Polycyclic aromatic hydrocarbons (PAHs), particularly
benzo[
The early 21st century is characterized by a significant exacerbation of environmental problems, especially in large industrial and urban centres. According to demographic projections, by 2050 about 66 % of the 9 billion people on Earth will be living in cities (UN DESA, 2015), which will lead to an increase in the anthropogenic impact on the environment and its further contamination.
Toxic substances, like polycyclic aromatic hydrocarbons (PAHs) and their
most dangerous member, benzo[
BaP is released from industrial, heating, vehicle and domestic waste. It is also a byproduct of organic waste and fuel combustion (Larsen and Baker, 2003; Pergal et al., 2015). BaP is added to the urban environment from polluted air with dust, precipitation and aerosols and accumulates in the surface layers of soils (Wania and MacKay, 1996; Trapido, 1999; Fernández et al., 2000; Nam et al., 2009). In the United Kingdom, over 90 % of all PAH reserves in the environment are limited to the surface layer of soil (Wild and Jones, 1995). PAH concentration in the air is generally significantly higher in winter than in summer because of greater fuel combustion (Ollivon et al., 2002; Gaga et al., 2009; Birgül et al., 2011).
In many major cities of the world, BaP concentrations in the environment are 10 to 100 times higher than the regional values (Trapido, 1999; Ma et al., 2005). Long-term (1990–2006) monitoring of BaP concentrations in the Eastern Administrative District (EAD), one of the most polluted parts of Moscow, demonstrates a steady increase in pollution and related deterioration of soil functioning (Kosheleva and Nikiforova, 2011).
The rate of BaP accumulation in soil depends on the balance between its fallout from the atmosphere and the intensity of removal and decomposition. BaP addition to urban soil is almost entirely derived from anthropogenic sources, whereas BaP removal through volatilization, degradation and leaching depends on landscape, geochemical and bioclimatic factors (Morillo, 2008; Kosheleva and Nikiforova, 2011). BaP input is measured by the rates of its deposition with road and industrial dust (Yu et al., 2014). In cities with cold climates, the BaP deposition from the atmosphere can be determined by examining BaP reserves in the snow cover (Haglund et al., 1987; Kasimov, 1995; Sharma and McBean, 2001). However, particulate matter and the water-soluble fraction of snow are rarely analyzed for BaP contents, so the contribution of these fractions to the formation of BaP anomalies in the urban environment has so far been poorly studied.
Among the individual PAHs, BaP is the least soluble in natural fresh waters and has a limited ability to be leached from the soils. Therefore, the losses of BaP from the soils are mostly determined by its destruction, which is enhanced by ultraviolet radiation (Gennadiev and Pikovskii, 1996). The rates of PAH and BaP molecule transformation are also heavily dependent on soil properties, such as pH, soil organic matter (SOM) content, particle aggregation, texture, redox conditions and heat and water fluxes and regimes. PAH and BaP biodegradation is affected by the molecular structure of specific compounds (Wild and Jones, 1995; Doelman, 1995; Kleeman et al., 2000; Johnsen and Karlson, 2007; Jiang et al., 2009; Birke et al., 2011; Kosheleva and Nikiforova, 2011; Kasimov et al., 2014).
The determination of BaP input and losses makes it possible to evaluate the
resilience of urban soils to organic pollutants and to assess the
possibility of their self-purification. The concept of critical loads on the
urban environment provides a methodological framework for this kind of
research. The critical load of a pollutant is defined as the maximal input
to an ecosystem that does not lead to irreversible changes in its
biochemical structure, biodiversity or productivity over a long period
of time (Vries et al., 1997; Baskhin et al., 2004). So, the aim of the
present study is to analyze the fate of BaP in the urban environment of
the EAD of Moscow using the concept of critical loads. The particular tasks
are as follows:
to identify the intensity of atmospheric BaP addition to the snow cover in
the winter period on the basis of dust deposition rates and the BaP contents in
the solid fraction of snow; to determine BaP concentrations and to reveal the specific features of BaP
spatial distribution in the soil cover of the EAD, including the localization and
size of anthropogenic anomalies; to assess the ecological risk of BaP pollution on the basis of sanitary
and hygienic standards for BaP concentration in soils, as accepted in Russia by
legislation; and to calculate BaP critical loads on urban soils with respect to BaP
degradation rates and exposure periods.
Land-use zoning of the study area with main sources of BaP and locations of the sampling points; the UTM coordinate system.
The work was conducted in the territory of Moscow, one of the largest cities in the world with a population of about 12 million. The city of Moscow is divided into 12 administrative districts. The study area is located in the eastern part of Moscow, which is considered to be one of the most polluted in the city because of its large industrial sector and concentration of large processing enterprises. The study area belongs to the southern taiga landscapes of the Meschera lowland, which is a flat outwash plain with mean altitudes of about 150 m (Fig. 1). The land surface is dominated by urban structures. Urbic Technosols dominate in the soil cover; their morphological, physical and chemical properties differ greatly from those of the background Retisols developing under coniferous–deciduous forests on loamy deposits (Kosheleva and Nikiforova, 2011). Anthropogenically modified and artificial soils are also represented by Eutric Retisols, Transportic Technosols, Ekranic Technosols (sealed by asphalt) and by several other soil types developed on man-made sediments and the cultural layer (Prokof'eva et al., 2014).
Traffic releases more than 90 % of pollutants in the aerosol and gas phase, including PAHs, into the air and thus defines the ecological situation in the district (Kasimov et al., 2014). Car exhaust emissions from large highways cause a significant adverse effect on urban soils (Fig. 1). A few large industrial zones with chemical and petrochemical plants and two thermal power plants are also sources of BaP pollution.
BaP contents in the solid fraction of snow and BaP fallout intensities in the winter period for the background territory and the land-use zones in the EAD of Moscow (2010).
The snow cover in the EAD was surveyed in early March when the snow depth was maximal, while the soil sampling was carried out in June 2010 using the methods recommended in Revich et al. (1982) and Kasimov (1995). Land-use zoning was accomplished and traffic, industrial, residential, recreational and agricultural zones were distinguished (Nikiforova et al., 2014). Overall, 50 snow and 50 topsoil (0–10 cm) samples were collected at the same locations at a regular grid with 800 m intervals. Composite snow samples were formed by mixing 10 replicate samples taken with a plastic tube 5 cm in diameter; the composite soil samples were compiled from 3–4 individual samples. Due to a prevailing western atmospheric transfer, background (reference) snow samples were collected 50 km west of Moscow, near the city of Zvenigorod. The background area for urban soils was located in the Meschera lowland, 40–50 km east of Moscow, since the natural soils in that area (Retisols) were formed on the same parent materials (sandy clays and light clay loams) as the urban soils.
The solid fraction (particulate matter) was separated from snow samples
using a membrane filter with a pore diameter of 0.45
The geochemical data treatment included the calculation of the enrichment factor
(EF), which compares the abundances of BaP in urban soils to the
reference value. For assessing the risk of contamination, the ecological
risk index (PI) was estimated on the basis of the maximum permissible
concentration (MPC), which is a general BaP sanitary level equal to 20 ng g
BaP is commonly added to soil cover through atmospheric precipitation, falling either in liquid or solid phases. Since the velocity of snowfall is slower and snowflakes have larger surfaces compared to raindrops, they become rich in BaP faster (Vasilenko et al., 1985).
In the
The estimations performed by Gabov et al. (2008) for the undisturbed
territories of the East European Plain in the taiga zone showed that the
annual average rate of BaP deposition with dust and snowfall to the soil
surface is 30–40 ng m
In this study, snow water in
BaP delivery from the atmosphere is defined not only by its content in solid
particles of snow, but also by its fallout intensity. In the EAD, the BaP
fallout with the snow dust during the winter period was quite significant,
and it averaged 75.6 ng m
BaP fallout, measured as the loading factor (LF), in the two most polluted zones was much higher than in the
background territories (LF
BaP content in the topsoils in some cities of the world.
The mean anthropogenic BaP addition to soil cover in Moscow's EAD (75.6 ng m
The surface horizons of the background soils, which are represented by
Retisols, were acidic (pH 5.5) with low BaP concentrations that were 34
times lower than the mean BaP value in the solid fraction of snow (Table 1).
Still, like dust in the snow cover, the natural soils showed typically high
variability of BaP contents (
In the majority of land-use zones, the solid fraction of snow had higher (4–7 times) BaP concentrations than the surface soil layers. This situation is typical for cases where relatively high BaP concentration is observed for the airborne particles (Amagai et al., 1999). In the agricultural zone, the relative enrichment of snow dust with BaP increased up to 42 times with respect to soil material. The difference between the BaP content in the snow and in the soils can be attributed to two factors. First, nearby sources, such as the Kozhukhovo settlement with homes individually heated by furnaces and the Rudnevo industrial zone with a waste incineration plant, probably released a considerable amount of BaP. Second, the cultivated soils in this zone had higher BaP decomposition rates compared to the soils in other zones due to favourable moisture and air regimes (Nikiforova and Alekseeva, 2005). The ratio between the average BaP concentrations in snow and in the surface soil horizons indicated a significant BaP addition from the atmosphere and its progressive accumulation in the soils, especially in the industrial and traffic zones.
In order to evaluate the ecological risk associated with BaP soil pollution,
the average concentrations of BaP in the district's land-use zones were
compared with the sanitary standards (MPC) accepted in Russia. The highest
risk index (PI) was revealed in the industrial zone
(PI
The measured BaP contents in the soils of Moscow's EAD were much higher than the BaP levels recorded in the majority of the other cities (Table 2). However, the comparison of our data with the results of BaP surveys in Moscow, as well as in a number of Russian and Belarusian cities, indicates similarity in the BaP levels. As a rule, the average BaP concentrations in the soil and in the atmospheric fallout make up no more than 10–15 % of the total PAH content (Garban et al., 2002), and maximum concentrations are usually observed in the soils in the industrial and traffic zones.
In order to determine the critical loads of BaP on urban soils in the
various land-use zones of the EAD, the topsoil reserves of BaP were calculated.
In the calculation, we considered the uppermost soil layer with a thickness
of 10 cm, an area of 1 m
The MPC of 20 ng g
Because the BaP concentrations exceeded the MPC in the majority of the EAD land-use zones, we considered a load critical if after a specific time interval (5, 10 or 25 years, etc.) and given the rate of the pollutant's degradation the BaP content would decrease to the permissible level.
The long-term dynamics of BaP reserves in the surface soil horizon
(
Equation (1) can be solved by dividing the variables with
Then, the BaP critical load is determined by the formula
According to the
The analysis of Fig. 2 demonstrates that the critical loads
(
The critical loads of BaP (ng m
The number of years required to lower the BaP content in the soils of
the EAD to the MPC (20 ng g
As the BaP degradation rate increases, so does the value of the critical
loads (
The critical BaP load values calculated using this model demonstrate that at
a certain degradation intensity the BaP concentrations in the EAD soils can
decrease to the level of MPC, although this will take a relatively long
time. Based on the above formulas, the time necessary for BaP content in the
soil to reach the MPC can be calculated as follows:
At existing levels of BaP deposition with insignificant degradation intensity (1–2 %), the BaP concentrations will reach the MPC in high-rise residential areas in 28–59 years. The BaP concentrations will lower to the MPC in the recreational zone in 106–214 years based on our calculations. It is worth noting that the MPC will never be reached in the other zones because the input of BaP into the soils exceeds its losses through decomposition.
As the degradation intensity grows, the time period necessary for a decrease in BaP contents to the permissible level becomes shorter. At BaP degradation rates of 3–5 % in traffic, industrial and low-rise residential zones, soil BaP content may gradually decrease to the MPC after a few decades or centuries. The drop in the BaP content in the soils of the mid-rise residential quarters can only occur if the rate of degradation is 6 % or more. In the agricultural zone, the BaP concentrations cannot exceed the permissible level even under low-intensity degradation because of the insignificant addition of BaP from the atmosphere.
Thus, at the existing emission rates of BaP into the atmosphere and its subsequent accumulation in the soil, its content in the soils in all of the land-use areas (except the agricultural zone) may come to the permissible level after decades or hundreds of years. In this situation, to achieve and to maintain environmentally acceptable BaP levels in urban soils, it is necessary to remove the contaminated soil surface layer and replace it with unpolluted soil material as well as to reduce BaP emissions.
The intensity of the BaP fallout from the atmosphere onto urban soils with the
solid fraction of snow in the EAD territory of Moscow ranged from 0.3 to
1150 ng m
The average BaP content in the soil surface layers of Moscow's EAD in 2010
was 409 ng g
The calculations of the BaP critical loads on the soils in the EAD showed that, at degradation intensities ranging from 1 to 10 % per year, the BaP concentrations may decrease to the permissible level only after a relatively long period of time up to many decades or centuries. Because the urban soils have limited self-purification abilities, a combination of remediation measures must be taken to reduce the ecological risk posed by BaP soil pollution.
The first experience of the application of the critical loads approach for BaP demonstrated its feasibility for contamination control in the urban environment. It also showed the need for further research on BaP entry and loss rates from urban soils, especially in relation to the conditions and rates of BaP decomposition. A reliable assessment of the pollutant's input and output is the basis for a sound prediction of the long-term dynamics of contamination with highly toxic organic compounds, such as BaP, in the urban environment.
The data used are available in the Supplement.
All authors cooperated in designing the experiments. E. Nikiforova, N. Kosheleva and D. Vlasov carried out the fieldwork. D. Vlasov and N. Koshleleva calculated the critical loads of BaP. All authors calculated the pollution levels of BaP in urban soils and snow cover, and all authors contributed to preparing the paper.
The authors declare that they have no conflict of interest.
This study was conducted with financial support from the Russian Science Foundation (project no. 14-27-00083). The authors would like to thank the staff of the Department of Landscape Geochemistry and Soil Geography at the Faculty of Geography, Lomonosov Moscow State University: N. I. Khlynina for her analytical work as well as M. Y. Lychagin, I. N. Semenkov, T. S. Koshovsky and G. L. Shinkareva for their participation in the fieldwork. We are grateful to E. N. Aseyeva, who helped us to prepare the paper for publication. This work contributes to the Pan-Eurasian Experiment (PEEX) research agenda. Edited by: V.-M. Kerminen Reviewed by: A Khaustov and one anonymous referee