Environ. Sci. Technol. 1997, 31, 3274-3280
Depositional Time Trends and Remobilization of PCBs in Lake Sediments BONDI GEVAO, JOHN HAMILTON-TAYLOR,* CHRIS MURDOCH, KEVIN C. JONES, MIKE KELLY, AND BRIAN J. TABNER Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA1 4YQ, United Kingdom
The vertical distribution of PCBs was measured in a dated core from the profundal sediments of Esthwaite Water, U.K. The resulting historical record of ∑PCB deposition agreed well with published U.K. production data. Ratios between the various PCB homolog groups revealed significant compositional variations with depth. Sediments deposited in the last ∼10 years and sediments deposited before PCB production began are both relatively enriched in the less chlorinated homolog groups. When considered with other available data, these enrichments provide evidence for postdepositional mobility of PCBs favoring the more aqueous soluble and volatile compounds. The results lend further support to a growing body of evidence that recyling of PCBs is occurring between the sediments and overlying waters, associated with solubilization and diffusive transport.
Introduction Polychlorinated biphenyls (PCBs) have been widely used in a variety of industrial applications since their first synthesis in 1929. As a result of their persistence, toxicity, and bioaccumulation, the manufacture of PCBs was discontinued in the early 1970s, and their use in ‘open’ applications was banned in the late 1970s (1). Although PCB concentrations in the atmosphere have subsequently undergone a marked decrease (2, 3), significant quantities continue to be detected in all environmental systems. It is believed that soils and sediments, which were once thought to act as permanent repositories, have begun to act as significant secondary sources with the decline in atmospheric concentrations (25). PCB volatilization from sands and soils was first reported by Haque et al. (6), who found significant losses at temperatures above ambient and that the losses were congener dependent. Release of PCBs from aquatic sediments to overlying water has been demonstrated in laboratory microcosm experiments (7, 8), while volatilization from water bodies has been demonstrated in both laboratory (9) and field (10) studies. In a mass balance study of PCBs in Lake Superior, Jeremiasson and others (11) showed that between 1980 and 1992, the mass of PCBs lost from the water column was far in excess of the total sedimentary burden accumulated since 1930. They showed that the predominant loss occurred by re-emission to the atmosphere, pointing to the idea of recycling PCBs in the water column and/or remobilization from buried sediments. The main aims of this study were to investigate possible biogeochemical transformations affecting the concentrations * Corresponding author e-mail:
[email protected]; tel: +44 1524 593893; fax: +44 1524 593985.
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and vertical distributions of PCBs in the profundal sediments of Esthwaite Water (EW), a small, biologically productive, semirural lake in northwest England. EW has a surface area of 1 km2, a maximum depth of 15.5 m, a mean depth of 6.4 m, and a hydraulic retention time of ∼13 weeks. PCB input to the lake is predominantly from atmospheric deposition and catchment runoff (12). There are no known point sources of PCBs, and motorized boats are banned from the lake.
Materials and Methods Sampling and Preliminary Measurements. Two sediment cores were obtained ∼15 m apart from the deepest part of EW with a Mackereth minicorer (13) in July 1995. Prior to use, the Plexiglas core barrels were washed thoroughly with detergent and rinsed with Milli-Q water. The cores were stored at 4 °C within 4 h of collection. Downcore variations in magnetic susceptibility were measured with a Barrington MS-2 meter in order to provide a stratigraphic correlation between the two cores (14). One core (core I) was used for most of the subsequent chemical and radiometric analyses, while the second core (core II) was used for determining the amount of “supported” 210Pb in the sediment. Three days after sampling, core I was sectioned at 1-cm intervals to a depth of 20 cm and at 2-cm intervals thereafter. The sectioning utensils were washed and solvent-rinsed between each section. Sediment from each section was immediately transferred to solvent-rinsed glass jars lined with solventrinsed aluminium foil. Core II was sectioned at 5-cm intervals. All subsamples were then frozen until required. Dating. The total 210Pb activity concentration in sediment subsamples of core I was determined by analysis of its decay product 210Po, on the assumption that the two were in equilibrium. The Po was extracted, purified, and self-plated onto silver disks according to the method of Flynn (15), with 208Po used as a yield monitor. Counting was by R-spectrometry with silicon surface barrier detectors. Due to the limited amount of core I material available, the supported 210Pb fraction was measured on core II material, and the two cores were correlated using the magnetic susceptibility profiles. Supported 210Pb was obtained by indirectly determining the activity concentration of the supporting parent 226Ra. Sediment subsamples were sealed in air-tight containers to prevent loss of 222Rn and counted by γ-spectroscopy with HPGE detectors after a sufficient period of radon ingrowth to achieve equilibrium with 226Ra. For the analysis, the efficient γ-emitter 214Pb was used, itself in equilibrium with 222Rn. The 137Cs activity concentration was determined on core I material by γ-spectrometry, using efficiency calibration standards of different masses of a sediment of similar bulk density to the Esthwaite sediment, spiked with 137Cs. Determination of Carbon and Nitrogen. The determination of organic C and N in the sediment core was carried out by the use of a Carlo Erba 1108 elemental analyzer. The instrument was routinely calibrated with a blank and two sulfonylamide standards (C, 41.84%; N, 16.27%) with a standard reference material for further confirmation. Five replicate analyses gave a relative standard deviation of 1.4% for C and 0.68% for N. Extraction and Analysis of PCBs. Sediments for PCB analyses (15-20 g wet weight) were mixed with anhydrous sodium sulfate that had been previously baked out at 450 °C to remove residual water. The mixture was placed in preextracted thimbles and extracted in a Soxhlet apparatus with pesticide-grade hexane for 12-16 h on a Buchi 810 fat extraction unit. Copper turnings were added to the beakers during the extraction stage to remove elemental sulfur.
S0013-936X(97)00276-9 CCC: $14.00
1997 American Chemical Society
FIGURE 1. Vertical distributions of the activity concentrations of (a) unsupported Further cleanup was similar to that described by Tremolada et al. (16). The sample extracts were analyzed on a Hewlett-Packard HP5890 gas chromatograph equipped with a 63Ni electron capture detector using splitless injection on a cross-linked 5% phenyl methyl silicone chromatographic column for separation (50 m × 0.2 mm i.d, and 0.11 mm film thickness). Operating conditions were identical to those of Lead et al. (17). Congener 209 was used as a retention time marker and added to each extract prior to GC-ECD analysis. Identification and quantification of PCBs was carried out by overlaying the chromatogram of a standard mix of 52 congeners onto the sample chromatogram and matching peaks by their retention times with respect to that of 209. This step was carried out using a VG Data Systems-Minichrom data handling package. ∑PCB is defined as the sum of the following IUPAC congeners: 30, 18, 54, 31, 28, 33, 52, 49, 47, 104, 44, 37, 61/74, 66, 155, 60, 101, 119, 81/87, 110/77, 82, 151, 123/149, 118, 114, 188, 153, 105, 141, 138, 126, 187, 183, 167, 185, 202, 156, 157, 204, 180, 193, 191, 170, 189, and 194. The concentrations of PCBs were calculated by dividing the masses by the dry weight of the extracted sediment, determined gravimetrically after drying separate subsamples of the sediment to constant weight at 105 °C. An analytical blank was processed for every five samples with the same weight of sodium sulfate used to dry the wet sediments. The PCBs present in the appropriate blank were subtracted from those in the sample extracts. Sample peaks are reported only if the signal exceeded three times the baseline noise. The mean, standard deviation, and range of the sum of the seven ICES congeners in the 12 blanks were 2.3 ( 1.1 and 0.4-6.2 of ng/sample. Analytical recoveries were carried out by spiking the samples before extraction with IUPAC congeners 40, 128, and 198. Average recoveries were 84% ((10 SD), 94% ((4 SD), and 87% ((5 SD), respectively. Reported values are not recovery corrected. The accuracy and precision of the method employed in the analysis were assessed by extracting three marine sediment reference materials that have been certified for certain PCB congeners. The results obtained were within 6-15% of the quoted value for >95% of the certified congeners in the three reference materials.
Results and Discussion Sediment Dating. The unsupported (total - supported) 210Pb activity concentration shows a relatively uniform rate of
210Pb
and (b)
137Cs
in Esthwaite Water sediments.
decrease from the surface when plotted on a logarithmic scale against cumulative mass per unit area (Figure 1a). The 2-σ counting errors are also shown. A regression line was fitted to the data, weighted for their counting errors in order to decrease the influence of the values from the lower part of the core, which inevitably have low unsupported 210Pb values with larger associated errors. The slope of the regression line gives a mean sedimentation rate for the dated length of the core of 0.995 kg m-2 yr-1 with an uncertainty (2 σ) of ( 0.153 kg m-2 yr-1. This gives the period covered by this upper core section as 93 ( 14 yr. Different mean sedimentation rates have possibly existed over shorter periods, but these cannot be quantified precisely. The condition represented by the regression line corresponds to the simplest dating model for unsupported 210Pb profiles in sediments (CFCS model), with a constant flux of atmospherically derived 210Pb to the sediment surface and a constant sedimentation rate (18). The distribution of 137Cs activity concentration in the core (Figure 1b) is related to fallout from nuclear weapons testing and from the 1986 nuclear power plant accident at Chernobyl. The two associated peaks in 137Cs deposition (1963 and 1986) are reflected in the maxima at depths of ∼25 cm and ∼6 cm in the sediments. However, the maxima cannot be dated precisely because the depth and hence temporal relationships to the peak fallout dates will be affected by post-depositional redistribution processes (e.g., bioturbation) and by secondary inputs of 137Cs from the catchment. The latter is indicated by the persistence of relatively high levels of 137Cs above the 1963 maximum (Figure 1b). The net result of the redistribution processes and secondary inputs is the possibility of displacement of the fallout maxima in the sediments (upwards in the case of the 1963 maximum). Notwithstanding these limitations, the 137Cs profile is compatible with the sedimentation rate given by 210Pb. The post-depositional redistribution processes in EW sediments have been clearly demonstrated by the downcore penetration of short-lived 134Cs from the Chernobyl fallout (19), now decayed to below detection. For example, 2 months after the fallout event, 134Cs was present in decreasing amounts to a depth of 4.5 cm below a peak at the sediment surface (19). Nevertheless, the redistribution processes are insufficient to give a uniform (i.e., well-mixed) surface layer at a resolution of 1 cm, as indicated by the present 137Cs profile (Figure 1b). The dates shown in subsequent figures are those based on the 210Pb-derived mean sedimentation rate and its
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FIGURE 2. Vertical variations in C, N, susceptibility, and C:N ratio in Esthwaite Water sediments. extrapolation to the lower section of the core. Mean sedimentation rates of 0.699 ( 0.077 and 0.708 ( 0.081 kg m-2 yr-1 for the upper 1 m of sediment from EW have previously been published by Sanders et al. (20) based on 210Pb and 137Cs, respectively. These are for a core from a shallower part (∼11 m) of the basin with a somewhat lower sedimentation rate as compared to the deep station (21). Organic Carbon and C:N Ratio. The C and N contents of the sediments in EW are highly correlated (r 2 ) 0.97) and probably occur predominantly as organic matter (OM). The sediments may contain a small amount of a mixed carbonate (equivalent to e1.5 wt % C), based on loss of weight between 550 and 1100 °C, but the data (22) are equivocal. EW is a soft-water lake, generally regarded as being free of carbonate precipitation and with no significant carbonate rocks in its catchment. The C and N concentrations in core I show a minimum around the turn of the century and thereafter a steady increase to a maximum at the sediment-water interface (Figure 2, panels a and c). Despite the high degree of correlation between C and N, there are clear depth trends in the C:N ratio, which reaches a minimum at the sedimentwater interface (Figure 2d). Given the magnitude of the associated concentration changes, the C:N ratio probably reflects changes mainly in the relative contributions from vascular and non-vascular plants of allochthonous and autochthonous origin, respectively (23). A range in the C:N ratio of 11-13 (Figure 2d) is typical of a mixture of such material (23). The steady decrease in C:N ratio over the last 50 years probably reflects the combined effects of changes in erosional characteristics in the catchment (24) and increased autochthonous productivity due to lake eutrophication (25). Preferential diagenetic breakdown of proteinaceous OM may also contribute in some small way to the increase in C:N with depth in the upper part of the core, but carbonate effects, if any, are unlikely to be important. ∑PCB Trends and Fluxes. The ∑PCB flux profile in core I is broadly consistent with PCB production data in the U.K.
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(1) (Figure 3a). The sediment maximum occurs at a depth corresponding to 1979 ((2 yr), compared to a production peak between 1965 and 1975, suggesting a time lag of a few years between production and incorporation into EW sediments. A sediment mixing effect cannot be discounted however. U.K. production commenced in 1954, although up to 100 t was imported prior to this date, and was discontinued in 1977. Pre-1950 sediments in core I exhibit low but measurable PCB fluxes, followed by an exponential rise to the maximum in the late 1970s and then a gradual decrease toward the sediment-water interface. The results of an earlier study of EW sediments (20) differ from the present PCB data in that the sediment maximum predated the production peak. The topmost data point of this earlier profile is plotted in Figure 3a at the time of coring (1990), while the other data points are plotted at the appropriate relative depths. That the sediment maximum predated the production peak was interpreted as indicating possible long-range atmospheric transport from North America and mainland Europe. We believe that this and other differences between the two profiles are probably caused by differences in the sampling and analytical methods employed in the two studies. The combining of material from the same depths in adjacent cores in the previous study is likely to have corrupted the historical record. The magnetic susceptibility profiles (Figure 2b) indicate that cores taken as close as 20 m from each other can have sedimentation rates that differ by ∼30%, so that mixing of subsamples from the same depths in adjacent cores is inappropriate. In another study, it has been shown that the sedimentation rate varies by as much as a factor of ∼2 throughout the profundal region of this part of the lake (21). Another issue is that the sediment samples analyzed by Sanders et al. (12) were air dried in the laboratory, which we now know has the potential to cause contamination (17, 26-28) or loss of the more volatile components (8, 28). We therefore conclude that the PCB chronology derived in the present study provides the more reliable basis for interpretation. PCBs are thought to be associated mainly with organicrich particles, and for this reason their concentrations are often normalized with respect to organic C (12, 29). In EW, however, it appears that PCB concentrations and loadings are effectively independent of those of OM. For example, the contrasting sediment profiles of C and ∑PCBs (Figures 2a and 3a) indicate that the historical records of PCB and OM deposition are quite different and that normalizing to C would in fact confuse the true PCB historical record. Additionally there is a total absence of any correlation between the depositional flux of organic C and ∑PCB in EW based on seasonal sediment trapping (12). This statistical independence is attributed to two main factors. Firstly OM is effectively present in excess due to the eutrophic nature of the lake, resulting for example in high OM concentrations of ∼14-25% by weight in the profundal sediments. Secondly the concentrations (per unit mass of particles) and fluxes of trace substances tend to be dependent on carrier-phase behavior (i.e., scavenging) only in large systems with long residence times, such as the oceans and large inland waters. In smaller water bodies with short particle, pollutant, and hydraulic residence times, temporal variations in external inputs are more likely to be important, as seen with trace metals (30). Homolog Trends. Depth profiles of the various homolog groups are presented in Figure 3b-f. Di-, octa-, and nonaPCBs were either below or around their detection limits and are not presented. The plots show that the individual homolog groups generally follow the ∑PCB profile. The tri-PCBs depart most noticeably from this trend with a gradual increase toward the surface in the upper 10 cm of the sediment. More detailed insights into PCB behavior were obtained by ratioing the concentrations of the various homolog groups. Many of the
FIGURE 3. Homolog and ∑PCB fluxes to Esthwaite Water sediments (µg m-2 yr-1) compared to the history of UK PCB production. ratios display clear depth trends and divide the sediment into three distinct zones, A, B, and C (Figure 4). Processes that may have contributed to the observed compositional variations with depth include (a) changes in the congener composition of Aroclors released to the environment over time; (b) changes in the sources of PCBs to the lake with time; (c) selective redistribution within the sediment; (d) selective destruction or transformation with time; and (e) artifacts linked to the analytical methodology. In zone A, the less chlorinated homolog groups tend to be relatively enriched as compared with the succeeding groups in the series (Figure 4e-h). The clearest trend of all is seen with the profile of the ratio of tri-PCBs:(∑PCB - tri-PCBs), which shows a maximum at the surface and decreases with depth. These trends suggest the possibility of rapid selective loss with depth and/or upward remobilization of the less chlorinated, and hence more aqueous soluble and volatile, congeners in the sediments. In contrast, when the homolog groups were ratioed against the hepta-PCBs, it was observed that there was generally a gradual increase with depth in zone A, which extended uninterrupted into zone B (Figure 4b-d). The 3Cl:7Cl ratio was an exception (Figure 4a). The
hepta-PCBs comprise the most heavily chlorinated group of those that were routinely detectable and were therefore expected to be the least affected by post-depositional solutionphase mobility. Some factor other than selective loss of the less chlorinated groups with depth must be controlling these ratios. The fact that the 3Cl:7Cl ratio stays more or less constant through zones A and B again reflects the fact that zone A sediments contain relatively high tri-PCB concentrations as compared with older sediments. The ratios between adjacent homolog groups are relatively constant in zone B, apart from 6Cl:7Cl (Figure 4d-g). Zone B straddles the period of peak input and highest PCB concentrations. Toward the base of zone B, at a depth of ∼30 cm, the ratios of the less chlorinated groups to the heptaPCBs either change abruptly or become increasingly noisy (Figure 4a-d). This is attributed to the very low concentrations of the hepta-PCBs below this depth. Many of the heptaand to a lesser extent hexa-PCB concentrations were below detection at depth and therefore do not appear in Figure 4. The observations in zone C are especially significant because the sediments predate the start of industrial PCB synthesis. Most notably there is clear evidence of detectable
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FIGURE 4. Vertical variations in homolog ratios in Esthwaite Water sediments. levels of PCBs in these sediments. Furthermore, there are certain trends indicating fractionation of the various homolog groups (Figure 4f-h). The presence of PCBs in preproduction sediments has been reported before for EW (20) and for other lake sediments, but the origins remain obscure. Possible causes acting individually or in combination to account for the presence of PCBs in preproduction sediments include (a) incorrect dating of the core (31); (b) physical mixing by bioturbation or by smearing during core extrusion (32); (c) sample contamination during preparation and analysis (26): (d) selective downward migration of PCBs in the sediment column (29, 31); (e) production from natural and/or anthropogenic combustion sources (33, 34): and (f) other ‘natural’ production (26). Incorrect dating can be discounted as the sedimentation rate obtained in this study is similar to those obtained for EW sediments by a number of workers, using a range of techniques (20, 21). The increasing downward enrichment of the less chlorinated groups (Figure 4f-h) indicates that physical mixing alone cannot explain the trends because all congeners would be affected equally. Since the absolute concentrations of the individual congeners in this zone are low, the possible
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effects of laboratory contamination might be important. However, it is reasonable to suggest that all preproduction sediments would have been contaminated to the same relative extent by different homolog groups since the entire analysis was carried out using the same procedure, in the same laboratory, around the same time. It is worth noting also that the concentrations reported were blank corrected. Contamination during sample preparation should therefore have been removed since the blanks were taken through the entire analytical procedure. Notwithstanding these arguments, an experiment was set up to investigate further whether contamination might have been responsible for the PCBs in preproduction sediments and also to discount smearing completely as a factor. Sediment dated as pre-1885, based on the dating of core I and susceptibility measurements, was obtained from an additional core taken at the same site. The outer 1 cm of the sediment was discarded to avoid contamination from smearing. A subsample was transferred immediately to a solventrinsed glass jar, lined by aluminium foil, and stored at -17 °C until required. The sample was thawed and placed in a glass chamber, which was then sealed at both ends. The
TABLE 1. Concentrations (ng/g) of Selected Congeners in pre-1929 Sediments in Main Study and Contamination Experiment congener
experimenta
main study (pre-1900)b
18 28 31 33 61/74 81/87 101 104 119 110/77 118 81/151 155 202/156 193 ∑PCBs
0.45 0.57 0.44 0.21 0.12 ndc 0.14 0.16 nd 0.21 0.12 0.13 0.12 nd 0.14 4.73
0.26 ( 0.22 0.58 ( 0.10 0.50 ( 0.16 0.56 ( 0.11 0.21 ( 0.10 0.17 ( 0.10 0.22 ( 0.17 0.19 ( 0.08 0.01 ( 0.03 0.31 ( 0.24 0.24 ( 0.14 0.21 ( 0.14 0.36 ( 0.07 0.05 ( 0.12 0.07 ( 0.11 6.42 ( 1.87
a Mean of duplicate analysis. b Mean ( SD for core sections dated pre-1900. c nd, below limit of detection.
sample was dried in a room held constantly at 30 °C with a stream of air that had been passed through a precleaned polyurethane foam (PUF) plug to remove any pre-existing PCBs in the air. After passing over the sample, the exhaust was passed through a second PUF plug to trap any PCBs volatilizing from the sediment. The second PUF trap and sediment were transferred with minimal delay to a Soxhlet apparatus. A drying control was also carried out with no sediment in the chamber to check for any possible breakthrough from the first PUF trap. Following Soxhlet extraction, the second PUF trap and sediment extracts were combined. The extracts were then cleaned up and analyzed as previously described. The PCB chromatogram for the extract from the drying control was similar to that of the analytical blank, indicating that breakthrough into the chamber did not occur during the drying step. In contrast, PCBs of all homolog groups (i.e., tri-PCBs to hepta-PCBs) were above detection in the sediment extract. Both the concentrations and skewness toward lighter congeners in the sediment extract were similar to those reported in zone C (Table 1). It was therefore confirmed that preproduction sediments in EW do contain PCBs that are relatively enriched in the less chlorinated homolog groups. This leaves selective downward migration and the existence of an unknown source as the only likely explanations. The observed trends in which the lower chlorinated homolog groups tend to be increasingly enriched with depth favor a selective migration of PCBs away from the zone of maximum input (zone B). The selective migration hypothesis is likely to favor the more soluble congeners, as was observed. Evidence of Recycling of PCBs to the Water Column. In addition to the fact that there are depth trends in the relative enrichment of the less chlorinated homolog groups in zone A (e.g., tri-PCB profile in Figure 3b), the magnitude of the enrichment argues against contamination during sample
preparation as the cause. For example, the surface sediment has the highest absolute concentration of tri-PCBs (Figure 3b) as well as the highest relative enrichment, which are therefore unlikely to be the result of any cross-contamination as all the samples were present in the laboratory around the same time. Further evidence concerning the selective PCB enrichment in zone A is available from other data sources. The changes in homolog ratios are summarized in Table 2 and comparisons are drawn with data from a sediment trap study in EW and from continuous air sampling at the lakeshore (our unpublished data). The trap site was the same as that from where cores I and II were taken, and the deployment depth was ∼2 m above the lake bottom. The ratios in Table 2 are the mean of two month-long (August and November) trap deployments in 1995. Continuous fortnightly air samples were collected between September 1995 and August 1996 using a high-volume sampler and analyzed for ∑PCBs (vapor + particulates). A common set of representative congeners in each homolog group was used for the three data sets to improve the comparison. The ratios are lowest around the time of maximum input (i.e., zone B), while the overlying air, the trap material, the topmost sediments, and the preproduction sediments all show similar relative enrichments of the less chlorinated homolog groups (Table 2). These comparisons suggest a number of possible scenarios, of which we describe the two most likely. The first is that PCB deposition to and within the lake is influenced by the atmospheric PCB composition and that this has changed with time (i.e., zone A sediments compared with zone B sediments). The second scenario differs in that, rather than a change in PCB composition with time, the lower chlorinated congeners are preferentially remobilized from the sediments back into the overlying waters and thence probably the atmosphere. Thus there is a partial recycling of PCBs in the sedimentlake water-atmosphere system favoring the more aqueous soluble and volatile compounds. Remobilization of PCBs from EW sediments has been demonstrated in the laboratory, and the composition of the outgassed PCBs was biased toward the lighter congeners (7). An earlier EW sediment trap study found that annual trap fluxes of PCB congeners were between 6.5 and 23 times their surface sediment accumulation rates, in contrast to the trap fluxes of both particle mass and PAHs, which were similar to their sediment accumulation rates (12). Based on this and other findings, it was concluded that largescale PCB recycling was occurring between the sediments and water column, involving remobilization into solution and diffusive release. The findings of this previous study therefore favor the second scenario given above. A similar recycling mechanism appears to be operating in the water column and sediments of Lake Superior (35). Outgassing of PCBs from other lakes, mainly during the summer period, has also been reported (36, 37). The steadily decreasing concentration with depth of heptaPCBs relative to other homolog groups, apparent in zones A and B (Figure 4b-d), is less easily explained. Selective redistribution does not appear to provide a plausible explanation given that hepta-PCBs have the lowest aqueous solubilities of all the homolog groups reported. Although
TABLE 2. Comparison of Homolog Ratios in Air, Sediment Traps and Sediments from Esthwaite Watera
3Cl:4Cl 4Cl:5Cl 5Cl:6Cl
zone A sediments
zone B sediments
zone C sediments
air (n ) 24)
traps 1995 (n ) 2)
0-3 cm depth (n ) 3)
max. input (n ) 7)
pre-1929 (n ) 10)
3.13 ( 0.89 1.69 ( 0.35 2.85 ( 0.56
4.8 0.7 4.4
7.9 ( 3.1 0.7 ( 0.8 3.5 ( 3.1
1.5 ( 0.4 0.4 ( 0.2 2.7 ( 1.1
1.7 ( 0.4 0.7 ( 0.2 6.2 ( 1.8
a The ratios (means ( SD) are based on the following congeners: tri-PCBs: 18, 28, 33, 37; tetra-PCBs: 44, 49, 52, 66; penta-PCBs: 82; 81/87, 110/77, 119; hexa-PCBs: 138, 141, 156, 157.
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there have been changes in the marketed Aroclor mixtures, it seems unlikely that there have been monotonic changes with time over the last ∼40 years in the source concentrations of the various homolog groups relative to the hepta-PCBs. The observed trends may be related to various poorly understood processes, including the possibility of anaerobic dechlorination (38) and changes in the relative extractibilities of the homolog groups with aging in the sediment (39, 40). In summary, this study supports the view that PCB inputs to the environment have declined in recent years. It also provides evidence that there may be a net transfer of some lighter PCBs from the bottom sediments to the overlying water column and that the rates of supply and exchange between the sediments, water, and air are different for the different homolog groups. Differential migration of PCBs downward into the pre-1929 sediments also appears to occur, again favoring the more aqueous soluble, less chlorinated congeners.
Acknowledgments We wish to thank Lancaster University for funding B.G. to carry out this work and Mr. Andy Quin of the Geography Department, Lancaster University, for his assistance with the use of the susceptibility meter. We also thank the Institute of Freshwater Ecology, Windermere, for allowing us to use their boating facilities.
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Received for review March 25, 1997. Revised manuscript received July 10, 1997. Accepted July 17, 1997.X ES970276F X
Abstract published in Advance ACS Abstracts, September 15, 1997.
