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RADIOACTIVITY IN VIRGIN SOILS AND SOILS FROM SOME AREAS WITH CLOSED
URANIUM MINING FACILITIES IN BULGARIA
Ivanka Yordanova, Lidia Misheva, Martin Banov, Donka Staneva, Tsvetanka Bineva
Institute of Soil Science, agro technology and plant protections “N.Poushkarov”
Shousse Bankya Str.,1080 Sifia, Bulgaria
e-mail: [email protected]
The soil radioecology is an important part of the environmental research in the country. Since the beginning of the 1970's regular monitoring of the content of different radionuclides in Bulgarian soils has been done. Objective of the studies were virgin soils from high mountain areas, hills and plains (the region of Kozloduy NPP and the
Danube river valley). Natural and men-made radionuclides were observed. In the 25-year period after the contamination with radionuclides due to the 1986 Chernobyl NPP accident a rich data base has been collected, recording the radiation status of the soils in Bulgaria. Special attention has been paid to the contamination with the long-lived technogenic
radionuclides caesium-137 and strontium-90. This paper presents a summary of the obtained results.
Caesium-137 and strontium-90 were the main men-made radionuclides detected in the examined Bulgarian soils few years after the Chernobyl NPP accident. Their content in the soils from high mountain areas (Rodopa and Rila mountains) is several times higher than that in the soils from North Bulgaria and Sofia fields. High non-homogenity
in the pollution within small areas (even as small as several square meters) has been observed.
Natural radioactivity was also studied. Averaged values for natural radionuclides like uranium-238, thorium-232, and radium-226 in virgin soils from different areas in the country are presented. A comparison of the dynamics of their behavior throughout the years is done. Bulgaria is a country with intensive uranium mining activities in the past
years. That is why radiological monitoring of closed uranium mining facilities in different regions of the country are obligatory and of great interest. This work presents results from such investigations made in regions where remediation has been done. The results have been evaluated according to the Bulgarian radionuclide environment contamination
legislation. The necessity of permanent environmental monitoring is assessed.
Contamination with men-made radionuclide
Cs-137
350
300
Bk/kg
250
1985
200
1986
150
1996
100
2006
50
2010
0
North Bul
South Bul
Sofia plain
Besides the data for 1985 and 1986, results for the period after are also presented. Cs and Sr activity is calculated to
reference date 1986 in order to discount the decay effect when studying the dynamics of the technogenic radioactive
pollution of soils. The number of samples for each group from which the average values were calculated were as
specified above in “Materials and methods. While summering the data and calculating the average values high nonhomogeneity was ascertained. The variances were between 20% and 80% for Strontium-90 and between 30% and
70% for Cesium-137 in the different years and different groups. Considering this we may state that there are no
statistically significant changes during the years after the Chernobyl’s accident. The surface layer soil radioactivity
concerning the men-made radionuclide is not affected by migration processes down the soil profile. The migration
processes are discussed in other publications
Bq/kg
120
70
60
1996
50
2001
40
30
2006
2010
100
1996
20
80
2001
10
60
2006
0
40
2010
20
0
S Bulgaria River Mesta
Valley
238U
activity concentration in soil samples
From different regions (1996-2010)
10000
1000
100
238U
226Ra
1
B alkan B uhovo
S liven
238U, 232Th
80
Bq/kg
140
10
15
1985
10
1986
1996
2006
5
2010
0
North Bul
II. Natural radioactivity
Fig. 3.
Sr-90
20
Valley of Mesta
river
Fig. 1. Averaged values for 137Cs activity in soil
samples from different regions [Bq.kg-1]
(1985 – 2010)
N Bulgaria
25
Bk/kg
400
N
Bulgaria
S
Bulgaria
River
Mesta
Valley
Fig. 4. 226Ra activity concentration in soil
samples from different regions(1996-2010)
Especially large disturbances and changes in the landscape and
soil layer at the site are caused by the geo-technological method
for extraction of uranium - on one hand the integrity of the soil is
mechanically disturbed during the course of drilling and blasting,
the construction of the sorption system, the deployment of
drilling pipes and other technological equipment and on the other
hand - the soils are subject to the chemical effects of the
solutions used during the mining process.
These changes and disturbances in the environmental
components require precise and proper planning of the
reclamation and restoration activities, combined with the
recommended activities for usage of the damaged land. This is a
complex and lengthy process that begins with a detailed survey
of the area affected by mining works.
We have studied some of the objects with former uranium mining
facilities. The sampling included virgin soils and soils disturbed by
the mining process activities. Information about the results
obtained from 3 such areas is presented on figures 6.
On fig. 6 (B) Beli iskar is included as an example of a virgin soil
from a mountain region (Rila mountain) with high natural activity
concentration (238U – 650 Bq.kg-1; 226Ra 80 Bq.kg-1) and ratio
226Ra/238U ≈ 8. This disequilibrium is most probably caused by
the high amount of organic matter in this soil that makes
relatively stable compounds with uranium.
The concentration of
and
as long-lived
natural radionuclide's of significance in the soil will be
discussed in this paper. Summary of the data obtained
for the period (1996-2010) is presented on figures 3, 4
and 5. Here again averaged values are calculated for
the same groups of samples described above. The
registered concentration of 238U , 232Th in the soils of
North Bulgaria are in good agreement with the value of
40 Bq.kg-1 estimated as average concentration of these
radionuclide's in soil in UNSCEAR, 1993 report. Activity
concentrations of 226Ra are also in the range of values
stated in the same report. The slightly higher
concentrations in soils from South Bulgaria and the of
Mesta river valley are also logical as the soils in these
areas are on rocks containing shale, gneiss with higher
concentration of natural radioactivity
South Bul
Sofia plain
Valley of Mesta
river
Fig. 2. Averaged values for 90Sr activity in soil
samples from different regions [Bq.kg-1]
(1985 – 2010)
100
80
1996
60
2001
40
2006
2010
20
0
N Bulgaria
S Bulgaria
River mesta
valley
Fig. 5. Content of 232Th activity concentration in soil from
different regions (1996-2010)
1000
100
10
238U
226R a
1
Balkan Buhovo Sliven
226Ra
Bq/kg
I.
Beli
iskar
A (disturbed soils)
B (virgin soils)
Fig. 6. 238U and 226Ra concentrations in disturbed (A) and virgin (B) soils from areas with former uranium mining facilities [Bq.kg-1].
CONCLUSIONS
Caesium-137
and
Strontium-90
are
the
main
radionuclides of significance, characterizing the soil pollution
with man-made radionuclides in all the areas investigated as
part of this study and originate from the Chernobyl accident;
Natural radioactivity concentrations in the investigated
virgin soil are in good agreement with the values presented in
the UNSCEAR reports for different soil types in the Nordic
countries;
The isotope ratio 226Ra/238U may be used for specifying
the origin of higher natural activity concentrations in soils;
The appropriate measures chosen for soil fertility
remediation have prevented additional pollution and human
exposure in the observed areas with former uranium mining
facilities.