Volume 52, Numéro 1, January-March 2017
|Page(s)||29 - 36|
|Publié en ligne||27 février 2017|
Assessment of 137Cs contamination of combustion products and air pollution during the forest fires in zones of radioactive contamination
Institute of Radiobiology of National Academy of Sciences of Belarus,
4, Fedjuninskogo str.,
2 Gomel State University, Sovetskaya St. 104, 246019 Gomel, Belarus
⁎ Corresponding author: firstname.lastname@example.org
Accepted: 28 November 2016
The results of the investigation of radiation hazard of forest fires in the radioactive contaminated areas in the article are presented. The activity concentration of long-lived radionuclides in forest fuel materials is shown. Airborne concentration of 137Cs produced by the combustion of forest fuel materials with different contamination density can exceed the permissible activity levels of 137Cs in air. During the large wildfires on the territories with high values of radioactive contamination density, the airborne concentration of radionuclides may be three or four times higher than the background values. Seasonal fluctuations of average volume activity were estimated by using the ratio between the values of aerosols activity in the atmosphere on the day of the fire (fire season) and the reference values.
Key words: forest fires / radioactive aerosols / seasonal fluctuations / activity index
© EDP Sciences 2017
After the Chernobyl Nuclear Power Plant (ChNPP) accident in 1986, more than 4 million hectares of forestland in Belarus, Ukraine and Russia were contaminated by radionuclides. In Belarus, more than 1.7 million hectares of forested areas having soil contamination density exceeding 37 kBq m−2 were contaminated by 137Cs. The southern part of Gomel region was also highly contaminated by long-lived radionuclides such as plutonium (238Pu, 239,240Pu) with the half-life time ranging around one hundred to thousands of years, 241Am and 90Sr with the half-life time 432.8 and 29.1 years, respectively. In 2014, the area of radioactively contaminated forests in Gomel region was about 49% of total forestland area. Pine trees cover more than 60% of forestland area in the Gomel region. According to a lot of research studies (Fawaris and Johanson, 1994; Pietrzak-Flis et al., 1996; Yoshida et al., 2004), the forest litter retains most of 137Cs activity in a forest phytocenosis. The contribution of litter to the total radioactive contamination of phytocenosis can range from 40% to 70% (Teramage et al., 2014).
The threat of forest fires revealed after numerous fires in the Chernobyl exclusion zone (CEZ) in 1992 on the border with Ukrainian part of exclusion zone (Hao et al., 2009). As a result of large forest fire at the territory of Polesie State Radiation-Ecological Reserve (Gomel Region, Belarus) in 1992 the level of 137Cs in smoke aerosols was increased 10 times within the 30 km exclusion zone (Dusha-Gudym, 2005). Several studies have noted that during the forest fire in CEZ in May 1992 volume activity of 137Cs in the air has increased by two orders of magnitude compared with normal values (about 3 × 10−3 Bq m−3) (Lujaniene et al., 1997; Yoschenko et al., 2006). The large amount of wildfires in Belarus also have been registered in 1996, 1999 and 2002. Between 1993 and 2013 over 1147 of forest and grasslands fires occurred in the CEZ (Zibtsev et al., 2015).
During forest fires on contaminated territories, radionuclides deposited in forest fuel materials are released in the air with smoke. Radioactive smoke plumes may have relatively high values of volume activity, which is able to exceed the allowable levels of radioactivity in the air. The emission containing radionuclides deposited on the fine aerosol fractions is particularly dangerous.137Cs and 90Sr are the main dose-forming isotopes after 30 years after the Chernobyl disaster. These radionuclides are dangerous both for external exposure and for internal exposure pathway (inhalation and ingestion) (Hohl et al., 2012). Inhalation of radionuclides (especially 238Pu, 239,240Pu, 241Am) can generate additional internal doses both for firefighters, close to the ignition source, and for citizens far from the source. In addition, radionuclides released from the fire source may be transferred with the airflows over long distances, which can lead to a secondary radioactive contamination, which may be significant especially when it spreads over territories that do not exhibit high contamination level, i.e., that remain safe of contamination.
Some studies indicate the increase of volume activity of 137Cs in the summer months (Kulan, 2006). The risk of forest fires is higher in the period from April to September (Zibtsev et al., 2015). In the winter season, forest fires are completely absent. Therefore, any increase in activity of 137Cs in the air in the winter season is not related to forest fire on contaminated territories and depends on other causes. Fire statistics performed by the National monitoring service of Belarus shows that more than 2480 forest's and grassland's fires leading to burning area of more than 3600 ha occurred in Gomel region between 1996 and 2014 (Fig. 1a), including areas with the level of contamination by 137Cs above 555 kBq m−2. Time series of the number of wildfires in CEZ between 1996 and 2012 is shown in Figure 1b. More than 90% of all cases occurred because of anthropogenic factors. In addition, sanitary cutting in the contaminated areas was not conducted for more than 20 years. This led to a significant accumulation of dead organic matter in the restricted access areas (Mousseau et al., 2014).
The objective of this study is to analyze the air pollution by 137Cs as the result of biomass burning emission and transport of radionuclides during forest fires and to estimate a potential threat for human health. We also present an assessment method the transfer of radioactive pollutants with smoke of forest fires on the territories contaminated by radionuclides. The estimation process consists of two stages. The first one based on empirical data obtained from the fire experiment. The second stage based on analysis of climatic, environmental and historical data obtained from continuous datasets in Belarus since 2005.
Long-term dynamics of fires in the areas of radioactive contamination of the Gomel region (a) and in the CEZ (b).
The research methodology is based on the determination of the volume activity of 137Cs in smoke emissions close to the fire and 10 km away. In order to study the determination of the volume activity of 137Cs in smoke emissions, the fire experiment was applied.
Sampling of fuel materials was carried out at three experimental forest sites located in areas with different levels of radioactive contamination density. The first site is located near the former village Kruki (N 51°31′24,6″, E 30°19′19,6″), at a distance of approximately 8 km north of the ChNPP. Dose rates were 5.0 ± 0.19 μSv h−1 in Kruki (air dosimetric survey measured at 1 m above ground). The second and the third experimental forest sites are located 150 km NE (N 52°61,941′, E 31°27,708′) of the ChNPP and belongs to the forestry of Vetka. Dose rates were 0.82 ± 0.02 μSv h−1. All experimental sites are largely characterized by dry sandy soils, and Scotch pine (Pinus sylvestris) forests. The average age of the trees in the forest was 50 ± 5 years. The general characteristics of the three sites are summarized in Table 1.
To collect the fuel material samples, plots (50 by 50 m) were selected for each forest site. The fuel materials include samples of forest litter with two horizons (А0L, and A0F + A0H). It also includes dry plants, branches with needles (diameter less than 10 mm) and fragments of dry bark.
For the determination of contamination density, 10 soil samples were collected from the each site. Samples were collected in the depth of 20 cm with a 5 cm diameter core sampler. Samples of forest litter were collected with square sampler (50 by 50 cm) at ten points on each plot. Litter was separated into layers: А0L (light fraction of forest litter), and A0F + A0H (mixture of the slightly decomposed forest litter and well humified soil organic matter). A0F and A0H layers were collected together. Samples of plants, small branches with a diameter less than 10 mm and dry cones were collected over 1 m2 area in three replicates. All samples were dried and weighted in the laboratory. The stock of fuel for each experimental site was calculated by summing the dry weight of all samples to 1 m2.
Characteristics of the experimental sites.
The experiment was carried out in controlled conditions using the smoke chamber. The smoke chamber was designed by employees of laboratory of radioecology specially (Institute of radiobiology of NAS of Belarus) for sampling and analysis of substances in smoke including radionuclides. The smoke chamber is a metallic cylinder with two compartments inside (Fig. 2). The upper compartment is used for the combustion of fuel materials with different levels of radioactive contamination. The lower compartment is used for collecting ash. The chamber is also equipped with an exhaust pipe used to collect aerosols.
Combustibles materials are incinerated at temperatures above 600 °C. At this temperature, a concentrated stream of smoke is created. For radioactive aerosol sampling, an aspirator with an output of 200 L min−1 was used. Duration of sampling was 10 ± 2 min (flow rate 200 L min−1). Uncertainty was equal 5%. Aerosols were sampled on perchlorovinyl fiber filters with a filtration surface of 10 cm2. After combustion of the material ashes were removed and placed in glass vessels. Fiber filters were placed in a clean polybags. To minimize the possibility of cross-contamination, the chamber was thoroughly cleaned of combustible materials residues and ash.
During the experiment three combustion cycles of materials from the each site was made. The total amount of samples was 9 for ashes and 27 for aerosols. The values of airborne activity concentration AV of 137Cs is calculated as follows: (1) where Af is the activity measured in the analytical filter (Bq) and V is the air volume passed through the filter (m3).
Scheme of the smoke chamber.
Aerosol filters were prepared by serial (three times) dissolving in a solution of 7M HNO3. After each iteration, the separation of the filtrate was made. After that, the filtrate was adjusted to a volume of the measuring cuvette with 1M HNO3.
Soil, plant and forest litter samples were measured air-dried. Each sample was placed in cylindrical plastic vessels with complete filling of geometry. Two geometries were used for measurements: one with a diameter of 14.5 cm and 11 cm height and the other with a diameter of 7 cm and 3.2 cm height.
The activity concentration of 137Cs in all samples was measured with the γ-spectrometric complex CANBERRA (USA), which is equipped with the Ge-detector (model GX2018). The energy resolution of the detector is 1.8 keV for the 60Co-line at 1.33 MeV. Detection efficiency of spectrum for energy 1.33 MeV is 22.4%.
Determination of air pollution by 137Cs from forest fires on analysis of radiation monitoring data for the period from 2003 to 2013 was based. The data on the dynamics of forest fires in the zones of radioactive contamination of the Gomel region, and information on meteorological conditions for interpretation were also used. Data were compared in compliance with:
distance from the fire site to the radiation control station shall not exceed 10 km;
fire area shall not be less than 0.1 ha;
fire site must be located on the territory having a level of radioactive contamination by 137Cs over 555 kBq m−2;
wind direction must be the same with the direction from forest fires to the radiation control center.
The atmospheric pollution by radionuclides in Belarus is controlled by center of radioactive contamination control and environmental monitoring. In accordance with the National monitoring network, there are a few stations of radiation control in the Gomel region. Aerosol sampling stations in Gomel and Mozyr are equipped with Petryanov filters and air pump with a flow rate of 1000 m3 h−1. The stations in other cities have only horizontal plates (Fig. 3).
In comparative database, we generally use the data on the density of radioactive contamination of forest site, area, type and intensity of the fire and the wind direction. The number of entries in the database was 426. The approach that we used is also based on calculation of an activity index defined as the ratio between airborne activity concentration during fire and a reference background concentration. The activity index represents the excess of volume activity of 137Cs in the air on the day of the fire over the reference values of volume activity of 137Cs in the air (in the days without fire). The absence of fire in the winter season allowed us to use data on the radioactive contamination of the air by 137Csin winter season of each year as a reference values.
The activity index values were achieved by using the formula: (2) where AVi is the activity concentration of 137Cs in air on the day of the forest fire (Bq m−3) and A0 is the reference values of 137Cs volume activity in air in the days without fire (Bq m−3).
Monitoring network in Gomel region.
The forest litter is the primary main combustion conductors during the forest fires (Kurbatskiy, 1962). It also includes dry grass and mosses, lichens, fallen leaves, branches, bark fragments, etc. Pine phytocenoses have well-developed forest litter (3–6 cm). Characteristics and the measured 137Cs activity in the samples from the experimental forest sites are presented in Table 2.
Data in Table 2 show that more than 90% of 137Cs is associated with the forest litter. The main source of litter is litterfall, which is also a good conductor of burning. The contents of 137Cs in the components of the litterfall vary from 10% to 15% of its total activity in combustible materials. More than 50% of cesium activity in the litterfall are associated with needles.
Characteristics of various compartments of forest fuel materials.
The data shows that airborne activity of 137Cs increase with increasing of 137Cs activity concentration in combustible materials (Fig. 4). The experimental data are fitted with a two-order polynomial (r2 = 0.89). Averaged values of airborne 137Cs concentrations during combustion of fuel materials taken from territory with contamination density of 560 kBq m−2 was one order of magnitude lower than those taken from territory having contamination density of 2690 kBq m−2, i.e., 4.8 times more. Activity concentration of radionuclides in aerosols depends on several factors: the contamination density of fuel materials, combustion intensity and duration.
According to the radiation safety standards the average permissible annual volume activity for workers and for population is 1.7 kBq m−3 and 27 Bq m−3, respectively (Radiation Safety Standards in Belarus, 2000). Airborne concentration of 137Cs produced by the combustion of forest fuel materials with contamination density over 400 kBq m−2 can exceed the permissible activity levels of 137Cs in air for population (46.7 Bq m−3), but not enhance the permissible activity levels of 137Cs for workers and firefighters. During the combustion of forest materials with radioactive contamination density 154 kBq m−2 a significant enhancement of allowable activity levels of 137Cs in air was not detected.
Airborne activity concentration of 137Cs during the experiment.
Comparative analysis of 137Cs fallout on horizontal plates shows that there is no significant difference between summer and winter seasons. Thus, the data show that the mean monthly activity of fallout on horizontal plates near Gomel in winter and summer during 2008 were equal (0.57 ± 0.09) and (0.53 ± 0.08) Bq m−2 per day, respectively (t05 = 1.75, df = 10). Since 2006, the fluctuations in the density of radioactive contamination fallout in summer and winter are similar (see Tab. 3).
On the other hand, the mean year fluctuations of 137Cs airborne concentration show a significant increase of cesium contents in air from April to September each year. As it shown in Figure 5, the ratio between the volume activity of 137Cs in the air in summer and winter season was 1.26 ± 0.06 for more than 7 years. The single values of monthly volume activity in comparative database exceeded reference values by 1.5 or even 2 times. The main reason of such ratio is the full absence of forest fires in the winter.
The only possible source of the air contamination can be associated with wood heaters activity and stove heating usage during the winter season. However, the contribution of this factor in the air radioactive contamination cannot be estimated authentically, because of the absence of significant statistical data. In addition, according to the hygienic standards in Belarus (National Hygienic Standards, 2001), the activity concentration of 137Cs in the firewood cannot exceed the permissible level of 740 Bq kg−1. Usage of any materials with more or less large quantities of 137Cs is strongly forbidden. However, the issue may be of interest for further research.
The activity index indicates significant enhancement of airborne concentration of 137Cs during the large wildfires on the territories with contamination density over 370 kBq m−2. As a result of ground and crown forest fires with high intensity, the background values of 137Cs airborne concentration may be increased by three or four times. Figure 6 shows the time series of the activity index during forest fires in Gomel region.
In all cases with high 137Cs activity index, the distance from the source of forest fire did not exceed 10 km. Thus, during the crown fire near the village of Tumarin (Vetka forestry, 52°30′12.2″N, 31°13′34.6″E) over 60 ha of pine forest were burned during the first day and 45 ha – in the second day of fire. The total burning area was larger than 100 ha. 137Cs contamination density of territory around the fire was 370 kBq m−2. The wind direction during the fire was the same than the direction from the source of fire to the stations of radiation control (Gomel). The average daily activity of 137Cs in the air on the stations of radiation control was (47 ± 5) × 10−5 Bq m−3, which exceeds four times the background values (the average monthly values of background activity by 137Cs was (12 ± 2) × 10−5 Bq m−3). The lowest background value along 2008 counted by using percentile 10 thresholds was equal to 4.0 × 10−5 Bq m−3.
Another case of crown forest fire was registered on territory with contamination density by 137Cs over 555 kBq m−2 (Narovlya Forestry, 51°42′5″ N, 29°37′35″ E). The activity index was 3.5 (the stations of radiation control in Mozyr).
The analysis of a year-to-year fluctuations of activity index show that its value depends on several factors such as radioactive contamination density of territory and forest fuel components (forest litter, small branches, needles and pinecones), forest fire area and its intensity, wind speed and direction.
In this way, experiments showed that the higher the radioactive contamination density of the fuel components, the higher the airborne concentration of radionuclides in the case of forest fire. Radionuclide fallout from the smoke plume on horizontal plates is low and the level of radioactive contamination is not changed practically. However, during the large wildfires on the territories with high values of radioactive contamination density the airborne concentration of radionuclides may be three or four times higher than the background values and exceed the average permissible annual volume activity for population. With a fair wind, the high values of airborne concentration of radionuclides also can be detected at medium range distance (10 km) from the fire source.
The fire source can be located at the distance up to 10 km from the station of radiation control. Additionally, dispersal of the smoke cloud by wind can lead to a decrease of radionuclides concentration in depositions. For these reasons, the horizontal plates sampling method has low accuracy at medium range distance. The data of radioactive fallout density in winter and summer can be similar within the statistical error. To obtain significant values of radioactive fallout density the horizontal plates must be set on short distance (up to 100 m) from the fire source as it indicated in Yoschenko et al. (2006).
The methodological approach of estimation of radionuclides release from fire source we presents above allowed us to use the empirical data for annual effective dose calculation for firefighters and for local population at the medium range distance. Prediction of doses for firefighters and amount of radionuclides that potentially can migrate with air masses outside of fire location are the main actions of the project in the framework of Belorussian State Program of scientific research 2016–2018. It is necessary to develop scientific recommendations to improve the efficiency of wildfires prevention at the territories contaminated by radionuclides (especially for territories near large cities with more or less low levels of contamination density). The outcomes data are also of great importance to inform the population and reducing of social and psychological stress in the society caused by the influence of mass media and the lack of scientifically based models of wildfire and radionuclides release.
Monthly activity of fallout on horizontal plates (stations of radiation control in Gomel), Bq m−2.
A year-to-year variation of 137Cs airborne concentration in Gomel (a) and Mozyr (b).
Time's series of activity index of 137Cs after the forest fires (in the brackets: the location of fire source, year, and contamination density by 137Cs).
The presented research was carried out within the framework of the project supported by the Belorussian State Program for Basic Research “The potential of natural resources”. We gratefully acknowledge the anonymous reviewers for their careful reading, critical comments and helpful suggestions on the paper.
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Cite this article as: Dvornik AA, Klementeva EA, Dvornik AM. 2017. Assessment of 137Cs contamination of combustion products and air pollution during the forest fires in zones of radioactive contamination. Radioprotection 52(1): 29–36.
Long-term dynamics of fires in the areas of radioactive contamination of the Gomel region (a) and in the CEZ (b).
|In the text|
A year-to-year variation of 137Cs airborne concentration in Gomel (a) and Mozyr (b).
|In the text|
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