Survey of Natural Radioactivity of Soil and Rock Samples and their Radiological Implications to Human Health in Ugwuele Quarry Mining Site, Uturu, Abia State, Nigeria

: This work measured the activity concentration of natural radionuclides 238U, 232Th and 40K in soil and rock samples collected in Ugwuele mining site and it surrounding communities in Uturu, Abia state, Nigeria. Fifteen different samples collected were analyzed. A NaI (Tl) spectrometry detector system was adopted for this research. The results obtained were used to estimate the radiological implication of the quarry mining in the study area. The average gamma activity obtained for 238U, 232Th and 40K in soil were 53.39 ± 3.08, 76.79 ± 9.66 and 1061.02 ± 15.72 Bq/kg respectively and 55.09 ± 5.71, 90.96 ± 16.10 and 1281.78 ± 6.38 Bq/kg respectively for rock samples. The calculated radiological hazard indices from Absorbed dose rate, Annual effective dose, Radium equivalent, internal hazard index and External hazard index gave 120.25 nGy/h, 0.147 msv/yr, 247.5 Bq/kg, 0.833 and 0.687 respectively. The average dose rate was higher than the recommended limit. Therefore, soil and rock materials found in Ugwuele could increase the radiological hazards on miners and residents of the area due to long time cumulative exposure.


INTRODUCTION
Natural radioactivity has become a burning issue for most researchers, due to the health implication of natural occurring radioactive materials (NORMS) on man.Human are regularly exposed to natural radioactivity through divers sources as well as some special human activities.Among these human activities are mining of solid minerals and introduction of fertilizers to farmlands which have been associated with increase in natural radioactivity concentration of any location (Ibikunle et al, 2013).The continuous distortion of geological formation of the earth arrangement through the process of mining has in no small measure contributed to increase in radioactivity concentration in our environment especially around the mining area.(Echeweozo and Igwesi, 2021).In addition to that, most inorganic fertilizers used in farmlands, produced from Phosphate material have high concentration of naturally occurring radionuclides.Therefore, these farmland are expected to exhibit elevated level of radioactivity when compared with virgin farmlands (Fasasi et al., 1999).The radiological implication of elevated radioactivity concentration of natural occurring radioactive materials in mines and adjoin farmlands cannot be over emphasized due to associated risk to miners and the teaming farming population (Oladapo et al., 2012).Although many studied have considered natural radioactivity in different mining sites and communities but for the first time, this study considered the radiological implication of the mining activities in Ugwuele mining site in Uturu, Abia State Nigeria and its impact on the adjoining farmlands and village quarters.The study also evaluated the radiological hazard indices of contiguous farmlands and village quarters with respect to the mining site.The knowledge of distribution pattern of these natural radionuclides is necessary for the determination the level of exposure of humans around these locations and estimating the associated health risk.Hence, the objective of this paper is to survey the natural radioactivity of soil and rock samples and their radiological implications to human health in Ugwuele Quarry Mining Site, Uturu, Abia State, Nigeria.

MATERIALS AND METHOD
Study Area: A cross sectional survey study approach was adopted in this work.Soil and rock samples were collected across the vicinity of Ugwuele quarry mining site and the host community in Uturu Isiukwuato L.G.A in Abia State which lies between latitude 5 o 33'N and 5 o 55'N to longitude 7 o 22'E and 7 o 42'E.The soil type in Ugwuele is characterized with false bedded sand stones of the Mastrichitan geological era with igneous outcrop (Anozie et al 1978).Three sites chosen for this study include Ugwuele quarry site, the village quarters (Ugwuele community) and the adjoining farmlands.The village quarters is about two kilometers from the mining site while the farmland is about one kilometer from the mining site.Sample collection and preparation: The choice of sampling locations were based on their proximity to the mining site and the determination to compare the rate radionuclide mobility within the mining site.Five soil samples were collected from each of the sampling area while four rock samples were collected from their bedrock in the quarry site.At each location soil samples were collected from approximately 1-2 meters apart using hand auger.Samples were obtained at depth of 5 cm from the top surface soil layer (Abaddy et al, 2014).Samples were collected on 13 th June, 2023.300 g of each of the soil samples were labeled and packaged in a plastic bag.Rock samples collected from different sections of the quarry pit were crushed and 300 g of each of the samples were also packaged and labeled.At the laboratory, soil and rock samples were placed in an oven at temperature of 60 o for 24hours to remove residual moisture.The dried samples were pulverized into fine powder from 1mm mash size (AST M No. 18).Sample were homogenized with electric stirring device.250 g of each of homogenized samples were then packed in a well-sealed 7 x 9 cm marinelli beaker and stored for 28days to attain radioactive equilibrium between radon 222 and radium 226 prior to spectroscopic measurement and analysis.(Avwiri, et al. 2013).The activity concentration of 238U, 232Th and 40K were measured with thallium scintillated 3cm x 3 cm Sodium Iodide (NaI(Ti) detector (model no 3M3/3) at the Center for Energy Research and Development, Obafemi Awolowo University , Ile-Ife.The detector was enclosed in a 5cm thick cylindrical lead shield to reduce the background radiations from possible natural radiation sources and to isolate it from other radiation sources used in the surroundings.

Energy and Efficiency calibrations:
The detection energy calibration of the system was done with reference standard sources (IAEA-444) Prepared from the radiochemical center Amersham, England.The 1.460MeV photo peak was used for the measurement of 40K while the 1.120MeV photo peak from 214Bi and the 0.911MeV photo peak from 288Ac were used for the measurement of 238U and 232Th respectively.Each of the samples was counted for 36000 seconds (10 hours).The background spectrum distribution was determined by counting an empty container with the same geometry as the measured samples.Standard sources with known gamma energies and activity similar to the measured samples were used for energy calibration (see Table 2).These standard sources were prepared by Isotope product laboratories Burbank California, USA.The spectrometer system was adjusted to position the 662KeV photo peak of the 137Cs gamma line at about one third full scale and also to obtain the 0.5KeV per channel in order to obtain the highest resolution of the measuring system.The gain range and LLD of the ADC were positioned at 4K, 8K, 0.3V respectively while the amplifier coarse gain and the shaping time were positioned at 20, 0.5 and 4µs respectively.This settings were kept constant throughout the period of measurement.The absolute efficiency Eabs of a detector for each photo peak representing a particular photo energy of a standard source whose emission probability Pr was calculated with Equation 1 as shown in Table 2 Where (  ) is the peak area and tc is the counting time.Results from the calculations are shown in Table 2 The dictation limit which is the level the true net count will be dictated above acceptable level (Lo) with a given probability when real activity is present was calculated with Where   is the standard deviation of the number of counts when a blank sample is measured to determine the background count.
The minimum detectable amount of concentration is given by Equation 3 Where Sn = the estimated standard error of the net count rate of the nuclide, n; Pγ = the absolute γ ray emission probability of nuclide n; ∑   = the absolute efficiency; M = the mass of the sample in Kg In order to determine background radiations around the detector, background measurement was counted for 10hours with no radiation source around the NaI(Ti) detector.

Measurements of radionuclide concentration in all samples:
The measurement of radioactivity concentrations of all samples was carried out with the calibrated 3" x 3" NaI(Ti) detector with PMT base coupled to a Canberra series 10 plus multi-channel analyzer (MCA) which was linked to a preamplifier base.All samples were counted at a constant geometry and for a constant time of 10 hours.The counting technique applied in this work is described in (Echeweozo and Okeke 2021).The activity concentrations of radionuclides in each sample were obtained from total counts at peak emissions by subtracting background counts and divided by efficiency of the photopeak, gamma intensity of the radionuclide, mass of the sample and counting time.
The activity concentrations (Ci) of 40K, 238U and 232Th in Bq/kg were estimated using Equation 4  = Cn is the net count rate under the corresponding peak.  is the absolute transition probability of specific ray.M is the mass of samples (kg) and ∈ is the detector efficiency at a specific gamma energy.The total error in the determination of the specific activity depends on the error in determination of nuclide specific counting efficiency and the statistical counting errors error in determination of counting efficiency ranged from 1-4%.
Absorbed dose rate in air (D): When radiation passes through an object, part of the energy will be transferred to the material.These transferred radiations is referred to as absorbed dose.The permissible absorbed dose rate of 60 nGy/h was prescribed by UNSCEAR (2000).The mean activity concentration of 40K, 238U and 232Th in Bq/kg in the soil and rock sample were computer with Equation 5 (Abojassim et al, 2014).
Where D is the absorbed dose rate in nGy/h and Ak, AU and ATh are the activity concentration of 40K, 238U and 232Th respectively.

Annual effective doses exposure (AEDEs):
This measures the health risk to human beings due to radiation exposure.In this study the outdoor annual effective dose exposure was computer with Equation 6 (Ugbede, and Echeweozo 2017).The worldwide range of the annual effective dose is 0.3 -0.6 mSv with average of 0.48 mSv (UNSCEAR 2000).
Radium equivalent dose (Req): Radium equivalent dose gives the combined effects of specific activities of (226Ra, 232Th and 40K) in a measured sample.The published maximum admissible value is 370 Bq/Kg (UNSCEAR (2000).It was deduced with Equation 7.
External and Internal Hazard Indices: External radiation exposure which is the total radiations dose due to the presence of 226Ra, 232Th and 40K is assessed by external hazard index, Hex.Its level is calculated by Equation ( 8 The internal exposure to radon and its progenies is evaluated with the internal hazard index, Hin, as shown in Equation ( 6).

RESULTS AND DISCUSSION
Radioactivity in samples: Table 3 represents the specific activity concentration of natural radionuclides in rock and soil samples from the studied area.The mean activity concentration of 40K, 238U and 232Th were found at 1061.02 ± 15.72, 53.39 ± 3.08 and 76.79 ± 9.61 Bq/kg respectively for all samples.These values were significantly higher than the world average values of 400, 35 and 30 for 40K, 238U and 232Th respectively.It can also be observed that the highest activity concentration of these radionuclides were observed from the quarry rock sample followed by farm land and the Village quarters.These large variation could be attributed to heavy quarry activities at the quarry site when compared to the passive farm land as shown in Figure 2. The elevated values of activity concentration observed in all sample were suspected to be influenced by geological nature of site, heavy quarrying activities and high usage of potassium fertilizer in the farmland region of the studied area.

Assessment of radiological hazard:
The assessment of radiological hazard could be considered in different ways to estimate the radiation exposure dose and to assess the biological and radiological effect of these natural occurring radionuclides to humans and the environment.In this study, five radiological hazard indices considered are (i) Absorbed dose rate (D) in air, (ii) Annual effective dose equivalent (AEDE) (iii) Radium equivalent dose.(iv) External hazard index (Hex) (v) Internal External hazard index (Hin) as shown in Table 5.The absorbed gamma dose rate in air due to the distribution of 40K, 238U and 232Th were computer and compared with recommended limit of 55nGy/h (UNSCEAR 2000).It was observed that the highest contributors to the total dose rates in the study area was from the rock samples from the quarry site.
The mean absorbed dose rate in the air of the study area is 120.55nGy/h.it was also observed that this mean value was greater than acceptable dose level.The absorbed dose is a function of gamma rays (γ) emission and activity concentrations of natural occurring radionuclides.Therefore, the radiological hazard of mining activities on workers and residents of the study area is high.
Conclusion: 238U, 232Th and 40K gave 53.39 ± 3.08, 76.79 ± 9.66 and 1061.02 ± 15.72 Bq/kg respectively for soil samples and 55. 09 ± 5.71, 90.96 ± 16.10 and 1281.78 ± 6.38 Bq/kg respectively for rock samples.The Absorbed dose rate, Annual effective dose, Radium equivalent, internal hazard index and External hazard index gave 120.25 nGy/h, 0.147 msv/yr, 247.5 Bq/kg, 0.833 and 0.687 respectively.The average dose rate was higher than the recommended limit.Therefore, rock materials found in Ugwuele could increase the radiological hazard on miners and residents.Conflicts of interest: There are no conflicts of interest.

Fig 1 :
Fig 1: Map of Isiukwuato L.G.A of Abia State, Nigeria with Ugwuele community in circle (Source: Anozie et al, 1978) )(El-Taher et al, 2010) hazard index which are the measure of hazardous effects of radon and its shortlived products to the respiratory organs in the body was evaluated with Equation (8 and 9).

Fig. 2
Fig. 2 Comparison Activity concentration of 40K, 238U and 232Th for all sample sites

Table 1 :
Channel for various energies of the Energy calibration sources