Correspondence Comment on “Urban Sprawl Leaves Its PAH Signature” SIR: Van Metre et al. in a recent paper (1) report that concentrations of PAHs in sediments in a study group of 10 reservoirs and lakes in six metropolitan areas have increased over the past 20-40 years. The authors state that these increases parallel increasing urbanization in these areas, and they conclude that there is an association between the increasing levels of PAHs in sediments and increased automobile use in the watersheds. These results are startling because they contradict the widespread belief, noted by the authors, that PAH emissions have declined in recent decades. Indeed, there is evidence that atmospheric PAH concentrations in one region studied (northern New Jersey) have declined by a factor of 10 since the early 1980s (2) due to increased emission controls. It can be expected that PAH deposition in this region has declined concomitantly with the decline in air concentrations. It is also likely that national PAH emissions and deposition are correlated with emissions of directly emitted particulate matter (PM10), which are estimated to have declined in the United States by a factor of 4 since a peak in the early 1950s (3). There is another explanation for the finding of higher concentrations of PAHs in more recently deposited sediments, and that is that the concentrations in the older layers are lower not because deposition was less in earlier years but because of in situ degradation. It is known that PAHs degrade in both aerobic and anaerobic environments. Pyrene, for instance, has an estimated half-life in anaerobic media of 2.3-20.8 yr (4). The authors’ observation that there is a higher proportion of four- and five-ringed species (vs. two- and three-ringed species) in the newer sediments also suggests that degradation is a factor since PAHs can be expected to lose carbon rings as they age. As shown in a hypothetical example (Figure 1), if one assumes first-order exponential degradation of PAHs in sediments with a 15-yr half-life, a curve very similar to those reported by the authors is generated by PAH deposition quantities that are identical in relative magnitude to national PM10 emissions estimates (5). In the example in Figure 1, the peak in the sediment concentrations occurring in the mid1970s is a pure artifact; the result of the overlap of the declining curve of PAH emissions and the increasing amounts of more recently deposited PAHs that have not yet degraded.
10.1021/es0019272 CCC: $20.00 Published on Web 03/23/2001
2001 American Chemical Society
FIGURE 1. Hypothetical PAH deposition to a lake, and deposition remaining in sediment with PAH half-life in Sediment of 15 yr. It may be, as the authors suggest, that new, heretofore poorly characterized sources of PAHs exist. However, future research on trends in PAH emissions inferred from concentrations in sediment should recognize that PAHs in sediments, unlike metals and some heavily chlorinated organics, can be expected to measurably degrade over time and eventually disappear. Inferences of deposition rates based on sediment concentrations, to be realistic, must take into account this degradation.
Literature Cited (1) Van Metre, P. C.; Mahler, B. J.; Furlong, E. T. Environ. Sci. Technol. 2000, 34, 4064-4070. (2) Eisenreich, S. J.; Gigliotti, C. L.; Brunciak, P. A.; Dachs, J.; Glenn, T. R., IV; Nelson, E. D.; Totten, L. A.; Van Ry, D. A. Persistent Organic Pollutants in the Coastal Atmosphere of the MidAtlantic States-USA; American Chemical Society Symposium Series; American Chemical Society: Washington, DC, 2000. (3) U.S. EPA. National Air Pollution Emission Trends, 1900-1998; Tables 3-5, Total Emissions of Directly Emitted Particulate Matter (PM10), 1940 through 1998; EPA- 454/R-00-002; U.S. Government Printing Office: Washington, DC, 2000. (4) Howard, P. H.; Boethling, R. S.; Jarvis, W. S.; Meylan, W. M.; Michalenko, E. M. Handbook of Environmental Degradation Rates; Lewis Publishers: Chelsea, MI, 1991. (5) U.S. EPA. 2000.
Michael Aucott New Jersey Department of Environmental Protection P.O. Box 409 401 East State Street Trenton, New Jersey 08625-0409 ES0019272
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