Natural radioactivity and its radiological implications from soils and rocks in Jaintiapur area, North-east Bangladesh.

Natural radioactivity concentrations in recent alluvial soils from swampy areas and Tertiary rocks from Jaintiapur were measured using gamma-ray spectrometer equipped with HPGe detector. The average radioactivity concentration of 226Ra, 232Th and 40K were 47 ± 6, 64 ± 5 and 762 ± 40 Bqkg-1 in soils, whereas, 25 ± 2, 37 ± 4 and 884 ± 41 Bqkg-1 in rock samples, respectively. Average radioactivity concentrations of studied soil and rock samples exceeded the world average except 226Ra for rocks. Radio-elemental ratios suggest that an oxic depositional environment with low uranium and high thorium content. Regarding radiological hazard indices, radium equivalent activities (Raeq), external hazard index (H ex) and internal hazard index (H in) was found to be below the world permissible limits. Whereas, absorbed dose rate (D), and annual effective dose equivalent (AEDE) averages exceeded the world admissible values. Statistical studies show that radioactivity for 226Ra and 232Th linked to a source enriched in radioactive minerals and 40K related to a different sources high in K enriched minerals. © Akadémiai Kiadó, Budapest, Hungary 2022, Springer Nature or its licensor holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Introduction

Human exposures are subjected to numerous degree of ionizing radiation from extra-terrestrial sources primarily include cosmic radiation from earth’s outer atmosphere and terrestrial or naturally occurring radioactive sources such as gamma rays released from 40K and radionuclides of 238U and 232Th through decay series present in soil, rocks and water [1-4]. Moreover, anthropogenic origins, such as weapon testing, nuclear treatment, nuclear incidents and nuclear power cycle are responsible for artificial radioisotopes in the environment. Natural and extra-terrestrial radiation exposure occur at various degrees in nature, and they differ regionally caused by variation in geological and radiochemical properties in each region [5, 6].

Natural background radiation from primordial radionuclides such as 226Ra, 232Th, and 40K in sediment, soil, water, and rock accounts for around 80% of the total radiation dosage a person receives in a year, making them the topic of most radioactivity measurement studies [7]. Of these, soil plays a major determinant of radioactive pollution in the environment because it operates as a means of transportation of radionuclide to biological systems [8]. In addition, soil radioactivity is frequently used to generate a benchmark for future radiation hazard analysis, nuclear safety, and exploration [9]. The distributions of 226Ra, 232Th, and 40K in rock are influenced by the radionuclide distribution in parent rocks as well as the physicochemical processes that concentrate them. Higher levels of radiation are found in igneous rocks including dark colored heavy minerals, while lower levels of radiation are found in sedimentary rocks. The major sources of high natural background radiation are 238U and its decay products in soils and rocks, as well as 232Th in monazite sands [8]. Radioactive particles in phosphate rocks can reach the environment through a variety of methods, including the usage of phosphogypsum in construction and agriculture, as well as fertilization of agricultural fields [9]. Thus, through a systematic study of determination of the levels of different radionuclides (226Ra, 232Th and 40K) and their associated health risks for human beings can act as a vital part in radiation protection, geo scientific studies and in establishing guidelines for the alleviation of these radionuclides [2]. Geologically, Bangladesh has already been categorized to four zones: (1) Eastern Mobile Belt (EMB), (2) Stable Platform (SP), (3) Dauki Fault Belt (DFB) and (4) Dinajpur Slope (DS) considering the favorable criteria of uranium formation [10]. A considerable amount of radiometric irregularities has subsequently been discovered in sandstone of Tipam and Dupitila Formations at the surrounding of Jaintiapur within the DFB zone. Moreover, this zone is found adjacent to the Mahadek uranium belt on the Shillong plateau's southern border. As a result, it's reasonable to believe that the uranium bearing solution has been streaming for a prolonged geologic period and that ore has accumulated within the DFB zone [10]. Geologically, Bangladesh is built up entirely of sedimentary rocks, and the ultimate prospect of uranium mineralization under favorable reducing conditions is sedimentary type uranium mineralization, which likely to be precipitated as economic uranium ore [10]. The reason for the study in this region of Jaintiapur is because the DFB zone has been demonstrated as potential area for uranium exploration, and data about natural radionuclides is currently lacking in this zone. Previous researches [10-13] have been focused to certain locations, radionuclides, and/or geological formations. As a result, a detailed analysis is highly needed to thoroughly estimate the dose of radiation exposure from environmental sources, the potentiality of uranium deposition, and assess the health concerns caused by radiation.

The purpose of this present work is to investigate the existence of natural 226Ra, 232Th and 40K, and artificial 137Cs radionuclide, as well as their activity concentration levels in the collected recent alluvial soils and Tertiary rocks samples from the geologic structure of Jaintiapur and the adjoining area of the DFB zone. Absorbed dose rate, radium equivalent, annual effective dose rate, as well as external and internal hazard index are calculated for estimating the radiological impact on the population and the environment related to these radionuclides. Additionally, these results are compared to values from other countries throughout the world as well as the UNSCEAR’s reference value [6]. Therefore, the elemental concentration of these radionuclides, which provides information on the paleo-oxygenation condition of the investigated area, was calculated to find out the potentiality of uranium deposition. In addition, statistical studies are performed to understand the relationships between the radionuclide and radiation hazard indices. Moreover, this evaluation would serve as a reference data for assessing variations in environmental radiation.

Study area

The Bengal Basin is the northeastern part of Indian subcontinent bounded by the Precambrian Indian Shield platform in the west, the Precambrian Shillong Plateau in the north, Indo-Burman Ranges to the east and on the south it is plunges in to the Bay of Bengal. The sedimentation of the Bengal Basin has been controlled by the movement and collision pattern of the Indian plate with the Burmese and Tibetan plates, as well as the uplift and erosion of the Himalayas and Indo Burman Ranges [14]. Surma Basins situated in the north-eastern part of Bangladesh (Fig. 1a), floored by an enormous thickness of sediments about 12 to 16 km from late Mesozoic to Cenozoic [15]. The Surma Basin is bordered to the north by the sole elevated topography, the Shillong Massif [16] (Fig. 1a). The contact between the Surma Basin and the Shillong Plateau is marked out by the E-W-trending Dauki Fault system with huge vertical displacement [15, 17]. The basin is bordered on the west by the Indian Shield Platform and on the east and southeast by the Chittagong–Tripura Fold Belt of the Indo-Burman Range [16] (Fig. 1a). The basin is open to the south and southeast to the Bengal Basin. Maximum number of litho-formations from the Eocene to recent is exposed in the Surma Basin. Stratigraphically, Sylhet Trough holds Tertiary Jaintia Group, Barail Group, Surma Group, Tipam Group, Dupi Tila Formation and Dihing Formation from older to younger [15, 18–20]. The possible source areas of the Surma Group sandstone may be the eastern Himalayan and/or from the indo-Baurman ranges and less commonly the Shillong Plateau [21].

Fig. 1

a The regional geo-tectonic frameworks of the Bengal Basin (modified after [16, 22–24]) b Geological map of Jaintiapur and its adjoining areas showing the exposed geologic units and sample locations (modified after [12, 16])

The current research area, Jaintiapur and its surrounding areas in the Sylhet district, is located in Bangladesh's north eastern region (Fig. 1b). It covers Jaintiapur hill, Sripur Tea garden, Harafkata, Lalakhal Tea garden, as well as some bils and haors including Tama Bil, Kendri Bil, Yam Bil, Dibir haor and is located between longitude 92°04′E–92°12′E and latitude 25°04′N–25°16′N.

Experimental

Sample collection and preparation

The soil and rock samples were taken from 13 randomly chosen points to determine the natural radioactivity concentration. The Location data of these sites were noted and defined as regards degree- minute- second (latitudinal and longitudinal position) with a hand held global positioning device (Model: Magellan-Map-410) units. For soil samples around 2 kg of soil was collected by removing the 5 cm surface soil. The rock samples were crushed to fine grains using a grinding machine after being cleaned and air dried. These samples were sieved with 2-mm mesh-sized sieve to produce homogeneous fine-sized particles and dried at 110 °C, over 24 h in a temperature-controlled furnace. The specimen was placed in an impervious 180 ml PVC container to avoid the get out of radiogenic gases radon (222Rn) and thoron (220Rn).

Sample counting and measurements

The radiological characterizations of the prepared samples were performed using γ-ray spectrometric analysis. The CANBERRA (Model GC‑2018 and serial No. 0408941) spectrometer has a p-type coaxial high-purity germanium (HPGe) γ-ray detector of 93 cm3 active volume and 20% relative efficiency. The resolution of the detector was 2 keV (FWHM) at 1332 keV photo peak of 60Co. A multichannel analyzer of 16 k was coupled with the detector. Genie-2000 spectra analysis software was operated to explain the spectra of all samples leading to determine the activity concentration. The energy regions selected for the corresponding radionuclides were 295.2 and 351.9 keV of 214Pb; 609.3 keV of 214Bi for 226Ra; 238.6 and 300.1 keV of 212Pb; 583.2 keV of 208Ti; 911.1 and 969.1 keV of 228Ac for 232Th; and 1460.8 keV for 40K [25]. The activity concentrations of 40K were calculated from its own γ-rays while the activity of 226Ra and 232Th was measured from the γ-rays of their decay products. In the spectrum no peak was available at energy of 661.6 keV, which is because of decay of 137Cs [26].

Calibration of Gamma ray spectrometer

The detector's efficiency was calibrated with homogenously integrated standard solutions of 226Ra into inactive matrices (e.g., Al2O3). 180 ml Teflon container was used for formulating the standard source. A volume of the solution was pipetted into the matrices to generate the standard, which was then dried for 24 h at 408 °C [27]. Before measurement, the sample was thoroughly mixed to confirm that the radionuclides were uniformly distributed all across the source. The test uniformity was conducted on various aliquots of the 226Ra solution mixed Al2O3 matrices [28, 29]. Figure 2 represents the efficiency calibration curve for solid matrix as a function of energy. Moreover, 137Cs and 60Co point source was used for the energy calibration of the detector.

Fig. 2

a Energy diagram of the sample in HPGe detector with 20% efficiency b Efficiency curve of the HPGe detector in 4π geometry

The background radiation spectrum was used to calculate the lowest detectable activity of each radionuclide for the same counting time as for soil and rock samples and was estimated as 2.1, 4.2, and 59.1 Bqkg−1 for 226Ra, 232Th, and 40K respectively.

Calculation of activity concentrations

The activity concentration (A) of each radionuclide in the studied samples was computed following equation [30]:where, A = activity concentration in Bqkg.−1, cps = net counts per second, E = photo-peak efficiency I = gamma intensity and W = samples weight (in kg). The relative combined standard uncertainty (CSU) of the activity concentration was calculated (considering all known uncertainty components at coverage factor k = 1) by using equation given below [31, 32]where, N, T, Iγ, m and E are the sample counts, counting time, gamma-ray emission probability, sample weight, and counting efficiency, respectively and u(N), u(T), u(Iγ), u(m) and u(E) are their respective uncertainties. In this study, the minimum detectable activity (MDA) of the gamma-ray measurement system was calculated by the following equation [33-35]where, B, is the background counts over the region of interest for each radionuclide and T, is the measurement time in seconds.

Radio-elemental ratios

The International Atomic Energy Agency [30] provides conversion factors to converts the measured 226Ra, 232Th, 40K activity concentrations in Bqkg−1 to eU, eTh elemental concentrations in ppm and potassium in %. These recommended conversion factors are: 1 ppm U = 12.35 Bqkg−1 of 238U; 1 ppm Th = 4.06 Bqkg−1 of 232Th; and 1% K = 313 Bqkg−1 of 40K. The ratio of eTh/eU activity concentration is important in identifying "geochemical faces." The viability of uranium to thorium ratio is employed as an indicator of relatively oxidizing or reduction circumstances based on examinations of several soil and sediment samples [36]. Under reducing circumstances, uranium has an immobile tetravalent phase that is stable, but it transforms into soluble hexavalent phase that can be transported in the solution. Thorium, on the other hand, has only one insoluble tetravalent form that is geochemically associated to uranium [37].

Radiological hazard

Radium equivalent activity concentration index (Raeq)

Radium equivalent activity Raeq can be represented as the Eq. (4), the weighted summation of three radionuclides 226Ra, 232Th and.40K [38]Here, CRa, CTh and C are the activity concentrations of 226Ra, 232Th and 40K, respectively in Bqkg−1.

Absorbed dose rate (D)

Absorbed dose rate (D) is calculated to assess the radiation exposure to gamma radiation at 1 m above the earth surface [39]Here CRa, CTh and C are the activities of 226Ra, 232Th and 40K in Bqkg−1 and 0.462, 0.604 and 0.0417 are the conversion factors that convert the following activities in to doses [40].

The annual effective dose equivalent (AEDE)

AEDE was computed to estimate the health effects of the absorbed dose, which was calculated by using the following Eq. (6)D is the absorbed gamma dose rate calculated in Eq. (3); According to UNSCEAR 2000, the DCF (dose conversion factor) and OF (outdoor occupancy factor) values are 0.7 SvGy−1 and 0.2, respectively, as well as T is the time factor (8760 h). OF = 0.2 was derived from the assumption that human roughly spend 20% time outside [41].

External hazard index

The external and internal hazard indexes was calculated to make sure that radiation exposure due to 226Ra, 232Th and 40K in the analyzed samples are within the allowable dose equivalent of 1 mSvy−1 [38]. Equation (7) is used to determine the external hazard index.

Internal hazard index

Internal hazard index (Hin) was calculated using the following formula (8): [42],Here, CRa, CTh and CK having the same meaning as in Eq. (4). Radiation hazard are considered insignificant when the value of this index value is lower than 1.

Statistical analysis

Statistical studies including Pearson correlation coefficient analysis, and Principal component analysis were performed with SPSS (version 23) to understand the overall relationships between the combinations of radionuclides and the variables derived from activity concentration, with a view to notice the magnitude of the existence of these radionuclides collectively in positive vicinity.

Results and discussion

Activity concentrations

The activity concentrations of 226Ra, 232Th and 40K computed in Bqkg−1 for all samples are presented in Table. 1. 226Ra activity concentrations ranged from 22 ± 2 to 66 ± 2 Bqkg−1 (soil samples) with an average activity of 47 ± 6 Bqkg−1 and from 14 ± 2 to 51 ± 2 Bqkg−1 (rock samples) with an average activity of 25 ± 2 Bqkg−1. For 232Th the values ranged from 25 ± 3 to 92 ± 4 Bqkg−1 (soil samples) with an average activity of 64 ± 5 Bqkg−1 and from 20 ± 3 to 57 ± 3 Bqkg−1 (rock samples) with an average activity of 37 ± 4 Bqkg−1. The activity concentrations for 40K ranged from 408 ± 34 to 1006 ± 40 Bqkg−1 (soil samples) with an average activity of 762 ± 40 Bqkg−1 and from 544 ± 36 to 2060 ± 54 Bqkg−1 (rock samples) with an average activity of 884 ± 41 Bqkg−1 (Table 1). The special distribution map represented the activity concentrations of all radionuclides detected in the analyzed samples (Fig. 3). The higher concentration of 226Ra and 232Th is mainly found in the soil samples, whereas R-7 rock sample shows higher 40K concentrations (Fig. 3). The average activity concentration was followed an ascending order as 226Ra < 232Th < 40K. In all of the studied soil and rock samples, 40K activity was usually the largest contributor to the particular activity when compared to 232Th and 226Ra. The world's average activity concentrations for 226Ra, 232Th and 40K are accordingly 35, 30, and 400 Bqkg−1 [6].The average activity concentration values of 232Th and 40K for both the soil and rock samples of the studied area were greater than the world's average values [6], while the mean concentration of 226Ra was higher compare to the world's average values for soil, however, the value for rock samples lie within the limit. In comparison to the mean activity concentration of 226Ra and 232Th for rock samples, it was higher in soil samples from the research area. Moreover, the studied confirmed the absence of artificial 137Cs radionuclide, the activity concentrations of all the analyzed samples were found beneath the minimum detection limit of 0.18 Bqkg−1(Table 1).

Table 1

Activity concentrations of 226Ra, 232Th and 40K, absorbed dose rate (D), annual effective dose equivalent (AEDE), radium equivalent activity (Raeq), external (Hex), and internal (Hin) hazard index in soil and rock samples from Jaintiapur area as well as comparison of this studies with other areas of Bangladesh and worldwide

Sample ID	Sample Type	226Ra	232Th	40K	137Cs	Raeq	Hex	Hin	D	AEDE
		(Bqkg−1)	(Bqkg−1)	(Bqkg−1)	(Bqkg−1)	(Bqkg−1)			(nGyh−1)	(mSvy−1)
	Soil
S-1	Alluvial Soil	55 ± 3	83 ± 6	750 ± 40	Below detection limit (0.18)	231	0.62	0.77	107	0.13
S-2	Alluvial Soil	66 ± 2	92 ± 4	1006 ± 40		274	0.74	0.92	128	0.16
S-3	Alluvial Soil	54 ± 3	59 ± 6	847 ± 46		204	0.55	0.70	96	0.12
S-4	Alluvial Soil	37 ± 20	61 ± 4	799 ± 41		186	0.50	0.60	88	0.11
S-5	Alluvial Soil	22 ± 2	25 ± 3	408 ± 34		88	0.24	0.30	42	0.05
Maximum		66 ± 2	66 ± 2	92 ± 4		1006 ± 40	274	0.74	0.92	128
Minimum		22 ± 2	22 ± 2	25 ± 3		408 ± 34	88	0.24	0.60	42
Average		47 ± 6	64 ± 5	762 ± 40		197	0.53	0.66	92	0.11
	Rock
R-1	Barail Sandstone	24 ± 2	39 ± 3	600 ± 36		125	0.34	0.40	60	0.07
R-2	Barail Sandstone	14 ± 1	20 ± 2	544 ± 36		83	0.23	0.26	41	0.05
R-3	Bokabil Sandstone	27 ± 2	39 ± 5	565 ± 42		126	0.34	0.42	60	0.07
R-4	Tipam Sandstone	24 ± 2	28 ± 5	907 ± 44		133	0.36	0.42	66	0.08
R-5	Tipam Sandstone	14 ± 2	23 ± 3	873 ± 41		114	0.31	0.35	57	0.07
R-6	Bhuban Shale	51 ± 2	57 ± 3	872 ± 40		200	0.54	0.68	95	0.12
R-7	Bokabil Sandstone	19 ± 2	54 ± 4	2060 ± 54		255	0.69	0.74	128	0.16
R-8	Barail Sandstone	24 ± 2	36 ± 3	654 ± 38		126	0.34	0.41	61	0.07
Maximum		51 ± 2	51 ± 2	57 ± 3		2060 ± 54	255	0.69	0.74	128
Minimum		14 ± 1	14 ± 2	20 ± 2		544 ± 36	83	0.23	0.26	41
Average		25 ± 2	37 ± 4	884 ± 41		145	0.39	0.46	71	0.09
World average	[6]	35	30	400		370	1	1	55	0.07
Inani Beach	[51]	44	70	1007		222	0.60	0.72	105	0.13
Chittagong	[11]	35	60	438		154	0.42	0.51	71	0.09
Pabna	[52]	33	47	449		135	0.36	0.45	62	0.08
Dhaka	[53]	33	55	574		156	0.42	0.51	73	0.09
Jessore	[54]	48	53	481		161	0.43	0.56	74	0.09
Sitakunda	[55]	31	62	467		155	0.42	0.50	71	0.09
Sylhet	[11]	55	125	491		271	0.73	0.88	122	0.15
Habiganj	[13]	11	22	227		60	0.16	0.19	28	0.03
Egypt	[56]	17	18	320		67	0.18	0.23	32	0.04
USA	[6]	40	35	370		118	0.32	0.43	55	0.07
China	[57]	32	41	440		124	0.34	0.42	58	0.07
Japan	[6]	33	28	310		97	0.26	0.35	45	0.06
Malaysia	[58]	66	82	310		207	0.56	0.74	93	0.11
India	[59]	29	64	400		151	0.41	0.49	69	0.08
Iran	[6]	28	22	640		109	0.29	0.37	53	0.07
Saudi Arabia	[60]	15	11	225		48	0.13	0.17	23	0.03
Turkey	[61]	86	51	772		218	0.59	0.82	103	0.13

Fig. 3

Spatial distribution map of 226Ra, 232Th and 40K activity concentrations (BqKg−1) in the study area

Heavy minerals are incorporated with 226Ra and 232Th radionuclides in their crystal structure whereas light minerals like quartz and feldspar can have comparatively high quantities of 40K [43]. Heavy minerals might be present at sample locations with high 226Ra and 232Th concentrations, while light minerals might be present in sampling locations with high 40K concentrations. The probable reasons for higher activity concentration in soil include presence of higher organic matter content, dominance of minerals (smectite, clays, and carbonates), the difference in the underlying bedrocks, and the inundation of these area during flooding through several streams and channels flown through the different formations exposed in nearby highly radioactive Khasi and Jaintia hills of Shillong Plateau, Meghalaya, India and Tertiary succession of Jaintiapur Area of Surma basin which carry various sediments from different formations having diverse age range [12, 44, 45]. The activities of radionuclides in the soil samples may vary locally even in shorter distance due to multi-channel sediments deposition which we have observed during in-situ field measurements. Similar studies from Shillong Plateau, Meghalaya, India also showed that the variation of the radioactivity content from location to location is because of the variation of these elements in different geological formations of oldest Precambrian gneissic complex to Recent alluvium soil [46, 47]. Therefore, the reasons for fluctuating activity concentrations in the studied area include the variation of mineralogical and chemical composition of rock formations exposed in the studied area which were deposited in varied depositional settings such as continental to marine in the geological past, the presence or absence of alluvium covering and compactness of sandstone [12, 16, 48]. A close observation during the field survey revealed that black patches on the soils indicating the presence of thorium in the studied samples and the variation arises due to varying grades of monazite deposition in the samples. These were further confirmed from the previous mineralogical studies of this area available in literatures [16, 48–50]. Previous XRD analysis of Neogene Surma Group sedimentary rocks from the current study area confirmed the presence of kaolinite, illite, chlorite, illite/smectite and kaolinite/smectite mixed layers. Petrographic studies show that sandstones consist dominantly quartz (61%), feldspar (8%) with lithic fragments (12%) and the shales contain mainly quartz, feldspar and clay minerals with minor carbonates. In Meghalaya, the uranium containing host rock is feldspar rich Arkosic sandstone and the thorium containing host rock contain monazite [46]. Besides, the anthropogenic activities such as use of fertilizer containing potassium in the haor and bills area during agricultural activities and presence of stone crushing site nearby the studied area might result in variation of activity concentrations [3, 12, 45]. Thus, there is no significant correlation exist between radionuclides in soil samples and activity of radionuclides in rock samples of the study area.

Mean activity concentration of 226Ra, 232Th and 40K in studied area was compared with the mean activity concentration of soil from different parts of Bangladesh and with different countries of the world (Table 1). Where the average values of 226Ra, 232Th and 40K in samples were within the range or exceed the respective values of the countries listed.

Radio-elemental concentration with ratios

The calculated uranium and thorium concentration in ppm and potassium in weight % is shown in (Table 2). Uranium concentration varied from (1.7 to 5.3) ppm and (1.1 to 4.1) ppm and Thorium concentration was in the range of (6.06 to 22.6) ppm and (4.8 to 14) ppm for soil and rock samples respectively, 40K concentration was in the range of (1.3 to 3.2) % and (1.7 to 6.5) % for soil and rock samples. The elemental concentration of uranium and thorium in soil samples were just exceed the average UCC value [62]. In contrast, potassium % in rock samples largely exceed the UCC value. The radioactive minerals could be the cause of higher value of radioactivity concentrations of the studied samples.

Table 2

Elemental concentrations of Uranium, Thorium (ppm), Potassium (%) and elemental ratio of eU/eTh, eTh/eU, in soil and rock samples

Sample ID	U (ppm)	Th (ppm)	K%	eU/eTh	eTh/eU
S-1	4.47	20.37	2.39	0.22	4.56
S-2	5.32	22.61	3.21	0.24	4.25
S-3	4.38	14.57	2.71	0.3	3.33
S-4	2.99	15.09	2.55	0.2	5.05
S-5	1.77	6.07	1.3	0.29	3.43
Maximum	5.32	22.61	3.21	0.3	5.05
Minimum	1.77	6.07	1.3	0.2	3.33
Average	3.79	15.74	2.43	0.25	4.12
R-1	1.91	9.61	1.92	0.2	5.03
R-2	1.11	4.82	1.74	0.23	4.34
R-3	2.2	9.6	1.81	0.23	4.35
R-4	1.92	6.78	2.9	0.28	3.53
R-5	1.12	5.64	2.79	0.2	5.02
R-6	4.15	14.01	2.78	0.3	3.37
R-7	1.54	13.37	6.58	0.11	8.71
R-8	1.94	8.96	2.09	0.22	4.61
Maximum	4.15	14.01	6.58	0.3	8.71
Minimum	1.11	4.82	1.74	0.11	3.37
Average	1.99	9.1	2.83	0.22	4.87
UCC [47]	2.7	10.5	2.8	0.26	3.89

The eU/eTh ratio was being used as a redox indicator with a view to determine the paleo-oxygenation status of the studied deposits. A eU/eTh ratio less than 1.25 indicates oxic depositional conditions, as well as ratio more than 1.25 indicates sub-oxic and anoxic conditions [63-65]. In the present study, average eU/eTh ratio is 0.24 and 0.22 for soil and rock samples respectively, which suggesting the samples were deposited in an oxic environment.

eTh/eU ratio < 2 implies relative uranium enrichment and the presence of reducing conditions. The intermediate (2 to 7) facies is assumed to suggest poor weathering and quick deposition of igneous rock debris, as well as eTh/eU > 7, which indicate the eliminations of favorable enrichment, possibly due to leaching [36]. In the present study, Table 2 shows a wide range of eTh/eU ratio (3.32 to 5.05) for soil samples and (3.37 to 8.71) for rock samples. The average eTh/eU ratio was 4.12 for soil samples and 4.87 for rock samples which exceed the average UCC value (3.8) [66, 67], indicating intermediate to high facies, suggesting poor weathering and quick deposition of igneous rock detritus and low uranium content over the crust composition and both soil and rock samples are enriched in Th.

Radiological hazards

The measured Raeq (Table 1) was ranges from 204 to 274 Bqkg−1and 126 to 133 Bqkg−1with an average of 236 and 130 Bqkg−1 for soil and rock samples. Average Raeq value was below the maximum admissible value of 370 Bqkg−1 [4].

The estimated absorbed dose rates listed in (Table 1), range from 96 to 128 nGyh−1 and 60 to 65 nGyh−1with the average values of 110 nGyh−1and 63 nGyh−1 for soil and rock samples and this values exceed the accepted limit 55 nGyh−1 [4]. Absorbed dose rates vary according with the spatial variations of activity concentrations of radionuclide in minerals of soil and rock shown in (Fig. 4). Absorbed dose rates are found highest in S-2 location where 226Ra and 232Th concentrations are highest, and in R- 7 location the 40K concentrations is also reached to highest of the study.

Fig. 4

Absorbed dose rates due to the natural radioactivity in different samples sites of the study area

The outdoor annual effective dose equivalent values (Table 1) varied from 0.01 to 0.15 mSvy−1 and 0.05 to 0.157 mSvy−1 for soil and rock samples, respectively, with average of 0.113 mSvy−1 and 0.086 mSvy−1. The values for both soil and rock samples are somewhat exceed the world wide average of 0.07 mSvy−1 for outdoor annual effective dose [6].

External hazard index (Table 1) values for soil and rock samples varied from 0.55 to 0.74, and 0.34 to 0.35, including a mean of 0.63 and 0.35, whereas internal hazard index values for soil and rock samples ranged from 0.60 to 0.91 and 0.20 to 0.74, with a mean of 0.65 and 0.45. The hazard indexes were below the world permissible value 1 [6].

Statistical analysis

Pearson’s correlation coefficient analysis

Pearson correlation coefficient analysis has been used as a bivariate statistic to evaluate the mutual relationships and strength of the association between pairs of variables [68]. Pearson correlation coefficients among radionuclides and radiological hazard indices for all the studied samples are presented in Table 3. There is a strong positive statistical relationship between 232Th and 238U in all hazard indices Raeq, Hex, Hin, D, and AEDE with a P value < 0.02. A significant positive statistical relationship is observed between 226Ra and 232Th with correlation coefficient equals 0.891 because uranium and thorium decay series arise simultaneously [69]. In contrast, 40K showed a very weak correlation with 226Ra and 232Th (Bqkg−1), would be related to the high potassium solubility [70]. The competing chemical properties could affect the adsorption of these ions on clay particle and describe the association of 40K with 226Ra and 232Th series [71]. Furthermore, 40K has a strong positive correlation with D, and AEDE. Therefore, it can be presumed that, all the radionuclides 226Ra, 232Th and 40K, influence to the gamma emission in all places.

Table 3

Pearson correlation coefficients between radioactive variables in studied samples

Variables	238U	232Th	40K	Raeq	Hex	Hin	D	AEDE
238U	1
232Th	0.891**	1
40K	0.019	0.313	1
Raeq	0.744**	0.915**	0.658*	1
Hex	0.741**	0.912**	0.663*	1.000**	1
Hin	0.838**	0.951**	0.543	0.988**	0.987**	1
D	0.692**	0.879**	0.718**	0.997**	0.997**	0.973**	1
AEDE	0.691**	0.871**	0.723**	0.994**	0.995**	0.971**	0.999**	1

*Correlation is significant at the 0.05 level (2-tailed)

**Correlation is significant at the 0.01 level (2-tailed)

Principal component analysis

Factor analysis is a statistical tool for determining interconnections between radioactive variables and describing them in regards of their common underlying dimensions (factors) [72]. Only factors with eigenvalues greater than 1 are taken into account. Table 4 represents the varimax rotated factor loadings, including the eigen values and eigen communalities. The findings revealed that there were two eigen values greater than one, implying that these two elements could be related over 99.27% of the total variance is explained. PC1 accounted for 86.49% of the overall variation and all the hazard parameters originated from 232Th and 238U series are significantly loaded. The second component PC2, loaded with potassium and radiological hazard effects (D, and AEDE), which explain 12.779% percent of the total variance. The rotating factor loading of radiological parameters is shown in Fig. 5.

Table 4

Rotated factor loadings of PCA 1 and PCA 2

Variables	Component
	1	2
238U	0.987	-0.009
232Th	0.936	0.306
40K	0.019	0.998
Raeq	0.763	0.646
Hex	0.759	0.650
Hin	0.850	0.526
D	0.708	0.706
AEDE	0.703	0.710
Eigen value	7.780	1.150
Variance explained in%	86.49%	12.77%

Fig. 5

Rotated factor loadings of PCA 1 (86.49%) and PCA 2 (12.77%)

Conclusions

The radioactivity concentration of primordial 226Ra, 232Th, and 40K radionuclides was analyzed using a gamma-ray spectrometer for soil and rock samples collected from Swampy land and adjacent area of Jaintiapur, Sylhet, Bangladesh. The mean activity concentrations of 232Th and 40K in the studied soil and rock samples, as well as 226Ra in soil, were all above the world permissible value [6]. Whereas, the measured mean activity of 226Ra for rocks samples was within the world admissible levels of 35. The extracted values were, in general, higher than international reference value, however corresponding to other countries of the world didn’t significantly vary. Radiological safety impact parameters inclusive of estimated average values of Raeq as well as Hex and Hin were less than the sort of international reference value [6]. In radiological hazards analysis, absorbed dose rate D, and AEDE in studied samples were found exceed the standard limits for radiological safety. The processed statistical methods were also confirmed that these D, and AEDE are comes from both 232Th, 226Ra series and 40K series. This suggests that local residents of the studied area are exposed to radiation, indicating the need for further research in this area. Elemental eU/eTh and eTh/eU ratios indicated an oxic depositional environment, along with low uranium and high thorium concentration. So, further investigation is still needed to find out the potentiality of uranium deposition in this studied area of Dauki Fault Belt (DFB) zone. Moreover, this research work might be applicable for natural radioactivity mapping and will establish a reference data for future assessment of the surroundings of Jaintiapur.