Environ. Sci. Techno/.1995, 29, 2090-2098
Estimatiw the A m h e r i c
Diknzogdbxins and Dibenrofurans from Soils LOUIS P. BRZUZY AND RONALD A. HITES' School of Public and Environmental Affairs and Department of Chemistiy, Indiana University, Bloomington, Indiana 47405
Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) were studied in soils to determine if, and under what conditions, soil acts as a conservative matrix for the collection of atmospheric deposition. Studies of four soil cores showed that 80% of the PCDD/F load was contained in the top 15 cm of the core. Concentrations were highly correlated with organic carbon, indicating that sorption to organic carbon is the dominant mechanism. Study of two other cores did not exhibit this behavior due to low soil organic carbon and to heavy PCDD/F loading from the atmosphere. Soils and lake sediments were collected in similar geographic regions, and the soil-derived and lake sediment-derived PCDD/F fluxes were compared. The percent relative standard deviation of four soil-lake sediment pairs ranged from f l %to f26%, and the fluxes ranged from 180 to 990 ng m-2 yr-l. Comparison of homologue profiles also showed good agreement. Analysis of field duplicate soil samples gave percent relative standard deviations ranging from f5% to f59% with fluxes ranging from 2 to 470 ng m-* yr-l, The results of this study suggest that soils can be used to estimate the deposition of PCDD/F from the atmosphere. This will allow us to easily expand the worldwide database of PCDDF flux estimates.
Introduction Chlorinateddioxinsand dibenzofurans (PCDDlF)are wellknown environmental contaminants that have received prolonged attention by the scientific community and by environmental regulators. PCDDlF enter the atmosphere primarily as combustion byproducts from incineration, metal production, andautomobile emissions (1-1 1). These compounds are semivolatileand hydrophobic;hence, they accumulate in organic rich media such as soils, sediments, and biota (12-20). In spite of our knowledge about the environmental behavior of PCDDlF, a significant problem remains: Deposition from the global atmosphere is estimated to be roughly20 times higher than estimated atmospheric inputs * E-mail address:
[email protected].
2090 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 8, 1995
from known sources (2,211.There are three explanations for this mass balance discrepancy: (a) The depositional fluxes are overestimatedbecause of samplingbias. (b)The input estimates are low because of unrecognized sources. (c)Both the sources are underestimated, and the deposition is overestimated. We believe that depositional fluxes are overestimated. Most PCDDlF measurements have been made in places of significant human industrial activity; 54% of such measurements are from northern and central Europe, 21% are from Canada, and 14% are from the United States east of the MississippiRiver. Deposition estimates may only apply to these regions, and they may not be representative of parts of the world less impacted by industrial activity. Clearly, an expanded set of PCDDlF measurements from many geographical regions must be established before a realistic assessment of their mass balance can be made. Measuring PCDDlF deposition directly is difficult, and there are few such measurements in the literature (22,231. Instead, deposition is estimated from concentration measurements in other environmental matrices (16, 17,2426). For example, concentrations of PCDD/F in air and precipitation (usually rain) are used to estimate dry and wet deposition, respectively (25,26). Atmospheric deposition can also be estimated from PCDDlF concentrations in lake sediments (16, 17). This technique is based on the partitioning of hydrophobic substances onto aquatic particulatematter, which is rapidly deposited with velocities of 0.5-2.0 m day-' (27).If the compound is stablein the sediment,a record of atmospheric deposition is preserved. A sediment-derived flux yields a s u m of both wet and dry deposition. Each of these techniques has limitations. Using air and rain to estimate deposition requires that a large number of measurements be made over an extended period of time (at least 1 yr) to obtain an average deposition estimate. Sampling for shorter times can result in substantial errors due to the climatic factors that influence PCDDlF air and precipitation concentrations. In addition, transporting, operating, and maintaining air and precipitation sampling equipment at remote locations is not practical. A major limitation in using lake sediments to estimate global deposition is that lakes are not uniformly distributed worldwide. Furthermore,collectingundisturbed sediment cores from lakes in remote regions is difficultand expensive. In order to estimate fluxes on an expanded geographical scale, we need a technique that will overcome these limitations. This technique must rely on an environmental matrix that (a) is uniformly distributed both regionally and globally, (b) is straightforward to collect and handle, and (c) acts as a conservative matrix for atmospheric PCDDlF deposition. Soils may meet these criteria. Soils are uniformly distributed, and they are relatively easy to collect and handle; however,it is not clear if they are a conservative matrix for PCDDlF deposition. If soils are conservative deposition collectors, then a predictable sorption mechanism of PCDDlF to soils must exist,percolation of PCDDlF through the unsaturated zone or losses by erosion must not be significant, and the degradation of PCDDlF in soils must be minimal. Let us address these criteria sequentially.
0013-936x/95/0929-2090$09.00/0
0 1995 American Chemical
Society
The deposition of PCDDlF to soils occurs by the processes of wet and dry deposition. For each process, the PCDDlF can be in two phases. In the case ofwet deposition, PCDDlF are dissolved in the precipitation, and they are associated with atmospheric aerosols scavenged by the precipitation. In the case of dry deposition, PCDDlF are deposited to soils by vapor-phase diffusioninto the soil, or they are associated with particles that deposit to soils by gravitational settling or impaction. M e r deposition, vapor-phase and dissolved-phase PCDDIF movement into soils is controlled by the equilibrium sorptionldesorption processes between the soil compartments (air, water, mineral, and organic matter). For nonpolar organic compounds, the equilibrium favors sorption to the organic carbon in the soil (28). Soil distribution coefficients (&) for PCDDlF can be predicted from GWvalues, which range from about 106-108 for the various PCDDlF (29). Using these values and organic carbon concentrations as low as 1%, we calculate & values from lo4 to lo6 for the tetra- through octachlorodioxins and furans. These high distribution coefficients suggest that PCDDlF sorb strongly to soils with typical organic carbon concentrations. Because PCDDlF are nonpolor and nonionic, the pH of the soil will not affect the sorption process. Predictingthe movement of particle-associated PCDDlF through soils is not as clear. The movement of small particles through a particular soil will be highly dependent not only on the organic carbon content of the soil but also on the nature of the particle itself and on the soil texture. Generally PCDDlF are associated with aerosols in the atmosphere that are between 0.1 and 1.0pm in diameter (30). This type of particle, once deposited,can bind strongly to the organic matter in soil. Also, the better a soil retards the movement of small particles, the better it will retain particle-associated PCDDlF deposition. For example, a soil with a course sandy texture may allow movement particles deep into the soil as water moves through it. Clearly, these vapor- and particle-phase deposition processes are not completely decoupled in the soil matrix. Once a particle deposits to the soil, the associated PCDDlF can partition from the particle to the various soil phases. Also vapor- and dissolved-phasePCDDlF that sorb to soil particles can then be transported through the soil associated with soil particles. Limited studies of PCDDlF behavior in soil indicate that these compounds are highlyimmobile once deposited (3135). This behavior has been observed by Hagenmaier etal. (34)in soils contaminatedthrough atmosphericdeposition. Other studies have concluded that PCDDlF sorption to soils may approach irreversibilitydue to the encapsulation of the compounds in the soil organic and mineral matter (35)* Losses due to erosion or degradation of PCDDlF in the soil must also be considered. Loss of PCDDlF due to erosion of the soil can occur, but in a soil stabilized by vegetation, such losses are generally much less than 1% per year (27).Degradation of PCDDlF in soil is considered to be slow or nonexistent under natural conditions. Hagenmaier et al. (34) showed that PCDDlF are stable for at least 9 yr in soils. Other half-life estimates for PCDDlF in soils are highly variable, but they are generally in the range of 25-100 yr in subsurface soil and 9-15 yr in the top 1 mm of soil (33).
Based on these considerations,it seemedlikely that soils could be used to measure the deposition of PCDDlF from the atmosphere. However, given the complexity of the interactions of atmospherically deposited PCDDlF with soils, we designed and conducted a series of field experiments to test this supposition. The experiments were designed to test both the accuracy and precision of our proposed technique. We measured the concentration of PCDDlF in soil as a function of depth to evaluate their mobility in the soil. To determine the accuracy of using soils to estimate fluxes, soil flux estimates were compared to lake sediment-derived fluxes (an accepted technique) from similar geographical areas. To test the precision of our method, field duplicates were collected and analyzed in order to reveal possible errors resulting from variability of the soil matrix and to test the reproducibility of our sample collection protocol.
Experimental Methods Sample Collection. (A) Soil Collection. Soil cores were collected with a 2.54 cm 0.d. x 30.48 cm soil core probe (Clements Associates Inc., Newton, IA) using precleaned stainless steel liners. Approximately 1 m long cores were collected in three sections. The first section was collected by pushing the probe into the soil to a depth of 30 cm. If the soil was hard, the corer had to be forced into the soil with the aid of a rubber mallet. Once the corer was to the proper depth, it was twisted to break the soil loose and slowly brought to the surface. The stainless steel liner, containing the sample, was removed and capped at both ends. A new liner was placed in the sampler, and the sampler was reintroduced into the hole and inserted to a depth of 60 cm. The process was repeated to collect the third section to a depth of approximately 90 cm. Soil cores were collected near Shingleton, Grayling, and Verona, MI; Mitchell, IN; and on the island of Guam. The stainless steel tubes containing samples were transported to the laboratory and immediately frozen. Once the sample had frozen completely, it was removed from the freezer and allowed to thaw just enough to let the core slip easily from the tube. The partially frozen core was then cut into 2-6 cm sections. The sections were weighed and placed in precleaned glass jars. They were then stored at -18 "C until analyzed. Bulk soil samples were collected in 475 mL (8.25cm diameterby8.25cmhigh) precleaned glass jarswithTeflonlined lids. The jar opening was impressed into the soil coveringan area of 53.4 cm2. Alternatively,an area of known dimension was marked off. The area was dug out to a depth that corresponded to the organic layer depth of the particular soil type (the A horizon). Generally, this depth was between 5 and 15 cm. In areas where an appreciable A horizon could not be identified, soil to a depth of 10 cm was collected. Once samples were collected, they were stored in a cool dark place until delivered to our laboratory. Once in the laboratory they were stored at -18 "C until analyzed. All soil samples were collected from areas that had not been disturbed (to the best of the collectors knowledge) for the past 75yr. Additionally,landscape considerationswere taken into account: Areas of potentially significant erosion were avoided (that is, steep sloped areas). Flat, highly vegetated areas were targeted when possible. (B)Sediment Collection. Lake sediment sampleswere collected using a 28 L Eckman box sampler Widlife Supply VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2091
TABLE 1
Location, Sampling Infomation, Classification, and Properties for Soils from Four Geographical Regions typical A horizon propartias location
lat.
Lewis and Clark National Forest, MT Bigfork, MT Superior, MT Missoula, MT Bear Tooth Pass, WY West Yellowstone, WY Shelly, ID Pocotello, ID Georgetown, ID Raft River, ID
48.35 48.04 47.1 1 46.52 44.58 44.39 43.22 42.52 42.29 41.55
Barnsville, MN Shingleton, MI Grayling, MI
long.
texture
pH
% OC
Northern Rocky Mountain 113.40 bulk Orthic Podzol 114.04 bulk Orthic Podzol 114.53 bulk Dystric Cambisol 113.59 bulk Haplic Kastanozem 109.28 bulk Luvic Kastanozem 111.06 bulk Orthic Podzol 112.07 bulk Calcic Kastanozem 112.26 bulk Calcic Kastanozem 111.2 bulk Calcic Kastanozem 113.25 bulk Calcic Kastanozem
silt loam silt loam loam sandy loam loam silt loam clay clay clay clay
3.3 3.3 5.3 7.1 6.4 3.3 7.9 7.9 7.9 7.9
2.0 2.0 2.0 2.0 4.0 2.0 2.0 2.0 2.0 2.0
46.23 46.23 44.39
Upper Michigan 96.25 bulk Mollic Gleysol 86.30 core Gleyic Podzol 84.42 core Eutric Podzuluvisol
clay loamy sand silt loam
7.0 4.5 5.7
2.0 10.0 2.0
Wyoming, MI Verona, MI
42.54 43.48
Lower Michigan 85.42 bulk Gleyic Podzol 83.00 core Gleyic Podzol
loamy sand loamy sand
4.5 4.5
10.0 10.0
Lake District, near Sickle Tarn
54.30
m ENVIRONMENTAL SCIENCE &TECHNOLOGY
soil classification
Lake District, United Kingdom 3.00 bulk
Company, Saginaw, MI). Additional weight (10 kg) was added to the sampler to ensure penetration into the sediment. When possible, a boat equipped with a depth finder was used to identify the flat, soft areas of a lake most suitable for sedimentcollection. The samplerwas attached to a winch assembly and slowly lowered to the lake bottom to minimize the disturbance of the sediment-water interface. A "messenger" was used to trip the springactivated jaws, causing them to clamp shut. The sampler was then winched to the surface. If the sediment surface was covered with clear water and sufficient material was collected, two or three subcores were taken using 6.5 cm i.d. x 25 cm polycarbonatetubes. The tubes were capped, tape sealed, stored upright in ice, and transported to shore. After collection, the cores were chilled overnight before sectioning. Cooling thickened the core material, making the sectioning process more precise. Cores were sliced into 1 cm sections using an apparatus designed by Kemp et al. (36). The sections were placed in precleaned glass jars with Teflon-linedlids and transported to the laboratory where they were stored at -18 "C until analyzed. (C)Soiland Lake Sediment GeographicalReglons. Soilderived and lake sediment-derivedflux estimates from four regions were compared. We designated these regions the northern Rocky Mountains region, the Upper Michigan region, the Lower Michigan region, and the Lake District of the United Kingdom region. Northern Rocky Mountains Region. Sediment cores were collected from Swan Lake in northwest Montana and from Yellowstone Lake in Yellowstone National Park, Wyoming. One core was collected from each lake. The geometric average PCDD/F flux of these two lakes was compared to the geometric average flux of 10soil samplescollected from a region between the two lakes, an area of approximately 50 000 km2. Lower Michigan Region. The geometric average flux of PCDDlF for Lake Huron estimated from four sediment cores studied by Czuczwa and Hites (16) was compared to the geometricaverage flux estimated for the soil core collected 2092
type
/ VOL. 29, NO. 8,1995
muck
near Verona, MI, and to a bulk soil sample collected near Wyoming, MI. Flux estimates to the soil cores were determined by treating the cores as bulk soil samples collected to a depth of 15 cm. UpperMichigan Region. The flux of PCDD/F to Siskiwit Lake, Isle Royale, MI, estimated from the data in Czuczwa and Hites (16) was compared to the geometricaverage flux estimated for the two soil cores collected near Shingleton and Grayling, MI. Lake District of the United Kingdom Region. A sediment core fromwmdermere Lake was analyzed for PCDDIF, and an average flux was estimated. This was compared to a soil-derivedflux for a bulk soil collected in the Lake District, near Sickle Tarn Lake. Table 1 lists the locations of the soil samples collected, how they were collected (bulk or core), soil classification, and representative properties of the A horizon. These properties were compiled from the Food and Agriculture Organization World soil database (37). (D) Field Duplicates. All duplicates were collected as bulk samples. They were collected either at the same time by the same individual or at different times by different individuals. Some duplicates were collected within less than 50 m of each other, while others were collected up to 80 km from each other. Analytical Methods. (A) Sample Preparation. All samples were mixed, and in the case of soils, any organic material was ground with a mortar and pestle. Between 2 (sediment) and 25 g (soil) of each sample was transferred to a beaker and mixed with enough preextracted Na2S04 (Fisher Scientific, St. Louis, MO) to make a loose, friable mixture. Between 0.04 and 0.1 ng of [13C1~-1,2,3,7,81pentachlorodibenzofan and between 0.8 and 1.6 ng of [13C~~1~ctachlor~dibenz~-p-dioxin (Cambridge Isotopes, Cambridge, MA) were then added to the mixture to serve as internal standards. The samples were then transferred t o a glass Soxhlet thimble and extracted with 300 mL of 2-propanol (EM Science, Ralway, NJ) for 24 h and then with 300 mL of dichloromethane (EM Science) for an additional 24 h. The two fractions were combined, rotary
evaporated to less than 2 mL, solvent-exchanged (three times with 75 mL) into hexane (EM Science), and concentrated to less than 5 mL. (B)Extract Fractionation. Fractionation procedures followed to isolate the PCDDlF from interferingsubstances were those used by Czuczwa and Hites (16). Briefly, preextracted silica gel (100-200 mesh Fisher Scientific,St. Louis, MO) was activated at 160 "C for 16 h, deactivated with 1%water by weight, and equilibrated for 16 h. It was then loaded into a 1.3cm i.d. x 25 cm column in a hexane slurry to a height of 20 cm. The silica was capped with 1 cm of Na2S04. The samplewas transferred onto the column and eluted with 75 mL each of hexane, 15% dichloromethane in hexane, and dichloromethane. PCDDlF eluted in the second fraction. The solvent from this fraction was exchanged (three times with 50 mL) into hexane and reduced to less than 1 mL total volume. Alumina (BrockmanSuper ActivityI, 50-200 mesh, ICN Biomedicals, Inc., Costa Mesa, CA) was activated for 12 h at 160 "C. The alumina was loaded dry into a 0.5 cm i.d x9.5 cm Pasteur pipette to a height of 6.5 cm. The alumina was cappedwith 0.5 cm of Na2S04, and wettedwith hexane. The sample was transferred onto the column and eluted with 8 mL each of hexane, 2% dichloromethanein hexane, and 40% dichloromethanein hexane. The PCDDlF eluted in the third fraction, which was concentrated to less than 50 pL by slowly passing purified nitrogen over the sample. (C) Analysis. All analyses were performed on a HewlettPackard 5985B G U M S system operated in the electron capture (EC) mode. The gas chromatographon the system had been upgraded to a Hewlett-Packard 5890 Series 11. Chromatographicseparations were achieved using a 30 m x 250pm i.d. DB-5MS fused silica column (5%phenyl, 95% dimethylpolysiloxane stationary phase, JSCW Scientific, Folsom, CA) with helium as the carrier gas flowing at a linear velocity of 22 cmls measured at 200 "C. Samples were injected into the column through a splitlsplitless injector operated in the splitless mode. The followingtwostep linear temperature program was used. A n initial oven temperature of 40 "C was held for 2 min then ramped at 30 "Cl min to a temperature of 210 "C. The oven temperature was then ramped at 2 "Clmin to a final temperature of 285 "C and held for 10 min. The transfer line between the GC and the MS was held at 300 "C, and the ion source temperature was maintained at 150 "C. The pressure of the reagent gas, methane, in the ion sourcewas maintained at 0.43 Torr, as measured by a probe-mounted capacitance manometer. For maximum sensitivity, care was taken to be sure that the system was leak free to avoid the formation of [(M - C1 01-1 ions. The mass spectrometer was controlled by an HP Series 1000computer running RTE revision F software. Data files collected on the RTE system were translated to a DOScompatibleformat and transferred to a personal computer (PC)using Hewlett-PackardChemLan software. Data were then further reduced on the PC using Hewlett Packard Chemstation software version G1032C. (D)Cesium-137 Measurements. In order to determine the sedimentation rates for the various lakes studied, the sediment core sections had to be dated. This was accomplished using the Cs-137 dating technique (38,39). Between 2 and 4 g of dried sediment was ground to a powered consistency using a mortar and pestle. The powdered sediment was then packed into a 4 cm diameter weighmg bottle. This diameter was chosen to match the
+
effectivediameter of the detector. Cesium-137 disintegrations, measured by the detection of y-rays at 661 KeV, were counted for 24-48 h depending on the sample size and on the concentration of Cs-137. The detector was a lithiumdrifted germanium crystal connected to a multichannel analyzer. (E) Organic Carbon Measurements. Organic carbon was measured using a LECO 6000 CIS analyzer (LECOCorp., St.Joseph,MI) equippedwithaninfrared detector. Samples were prepared as follows: Soil samples were dried in an oven at 105"C for 24 h. They were then powdered to a fine consistency. Approximately 0.25 g of sample was weighed out and placed in a 250 mL plastic beaker. Removal of inorganic carbon (carbonates)was accomplished by adding about 50 mL of 0.1 N HC1to each beaker and allowing them to sit for 24 h. The samples were then filtered through Whatman GFlF filter paper and rinsed with about 250 mL of deionized water. The filters were dried for 24-48 h at 60 "C, and then combusted in the instrument. Percent carbon was determinedby comparisonof detector response to a calibration curve generated by the combustion of standards of known organic carbon content. All samples were analyzed in duplicate. (p) Quantitation. PCDDlF were quantitated by homologues. In this case, a "homologue"is the sum of the concentrations of each isomer at a particular level of chlorination. Thus, there are 10 homologues for the tetrachloro- through octachlorodioxinsand dibenzofans. For increased sensitivity, selected ion monitoringwas used. Quantitation was performed by finding the ratio of the appropriate peak area to that of the internal standard and correcting for relative response factors that were obtained from running a standard mixture of PCDDlF. The [13C121pentachlorodibenzofan internal standard was used to quantitate the tetra through hexa homologues, and the [13C1~]octachlorodioixn was used to quantitate the hepta and octa homologues. Overall reproducibility of the method was &30%. The quantitation procedure is described in detail in ref 16. Convertinga soil concentrationto a deposition estimate is straightforward. First, we assume all the PCDDlF in the soil has been collected. Second,the surfacearea fromwhich the soil was collected has been measured. Third, the mass of the entire sample collected has been determined. Once a soil concentration of PCDDlF has been measured, the depositional flux is calculated by the following equation: flux (ng rn-' yr-') = crn/(At) were cis the PCDDlF concentration (ng g-9, m is the mass of soil collected (g),A is the area of sample collected (m2), and t is the time of accumulation (yr). It is clear from sediment core data that PCDDlF accumulation in the environment began around 1935, and this trend is highly correlated to the rise in the commercialuse of chlorinated compounds worldwide (16,17).From these data, we can set t at 60 yr. Sediment fluxes were estimated by the methods outlined in ref 40.
Results and Discussion Soil Core Studies. Figure 1 shows the soil PCDDlF concentrations as a function of depth for the three Michigan cores and the Guam core. PCDDlF concentrations approach detection limits at about 20-25 cm in depth. The Guam core was only collected to a depth of 18 cm, and VOL. 29, NO. 8, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY 2099
cone (WQ) 100 150
Y 50
0
0 6 12
18 A
E 24
10000
20000
0
250 500
750 1000
i :: 42
48 54
83
63
72
72
60 Shingleton, Michigan
Verona, Michigan
fW9)
0 5
cone
0 20 40 60 8 0 1 0 0 1 2 0
15
20
40
fwQ) 30 40 50
i
1:
0.0
12
18
Guam
-g
75 50 0 25
3
6
8
1 2 i 6
0.0
0.4
0.8
1.2
1.6
2.0
Percent Organlc Carbon
Percent Organlc Carbon Verona.. MlChlMn -
Guam
70
0
2
4
6
8
Percent Organlc Carbon
10
0.0
0.1
0.2
0.3
0.4
Percent Oganlc Carbon
FIGURE2. Total PCDDFconcantration versus percent organic carbon for soil cores from Michigan and Guam. All correlations are significant at the 95% level.
concentrations are near detection limits at this depth. Cumulative inventories of PCDDlF can be computed by multiplyingthe PCDD/F concentrations by the total weight of each core section. In all cases, greater than 80% of the total PCDDlF inventory is found in the upper 15 cm of soil. Figure 2 shows the soil core PCDD/F concentrations as a function of organic carbon content. As the plot shows, the two variables are highly correlated. All correlation coefficients are significant at the 95% significance level. This indicates that organic carbon is an important factor in PCDD/Fsorption to soils for both the vapor- and particlephase components of deposition. It is also important to notice that organic carbon content does not have to be high to observe this behavior: the organic 2094
0,s
0.0
1.2
1.6
0.00 0.25 0.10 0.75
1.00 1.25
Percent Organlc Carbon
FIGURE4. Total PCDDF concentration varsus percent organic carbon for soil cores from Mitchell, IN. There is no significant correlation ( p 0.05) between the two variables in either core.
Gnyllng, Mlchlgan
0
0.3
Percent Omanlc Carbon
18
ShlnQlOtOn,Mlchlgan
8
Mitchell, lndlani 2
E
FIGURE 1. Total PCDD/F concentration (the sum tetrachloro- through octachloro- homologues) varsus average depth below the surface for soil corns from Michigan and Guam.
0
Mitchell, Indiana 1
- 6
Grayling, Michigan
50
FIGURE 3. Total PCDDF concentration versus average depth below the surface for soil cores from Mitchell, IN.
7
14
45
Mitchell, Indiana 2
4
ri :: 35
10 20
B Ij'i 0 2
10
0
--
Mitchell, Indiana I
ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29, NO. 8, 1995
carbon content in the Guam soil core ranges between just 0.05 and 0.4%. If we consider soil analogous to the stationaryphase in a chromatographic system,the capacity ofthesoil toretainPCDDlFwiUdependbothontheamount of PCDDlF being deposited to the soil and the amount of organic carbon in the soil (stationaryphase thickness).For the Michigan and Guam soil cores, the capacity of the organic carbon in the soil has not been exceeded, and PCDDlF are effectively retained in the soils' A horizon. This behavior was not observed for some other soil cores we studied. Figure 3 shows the soil PCDDlF concentrations as a function of depth for the Mitchell, IN, core. These samples were collectedfrom a state park in an area of virgin forest; they were dystric cambisol soils with a silt loam texture. In these two cores, high levels of PCDDlF (predominately octachlorodioxin)are present to depths as deep as 90 cm with a subsurface maximum at about 40 cm for core 1 and 50 cm for core 2. Figure 4 shows the PCDDlF concentrations as a function of percent organic carbon for these two cores. There is no significant correlationbetween the two variables in either core. We note that the organic carbon content is low in both cores (relativeto the Michigan cores) and that the A horizon in these cores is only about 5 cm thick. The PCDDlF concentrations are also unusually high, indicatingheavy loading from the atmosphere. These observations suggest that the capacity of the A horizon to retain the PCDDlF has been exceeded. The low soil organic carbon content and heavy deposition are the worst case for PCDDlF immobilization in soils,and accuratefluxes cannot be predicted for such cases. It is not clear why these octachlorodioxin soil concentrations are so high at this location. Lake Sediment Fluxes. On the basis of the profile of Cs-137in the sediment cores, dates were assigned to depth, and sedimentation rates were determined for each of the lakes studied. To determine if sediment focusing (or defocusing) had occurred, the total Cs-137 inventory in
TABLE 2
Data Used to Estimate PCDDF Fluxes to Three Lakes Samglcd in This Studf cesium-137 location
tat.
long.
sed. rate (cm s-l)
Swan Lake, MT Yellowstone Lake, WY Windermere Lake, U.K.
48.04 44.25 54.22
114.05 110.22 2.56
0.29 0.35 0.30
inventory (dpm
deposition (dpm cm-2)
focusing factor
flux (ng m-2 yr-1)
10.2 4.9 33.8
7.5b 7-56 24.8c
1.4 0.7 1.4
171 194 990
a The focusing factor is the ratio of Cs-137 inventory to the Cs-137deposition. Flux of PCDD/F to the sediment is adjusted by this factor. Estimated from strontium-90 deposition reported by Larson ( 4 7 ) for Helena, MT. Estimated from data in Cambray et a/. (42).
-
Soil Northern Rocky Mountaine, USA
TABLE 3
Soil= and Sediment-Derived Fluxes (in ng m-* from Different Geographical Regions
yrl)
region
sediment
soil
av
YO RSD
Northern Rocky Mountains Lower Michigan Upper Michigan Lake District, U.K. mean RSD
181 850 183 990
183 663 264 795
182 756 223 893
1 17 26 15 15
A
F4
each core was ratioed to estimates of Cs-137 atmospheric deposition to give a focusing factor. Table 2 lists the sedimentation rates, the total Cs inventory, the estimated Cs inventory, the focusing factor, and the average PCDDlF flux to sediment for the lakes studied. In each lake, some focusing or defocusing occurred, and fluxes were adjusted by dividing by this factor. Comparison of Soil- and Lake Sediment-Derived Fluxes. A soil-derived flux yields an average flux over the time of deposition. To compare these values to lake sediment-derivedfluxes, an analogous average flux to the lake sedimentswas determined. This may be slightlyhigher or lower than the present-day flux, depending on the depositional history revealed in the sediment core. Table 3 shows the soil and sediment flux estimates for each region, their average, and the percent relative standard deviation (RSD) between the values. For all the regions, the percent RSD is well within our measurement error of &30%,with an overall percent RSD of f15%. This good agreement indicates that soils and sediments produce comparable flux estimates for the same geographic region and validates the accuracyof using soils to estimate fluxes. If soils and sediments are equivalent matrices for collecting atmospheric deposition of PCDD/F, then their homologue profiles should also be similar. A homologue profile is the relative distribution of the tetrachloro through octachloro homologues in a sample. Figures 5 through 8 show the homologue profiles for the four sediment-soil sets. In all regions, except for the Lake District of the United Kingdom, the homologue profiles in the lake sedimentcores did not change significantlyas a function of sediment depth. Thus, we have chosen to represent these profiles as an average over the sediment depth. The homologue profiles for the northern Rocky Mountain region (see Figure 5) show good agreement between soils and sediment. There is a slight enhancement in the lake sediments of the heptachlorinated dioxin homologue relative to the soil. We do not have an explanationfor this, but the difference is minor. For the Lower Michigan region (see Figure 6)and the Upper Michigan region (see Figure 71, the agreement in the homologue profiles between soils
F5
F6
F7
F8
D4
D5
D6
D7
D8
Lake SedlmenbHorthem Rocky Mountains, USA
B
F4
F5
F6
F7
F8
D4
D5
06
D7
D8
Homologue
FIGURE 5. Homologue profiles (normalized such that the octachlorodioxinconcentrationis lo%)for northern Rocky Mountains region soils (A) end lake sediments (B). The lakes sediment profile is the average profile for Swan Lake, MT, and Yellowstone Lake, Yellowstone NP, WY. The letter F refers to furans, and the letter D refers to dioxins. The number beside each letter indicatesthe level of chlorination.
and sediment is reasonable. In both cases, there is a slight enhancement of the hexachloro- and heptachlorodioxin homologuesin the soil relative to the sediments. This might be explained by the difference in surface roughness between the two matrices (soiland water surface). The high organic carbon in the soils of this region may act as a slightly more efficient scavenger of PCDD/F than do water surfaces.Again these differences are minor, and flux estimates are still comparable because the deposition is dominated by the octachlorinated homologue. In the case of the Lake District of the United Kingdom, there is a significant difference in the homologue profiles of the soil (see Figure 8A) and the top sections of the lake sediment. The soil is greatlyenhancedin the dibenzofurans, relative to the sediment (see Figure 8B). Figure 8C also shows the homologue profile for the core section dated at 1946- 1950, which indicates that the dibenzofuranswere a significant fraction of the total PCDDlF deposition to Windermere Lake in these years. Although we do not see this trend in the Great Lakes sediments (1 61,it was observed VOL. 29. NO. 8,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2091
-
Soil Lake Dirtrlct, United Kingdom
A
A
F4
FS
F6
F7
F8
D4
DS
D6
D7
D8
-
Sedlment Windermere Lake, 1988.1992
B F4
F5
F6
F7
F8
04
D5
D6
D7
D8
F4
F5
F4 F5
F6
F7
F8
D4
D5
06
07
FIGURE 6. Homologue profiles for lower Michigan region soil (A) and Lake Huron, Michigan, sediment (8).
F8
M
DS
D6
D7
Sedlment Wlndrrmen Lake, 19464950
F5
F6
D8
Homologue
F7
-
c
F4
F6
F7
F8
D4
D5
D6
D8
I
D7
D8
Homologue
FIGURE 8. Homologue profiles for Lake District, United Kingdom soil (A), a Lake Windermere 1988-1992 sediment core section (E), and a Lake Windermere 1948-1950 sediment core section (C).
TABLE 4
Total PCDD/F Flux (in ng m-z p-l), Mean, Relative Standad Deviation, and Distance (0)between Samples for Field Duplicate Measurements Yo
region
F4
F5
F6
F7
F8
04
05
D6
D7
D8
-
Lake Sedlment Upper Mlchlgm, USA
B
F4
F6
F7
Fa
D4
D5
D6
D7
D8
Homologue
FIGURE 7. Homologue profiles for upper Michigan region soil (A) and Siskiwit Lake, Isle Royala, MI (8).
by Smith et al. for Green Lake near Syracuse, NY (17).This trend indicates that the input of PCDDlF to the region has changed over time. Sources that emitted significant quantities of the lower chlorinated homologues probably have been reduced. Since the soil represents an integrated 2096
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 8,1995
dup2 mean RSD
Ponta-de Pedroz-PA, Brazil 41 44 43 5 Flaming Gorge, WY 2.1 1.9 2.0 7 St. Croix, Virgin Islands 14 16 15 9 Stockholm, Sweden 438 508 473 10 Black Forest, 227 551 389 59 Triberg, Germany 80 17 La Curina, Spain 70 89 Eagle, COB 12 14 13 14 Ruby Lake, NV 6.3 11.8 9.1 43 21 mean RSD a
F5
dupl
D (km) 10.05
