Introduction

In an environment, human beings are harmed by radiation from three types of isotopes: radioactive isotopes (naturally occurring isotopes of the earth's crust), cosmogenic isotopes, and manmade isotopes [1, 2]. Particularly, in natural radioisotopes, the uranium series originates from 238U, the thorium series originates from 232Th, and the actinium series originates from 235U. There is most important singly occurring radionuclide is 40K because it is a gamma-ray emitter in addition to beta decays and therefore contributes significantly to gamma radiation exposure [3]. Exposure of ionizing radiation to humans is due to the presence of a significant amount of natural radionuclides 238U, 232Th, and 40K and their progenies in the environmental matrix such as soil, sediment, rock, water, etc. The concentration of these radionuclides is enhanced along the coastal area due to natural activities such as the leaching from parent rocks through both erosion and dissolution and anthropogenic activities such as nuclear accidents, nuclear weapons, mining, fertilizers derived from phosphate rock, drilling, transportation, and burning of fossil fuels [4, 5]. Such an enhanced activity concentration (monazite-bearing sands) was identified along the east coastal zones of India namely Orissa, Tamil Nadu, and Kerala.

In the coastal area, sediments are inorganic silicon-rich coarse materials that are derived from weathering of parent rocks. They may have deposited to their place after transport by winds, rivers, and glaciers due to the actions of waves and currents [6]. Also, mineral tracers are deposited and distributed throughout the beach during rainfall and tsunamis. This causes the accumulation of radioactive minerals and hence natural radioactivity increases in the area [7]. The coastal area has attracted all types of people across the world. With high accessibility, people settle on the coasts to live as well as leisure, recreational activities, and tourism [8]. Assessment of natural radioactivity is the most important work towards the health concern because it could cause some health issues such as bone, liver, lung, and breast cancers, anemia, and cataracts [9, 10]. Especially, cancer tissues are introduced into the human body by exposure to gamma radiation from 238U, 232Th, and 40K. For these reasons, there is a need to measure and monitor the natural radioactivity along the coastal area. Recently, many potential researchers have carried out extensive work on the natural radioactivity level on beaches across the world [11,12,13,14,15,16,17,18,19,20,21,22,23], while the data about the natural radioactivity of beach sand along the North Chennai to Pondicherry coastal area, India is not available. A few regions of India's western coast are also high in natural radioactivity, while the country's southeast coast is known as one of the locations with the highest natural background radiation levels around the world. Despite the abundance of studies conducted around the world, there is a scarcity of radionuclide research on the North Chennai to Pondicherry coastal area. Therefore, the current investigation focused on radioactivity in coastal sediments because it might have a very large background level due to geology of the location. Hence, the main objectives of the present work are (i) to determine the activity concentration of radionuclides (238U, 232Th, and 40K) in the North Chennai to Pondicherry coastal sediments by NaI(Tl) gamma-ray spectrometry, (ii) to assess the radiological risks in coastal sediments by calculating the radium equivalent activity (Raeq), gamma dose rate (DR), annual effective dose equivalent (AEDE), external hazard index (Hex), excess lifetime cancer risk (ELCR), annual gonadal dose equivalent (AGDE), (iii) to assess the relationship between the radioactive variables using Pearson correlation analysis.

Study area

The sprawling study area of ⁓160 km in length was surveyed along the North Chennai to Pondicherry coastal area for sample collection. The sampling points along the study area are shown in Fig. 1. The cumulative study area spread from Kalanji (13°19ʹ8.94ʺN; 80°20 ʹ 33.7554ʺE) Thiruvallur district of Tamil Nadu to Kalapet (12°0ʹ53.496ʺN; 79°51ʹ41.1474ʺE) near Pondicherry university at Pondicherry. This area covers mainly 4 coastal districts of Tamil Nadu such as Tiruvallur, Chennai, Chengalpattu, and Villupuram. During the last few decades, the study area is fully dominated by tourism, transportation, seaports, urbanization, industry, and aquaculture activities. It also covers some famous beaches like Mahabalipuram Beach (A historical place), Edward Elliot's Beach, and Marina Beach (2nd longest urban beach in the world) which is visited by many tourists [24, 25] from across the world, and also has an active atomic power station. The Palar River runs into the Bay of Bengal at Vayalur, approximately 70 km south of Chennai (near the Indira Gandhi Centre for Atomic Research–IGCAR, Kalpakkam) [26]. Thus, the study area pays more attention to measuring the level of radiation exposure due to the presence of natural radionuclides.

Fig. 1
figure 1

A map of the North Chennai to Pondicherry, India (Study area) showing the sampling points

Full size image

Experimental methods

Sample collection and preparation

The 21 sediment samples were collected from twenty-one locations along the North Chennai to Pondicherry coastal area, India (from the period March–July 2022). The sampling pathway covers an interval of 5–10 km for urban coasts and 10–15 km for non-urban coasts. The geographical coordinates of each location were noted using a hand-held Garmin global positioning system and given in Table 1. Each location was 0–20 m away from the high tide and (Fig. 1), about 2 kg of sediment samples were collected at 40 cm depth from the surface level using a stainless-steel T-rod mud auger [27]. The collected samples were placed in polythene covers and properly labelled and transported to the laboratory.

Table 1 Geographical co-ordinates of the sampling locations along the study area
Full size table

Other unwanted substances like stones, shells, pebbles, and macro impurities present in sediment samples were completely removed. In order to obtain a constant weight, samples were allowed to dry under direct sunlight, and also each sample was oven-dried at 110 °C for 2 h to remove the moisture content. Before radionuclides measurements, samples are mashed, dehydrated, and sieved, then samples were tightly stuffed with a radon-impermeable 250-cc volume of trap-shaped polyethylene containers with uniform size (dia: 60 mm; height: 120 mm) and hermetically sealed for 2 fortnights to reach secular equilibrium between 238U, and 232Th series and their respective progenies [28, 29]. It was assumed 222Rn, and 220R could not escape from the containers.

γ-ray spectrometry

The gamma-ray spectrometry equipment at the sophisticated “Radiation physics laboratory” in the Department of Physics, Sri Sivasubramaniya Nadar College of Engineering, Chennai, India, was used for this research. A gamma-ray spectroscopy system consists of a Sodium Iodide Thallium (NaI-Tl) scintillation detector with a 98% effectiveness in counting which is paired with a 1024-channel computerized multi-channel analyzer (MCA). A (NUCLEONIX, GR611M) detector with Anuspect (version 1.0) software was employed for these measurements, with a resolution for the energy of FWHM is 3.398 keV at 1332 keV of gamma line for 60Co. The detector and pre-amplifier were placed inside a lead shield containing an inner concentric cylinder of Cu foils (0.3 mm) to absorb X-rays generated in the lead [28]. This entire structure was contained under a 15 cm squared lead shield to minimize ambient noise in the system. For efficiency calibration, the approved Standard International Atomic Energy Agency (IAEA) sources of reference-grade materials such as RG-U (4940 ± 30 Bq kg−1), RG-Th (3250 ± 90 Bq kg−1), and RG-K (14,000 ± 400 Bq kg−1) were used [30,31,32]. The energy calibration was accomplished by inserting known-energy gamma sources, 137Cs (662 keV) and 60Co (1173–1332 keV), into the detector.

Considering that 238U, and 232Th, as well as their decay products, are in secular equilibrium, the concentration of 238U, and 232Th were calculated from their progeny photopeak of 1764 keV for 214Bi and 2614 keV for 208Tl were used for determining the activity concentrations of 238U and 232Th respectively, and the gamma-ray transition of 1460 keV was used to determine 40K concentrations. A similar geometry was maintained for counting and standard sample analysis and the counting time for all the samples was 10,000 s.

Pearson correlation analysis

Correlation analysis was performed as a bivariate statistic to identify the mutual linkages and strength of association between two variables using the linear Pearson correlation coefficient (r). It is a number between -1 and 1 that measures the strength and direction of the relationship between two variables. The statistical software IBM-SPSS version 20 was used to perform Pearson’s correlation analysis among radioactive variables [22] and the ORIGIN PRO (v2022) was used for graphing analysis.

Results and discussion

Distribution of activity concentration of 238U, 232Th, and 40K in sediments

The specific activity of 238U, 232Th, and 40K for the collected sediment samples is determined using the following formula [33],

$${\text{A }} = { }\frac{{{\text{NCPS}}}}{{{\text{W }} \times \eta { }}}$$
(1)

where A is the activity concentration of radionuclide (238U, 232Th, and 40K) which is typically expressed in Becquerel per kilogram (Bq/kg), NCPS represents the total gross counts of corresponding photo peak from the spectrum subtracted from background counts and divided by counting time i.e., 10,000 s. W represents the total mass of each sample in (kg). η denotes the photo peak's efficiency as determined via efficiency calibration. In the present work, the activity concentration of 238U, 232Th, and 40K was measured for sediment samples using NaI(Tl) detector and given in Table 2 with their respective uncertainties (± 2σ). As can be seen from Table 2, except for four (S6, S7, S11, and S16), all other sediment samples show a high concentration of 238U in the study area. This may be due to uranium mobility [34]. Therefore, uranium migrates sequentially in the study area. On the other hand, the concentration of 232Th, and 40K seem to be homogeneously distributed all sampling points. The high activity concentration of 238U and 232Th (87 ± 3 Bq kg−1 and 129 ± 3 Bq kg−1) was found at Thazhankuppam Beach (S2), while 40K activity (692 ± 10 Bq kg−1) was also high at Thiruvottiyur Beach (S3). These high activity concentrations are mainly due to their geochemical origin and they could be related to geological conditions or other factors such as rainfall, temperature, and human activities in the studied location [35]. At the same time, the existence of black sands (Fig. 2) found in these locations, which are rich in a phosphate mineral that is predominantly reddish-brown in colour and includes rare-earth elements (Ce, La, Nd, Th) PO4, which contains a considerable level of 232Th [36]. The enrichment arises because monazite's specific gravity permits it to concentrate along beaches where lighter elements are washed away [37]. Figure 3 shows the distribution of activity concentration for 238U, 232Th, and 40K in the sediment samples.

Table 2 Complete activity concentrations and radiological parameters associated with radionuclides 238U, 232Th, and 40K
Full size table
Fig. 2
figure 2

The existence of black sands in each location along the study area

Full size image
Fig. 3
figure 3

Box plot showing the distribution of activity concentrations for 238U, 232Th, and 40K in the sediment samples

Full size image

It is clear that the activity concentration of 238U, 232Th, and 40K increased from Kalanji Beach (S1) to Thazhankuppam Beach (S2) in the study area. However, the 238U, 232Th, and 40K concentrations ranged from 22 ± 2 to 87 ± 3 Bq kg−1 with an average value of 50 Bq kg−1, BDL to 129 ± 3 Bq kg−1 with an average value of 32 Bq kg−1, 368 ± 10 to 692 ± 10 Bq kg−1 with an average value of 543 Bq kg−1 respectively. The mean activity concentrations of 238U, 232Th, and 40K in the sediment samples are higher than the world average value [38]. This could be due to weathering of parent rock materials or anthropogenic activities like fishermen, tourists, and effluents from industries. Moreover, the contamination due to these activities may hold radioactive materials which are significantly deposited in the sediment samples.

In addition, the mean activity concentration of the radionuclides is compared with previous studies [6, 20, 39,40,41,42,43,44,45,46,47,48,49,50,51] with other coastal areas across the world and given in Table 3. From this comparison table, in the present study, the mean concentration of 238U is significantly lower than the Penang from Malaysia [46], Preta beach of Brazil [47], while the mean 232Th concentration is higher than Patras Coast of Greece [41], Red Sea from Saudi Arabia [44] and Sudan [45], Xiamen Island from China [6]. Similarly, the activity concentration of 40K is lower than the Southern Coast from Albania [39], Tyrrhenian Sea of Italy [43], and Penang from Malaysia [46]. Hence, the distribution of 238U and 40K is homogeneous and 232Th is inhomogeneous along the coastal area of North Chennai to Pondicherry, India.

Table 3 Comparison of activity concentrations of the present study with similar studies in the world
Full size table

Evaluation of radiological parameters

The standard radiological parameters are essential to assess the potential ecological risk and also human health risks, once the radiation is absorbed by living organisms or people. The significant radiological parameters such as Raeq, DR, AEDE, Hex, ELCR, and AGDE are calculated in the sediments samples, and obtained results are compared with the international recommended value given by UNSCEAR [38] and ICRP [52]. For all calculations carried out in this section, we have used the symbols: AU, ATh, and AK to represent the activity concentrations of 238U, 232Th, and 40K radionuclides respectively.

Radium equivalent activity (Bq kg−1)

In order to express the activity levels of 238U, 232Th, and 40K by a single quantity that takes into consideration the radiation risks associated with them, a common radiological index has been developed based on the presumption 10 Bq kg−1 of 226Ra, 7 Bq kg−1 of 232Th, and 130 Bq kg−1 of 40K produce the same gamma dose rates [53,54,55,56]. The elevated concentrations of radium isotopes in sediments can enhance the high radium equivalent activity (Raeq), which causes harmful effects on marine organisms and human health if they are exposed to gamma radiation over an extended period of time. Also, it can be used to assess whether the radium activity in the sediment is within a safe level or not, as determined by regulatory standards. The Raeq activity is mathematically defined as follows [54],

$${\text{Ra}}_{{{\text{eq}}}} = {\text{A}}_{{\text{U}}} + {1}.{\text{43A}}_{{{\text{Th}}}} + 0.0{\text{77A}}_{{\text{K}}}$$
(2)

The Raeq values are calculated and given in Table 2. The radium equivalent activity (Raeq) in the sediment samples ranges from 68 ± 14 to 319 ± 15 Bq kg−1 with a mean value of 137 Bq kg−1 in sediment samples. As seen from Table 2, the highest Raeq was 319 Bq kg−1 observed at only one location S2 (Thazhankuppam Beach) where sediments are contaminated due to harbour activities. However, the obtained average value seems to be less than the recommended maximum value of 370 Bq kg−1 [28, 57]. Therefore, regular monitoring of radium equivalent activity is essential in these coastal sediments, which can help to ensure the safety of the people who are living in the study area.

γ-absorbed dose rate (DR)

In order to provide a characteristics of external gamma radiation, it is necessary to calculate the absorbed dose rate for sediments above the ground surface in the study area. The conversion coefficients for calculating outdoor absorbed gamma dose rate (DR) in the air per unit activity concentration in Bq kg−1 (dry weight) are 0.92 nGyh−1 for 238U, 1.1 nGyh−1 for 232Th, and 0.0807 nGyh−1 for 40K. The absorbed gamma dose rate was calculated for all locations using Eq. 3 given by UNSCEAR as follows [38, 52].

$${\text{D}}_{{\text{R}}} = 0.{\text{92A}}_{{\text{U}}} + {1}.{\text{1A}}_{{{\text{Th}}}} + 0.0{8}0{\text{7A}}_{{\text{K}}}$$
(3)

The calculated values of the absorbed dose rate for sediments are given in Table 2. From obtained results of Table 2, the lowest dose rate was 68 ± 6 nGyh−1 for the sediments of Koovathur Beach represented by sample S16, while the highest dose rate was 272 ± 6 nGyh−1 for the sediments of Thazhankuppam Beach represented by sample S2. The mean value was 124 nGyh−1 which is greater than the global average value of 59 nGyh−1 [38]. In addition to that, all studied locations possess a higher value of gamma dose rate and it may be due to the significant amount of uranium and thorium in the samples. These high levels of gamma absorbed dose rate in sediment can pose a significant health risk to the environment and human population living near or working with the sediments. Figure 4. Shows the variation of absorbed dose rate in the samples.

Fig. 4
figure 4

Variation of the gamma absorbed dose rate in the study area

Full size image

Annual effective dose equivalent (AEDE)

An outdoor annual effective dose equivalent was calculated for sediment samples by using the conversion factor of 0.7 Sv Gy−1 and outdoor occupancy factor (20%) to convert the total absorbed gamma dose rate in the air to the human effective dose equivalent [58]. The annual effective dose equivalent in the unit of (mSv) was calculated by the following equation:

$${\text{AEDE}}_{{{\text{out}}}} \left( {{\text{mSv}}} \right) = {\text{D}}_{{\text{R}}} \left( {{\text{nGyh}}^{{ - {1}}} } \right) \times {876}0\;{\text{h}} \times 0.{2} \times 0.{7}\;{\text{SvGy}}^{{ - {1}}} \times {1}0^{{{-}{6}}}$$
(4)

The mean computed annual effective dose equivalent value is 0.15 mSv for collected samples, which is greater than the world average value of 0.07 mSv [38]. From obtained results, it is clearly indicated that the AEDE of all sampling locations has less than the recommended limit of 1 mSv of radiation exposure to the population [52]. Hence the emission of gamma radiation and exposure to the human population in the study area are insignificant due to the presence of 238U, 232Th, and 40K.

External hazard index (Hex)

The external hazard index was proposed by Krieger [59] to ensure the harmful effects on marine biota and human populations due to natural radionuclides in the sediment samples. Therefore, the risk of gamma radiation from the environment and health effects from the activity concentration of radionuclides present in the sediment sample was assessed using the external hazard index which is calculated using the following equation [60, 61],

$${\text{H}}_{{{\text{ex}}}} = \frac{{{\text{A}}_{{\text{U}}} }}{{370{ }\left( {{\text{Bq kg}}^{ - 1} } \right)}} + \frac{{{\text{A}}_{{{\text{Th}}}} }}{{259{ }\left( {{\text{Bq kg}}^{ - 1} } \right)}} + \frac{{{\text{A}}_{{\text{K}}} }}{{4810{ }\left( {{\text{Bq kg}}^{ - 1} } \right)}}$$
(5)

According to the UNSCEAR report [38], the Hex value of all the sampling points must be less than unity for safe from radiation exposure of humans. The calculated external hazard index for these sediment samples is given in Table 2. It is noticed that Hex value of sample S1(Kalanji Beach) is 0.75 ± 0.02 and sample S2 (Thazhankuppam Beach) is 0.86 ± 0.02. These two locations show the nearest to the recommended limit of unity due to significant activity concentration of 238U and 232Th in these samples. However, the lowest value 0.18 of Hex was noted at the sampling location of Koovathur Beach (S16), while the highest value 0.86 was observed at the sampling location of Thazhankuppam Beach (S2), with a mean value of 0.37. This mean value is less than the recommended limit of 1. Hence, the sediments in the study area may do not harm to biota, fishermen, tourists and people who are living in this region.

Excess lifetime cancer risk (ELCR)

Humans have a risk of getting cancer due to long-term exposure of even low doses of ionizing radiation from natural radionuclides in the sediment samples. The risk of cancer increases as the dose of radiation increases [62]. According to the National Cancer Institute report [63], 33% of the population will get some type of cancer during any stage of their lifetime. Hence, the additional risk parameter ELCR was calculated using Eq. 6 [64],

$${\text{ELCR}}_{{\text{outdoor }}} = {\text{AEDE}}_{{{\text{outdoor}}}} { } \times {\text{LE }} \times {\text{RF}}$$
(6)

In this equation, LE stands for average life expectancy (70 years), and RF stands for risk factors such as deadly cancer risk (per sievert). In the case of stochastic effects, the International Commission on Radiological Protection (ICRP) recommends RF level is 0.05 to the public [52]. As seen from Table 2, the ELCR value of S1(1.01 ×10−3 ± 0.03 mSvy−1) and S2 (1.17 ×10−3 ± 0.03 mSvy−1) are nearly 4 times greater than the world average value of 0.29 ×10−3 mSvy−1 due to presence of a high concentration of 238U, and 232Th in the sediment samples. It is observed that a similar world average value is found in only one sample S16 and the other samples show nearly two times greater than the world average value. This may be due to the deposition of heavy minerals-rich black sands in the study area (Fig. 2).

Though, this study reveals that the minimum value was found in sample S16 (Koovathur Beach) and the maximum value was found in S2 (Thazhankuppam Beach) with a mean value of 0.53 ×10−3 mSvy−1 as shown in Table 2. This mean value is greater than the world average value but less than the high background radiation area (HBRA) of Kerala reported by Ramasamy et al. [8] and more or less equal to Kirklareli region, Turkey reported by Taskin et al. [65]. Therefore, continuous assessment of cancer risk is necessary for the study area to protect the human population from gamma-ray exposure due to natural radioactivity.

Annual gonadal dose equivalent (AGDE)

A high (AGDE) concentration in the gonads causes negative health problems, so it is appropriate to measure the annual gonadal dose (AGDE) concentration in 238U, 232Th, and 40K. UNSCEAR [38] considers the thyroid, lungs, female breast, gonads, active bone marrow, and bone surface cells to be the organs of interest. Hence, the annual gonadal dose equivalent (AGDE, µSv y−1) was calculated due to activity concentration of 238U, 232Th, and 40K using the following equation,

$${\text{AGDE}}\left( {\mu {\text{Sv}}\;{\text{y}}^{{ - {1}}} } \right) = {3}.0{\text{9A}}_{{\text{U}}} + {4}.{\text{18A}}_{{{\text{Th}}}} + 0.{\text{314A}}_{{\text{K}}}$$
(7)

It is particularly noted that the AGDE value of S14 and S16 is less than the world average value of 300 μSv y−1 due to the absence of 232Th in these samples. This is clearly indicating that there is not much black sand deposition in these two locations. On the other hand, a high value of AGDE is found in samples S1 and S2 due to the significant concentration of uranium and thorium. However, the AGDE value for all other samples shows slightly greater than the world average value [62]. These elevated levels of AGDE are also known to affect the bone marrow that produces red blood cells. This may lead to cancer of the blood called leukaemia, which is often fatal. Therefore, it is necessary to study the various biological effects of ionizing radiation due to natural radionuclides in the sediment samples.

Pearson’s correlation coefficient analysis

The activity concentration of radionuclides can be affected by a number of factors such as the geological composition of the sediments, the composition of minerals, and the chemical behaviour of radionuclides. Pearson correlation coefficient matrix between radionuclides (238U, 232Th, 40K) and radiological parameters (Raeq, DR, AEDE, Hex, ELCR, and AGDE) in sediments was measured and given in Table 4. In this study, the terms strong, moderate, and weak correlation coefficients refer to > 0.70, 0.70–0.50, and 0.50–0.36 respectively were identified at p < 0.05 for samples (n = 21).

Table 4 Pearson correlation matrix among the variables in the samples
Full size table

The obtained results show a moderate correlation between 232Th and 238U in the samples with an ‘r’-value of 0.527 (Fig. 5). This indicates that the 232Th decay series and the 238U decay series have a significant link and co-occur in sediment samples. 40K has a very weak correlation between 238U and 232Th (r = 0.039 and r = 0.027) respectively. Hence it indicates that 40K occurs in different decay series in nature. As seen from Table 4, all the radiological parameters strongly correlate with 238U and 232Th whereas a weak correlation with 40K. This clearly suggested that natural radioactivity in the beach sediments is due to presence of uranium and thorium. The contribution of 40K is insignificant in the samples.

Fig. 5
figure 5

Correlation between 232Th and 238U activity in the samples

Full size image

Conclusion

Determination of activity concentration of 238U, 232Th, 40K and the associated radiation hazards were carried out for sediment samples collected from North Chennai to Pondicherry coastal area, India using gamma-ray spectrometry. From obtained results, the main observation is high activity concentration and significant radiological hazards are found in samples S1 (Kalanji Beach) and S2 (Thazhankuppam Beach) due to the deposition of black sands. Also, the mean activity of 238U, 232Th, and 40K is greater than the world average value. This may be attributed to tourists, fishermen, harbour, and industrial activities. The significant correlation of all the radiological parameters with 238U, and 232Th implies that, exposure of radiation is due to only uranium and thorium. The contribution of 40K is insignificant in the study area. Also, radium equivalent activity, annual effective dose equivalent, and external hazard index are less than the world-recommended limit for all the studied samples. Hence, the study area doesn’t possess any radiological hazards to human populations.