Estimation of the atmospheric and nonatmospheric contributions and

Amounts of PCBs accumulated in sediments of Lake. Michigan and four Wisconsin lakes, isolated from point sources, were measured. The flux of PCBs to L...
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Environ. Sci. Technol. 1986,20,879-883

Estimation of the Atmospheric and Nonatmospheric Contributions and Losses of Polychlorinated Biphenyls for Lake Michigan on the Basis of Sediment Records of Remote Lakes Deborah L. Swackhamer” and David E. Armstrong

Water Chemistry Program and Department of Civil and Environmental Engineering, University of Wisconsin, Madison, Wisconsin 53706 Amounts of PCBs accumulated in sediments of Lake Michigan and four Wisconsin lakes, isolated from point sources, were measured. The flux of PCBs to Lake Michigan sediments was 4 times greater than the average loading of 1.9 f 1.1pg/m2 per year to the remote lakes and has been occurring longer. Comparisons of the remote lakes and Lake Michigan were used to estimate the net atmospheric and nonatmospheric contributions of PCBs to Lake Michigan. The net atmospheric input and the total atmospheric flux, reported previously, were used to calculate the net vapor-phase transfer. Total atmospheric (430 f 130 kg/year) and net nonatmospheric (330 f 140 kg/year) contributions to Lake Michigan were of similar magnitude. Volatilization losses (320 f 140 kg/year) were similar to sedimentation losses (440 f 130), indicating that volatilization may be a major removal mechanism of PCBs from lakes.

Introduction Polychlorinated biphenyls (PCBs) enter lake systems from both diffuse and point sources. While measurements of direct input from point sources (e.g., industrial discharges and waste treatment plant effluents) are possible, transport from diffuse sources by water or air is difficult to measure. Airborne PCBs can be transported over long distances and eventually enter lakes by washout and dry deposition ( I ) . While reasonable estimates exist for many of the depositional processes (2), a clear understanding of the exchange across the air-water interface is lacking (3). Because data on PCB concentrations in the atmosphere are limited and models for calculating input to lakes from concentrations in air may be inaccurate (2),the contribution of atmospherically derived PCBs to lakes remains uncertain. Lake sediments provide a valuable record of the historical input of contaminants to lakes. Since PCBs and other hydrophobic organic microcontaminants tend to associate strongly with particulate matter, their primary sink is thought to be the bottom sediments. Lake Michigan has accumulated PCBs from atmospheric, tributary, and point sources, resulting in PCB levels in lake trout and salmonids that exceed guidelines for human consumption. The sediment record in Lake Michigan reflects the net accumulation from all sources of PCBs to the lake. In this paper we report estimates of the atmospheric and nonatmospheric contributions of PCBs to Lake Michigan by assuming that the net accumulation of PCBs in sediments of lakes receiving PCBs only from atmospheric sources provides an estimate of the net flux of PCBs from the atmosphere to Lake Michigan. The approach involved comparisons of several “remote” lakes in Wisconsin with Lake Michigan. The term remote was used to represent lakes with no influent tributaries *Address correspondence to this author at Geology 541, Indiana University, Bloomington, IN 47405. 0013-936X/86/0920-0879$01.50/0

and distant from urban areas. Remote lakes and Lake Michigan were assumed to be exposed to similar PCB atmospheric concentrations. Measurements were made of the PCB concentration and accumulation with depth in the sediments of Lake Michigan and the remote lakes. The total accumulation and flux of PCBs to the lake sediments was then used to evaluate the role of atmospheric deposition of PCBs to Lake Michigan. Sources and Sinks. To evaluate the importance of the atmospheric source, consideration of all the sources and sinks of PCBs to lakes is necessary. In general, major sources to the water column include wet and dry atmospheric deposition, industrial point sources, river inflow, and resuspension of deposited sediments. Possible sinks are sediment deposition, river outflow, weathering processes such as degradation and photolysis, and volatilization back to the atmosphere. In Lake Michigan, atmospheric deposition and industrial discharges to tributaries entering the lake are the primary sources ( 4 , 5 ) . Although sediments are a net sink, both particulate and dissolved PCBs are transported from sediments into lake water (6). We assume accumulation in sediments is large in comparison to removal by weathering or river outflow. Recent evidence suggests that volatilization of PCBs may be important during the warmer times of the year because of the temperature dependence of the air-water partition coefficient (7, 8) but the net flux of PCBs is from the atmosphere to the lake as evidenced by the presence of PCBs in remote lake sediments, as discussed below. For the remote lakes considered in this study, atmospheric input was the dominant source, and sediments were the primary sink.

Methods The remote lakes chosen for study were all seepage lakes (tributary-free) located in mid and northern Wisconsin. They received no point source inputs of PCBs due to their physical isolation from industrial sources and singular use for recreational purposes. Sites included Crystal Lake (46’00’00”, 89’36’45’’) in Vilas County, Dark (45”6’30”, 91’28’30’’) and Little Pine (45”7’00”, 91O29’30’’) Lakes in Chippewa County, and Emrick Lake (48’52‘30”, 89’36/00”) in Marquette County. Surface areas and depths of the remote lakes varied between 1 X lo5 and 3 X lo6 m2 and between 21 and 24 m. The sediments of all the remote lakes except Crystal were clearly varved and unmixed, resulting in an undisturbed sediment record. Crystal Lake sediments were mixed. Sediments from the remote lakes were collected during 1981-1982 by using a Jenkins corer (7-cm diameter). Multiple cores were obtained from the central depositional zones of each lake, sectioned into 2- or 3-cm vertical intervals, and the corresponding sections were combined to form a composite sample. A core obtained from the deepest part of the major depositional zone of the southern basin (station 18, 42’44’,87’00’)) was used as a representative core for Lake

0 1986 American Chemical Society

Environ. Sci. Technol., Vol. 20, No. 9, 1986 879

Table I. Concentration, Areal Burden, and Flux of PCBs with Depth in Sediment Cores from Four Remote Lakes and One Site in Southern Lake Michigan

lake” Crystal

core section, cm 0-3 3-6 6-9 0-2 2-4 4-6 6-8 0-2 2-4 4-6 6-8 8-10 0-2 2-4 4-6

total PCB concn, ng/g dry wt (SD)b

areal burden, wg of PCB/m2

flux, pg of PCB/(m2 year)

15.2 (2.3) 16.2 (2.9) 15.3 (7.5) 19.2 (2.1) 20.4 (0.7) 8.8 (0.9) 2.2 (1.2) 89.0 (5.9) 12.8 (1.1) 12.7 (1.3) 26.3 (1.9) 13.3 (1.6) 2.6 (0.2) 7.8 (0.2) 2.8 (0.3) 91.1 (0.6) 54.8 (1.0) 42.0 (0.7) 18.6 (0.6) 7.6 (0.3)

30 29 29 16 27

3.2 3.1 3.1 1.6 3.0

time interval

1954-1982‘ 1954-1982 1954-1982 Dark 1971-1981 1962-1971 12 0.8 1948-1962 3 0.2 1935-1948 Emrick 16 6.2 1978-1983 6.9 2.1 1975-1978 11 2.7 1971-1975 17 2.2 1963-1971 11 1.2 1954-1963 Little Pine 2.1 0.5 1978-1982 8.4 2.0 1974-1978 3.0 0.3 1965-1974 Michigan 18d 0-1 210 14.0 1964-1980e 1-2 130 7.0 1949-1967 2-3 110 6.2 1934-1952 3-4 47 2.7 1919-1937 4-5 20 1.1 1904-1922 “ A fifth lake, Dudley Lake (45°25’00’’, 89O29’00”), was also sampled and found to have an unusually high PCB areal burden (200 pg/m2) and flux (10 pg/(m2 year)). Because this lake had more human activity within its watershed and was less isolated than the other four lakes, it may have received PCBs from sources other than the atmosphere. These data were not included in the final data set. * SD = standard deviation of the regression analysis. CSedimentmixing depth in Crystal Lake is 4-6 cm. dStation location, 42O44’, 87’00‘. eDeposition periods represent the maximum age of the sediment since the surface mixed layer depth is 1.2 cm. Some sediment within an interval could be up to 18 years more recent.

Michigan. This core was collected by a box corer (9 X 24 cm X 25 cm deep) and sectioned by 1-cm intervals. The box corer and Jenkins corer both obtained sediments with a relatively undisturbed surface. Sediments (100-200 g wet weight) were extracted by steam distillation (9) into hexane, and the resulting extracts were cleaned by column chromatography. Columns (1.5-cm diameter) contained 7 g of alumina (deactivated 10%) over 7 g of silica gel (deactivated 3%) over 2 g of acid-washed copper grains (20-80 mesh). Extracts were reduced to a final volume of 1-10 mL and analyzed by capillary column gas chromatography (GC). The instrument used was a Hewlett-Packard 5830 equipped with a 63NiEC detector, microprocessor, Grob splitless injector, and autosampler. Conditions were as follows: H2carrier gas, 2.5 mL/min; 95/5% argon-in-methane makeup gas, 20 mL/min; injection port 225 OC; detector 325 OC; 50 m X 0.2 mm i.d. SP2100 WCOT column; initial column temperature 50 “C, time-temperature programmed from 180-240 “C. The gas chromatographic data was analyzed by a multiple linear regression computer program (COMSTAR) which gives the “best fit”of Aroclor standards to the sample GC data (peak area and retention time) (10). Samples were quantified with respect to Aroclors 1242, 1248,1254, and 1260, and their sum was total PCBs. The recovery by steam distillation is >90% for Aroclors 1242, 1248, and 1254 and about 60% for Aroclor 1260. Because Aroclor 1260 was a minor component, the lower recovery was unimportant. The coefficient of variation (C,), calculated as 100 X SD/mean PCB concentration, was 11f 14% for total PCBs. The r2 for the regression fit of the sediment data was greater than 0.88 in all cases. Sedimentation rates for Emrick, Dark, and Little Pine Lakes were determined by measuring the thickness of intervals between annual varves (11,12). Crystal Lake and Lake Michigan core 18 were dated by the 210Pbtechnique (13, 14). Results The data are presented as the sediment PCB concen880

Environ. Scl. Technol., Vol. 20, No. 9, 1986

Table 11. Total Areal Burden, Years of Accumulation, and Flux of PCBs for Sediments of Four Remote Lakes and Lake Michigan

lake Crystal Dark Emrick Little Pine Michigan 18 Michigan S. basin

time total areal period of mean burden, accumulation, total flux, wg/m2 years rg/(m2 year) 88 57 62 13 517” 390b

28 46 27 17 5lC 51‘

3.1 1.3 2.3 0.8 10.1 7.6

average flux (SD) for remote lakes = 1.9 (1.1)pg/(m2 year) Station 18 (see text for location). Weighted average for entire southern basin (22, 23) CMaximumperiod (see also footnote e of Table I).

tration (dry weight basis) in nanograms per gram (Zf 1 SD), the total and annual areal loading for each sediment core section, and the total and mean annual areal PCB loading and period of accumulation for each lake (Tables I and 11). The net areal loading, mass of PCB/m2, was calculated for a given depth of sediment core by summing the product of PCB concentrations (mass/m3) and interval thickness (m) over the sediment sections comprising the depth interval. PCBs were not detected in depth intervals below those reported. The flux, mass of PCB/(m2 year), was calculated from the period of accumulation (years) and areal PCB loading for the respective sediment depth interval. Mixing was assumed to be absent for varved sediments. For Crystal Lake and Lake Michigan, mixing will not affect calculated total PCB accumulation but may bias calculated fluxes for a given sediment (time) interval. Three of the sections from Crystal Lake were analyzed in duplicate. The average coefficient of variation (C,) for these analyses was 25 f 11%. The total error (analytical replication and quantitation) for sediment samples, expressed as C,, was 28%.

Discussion Assumptions. A central premise of this work is that the remote lakes and Lake Michigan would receive similar atmospheric loadings. Their geographical proximity suggests they would have similar meteorological conditions and annual precipitation. Measured air PCB concentrations above Lake Michigan and Lake Superior are also similar (4,15). While air concentrations near Chicago were higher than over the open waters of Lake Michigan (15), rain samples taken from a remote site in northern Lake Michigan had similar concentrations to those taken in Chicago ( 5 ) . Rain washout is thought to be the primary atmospheric deposition mechanism to the lake ( 2 ) . Another assumption made is that secondary losses such as biodegradation and photolysis are negligible compared to loss to the sediments. Photolysis has been documented in laboratory and field studies but is generally thought to be insignificant compared to sedimentation (16). While laboratory results extrapolated to natural environmental conditions indicate that biodegradation is negligible ( I 7), recent field evidence suggests that biodegradation may be significant over time (18). However, this is limited to the di-, tri-, and tetrachlorinated congeners. This would lead to an underestimation of nonatmospheric loading and overestimation of the volatilizaton losses presented below. Areal PCB Loading. The total accumulation of PCBs in sediment cores ranged from 13 pg/m2 for Little Pine Lake to 520 pg/m2 for Lake Michigan station 18 (Table 11). In the remote lakes, areal loadings were less than 90 pg/m2, resulting in an average areal loading of 55 f 31 pg/m2 (1SD). The estimated mean for the southern basin of Lake Michigan (390 pg/m2) was 7 times the remote lake average. Areal loadings calculated from cores taken in depositional zones may overestimate the areal loading for the lake basin because of sediment focusing, or movement of sediment from nondepositional areas to depositional zones by resuspension and gravitational settling processes. The heterogeneity of bottom topography and sediments in Lake Michigan (13,19)results in substantial sediment focusing in depositional zones. Sediments are more uniform in the remote lakes. The average areal loading for the southern basin of Lake Michigan has previously been estimated to be 230 pg/m2 (20). This estimate is probably low because all the PCB was assumed to be present only in depositional regions. Another estimate of 390 pg/m2 (Table 11) was obtained by summing the total PCB areal concentrations (pg/m2) of the nondepositional, transitional, and depositional zones of the southern basin (21, 22). The latter estimate is considered more accurate because it addresses the different sedimentation zones of the lake and will be utilized in the remaining discussion. Comparison of this value to the areal loading at station 18 (520 pg/m2) indicates that sediment focusing is occurring. The reported areal loadings of the remote lakes should represent maximum values with respect to focusing, because cores were obtained from the center of the basin. The possible effects of sediment focusing were estimated by two methods. Comparisons were made of sediment trap mass flux to the mass sedimentation rate in the depositional zone for Crystal and Emrick Lakes. For both lakes, the mass sedimentation rate was greater by a factor of 1.4-1.6 (23),indicating the occurrence of some focusing or postdepositional decomposition. Previous measurements in Crystal.Lake also showed that the trap flux was equal to or greater than the mass sedimentation rate (24-26). However, the mean annual atmospheric flux and annual sedimentation rate of 210Pbin the depositional zone were

approximately equal (24),indicating sediment focusing in Crystal Lake may be minor. An extreme effect of sediment focusing would be seen if material deposited over the whole lake surface was accumulated entirely in the depositional zone. The ratio of depositional area to total surface area was estimated for Crystal, Emrick, and Little Pine Lakes to range from 0.20 to 0.25. Thus, the areal loadings reported for the remote lakes are considered maximum values and may be high depending on the amount of focusing actually occurring in the lakes. PCB Sediment Fluxes. Lake Michigan apparently received PCBs earlier than the remote lakes (Table I). Measureable amounts were detected in the 3-4-cm layer deposited between about 1920 and 1950, indicating that input of PCBs to Lake Michigan began soon after PCBs were first sold commercially in the US. in 1929. The range in possible age of material in this layer is based on the depth of mixing at the sediment surface (1.2 cm) and the sedimentation rate (0.066 cm/year), as discussed elsewhere (20). The remote lakes appeared to differ somewhat in their sediment histories, with the earliest deposition data ranging from 1965 for Little Pine Lake to 1935 for Dark Lake. These dates are based on the age of the lowest sediment depth interval that contained detectable PCBs. The differences between lakes are probably due to the errors associated with sectioning the cores, the low sedimentation rates of the remote lakes, and PCB levels in the lower sections which approach the analytical detection limit. The average date for the appearance of PCBs in the remote sites is 1952. This is similar to the deposition record for Lake Superior, where significant accumulation of PCBs has been occurring since approximately 1955 (27). The onset of deposition recorded in the sediments is consistent with the atmospheric transport of PCBs to these lakes. A lag between initial sales and transport to the atmosphere would be expected. PCB deposition may have occurred earlier, but in amounts too low to be detected. The estimated total annual flux of PCBs to the sediments of each of the lakes is shown in Table 11. Fluxes ranged from an average of 1.9 f 1.1pg/(m2 year) in the four remote lakes to 10 pg/(m2 year) for station 18 in southern Lake Michigan. The basin-wide PCB flux to Lake Michigan sediments is estimated to be 7.6 f 2.3 pg/(m2 year). The flux to Lake Michigan is based on the maximum period of accumulation and thus represents the minimum average flux per year. This flux is about 4 times greater than the PCB flux to the remote lake sediments. The accumulation of PCBs in sediments varied with time (Table I). In general, the flux decreased with depth in the core, with the largest flux occurring from the 1960s into the 1970s. The PCB loading before this time was much lower. While these trends hold for both Lake Michigan and the remote lakes, the magnitude of accumulation and the flux are much greater for Lake Michigan. During the time period of greatest flux to the remote lakes, Lake Michigan received 4-5 times more PCBs. From the mid-seventies to the early 1980s, the PCB flux appeared to decrease in the remote lakes, with the exception of Emrick Lake. While some cores from Lake Michigan show a subsurface maximum of PCB accumulation, possible recent decreases in PCB flux to the core taken from station 18 are masked by mixing of the top 1.2 cm. In Crystal Lake, similar fluxes were obtained in all three core sections, as expected due to mixing of the surficial sediment over 4-6 cm (24). Atmospheric PCB Fluxes. A further understanding of the sources and sinks of PCBs to Lake Michigan can Environ. Sci. Technol., Vol. 20, No. 9, 1986 881

be gained by comparing the average remote lake sediment flux to that in Lake Michigan. The assumption is made that the rate of accumulation in sediments of the remote lakes is an approximation of the net atmospheric flux to the lake, Le., the sum of wet and dry deposition by both aerosols and vapor minus the losses due to vapor exchange back to the atmosphere: Anet = W + R + D + VI - Vo where Anetis the net atmospheric deposition, W is vapor washout, R is particle rainout, D is dry deposition, and VI and Vo are vapor exchanges in and out of the lake, respectively. Other removal processes (microbial degradation, photodegradation) are assumed to be unimportant. Andren (2) calculated the atmospheric input of PCBs to Lake Michigan, excluding vapor exchange, in a critical review of PCB flux across the air-water interface. Particle rainout is the dominant flux, estimated to be 4.7-8.8 pg/(m2 year). (Lake Michigan surface area is 5.8 X 1O1O m2.) Vapor washout was estimated to be 0.086 pg/(m2 year) and dry deposition to range from 0.43 to 0.86 pg/(m2 year), yielding a total atmospheric flux of 7.5 f 2.2 pg/(m2 year). Using the average remote lake sediment flux of 1.9 f 1.1pg/(m2 year) to approximate the net atmospheric flux together with these values in the previous equation yields a net vapor exchange (Vo- VI) of 5.6 f 2.5 pg/(m2 year) from the lake to the atmosphere. Net vapor exchange would also include processes such as bubble stripping and ejection. This estimate suggests that for the remote lakes, the atmosphere is a sink for PCBs of greater magnitude than the sediments. On the basis of PCB air concentrations, the net sediment accumulation of PCBs in the remote lakes would represent a minimum net atmospheric deposition to Lake Michigan. While atmospheric PCB concentrations are similar in remote sites and above the open waters of Lake Michigan, concentrations are higher near urban areas such as Chicago and Milwaukee ( 1 , 1 5 ) . Thus, the estimated vapor losses from the lake of 3.1-8.1 pg/(m2 year) are interpreted as maximum losses. For Lake Michigan, a net vapor flux of 5.6 f 2.5 pg/(m2 year) corresponds to a loss to the atmosphere of 180-470 kg/year. The possibility of a net vapor phase loss of PCBs from Lake Michigan to the atmosphere is supported by the recent data of Burkhard et al. (8),Murphy et al. (7), and Mackay et al. (28). Burkhard et al. determined Henry's laws constants (H) for all 209 PCB congeners (8) and used them to calculate the thin film resistances (29) from the Whitmann two-layer model (30) for vapor flux across the air-water interface (31). While fluxes obtained on the basis of this model may be inaccurate, the model is useful for predicting the direction of transport. By use of the two-layer model, the direction of transport was calculated for three PCB congeners, each a major component of Aroclors 1242,1254, and 1260, at the temperature extremes of 0 and 25 "C. A total dissolved waterphase PCB concentration of 0.5 ng/L composed of 31% 1242,22% 1248,25% 1254, and 22% 1260 (23,32)and a total vapor-phase PCB concentration of 0.9 ng/m3 composed of 75% 1242 and 25% 1254 (15) were assumed. During the colder months the lower chlorinated congeners (1242) were predicted to go from air to water, while the reverse was true for the warmer months. This change in direction occurs because H is a function of temperature. Values of H increase by an order of magnitude as temperature increases from 0 to 25 "C (8). For the more highly chlorinated congeners (1254 and 1260), the direction of vapor transfer was predicted to be from water to air at both temperatures because these congeners are present at low concentrations in vapor relative to Lake Michigan water. 882

Environ. Sci. Technol., Vol. 20, No. 9, 1986

Table 111. PCB Mass Balance for Lake Michigan

kg/year inputs atmospheric deposition" wet washout (vapor) rainout (particles) dry deposition (particles) total nonatmospheric, netb total

5

270-510 25-50 300-560

190-480 490-1040

0utp ut s

atmospheric:c vapor loss outflowd sedimentatione

180-470

_310-570

total

550-1100

60

=From ref 1. *Sedimentation flux-net atmospheric input = 5.7 2.3 pg/(m2 year). CVaporloss = total atmospheric deposition net atmospheric deposition = 3.1-8.1 pg/(m2 year) (see text). Water outflow to Lake Huron [49 km3/year (33)]X average PCB water concentration [1.2 ng/L ( 3 2 ) ] . eEstimated lake mean flux X lake area; approximately 30% error (22, 23). f

The relative magnitudes of these exchanges suggest that, for the concentrations used for air and water, the total PCB net vapor-phase transfer is from water to air. This would hold true for both Lake Michigan and the remote lakes because dissolved PCB concentrations are similar for all the lakes, even though total PCB concentrations are much lower in the remote lakes (23,32)and PCB concentrations in air are assumed to be similar between sites (5, 15). Lake Michigan Mass Balance. The sediment data and the estimated atmospheric contribution can be used to construct a PCB mass balance for Lake Michigan. As described above, the estimated total and net atmospheric contributions are 7.5 f 2.2 and 1.9 f 1.1pg/(m2 year), respectively. The difference between these values provides an estimated net nonatmospheric contribution of 5.7 f 2.5 pg/ (m2year). Volatilization and sedimentation are estimated to be 5.6 f 2.5 and 7.6 f 2.3 pg/(m2 year) (see above). Corresponding fluxes (kg/year) for Lake Michigan are calculated by using an area of 5.8 x 1O1O m2 (Table 111). The derived budget applies to Lake Michigan excluding Green Bay. The nonatmospheric contribution includes inputs from tributaries and point sources. The estimate of 190-480 kg/year (Table 111) is consistent with an estimate of 83-740 kg/year obtained from measured concentrations of PCBs in the major tributaries to Lake Michigan, excluding those discharging into lower Green Bay (34). PCBs discharged into Green Bay through tributaries are partially retained in sediments of the bay. Thus, the nonatmospheric input estimated from sediments in the southern basin would not apply to Green Bay. The constructed mass balance provides a useful framework for comparing the relative importance of the different sources and sinks. The atmospheric and net nonatmospheric contributions to the lake are of similar magnitude. Nonatmospheric inputs account for 39-46% of the input budget. Volatilization is a major loss mechanism, accounting for 33-4370 of the total lake sinks. Approximately 56% of the losses are due to sedimentation. This lake-wide budget, derived in part from measured sediment fluxes in Lake Michigan and the remote lakes, is useful for estimating additional fluxes that are difficult to measure directly. However, several pitfalls which may lead to biased estimates should be recognized. Areal concentrations of PCBs in sediments of the depositional, transitional, and nondepositional zones of the northern basin are assumed to be similar to those in the corre-

sponding zones of the southern basin. Our data for a few stations in the northern basin indicate this assumption is satisfactory. Volatilization, as estimated from total and net atmospheric deposition in the remote lakes, is assumed to be independent of the nonatmospheric contribution to Lake Michigan. The higher loading to Lake Michigan might lead to a higher water-to-air concentration gradient and a higher volatilization rate. In spite of these possible biases, the budget provides an important perspective on the sources and sinks of PCBs and a basis for comparison with independent estimates.

Summary and Conclusions The total areal loading of PCBs to southern Lake Michigan sediments (390 pg/m2) is 7 times greater than the average areal loading to four remote lakes in mid and northern Wisconsin. Significant deposition has been occurring since approximately 1950 in the remote sites and as early as 1930 in Lake Michigan. Estimated fluxes of PCBs to bottom sediments have averaged 1.9 f 1.1 pg/(m2year) for the remote lakes and approximately 7.6 f 2.3 pg/(m2 year) for southern Lake Michigan. The assumption is made that the remote lake sediment flux is a good approximation of the net atmospheric flux to the remote lakes and to Lake Michigan. This estimated net flux, combined with other source data, indicates that the atmosphere is a sink for PCBs (vapor/ dissolved phases) as well as a source. From the net atmospheric flux of 1.9 f 1.1 pg/(m2 year) and the reported atmospheric flux to the lake of 7.5-2.2 pg/(m2 year) (2), the flux back to the atmosphere was calculated to be 5.6 f 2.5 pg/(m2 year). In the remote lakes with only atmospheric inputs, volatilization back to the atmosphere apparently accounts for more than half the losses from the water column, with the rest being lost to the sediments. On the basis of a mass balance constructed from measured and estimated fluxes, net atmospheric deposition and nonatmospheric sources each account for approximately half of the total PCB input, about 430 f 130 and 330 f 145 kg/year, respectively. Estimated losses are approximately 320 f 145 kg/year to the atmosphere and 440 f 130 kg/year to the sediments. Acknowledgments

(8) (9) (10) (11)

(12) (13) (14) (15) (16)

(17) (18) (19)

(20)

(21) (22) (23) (24) (25) (26) (27)

We thank A. W. Andren for helpful discussions and H. Grogan and J. Schneider for typing the manuscript. We also thank D. Liebl for his assistance in sample collection.

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(30) (31) (32) (33) (34)

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Received for review August 2,1985. Revised manuscript received March 26, 1986. Accepted April 11, 1986. This research was supported by a n institutional grant t o the University of Wisconsin from the Sea Grant Program of the National Oceanic and Atmospheric Administration, U.S. Department of Commerce, and from the State of Wisconsin, Federal Grant NABOO-AA0-00086, Project RIMW-23, and by the University of Wisconsin Water Resources Center, Project A-084.

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