Environ. Sci. Technol. 1996, 30, 3609-3617
Spatial Trends and Historical Deposition of Polychlorinated Biphenyls in Canadian Midlatitude and Arctic Lake Sediments DEREK C. G. MUIR,* ALEX OMELCHENKO, NORBERT P. GRIFT, DAN A. SAVOIE, W. LYLE LOCKHART, PAUL WILKINSON, AND GREGG J. BRUNSKILL† Freshwater Institute, Department of Fisheries and Oceans, Winnipeg, Manitoba, R3T 2N6 Canada
A study of PCB concentrations and fluxes in lake sediments was conducted to test the “global fractionation” hypothesis that deposition of semivolatile organics will decline while more volatile congeners will be enriched in polar regions. Sediment cores were collected from 11 remote lakes in Canada ranging from 49° N to 82° N and were dated using excess 210Pb and 137Cs. Sediment extracts were analyzed for up to 90 PCB congeners by capillary GC-ECD with confirmation by GC/high-resolution MS. Total PCB (∑PCB) concentrations in surface slices ranged from 2.4 to 39 ng g-1 (dry wt) and showed no latitudinal trend. Fluxes (ng m-2 yr-1) and inventories of ∑PCB as well as total tetra- to octachlorobiphenyls declined with increasing north latitude while those for di/trichlorobiphenyls showed no latitudinal trend. The proportion of di/trichloro congeners of ∑PCB also increased significantly with latitude, while total octachlorobiphenyls declined. Maximum ∑PCB concentrations were observed in subsurface slices dating to the 1960-1970s in most lakes except those in the high Arctic, where maxima were generally in surface slices. The onset of elevated ∑PCB deposition was delayed in the high Arctic (1950-1960s) relative to the midlatitude and sub-Arctic lakes (1930-1940s). The high proportions of lower chlorinated congeners and the delayed appearance of PCBs are consistent with predictions of the global fractionation model.
* Author to whom correspondence should be addressed; telephone: 204-983-5168; fax: 204-984-2403; e-mail address:
[email protected]. † Present address: Australian Institute of Marine Science, PMB 3, Townsville, Queensland 4810, Australia.
S0013-936X(96)00393-8 CCC: $12.00
Published 1996 by the Am. Chem. Soc.
Introduction The presence of polychlorinated biphenyls (PCBs) in Arctic and sub-Arctic freshwater and terrestrial environments in North America has been well documented in recent years (1-4). Although there have been significant local sources such as military radar sites (5, 6), the predominate pathway for PCB inputs to this region is thought to be atmospheric transport in gas phase and aerosols. PCBs are deposited in snow pack (7, 8) or on plant/soil surfaces (5, 9). Once deposited, PCBs may be partially revolatilized, buried, or transported to the aquatic environment during snowmelt (10, 11). Low temperatures may reduce revolatilization of PCBs giving rise to the “global distillation” or “cold condensation” effect (12). The global fractionation model (12) predicts that persistent, semivolatile organics will be more prominent in polar regions and temporal trends in deposition will be delayed and prolonged relative to temperate regions. Analysis of dated sediment cores has been used to infer the depositional history of PCBs in the Great Lakes (13-15) and in lakes in western Europe (16 -18). The sediment records from Arctic lakes therefore could provide information on temporal trends of deposition of these hydrophobic contaminants in the Arctic. However, factors unique to Arctic lakes such as long periods of ice cover and low sedimentation rates may limit inputs to bottom sediments and make them a less significant reservoir for hydrophobic organics than temperate lakes (19). There is very little published information on the concentrations or fluxes of PCBs in lake sediments in Arctic and sub-Arctic regions of North America. Low concentrations of total PCB (∑PCB; 0.3 to 30 ng g-1 dry wt) were reported in sediment cores from two Alaskan lakes (4). Mudroch et al. (20) found maximum concentrations of ∑PCBs ranging from 2-6 ng g-1 in Great Slave Lake sediments but were unable to assess historical trends due to high sedimentation rates and mixing. Analysis of glacial snow cores from northern Ellesmere Island has indicated declines in fluxes of hexachlorocyclohexane and dieldrin (10) but quite variable deposition of PCBs during the past 20-30 years (8). To date there have been no reports of spatial or historical depositional trends of PCBs in Canadian Arctic and sub-Arctic lakes. However, we have previously reported on the spatial trends and historical profiles of organochlorine pesticides in eight Canadian lakes along a mid-continental transect from 49° N to 81° N (21). Other studies on sediment cores from these lakes have examined profiles of mercury, polyaromatic hydrocarbons, and radionuclides (22-24). In this paper, we examine the sedimentary profiles and geographical trends of PCBs in sediment cores from nine Arctic and three midlatitude lakes in Canada. We hypothesized that fluxes of ∑PCBs would decline with latitude and distance from midlatitude sources in North America and that the more volatile congeners would be enriched in high Arctic samples as predicted by the global fractionation model (12). All the lakes were remote from permanent settlements. They had little or no human activity on them, or within their drainage basins, except in some cases for subsistence or sports fishing, so that inputs were directly or indirectly from the atmosphere.
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TABLE 1
Locations, Limnological Characteristics, Net Sedimentation Rates, and ∑PCB Levels (ng g-1 in Surface Slices) of 11 Midlatitude and Arctic Lakes lake
latitude
L375 L382 Trout Kusawa Hawk Far Ste Therese Belot Amituk Sophia Hazen
49°45′ N 49°42′ N 51°15′ N 60°20′ N 63°38′ N 63°42′ N 64°35′ N 66°53′ N 75°03′ N 75°07′ N 81°45′ N
mean max lake depth, m depth, m area, ha longitude (Z) (Zmax) (Ao) 93°47′ W 93°4′ W 93°15′ W 136°22′ W 90°42′ W 90°40′ W 121°20′ W 126°16′ W 93°46′ W 93°35′ W 71°30′ W
12 6 14 54 12 3.6 ∼10 16 10 20 85
30 13 49 140 34 8.9 18 57 42 55 263
drainage area, ha (Ad)a
vol × m3
106
19 210 2.17 37 205 2.15 34 700 71 800 4 720 14 220 420 000 7 720 24 299 2.83 3.7 16.6 0.134 11 840 1 742 000 118 30 250 135 000 4 860 59 2 600 5.9 330 6 120 66 53 570 400 000 45 500
core typeb box box KB KB box box KB KB KB box box
sedimentation,c g m-2 yr-1
RSSM 210 ( 6 111 ( 2 244 ( 5 218 ( 15 70 ( 2 97 ( 1 367 ( 9 121 ( 5 278 ( 24
mixed ∑PCB,e depth focusing ng g-1 regress (cm) factord dry wt
264 227 51 77 429 70 389 177
3.8 10.4 7.2 0.9 1.3 2.8 90%, and no corrections were made for recoveries. 137Cs and 226Ra were determined by direct counting of 5-10 g of sediment sealed in 60 × 15 mm plastic petri dishes on a γ-spectrometer (Ge-Li) semiconductor detector. In some cases, counting was done on a hyperpure germanium crystal, and 210Pb was determined directly along with 137Cs and 226Ra. Samples of 1-3 g were analyzed for 210Pb by leaching in 6 N HCl in the presence of a 209Po tracer, autoplating Po onto a silver disk (32), and counting the disk on an R-spectrometer to determine 210Pb via its 210Po daughter. Sediment Core Dating. Excess 210Pb in each slice was calculated by subtracting the total 210Pb activity from the activity of the measured 226Ra. Sedimentation rates were calculated using the rapid steady-state mixing (RSSM) model (33), where a deep mixed layer was encountered, or by least squares fit of loge excess 210Pb versus total accumulated sediment mass. Both models assume a constant input flux of 210Pb. Sedimentation rates and mixed depths were also estimated using the 137Cs profile as a discrete time marker (34). Recent PCB fluxes (ng m-2 yr-1) were calculated by multiplying the sedimentation rates by the PCB concentration in surface slices, and the inventories (ng m-2 ) by summing annual fluxes. Focusing Factors and Data Analysis. Sediment focusing factors were calculated as ratios of the depositional 210Pb fluxes or total sediment inventories of radionuclides to the regional atmospheric 210Pb fluxes or decay-corrected regional radionuclide inventories. The 210Pb atmospheric depositions were determined, where possible, from soil profiles not influenced by focusing. In other cases, the predicted values of atmospheric radionuclides deposition based on historical fallout were used (24). Areal normalized fluxes of PCBs were calculated by dividing the PCB flux to sediment by the corresponding focusing factor. Simple linear regression of log10 transformed concentrations of ∑PCBs and homologs versus latitude was used to assess latitudinal trends. Results expressed as percent of ∑PCBs were not log transformed. The statistical significance of the regression model was tested using the F-test.
Results and Discussion Sedimentation and Mixing in the Lake Sediments. A wide range of sedimentation rates, focusing factors, and mixed depths were observed among the 11 lakes (Table 1). Highest sedimentation rates were found in Lakes Amituk and Ste Therese (389-429 g m-2 yr-1). Amituk is a narrow lake within a relatively steep-sided valley that is virtually vegetationless (27). 137Cs sedimentary profiles in five of the lakes, L375, Kusawa, Ste Therese, Hawk, and Hazen, exhibited subsurface maxima corresponding to slices dated to the 1960s using 210Pb. The Far Lake sediment core showed rather uniform 137Cs activity in the upper 3-4 cm, which is in agreement with the computed mixed depth (Table 1). The 137Cs profiles in lakes Trout, Belot, Sophia, and Amituk did not exhibit subsurface maxima. The activity decreased with depth from the sediment-water interface, although a subsurface maxima would be expected from the 210Pb data. Thus 137Cs was not suitable for dating of these cores.
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FIGURE 2. GC/high-resolution MS chromatograms of PCB congeners in the first slice from five lake sediment cores collected from midlatitude and Arctic lakes.
Knowledge of mixed depth allows the determination of the intrinsic time resolution [t* ) mixed depth (cm) ÷ sedimentation rate (cm yr-1), 13], i.e., the time span over which an alteration in the input rate of contaminants will not be observed in the sediment record. The largest mixed depths were found in cores from the more southerly lakes (L375, L382, Trout, and Far). Sediments from these lakes had high porosities (0.94-0.98). In cores from Lakes Amituk, Belot, Hawk, and Kusawa, where mixed depths were small, the values of t* ranged from 0 to 4 yr. Other cores (L375, L382, Far, and Hazen) had sizable mixed zones, with values of t* for these cores of 14, 52, 20, and 18 yr, respectively, based on 210Pb chronology. The mixed depth for core from Sophia Lake, computed by RSSM, was only 0.6 cm, but due to high sediment compaction (mean porosity based on the upper 5 cm ) 0.86) the intrinsic resolution for this core was about 12 yr. However, PCB sedimentary profiles may not be destroyed in the mixed zone (even if the 210Pb or 137Cs profiles demonstrate mixing) because organochlorines enter the sediments with a source function half-life shorter than those for 210Pb and 137Cs (13). Concentrations and Proportions of PCBs in Lake Sediments. The pattern of PCB congeners in the surface slice from five of the lakes is shown in the GC-HREIMS chromatograms (Figure 2). Di, tri- (CB 8,18, 28), and tetrachlorocongeners (CB 52, 70/76, 66) predominated in most samples especially in those from Amituk Lake in the high Arctic. GC-HREIMS profiles for L375 and Trout in northwestern Ontario, Kusawa (southern Yukon), and Belot (northwestern NWT) show that these sediment extracts contained significant amounts of pentachlorobiphenyls (CB101, CB118) and hexachloros (CB153 and CB138) relative to high Arctic lakes (Figure 2). Concentrations of individual
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FIGURE 3. Relationships between concentrations, proportions, inventories, and latitude for PCBs and organic carbon in lake sediment. (A and B) Percent organic carbon in sediment and organic carbon inventory. (C and D). Percent ∑di/tri- and ∑octaCBs. (E and F) Inventories of ∑di/tri, hepta, octa, and PCB.
congeners were consistently
