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Year : 2012  |  Volume : 35  |  Issue : 2  |  Page : 90-95  

Environmental radioactivity evaluation in the coastal stretch of Bay of Bengal from Pondycherry to Velanganni (South East coast of India)

1 Department of Zoology, Periyar E.V.R. College, Tiruchirappalli, Tamil Nadu, India
2 Environmental Research Centre, J.J. College of Engineering and Technology, Tiruchirappalli, Tamil Nadu, India

Date of Web Publication21-May-2013

Correspondence Address:
G Satheeshkumar
Research Scholar, PG and Research Department of Zoology, Periyar E.V.R. College, Tiruchirappalli - 620 023, Tamilnadu
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-0464.112351

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Background: Since the Madras Atomic Power Station, Kalpakkam and Kudankulam Nuclear Power Plant, Kudankulam are sited in Bay of Bengal coast, radiological study of this coastal environment is imperative to assess the impact of operation of nuclear power plant, if any, on the radioactivity profile of the coast. Materials and Methods: In the present study the environmental radioactivity was evaluated by measuring activity concentrations of primordial radionuclides namely 238 U, 232 Th and 40 K. in the beach sand samples using high purity germanium gamma ray spectrometry. 10 sampling stations were fixed along a 290 km coastal stretch of Bay of Bengal from Pondycherry to Velangannni. Results: The activity concentration of 238 U is always much less than that of 232 Th and 40 K and it ranged from 9 Bq/kg (Pondicherry coast) to 93 Bq/kg (Karaikal coast). However, 232 Th concentration fluctuated from 12 Bq/kg (Pichavaram Mangrove) to 1075 Bq/kg (Karaikal coast). The activity concentration of 40 K varied within a narrow range from 162 Bq/kg to 474 Bq/kg. The distribution of primordial radioactivity was non-uniform along the entire stretch of Bay of Bengal. The elevated levels of 232 Th were recorded in Karaikal coast (S7) (1075 Bq/kg), Tirumullaivasal coast (S4) (215 Bq/kg), and Nagapattinam coast (S9) (146 Bq/kg) and attributed to the presence of thorium-rich monazite as beach placer in coastal sands. Conclusion: The total absorbed gamma dose contributed by the activity of 238 U, 232 Th, and 40 K varied between 29 nGy/h and 308 nGy/h with a GM value of 64.4 nGy/h. Similarly, the annual effective dose recorded for Bay of Bengal ranged from 0.04 mSv/year to 0.38 mSv/year, indicating that the Bay of Bengal, in general, falls under Normal Background Radiation Area.

Keywords: Absorbed gamma dose rate, Bay of Bengal, environmental radioactivity, high background radiation areas, high purity germanium gamma ray spectrometry, primordial radionuclides

How to cite this article:
Satheeshkumar G, Hameed P S, Pillai G S, Anbusaravanan N. Environmental radioactivity evaluation in the coastal stretch of Bay of Bengal from Pondycherry to Velanganni (South East coast of India). Radiat Prot Environ 2012;35:90-5

How to cite this URL:
Satheeshkumar G, Hameed P S, Pillai G S, Anbusaravanan N. Environmental radioactivity evaluation in the coastal stretch of Bay of Bengal from Pondycherry to Velanganni (South East coast of India). Radiat Prot Environ [serial online] 2012 [cited 2022 Jun 28];35:90-5. Available from: https://www.rpe.org.in/text.asp?2012/35/2/90/112351

  Introduction Top

Radioactive materials have become part of the earth at its very formation and therefore all organisms including man are exposed to natural radiation. The activity concentrations of primordial radionuclides namely 238 U and 232 Th and their decay products are the primary sources of natural radiation in addition to the presence of singly occurring radionuclide, 40 K in the earth crust. Natural radiation contributes about 87% total radiation dose received by the mankind and the remaining 13% is from man-made sources. [1] Natural radiation levels are found to be non-uniformly distributed in the earth crust. However, a few regions of the world have recorded higher levels of radiation and hence known as high background radiation areas (HBRAs) due to local geological and geochemical factors. [1],[2],[3] Notable HBRAs of the world are the coastal regions of Guarapari and Morro Do Forro in Brazil, Yangjiang in China, Nile Delta in Egypt, Southwest in France, Ramsar and Mahallat in Iran, Cox's Bazar in Bangladesh, and in certain regions of the USA and Canada. [4] The HBRAs are identified in coastal regions of India by several authors, e.g., Ullal coast in Karnataka, [5] Southwest coast of India, [6] Gulf of Mannar in the East coast of India, [7] Kalpakkam coast in Tamil Nadu, [8] Chhatrapur coast in Orissa, [4] and Visakhapatnam coast. [9] Evaluation of coastal environmental radioactivity is imperative in order to identify HBRAs and it is also a pre-requisite for a meaningful assessment of impact on environmental radioactivity profile due to operation of Nuclear Power Plants sited in the coastal regions.

The south east coast of India has three distant provinces, namely, Bay of Bengal, Palk Strait, and Gulf of Mannar [Figure 1]. Somasundaram et al., [10] and Shahul Hameed [11],[12] have studied the environmental radioactivity status of Gulf of Mannar and also identified HBRAs in the Gulf. The radiation ecology of the Palk Strait, which is located between Gulf of Mannar and Bay of Bengal, is investigated by Sadiq Bukhari, [13] and Hameed et al. [14] However, no adequate work was available on the environmental radioactivity states of Bay of Bengal except preliminary reports on Nagapattinam coast [15] and Kalpakkam coast.[8] Hence, this work is undertaken in a 290 km coastal stretch of Bay of Bengal with the objective of generating a baseline data on the environmental radioactivity and such data would be valuable in assessing the variation in natural radioactivity levels, if any, due to the operation of two nuclear power plants: One in Kalpakkam (Bay of Bengal) and another in Kudankulam (Gulf of Mannar) [Figure 1].
Figure 1: The map showing the study area

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South east coast of India runs along the Eastern boundary of Tamil Nadu state, India. It has three distinct marine zones, namely, Gulf of Mannar, Palk Strait, and Bay of Bengal. For this study, 10 stations were chosen along a 290 km stretch of Bay of Bengal extending from Pondicherry (S1) (Lat 11° 93', Long 79° 78') to Velankanni (S10) (Lat 10° 68', Long 79° 84'). Except Pichavaram Mangrove (S3), all other stations are sandy beaches. The stations are fixed based on accessibility, fishing activity, and coastal human population [Figure 1].

  Materials and Methods Top

The beach sand samples were collected from spots recording higher gamma radiation as determined from Scintillometric measurement. Three such samples were collected from the sampling spot of about 1 m 2 area. The total sample collected from each station was about 2 kg. The sand sample was first air-dried and later dried in an air oven at temperature of 105-110°C till the sample attained weight constancy. The samples were cooled, sieved in 300 μm mesh, and stored in air-tight cylindrical plastic container of 300 ml volume for a period 30 days to allow radioactive equilibrium of 238 U and 232 Th with corresponding progenies. The concentration of daughter products of uranium and thorium gives the concentration of the parents which are determined using High Purity Germanium (HPGe) detector (Type :Intrinsic Germanium Crystal 30; Volume: 133 cc; Princeton Gamma-Tech make). The detector was protected by a lead shield to reduce the background radiation noise. A standard solution of 226 Ra in equilibrium with its daughters was used to calibrate the detector. Further, International Atomic Energy Agency (IAEA) standard reference material was also used to check the calibration of the system. The system maintained the energy resolution of 2.0 keV and relative efficiency of 33% at 1.33 MeV. The samples were counted in identical (300 ml) containers to maintain constant counting geometry for a period of 20 h to minimize the counting error. The activity of 238 U was evaluated from the 609 keV gamma line of 214 Bi and that of 232 Th from 911 keV gamma line of 228 Ac. The activity of 40 K was determined from 40 K peak at 1,461 keV. It was assumed that these radionuclides were in secular equilibrium with their corresponding decay products. [16] The activity calculations were carried out by following the method of Lalit and Ramachandran [17] and Mishra et al. [18]

  Results and Discussion Top

Primordial radionuclides

The data on the concentrations of primordial radionuclides such as 238 U, 232 Th, and 40 K in the beach sand samples collected from the 10 sampling stations located in a 290 km stretch of Bay of Bengal are presented in [Table 1]. The activity concentrations of 238 U are always much less than that of 232 Th and 40 K. It ranged from 9 ± 1.5 Bq/kg (Pondicherry coast) to 93 ± 7.5 Bq/kg (Karaikal coast) with geometric mean (GM) value of 18.4 ± 1.91 Bq/kg. However, the lowest concentration of 232 Th (12 ± 4.2 Bq/kg) was recorded in Pichavaram Mangrove, whereas the highest concentration of 1075 ± 30 Bq/kg was recorded in Karaikal coast and GM value of 232 Th in this stretch of Bay of Bengal was estimated to be 56.5 ± 4.08 Bq/kg. In all the 10 sampling stations studied, a higher range of 238 U (16 ± 4.2-93 ± 7.5 Bq/kg) was registered in Velankanni coast (S10), Nagapattinam coast (S9), Nagore coast (S8), and Karaikal coast (S7). Similarly, elevated levels of 232 Th were recorded in Karaikal coast (S7) (1075 ± 30 Bq/kg), Tirumullaivasal coast (S4) (215 ± 8.7 Bq/kg), and Nagapattinam coast (S9) (146 ± 8.1 Bq/kg). It is interesting to note that, Pichavaram Mangrove coastal environment maintained a minimal level of 12 ± 4.2 Bq/kg and 12 ± 2 Bq/kg for 238 U and 232 Th, respectively. It may be attributed to the reduced level of primordial-rich monazite mineral in the mud flats of mangroves in contrast to the coastal sands. In contrast, Karaikal coast (S7) station registered a maximum levels of 238 U (93 ± 7.5 Bq/kg) and 232 Th (1075 ± 30 Bq/kg). Thorium-rich monazite present in the black sand of this station is responsible for the elevated levels of 238 U and 232 Th. The activity ratio of uranium to thorium was found to be maximum (14.1) in Tirumullaivasal and minimum (0.9 and 1.0) in Devanampattinam coast and Pichavaram Mangrove and the GM activity ratio for the entire study area was found to be 3.0. This means 232 Th concentration was three times higher than 238 U concentration. Schulz [19] classified radioactive elements into two groups, namely mobile group, (e.g., uranium) and immobile group, (e.g., thorium). The mobility of uranium is due to uranyl ions which are able to form complex compounds, and are generally soluble in nature. But thorium does not form any soluble complexes. Therefore, uranium can be carried in an oxidizing environment, whereas thorium isotopes cannot. 238 U is not accumulated in a particular place and it moves faster than 232 Th. The activity concentrations of 40 K fluctuated within a narrow range from 162 ± 44 Bq/kg (Nagore coast) to 474 ± 57 Bq/kg (Velankanni coast) with the GM activity concentration of 302 ± 104 Bq/kg.
Table 1: Data on levels of primordial radionuclides, 238U, 232Th and 40K in coastal sand samples from Bay of Bengal

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Data on the activity concentrations of primordials in various regions of Indian coast and international coast are presented in [Table 2]. The activity concentrations of 238 U, 232 Th, and 40 K recorded in beach sand samples of Bay of Bengal are distinctly higher than the levels recorded in Palk Strait by Shahul Hameed. [14] However, the data generated in this study are much lower than the concentrations of primordials recorded in Gulf of Mannar by Somasundaram. [7] The situation establishes the fact that the environmental radioactivity levels are non-uniformly distributed in South east coast of India. The situation also holds in good for the entire stretch of Bay of Bengal. HBRAs identified in Bay of Bengal are Karaikal coast (present study), Kalpakkam [8] and Chhatrapur coast in Orissa. [20] In all these areas, the elevated background levels are attributed to enriched concentration of thorium in the beach sand. The south west coast of India (Kerala coast) maintains a distinctly higher radiation levels [6] as compared to south east coast of India.
Table 2: Comparison of activity concentrations of 238U, 232Th and 40K is surface beach sand samples of Indian coast and different areas in the world

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The GM activity concentration of 238 U (18.4 ± 1.91 Bq/kg) is found to be slightly higher than that of the all India average value of 14.8 Bq/kg, whereas the GM 232 Th activity (56.5 ± 4.08 Bq/kg) in Bay of Bengal is found to be about three times higher than the all India average 18.3 Bq/kg and the GM 40 K activity of (285 ± 1.42 Bq/kg) is seven times higher than that the all India average value of 40.7 Bq/kg. [21] When the world average values of the primordial [Table 2] are compared with the GM values recorded in sand samples of Bay of Bengal, it can be observed that the GM activity of 238 U (18.4 ± 1.91 Bq/kg) for Bay of Bengal is found to be less than the world average (25 Bq/kg), whereas thorium GM activity of Bay of Bengal coast (56.5 ± 4.08 Bq/kg) is about 2.3 times higher than that of world average (25 Bq/kg). However, the 40 K GM activity of Bay of Bengal (285 ± 1.42 Bq/kg) is found to be less than the world average (370 Bq/kg). [22]

Absorbed dosed and annual effective dose

The activity concentration of 238 U, 232 Th, and 40 K were converted into absorbed dose values by applying dose conversion formula of UNSCEAR [22] as given below:
D = (0.427 CU + 0.662 CTh + 0.043 CK ) nGy/h,
where D = Absorbed dose rate;
CU = Activity concentration of 238 U;
CTh = Activity concentration of 232 Th;
CK = Activity concentration of 40 K.

The absorbed gamma dose rate due to the presence of 238 U, 232 Th, and 40 K in the beach sand of 10 sampling of the study area is presented in [Table 1]. It is found to be maximum in Karaikal coast (S7) (308 nGy/h) and minimum in Pichavaram Mangrove (S3) (29 nGy/h). The entire stretch recorded the GM value of 64.4 ± 2.17 nGy/h. About 63% of the absorbed gamma dose was contributed by the activity of 232 Th and 40 K contributed 22%, and 238 U contributed 15%. The total absorbed gamma dose rate for entire stretch was calculated from the GM values of 238 U, 232 Th, and 40 K and adding the cosmic radiation factor 34 and it was found to be 91.5 nGy/h. The absorbed dose rate of each sampling station was converted into annual effective dose rate (out-door) by applying the following equation proposed by UNSCEAR. [1]

Out-door annual effective dose (mSv/year) = Absorbed dose rate nGy/h × 8760 nGy/h × 0.2 × 0.7 Sv/Gy × 10 -6 .

The annual effective dose rate varied from 0.04 mSv/year (Pichavaram Mangrove) to 0.38 mSv/year (Karaikal coast) with a GM value of 0.07 mSv/year. However, the value for the entire stretch was calculated to be 0.11 mSv/year after adding cosmic radiation factor. In normal background radiation areas (NBRAs), the average annual external effective dose from terrestrial radionuclides is 0.46 mSv/year. [2],[3] The value recorded for Bay of Bengal coast (0.11 mSv/year) was much lower than the world average. Hence, it is evident that, in general, the study area falls under NBRAs.

  Conclusions Top

This study indicated that the gamma radiation regimes in the Bay of Bengal coastal stretch were non-uniform and fluctuated between 29 nGy/h and 308 nGy/h. However, the major contribution (63%) to the gamma dose was from 232 Th which was followed by 40 K (22%) and 238 U (15%). The thorium enrichment of beach sands of the study area was attributed to the presence of high concentration of monazite as beach placer. Karaikal coast (S7) was identified as HBRA since it registered elevated levels of 232 Th (1075 ± 30 Bq/kg). Among the ten beaches surveyed, Karaikal beach recorded a maximum absorbed dose rate value of 308 nGy/h and a maximum annual effective dose of 0.38 mSv/year. Except Karaikal beach, the remaining coastal stretch falls under NBRAs.

  Acknowledgments Top

The authors gratefully acknowledge Dr. V. Kannan, Ex-Officer-in Charge, Environmental Survey Laboratory, Department of Atomic Energy, Kalpakkam, for the technical support in HPGe gamma ray spectrometric analysis of beach sand samples and Prof. K. Ponnusamy, Chairman, J.J. Group of Institutions for support and facilities.[30]

  References Top

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