigan, possibly resulting in higher atmospheric inputs of PCBs to Lake Michigan than to the remote lakes. However, the major portion of the Lake’s surface is apparently influenced by air concentrations similar to those in remote areas. We reemphasize that assumptions made in comparing Lake Michigan and the remote lakes will lead to some uncertainty in the calculated fluxes. We encourage continued examination of the air-water transfer processes to improve the reliability of flux estimates. However, we believe the comparison clearly shows the loss of PCBs from the lake to the atmosphere is significant and that atmospheric and net nonatmospheric inputs of PCBs to Lake Michigan have been of similar magnitude.
Literature Cited (1)
Swackhamer,D. L.; Armstrong,D. E. Enuiron. Sci. Technol.
1986, 20, 879-83. (2) Andren, A. W. In Physical Behavior of PCBs i n the Great
Lakes; Mackay, D., Paterson, S., Eisenreich,S. J., Simmons, M. S., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; Chapter 8. (3) Eisenreich, S. J.; Looney, B. B.; Thornton, J. D. Environ. Sci. Technol. 1981, 15, 30-38.
Not subject to U.S. Copyright. Published 1988 by the American Chemical Society
