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ORIGINAL ARTICLE
Year : 2018  |  Volume : 41  |  Issue : 3  |  Page : 152-159  

Enrichment pattern and depth profile of natural radionuclides in monazite areas of coastal Karnataka


1 Department of Studies and Research in Physics, Payyanur College, Kannur University, Kannur, Kerala, India
2 Department of Physics, Mangalore University, Mangaluru, Karnataka, India

Date of Submission15-Mar-2018
Date of Decision14-Apr-2018
Date of Acceptance03-Sep-2018
Date of Web Publication19-Nov-2018

Correspondence Address:
Dr. V Prakash
Department of Studies and Research in Physics, Payyanur College, Kannur University, Edat, Kannur - 670 327, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_11_14

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  Abstract 


The activities of radionuclides 232Th, 226Ra, and 40K have been measured in sand samples of Ullal beach area, where presence of low-level monazite has been reported. The sand samples collected from the region, at different distances from sea waterline and at different depths, were analyzed for radionuclide activity by gamma spectrometry to study the distribution, enrichment pattern, and vertical profile of the radionuclides in the region. The study on the enrichment of radionuclides in different size fractions shows highest activity confined in <125 μm and lowest activity confined in 1000–500 μm particle size fractions. The minimum 232Th activity was 1.1 Bq/kg, found in Ombattu Kere beach at a depth of 10–20 cm, at waterline in 1000–500 μm particle size fraction and maximum activity of 6690.7 Bq/kg was found in Ombattu Kere beach at a depth of 10–20 cm, at 50 m away from waterline in grains of size 250–125 μm. The lowest 226Ra activity was 25.1 Bq/kg, found in Ombattu Kere beach at a depth of 10–20 cm, at waterline in grains of size 1000–500 μm and highest 226Ra activity was 1286.0 Bq/kg, found in Ombattu Kere beach in grains of size <125 μm for sample collected at a distance of 50 m away from waterline and at a depth of 10–20 cm. The minimum 40K activity was 130.5 Bq/kg, found in Ombattu Kere beach at a depth of 20–30 cm, at 50 m away from waterline in grains of size 500–250 μm and maximum activity of 40K was 5686.2 Bq/kg, found in Ombattu Kere beach for sample collected at waterline at a depth of 20–30 cm in <125 μm particle size fraction. The dose rate measured using plastic scintillometer at 1 m above the ground level at Ullal is having the range 39.4–459.9 nGy/h with a mean value of 193.2 nGy/h. The results of these investigations are presented and discussed in this article.

Keywords: 232Th, enrichment, gamma spectrometry, natural radionuclides, vertical profile


How to cite this article:
Prakash V, Mahamood K N, Narayana Y. Enrichment pattern and depth profile of natural radionuclides in monazite areas of coastal Karnataka. Radiat Prot Environ 2018;41:152-9

How to cite this URL:
Prakash V, Mahamood K N, Narayana Y. Enrichment pattern and depth profile of natural radionuclides in monazite areas of coastal Karnataka. Radiat Prot Environ [serial online] 2018 [cited 2018 Dec 10];41:152-9. Available from: http://www.rpe.org.in/text.asp?2018/41/3/152/245792




  Introduction Top


Radiation is omnipresent in the environment of earth's surface. The greatest contribution to humankind's exposure comes from natural background radiation. However, much higher levels of exposure are usual for inhabitants of natural high background radiation areas. The major sources responsible for exposure are naturally occurring radionuclides in the earth's crust such as 232Th, 226Ra, and 40K, which occur associated in minerals such as monazite and zircon. There are few regions in the world known as high-background-radiation areas due to local geology and geochemical effects that cause enhanced levels of terrestrial radiation.[1] High-background-radiation areas are located on the geochemical provinces where endogenous or sedimentary processes cause significant enrichment of radioactive minerals.[2] Major anomalies in the concentration of radioactive minerals in soil and sand are found in Brazil and India. One of the prime sources for high radiation background is radioactive monazite, and the presence of these mineral deposits in certain beaches of these two countries has been reported by several investigators.[3],[4],[5]

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],[6],[7],[8],[9],[10] In India, there are quite a few monazite sand-bearing placer deposits causing high background radiation along its long coastline such as Chavara[5] in Kerala, Kalpakkam[11] in Tamil Nadu, and eastern coast of Chatrapur[12] in the state of Odisha.

One of the areas along Southwest coast India where high radiation level has been reported was in Ullal of coastal Karnataka.[13] Though the distribution of radionuclides in different environmental matrices of the region has been studied extensively by different investigators, studies on enrichment of radionuclides in different depth intervals and in different size fractions of sand samples of the region are sparse. In view of this, detailed studies on radionuclide concentration in different environmental matrices of monazite areas were undertaken to study the distribution, enrichment pattern, and vertical profile of the radionuclides in the region. The gamma-absorbed dose prevailing in the region is measured using plastic scintillometer. As part of the study, sand samples were collected from waterline, 50 m away from waterline, and 100 m away from waterline in each sampling location. The activity of radionuclides was determined in different size fractions of the sample to study the enrichment pattern. The samples collected from three different depths, namely 0–10, 10–20, and 20–30 cm were analyzed for radionuclide activity in different depth intervals and different size fractions. The results of these investigations are presented and discussed in this article.


  Materials and Methods Top


Sampling and sample preparation

Standard procedures were followed in the collection of samples.[14] Samples were collected from waterline, 50 m away, and 100 m away from waterline in each sampling location, namely Ombattu Kere beach, Summer Sand beach, and Bhagavathi Temple region of Ullal beach area. The sampling locations are depicted in [Figure 1].
Figure 1: Map showing sampling locations

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The samples were also collected from three different depths, namely 0–10, 10–20, and 20–30 cm from each sampling location. After collection, the samples were brought to the laboratory and dried in an oven at 110°C till a constant dry weight is obtained. The dried samples were passed through different sieves to obtain four particle size fractions, namely 1000–500, 500–250, 250–125, and <125 μm. The samples were stored in airtight plastic containers of 300-ml volume for 30 days to ensure secular equilibrium between 226Ra and its short-lived progeny. The containers were sealed carefully to avoid the escape of gaseous 222Rn and 220Rn.[15] These samples are subjected to gamma spectrometric analysis.

Experimental setup

The concentration of radionuclides in the samples was determined employing high-efficiency 5” × 5” NaI(Tl) gamma ray spectrometer. The gamma ray spectrometer consists of NaI(Tl) detector coupled to a 4K MCA. The spectrometer was calibrated using different standards. The standards for uranium and thorium were prepared using known weight of analyzed ore samples that are in radioactive equilibrium and mixing them with silica powder simulating the sample matrix. 40K standard was prepared by taking oven-dried analar potassium chloride.[15] The efficiency of the detector was determined experimentally using the above standard radioactive sources. The spectrum obtained from NaI (Tl) is more complex to analyze. The Compton continuum of higher energy peak contributes to the count rate observed in the lower energy peak. Therefore, Compton contribution from the high energy peak to the lower energy peak should be evaluated to determine the activity. In the present work, simultaneous equation method[16] was employed for the analysis of the spectrum and to determine the activity concentration of various radionuclides. The activity of 40K was evaluated from the 1461 keV photopeak, the activity 226Ra from 1764 keV gamma line of 214Bi, and that of 232Th from the 2614 keV gamma line of 208Tl. The activity was counted for sufficiently long time (36,000 s) to reduce the counting error. The activity reported corresponds to the dry weight of the sample. The minimum detectable activity at 95% confidence level for 36,000 s counting time and 500 g sample weight was found to be 2.9 Bq/kg for 40K, 1.0 Bq/kg for 226Ra, and 0.6 Bq/kg for 232Th.


  Results Top


Variation of 232Th activity with depth and grain size

The activities of 232Th in different grain sizes and for different depths at water line, 50 m away from waterline, and 100 m away from waterline for all the three sampling locations are summarized in [Table 1]. The variation of activity with depth and grain size is shown in the form of histograms [Figure 2], [Figure 3], [Figure 4].
Table 1: Variation of 232Th activity with depth and grain size

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Figure 2: Variation of 232Th activity with depth at Ombattu Kere beach

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Figure 3: Variation of 232Th activity with depth at Summer Sand beach

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Figure 4: Variation of 232Th activity with depth at Bhagavathi Temple

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At Ombattu Kere beach, the lowest activity of 1.1 Bq/kg was observed at waterline in 1000–500 μm particle size fraction and at a depth of 10–20 cm and the highest activity of 6690.7 Bq/kg was found at a depth of 10–20 cm for sample taken 50 m away from waterline in 250–125 μm fraction. At Summer Sand beach, minimum activity of 2.1 Bq/kg was found at waterline in 1000–500 μm particle size fraction and at a depth of 10–20 cm and the maximum activity of 3834.5 Bq/kg was observed at a depth of 20–30 cm for sample collected 100 m away from waterline in grains of size <125 μm. At Bhagavathi temple, minimum activity of 1.8 Bq/kg was found at a depth of 10–20 cm, at waterline in 1000–500 μm particle size fraction and the maximum activity of 3318.2 Bq/kg was observed in grains of size <125 μm at a depth of 0–10 cm for sample collected at waterline.

Variation of 226Ra activity with depth and grain size

The activities of 226Ra in different grain sizes and for different depths at waterline, 50 m away from waterline, and 100 m away from waterline for all the three sampling locations are summarized in [Table 2]. The variation of 226Ra activity with depth and grain size is shown in the form of histograms [Figure 5], [Figure 6], [Figure 7].
Table 2: Variation of 226Ra activity with depth and grain size

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Figure 5: Variation of 226Ra activity with depth at Ombattu Kere beach

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Figure 6: Variation of 226Ra activity with depth at Summer Sand beach

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Figure 7: Variation of 226Ra activity with depth at Bhagavathi Temple

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At Ombattu Kere beach, the lowest activity of 25.1 Bq/kg was observed for sample collected at waterline at a depth of 10–20 cm in grains of size of 1000–500 μm and the highest activity observed was 1286.0 Bq/kg in grains of size <125 μm for sample collected at 50 m away from waterline at a depth of 10–20 cm. At Summer Sand beach, the lowest activity of 25.5 Bq/kg was observed for sample collected at waterline and at a depth of 0–10 cm in grains of size 1000–500 μm and the highest activity observed was 971.8 Bq/kg in grains of size <125 μm for sample collected at a distance of 100 m away from waterline and at a depth of 20–30 cm. At Bhagavathi Temple, the lowest activity of 28.2 Bq/kg was observed at waterline at a depth interval 0–10 cm in grains of size 1000–500 μm and highest activity observed was 1270.4 Bq/kg in grains of size <125 μm for sample collected at waterline at a depth of 0–10 cm.

Variation of 40K activity with depth and grain size

The activities of 40K in different grain sizes and for different depths at waterline, 50 m away from waterline, and 100 m away from waterline for all the three sampling locations are summarized in [Table 3]. The variation of 40K activity with depth and grain size is shown in the form of histograms [Figure 8], [Figure 9], [Figure 10].
Table 3: Variation of 40K activity with depth and grain size

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Figure 8: Variation of 40K activity with depth at Ombattu Kere beach

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Figure 9: Variation of 40K activity with depth at Summer Sand beach

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Figure 10: Variation of 40K activity with depth at Bhagavathi Temple

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At Ombattu Kere beach, the lowest activity of 130.5 Bq/kg was observed for sample collected at 50 m away from waterline at a depth interval 20–30 cm in grains of size 500–250 μm and the highest activity observed was 5686.3 Bq/kg in grains of size <125 μm for sample collected at waterline at a depth of 20–30 cm. At Summer Sand beach, the lowest activity of 158.4 Bq/kg was observed for sample collected at 100 m away from waterline and at a depth interval 0–10 cm in grains of size 500–250 μm and the highest activity observed was 3716.7 Bq/kg in grains of size <125 μm for sample collected from waterline and at a depth of 10–20 cm. At Bhagavathi Temple, the lowest activity of 199.1 Bq/kg was observed for sample collected at 50 m away from waterline at a depth interval 20–30 cm in grains of size 500–250 μm and the highest activity observed was 3465.6 Bq/kg in grains of size <125 μm for sample collected at 50 m away from waterline at a depth of 10–20 cm.

The dose rate at Ombattu Kere beach varies from 52.6 to 219.0 nGy/h with a mean value of 163.5 nGy/h, the dose rate at Summer Sand beach varies from 39.4 to 459.9 nGy/h with a mean value of 261.3 nGy/h, and at Bhagavathi Temple, it varies from 78.8 to 219.0 nGy/h with a mean value of 154.7 nGy/h.


  Discussion Top


It may be noted from [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10] that the lowest activity for 232Th and 226Ra was observed in 1000–500 μm particle size fraction and the lowest 40K was observed in 500–250 μm. The activities found to increase in the subsequent 500–250 μm, 250–125 μm, and <125 μm fractions. From the figures, it is clear that the increase of activities of radionuclides in 1000–500 μm and 500–250 μm fractions is rather narrow and the highest activity was found to be confined in <125 μm particle size fractions. This indicates the selective enrichment of radioactive mineral in this fraction. Ombattu Kere beach sand samples have shown high 232Th concentration. This may probably be related to the predominance of fine sand fraction in the beach that is enriched with radioactive minerals. This may also be due to the net movement of heavy minerals toward the near shore zone of Ombattu Kere beach.[17]

It is clear from [Figure 2] that the activity of 232Th in Ombattu Kere beach is high at 50 m away from waterline and the activity is very less at waterline. Depth profile study shows no systematic variation of 232Th activity with depth at Ombattu Kere beach and except at waterline the enrichment was more for the samples collected at a depth of 10–20 cm. From [Figure 3], it is clear that the 232Th activity in Summer Sand beach is high at 100 m away from waterline. The depth profile study shows that at waterline and 100 m away from waterline, the activity increases with depth and at 50 m away from waterline, there is no systematic variation with almost uniform activity for all depths. It is clear from [Figure 4] that 232Th activity in Bhagavathi Temple is high at waterline and the depth profile shows that at waterline, the activity decreases with depth and at 100 m away from waterline, the activity increases with depth. There is no systematic variation at 50 m away from waterline with almost uniform activity for all depths.

It is evident from [Figure 5] that 226Ra activity in Ombattu Kere beach increases with depth at waterline and decreases with depth at 100 m away from waterline. There is no definite variation at 50 m away from waterline. From [Figure 6], it is clear that 226Ra activity in Summer Sands increases with depth at waterline and 100 m away from waterline. Depth profile study at 50 m away from waterline shows no systematic variation with almost uniform activity for all depths. From [Figure 7], it is evident that 226Ra activity in Bhagavathi Temple decreases with depth at waterline and 100 m away from waterline. Depth profile study at 50 m away from waterline shows no systematic variation with almost uniform activity for all depths.

It is clear from [Figure 8] that the 40K in Ombattu Kere beach increases with depth at waterline and decreases with depth at 100 m away from waterline. There is no definite variation at 50 m away from waterline. [Figure 9] shows that in Summer Sand beach, 40K activity decreases with depth at 100 m away from waterline and no systematic variation is observed for 40K activity at waterline and 50 m away from waterline with almost uniform activity for all depths. [Figure 10] shows that at Bhagavathi Temple, there is no systematic variation of 40K activity with almost uniform activity for all depths.

The activity of 232Th was high in Ombattu Kere beach samples compared to Summer Sand beach and Bhagavathi Temple. This is due to the presence of black sand deposit along the beach. Significantly large deposition of black sand observed in the beach area indicates the presence of significant amount of enriched radioactive minerals, i.e., monazite and zircon. The less 40K activity at Ombattu Kere beach compared to Summer Sand and Bhagavathi Tempe region may be due to the measurement error while analyzing monazite- and zircon-rich samples. This is because the activity concentrations of 40K were measured directly by its own gamma rays (1460.8 keV). However 228Ac, a daughter nuclide of 232Th, produces 1459.2 KeV gamma rays, which interferes with 40K. Therefore, when the 232Th content was very high, it becomes difficult to determine the 40K content accurately.[12] Studies indicate that the monazite deposit in Ullal beaches is less extensive than those in Kerala.[5]

A comparison study of activity obtained for Ullal region with other beaches of India and world reveals that the average activities of 232Th, 226Ra, and 40K are high compared to other beaches [Table 4]. The 232Th activity reported for two island beaches of Brazil namely Preta beach and Dois Rios beach is 239 Bq/kg and 48 Bq/kg and that of 238U is 121 Bq/kg and 39 Bq/kg, respectively.[8] Mohanty et al.[12] have reported the mean values of 232Th, 238U, and 40K concentration as 2500, 230, and 120 Bq/kg, respectively, for Chatrapur beach sand, Odisha, India.
Table 4: Comparison of activities with other environs

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There exists a good correlation between 232Th and 226Ra activities (r = 0.9) at Ombattu Kere and 226Ra and 40K activities (r = 0.8) at Bhagavathi Temple region. The correlation between 232Th and 226Ra and 226Ra and 40K was significant for all locations. Similar findings were also reported in beach sand samples of Kalpakkam.[11] A weak correlation was observed between activities of 232Th and 40K in Ombattu Kere and Summer Sand beaches. It indicates that 40K activity cannot not be related to the presence of 232Th, bearing mineral sands.

The dose rate measured at Ullal varies in the range of 39.4–459.9 nGy/h with a mean value of 193.2 nGy/h. The high dose rate observed at Ullal beach area is due to the presence of radioactive mineral, namely monazite prevailing in the region. The sand in this area was found to be of shining black color, a characteristic nature of monazite.[26] A more extensive survey of gamma dose rate carried out in Ullal beach area at different distances from the waterline showed high value of dose rate at 100 m away from waterline and low value of dose rate at waterline. There was an increasing trend of dose rate starting from waterline along the middle (50 m away from the waterline) up to 100 m away from the waterline. This indicates that there is an attenuation of gamma rays near the waterline and the continuous wave action results in the deposition of possible radioactive mineral around 50–100 m distance away from the waterline.


  Conclusions Top


The lowest 232Th activity concentration is observed in the samples collected at sea waterline. The present activities of natural radionuclides 232Th, 226Ra, and 40K at Ullal beach area are high compared to that of other normal background areas. The variation of the concentration of 232Th, 226Ra, and 40K with depth depends on the beach morphology. The lowest activity was observed in 1000–500 μm particle size fraction in sand. The activities were found to increase in the subsequent fractions. The highest activity was found to confined in 250–125 μm and <125 μm particle size fractions, indicating the selective enrichment of radioactive minerals in these fractions. Systematic variation of radionuclides activity with depth is observed at waterline and 100 m away from waterline. There was no regular variation of activity at 50 m away from waterline with almost uniform activity for all depths. In beach sands, which are rich in monazite and zircons, the 40K level is found to be low. A good correlation exists between the activities of 232Th and 226Ra.

Acknowledgment

The authors are grateful to Prof. K.M. Kaveriappa, Vice-Chancellor, Mangalore University. and Prof. K. Siddappa, former Vice-Chancellor, Bangalore University. for their encouragement. The authors are also grateful to Prof. K.M. Balakrishna, Chairman, Department of Physics, Mangalore University, for his support. The help received from Dr. P.K Shetty, Mr. K.M Rajashekara, and Mr. Praveen Joseph is thankfully acknowledged.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
 
 
    Tables

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



 

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