1Ivane Javakhishvili Tbilisi State University, 13 Chavchavadze av., Tbilisi, 0179, Georgia,
2Ferdinand Tavadze Metallurgy and Materials Science Institute, 10 Mindeli str., Tbilisi, 0186, Georgia,
3Georgian Technical University, 77 Kostava str., Tbilisi, 0175, Georgia

  1. Corresponding author email

Associate Editor: Dr. Noor Danish Ahrar Mundari
Science and Engineering Applications 2017, 2, 181–192. doi:10.26705/SAEA.2017.2.14.181-192
Received 28 Nov 2017, Accepted 06 Dec 2017, Published 18 Dec 2017

Abstract

Natural and technogenic environmental radioactivity is one of the main problems in ecological research. This study investigates the distribution of natural and technogenic radionuclides in rock samples in the northern part of the Mtskheta-Mtianeti region which occupies territory along the Georgian Military road up to the border with the Russian Federation. This area is characterized by a complex geotectonic structure, in particular, Main Range zone of the Greater Caucasus fold system, Kazbek-Lagodekhi zone and Mestia-Tianeti zone are identify here. Radioactivity of rocks in this region has not been investigated. Using gamma-spectroscopic method we investigated 15 samples of rocks of various types: igneous, sedimentary and metamorphic. Up to 21 naturally occurring radionuclides and one technogenic radionuclide were identified in samples. Activity concentration of radionuclides of Th-232 family varied from 0.39 to 53.8 Bq/kg; U-238 family from < Minimal Detectable Activity to 35.2 Bq/kg; U-235 family from 0.07 to 1.52 Bq/kg. The highest activity concentration was observed for K-40 (maximum value of 845 Bq/kg). In some samples the technogenic radionuclide Cs-137 was observed the activity of which varied from 0.2 to 1.3 Bq/kg. There are some marked features in radionuclides distribution, depending, in particular, on rock type, tectonic zone and age. Some activity ratios were determined, in particular, U-238/U-235, U-238/Th-232, Ra-226/U-238 and Pb-210/Ra-226. These ratios allow to estimate condition of system (closed or opened) and to receive a certain notion about character of relevant geochemical processes. Comparison was carried out with existing data in the literature.

Keywords: radionuclides; rock; radioactivity; activity ratios; Georgia.

Introduction

Research of natural and technogenic radioactivity in various environmental situations is a significant problem in ecology research. Natural radioactivity in the environment is caused by radionuclides of three families: Th-232, U-238 and U-235, and also K-40 (they all are naturally occurring radioactive materials – NORM), which are so-called alpha-, beta- or gamma-emitters. The long-lived radionuclide Cs-137 (as well as Sr-90) is the most widespread technogenic radionuclides. Rock radioactivity is the subject of numerous works.

Previous work [1] has focused on samples of igneous rocks (dolerite, granite, and granitic gneisses), sedimentary rocks (shale, limestone, sandstone) and metamorphic rocks (black shale, slate, graphite, marble, quartz mica, calcareous schist, and quartz) selected in the state of Azad Kashmir, Pakistan. As a result of this work it was determined that, in samples of metamorphic rocks in particular activity concentration of Ra-226 varied from 5.4 to 100 Bq/kg; concentration of Th-232 varied from ≤1.8 to 111 Bq/kg; and K-40 varied from 9.6 to 1055 Bq/kg. An estimation of radiation hazard was made; in particular the value of the radium equivalent activity of all tested samples varied in the range from 20.6 to 294 Bq/kg, which is sufficiently lower than the recommended limit of 370 Bq/kg [2-3].

In a different study [4] samples of rocks from the West of Saudi Arabia were investigated, in particular, basalt, granite, andesite and marble. The concentration of Ra-226 in basalt was 3.0 to 10.9 Bq/kg, in granite was 28.4 to 113 Bq/kg, in andesite was 5.7 Bq/kg (one sample) and in marble was 1.6 Bq/kg (one sample). The concentration of Th-232 in basalt varied from 3.2 to 13.3 Bq/kg, in granite from 33.8 to 89.1 Bq/kg, in andesite it was 3.7 Bq/kg and in marble it was 1.4 Bq/kg. Concentration of K-40 in basalt ranged from 95.6 to 362 Bq/kg, in granite from 1260 to 1629 Bq/kg, in andesite it was 472 Bq/kg and in marble it was 10.1 Bq/kg. The activity ratio Ra-226/Th-232 for basalt was in the range 0.62 to 0.97, for granite was 0.84 to 1.36, for andesite was 1.53 and for marble was 1.13. Also, the radium equivalent activity for basalt ranged from 15.0 to 57.4 Bq/kg, for granite from 174 to 349 Bq/kg, for andesite it was 47.4 Bq/kg and for marble it was 4.3 Bq/kg, which is sufficiently lower than recommended limit value of 370 Bq/kg.

Another study [5] carried out comparative research of radioactivity content in samples of igneous rocks from various deposits in Egypt and Germany. Concentration of Th-232 in igneous samples collected in Egypt varied from 3.2 to 53.4 Bq/kg, and in samples from Germany from 5.8 to 70 Bq/kg. Concentration of Ra-226 in Egyptian igneous samples rangeв from 3.9 to 57.4 Bq/kg and in samples from Germany from 11.2 to 76.1 Bq/kg. Activity of K-40 ranged from 203 to 1041 Bq/kg in samples from Egypt and from 177 to 1465 Bq/kg in samples from Germany. The authors noted that even though these radionuclides are widely distributed, their concentrations depend on the local geological conditions.

A study in the Modane-Aussois region (Western Alps, France) [6] studied radionuclides content of various characteristic rocks, in particular, sedimentary (carbonaceous breccia, limestone dolomite, dolomite) and metamorphic (calcschist, marble, quartzite) rocks. The activity concentrations of K-40 varied from 18 Bq/kg (limestone dolomite) to 572 Bq/kg (quartzite). The activity concentration of Th-232 varied from < 1 Bq/kg (limestone dolomite) to 18 Bq/kg (calcschist). The highest concentration of U-238 was 29 Bq/kg (dolomite) and the lowest was 9.5 Bq/kg (quartzite).

Researches of radioactivity of various environmental objects in Georgia were carried out in the past, stimulated by failure of the Chernobyl atomic power station in 1986. A raised concentration of various technogenic radionuclides (up to several thousands of Bq/kg) was observed, especially in soil of coastal part of Western Georgia; however, rocks were not investigated.

The present study investigates natural and technogenic radioactivity of rocks in the high-altitude region of Mtskheta-Mtianeti, Georgia, which is located in the central part of the Main Caucasian ridge. Some results of soil radioactivity studies in this region are given in the work [7] .

MATERIALS AND METHODS

Study area

The Mtskheta-Mtianeti region is located north of Tbilisi, the capital of Georgia, along the Georgian Military road to the border with Russian Federation. Geologically, the territory can be divided into northern and southern areas. The northern area covers a small area of the northern slopes of the Great Caucasus Main Ridge, in the river Tergi gorge (latitude up to 42.70) and southern slopes in river Aragvi gorge (latitude up to 42.10) to settlement Bodorna.

This area is located in the fold (fold-napped) system of the Greater Caucasus (Kavkasioni, I) [8] and includes some tectonic zones, in particular:

  • Main Range zone (I1, Figure 1): northern high-altitude region (from 2400 m up to approximately 1700 m above sea level), situated on the northern slopes of the ridge; one sample (92, Table 1) was investigated from this area; the zone is characterized by a complex geological structure, in particular, represents in the modern structure large-scale horst-like high, overturned to the south and partially thrust on the located to the south folded zones of the southern slope of the Greater Caucasus;
  • Kazbek-Lagodekhi zone (folded-flaky) (I2): high-altitude region including the main pass on the Kavkasioni (so-called Cross pass) (2400 m above sea level); five samples (1, 3, 4, 6 and 7) from this area were investigated; the zone is characterized by a sufficiently complex geological structure, in particular, it is composed of a thick sandy-shale set of lower and middle Jurassic age and mainly represents the development strip of numerous compressed folds of common Caucasian strike;
  • Mestia-Tianeti zone (fold-napped) (I3): rather low-altitude area reaching from Cross pass to the settlement Bodorna (approximately 600 m above sea level); nine samples (9, 11, 12, 14, 15, 17, 18, 19 and 21) from this area were investigated; the zone is composed of flysh formation (silty sandstone and clastic-lime turbidites and pelagic deposits) of upper Jurassic, Cretaceous and Palaeogene age.

In these areas there are widely-distributed igneous rocks (andesite, basaltic-andesite), as well as and sedimentary (limestone, sandstone, argillite, clay, slate) rocks.

Types of rock samples are the following (Table 1):

  • five igneous (Ig) samples: intrusive (It) – one sample, effusive (Ef) – four samples; by acidity they pertain to the intermediate group;
  • six sedimentary (Sd) samples: carbonate (limestone) (Cr-Ls) – four samples; chemogenic (silicon) (Slc) – one sample; mixed carbonate (sandstone-marl) (Ss (Cr)) – one sample;
  • four metamorphic sedimentary (Mt (Sd)) samples.

Table 1: List of control points (CP), field number (FN) of investigated samples, their types (ST) and age (T)

# Tct CP FN Lt(N); Ln(E) ST T
1 I1 Mt-2 92 42.73510; 44.63530 Diorite quartzous (Dt (Q)) Paleozoic
2 I2 St-1 1 42.66704; 44.62976 Andesite (An) uaternary
3 -“- St-3 3 42.63812; 44.63520 Clay-shale (argillite) (Cl-Sl (Ar)) Jurassic
4 -“- Ar-1 4 42.62355; 44.60974 -“- -“-
5 -“- Tk-1 6 42.60987; 44.57404 Andesite (An) Quaternary
6 -“- Kb-1 7 42.56038; 44.51210 -“- Neogene
7 I3 Sk-1 9 42.43767; 44.49316 Andesite-basalt (An-Bs) -“-
8 -“- Km-1 11 42.43244; 44.50319 Bituminous shale (Sl (Bt)) Cretaceous
9 -“- Kv-1 12 42.37647; 44.67817 Clay-shale greyish (Cl-Sl (Gr)) Jurassic
10 -“- An-1 14 42.17771; 44.68455 Carbonate limestone (whitish) (Cr-Ls (W)) Cretaceous
11 -“- -“- 15 -“- Carbonate limestone (cinnamonic) (Cr-Ls (Cn)) -“-
12 -“- An-3 17 42.16147; 44.69986 -“- -“-
13 -“- An-4 18 42.16223; 44.70038 Chemogenic, Silicon (Chem, Slc) -“-
14 -“- An-5 19 42.16351; 44.70179 Sandstone (Ss (Cr)) -“-
15 -“- Zh-2 21 42.13761; 44.76942 Carbonate limestone (greyish) (Cr-Ls (Gr)) Paleogene
Note. Tct – tectonic zone; geographical coordinates (degrees with decimal fractions); Lt(N) – latitude (north); Ln(E) – longitude (east).

Figure 1 shows sampling point location (close by the monastic building (Mt), the settlements of Stepantsminda (St), Arsha (Ar), Tkarsheti (Tk), Kobi (Kb), Sakuriani (Sk), Kvemo Mleta (Km), Kavtarani (Kv), Ananuri (An) and Zhinvali reservoir (Zh)). Figure 2 shows exterior view of samples 14 and 15 in the location of the control point of their sampling (An-1).

[2456-2793-2-14-1]

Figure 1: Layout of control points (symbols are in compliance with Table 1).

[2456-2793-2-14-2]

Figure 2: Control point An-1 where two samples – 14 and 15 – were selected.

Sampling

Samples were selected from the outcropped rocks and put in plastic containers (volume up to 2.0 L). After drying in the laboratory, samples were broken into pieces < 40 mm and were then crushed using a special crusher (jaw crusher Retsch) to a size of approximately 1 mm. Then samples were dried at 105 - 1100C to constant weight and their bulk density was then determined. These values were used for the description of sample geometry. The samples were sealed in Marinelli beaker and stored for more than four weeks to achieve secular equilibrium between Ra-226 and Rn-222.

Measurement of gamma radiation activity

Measurements were carried out using a gamma spectrometer Canberra GC2020 with a semi-conductor germanium detector with relative efficiency of 24%. Gamma spectra acquisition time was 72 hour. Genie-2000 S500 software with additional modules was used for the analysis, in particular, S506 Interactive Fit Program. By means of this program, for all spectra a “decomposition” of the interference peak in the area of 186 keV was carried out (program identifies one peak in this area that is growing out of an interference of two closely spaced peaks of U-235 (185.715 keV) and Ra-226 (186.211 keV)). The program S506 processes the spectral curve mathematically; therefore, in this area two peaks are created with energies corresponding to U-235 and Ra-226. During the program identification of peaks and calculation of activity concentration a tolerance value was established in such a manner that low-energy peak was compared only with U-235 and high-energy peak only with Ra-226. Results show that, in particular, determination error of the activity concentration of Ra-226 was within 9 and 24%. Its activity was compared with the activity of its daughters, Pb-214 and Bi-214, which had a determination error between 3 and 9%. Values of Ra-226, Pb-214 and Bi-214 activity did not differ sufficiently. Thus, it is possible to consider that similar determination error of Ra-226 concentration is satisfactory; thus, this method was also used for the determination of U-235 activity concentration by the 185.715 keV line. Received values of U-235 activity (which activity determination error range was 8 - 16%, and in individual cases for low-activity samples increased to the range 18 – 31 %) were compared to values of U-238 activity (which were determined by the line 63.3 keV line of Th-232, with an error range of 6.3-14%). The value of their activities ratio U-238/U-235, which is considered as constant (21.7) for natural objects [9], was used as a criterion. When deviation from this value is big (more than 10 %) reanalysis of 186 keV line (as well as 63.3 keV line) by means of the program S506 was carried out. Concentration of Pb-210 was determined by the 46.54 keV line (with error range of 8.3 - 20.6 %). For Th-232 activity determination average values for Ac-228, Ra-224, Pb-212, Bi-212, and Tl-208 were used, which had determination uncertainties limits between 1.4 and 7.0%. Activities ratios were also determined U-238/Th-232 (which is accepted as being equal 0.81 for the closed systems [10-11], Ra-226/U-238 and Pb-210/Ra-226 (equilibrium value 1.00), which are used to estimate the mechanism of various geochemical processes.

Taking into account the influence of matrix composition, the chemical composition of samples was determined on the basis of literary data [12-13], which were then used in the special software (LabSOCS) for efficiency calibration of the activity concentration calculation. System LabSOCS allows to create calibrations by laboratory quality efficiency without application of radioactive calibrate sources. For radionuclides identification a special library was used that contains lines of 41 radionuclides and other specific sources (in total 351 lines). Database NuDat [14] was used for library compiling. For activity (A) calculation, the background radiation was subtracted.

The assessment of values of radium equivalent activity Raeq (Bq/kg) was carried out using the formula [15]:

[2456-2793-2-14-i1]
(1)

where AU, ATh, and AK are the activity concentrations (Bq/kg) of U-238, Th-232 and К-40, respectively.

For samples characterization by radioactivity degree, taking into account accepted limit value of Raeq(370 Bk/kg; equivalent to the annual γ-radiations dose of 1.5 mSv/y) [16]) some groups were established according to their value of equivalent activity, in particular:

1st group: nonradioactive samples (activity is low and did not exceed 30 Bq/kg);

2nd group: samples with low radioactivity (activity is in the range of 30 to 100 Bq/kg);

3rd group: samples with average radioactivity (activity is in the range of 100 to 300 Bq/kg);

4th group: samples with high radioactivity (activity is in the range of 300 to 1000 Bq/kg; note: in the present work such samples have not been observed.).

Results

Up to 22 radionuclides were identified from results of the analysis of gamma spectra in rock samples: Th-232 family (Ac-228, Th-228, Ra-224, Pb-212, Bi-212, Tl-208); U-238 family (Th-234, Pa-234, Th-230, Ra-226, Pb-214, Bi-214, Pb-210; U-235 family (U-235, Th-231, Th-227, Ra-223, Rn-219, Pb-211; radionuclides Be-7, K-40, Cs-137 (some specific lines were also identified that are incipient as a result of interaction of cosmic rays with the material of the detector or the sample).

Average activity of identified families’ radionuclides varied widely, from 0.07 Bq/kg (for U-235 family) to 53.8 Bq/kg (for Th-232 family). Among individual radionuclides, K-40 has the highest activity (up to 845 Bq/kg). In some samples, activity of some radionuclides (particularly the U-235 family) was lower than the Minimal Detectable Activity (MDA).

Activity concentration of main radionuclides of the investigated samples, equivalent activity, activity ratios and their average (av), minimal (mn) and maximal (mx) values, among other data, are given in Table 2, Table 3, Table 4, Table 5, and Table 6.

Table 2: Activity concentration (Bq/kg) of radionuclides of families Th-232, U-238 (Th-234), U-235, Ra-226,Pb-214, Bi-214, Pb-210, radionuclides Be-7, K-40 and Cs-137, equivalent activity (Raeq, Bq/kg), activity ratios U-238/U-235; U-238/Th-232, Ra-226/U-238 and Pb-210/ Ra-226, their average (av), minimal (mn) and maximal (mx) values for rocks of various types

# CP FN ST Th-232 U-238 Ra-226 Pb-214 Bi-214 Pb-210 U-235 Be-7 K-40 Cs-137 Raeq U-238/U-235 U-238/Th-232 Ra-226/U-238 Pb-210/Ra-226
1 Mt-2 92 Dt(Q) 45.1 33.7 39.8 35.7 34.8 33.4 1.49 <M 631 148 22.6 0.75 1.18 0.84
2 St-1 1 An 30.8 29.3 24.9 27.1 25.5 20.6 1.30 443 <M 100 22.5 0.95 0.85 0.83
3 St-3 3 Cl-Sl (Ss,Ar) 45.1 33.4 39.5 35.7 34.8 33.4 1.52 <M 631 148 22.0 0.74 1.18 0.84
4 Ar-1 4 Ar 53.8 35.2 32.9 32.4 31.0 24.9 1.47 <M 845 1.31 169 23.9 0.65 0.94 0.75
5 Tk-1 6 An 39.2 31.2 35.2 32.3 30.9 34.1 1.32 582 132 23.6 0.79 1.13 0.97
6 Kb-1 7 “—“ 37.3 27.8 28.4 29.6 28.7 24.6 1.36 501 0.19 117 20.4 0.74 1.02 0.87
7 Sk-1 9 AnBs 29.5 21.9 22.5 21.8 20.7 28.7 1.00 3.88 450 96 21.9 0.74 1.03 1.27
8 Km-1 11 Sl (Bt) 14.1 20.1 16.2 17.7 17.5 20.2 0.90 262 55 22.3 1.43 0.81 1.24
9 Kv-1 12 Cl-Sl (Gr) 45.2 25.4 25.3 26.8 25.3 30.1 1.21 <M 784 145 21.0 0.56 1.00 1.19
10 An-1 14 Cr-Ls(St) 3.3 <M 1.51 1.5 <M 0.11 <M 51.7 - - - - -
11 “— 15 Cr-Ls (Sl) 22.8 7.7 8.3 7.4 7.6 <M 0.33 <M 644 86 23.8 0.34 1.07 -
12 An-3 17 Cr-Ls 22.6 9.6 10.7 9.28 8.9 <M 0.45 470 76 21.3 0.43 1.11 -
13 An-4 18 Slc 0.73 2.6 3.0 3.2 2.8 <M 0.13 9.0 5 20.6 3.59 1.16 -
14 An-5 19 Cr-Mx (Ss,Mr) 14.7 14.9 14.0 11.9 11.7 18.1 0.65 <M 269 54 23.0 1.01 0.94 1.29
15 Zh-2 21 Cr-Ls 0.39 <M <M 1.25 1.3 <M 0.07 <M 3.9 - - - - -
    av   27.0 22.5 23.1 19.6 18.9 26.8 0.89 3.9 438 0.7 102 22.2 0.98 1.03 1.01
    mn   0.39 2.6 3.0 1.2 1.3 18.1 0.07 - 3.9 0.2 4.7 20.4 0.34 0.8 0.75
    mx   53.8 35.2 39.8 35.7 34.8 34.1 1.52 - 845 1.3 169 23.9 3.6 1.2 1.29
Note. M – Minimal Detectable Activity (MDA)

Table 3: Generalized data – average (av), minimal (mn), maximal (mx) values of concentration of radionuclides of families (Th-232, U-238 and Ra-226, U-235) and radionuclide K-40, equivalent activity (Raeq, Bq/kg), activity ratios – depending on origin.

# G GR ST Th-232 U-238 Ra-226 U-235 K-40 Raeq U-238/Th-232 Ra-226/U-238 Pb-210/Ra-226
av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx
1 Ig     36.4 29.5 45.1 28.8 21.9 33.7 30.2 22.5 39.8 1.3 1.0 1.5 522 443 631 119 96.2 148 0.80 0.74 0.95 1.04 0.85 1.18 0.95 0.83 1.27
    It Dt(Q) (92) 45.1 - - 33.7 - - 39.8 0.0 0.0 1.5 - - 631 - - 148 - - 0.75 - - 1.18 - - 0.84 - -
    Ef   34.2 29.5 39.2 27.5 21.9 31.2 27.8 22.5 35.2 1.2 1.0 1.4 494 443 582 111 96.2 132 0.81 0.74 0.95 1.01 0.85 1.13 0.98 0.83 1.27
      An (1;6;7) 35.8 30.8 39.2 29.4 27.8 31.5 29.5 24.9 35.2 1.3 1.3 1.4 509 443 582 116 100 132 0.83 0.74 0.95 1.00 0.85 1.13 0.89 0.83 0.97
      An-Bs (9) 29.5 - - 21.9 - - 22.5 - - 1.0 - - 450 - - 96.2 - - 0.74 - - 1.03 - - 1.27 - -
2 Sd     10.8 0.39 22.8 8.7 2.6 14.9 9.0 3.0 14.0 0.29 0.07 0.65 241 3.9 644 55.2 4.7 86.0 1.34 0.34 3.59 1.07 0.94 1.16 1.29 - -
    Ss Ss (Cr) (19) 14.7 - - 14.9 - - 14.0 - - 0.65 - - 269 - - - - - 1.01 - - 0.94 - - 1.29 - -
    Cr(Ls)   12.3 0.39 22.8 8.7 7.7 9.6 9.5 8.3 10.7 0.24 0.07 0.45 292 3.9 644 81.0 76.0 86.0 0.38 0.34 0.43 1.09 1.07 1.11 - - -
      Cr-Ls(Sl) (15),Cr-Ls (Cn)(17) 22.7 22.6 22.8 8.7 7.7 9.6 9.5 8.3 10.7 0.39 0.33 0.45 557 470 644 81.0 76.0 86.0 0.38 0.34 0.43 1.09 1.07 1.11 - - -
      Cr-Ls(St) (14), Cr-Ls (21) 1.8 0.4 3.3 <M - - <M - - 0.09 0.07 0.11 27.8 3.9 51.7 4.7 - - - - - - - - - - -
    Chem Slc (18) 0.73 - - 2.6 - - 3.0 - - 0.13 - - 9.0 - - 53.9 - - 3.59 - - 1.16 - - - - -
3 Mt (Sd) Cl-Sl   39.5 14.1 53.8 28.5 20.1 35.2 28.5 16.2 39.5 1.3 0.9 1.5 630 262 845 129 54.7 169 0.85 0.56 1.43 0.98 0.81 1.18 1.01 0.75 1.24
      Cl-Sl(Ss, Ar) (3) 45.1 - - 33.4 - - 39.5 - - 1.5 - - 631 - - 148 - - 0.74 - - 1.18 - - 0.84 - -
      Cl-Sl(Ar) (4) 53.8 - - 35.2 - - 32.9 - - 1.5 - - 845 - - 169 - - 0.65 - - 0.94 - - 0.75 - -
      Cl-Sl (Gr) (12) 45.2 - - 25.4 - - 25.3 - - 1.2 - - 784 - - 145 - - 0.56 - - 1.00 - - 1.19 - -
      Sl (Bt) (11) 14.1 - - 20.1 - - 16.2 - - 0.9 - - 262 - - 54.7 - - 1.43 - - 0.81 - - 1.24 - -
Note. G - Genesis, GR – group of rocks.

Table 4: Generalized data – average (av), minimal (mn), maximal (mx) values of concentration of radionuclides of families (Th-232, U-238 and Ra-226, U-235) and radionuclide K-40, equivalent activity (Raeq, Bq/kg), activity ratios – depending on origin.

# Tct Th-232 U-238 Ra-226 U-235 K-40 Raeq U-238/Th-232 Ra-226/U-238 Pb-210/Ra-226
av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx
1 I1 45.1 - - 33.7 - - 39.8 - - 1.5 - - 631 - - 148 - - 0.75 - - 1.18 - - 0.84 - -
2 I2 41.2 30.8 53.8 31.4 27.8 35.2 32.2 24.9 39.5 1.4 1.3 1.5 601 443 845 133 100 169 0.78 0.65 0.95 1.02 0.85 1.18 0.85 0.75 0.97
3 I3 17.0 0.39 45.2 14.6 2.6 25.4 14.3 3.0 25.3 0.54 0.07 1.2 327 3.9 784 73.7 4.7 145 1.16 0.34 3.59 1.02 0.81 1.16 1.25 1.19 1.29

Table 5: Generalized data – average (av), minimal (mn), maximal (mx) values of concentration of radionuclides of families (Th-232, U-238, U-235) and radionuclide K-40, equivalent activity (Raeq, Bq/kg), activity ratios – depending on the sample age (T)

# T (FN) Th-232 U-238 Ra-226 U-235 K-40 Raeq U-238/Th-232
av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx av mn mx
1 Quaternary (1;6) 35.0 30.8 39.2 30.2 29.3 31.2 30.1 24.9 35.2 1.3 1.3 1.3 513 443 582 116 100 132 0.87 0.79 0.95
2 Neogene (7;9) 33.4 29.5 37.3 24.8 21.9 27.8 25.5 22.5 28.4 1.2 1.0 1.4 476 450 501 106 96.2 117 0.74 0.74 0.74
3 Paleogene (21) 0.39 - - <M - - <M - - 0.07 - - 3.9 - - - - - - - -
4 Cretaceous 13.0 0.73 22.8 1.0 2.6 20.1 10.5 3.0 16.2 0.43 0.11 0.90 284 9.0 644 47.3 4.7 86.0 1.36 0.34 3.59
(14;18) 2.0 0.73 3.3 2.6 - - 3.0 - - 0.12 0.11 0.13 30 9.0 51.7 6.5 4.7 8.3 3.59 - -
(11;15;17;19;) 18.6 14.1 22.8 13.1 7.7 20.1 12.3 8.3 16.2 0.58 0.33 0.90 411 262 644 67.6 53.9 86.0 0.80 0.34 1.43
5 Jurassic (3;4;12) 48.0 45.1 53.8 31.3 25.4 35.2 32.6 25.3 39.5 1.4 1.21 1.5 753 631 845 154 145 169 0.65 0.56 0.74
6 Paleozoic (92) 45.1 - - 33.7 - - 39.8 - - 1.5 - - 631 - - 148 - - 0.75 - -

Table 6: Distribution of average value Raeq-av of equivalent activity Raeq by the activity level group (GA), their quantity (Ns) and percentage (r, %).

# GA Raeq, Bq/kg Raeq-av, Bq/kg Ns r, %
1 I <30 3.4 2 13.3
2 II 30-100 62.4 6 40.0
3 III 100-300 136 7 46.7

General characteristics

Activity of families’ radionuclides varied in various samples more than two orders of magnitude (Table 2), in particular, Th-232 – from 0.39 to 53.8 Bq/kg (average value of 27.0 Bq/kg); U-238 – from < MDA (2.38 Bq/kg) to 35.2 Bq/kg (average value of 22.5 Bq/kg); Ra-226 – from 3.0 to 39.8 Bq/kg (average value of 23.1 Bq/kg); U-235 - from 0.07 to 1.52 Bq/kg (average value of 0.89 Bq/kg). Activity of K-40 varied more than two orders of magnitude - from 3.9 to 845 Bq/kg (average value of 438 Bq/kg). Be-7 was fixed in one sample (9) – 3.9 Bq/kg (and in trace amounts in some samples). Technogenic radionuclide Cs-137 was fixed in two samples (4; 7) in small amounts (1.3 and 0.2 Bq/kg, respectively), and in one sample (1) in trace amounts. For the activity ratio of U-238/U-235 all obtained values correspond (in limits ±10%) to the value of 21.7 (accepted for natural objects). Activity ratio of U-238/Th-232 showed marked deviation (more than ±10%) from the average value 0.81 (for closed systems); an increase was observed in four samples (1, and especially 11, 18, and 19, ranging from 1.01 to 3.59), and a decrease in four samples (4, 12 and especially 15 (0.34) and 17 (0.43)). Value of the ratio of Ra-226/U-238 differs (more than ±10%) from the equilibrium value for some samples – in two samples (92; 3) is greater than equilibrium, and in two samples (1; 11) is lesser. The ratio of Pb-210/Ra-226 differs also from the equilibrium value (more than ±20% - the range of limits is expanded, because determination uncertainty of Pb-210 reached up to 20%) in some samples – in three samples (9; 11; 19) is greater than equilibrium and in one sample (4) is lesser (note: activity ratios were not determined for all samples because in some samples (14; 15; 17; 18; 21) activities of corresponding radionuclides were below the MDA or were not fixed). Within chain Th-232 - Tl-208, basically, it was observed equilibrium (except for Th-228, which determination error is appreciable more than for other radionuclides). The main quantity of samples (86.7%) by the level of radium equivalent activity applies to the groups with low and average radioactivity, the small quantity of samples (13.3%) applies to the group of nonradioactive samples (table 6).

Dependence on the genesis (type)

Average activity of families’ radionuclides and radionuclide K-40 in igneous and metamorphic sedimentary rocks (Table 3) are of the same level (except sample 11, which activity corresponds to sedimentary rocks), and in sedimentary rocks is sufficiently lower – approximately 2-3 times, and they can be divided into two groups: with relatively low activity values – from 0.73 to 3.3 Bq/kg for Th-232 and U-238 families, and from 9.0 to 51.7 Bq/kg for radionuclide K-40 (samples 14, 18 and 21); with relatively high activity values – from 8.7 to 22.7 Bq/kg for Th-232 and U-238 families, and from 269 to 557 Bq/kg for radionuclide K-40 (15, 17 and 19) (note: it is necessary to notice, that samples 14 and 15, selected at the same control point (An-1) differ from each other by appearance and almost 10 times differ by activity of radionuclides of Th-232 and U-238 families and radionuclide K-40). Calculated values of U-238/Th-232 for igneous rocks ranged from 0.74 to 0.95 (insignificant excess concerning average value can be noted only in one sample (1)); ratios Ra-226/U-238 and Pb-210/Ra-226 are sufficiently close to equilibrium or a little higher. For the group of sedimentary rocks with relatively high activity values it was observed wider diapason for ratio U-238/Th-232 – from 0.34 to 1.01; ratios Ra-226/U-38 are close to equilibrium, and ratio Pb-210/Ra-226 was calculated only for one sample (19), and it was higher than equilibrium (Note: for the group of sedimentary rocks with relatively low activity values these ratios have not been calculated because of their small values and accordingly big error). For metamorphic rocks ratio U-238/Th-232 for one sample (3) corresponds to equilibrium, for two samples (4; 12) is lower (0.56 and 0.65), and in one sample (11) is much higher (1.43); for ratio Ra-226/U-238 it was observed deviations from equilibrium value both in greater way and in smaller way; similarly for Pb-210/Ra-226.

Dependence on the tectonic zones

The certain tendency of activity (as well as equivalent activity) decrease from zone I1 to zone I2 was observed, and, is especially appreciable – to zone I3 (Table 4). For values of activity ratio U-238/Th-232 for zones I1 and I2 (where all igneous samples have been selected) there were observed insignificant deviations from the average value both in greater and in smaller way (they ranged from 0.65 to 0.95), and in zone I3 (where sedimentary and metamorphic samples have been selected) this range is bigger – from 0.34 to 1.01. Values of the ratio Ra-226/U-238 for zones I2 and I3 insignificant deviates from equilibrium value both in greater and in smaller way (they ranged from 0.81 to 1.18), and in zone I1 received value slightly exceeds the equilibrium. For activity ratio Pb-210/Ra-226 for zones I2 and I3 the similar picture was observed, and in zone I1 the received value corresponds to the equilibrium.

Dependence on the age of rocks

The highest activity values of families’ radionuclides and K-40 (Table 5) take place for rocks of Paleozoic era and Jurassic, Neogene and Quaternary periods (equivalent activity ranged from 96.2 to 169 Bq/kg), and is much lesser for rocks of Cretaceous and Paleogene (from 4.7 to 86.0 Bq/kg). For rocks of Cretaceous period it is possible to allocate two groups of samples by activity, in particular, with low values – equivalent activity ranged from 4.7 to 8.3 Bq/kg (samples 14, 18), and this group includes also sample 21 (Paleogene); with rather high values – equivalent activity ranged from 53.9 to 86.0 Bq/kg (samples 11, 15, 17 and 19). For activity ratio U-238/Th-232 in rocks of Paleozoic and Neogene all values are within average value, in Jurassic rocks – within average value and lower, and in Cretaceous rocks (with rather high activity values) appreciable deviations take place both in smaller, and in greater way, in Quaternary rocks – within average value and higher (Note: corresponding ratio values have not been calculated for Cretaceous samples (with low activity values) and Paleogene samples because of their low activity values (ratios Ra-226/U-238 andPb-210/Ra-226 have not been analyzed because the age does not influence on their value)).

DISCUSSION

The concentration of radioactive elements in rocks and soils is formed by the radioactivity of original structures and the whole set of subsequent processes of rock and soil formation. The content and concentration of NORM identified in the investigated samples generally correspond to those observed [17] for various rocks and soils. For the study region, this is the first time such analysis has been carried out (there are also no similar data for other regions of Georgia). All these radionuclides, except Cs-137, are of natural origin. They are also characteristic for the region of East Georgia, in particular, for soil in the river Mtkvari region [18].

As stated above, rock samples were selected in a geological area characterized by a sufficiently complex geotectonic structure. Types of the selected samples corresponded to all three main groups of rocks: igneous (in particular, intrusive and effusive), sedimentary and metamorphic (meta-sedimentary). Each of these groups has specific mineralogical and chemical composition as well as rock-forming mechanism, which is connected to the wide range of radioactivity concentration values, taking place practically for all identified radionuclides, and also for activities ratios of some radionuclides.

In all samples the U-238/U-235 ratios observed correspond, within error, to the natural value, that, besides methodological aspect, allows making the conclusion about absence of pollution by anthropogenous U-235. For igneous samples activity ratios U-238/Th-232 and Ra-226/U-238 were close to average and equilibrium values, indicating that these primary rocks represent closed systems. To a certain degree there seems to be an insignificant deviation of the Pb-210/Ra-226 ratio from the equilibrium value, which can be explained by the condensed structure of these rocks.

However, in sedimentary and metamorphic samples (being secondary rocks) peculiarities of activity ratios, in particular deviations from the average U-238/Th-232 ratio value and equilibrium value of Ra-226/U-238 ratio, imply that these systems were not closed and that various geochemical processes took place in them in past.

As an example, previous works [17, 19] noted that Th isotopes, which are found in natural environments only in the tetravalent form, generate compounds practically insoluble in waters and that are transferred mechanically in the form of stable minerals, while U isotopes occur in nature in tetravalent and hexavalent form: in tetravalent form they have chemical properties similar to Th, while in hexavalent form they have large chemical activity and can migrate large distances in a soluble form in water. These processes can lead to a decrease in the U-238/Th-232 ratio relative to the average value, especially in sedimentary and metamorphic rocks where the raised porous structure could favour their greater intensification (as was observed, for example, in samples 4, 12, 15, 17) . On the other hand it appears that under profound chemical aeration of the parent rocks Th can migrate in colloid form. Such processes can lead to an increase in the U-238/Th-232 ratio relative to the average value (as was observed in sedimentary and metamorphic rocks in samples 11, 1, etc.). It is necessary to note that such processes can proceed simultaneously, causing a reduction in U-238 (and U-235), as well as Th-232. In this case their concentration can decrease strongly, as was also observed in a number of samples (for example, 14, 18, 21).

The same factors, and also influence of hypergenesis (weathering) can lead to disequilibrium in radioactive families, in particular, between U and Ra. Here, in addition to the reasons specified above, influence of distinctions of chemical properties of elements is essential. In particular, the Ra isotope is easy leached and washed away by water: in natural formations Ra-226 often accumulates in quantities exceeding equilibrium with uranium. Deviations from the equilibrium values for the investigated samples are rather small, and it is possible to conclude that above-mentioned factors are insignificant in this region.

Slight hyperactivity of samples observed in tectonic zones I1 and I2, in comparison with samples in zone I3, may be connected with the complex geotectonic orogenic processes that took place during the formation of the Caucasian mountain system many millions of years ago (Neogene and Quaternary periods). During this period volcanoes operate in this region (located, in particular, near to sampling locations of Kazbek, Tkarsheti, Kabarjina, which represent the palaeovolcanoes) and produced various igneous (volcanic) rocks in this region, which included substances from deep layers of magma with a raised concentration of radioactive elements.

In zone I3 sedimentary rocks prevail, along with metamorphic sedimentary rocks, that involved rock-forming process that resulted in a reduction (as a result of noted above processes of weathering and leaching) of compounds with a raised concentration of Th-232, U-238, U-235 and K-40 activity and caused an observable decrease of radioactivity in the rocks of this zone. In several rock types (for example, silicons and sandstones, in which rather insignificant activity concentrations were observed) these processes had a specific character indicated above that caused an observable decrease in activity.

The time of rock formation also would have an influence on radionuclide activity, for example in the case of samples 14 and 15 (limestones), which were sampled at the same location but were in different horizons (Figure 2) that could cause an appreciable difference in radionuclide concentrations caused by local changes of rock-forming conditions during the formation of these horizons. The results given in table 4 confirm the influence of the time of rock formation on activity, as there is an appreciable reduction of activity in samples from the Cretaceous period (and also the Paleogene) in comparison with samples from other periods.

More appreciable deviations from average value of U-238/Th-232 activity ratio are observed in sedimentary and metamorphic rocks, apparently due to processes of hypergenesis. The Cretaceous (when the majority of sedimentary rocks were formed and from there was a metamorphic sample 11) was characterized by land domination on the Earth, the climate was hot and dry and deposits of chalk, carbonates and clay slates, among others, were formed. Mountain systems were formed, and heretofore the midland seas and extensive bogs were formed. This all promoted intensification of process of hypergenesis and intensive leaching of radioactive elements during rock-forming or metamorphism processes. As a result, in these rocks there is an appreciable decrease in content of radioactive elements. After the Cretaceous the climate changed, the temperature decreased and this factor promoted a change of the conditions of hypergenesis, which promoted an increase of radionuclide content in rock-forming process (which we observe for samples of Neogene and Quaternary age).

We do not observe the same deviation of ratio Pb-210/Ra-226 from the equilibrium that is reported in previous work [19]. It is possible that absence of super equilibrium (allochthonous) Pb-210 in rocks (in comparison with soils, where its appreciable presence is often fixed) is connected with their condensed structure, which renders the radionuclides within them in a “sealed” condition and, as such, radon migration does not take place within them. The insignificant excess observed in several samples can be connected with deposits of Pb-210 from the atmosphere.

In some samples an insignificant concentration of naturally occurring radionuclide Be-7 (a so-called cosmogeneous radionuclide that is formed as a result of nuclear reactions in an upper atmosphere) are observed as a result of precipitation and in some cases can be identified in gamma-spectra.

The technogenic radionuclide Cs-137 also gets into the samples as a result of atmospheric precipitation. Usually, the presence of Cs-137 in natural objects (for example, in soil) is linked to the Chernobyl disaster. After that it decreased in quantity as a result of decay and migration. In several samples an insignificant concentration (in comparison with soils where its concentration is considerably higher) was observed that, apparently, is connected with intensive process of washing from a sample surface.

The obtained values of radium equivalent activity are appreciable below the recommended value of 370 Bq/kg [2-3].

In Table 7 some literary data is cited from other regions of the world. The values produced in this work are, on average, much lower than in other regions and also in comparison with global average values.

Table 7: Activity concentration (Bq/kg) of radionuclides in rocks and other data in various regions of the world

G GR ST SR Th-232 U-238 Ra-226 U-235 K-40 Cs-137 Raeq U-238/U-235 U-238/Th-232 Ra-226/U-238 Pb-210/Ra-226 Ref.
Ig     Ww 7-480 0.4-740 7-115   124-2790             [20]
  Ef   -“- 39
48-130
30
18.5-52
-
48-137
  815
658-925
            -“-
    Im -“- 60
10-128
41
6-160
-   837
465-1860
            -“-
      Ge 36.4
29.5-45.1
28.8
21.9-33.7
30.2
22.5-39.8
1.3
1.0-1.5
522
443-631
  119
96.2-148
22.2
20.4-23.6
0.80
0.74-0.95
1.04
0.85-1.18
0.95
0.83-1.27
Present study
  It Dt Eg 8.0
7.0-9.9
  12.4
12.1-12.7
  380             [5]
      Gm 5.8   11.2   180             -“-
    Dt(Q) Ge 45.1 33.7 39.8 1.5 631   148 22.6 0.75 1.18 0.84 Present study
  Ef An SA 3.7   5.7   472       1.531     [4]
      Ge 35.8
30.8-39.2
29.4
27.8-31.5
29.5
24.9-35.2
1.3
1.3-1.4
509
443-582
  116
100-132
22.2
20.4-23.6
0.83
0.74-0.95
1.00
0.85-1.13
0.89
0.83-0.97
Present study
    Bs SA 9.1
3.2-13.0
  7.9
3.0-10.9
  235
95.6-362
      0.831
0.62-0.97
    [4]
    An-Bs Ge 29.5 21.9 22.5 1.00 450   96.2 21.9 0.74 1.03 1.27 Present study
Sd     Ww 0.8-120 1.2-1107 15-1720   15.5-1270             [20]
  Cr   Ww 7
3-22
30
11-148
-   -
31-372
            -“-
  Cr Ls Pk 30.9
28.7-32.4
  22.2
18.1-25.4
  237
224-251
≤0.62 84.7
76.4-91.0
        [1]
      Ge 12.3
0.39-22.8
8.7
7.7-9.6
9.5
8.3-10.7
0.24
0.07-0.45
292
3.9-644
  81.0
76.0-86.0
22.5
21.3-23.8
0.38
0.34-0.43
1.09
1.07-1.11
- Present study
  Ss   Ww 12
26-120
19
3.6-98
26   527
34-930
            [20]
  Ss Cr Pk 66.3
64.5-68.9
  42.4
40.7-45.2
  500
440-610
≤0.62 176
169-180
        [1]
      Ge 14.7 14.9 14.0 0.65 269     23.0 1.01 0.94 1.29 Present study
Mt     Ww 0.4-333 0.24-1970     148-1517             [20]
      Ge 39.5
14.1-53.8
28.5
20.1-35.2
28.5
16.2-39.5
1.3
262-845
630
262-845
  129
54.7-169
  0.85
0.56-1.43
0.98
0.81-1.18
1.01
0.75-1.24
Present study
  Sl   Ww 40 31     -             [20]
    Sl (Cr,Cl) Fr 18.0   14.4   392             [6]
  Ig Cl-Sl (Ss,Ar) Ge 45.1 33.4 39.5 1.50 631   148 22.0 0.74 1.18 0.84 Present study
    Cl-Sl (Ar) -“- 53.8 35.2 32.9 1.50 845   169 23.9 0.65 0.94 0.75 -“-
    Cl-Sl (Gr) -“- 45.2 25.4 25.3 1.20 784   185 21.0 0.56 1.00 1.19 -“-
Note.
  1. In the paper there are given activity ratio of Ra-226/Th-232.
  2. G- genesis; GR – group of rocks; SR – studied region; Im – intermediate magmatic rocks; Eg – Egypt; Gm – Germany; Pk – Pakistan; SA – Saudi Arabia; Fr – France; Ge – Georgia; Ww – Worldwide.

In conclusion it is necessary to note that the received results represent doubtless scientific evidence that confirms the urgency of research such as this and the necessity of their regular conduct.

Conclusions

  1. 1. It was established that in rock samples there are up to 22 detected radionuclides, in particulat: the Th-232 family – Ac-228, Th-228, Ra-224, Pb-212, Bi-212, Tl-208 (in total six radionuclides); the U-238 family – Th-234, Pa-234, Th-230, Ra-226, Pb-214, Bi-214, Pb-210 (in total seven radionuclides); U-235 family – U-235, Th-231, Th-227, Ra-223, Rn-219, Pb-211 (in total six radionuclides); other naturally occurring radionuclides – Be-7, K-40 and the technogenic radionuclide Cs-137.
  2. 2. The main features and regularities of samples radioactivity were established, in particular:
    • activity of families radionuclides and the radionuclide K-40 varied in various samples by more than two orders of magnitude; the U-238/U-235 activity ratio corresponds to a value of 21.7 (accepted for natural objects); U-238/Th-232 ratio deviations (more than ±10%) from the average value of 0.81 (for closed systems) were observed as both increases and decreases; deviations of ratios Ra-226/U-238 and Pb-210/Ra-226 from the equilibrium value (1.0) were insignificant;
    • radionuclide Be-7 was fixed in some samples at trace amounts;
    • the technogenic radionuclide Cs-137 was fixed in two samples.
  3. Some features of activity distribution were determined depending on genesis and sample type and depending on geotectonic zones and the age of samples:
    • average activities of families radionuclides and the radionuclide K-40 in igneous and metamorphic sedimentary rocks were at the same level and were appreciably lower in sedimentary rocks, by approximately two-three times;
    • the highest activity values were observed in zones I1 and I2 and appreciably lower activity in zone I3;
    • the highest activity values of families’ radionuclides and the radionuclide K-40 occurred in rocks from the Palaeozoic era and Jurassic, Neogene and Quaternary periods, while activity was much lower for rocks from the Cretaceous and Paleogene.
  4. Analysis of obtained results and some of their features was carried out, as well as comparison with literary data.

Acknowledgements

This work was supported by the Shota Rustaveli National Science Foundation, Georgia [grant number FR/49/9-170/14].

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