Environ. Sci. Technol. 1998, 32, 3491-3495
PCB Concentration in Fish in a River System after Remediation of Contaminated Sediment GUDRUN BREMLE* AND PER LARSSON Ecotoxicology, Department of Ecology, Lund University, Ecologybuilding, 223 62 Lund, Sweden
An 1991 investigation of PCB concentrations in water and fish along a river was repeated in 1996 after the completion of a remediation of PCB-containing sediment in a lake within the river system. The PCB concentrations in the lake water decreased significantly (from 8.6 to 2.7 ng/ L), and the concentrations in fish was halved (significantly lower) after remediation. PCB still remaining in littoral sediment was probably the cause for a recorded gradient of PCB concentrations in fish from the lake and downstream. The PCB concentrations in water and fish was lower in 1996 compared to 1991 in all locations studied. The decreased levels of PCB in fish between the years for the two upstream locations, probably results from a decline in background PCB exposure. Monitoring data from 30 years back, recorded in a south Swedish lake, has shown a 6% decline per year in the PCB concentration in pike (Esox lucius) (Bignert, A.; Olsson, M.; Persson, W.; Jensen, S.; Zakrisson, S.; Litze´ n, K.; Eriksson, U.; Ha¨ ggberg, L.; Alsberg, T. Environ. Pollut. 1998, 99, 177-198). The data on PCB concentration in fish (perch, in the present study) from the two upstream locations were recalculated on the basis of this yearly decline and resulted in concentrations close to those measured in 1996. The results indicated, that changes in background exposure must be taken into account when evaluating the success of remedial actions measures carried out over several years.
Introduction In 1991 we investigated the PCB concentration in the water and in one-year-old perch (Perca fluviatilis L.) in a river upstream and downstream from a PCB-contaminated lake of southern Sweden (2). The PCB concentration was significantly higher downstream of the lake in the water as well as in fish. The PCB contamination originated from the sediment of Lake Ja¨rnsjo¨n, a result of PCB discharges from a paper mill. In the summer of 1993 and 1994, the PCB-contaminated lake was remediated. About 400 kg of PCB in 150 000 m3 sediment was dredged and landfilled (3). The results of the remediation and the PCB concentration in water during remediation was reported in Bremle et al. (4). The PCB concentration in the sediment was before remediation 0.00231 µg/g of dry weight (dw) and 54 ng/g of dw (geometric mean, range 6-850 ng/g) after remediation, respectively. The remediation was successful in that over 95% of the estimated total amount of PCB in the sediment was landfilled. * Corresponding author e-mail:
[email protected]; fax: +46 +46 222 37 90; telephone: +46 +46 222 45 98. 10.1021/es971009c CCC: $15.00 Published on Web 09/30/1998
1998 American Chemical Society
The goal of the remediation was to reduce PCB concentration in biota of the river. This task has previously been attempted in other river systems, mainly in North America. The PCB availability before, during, and after dredging of contaminated sediments in the Shiawassee River was studied using caged clams and fish (5). During dredging significant amounts of PCB were released from the sediments. At the site of dredging and downstream, there were increases in availability of PCBs for at least six months, resulting in higher levels in fish and clams. No noticeable change in PCB concentration in the water was detected after dredging. In the Hudson River, removal of contaminated sediment from parts of the river may have caused decline of PCB in water and fish (6), but the results could also be explained by elimination of direct discharges, stabilization of remnant deposits, and increased transport of PCB-containing sediment downstream. To predict the bioavailable amount of contaminant, and the resulting contaminant concentration in biota, detailed knowledge of exposure routes in the ecosystem is needed. Few models that adequately predict contaminant uptake and exposure in a field situation has yet been developed. An even more complicated task is predicting the outcome of a remedial action. If contaminated sediment is dredged, buried sediment will be exposed which may enhance the bioavailable amount of contaminants. The resulting contaminant level in fish will be hard to predict. Additionally, the change must be validated together with the possible alteration of the background contaminant within that time period. In this study we follow up the investigation performed in 1991, before the remediation (2), with results from after the restoration. The focus of the investigation was devoted to an evaluation of the effects on PCB uptake in fish, effects on concentration of PCB in river water, and congener-specific changes in biota.
Materials and Methods A detailed description of sampling and methods (which were the same in 1991 and 1996), including a map, is provided in ref 2, together with data from the study before the lake remediation. They will, therefore, only be summarized here. Water was sampled at five locations; two upstream (35 and 9 km) from the contaminated lake (nos. 2 and 4), one at the outlet of the lake (no. 5) and two downstream (16 and 78 km, nos. 6 and 8). Samples were taken weekly from August 5 to September 30 1996. Ten samples per location were chosen (and representative of a larger time serie). For each water sample, about 100 L was continuously pumped through polyurethane columns (PUC) immeresed in the river water at a flow rate of 10 mL/min and represented a one-week integrated sample. The peristaltic pump was running continuously under the sampling period. Both dissolved PCB and PCB adsorbed to particles in the water phase were collected by the columns. Samples were stored in a freezer until analysis. Fish were caught with gill nets during August 1996 at the same four sites as in 1991, as close to the water sampling stations as possible. Lake 1 corresponds to water sampling site 2, lake 3 to site 4, and lake 7 to site 8. In the remediated Lake Ja¨rnsjo¨n (no. 5) both water and fish were sampled, whereas no fish from water sampling site 6 was analyzed. Ten perch (Perca fluviatilis L., 1-year-old, 5 females and 5 males, weighing 4.4-11.1 g) were taken from each lake. Whole individual fish were cut into pieces and freeze-dried. Detailed methods for sample preparation and analysis are described in Bremle et al. (2). In short PUCs and dry fish VOL. 32, NO. 22, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3491
TABLE 1. Concentration of ∑PCB in the Water of Ema˚n River after the Remediation in 1996 at Different Stations (Geometrical Means, n ) 10)a water
fish
station, river sites (number) concentration (ng/L) station, lakes (number) concentration (ng/g of wet wt) fat (%) concentration (ng/g of fat) 35 km upstream (2) 9 km upstream (4) Lake Ja¨ rnsjo¨ n (5) 16 km downstream (6) 78 km downstream (8)
0.2 (0.1-0.2) 0.9 (0.6-1.3) 2.7 (2.2-4.0) 2.3 (1.6-2.8) 1.1 (0.6-1.3)
(1) (3) Lake Ja¨ rnsjo¨ n (5)
25.9 (20.3-35.4) 170 (130-220) 476 (387-563)
2.9 3.0 2.9
960 5760 16110
(7)
128 (55-278)
2.5
5120
Figures within parentheses refer to range. PCB concentrations are reported for different stations as geometrical means (n ) 10). Percentage of fat are on a fresh weight basis (mean, n ) 10). a
were Soxhlet extracted in hexane/acetone and evaporated in a vacuum centrifuge. Concentrated extracts were purified on acid/basic double layer silica gel columns. Eluates were evaporated in a vacuum centrifuge and redissolved in isooctane. Samples were then analyzed for PCBs by capillary gas chromatography/ECD (Shimadzu (GC-14A) with split/ splitless injector, 20 m DB 5-quartz capillary column (i.d. 0.18 mm)). PCB components were identified and quantified according to Mullin et al. (7) and Schulz et al. (8). Pentachlorobenzene was used as a chromatographic standard (to check retention and response). The analytical performance was regularly checked against PCB standards such as Aroclor 1242 and Clophen A60. Concentration of total PCB was calculated from the sum of concentrations of 54 identified peaks (IUPAC no. 10/4, 7/9, 18/17/15, 24/27, 16/32, 29, 26, 25, 31/28, 20/ 33/53, 51/22, 45, 46, 52, 49, 44, 37/59/42, 41/64, 40, 63, 74, 70, 66/95, 91, 90/101, 99, 83, 97, 87/115, 85, 77/110, 82/151, 135, 123/149/118, 134, 146, 132/153/105, 141/179, 176/137, 160/138/158, 129/126/178, 187, 183, 128, 185, 174, 177, 202/ 171/156, 172, 180, 170/190, 203/196, 208/195, and 194). Polyurethane filters were precleaned by Soxhlet extraction with acetone and hexane and found to be clean after this treatment just as unexposed filters returned from the sampling area were found to be uncontaminated. Extraction efficiency of the internal standard, octachloronaphthalene was determined for this method in Agrell et al. (9) and recoveries for river water samples found to be 97 ( 26%. Samples were not corrected for extraction efficiencies. Fish fat was determined gravimetrically from a portion of the Soxhlet extract. Statistical computations were performed on 10 log transformed values due to nonhomogeneous variance of the concentration data (10). Statistics were carried out using StatView 4.02 (11) for geometric mean and two-sample t tests, and SPSS 6.1 (12) computer packages for the PCA (principal component analysis). For pattern analysis with PCA the congeners data was normalized to unit concentration, based on the sums of the congeners and congeners with missing values were omitted.
Results Water. The highest PCB concentration in the river system was measured in water at the outlet of the remediated lake (station 5, Table 1). This concentration (2.7 ng/L) was significantly higher (t test, p < 0.05) than at upstream locations, which was also the case before remediation. The PCB concentration in the water decreased with distance from the source (Lake Ja¨rnsjo¨n) in 1991 and in 1996, but it was less pronounced after the remediation in 1996 (Figure 1). The PCB concentration in the water was significantly lower (t test, p < 0.05) in 1996 than in 1991 for all stations except location 8, at the river mouth. The largest decrease of PCB concentration over time was in the water of the remediated lake (station 5) and in the uppermost upstream location (station 1). The pattern of PCB congeners in the river water 3492
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 22, 1998
FIGURE 1. Boxplot of the PCB concentration in water in 1991 and in 1996 at the different locations plotted on a distance scale. At zero distance is the contaminated Lake Ja1 rnsjo1 n (no. 5). Each box displays the 25th to 75th percentile, the line in the middle is the 50th percentile or median. The lines from the boxes show the 10th and 90th percentiles, and values outside this range are plotted as circles.
FIGURE 2. Boxplot of the PCB concentration in fish fat at the four locations in 1991 (open boxes) and in 1996 (shaded boxes) plotted on a distance scale. differed between upstream and downstream locations. Patterns downstream of the former PCB-contaminated lake resembled the low-chlorinated mixture (Aroclor 1242), found in the contaminated sediment (2), whereas it was relatively more highly chlorinated at the upstream locations. Fish. One-year-old perch in Lake Ja¨rnsjo¨n (station 5) two years after the remediation contained 476 ng of PCBs/g of fresh weight (16.1 µg/g of fat, Table 1), and this was significantly (t test, p < 0.001) higher than in fish from all other sampling locations. The PCB concentration in Lake Ja¨rnsjo¨n fish was approximately reduced by half in 1996 and significantly lower (t test, p > 0.001) compared with before remediation in 1991 (825 ng/g of fresh weight, 34.2 µg/g of extractable fat, Figure 2). Fish at the two upstream locations also had lower PCB concentrations in 1996 than in 1991 (t test, p < 0.05). The concentration in fish about 80 km downstream was not significantly lowered after the remediation. Generally, the compositions of PCB congeners in
FIGURE 3. The PCB congener pattern in the fish analyzed with principal component analysis and plotted as scores on PC1 and PC2. The pattern is grouped for lake and year. Groups are visualized with shading and circles and denoted with lake number and year. On PC1 which explained 76% of the total variance, low loading refers to a relatively low chlorinated pattern and high loading to a greater high chlorinated pattern, see loadings plot.
FIGURE 4. The loadings plot from the principal component analysis. The eigenvalues for different congeners [as domain, number according to Schulz et al. (8)] on the principal component 1 and 2. The more low chlorinated congeners (domains with lower number) have low loading, and the relatively more high chlorinated congeners have high loading on PC 1. fish were more highly chlorinated compared with that of the water, as was also the case in 1991. Fat content (percentage based on fresh weight) in perch did not differ between sites or years (t test, p > 0.1), but for fish in Lake Ja¨rnsjo¨n and the site 80 km downstream (t test, p < 0.05). Fish from the different lakes in 1991 and 1996 grouped individually in the PCA score plot (Figure 3). Principal component one that explained most (76%) of the variation in the PCB congener pattern separated congeners from lowto high-chlorinated ones (PCA loadings plot Figure 4). The composition of PCB congeners in fish from Lake Ja¨rnsjo¨n in 1996 was similar to the composition from 1991 and relatively low chlorinated. Downstream fish, from lake 7, clustered together with Lake Ja¨rnsjo¨n fish in 1991 but grouped individually in 1996. The congener composition change over the years was largest in the upstream locations, which both became relatively more highly chlorinated.
Discussion The PCB concentration in river water during summer had decreased from 8.6 ng/L in 1991 to 2.7 ng/L in the water of Lake Ja¨rnsjo¨n two years after the completed remediation. A more thorough investigation of the PCB concentration in
the water of the lake was presented Bremle et al. (13) and, briefly, did the PCB concentration decrease during the two years varying seasonally (range 0.4-8.2 ng/L, n ) 57). One conclusion from that study was that removal of the contaminated sediment of the lake considerably lowered PCB concentrations in the water. The decrease of PCB in the water from 1991 to 1996 for the two upstream locations could be attributed to an overall decrease in background exposure, similar to that found for fish at these locations (see further below). The main source of background exposure has been shown to be the atmosphere (14, 15). In Sweden, a major decline in DDT fallout from the atmosphere was registered from 1973 to 1985 [to about 10-20% of the values registred in 1973 (16)], but the PCB deposition was similar between the years. It is possible, however, that a similar decline in PCB fallout may have occurred during the past decade. In Norway the PCB concentration in moss declined between 1977 and 1990 due to decreased atmospheric deposition (17). The decrease in PCB concentration in the water may also have been caused by reduction of a point source discharge. It could further be explained by variables such as temperature, water discharges, or organic matter content that differed for the river between VOL. 32, NO. 22, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3493
TABLE 2. Geometric Mean of the PCB Concentration in Fish in 1991 and in 1996a PCB concentration (ng/g of fat) station
measured 1991
1992
1993
calculated 1994
1995
1996
measured 1996
ratio measured/calculated
Upstream (1) Upstream (3) Lake Ja¨ rnsjo¨ n (5) Downstream (7)
1327 8869 34225 6224
1247 8337 32172 5851
1173 7836 30241 5500
1102 7366 28427 5170
1036 6924 26721 4860
974 6509 25118 4568
922 6071 16220 5165
1.06 1.07 1.55 0.88
a Stations 1 and 3 are situated about 40 and 10 km upstream of the remediated Lake Ja ¨ rnsjo¨ n (5); site 7 is around 80 km downstream at the river mouth. From the values in 1991, a 6% decrease per year was calculated and shown in italics. The ratio is the measured value in 1996 divided by the calculated value for 1996.
years. These variables are known to affect concentration of e.g. PCB, by altered desorption from sediment (temperature), dilution and particle transport (water discharge) and increased association with dissolved and particulate organic matter (18-20). The concentration of PCB declined with distance from the source (Lake Ja¨rnsjo¨n) in 1991 but this was less pronounced after the remediation in 1996. As the PCBcontaining sediment was removed from the lake, transport across the sediment/water interface decreased. Downstream (ca. 80 km), where PCB-containing sediment originating from the lake still resided in the river, concentrations of PCB in the river were similar in both years. During remediation, an increased particle transport was registred that may affect PCB concentration downstream (4). As in 1991, the PCB congener pattern in 1996 in the water downstream was still dominated by the low-chlorinated PCB mixture, similar to that of the contaminated lake sediments. Fish from all the locations in 1996 had lower PCB concentration than in 1991. The most pronounced decrease was observed in the remediated lake, where levels in fish were halved. The main reason for the reduced levels was the remediation. The removal of contaminated sediment from the lake, reduced the major source of PCB to the river ecosystem. Earlier studies of the lake showed that the sediment released PCB to the overlying water, mainly as a result of desorption (21). The present results also showed that the decrease of PCB in fish, or the response to the remedial action, was considerably slower in fish than the direct effect on concentrations in water. The one-year-old perch were hatched in the early summer 1995 and had used the river habitat for 1 year. During this period PCB concentration in the water fluctuated and higher concentrations in the summer of 1995, the first summer after completed remediation, led to higher exposure than predicted from the lower PCB concentration in water during the summer of 1996. It is not only the direct partitioning effect of PCB from water to fish that affects the uptake in perch. The uptake via food is also important. PCB concentration in prey probably declined more slowly than in the water phase. Juvenile perch feed mainly on zooplankton (22), both in pelagic and benthic habitats. Since some of the contaminated sediment was left in the lake after remediation and this sediment was mainly located in the most shallow, littoral areas, these sites constituted a probable source of PCB to zooplankton and fish. PCB still remaining in littoral sediment was probably the cause for a recorded gradient of PCB in fish from the lake and downstream the river. Furthermore, older fish present in the lake before and during remediation probably show an even slower decline in PCB concentration than juvenile perch, since they were subjected to higher exposure during their lifetime. Elimination half-lives of PCBs were found to be in the order of years for eels (Anguilla anguilla ), which were transferred from a contaminated site in the River Rhine to a lake (23). Species-specific factors, such as fat content, age 3494
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 22, 1998
distribution, growth rate, and prey selection will also have an effect on PCB concentrations (24). Shorter-lived fish or young individuals, with a trophic position lower in the foodchain and containing low lipid levels, are usually not chosen in chemical-monitoring studies because of their lower sensitivity as indicators (25). However, they provide a “short-term” time-averaged measure of recent PCB exposure such as after remedial actions at contaminated sites. Furthermore it is an advantage if the fish is nonmigrant and is abundant in a wide range of habitats. These requirements were met by the sunfish (Lepomis auritus and L. macrochirus), which was used to monitor PCB contamination in small streams (25). PCB concentration in the sunfish reflected the exposure concentration at the site of capture and responded rapidly to changes in exposure. Young-of-the-year fish (spottail shiner, Notropis hudsonius) has also been successfully used to show temporal trends of organochlorine contaminats (26). Juvenile perch, used in this study, fulfill most of these criteria and seem to indicate the change in PCB exposure. There is, however, a time delay in response to decreasing PCB concentrations in the water for the fish, probably emanating from pollutants in the prey. The PCA analysis revealed that fish from each site showed a unique PCB congener pattern. Lower chlorinated PCB congeners were relatively more predominant in fish from Lake Ja¨rnsjo¨n (no. 5) than the upstream lake 3. The pattern did not change in Lake Ja¨rnsjo¨n fish after remediation. Fish from the remediated lake still showed similarities with the congener pattern registered from sediment, which strengthened the hypothesis that litoral, contaminated sediment, not affected by remediation, controlled PCB congener distribution in fish. The pattern changed somewhat in the downstream, lake 7, indicating a shift in uptake or load of PCB congeners in this lake after remediation. The most upstream location (no. 1) showed the largest difference in PCB congener patterns between the years, becoming relatively more highly chlorinated and similar to the other upstream lake (no. 3). The reason for this could be a change in atmospheric deposition. In Norway, the PCB congener pattern in moss changed between 1977 and 1990 and the relative importance of the more highly chlorinated congeners increased in the colder areas (17). The reason for the decline of PCB in fish could be decreased atmospheric deposition and thus lowered loadings of PCB to the freshwater. A decrease in PCB concentration in fish has also been shown in, e.g., Lake Michigan salmonids (Oncorhynchus kisutch and O. tshawytscha) (27) and in Lake Ontario lake trout (Salvelinus namaycush) (28). Stow et al. (27) suggested that the decrease was the result of an interaction in two phases: a initial fast decrease as the result of direct releases of PCB to the environment being cleaned, and a slower decrease depending on background sources such as the atmosphere and sediment slowly responding to primary sources. As pointed out in both studies, ecological processes such as increased growth rate of the predator may
lower levels of PCB, as will growth rate and age of their major prey. Fish from the lakes upstream of the remediated lake had lower PCB levels in 1996 than in 1991. This was also the case for fish near the river mouth. The reason for the decrease in concentrations in fish over years could be an effect of lowered background contamination. PCB concentration in northern pike (Esox lucius L.) was shown to decline with about 6% per year in a southern Swedish lake investigated over 30 years (1). When using the measured PCB levels in fish from 1991, and assuming a 6% yearly decrease, the calculated and measured PCB concentration in perch from the Emån River locations in 1996 were similar (Table 2). The results show that if a remedial action is to be evaluated and the process is extended over several years, changes in background contamination must be taken into account. After a remedial action, the result need to be followed over several years to show if it has been successful, which has not yet been the case in the present study. It also stresses the importance of using reference sites, to compare the results from the remedial area. A decrease in overall background contamination could otherwise well be interpreted as a result of the remedial action only.
Acknowledgments Thanks to all involved in the Lake Ja¨rnsjo¨n project, especially A. Helgee, B. Troedsson, and T. Hammar. Funding was received from Swedish Environmental Protection Agency.
Literature Cited (1) Bignert, A.; Olsson, M.; Persson, W.; Jensen, S.; Zakrisson, S.; Litze´n, K.; Eriksson, U.; Ha¨ggberg, L.; Alsberg, T. Environ. Pollut. 1998, 99, 177-198. (2) Bremle, G.; Larsson, P.; Okla, L. Environ. Sci. Technol. 1995, 29, 2010-2015. (3) Gullbring, P.; Hammar, T. Water Sci. Technol. 1993, 28, 297306. (4) Bremle, G.; Larsson, P.; Hammar, T.; Helgee, A.; Troedsson, B. Water, Air, Soil Pollut. 1998, 107, 237-250. (5) Rice, C. R.; White, D. S. Environ. Toxicol. Chem. 1987, 6, 259274. (6) Brown, M. P.; Werner, M. B.; Sloan, R. J.; Simpson, K. W. Environ. Sci. Technol. 1985, 19, 656-661.
(7) Mullin, M. D.; Pochini, C. M.; McCrindle, S.; Romkes, M.; Safe, S. H.; Safe, L. M. Environ. Sci. Technol. 1984, 18, 468-476. (8) Schulz, D. E.; Petrick, G.; Duinker, J. C. Environ. Sci. Technol. 1989, 23, 852-859. (9) Agrell, C.; Larsson, P.; Okla, L.; Bremle, G.; Johansson, N.; Zelechowska, A. In Large Scale Effects in the Baltic Sea; Wulff, F., Rahm, L., Larsson, P., Eds.; Springer-Verlag: Germany, submitted for publication. (10) Berthouex, M. P.; Brown, L. C. Statistics for Environmental engineers; Lewis Publisher, CRC Press: Boca Raton. 1994. (11) StatView 4.02. computer package; Albacus concepts. Inc.: Berkeley, CA 1992-1993. (12) SPSS professional statistics 6.1 computer package; SPSS Inc.: Chicago, IL, 1995. (13) Bremle, G.; Okla, L.; Larsson, P. Ambio 1998, 27, 398-403. (14) Johnson, M. G.; Kelso, J. R. M.; George, S. E. Can. J. Fish Aquat. Sci. 1988, 45, 170-178. (15) Swackhamer, D. L., Hites, R. A. Environ. Sci. Technol. 1988, 22, 543-548. (16) Larsson, P.; Okla, L. Atmos. Environ. 1989, 23, 1699-1711. (17) Lead, W. A.; Steinnes, E.; Jones, K. C. Environ. Sci. Technol. 1996, 30, 524-530. (18) Bremle, G.; Larsson, P. Environ. Sci. Technol. 1997, 31, 32323237. (19) Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Environ. Sci. Technol. 1986, 20, 502-508. (20) Mackay, D. J. Great Lakes Res. 1989, 15, 283-297. (21) Larsson, P.; Okla, L.; Ryding, S.-O.; Westo¨o¨, B. Can. J. Fish. Aquat. Sci. 1990, 47, 746-754. (22) Persson, L. Oikos 1983, 40, 197-207. (23) de Boer, J.; van der Valk, F.; Kerkhoff, M. A. T.; Hegel, P. Environ. Sci. Technol. 1994, 28, 2242-2248. (24) Larsson, P.; Backe, C.; Bremle, G.; Eklo¨v, A.; Okla, L. Can. J. Fish. Aquat. Sci. 1996, 53, 62-69. (25) Southworth, G. R. Water, Air, Soil Pollut. 1990, 51, 287-296. (26) Suns, K.; Hitchin, G., Adamek, E. Can J. Fish Aquat. Sci. 1991, 48, 1568-1573. (27) Stow, C. A.; Carpenter, S. R.; Amrhein, J. F. Can J. Fish Aquat. Sci. 1994, 51, 1384-1390. (28) Borgmann, U.; Whittle, D. M. J. Great Lakes Res. 1991, 17, 368381.
Received for review November 18, 1997. Revised manuscript received August 4, 1998. Accepted August 5, 1998. ES971009C
VOL. 32, NO. 22, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3495
