Polycyclic aromatic
hydrocarbons (PAHs) are priority pollutants that arrive in the environment
from numerous anthropogenic and natural sources, but the data on their
natural sources including wildfires remain insufficient. The level of
contamination and the composition of PAHs in soils of the areas affected by
wildfires were studied in this work. The study was conducted in the Moscow
region (Russia) in areas occupied by drained peatland and strongly damaged by
fires in 2002, 2010 and 2012. The features of PAH accumulation and the
profile distributions in histosols and histic podzols after the fires of
different times were analyzed. It was shown that new soil horizons formed
after the fires – Cpir, Hpir and incipient O horizons – and that these
horizons differ in PAH accumulation rate. Maximal total concentrations of 14
PAHs were detected in charred peat horizons Hpir (up to 330 ng g
Polycyclic aromatic hydrocarbons (PAHs) are a group of high molecular weight organic compounds, which include carcinogens and mutagens. PAHs are formed in natural and technogenic processes, and are ubiquitous in different landscape components. Scientific interest in PAHs has remained high during recent decades (Wilcke, 2000; Haritash and Kaushik, 2009; Cai et al., 2008; Gennadiev et al.,1996; Bandowe et al., 2014, Maisto et al., 2006, Baek et al., 1991). Most PAHs in the environment arrive from pyrogenic sources. So far, pyrogenic anthropogenic sources of PAHs (automobile exhaust, various industrial and power plant emissions) are well studied (Khan et al., 2008; Mastral and Callen, 2000; Tsibart and Gennadiev, 2013; Wilcke, 2000, 2007; Agarwal, 2009; Kwon, 2014; Mu et al., 2013) and, nowadays, in developed countries, there is a trend of reducing environmental contamination with PAHs because of improvement in technologies (Guo et al., 2011).
The number of PAHs arriving in the environment from natural pyrogenic sources (wildfires, volcanism) remains uncertain. Numerous works are devoted to PAH formation from vegetation components influenced by flaming and smoldering combustion. Burning conditions (the amount of available oxygen, the duration of heating, the temperature) and the type of vegetation define the amount and composition of PAHs (Ramdahl and Bechler, 1982; Jenkins, 1996; Nussbaumer, 2003; Medeiros and Simoneit, 2008; Simoneit, 1999; Schauer et al., 2001; Nakajima et al., 2007; Fitzpatrick et al., 2008; Kakareka et al., 2004).
The publications devoted to pyrogenic PAHs in the territories affected by wildfires do not cover the range of questions related to this problem. For instance, the peculiarities of PAH composition in the air after a wildfire were studied in several works (Radojevic, 2003; Masclet et al., 1995, Maioli et al., 2009; Freeman and Cattell, 1990; Yuan et al., 2008), but there is an obvious lack of information on pyrogenic PAH accumulation in soils. Some publications are focused on PAHs in different organo-mineral soils that were subjected to fire (Gennadiev and Tsibart, 2013; Dymov et al., 2014; Gonzalez-Vila et al., 1991; Garcia-Falcoan et al., 2006; Vergnoux et al., 2011). It was shown that PAHs accumulated in these soils in small amounts and posed no danger to humans. However, PAH accumulation in soils after smoldering fires causing deep changes in soil profiles was investigated insufficiently (Vane et al., 2013; Gennadiev and Tsibart, 2013; Bojakowska and Sokołowska, 2003).
Peat fires differ from other fire types, because the burning material in this
case is not only the vegetation, but also the soil organic matter. The
conditions present in peat smoldering favor
PAH formation because fires propagate slowly and deep soil horizons are
affected by high temperature (Rein et al., 2008; Hartford and Frandsen,
1992). Moreover, in comparison to flaming fires, smoldering consumes most of
the peat. Also, smoldering drives the spread of the pyrolysis front where
PAHs are produced, and the smoldering process occurs under conditions of low
oxygen and temperature (500–700
It is worth noting that the combustion products differ depending on the peat
type, moisture and the completeness of combustion. Products of pyrolysis are
a gaseous mixture of organic species released into the air. They include
volatile organic compounds, hydrocarbons (CH
In most cases, the scientific literature contains data on pyrogenic PAHs in undrained peat soils. The drained soils, following fire, are not studied from this context, although they are very vulnerable to wildfires (Blake et al., 2009; Zaidel'man et al., 2007). Large amounts of organic matter burn out during the fires, and deep transformations occur in the profiles, in comparison with the soils of undrained territories (Zaidel'man et al., 2007, 1999). The drained peat soils are widespread in densely populated areas of the European part of Russia, and an important task is to reveal levels of accumulation of PAHs in these areas.
The aim of this study was to reveal the features of PAH accumulation in podzols and histosols of drained peatlands affected by wildfires. Study objectives included (1) the comparison of PAH distributions and levels of accumulation in different post-fire soils, histosols and podzols, (2) the identification of the parts of post-fire soil profiles with maximal PAH accumulation, (3) the detection of the trends of different PAH group accumulation, and (4) the comparison of PAH accumulation in soils after fires of different times.
The location of the research site.
The location of the investigated soil pits.
This study was conducted in the soils of Shatura district (Moscow region,
Russia). The burned area is located at coordinates 55
The territory represents the western Meshchera fluvioglacial plain. The major part of the area is covered by fluvial–glacial deposits, although alluvial deposits also occur. The elevation is within the limits of 120–126 m, and the relief is low (Wetlands of Moscow Region, 2008; Zonov and Konstantinovich, 1932).
The investigated area is covered with peatlands overlying ancient alluvial deposits, which are located above the confining clay layer. The development of these wetlands is caused by a flattened relief and a shallow horizon of waterproof clay. The area covered with peat bogs in this part of the Meshchera plain is 15–16 % (Wetlands of Moscow Region, 2008; Zonov and Konstantinovich, 1932; Kudravtseva, 1973).
The investigated plot belongs to the Petrovsko–Kobelevskoe high-moor peat, which is a part of the Shatura wetland area. Its total area is 6443 ha; before the era of peat extraction, the thickness of the peat layer reached 7.5 m, and its average thickness was 2.5–4 m (Zonov and Konstantinovich, 1932).
These large areas were transformed during the drainage and peat mining. Beginning at the end of the eighteenth century, the peat bogs of the Moscow region were used as peat fields. The demand for peat increased in the 1920s, when peat-burning power stations including Shaturskaya station were constructed (Wetlands of Moscow Region, 2008; Simakin, 1958). The peat deposits in the Shatura district were mined beginning in the 1920s (Timashev, 1932). Now mining no longer occurs, and most of the peat bogs in Meshchera bog province are in a stage of recovery (Wetlands of Moscow Region, 2008).
As a result of peat mining, natural bog complexes were changed to large open pits connected to systems of channels and distributaries and to fields of peat mining at different stages of recovery, with birch–aspen forests occurring on their banks. On the plots with flooded peat pits, the process of bog restoration has started, but it will take several centuries before peat deposits will start to accumulate (Sushkova, 2008).
Because of changes in hydrological regime, the number of areas of peat fires has increased dramatically in this region (Wetlands of Moscow Region, 2008). According to Zaidel'man (2003), there is a repeating pattern of the fires in drained peatlands. Almost every 10 years, large fires occur (1972, 1982, 1992, 2002). In 2010, wildfires in Russia damaged large areas and, in the Moscow region, they caused significant air pollution, degradation of ecosystems and health impacts for the population (Donkelaar et al., 2011; Shvidenko et al., 2011).
The soil cover of the area is presented by gleyic histosols, histic podzols and sod podzols (WRB, 2006) variously altered by the wildfires. Podzols are formed under forest vegetation on sandy parent material in terms of water percolation. These soils have an elivio-illuvial distribution of organo-mineral complexes within the profile. Histosols are characterized by the accumulation of organic matter and various compounds in thick peat horizons, and they contain more material available for burning. One important goal of our study was to establish the patterns of PAH accumulation in different soil types and to check if post-fire PAH distribution follows general soil-forming processes.
Background soils are represented by histosols and histic podzols. The
histosols have the following horizons, O–H–Hp–He–Ha–C, and a typical
profile of histic podzols has O–A–E–EBhs–Bhs–Cs horizons. After the
smoldering fire and burning out of the peat layer, the new ash horizons
(Cpir) up to 5 cm in depth form; they are underlaid by
Post-pyrogenic histic podzols are confined to the peripheral part of peatlands; the parent material for them is bare quartz sands, and typically their profile has the following horizons: O–Cpir–Hpir–He–Ha–E–EBhs–Bhs–Bh–BC–C. Post-pyrogenic gleyic histosols occupy large areas and form in the central parts of peatlands; their profile consists of O–Cpir–Hpir–He–Ha–Ch–Cs–Cg (Figs. 3, 4).
It should be pointed out that the thickness of horizons in soils affected by the fire varies, depending on the intensity of the pyrogenic impact. The depth of organogenic horizons in soil pits ranges from 10 to 30 cm at a distance of several meters.
The profile characteristics of the investigated soils.
In order to study PAH accumulation in post-fire soils of different types, the areas covered with histosols (pits 7, 8, 9, 11, 12) and sod podzols (pits 2, 3, 4, 5, 6) were sampled, as well as areas with analogous background soils (pits 1, 10) (Fig. 3).
For the comparison of PAH accumulation in soils after fires of different times, the areas affected by the fires of 2002, 2010 and 2012 were studied. The field work and sampling were conducted in 2012 and, in total, we studied 12 soil pits (Fig. 2).
Soil pits were excavated to a depth of 50–100 cm, depending on the depth of the parent material. Detailed morphological descriptions of soil profiles were made.
Soil profiles investigated. 1-post-pyrogenic histic podzol (pit 6), 2-post-pyrogenic histosol (pit 8).
For the identification of the parts of post-fire soil profiles with maximal PAH accumulation, the samples were collected from each genetic horizon and, in cases of thick horizons (more than 10 cm), samples were taken at 10 cm intervals. Samples were stored in plastic bags and taken to the laboratory, where they were air dried, homogenized and sieved through a 0.25 mm sieve.
The target PAHs in this study were naphthalene, phehanthrene, chrysene, pyrene, anthracene, benz(a)anthracene, benz(a)pyrene, benz(ghi)perylene, fluorene, dibenztiophene, triphenylene, benz(e)pyrene, benz(k)fluorantene, and coronene. This group of compounds includes PAHs from low-molecular-weight to high-molecular-weight compounds (2–7 benzene rings in their structure), which are widespread in the environment.
The quantitative analysis was conducted with the specrtofluorometry method at the temperature of liquid nitrogen (Spolskii spectroscopy) (Alexeeva and Teplitskaya, 1988; Gooijer et al., 2000; Personov, 1981; Gennadiev et al., 1996).
Liquid extraction was used in the analysis. 3 g of air-dried soil samples
were extracted with
The measurements were done on a Jobin Yvon Fluorolog-3-22 spectrofluorimeter. The extract was frozen in liquid nitrogen (77 K). Then, the mixture of PAHs in the frozen extract was irradiated by light with optimum wavelengths for each compound, and the PAH luminescence spectra were recorded (Fig. 5).
The wavelengths of the excitation and emissions of luminescence used for the PAH identifications.
The wavelengths of the excitation and emissions of luminescence used for the
PAH identifications are given in Table 1. Spectral fractionation
(identification of each hydrocarbon by the most optimum excitation and
luminescence wavelengths) was used. High selectivity of the method is
obtained by using a spectra selection of PAHs in multicomponent solutions by
scanning the narrow excitation wave band (Alekseeva and Teplitskaya, 1981).
Identification and quantitative estimations of PAHs were made by comparison
of fluorescence and excitation spectra with the SRM NIST 2250a (36 PAHs
mixture) reference standard solution. Limits of detection (LOD) for each PAH
were
The results were analyzed with STATISTICA 8.0. The distribution of total PAHs
in each horizon type was tested for normality with the Kolmogorov–Smirnov
test, and each group included similar horizons from a similar soil type, so
sample replicates were considered; the number of values in each group was
from 6 to 14. The data did not follow the normal distribution at significance
level
Cluster analysis was conducted to find similarities in the distributions of
the individual PAH compounds. In this analysis, complete linkage was used as
the amalgamation rule, and the distance metric was 1-Pearson
PAH luminescence spectra for SRM NIST 2250a.
PAH concentrations (ng g
The studied soils varied highly in PAH concentrations; their total content
changed from 5 to 330 ng g
PAH concentrations (ng g
PAH concentrations (ng g
The common features of PAH distribution in
Post-fire histosols had various differences in PAH distribution. In pit 9,
the thicknesses of undestroyed peat horizons were greatest among the
investigated soils (approximately 30 cm), and PAH concentrations were also
highest in this case – up to 255 ng g
The profile distribution and composition of PAHs in post-fire histosol in the case of a thick organogenic horizon (pit 9).
The profile distribution and composition of PAHs in post-fire histosol in the case of almost complete burning out of the organic layer (pit 8).
The categorized box and whisker plot for total PAHs in different horizons of histosols (pits 7, 8, and 9). The number of sample replicates: O-3, Cpir-5, Hpir-6, He-14, Ha-5, Chs-5, and C-12. Horizons Cs and Ch were combined for this analysis, indicated by Chs.
The profile distribution and PAH composition in post-pyrogenic histic podzols, wildfire 2010 (pit 6).
The profile distribution and PAH composition in post-pyrogenic histic podzols from the wildfire of 2002 (pit 5).
It may be noted that total PAH concentrations in residual peat horizons of
post-fire histosols changed from 10 to 255 ng g
The Kruskal–Wallis ANOVA test indicated significant differences in total PAH
concentrations between different horizon types of histosols. Significant
differences were detected between mineral C and post-fire Hpir horizons, and
between mineral C and organogenic H horizons at
Furthermore, occasionally, Cpir and post-pyrogenic O horizons of pits 4 and
5, closer (50 m) to the burned area of 2010, contained up to
150 ng g
At a distance from the site of the last fire, the Cpir horizons (pits 2 and
3) (Table 3), formed in the fire of 2002, contain PAHs in low amounts – up
to 2–3 ng g
In contrast to histosols, the sod podzols are characterized by
eluvio-illuvial translocation of hydrocarbons. For instance, eluvial horizons
E contained 0.7 to 5 ng g
As opposed to histosols O, Cpir and Apir, horizons of histic and sod podzols
had high levels of PAHs that exceeded the range in pyrogenic organogenic
horizons (Fig. 11). In podzols, significant differences in PAH concentrations
were identified between the A and E and between the A and B horizons (
In the investigated soils, PAHs tended to form groups different in their
distribution, which was confirmed by cluster analysis (Fig. 12). PAHs were
combined in clusters if the coefficient of correlation in every group was
significant at
Among the samples, the coefficient of variation was high for high-molecular-weight compounds, especially in Hpir horizons, for benz(a)pyrene (283 %), benz(ghi)perylene (283 %), benz(k)fluorantene (437 %), and chrysene (195 %). The variation of low-molecular-weight compounds was lower; in H horizons, coefficients of variation for naphthalene were 131 and 126 % for phenanthrene.
Certain differences in the intensity of PAH accumulation could be noted in
soils of different age burned sites. On the site burned in 2012, the
post-fire histosol within the burned area (pit 12) and the histosol situated
nearby the burned site and containing charred peat layers from previous fires
(pit 11) were studied. In the soil of this most recently burned site, the
total PAH concentrations were the highest among all the sites studied. Here,
PAH concentrations were 330 ng g
In the case of sod podzols affected by the fires of 2002 and 2010 (pits 5 and 6), the pyrogenic organogenic horizons had similar PAH concentrations (Fig. 9, 10). The differences in the intensities of eluvio-illuvial processes were not detected.
The Kruskal–Wallis ANOVA test indicated significant differences in PAH
concentrations, depending on the number of years following the fire. Soils
affected by the fire of 2012 differed significantly from years 2002 and 2010,
but the differences between 2002 and 2010 were not detected (
The data on PAH concentration and composition in soils not affected by fires corresponded to results obtained for soils of other territories distant from PAH sources (Gennadiev and Tsibart, 2013; Wilcke, 2007; Rovinskii, 1988; Gabov et al., 2007; Krasnopeeva, 2008).
The categorized box and whisker plot for total PAHs in different horizons of podzols (pits 2, 3, 4, 5, and 6). The number of sample replicates: O-6; Cpir-4 Apir-3; A-3; Hpir-3; Ha-6; E-9; EB-5; Bhs-15; BC-4. Horizons Bh, Bs, and Bhs were combined for this analysis into Bhs.
The production of PAHs in Hpir horizons of
In addition, a possible factor causing different PAH concentrations in peat horizons was the amount of peat remaining after the burning. The greater the residual mass of peat, the greater the sorption of PAHs that was observed to take place. At the same time, allowing for the high variability of absolute PAH concentrations in residual peat horizons, it could be surmised that they tend to accumulate 5-6-nuclear compounds, which could indicate processes of peat combustion. Vane et al. (2013) also pointed out the presence of low- and high-molecular PAHs in soils after peat fire. However, 5-6-nuclear compounds (benz(ghi)perylene, benz(a)pyrene, benz(e)pyrene, benz(k)fluotanthene) could be considered the indication group marking the peat combustion.
The cluster analysis (complete linkage; 1-Pearson
The soils with very shallow (up to several cm) peat horizons remaining following fire had low PAH concentrations due to the fact that almost all organic matter had been totally burned out. High coefficients of variation of PAHs in residual peat horizons could be caused by different durations and depths of heating of these horizons at different sites. The depth of changes in soils differs depending on the fire intensity and location characteristics (varying moisture content, microrelief and peat thickness) (Efremova and Efremov, 2006; Grishin et al., 2013).
The categorized box and whisker plot for total PAHs in horizons Hpir, Apir, O and Cpir of soils affected by the fires of 2002, 2010, and 2012 (pits 2, 3, 4, 5, 6, 7, 8, 9, 11, and 12). The number of sample replicates: 2002 year -15, 2010 year -19, 2012 year -11.
In the surface incipient horizon O (20–70 ng g
The PAH concentrations in pyrogenic organogenic horizons (Hepir, Hapir, Apir, He, Ha) of podzols were lower than in analogous horizons of histosols, as these soils had a lesser amount of available organic material for burning and PAH formation.
Also, in comparison with histosols, Cpir and post-fire O horizons of podzols had rather high PAH concentrations. These findings could be explained by the fact that these sites were affected by fire in 2002, but were also directly nearby the site of a burned area of 2010 (Fig. 3). These horizons have a sandy loam texture (Zaidel'man et al., 2006); so, they have a high sorption capacity. Presumably, the accumulation of low-molecular-weight compounds formed during the last fire could take place at these sites.
Also, probably, as the accumulation of heavy PAHs takes place in situ in char and ash horizons, PAHs are translocated across them to the atmosphere, because these horizons act as filters of gases. The accumulation of low-molecular-weight PAHs could be explained by atmospheric input; these compounds are released into the atmosphere with other combustion products, where they are translocated and then deposited at other plots (Rein, 2013).
In contrast to histosols, the sod podzols were characterized by eluvio-illuvial translocation of hydrocarbons, which was also found in the study of Gabov (2007); so, PAH distribution follows the soil-forming processes typical of the wet climates of taiga biomes. The increased migration of PAHs after the fire was, probably, caused by vegetation destruction during the fire and following intense percolation through the soil profile.
The histosols affected by the fire in 2012 had a higher PAH concentration in comparison with the histosols after the fire of 2010. On the one hand, it caused arriving PAHs, especially low-molecular-weight compounds, to possibly degrade after the fire. On the other hand, in the present case, the organic horizons of the most recently burned site were not completely burned out, which favors the intense sorption and accumulation of polycyclic aromatic hydrocarbons in these soils, but it is worth mentioning that the time of the pyrogenic PAH presence in the soil could differ, depending on the conditions. Thus, even on a geological scale, the elevated PAH concentration in the deposits of the Cretaceous and Jurassic periods could be explained by ubiquitous occurrence of wildfire (Killops and Massourd, 1992; Marynowski et al., 2011; Belcher, 2006). However, according to Garcia-Falcoan (2006), the concentrations in burned organo-mineral soils remained high only during the first 3 months after the fire and, then, PAHs were reduced by soil processes including degradation and migration.
In the case of sod podzols from the 2002 fire, the concentrations of PAHs could also be influenced by the 2010 fires; therefore, trends of PAH degradation with time were not so prominent.
Considering the soils of drained peatlands, it is necessary to emphasize
that, in both post-fire sod podzols and histosols, the total PAH
concentrations could be rather high – up to hundreds of ng g
However, it should be noted that even under conditions of catastrophic fires
on the drained peatlands in the studied cases, the PAH concentrations,
especially high-molecular-weight compounds, were relatively low: for example,
benz(a)pyrene concentrations did not exceed 3 ng g
The findings from this study correspond to some previously reported results in the literature. Vane et al. (2013) note that PAH concentrations in peat soils after fire are lower than the amount dangerous for biota. Also, there is no evidence that amounts of PAHs accumulated after the fire are dangerous to humans, as they are lower than hazardous levels determined for soils, but the variation of high-molecular-weight PAHs in soil horizons was high, as these compounds do not occur in all horizons, and their formation is a more complicated process in comparison with low-molecular PAHs.
The conclusions of our study are the following:
The wildfires on the drained peatlands caused the change in the morphological properties of
soils; new soil horizons Cpir, O, Hpir, and Hapir were formed with different
accumulation rates of PAHs. The highest PAHs concentrations were seen in
charry peat Hpir horizons and in post-fire incipient O horizons. Post-fire histosols and histic podzols differed in their PAH distribution. In both cases, the highest
PAH concentrations occurred in the organogenic post-fire horizons, but in
cases of histic podzols, their slight accumulation of PAHs in illuvial
horizons was possible. 5-6-nuclear compounds were formed in pyrogenic horizons Ha,pir and He,pir. Their production was
facilitated in smoldering processes due to a lack of oxygen; this group could
be considered an indicator group of peat combustion. 2-3-nuclear PAHs
occurred within the whole profile; in sod podzols and histic podzols, their
migration to illuvial horizons was observed. In the cases we studied, PAH
composition in pyrogenic horizons did not change in different soil types. The trends of higher PAH accumulation in soils were observed in cases of incomplete burning out of
peat horizons. PAH sorption in upper horizons of soils near the sites of the
most recent fires was observed.
This work was supported by the Russian Geographical Society and by the Russian Foundation for Basic Research, project 12-05-31314. Edited by: P. Pereira