Radiation Protection and Environment

ORIGINAL ARTICLE
Year
: 2018  |  Volume : 41  |  Issue : 1  |  Page : 3--7

Concentration of uranium in groundwater and its correlation with the gamma activity of primordial radionuclides in the bedrock samples: A study from northeastern part of Bengaluru city, India


Gladys Mathews1, N Nagaiah1, MB Karthik Kumar1, MR Ambika1, N Karunakara2, BC Prabhakar3,  
1 Department of Physics, Bangalore University, Bengaluru, Karnataka, India
2 Centre for Advanced Research in Environmental Radioactivity (CARER), Mangalore University, Mangalore, Karnataka, India
3 Department of Geology, Bangalore University, Bengaluru, Karnataka, India

Correspondence Address:
Dr. N Nagaiah
Department of Physics, Bangalore University, Bengaluru - 560 056, Karnataka
India

Abstract

The present investigation aims to study the incorporation of uranium from the bedrock to groundwater through leaching. In view of this, rock powder samples were collected in the form of slurry from the bedrock of freshly drilled borewells of the study area. The rock powder and the supernatant were separated. The gamma activity concentrations of primordial radionuclides-238U (226Ra), 232Th, and 40K in the rock powder samples were measured using HP-Ge gamma-ray spectrometer. The supernatant was analyzed for the concentration of Unatusing laser-induced fluorimeter. A strong positive correlation was observed between the concentration of uranium in the rock powder and the corresponding concentration in the water samples indicating the possible leaching of uranium from the bedrock to groundwater under favorable conditions.



How to cite this article:
Mathews G, Nagaiah N, Karthik Kumar M B, Ambika M R, Karunakara N, Prabhakar B C. Concentration of uranium in groundwater and its correlation with the gamma activity of primordial radionuclides in the bedrock samples: A study from northeastern part of Bengaluru city, India.Radiat Prot Environ 2018;41:3-7


How to cite this URL:
Mathews G, Nagaiah N, Karthik Kumar M B, Ambika M R, Karunakara N, Prabhakar B C. Concentration of uranium in groundwater and its correlation with the gamma activity of primordial radionuclides in the bedrock samples: A study from northeastern part of Bengaluru city, India. Radiat Prot Environ [serial online] 2018 [cited 2021 Apr 13 ];41:3-7
Available from: https://www.rpe.org.in/text.asp?2018/41/1/3/233642


Full Text

 Introduction



Natural radiations originate from many sources, including more than 60 naturally occurring radioactive materials found in rock, soil, water, and air. The maximum contributions to these invisible radiations originate from the decay series of 238U and 232Th and the singly occurring isotopes like 40K.[1] The spatial distributions of these radionuclides show a varying pattern which depends on the nature of the parent rock and soil. Radionuclides can transfer from soil to the human system through various pathways, out of which ingestion plays a major role. Groundwater when it reacts with the soil and bedrock, the latter releases the dissolved components which in turn depends on the geochemical composition of the soil and the rock, chemical composition of water, degree of weathering of rock, redox conditions, and the residence time of groundwater in the soil and the bedrock.[2] Hence, one can presume that the activity concentration of natural radionuclides in groundwater has a direct bearing with the concentrations of uranium and thorium and their decay products in the soil and the bedrock.[3]

The present study has a two-fold approach. In the first part, bedrock samples collected from the borewells of the study area were analyzed for the gamma activity of the primordial radionuclides (238U (226Ra),232Th, and 40K) and the mass elemental concentration of these elements was estimated. In the second part, the concentration of natural uranium in the first spring up water of the same freshly dug borewells was measured. To find the dependence of the concentration of Unat in the water samples on the mass elemental concentration of uranium (238U) in the bedrock, the correlation study was carried out.

Study area

The study area [Figure 1] of the present investigation covers an area of approximately 20 km 2 in the northeast region of Bengaluru urban district. As this area is a newly developed residential layout, treated water supply system of the Bengaluru City Corporation is not accessible to the residents and hence borewells are dug and people rely on the groundwater for drinking and other purposes.{Figure 1}

Experimental Methods

In the present study, samples were collected in the form of slurry from the bedrock of the freshly drilled borewells during the time of drilling along with the first spring up water. The slurry was allowed to settle completely. The settled rock powder was used to measure the gamma activity of 226Ra (238U),232Th, and 40K whereas the supernatant was used for measuring the concentration of uranium. The depth of the borewells ranged from 183 to 381 m bgl (meters below ground level).

Gamma activity of radionuclides in bedrock samples

The rock powder samples obtained from the residue of the slurry were dried in the sunlight to remove the water content and then kept in hot air oven at 110°C overnight. The dried samples were then filled in 250 ml airtight plastic containers having threaded lids. The lids were tightened so as to avoid the leakage of Rn gas. The samples were then stored for over a month to achieve the radioactive equilibrium between the parent and the daughter nuclides.[4] Since it is difficult to measure the concentration of 238U directly, as the gamma lines of this element are weak, the gamma lines of energy 609keV and 1120 keV of 214Bi were used to measure the same by assuming secular equilibrium between the parent (238U) and the daughter nuclide (226Ra). In the present study, as the samples were collected from the highly undisturbed environment, the above consideration is more valid. To determine the gamma activities of (226Ra),232Th and 40K in the natural environmental samples, the hyperpure germanium (HP-Ge) gamma ray spectrometer is used. The HP-Ge gamma ray spectrometer used in the current study is an n-type closed end coaxial detector (Model GR 4021, Canberra USA) with a relative efficiency of 42%. Before the measurement of activity of radionuclides in the samples, the HP-Ge gamma ray spectrometer was calibrated for the energy and efficiency. Because of the low activity of the samples, the data were accumulated in each case for a period of 60000 s. The background count was also recorded for the same counting period. The gamma lines of 911 keV (228Ac) and 2618 keV (208Tl) were used to determine the activity of 232Th. The activity of 40K was determined from its gamma line of energy 1460.8 keV. The activity (Bq/kg) of each radionuclide was calculated using the standard relation (IAEA, 1989).[5]

[INLINE:1]

where,

S is the Compton corrected background subtracted counts per se cond under the photopeak used,

SD is the standard deviation,

E is the efficiency of the detector for the corresponding gamma energy (%),

W is the weight of the sample taken for the analysis (g) and

A is the branching intensity of gamma ray energy (%).

Mass concentration of radioisotopes in the bedrock samples

The mass concentration of uranium (in ppm) was calculated from the activity concentration of 238U using the following equation [6]

[INLINE:2]

Where,

FE is the fraction of element in the sample (ppm),

ME is the atomic mass (kg/mol),

NA is the Avagadro's number (6.023 × 1023 atom/mol),

fE is the fractional atomic abundance in nature,

T½ is the halflife of the radio nuclide (s),

C is a constant with a value 106 for 238U (226Ra) and 232Th for concentration in ppm and 102 for concentration of 40K(%),

AE is the measured activity of the radionuclide (Bq/kg).

Measurement of uranium concentration in water samples

The water samples (supernatant) drained off from the slurry were filtered through Whatman 42 filter paper. Fluran (1 mL) was added to 6 mL of each water sample and analyzed for the concentration of uranium using pre-calibrated laser-induced flourimeter [7] having a minimum detection limit of 0.2 ppb.

Annual effective dose

The annual effective dose (μSv/y) due to the ingestion of uranium through drinking water was calculated as the product of activity concentration (Bq/L) of the element in water, the annual intake of water (L/y), and the dose conversion factor (Sv/Bq). In the present study, the annual intake of water was taken as 730 L/y, at the rate of 2 L/day,[8] and the dose conversion factor as 4.62 × 10−8 Sv/Bq obtained as the average of the dose coefficients for 234U,235U, and 238U isotopes based on ICRP publications.[9]

 Results and Discussion



The results obtained in the present investigation reveal that the gamma activity concentrations of 238U (226Ra) in the rock powder samples is found to vary from 4.8 ± 0.5 – 31.9 ± 1.2 Bq/kg with a mean of 16.04 ± 0.82 Bq/kg whereas that of 232Th and 40K are found to be in the range 13.2 ± 0.6–82 ± 1.4 Bq/kg and 499.1 ± 8.3–783.1 ± 10.3 Bq/kg, respectively, and are shown in [Table 1].{Table 1}

The observed values are well within the global range [10] for 238U (226Ra). Three samples (S8, S10 and S11) show higher concentration of 232Th values compared to the global average. The average gamma activity of the 40K (669 ± 9.8 Bq/kg) is high compared to the global average of 420 Bq/kg. Among all the samples, sample no. S12 exhibits the lower activity of all the three radionuclides, whereas sample no. S8 shows the highest activity for 238U (226Ra) and 232Th with quite high value (760.2 ± 10.4 Bq/kg) for 40K. The gamma activity of 232Th is considerably large compared to that of 238U (226Ra) in all the samples and this may be due to the fact that uranium, which is the parent nuclide of radium is more susceptible to solubility compared to thorium which is less soluble and hence adsorbed to the rock system.[11]

The correlation analysis (Pearson correlation coefficient) between the gamma activity of the three radionuclides was carried out, and the positive and strong correlation coefficient (r = 0.66, 0.57, and 0.56 for 232Th and 226Ra,40K and 226Ra and 40K and 232Th, respectively) obtained among the three radionuclides indicate that the rock samples belong to a bedrock system that are geochemically coherent.[12]

The mass concentration of 238U,232Th, and 40K(%) calculated using Eq (2) are presented in [Table 2]. The concentration of uranium (ppb) in the water samples (obtained from the supernatant) measured using laser fluorimeter and the estimated annual effective dose (μSv/y) is also included along with the concentration of uranium in the water samples collected from the same borewells when they are put into regular use.{Table 2}

In the present investigation, the mass concentration of 238U in the bedrock samples collected from the study area was found to lie in the range 0.39 ± 0.04–2.58 ± 0.09 with a mean of 1.3 ± 0.07 which is well within the average concentration of uranium (2.64 ppm) in the earth's crust.[13] It can also be seen from the table that the sample S8 exhibits the highest mass concentration of uranium in the rock powder sample (2.58 ppm) as well as in the water sample obtained from the slurry (172.4 ppb). A similar result is noticed in the sample S6. The sample S12 having the least mass elemental concentration of uranium (0.39 ± 0.04 ppm) exhibits the lowest concentration of uranium in water samples (1.0 ppb) among all the samples. Therefore, to understand the possible dependence of uranium concentration in water samples on the concentration of the same in the bedrock system (source), the correlation study between the concentration of uranium in the collected rock powder samples and its concentration in the corresponding water samples was performed and is shown graphically in [Figure 2].{Figure 2}

The figure clearly indicates that there is a strong positive correlation (r = 0.87) between concentration of 238U in rock powder samples and the concentration of uranium in water samples. This certainly shows the possibility of leaching of uranium from the rock system into the water body. The observed correlation between the concentrations of uranium in water to that in the rock powder has again been confirmed by measuring the concentration of uranium in the water samples collected from the same borewells after they were put into domestic use. It is also observed that the concentration of uranium in the water samples collected initially (supernatant) and when they are in regular use does not show much variation [Table 2].

The leaching of uranium from the rock system to the water body can be understood as follows. The flow of groundwater to a drilled well in crystalline rock normally follows through the fracture zones and other rock structures such as rock contacts.[14] Due to the hydro geochemical processes occurring in the deep subsurface, uranium and its decay products such as radium and radon may be enriched on the surface of the fractures.[15] Uranium in rocks normally occurs in tetravalent (immobile) state. If the surrounding environment is oxidizing, uranium is easily oxidized to the more mobile hexavalent state. The leached uranium gets into solution and is transported together with groundwater.[16] Under chemically reducing conditions, the dissolved uranium precipitates out of the solution. Thus, the observed trend of variation in the concentration of uranium in the groundwater samples can be correlated mainly with that in the bedrock system.

The typical concentration of uranium in granitic rocks is 3–5 ppm whereas in sedimentary rocks, it is 2–3 ppm. The observed high elemental concentration of uranium in few rock powder samples could suggest pockets of relatively higher concentration in the bedrocks. The permissible limit of concentration of uranium in drinking water as set by the World Health Organization is 30 ppb and the annual effective dose due to its ingestion is 100 μSv/y,[8] whereas the Atomic Energy Regulatory Board, India has set 60 ppb as the safe limit for the Indian environment.[17] Thus, in the present investigation, the concentration of uranium in two water samples collected from the borewells (S6 and S8) is much higher compared to these permissible limits.

 Conclusions



The data obtained in the present study reveal that the water originating from borewells drilled in uranium rich bedrock has a high concentration of uranium due to the leaching of the element under favorable conditions. A positive correlation (r = 0.87) observed between the concentration of 238U in rock powder samples and that of Unat in water samples collected from the same borewells substantiates this fact. Although the study area covers a ground area of about 20 km 2, the mass concentration of uranium in the rock powder samples shows considerably wide range (0.39 ± 0.04–2.58 ± 0.09 ppm) with an average of 1.3 ± 0.07 ppm. This is well within the world average of 2.64 ppm for concentration of uranium in the earth's crust. The concentration of uranium in the water samples lies in the range 1.0 ± 0.01–172.4 ± 0.03 ppb with an average of 40.4 ppb. This corresponds to the annual effective dose of 0.82–147.34 μSv/y with a mean of 34.54 μSv/y. The exact reason for the wide range of concentration of uranium in the water samples collected from the borewells of the study area require a better understanding of the factors such as the degree of leaching, the properties of the rock, the flow path, fracture system, pH, transport mechanism, and many others which influence the concentration of the element.

Acknowledgments

The authors are thankful to the residents of the study area for their cooperation during the sample collection. One of the authors (Gladys Mathews) also thank the University Grants Commission for granting permission under Faculty development Program to pursue this research work.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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