Environ. Sci. Technol. 1997, 31, 2903-2909
Concentrations, Accumulations, and Inventories of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans in Sediments of the Great Lakes ROGER F. PEARSON,† D E B O R A H L . S W A C K H A M E R , * ,† S T E V E N J . E I S E N R E I C H , ‡,§ A N D DAVID T. LONG| Environmental and Occupational Health, School of Public Health, Box 809 UMHC, University of Minnesota, Minneapolis, Minnesota 55455, Gray Freshwater Biological Institute, University of Minnesota, Navarre, Minnesota 55392, and Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824
The chronology of PCDD/F accumulation was determined in sediment cores from Lakes Superior, Michigan, and Ontario and two small “control” lakes. Accumulation rates began increasing about 1940, increased to maxima at 1970 ( 10 yr, and then declined to the present rates that are 30-70% of the maxima. The observed accumulations ranged from 90% of their current PCDD/F inputs from subregional atmospheric (air parcels having higher concentrations of PCDD/F than remote air due to local sources) and/or non-atmospheric sources. We also compared the sedimentary accumulations to estimates of atmospheric deposition. The PCDD/F accumulation rates in the control lakes were similar to atmospheric deposition of PCDD/Fs from remote air in the Great Lakes basin. Atmospheric deposition from suburban air can support the accumulation of PCDD/F in southern Lake Michigan but some non-atmospheric sources to northern Lake Michigan are implicated. Lake Ontario may be receiving >70% of its current inputs of PCDD/F from non-atmospheric sources.
Introduction Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/ F) enter the environment primarily as a result of anthropogenic activities. Many of the congeners bioaccumulate and are considered potent toxicants capable of producing a wide spectrum of adverse health effects in biota and humans such as immunotoxicity, chloracne, carcinogenicity, reproductive and developmental toxicity, disruption of the endocrine system, induction of enzymes, and antiestrogenic effects (1). * To whom correspondence should be addressed; phone: (612) 626-0435; fax: (612) 626-0650; e-mail:
[email protected]. † School of Public Health. ‡ Gray Freshwater Biological Institute. § Present address: Department of Environmental Sciences, Cook College, Rutgers University, New Brunswick, NJ 08903-0231. | Michigan State University.
S0013-936X(97)00101-6 CCC: $14.00
1997 American Chemical Society
As a chemical class, they occur as byproducts of chemical manufacturing and incineration processes (1-3). It is currently thought that emissions from incineration of medical, municipal, and chemical wastes provide the major source of PCDD/F to the environment (4). The Great Lakes represent a significant resource of freshwater in the United States and Canada and have been found to be particularly sensitive to contamination by hydrophobic organochlorine compounds such as PCDD/F. Based on minimal sampling, PCDD/Fs have been found in varying concentrations in Great Lakes fish (5), sediments (6), and waterfowl (7). If incineration does provide the major source of PCDD/F to the environment, it follows that atmospheric delivery is the dominant input route of PCDD/F to the Great Lakes. However, the variability observed in sediment and fish concentrations implies that localized source(s), either atmospheric or non-atmospheric, may be important. To help establish strategies that will effectively control the inputs of PCDD/F to the Great Lakes, it is important to identify and characterize their past and current input fluxes and to determine how the lakes have responded to changing inputs historically. The sedimentary history of PCDD/F provides direct evidence of what the relative inputs of PCDD/F to lakes currently are and how they have changed over time. Comparison of the sedimentary records among the Great Lakes and control lakes, which can receive inputs from only atmospheric sources, provides evidence for the importance of long-range regional atmospheric sources as compared to more localized atmospheric or non-atmospheric sources. PCDD/Fs are very hydrophobic chemicals (Kow values range from 106.1 to 108.2) (8). A significant fraction of the total burden of PCDD/F in the water column (independent of source) will be adsorbed to particles and delivered to the sediments even at the relatively low particle concentrations (0.5-10 mg/L) in the waters of the Great Lakes (9). Thus, the chronology of PCDD/F preserved in lake sediments reflects the relative changes in the flux of the chemicals to the lakes over time (10, 11). The purpose of this study was to determine the concentrations, accumulations, and inventories of PCDD/F in a series of dated sediment cores taken from three of the Great Lakes: Superior, Michigan, and Ontario. Cores were also taken from two control lakes located in a remote area of the Great Lakes Basin, which could receive inputs of PCDD/F from only the atmosphere. From the sediment records, our objectives were to (a) determine the concentration of PCDD/F over time in each of the lake cores; (b) determine the dates of onset and maximum accumulation of PCDD/F in each core; (c) determine the rates of accumulation and changes in the same over time and from that assess how the relative rates of inputs have varied spatially and temporally in the Great Lakes; (d) determine the total mass of chemical sequestered in the sediments of the Great Lakes; (e) estimate the relative importance of atmospheric inputs of PCDD/F to the Great Lakes by comparison to the control lakes.
Experimental Methods Sample Collection. Sediment cores were taken (1991-1994) from three of the Great Lakes and two control lakes (Figure 1). The two control lakes were Siskiwit Lake (an inland lake from Bayfield Peninsula, WI, near the Apostle Islands of Lake Superior) and Outer Island Lake (on Outer Island of the Apostle Islands). The sites in the Great Lakes included two cores from Lake Superior, a core off Basswood Island of the Apostle Islands and a core from a representative depositional zone; three cores from Lake Michigan, two in the northern
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FIGURE 1. Locations, mass sediment accumulation rates, mixing depths, and focusing factors of the sediment cores. depositional zones and one in the southern depositional zone; and three cores from Lake Ontario, one each from the western, central, and eastern depositional zones. Tube cores (i.d. 6.7 cm) from the control lakes and the Basswood site were taken by a scuba diver courtesy of the U.S. Geological Survey. Cores from the depositional zones of the Great Lakes were taken by surface deployment of a box corer (70 × 30 × 40 cm) from the U.S. EPA R/V Lake Guardian or by using a box corer (40 × 15 × 15 cm) deployed from the Johnson Sea Link II or Clelia submersibles (Harbor Branch Oceanographic Institute, Ft. Pierce, FL). Three to five subcores (i.d. 6.8 cm, length 30-40 cm) were taken from each of the box cores and sectioned to get adequate time resolution. The subcores were sectioned using a hydraulic extruder aboard ship within 2 h of sample collection, and the sections were placed into solvent-rinsed glass jars with aluminum foil-lined lids and stored at 4 °C in the dark until analysis. To minimize smearing effects during subcoring, sediment in contact with the sides of the tube was trimmed and discarded during sectioning. One set of subcores was used for 210Pb dating, and the remainder was used for chemical analyses. Subsamples of the sections of the USGS cores were taken for 210Pb dating. Extraction and Analysis. Sediment samples (10-50 g wet weight) were mixed with pre-ashed anhydrous sodium sulfate (ca. 7:1 wt/wt) and extracted by Soxhlet apparatus with a 1:1 (v/v) mixture of acetone and hexane (Fisher optima grade) for 24 h. All samples had a mixture of 13C-labeled dioxins and furans containing one 2,3,7,8-substituted PCDD/F congener for each of the homologs (EDF-957-A, Cambridge Isotope Laboratories, Cambridge, MA) added as surrogates prior to extraction (1.0-5.0 ng of each congener). Following extraction, the extracts were solvent reduced and exchanged to hexane (∼1 mL) using a Kuderna-Danish apparatus. Interferences were removed using liquid-solid chromatography columns (i.d. 1.5 cm × 25 cm) containing 2 g of ashed sodium sulfate over 10 g of activated silica gel over 1
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g of ashed sodium sulfate. The columns were eluted with 75 mL of hexane followed by 75 mL of 30% (v/v) DCM in hexane. The fractions were combined, volume reduced, exchanged to hexane (∼1 mL), and loaded onto a microcolumn (Pasteur pipet) containing 0.5 g of 1% water-deactivated neutral alumina. The extracts were eluted with 8 mL of 2% DCM in hexane (v/v) followed by 8 mL of 40% DCM in hexane (v/v). These fractions were combined, solvent reduced, exchanged to DCM, and passed over a microcolumn of acid-cleaned copper filings to remove sulfur interferences. These extracts were exchanged to hexane and volume reduced to ∼100 µL by gently passing purified nitrogen over the sample. Prior to instrumental analysis, all extracts were spiked with 2,2′,3,4,4′,5,6,6′-octachlorobiphenyl (IUPAC PCB No. 204) or 13C-labeled 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin as internal standards. All analyses were performed on a Hewlett-Packard 5988A gas chromatographic mass spectrometer (GC/MS) with the mass spectrometer operated in the electron capture negative ionization mode (ECNI) after the method of Czuczwa and Hites (10). The GC used a 60 m × 0.25 mm, 0.25 µm film thickness, fused silica capillary gas chromatographic column (J&W DB-5MS) with helium (∼40 cm/s linear velocity) carrier gas. The operating conditions of the GC and MS have been described elsewhere (12). PCDD/PCDF quantitation was done on an individual peak basis by monitoring the most intense ion of the molecular ion chlorine isotope cluster to the nearest 0.1 ( 0.1 amu (selected ion monitoring mode) for each chemical class homolog in the appropriate retention time window. An additional ion of the molecular ion chlorine isotope cluster of each homolog was also monitored for confirmation. The ions monitored, confirmational criteria, and protocol for establishing the retention time windows are reported elsewhere (12). Quantitation of samples was done by the internal standard method using response factors determined daily. Individual peak concentrations were summed within a given homolog class to arrive at the homolog total, and the homolog totals were summed to arrive at total PCDD and PCDF. Method recoveries as determined by spikes of PCDD/F to deep sediment samples shown to be free of analytes averaged 72 ( 31% before normalization to the appropriate 13C surrogate recovery and 98 ( 26% after surrogate normalization. All data have been corrected to surrogate recoveries. Sample surrogate recoveries averaged 80 ( 20% (n ) 107). The coefficient of variation (CV) for duplicate analysis of core sections (n ) 4 duplicates) averaged 18% for ∑PCDD/F and 44% for individual homologs. For reporting purposes, congeners detected at masses below the method detection limit were taken as zero. Sediment Dating and Focus-Correcting. The core sections were dated using a rapid steady-state mixing model based on the activity of 210Pb in the core sections (13, 14). PCDD/F accumulation rates were corrected for sediment focusing using a focusing factor (FF), which is the ratio of the observed unsupported 210Pb inventory in a given core to that expected in this geographic region (15.5 pCi/cm2) (15, 16). Mass sedimentation rates, mixing depths, and focusing factors are given in Figure 1.
Results Concentrations and Accumulations of PCDD/F. Concentrations and accumulations vs core section date are shown for each of the cores in Figures 2 and 3, respectively. Control Lakes. The two control lakes are located away from urban activity and receive their input of PCDD/F from only atmospheric deposition. The concentrations are low in both lakes (0.3-1.1 ng/g) as are the accumulation rates (0.007-0.03 ng cm-2 yr-1). Both are greater in Siskiwit Lake than in Outer Island by about a factor of 2-3. The onset of appearance of PCDD/F in Siskiwit Lake was earlier than 1954 ( 5 yr based on the age of the deepest section analyzed. The
FIGURE 2. Total concentrations (ng/g) of polychlorinated dibenzop-dioxins and dibenzofurans (∑PCDD/F) in sediment cores taken from the Great Lakes and control lakes. The locations of the cores are given in Figure 1. first appearance of PCDD/F in the Outer Island core occurred at 1937 ( 6 yr. After onset, the Outer Island core had constant concentrations and accumulation rates throughout the core. Concentrations and accumulation rates in the Siskiwit core increased by less than a factor of 2 from the deepest section to a subsurface maximum at 1980 and then declined slightly at the surface. The observed surface concentrations are similar to those previously reported for the Siskiwit Lake located on Isle Royale in Lake Superior (6, 17), but that core exhibited more temporal change than did these cores. Lake Superior. The concentrations of PCDD/F in the depositional core (NOAA3) are 5-10 fold higher than in the nearshore Basswood core. Their accumulation profiles are more similar because of the higher mass sedimentation rate at the nearshore site (Figure 1). The Basswood core was collected off the Apostle Islands, proximate to the control lake cores, and has concentrations and accumulations throughout the core similar to those of Outer Island. The first appearance of PCDD/F was observed at 1922 ( 14 yr. The NOAA3 site has been shown to be a representative sediment depositional site for the accumulation of compounds from the open waters of Lake Superior (18-20). The appearance of PCDD/F in the NOAA3 core was first observed in the deepest section of the core analyzed, estimated at 1909 ( 9 yr. Actual dating of this core was not done, and the reported dates were estimated by applying the mass sedimentation rates from a core extracted at the same coordinates 4 yr prior (21). A mixing depth of 1.0 cm would place the date of the deepest core section as recent as 1932 ( 9 yr. Following onset, PCDD/F increased to a maximum concentration of 4 ng/g at 1973 ( 4 yr and subsequently declined to the surface concentration of 0.97 ng/g. The maximum concentration is elevated over any of the control lake concentrations, but the accumulation rates in the NOAA3 core are similar to those
FIGURE 3. Focus-corrected accumulations (ng cm-2 yr-1) of ∑PCDD/F in sediment cores taken from the Great Lakes and control lakes. See text for discussion of focus-correcting and Figure 1 for the sitespecific focusing factors. of the Siskiwit Lake core. The NOAA3 and Basswood cores have similar accumulations of PCDD/F in early and recent times (∼0.01 ng cm-2 yr-1), but the NOAA3 core has increased accumulation rates in the 1950-1970 horizon. Lake Michigan. The two northern cores (LM-68k and LM47s) had nearly identical surface concentrations and accumulation rates of PCDD/F, which were 1.5-2 times that of the LM-18 southern core (Table 1). PCDD/F were first detected at 1906 ( 9 yr in the northernmost core, LM-68k, and at 1926 ( 4 yr in the other northern basin core, LM-47s. Concentrations increased linearly in the LM-68k core to 3.0 ng/g by 1960 and then were constant to the surface. The increase was exponential in the LM-47s core reaching a maximum concentration of 4.5 ng/g in 1973 ( 2 yr and then declining by about 30% to its surface value. In the southern basin core, the onset occurred at 1935 ( 4 yr. Concentrations increased to a maximum at 1963 ( 4 yr (3.3 ng/g), declined by ∼10% over the next 20 yr, and then decreased to the surface concentration of 2.1 ng/g. These data are consistent with concentrations previously reported for two sediment cores taken from Lake Michigan in 1982 (6). Lake Ontario. Detectable concentrations of PCDD/F were found in the deepest sections analyzed in all three of the cores (before 1838 ( 25 yr for LO-19, before 1936 ( 3 yr for LO-40a, and before 1911 ( 4 yr for LO-E30). The western core, LO-19, had both a significant mixed depth (2.3 cm) and two distinct eras of constant sedimentation rates. The combination of these two factors places the date of the deepest section as recent as 1900. PCBs, DDT, mirex, and HCB were all detected in the LO-19 core at dates significantly earlier than what would be predicted based on their production and use curves; but, in the LO-E30, LO-40, and two other cores, onset was consistent with their use history (22). This indicates that the observance of PCDD/F at the early date in the LO-19 core may be a core-specific bias. However, samples of
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TABLE 1. Summary of ∑PCDD/F Sediment Concentrations, Accumulations, and Inventoriesa
control lakes Siskiwit Outer Island Lake Superior Basswood NOAA3 Lake Michigan LM-68k LM-47s LM-18 Lake Ontario LO-19 LO-40 LO-E30
onset date b
date of max accum
max concn (ng/g)
surface concn (ng/g)
max accum (ng cm-2 yr-1)
surface accum (ng cm-2 yr-1)
inventory (ng/cm2)
