Environ. Sci. Technol. 2000, 34, 1707-1714
Vegetation/Soil Distribution of Semivolatile Organic Compounds in Relation to Their Physicochemical Properties PETER WEISS* Federal Environment Agency, Spittelauerla¨nde 5, A-1090 Wien, Austria
The concentrations (C) of several semivolatile organic compounds (SOCs) in Norway spruce needles (N) and in the local humus horizon (O) of 25 remote Austrian forest sites were used to calculate an ecosystem-oriented partition coefficient needles/humus horizon (CN/CO). Between 66 and 78% of the compounds’ variation of this quotient could be explained by each of the following physicochemical parameters: vapor pressure (pS) and the partition coefficients n-octanol/water (KOW), n-octanol/air (KOA), and adsorbed/ dissolved in soil (KOC) of the compounds. This result further underlines the usefulness of these parameters for predicting the behavior of SOCs in terrestrial ecosystems. Compounds with low pS and high KOW, KOA, and KOC show a very low CN/CO quotient, which implies a higher accumulation of these compounds in the O horizon than in the needles. The role of forest soils as sink for these SOCs is demonstrated. Alternatively, CN/CO > 1, due to higher concentrations in the needles than in the O horizon, have been shown for SOCs with comparably high pS and low KOW, KOA, and KOC. In this respect, the possible role of revolatilization of the more volatile SOCs from soils to needles is discussed. In the mineral soil layers below the O horizon, SOCs with lower KOC and better water solubility tend to be less accumulated. However, if all investigated compounds are taken into consideration, accumulation in the mineral soil layers showed no general trend in relation to the selected physicochemical parameters.
Introduction Vegetation and soil represent important sinks for semivolatile organic compounds (SOCs). It has been estimated that several times the amount of current annual Austrian emission of SOCs has accumulated in these two compartments of the Austrian forests (1, 2). Measurements of the deposition of SOCs to forest canopies and forest soil underline the role of forests as filters of airborne SOCs (3-6). In a recently published model, this filter effect has been explained by the physicochemical properties of these chemicals (7). Uptake of SOCs via cuticula from the surrounding atmosphere is an important reason for the contamination of aerial plant parts with these compounds (8-12). Several working groups found relationships between bioconcentration factors in leaves and vapor pressure (p) and n-octanol/ water partition coefficients (KOW) of the SOCs (13-17). More recent studies used the octanol/air partition coefficients (KOA) * Corresponding author e-mail:
[email protected]; tel: ++431-31304/3430; fax: ++43-1-31304/5400. 10.1021/es990576s CCC: $19.00 Published on Web 03/28/2000
2000 American Chemical Society
of SOCs to characterize the measured partitioning plant/ atmosphere of these compounds (18-25). According to these studies, compounds with lower p and higher KOA and KOW need a comparably longer time to reach an equilibrium between plant and surrounding air and are more strongly accumulated in plants. Forests are characterized by a rare biomass harvesting, which implies that the forest soil receives the total SOC input by dry and wet deposition and litterfall. Several processes lead to losses or accumulation of these compounds in soil. SOCs with lower p show less volatilization from soil (26-28). Excellent log/log-linear relationships between the soil/air partition coefficients and KOA of SOCs have recently been identified (28, 29). More hydrophobic compounds (higher KOW) and less water-soluble SOCs can be less easily translocated by soil water in deeper soil layers, which results in a comparably stronger enrichment in the upper-most soil layers, too (30-32). SOCs with a stronger sorption to organic carbon (higher partition coefficients adsorbed/dissolved in soil, KOC) are more strongly bound to the organic matter in soil (26), which is enriched in the upper-most layer of forest soils. Therefore, SOCs with lower p and higher KOW, KOA, and KOC are favored for accumulation in the upper-most soil layer over time (30, 31, 33, 34). Several studies confirmed that vapor phase/particle partitioning of SOCs in the atmosphere is influenced by the physicochemical properties of SOCs, too. Compounds with lower p and higher KOW are more associated with particles, whereas compounds with high p and low KOW are more present in the vapor phase (35-40). In this way, the different physicochemical properties of SOCs have an additional influence on the characteristics of the deposition to plants and soil (4, 5, 7, 10, 24, 25, 41). These findings in the literature indicate similarities in accumulation trends of SOCs in plants as well as in soil in relation to the different pS, KOW, KOA, and KOC of the compounds. However, it has not been proved so far that these physicochemical parameters are suitable to explain the differences in accumulation between SOCs in plant and soil. To clarify this, a simultaneous investigation of plants and soils in environments, where the atmosphere is the main source for the detected loads in both compartments, is required. Only few field studies deal with an observation of SOCs in such atmosphere/plant/soil or plant/soil systems (6, 23, 42). Particularly for forests, there still exists a lack of information in this field (6, 42). This paper deals with an investigation of the concentrations of several SOCs in Norway spruce needles and in the parent forest soils of 25 remote Austrian forest sites. The chosen sites have not been sprayed with pesticides. Thus, long-range transport of polluted air masses and input via atmosphere are the dominant sources for the concentrations of all investigated SOCs in the sampled compartments. This approach allowed the study of the fate and accumulation of SOCs in these compartments. An ecosystem-oriented partition coefficient vegetation/soil is introduced and used to characterize the different enrichment behavior between SOCs in needles/soil. The relations of this coefficient to the physicochemical properties of the compounds are discussed.
Methods A detailed description is given in ref 43. Twenty-five remote forest sites from all over Austria were investigated. The altitudes of the sites range from 450 to 1750 m above sea level (asl), and so they are exposed to a wide variety of climates. The chosen forests were all Norway spruce moVOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. HCB, r-HCH, γ-HCH, PAH, and PCDD/F in Norway spruce needles (1st needle age class, N1) and the humus horizon (O) of 25 remote forest sites. Boxes show interquartile range with the median as line; whiskers show extraquartile range except near and far outliers indicated as circles and asterisks, respectively. nocultures to guarantee that the humus horizons of all sites have the same source material and similar conditions for turnover and that the forest canopies represent comparable conditions for deposition. Norway spruce needles of the 1st (half-year-old, N1), 2nd (1.5-year-old, N2), and 3rd (2.5-year-old, N3) needle age classes were collected from the range of the 7th branch whorl from the top (within the sun crown) and from all wind directions of two dominant trees at each site in October 1993. At the same sites, 10 samples of each, the humus horizon (whole L, Of, and Oh horizons) and the underlying mineral 0-5- and 5-10-cm soil layers, were taken of a defined volume and at random along a rectangle of 25 × 5 m. N1 (whole needles) and the whole humus horizon (O) without living roots and stones of all 25 sites were analyzed for their concentrations of polychlorinated dibenzo-p-dioxins and -furans (PCDD/F), polychlorinated biphenyls (PCB), hexachlorocyclohexanes (HCH), hexachlorobenzene (HCB), and polycyclic aromatic hydrocarbons (PAH). In addition, at five of these sites the mineral soil layers (fraction 1, Figures 2 and 3). It is remarkable that the KOW value of HCB does not adequately reflect the CN/CO > 1 of this SOC, whereas the other parameters do (Figure 2). Measurements of PAHs and PCBs in a Swedish Norway spruce forest ecosystem showed concentration increases of more volatile PAHs in the following way: needles of different ages (half- and 1-year-old) < litterfall < humus layer (6). The findings of the present study, CN/CO > 1 for the more volatile SOCs, are the opposite. There are several possible explanations for CN/CO > 1 and the ascertained high quotients for the more volatile SOCs: (i) Artifacts and losses of SOCs in O horizon during sample treatment and analyses: The samples were kept in freeze boxes during transport and at -20 °C until analyses. Furthermore, different procedures of chemical analyses were used to analyze PAH and HCH + HCB. Therefore, such artifacts are considered unlikely. Nevertheless, the differences in the results between individual studies might be influenced by the fractions of the humus layer that were analyzed. In
the present study, the whole humus layer, which also includes coarse fractions and fractions with a lower storage capacity for SOCs, was analyzed. These fractions and the upper-most layer of the humus horizon are excluded by an analysis of a sieved fraction of the humus horizon, which due to the concentration increases during decomposition (30, 31) leads to higher detected concentrations in the humus horizon. (ii) The properties of the more volatile SOCs allow quicker losses by runoff and degradation in the O horizon. This has likely contributed to the observed quotients CN/CO > 1. However, it has been shown that even the less hydrophobic PAHs show enrichment factors of about 3 during litter decomposition in several forest humus types of central European forests for both quickly decomposing mull of deciduous forests and slowly decomposing mor of Norway spruce stands (30, 31). Therefore, even these PAHs tend to accumulate in the whole O horizon over time. Hence, it is very unlikely that degradation as well as translocation to deeper soil layers alone can lead to the observed situation CN/CO > 1. (iii) Root uptake and translocation to the needles: The KOW values of HCB and the PAHs under consideration are in a range where a significant contribution of this pathway can be widely excluded (58, 59). (iv) Where the needles are in an equilibrium with the surrounding air, which is not unlikely for the more volatile SOCs, the concentrations of these SOCs in the needles can vary due to the change of temperature (11, 22). Hence, a drop of air temperature in October could have been responsible for higher concentrations in N1, N2, and N3. However, this situation would occur every year in the cold season and would consequently have an influence on the concentrations of the following compartments, litterfall and humus layer. Litterfall in Norway spruce forests (mainly needles) occurs throughout the year with peaks during the first weeks of fall and spring (60). The mean temperature during the month of sampling was above the mean annual temperature (Table 2). It is therefore unlikely that temperature mediated concentration increases in the needles alone can lead to CN/CO > 1. (v) The N1, N2, and N3 concentrations of the more volatile SOCs were influenced by a comparably higher load of the surrounding atmosphere and are higher than the mean load of needles throughout the exposure period of the sampled O horizon: Such a situation would be caused by higher emissions and atmospheric transport of these SOCs to the remote sites during the exposure time of the needles. The SOCs with CN1/CO > 1 are emitted by completely different primary sources: the emissions of PAHs are higher during wintertime (house heating) (61), the use of HCB was legally banned in Austria several years ago, and the agricultural use of γ-HCH was reduced by law and has steadily decreased during the last years also. Hence, the sampled O horizons and their source material were exposed during times of higher primary emissions of these SOCs than N1, N2, and N3. The remaining emission sources, which could have contributed to the high CN1/CO quotients, are revolatilizations from soils. The Austrian forest soils (46% of total area) contain several times less HCH and HCB than agricultural soils (42% of total area) (43). Hence, there might still be a diffusion gradient from agricultural areas to forest areas. Revolatilization is strongly dependent on temperature (26), which is a further reason it is potentially greater from agricultural soils and in the warm season. A further aspect could be the observed higher mean annual temperatures during the exposure period of the needles than the average between 1961 and 1990 (62). In addition, losses from the O horizon by volatilization could have contributed to the ascertained high CN/CO quotients for the more volatile SOCs. Several studies indicate that revolatilization of SOCs can significantly contribute to the VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Relative patterns of HCH + HCB, PAH, PCB. and PCDD/F in the humus horizon (O) and in the mineral soil layers 0-5 and 5-10 cm. The numbers above the bars represent the quotients of the concentrations in the 0-5-cm layer divided by the concentrations in O and of the concentrations in the 5-10-cm layer divided by the concentrations in O, respectively (all based on kg dw). load of the atmosphere (16, 23, 26-29, 63). This discussion underlines the indispensability of further studies to clarify the role of SOCs’ revolatilization for the concentrations in plants and the redistribution of SOCs in the environment. SOCs in the Forest Soil Profiles. To study the fate of SOCs in soil profiles, mean relative SOC patterns for each soil layer were calculated using the data of those five sites, 1712
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where the mineral soil layers were analyzed. In addition, mean concentration quotients of 0-5- and 5-10-cm layer/O horizon were calculated. Nearly all compounds decreased in their concentrations from the O horizon to the 5-10-cm mineral soil layer (Figure 4). R + γ-HCH and HCB were below detection limit in the 5-10-cm layer and showed the strongest decrease with soil depth. β-HCH had similar concentrations
in the O horizon and in the 0-5-cm layer and, therefore, a dominant share in the relative HCH + HCB pattern of the 0-5-cm layer (Figure 4). HCB shows a more similar result to R- and γ-HCH although it is even less water-soluble and has a higher KOW than β-HCH (Figure 4, Table 1). According to several studies (64, 65) and our results, β-HCH shows a comparably stronger enrichment in soil. The lower chlorinated PCB28 and PCB52 were detectable in the mineral soil layers but not in the O horizon, which is expressed by the mineral soil layers/O horizon concentration quotients >1 and higher shares in the pattern (Figure 4; detection limits were the same for the O horizon and the mineral soil layers). A similar result has been reported for soil profiles of two forest sites close to a conurbation (66). These two less hydrophobic congeners seem to be more rapidly translocated down the profile. No trend was observed for the concentration quotients from PCB28 to PCB180 and from PAHs with three rings to PAHs with seven rings (Figure 4), which have widely differing S, KOW, and KOC (Table 1). The relative shares of the low chlorinated PCDD/Fs decrease from O to 5-10 cm. The shares of PAHs with seven rings (coronene) and of the highly chlorinated furans (HpCDF and OCDF) tend to increase with increasing soil depth (Figure 4). This is partly caused by lower decreases or even increases (OCDF) of the concentrations from the O horizon to the mineral soil layers. However, OCDD and PCB180 with similar physicochemical properties did not show such a trend or even the opposite (Figure 4, Table 1). The concentration quotients of the PCDD/F give evidence for a trend: The 0-5- and the 5-10-cm/O quotients tend to increase from TCDD to OCDD and from TCDF to OCDF, respectively (Figure 4). Both of these profiles are analogous with a decline in properties of the homologues that favor translocation in soil (e.g., decrease in S, Table 1). The quotients of each dioxin homologue are smaller than the quotients of each corresponding furan homologue with the same degree of chlorination (Figure 4). This indicates that the decrease in concentrations from the O layer to the mineral soil layers is higher for PCDD than for PCDF. However, PCDD tend to be less water-soluble and more hydrophobic than PCDF (Table 1). This conflicts to the increase of the quotients from TCDD to OCDD and from TCDF to OCDF with decreasing S and increasing KOW as described above. Hence, the results for all SOCs give evidence for differences in the accumulation of these compounds in forest soil profiles that cannot be explained by the chosen physicochemical parameters but have reasons that remain to be identified. Besides processes in soil, a possible change in SOC patterns of emission during the past decades could have partly influenced these findings.
Acknowledgments I thank C. Collins, C. Trimbacher, and three anonymous reviewers for their helpful comments on the manuscript. I would also like to acknowledge the efforts of my colleagues from the analytical departments of the Federal Environment Agency who performed the chemical analyses.
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Received for review May 24, 1999. Revised manuscript received February 2, 2000. Accepted February 10, 2000. ES990576S
