Correspondence Comment on “Delayed Deposition of Organochlorine Pesticides at a Temperate Glacier” SIR: The paper by Donald et al. (1) describes the results of a study of the organochlorine pesticide residues in glacial ice in a temperate glacier at Snow Dome Mountain in the Rocky Mountains of Canada. The data presented indicate an intuitively logical result. However, we believe that the conclusions drawn by the authors would benefit through the consideration of additional and more recent literature than that which was cited in this paper. We wish to draw your attention to some specific items. In the Introduction, the authors refer to the measurement of “... the long term depositional history of organochlorine pesticides ...”. The key word that we take exception to is deposition. Measuring semivolatile contaminants in glacial snow is complex (see below), and we believe that the authors are not measuring deposition but are measuring contaminant residues, i.e., residual concentrations, with little consideration of the processes that determine the resultant residues. Moreover, flux values for individual pesticides are only reported two times in the text. The assumption that they are in fact measuring deposition (presumably from the atmosphere to the glacier) as opposed to accumulation is carried throughout the text and causes us to question their interpretation, particularly their conclusion that deposition is declining. The authors noted that the “annual strata in temperate glaciers are recognized in alternating bands of opaque and clear ice”. This clearly demonstrates that extensive melting occurs on this glacier, which will significantly affect the concentration of the contaminant residue detected in the layer. Melting may distribute meltwater and contaminants among more than one layer of the glacier or in fact remove contaminants from the layer, if surface runoff occurs. The authors themselves note that the sampling was conducted by “entering a crevasse” and by “climbing down an ice cliff” to collect their samples, thus suggesting that the sampled layers of firn/ice may have been exposed to significant weathering processes, including melt-refreeze cycles. We have noted extensive migration of meltwater in some annual layers in the Agassiz Ice Cap, a polar ice cap in the Canadian Arctic (2). Here the average summer snowmelt affects only about 3% of the winter snow layer (3), and the process can be even more significant in temperate glaciers (4). Without an indication of the absence of interlayer migration (e.g., from analysis of conservative ionic species), the assumption of preserved annual records remains questionable. Donald et al. (1) refer to the “cold condensation effect” as a conceptual mechanism for the global redistribution of contaminants but fail to recognize the significance of this phenomenon upon the interpretation of their own data. Ref 5 noted that the PCB congeners in the Agassiz Ice Cap were dominated by the lower chlorinated PCBs, indicating longrange atmospheric transport. Vapor pressure, ambient temperatures, aerosol concentrations, and other factors will all determine the extent of volatilization of semivolatile contaminants both from source areas and from glaciers. Organic compounds, such as hexachlorobenzene (HCB), and R-HCH (hexachlorocyclohexane), and γ-HCH with subcooled liquid vapor pressures >10-3 Pa at 0 °C may volatilize substantially 10.1021/es991370u CCC: $19.00 Published on Web 05/27/2000
2000 American Chemical Society
from the snowpack; while PCBs and PAHs, with lower vapor pressures and a greater tendency to be associated with particles, may not (6). On the basis of data from the Agassiz Ice Cap, Franz et al. (6) reported approximate snowpack losses, based on replicate measurements of concentrations, as follows: compound
% loss
compound
% loss
HCB R-HCH γ-HCH heptachlor epoxide
69 53-92 67-97 87-100
γ-chlordane R-chlordane R-endosulfan dieldrin
96-100 80-94 88-100 37-84
The loss mechanism is likely to be primarily a result of the capacity of snow for sorbed vapor-phase organic compounds being a function of the snow surface area (7). As fresh snowfall settles, it compacts and undergoes a process of metamorphism ending in dense glacial ice. During the first stages of this process there is a dramatic decrease in snow surface area, leading to the possible displacement of sorbed semivolatile compounds back into the gas phase where they are then available for advective transport within and out of the snowpack and ultimately for recycling back into the atmosphere. Modeling of this process for γ-HCH suggests that a substantial fraction of the original amount deposited can revolatilize after a process of fernification lasting for a period of just 1 year (8). Finally, concentration peaks, which are the basis for Figure 1 in ref 1, can be misleading when referring to precipitation records. Our extensive experience in polar snow packs has demonstrated that high concentrations in annual layers may more appropriately correlate with reduced annual snowfall. As snowfall is the primary determinant in contaminant flux, a high contaminant concentration may in fact be indicative of reduced flux. If actual annual snowfall and snow/water equivalents are used to calculate annual contaminant flux (see ref 9) (as opposed to assuming “... a uniform 1 m annual snowpack and a 3/10 ratio for water to snow”), we would have much more confidence in any reported trend. One possible indication of this inappropriate conclusion is indicated in Figure 1 of ref 1. Specifically, if the trend is being determined by source emissions and revolatilization from these source regions, why would one expect a similar pattern for both dieldrin and endosulfan? Similarly, we do not see a great deal of difference between the residue trend reported for Σ-chlordane and heptachlor epoxide and that reported for dieldrin and endosulfan. Dieldrin and heptachlor were banned from agricultural use in the mid-1970s in the United States while endosulfan continues to be used worldwide. Chlordane has been used in the United States for agricultural and domestic purposes since the 1960s but has been restricted since 1988 and 1990 in the United States and Canada, respectively (10). Perhaps this similarity in concentration residues is therefore more indicative of similar spring and summer loss processes, which are controlling the residue profiles, rather than similar deposition patterns. In conclusion, while we agree that the data suggest a general decline in pesticide concentrations in the 30+ years of data from Snow Dome Mountain, we do not believe that the authors have substantiated their conclusion that this reflects decreased deposition as a result of decreased use. Although the discussion of trends and the conclusion that many of the compounds reveal decreased residues in the glacier in latter years agrees with the expected outcome, we VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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do not believe that these data alone can be used to conclude a depositional decline.
Literature Cited (1) Donald, D. B.; Syrgiannis, J.; Crosley, R. W.; Holdsworth, G.; Muir, D. C. G.; Rosenberg, B.; Sole, A.; Schindler, D. W. Environ. Sci. Technol. 1999, 33, 1794-1798. (2) Peters, A. J.; Gregor, D. J.; Teixeira, C. F.; Jones, N. P.; Spencer, C. Sci. Total Environ. 1995, 160/161, 167-179. (3) Barrie, L. A.; Gregor, D.; Hargrave, B.; Lake, R.; Muir, D.; Shearer, R.; Tracey, B.; Bidleman, T. Sci. Total Environ. 1992, 122, 1-74. (4) Wagenbach, D. In The Environmental Record in Glaciers and Ice Sheets; Report of the Dahlem Workshop, Berlin, 1988; Oeschger, H., Langway, C. C., Eds.; Wiley-Interscience: New York, 1989. (5) Gregor, D. J.; Peters, A. J.; Teixeira, C.; Jones, N.; Spencer, C. Sci. Total Environ. 1995, 160/161, 117-126. (6) Franz, T. P.; Gregor, D. J.; Eisenreich, S. J. Snow deposition of atmospheric semivolatile organic chemicals. In Atmospheric Deposition of Contaminants to the Great Lakes and Coastal Waters; Baker, J. E., Ed.; Society for Environmental Toxicology and Chemistry, Proceedings from a session at the SETAC 15th Annual Meeting, October 30-November 3, 1994, Denver, CO; SETAC Technical Publication Series; SETAC Press: Pensacola, FL, 1997; pp 73-107.
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(7) Hoff, J. T.; Wania, F.; Mackay, D.; Gillham, R. Environ. Sci. Technol. 1995, 29, 1982-1989. (8) Wania, F. Chemosphere 1997, 35, 2345-2363. (9) Gregor, D. J.; Teixeira, C.; Rowsell, R. Chemosphere 1996, 33 (2), 227-244. (10) AMAP (Arctic Monitoring and Assessment Program). AMAP Assessment Report: Arctic Pollution Issues; Arctic Monitoring and Assessment Program: Oslo, Norway, 1998; xii + 859 pp.
Dennis J. Gregor* MDA Consulting Limited No. 163, 3017 St. Clair Avenue Burlington, Ontario, Canada L7N 3P5
Andrew J. Peters Centre for Ecology and Hydrology Natural Environment Research Council Winfrith Technology Centre Dorchester, Dorset, U.K., DT2 8ZD ES991370U
