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ORIGINAL ARTICLE
Year : 2017  |  Volume : 40  |  Issue : 1  |  Page : 27-33  

Assessment of natural radioactivity and associated radiation indices in soil samples from the high background radiation area, Kanyakumari district, Tamil Nadu, India


1 Department of Physics and Research Centre, Women's Christian College, Nagercoil, Tamil Nadu, India
2 Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India
3 Radiological Safety Section, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India
4 Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India

Date of Submission15-May-2016
Date of Decision11-Dec-2016
Date of Acceptance20-Jan-2017
Date of Web Publication24-Apr-2017

Correspondence Address:
A K Ajithra
Department of Physics and Research Centre, Women's Christian College, Nagercoil, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_31_16

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  Abstract 

Assessment of natural radioactivity is very important from different points of view, especially for assessment of radiation exposure to human. In the present study, natural radionuclide concentrations of 238U, 232Th, and 40K were measured by gamma spectrometry using HPGe detector in soil samples collected from Southwest coast of Kanyakumari district, Tamil Nadu. The radiological index parameters due to natural radionuclides such as radium equivalent activity, absorbed dose rate, annual effective dose rate, external hazard index, internal hazard index, and gamma index were calculated for the soil samples. All the calculated radiological index values are higher than world average values and the recommended safety limits. Multivariate statistical techniques such as Pearson correlation, principal component analysis, and cluster analysis were applied to know the relation between radionuclides and radiological parameters and to study the spatial distribution of radionuclides.

Keywords: Gamma ray spectrometry, multivariate statistical techniques, natural radioactivity, soil


How to cite this article:
Ajithra A K, Venkatraman B, Jose M T, Chandrasekar S, Shanthi G. Assessment of natural radioactivity and associated radiation indices in soil samples from the high background radiation area, Kanyakumari district, Tamil Nadu, India. Radiat Prot Environ 2017;40:27-33

How to cite this URL:
Ajithra A K, Venkatraman B, Jose M T, Chandrasekar S, Shanthi G. Assessment of natural radioactivity and associated radiation indices in soil samples from the high background radiation area, Kanyakumari district, Tamil Nadu, India. Radiat Prot Environ [serial online] 2017 [cited 2018 Aug 18];40:27-33. Available from: http://www.rpe.org.in/text.asp?2017/40/1/27/205052


  Introduction Top


Natural occurring radionuclides, also called terrestrial or primordial radionuclides, are present in varying amounts in the earth's crust (soil sand rocks). Terrestrial radionuclides includes the decay radionuclides in the series of uranium (238 U) and thorium (232 Th) and a nonseries decay natural radionuclides such as 40 K,87 Rb,138 La,147 Sm, and 176 Lu. The main contribution to external exposure in outdoor is from gamma radiation emitted by these terrestrial radionuclides mainly 238 U and 232 Th series and 40 K.[1] Artificial radionuclides can also be present from the testing of nuclear weapons and the accidents at nuclear reactors, and discharges of radioactive waste from the nuclear installations.

There are few regions in the world known to be high background radiation areas due to local geology and geo chemical effects that cause enhanced levels of terrestrial radiation.[2] In the high background areas of the countries such as Austria, Brazil, China, France, India, and Iran, the radiation levels were found to be high varying over an order of magnitude depending on the site-specific terrestrial radioactivity.[3] In India, there are quite a few monazite sand bearing placer deposits causing high background radiation along its long coastal line. Ullal in Karnataka, Kalpakkam in Tamil Nadu, coastal parts of Tamil Nadu and Kerala state and Southwestern coast of India are known to be high back ground radiation areas.[4] Coastal region of Kanyakumari district, Tamil Nadu was one of the areas in the Southwest coast where high radiation level has been reported. Beach sand in these areas contains heavy minerals such as ilmenite, rutile, zircon, monazite, and sillimanite.232 Th and 238 U are reported from these regions, caused mainly due to the monazite bearing black sands. Natural radiation levels in this region are higher than normal which are believed to be emitted from the rich deposits of the monazite bearing beach sands. The mineral monazite contains radio elements, which is the main cause for natural radiation in the Southwest coastal belt. Natural radionuclides in soil are responsible for the background radiation exposure to the population.

Natural radio nuclides in soil generate a significant component of the background radiation exposure to the population and gamma-radiation intensity in a region depends on soil and geomorphology. Therefore, measurements of natural radioactivity in soil are of an interest for many researchers worldwide, which has led to worldwide national surveys in the past two decades. This study has been undertaken to measure the activity concentrations of 40 K,232 Th, and 238 U in high background radiation area soil samples collected from Thoothoor to Chettikulam of Southwest Coast Tamil Nadu, India and to assess the radiological conditions with statistical approach.


  Materials and Methods Top


Description of study area

The area chosen for this study was Southwest coastal stretch of Tamil Nadu Kanyakumari district. The sampling locations are located between 77°8' and 77°38'of East of longitude and 8°16' and 8°19' North of latitude. The district has 62 km of coast on the Western side (Arabian Coast) and 6 km of coast on the Eastern side (Gulf of Mannar/Bay of Bengal Coast). The investigated locations are Thoothoor, Eramanthurai, Colachel, Mandaikadu, Manavalakurichi, Pillaithoppu, Azhickal, Muttom, Kootapuli, and Chettikulam beach areas of Southwest coast of Tamil Nadu, India.

Sample collection

The soil samples collected at different locations of Southwest coast of Tamil Nadu are shown in [Figure 1]. The locations selected for sampling were uncultivated areas and each soil sample collection an area of 1 m × 1 m was marked and carefully cleared of debris to a few centimeters depth. Sampling site was chosen away from field boundary, a road, or other obstruction. The collected samples were then placed in labeled polythene bags and transferred to the laboratory for preparation and analysis. These samples were mixed together thoroughly, to obtain a representative sample of that area. Approximately 2 kg of wet weight per sample obtained at a depth of 5 cm from the top surface layer. After removing stones and vegetable matter, each soil sample was packed into a water-tight bag to prevent cross contamination and shifted to the laboratory. Sample preparation was carried out by placing each soil sample in an oven, drying at a temperature of 105°C to achieve a constant weight and pulverizing the sample into a fine powder to pass through a standard 1 mm meshsize. The homogenized samples were placed in 250 ml Marinelli beakers, sealed with a polyvinyl chloride tape and stored for at least 1 month prior to measurement to attain radioactive equilibrium between 226 Ra and 228 Ac and their short lived progeny (>7 half-lives of 222 Rn and 220 Rn).
Figure 1: Location map

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Radiometric analysis

The soil samples collected from the locations indicated on the data acquisition were subjected to gamma spectroscopic analysis. Measurements of the radionuclide activities in the soil samples were under taken using a high resolution, low background, hyper pure P-type coaxial gamma ray detector (ORTEC HPGe) coupled to an ORTEC analyzer and an 8 K multi-channel analyzer. The system has relative efficiency of 50% and resolution of 1.85 at a 1332 keV gamma line of 60 Co. The spectrometer is calibrated using standard samples 137 Cs and 60 Co supplied by International Atomic Energy Agency, Vienna. The minimum detectable activity for 232 Th,238 U, and 40 K are 1.0, 3.5 and 12.25 Bq/kg at a back-ground shielding factor of 95%.

The analysis of the gamma spectra obtained is performed with dedicated software, and the choice of the reference peak is made in such a way that they are sufficiently discriminated. Of the peaks that could be identified through the software, reference is made to that at 1.764 MeV for 214 Bi in the 238 U decay chain, that at 2.614 MeV for 208 Tl in the 232 Th decay chain, and one at 1.460 MeV of 40 K. Each measurement is performed with a counting time of 10,000 s.


  Results and Discussions Top


Activity concentrations of 238 U,232 Th and 40 K in the soil

The activity concentrations of 238 U,232 Th and 40 K together with their average values for the soil samples are shown in [Table 1]. All values are given in Bq/kg of dry weight. The range of activities for 238 U,232 Th, and 40 K are 5.1–158.2, 27.3–794.3 and 44.1–251.4 Bq/kg -1, respectively. The data set includes high background and also a few normal background locations such as PHU, AKL, MTM, KPL and CKM.. [Figure 2] shows the variation of activity concentration at different sampling locations.
Table 1: Geographical information of sampling points, Kanyakumari, Tamil Nadu

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Figure 2: Locations versus activity concentrations and radium equivalent activity (Bq/kg)

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232 Th was higher than 238 U in all samples. This could be related to their difference in chemical speciation and solubility in a natural environment.232 Th is insoluble and also preferentially accumulated on the particular phases relative to 238 U.[5],[6] The high value for 232 Th activity concentration observed at Mandaikadu (MDU), Colachel (CLL) and Thoothoor (THR) in the study area could be explained due to the presence of black sands, which are enriched in the mineral monazite containing a significant amount of 232 Th. The enrichment occurs because the specific gravity of monazite allows its concentration along beaches where lighter materials are swept away [7] and anthropogenic inputs can release the additional amounts of natural radionuclides into the environment. From the results, it is clear that the activity of 238 U and 40 K are lower while that of 232 Th is higher when compared with worldwide average value (UNSCEAR, 2000) for this selected high background radiation area (HBRA) regions of this study (MDU, CLL & THR).

To determine the radiation hazard index due to the natural radioactivity associated with the soil, different radiological parameters are estimated and the obtained values are compared with internationally recommended safe limits.

Determination of radiological hazard indices

Radium equivalent activity

It is well known that natural radionuclides 226 Ra,232 Th, and 40 K are not uniformly distributed in soil. The nonuniform distribution of these naturally occurring radionuclides is due to several reasons such as land use patterns. For uniformity in exposure estimates, the concentrations of radionuclides have been defined in terms of radium equivalent activity (Raeq) having units Bq/kg.



Where AU, ATh and AK are the activity concentrations of 238 U,232 Th and 40 K, respectively (in Bq/kg). The assumption is that 370 Bq/kg of 226 Ra or 238 U, 256 Bq/kg of 232 Th and 480 Bq/kg of 40 K produce the same gamma-ray dose rate. As can be seen from [Table 1], the Raeq values for the soil samples varied from 51.35 (MTM) to 1296.36 (MDU) Bq/kg with the average of 471 Bq/kg. The mean value of the Raeq obtained for the soil sample is 471 Bq kg-1,(when all locations are considered), which is higher than the recommended safe value of 370 Bq kg-1 (OECD (1979). [Figure 2] shows the locations and Raeq.

Absorbed dose rate

The absorbed dose rate (DR) enables estimation of radiation exposure from natural gamma radiation on the terrestrial mainland. To assess the exposure to natural radiation and convert activity concentrations of natural radionuclides into dose rates, the DR in the air at 1 meter above ground surface was calculated. Therefore, DR was estimated using dose conversion factors 0.462 nGy/h for 238 U series, 0.604 nGy/h for 232 Th series and 0.042 nGy/h for 40 K (UNSCEAR 2000).



Where, AU, ATh and AK represent the activity concentrations of 238 U,232 Th and 40 K in Bq/kg, respectively in the samples. Using the above formula DR had been evaluated and tabulated in [Table 1]. The absorbed dose rate values ranged between 42.2 (MTM) and 1022.9 (MDU). The Global and Indian average dose rates are 84 nGy/h and 90 nGy/h respectively as reported by UNSCEAR (2000). The higher absorbed dose rates in few locations are due significant amount of 232 Th in the study area. [Figure 3] shows the variation of absorbed gamma dose rate in different locations.
Figure 3: Locations versus absorbed gamma dose rate (nGy/h)

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Annual effective dose rate

To estimate the annual effective dose (HR), one has to take into account the conversion factor from absorbed dose in air to effective dose and the outdoor occupancy factor. In the recent UNSCEAR (2000) reports, a value of 0.7 Sv/Gy was used for the conversion factor from absorbed dose in air to effective dose received by adults, and 0.2 for the outdoor occupancy factor, implying that 20% of time is spent outdoors on average around the world. The effective dose rate was calculated from the formula:



Where DR (nGy/h) is given by the Equation 2. The estimate result for HR is given in [Table 1]. The estimated HR values for all the studied samples ranged from 0.052 to 1.255 mSv/year with a mean of 0.459 mSv/year. In areas with the normal background radiation, the average annual external effective dose from terrestrial radionuclides is 0.07 mSv/year.[9] Therefore, the obtained mean value from this study area (0.459 mSv/year) is higher than the world average value due to high 232 Th concentration in soil, as it is a known HBRA. [Figure 4] shows the locations and HR of soil samples.
Figure 4: Locations versus internal hazard index and external hazard index

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External radiation hazard indices

The external hazard index (Hex) is an important parameter to evaluate the radiation dose expected to be delivered externally if these materials are used for construction of buildings. This index value must be less than unity for the radiation hazard to be negligible. This Hex was calculated from the formula:[10]



Where AU, ATh, and AK are the activity concentrations of 238 U,232 Th and 40 K, respectively. The calculated external hazard values are between 0.14 (MTM) and 3.53 (MDU). [Figure 5] shows the variation of Hex with sampling locations.
Figure 5: Clustering of radioactive variables

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Internal radiation hazard indices

The internal hazard index (Hin) is used to reduce the internal exposure to 222 Rn and its radioactive progeny. For safe use of soils for building constructions, it should be noted that Hin is ≤1. Hence, the Hin, was calculated using the following formula:[10]



The calculated radiation Hin of soil samples are given in [Table 2]. This Hin values vary from 0.152 (MTM) to 3.900 (MDU) with an average of 1.442 which is higher than the recommended value of unity. [Figure 5] shows the variation of Hin with sampling locations.
Table 2: Activity concentration and radiation indices in soils of Kanyakumari, Tamil Nadu

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Multivariate statistical analysis

Multivariate statistical methods are successfully used to interpret the relationships among variables in the environmental studies (Liu et al., 2003).[11] Multivariate analysis such as principle component analysis (PCA), and cluster analysis (CA) is used to explain the correlation amongst a large number of variables in terms of a small number of underlying factors without losing much information.[12],[13] This method can also help to simplify and organize large data sets to provide meaningful insight (Laaksoharju et al., 1999),[14] and can help to indicate natural associations between samples and/or variables.[15] The main Statistical Software Statistical Program for the Social Science (SPSS/PC, Chicago, Illinois, United States) was used for statistical analysis.

Basic statistics

The average activity concentration with minimum, maximum, and standard deviation, kurtosis, and skewness are presented in [Table 3]. In general, if the standard deviation is higher than the mean value then it indicates that low degree of uniformity and vice versa. In the present study, the standard deviation value of 232 Th higher than the mean value indicates that low degree of uniformity, whereas 238 U and 40 K standard deviations lower than the mean value indicates the that high degree of uniformity in the soils.
Table 3: Summary of basic statistics of natural radionuclides (Bq/kg) in soils

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According to Adam and Eltayeb,[16] skewness data of natural radionuclide describes the degree of asymmetry of a distribution around its mean. Positive skewness [Table 3] indicates a distribution with an asymmetric tail extending toward values that are more positive. Negative skewness indicates a distribution with an asymmetric tail extending toward values that are more negative. Kurtosis is a measure of the peakedness of the probability distribution of a real-valued random variable. It characterizes the relative peakedness or flatness of a distribution compared with the normal distribution. Positive kurtosis indicates a relatively peaked distribution. Negative kurtosis indicates a relatively flat distribution. From [Table 3], kurtosis of 40 K is positive indicate its peaked distribution whereas kurtosis of 238 U and 232 Th is negative shows that flat distribution in the soil samples.

Pearson correlation analysis

Pearson correlation analysis has been carried out to determine the mutual relationships and strength of association between radionuclides and radiological parameters. The results of the Pearson correlation co-efficient of variables are given in [Table 4]. A very strong positive correlation (R = 0.991) was found between the 238 U and 232 Th. This may indicate the strong relationship between 238 U decay series and 232 Th decay series in soils and occur together in nature.[16]40 K had moderate correlation with uranium/thorium indicates that origin of 40 K occur in different decay series in nature. The calculated radiological parameters Raeq, DR, Hin, Hex showed that very strong positive correlation with 238 U and 232 Th. This indicates that all the radiological parameters strongly associated with concentration of uranium and thorium in the soils and these radionuclides mainly contribute the emission of gamma radiation in the study area.
Table 4: Pearson correlations between radionuclides and associated radiological hazards

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Principal component analysis

PCA is the most common multivariate statistical method used in environmental studies. This is widely used to reduce data and to extract a smaller number of independent factors (principal components) for analyzing relationships among observed variables.[17] In the present study PCA was applied to identify variables by applying varimax rotation with Kaiser normalization. By extracting the Eigen values and Eigen vectors from the correlation matrix, the number of significant factors and the percent of variance explained by each of them were calculated. [Table 5] gives the results of PCA with varimax rotation. The results show that these two factors could explain over 99.88% of the total variance. Normally, an ordination result was good if the value was 75% or better.[18],[19]
Table 5: Rotated factor loadings of variables

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Component 1 is the most widespread within the determined natural radionuclides and radiological parameters describing 74.92% of the data variability. It is characterized by high positive loading of 238 U and 232 Th and strong positive loadings of all radiological parameters.

This Component 1 shows that the concentration uranium and thorium plays the main role in natural radioactivity. Component 2 loaded due to high positive loading of 40 K with accounts for 24.96% of total variance. This shows that concentration of 40 K not contribute in the total natural radioactivity.

Cluster analysis

CA is one of the multivariate techniques used to identify and classify groups with similar characters in a new group of observations. CA is often coupled with PCA to check results and to group individual parameters and variables. The purpose of CA is to discover a system of organizing observations where a number of groups/variables share observed properties. A dendrogram is the most commonly used method of summarizing hierarchical clustering. CA carried out through axes was to identify similar characteristics among natural radioisotopes and radiological parameters in the soils.[19] In CA, the average linkage method along with correlation coefficient distance was applied and the derived dendrogram is shown in [Figure 5].

In this dendrogram, all radiological parameters and three radionuclides were grouped into three statistically significant clusters. Cluster-I consists of Hex, Hin, HR,40 K and 238 U. Cluster-II consists of 232 Th and main radiological parameters such as DR and Raeq. This is due to the high activity concentration of thorium in the soil samples. From this CA, Hex and Hin in the study area are due to the concentration of 238 U and dose absorbed by the human beings are due to high concentration of 232 Th. This CA results are in good agreement with PCA and correlation analysis.


  Conclusions Top


The measured activity concentrations of 238 U,232 Th, and 40 K in soil samples from the high HBRA region for a few locations are higher than the world average value reported by UNSCEAR (2000). The estimated gamma DR is higher than the recommended safe limit. Hence, it is observed that the soil samples may not be suitable for building materials. The radiological data were processed using multivariate statistical methods, Pearson correlation coefficient, cluster, and factor analyses determined the similarities and correlation between the various samples in more systematic way.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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Papaefthymiou H, Athanasopoulos D, Papatheodorou G, Iatrou M, Geraga M, Christodoulou D, et al. Uranium and other natural radionuclides in the sediments of a Mediterranean fjord-like embayment, Amvrakikos Gulf (Ionian Sea), Greece. J Environ Radioact 2013;122:43-54.  Back to cited text no. 5
    
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Meglen RR. Examining large databases: A chemometric approach using principal component analysis. Mar Chem 1992;39:217-37.  Back to cited text no. 13
    
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Chandrasekaran A, Ravisankar R, Rajalakshmi A, Eswaran P, Vijayagopal P, Venkatraman B. Assessment of natural radioactivity and function of minerals in soils of Yelagiri hills, Tamilnadu, India by Gamma Ray spectroscopic and Fourier Transform Infrared (FTIR) techniques with statistical approach. Spectrochim Acta A Mol Biomol Spectrosc 2015;136:1734-44.  Back to cited text no. 17
    
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Ravisankar R, Sivakumar S, Chandrasekaran A, Prince Prakash Jebakumar J, Vijayalakshmi I, Vijayagopal P, et al. Spatial distribution of gamma radioactivity levels and radiological hazard indices in the East coastal soils of Tamil Nadu, India with statistical approach. Radiat Phys Chem 2014b; 103:89-98.  Back to cited text no. 19
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]


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