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Response to Comment on “Is the Hyde Park Dump, near the Niagara River, Still Affecting the Sediment of Lake Ontario?” SIR: In 1986, we reported that several polyfluorinated compounds were leaking from the Hyde Park Dump and contaminating Lake Ontario (1). At that time, based on an extrapolation of our sediment core data, we suggested that surficial sediment concentrations of these compounds would “reach negligible levels in the early 1990s”. When we recently re-investigated this issue, we found that this extrapolation was not correct and that the current surficial sediment concentrations were about 20% of the maximum concentrations (which occurred in about 1970) (2). From this observation, we concluded that “it is probable that leachate [from the Hyde Park Dump] is still migrating into the Niagara River” (2). This was not a definitive conclusion, and we suggested that there were “many reasons to explain why the concentrations are not yet at background levels” (2). In their comments on our paper, Smith and Weston suggest that the Hyde Park Dump is “not leaking chemicals to Lake Ontario”. Later in their comments, they modify this unequivocal statement to suggest that leakage from the Hyde Park Dump is either “unlikely” or “negligible”. Smith and Weston base their polemic on two lines of reasoning; we will comment on these in turn. Core Averaging. Smith and Weston suggest that the concentrations have not leveled off at about 20% of their maximum values and that our observations are really just “an optical illusion of plotting”. Furthermore, Smith and Weston state that we did “not provide a methodology for [core] averaging” to get the data used in our Figure 6 (2). While we did explain that the data were “normalized to the highest concentration in each core”, we were negligent in explaining the methodology by which the various sediment core sections (corresponding to different times) were composited. Figure 6 showed that these core sections were composited at time intervals ranging from 7 years at the bottom of the core to 2 years at the top (2). Upon reflection, we decided to repeat these calculations using a constant compositing time of 3 years. This time corresponds to the time interval covered by the average 1-cm‚ thick core section, given that the sedimentation rate averaged 0.2-0.4 cm/year. The following calculation procedure was used: The year-concentration data pairs for each compound were first normalized by dividing each concentration by the maximum concentration of that compound in a given core. The normalized concentrations of each compound were then averaged in 3-year windows. These time windows started with 1993 (when the samples were collected) to 1991, and moved back in 3-year increments to 1942-1940. For example, there were six measurements of compound 1 in the 1993-1991 time period; the normalized concentrations were 0.099 (core 23), 0.115 (589), 0.467 (597), 0.570 (602), 0.342 (603), and 0.163 (609). The average of these values was 0.293; this value was assigned to 1992, which was the middle year of the 3-year interval. Incidentally, the complete data set is available elsewhere (3). This calculation was repeated for each compound and for each 3-year time interval. The results are plotted in Figure 1. We have not fitted any curve to these data; the points are merely connected by a cubic spline line. There are some minor differences between this figure and Figure 6 in our
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FIGURE 1. Relative concentrations of compounds 1, 2, and 3-2 normalized to the maximum concentration in each core composited in 3-year intervals. Kingston Basin cores are not included. See ref 2 for compound structures. previous paper (2). For example, the relatively high surficial values (see point A in Smith and Weston’s Figure 1) are now lower. In terms of current surficial sediment concentrations, it is a fact that the levels of these fluorinated compounds coming from the Hyde Park Dump have not yet reached background levels as we had suggested in our 1986 paper (1). It is also a fact that the surficial sediment concentrations of these compounds are 20-30% of their maximum concentrations in 1970. First-Order Decrease. Smith and Weston suggest that the correct model for the decline of the concentrations of these pollutants as a function of depth in the lake’s sediment is a first-order exponential model rather than the normal or log-normal curves we had used. As Smith and Weston note, we had not intended either of these two curves to “imply a model for the input function” of these pollutants. We used these curves only to smooth the data; we stated this explicitly in our 1986 paper (1), but we failed to make this clear in our 1996 paper (2). A first-order model requires that a plot of the logarithm of concentration vs time of deposition in the core should be linear. We tested Smith and Weston’s suggestion by plotting the post-1970 concentrations on a logarithmic scale; see Figure 2. Clearly, their suggestion is valid, and it allows us to make some extrapolations into the future. For example, these data indicate that it will take at least another 15 years for the surficial sediment concentrations of these
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1997 American Chemical Society
about the source function of these compounds. For example, first-order behavior could be observed if the leak rate from the Hyde Park Dump had been decreasing exponentially since about 1970. We think that this is a likely possibility. In any case, a first-order rate of decrease of the concentrations of these polyfluorinated compounds with depth in the sediment core does not indicate that the Hyde Park Dump is no longer leaking. Summary. The overall decrease in the sediment concentrations from 1970 to the present is a testament to the effectiveness of the remediation efforts carried out by the OxyChem Chemical Co. However, we note that Smith and Weston state that “minor adjustments to the bedrock remedy, which will eliminate flow through the bedrock toward the Niagara River, are ongoing”. We presume this means that this leakage pathway is not yet completely eliminated and that leakage from the Hyde Park Dump is still possiblesa statement fully compatible with our conclusions. Thus, despite Smith and Weston’s statement to the contrary, it is fair to say that no one knows if the Hyde Park Dump is still leaking or not. Our data indicate that this is a possibility.
Literature Cited (1) Jaffe, R.; Hites, R. A. Environ. Sci. Technol. 1986, 20, 267274. (2) Howdeshell, M. J.; Hites, R. A. Environ. Sci. Technol. 1996, 30, 969-974. (3) Howdeshell, M. J. Ph.D. Thesis, Department of Chemistry, Indiana University, 1995.
FIGURE 2. Post-1970 data from Figure 1 replotted on a semilogarithmic scale. The correlation coefficients (r 2) for the regression lines are 0.956, 0.952, and 0.933, all of which are significant at the 99% confidence level. compounds to reach 5% of their maximum levels. The accuracy of this model, however, does not tell us anything
Michael J. Howdeshell and Ronald A. Hites* School of Public and Environmental Affairs and Department of Chemistry Indiana University Bloomington, Indiana 47405 ES962017G
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