Environ. Sci. Technol. 1987, 21, 709-712
NOTES Polychlorinated Biphenyl Accumulation in Tree Bark and Wood Growth Rings Marcia L. Meredith and Ronald A. Hites”
School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Polychlorinated biphenyls (PCBs) were found in the bark of black walnut and tulip poplar trees growing near a PCB-contaminated landfill. PCBs were also found in the bark of white oak trees growing 14 km away from the landfill. The concentration of individual congeners in the bark averaged 18 ppb at the landfill and 0.5 ppb at the other site. The PCB congeners were accumulated into the bark in proportion to their lipophilicity (as measured by octanol-water partition coefficients). Our findings suggest that tree bark could be used for biomonitoring of lipophilic organic pollutants in the atmosphere. There is little evidence that PCBs are present in the wood of trees. The signal to blank ratios are always less than 3, and the relative concentrations between 20-year time intervals do not show trends that correlate with the known inputs of PCBs in Bloomington, IN. ~~
~
Introduction The uptake and accumulation of heavy metals by trees have been well studied (1-3). The concentrations of metals have been measured in growth rings and have been correlated with input functions of pollutants and with distances from sources. Data such as these have been used to evaluate the effects of air pollution on forests and to determine pollution histories of trace metals (1, 3). For example, high concentrations of lead have been found in rings of several species of trees, and the increase in concentration as a function of time has corresponded to the rise of local industrial inputs (2). Lead concentrations in trees also correlate with traffic density and with distance from roads (4, 5). Tree bark also has been studied to determine the fate of pollutants taken up by trees. Bark accumulated lead, and it may be used as a sensitive indicator of the degree of lead pollution (6). Bark has also been used as a indicator of environmental acidity and SO2 emissions (7). It is not clear if trees can be used for biomonitoring of toxic organic compounds. We are aware of only one study on this subject (8). In that study, the uptake of 14C-labeled polychlorinated biphenyls (PCBs) by 4-year-old conifers grown in contaminated sludge was investigated. About 30 ppb PCBs were found in the needles and 5 ppb in the stem,but the uptake, relative to the soil concentration, was only 0.8%. We formed two hypotheses: (a) tree bark can be used for biomonitoring lipophilic organic pollutants, and (b) tree rings can be used for reconstructing pollution histories for these organic contaminants. Thus, we analyzed for PCBs in both the bark and wood of several tree species. PCBs were chosen as the analyte because there is evidence they can be taken up by plants (9,10) and because tree samples were available at a local site that was contaminated with PCB wastes from a nearby capacitor manufacturer (11). We measured the concentrations of PCBs in mature trees growing next to this site, an uncontrolled landfill that 0013-936X/87/0921-0709$01.50/0
was used in 1966 and 1967 for the disposal of capacitors filled with Aroclor 1242 and other contaminated material (12). Ambient air concentrations of Aroclor 1242 were measured in the summer of 1983 (11). Concentrations measured 1-2 m above contaminated soil ranged from 0.4 to 18.0 wg/m3. The concentrations measured along the downwind perimeter of the contaminated area were 0.2-1.8 pg/m3. On the basis of these data, we know that the landfill was a significant source of Aroclor 1242 over 15 years after the disposal of PCB wastes. Experimental Section Sampling. Trees were sampled at two sites in Bloomington, IN, in October 1985 and March 1986. Site I was near the landfill, which was 8 km west of Bloomington (39” 10’ 00’’ N, 86” 38’ 30” W). Site I1 was in a forested area next to a recreational lake. The area was about 14 km downwind from the landfill (39” 11’50” N, 86” 31’ 20’’ W). At site I, black walnut (Juglaus nigra) and tulip poplar (Liriodendron tulipifera) trees were chosen randomly from a clearing just west of a fence surrounding the contaminated area. At site 11,white oak (Quercus alba) trees along the forest edge were sampled. See Table I for the details of the trees sampled. Six cores were taken from each tree at breast height and at evenly spaced intervals around the whole circumference. A 25 cm X 5 mm diameter, stainless steel, Teflon-coated increment borer was used. The borer was rinsed with acetone and wiped clean with a lint-free cloth between each sample. The cores were sealed in ashed, glass tubes and stored at -18 “C. Composite bark samples were taken from the side of the tree facing the contaminated area. The bark surface was cleaned of lichens and particles with rough sand paper, and the sample was scraped away from the tree with a knife. Inner and outer bark samples were obtained with a 12-mm cork borer and separated in the field. Extraction and Sample Preparation. The cores were removed from the glass tubes and thawed on a piece of clean foil. Wood rings were dated by comparing cores taken 180” apart from the same tree. The cores were sectioned into 20-year intervals, and the sections from all six cores were composited to yield three representative time intervals from each tree. Twenty-year intervals were a compromise between time resolution and having enough sample for analysis. The samples were ground to a fine dust in a modified coffee grinder. Samples (4-12 g) were ultrasonically extracted in an ice bath at 70% continuous power for 5 min. The wood was extracted once with 80 mL of 1:l hexane/acetone and once with 75 mL of hexane. The solvent was decanted through glass wool into a round-bottom flask, and the wood was rinsed twice with hexane. The extract was solvent-exchanged with hexane and rotary-evaporated to about 5 mL. The extract was passed through a 1 x 5 cm column of preextracted silica gel deactivated with 3 % by weight
0 1987 Amerlcan Chemical Society
Environ. Sci. Technol., Vol. 21, No. 7, 1987 709
Table I. Tree Sampling Details
site I walnut 1 walnut 2 walnut 3 walnut 4 poplar 1 site I1 oak 1 oak 2
radius, cm
section 1
dates of interval section 2
26 29 29 21 38
1985-1965 1985-1965 1985-1961 1985-1965 1985-1965
1965-1945 1965-1945 1961-1941 1965-1945 1965-1945
1945-1930 1945-1925 1941-1921 1945-1925 1945-1930
Yes Yes no no Yes
35 35
1985-1960 1985-1960
1960-1935 1960-1935
1935-1915 1935-1910
yes
water. Chlorinated organics were eluted with 30 mL of 9 1 hexane/methylene chloride. A methylene chloride flush fraction was also collected. The fractions were solventexchanged with 20 mL of hexane and transferred to 4-mL amber vials. The samples were further reduced to 1mL for gas chromatography quantification and to 100 pL for mass spectrometry analysis by using a stream of ultrapure dry nitrogen. All glassware was acid-washed and ashed at 460 "C in a muffle furnace. All solvents were Omni-Solv MCB reagents. Three replicate solvent blanks were run with each set of samples extracted. In a separate experiment, procedural blanks were run on wood that was cleaned by a 24-h Soxhlet extraction and dried at 150 "C for 2 h. Three procedural blanks and three solvent blanks were worked up simultaneously to confirm the reliability of the routine solvent blanks. Procedural recoveries of the spiked PCB quantitation standard were 98-129%. GC/ECD Analyses. Quantitative analyses were carried out on a Hewlett-Packard 5890 gas chromatograph with electron capture detection. A J&W, fused silica, 30-m DB-5 column was used for all chromatographicseparations. The GC oven was held at 100 "C for 1 min for splitless injection: programmed at 30 deg/min to 160 "C; then at 2 deg/min to 280 "C and held for 5 min. Hydrogen was used as the carrier gas at a velocity of 40 cm/s. The injector and detector temperatures were 225 and 325 "C, respectively. 4,4'-Dibromobiphenyl was used as the internal standard, and 23 PCB congeners were quantitated. Integration was done with a Hewlett-Packard 3390 integrator; response factors were obtained from the average of two standard runs each day. All samples from one tree were run on the same day. GC/MS Analyses. The identification of PCBs was done by electron capture, negative ion, mass spectrometry. A Hewlett-Packard 5985B gas chromatographic mass spectrometer system with the GC conditions listed above was used. The ion source temperature was 100 "C, and it was held a t 0.4 Torr of methane. Retention times and spectra of standards were used to confirm the presence of PCBs. Data Reduction. The concentrations of PCBs in the samples are reported as nanograms of an individual congener per dry weight of sample. A total PCB concentration is not reported. In some instances, the geometric average of the concentrations of the individual congeners has been used to describe general differences between the samples and the sampling sites. The geometric average was used because of the variability among the congener concentrations within a sample.
Results and Discussion PCBs in Bark. The protective function of the outer bark is derived from the physical nature of its cork cells. The cells are dead and highly deposited with suberin, a 710
Environ. Sci. Technol., Vol. 21,
No. 7, 1987
section 3
bark samples
distance from fence, m 90 270
275 150 180
Yes
-
complex fatty substance that makes them somewhat impervious to water (13). Thus, outer bark should absorb PCBs from the atmosphere because the PCBs would have an affinity for the lipophilic suberin. This lipophilic attraction could also deter the translocation of PCBs through the bark into the wood. The concentrations of selected PCB congeners in several bark samples are given in Table 11. These congeners were chosen because they do not coelute with any other congener and they represent a wide range of vapor pressures and lipophilicities. The congeners given in Table I1 are also found in Aroclor 1242; other congeners were included in the quantitation standard but were not found in the samples. The congeners are identified by their IUPAC number. The concentrations in the bark ranged from 180 to 0.04 ppb for individual congeners. When the bark PCB values were normalized to the relative abundance of the correspondingcongener in Aroclor 1242 (see Table 11,rows labeled "ratio"), it was obvious that the more highly chlorinated congeners are relatively more abundant in the bark samples than in Aroclor 1242. For example, the penta- and hexachloro congeners (97-153) are, on average, 3 times more abundant (relative to Aroclor 1242) than the tri- and tetrachloro congeners (18-70). These ratios are highly correlated with the PCB congener's octanol-water partition coefficients (a measure of lipophility) (14);the correlation coefficients range from 0.71 for poplar 1to 0.95 for oak 2 when calculated on a log-log basis (see Table 11). This is just the behavior one would expect if tree bark was acting as a lipophilic absorbent for organic compounds in the atmosphere. Another mechanism by which PCBs could get into bark is uptake through the leaves and incorporation into the dead cell wall as the bark forms. To investigate this idea further, the surface and the inner bark layers of the most contaminated walnut were analyzed separately. On averge, the concentration in the outer 3-4 mm of bark was 4 times greater than that in the inner bark (see Table 11). Since the surface was the most concentrated, it seems that direct absorption from the atmosphere is a more efficient process than uptake through the leaves. We found about 40 times more PCBs in bark from the trees near the dump than in those 14 km away. While we recognize that the species of trees sampled are different at the two sites, it seems unlikely that this factor of 40 difference can be explained on this basis alone. I t seems more likely that trees near the dump are living in air that is more contaminated with PCBs. Our data have some interesting applications. For example, tree bark monitoring could be used to estimate the level and geographic distribution of lipophilic air pollutants. Clearly, bark can be used to estimate the differences between highly contaminated areas and regional background, but bark monitoring may be sensitive enough to detect differences between trees near one another. For
w
w
u
w
w m
w w
T'u Tu?
m m ~ o o
m
coo
rnm
m tm- 0
u
u
w
u
mcomri
u '99u??C?LL? ~ c o m 0 0 0 0
*: "
-*mc1 2c o w"c - m * o o * i o r i * h lr ci m ri
0
example, at the contaminated site, there is a decrease in PCB concentration in the bark with distance from the fence. The average congener concentration in the bark of the walnut closest to the fence is 39 ppb; the average concentrations decrease to 17 and 9 ppb in trees 2 and 3 times farther away. More samples are needed for a statistical correlation, but the initial results encourage more work. The use of tree bark as a bioindicator of toxic organic compounds has several advantages. Trees are stationary and well dispersed geographically. For most species, one can easily get enough sample for analysis without harming the tree. Since it seems that direct absorption is the mechanism by which PCBs are incorporated in bark, the concentration in the bark should be related to atmospheric concentrations. PCBs in Wood. We also hoped to determine if tree ring (xylem) analysis could be used to develop pollution histories of PCBs. We formed the hypothesis that the concentration of PCBs in the trees near the contaminated site would reflect the landfill's pollution history. The first question we addressed was whether PCBs are even present in the wood of the tree. The gas chromatograms of the PCBs in the wood and the blanks were very similar, and the signal to blank ratio was low. Congeners 8, 18, 33, 44, 52, 66, 70, and 97 were higher in the wood samples than in the blanks for site I; the concentrations of these congeners are given in Table 11. However, the concentrations were about 30 times less than in the tree's bark, and the signal to blank ratios were always less than 3. Also, many of the congeners found in Aroclor 1242 were not greater in the wood samples than in the blanks (congeners 4, 49, 101, 118, 138, and 153). Relative amounts can often be compared even in the face of high backgrounds. Thus, the change in concentration between each 20-year interval for each specific congener was compared to see if there were any trends that related to the chemical properties of the congeners. Ideally, the highest concentration would be in the 1985-1965 segment, reflecting when PCB wastes were paced into the landfill. Unfortunately, the specific congener concentrations (see Table 11) showed that, in general, the PCB concentration in both the 1985-1965 and 1945-1925 sections was higher than that in the 1965-1945 section. This is not a reasonable trend because no PCBs were in use in Bloomington in the period 1925-1945. It seems likely that the concentrations in the wood are simply noise around the detection limits of the analytical method. We must draw the conclusion that, within these limits, PCBs are probably not present in the wood rings of trees.
Acknowledgments We thank I. Basu for technical assistance, D. Liebl for sampling, and D. Swackhamer for helpful discussions. Registry No. PCB, 92-52-4; 2,2'-4, 13029-08-8;2,4'-8,3488343-7; 2,2',5-18, 37680-65-2; 2',3,5-33, 37680-68-5; 2,2',3,5'-44, 41464-39-5; 2,2',4,5'-49, 41464-40-8; 2,2',5,5'-52, 35693-99-3; 2,3',4,4'-66, 32598-10-0; 2,3',4',5-70, 32598-11-1; 2,2',3',4,5-97, 41464-51-1;2,2',4,5,5'-101,37680-73-2;2,3',4,4',5-118, 31508-00-6; 2,2',3,4,4',5-138, 35694-06-5; 2,2',4,4',5,5'-153, 35065-27-1.
Literature Cited (1) Berish, C. W.; Ragsdale, H. L. Can. J. For. Res. 1985, 15, 477-483. (2) Robitaille, G. Enuiron. Pollut., Ser. B 1981, 2, 193-202. (3) Baes, C. F.; McLaughlin, S. B. Science (Washington,D.C.) 1984,224, 494-496. (4) Baes, C. F.; Ragsdale, H. L. Enuiron. Pollut., Ser. B. 1981, 2, 21-35. Environ. Sci. Technol., Vol. 21, No. 7, 1987
711
(5) Kardell, L.; Larsson, J. Arnbio 1978, 7, 117-21. (6) Barnes, D.; Hamadah, M. A.; Ottaway, J. M. Sci. Total Environ. 1976, 5, 63-67. (7) Grodzinska, K.; Hartel, 0. In Monitoring of Air Pollutants by Plants; Steubing, L.; Jager, H. J., Eds.; Junk The Hague, 1982; pp 33-42, 137-147. (8) Moza, P. N.; Scheunert, I.; Klein, W.; Korte, F. Chemosphere 1979,8, 373-382. (9) Strek, H. J.; Weber, J. B. Enuiron. Pollut., Ser. A 1982,28, 291-312. (10) Pal, D.; Weber, J.; Overcash, M. Residue Rev. 1980, 74, 45-98. (11) Lewis, R. G.; Martin, B. E.; Sgontz, D. L.; Howes, J. E.
712
Environ. Sci. Technol., Vol. 21, No. 7, 1987
Environ. Sci. Technol. 1985, 19, 986-991. (12) Sunday Herald Times, Bloomington, IN, Feb. 19, 1984, p 6. (13) Zimmermann, M. H.; Brown, C. L. Trees Structure and Function; Springer-Verlag: New York, 1971; pp 87-91. (14) Rappaport, R. A,; Eisenreich, S. J. Environ. Sci. Technol. 1984, 18, 163-170. Received for review June 30, 1986. Revised manuscript received February 3, 1987. Accepted March 15, 1987. This work was supported by a grant from the U.S. Department of Energy (80EV-10449).
