Correspondence Comment on “Is the Hyde Park Dump, Near the Niagara River, Still Affecting the Sediment of Lake Ontario?” SIR: In a recent paper, Howdeshell and Hites (1) conclude that Lake Ontario sediment data for several fluorinated compounds present evidence of continued leaking of those compounds from Hyde Park, a landfill on the Niagara River. However, we believe that a closer look at the data and methods of Howdeshell and Hites suggests a conclusion opposite to theirs: Hyde Park is not leaking chemicals to Lake Ontario. Howdeshell and Hites’ erroneous conclusion of continued leakage is primarily based on the observation that most recent declines in sediment concentrations did not follow declines predicted by a Gaussian or normal distribution (see Figure 1, which is based on data from Figure 6 of ref 1). Instead, declines followed a “log-normal curve”, which is slower than the back half of the Gaussian distribution (Figure 1). There is no theoretical justification for the use of the back half of a Gaussian distribution as an appropriate model for declines of chemical concentrations in sediments, nor is there any reason to believe that concentration time series should fit any probability distribution. The increasing side of a concentration time series is primarily a function of the level of external loading over time. In contrast, the latter half of the concentration time series, the period of chemical decline after cessation of loading, primarily depends on the interaction of the chemical’s physical-chemical properties with the limnology of the lake. As the periods of increase and decrease are based on independent phenomena, it would only be a rare coincidence when they would, when pieced together, be mirror images of each other as assumed with the Gaussian distribution or be matched ends of any other probability distribution. Their use of the Gaussian distribution is based on an earlier paper by Jaffe and Hites (2). However, this earlier paper used the Gaussian distribution “to establish the yearly maximum deposition, the corresponding concentration maximum, and the year of advent. The Gaussian curve, however, does not imply a model for an input function.” (Ref 2, bolding added for emphasis.) Jaffe and Hites, therefore, apparently used the Gaussian distribution as a data smoothing technique, explicitly not as a model for external loading as later used by Howdeshell and Hites. The appropriate model for declines of these compounds in sediments is first-order decline. The loss processes for these chemicalssburial, desorption to the water column, and biodegradationsdepend on the ambient concentration, i.e., dC/dt ) f(C). First-order decline was used in all of the whole lakes models we surveyed (3-5) as well as in research describing declines of chemicals in Great Lakes fish (6), gull eggs (7), water column (8), and sediments (9). The data presented in Howdeshell and Hites adhere closely to a model of first-order decline (Figure 1), indicating that the shape of observed declines of these compounds is consistent with expected declines. Howdeshell and Hites also assert that concentrations of these compounds have leveled off at about 20% of the maximum values, and their Figure 6 does show an apparent stabilization of concentration for the three fluorinated
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FIGURE 1. Relative concentrations of chemical 1 in Lake Ontario sediments versus time. These data points are taken from Figure 6a of ref 1. The dashed line is the Gaussian distribution fit to data up to 1978, and the solid line is the best fit line assuming a first-order decline. compounds. However, their conclusion of stabilization is problematic because it appears based on visual inspection of the data as opposed to a statistical analysis. Visual analysis of concentration data over time is particularly unreliable in this case because decline of chemicals in sediments is a firstorder loss process that always traces a concave-up path over time when plotted on a linear y-axis. Much of what appears to be an approach to equilibrium in their Figure 6 and our Figure 1 is really an optical illusion of plotting a first-order relationship on a linear y-axis. In addition, changes in sediment concentrations, unrelated to source dynamics, are to be expected due to analytical and sampling variability and/ or real environmental events like extreme mixing events. Given an expected high level of data variability, potential deviations from long-term trend lines must be based on appropriate statistical methods (10). Without access to the actual data, we cannot test these data statistically. However, inspection of the individual core data presented in Howdeshell and Hites’ Figures 3-5 suggests that few if any cores would have most recent data that are significantly above the 95% confidence interval for the regressions of previous data. Equally important, Howdeshell and Hites’ conclusion of stabilization is based on what is described as “average” data, but their averaging methodology significantly distorts their data. Howdeshell and Hites do not provide a methodology for averaging. Whatever the method, the averaged data do not represent average behavior of individual cores. For example, the average data in Howdeshell and Hites’ Figure 6 show a sharp increase in concentrations between the last date and next to last date for compounds 1 (between points A and B in Figure 1) and 3. However, as shown in their Figures 3 and 5, concentrations of these chemicals in individual cores generally show ongoing declines between the two most recent data points. For 1, individual cores indicate that most recent concentrations in seven cores decline compared to the second most recent concentration, concentrations in one core stays
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1997 American Chemical Society
constant at near zero (core 23), and four cores show increases in concentration (cores 600, 602, 596, and 609). Thus, cores with continuing decreases between the two most recent points are about twice as frequent as cores with increases. The sharp spike in average concentration of 1 after 1990, from about 20% to 40% of the maximum concentration (points A and B in Figure 1), really does not occur in any core at all. This spike appears to be totally an artifact of the averaging method. Similarly, 2 is depicted in Figure 6 as having an increase in concentration between the last two dates and the previous two dates. According to their Figure 4, the individual core data again show a very different picture. In all but one of 12 cores, the mean concentration for two most recent data points is less than the mean for the third and fourth most recent points. Thus, the perception of stabilizing concentrations is artificially enhanced by Howdeshell and Hites’ “averaged” data, which show increases in concentration for most recent data. However, most individual cores show continuing declines in concentration up to and including most recent data. Finally, the data presented in Howdeshell and Hites do not support their contention that most recent concentrations are higher than would be expected from the rates of decline predictable from data collected earlier by Jaffe and Hites (2). According to the data presented in Figure 9 of Jaffe and Hites, 2 appeared to be declining at about 5% per year from its peak concentration in 1971 onward. More recent data presented in Howdeshell and Hites suggest that the decline since 1971 has been about 8% per year, nominally faster than would have been predicted by the earlier data. We also provide here information concerning remediation conducted at the Hyde Park Landfill. The Hyde Park Landfill was operated by Hooker Chemical from 1953 to 1975. The site was closed in 1975, and an impermeable clay cap was installed over the site. An overburden drain tile collection system was installed in 1978. These measures precluded groundwater movement through the waste and, therefore, significantly reduced chemical loading to the Niagara River and Lake Ontario from the site. With site wastes secure, a series of studies were performed over the next 10 years to determine the final remedy. The final remedy was installed, in stages, between 1989 and 1994. At this time, it prevents overburden groundwater from exiting the site. Minor adjustments to the bedrock remedy, which will eliminate flow through the bedrock toward the Niagara River, are ongoing. Thus, our geology and engineering data indicate that continual significant leakage from Hyde Park is unlikely. These data are consistent with our re-interpretation of Howdeshell and Hites’ data. In conclusion, Howdeshell and Hites’ conclusion of continued leakage from Hyde Park was based on the following three lines of evidence. First, most recent declines in the sediment concentrations of these compounds did not follow declines predicted by a Gaussian or normal distribution. Instead, declines followed a log-normal curve, which is slower than the back half of the Gaussian distribution (Figure 1).
Second, the authors assert that their data show chemicals stabilizing at about 20% of their historical maxima. Third, the authors contend that recent concentrations are higher than would be predicted from samples taken earlier by Jaffe and Hites (2). However, our re-analysis of their data and methods indicates the following. Declines of chemicals in sediments are correctly modeled as first-order decline, and this appropriate model adequately describes the concentration time series presented in Howdeshell and Hites. The approach to equilibrium seen by Howdeshell and Hites has no statistical basis, appears to be based on visual observation of data on an inappropriate y-axis, and is partially an artifact of their averaging method. Most recent concentrations are actually nominally lower than would be predicted based on data collected earlier by Jaffe and Hites. Thus, rather than indicating continuing leakage from Hyde Park Landfill, the data presented in Howdeshell and Hites actually indicate the opposite: loading from Hyde Park is negligible.
Acknowledgments Both authors’ work was funded by the Occidental Chemical Corporation.
Literature Cited (1) Howdeshell, M. J.; Hites, R. A. Environ. Sci. Technol. 1996, 30, 969-974. (2) Jaffe, R.; Hites, R. A. Environ. Sci. Technol. 1986, 20, 267-274. (3) Gobas, F. A.; Z’Gaggen, P. M.; Zhang, X. Environ. Sci. Technol. 1995, 29, 2038-2046. (4) Endicott, D. D.; Richardson, W. L.; DiToro, D. M. Lake Ontario TCDD Modeling Report. In Lake Ontario TCDD Bioaccumulation Study: Final Report; EPA, NY Department of Environmental Conservation, NY Department of Health, and Occidental Chemical Corporation: May 1990. (5) Mackay, D.; Sang, S.; Vlahos, P.; Diamond, M.; Gobas, F.; Dolan, D. J. Great Lakes Res. 1994, 20, 625-642. (6) DeVault, D. S.; Wilford, W. A.; Hesselberg, R. J.; Northrupt, D. A.; Rundberd, E. G. S.; Alwan, A. K.; Bautista, C. Arch. Environ. Contam. Toxicol. 1986, 15, 349-356. (7) Smith, D. W. Environ. Sci. Technol. 1995, 29, 740-750. (8) Jeremiason, J. D.; Hornbuckle, K. C.; Eisenreich, S. J. Environ. Sci. Technol. 1994, 28, 903-914. (9) Pearson, R. F.; Hornbuckle, K. C.; Eisenreich, S. J.; Swackhamer, D. L. Environ. Sci. Technol. 1996, 30, 1429-1436. (10) Smith, D. W. Environ. Sci. Technol. 1995, 29, 42A-46A.
Daniel W. Smith* SMITH Technology Corporation One Plymouth Meeting Plymouth Meeting, Pennsylvania 19462
Alan P. Weston Occidental Chemical Corporation 5005 LBJ Freeway Dallas, Texas 75380 ES960548Z
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