Research Siskiwit Lake Revisited: Time Trends of Polychlorinated Dibenzo-p-dioxin and Dibenzofuran Deposition at Isle Royale, Michigan JOHN I. BAKER AND RONALD A. HITES* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
We have investigated the time trends of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/ F) in two dated sediment cores from Siskiwit Lake, a remote lake on Isle Royale in Lake Superior. Both cores indicated that atmospheric deposition of PCDD/F to Siskiwit Lake increased sharply in the 1930s and peaked in the late 1970s at around 9.5 pg cm-2 yr-1. Since then, atmospheric deposition to Siskiwit Lake has continued to decline. Analysis of the top 1.0 cm of these cores showed that in 19961998, the PCDD/F flux to Siskiwit Lake was about 4.5 pg cm-2 yr-1. We compared this sediment-derived flux to a soilderived flux from Ryan Island, the largest island in Siskiwit Lake, and found a similar value. Thus, the atmospheric deposition of PCDD/F has decreased about 50% since the late 1970s. This decline is slower than might be expected as a result of combustion source regulations alone. Examination of the PCDD/F homologue distributions throughout the sediment cores showed that the relative percent of the hepta- and octachlorinated dibenzo-pdioxins to total PCDD/F increased with time. We propose that de novo synthesis of these compounds from pentachlorophenol in the atmosphere explains these data.
lake sediment cores is a less time-consuming method of determining time trends and is a valuable supplement to air monitoring. As lipophilic pollutants enter a lake, they travel rapidly through the water column and deposit in the sediment (2). If the sediment has been relatively undisturbed, it can be cored, sectioned, and analyzed to estimate the time course and the quantitative input of a specific pollutant, such as PCDD/F, into that lake as a function of time. The absolute assignment of dates to the different layers in a sectioned sediment core can be done over the last 100 yr using a variety of radioisotopic techniques (5-7). Thus, one can develop historical data on the inputs of pollutants to the lake. To determine only atmospheric deposition, sediment from a lake with relatively low direct inputs from the shoreline or from rivers and streams should be collected. Since atmospheric deposition would be the primary source of pollutants in such a lake, one could subsequently infer atmospheric trends from its sediment. In the past, we have found sediment from Siskiwit Lake, a remote lake located on Isle Royale in Lake Superior, to give particularly valuable information on the history of PCDD/F deposition (1). The water level in Siskiwit Lake is 17 m higher than Lake Superior, and there are no significant anthropogenic sources of PCDD/F in the drainage basin of this lake (1, 8). Therefore, atmospheric deposition is the primary source of contamination to this lake (1, 2, 8). Previous work in our laboratory measured PCDD/F in sediment cores obtained in 1982 (1, 2) from Siskiwit Lake. The concentrations of PCDD/F were near zero before about 1935, and these concentrations maximized in about 1970 before declining to about 80% of their maximum values. We thought it would be useful to sample the sediment in Siskiwit Lake 16 yr later to see if this decline had continued. This extended period allowed us to find evidence for a noncombustion-related source of PCDD to the atmosphere. We also sampled soil from Ryan Island, the largest island in Siskiwit Lake, and compared our sediment-derived and soil-derived PCDD/F fluxes to validate the use of soil samples for estimating current PCDD/F deposition.
Introduction
Experimental Section
Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) are emitted into the environment by the combustion of municipal, hospital, chemical, and industrial waste (1, 2); thus, efforts to reduce the levels of these compounds in the environment have focused on emission control technology (3). The implementation of such technology should lead to decreasing PCDD/F emissions and eventually to the virtual elimination of these contaminants from the environment. To monitor the effectiveness of pollution control technology and to investigate half-lives of banned chemicals, long-term air-monitoring programs such as the Integrated Atmospheric Deposition Network have been implemented (4). Knowing air concentrations is necessary to estimate the stock of pollutants in the atmosphere, and long-term monitoring of these concentrations is necessary to investigate the time trends of this stock. Unfortunately, this method of monitoring concentration trends can take several years for the collection and analysis of the many air samples necessary before accurate time trends can be extracted from the data. Alternately, the analysis of dated
Sediment Collection. Two sediment samples were obtained in June 1998 from sampling sites 3 and 5, as shown in Figure 1. A rowboat equipped with a depth finder was used to find the deepest part of the lake (site 5) and a large deep flat location (site 3). Sediment cores were collected using a commercial piston corer (WildCo Wildlife Supply, Saginaw, MI); this is a stainless steel tube lined with a 5 cm diameter polycarbonate tube. A winch was used to slowly lower the corer to the bottom of the lake. A messenger was then dropped to trigger a suction cup, which plugged the top end of the core tube and served to hold the sediment undisturbed inside the core tube. Following collection, the sediment was extruded by hydraulic pressure and sectioned in 0.5-cm intervals to a depth of 5 cm and then in intervals of 1 cm to a depth of 10 cm. The rest of the core was sectioned in 2-cm intervals. The outside edge of each core section was scraped off to minimize smearing effects. At each site, three sediment cores were sectioned and composited in order to obtain a large enough sample for analysis. A fourth core was taken and sectioned at each site for radioisotopic lead dating. Core sections were placed in precleaned amber jars and stored at -18 °C until analyzed.
* Corresponding author e-mail:
[email protected]. 10.1021/es991280p CCC: $19.00 Published on Web 06/09/2000
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FIGURE 1. Map of sampling locations. Sediment samples were taken from sites 3 and 5. The site numbers were retained from previous work (8). The soil sample was from Ryan Island, the largest island in Siskiwit Lake (20). Soil Sampling. A soil sample was collected from Ryan Island (see Figure 1) using a soil corer (Clements Associates Inc., Newton, IA) fitted with a 30 cm long by 2.3 cm diameter PETG Copolyester core tube. The soil sample was collected to a depth of about 15 cm to ensure recovery of the entire organic layer. Following collection, the core tube was capped and wrapped in aluminum foil. Upon arrival in the laboratory, the soil sample was stored at -18 °C until analyzed. Details of the extraction and cleanup procedures are described elsewhere (9). Quantification of PCDD/F in the soil was the same as for sediment; see below. Extraction and Analysis. Each sediment section was removed from the freezer and mixed thoroughly after being warmed to room temperature. Between 10 and 20 g of wet sediment was weighed and transferred to a 500-mL beaker, where the sediment was mixed with enough precleaned anhydrous Na2SO4 to obtain a loose, friable mixture. Between 1 and 2 g of each section was transferred to a 50-mL beaker, weighed, and placed in a drying oven overnight at 100 °C. The dried sediment was then weighed to obtain the percent dry weight of the sample. The sediment/Na2SO4 mixture was spiked with 100 pg each of [13C12]-1,2,3,4-tetrachlorodibenzofuran, [13C12]-1,2,3,7,8-pentachlorodibenzofuran, [13C12]1,2,3,6,7,8-hexachlorodibenzo-p-dioxin, [13C12]-1,2,3,4,6,7,8heptachlorodibenzo-p-dioxin, and [13C12]-1,2,3,4,6,7,8,9octachlorodibenzo-p-dioxin (Cambridge Isotopes, Inc.) to act as internal standards. The samples were then transferred to a glass Soxhlet thimble and extracted with 50% hexane in acetone for 24 h. The extracts were reduced to a volume of about 2 mL, and any residual water was removed using Na2SO4. The extract was then solvent exchanged to hexane and reduced to about 0.5 mL for silica cleanup. Pre-extracted silica gel (Davidson Chemical, Baltimore, MD) was activated at 160 °C for 24 h, allowed to cool, and then deactivated with 1 wt % HPLC grade water. A 15-mm i.d. glass column fitted with a Teflon stopcock was then filled with 15 cm of the silica in a hexane slurry. A 1-cm layer of anhydrous Na2SO4 (Fisher Scientific, Pittsburgh, PA) was placed on top of the silica to absorb residual water in the extract. The sample was then loaded on the column and eluted with 75 mL each of hexane, 15% dichloromethane in hexane, and dichloromethane. The flow rate of these columns was approximately 0.5-1.0 mL/min. PCDD/F eluted in the first two fractions, so these two fractions were combined, solvent exchanged 3 times with 50 mL of hexane each, and reduced to a volume of approximately 200 µL for alumina cleanup. All solvents were purchased from EM Science, Gibbstown, NJ. 2888
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A 5 × 95 mm Pasteur pipet was dry loaded to a height of 6.5 cm with neutral alumina (Brockman Activity I, Costa Mesa, CA), which had been activated by heating it at 160 °C for 24 h, and topped with 0.5 cm of anhydrous sodium sulfate. The 200-µL extract from the silica cleanup was loaded onto the column and eluted with 8 mL each of hexane, 2% dichloromethane in hexane, and 40% dichloromethane in hexane. The PCDD/F were contained in the 40% fraction, so this fraction was reduced to a volume of less than 25 µL under a gentle stream of nitrogen before analysis by GC/MS. PCDD/F were quantitated using the internal standard approach, and analyses were performed on a HewlettPackard 5973 GC/MS operating in the electron capture, negative ionization mode. Chromatographic separation was achieved using a 3-m, DB-5MS, capillary column (250 µm i.d.; 0.25 µm film thickness; J&W Scientific, Folsom, CA). Helium was used as the carrier gas. The sample (2 µL) was injected with a 25 psi pulsed injection in the splitless mode. Temperature programming was as follows: isothermal for 2 min at 110 °C, 30 °C/min to 210 °C, 2 °C/min to 280 °C, isothermal for 10 min. The flow of the reagent gas, methane, in the mass spectrometer’s ion source was maintained at 40% of the total flow (2 cm3/min), which gave a manifold pressure of 2 × 10-4 Torr. The ion source temperature was held at 150 °C. Two ions from each homologue group were monitored using selected ion monitoring to enhance sensitivity, and a peak was only classified as a PCDD/F if its mass spectral intensities were in the correct, predicted isotopic ratio (10). Individual peak areas from each homologue group were added together to give the total area for each PCDD/F homologue from which we subsequently calculated the total mass for each PCDD/F homologue. Total PCDD/F represent the sum of the tetra- through octa PCDD/F homologue group concentrations. Detection limits for individual PCDD/F congeners were on the order of 0.1 pg/g dry sediment. One procedural blank sample was included with each set of extractions. In this case, 100 pg of each of the internal standard compounds was spiked onto 50 g of Na2SO4, and this “sample” was taken through the same extraction and cleanup procedure as described for the sediment samples. No PCDD/F were detected in any of the blank experiments. Recoveries were measured by spiking a known concentration of PCDD/F, with congeners representing each homologue group, onto 50 g of Na2SO4. These samples were then taken through the same extraction and cleanup procedure as described for the sediment samples; however, the internal standard was not spiked onto the extract until just before analysis. PCDD/F recoveries were between 80 and 120%. Sediment Fluxes and Inventories. Both cores were dated using a constant flux, constant rate of supply model using the activity of 210Pb in the core sections (5-7). The dating results from site 5 revealed that the dated core was not representative of the composite used for PCDD/F analysis. The sedimentation rate calculated from this core was much lower (by about a factor of 5) than previous studies of this lake (11) and much lower than the sedimentation rate from site 3. Since the sedimentation rates and the focusing factor for core 3 closely resembled the dating from an earlier core taken at site 5 (11), we assumed these were the same at both sites and applied the dating results from site 3 to the composite core from site 5. The mass sedimentation rate was 0.018 g cm-2 yr-1 for the first 6.0 cm and 0.007 g cm-2 yr-1 below 6.0 cm. The inventory of unsupported 210Pb in the dated sediment core was 23.0 pCi/cm2. Since the expected inventory in this region is 15.5 pCi/cm2, we calculated a focusing factor of 1.48 (12). Multiplying the concentration of PCDD/F in each sediment section by the mass sedimentation rate and then dividing this value by the focusing factor gave PCDD/F fluxes normalized to lake surface area as a function
TABLE 1. PCDD/F Concentrations versus Average Core Section Date for Cores 3 and 5a 1998
1997
1994
1992
1988
1984
1980
1975
1967
1954
1935
1910
1888
52 41 27 20 15
