PCBs in Lake Superior, 1978-1992: Decreases in ... - ACS Publications

Environ. Sci. Technol. 1994, 2S, 903-9 14

PCBs in Lake Superior, 1978-1 992: Decreases in Water Concentrations Reflect Loss by Voiatllization Jeff D. Jeremiason, Keri C. Hornbuckle, and Steven J. Eisenreich’

Gray Freshwater Biological Institute, University of Minnesota, P.O. Box 100, Navarre, Minnesota 55392 Polychlorinated biphenyl (PCB) water concentrations in Lake Superior from 1978 to 1992 have been compiled in order to determine long-term trends. Based on the concentrationsof the same 25 PCB congeners from surface water samples collected between 1980 and 1992, CPCBs have decreased with a first-order rate constant of 0.20 y r l . The concentration of CPCBs in 1980 based on 82 congeners was 2.4 ng/L, by 1992 the concentration had decreased to 0.18 ng/L, indicating a loss of -26 500 kg of PCBs from Lake Superior during this 12-yr period. PCB concentrations in dated sediment cores collected in 1986 and 1990 indicate that the sediments have accumulated -4900 kg of PCBs since 1930when PCB production began in the United States. A mass balance of PCBs in Lake Superior for 1986 predicts that volatilization, not sedimentation, is the dominant loss process of PCBs from the lake. Net volatilization of PCBs from Lake Superior is estimated to be -1900 kg in 1986, while the loss due to burial in the sediments was 110 kg. A congener-specific first-order rate constant model predicts sedimentation to be a minimal loss process in Lake Superior and volatilization to dominate. The pseudo-first-order rate constants for volatilization and sedimentation of CPCB are predicted by the rate constant model to be -0.24 and -0.004 y r l , respectively. The model estimated PCB inputs in 1986 of 500 kg.

-

-

Introduction Since production began in the United States in 1930, polychlorinated biphenyls (PCBs) have been globally distributed as evidenced by their detection in remote regions (1-9). PCBs can be found in all components of the ecosystem including air, rain, snow, water, sediment, fish, wildlife, and humans (10-17). The Great Lakes, in particular, are susceptible to hydrophobic organic contaminants such as PCBs, due to their large surface areas, long water residence times, proximity to urban/industrial centers, and long aquatic food chains, which allow for substantial biomagnification of such chemicals (18-21). The discharge and the deposition of PCBs to the Great Lakes and subsequent bioaccumulation have resulted in fish consumption advisories being issued throughout the Great Lakes. The production of PCBs was voluntarily restricted in 1971 and banned in 1979 (22). The ecosystem’s response to decreased loadings of PCBs resulting from the halt in production is still unclear. PCB levels in air have remained relatively constant since -1980 (10, 23-26). Dated sediment cores from Lake Ontario and Lake Michigan show PCB concentration profiles reminiscent of PCB production in the United States through 1980,but show no or limited decreases since that time (27-29). Great

Lakes fish studies show decreasing levels of PCBs in the lower Great Lakes and Lake Michigan through the early 1980s (30-35). Since then, however, PCB concentrations in fish throughout the Great Lakes have not decreased (36). The decrease in PCB levels in fish until the early 1980s from the lower Great Lakes reflects a decrease in point source loadings and improved disposal practices. None of these studies give evidence of a continuing ecosystem reduction of PCBs in response to decreased production in recent times. In this study, Lake Superior is utilized as a surrogate of ecosystem response to decreased loadings and continued environmental cycling of PCBs. Due to its remote location and minimal point source loadings, Lake Superior receives most of its chemical inputs via the atmosphere (18,37). PCBs are removed from the water column of Lake Superior by outflow via rivers, sedimentation, volatilization, and chemical and biological transformations. Due to their hydrophobic nature, PCBs are classified as “particlereactive”, and their main removal process from large and small water bodies is thought to be sorption to settling particles, delivery to the sediments, and subsequent burial (28, 38-40). Our laboratoryhas collected air, water, sediment trap, and sediment core samples from Lake Superior since 1977 (18,23,40-44). This large and unique data set allows the determination of long-term trends in the concentrations, inventories, and loadings of PCBs in and into Lake Superior and the relation of these trends to decreases in PCB inputs. It will be shown that PCBs are decreasing in the water column of Lake Superior at the rate of - 4 . 2 0 yr-l since 1980 and that this decrease can be attributed primarily to volatilization from the lake. A mass balance of PCBs constructed about the water column and a pseudofirst-order rate constant model will also demonstrate volatilization as the major loss process for PCBs from Lake Superior.

Lake Superior

* To whom correspondence should be addressed; e-mail address: [email protected].

Lake Superior is a large, oligotrophic lake located in a relatively remote area of North America. It has an average depth of 145 m, maximum depth of 405 m, surface area of 8.21 X 1O1Om2,volume of 1.21 X 1013m3 (451,and a flushing time of 177 years (46). The suspended solids range from 0.1 to 0.5 mg/L in the open waters and from 0.1 to 4 mg/L in the western basin (42). The organiccarbon concentration (DOC) of the water is approximately 1.1 mg/L with particulate organic carbon accounting for about 10-30% of the organic carbon (47). Primary productivity in the summer months ranges from -1 to 2 mg m-3 h-l (48). Only 1% of the drainage basin is residential or industrial (49). The remote location, large surface area, andlong water residence time implies that the atmosphere is the main source of many chemicals to the lake (18,37).

0 1994 American Chemlcal Society

Envlron. Scl. Technol., Vol. 28, No. 5, 1994 903

-

0013-938X/94/0928-0903$04.50/0

Fbur 1. Lake SuperIw sampling sites, 1984-1992.

Sampling Methods Figure 1 shows the surface water sampling sites and sediment coring sites for the years 19861992. Concentrations of PCBs in water samples collected in 1986 are reported by Baker and Eisenreich (23). PCB concentrations in sediment cores collected in 1986 and 1990 are reported elsewhere as well (44, 50-52). Lake Superior water samples from 1988 and 1990 were collected from the RV Seward Johnson in the same manner as described byAchmanetal. (13)using293-mmglassfiberfilters(GFF, Schleicher and Schuell no. 25, 0.7 pm) to collect the “particulate PCB fraction” and XAD-2 resin (Sigma Chemical Co., A-7643)to collect the “dissolved PCB fraction”. The sample volumes ranged from 290 to 486 L for the particulate fraction and from 50 to 70 L for the dieaolved fraction. The 1992water sampleswere collected from aboard theEPARV Lake Guardian. The particulate fraction was obtained in the same manner as above. The filtrate was collected by extraction with dichloromethane employing a Goulden-type liquid-liquid continuous flow extractor (53-56). The Goulden extractor was operated at 20 “C and at an average flow rate of 0.95 L/min. Sample volumes in 1992 ranged from 476 to 952 L for both the dissolved and particulate fractions. Table 1 includes details of all the surface water collection methods and the appropriate references for samples collected since 1978. All sediment cores were retrieved using the Johnson Sea Link I1 submersible [Harbor Branch Oceanographic Institute under contract to the National Underseas Research Center (UCONN/NOAA)I. Cores were collected using a box corer measuring 15.4 X 15.4 X 30.5 cm deep. The mechanical ann of the Johnson Sea Link I1 submersible was used to extract each core from the lake bottom. Upon returning to the ship, each undisturbed box core was immediately extruded using a mechanical box core extruder. The cores were sectioned into 0.251.0-cmincrementa while generally removing the outer -0.5 cm to prevent contaminationby smearing. The individual sections were placed in cleaned glass jars with aluminum foil-lined lids. The samples were refrigerated until ready for laboratory analysis. Laboratory Procedure The GFF and XAD-2 resin for water sampled in 1988 and 1990 were rinsed with 200-500 mL of methanol and extracted with methanol for 24 h followed by a 2 4 h dichloromethaneextraction. PCB surrogates were added 001 Ewbm. Sd. Tsomd.. Vol. 28. No. 5 . l W 4

to all samples prior to extraction (3,5-dichlorobiphenyl, IUPAC no. 65; IUPAC no. 14;2,3,5,6-tetrachlorobiphenyl, and 2,3,4,4‘,5,6-hexachlorobiphenyl, IUPAC no. 166). All three extracts were combined, and the methanol was removed by hack-extraction with water and dichloromethane. The extracta were concentrated and transferred to hexane using a Kuderna-Danish apparatus or a Buchi Rotavap (Model RE111). Interfering compounds were removed by liquidaolid chromatography using 1.25% water deactivatedFlorisilas thesolid adsorbentasoutlined by Baker and Eisenreich (23) or with 10% deactivated alumina and 6% deactivated silica as described elsewhere (IO, 13). The samples were then concentrated to 2C-100 pL under a gentle stream of prepurified nitrogen before gas chromatographic (GC) analysis. Internal standards (PCB congeners 30, 2,4,6-trichlorobiphenyl,and 204, 2,2’,3,4,4’,5,6,6’-&achlorobiphenyl) were added prior to the final nitrogen blow down. PCBs in the final extract were analyzed using a Hewlett-Packard 5890 gas chromatographequippedwith ae3Nielectron capture detector (IO, 13). The column was a 30 m X 0.32 mm i.d. dimethyl diphenyl polysiloxane capillary column with a 0.25-pm film thickness (DB-5, J&W Scientific). Laboratory procedures for water samplescollected between 1978and 1986 were similar to those described above. Table 1 reports laboratory methods, chromatographic conditions, and appropriate references for water samples collected from 1978 to 1992. The laboratory procedure for sediment core samples is very similar to the procedure used for water samples. A brief description will be given here; a more detailed procedure is reported elsewhere (51,52). Subsamples of each individual box core section were used for 21”Pbdating and bulkdensitymeasurementa. Theremaining sediment was used for organic contaminant analysis. The zloPbcontent of each section was measured using amodified version of the method described by Eakins and Morrison (57). The dating of the core was determined using the constant rate of supply (CRS) model (58,59). The fmt step in isolating HOCs from the sediment matrix was the removal of residual water to allow for maximum extraction efficiency. Granular anhydrous NazS04 was added in a mass ratio of approximately5 1 anhydrousNar SO4 to wet sediment (25-50 g). The NazSO4 and sediment were mixed together in a beaker and ground with a mortar and pestle until themixture was free-flowing. The sample was quantitatively transferred to a Soxhlet extractor, surrogate standards were added, and the sample was extracted for 24 h with dichloromethane. The rest of the sediment laboratory procedure is the same as the water sample procedure with the exception of the addition of a small amount of activated copper filings prior to gas chromatographic (GC) analysis to remove sulfur from the sediment extracta. Quality AssurancelQuality Control In the 1992 field studies, distilled water spiked with surrogate standards was passed through the GFF/XAD apparatus and the Goulden liquid-liquid extractor to account for any contamination resulting from isolation of organics in the field. Several XAD samples and all Goulden samples were spiked with surrogate standard while water was being collected. XAD resin was prepared by sequential 24-h extractions with methanol, acetone,

Table 1. Collection Methods and Laboratory Techniques, 1978-1992

chromatographic conditions

collection method

extraction conditions

1978" XAD-a/filter XAD vol = 40 L filter vol = 90-163 L

Soxhlet extraction, 1:l acetone:hexane t 18-24 h

reversed-phase HPLC

1979'

XAD-a/filter XAD vol = 20 L filter vol = 90-163 L

Soxhlet extraction, 1:1 acetone:hexane t = 18-24 h

micro florisil (0% deactivated) column (0.4 g) eluted with 4 mL of hexane followed by 4 mL of 9:l HEX-DEE (diethyl ester)

1980'

XAD-a/filter XAD vol = 40 L filter vol = 90-163 L

Soxhlet extraction, 1:l acetone:hexane t = 18-24 h

XAD-2, same as 1979 filters: 13 g of florisil (1.25% deactivated) eluted with 60 mL of hexane followed by 50 mL of 91 HEX.DEE

1983b XAD-a/filter XAD vol = 20 L filter vol = 70-120 L 1986c XAD-a/filter XAD vol = 67 L filter vol = 300-500 L

Soxhlet extraction, 1:l acetone:hexane t=48h Soxhlet extraction, methanol followed by dichloromethane t = 48 h each

13 g of florisil(l.25% deactivated) eluted with 60 mL of hexane followed by 50 mL of 91 HEXDEE 13 g of florisil(l.25% deactivated) eluted with 50 mL of hexane followed by 60 mL of 9 1 HEXDEE

1988d XAD-a/filter XAD vol = 50-70 L filter vol = 290-486 L

Soxhlet extraction, methanol followed by dichloromethane t = 24 h each

1990d XAD-2lfilter XAD vol = 65 L filter vol = 305-330 L

Soxhlet extraction, methanol followed by dichloromethane t = 24 h each Soxhlet extraction, methanol followed by dichloromethane t = 24 h each

year

1992d Goulden extractor/filter V O = ~ 476-952 L

a

cleanup procedure

HP 5840A GC with 63Ni ECD packed column, 1.5% OV-17 plus 1.95% QF-1 length = 2.4 m run time = 65 rnin HP 5840A GC with 6*NiECD stationary phase, SP-2100 length = 25 m i.d. = 0.22 mm run time = 75 min HP 5840A GC with 6sNi ECD stationary phase, SP-2100 length = 25 m i.d. = 0.22 mm run time = 75 min same as 1979

HP 58906 GC with '33Ni ECD stationary phase, 5% phenyl methyl silicone length = 25 m i.d. = 0.31 mm run time = 120 min HP 5890A GC with 63Ni ECD 13g of florisil 1.25% deactivated) eluted with 50 mL of hexane followed by 60 mL stationary phase, 5% dimethyldiphenyl polysiloxane of 9 1 HEXDEE (1.25% deactivated) length = 30 m eluted with 50 mL of hexane followed i.d. = 0.32 mm by 60 mL of 9:l HEXDEE run time = 145 rnin same as 1988 3 g of silica (6% deact.) and 10 g of alumina (10% deact.) eluted with 50 mL of hexane then 60 mL of 9:l HEX:DEE 8 g of florisil(l.25% deactivated) eluted same as 1988 with 90 mL of hexane followed by 60 mL of 9:l HEXDEE

Ref 42. b Ref 43. Ref 23. This study.

hexane, and dichloromethane, followed by 24-h extractions with hexane, acetone, and methanol to ensure a wettable surface. Glass fiber filters were baked at 450 "C for approximately 12 h. Several solvent blanks were run through the same procedure as water samples to quantify a laboratory blank. Several procedural blanks were also spiked with a calibration standard containing Aroclors 1232, 1248, and 1262 mixed in the ratio of 25:18:18to form a solution containing 610 ng/mL of PCBs to quantify the procedural recovery of all PCBs analyzed in our laboratory (60,61,13). These calibration standard spikes were also used to determine which surrogates to use for surrogate recovery correction. Average recoveries (% ) f1standard deviation for surrogate standards for 1988 and 1990 were as follows: no. 14,79 f 20; no. 65,102 f66; no. 166,83f 11. In 1992,the recoveries were 102 f 3, 101 f 5, and 99 f 5, respectively, for nos. 14,65, and 166for the filter samples. Recoveries from the Goulden extractor were 56 f 1476, 56 f 12%, and 62 f 14% ,respectively, for congeners 14,65, and 166. All water samples were corrected to no. 166 based on calibration standard spikes and since it was least prone to co-elution problems and demonstrated the least amount of variability. For the sediment core samples several matrix blanks, consisting of pre-1800 sediment, were spiked with only the surrogate standard. A t least one matrix blank was

analyzed for each sediment core. Several deep-core (pre1800) samples were also spiked with the calibration standard to quantify the sediment procedural recovery of each individual PCB congener. Average surrogate recoveries were 109f7 % ,106 f6 % ,and 107f 8 % for congeners 14,65,and 166,respectively. Congener 166was again used to correct all sediment samples for recovery.

Quantification of PCBs The mass of individual PCB congeners present in environmental samples was determined by comparison to a standard calibration mix containing internal standards and Aroclors 1232,1248, and 1262 mixed in the ratios of 25:18:18, respectively (60-62). This standard calibration mix was run daily to calculate a relative response factor (RRF) for each PCB congener. The RRF was determined as follows:

where IS corresponds to internal standard. The mass of a PCB congener present in an environmental sample was determined as: Envlron. Scl. Technol.. Vol. 28.

No. 5, 1994 005

mas+.,

= RRF(massdarea,)area,,

0 I.FCB rn

Surface water PCB congener concentrations were determined by summing the dissolved phase concentration (ng/L) and the particulate phase concentration (ng/L). Total PCB concentrations (EPCB) were calculated by adding all of the congeners quantified in a given year. Due toceelutionof contaminants, the peaks for PCB congeners 198,158, and 41+64+71 were not quantified.

= I JCB, crp(-O.ZW R’= O.% EJCB = I.FCB, crp(-0.221) R’ = 0.98

I 1978: Paeked Column Chromatography

. .

0 1986: Stevens and Nielsen (55)

X 1989 Skoglund and Swaclrhamr (63)

Lake Superior Water Column PCB Trends Figure 2showsthe trendinPCB eoncentrationsobserved inLake Superior surface waterscollected by our laboratory since 1980. To accurately establish this trend, the same 25 PCB congeners were selected from every surface water sample collected by our laboratory since 1980. These 25 congeners, represented by & P C B (ng/L) and listed in Table 2, were chosen because they constitute the congeners analyzed every year in our laboratory since 1980. The pseudo-first-order equation describing the decrease of ZPCB26 in Lake Superior is

where t is time in years (t is arbitrarily set equal to 0 in 1978). Since these 25 PCB congeners do not represent the actual total PCB concentration, EmPCB was also calculated based on 82 congeners (also plotted in Figure 2). As shown in Table 3,82 congeners were quantified in 1988,1990, and 1992. In the years 1980,1983, and 1986, less than 82 congeners were quantified. The ratio, &PCB/EjPCB, where j is the number of congeners analyzed in a given year, was 2.1,1.9, and 1.5,respectively, for the years 1980,1983, and 1986 based on the 1988-1992 data; 1980-1986 EPCB concentrations reported for these years were adjusted to represent 82 congeners. These ratios, or correction factors (CFs), were calculated as follows:

1975

1980

1985

1990

Is95

Ycar

Flpun 2. PCB decline In Lake Superior surfam waters from IS80 to

1992.

Table 2. Loe IG-.,and Rate Constant8 for PCB Congenera in Lake Superior congener(s)a 005 + 008

016 + 032 017 033”+ 053 + 021 022 031 028 044 037

+ 042

+

+

041d 071 064 049 047+048 076 + 07od 056 060

+ 066 + 095

074 101 llod + 077 099 118 141 146 13Ed+ 163

lop Kmb

k CVr9

5.02b 5.3w

-0.13

5.25 5.60. 5.58 5.67 5.67 5.75 5.801 5.69. 5.85 5.82’ 6.20.

-0.09 -0.21

6.1le

-0.22 -0.15 -0.18 -0.26 -0.22 -0.14 -0.38 -0.51

6.20. 6.20

6.38

6.48. 6.39 6.74 6.82 6.89 6.83‘

-0.16 -0.17

-0.20 -0.20

-0.19 -0.20

-0.W -0.17

-0.17 -0.27

-0.52

-0.21

a Congeners 18 and 97 included in

change. Ref 79. Average K , nant congener. f 19W1986 onlv.

where Y is the year for which the CF is being calculated (1980,1983, or 1986), EszPCB(X) is the E:szPCB concentration in year X (1988,1990, and 1992),j is the number of congeners analyzed in year Y,and EjPCB(X) is the sum of the same j congeners in year X. Applying these correction factors, the adjusted average E8zPCB concentrations were determined as 2.37, 1.52, and 0.84 ng/L, respectively, for the years 1980, 1983, and 1986. The equation describingthe decrease of Z&CB concentrations shown in Figure 2 is

where t is time in years. The first-order rate constant predicted by eq 3is similar to that described by eq 1(-0.22 vs. 4.20 yrl). Furthermore, the 1979 average EPCB concentration generatedwiththe modelPCBQ (42)agrees with the concentration predicted by eq 3. The 1978PCB samples, separated on a packed gas chromatographic column and quantified manually, do not correspond to the line generated by 82 congeners. 900

EnvtOn. Sd. TeCbzi.. Vd. 28. No. 5. 1994

Zd’CB showed no significant Dominant congener. * K, of domi-

Also included in Figure 2 are recent ZPCB concentrations in Lake Superior reported by other laboratories. Stevens and Nielsen (55) reported an average ZPCB concentration of 0.34 ng/L ( n = 19) in 1986 for samples collected with the Goulden liquid-liquid extractor in the open waters of the lake. Skoglund and Swackhamer (63) measured a EPCB concentration of 0.55 ng/L ( n = 1)in the western basin of Lake Superior in 1989. Historical PCB measuremanta can be compared to concentrations predicted by eq 3. The &PCB concentration in Lake Superior calculated by eq 3 is -8 ng/L in 1975. Swain (64)reported PCB concentrations of 5-7 ng/L in 1974and 10-20 ng/L in 1976. In eq 1, as well as using the same 25 congeners from each year, only surface water samples were included in the data set. Seasonal and spatial variations in ZPCB concentrations need to be considered when dealing with a time-dependent data set in a large dynamic system such as Lake Superior. In 1983, extensive sampling was conducted during the ice-free season in the western basin

Table 3. Summary of PCB Concentrations in Lake Superior since 1978 year (n)

sampling dates

1978*(10) 1979*(13) 1980b (20) 1983O (40) 1986d(5) 198EE(5) 1990E(6) 1992e (5)

June Aug Jun-Oct Aug 3-10 Jull2-16 Aug 26-Sep 1 May 8-12

a

July

Burden = av IPCB

X

no. of congeners

av IPCB f std err (ng/L)

av ZPCBa f std err (ng/L)

burdena (kg)

1.73 f 0.65

N/A N/A 0.99 0.10 0.73 f 0.06 0.55 f 0.15 0.20 0.01 0.21 k 0.01 0.09 k 0.01

20 900 48 900 28 70W 18 2001 10 low 4 000 3 800 2 150

N/A N/A 27 28 35 82 82

4.04 f 0.56

* *

1.13 0.11 0.80 0.07 0.56 f 0.16 0.33 h 0.04 0.32 f 0.03 0.18 f 0.02

82

*

*

vol (1.21 X 1013 m9). Ref 42. c Ref 43. d Ref 23. e This study. f Burden based on &PCB.

of Lake Superior to determine seasonal variations in CPCB concentrations (43). CPCB concentrations did not change significantly over the study period, but higher molecular weight congeners were lost from the surface waters at a rate of -0.18- to -0.28 day1 and were replaced by lower molecular weight congeners. This observed phenomenon was attributed to the differential settling of particle-bound higher molecular weight congeners (43). Similar trends are observed in the 1986-1992 CPCB concentrations, with samples collected after thermal stratification (1986,1988, and 1990) exhibiting congener distributions dominated by lower molecular weight PCBs. In 1986, congeners with six or more chlorines accounted for 19% of PCBs in the water column (23). In 1988and 1990,congeners containing six or more chlorines accounted for 12% and 7 % of the total water column PCB burden, respectively. On the other hand, the 1992 samples were collected in May when the lake was not yet stratified, and 33% of the total burden consisted of PCBs with six or more chlorines. We conclude that the seasonal variations in CPCB concentrations in a given year will be minimal in comparison to the total change between years; however, the congener distributions may be altered by seasonal variations. Note that after 1980, there were at least 2 yr between sample collection. Sampling location may also influence CPCB concentrations. During the 1992sampling expedition, five sites were visited spanning the open waters of the lake (see Figure 1). The small range of concentrations (0.14-0.26 ng/L) and very low standard error between these samples (Table 3) show that there was no spatial difference in CPCB concentrations in the open waters of Lake Superior at this time. Samples collected in the western basin of the lake might be expected to be higher in CPCB concentrations due to the proximity of the Duluth, MN/Superior, WI, urban and port areas. In years when samples were collected from the western basin and other areas of the lake (1990 and 1980), statistical comparisons show no statistical increase in PCB concentration in the western basin. In 1990, the three samples collected in the western basin exhibited an average C25PCB concentration of 0.22 ng/L, while the three samples collected in the open lake had an average CzsPCB concentration of 0.21 ng/L. In 1980, the western basin actually had significantly lower CPCB concentrations (0.74 ng/L) than both the central (1.2 ng/ L) and eastern (1.1 ng/L) regions of the lake at the 95% confidence interval (42). The burden, or mass of PCBs in Lake Superior reported in Table 3, is calculated as burden = vol X C P C B

(4)

where burden (kg) is the mass of PCBs in Lake Superior, the volume of Lake Superior is 1.21 X 1013m3, and CPCB

is the model-generated CPCB concentration (1978-1979) or the &PCB concentration (1980-1992). In 1980, the PCB burden was -28 700 kg (Table 3); by 1992, the PCB burden had decreased to -2150 kg, reflecting a decrease of -26 500 kg of PCBs between 1980 and 1992. PCBs in the surface waters of Lake Superior exhibit a pseudo-first-order rate constant, k, of -0.20 yr-l, leading to a half-life (tl/z) of 3.5 yr in the water column. The 95% confidence interval for the slope, based on the fit of the line, is -0.11 to -0.28 yr-l corresponding to tl/z values of 2.5-6.3 years. Pearson and Swackhamer (65)report PCBs in Lake Michigan waters, based on data collected 11 yr apart, have decreased from 1.2 ng/L in 1980 to 0.63 ng/L in 1991, corresponding to a rate of -0.059 y r l . The rate constant characterizing the decrease of PCBs in the surface waters of Lake Superior is comparable to pseudo-firstorder rate constants describing the decrease of PCBs in lake trout from Lake Superior and other species of fish collected from the other Great Lakes (35). DeVault (35) reported a rate constant of -0.13 yr-l corresponding to a t l p of 5.4 yr for lake trout (Saluelinus namaycush) in Lake Superior. Rodgers and Swain (30)calculated a rate constant for decrease in PCB concentrations of -0.12 yr-l in bloaters (Coregonus hoyi) in Lake Michigan from 1972 to 1980, while DeVault et al. (33) and Miller et al. (32) calculated rate constants of -0.18 (1974-1982) and -0.11 yr-l (1975-1990), respectively, for PCB decreases in lake trout in Lake Michigan. DeVault et al. (33)also calculated rate constants for PCB decreases of -0.47 and -0.26 yr-l in coho salmon (Oncorhynchus kisutch) from Lake Michigan and Lake Erie, respectively, for the years 19801984. Borgmann and Whittle (34)reported tl/z values of 8-12 yr describing the decrease of DDE, PCBs, mirex, and chlordane in Lake Ontario lake trout from 1977 to 1988, corresponding to k values ranging from -0.058 to -0.087 yr-l. Borgmann and Whittle (34) noted, however, that the R2 values in their study ranged from 0.40 to 0.48. The slower decline in Lake Ontario may be due to the continued impact of Niagara River inflow, urban and industrial activities on the lake, and chemical recycling from the bottom sediments. The rate constants calculated for fish from the Great Lakes and from the surface waters of Lake Superior and Lake Michigan are a result of decreased PCB loadings to the lakes and in-lake loss processes such as sedimentation and volatilization. The lower Great Lakes are directly impacted by major industrial centers and a concentrated population. On the other hand, Lake Superior is relatively unimpacted by point source loadings and receives the majority of its contaminant loading via the atmosphere (18, 66, 67). Thus, Lake Superior is representative of a continental atmospheric response to decreased PCB Environ. Sci. Technol., Vol. 28, No. 5, 1994 907

Table 4. 1990 and 1986 Lake Superior Sediment Cores core (no. of congeners)

focusing factop

corrected core inventory (ng/cm2)

surface PCB concn (ng/g)

accumulation rate (mg/cm-2y r l )

coordinates (latitude/longitude)

site DTL (82) site SJEII (82) NOAA site 3 (82) 1383d(82P 1387b(82P 139011391" (82)c

1.48 28.47 15.28 34 46'41', 1.26 8.76 7.78 27 47'02', 1.14 4.42 7.99 12 47'20', 3.21 5.89 13.25 39 47'39', 0.85 5.82 17.60 7.3 47'22', 1.79 4.55 7.98 20 46'45', Based on 15.5 pCi/cm2 unsupported zlOPb. Ref 44. Multiplied by a factor of 1.5 to adjust for 82 congeners. d Ref 50.

loadings, while the picture for the other lakes is more complex. Mass Balance

Atmospheric Deposition Wet 125 kdyr

Other dischiuges

IW32kd~ Total -157 kdyr

/

91'46' 91'18' 89'15' 87'58' 86'58' 84'47'

.. . -1900 kg/y

tkLhhWm

I

We have shown that the mass of PCBs in Lake Superior has decreased by -26 500 kg since 1980. Where did the PCBs go? Given the hydrophobic and particle-reactive nature of PCBs, a likely choice is the bottom sediments (40,681. Table 4 lists the important characteristics of six sediment cores collected in 1986 and 1990 (44,50-52). In order to answer the question 'Where did the PCBs go?', the mass of PCBs lost from the water column will be compared to the mass which has accumulated in the sediments. The mass of PCBs accumulated in the sediments is determined based on the areal concentration (ng/cm2) or core inventory. The areal burden or core inventory of PCBs [CPCB (ng/cm2)1was calculated as C P C B (ng/cm2)= C P C B , ( l - 4,)pBzi

(5)

where CPCBi (ng/g) is the PCB concentration in nanograms per gram of dry sediment in depth increment i, 4i is the porosity in increment i, and zi is the thickness of depth increment i. The core inventories have been corrected for sediment focusing by a focusing factor (FF). The FF was determined as the areal burden of unsupported 210Pbdetected in the core divided by the areal burden expected based on a historical deposition of 210Pbof 15.5 pCi/cm2 (69, 70). The FF corrects for the "focusing" of sediments from nondepositional to depositional zones of the lake. Lakewide core inventories (based on the sum of 82 PCB congeners) not corrected for focusing ranged from 5.0 to 42.1 ng/cm2 in cores collected in 1986 and 1990. Core inventories corrected for focusing ranged from 4.4 ng/cm2in the open lake to 28.5 ng/cm2in the Duluth basin. The total burden of PCBs in the sediments of Lake Superior based on an open-lake average core inventory of 5 ng/cm2 and the surface area of the lake (8.21 X 1O1O m2) is -4100 kg. Correcting for the higher inventories found in the western basin of the lake (Site DTL and SJE 111, the total burden of PCBs in the Sedimentsof Lake Superior is determined to be -4900 kg (51,52).The mass of PCBs accumulated in the sediments (4900 kg) since 1930 is significantly less than the mass of PCBs lost from the water column in just 1 2 yr from 1980 to 1992 (-26 500 kg). Therefore, the PCBs lost from the water column of Lake Superior since 1980 have not accumulated to a significant extent in the sediments. The other possible loss mechanisms which could explain the decrease of PCBs in Lake Superior are volatilization and degradation. Evidence does not exist to support PCB degradation in Lake Superior or any other oligotrophic, aerobic system exhibiting low ambient concentrations. Thus, we hy908

Environ. Sci. Technol., Vol. 28, No. 5, 1994

Rivers -b

-3000 kg/Y

Recycling -2890 kg/yr

-110 kg/p Burial

-60 kg/p

Sediment -4900 kg

Net Volatilization = Inputs - Outputs + M, 1st Order Loss Rate = -0.20yi' 1986MM=-1800kg Flgure 3. Mass balance of PCBs in Lake Superior, 1986.

pothesize that the loss of PCBs from the water column must be due to volatilization. To support this hypothesis and to further examine volatilizationas the major PCB loss process, a mass balance of PCBs in Lake Superior has been constructed to quantify the inputs and outputs (Figure 3). The mass balance is constructed about the water column in 1986 with the net mass lost, Mlost in 1986 determined to be -1800 kg

Mlost= M , - MoekAt where M , is the mass of PCBs in Lake Superior in 1986, k is the measured first-order rate constant (-0.20 yr-9, and At is equal to 1year. Mo is 10 100 kg in 1986 based on a CPCB water concentration of 0.84 ng/L. The value of 0.84 ng/L was determined by multiplying the measured &,PCB (35 congeners) value of 0.56 ng/L (43) by a correction factor of 1.5 to adjust to 82 congeners (eq 2). The inputs of PCBs to the water column of Lake Superior include wet and dry atmospheric deposition, riverine inputs, and industrial and municipal discharges. The loss processes of PCBs from the water column include riverine flow, sedimentation,degradation, and volatilization (Table 5). Chemical or biological degradation of PCBs is assumed to be negligible. Also included in Figure 3 are the vertical PCB fluxes derived from sediment trap studies which

-

Table 5. 1986 Lake Superior Inputs and Outputs of PCBs inputs

WYr

outputs

riverine othera dry deposition wet deposition total

110 41 32 125 -308

outflow sedimentation volatilizationb

kdyr

-

-

60 110 19ooc

2070

a Municipal and industrial discharges. Net volatilization = volatilization - absorption. c Net volatilization = inputs - outputs + M1mt (-1800 kg).

Table 6. Volatilization Fluxes of PCBs

0

location

volatilization flux (ng m-2 day-1)

Lake Superior Lake Superior Lake Superior Lake Superior Lake Superior Siskiwit Lake Green Bay Green Bay Lake Michigan Lake Michigan Lake Michigan Lake Ontario

63" 8.3b 19c 14lC(5 m/s = 11mph) 63d 23* 15-3OOf (1-3 m/s) 50-13OOf (4-6 m/s) 0-13 OOW 24oh 15d 81'

cm/s), 4 is the fraction of PCBs associated with particles, SA is the surface area of the lake (8.21 X 1O1O m2),and fd is the fraction of the year when it is not raining or snowing (0.9). Seasonal variations in PCB air concentrations as well as changes in the fraction of PCBs associated with particles due to the effect of temperature on vapor pressure were also incorporated into the model with the following results (67): season

PCB concn (ng/m3)

@

PCB flux (ng m-2 season-2)

summer fall winter spring

0.40 0.20 0.10 0.20

0.01 0.03 0.11

50 90 160 90

0.03

Dry deposition is estimated to contribute 32 kg/yr of PCBs to Lake Superior. Outputs. Riverine output of PCBs from Lake Superior is estimated to be 60 kg/yr due mainly to outflow via the St. Mary's River. This estimate is based on an outflow rate of 7.1 X 1013L/yr and a CazPCB water concentration of 0.84 ng/L in 1986. The loss of PCBs due to sedimentation is 110 kg/yr (Table 4) based on surficial sediment PCB accumulation rates in six dated sediment cores collected in 1986 and 1990 (44, 52-52). Volatilization. In the construction of the mass balance, volatilization was assumed to account for the difference between the inputs and outputs of PCBs to Lake Superior. The net volatilization (volatilization - gas absorption) estimate of 1900 kg/yr (63ng m-2 day1) for 1986 (Figure 3) was calculated as follows:

-

-

This study. Ref 85. Ref 23. Ref 37. e Ref 75. f Ref 13. g Ref

76. Ref 77. Ref 78.

reflect intense internal recycling (44). Internal recycling results in no net change in the burden of PCBs in Lake Superior, but influences the magnitude of PCB lossesfrom the water column due to sedimentation and volatilization. Inputs. The riverine input of PCBs to Lake Superior is estimated to be 110kg/yr, assuming a PCB concentration of 2 ng/L in all tributaries and a total water inflow rate of 5.4 X 1013L/yr. The estimate of 2 ng/L is based on PCB concentrations in rain in relatively remote, unimpacted regions of North America (12,67,71-74). Estimates based on minimum monitoring of 20 tributaries of Lake Superior and assuming a PCB concentration of 0.1 pg/g on suspended solids yields an input of 73 kg/yr (66),similar to our estimate of 110 kg/yr. Industrial and municipal effluents, runoff, and storm sewer overflows are estimated to contribute a combined total of 41 kg/yr of PCBs to Lake Superior (66). Eisenreich and Strachan (67)calculated wet deposition of PCBs to Lake Superior based on PCB concentrations in rain and snow (CPCBT,rain) of 2 ng/L and annual precipitation (P)of 76 cm: (7) where Fwet (mass/yr) is the flux of PCBs to Lake Superior due to precipitation and SA is the surface area of the lake (8.21 X 1O1O m2). The estimate of 2 ng/L in snow is based on limited snow concentration data in the Great Lakes region provided by Franz et al. (11). Input of PCBs due to wet deposition is estimated to be 125 kg/yr. Dry deposition of PCBs to Lake Superior was estimated (67) as

where FdIy is the dry depositional flux (mass/yr) of PCBs to Lake Superior,CPCBT,dr,is the total PCB concentration in the air, v d is the dry particle deposition velocity (0.2

-

net volatilization = inputs - outputs

+ Mlost

Baker and Eisenreich (23) calculated PCB volatization fluxes in still air of 19f 24 ng m-2 day1from Lake Superior surface waters in 1986 leading to an average of -570 kg/ yr of PCBs volatilizing. At a wind speed of 5 m/s, annual volatilization flux is -4200 kg/yr (23). Volatilization fluxes of PCBs from Siskiwit Lake, Isle Royale, in Lake Superior were estimated to be 23 ng m-2 day1 (751,leading to an annual volatilization loss of -690 kg/yr if extrapolated to Lake Superior. Table 6 lists recent volatilization flux estimates from areas throughout the Great Lakes region. By comparison, PCB volatilization fluxes in the oceans have been estimated to range from --140 to 40 ng m-2 day-l, with higher latitudes tending toward more negative (gas absorption) values (9). In the current study, volatilization accounts for -92% of the PCB losses from Lake Superior in 1986, while sedimentation accounts for - 5 % . This compares to the 87% and 11% lost by volatilization and sedimentation, respectively, calculated by Strachan and Eisenreich (37). PCB Congeners Since the magnitude of KO, (-104-108) and Henry's law constant (10-3-10-5.6atm m3mol-l) varies among PCB congeners,the loss of individual PCB congeners from Lake Superior should differ. Recent studies have shown that the less hydrophobic, more volatile PCB congeners are more effectivelyvolatilized from surface waters compared tomore hydrophobic, less volatile PCBs (13,231. We have shown that volatilization is the dominant loss process of Environ. Sci. Technol., Vol. 28, No. 5, 1994 SO9

-0.6

-0.5

1 1

,

1

k = -0.1SlogK,,

+ 0.66

1

0

R’=0.71

n

enon operating over time would result in the lower molecular weight PCBs decreasing at a slower rate in the water column than the higher molecular weight PCBs. Pseudo-First-Order Rate Constant Model To determine the roles of sedimentation, volatilization, gas absorption, and other processes in controlling the concentration of PCBs in Lake Superior, a pseudo-firstorder rate constant model was developed. The model is based on the following equation:

!@!=I dt

I

00 5

6

7

Log KO, Figure 4. Rate constants of PCB congeners in Lake Superior, 19801992.

PCBs from Lake Superior. We hypothesize that the lower molecular weight PCBs should be decreasing at a faster rate than higher molecular PCB congeners if external inputs are negligible. In Figure 4, the measured rate constants for the individual congeners listed in Table 2 are plotted versus the octanol-water partition coefficient (KO,), a measure of chemical hydrophobicity (79). The 95% confidence interval of the slope (-0.15 & 0.06) of Figure 4 indicates there is a significant difference in the rate of decrease between higher and lower molecular weight PCBs in the surface waters of Lake Superior, with the higher KO,congeners actually decreasing at a faster rate. The seasonal variations in PCB congener distributions discussed earlier contribute to the variation in Figure 4. Figure 4 demonstrates that higher molecular weight PCBs have decreased faster than the lower molecular weight congeners despite volatilizationbeing the dominant loss process. In order to interpret the trend observed in Figure 4 and support the hypothesis that lower molecular weight PCBs are more effectively volatilized, the input of PCBs to Lake Superior must be significant and dominated by lower molecular weight PCBs. Baker and Eisenreich (23) and Achman et al. (13) have shown that lower molecular weight PCBs are more important in air-water transfer from the atmospheric gas phase than higher molecular weight congeners (13, 23). This suggests that the absorption of PCBs may be an important input of PCBs to Lake Superior. PCBs transported over long distances in the atmosphere also contain relatively greater proportions of the lower molecular weight PCB congeners, as indicated by the PCB congener pattern over Lake Superior (23). Another factor contributing to the trend observed in Figure 4 is the differential settling of PCB congeners. Baker et al. ( 4 4 ) calculated recycling ratios (RRs) of PCB congeners in Lake Superior as the ratio of the surficial sediment accumulation rate in cores collected in 1986 and the settling fluxesof individual PCB congeners determined by sediment traps deployed in 1984-1985 (see Figure 1). The RR is defined as the downward flux of a PCB congener in the water column (sediment trap) divided by the accumulation of that congener in the surficial sediments (44). PCB congeners with lower KO, values exhibited higher recycling ratios (RR = -200-1000) than the PCBs with higher KO,values (RR = -20-100). This phenom910

Envlron. Scl. Technol., Vol. 28, No. 5, 1994

+ k’M

whereMis the mass of PCBs in the lake (kg),I is the input of PCBs to the lake including direct inputs and gas absorption (kg/yr), and k’ is the rate constant describing PCB losses from the lake (yr-l). The overall first-order rate constant, k [ k = k’ + I(t)/M(t)], describing the decrease in water concentrations of PCBs in Lake Superior is -0.20 yr-’ as determined earlier (Figure 2). In the construction of the model, k’,the rate constant describing PCB losses is defined as

where Itf (yr-l) is the first-order loss rate of PCBs due to flushing, ksed (yr-l) is the loss rate due to sedimentation, kvol (yrl) is the first-order rate constant describing the loss rate due to volatilization, and k d e g (yr-l) is the loss rate due to degradation. The rate constant describing the loss of PCBs due to flushing of the lake, kf,equals -0.006 yr-l, calculated as outflow from the lake (Q, 7.1 X 1O1O m3/yr) divided by the volume (1.21 X 1013m3). This rate constant contributes less than 3% to the overall rate constant of -0.20 yr-l describing the loss of PCBs from the water column in Lake Superior from 1980 to 1992. The rate constant describing the loss rate of PCBs from a lake due to sedimentation, ksed, is calculated for each of the PCB congeners from Table 2 and is calculated as

where Wsed is the mass sedimentation rate (mg cm-2 yr-l), INV, is the inventory or areal total suspended sediment matter (TSM) concentration (mg/cm2),fp is the fraction of the chemical existing in the particulate phase in the water, and RR is the recycling ratio of the PCB congener. WSed is estimated as 9.1 mg cm-2y r 2based on the average sedimentation rate determined in 11dated sediment cores collected between 1977 and 1990 (51, 52). INV, is calculated as the average suspended sediment concentration (0.2 mg/L) multiplied by the mean water depth (144 m). The RR of individual PCB congeners decreases with increasing KO,and ranges from 20 to 1000. The fraction in the particulate phase (fp) was determined based on the 1988-1992 water column PCB data as well as the 1986 PCB concentrations (50). These field data indicate that fp is independent of molecular weight (80). Thus, fp is determined to be 0.13 based on an average log K p (solid/ dissolved partition coefficient, L/kg) of 5.89. The values of ksed calculated using eq 11are shown in Table 7. The average ksed based on eq 11 is -0.004 yr-l, contributing approximately 2 5% to the overall k value of -0.20 yr-l.

-

that the values Of ksed are significantly less than the overall measured rate constants for individual PCB congeners in Lake Superior. This supports the earlier conclusion that the removal of PCBs by sedimentation cannot account for the actual losses of total PCBs observed in the water column from 1980 to 1992. Thus far, the rate constants determined for outflow and sedimentation cannot account for the observed loss of PCBs from Lake Superior. Assuming that degradation of PCBs is negligiblein Lake Superior, the only other process that can account for the observed PCB losses is volatilization. Based on the physical-chemical properties of PCBs, wind speed, and water temperature, an overall air-water mass transfer coefficient,Kol,can be calculated. The mass transfer coefficient can be estimated as a first-order rate constant describing volatilization as follows:

Table 7. kmd,KO], and kvolof PCB Congeners Determined for Lake Superior congener(@

005 + 008 017 016 + 032 022 033 + 053 + 021 028 031 041 + 071 + 064 044 037 t 042 047 + 048 049 056 t 060 066 + 095 074 070 + 076 101 099 110 + 077 118 141 138 + 163 146

-"O

ksed

(Yr')

-4.1 X 1V -1.4 x 10-3 -1.1 x 10-3 -2.4 X lo-" -5.4x 104 -4.7 x 10-4 -9.5 x 10-4 e -5.9 x l -1.5 x 10-3 -2.1 x 10-3 -8.2 x 10-4 -4.3 x 103 -3.6 x 10-3 -3.5 x 10-3 -1.8 x 10-3 -3.0 x 10-3 -7.1 x 10-3 -6.1 x 10-3 -7.1 x 10-3 -7.1 x 10-3 -0.021 -0.021 -0.021

KO]W d a y )

kvolW-9

0.14 0.14 0.14 0.12 0.13 0.14 0.14 0.12 0.12 0.11 0.13 0.13 0.10 0.11 0.11 0.11 0.10 0.09 0.08 0.07 0.08 0.03 0.05

-0.32 -0.32 -0.30 -0.27 -0.29 -0.30 -0.30 -0.27 -0.25 -0.24 -0.29 -0.29 -0.22 -0.25 -0.23 -0.23 -0.22 -0.21 -0.18 -0.14 -0.18 -0.07 -0.11

kvol

l/Kol RT/k,H + l / k , (14) I =

00

-0.5

Y

0.0

(13)

where Kol is the average yearly total mass transfer coefficient (m/yr), h is the depth of the lake (144 m), and f, is the fraction of the chemical in the dissolved phase. Using the stagnant two-film model (811,Kolwas calculated as follows:

h

v

= (Ko,/h)fw

ti-

l

where k, is the air-film mass transfer coefficient, k, is the liquid-film mass transfer coefficient, H is Henry's law constant (82),Tis temperature, and R is the universal gas constant. The mass transfer coefficients k, and k, were calculated on an individual congener basis in a similar fashion to Achman et al. (13). Themass transfer coefficient describing the rate of transfer across the air film, ka,pCB, is determined by correlation with the mass transfer coefficient of water vapor across the air film, ka,HsO. The value of k,&o is determined by the following empirical relationship (68): ka,HzO = 0.2U10

Figure 5. k,# as compared to the actual measured rate constants.

The rate constant due to sedimentation, kaed, may also be calculated based on PCB data collected from Lake Superior using the following equation: ACCUMpcBAt = INV, - INV, eXp(ksedAt) (12) where ACCUMpcB(pg m-2 yr-l) is the focusing-corrected accumulation rate of PCBs in the sediment, At is equal to 1 yr, and INV, (pg/m2) is the areal concentration of PCBs in the water column of Lake Superior. The present accumulation rate of PCBs in the surficial sediment of Lake Superior in the open waters is 0.84 pg m-2 yr-1 (51, 52). INV, was 46.0 and 25.9 pg/m2, respectively, for the years 1990 and 1992. The ksed values of -0.02 and -0.03 yr-l determined, respectively, for the years 1990 and 1992 are larger than ksed = -0.004 yr-' determined by eq 11. These differences may be due to an underestimation of f p for the higher molecular weight PCBs, which are present in the sediments of the lake, but rarely detected in the water column. Note, however, that all of the values determined for ksed are significantly less than the overall rate constant of -0.20 yr-l. In Figure 5, ksed values determined by the model (eq 11) are plotted with the measured k values from Table 2 (Figure 4). Figure 5 shows

+ 0.3

(15)

where UIO is the wind speed (m/s) 10 m above the water surface. The value for ka,PCB is then calculated ka,PCB

= ka,HzO(DPCB,eir/DHzO,air)

0.61

(16)

where DPCB,air and DHzO,air are the molecular diffusivities of a PCB congener and HzO vapor in air, respectively. The mass transfer coefficient describing the rate of PCB transfer through the water film, kw,pcB, is determined relative to the rate coefficient controlling the transfer of 6 0 2 across a stagnant water film (kw,coz).The following empirical relationship was generated based on experiments conducted with SFG,a volatile compound exhibiting only water-film resistance (83):

kw,co2= o.45U101'64 The value for kw,pCB is determined as

where SCPCB and Sccozare the respective Schmidt numbers for a PCB congener and COZ. Finally, an average yearly Kol was determined by correcting H on a semimonthly basis for measured water temperatures (84) and using Environ. Sci. Technol., Vol. 28, No. 5, 1994 911

f 0.5 ng/m3. In 1992, C , calculated by eq 21 is -0.2 ng/ m3. Actual measurements in 1992 for C82PCB in air over Lake Superior are 0.3 f 0.1 ng/m3 (85). The close agreement in calculated and measured air concentrations of C82PCB in 1986 and 1992support the assignment of the air-water mass transfer coefficients.

-1.0

I 6

5

7

Log K..

Figure 6. kvolas

compared to the actual overall measured rate

constants. seasonal wind speeds to determine semimonthly average values for u10 (85). Table 7 lists the calculated values of k,l and Kol for the 23 congeners listed in Table 2. Figure 6 demonstrates that the magnitude of kvol is more than sufficient to account for the measured rate constants describing the decrease in PCB congener concentrations in Lake Superior from 1980 to 1992. The average kvol is -0.24 yr-l. The calculated value for k’ is -0.25 yr-l (0.006 + 0.004 + 0.24). By use of the equation k = k’ + I ( t ) / M ( t ) , the inputs for any given year can be estimated. For example, setting k = -0.20 yr-l and using the 1986 PCB burden of 10 100kg, the input of PCBsin 1986is estimated to be 500 kg. Using the average calculated value of -0.24 yr-1for kvol, the mass of PCBs lost by volatilization can be estimated as follows:

-M

Mlost,vol -

-M 0

e-kvolAt

(20)

0

where Mlost,vor(kg) is the mass of PCBs lost from Lake Superior by volatilization over a given time period ( A t ) , and Mo (kg) is the mass of PCBs in the lake at a designated time zero. In 1986, Lake Superior contained 10 100 kg of PCBs. Setting M , at 10 100 kg and At equal to 1 yr, Mlwt,volin 1986is estimated as 2200 kg. The mass balance of PCBs in Lake Superior in 1986 found net volatilization to be 1900 kg, thus the magnitude of gas absorption must have been -300 kg (2200-1900 kg). Furthermore, based on the calculated values of Kol and using the average &PCB water concentration in 1986 (assuming 100% in the dissolved phase), the air concentration of PCBs above Lake Superior can be estimated using the following equation:

-

-

F = K,,(C, - C,RT/H) where F is the net volatilization flux (mol m-2 yr-9, C, is the measured water concentration (mol/m3),C , is the air PCB concentration (mol/m3), R is the universal gas constant (atm m3 mol-1 K-l), T is absolute temperature (298 K), H is Henry’s law constant (atm m3 mol-l), and Kol (40 miyr) is the average yearly overall mass transfer coefficient (Table 7). Then, using an average PCB molecular weight (302 g/mol), an average H for PCBs (1 X lo4 atm m3mol-I), and the net volatilization flux (1900 kg/yr = 7.2 X 10-8 mol m-2 yr-l), C, is determined to be -1.1 ng/m3 in 1986. Baker and Eisenreich (23) report &PCB ( f l standard deviation) air concentrations of 1.2 912

Environ. Sci. Technol., Vol. 28, No. 5, 1994

Conclusions PCBs have decreased at a rate of -0.20 yr-l in the surface waters of Lake Superior from 1980 to 1992. Due to the remote location of Lake Superior and the absence of significant point source loading to the lake, this decrease is representative of a continental decrease in the atmospheric loadings of PCBs. We have shown, using three different approaches, that volatilization is the dominant loss process of PCBs from Lake Superior. The first approach showed that the total amount of PCBs lost from the water column of Lake Superior between 1980 and 1992 was -26 500 kg, which is significantly greater than the -4900 kg accumulated in the sediments of Lake Superior since N 1930 (44,50-52). This indicates that only a small fraction of the PCBs lost from the water column is due to sedimentation. Secondly, based on a mass balance constructed about the water column in 1986,the dominant loss process of PCBs is again volatilization with 1900 kg of PCBs predicted to have volatilized in 1986,while 110 kg/yr are predicted to have been lost to the sediments. The third approach utilized a pseudo-first-order rate constant model which predicted volatilization to have a rate constant of -0.24 yr-l and sedimentation to have a rate constant of -0.004 yr-l for CPCB, again supporting the hypothesis that volatilization is the dominant loss process of PCBs from Lake Superior. The results of this study indicate that large oligotrophic lakes in the mid-latitudes do not always act as sinks for many hydrophobic organic compounds, but actually are sources in present times acting to increase the residence times of these chemicals in the atmosphere and buffering their concentrations. The net effect may be that the higher latitudes act as the ultimate sink for these chemicals due to the effect of lower temperatures on the air-water and air-terrestrial exchange processes (9, 86,87).

--

Acknowledgments We gratefully acknowledge the assistance and expertise of the Captains and crews of the RV Seward Johnson, the Johnson Sea Link II submersible (both, Harbor Branch Oceanographic Institution), and the R/V Lake Guardian, (U.S. EPA). We thank D. Achman for sharing her laboratory expertise, J. Hallgren and M. Simcik for their laboratory assistance, D. Engstrom for 210Pbdata, and E. Lipiatou for her advice. A special thanks to J. Baker and P. Cape1for providing essential published and unpublished data used in this work. This paper benefited immensely from two thorough, helpful reviews and exchanges with D. Mackay and P. Vlahos. This research was funded in part by the MN Sea Grant College (NOAA Office of Sea Grant) Publication No. 315, the National Underseas Research Center of NOAA (at the University of Connecticut), and the GLNPO/US EPA (Grants EPA R995233-01 and -02). One of us (K.C.H.) was supported by a Mott Foundation Fellowship through the International Association for Great Lakes Research. Literature Cited (1) Risebrough, R. W.; Rieche, P.; Herman, S. G.; Peakall, D. B.; Kirven, M. N. Nature 1968, 220,1098.

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Received for review September 2, 1993. Revised manuscript received January 17, 1994. Accepted January 26, 1994.' @Abstractpublished in Advance ACS Abstracts, March 1,1994.