Sample collection and preparation

The sampling stations along Cauvery River are indicated in Figure 1. Sampling station K1 corresponds to the upper reaches of the river and K14 corresponds to the lower reaches. The soil samples from the river basin were collected following standard procedures.² The collected soil samples were brought to the laboratory and oven dried at 110°C till constant dry weight was obtained. The dried samples were sieved through 250-µm mesh and then stored in a sealed polyvinyl chloride container for 30 days to attain secular equilibrium between radium and its progeny.³

Figure 1.

Cauvery River basin map.

Activity measurement

The activity concentrations of ⁴⁰K, ²²⁶Ra, and ²³²Th in soil were measured using a high-resolution N-type HPGe (NGC 3019, DSG) detector–based gamma spectrometry system. The detector was shielded using thick lead blocks to avoid interference of external gamma radiations. The output of the detector was analyzed using a 16-K multi channel analyzer (MCA-3 series/P7882; FAST ComTec). The spectrometer was calibrated using International Atomic Energy Agency (IAEA) standard reference materials. The standards used were RG-U, RG-Th, and RG-K for uranium, thorium, and potassium, respectively. The gamma spectrum was obtained with a counting period of 20 000 seconds. The peaks corresponding to 1.46 MeV (⁴⁰K), 609.31 keV (²¹⁴Bi), and 911.07 keV (²²³Ac) were considered for evaluating the activity levels of ⁴⁰K, ²²⁶Ra, and ²³²Th, respectively.⁴

Radon exhalation rate

The radon exhalation rate in the soil samples of the Cauvery River was determined by “Sealed Can Technique” using SSNT detectors. About 100 g of the dried and sieved (250 µm) soil sample was taken in each “Can” (diameter: 7.0 cm and height: 10.5 cm), and LR-115 Type II SSNT detector (3 cm × 3 cm) was fixed on the top inside of each “Can.”⁵ The “Can” was kept airtight for 4 hours to allow the radon and its progeny to reach equilibrium. The equilibrium activity of the emergent radon was calculated based on the geometry of the “Can” and the time of exposure. The LR-115 film inside the can was exposed to radon for 90 days. The films (detectors) were removed and then etched in 2.5 N NaOH at 60°C ± 1°C for a period of 60 minutes in an etching bath at constant temperature to enlarge the tracks produced from the alpha particles emitted from the decay of radon.⁶ The background track density of the detector was measured using unexposed detectors under the same etching condition. The enlarged alpha tracks were counted using spark counter. Precounting was done at 900 V and finally track density was determined at 450 V, in the plateau region of the counter.

The effective radium concentration C_Ra can be calculated using the following formula⁷:

where ρ is the track density (0.056 tracks cm⁻² d⁻¹ [Bq m⁻¹]), T_e is the effective exposure time in an hour, h is the distance between detector films and surface of the specimen sample, M is the mass of the sample, A is the area of cross section of the cylindrical can, and k is the sensitivity factor and its value is k = 0.0312 tracks m⁻² d⁻¹ Bq⁻¹ m⁻³.

Surface exhalation rate “E_A” is obtained from the following expression⁸:

This formula is also modified to calculate the mass exhalation rate “E_M”:

where E_A is measured in Bq m⁻² h⁻¹ and E_M in Bq kg⁻¹ h⁻¹, V is the effective volume of can (m³), C is the integrated radon exposure as measured by LR-115 solid-state nuclear track detectors (Bq m⁻³ h), T is the exposure time (h), l is the decay constant for radon (h⁻¹), A is the area of the can (m²), and M is mass of the sample.

Activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K

The activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in soil samples of Cauvery River basin were measured using HPGe gamma-ray spectrometer, and the results are presented in Table 1. The measured activity ranged from 18 ± 1 to 65 ± 3 Bq kg⁻¹ for ²²⁶Ra, 3 ± 0.5 to 47 ± 2 Bq kg⁻¹ for ²³²Th, and 14 ± 1 to 531 ± 7 Bq kg⁻¹ for ⁴⁰K. The measured activity of the radionuclides varied from location to location. These variations are mostly due to the changes in local geology. The physical and chemical sorting processes also contributed to the variation of radionuclides in the soil from different locations.⁹ The mean activity of ⁴⁰K, ²²⁶Ra, and ²³²Th is 182 ± 4, 34 ± 2, and 19 ± 1 Bq kg⁻¹, respectively. In all the locations, activity concentration is in the order of ⁴⁰K > ²²⁶Ra > ²³²Th; the concentration of ²³²Th is much lower than the ²²⁶Ra and ⁴⁰K. The ⁴⁰K activity concentration dominates over ²²⁶Ra and ²³²Th activity. The activity concentrations of ⁴⁰K were found to be lower in upper reaches of the river and higher values were observed in lower reaches. The ⁴⁰K in the upper reaches decreases by leaching process and may also be diluted with increase in organic matter and soil water contents.^10,11 In all the locations, the average concentration of ²²⁶Ra, ²³²Th, and ⁴⁰K is lower than the world average values (world average of ²²⁶Ra, ²³²Th, and ⁴⁰K is 35, 30, and 400 Bq kg⁻¹, respectively, United Nations Scientific Committee on the Effects of Atomic Radiation [UNSCEAR]).¹ The average concentration of ²²⁶Ra is higher than the concentration of ²³²Th; ⁴⁰K is lower than the Indian average values (29, 64, and 400 Bq kg⁻¹ for ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively, UNSCEAR, 2000).

Table 1.

Activity of ²²⁶Ra, ²³²Th, ⁴⁰K in Cauvery River soil samples.

The measured activity concentrations of Cauvery River soil samples were compared with other river environs, which is presented in Table 2. The activity of ²²⁶Ra in Cauvery River basin soil sample is less than the activity of ²²⁶Ra in Netravathi River of Karnataka and is also below the world average value.¹ The activity concentration of Kallada River basin shows (343.4, 60.3, and 98.1 Bq kg⁻¹ for ⁴⁰K, ²²⁶Ra, and ²³²Th, respectively) higher activity than Cauvery River activity concentration.⁹ The activity concentration of Cauvery River is comparable with Wei River of China and Büyük Menderes River of Turkey.^13,17 The values are comparable with Chao Phraya, Thailand (364.49 and 42.95 Bq kg⁻¹ for ⁴⁰K and ²³²Th, respectively); Dez, Iran (289, 25, and 33 Bq kg⁻¹ for ⁴⁰K, ²²⁶Ra, and ²³²Th, respectively); and Pra River, Ghana.^14,16,18 The activity concentration of ⁴⁰K was found to be higher in all river environments compared with this study.

Table 2.

Comparison of ⁴⁰K, ²²⁶Ra, and ²³²Th activity in soil with other river environs.

The statistical analysis of ²²⁶Ra, ²³²Th, and ⁴⁰K activity is shown in Table 3. Figure 2 shows the frequency distribution of natural radionuclides in soil samples of Cauvery River. The kurtosis values which are positive indicate peaked distribution and negative kurtosis indicates the flat distribution. The skewness for a normal distribution is 0 and indicates the values are near to the mean value. Positive skewness indicates the few values are above the median values. The positive skewness and kurtosis values are in good agreement with the theoretical frequency distributions. The skewness and kurtosis values which are near to 0 indicate that there is no extreme deviation from the mean value.

Table 3.

Statistical data for the activities of ²²⁶Ra, ²³²Th, and ⁴⁰K in Cauvery River soils.

Figure 2.

Frequency distribution of ²²⁶Ra, ²³²Th, and ⁴⁰K in monsoon soils of Cauvery.

To assess the radiologic risk of the river basin soil, various radiological hazard indices were computed and the results are shown in Table 4. The hazard indices were calculated using the following equations.

Table 4.

Radiological hazard indices of soil sample.

Absorbed dose rate (D)

To calculate the dose rate in the air using the activity concentrations and conversion factors of ²²⁶Ra, ²³²Th, and ⁴⁰K, the absorbed dose rate (D in nGy h⁻¹) was calculated using the following equation^19,20:

where C_Ra, C_Th, and C_K are the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively. The mean dose rate was found to be 35 nGy h⁻¹, which is lower than the world (57 nGy h⁻¹) and Indian (56 nGy h⁻¹) average calculated by UNSCEAR.¹

Annual effective equivalent dose

The annual effective dose rate for indoor and outdoor in units of µSv y⁻¹ was calculated using the following formula²¹:

where D is the dose rate. The mean annual effective equivalent dose (AEED) for outdoor and indoor was 43 and 170 µSv y⁻¹, respectively. The mean value of AEED is lower than the recommended value (outdoor: 70 µSv y⁻¹ and indoor: 450 µSv y⁻¹).

Radium equivalent activity (Ra_eq)

The radium equivalent is a single index or number which describes the total gamma output from the combination of ²²⁶Ra, ²³²Th, and ⁴⁰K radionuclides in the sample from individual location. The radium equivalent (Bq kg⁻¹) was calculated using the following equation (UNSCEAR, 2010)²²:

where C_Ra, C_Th, and C_K are the activity of ²²⁶Ra, ²³²Th, and ⁴⁰K in Bq kg⁻¹, respectively. The mean radium equivalent value was found to be 75.28 Bq kg⁻¹ for soils of Cauvery River basin, which is lower than the safety limit (370 Bq kg⁻¹).

Hazard index

The external and internal external hazard indices were calculated using the following equations^22,23:

where C_Ra, C_Th, and C_K are the activity of ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively. The mean external hazard index and internal hazard index values were 0.20 and 0.29, respectively.

Annual gonadal dose equivalent

The UNSCEAR has formulated equations to estimate the dose received by the body organs such as the thyroid, lungs, bone marrow, bone surface cell, and the gonads. The annual gonadal dose equivalent (AGDE) (µSv y⁻¹) was calculated using the following equation²³:

where C_Ra, C_Th, and C_K are the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in Bq kg⁻¹, respectively. The average value of AGDE is 241.94 µSv y⁻¹, which is lower than the world average value of 0.30 mSv y⁻¹.

Excess lifetime cancer risk

The mean value of the estimated excess lifetime cancer risk (ELCR) is 0.149 × 10⁻³, which is lower than the world average 1.45 × 10⁻³.²⁴ The ELCR was estimated using the following equation and is presented in Table 4:

where AEDE is the annual effective dose equivalent, DL is the duration of life (70 years), and RF is the risk factor (Sv⁻¹) fatal cancer risk per sievert. As per the International Commission on Radiological Protection (ICRP) recommendation, the risk factor for stochastic effect to the public is 0.05.

Activity utilization index (I)

In South India, the river soil is used as construction material for plastering. Therefore, the Cauvery River soil was also examined via calculating the activity utilization index (AUI) (I). The AUI (I) was calculated using the following equation²³:

where C_Ra, C_Th, and C_K are the mean activity of ²²⁶Ra, ²³²Th, and ⁴⁰K in Bq kg⁻¹ in the soil and f_Ra, f_Th, and f_K are the fractional contributions to the total dose rate of ²²⁶Ra, ²³²Th, and ⁴⁰K. The mean AUI is 0.55, which is below the recommended safety limit (<2). Therefore, the soils may be used as construction material.

Radon exhalation measurement

The radium activity and radon exhalation rates in the soil samples were measured using “Can technique” and the results are given in Table 5. The activity concentration of effective radium content (C_Ra) ranged from 25.7 to 394.02 mBq kg⁻¹, radon surface exhalation (E_S) ranged from 63.61 to 973.23 mBq m⁻¹ h⁻¹, radon mass exhalation (E_M) ranged from 23.45 to 359.49 mBq kg⁻¹ h⁻¹, and radon activity ranged from 26.39 to 403 Bq m⁻³, respectively. It can be observed from the results that the radon exhalation rate varied appreciably from one sample to another. The high radium concentration in soil was found at locations K5 and K9. The mass exhalation rate and surface exhalation rate in soil samples were found to be high at K5 and K9. The variation in exhalation depends on the type of soil, emanation factor of radon from them, radium content, and diffusion coefficient of radon.

Table 5.

Radon activity and radon exhalation rate in Cauvery River soil samples.