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 Table of Contents 
ORIGINAL ARTICLE
Year : 2020  |  Volume : 43  |  Issue : 3  |  Page : 162-170  

Spatial mapping and radiometric survey of high background radiation areas in Southern Tamil Nadu, India


1 Center for Environmental Nuclear Research, Directorate of Research, SRM Institute of Science and Technology, Chennai, Tamil Nadu; Dr KC Patel Research and Development Center, Charotar University of Science and Technology, Anand, Gujarat, India
2 Center for Environmental Nuclear Research, Directorate of Research, SRM Institute of Science and Technology, Chennai, Tamil Nadu, India

Date of Submission26-Aug-2020
Date of Decision21-Sep-2020
Date of Acceptance23-Sep-2020
Date of Web Publication6-Jan-2021

Correspondence Address:
Kantha Deivi Arunachalam
Center for Environmental Nuclear Research, Directorate of Research, SRM Institute of Science and Technology, Chennai - 603 203, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.rpe_47_20

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  Abstract 


Southern Tamil Nadu has placer deposits, which is rich in minerals like zircon, garnet, monazite and rutile containing with radionuclides of U, Th and their decayed products. The present study is to identify the high background radiation areas in the regions of South west–east coast of Tamil Nadu. The ground radiometric survey was conducted in different seasons using portable radiation survey meter and global positioning system. Total of 137 observation spots were marked from 5 blocks for repeat measurements. Then calculated for the outdoor effective external terrestrial dose to the public residing in these blocks. The survey found average absorbed dose rate of block in this order 3>2> 4>1>5 (1394, 641, 247, 225, 167 nGy/h, respectively). From the data, a spatial mapping was performed for seasonal variation and interpolated for the unknown areas of dose rate. The outdoor effective dose to the public was found to be 1.66> 0.79>0.31>0.27>0.20 mSv/y with respective blocks. Conclusion: The dose to public was found to be lower, when compared with other high background radiation areas in the world. Therefore, there is no significant radiological risks to the public from natural radiation exposure.

Keywords: Absorbed dose rate, high background radiation area, inverse distance weighting, radiometric survey


How to cite this article:
Baskaran KV, Arunachalam KD. Spatial mapping and radiometric survey of high background radiation areas in Southern Tamil Nadu, India. Radiat Prot Environ 2020;43:162-70

How to cite this URL:
Baskaran KV, Arunachalam KD. Spatial mapping and radiometric survey of high background radiation areas in Southern Tamil Nadu, India. Radiat Prot Environ [serial online] 2020 [cited 2021 Jan 22];43:162-70. Available from: https://www.rpe.org.in/text.asp?2020/43/3/162/306281




  Introduction Top


The presence of natural radionuclides in the earth will lead to certain amount of radiation. The external radiation contribution for humans was 0.48mSv/y.[1] Most part of the terrestrial radiation contain natural radionuclides, which are in the form of minerals. Minerals such as uranite for uranium (238U, 234U, and 235U)[2] and monazite for thorium (232Th)[3] are the main sources of terrestrial radiation. These minerals are fused in the rocks by several geochemical processes which occurred in the earth's core during evolution by volcanic or other geochemical changes when the rocks are exposed. Due to the weathering of rocks in the earth's surface, these minerals would leach out and deposit in an area called placer deposits.[4],[5]

In placer deposits, the mineral contains rare earth element (REE) such as ilmenite, monazite, zircon, rutile, and garnet.[6] The mineral “monazite” is the prime source of thorium and some amounts of uranium, which exist all over the world such as China, Australia, India, Brazil, and Malaysia.[7] In India, the placer deposits are located in Manavalakurichi, Chavara,[8] Chatrapur,[9] and Bhimunipatnam,[10] in which the monazite content is in the form of thorium 9% and uranium 0.3% as phosphate combination.[6] The reported absorbed dose rate found in Manavalakurichi and Chavara was 11.40 μGy/h and 9.7 μGy/h, respectively,[5],[11] in Tamil Nadu and Kerala, South India. The world average absorbed dose rate is 60 nGy/h.[1] Because Manavalakurichi and Chavara are known high background radiation areas (HBRA) having higher than the average absorbed dose rate, it is necessary to monitor the terrestrial gamma radiation in this region, which will provide the information whether the minerals containing decay product of thorium and uranium have an impact on terrestrial gamma radiation of outdoors in the placer deposit soil. The concentration of natural radionuclides in the soil of Chavara region (placer deposit in Kerala) was 2355 Bq/kg of uranium and 14,000 Bq/kg of thorium.[11]

Radiation exposure to the public from soil may occur in three ways such as inhalation, ingestion, and dermal contact. Along with uranium and thorium, decayed products from their series will also occur in the environmental chain. Inhalation of radon and thoron[12] studies are carried throughout the decades in the world. Because soil has the concentration of natural radionuclide, migration of radionuclide is seen in food products, which leads to the direct consumption of radionuclide by the public in the placer deposits. Hence, natural radionuclide plays an important role in the environmental chain in the exposure of radiation to the public. Through ingestion studies, the presence of natural radionuclides such as 238U, 232Th, 226Ra, 228Ra, 210Po, 210Pb, and 40K in food products obtained from plants and animals, which originate in the HBRA regions, has been reported by various researchers.[13],[14]

Terrestrial radiation varies from place to place based on the concentration of the natural radionuclides in the soil. Hence, it is important to find out the terrestrial radiation levels and map the region; hence, it was planned to conduct a radiometric survey in the south coast region of Tamil Nadu, nearer to the placer deposit of Manavalakurichi. The terrestrial gamma radiation spots were observed in beach shores and inlands of the study regions during our survey. Based on the absorbed dose rate from the survey, a spatial mapping analysis was carried out and the dose to public has been computed. The annual terrestrial radiation exposure to humans was 0.07 mSv for outdoors and 0.14 mSv for indoors, which is from the contribution of annual radiation exposure levels of 2.4 mSv to the humans.[1] From this study, the outdoor terrestrial radiation dose to the public has been estimated to check any significance of radiological risk to the public due to the background terrestrial gamma radiation in this sector.

Geology of the study area

The ground radiometric survey was performed in the south coast of Tamil Nadu from west to east enclosed by 8.1–8.2°N latitude and 77.2–77.8°E longitude. Manavalakurichi is located in the south west coast of Tamil Nadu in the latitude of 8.15°N and longitude of 77.29°E. There is a mineral separation plant in the region for extracting minerals such as ilmenite, monazite, zircon, rutile, and garnet by M/s Indian Rare Earths Limited.[8] The presence of minerals and mineral deposit is attributed to the geology of this region. This is a granulite terrain region, which consists of crystalline rocks such as charnockite, khondalite, biotite, syenite, and migmatitic gneisses.[13] The characteristic elements present in the rocks of this region are Mg, Fe2+, B, Zr, B, Al, Ti, U, Th, etc., which are obtained in the form of mineral ore.[14] Due to erosion of the rocks, the minerals are deposited in the nearby coastal beaches and beach ridges of this region. The annual erosion rate for this domain was found to be in the range of 1000–11,000 m2/y.[15] Therefore, the study area was performed the radiometric survey in different seasonal conditions for any variations in the background radiations.


  Materials and Methods Top


Ground radiometric survey

The dose rate was measured above 1 m height from ground surface level using a portable radiation survey meter (Model: Instrument Model No: AT1123, Atomtex SPE, Minsk, Republic of Belarus). The survey meter was calibrated according to manufacturer guidelines before the survey and the survey was carried out in two physiographic regions such as beach shores and inlands for a total stretch of 73 km from south west to south east coast of Tamil Nadu. Regarding the west coast side, the distance between Tengapattinam and Kanyakumari was 45 km and in the east coast side the distance between Kanyakumari and Idindakarai was 28 km. The distance between shore and inlands was approximately in the range of 2–7 km, throughout the survey regions. The spots were located and marked in the Tremble global positioning system having an error of ~2 m distance from one point to another. The readings were consecutively taken for four different seasons such as survey 1 in winter (January–February), survey 2 in premonsoon (March–May), survey 3 in monsoon (June–September), and survey 4 in postmonsoon (October–December). Totally 137 observation/survey spots were taken and divided into five blocks, as shown in [Figure 1] and [Figure 2]. The details of the villages surveyed in our region are shown in [Table 1].
Figure 1: Overview of survey spots and location of Manavalakurichi in India

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Figure 2: Survey spot location in each block

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Table 1: List of villages surveyed in various blocks

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Spatial and hotspot analysis

The spatial mapping was computed to find out the dose rate in nonidentified areas within our survey area. In our study region, the interpolation technique was used to analyze the survey data using an algorithm called inverse distance weighting (IDW). This was computed in Arc GIS 9.3 software (Software Name and Version No: Arc GIS 9.3, ESRI India Technologies Ltd, Noida, Uttar Pradesh, India) and the area covered was 300 km2 for the spatial mapping. The IDW method has been predominantly used in the interpolation of the radiation studies and also in pollution studies.[16],[17],[18] The four different seasonal variations of the survey were plotted in the map to show the trend in the mapping pattern of our survey area.

The spatial mapping was analyzed using the spatial autocorrelation method and hotspot analysis (Getis-OrdGi) for the conceptual relationship in the maps. Spatial autocorrelation helps in analyzing the relationship between location variable values and the neighboring values. This will be reflected in the degree of Moran's I index of statistical significance and the differences in spatial cluster patterns of the survey spots in different seasonal surveys.

The Morans's I index value will be in the range of - 1 to + 1, and a value close to 0 means spatial randomness, whereas a positive value indicates the positive spatial autocorrelation and vice versa for negative value. The Z-score and P value computed were also in association with the Moran's I index. This will show the relationship of the spatial sampling points in the survey mapping, which are plotted by IDW and widely used in environmental studies.[19],[20]

Hotspot analysis (Getis-OrdGi) helps in the analysis of survey to show the cluster pattern of the absorbed dose rate of high and low values depending on the GiZ-score values in different blocks. The higher positive GiZ-score means the significant hotspots (high absorbed dose rate) and low negative GiZ-score means significant cold spots (less absorbed dose rate).[21] This shows the spread of the hotspots and cold spots in different blocks of each seasonal variation.

Annual effective dose from absorbed dose rate for outdoors

The annual effective dose was estimated for the outdoor radiometric survey by using the relation 1:



where

D is the annual effective dose in a year nSv (final result given in mSv), Dr is the absorbed dose in air per hour nGy/h, T is the total time spent in hours per year (8760 h), Df is the outdoor dose coefficient from absorbed dose in air to effective dose (0.7 Sv/Gy), and F is the outdoor occupancy factor (0.2) (unit less).[1]


  Results Top


The in situ ground radiation survey conducted in south east to west of Tamil Nadu was found to be in the range of 24–14,058 nGy/h during different seasonal surveys from 137 radiometric spots. The average absorbed dose rates for each block along with minimum and maximum dose rate are shown in [Table 2]. The average absorbed dose rate for each block was obtained from the values of 137 observation spots during different seasonal survey as shown in [Supplementary Table 1], along with the geographical coordinates. The highest mean absorbed dose rate was found in block 3 in the west region of Tamil Nadu. In this block, the minimum dose rate was 30 nGy/h in premonsoon season. The maximum dose was 14058 nGy/h in winter season. The lowest absorbed dose rate was found in block 5 in the east region of Tamil Nadu. In this block, the minimum dose rate was 24 nGy/h in premonsoon season. The maximum absorbed dose rate was 500 nGy/h in monsoon season.
Table 2: Mean absorbed dose rate of five blocks in different seasons along with minimum and maximum dose rates

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The annual effective dose for humans was calculated from the outdoor terrestrial gamma radiation by radiometric survey. The annual effective dose in block 3 was high with an average of 1.66 mSv and 0.79, 0.31, 0.27, and 0.20 mSv for blocks 2, 4, 1, and 5, respectively.


  Discussion Top


The present study revealed that block 3 consistently showed the higher absorbed dose rate (1394 ± 236 nGy/h) as the HBRA of our survey region during all the four different seasonal surveys and contributes to 52% of outdoor radiation, as shown in [Figure 3]. The other HBRA places such as Chavara in India and coastal zones of Guarapuri in Brazil, where the sand is rich in monazite mineral, showed the absorbed dose rate in the range of 1475–28,388 nGy/h[11] and 90–90,000 nGy/h,[22] respectively. Ghiassi-nejad et al. reported an absorbed dose rate of 17,000 nGy/h[23] for the hotspot of spring water in Iran.
Figure 3: Percentage contribution of absorbed dose rate in the study area

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The block 3 dose rate of this study region was considerably lower, when compared in terms of coastal HBRA's like Chavara (India) and Guarapuri (Brazil). The absorbed dose rate in Manavalakurichi area was on an average of 3135 nGy/h from the observation spots of 71–81 of all seasonal surveys, as shown in [Supplementary Table 1]. The highest dose rate of 14,058 nGy/h was found in the observation spot no. 73 (latitude 8.1532°N, longitude 77.2919°E) in the survey 1 (January–February) winter season. This spot is a beach shore. The reports from previous studies conducted in Manavalakurichi area showed an average dose rate of 3780 nGy/h [24] with the highest dose rate of 13,000 nGy/h. This proves Manavalakurichi area is consistent in radiation exposure from the minerals.[5]

As shown in [Figure 3], the second higher average absorbed dose rate for our survey was 641 ± 84 nGy/h in block 2, contributing to 24% of radiation survey for the study region. It is also clear from [Figure 3] that the villages from block 1 and 4 had an average dose rate of 225 ± 50 and 247 ± 44 nGy/h, contributing to 8.41% and 9.24% of the radiation survey, respectively. In block 5, the mean absorbed dose rate was 167 ± 29 nGy/h, contributing to 6.25% of outdoor radiation which is normal for block 5 compared to blocks 1 and 4. However, the radiation dose rate for blocks 1 and 2 may be due to the presence of minerals such as charnockite, khondalite, biotite, syenite, and migmatitic gneisses in the form of rocks. This may be the prime sources of U and Th radionuclides containing with REEs along with Fe2+, Zr, Al, and Ti.[13],[14] Based on the radiometric survey, a possible anomalous zone can be detected from the rocky minerals containing uranium and thorium in the survey region. Abouelnaga et al. also reported radioactivity due to secondary uranium minerals found in the altered granite regions of Gabal El Sela, Southeastern Desert, Egypt.[25]

Even though our study areas are rocky in blocks 1, 2, and 3, the blocks 1 and 2 showed lower dose rate compared to that of block 3. This was confirmed with the spatial mapping of the unknown areas which was plotted by following the IDW method using the absorbed dose rates obtained from the 137 survey spots and represented in [Figure 4]. The spatial pattern was analyzed by spatial autocorrelation for different seasonal surveys, which showed a spatially clustered pattern at the significance level of P = 0.01, as shown in [Table 3]. The highest positive Z-score of 5.06 was seen in survey 1 (January–February) and lower positive Z-score of 4.46 was seen in survey 3 (June–September). However, the Moran's I Index showed spatial randomness, which is closer to 0. In this aspect, Moran's I index was 0.08 for all seasons except for survey 3 of monsoon period, which showed 0.07.
Figure 4: Spatial mapping and hotspot analysis of absorbed dose rate in different blocks

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Table 3: Spatial autocorrelation for the study region of four seasonal survey

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The hotspot analysis was plotted in a map, as shown in [Figure 4], indicated with GiZ-score (standard deviation) for cold and hotspot clusters. The seasonal GiZ-score for each individual survey spot is given in [Supplementary Table 2] along with the geographical coordinates for calculating the average GiZ-score, which is given in [Table 4]. The high absorbed dose rate was observed in block 3, showing positive GiZ-score as hotspot, and the red color dots in the map reveal hotspot clusters. The cold spot representing low absorbed dose rate was distributed in the blocks 1, 2, 4, and 5, showing negative GiZ-score during all different seasonal surveys as shown in the map. Among them, the blocks 4 and 5 showed consistently very high cold spot for low absorbed dose rate in different seasonal surveys.



This is due to the weathering or leaching of minerals from the rocks present in block 1 and 2, which may have deposited in between the coastal zone of Manavalakurichi and estuarine area present near the village of block 3 region. Thus, the block 4 found with the low levels of absorbed dose rate, where the minerals deposited in block 3 regions. In the case of block 5, there was no rocky geological composition found compared to blocks 1, 2, 3, and 4, which contributes the least dose rate than the other blocks. The weathering of soils may affect the mineral deposition in the soils for the blocks 1, 2, 3, and 4. Padua et al., 2013, also found the same results in between the regions of blocks 3 and 4. However, they stated that Kanyakumari village had 1800 nGy/h, and similar results were also found in the survey spot nos. 90 and 93 [Supplementary Table 1] (latitude 8.0943, longitude 77.5579, latitude 8.0929, longitude 77.5644) during survey 1 of winter season with doses of 1146 and 1074 nGy/h, respectively.[26] The variation of dose rate might be due to the several environmental factors affecting the terrestrial area such as erosion and accretion.
Table 4: Hotspot analysis (Getis.OrdGi) for the study region of four seasons

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Kaliraj et al., 2013, reported erosion rates of 11,000 m2/y for Manavalakurichi, which comes under block 3 in our study. Villages such as Tengapattinam, Medalam, and Enayam Puthenthurai in block 2 of our study area showed both erosion rate of 7000 m2/y and accretion rate of 7000 m2/y, whereas the villages of block 4 such as Rajakkamangalam, Kanyakumari, and Kovalam showed only high accretion rate 23,000 m2/y.[15] Thus, the weathering of minerals and the erosion rate in the coastal zone favors the high absorbed dose rate, where the outdoor gamma radiation emitting from the minerals was observed.

The seasonal radiometric survey showed maximum average dose rate and highest dose rate in block 3 of our study area during winter season of survey 1 (January–February). During this period, the rainfall from the southwest and north east monsoon was settled and the minerals leached out from the rocks which would have deposited in the respective zones, contributing to more absorbed dose rate in the survey region. The minimum average absorbed dose rate and the lowest dose rate in block 3 of the survey were observed during premonsoon period (March–May). The natural background radiations in various regions of the world are summarized in [Table 5], as a comparison to our study area. The block 5 of our survey has indicated the dose rate of 167 nGy/h, which falls on the east coastal region of Tamil Nadu, and similar results were also observed by Lakshmi et al.[27] However, the world average dose rate is lower compared to that of all blocks in our study. Continuous monitoring of radiometric survey revealed the consistent natural background radiation of this region.
Table 5: Comparison of absorbed dose rate of our radiometric survey and other reported dose rate around the world

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Thus, it helps us to identify the higher annual effective dose from external outdoor terrestrial radiation to public residing in these areas. The present study shows the outdoor external terrestrial radiation to public was higher in all the blocks (blocks 1 – 0.27 mSv, 2 – 0.79 mSv, 3 – 1.66 mSv, 4 – 0.31 mSv, and 5 – 0.20 mSv), when compared to UNSCEAR data of 0.07 mSv. However, compared to the UNSCEAR indoor external terrestrial radiation level of 0.41 mSv, blocks 3 and 2 are significantly higher and rest were considered as a normal background radiation area.[1]

Other HBRAs in the world such as Ramsar, Iran, has an annual effective dose of 30 mSv, which is thirty times higher than our values.[28] In Guarapuri, Brazil, of south American continent, the annual effective dose was 5.5 mSv[22] and in Lambwe East, Kenya, of African continent, the yearly dose was 5.7 mSv.[29] In Nigeria of the same continent, the annual effective dose was reported to be 8.7 mSv.[30] The annual effective dose in Indian HBRA scenario for other places such as Chavara, Kerala, was 3.8 mSv[29] and 2.0 mSv for Chatrapur, Odisha.[9]

However, the annual world average natural radiation exposure by UNSCEAR 2000, for humans, is 2.4 mSv, which includes cosmic, terrestrial, and human-made radiation.[1] In our survey region, block 3 showed the higher dose compared to the other blocks of our survey. When compared to other HBRAs in India and world, it is far lower for the public in this region.





The radiometric survey of our study area showed that the average external outdoor dose rate in block 3 is higher than that of blocks 1, 2, 4, and 5. Consistently, in all seasonal surveys, block 3 showed the higher mean absorbed dose rate of 1394 ± 236 nGy/hand its annual effective dose was 1.66 mSv. Manavalakurichi from block 3, is the highest background radiation area compared to the other places of Tamil Nadu. But in comparison with the other places of the HBRA in the world and India, it showed lower values to the public residing in this region. Hence, there are no significant radiological risks to the public residing in these regions due to the outdoor external terrestrial radiation exposure from natural source.



Acknowledgments

This work was supported by the Board of Research in Nuclear Science (BRNS), Department of Atomic Energy (DAE), Government of India, for the research funding (grant no. 2008/36/58–BRNS), Environmental Survey Laboratory (ESL), Kalpakkam, and SRM University. The authors thank Dr. D. D. Rao, project collaborator, for the technical support.

Financial support and sponsorship

This work was supported by the Board of Research in Nuclear Science (BRNS), Department of Atomic Energy (DAE), Government of India, for the research funding, grant no. 2008/36/58–BRNS.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

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