|Year : 2019 | Volume
| Issue : 1 | Page : 22-27
Effect of natural radioactivity content in the beach sands along the east coast of Tamil Nadu, India, due to tsunami
KS Lakshmi, V Meenakshisundaram, J Punniyakotti
Department of Physics, Meenakshi Sundararajan Research Centre, Meenakshi College for Women, Chennai, Tamil Nadu, India
|Date of Submission||08-Mar-2019|
|Date of Decision||14-Mar-2019|
|Date of Acceptance||14-Mar-2019|
|Date of Web Publication||3-Jun-2019|
Department of Physics, Meenakshi Sundararajan Research Centre, Meenakshi College for Women, Kodambakkam, Chennai - 600 024, Tamil Nadu
Source of Support: None, Conflict of Interest: None
In the aftermath of devastating tsunami, studies were undertaken to assess its impact on the environmental radioactivity profile in the beach sands along the entire east coastal region of Tamil Nadu state (~1000 km) and compared with previously established pretsunami distribution profile. The results of the present study clearly indicate that the radioactivity content of 232Th and 238U in the beach sands registered a steep fall (52% and 41%, respectively), whereas the 40K activity has increased by 26%. This is attributed to the hydrodynamics of tsunami waves, which removed huge quantities of beach minerals that have been built-up over a few decades. The individual activity content of the three primordial radionuclides, total activity content, comparison between pre- and post-tsunami data for all the 32 sites, and their statistical analyses are covered in the paper.
Keywords: Beach sand, natural radioactivity, pre-post tsunami, Tamil Nadu coast
|How to cite this article:|
Lakshmi K S, Meenakshisundaram V, Punniyakotti J. Effect of natural radioactivity content in the beach sands along the east coast of Tamil Nadu, India, due to tsunami. Radiat Prot Environ 2019;42:22-7
|How to cite this URL:|
Lakshmi K S, Meenakshisundaram V, Punniyakotti J. Effect of natural radioactivity content in the beach sands along the east coast of Tamil Nadu, India, due to tsunami. Radiat Prot Environ [serial online] 2019 [cited 2019 Oct 18];42:22-7. Available from: http://www.rpe.org.in/text.asp?2019/42/1/22/259676
| Introduction|| |
It is well known that the coastal areas of southern part of India are interspersed with surface deposits of monazite, a thorium-bearing mineral. An extensive environmental radioactivity monitoring of entire east coastal region of Tamil Nadu state, India, covering around 1000 km was done earlier in 2002 before tsunami., However, the catastrophic tsunami waves that swept the Tamil Nadu coast could have completely or at least partially altered the natural radioactivity profile picture. Hence, the same study area was chosen and investigated whether the profile has changed as a consequence of devastating tsunami. The study area covers from Kanyakumari to Pulicat (situated 30 km north of Chennai). Thirty-two sites were selected, and the distance between each site is about 30 km. It may be noted that all the 32 sampling sites selected for both the studies, that is, before and after the tsunami, are exactly the same. Among other investigations done as part of this posttsunami study, the natural radioactivity profile in the beach sands, one of the prime objectives, is covered in this paper.
| Materials and Methods|| |
Description of study area
The study area, that is, east coast of Tamil Nadu, State of India, is given in [Figure 1]. The length of the coast covered in this study area is little more than 1000 km. The study area starts right from Kanyakumari at the southern tip of Indian peninsula to Pulicat situated 30 km north of Chennai city, the capital of Tamil Nadu. This coastal area lies between 8°11' N and 13°04' N latitude and 77°29' E and 80°17' E longitude and covers east coastal region of entire Tamil Nadu of approximately 1000 km. Along the Tamil Nadu coastal area, there are several tourist places. Some notable ones are Mamallapuram, Puducherry, Pichavaram, Rameshwaram, Tiruchendur, and Kanyakumari. Furthermore, there are major ports such as Chennai and Tuticorin besides the minor ones such as Nagapattinam and Cuddalore. Industrially, this area is a well-developed one. There are many chemical and petrochemical industries in places such as Chennai, Tuticorin, Puducherry, and Cuddalore. A Heavy Water Plant is situated at Tuticorin. At Kalpakkam, two nuclear power reactors are in operation. Two nuclear power reactors of 1000 MWe each are in operation at Kudankulam site near Kanyakumari, and there is a distinct possibility of commissioning of additional power reactors at both Kalpakkam and Kudankulam. Being a coastal area, the critical population here is fishermen community. It is also known that some regions of the east coast have a significant distribution of monazite. It would be of interest to determine the profile of natural radioactivity distributions all along the east coast of Tamil Nadu and their contribution to the background radiation levels, especially the external component.
Sample collection and preparation techniques
The present study area of east coast of Tamil Nadu covers a total length of 1000 km, from which, 32 successive sites were selected as shown in [Figure 1]. The beach sand samples have been manually collected from all the 32 sites with the help of a plastic spade in polyethylene bags from July 2007 to January 2009. The collected samples were uniformly mixed, sieved, and air dried. The samples were further dried in an oven at temperature of 100°C–120°C for an hour to remove the moisture content and stored in an airtight 250-ml plastic container for 1 month before subjecting them to gamma-ray spectral analysis. This is to ensure attaining secular equilibrium between radium and its short-lived daughter products. The net weight of the samples was determined before counting.
A 3“×3” NaI (Tl) scintillation detector-based gamma-ray spectrometer is used for spectral measurements to enable one to cover the entire energy range of the naturally occurring radionuclides up to 2.6 MeV (208 Tl, one of the daughter products of 232Th). The detector is shielded by 15-cm thick lead on all sides, including at the top to reduce background due to the cosmic-ray component by almost 98%. The inner sides of the lead shielding are lined by 2-mm thick cadmium and 1-mm thick copper to cutoff lead X-rays and cadmium X-rays, respectively. These graded lining shield materials further reduce the background, especially in the low-energy region. Standard sources of the primordial radionuclides, obtained from IAEA in the similar geometry and having soil equivalent matrix, were used to determine the efficiency of the detector for various energies. The sealed sand samples were placed on the top of 3”×3” NaI (Tl) detector, and count spectra were obtained for each of the sand sample. The net radioactivity content of the three primordial nuclides, namely40 K, 232Th, and 238U are deduced from the count spectra. The region under the photopeaks corresponding to 1.46 MeV (40 K), 1.764 MeV (214 Bi), and 2.614 MeV (208 Tl) energies are considered to arrive at the radioactivity levels of 40 K,238U, and 232Th, respectively. The below detectable limit (BDL) of each of the three primordial radionuclides is determined from the background radiation spectrum obtained for the same counting time as was done for the sand samples. The estimated BDL values are 2.22 Bq/kg for 238U, 2.15 Bq/kg for 232Th, and 8.83 Bq/kg for 40 K.
| Results and Discussion|| |
The radioactivity contents of 238U, 232Th, and 40 K have been estimated in the beach sand samples collected at all the 32 sites of Tamil Nadu coast for posttsunami era, and the same is given in [Table 1]. For comparative analysis, the pretsunami values of 238U, 232Th, and 40 K activity levels are taken from Lakshmi, Ph. D. thesis, 2001 and are also given in [Table 1]. As per pretsunami data, the radioactivity contents of 238U, 232Th, and 40 K are in the range from BDL to 254 ± 11 Bq/kg with an average value of 58 ± 6 Bq/kg and 30 ± 5–3576 ± 26 Bq/kg, with an average value of 484 ± 10 Bq/kg and 15 ± 12–524 ± 49 Bq/Kg, and with an average value of 212 ± 22 Bq/Kg, respectively. Similarly, for posttsunami, the radioactivity contents of 238U, 232Th, and 40 K are in the range from BDL to 192 ± 14 Bq/kg with an average value of 34 ± 6 Bq/kg and 6 ± 4–1578 ± 28 Bq/kg, with an average value of 230 ± 10 Bq/kg and 52 ± 21–657 ± 63 Bq/kg, and with an average value of 287 ± 31 Bq/kg, respectively.
|Table 1: Comparison for radioactivity content of primordial radionuclides pre- and post-tsunami|
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If one compares the data given in [Table 1], the radioactivity content of both238U and 232Th found to decrease during posttsunami by around 41% and 52%, respectively, as compared to pretsunami; however, the activity concentration of 40 K is found to increase (26%) as compared to pretsunami as shown in [Figure 2]. To have better clarity, an attempt is also made to compare the radioactivity content of the three individual radionuclides for all the 32 sites by juxtaposing the pretsunami data over the posttsunami data as shown in [Figure 3], [Figure 4], [Figure 5]. As shown in [Figure 3], it may be noted that the radioactivity content of 238U during the posttsunami era is generally lower as compared to pretsunami era excepting at four out of 32 sites namely, Mariyur (S6), Mamallapuram (S29), Kovalam (S30), and Pulicat (S32). 232Th activity levels are compared in [Figure 4], and it may be seen that during posttsunami era, 232Th activity levels are generally lower in most of the sites excepting for the same four sites, namely, Mariyur (S6), Mamallapuram (S29), Kovalam (S30), and Pulicat (32), where238U activity was also higher. The activity levels of 232Th are almost the same at Mimisal (S11) and Pazhayar (S23). The radioactivity content of 40 K in majority of the sites is generally higher as compared to pretsunami levels as shown in [Figure 5]. However, at seven sites, namely, Tuticorin (S4), Mariyur (S6), Rameshwaram (S8), Muthupet (S15), Kodiyakarai (S16), Thopputhurai (S17), and Pulicat (S32) sites, the activity content of 40 K is higher during pretsunami era and at Marakkanam (S27), both pre- and post-tsunami levels are almost the same.
|Figure 2: Comparison between pre- and post-tsunami radioactivity values of 232Th,238U, and 40K|
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The reduction in thorium and uranium activity content and the variation in the activities of 40 K must be due to the washing of top layer sand, and other particulates containing heavy minerals by tsunami waves and replacing with the bottom layer of sand. Several investigators have reported the depth distribution of radionuclides in the beach sand.,,, Iyer et al. have studied the variation of monazite content in the beach sand of Kalpakkam in the depth intervals of 0–20 cm and 20–40 cm and observed a decrease in monazite content over 30% from the first interval to the second interval. Narayana et al. had studied the radioactivity content in the beach sand samples of high background radiation area in Ullal, southwest coast of India, for different depth intervals. They had observed a decrease in the activity content of 238U and 232Th, while40 K activity is slightly increasing with depth. Shetty et al. observed a decrease in the activity content of 238U and 232Th up to 30 cm, whereas for 40 K, it was slightly increasing with depth. Ramasamy et al. also reported a marginal increase of potassium content with depth in river sediments. Lakshmi et al. reported that40 K concentration increases with depth in sand though marginally.40 K is generally associated with an ordered crystalline oxide material, resistant organic matter, and clay lattices. The lower concentration of 40 K in the upper layers and might be due to physical removal of the above fractions and the mineral muscovite, which is enriched in potassium, due to wind and wave action. Thus, it can be postulated that the decrease in the activity of 238U and 232Th and increase in the 40 K activity must be due to the washing of top layer of sand containing heavy minerals during the tsunami and replacing with bottom layer of sand. The present posttsunami data reiterate the fact, as can be inferred from [Table 1], that the radioactivity content of 232Th and 238U in the beach sand registered a lower value, whereas40 K activity has increased. Posttsunami studies on the distribution and bioaccumulation of natural radionuclides in southeast coast of Tamil Nadu, India, were undertaken by Satheeshkumar et al. at Pitchavaram site alone; they reported that one of the major reasons that can be attributed for the reduced level of radioactivity in the beach sands of east coast of Tamil Nadu is the hydrodynamics of the tsunami waves, which removed huge quantities of beach materials that have been built up over a few decades; they also reported that there was no enrichment of monazite in the tsunamigenic sediment and hence the reduction in the background radiation of the ecosystem. It may be noted that the Pichavaram site is one of the 32 sites of the present study area too. Hence, the radiological profile becomes modified reducing drastically background radiation levels in the environs of east coast of Tamil Nadu in the posttsunami era.
| Conclusions|| |
The radioactivity content of 238U, 232Th, and 40 K in the beach sand samples of the entire east coast of Tamil Nadu as part of posttsunami era has been experimentally quantified by gamma spectral analysis. The study revealed that the average radioactivity content of 238U, 232Th, and 40 K in the beach sand samples was found to be 34 ± 6, 230 ± 10, and 287 ± 31 Bq/kg, respectively. These results indicate that activity content of 238U and 232Th in the beach sand samples of east coast of Tamil Nadu in posttsunami era is much lower than that were prevalent during the pretsunami era. The catastrophic tsunami had reduced the mean activity level of thorium and uranium content to a very large extent (52% and 41%, respectively) in the beach sands of Tamil Nadu coast.
The authors would like to acknowledge Board of Research in Nuclear Sciences, DAE, Government of India, for funding the major research project (Sanction No. 2006/36/21-BRNS) on “studies on Environmental Radiation along the East Coast of Tamil Nadu after Tsunami” and the authors are very much thankful to Director, IGCAR and Head, RSD, IGCAR, Kalpakkam, Tamil Nadu, for granting permission to use some of their facilities for undertaking measurements.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]