Environ. Sci. Technol. 1900, 24, 666-671
Polychlorinated Biphenyls in Tree Bark Mark H. Hermansont and Ronald A. Hites'
School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Polychlorinated biphenyl (PCB) concentrations in tree bark vary with proximity to a source. Higher total PCB barklair ratios in areas near contamination show that bark may retain PCB from prior periods of high atmospheric concentrations. Bark is enriched in the more chlorinated PCB homologues relative to air. Congener-specific analyses show that, when compared with air, bark favorably accumulates the less volatile congeners. Lipophilicity is not a good indicator of bark PCB concentrations, but vapor pressure is.
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Introduction
Polychlorinated biphenyls (PCB) are distributed throughout the earth's atmosphere, arising from scattered dumps and industrial equipment that still contain PCB (1-4). The global distribution of atmospheric PCB suggests that a relationship may exist between the concentration of these compounds in the air at a given location and the amount observed in plants living there. For example, Meredith and Hites (5) observed high concentrations of certain PCB congeners in tree bark near a Superfund PCB dump site and lower values in trees 14 km away. This finding suggested that there may be a relationship between PCB concentrations in tree bark and PCB concentrations in air. Tree bark has been used to assess pollutants in air, including metals (6, 7), acid gasses (8,9),and radioisotopes (10). Plant leaves have been found to contain PCB absorbed from atmospheric vapors (11-14). Translocation of PCB from soils to leaves through the root and vascular systems of plants contributes little to the PCB found in foliage, even in plants grown in soil inoculated with PCB (15). Absorption of PCB into foliage is species dependent: Buckley (16) showed that PCB concentrations in different species of plants growing at a given site varied by an order of magnitude. Garty et al. (17, 18) used lichens as indicators of atmospheric PCB, but concentrations were never correlated with air data. Bush et al. (19) attempted to relate air concentrations of PCB to plant tissue levels and concluded that PCB in plants were not related to air concentrations but instead were translocated from soil, the opposite of findings reported in other studies (11-14). This odd conclusion may have resulted from a detection limit for air samples that was more than 1 order of magnitude greater than PCB vapor concentrations often observed in the atmosphere (1, 20). Based on the findings of Meredith and Hites ( 5 ) ,our objective was to further study the interaction of tree bark with atmospheric PCB. Thus, we measured PCB concentrations in bark and air near PCB dumps, a t sites within 15 km of known contaminated areas, and at remote areas at least 35 km away from possible PCB sources. PCB concentrations in bark were also compared to those in other plant types from PCB-contaminated and noncontaminated areas to determine if other plant materials show similar results. Present address: Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495. 666
Environ. Sci. Technol., Vol. 24, No. 5, 1990
We investigated bark/air relationships using PCB data in three forms: total PCB concentrations, PCB homologue concentrations, and PCB congener concentrations. We also related lipophilicity and vapor pressures of PCB congeners to congener partitioning between bark and air. These relationships help to explain the observed patterns of PCB absorption by bark. Methods
Study Sites. We collected bark samples from four sites in and around Bloomington, IN, an area near sites with significant PCB contamination (see Figure 1). Bark sampling sites corresponded to our air sampling sites (21) and to places of known contamination. We collected air and bark samples at the Monroe County Courthouse in downtown Bloomington (within 3 km of two Superfund PCB sites) and 13 km south in a rural area near the former Sanders School. Bark samples were also collected at Neal's Landfill, a Superfund PCB site 13 km west of the courthouse where air samples were collected by others in 1984 (2). Bark and air samples at Neal's Landfill were collected 40 m downwind from the contaminated area. Although air samples were collected 4 years prior to bark sampling, we assume that atmospheric PCB concentrations at this site in 1988 were similar because the condition of the dump site (the source of most atmospheric PCB in the area) remained unchanged from 1984 to 1988. We also collected bark a t the abandoned Winston-Thomas Wastewater Treatment Plant in Bloomington, which is not on the Superfund list but which contains stockpiled PCB-contaminated sewage sludge (22, 23). Winston-Thomas is within 1 km of the Batchelor Middle School where we collected air samples; we consider these to be one site. We also collected bark samples at Isle Royale National Park (see Figure 2) to compare to atmospheric PCB concentrations measured there earlier (24). Isle Royale has been a national park since the 19409,and during its history, it has had no industry other than fishing and copper mining, which ceased before 1940. PCB present on the island are therefore transported atmospherically from sources on the mainland, no closer than 35 km distant. Since recent long-term air vapor PCB concentrations vary only by a factor of 2 between the Bloomington and Isle Royale sites (21,24), bark PCB should be similar at the two locations or they will reveal different histories of atmospheric PCB. We collected duplicate bark samples from six trees in the Bloomington area in the summer of 1988 during our 2-year air sampling project. Thirty-one trees were sampled on Isle Royale (16 duplicates reported here) in September of 1987. Three general areas forming a north-south transect across the western end of Isle Royale were sampled to examine geographic variability of PCB concentrations (see Figure 2). Higher concentrations in one area would suggest the direction of an atmospheric PCB source. No variations were observed. In consideration of the order of magnitude differences in PCB levels found in different plant species investigated by Buckley (161,we sampled three different species on Isle Royale, red maple (Acer rubram), white pine (Pinus strobus), and paper birch (Betula alleghaniensis) in order
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0 1990 American Chemical Society
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Superlund Sites Treebark sampling sites Air sampling sites
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Figure 1. Bark and air sampling sites and Superfund PCB dumps sites near Bloomington, IN.
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Figure 2. Bark and air sampling sites on Isle Royale.
to observe any significant variation of PCB absorption. All Bloomington trees were either sugar maple (Acer saccarum) or red maple. Since we collected bark from the genus Acer at both Isle Royale and Bloomington, we can compare PCB concentrations among trees with the same types of bark. Sample Collection. Garty observed PCB in lichens ( I 7, 18),so we specifically selected trees with few or no lichens growing on the bark to avoid this interference with bark PCB absorption. The oxidized bark layer and other foreign material, including lichens and moss, were removed with sandpaper from an area 5 cm wide by 20 cm high, similar to sampling methods used by Grodzinska (6) and by
Meredith and Hites (5). All bark from this area was removed, leaving a thin layer over the vascular system to minimize damage to the tree. We restricted ourselves to sampling trees with circumferences greater than 90 cm. Thus, within a given forested area such as Isle Royale, the size and age of trees within a species was similar, so the time exposure of trees to atmospheric PCB was also similar. The bark was chiseled into a samplingjar precleaned by heating to 400 "C for 4 h. Chisels were rinsed with dichloromethane between samples. The jar was labeled and sealed with clean aluminum foil. Areas where bark was sampled were covered with tree patch. Samples were frozen at the end of each field day to prevent PCB losses Environ. Sci. Technol., Vol. 24, No. 5, 1990 667
by vaporization. Air sampling methods and results were reported by Hermanson and Hites (21). Sample Analyses. Bark samples were ground into particles of
