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 Table of Contents 
ORIGINAL ARTICLE
Year : 2016  |  Volume : 39  |  Issue : 4  |  Page : 204-211  

Assessment of natural radioactivity levels and identification of minerals in Brahmaputra (Jamuna) river sand and sediment, Bangladesh


1 Nuclear Minerals Unit, Atomic Energy Research Establishment, Ganakbari, Savar, Dhaka, Bangladesh
2 Material Science Division, Atomic Energy Center, Bangladesh Atomic Energy Commission, Ramna, Dhaka, Bangladesh
3 Health Physics and Radioactive Waste Management Unit, Atomic Energy Research Establishment, Ganakbari, Savar, Dhaka, Bangladesh
4 Department of Physics, Jahangirnagar University, Savar, Dhaka, Bangladesh

Date of Web Publication13-Feb-2017

Correspondence Address:
Md. Ibrahim Khalil
Nuclear Minerals Unit, Atomic Energy Research Establishment, Dhaka
Bangladesh
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.199980

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  Abstract 

Distribution of the natural radionuclides (238 U,232 Th, and 40 K) and their specific activities in sands and sediments of the Brahmaputra (Jamuna) river of Bangladesh together with mineral characteristics has been studied to assess the radiation levels as well as to develop a baseline database for comparison in the future in case of any change in the area under study due to anthropogenic activities. The radiological parameters of natural radioactivity were assessed calculating the radium equivalent activity, hazard index, the absorbed dose rate, and annual effective dose. The average activity concentrations of 226 Ra (238 U),232 Th, and 40 K in sand and sediment were found to be 59 ± 2 & 60 ± 2 Bq/kg, 113 ± 5 & 135 ± 5 Bq/kg, and 983 ± 42 & 1002 ± 43 Bq/kg, respectively. The calculated average absorbed dose rate and annual effective dose were found to be 150 nGy/h and 0.18 mSv/year respectively. These high values are associated with mineral content of the sediment. X-ray diffraction peaks of sand and sediment samples identify quartz, feldspar, rutile, zircon, monazite, uranium fluoride, hematite, kyanite, and uranium arsenide minerals to be present in the samples.

Keywords: Brahmaputra river, minerals, natural radioactivity, radionuclide, sand, sediment


How to cite this article:
Khalil MI, Majumder RK, Kabir MZ, Deeba F, Khan MN, Ali MI, Paul D, Haydar MA, Islam SM. Assessment of natural radioactivity levels and identification of minerals in Brahmaputra (Jamuna) river sand and sediment, Bangladesh. Radiat Prot Environ 2016;39:204-11

How to cite this URL:
Khalil MI, Majumder RK, Kabir MZ, Deeba F, Khan MN, Ali MI, Paul D, Haydar MA, Islam SM. Assessment of natural radioactivity levels and identification of minerals in Brahmaputra (Jamuna) river sand and sediment, Bangladesh. Radiat Prot Environ [serial online] 2016 [cited 2020 Sep 19];39:204-11. Available from: http://www.rpe.org.in/text.asp?2016/39/4/204/199980


  Introduction Top


Naturally occurring radioactive isotopes in the environment come from 226 Ra (238 U),232 Th, and 40 K in rocks, sand, soils, and sediments. Artificial isotopes in the environment arise from anthropogenic sources, such as weapon testing, nuclear medicine, nuclear accidents, and nuclear fuel cycle. Environmental radioactivity and external exposure due to gamma radiation occurs at different levels in nature and it varies geographically due to geological changes in each region in the world.[1]226 Ra (238 U),232 Th, and 40 K distributions in sands depend on the radionuclide distribution in parent rocks together with the chemical and mechanical processes through which the sands are concentrated. The specific levels of terrestrial background radiation are related to the types of rocks from which the sediments originate. Igneous rocks containing dark-colored heavy minerals usually show higher radiation whereas lower level radiation comes from sedimentary rocks.

The Brahmaputra river originates at Kailash mountain, at an elevation of about 5200 m in a great glacier mass in the north of the Himalayas. The vast amount of sediment carried by the Brahmaputra river, derived from intense erosion of the Himalayas (in the north), is delivered into the Bay of Bengal. Brahmaputra river diverted discharge from its “Old Brahmaputra” course from east of the Madhupur terrace into the present Brahmaputra–Jamuna river course to the west of the Madhupur terrace in the late 18th to early 19th century.[2],[3] Within Bangladesh, the current course is called the Jamuna River. The Brahmaputra flows through various rock types including Precambrian metamorphics (high-grade schists, gneisses, quartzites, metamorphosed limestones), felsic intrusive, and Paleozoic–Mesozoic sandstones, shales and limestones.[4] Sand deposits usually originate from the weathering and erosion of both igneous and metamorphic rocks in the environment, some constituent minerals of which bear natural radionuclides from the uranium and thorium series as well as potassium. Detritus products of these rocks containing the radionuclides are accumulating as river sediment where they are contributing significantly to the natural radioactivity. The Brahmaputra–Jamuna river channel has an estimated average sediment load of 590 m/year [5] originating from the igneous and metamorphic source rocks. To the best knowledge of the authors, there are no reports available on the natural radioactivity and radiological hazard due to the concentrations of radionuclides of these river sediments. An attempt has been made by this study to assess the natural radioactivity and radiological hazard due to the exposure to gamma radiation. In addition, constituent minerals of the sediment contributing to the radioactivity have also been determined.

The objectives of the present study, therefore, are to measure the natural radioactivity levels and to estimate the hazard indices; radium equivalent activities (Raeq), representative level index, absorbed dose rate, and annual effective dose in the Brahmaputra–Jamuna river system. For this purpose, sand samples were analyzed by high-resolution gamma spectrometry system, and the specific activities of 226 Ra (238 U),232 Th, and 40 K were determined. X-ray diffraction (XRD) was employed for the mineralogical analysis of the sand and sediments. The data generated in this study will provide baseline data of natural radioactivity in the area under study and will be useful for authority in charge of implementation of radiation protection standards in the region and on the general population of host communities.


  Materials and Methods Top


Study area

The present study area covers Kurigram Sadar, Ulipur, and Chilmari Upazila of Kurigram districts [Figure 1] which is traversed by Brahmaputra–Jamuna river system (25° 48'26”; 89° 45'09”– 25° 33'32”; 89° 41'01”). The study area covers stable sand bars in an area of 30 × 4 km from upstream to downstream of the Brahmaputra river in the Kurigram district located in the northeastern part of Bangladesh. The total length of the river from its source in southwestern Tibet to the mouth in the Bay of Bengal is about 2850 km (including Padma and Meghna up to the mouth). Within Bangladesh territory, Brahmaputra–Jamuna is 276 km long, of which Brahmaputra is only 69 km. The Jamuna is a very wide river. During the rains, it is about 5–8 miles (8.0–12.9 km) from bank to bank. Even during the dry season when the waters subside, the breadth is <2–3 miles (3.2–4.8 km).
Figure 1: Location map of the area under study and sampling points

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Sample collection

The present study covers about 120 km 2 area, from which 14 successive locations were selected and numbered as DK1-DK14. Two samples were collected from each location one as sand from the sand bars of the river and the other as sediment from the river bed which is submerged 1 m in the river. The sampling locations were recorded in terms of degree-minute-second (latitudinal and longitudinal position) using a handheld global positioning system (Model: Magellan-Map-410) unit. Each location is separated by a distance of approximately 2 km (approximately 1 minute in geographic co-ordinate system). The sand samples were collected at the depth of 0–5 cm in sand bars by plastic spade during summer period of 2014, and collected samples were packed in polyethylene bags and labeled with specific identification numbers. Sediment samples were also collected using the plastic spade at the depth of about 1 m from the river water level. Each sample had a weight of about 2 kg. The collected samples were cleaned and air-dried at room temperature in open air.

Radioactivity measurements

Sample preparation

The collected samples were homogenized and oven dried at 110°C for 24 h. The samples were then packed and sealed in an impermeable air-tight 180 ml PVC container (height: 7 cm, diameter: 6.5 cm) to prevent the escape of radiogenic gases radon (222 Rn) and thoron (220 Rn). About 450–500 g of samples were used for measurements. Before measurements, the containers were kept sealed hermetically and externally for about 4 weeks to reach secular equilibrium among 238 U,232 Th, and their decay products. The exact net weight of the samples was determined using an electronic balance before measurement.

Instrument used and procedure

The radiological characterization of the prepared samples was performed using a p-type coaxial high-purity germanium gamma-ray detector of 93 cm 3 active volume and 20% relative efficiency supplied by CANBERRA (Model GC-2018 and serial No. 0408941)with a resolution of 2 keV (FWHM) at 1332 keV of 60 Co gamma-ray line. The detector was coupled to a 16 k multichannel analyzer. The spectra of all samples were analyzed using Genie-2000 spectra analysis software to calculate the concentrations of 238 U,232 Th, and 40 K. The detector was enclosed in a cylindrical shielding container made of lead and iron with 11.3 cm thickness, 51 cm height, and 28 cm internal diameter, and having a fixed bottom and moving cover to reduce the external gamma-ray background.[6] All the samples were counted for 10,000 s, where the environmental gamma background at laboratory site was determined with an identical plastic container used for sample preparation prior to the measurement. The energy regions selected for the corresponding radionuclides were 295.2 and 351.9 keV of 214 Pb; 609.3 keV of 214 Bi for 226 Ra; 238.6 and 300.1 keV of 212 Pb; 583.2 keV of 208 Tl; 911.1 and 969.1 keV of 228 Ac for 232 Th; and 1460.8 keV for 40 K.[7]

The efficiency calibration of the detector was performed by standard solid sources prepared using 226 Ra standard solutions.[8] The standard sources were prepared using 180 ml plastic container used for the measurement of the samples. The preparation process of standard sources had been reported elsewhere.[9] The detector efficiency calibration curve as a function of energy is shown in [Figure 2]. The energy calibration of the detector was performed by 137 Cs and 60 Co point sources. The minimum detectable activity of each radionuclide was determined from the background radiation spectrum for the same counting time as for sediment samples and was estimated as 2.1, 4.2, and 59.1 Bq/kg for 238 U,232 Th, and 40 K, respectively.
Figure 2: Efficiency curve of the HPGe detector in 4π geometry

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In situ gamma radiation dose rates were measured using an advanced survey meter (model: Fluke Biomedical 993) in each location under the study. For statistical purposes, 3–4 readings were taken at each location, and the average was recorded.

Characterization of minerals

The XRD powder pattern was recorded at room temperature using A Philips X'Pert Pro XRD system. For each composition, the cylindrical samples of weight more than 2 g were converted into powderulated. For XRD experiment, each powder sample was put on a special holder for recording the X-ray data. The powder specimens were exposed to CuKα radiation with a primary beam of 40 kV and 30 mA with a sampling step of 0.02°, and the time for each step of data collection was 1.0 s. A 2θ scan was taken from 15° to 70° to get possible fundamental peaks where Ni filter was used to reduce CuKb radiation. All the data of the samples were analyzed using computer software “X'pert Highscore.” Observed peak positions were matched against the International Centre for Diffraction Data Joint Committee on Powder Diffraction System (JCPDS) card database.


  Results and Discussion Top


Activity concentration of radionuclides

Sand and sediment samples were subjected to gamma-ray spectral analysis to measure the activity concentrations of 226 Ra (238 U),232 Th, and 40 K. The peaks of gamma-ray spectrum found in each of the investigated sample are attributed to the decay products of 238 U and 232 Th series and to 40 K. The 226 Ra (238 U) activity was determined individually from the net area of peak at energies of 295.2, 351.9, (due to 214 Pb), and 609.3 keV (due to 214 Bi). Similarly, the counts at peak energies of 238.6 and 300.1 keV (due to 212 Pb), 583.2 keV (due to 208 Tl), and 911.1 and 969.1 keV (due to 228 Ac) were used to determine the 232 Th activity. Finally, the values obtained from different peak energies were averaged to get the final value. The activity of 40 K was determined from its 1460.8 keV gamma-line. It should be mentioned that no peak appeared at energy of 661.6 keV in the spectrum which is due to decay of 137 Cs and it confirms that the artificial radioactivity in the investigated samples was below the detection limit of 0.07 Bq/kg.

From the measured gamma-ray count rate (CPS), the activity concentration of the 226 Ra,232 Th, and 40 K radionuclides were calculated by the following formula:[10]

A = (CPS × 1000/(ε × Iγ × M)

Where, A represents the specific activity (Bq/kg), ε is the efficiency of the detector for the corresponding peak, Iγ is the gamma-ray emission probability, and M is the mass of the sample in g.

The activity concentration of the radionuclides 226 Ra (238 U),232 Th, and 40 K for all the samples is presented in [Table 1] and [Figure 3]. It is observed from the results that the average activity concentration of 226 Ra (238 U),232 Th, and 40 K in sand is 59 ± 2 Bq/kg, 135 ± 5 Bq/kg, and 982 ± 42 Bq/kg, respectively, while in sediment, it is 60 ± 2 Bq/kg, 113 ± 5 Bq/kg, and 1002 ± 43 Bq/kg, respectively. Measured radionuclide concentrations vary from site to site following 226 Ra (238 U) <<sup>232 Th <<sup> 40 K order. From [Figure 3], it is observed that the 226 Ra (238 U) and 232 Th concentration in sands follows the same trend toward downstream while 40 K follows the reverse trend attributing the variation in heavy and light mineral concentration. These variations in different locations are related to the geological process, particularly physical and chemical sorting processes from location to location and drainage pattern of the study area. A detailed analysis of the results indicated that there was a good correlation between the activity concentrations of 232 Th and 226 Ra (238 U) in both sand and sediment samples (R2 = 0.6821 and 0.6904, respectively) collected from the area under study as has been shown in [Figure 4]. The sand sample in site number DK7 produces the highest concentration of 226 Ra and 232 Th while the highest concentration of 40 K is found in DK1 sand. The second highest concentration of 238 U and 232 Th is found in site number DK3 while minimum concentration of 226 Ra and 232 Th is found in site DK14. Carvalho et al.[11] noted that heavy minerals tend to incorporate high concentrations of naturally occurring radionuclides such as 238 U and 232 Th decay series in their crystal structure. They also stated that light minerals such as quartz and feldspar may contain relatively high concentrations of 40 K. Other researchers [12],[13],[14] noted that thorium, uranium, and potassium contents tend to be high in felsic rocks, and potassium is usually found in potash feldspars or in micas. They also noted that most uranium and thorium atoms are bound in accessory and dark-colored minerals (red, purple, or black), known as heavy minerals. Sandy mineral substances with these heavy minerals show radiometric signatures. Both thorium and potassium are associated with clay minerals by the process of adsorption and its chemical composition, respectively. In the study area, heavy minerals such as ilmenite, zircon, rutile, monazite, magnetite, and uranium arsenide were encountered as obtained from XRD studies. Zircon typically contains 5–4000 ppm of 238 U and 2–2000 ppm of 232 Th,[15] and magnetite contains considerable amount of 238 U and 232 Th [16] while monazite is a thorium-bearing mineral. Thus, the presence of heavy minerals in the sampling locations may have relation for having higher 238 U and 232 Th concentrations in these samples. Similarly, the light minerals in the sampling sites may play a role for having high concentrations of 40 K in the samples. Average concentrations of 238 U,232 Th, and 40 K are higher than world average values in both sand and sediment in the area under study. These values are compared with the values of other rivers [17],[18] in Bangladesh and found to be higher. Average concentration of 238 U,232 Th, and 40 K is, respectively, 1.97, 3.86, and 2.45 times higher than those of world average values in sand, while in sediment samples, it is 1.89, 3.24, and 2.50 times higher, respectively. Therefore, both heavy and light minerals might have contribution to fix the high-activity concentrations in the current samples.
Table 1: Activity concentration of 226Ra (238U), 232Th, and 40K in sand and river sediment samples collected from the study area

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Figure 3: 226Ra (238U), 232Th, and 40K concentrations and absorbed dose rate of all sites. Arrow mark represents direction to the downstream

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Figure 4: Correlations between the activity concentration of 226Ra (238U) and 232Th in (a) sand and (b) sediment samples

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Radiological parameters

Radiological effect of the Brahmaputra (Jamuna) river sediments due to the presence of 226 Ra,232 Th, and 40 K has been determined through the calculation of radiological indices. To assess the radiological risk of the Brahmaputra river sand, which are mainly used as the building construction material, different radiological parameters such as Raeq, absorbed dose rate (D), annual effective dose equivalent (AEDE), and external hazard index (Hex) are calculated.

Radium equivalent activities

The Raeq is widely used as an index to describe the gamma output from different mixtures of uranium, thorium, and potassium in the sediments sampled from different locations. It is calculated using the following equation.[19]

Raeq (Bq/kg) = Cu + 1.43CTh + 0.077Ck

Where CU, CTh, and CK are the mean activity concentrations of 238 U,232 Th, and 40 K in Bq/kg, respectively. [Table 2] summarizes the value of Raeq. These values varied from 163 to 566 Bq/kg for sand. The average value is 328 which is lower than the recommended maximum value of 370 Bq/kg for building materials,[1] while in some locations, it is higher than the recommended value. Lower Raeq may be due to the leaching of heavy minerals by continuous flow of water in the river.[20]
Table 2: Absorbed dose rate, annual effective dose equivalent, radium equivalent activity, and external hazard index in all sand (SMa) samples collected from the study area

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Absorbed dose rate

Determination of absorbed dose rate is the first major step for evaluating the radiation exposure to gamma radiation. The measured activity concentrations of 238 U,232 Th, and 40 K are converted into doses by applying the conversion factors 0.462, 0.604, and 0.042 for uranium, thorium, and potassium, respectively.[1] These factors are used to calculate the total dose rate, D (nGy/h) using the following equation; generally for undisturbed soil radioactivity. However, the same factors are used for sand bed in the present study.

D = (0.462Cu + 0.604CTh + 0.042CK) nGy/h

Where CU, CTh, and CK are the activity concentrations (Bq/kg) of 238 U,232 Th, and 40 K in river sediments, respectively. The calculated values are shown in [Table 2], and range of absorbed dose rate is found to vary from 78 to 252 nGy/h. Average absorbed dose rate for all samples is higher than the world average value (57 nGy/h) [Table 2].[1] From [Figure 3], it is observed that the absorbed dose rates are changed according to the spatial variations of radionuclide concentrations in minerals of sediments. It is found highest in DK7 location where 226 Ra (238 U) and 232 Th concentrations are also reached to highest of the study.

In situ measurement of the gamma dose rate values was found to vary from 180 to 432 nGy/h and these values were higher than those of calculated values. These differences between the measured and calculated dose rates are probably due to processing and treatment of the samples prior to gamma spectrometric analysis in addition to the cosmic ray component. Physical properties and ambient environment of the in situ measurement locations such as density, humidity, sample size, compactness degree in situ as well as ambient scattering are different from the samples prepared for laboratory analysis which might have also been contributed in lowering the calculated values.[21]

The annual effective dose equivalent

Since the present sand are mainly used as building materials, the determination of AEDE of each site sample is important. The AEDE was calculated from the absorbed dose by applying the dose conversion factor of 0.7 Sv/Gy and the outdoor occupancy factor of 0.2.[1] The annual effective dose is determined using the following equation:



From the calculation, it is observed that values of AEDE [Table 2] are found to vary from 0.10 to 0.31 mSv/y for sand samples. The average values of outdoor AEDE for all samples are higher than the world average values (0.07 mSv/y for outdoor) (UNSCEAR, 2000). AEDE values have exceeded the world average due to the presence of high-activity concentration of some radionuclides, however they are well within the observed range of world averages.

External hazard index

Another radiological index, Hex, is calculated by the following equation:[22]



Where CU, CTh, and CK are the activity concentrations of 238 U,232 Th, and 40 K in Bq/kg, respectively. The value of this index must be less than the unity to keep the radiation hazard to be insignificant. The maximum value of Hex equal to unity corresponds to the upper limit of Raeq (370 Bq/kg). The calculated values of Hex for the sand samples ranged from 0.4 to 1.5 with an average value of 0.9 [Table 2].

Mineral analysis – X-ray diffraction

To determine the mineralogical composition, XRD spectra are recorded for both sand and river sediment samples. A representative diffractogram is shown in [Figure 5] along with name of the minerals present. The minerals contained in the samples were identified using the JCPDS file. It was observed from the analysis of the XRD that the positions of the peaks comply with the reported value. All diffraction peaks of the studied samples are compared to the reported structure for relevant base sand samples. The observed peaks confirmed that the sand and sediments mainly contained quartz, feldspar, rutile, zircon, monazite, uranium fluoride, hematite, kyanite, and uranium arsenide.
Figure 5: Representative X-ray diffraction pattern of sand samples from Brahmaputra (Jamuna) river

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  Conclusion Top


Natural radiation level and mineralogical (light and heavy) characteristics of Brahmaputra (Jamuna) river sediments in 14 samples have been analyzed successfully. The mean activity concentrations of 226 Ra (238 U),232 Th, and 40 K of the river sand and sediment samples are higher than the world average values.[1] The highest values of 226 Ra (238 U) and 232 Th activity concentrations were found due to the presence of elevated radiogenic heavy mineral content in the samples while highest 40 K values are due to the presence of light minerals. These light and heavy minerals might be originated from the source rocks as the river travels a long-way path containing igneous, metamorphic and sedimentary rocks. Brahmaputra (Jamuna) river sediment is found to contain 8%–15% heavy mineral concentration in an average [23],[24] which is ultimately contributing to the natural radioactivity. XRD peaks confirmed the presence of quartz, monazite, feldspar, uranium fluoride, rutile, zircon, hematite, kyanite, and uranium arsenide minerals. Radiological hazard indices except Raeq are higher than the recommended level proposed by [25] the Organization for Economic Cooperation and Development while Raeq exceeded the recommended limit in some locations. There is no report on the probability of immediate health effect due to the use of these sand and sediment as building materials as the levels are within the variations of world averages.

Ministry of Science and Technology, Government of The People's Republic of Bangladesh under the Special Allocation program 2014-2015.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2]


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