Environ. Sci. Technol. 2001, 35, 1890-1891
Response to Comment on “Urban Sprawl Leaves Its PAH Signature” SIR: Although Aucott raises an interesting question regarding PAH trends in sediment cores (i.e., could increasing trends be an artifact of degradation of PAH in older sediment layers?), we think that the evidence supporting our interpretation is clear and overwhelming. Aucott is correct in pointing out that (i) there has been a recent decline in PM10 emissions trends and that (ii) PAH can be biodegraded. However, from both a chemical and accumulation-modeling standpoint, he is incorrect about the importance and/or the consequences of these processes. First, PM10 emissions, while declining, are only one component of the total PAH burden to reservoir watersheds. Numerous researchers have investigated and documented urban sources of PAH, many of which do not have atmospheric pathways (e.g., leaking motor oil, asphalt and tire wear). Our research highlights the probability that these sources play a significant role in the observed increasing trends. For example, Tokyo street dust, containing PAH from asphalt, automobile exhaust, and tire wear residue, with contributions from used motor oil and gasoline spills, was a rich (ppm by weight) source of PAH (1). In their study, the dominant source of PAH in street dust was automobile exhaust. These street dust contributions would not be reflected in PM10 emission data because their particle size is much larger. If street dust is a substantial component of the total PAH deposition observed in urban reservoirs, then our interpretation of our results is consistent with increasing vehicular traffic in the urban watersheds in our paper (see Figure 4 and ref 2). If the increasing trends cited in our paper were simply a result of atmospheric inputs followed by degradation, as suggested by Aucott, all the lakes in a metropolitan area should show similar trends. In fact, we often see contrasting trends in lakes in neighboring watersheds, if the watersheds have undergone different levels of development. For example, as shown in Figure 1, trends in two Minneapolis lakes, Harriet and Palmer, are very different even though these lakes are only about 17 km apart. Second, Aucott suggests that the PAH assemblages seen (higher concentrations of the four- and five-ringed or “combustion” PAHs) in more recent sediments are further evidence for degradation, because the four- and five-ringed PAHs have degraded to two- and three-ringed PAHs in the older sediments. In fact, lower-ringed PAHs degrade faster than higher-ringed PAHs (3, 4). If degradation were controlling the trends seen in the cores analyzed, we would expect to see the older sediments enriched in the four- and fiveringed PAHs. The fact that we see the opposite supports our belief that the change in assemblage reflects a shift from noncombustion to combustion sources coincident with urbanization and increasing trends. Furthermore, when PAH degrade, they are not converted to other unsubstituted PAH with lower ring numbers but rather degrade first to diolsubstituted PAH and then, as the aromatic rings are opened, to alcohol, aldehyde, and acid-substituted derivatives (5). Third, Aucott cites degradation as the most likely cause of the overall trends in total PAH seensthat decreasing PAH concentrations would be seen as increasing because of degradation of PAHs over time. In that case, it is difficult to understand why in every case the sharp increase in PAH concentrations coincides with the onset of urbanization. For 1890 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 9, 2001
FIGURE 1. Measured total PAH in sediment cores from Lake Harriet and Palmer Lake.
FIGURE 2. Lake Harriet core with measured and modeled total PAH concentrations at time of deposition, based on the degradation model proposed by Aucott. example, development began in the Lake Harriet watershed around 1920 and in the Palmer Lake watershed around 1970 (Figure 2). Furthermore, the flaw in the degradation theory can be demonstrated by applying Aucott’s degradation model to real data. Applying his model to the total PAH concentrations analyzed in the core from Lake Harriet, a concentration of 1 500 000 µg/kg at time of deposition would be necessary to result in the concentration of 115 000 µg/kg seen in the core interval corresponding to the 1950s andseven more unbelievablysan original concentration of 3 000 000 µg/kg would be necessary to result in the concentration of 400 µg/kg corresponding to the early 1800s. These concentrations are an order of magnitude higher than those seen in some of the nation’s most polluted water bodies. The controlling variable for PAH bioavailability, and thus biodegradation, is the partitioning of PAH between sediment and the aqueous phase (6). The data available suggest that PAH and other hydrophobic contaminants partitioned onto natural sediment and soil become less bioavailable as the solids age (7) and that decreased desorption of PAH as sediments age may control bioavailability and hence degradation (8). As Carmichael et al. (8) note, this can explain the persistence of PAH in soils and sediment, even in the presence of an active community of PAH-degrading microorganisms. In addition, recent evidence suggests that a substantial fraction of sedimentary PAH are not available for partitioning and hence do not partition into the aqueous phase and are not available for degradation (9). Contrary to Aucott’s assertion, we do not suggest that “new, heretofore poorly characterized sources of PAH exist”. 10.1021/es010658s Not subject to U.S. copyright. Publ. 2001 Am. Chem.Soc. Published on Web 03/23/2001
We suggest rather that as regional atmospheric emissions (e.g., power plants) and urban emissions (e.g., the switch from using coal for home heating) have declined in the United States, large increases in vehicle traffic accompanying the growth of urban “sprawl”-type development have replaced them as major sources of PAH in the urban environment and are leading to increasing trends in PAH in urban (and urbanizing) waterbodies.
(6) McElroy, A. E.; Farrington, J. W.; Teal, J. M. Bioavailability of polycyclic aromatic hydrocarbons in the aquatic environment. In Metabolism of Polycyclic Aromatic Hydrocarbons in the Aquatic Environment; Varanasi, U., Ed.; CRC Press: Boca Raton, FL, 1989; pp 1-39. (7) Alexander, M. Environ. Sci. Technol. 2000, 34 (20), 4259-4265. (8) Carmichael, L. M.; Christman, R. F.; Pfaender, F. K. Environ. Sci. Technol. 1997, 31 (1), 126-132. (9) McGroddy, S. E.; Farrington, J. W.; Gschwend, P. M. Environ. Sci. Technol. 1996, 30 (1), 172-177.
Literature Cited (1) Takada, H.; Onda, T.; Harada, M.; Ogura, N. Sci. Total Environ. 1991, 107 (1), 45-69. (2) Van Metre, P. C.; Mahler, B. J.; Furlong, E. T.; Environ. Sci. Technol. 2000, 34 (19), 4064-4070. (3) MacRae, J. D.; Hall, K. J. Water Sci. Technol. 1998, 38 (11), 177-185. (4) Potter, C. L.; Glaser, J. A.; Chang, L. W.; Meier, J. R.; Dosani, M. A.; Herrmann, R. F. Environ. Sci. Technol. 1999, 33 (10), 1717-1725. (5) Cerniglia, C. E.; Heitcamp, M. A. Microbial degradation of polycyclic aromatic hydrocarbons (PAH) in the aquatic environment. In Metabolism of Polycyclic Aromatic Hydrocarbons in the Aquatic Environment; Varanasi, U., Ed.; CRC Press: Boca Raton, FL, 1989; pp 41-68.
Peter C. Van Metre* and Barbara J. Mahler U.S. Geological Survey 8027 Exchange Drive Austin, Texas 78754-4733
Edward T. Furlong U.S. Geological Survey P.O. Box 25046, MS 407 Denver Federal Center Lakewood, Colorado 80225 ES010658S
VOL. 35, NO. 9, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1891
