Environ. Sci. Technol. 2006, 40, 759-763
Evolution of the Concentrations of Polycyclic Aromatic Hydrocarbons in Burnt Woodland Soils M . S . G A R C ´ı A - F A L C O Ä N,† B . S O T O - G O N Z AÄ L E Z , ‡ A N D J . S I M A L - G AÄ N D A R A * , † Nutrition and Bromatology Group, Analytical and Food Chemistry Department, and Soil and Agricultural Science Group, Plant Biology and Soil Science Department, Faculty of Food Science and Technology, Ourense Campus, University of Vigo, E-32004 Ourense, Spain
Little is known of the fate of polycyclic aromatic hydrocarbons (PAHs) in soils under burnt woodland. It is not clear what the behavior of the overlying wood ash layer will be along months. In this study, the levels of eight representative PAHs in the 1-5 cm layer of a periurban woodland soil that had undergone wildfire were compared with those measured in nearby and distant unburnt periurban woodland soils and in a distant unburnt rural woodland soil, and the levels at the burnt site were monitored during some 10 months. The analytical method optimized for the purpose afforded recoveries of 74-111% (depending on PAH) and repeatabilities (RSDs) better than 9%, with limits of detection ranging from 1 to 7 µg/kg. PAH levels in the 1-5 cm layer of the burnt periurban soil were very similar to those of distant unburnt periurban soil (188 vs 173 µg/kg), about seven times the 26 µg/kg measured in unburnt rural soil, which furthermore contained no detectable quantities of the highest molecular weight PAHs typical of traffic and other urban sources, as the periurban soils did. At the burnt site, PAH levels fell along the months (the total PAH level from 188 to 119 µg/kg), apparently as the result of rainfall and the prevention of further input from the atmosphere by the overlying layer of wood ash, which had a very high PAH adsorption capacity (1169 µg/kg) and did not itself appear to act as a source of PAHs. PAH transport may have been assisted by increased mobilization of PAHs associated with dissolvable organic matter due to an increase in soil pH due to alkaline ash components.
Introduction Due to their ubiquity and persistence, polycyclic aromatic hydrocarbons (PAHs) are present in most soils, where they accumulate because of their low solubility in water (1-9). Forest soils, in particular, receive high inputs of PAHs (and other organic contaminants) because of the large intercepting surface constituted by foliage that eventually falls, leading to the accumulation of PAHs in the organic layer of the soil (10-12). * Corresponding author phone: +34 988 38 7060; fax: +34 988 38 7001; e-mail:
[email protected]. † Nutrition and Bromatology Group, Analytical and Food Chemistry Department. ‡ Soil and Agricultural Science Group, Plant Biology and Soil Science Department. 10.1021/es051803v CCC: $33.50 Published on Web 12/21/2005
2006 American Chemical Society
Although the combustion of wood is a source of PAHs, and high concentrations have been measured in the fumes and particles emitted by wood fires (13, 14), there has been little research on how forest soil PAH content is affected by forest wildfires (15) or by wood ash (16-18), although it has been hypothesized that the increase in soil pH following fire might increase dissolved organic matter (DOM) and hence the mobilization and redistribution of PAHs associated with organic matter (19, 20). Unfortunately, some of this research (15, 17) has been carried out by quantifying PAHs in wood ash by means of methods designed for quantifying PAHs in soil, without any information being available as regards the recovery, precision, or quantification limits of these methods for such an active-carbon-rich matrix as wood ash. In the work described in this paper, we compared PAH levels measured in a periurban woodland soil after wildfire with those measured in nearby and distant unburnt periurban woodland soils and in a distant unburnt rural woodland soil (for this purpose we optimized the extraction and quantitation of representative PAHs from soil) and monitored PAH levels at the burnt site during some 10 months, relating this evolution to rainfall and pH and to the evolution of the overlying ash layer and the pH of the ash-soil interface.
Materials and Methods Chemicals and Other Materials. The eight PAHs studied (chosen because of their proven toxicity and/or carcinogenicity) were fluoranthene (99% pure; F), benzo[b]fluoranthene (98%; B[b]F), benzo[k]fluoranthene (98%; B[k]F), benzo[a]pyrene (97%; B[a]P), benzo[ghi]perylene (98%; B[ghi]Pl), indeno[1,2,3-cd]pyrene (98%; I[1,2,3-cd]P), benzo[a]anthracene (98%; B[a]A), and dibenzo[ah]anthracene (97%; DB[ah]A); all were purchased from Aldrich or Supelco. HPLCgrade acetonitrile, water, dichloromethane, acetone, ethyl acetate, and hexane were supplied by Merck. Potassium hydroxide (85%) and hydrochloric acid (37%) were purchased from Panreac. Samples were purified and concentrated by solid-phase extraction (SPE) on Waters Sep-Pak silica (690 mg) or octadecylsilica (360 mg) cartridges using a Visiprep solidphase extraction vacuum manifold with a Visidry drying attachment for simultaneous processing of up to 24 SPE cartridges. Analytical grade C-45 nitrogen was supplied by Carburos Meta´licos (Vigo, Spain). Conventional apparatus included a rotary evaporator, an ultrasonic bath, an oven, an analytical scale, an up-and-down shaker, and a vortex shaker, and disposables included a sieve (0.2 mm), nylon filters (0.45 µm), micropipets (200-1000 µL), and injection vials (2 mL) provided with screw caps and PTFE-lined butyl rubber septa and inserts (0.35 mL). Study Area and Samples. The study area, in the vicinity of Ourense (northwestern Spain) has a mean annual rainfall of 817 mm and an annual mean temperature of 14.5 °C. Woodland soils in this area are sandy loam textured Umbric Leptosols (21) under Atlantic woodland with a predominance of Pinus pinaster and Pinus sylvestris together with Quercus robur, Quercus suber, Arbutus unedo and the shrubs Ulex europaeus, Erica umbellata, Erica cinerea, and Erica ciliaris. Because of periodic wildfires, their biomass density is generally less than 15 Tm/ha (22). Samples were taken from four woodland sites in this area, as follows. Burnt Periurban Woodland. On June 22, 2003, wildfire burnt an area of about 2 ha close to a busy road, constituting a pre- and postfire source of PAHs. Samples of ash, the ashsoil interface (0-1 cm deep), and soil (1-5 cm) were taken on July 2, July 22, September 18, and December 19, 2003, and VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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on May 6, 2004. In each case, three samples of each kind were collected from 50 × 50 cm squares using a stainless steel trowel. Distant Unburnt Periurban Woodland. On the first of the dates on which the burnt area was sampled, July 2, 2003, samples of unburnt soil were collected at depths of 0-5, 5-7 and 7-10 cm (three samples for each depth) from a site that was likewise close to a busy road but about 5 km from the burnt area. Unburnt Rural Woodland. Likewise, on July 2, 2003, samples of unburnt soil were collected at the depth of 0-5 cm (three samples) from rural woodland about 20 km from the burnt area. Nearby Unburnt Periurban Woodland. On the last two of the dates on which the burnt area was sampled, December 19, 2003, and May 6, 2004, three samples of unburnt soil (1-5 cm) were collected approximately 500 m from the burnt area, at a site that had been downwind of the burnt site at the time of the fire. All samples were transported in plastic bags to the laboratory, minimizing the headspace air in contact with the samples. The sample contact time in the bag was not more than 1 h. In the laboratory, samples were air-dried and stored at 0-4 °C pending analysis. Air-drying and refrigeration of soils had no effect in losses of the studied PAHs. Characterization of Soils. Soil texture was determined by measuring the proportions of sand (the 0.05-2.00 mm fraction), silt (0.002-0.05 mm), and clay ( 0.9997). Quality Parameters of the Soil Analysis Procedure. Because extraction with dichloromethane followed by purification with a silica cartridge fully recovered all PAHs in burnt woodland soil, method characterization was performed with PAH-spiked rural woodland soil. Rural woodland soil (0.5 g) was then fortified with all PAHs in an hexane solution (0.5 mL), and the spiked sample was analyzed after being left 12 h overnight protected from light under refrigeration. The complete evaporation of the hexane was reached with the help of a nitrogen purge if necessary. The results are summarized in Table 2; all recoveries percentages were higher than 74%, while percent RSDs were lower than 9%. There
TABLE 3. Individual and Overall PAH Levels in Woodland Soils at a Depth of 1-5 cm (µg/kg), Together with Nitrogen, Carbon, and Calculated Organic Matter Contents (mean ( SD; n ) 3) and Ash Layer Burdens unburnt rural woodland sampling datea ash layer (kg/ha) ash pH ash N% ash C% F B[a]A B[b]F B[k]F B[a]P DB[ah]A B[ghi]Pl I[1,2,3-cd]P ∑PAHs N (%) C (%) organic matter (%) a
02/07/03
unburnt, distant periurban woodland
02/07/03 7103 10.7 1.3 ( 0.1 28.9 ( 9.0 13 ( 1 55 ( 2 64 ( 9 ndb 14 ( 1 15 ( 3 7(1 32 ( 1 35 ( 4 4(1 16 ( 1 17 ( 3 2(1 9(1 8(1 nd nq nq nd 22 ( 3 22 ( 2 nd 25 ( 1 27 ( 4 26 173 188 0.4 ( 0.01 0.9 ( 0.02 0.5 ( 0.01 7.3 ( 0.3 13.6 ( 0.5 8.7 ( 0.4 13 23 15
Format: day/month/year.
b
02/07/03
unburnt nearby periurban woodland
burnt periurban woodland 22/07/03 6896 10.2 1.7 ( 0.2 27.3 ( 6.3 75 ( 16 19 ( 3 38 ( 5 18 ( 2 9(1 nq 17 ( 2 28 ( 4 204 0.6 ( 0.01 8.6 ( 0.4 15
18/09/03 6790 9.1 1.3 ( 0.1 19.6 ( 13.2 66 ( 14 17 ( 3 40 ( 6 22 ( 2 9(1 nq 23 ( 3 32 ( 4 209 0.6 ( 0.01 8.5 ( 0.4 15
19/12/03 4518 7.4 1.6 ( 0.1 23.2 ( 5.1 52 ( 5 14 ( 2 34 ( 4 17 ( 2 8(1 nq 21 ( 2 19 ( 2 165 0.5 ( 0.01 7.6 ( 0.3 13
06/05/04 0
19/12/03
06/05/04
31 ( 10 13 ( 3 28 ( 4 13 ( 2 7(1 nq 12 ( 2 15 ( 3 119 0.6 ( 0.01 7.4 ( 0.3 13
179 ( 10 69 ( 12 109 ( 11 57 ( 5 39 ( 5 nq 68 ( 12 80 ( 9 601 0.4 ( 0.01 5.7 ( 0.2 10
255 ( 50 95 ( 21 129 ( 29 73 ( 14 48 ( 9 nq 76 ( 28 115 ( 25 791 0.5 ( 0.01 5.8 ( 0.2 10
nd, not detected; nq, not quantifiable.
were no matrix effects affecting results when spiked rural woodland soil samples collected at different places with a varying degree of carbon content (5-13%) were analyzed according to the proposed procedure, since the results obtained were not statistically significantly different at the 95% probability level. Table 2 also lists limits of detection (LOD) and quantitation (LOQ), calculated in accordance with ACS norms (23), as the analyte contents producing height signals respectively 3 and 10 times the average noise level recorded in analyses of uncontaminated real soil samples (seven in this study). LOD and LOQ were defined as the concentration of the analyte that produced a signal-to-noise ratio of 3 and 10, respectively, and were then tested experimentally by spiking blank samples at such levels. Determination of PAH Adsorption Capacity of Ashes and Ash-Soil Interface. Ashes. In each of five centrifuge tubes were placed 0.07 g of ashes. In the tubes were also placed increasing volumes (0.25, 0.5, 1, 3, and 5 mL, respectively) of a solution of eight PAHs in dichloromethane at individual levels of 1000-2500 µg/L, taking always the final solvent volume to 5 mL. These mixtures were shaken for 12 h, filtered, and concentrated to dryness, the residues were redissolved in acetonitrile, and PAHs were determined by HPLC as described in the previous subsection. Adsorbed amounts of PAH were calculated by difference. Ash-Soil Interface. The above procedure was repeated using 0.07 g of ash-soil interface per centrifuge tube.
Results and Discussion Adsorption of PAHs by Ashes and the Ash-Soil Interface. No PAHs were detected with this method at any date in either ashes or ash-soil interface, presumably because of their effectively irreversible adsorption by active carbon. However, experiments carried out as described in Materials and Methods to determine the adsorption capacity of ash and ash-soil interface showed that, given the failure to detect PAHs in the extractant, the total PAH contents of ash and ash-soil interface must have been less than 1169 and 130 µg/kg, respectively. The fact that other authors have been able to report the PAH contents of ash produced by wildfire may be attributed to their ashes doubtless having had a different constitution than ours, due to differences in the respective combustion processes (15, 18). Effects of Burning on Soil PAH Content and Its Evolution. Ten days after the wildfire, PAH levels in the 1-5 cm layer
of the burnt soil were very similar to those of distant unburnt periurban soil (Table 3), suggesting that almost all PAHs produced by the fire that remained at the burnt site had been adsorbed by the ash and ash-soil interface. The total PAH contents (∑PAHs) at these sites were 188 and 173 µg/kg, about seven times the 26 µg/kg measured in unburnt rural soil, which furthermore contained no detectable quantities of the highest molecular weight PAHs typical of traffic and other urban sources, as the periurban soils did. Ten months after the wildfire, the total PAH content of the 1-5 cm layer at the burnt site had fallen from 188 to 119 µg/kg. By contrast, the total PAH content in unburnt woodland 500 m downwind of the fire, 791 µg/kg, was almost 5 times greater than the 173 µg/kg that had been measured at the distant periurban site. The notion that this high level was due to PAHs in the fire smoke having been intercepted by the vegetation and subsequently incorporated into the organic layer of the soil is supported by the fact that the levels of the lower molecular weight PAHs typical of wood combustion were 4-7 times higher than at the distant site, whereas those of highest molecular weight PAHs were only 3-5 times higher. Note that the difference in organic matter content between the nearby and distant unburnt periurban soils (Table 3) is insufficient to account for the difference in PAH levels. PAH levels in the 1-5 cm layer of the burnt soil remained more or less constant during the first 3 months after burning (possibly due to lack of rain and to fresh PAH adsorbed by the ash layer) and then, over the following 7 months, fell to about 57% of their peak level (Table 3 and Figure 1). This decline is probably attributable to the existing PAH content being transported to lower soil layers by rainfall while the ash layer continued to prevent fresh input of PAHs from the atmosphere. Furthermore, this transport may have been assisted by increased mobilization of PAHs associated with dissolvable organic matter due to an increase in soil pHH2O from pH 4 to pH 5 that occurred toward the end of the study (Figure 2a), an increase attributable to the leaching of alkaline ash components from ash; such a role has previously been attributed to pH (19, 20, 24). In the 0-1 cm layer, pH rose from 4 to 7 as an immediate result of the fire due to input of alkaline ash components, but with the first rainfall the pH fell to about 6, thereafter falling only very slightly throughout the remainder of the study (Figure 2b). VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Variation of total soil PAH levels over time in relation to rainfall. active. As Bundt et al. (17) have pointed out, fast leaching of PAHs along preferential pathways in well-structured soils may endanger groundwater quality.
Acknowledgments This work was supported by the Spanish Ministry of Science and Technology through a Ramo´n y Cajal research contract awarded to M.S.G.-F.
Literature Cited
FIGURE 2. Variation of pH over time, in relation to rainfall, in the 1-5 cm soil layer (a) and the ash-soil interface (b). Black dots, pH in water; white dots, pH in KCl. There is evidence that the elimination of PAHs from the upper soil can occur largely in the neighborhood of preferential paths of water flow, not only because of relatively greater rates of transport in association with dissolved organic matter but also because these are the regions of greatest root growth and microbial degradation activity (25, 26). Given the low overall mobility of PAHs in soils (5, 27, 28), the decline in PAH levels observed in this study in the 1-5 cm layer of the soil protected from ongoing PAH input by an ash layer suggests that PAH sinks of these or other kinds were indeed 762
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(1) Sims, R. C.; Overcash, M. R. Fate of polynuclear aromatic compounds (PNAs) in soil-plant systems. Res. Rev. 1983, 88, 1-68. (2) Hartmann, R. Polycyclic aromatic hydrocarbons (PAHs) in forest soils: Critical evaluation of a new analytical procedure. Intern. J. Environ. Anal. Chem. 1995, 62, 161-173. (3) Wild, S. R.; Jones, K. C. Polynuclear aromatic hydrocarbons in the United Kingdom environment: A preliminary source inventory and budget. Environ. Pollut. 1995, 88, 91-108. (4) Meharg, A. A.; Wright, J.; Dyke, H.; Osborn, D. Polycyclic aromatic hydrocarbon (PAH) dispersion and deposition to vegetation and soil following a large scale chemical fire. Environ. Pollut. 1998, 99, 29-36. (5) Krauss, M.; Wilcke, W.; Zech, W. Polycyclic aromatic hydrocarbons and polychlorinated biphenyls in forest soil: Depth distribution as indicator of different fate. Environ. Pollut. 2000, 110, 79-88. (6) Howsam, M.; Jones, K. C.; Ineson, P. Dynamics of PAH deposition, cycling and storage in a mixed deciduous (QuercusFraxinus) woodland ecosystem. Environ. Sci. Pollut. 2001, 113, 163-176. (7) Ribes, A.; Grimalt, J. O.; Torres-Garcı´a, C. J.; Cuevas, E. Polycyclic aromatic hydrocarbons in mountain soils of the Subtropical Atlantic. J. Environ. Qual. 2003, 32, 977-987. (8) Nadal, M.; Schuhmacher, M.; Domingo, J. L. Levels of PAHs in soil and vegetation samples from Tarragona County, Spain. Environ. Pollut. 2004, 132, 1-11. (9) Wong, F.; Harner, T.; Liu, Q.-T.; Diamond, M. L. Using experimental and forest soils to investigate the uptake of polycyclic aromatic hydrocarbons (PAHs) along an urban-rural gradient. Environ. Pollut. 2004, 129, 387-398. (10) Matzner, E. Annual rates of deposition of polycyclic aromatic hydrocarbons in different forest ecosystems. Water Air Soil Pollut. 1984, 21, 425-434. (11) Berteigne, M.; Lefe`vre, Y.; Rose, C. Accumulation des polluants organiques (H. P. A.) dans les horizons humife`res des sols. Eur. J. For. Path. 1988, 18, 310-318. (12) Wilcke, W.; Zech, W.; Kobza, J. PAH pools in soils along a PAH deposition gradient. Environ. Pollut. 1996, 92, 307-313. (13) Freeman, D. J.; Cattell, F. C. R. Woodburning as a source of atmospheric polycyclic aromatic hydrocarbons. Environ. Sci. Technol. 1990, 24, 1581-1585.
(14) Baek, S. O.; Field, R. A.; Goldstone, M.; Kirk, P. W.; Lester, J. N.; Perry, R. A review of atmospheric polycyclic aromatic hydrocarbons: Sources, fate and behavior. Water Air Soil Pollut. 1991, 60, 279-300. (15) Kim, E. J.; Oh, J. E.; Chang, Y. S. Effects of forest fire on the level and distribution of PCDD/Fs and PAHs in soil. Sci. Total Environ. 2003, 311, 177-189. (16) Diebel, J.; McGinnis, G. M.; Pughani, J.; Shetron, S.; Jurgensen, M. The environmental fate of wood ash applied to soils, p 359368. In Proceedings of Waste Wood Processing and Combustion for Energy, 5th Annual National Biofuels Conference and Exhibition (19-22 Oct), Boston, MA, 1992. (17) Bundt, M.; Krauss, M.; Blaser, P.; Wilcke, W. Forest fertilization with wood ash: Effect on the distribution and storage of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). J. Environ. Qual. 2001, 30, 1296-1304. (18) Rey-Salgueiro, L.; Garcı´a-Falco´n, M. S.; Soto-Gonza´lez, B.; SimalGa´ndara, J. Procedure to measure the level of polycyclic aromatic hydrocarbons in wood ashes used as fertilizer in agroforestry soils and their transfer from ashes to water. J. Agric. Food Chem. 2004, 52 (12), 3900-3904. (19) Chiou, C. T.; Malcolm, R.; Brinton, T. I.; Kile, D. E. Water solubility enhancement of some organic pollutants and pesticides by dissolved humic and fulvic acids. Environ. Sci. Technol. 1986, 20, 502-508. (20) McCarthy, J. F.; Zachara, J. M. Subsurface transport of contaminants. Environ. Sci. Technol. 1989, 23, 496-502. (21) FAO. Leyenda revisada del mapa mundial de suelos. FAO, Rome, 1990.
(22) Diaz-Fierros, F.; Paz, A. and Calvo de Anta, R. M. As especies forestais e os solos de Galicia. Consello da Cultura Galega, Santiago de Compostela, Spain, 1982. (23) American Chemical Society: Subcommittee on Enviromental Analytical Chemistry. Guidelines for data acquisition and data quality evaluation in environmental chemistry. Anal. Chem. 1980, 52 (14), 2242-2249. (24) Garcı´a-Falco´n, M. S.; Pe´rez-Lamela, C.; Simal-Ga´ndara, J. Strategies for the extraction of free and bound polycyclic aromatic hydrocarbons in runoff waters rich in organic matter. Anal. Chim. Acta 2004, 508 (2), 177-183. (25) Pierret, A.; Moran, C. J.; Pankhurst, C. E. Differentiation of soil properties related to the spatial association of wheat roots and soil macropores. Plant Soil 1999, 211, 51-58. (26) Vinther, F. P.; Eiland, F.; Lind, A. M.; Elsgaard, L. Microbial biomass and number of denitrifiers related to macropore channels in agricultural and forest soils. Soil Biol. Biochem. 1999, 31, 603-611. (27) Jones, K. C.; Stratford, J. A.; Waterhouse, K. S.; Vogt, N. B. Organic contaminants in Welsh soils: Polynuclear aromatic hydrocarbons. Environ. Sci. Technol. 1989, 23, 540-550. (28) Guggenberger, G.; Pichler, M.; Hartmann, R.; Zech, W. Polycyclic aromatic hydrocarbons in different forest soils: Mineral horizons. Z. Pflanzenerna¨hr. Bodenkd. 1996, 159, 565-573.
Received for review September 11, 2005. Revised manuscript received November 15, 2005. Accepted November 17, 2005. ES051803V
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