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a Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL), Zürcherstr. 111, CH-8903 Birmensdorf, Switzerland
b Institute of Soil Science and Soil Geography, University of Bayreuth, 95440 Bayreuth, Germany
* Corresponding author (bundt{at}wsl.ch)
Received for publication August 14, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: ACE, acenaphtene • ACY, acenaphtylene • ANT, anthracene • BAA, benz(a)anthracene • BAP, benzo(a)pyrene • BBJK, benzo(b+j+k)fluoranthene • BEP, benzo(e)pyrene • BGHI, benzo(ghi)perylene • CT, chrysene + triphenylene • DBAH, dibenz(a,h)anthracene • FLA, fluoranthene • FLU, fluorene • IND, indeno(123-cd)pyrene • LCPCB, lower chlorinated polychlorinated biphenyl • LMPAH, low molecular weight polycyclic aromatic hydrocarbon • NP, naphtalene • PAH, polycyclic aromatic hydrocarbon • PCB, polychlorinated biphenyl • PER, perylene • PHE, phenanthrene • PYR, pyrene
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In most studies on PAH and PCB concentrations in soil, the homogenized bulk soil was analyzed. However, organic contaminants are heterogeneously distributed in soils. Wilcke and Zech (1997) reported higher PAH concentrations in soil samples taken from near the stem of beech (Fagus sylvatica L.) trees as compared with samples taken between trees, and attributed this to increased inputs with stem flow. On a smaller scale, Wilcke et al. (1996a)(b) found higher PAH concentrations at the surfaces of soil aggregates than in their cores. The authors explain their findings with preferential water movement along the aggregate surfaces leading to an increased sorption of the PAHs at the aggregate surfaces.
Preferential flow of water and solutes has been shown for a large variety of soils (Sollins and Radulovich, 1988; Ghodrati and Jury, 1992; Flury and Flühler, 1994b). The rapid water movement through a small fraction of the soil volume leads to a faster transport of solutes, particles, and contaminants. There is evidence that preferential flow paths are not only regions of increased transport, but also of enhanced root growth and microbial activity (Pierret et al., 1999; Vinther et al., 1999). Therefore, organic contaminants stored in or near preferential flow paths might represent an active pool, whereas contaminants stored in the matrix regions might have slower turnover rates.
The objective of this study was to examine the effects of an experimental application of wood ash on the concentrations, storage, and distribution of PAHs and PCBs in a Swiss forest soil. Special consideration was dedicated to the distribution of these contaminants between soil material from preferential flow paths and from the soil matrix.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil and organic layer samples were taken from all plots before the wood ash application in April 1998. In May 1999 (i.e., 1 yr after wood ash application), we again collected mineral soil and organic layer samples from the control subplots and the wood-ash subplots. To determine the depth distribution of organic contaminant concentrations, soil samples from one control subplot were used.
Sampling Procedure
The sampling procedure included an experiment to stain the preferential flow paths in the soil. With a field sprinkler, 45 mm of deionized water containing 3 g of the food dye Brilliant Blue (CI 42090) per liter was applied in 6 h (Flury and Flühler, 1994a,b). One day after dye application, a trench was opened to 1.2 m depth. A vertical soil profile of 1 by 1 m was prepared 0.3 m away from the plot's border. Photos were taken (Fig. 2) to estimate the dye coverage of each profile. These were used to determine the volumetric proportions of preferential flow paths and soil matrix. The blue stained areas were defined as preferential flow paths, the nonstained areas as soil matrix. As a mean of 40 profile photos, we calculated the volumetric proportions of preferential flow paths as 74, 34, 9, and 1% in 0 to 9, 9 to 20, 20 to 50, and 50 to 100 cm depth, respectively. Samples of the preferential flow paths and of the matrix were taken with a small spatula at various locations distributed over the whole width and depth of the horizon (0–9 cm). Additionally, we took representative bulk soil samples. For the depth distribution of PAHs and PCBs, soil material from 0- to 9-, 9- to 20-, 20- to 50-, and 50- to 100-cm depths was sampled. These sampling depths corresponded approximately to the morphological soil horizons. In April 1998, we sampled the total organic layer (bulk organic layer) and in May 1999 we sampled Oi, Oe, and Oa horizons separately.
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We quantified 20 PAHs and 14 PCBs: naphthalene (NP), acenaphthylene (ACY), acenaphthene (ACE), fluorene (FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLA), pyrene (PYR), benz(a)anthracene (BAA), chrysene + triphenylene (CT), benzo(b+j+k)fluoranthenes (BBJK), benzo(a)pyrene (BAP), benzo(e)pyrene (BEP), perylene (PER), indeno(1,2,3-cd)pyrene (IND), dibenz(a,h)anthracene (DBAH), benzo(ghi)perylene (BGHI), and PCB Congeners 1, 8, 20, 28, 52, 35, 101, 118, 138, 153, 180, 199, 206, and 209 (numbers according to Ballschmiter and Zell, 1980).
The air-dried and homogenized samples were extracted with hexane and acetone (2:1) in an accelerated solvent extractor (Dionex [Sunnyvale, CA] ASE 200) as described in Krauss et al. (2000). All samples were purified with a column filled with aluminum oxide and silica. Details of the purification procedure are found in Wilcke et al. (1999a). Extracts containing a high amount of waxes, which interfered with PER and IND signals in mass-selective detection, were further purified using columns filled with 1 g HR-P resin (polystyrene-divinylbenzene copolymer; Macherey–Nagel, Dueren, Germany). Aliphatic compounds were eluted with 10 mL hexane, PAHs were eluted with 20 mL toluene.
For the determination of PCBs, the extracts were additionally purified with an acid–base silica column (Wilcke et al., 1999b).
A Hewlett–Packard (Palo Alto, CA) 5890 Series II gas chromatograph equipped with a Hewlett–Packard 5-MS fused silica capillary column (30 m x 0.25 mm x 0.25 µm) was used with He as the carrier gas (constant pressure mode 80 kPA) and splitless injection. Compounds were detected with a Hewlett Packard 5971A mass selective detector with electron impact ionization in selected ion monitoring mode. Details of the gas chromatography (GC) program are given in Wilcke et al. (1999a) for PAHs and in Wilcke et al. (1999b) for PCBs.
Eight deuterated PAHs (NP-D8, ACE-D10, FLU-D10, ANT-D10, PYR-D10, chrysene-D12, PER-D12, BGHI-D12) and seven 13C-labelled PCBs (Congeners 28, 52, 101, 138, 153, 180, 209) were used as internal standards for quantification and spiked to the soil samples prior to extraction. To check the recovery of the internal standards and thus the quality of the analytical procedure, fluoranthene-D10 was spiked to the extracts prior to injection into the gas chromatograph. The average recoveries and standard deviations of the internal standards ranged from 74 ± 18% to 93 ± 16% (n = 94, silica–alox cleanup) and 48 ± 17% to 85 ± 10% (n = 36, HR-P cleanup) for the PAHs and from 83 ± 13% to 91 ± 11% (n = 125) for PCBs. Details on the principle of the quantification method are found in Kjeller (1998). The influence of background contamination as determined with seven blanks was negligible.
Calculations and Statistical Analysis
The increase or decrease in storage of PAHs and PCBs was calculated using the following equation:
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storagex is the increase or decrease in storage of x = PAHs or PCBs; h is the number of horizons for which the storage was calculated; ch is the concentration of organic contaminants in the respective horizon 1 yr after the wood ash application (ash) and without wood ash (control); zh is the thickness of a given horizon, and 
h is its bulk density. The differences in PAH and PCB concentrations between preferential flow paths and soil matrix and between the control and the wood ash treatments were tested with analysis of variance (Conover, 1980) using the software S+ (MathSoft, 1996). The residuals were checked for normality and independence using the normal probability plot and the Tukey–Anscombe plot.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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20 PAH) concentrations was lower than in the organic layer. The fraction of low molecular weight PAHs (LMPAHs: NP, ACY, ACE, FLU, PHE, ANT) decreased from the Oi horizon to the Oa horizon and increased slightly in the A horizon (Table 2), which is consistent with other studies (Pichler et al., 1996; Wilcke and Zech, 1997; Krauss et al., 2000). The increase of PAH concentrations and decrease of the fraction of LMPAHs with depth in the organic layer is explained by the faster mineralization of organic matter than of PAHs and by preferential degradation and leaching of the LMPAHs.
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14 PCB) concentrations in the bulk organic layer ranged between 21.7 and 48.8 µg kg-1 with a mean of 35.3 µg kg-1. This is considerably higher than values in forest soils in southern Germany (Krauss et al., 2000). The increase in the 14 PCB concentrations from the Oi to the Oe and Oa horizon was approximately 30% (Table 2). Similar to the PAH concentrations, the PCB concentrations in the mineral soil were substantially lower than in the organic layer. As for LMPAHs, there was a decrease of the fraction of lower chlorinated polychlorinated biphenyls (LCPCBs: PCB Congeners 1, 8, 20, 28, 52, 35) in the order Oi > Oe > Oa horizon. Lower chlorinated PCBs are more easily degradable, more volatile, and more water soluble than the higher chlorinated PCBs, which can hardly be degraded by microorganisms (Hankin and Sawheney, 1984; Mackay et al., 1992). Therefore, higher chlorinated PCBs become relatively enriched with increasing degradation of soil organic matter.
Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls in the Wood Ash
There is little information in the literature about concentrations of PAHs and PCBs in wood ash. Although PAHs are formed during the combustion process and high concentrations were measured in fumes and particles emitted from wood fires (Freeman and Cattell, 1990; Baek et al., 1991), the ash itself was seldom studied. In the wood ash used in our experiment, the 20 PAH concentration was 16.8 mg kg-1. This was rather high and nearly reached the proposed threshold value of 20 mg kg-1 for the application of secondary materials to agricultural field soils (Zollner et al., 1997). It was also high compared with the concentrations in the forest soil (Table 2, Fig. 3), and with annual deposition rates. These were estimated to be 2 to 4 mg m-2 yr-1 for Germany and approximately 0.8 mg m-2 yr-1 for the UK (Führ et al., 1986; Wild and Jones, 1995). Therefore, the PAH input with 8 Mg wood ash ha-1 (Table 3) equals approximately 3 to 16 times the annual atmospheric deposition rate.
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14 PCBs in the wood ash of 3.4 µg kg-1 were low compared with other soil amendments such as sewage sludge and compost with concentrations of 1200 to 1600 µg kg-1 and 260 µg kg-1, respectively (Fiedler, 1993). They were in the same order of magnitude as the mineral topsoil (Table 2). The PCBs 199, 206, and 209 could not be detected in the wood ash. Annual deposition rates for rural soils in the UK were estimated to be 14 µg m-2 yr-1 as the 8 PCBs and 48 µg m-2 yr-1 as the 44 PCBs (Harrad et al., 1994). Thus, the input of 3 µg m-2 with the wood ash (Table 3) was small in comparison with annual deposition rates in rural areas.
Effects of Wood Ash Application
In the organic layer, the PAH concentrations were significantly increased (P < 0.05) 1 yr after wood ash application in all of the four plots (Table 2, Fig. 3). The increase was most pronounced in the Oe and Oa horizon. The small change in the Oi horizon is not astonishing, because most of the leaves, twigs, and needles of the Oi horizon sampled in May 1999 reached the forest floor in autumn 1998, half a year after the wood ash application.
The PAH spectra of the three organic layers of the control plots were dominated by PHE and to a slightly smaller extent by FLA, BBJK, NP, PYR, and CT (Fig. 3). One year after the wood ash application, the spectra were still similar as before in the Oi horizon, but in the Oe and Oa horizon, they approached the wood ash spectrum (Fig. 3). The total input of PAHs with the wood ash was 13.4 mg m-2, while the increase in storage of the organic layer was 24.9 mg m-2 (Table 3). This large increase, however, was mainly due to the high storage increase of one of the four plots, which we cannot explain. Possibly, the plot was contaminated with PAHs from other sources than the experimental wood ash application. Omitting this plot, the increase in PAH storage in the organic layer was 12.5 mg m-2, corresponding well with the wood ash input. The input of individual PAHs with the wood ash significantly correlated with the increase in storage in the organic horizons (Fig. 4). This strongly suggests that the PAHs, which were applied to the forest soil with the wood ash, were stored efficiently in the organic layer without much alteration in the course of 1 yr. Consistently, total PAH concentrations in the A horizon were not significantly (P = 0.15) affected by the wood ash application (Table 2). Our findings agree well with those of Deschauer (1995), who found that 2 yr after compost application to a forest soil, the added PAHs were mainly retained in the organic layer and were not recovered in the soil solution or in mineral soil horizons.
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20 PAH concentrations remained almost constant, the fraction of LMPAH increased in the A horizon after wood ash application. This was mainly due to naphthalene, which contributed the largest proportion (14%) of all individual PAHs to the 20 PAH concentrations in the wood ash. Naphthalene is rather mobile in the soil and was probably leached through the organic layer into the mineral topsoil. At 0 to 9 cm depth, PHE, FLA, and BBJK showed the strongest decrease after the wood ash application. This might be due to facilitated transport with dissolved organic matter into deeper soil layers (McCarthy and Zachara, 1989).
In contrast to PAHs, PCB concentrations were decreased in the organic layer of the wood ash plots compared with the control plots (Table 2). Despite an input of 3 µg PCBs m-2, the 14 PCB storage in the organic layer decreased by 34 µg m-2 or by 3 µg m-2 when omitting the same plot as above for PAHs from the calculation (Table 3). The data are quite variable, but the decrease was consistent for all plots in the Oe horizon. Of particular interest was the decrease of higher chlorinated PCBs in the Oe horizon. These compounds are hardly degradable by microorganisms under aerobic conditions and are not volatile (Hankin and Sawheney, 1984; Dmochewitz and Ballschmiter, 1988; Abramowicz, 1990). Therefore, the decrease was probably due to transport into the mineral soil. Because the concentration of PCBs in the wood ash was small in comparison with the concentration already stored in the organic layer, the spectrum of the PCBs changed only slightly in the Oe horizon after the wood ash application (Fig. 5).
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While adding wood ash to the forest soil increased the storage of PAHs in the organic layer without much alteration during 1 yr, PCBs, which were stored in the organic layer of the forest soil, were mobilized and transported to deeper depths, or degraded. This was probably due to the enhanced mineralization after the wood ash application and/or due to a pH effect. The alkaline wood ash increased the pH in percolating water below the organic layer from 6.5 to above 9 two weeks after the wood ash application and increased concentrations of dissolved organic carbon (DOC) from 20 to more than 450 mg DOC L-1 (S. Zimmermann, Swiss Federal Institute for Forest, Snow, and Landscape Research, personal communication). However, the effects on PCBs were mostly restricted to the organic layer, and were not detectable in the mineral soil.
Distribution and Dynamics
In the mineral soil, the 20 PAH concentrations was higher in the preferential flow paths than in the matrix at all of the studied depths, except for the 50- to 100-cm layer (Table 4). The higher PAH concentrations in the preferential flow paths down to 50 cm were most likely due to the higher input into these zones. However, preferential flow paths are also zones of preferential leaching and degradation, which limits the size of the concentration gradient between preferential flow paths and matrix. The preferential degradation can be attributed to a higher microbial biomass in the preferential flow paths than in the soil matrix of the studied forest soil (unpublished data, 2000). Consistently, other authors also reported enhanced degradation of organic substances such as pesticides in macropores as compared with the soil matrix (Pivetz and Steenhuis, 1995; Mallawatantri et al., 1996). The lower 20 PAH concentrations in the preferential flow paths than in the matrix of the subsoil (50 to 100 cm) may be the result of higher degradation than input rates. This assumption is supported by the overall low mobility of PAHs in soils (Jones et al., 1989; Guggenberger et al., 1996; Krauss et al., 2000).
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20 PAH concentrations, except for perylene. The distribution of PAHs between preferential flow paths and the matrix was controlled by the octanol–water partition coefficient (Kow), which is a measure for the hydrophobicity of a given compound. For PAHs and PCBs, the Kow is correlated to the molecular weight of the compounds. The importance of the Kow for the distribution of PAHs was indicated by the increasing ratio of individual PAH concentrations in the preferential flow paths to those in the matrix at 0 to 9 cm depth with increasing Kow of the compounds (Fig. 6, top). After wood ash application, the slope of the regression line between Kow and the ratio of PAH concentrations between preferential flow paths and the matrix increased. This was due to higher concentration ratios for the higher molecular weight PAHs with high Kow values, while the concentration ratios for the LMPAHs with lower Kow values remained rather stable. There may be three reasons for our finding: (i) the leaching of the more mobile LMPAHs to greater depths, (ii) the enhanced degradation of LMPAHs in preferential flow paths, and (iii) the preferential sorption of PAHs with a higher affinity to soil organic matter in the preferential flow paths, since SOM is enriched in those flow regions (Table 1).
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The concentrations of PCBs at 0 to 9 cm depth were higher in preferential flow paths as compared with the soil matrix (Table 4). At 9 to 50 cm depth, no differences in PCB concentrations between preferential flow paths and soil matrix were observed, while at 50 to 100 cm depth, the matrix had higher PCB concentrations than the preferential flow paths. Thus, in the upper part of the mineral soil, PCB input rates in the preferential flow paths were higher than leaching and degradation rates, while in the subsoil the opposite was true. This can be due to the transport pathways being blocked at that depth and organic contaminants being laterally dispersed into the matrix, or due to higher degradation rates in the preferential flow paths at 50 to 100 cm depth. Similar to the PAHs, the ratio of PCB concentrations in preferential flow paths and in the matrix increased linearly with the log Kow of the compounds (Fig. 6, bottom). However, unlike the PAHs, the slope of the regression line did not change 1 yr after the wood ash application. This may be explained by the negligible input of PCBs into the mineral soil after the wood ash application compared with the PCB storage.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Both contaminant classes, in particular their high molecular weight representatives, were accumulated in the preferential flow paths of the A horizon compared with the soil matrix. The enrichment resulted from the higher input into the preferential flow regions and was present in spite of the higher leaching rates in the preferential flow paths and most likely also higher degradation rates. At greater soil depth, degradation or leaching became increasingly more important than accumulation in the preferential flow paths, resulting in lower concentrations in the preferential flow paths than in the matrix in the 50- to 100-cm depth layer. Our results strongly suggest that in the mineral soil, preferential flow paths represent the active and accessible regions for contaminants, whereas organic contaminants stored in the soil matrix probably exhibit slower transport and turnover rates. In well-structured soils, this may result in a fast leaching along preferential pathways and thus may endanger ground water quality. Thus, only wood ashes with low concentrations of organic contaminants should be used for fertilization in forests with low background concentrations of organic contaminants.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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