Research Polychlorinated Dibenzo-p-dioxin and Dibenzofuran Concentration Profiles in Sediment and Fish Tissue of the Willamette Basin, Oregon BERNADINE A. BONN* U.S. Geological Survey, Water Resources Division, 10615 S. E. Cherry Blossom Drive, Portland, Oregon 97216
Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) are highly hydrophobic compounds that have been implicated as carcinogens and, more recently, as estrogen disrupters. An occurrence and distribution study of these compounds in the Willamette Basin, Oregon, was conducted by the U.S. Geological Survey as part of the National Water-Quality Assessment Program. Bed sediment was collected from 22 sites; fish tissue was collected from eight sites. PCDD/F were found to be ubiquitous in Willamette Basin sediment. A distinct homolog profile, dominated by octachlorodibenzo-p-dioxin, was observed in sediment throughout the basin. The PCDD homolog profile was consistent at all sites, regardless of total PCDD/F concentration, presence of point sources, subbasin size, geographic location or land use. Principal components analysis revealed a gradient among the homolog profiles that showed increasing dominance of highly chlorinated congeners where human and industrial activity increased. Tissue and bed sediment obtained from the same site did not have similar PCDD/F concentrations or homolog profiles. Fish tissue showed enrichment in less chlorinated congeners and congeners with chlorine substitutions in the 2, 3, 7 and 8 positions.
Introduction Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/ F) are well-known environmental contaminants. Some PCDD/F are considered human carcinogens (1) and have been implicated as endocrine disrupters in humans (2) and other animals (3). They are inadvertently produced from a variety of processes, including municipal and medical waste incineration, production of chlorinated aromatic compounds (such as pentachlorophenol and PCBs), bleaching of kraft pulp, metals production, and chlorination of sewage effluent. A survey of PCDD/F occurrence and distribution in the Willamette and Sandy River Basins was conducted from 1992 to 1995 by the U.S. Geological Survey in cooperation with Oregon Department of Environmental Quality as part of the National Water-Quality Assessment Program (NAWQA). The NAWQA program is designed to assess water-quality conditions in representative watersheds throughout the country (4). Part of this assessment included the analysis of bed sediment and tissue samples for hydrophobic organic contaminants. Although PCDD/F are not target analytes for * Author fax: 503-251-3470; e-mail:
[email protected]. S0013-936X(97)00609-3 Not subject to U.S. Copyright. Publ. 1998 Am. Chem. Soc. Published on Web 02/03/1998
the national program, they were of enough local concern that the Willamette Basin study group included them in its bed sediment and tissue sampling survey (5). There are 75 PCDD and 135 PCDF congeners, compounds that differ in the number and/or position of chlorine atoms. PCDD/F can be grouped as homologs, or congener classes, compounds which have the same number of chlorines. Throughout this paper, homologs will be abbreviated as follows: D4, D5, D6, D7, and D8 for tetrachloro-, pentachloro-, hexachloro-, heptachloro-, and octachloro-dibenzop-dioxins and F4, F5, F6, F7, and F8 for tetrachloro-, pentachloro-, hexachloro-, heptachloro-, and octachlorodibenzofurans, respectively. Congeners that have chlorine atoms located in the 2, 3, 7, and 8 positions (e.g., 2,3,7,8tetrachlorodibenzo-p-dioxin or 1,2,3,7,8-pentachlorodibenzofuran) will be referred to as 2,3,7,8-substituted congeners.
Materials and Methods Sampling. (A) Sites. Twenty-three sites were selected from throughout the Willamette and Sandy River Basins (Figure 1). Upon the basis of nearby and upstream land use, sites were classified as reference (remote sites with no known PCDD/F sources and subject to little human activity), agricultural (sites with minimum 50% basin area used for agriculture), industrial/urban (sites in urban areas, and/or near industrial areas), and mixed-use (sites with urbanresidential and agricultural land) (6). (B) Sediment. Bed sediment was collected from 22 sites (Figure 1). For wadeable sites, a Teflon scoop was used to remove the top 1-2 cm of fine grained sediment from at least 10 wadeable depositional zones within a reach. For nonwadeable sites, the same approach was used to subsample sections of sediment that had been obtained with an Ekman dredge. Approximately 8 L of wet sediment were composited for each reach. A subsample (about 300 mL) was removed, sieved at 2 mm, and kept at 4 °C until analysis. The collection method is described in detail by Shelton and Capel (7). (C) Fish Tissue. Fish were collected from eight sites (Figure 1) by electrofishing and then euthanized by a sharp blow to the head. Species were chosen by availability. Sculpin were obtained from three sites, largescale sucker from two sites, and carp, bluegill, and cutthroat trout from one site each. Whole fish were wrapped in aluminum foil and frozen until analysis. Each sample consisted of 5-20 whole fish, which were homogenized, composited, and subsampled by Quanterra Environmental Services (Sacramento, CA). The collection method is described in detail by Meador, et al. (8). Analytical Methods. (A) PCDD/F Analysis. Samples were analyzed for PCDD/F (10 tetra- through octa-homolog totals and 17 2,3,7,8-substituted congeners) by Quanterra Environmental Services (Sacramento, CA) using isotope dilution gas chromatography/mass spectroscopy [EPA method 8290 (9)]. To permit quantification of low levels of PCDD/F in sediment samples from reference sites, the method was modified by increasing the sample size from 5 g to 10 or 30 g for select samples. The author reviewed the ion chromatograms, calibration checks, and associated calculations. Concentrations were calculated from all chromatographic peaks which had peak height greater than 2.5 times the signal-to-noise ratio and met all other criteria outlined in U.S. EPA method 8290. This resulted in quantification of concentrations that were less VOL. 32, NO. 6, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Sampling sites in the Willamette Basin. Primary land use at site is indicated by site id number: R ) reference, I ) industrial/ urban, M ) mixed use, A ) agricultural. than the minimum reporting level routinely used by Quanterra Environmental Services. Percent recoveries of sample fortification standards and laboratory reagent control spikes were within U.S. EPA approved limits. Method blanks generally contained octachlorodibenzo-p-dioxin (D8), but at concentrations no more than 2 pg/g; other congeners were present only sporadically and in much smaller concentrations. Method blank concentrations were subtracted from the determined sample concentrations. Interference from chlorinated diphenylethers (DPEs) was a problem for some samples, especially fish tissue. When DPE interference was present, it prevented the quantification of tetra- through hepta-chlorinated dibenzofuran congeners. DPE interference affected fish data from two sites; DPE was 730
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not evident in fish tissue samples from the other six sites. Triplicate sediment samples were obtained at three sites; one set of triplicate fish samples was collected. Agreement among replicates varied with congener class and site (Table 1). In general, variability was highest for the least chlorinated congeners, which were also present at the lowest concentrations: D4 and D5 for sediment and D6-D8 in fish tissue. (B) Moisture and Lipid Content. Moisture (sediment and fish tissue) and lipid content (fish tissue) were determined gravimetrically by Quanterra Environmental Services as described in U.S. EPA method 8290 (9). (C) Organic Carbon Content. Organic carbon content of sediment samples was determined as the difference of total carbon and inorganic carbon. Total carbon was determined using an induction furnace method (method
TABLE 1. Reproducibilitya of PCDD/F Analyses coefficient of variation (%) [mean value (pg/g)] sediment site D4 D5 D6 D7 D8 F4 F5 F6 F7 F8
A 23 [0.6] 18[0.9] 2[9.5] 2[30] 3[110] 13[0.8] 10[1.5] 5[3.6] 4[5.7] 7[3.9]
fish
B 13[2.5] 13[3.5] 10[25] 11[43] 18[130] 3[1.2] 4[1.0] 7[1.0] 18[1.2] 16[1.2]
C 63[0.8] 50[1.4] 8[27] 6[170] 4[700] 19[2.3] 20[6.5] 23[27] 6[57] 3[52]
23[2.0] 10[1.3] 30[1.7] 37[2.3] 41[11] DPEb DPE DPE DPE NDc
a Statistics shown are from triplicate samples (three sediment sites and one fish tissue site). b DPE indicates that diphenylether interference prevented quantitation of the compound in the sample from this site. Other sites were not affected. c ND indicates compound was not detected in the sample.
O-5101-83 (10)). Inorganic carbon was measured using a modified Van Slyke method (method O-5102-83 (10)). Inorganic carbon typically was negligible relative to total carbon. All carbon analyses were performed by the USGS National Water Quality Laboratory in Arvada, CO. Data Analysis. (A) Homolog Profiles. Homolog profiles are created by normalizing homolog concentrations to the total PCDD/F concentration for that sample. Homolog profiles allow the comparison of samples with a wide concentration range. Profile patterns have been used to link sites with potential sources (11-14). (B) Statistical Methods. Nonparametric methods were used for most statistical analyses. Differences among groups were assessed using the Kruskal-Wallis test followed by multiple comparisons tests (also using the Kruskal-Wallis statistic). The Kruskal-Wallis test compares group medians. The critical R value was modified at each step in the multiple comparison test to control the overall error rate (15). Correlations between homolog profiles were assessed using the Spearman rank correlation statistic (F). Parametric statistics (mean and coefficient of variation) were used to assess replicate data, as random errors can be expected to be normally distributed. (C) Principal Component Analysis. Homolog profile patterns were compared using principal component analysis (PCA). PCA is a multivariate technique that can be used to reduce the dimensionality of complex data. It has been described in a number of texts (16, 17) and has been applied to PCDD/F data by several researchers (11, 18-21). Principal components are linear combinations of the original variables and are ordered such that the first principal component accounts for the greatest fraction of the variance, and the last component accounts for the least. Applying PCA to homolog profiles presents some challenges. The data must be normalized to total PCDD/F to produce comparable profiles. However, normalization introduces dependence among the variables and, consequently, a negative bias among the correlations used to determine the principal components. Aitchison (22) showed that applying the logcontrast transformation could eliminate the problem of negative bias. The logcontrast transformation was applied to normalized homolog profiles before PCA. For 10-dimensional homolog profiles (Hi), the logcontrast transformation is given by
logcontrast (Hi) ) log Hi -
1
10
∑log H 10
j
j)1
FIGURE 2. Comparison of total PCDD/F concentration in sediment in nanograms per gram dry weight (O) and in nanograms per gram organic carbon (]) by land use at site (ref ) reference, agri ) agricultural, mix ) mixed use, ind/urb ) industrial or urban). Different letter designations indicate statistically significant differences among groups (r < 0.05). The groups designated b and b′ are different only at r < 0.1. Statistical tests were performed separately for the two different units of measure and are indicated by upper and lower case letters.
Results and Discussion General Trends in Sediment. PCDD/F were found in all sediment samples, including those from streams in old growth forests. Total PCDD/F concentrations in sediment varied by about 3 orders of magnitude across the basin (Figure 2). As might be expected, the lowest total concentrations (less than 0.1 ng/g) were found at the most remote sites where atmospheric deposition is the only known source. The highest total concentrations (20-50 ng/g) were found in industrial/urban areas; these concentrations were significantly higher (R < 0.05) than those from reference, agricultural, and mixed-use areas. Concentrations at reference, agricultural, and mixed-use areas were not significantly different from one another (R < 0.05), suggesting that they are largely influenced by atmospheric deposition. Total PCDD/F at these sites were usually less than 1 ng/g and similar to concentrations considered to be background levels due to atmospheric deposition: Siskiwit Lake, Isle Royale, MI, 0.6 ng/g (11); Shepaug River, CT, 0.9 ng/g (18); Elk River, MN, 0.7 ng/g (23); River Dane, U.K., 0.4 ng/g (24). When total PCDD/F concentrations were normalized to organic carbon content, neither the range of concentrations nor the standard deviation decreased. At first glance, this lack of relationship may seem surprising, but upon closer consideration it is unlikely that variation in organic carbon content could account for the wide PCDD/F concentration range found in the Willamette Basin. Source strength is probably the principal factor accounting for the differences among these samples. Strong correlations between total PCDD/F and organic carbon content have been found elsewhere (25), but the source strength was relatively constant in such cases. Normalizing to organic carbon did reveal a trend among site classifications. Total PCDD/F concentrations normalized to organic carbon increased with intensity of human activity: reference sites < agricultural sites < mixed-use sites < industrial/urban sites (Figure 2). The causes and implications of this trend are unclear. If the sediments are in equilibrium VOL. 32, NO. 6, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Distribution of homolog profiles (normalized to total PCDD/F) for sediment. The center line of the boxplot indicates the median, the box extends from the 25th to the 75th percentiles, and the whiskers extend to the 10th and 90th percentiles. Only the central 80% of the distribution is shown. Shaded boxplots to the right show the median pattern with expected variation due to analytical error. Error distributions were created by Monte Carlo simulation (n ) 100 000) using the average variance from three sets of triplicate samples. with the water column with respect to PCDD/F, aqueous PCDD/F concentrations would exhibit a similar trend and a range of source strengths is implied. However, organic carbon varied both qualitatively and quantitatively among the site classifications. For example, organic sediment collected from reference sites appeared to be less decomposed than that from agricultural sites. If such “fresh” organic matter was not in equilibrium with PCDD/F, it would have effectively diluted the PCDD/F concentration normalized to organic carbon content. It is also possible that organic partitioning differed among the sediments. Kile et al. (26) have shown that organic-carbon-normalized partition coefficients (KOC values) were greater for aged sediment than for recently eroded soils and greater still for sediment contaminated with hydrocarbons. It is likely that both variation in source strengths and qualitative differences in organic carbon act simultaneously. Homolog Profiles in Sediment. Figure 3 shows the homolog profile distribution in the Willamette Basin. Throughout the basin, homolog profiles were strikingly similar, regardless of the total PCDD/F concentration. Profiles at 17 of the 22 sites correlated very well (F g 0.9, R < 0.001) with the median pattern shown in Figure 3. At all but two sites, a single congener, OCDD, accounted for more than half of the total PCDD/F, and the combined furans accounted for less than 20% of the total. The PCDD portion of the profile was pronounced, spanning more than 2 orders of magnitude at every site. It was also remarkably robust. All sites had profiles that correlated well with the median PCDD profile (F g 0.9, R < 0.05); no exceptions were observed. In general, D8 ≈ 4 × D7, D7 ≈ 6 × D6, D6 ≈ 10 × D5, D5 ≈ D4. Among all the sites, the only variant in the PCDD profile was the relative ranking of D4 and D5, both of which had similarly low concentrations, often near detection limits. The consistency of this profile is all the more striking when the differences among samples are considered. The profile was invariant with respect to total PCDD/F concentration (range of 103), the relative amount of PCDF, different PCDF profiles, subbasin size, geographic location, and land use. The PCDF portion of the profile was more variable than the PCDD portion. F8 and F7 generally had higher concentrations than F4 and F5, but the entire PCDF profile spanned less than an order of magnitude. A few sites had unusual PCDF profiles which may have been related to nearby 732
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point sources (27), but their PCDD profiles were the same as those found throughout the basin. The shape of the PCDD/F homolog profile (Figure 3) is not unique to the Willamette Basin. It has been observed in sediment by other researchers and is often attributed to atmospheric deposition of combustion-derived PCDD/F (11, 18). Environmental processing in the atmosphere has been shown to lead to a profile of this shape. Because of their high hydrophobicity and low vapor pressure, highly chlorinated congeners (especially D8) accumulate on atmospheric particles. These particles accumulate a smaller fraction of the less chlorinated species, which have higher vapor pressures and also undergo photolysis (28, 29). Thus, atmospheric particles become enriched in higher chlorinated species and depleted in lower chlorinated species. Sediments become the eventual sink for these particles and the PCDD/F they carry. Although the homolog profiles observed in the Willamette Basin are consistent with an atmospheric source, it is unlikely that atmospheric deposition alone is the primary source of PCDD/F at all of the sites sampled. The wide range of total PCDD/F among these sites, and the particularly high concentrations at a few sites, argue against a single atmospheric source. Sources other than atmospheric deposition may produce profiles similar to those observed in this study. For example, technical-grade pentachlorophenol contains traces of PCDD/F and its homolog profile is dominated by D8 and F8 (30). It has been the presumed source of PCDD/F to sewage sludge and washing machine effluent, both of which have PCDD profiles that resemble those found in sediment (13). Pentachlorophenol contamination in sediment has been reported near some of the sites in this study (31). However, PCDD/F sources that are associated with different profile shapes, such as chlorine bleaching of paper pulp and PCB contamination, were also present at some sites sampled in this study (32), yet no such sites had distinctive PCDD profiles and only one had an unusual furan profile. Other workers have made similar observations. For example, Fiedler et al. (19) were unable to distinguish between homolog profiles in sediment obtained upstream and downstream of a paper mill, even though the profile of mill effluent was distinctive. Clearly, homolog profiles in sediment, especially the PCDD portion, are not sufficient to identify sources in many cases. The similarities among profiles from many sites may partially result from environmental processing in the aquatic environment as a consequence of the physical properties of these compounds. Water solubility decreases, and octanolwater partition coefficients increase as the number of chlorine substituents increase (33). Although this trend applies to both PCDDs and PCDFs, the degree of change differs for the two classes of compounds. From D4 to D8, water solubility of the PCDDs decreases by about 4 orders of magnitude. In contrast, water solubility of the PCDFs decreases by about 2 orders of magnitude from F4 to F8. Water solubilities of the D4 and F4 congeners are similar. These patterns in the physical properties are consistent with the observed sediment homolog profile: a pronounced PCDD profile strongly dominated by D8, and a similar but flatter PCDF profile. Profile Gradient in Sediment. A distribution of homolog profiles consisting of the median profile plus random analytical error was compared to the homolog profile distribution for Willamette basin sediments (Figure 3). This comparison shows that differences among the sediment profiles cannot be explained solely by analytical error (shaded boxplots). Principal components analysis (PCA) was used to explore the differences among homolog profiles. Because the purpose of this analysis was to search for systematic differences among profiles with similar shape, two profiles
FIGURE 4. First principal component as a function of total PCDD/F concentration in sediment. Homolog pattern ranges corresponding to land use and principal component groupings are shown at right. Solid lines encircle land-use categories; dashed lines encircle replicate samples. that exhibited unusual PCDF portions were omitted from the PCA. The first principal component accounted for 77% of the variance among profiles and revealed a profile gradient that appears to be related to the intensity of human activity (Figure 4). At reference sites, the PCDD portion of the profile spans about 2 orders of magnitude and the PCDF portion is approximately flat. At agricultural sites, the PCDD portion is essentially unchanged from that of the reference sites, but the PCDFs show increasing concentration with degree of chlorination from F4 to F7. This change in profile shape was not accompanied by a significant change in total concentration. At the sites nearest the most intense human and industrial activity, highly chlorinated species have become strongly dominant for the PCDDs and PCDFs. Both PCDD and PCDF profiles are about 10 times steeper at these sites than at more remote sites. The subtle change in profiles may be related to increased inputs near municipal and industrial sources. In particular, sources associated with pentachlorophenol could increase the relative concentrations of highly chlorinated congeners. Alternatively, the steeper profiles near centers of human activity may indicate that less chlorinated species (D4, D5, and F4) are transported longer distances in the atmosphere while highly chlorinated species (D8, F7, and F8) are rapidly transported with particulate to the sediments. The profile gradient could also reflect differences in the age of the deposited PCDD/F. The production of lesser chlorinated congeners from highly chlorinated congeners via dechlorination has been observed in sediment under anaerobic and aerobic conditions (34, 35). The profiles at industrial and urban areas could represent recently deposited PCDD/F that had little chance for dechlorination.
Tissue-Sediment Relationships. Fish tissue and sediment collected from the same site did not have similar concentrations of total PCDD/F. Sites with higher PCDD/F concentrations in sediment were not always associated with tissue samples with higher concentrations. Normalizing to organic carbon and lipid content did not increase the resemblance. Total concentrations of PCDD/F in tissue were much smaller, less than 20%, than those in sediment from the same site. In addition, sediment had a much larger range of total PCDD/F concentrations than tissue (80-7000 pg/g and 10140 pg/g, respectively). Some of these differences could be due to species and age differences among the fish samples. However, most of the difference between total PCDD/F concentrations in sediment and tissue can be traced to sharply different homolog profiles in these two media. For example, D8 accounted for more than 75% of the total PCDD/F in sediment, but less than 40% in tissue. Differences between the two media were large enough that they overshadowed differences due to fish species, age or other factors. Fish tissue was enriched in the less chlorinated homologs and depleted in the highly chlorinated homologs relative to sediment from the same site (Figure 5). At four of eight sites, D4 concentrations in tissue exceeded those in sediment by at least a factor of 2. Only one site had sediment D4 concentrations that exceeded those in tissue. D5 behaved similarly. In contrast, concentrations of the more highly chlorinated congeners were less in fish tissue than in sediment. The difference between sediment and tissue concentrations increased with the number of chlorine substituents; D6, D7, and D8 concentrations in tissue were 1-50, 5-100, and 6-200 times less than those in sediment, respectively. The same general pattern was observed for the VOL. 32, NO. 6, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Concentrations of 2,3,7,8-substituted congeners normalized to the concentration of their respective homolog class in sediment (O) and fish tissue (b) at the same sites. 3 is shown at the maximum possible value when the 2,3,7,8-substituted congener was not detected but other members of the congener class were detected. Points are not shown if there were no detections in the homolog class. Numbers adjacent to points indicate multiple incidences of the same value.
FIGURE 5. Three representative sets of site-matched homolog profiles for sediment (O) and fish tissue (b). The detection limit is shown for congener classes that were not detected (3 and 1 for sediment and fish tissue, respectively). PCDFs. Similar homolog profile differences between sitematched sediment and fish tissue samples were described by Sakurai et al., for freshwater lakes in Japan (36). The difference in homolog profiles between tissue and sediment may indicate that fish obtain these compounds from the water column rather than from sediment. Because octanol-water partition coefficients increase with number of chlorine atoms, lesser chlorinated species will tend to partition into the dissolved phase more than highly chlorinated species. D8 concentrations in tissue may be low because little D8 is dissolved and accessible to fish. Simple partitioning between sediment and water cannot entirely explain the different homolog profiles that were observed. The isomeric composition within the homolog classes differed between sediment and tissue (Figure 6). In every homolog class, tissue had significantly larger proportions of 2,3,7,8-substituted congeners than did sediment (R < 0.05). In several cases, the congeners detected in tissue were exclusively 2,3,7,8-substituted congeners.This was never the case for sediment and suggests specific interactions between 2,3,7,8-substituted congeners and tissue. The behavior of the D6 homologs was intriguing and should be noted. Only the 1,2,3,6,7,8-isomer appeared to accumulate in fish tissue collected in this study. It is not known if this is because too little of the other two isomers (1,2,3,4,7,8- and 1,2,3,7,8,9-) was present in the local environment or if fish preferentially accumulate the 1,2,3,6,7,8isomer. The differences between PCDD/F concentrations in fish and sediment have implications for monitoring strategies for these compounds. Sediment sampling is an attractive and often-used method for highly hydrophobic compounds. 734
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Compared to water, very small sample sizes are required. Sediment samples are relatively easy to obtain, contain the highest concentrations of total PCDD/F of any media, and have lower variability than tissue samples. In contrast, sampling tissue is inherently more difficult and introduces variables such as species, mobility, and age. However, the primary reason for monitoring concentrations of PCDD/F is the extreme toxicity of a select number of congeners. These data show that for the most toxic PCDD/F, congeners with five or fewer chlorines and 2,3,7,8-substituted congeners, concentrations in sediment are not a good indication of tissue concentrations.
Acknowledgments The author is grateful to the Oregon Department of Environmental Quality and the Biological Resources Division of the U.S. Geological Survey for their support of this work. In addition, I thank Dr. C. Henny of the Biological Resources Division for fish tissue samples from two sites. The use of trade, product, or firm names is for identification purposes only and does not imply endorsement by the U.S. Government.
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Received for review July 9, 1997. Revised manuscript received November 24, 1997. Accepted December 2, 1997. ES9706099
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