Environ. Sci. Technol. 1996, 30, 1429-1436
PCBs in Lake Michigan Water Revisited ROGER F. PEARSON,† K E R I C . H O R N B U C K L E , ‡,§ S T E V E N J . E I S E N R E I C H , ‡,| A N D D E B O R A H L . S W A C K H A M E R * ,† Environmental and Occupational Health, School of Public Health, Box 807 UMHC, 420 Delaware Street SE, University of Minnesota, Minneapolis, Minnesota 55455, and Gray Freshwater Biological Institute, University of Minnesota, P.O. Box 100, Navarre, Minnesota 55392
PCBs were determined in water samples from 11 open water sites in Lake Michigan and compared to data from 1980. PCB concentrations declined from 1980 to 1991 with a first-order rate constant calculated to be -0.078 yr-1. The most southern and southwesterly sites sampled were higher in concentration and exhibited different homolog profiles and partitioning behavior than samples from other sites. The organic carbon-normalized partition coefficients were not correlated to Kow. A simple mass budget about the water column for 1991 using current measurements of PCB inputs and outputs shows a significant imbalance of PCBs and indicates the need for further study of the roles that subregional atmospheric deposition and sediment-water interactions play as sources to the lake.
Introduction During the middle part of this century, mixtures of polychlorinated biphenyls (PCBs) were manufactured for large scale use in a number of industrial applications (1, 2). Environmental occurrences of PCBs were first reported in 1966, and they were soon established as one of the most widely distributed of the chlorinated aromatic pollutants (2-4). With increased evidence of ecological contamination by PCBs and concern for potential human health effects, their domestic production was voluntarily reduced in the early 1970s and banned in 1977 (5). PCBs are hydrophobic, semivolatile organic compounds that degrade slowly and tend to bioaccumulate (6, 7). Because of their extensive use in the Great Lakes basin, PCBs are of major concern to regulatory agencies that oversee the water quality of the lakes. While concentrations * To whom correspondence should be addressed. Telephone: (612)626-0435; fax: (612)626-0650; e-mail address: dswack@ mail.eoh.umn.edu. † School of Public Health. ‡ Gray Freshwater Biological Institute. § Present address: Department of Civil Engineering, University at Buffalo, State University of New York, Buffalo, NY 14260. | Present address: Department of Environmental Sciences, Rutgers University, New Brunswick, NJ 08903.
0013-936X/96/0930-1429$12.00/0
1996 American Chemical Society
of PCBs in predator fish have declined since their ban, consumption advisories are still in place for many species. It is hypothesized that water concentrations in the Great Lakes have also declined; however, the last spatial sampling of the open waters of Lake Michigan was done in 1980 (8). Mass balance models have been proposed as possible regulatory tools that could be used to adequately predict ecosystem responses to changing inputs of contaminants (9). In order to calibrate the models and to evaluate their parametrization, adequate data are needed on concentrations in inputs, outputs, and the water column. One objective of this study was to determine the current spatial distribution of PCB concentrations in the open waters of Lake Michigan and to use these data to evaluate time changes in PCB concentrations. Specifically, the concentrations are compared to the 1980 study of Swackhamer and Armstrong (8) to estimate a rate constant for the change of PCB concentration in water with time. A second objective was to use these data, in conjunction with sediment, atmospheric, and tributary concentrations, to construct a mass budget of PCBs for Lake Michigan. This budget is used to evaluate our current understanding of sources and sinks of PCBs to the lake.
Experimental Methods Field Collection. Samples were collected in September 1991 from the U.S. EPA R/V Lake Guardian (Bay City, MI) from 11 open water stations. Site locations and coordinates are given in Figure 1. These sites correspond to long-term monitoring sites established by the U.S. EPA Great Lakes National Program Office for nutrients, water chemistry, and physical limnological measurements. Dissolved and particulate phases were sampled at mid-epilimnion (8 m). Water was brought to the surface by a submersible pump through pre-equilibrated tygon tubing. Samples taken sideby-side using nylon tubing produced similar results (10). Water was subsampled by peristaltic pump (flow rate ca. 4 L/min) from the outflow of the submersible pump (flow rate 15 L/min) and was passed via Teflon tubing through a 293-mm pre-ashed (450 °C for 12 h) GF/F filter (Whatman; 0.7 µm nominal cutoff). Sample volumes were approximately 100 L, and material retained on the filters was operationally defined as the particulate phase. Filters were removed from the filter head, folded in quarters, wrapped in pre-ashed aluminum foil, and stored at -20 °C prior to extraction and cleanup. Filtrate (dissolved phase) was collected in 70-L Teflon-lined stainless steel tanks and extracted using continuous flow liquid-liquid extraction (see below). Sample Extraction and Cleanup. Organics were extracted from the filtered water using a continuous flow liquid-liquid Goulden large-sample extractor (GLSE), the detailed design of which is reported elsewhere (11, 12). Dichloromethane (DCM) was the extraction solvent. During the extraction, surrogates (3,5-dichlorobiphenyl, IUPAC No. 14; 2,3,5,6-tetrachlorobiphenyl, IUPAC No. 65; and 2,3,4,4′,5,6-hexachlorobiphenyl, IUPAC No. 166) were metered continuously in methanol carrier to monitor extraction efficiency. After the desired volume (100 L) of filtered lake water had been extracted, the mixer was shut off and the DCM phase was removed to a pre-ashed 1-L liter amber
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FIGURE 1. Summary of 1991 PCB concentrations in Lake Michigan. All samples were taken at mid-epilimnion depth (∼8 m). Ct is the concentration of total PCB in the water column; Cw is the concentration of total PCB in the dissolved phase; Cp is the concentration of total PCB in the particulate phase.
bottle along with 2 × 25 mL rinses of the extraction vessel. The samples were stored at 4 °C prior to cleanup and analysis. Sample filters were Soxhlet extracted for 12 h in 1:1 hexane:acetone (v/v). Prior to extraction, surrogate recovery standards were added to the extraction solvent (same compounds as for GLSE). The extract was transferred to a separatory funnel, the organic layer removed to a KudernaDanish (KD) apparatus, and any water layer was extracted once with 50 mL of hexane. The organic phases were combined after passing through a funnel containing ashed glass wool and anhydrous sodium sulfate (heated at 450 °C for 8 h). The extracts were solvent-exchanged to hexane and volume reduced to about 3 mL using a Snyder column and steam bath. Interferences were removed by passing the extracts through columns (1 cm × 12 cm) of 2 g of ashed anhydrous sodium sulfate over 5 g of 100% activated (450 °C for 4 h) neutral alumina (Brockman activity I) over 1 g of ashed anhydrous sodium sulfate. Extracts were eluted with 4 × 25 mL of 2% DCM in hexane (v/v). The column eluents were solvent-exchanged to hexane by evaporation over a steam bath using a KD-Snyder apparatus as above and volume reduced to 3 mL for storage at -20 °C prior to analysis. The GLSE extracts (dissolved phase) were transferred to separatory funnels, and the DCM phase was drained through a funnel containing ashed glass wool and anhydrous sodium sulfate to a KD apparatus. The DCM extract was solvent-exchanged to hexane and then processed in the same manner as the filter extracts.
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Gas Chromatography. Extracts were reduced to approximately 100 µL, and internal standards (2,4,6-trichlorobiphenyl, IUPAC No. 30, and 2,2′,3,4,4′,5,6,6′-octachlorobiphenyl, IUPAC No. 204) were added just prior to GC analysis. Samples were analyzed by capillary column gas chromatography (60 m × 0.25 mm, 0.25 mm film thickness J&W DB-5 column; Hewlett-Packard 5890 GC) with 63Ni electron capture detector and chromatographic data system (Maxima 820, Waters-Millipore). Operating conditions were as follows: carrier gas, H2, 40 cm/s; makeup gas, 5% methane in argon, 25 mL/min; injection port, 225 °C; variable temperature ramp, 100 °C to 280 °C over 170 min; detector at 325 °C. Quantitation. Quantitation of individual congeners was done by the internal standard method, and concentrations are reported corrected for surrogate recovery. Congeners with two and three chlorines were normalized to PCB surrogate 14, congeners with four and five chlorines were normalized to PCB surrogate 65, and congeners with six through nine chlorines were normalized to PCB surrogate 166 (13, 14). Response factors relative to internal standards were updated daily, and a calibration standard was quantitated daily to determine the accuracy of the calibration curves. The identification criterion for a given congener was that the retention time relative to the internal standard was (0.1%. Approximately 85 individual congeners or congener groups were analyzed. Suspended particulate matter (SPM) was determined gravimetrically by filtering bulk water through preweighed polycarbonate membrane filters (Nuclepore 0.4 µm). Particulate organic carbon (POC) measurements at each station were provided by the U.S. EPA (10). The average coefficients of variation for duplicate analysis of SPM and POC were 11 and 27%, respectively. Quality Control. A procedural blank was included with every five samples. Two GLSE field blank samples were included and had negligible PCBs. No particulate field blanks were included in the Lake Michigan study. However, laboratory filter blanks were negligible, and our sample results were comparable to those generated by EPA in parallel to ours (10). Two of the 11 sites were sampled in duplicate. For the GLSE samples, the average coefficient of variation (CV) for the duplicates was 15% for total PCB (sum of congeners), and the average CV for individual congeners was 62%. For filter sample duplicates, the average CV was 49% for total PCB and averaged 140% for individual congeners. The average recovery of three spike recoveries (300-600 ng of total PCB spike) was 75%. Surrogate recoveries averaged 75% for procedural blanks and filter samples and were slightly lower (57%, significant at p < 0.05) for the GLSE samples. Values reported for total PCB and individual congeners reflect surrogate recovery-corrected concentrations. Congener-specific limits of detection (LOD) were established for both media. Limits of detection were defined to be (15)
LOD ) X h + 3σ
n)7
where X h and σ denote the average area and standard deviation of seven injections of matrix blanks. Congener detection limits were about 0.5 pg/L for the dissolved phase and 1.0 pg/L for the particulate phase. For data reduction
FIGURE 2. PCB homolog concentrations at stations 11 and 17 and all stations exclusive of stations 11 and 17 of (a) the dissolved and (b) the particulate phases.
purposes, masses calculated from areas below the LOD were equated to zero.
Results and Discussion PCB Concentrations. Total PCB concentrations at each sampling site were determined by summing contributions of individual congeners. Total PCB ranged from 0.34 to 1.7 ng/L in the 11 open-water surface samples, and the lakewide total PCB concentration was 0.64 ( 0.43 ng/L. Concentrations at stations 11 and 17 (see Figure 1 for location) were significantly elevated (p < 0.01) compared to the other nine stations. The total PCB concentration was markedly constant if stations 11 and 17 were excluded and averaged 0.47 ( 0.06 ng/L. The nine stations (omitting 11 and 17) had a very similar congener composition. The average homolog concentrations for these stations and for stations 11 and 17 in the dissolved and particulate phase are shown in Figure 2. Homolog concentrations of the dissolved phase at stations 11 and 17, compared to the other nine stations, were elevated in all homologs except the di- (p < 0.10). Only the di- and tetra-homolog concentrations were significantly greater in the particulate phase. Figure 3 shows the percent homolog composition, calculated as a given homolog concentration divided by the total PCB concentration, multiplied by 100. The average percent homolog distributions for stations 11 and 17 relative to the average of the other stations show the dissolved phase to be comprised of a slightly different homolog composition, with the distribution maximum shifted toward the penta- and hexahomologs as compared to the remainder of the stations, which favored the tri-, tetra-, and penta-homologs (Figure 3). Comparison of the particulate homolog distribution does not show clear differences. In general, the lower chlorinated homologs (di- though tetra-) contribute more
FIGURE 3. Percent PCB homolog distributions at stations 11 and 17 and all stations exclusive of stations 11 and 17 of (a) the dissolved and (b) the particulate phases.
to the distribution at stations 11 and 17 than at the remainder of the stations. Because of their location, it seems possible that influence from the urbanized southern and southwesterly shores of the lake could account for the nonhomogeneity of concentrations and homolog profiles observed at stations 11 and 17 compared to the remainder of the sample sites. Concentrations of PCB measured in the air near Chicago have been found to be 20-100 times greater than those measured in more remote areas (16, 17). The prevailing winds during the sampling at stations 11 and 17 were southerly, and the observed vapor PCB concentrations measured concurrently at those sites were a factor of 2-5 times those measured over the remainder of the lake (16). The increased vapor and dissolved aqueous PCB concentrations at stations 11 and 17 perhaps indicate a dynamic interaction with an atmospheric source. If dynamic exchange with the vapor PCB were controlling the dissolved PCB, similarity in the homolog distributions of the two phases would be expected. This was not observed. The vapor phase homolog distributions were similar in all locations of the lake, and they were less similar to the pattern observed for dissolved phase samples at stations 11 and 17 and more representative of those observed at the remainder of the sampling sites (Figure 4). This observation argues against vapor phase control of the concentrations and profiles observed at stations 11 and 17 but does not exclude potential influence of air particle deposition. A Chicago study indicated that the majority of the measured PCB depositional flux to southern Lake Michigan was attributed to PCB associated with large air particles with high depositional velocities (18). During our sampling cruise, no attempt was made to determine large particle PCB concentrations, and the filter-retained material from
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TABLE 1
Comparison of 1980 and 1991 PCB Concentration Data from Open Waters of Lake Michigan cruise average 1980 1991a total PCB (ng/L) particulate PCB (ng/g) dissolved PCB (ng/L) % of total as dissolved SPM (mg/L) log Kd
1.2 431 0.56 50 1.46 5.85
0.47 142 0.34 75 1.01 5.39
significance level (p)b
