Radiological Assessment of Soil and Rock Samples from Stubla Village, Kosovo
Article information
Abstract
Background:
This study represents a thorough exploration of the radioactivity levels in soil and rock samples collected from Stubla Village, Kosovo. The main aim is to evaluate the potential hazards and risks associated with using these materials in construction, and this is a common practice among residents when building homes.
Materials and Methods:
Gamma spectrometry was employed to analyze 19 soil samples and seven rock samples, followed by the calculation of the primary radiological parameters.
Results and Discussion:
The activity concentration of 226Ra, 232Th, 40K, and 137Cs was determined in soil samples (95.84±67.5, 75.51±35.5, 12.87±23.5, and 738.87±186.7 Bq/kg, respectively) and natural rock samples (199.8±190.2, 147.0±14.3, 967.1±47.4 Bq/kg, and not detected, respectively). Almost all the calculated health radiological hazard factors exceeded the recommended limits by international professional organizations.
Conclusion:
The rocks from this specific region are unsuitable for construction work due to their high radioactivity levels. This emphasizes potential environmental and health risks. The findings underscore the importance of implementing alternative materials for construction. Additionally, further investigations into radioactivity risks in the area under study should be conducted, such as assessing radioactivity concentrations in drinking water, food, and indoor radon exposures, to ensure the safety and well-being of the community and the environment.
Introduction
Radiation exposure to human beings is a complex interplay of natural and artificial sources. Natural radiation primarily originates from cosmic rays in Earth’s outer atmosphere and naturally occurring radioactive materials, including gamma radiation from 40K and the radionuclides of 238U and 232Th present in soil, rocks, and water [1–4]. Conversely, artificial sources, like nuclear testing, nuclear treatments, incidents, and the nuclear power cycle, introduce artificial radioisotopes into our surroundings. This interplay results in varying degrees of radiation exposure, shaped by geological and radiochemical factors [5–8].
Among these sources, natural background radiation is a significant contributor, primarily arising from radionuclides such as 226Ra, 232Th, and 40K found in sediment, soil, water, and rock. These elements collectively constitute a substantial portion of the typical annual radiation dose for humans. Soil plays a central role in dispersing radionuclides into the environment and biological systems, making it a key determinant of radioactive pollution [9]. It also serves as a foundational benchmark for radiation hazard assessments, nuclear safety, and scientific explorations.
The distribution of radionuclides in rocks is influenced by parent rock composition and physicochemical processes that concentrate them. Igneous rocks, especially those rich in heavy minerals, typically exhibit higher radiation levels, whereas sedimentary rocks generally have lower levels. Principal contributors to high natural background radiation include 238U and its decay products in soils and rocks, as well as 232Th found in monazite sands.
Radioactive particles from phosphate rocks can enter the environment through various routes, such as the use of phosphogypsum in construction and agriculture and agricultural field fertilization [10]. A comprehensive study, focusing on the determination of different radionuclide levels (226Ra, 232Th, and 40K) and their associated health risks for human populations, plays a vital role in radiation protection, geoscientific investigations, and the establishment of guidelines for mitigating the risks posed by these radionuclides. Of equal importance is the assessment of the concentration of artificial radioisotopes in general. Particularly, the levels of the radioisotope 137Cs, the presence of which has been confirmed by other publication in different places like soil, plants, and foods [11].
Kadiri et al. [12] conducted similar studies covering the entire territory of Kosovo, generating maps that depict the concentration of radioactive elements. While their research covered the entire country, our study specifically reveals that the Vitia region exhibits significantly higher results than previously reported. The aim of this study is to explore the presence and quantify the activity concentrations of natural radionuclides, namely 226Ra, 232Th, and 40K, alongside the artificial radionuclide 137Cs, within recently collected soil and natural rock samples from the residence of Stublla, municipality of Viti, Republic of Kosovo. To evaluate the radiological impact on the populace and also on the environment from these radionuclides, there have been calculations of some relevant parameters commonly used worldwide and valid under current international standards for assessing radiological hazards. These include radium equivalent activity (Raeq), gamma index (Iγ), external hazard index (Hex), absorbed dose rate in the air (ADR), annual gonadal dose rate (AGDE), annual effective dose rate (AEDE), and excess lifetime cancer risk (ELCR). These parameters collectively provide insight into the potential risks posed by the presence of these radionuclides.
Materials and Methods
1. The Geology of Soil and Rocks under the Study
The study area is in the eastern part of the Republic of Kosovo (Fig. 1), belongs to the topographic map on a scale of 1:25,000 and lies between the coordinates 42° 15´ 00´´ N to 42° 22´ 30´´ N and 21° 22´ 30´´ E to 21° 30´ 00´´ E.
Study area and location of sampling points.
According to the geological map on a scale of 1:25,000, it turns out that the geological construction of the study area consists of ophiolitic melange with olistolites, marls, limestones, and flysch (conglomerates, breccias, sandstones, and clays), then with diabase and trachytes (Fig. 2). The specimens chosen for the investigation were of granitic composition.
Geological map of surveyed area (reference geological map in scale 1:25,000).
2. Sample Collection and Preparation
The locations of soil samples were recorded using a portable Global Positioning System device during the sampling process, and their spatial distribution is illustrated in Fig. 1. This study was conducted in a hilly-mountainous area characterized by limited agricultural activities. Soil samples were collected from yards and non-agricultural fields, taken at a depth ranging from 0 cm to 5 cm. Meanwhile, rock samples were gathered from the surface walls of houses and enclosures surrounding the yards. The soil samples, collected from a depth of 0 cm to 5 cm using a shovel in 19 measurement points, mostly agricultural land (90%), were subjected to a meticulous preparation process. Unwanted materials such as stones, vegetation, and roots were carefully removed, and the cleaned samples, weighing 1–2 kg, were then placed in sealed, zip-locked bags. Similarly, seven natural rock samples were randomly collected from the surface of the studied area and were ground in a special mill to ensure uniform granulation into powder form and stored in zip-locked bags.
Both types of samples were subjected to a standardized procedure: they were dried in an oven at 60 °C for 48 hours, subsequently ground, passed through 2 mm sieves, weighed, and carefully transferred into uncontaminated standard plastic containers with a 550 mL size, known as Marinelli beakers, and sealed to prevent 222Rn leakage. To ensure the stability of radioactive equilibrium, the samples were stored for four weeks, allowing daughter products to reach equilibrium with their parent isotopes, namely, 226Ra and 222Rn.
3. The Composition of the Elements
The calculation of the concentration of radioactive elements based on measured activity is a widely employed method that provides crucial information about the geological composition of rocks and soil. The concentrations of 238U, 232Th, and 40K in parts per million (µg/g) were determined based on the measured activities in Bq/kg, utilizing the following experimental formula [13], as Equation (1):
where the C is the activity concentration in Bq/kg, Mw is the molecular weight (g/mol), N is Avogadro’s number, and t1/2 is the half-life in seconds. The conversion factors λ were calculated for each radionuclide.
4. Experimental Method for Radioactivity Measurement
Radioactivity analyses of soil and rock samples taken from the study area were performed using a high-purity germanium detector (Model: GEM55P4-95; ORTEC) with a resolution of 1.9 keV and a relative efficiency of 55% at 1,332.5 keV [12]. The detector’s sample chamber is shielded with a 10 cm lead block against background radiation from construction materials and cosmic rays. Each sample was placed in the detector and counted for 50,000 seconds, and the spectra obtained as a result of counting were evaluated with the data analysis program Gamma Vision (AMETEK ORTEC).
The activity concentration of each sample was determined based on the corresponding gamma lines. To determine the activity concentration of 226Ra in the samples, a photo peak of 295.22 keV gamma-ray energy of 214Pb with an abundance of 19.2%, a photo peak of 351.9 keV gamma-ray energy of 214Pb with an abundance of 37%, and a photo peak of 609.32 keV gamma-ray energy of 214Bi with an abundance of 46.09% were used. For the measurement of 232Th activity concentration, a photo peak of 583.19 keV gamma-ray energy of 208TI with an abundance of 86% and a photo peak of 911.2 keV gamma-ray energy of 228Ac with an abundance of 29% were used. The activity concentration of 40K was used from its gamma-ray energy of 1,460.2 keV with an abundance of 10.7%. To determine the activity concentration of 137Cs, a photo peak of 661.7 keV with an abundance of 84.62% was used. The radioactivity concentrations of the soil and rock samples after determining the peak areas at these energies were calculated using Equation (2):
where N is the net area at the energy of interest, ε is the efficiency at the gamma energy of interest, Iγ is the abundance of the gamma ray at the energy of interest, t is the count time (s) and m is the sample mass (kg).
Detector efficiency should be determined for each observed peak in the spectrum to determine the actual value of the gamma counts that the detector has detected [14]. For this, the 152Eu standard source (Model: 7152, Seri no: 1280-68-2; Eckert&Ziegler) was placed in front of the detector and counted for 10 minutes, and then the count rate values of the formed spectra were obtained. Detailed descriptions of the calibration and operation of the detector used to determine the activities of the 226Ra, 232Th, and 40K and 137Cs radioisotopes were presented in previously published study [12].
5. Radiological Risk Parameters
Within this study, various radiological parameters have been meticulously evaluated. Raeq represents the total radioactivity from uranium, thorium, and potassium in a material, expressed as if it were only from 226Ra. This value was calculated by Equation (3) [15]:
In the given equation, CRa, CTh, and CK represent the individual activity concentrations of 226Ra, 232Th, and 40K, respectively, measured in units of Bq/kg. The referenced value recommended for Raeq value is 370 Bq/kq [16].
The hazard index helps determine if materials are radiation-safe for use in construction and can be expressed as follows Equation (4) [16]:
where CRa, CTh, and CK are the activity concentrations of 226Ra, 232Th, and 40K, respectively. This Iγ is also used to correlate the annual dose rate due to the excess external gamma radiation caused by superficial materials. The value of Iγ must be ≤1.
Potential Hex is an approach to determine whether materials containing naturally occurring radionuclides give a risk of radiation exposure to people while maintaining safe levels of exposure. It is given by Equation (5) [15]:
where CRa, CTh, and CK are the average activity concentrations of 226Ra, 232Th, and 40K in Bq/kg respectively. The value of the indices (Hex) must be less than unity (<1) for the radiation hazard to be negligible [4].
Calculation of the gamma dose rate absorbed in air represents the amount of radiation energy absorbed per unit of time in air. It is usually measured at 1 m above ground level providing an indication of the external exposure risk to humans by multiplying the unit activities of 226Ra, 232Th, and 40K with certain coefficients, as explained in Equation (6) [7]:
In the provided equation, CK, CRa, and CTh denote the respective specific activity levels of 40K, 226Ra, and 232Th, expressed in units of Bq/kg. It is recommended that the population-weighted world average of the ADR should not exceed 55 nGy/hr [7].
The projected annual cumulative radiation dosage that an individual receives from environmental exposure is calculated for the varying radiation sensitivity of various organs. The amount of dose that a person will receive in a year from the radiations emitted from different radiation sources is calculated by using the Equation (7) [17]:
In the given equation, it corresponds to a conversion coefficient of 0.7 Sv/Gy, an occupancy factor of 0.2 (considering that individuals spend 20% of their time outdoors), and represents 8,760 hours per year. The limit value of AEDE given as 70 µSv/yr [17].
AGDE stands for the annual radiation dose human reproductive organs (gonads) absorb and it is used to calculate the possible genetic effects of radiation exposure. The AGDE for the resident using such material for building materials is evaluated by the following Equation (8) [18]:
where CRa, CTh, and CK are the radioactivity concentration of 226Ra, 232Th, and 40K in samples. To obtain an indication of how exposure can affect overall health, the equivalent dose can be multiplied by a factor related to the risk for a particular tissue or organ. This multiplication provides an effective dose absorbed by the body. Annual gonadal equivalent dose value threshold is 300 mSv/yr [4].
Lifetime cancer risk is defined as the probability a person will get cancer over the course of their lifetime because of ongoing radiation exposure. It is a gauge of the radiation exposure of long-term cancer risk and is calculated by the Equation (9) [19]:
In the given context, AEDE represents the annual effective dose equivalent, while cancer risk coefficient (CRC) denotes the risk factor measured in 1/Sv. For stochastic effects, the International Commission on Radiological Protection recommends taking the CRC value as 0.057/Sv to represent the risk of fatal cancer in the population [19]. YS is the average life expectancy (70 years). The ELCR’s world mean value is determined as 0.29×10–3 [17].
Results and Discussion
1. Radionuclides in Soil Samples
The discerned order of mean activity levels was found to be 40K>226Ra>232Th, as evidenced by the comprehensive data presented in Table 1. A notable finding from this study indicates that 58% of the soil samples examined exhibited detectable levels of the artificial radioisotope 137Cs. The quantified activity concentrations within this subset of samples ranged from not detected to 81.22 Bq/kg. Remarkably, the derived average activity value across these samples was determined to be 12.8±23.5 Bq/kg. This discovery underscores the prevalence of 137Cs in the soil matrix, warranting further investigation into potential sources and environmental implications. The presence of the radionuclide 137Cs has been confirmed by other studies conducted on soil [12] and honey [9] throughout the country.
Comprehensive Analysis of Activity Concentrations of Natural and Artificial Radionuclides in Soil Samples, Including Comparative Data from Existing Literature
The concentration level of 226Ra in the studied area soil ranged from 20.7 Bq/kg up to 295.2 Bq/kg, with an average value of 95.84 Bq/kg. According to previous research, the average concentration level of 226Ra in the soil of Kosovo is lower than the results reported in Annex B in the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2000 report. However, the findings from this study indicate that the 226Ra level is four times higher than the reported average for Kosovo. Consequently, it is 2.8 times higher than the average reported by UNSCEAR.
A similar situation arises with 232Th in relation to 226Ra. The concentration level of 232Th in the soil of the studied area ranged from 18 Bq/kg to 128 Bq/kg, with an average value of 75.5 Bq/kg. Recently published literature, as well as data from neighboring states, indicates lower values of 232Th recorded in the soil of respective countries. Moreover, reports from earlier studies in Kosovo state that the national average level was 21 Bq/m3 [12]. So, the concentration of 232Th discovered the average values from this study were 3.5 times higher than the values reported so far. Also, the average concentration of 232Th in the analyzed soil results to be 1.5 times higher than the world average values reported in UNSCEAR 2000.
From the obtained results, it is confirmed that the opposite situation exists regarding the level of concentration of the artificial radioisotope 137Cs. Table 1 shows that neighboring countries such as Croatia and Italy have reported higher values of 137Cs concentration in their territories than the research we have conducted [7, 12, 20–22]. Additionally, the results obtained from this study are almost identical to the findings from previous studies in Kosovo. This is justified by the fact that contamination with the artificial radioisotope 137Cs is almost homogeneous at the national level. The contamination with artificial radioisotope of Kosovo has mostly been done by the nuclear accident from Chernobyl nuclear power plant [23].
The average concentration level of 40K from the soil samples in the area when this study was done was almost the same as the results reported in Italy and Croatia (Table 1), which refer to surveys recently conducted. However, the ratio of the average values from this study compared to the nationally reported data is twice as high. A similar ratio is also observed in comparison to the results published by UNSCEAR [7].
Based on the findings obtained through gamma-ray spectroscopy analysis of soil samples and Equation (1), the following results were discerned: the mean concentration for 226Ra was determined to be 7.24 ppm, exhibiting a range from 1.7 ppm to 23.7 ppm; for 232Th, the mean concentration was calculated to be 18.11 ppm, with a range spanning from 4.45 ppm to 31.54 ppm; based on the assessed 40K concentration, the mean concentration of K resulted to be 2.79%, demonstrating a variability from 1.48% to 3.9%.
2. Radionuclides in Natural Rock Samples
The distinct radiation levels are intricately linked to the geological origin of soils, primarily determined by the types of rocks present. Elevated radiation levels tend to be correlated with igneous rocks, exemplified by granite, while sedimentary rocks generally exhibit lower radiation levels. Nevertheless, exceptions exist, notably in certain shales and phosphate rocks that demonstrate a relatively high content of radionuclides. Thus, variations in soil composition and geology contribute to the nuanced distribution of radiation levels across different rock types.
So far, there has been no local publication regarding the levels of radionuclides in rocks. Therefore, the comparison will be made solely using data from other countries. In general, our research confirms that the activities of 226Ra and 40K are like the values reported in recent studies, as seen in Table 2 [13, 24–27]. Therefore, the asserted average activity of the rock samples included in this study is comparable to the published activity of marble by various authors.
Comprehensive Analysis of Activity Concentrations of Natural Radionuclides in Granite Rock Samples, Including Comparative Data from Existing Literature
This study has demonstrated higher values of 232Th concentration in the rock samples. The range of values has been from 122 Bq/kg to 161 Bq/kg, with an average value of 147.0 Bq/kg. This average value is higher than the majority of findings reported in previous studies. Although higher concentrations of 232Th in rock samples have been reported by Arafa [28] and Hanfi et al. [29] with 162 Bq/kg and 232.2 Bq/kg, respectively.
None of the rock samples evaluated in this study showed the presence of the artificial radioisotope 137Cs. However, soil samples taken from the same area as the rocks exhibited the presence of 137Cs. This can be elucidated by the fact that most rock samples were composed of one or two compact parts, thereby preventing the infiltration of 137Cs particles into the inner structure of the rock. The 137Cs radioisotopes were consistently detected in soil samples, a phenomenon attributed to the deposition of particles that have traveled through the atmosphere from nuclear weapon tests and nuclear accidents.
Gamma-ray spectroscopy analysis of rock samples revealed the elemental composition as follows: the mean concentration of 226Ra was 16.2 ppm, with a range from 8.2 ppm to 50.61 ppm; 232Th exhibited a mean concentration of 36.2 ppm, ranging from 30.1 ppm to 39.72 ppm; and 40K demonstrated a mean concentration of 3.73%, fluctuating between 3.42% and 3.88%. The high values of radium in the rocks of this area have led to high levels of radon concentration being recorded in indoor environments, for the construction of which these rocks have been used as building materials [30]. Until radon concentration levels in other parts of Kosovo are confirmed to comply with both local and international standards, specific reference is made to indoor environments such as residential homes [31] and open coal mine [32] and underground mines [33, 34].
3. The Radiological Hazard Parameters of Soil and Rock Samples
Radiological hazard and health risk parameters have been evaluated based on the concentration of radioactive elements in both soil and rock samples. The outcomes of these assessments are detailed in Table 3. These parameters have been assessed based on standards that regulate the use or nonuse of these materials for construction purposes. Given that, in the study region, it is a common practice to use the soil and rocks of this area for residential construction.
The Calculated Values of Radiological Hazard Parameters for Soil and Rock Samples
The average of Raeq was calculated because it is important to be used as an index number to encapsulate the specific activities of 226Ra, 232Th, and 40K into a single quantity. This index takes into consideration the radiation hazards associated with these elements. Considering the established limit for Raeq at 370 Bq/kg [7], only three of 20 soil samples indicated values above this threshold, with an average value of 260 Bq/kg. In contrast, eight of 10 rock samples indicate values above this threshold, with an average of Raeq 484 Bq/kg. A similar situation to Raeq was observed for Hex. The average results of the soil meet the standards, below 1 [35], whereas the results of rock samples exceed the standard by 30%. This parameter serves as a criterion for the indoor radiation dose rate resulting from external exposure to gamma radiation emitted by natural radionuclides present in building materials within dwellings [15]. The averages of Iγ calculated for the soil and rock samples were 1.89 and 3.45, respectively, higher than the recommendation of European Commission [35].
The average of ADR in the air, at 1 m above the ground, generated from gamma radiation of 226Ra, 232Th, 40K, and 137Cs in the soil and rock samples resulted to be 122 nGy/hr and 224 nGy/hr, respectively, thus resulting 2 and 3.7 times higher for soil and rock samples than the standard of 60 nGy/hr recommended by UNSCEAR [7]. A similar situation to ADR is observed for AEDE when the average of soil and rock samples is 2 and 3.9 times higher than the standard of 70 µSv/yr [7]. All results from both soil and rock samples for AGDE parameter resulted to be higher than the standard value of 289 μSv/yr [16]. The average values of ELCR, for soil and rock samples resulted to be 0.53×10−3 and 0.96×10−3, respectively. Both are higher than the average world value of ELCR, 0.29×10−3 [7].
Conclusion
The natural stones and soil in the studied area should not be used as construction material, as they do not meet the necessary conditions from a radiological perspective. For existing houses constructed with these materials, an additional evaluation based on internal dose measurements should be conducted, as these assessments were beyond the scope of this research. The consistency in results across all collected samples implies that the radiation levels in the research region are uniform.
It suggests that the region with similar geological characteristics is probably larger than our measurement points indicate, and further exploration is needed to precisely delineate its boundaries in future research. Furthermore, conducting detailed geological analyses is essential to assess the feasibility of initiating any mining activities in that area.
Upon reviewing the literature, it has been established that the radioactivity concentration in the soil of Kosovo does not present a radiological hazard to the population. However, this study highlights that none of the radiological hazard parameters in rock samples meet international standards for building material, a situation mirrored in soil samples. These findings are attributed to the relatively elevated average concentrations of 226Ra, 232Th, and 40K. In soil samples, these concentrations were calculated to be 16.2 ppm, 36.2 ppm, and 3.73%, respectively, while in rock samples, they were determined to be 7.2 ppm, 18.11 ppm, and 3.88%, respectively. Consequently, this research emphasizes the importance of utilizing high-resolution measurements in the preparation of a radiological map.
Notes
Funding
The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.
Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Ethical Statement
This article does not contain any studies with human participants or animals performed by any of the authors.
Data Availability
Data sharing is not applicable to this article as no new data were created or analyzed in this study.
Author Contribution
Conceptualization: Makolli S, Hodolli G. Methodology: Dizman S, Hodolli G. Formal analysis: Kadiri S. Supervision: Hodolli G. Project administration: Hodolli G. Visualization: Cadraku H. Validation: Makolli S, Dizman S, Cadraku H. Writing - original draft: Dizman S, Hodolli G. Writing - review & editing: Makolli S, Kadiri S. Approval of final manuscript: all authors.