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| ORIGINAL ARTICLE |
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| Year : 2022 | Volume
: 45
| Issue : 3 | Page : 170-182 |
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Assessment of environmental radioactivity and groundwater quality around Tummalapalle uranium mining site, Andhra Pradesh, India
Barendra Kumar Rana1, SK Jha1, Samim Molla2, MR Dhumale2, MS Kulkarni1
1 Homi Bhabha National Institute; Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
| Date of Submission | 25-Feb-2023 |
| Date of Decision | 09-Apr-2023 |
| Date of Acceptance | 10-Apr-2023 |
| Date of Web Publication | 18-May-2023 |
Correspondence Address:
Barendra Kumar Rana
Homi Bhabha National Institute, Mumbai . 400 094, Maharashtra, India. Health Physics Division, Bhabha Atomic Research Centre, Mumbai - 400 085, Maharashtra
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/rpe.rpe_10_23
A comprehensive background radiological status in the surrounding environment of the Tummalapalle U mining and processing facilities was evaluated. Radioactivity in soil and rock was estimated by high-resolution gamma spectroscopy with a high-purity p-type germanium detector. The external gamma radiation level, outdoor 222Rn concentration, radioactivity in the groundwater, and water quality parameters around the Tummalapalle site were studied. The terrestrial radioactivity in soil was higher than the national and global averages in the study region. Radium equivalent activity (Raeq) in soil and rock was much lower than the safe limit of 370 Bq/kg and therefore safe to use as a construction material. Hydrogeochemical analysis indicated that the groundwater in the study region is neutral to medium basic, oxic, fresh to brackish, and predominantly Ca2+-Mg2+-HCO3− (62.5%) type. The hydrogeochemistry of groundwater was primarily governed by rock-aquifer interaction in this region. The data generated in this study can serve as the baseline for this region to understand the change in environmental conditions, if any, due to prolonged anthropogenic activities.
Keywords: Annual effective dose, gamma-ray spectroscopy, radium equivalent activity, soil radioactivity, specific activities
How to cite this article:
Rana BK, Jha S K, Molla S, Dhumale M R, Kulkarni M S. Assessment of environmental radioactivity and groundwater quality around Tummalapalle uranium mining site, Andhra Pradesh, India. Radiat Prot Environ 2022;45:170-82 |
How to cite this URL:
Rana BK, Jha S K, Molla S, Dhumale M R, Kulkarni M S. Assessment of environmental radioactivity and groundwater quality around Tummalapalle uranium mining site, Andhra Pradesh, India. Radiat Prot Environ [serial online] 2022 [cited 2023 May 28];45:170-82. Available from: https://www.rpe.org.in/text.asp?2022/45/3/170/377228 |
| Introduction | |  |
Exploration of uranium, mining, and its extraction has taken place over the past six decades due to growing global demand and advantages over conventional energy sources.[1],[2] In India, uranium mining and extraction started in 1967 in the Jaduguda region of Jharkhand, and there are currently seven uranium mines (six underground and one open-pit) and two uranium extraction plants in operation using acid-leaching technology. To meet the country's need for additional uranium, the Atomic Mineral Directorate for Exploration and Research (AMD) has identified the Cuddapah Proterozoic Basin in southern India as an important uranium reserve outside the Singhbhum thrust belt of Jharkhand.[3] Uranium mining at the Tummalapalle site started recently for the extraction of uranium by adapting an indigenously developed alkali leaching process from the phosphatic dolomitic ore.[4] As per the recent evaluations, it has been estimated that the uranium mineralization in the strata-bound Vempalle dolostone represents more than 55% of the country's deposits and turned out to be the biggest U deposit in the country (>200,000 tons, with an average mineral content of 0.045% U3O8).[5] During underground mining, a substantial quantity of waste rock with low uranium content is brought to the surface and disposed of or used for its intended purpose. Part of the waste rock along with coarse tailings is mostly used to fill depleted mine workings to prevent their collapse. A significant amount of radionuclides and toxic elements may be released from waste rocks by atmospheric precipitation or wind dispersion of generated dust, which can contribute to the local atmosphere or the hydrosphere. A certain amount of radioactivity to the environment is also contributed by the exhalation of 222Rn and its daughter products from the ore and waste rock; the discharge of gaseous waste to the environment containing long-lived alpha activity, 222Rn, and its daughter products; and the discharge of treated liquid effluents, if any, to the environment.
Because the grade of ore processed in India is very low (0.05% eU3O8), almost all processed ore is generated as a waste called tailings and dumped in an engineering-designed tailings pond for longer-term confinement. Uranium mill tailings may be an environmental concern for several reasons: containing the entire long-lived radioactivity of the ore from which they were produced; containing biotoxic heavy metals and other chemical compounds; being susceptible to leaching, erosion, or collapse under different environmental conditions; having a larger surface area exposed to the natural elements and, therefore, increasing the risk of release of radon flux and its daughter products; dispersal of radioactive and other chemically toxic dust; and interacting with surface water systems. The fate of chemical contaminants and radioactivity that may leach from the tailings pond into the surrounding environment can be governed by the reagents used during the milling process, the characteristics and grade of the ore, the design of the tailings pond, and the local geology of the area in which they are dumped. Almost all of the 226Ra (T1/2: 1600 years) present in the tailings constantly produces 222Rn gas (T1/2: 3.824 days), escaping from the interior of the tailings matrices and being easily carried away from the tailings pond by the wind.[6]
Several studies have been conducted worldwide to estimate natural radioactivity around uranium mining and processing facilities, and the radiation dose has also been estimated for the surrounding population.[7],[8],[9],[10],[11],[12],[13],[14],[15] The uranium mining in Eastern Germany, Mexico, Hungary, Kyrgyzstan, Serbia, and Tajikistan has been shown to have a significant impact on the surrounding environment, and consequently, higher radiation doses have been received by the public.[7],[8],[9],[14],[15] To understand the impact of anthropogenic activities on the environment, it is essential to assess the natural radioactivity in different environmental matrices from a radiation protection perspective and an environmental management, health, and safety point of view around the uranium mining industry. This study can dispel some myths, fears, and misconceptions about radioactivity by establishing radioactivity levels in environmental matrices (soil, rock, groundwater, and ambient air) and groundwater quality. This study can also ensure that mining, milling, and waste management policies are technically best in terms of reducing the impact on the surrounding environment.
| Materials and Methods | | |
Study area
The current study focused on the southwestern portion of the mid-to-late Proterozoic crescent-shaped Cuddapah basin in Andhra Pradesh, India, which contains an unmetamorphosed to a slightly metamorphosed thick arenaceous and argillaceous sedimentary sequence that lies overlying the profound Eparchean unconformity. Under a thick sedimentary layer, highly metamorphosed and deformed late Achaean to early Proterozoic granite gneisses and Dharwarian schists lie on the basin's southwest boundary, the sediments are mostly undisturbed, whereas, on the eastern margin, Dharwarian schists and gneisses surged over them. Carbonate rock formations host the geological formations of this area. Lithologically, this area comprises purple shale, shale, massive limestone, cherty limestone, intraformational conglomerate, and dolostone (uranium-rich).[16] The Cuddapah region is well known for mineralizing barytes, limestone, asbestos, quartz, quartzite, uranium, granite, and limestone slabs (black). Uranium mineralization is hosted by phosphatic dolostone, which is confined between purple shale and dolostone.[17] The major mineral composition of the host rock is reported as 83.2% carbonate, 11.3% quartz and feldspar, 4.3% apatite, and 0.47% pyrite.[18] The climate is considered to be tropical and semi-arid, with more sunny days. The winter season is short, while the summer season is long and hot. The temperature in the region varies from 10.6°C to 40.6°C in winter (November–February) and in summer (March–June) it varies from 15°C to 46°C, respectively.[19] The average annual (2012–2020) rainfall in this area was 377–1379 mm.[20] The least rainfall is observed during January–May, whereas maximum rainfall is observed during August–November. The location map of the Tummalapalle region and the sampling points are shown in [Figure 1]. | Figure 1: Map showing sampling locations for soil and groundwater around the Tummalapalle site
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Experimental procedure
Assessment of radioactivity in soil and rock
Surface soil and rock samples were collected from the surrounding area of the Tummalapalle site and prepared for gamma-ray spectral analysis according to the standard procedure discussed elsewhere.[21],[22] 119 soil samples and 22 rock samples were collected. Unwanted objects such as rocks, pebbles, and other debris were separated from the soil before sample preparation. The moisture was subsequently removed by oven-drying the soil and rock samples for 24 h at 105°C. Finally, the samples were sieved through a 0.2-mm mesh, and approximately 300 g of each sample was sealed completely airtight in a cylindrical polyethylene container (2.6 inches in diameter and 3 inches in height) and stored for about 30 days to achieve radioactive secular equilibrium amongst the daughter products of the 238U and 232Th decay series. Radioactivity in the samples was analyzed using a high-resolution gamma-ray spectrometry system consisting of a carbon fiber (thickness: 0.6 mm) coaxial p-type high-purity germanium (HPGe) detector (energy range: 3-10000KeV) having a relative efficiency of 50% compared to a 3” ×3” NaI (Tl) detector. In terms of full width at half maximum, the resolution of the HPGe system was 2.1 keV at 1332.5 keV gamma energy of the 60Co standard source spaced 25 cm apart from the detector connected to an 8k MCA (PHAST, Electronics Division, BARC). In the energy range of 46.53–2614.53 keV, IAEA reference standards (RGU-1 and RGTh-1) are utilized for the calibration of the energy and efficiency of the detector. The activity of the radionuclide in the 100,000-second-count sample was estimated by the following equation:
where N is the total counts obtained from the sample and the background, B is the detector background reading, T is the spectrum acquisition time (s), γ is the escape probability of particular gamma radiation, ε is the ultimate efficiency of the HPGe detector at given gamma radiation, and w is the mass of the sample (kg). Using the two main gamma energies of 234Th, 63.3 keV (4.8%), and 92.59 keV (5.58%), the activity of 238U was estimated by gamma spectroscopy. The prominent gamma radiation of 1764.5 keV (15.4%) energy was used to estimate 226Ra because its self-attenuation is small compared to other energies. The 46.53 keV line was used to estimate 210Pb in the soil samples. The activity of 232Th was estimated using the gamma radiation emitted by 208Tl, such as 2614.53 keV (35.63%) and 583.2 keV (30.3%). 661.6 keV gamma radiation was utilized to analyze 137Cs. Quality assurance of the data was carried out by parallel analysis of IAEA-certified reference sources, and the estimated accuracy was within 5% of the reported value.
Measurement of gamma radiation level by a survey meter
The gamma radiation levels at different sampling sites were estimated using a NaI-GM tube dual detector-based portable survey meter (Make: Flir, Model: Identifinder R400 NG) in the study area. NaI (Tl) scintillator detector (35 mm × 51 mm) works in the low dose range (0–500 μSv/h) whereas the Geiger-Müller detector works in the relatively higher dose range of 100 μSv/h–10 mSv/h. The instrument generally works in a wide energy range of gamma radiation, starting from 20 keV to 3 MeV, and suitable for measuring low-level environmental gamma radiation. The accuracy of the dose rate as measured by the survey meter is about ±30% with respect to 137Cs source. It has 1024 channel MCA and the typical resolution is ≤8%. Sixty observation sites were selected in Tummalapalle area to evaluate outdoor gamma radiation levels at 1 m above the ground level during this investigation, and most of the sites are located in the populated zone. A representative gamma dose rate was obtained by averaging 10 data points for a particular location. The quality control and quality assurance of the data and performance of the instruments was evaluated by following alternate measurement techniques and periodical calibration of the instruments at the Radiation Safety Standard Division, BARC, Mumbai.
Assessment of 222Rn concentration
The AlphaGuard DF2000, a professional continuous radon monitor from Bertin Instruments, was used to measure radon concentration in the ambient air. The details and layout of the instruments are described elsewhere.[23] These instruments can be operated either in diffusion or flow mode. This device has an active volume of 0.56 L, and its metallic interior has a potential of +750 V with the ground when the instrument is turned on. Faster response, sensitivity (1 count per minute (CPM) at 20 Bq/m3), and a wider range of measurements (2 Bq/m3 to 2 M Bq/m3) are the major advantages of the instruments. In diffusion mode, only 222Rn gas enters the chamber through a glass fiber filter (retention coefficient >99.9%), which retains radon progeny. Filters protect the interior of the chamber from contamination by dust particles and aerosols. This instrument works by discriminating radon and thoron simultaneously and using pulse ionization chamber 3-D alpha spectrometry.[23] The instrument contains a series of environmental sensors to monitor air temperature, barometric pressure, and relative humidity near the instrument. 222Rn measurements at the Tummalapalle site were carried out at a set frequency of 60 min for 24 h.
Collection of groundwater samples and processing
Seventy-two subsurface water samples were taken from bore wells and hand pumps in precleaned plastic carboys all through the postmonsoon period of 2017. About 5 L of water samples from each location were collected, and the pH, temperature, oxidation-reduction potential (ORP), and electric conductivity (EC) of each sample were analyzed without delay, as these parameters can change over time because of exposure to air and microbe activity. The samples were then split into three parts for the study of radionuclides, cations, and anions.
Estimation of 222Rn and 226Ra concentrations in groundwater
222Rn in groundwater was estimated using a radon bubbler and a scintillation cell technique. About 80 mL of groundwater were directly taken into a radon bubbler, and immediately dissolved 222Rn gas was collected into the preevacuated scintillation cell. Three scintillation cells were used for a complete collection of the dissolved radon. After reaching equilibrium between 222Rn and its short-lived progeny (3 h), the scintillation cell was counted for alpha activity with a programmable alpha counter (Make: Polltech, Model: PSI-PCS 2). The 222Rn concentration in the water sample was estimated by using Eq. (1).[24]
Where C is net counts in T seconds of counting time, λ is the decay constant of 222Rn (s−1), E is the efficiency (%) of the scintillation cell, V is the volume of the sample (liters), and t is the delay between sampling and counting time (s). The efficiency of the system is 75%, and the minimum detectable limits for waterborne 222Rn are worked out to be 76 Bq/m3 for counting of 10 min, and 20 Bq/m3 for counting of >100 min.
Two liters of each filtered groundwater sample were reduced to 70 ml by evaporation. To remove any built-in 222Rn, the concentrated sample was placed into a radon bubbler and flushed the sample with fresh air. Eq. (2) was used to determine 226Ra activity in water samples by counting the scintillation cell for 10 min.[12] The activity concentration of 226Ra in the groundwater sample was estimated by the following equation:
Where C is the number of net counts in T seconds of counting time, λ is the decay constant of 222Rn (s−1), E is the scintillation cell's efficiency (percentage), θ is the build-up period (s), V is the sample volume (liters), and t is the time between sampling and counting (s). The method has a 75% efficiency, and the minimum detectable limits for waterborne 226Ra, after allowing for a maximum build-up period, are 4.0 and 0.8 mBq/L for 10 and 100 min counting periods, respectively.
Analysis of groundwater for physicochemical parameters
The physicochemical parameters (pH, temperature, ORP, EC, and TDS) were measured immediately following sample collection using a multiparameter water quality meter (make: Horiba, U53). The instrument was calibrated using NIST-traceable certified reference material (CRM) for pH, ORP, and EC (Make: Eutech Instruments, Singapore). The anion concentrations, such as SO42−, Cl−, PO43−, and HCO3−, were estimated using Bureau of Indian Standards techniques.[25],[26],[27],[28] The Atomic Absorption Spectrometer (AAS) (Make: GBC, Savanta Sigma) was used to estimate the contents of Ca2+, Mg2+, Na+, and K+. The AAS was calibrated for quality control and quality assurance of the data using a NIST-traceable CRM (Make: Inorganic Ventures, USA).
| Results and Discussions | | |
Distribution of radioactivity in rock and soil
Statistical analyses of radioactivity in rock samples and calculated gamma dose rates are presented in [Table 1]. The activity concentrations of 226Ra, 232Th, and 40K in the rock samples varied from 8.4 ± 0.1–663.0 ± 1.2, 3.8 ± 0.1–51.0 ± 0.3, to 38.0 ± 1.3–2074 ± 11 Bq/kg, with a mean of 60.4 ± 137.9, 17.0 ± 15.2, and 494.0 ± 550 Bq/kg, respectively. The Vempalle Formation is composed of purple shale, intra-formation conglomerate, dolostone (uranium-bearing), massive limestone, cherty limestone, and shale. The uneven distribution of radioactivity in rocks may result from the geological formation of the region and the types of rocks found in the Tumalapalle area. Further, the statistical analyses of soil radioactivity and gamma dose rates at different radial distances from the Tummalapalle site are presented in [Table 2], while the frequency distribution of different radionuclides analyzed is presented in [Figure 2]a, [Figure 2]b, [Figure 2]c, [Figure 2]d, [Figure 2]e, [Figure 2]f. The activity concentrations of 238U, 226Ra, 210Pb, 232Th, 40K, and 137Cs in the soil varied from 9 ± 1–106 ± 2, 4 ± 4–230 ± 1, 8 ± 1–308 ± 4, 5 ± 1–294 ± 7, 72 ± 6–1630 ± 9, to 0.1 ± 0.1–4.3 ± 0.1 Bq/kg with a mean of 50 ± 28, 47 ± 27, 90 ± 71, 56 ± 51, 621 ± 361, and 1.35 ± 1.1 Bq/kg, respectively. The mean activities of 238U, 226Ra, 232Th, and 40K were marginally higher than the respective worldwide average values of 33, 32, 45, and 420 Bq/kg.[29] 238U, 226Ra, and 40K specific activities were higher than the respective Indian mean values of 29, 29, and 400 Bq/kg[29] whereas 232Th activity was less than the national average of 64 Bq/kg. The activity concentration of 137Cs in the majority of the soil samples was much lower than that recorded in the Chamba and Dharamshala regions of Himachal Pradesh, India.[30] It was found that 23% of data from 238U and 24% of data from 226Ra showed activity below the national average of 29 Bq/kg, whereas 81% of data from 232Th showed activity of <64 Bq/kg. For 40K, 31% of the data was <400 Bq/kg. 57% of the data for 210Pb indicated the activity was <80 Bq/kg. The 210Pb's higher radioactivity than the parental radionuclides such as 226Ra and 238U is likely attributable to the atmospheric contribution through the wet and dry deposition of the 210Pb.[31] The radioactivity in the soil of the present study was found to be comparable to the results investigated around the Jaduguda uranium mining region of East Singhbhum in Jharkhand.[14] The heterogeneous distribution of radionuclides in soil may be influenced by the local geology and soil formation governed by various rocks in the study region. The weathering of rocks from small hills and rocks covering a part of the surface in the study area has governed soil formation that has resulted in the heterogeneous distribution of radionuclides in soil. Weathering of the parent rocks, which contained a wide range of radioactivity over time, was assumed to affect radioactivity content in the soil. This fact was well understood from the radial distribution pattern of radionuclide concentration in soil from the Tummalapalle site [Table 2]. The radioactivity ratio of 238U/226Ra (AR) in soil may help to understand the secular equilibrium within different radionuclides of the 238U decay chain. The existence of secular equilibrium in the 238U decay chain in soil indicates an AR of 238U/226Ra ≃1, whereas the deviation of the ARs from 1 suggests the occurrence of disequilibrium. The degree of ion mobility of radionuclides and major ions is a cause of disequilibrium in various environmental matrices.[32] The different physical and chemical properties of soil, such as pH, ion exchange capacity, the content of organic matter, percentage of moisture, and particle size, affect the disequilibrium between different radionuclides, particularly in the 238U decay chain present in soil.[29],[33] | Table 1: Specific activity concentration 226Ra, 232Th, and 40K in rock samples
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 | Figure 2: (a) Frequency distribution of 238U activity in soil, (b) Frequency distribution of 226Ra activity in the soil, (c) frequency distribution of 210Pb activity in soil, (d) frequency distribution of 232Th activity in the soil, (e) frequency distribution of 40K activity in soil, (f) frequency distribution of 137Cs activity in the soil
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The ARs of 238U/226Ra and 226Ra/210Pb in soil ranged from 0.52–2.89 to 0.2–3.28, with a respective mean of 1.07 ± 0.46 and 0.85 ± 0.68. The wide variation in ARs of 238U/226Ra and 226Ra/210Pb has indicated the presence of disequilibrium in 238U series radionuclides in soil, which may be attributed to the depletion of either parent or daughter radionuclides and has been governed by the extent of weathering, leaching characteristics of various radionuclides in soil, and parent rocks that contributed towards the formation of the soil. The mean ARs of 238U/226Ra were comparable with the reported values of Kazakhstan (1.06) and China (1.03)[29] and less consistent with the Dalhousie area of northern India (0.83).[34]
Radium equivalent activity of soil and rocks
Uniform radiation doses of different radionuclides were estimated in general terms of radium equivalent radioactivity (Raeq) expressed in units of Bq/kg. Knowing the activities of 226Ra, 232Th, and 40K in soil and rocks, Raeq was estimated according to the following equation.[35]
Here, ARa, ATh, and AK are the soil activities of 226Ra, 232Th, and 40K, respectively. Raeq was determined depending on the assessment concerning 370 Bq/kg of 226Ra, 259 Bq/kg of 232Th, and 4810 Bq/kg of 40K, which gives an equal gamma absorbed dose rate. Raeq varied from 41, with an average value of 175 Bq/kg, to 552 Bq/kg in the local soil. The average value of Raeq was within the safe limit set by the OECD of 370 Bq/kg.[29],[36] Approximately 6% of the data showed Raeq levels above the acceptable value of 370 Bq/kg. In the case of rocks, the Raeq varied from 19, with an average value of 123 Bq/kg, to 816 Bq/kg. Only one rock sample showed Raeq activity above the acceptable limit of 370 Bq/kg. The study result showed that both soil and rocks in the study region are suitable for use as construction materials and for other purposes, with no substantial radiation risks to the residents.
Assessment of gamma absorbed dose rate
The gamma radiation levels at 1 m above the ground can be computed by knowing the activities of 226Ra, 232Th, 40K, and 137Cs in the soil at different sampling sites. The conversion coefficients, such as 0.462 nGy/h per Bq/kg for 238U/226Ra, 0.604 nGy/h per Bq/kg for 232Th, 0.0417 nGy/h per Bq/kg for 40K,[29] and 0.136 nGy/h per Bq/kg for 137Cs,[37] were applied to determine the absorbed dose rate (D) using the equation as follows:
Here, the activities of 226Ra, 232Th, 40K, and 137Cs in soil are represented as ARa, ATh, AK, and ACs, respectively. The dose rate was calculated based on a secular equilibrium in the soil between 238U and 226Ra. According to Karunakara et al.,[38] the daughter products of the 226Ra sub-series of the 238U decay chain account for 98% of the reported gamma dose rate.[38] Hence, the existence of disequilibrium between 226Ra and 238U will not influence the quantification of the absorbed dose rate from soil radioactivity.[38] The absorbed dose rates estimated from the soil ranged from 20 to 245 nGy/h, with an average of 82 nGy/h [Table 2], which is 36% more than the 60 nGy/h global average recorded in the general areas.[29] The contributions of the 238U decay chain, the 232Th decay chain, 40K, and 137Cs in the soil to external gamma radiation levels in the Tummalapalle region were computed at 26.8%, 41.5%, 31.5%, and 0.2%, respectively. However, for Indian soil, these values were 17.7% (238U series), 33.6% (232Th series), and 48.7% (40K),[39] and the global values were 25% (238U series), 45% (232Th series), and 30% (40K), respectively.[40] The absorbed gamma dose rates evaluated from the 238U series, 232Th series, and 40K radionuclides in this study were equivalent to the reported values worldwide.[29] The absorbed dose rates estimated from the rocks ranged from 9 to 381 nGy/h with an average of 59 nGy/h [Table 3], which is comparable to the global average of 60 nGy/h recorded in the general areas.[29] The contributions of the 238U decay chain, the 232Th decay chain, and the 40K, in the rocks to external gamma radiation levels in the Tummalapalle region were computed at 48%, 17%, and 35%, respectively. The contribution of cosmic radiation to the mean global external gamma dose rate has been estimated at 31 nGy/h at sea level,[29] whereas the estimated value is 32 nGy/h for India.[41] Due to the altitude dependence of cosmic radiation, according to the research area, this value was calculated. For different altitudes, the impact of cosmic rays on the external gamma radiation can be corrected by using the following equation.[29] | Table 2: Estimated gamma dose rates and activities of 238U, 226Ra, 210Pb, 232Th, 40K, and 137Cs in soil around the Tummalapalle site
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 | Table 3: Statistical analysis results of major physicochemical parameters and radioactivity content in groundwater
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where kz is the external gamma absorbed dose rate from cosmic radiation at z (km) altitude and k0 is the external dose rate at Indian sea level. The average altitude of the survey area was 350 m, and the mean cosmic rays were estimated to be 33 nGy/h.
Comparison between measured and estimated gamma dose rate
The gamma dose rate estimated by the survey meters is compared with the soil radioactivity collected from the measured locations. The outdoor dose rates varied from 60 to 270 nGy/h with a mean value of 150 ± 35 nGy/h, whereas the gamma dose rates estimated by soil activity fluctuated between 20 and 245 nGy/h with a mean value of 82 ± 47 nGy/h. The exclusion of the cosmic ray component in the assessment of dose from soil radioactivity may explain the large deviation in the average absorbed dose rates estimated by these two techniques. Cosmic radiation increases with altitude, and its value can be nearly doubled at a height of 1500 m above sea level. The study area was about 350 m above mean sea level, and thus the cosmic ray dose rate was estimated to be 33 nGy/h. By adding the cosmic component to the average gamma radiation evaluated from terrestrial radioactivity, the total gamma dose rate was estimated to be 115 nGy/h, which is comparable to the measured dose rate by the radiation survey meters. Therefore, it is evident that soil radioactivity alone underestimates the gamma dose rate compared with the in situ measurement. The investigated terrain is unevenly covered with soil and rocks, which could be a factor for the lack of closeness between estimated gamma dose rates from soil activity and the survey meter. The observed gamma radiation level around the site was found to be comparable to the reported values (range: 87–220 nGy/h−1, and mean: 128 ± 18.5 nGy/h−1) around the mining environment of Narwapahar in the Jaduguda region of Jharkhand.[42]
Radon concentration
The overall outdoor 222Rn concentrations during the winter and summer seasons were found to vary from 5 to 117 and 4–80 Bq/m−3, with respective means of 26 ± 20.4 and 14 ± 8.1 Bq/m−3. The variation in the mean radon concentration during the winter and summer may be attributed to variations in temperature and other meteorological conditions during the measurements. Temperature, humidity, and pressure varied from 17°C to 43°C, 17%–96%, and 968–982 mbar during the winter season, with respective means of 26.6°C ± 7.6°C, 63% ± 26%, and 975 ± 3 mbar. Similarly, during the summer, the temperature, humidity, and pressure ranged from 24°C to 48°C, 8%–96%, and 939–978 mbar, with mean values of 35.1°C ± 4.8°C, 34.4% ± 15.9%, and 968 ± 3.4 mbar, respectively. The day-night measurements show that radon concentrations are higher at night than they are during the day. In the winter season, the mean radon concentration during daytime and nighttime was observed to be 21 ± 18 and 31 ± 20.4 Bq/m−3, respectively. Similarly, in the summer season, the respective means of radon for day and night times were observed to be 12 ± 5.6 and 15 ± 9.8 Bq/m−3. The observed radon concentration around the study region was found to be comparable to the reported values around the mining and milling environment of the Jaduguda region of Jharkhand.[42],[43]
Physico-chemical characteristics of groundwater
The summary of the statistical analysis results for pH, temperature, ORP, EC, TDS, Cl−, SO42−, PO43−, HCO3−, Na+, K+, Ca2+, and Mg2 + are presented in [Table 3]. The pH ranged from 6.6 to 7.8 with a mean of 7.3 (0.3), indicating the groundwater is neutral to slightly basic in the Tummalapalle area. The ORP of the groundwater samples varied from 41 to 252 mV, with a mean of 156 ± 39 mV. All groundwater samples showed positive ORP values, which suggested that the subsurface water was derived from an oxic environment. The EC varied from 0.31 to 2.84 mS/cm and showed a linear relationship between cation and anion concentrations. TDS varied from 200 to 1820 mg/L, having a mean of 779 ± 378 mg/L. Thus, the subsurface water was observed to be fresh to brackish in this region. The HCO3−concentration varied between 135 and 731 mg/L, having a mean of 451 ± 138 mg/L. The concentration of Cl−varied from 9 to 724 mg/L, with a mean of 98 mg/L. The PO43−content in groundwater ranged between 0.02 and 23.3 mg/L, with an average of 1.0 ± 3.4 mg/L. The Ca2+ concentration varied from 22 to 548 mg/L, having a mean of 96 ± 88 mg/L that could have originated from different minerals high in calcium, such as calcite or aragonite, limestone, and dolomite rock. The Mg2+ content in groundwater differed from 12 to 142 mg/L, with an average of 50 ± 26 mg/L. The Na+ and K+ concentrations of the groundwater ranged from 13 to 422 and 1–218 mg/L, with mean values of 83 ± 70 and 8.0 ± 26 mg/L, respectively. The decreasing trend in abundance of cation concentrations was Ca2+ > Na+ > Mg2+ > K+, and in the case of anions, the trend was HCO3− > SO42− > Cl− > PO43− [Table 3]. The distribution of major cation and anion concentrations with min, max, median, first quartile (Q1), and third quartile (Q3) is shown in a box-whisker plot [Figure 3]. | Figure 3: Box-Whisker plot of major cations and anions with other water quality parameters
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The Piper diagram is extensively used in water chemistry to understand the composition and possible source of origin. The Piper diagram is made up of two ternary diagrams for each cation (Ca2+, Mg2+, and Na+ + K+) and anion (Cl−, SO42−, and CO32− +HCO3−), as well as a diamond plot that is a matrix transformation of the two ternary diagrams.[44] The Piper plot [Figure 4] analysis shows that the concentration of alkaline earth metals (Ca2+ and Mg2+) in groundwater was greater than that of alkalis (Na+ and K+) in the study area, although the concentration of weak acids (HCO3− + CO32−) exceeded that of strong acids (Cl− + SO42−). The major hydrogeochemical facies in the groundwater was observed to be Ca-Mg-HCO3 (62.5%). The presence of calcareous rocks, namely, massive dolostone, phosphatic dolostone, purple shale, and chert dolostone, is the reason for the high abundance of Ca2+-Mg2+-HCO3−type in groundwater in this region.[5],[45] The other estimated hydrogeochemical facies were found to be mixed forms of Ca-Na-HCO3 (21%), Na-HCO3 (7%), Ca2+-Mg2+-Cl−-SO42− (4%), and the rest of the samples were Ca-SO42-, Na-Cl, and Ca-Mg-Cl. Sulfate is the second most predominant anion in the water samples. The abundance of sulfate in the subsurface water is due to the existence of sulfide minerals such as pyrite, chalcopyrite, and molybdenite in dolostone rock, which releases sulfate ions upon oxidation.[5]
Hydrogeochemical process controlling the chemistry of groundwater
To understand the various hydrogeochemical processes regulating the subsurface water chemistry in the Tummalapalle area. TDS were plotted on Gibbs diagrams using the ratios (Na+ + K−)/(Na+ + K+ + Ca2+) and Cl−/(Cl− + HCO3−) [Figure 5]a and [Figure 5]b. The Gibbs diagram analysis indicated that the hydrogeochemistry of most of the subsurface water samples was dominated by rock-aquifer interaction in the investigated area. Only a few samples fall under the evaporation dominance category, as observed in the Gibbs diagram [Figure 5a and b]. This can be attributed to the dry environment and low rainfall in this area. | Figure 5: (a) Scatter graph TDS versus (Na+ + K+)/(Na+ + K+ + Ca2+), (b) Scatter graph of TDS versus Cl−/(Cl− + HCO3−, (c) Scatter graph of Cl+ versus Na+, (d) Scatter graph of HCO3− versus Na+, (e) Scatter graph of Ca2+ + Mg2+ versus SO42-+ HCO3- and (f) Scatter graph of Ca2+ versus Na+
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Approximately 70% of the samples have Na+/Cl− >1, indicating that Na+ was released during silicate weathering and/or the ion exchange process.[46] Chloro-alkaline indexes were also studied in the groundwater samples.[47] 78% of the samples indicated both indices were negative, suggesting the ion exchange mechanism is the dominant process accountable for the higher Na+ ion content in the groundwater. The scatter graph between Na+ and Cl−shows the same fact [Figure 5c], where most of the groundwater samples are located below the 1:1 equiline, indicating the predominance of the Na+ ion. The groundwater in the carbonate and silicate weathering zones will be dominated by HCO3− ions.[48] The scatter graph between Na+ and HCO3− revealed that in the majority of the samples, HCO3− predominated [Figure 5d]. The scatter graph between Ca2+ + Mg2+ and SO42− + HCO3− ions showed about 24% of the samples falling on or approaching the 1:1 equiline [Figure 5e], indicating the weathering of calcite, dolomite, and gypsum in the aquifer. However, 17% of samples showed an excess of Ca2+ + Mg2+ over SO42− + HCO3− ion [Figure 5d], indicating a reverse ion exchange process in the Tummalapalle area.[49],[50] The same has been suggested by the chloro-alkaline indices study, where 22% of the water samples had positive indices.[51] As a result, the reverse ion exchange process raises Ca2+ and Mg2+ levels in subsurface water. The increase in Ca2+ concentration in a few groundwater samples is explained further by the exchange of Ca2+ ions with the host rock along with Na+ ions from the aquifer, as shown in the Ca2+ versus Na+ scatter plot [Figure 5f].
Distribution of dissolved 222Rn and 226Ra concentration
The statistical summary of the dissolved 222Rn and 226Ra concentrations in the groundwater is presented in [Table 3]. The concentration of dissolved 222Rn in groundwater samples ranged from 2 to 332 Bq/L with a mean, geometrical mean, and median of 98 ± 87, 60 (GSD: 3.12), and 72 Bq/L, respectively. The radon concentration in the study area is comparable to the radon concentration in Bengaluru city, where the radon concentration in groundwater has been reported to be 56–1189 Bq/L.[52] Using Origin Pro 2020 software, statistical analysis was carried out to assess the distribution pattern of dissolved radon using Kolmogorov–Smirnov hypothesis tests, which indicated the log-normal distribution of dissolved 222Rn in the studied area at a 95% confidence level. The study area, which includes Motunutalapalli, Tummalapalle, Koorli, Mavindlavaripalle, Chinnapalle, Balajipeta, Kanampalle, and Rachkuntapalle, had higher levels of 222Rn. The higher levels of dissolved radon in the study area may be attributed to its precursor radionuclides in the adjoining rocks of the groundwater aquifer. The presence of uranium mineralization passes through the study area, as it has been reported in the Vempalle dolostone of the Cuddapah Basin, which extends over a 160-km belt.[53] There is no guideline available for dissolved radon in the country. However, the WHO recommends setting a screening level for radon in water based on the national reference level for 222Rn in air and the distribution of 222Rn in the housing area. The WHO also recommends carrying out measurements where high radon concentrations are expected in groundwater. A safe limit of 100 Bq/L for 222Rn in drinking water has been prescribed by the WHO.[54] The European Union (EU) recommends using a reference level above 100 Bq/L in water used for commercial and public activities before considering any remedial actions to protect human health. The EU also recommends using the reference level of 1000 Bq/L in water used for individual activities.[55] In 64% of the studied samples, the radon level was below 100 Bq/L, and all samples were within 1000 Bq/L. Further, 94% of the groundwater data showed dissolved radon concentrations higher than the USEPA-recommended maximum contamination level of 11 Bq/L.[56] The United Nations Scientific Committee on the Effects of Atomic Radiation has recommended a permissible limit of 40.0 Bq/L for 222Rn in drinking water.[29] About 65% of the groundwater data showed a dissolved radon concentration greater than the UNSCEAR limit. The escape of radon gas from water during storage and cooking into the air reduces the risk from dissolved radon concentrations in groundwater from intake.
The concentration of dissolved 226Ra in groundwater samples ranged from 4 to 616 17 mBq/L, with a mean, geometrical mean, and median of 38 ± 83, 19 (GSD: 2.68), and 17 mBq/L, respectively, in the study area. A normality test using the Kolmogorov–Smirnov hypothesis tests was performed by the Origin Pro 2020 software, which shows the log-normal distribution of 226Ra at a 95% confidence level. Comparatively, higher levels of 226Ra concentrations were observed in the southwest direction of the study area, which comprises Tummalapalle, Koorli, and Motunutulapalli. These areas also showed higher dissolved radon concentrations in groundwater, implying a common mode of origin from uranium-rich rocks and soil. The 226Ra concentration was below 100 mBq/L in about 94% of the samples. The WHO recommended a guidance level of 1000 mBq/L for drinking water based on daily consumption of 2 L of water.[54] The International Commission on Radiological Protection recommended the annual limit on intake for ingestion of 226Ra as 90 kBq but not for 222Rn.[57] All the groundwater samples showed a 226Ra concentration, well below the WHO-recommended guidance level of 1000 mBq/L for drinking water. Besides, one sample of all groundwater samples showed a 226Ra concentration well below the USEPA limit of 500 mBq/L.[56] Again, the mean 226Ra concentration in groundwater was less than the derived water concentration (DWC) limit of 300 mBq/L.[58] However, the 226Ra content was found to be well within this DWC limit in 97% of the samples.
| Conclusions | | |
Natural radioactivity in the investigated area was found to be heterogeneously distributed, which may be governed by the geological formation and uranium mineralization in the region. The mean specific radioactivity of 232Th, 238U, 226Ra, and 40K in the study area was evaluated as being marginally higher than the world average due to the presence of enhanced uranium in the parent uraniferous rock. The activities of 226Ra, 232Th, and 40K were found to be widely distributed in the locally available rocks. According to the Raeq activity analysis, both surface soil and rocks were found to be suitable for use as construction materials, except for a few for dwellings and other human activities, without posing any significant radiation risks to the public. The mean outdoor radon concentration in the winter season was slightly higher than in the summer season, which can be attributed to the variation in temperature and other environmental factors. The estimated gamma radiation from the soil and rock radioactivity was comparatively less as compared to the radiation survey meter readings. The estimated activity ratio of 222Rn/226Ra indicated the existence of disequilibrium between daughter and parent radionuclides. Hydrogeochemical analysis of the groundwater in the study area revealed it to be neutral to moderately basic, oxic, fresh to brackish, and predominantly Ca2+-Mg2+-HCO3− (62.5%) type. The geochemistry of the groundwater in the Tummalapalle region is mainly controlled by rock-water interaction, silicate and carbonates weathering, ion exchange processes, and to a lesser extent, reverses ion exchange processes.
Acknowledgments
The authors would like to thank Dr. D. K. Aswal, Director of Health, Safety, and Environment Group, BARC for his inspiration and keen interest in the work. The chairman and managing director of UCIL, Dr. C. K. Asnani, is acknowledged by the author for his assistance, inspiration, and interest in the work. We sincerely appreciate the support of the additional team members from the Health Physics Unit, Tummalapalle.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3]
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