Radiological Hazard of Limestone at Tse-Kucha Mining and Processing Site, Gboko, Nigeria

Background and Objective: Mining activities expose buried unstable radionuclides to the subsurface which releases harmful energy in form of radiation to the environment during decay. This study examined the radiological risk from radionuclides in limestone samples at the Tse-Kucha mining and processing site, Gboko, Nigeria and proffer radiation safety advice. Materials and Methods: The radiological assessment was carried out using radiation alert inspector Exp + for ambient radiation measurement, a Global Positioning System (GPS) for mapping sampled points and Sodium Iodide Thallium Activated [NaI(Tl)] detector for measuring activity concentrations and distribution patterns of the radioisotopes ( 40 K, 238 U and 232 Th). The study used Microsoft Excel and SPSS for radiological and Pearson correlation analysis. Results: Despite the high mean background radiation measurement of 2.445 mSv yr G 1 , accepted limits of 339.34±18.01 Bq kg G 1 for 40 K, 8.41±1.02 Bq kg G 1 for 238 U and 10.99±0.69 Bq kg G 1 for 232 Th were recorded. Similarly, the estimated radiation hazard parameters recorded mean concentrations within the UNSCEAR recommended values except excess lifetime cancer risk with 73.1E-5 against 29E-5. Conclusion: The study shows that the radionuclides are not evenly distributed in the limestone. The work also shows that continuous radiation exposure will enhance the tendency of suffering from cancer. As a result, the study recommends regular radiological studies of the area and the mandatory use of personal protective equipment when accessing the environment.


INTRODUCTION
The activities of mining and processing of limestone uncover the subsurface, encapsulated naturally occurring radionuclides ( 238 U, 232 Th and 40 K) which are significant sources of radiation 1 .These associated natural gammas, beta and alpha-emitting radionuclides find their way into the air and other environmental components, thereby elevating the natural background radiation of the environment.Depending on the level of human exposure, radiation from these radionuclides can cause respiratory diseases, skin diseases 2 , cataracts, cancer and other health challenges.As a result of these effects, a radiological study of environments with such activities is highly recommended, hence, this study.
The West African Portland Cement Plc Ewekoro, located in Southwestern Nigeria where mining and processing of limestone is common, was evaluated unsafe with high activity concentrations of 904.42±39, 296.86±12 and 171.14±6Bq kgG 1 in limestone against 500, 50 and 50 Bq kgG 1 for 40 K, 238 U and 232 Th, respectively 3 .The corresponding mean absorbed dose rate of 264.96 nGy hG 1 or 2.444 against 1 mSv yrG 1 in the air at 1 m above the ground was estimated from the study.In Kogi state, another high activity concentration of 40 K and 238 U radionuclides in limestone was reported from evaluating natural radionuclide contents in raw materials 4 .The study reported a mean activity concentration of 4694.0±366.0,547.0±242.0 and 0.0±32.0Bq kgG 1 for 40 K, 238 U and 232 Th in limestone, respectively and a high absorbed dose rate of 3.797 mSv yrG 1 .In 2010, a study was conducted on radionuclide pollutants in bedrocks (limestone and shale) and soils from the Ewekoro cement factory in Southwest Nigeria 5 .The assessment reported average specific activity concentrations of 35.86±7.06,91.30±2.33 and 5.75±2.57Bq kgG 1 for 40 K, 238 U and 232 Th from the limestone bedrock type, respectively and an absorbed dose rate of 0.39 mSv yrG 1 .In this study, only the activity concentration of 238 U was found to exceed the safe limit of 50 Bq kgG 1 recommended by UNSCEAR 6 .Similarly, a radiological assessment of limestone samples from Sinai and Eastern desert in Egypt estimated high activity concentration of 212.41±0.64Bq kgG 1 for 238 U radionuclide 7 .The 40 K and 232 Th activity concentrations in this study were observed to be within the UNSCEAR accepted limit, with activity concentration values of 151.16±0.10 and 22.97±0.20Bq kgG 1 , respectively.Furthermore, Malczewski and Żaba 8 evaluated the natural radioactivity in rocks of the Modane-Aussois (SE France) and reported safe activity concentrations of 18 Bq kgG 1 for 40 K, 26 Bq kgG 1 for 238 U and 0.7 Bq kgG 1 for 232 Th in limestone.Kehinde et al. 9 , also assessed the radionuclides in limestone at Ewekoro, South Western Nigeria in 2019 and reported high mean concentrations for 238 U and 232 Th.The study reported mean activity concentrations of 158.47±5.86,121.30±14.80 and 112.25±6.73Bq kgG 1 for 40 K, 238 U and 232 Th, respectively.Annual effective dose equivalent from outdoor terrestrial gamma radiation of 0.166 mSv yrG 1 was estimated from this study.Najam et al. 10 evaluated natural radioactivity levels of limestone rocks at eleven different locations in Northern Iraq.The study revealed high mean activity concentrations of 578.43 and 51.94 Bq kgG 1 for 40 K and 238 U, respectively from Sadbakhma and a corresponding annual effective dose equivalent from outdoor gamma radiation of 0.244 mSv yrG 1 .Natural radionuclides in rock and radiation exposure index from uranium mine sites in parts of Northern Nigeria were studied 11 .The study which did not report any significant radiation exposure to the workers and dwellers did not include Benue, the North Central part of Nigeria where the Tse-Kucha mining and processing site is situated.
At Yandev (near Tse-Kucha Mining and processing site), Gboko, Nigeria, a study of limestone elemental constituents and concentration revealed a harsh relationship between the limestone deposit and health 12 .Though the authors suggested proper management of wastes and particulate emissions by the company to attain environmental safety, the radiation hazard analysis of radionuclides (which act as a significant source of radiation) in limestone at the mining site was not evaluated.Another environmental study conducted around the mining site (study area) is soil analysis.It indicated that the soils are polluted with some oxides and heavy metals originating from limestone mining and cement production 13 .This study only reported the chemical pollution of the soil, the radiological risk from the radionuclides in the environment was not reported.At the same location, an appraisal of the social and health impact of the Dangote cement plant was carried out and severe health-related impacts of limestone mining and cement production on the populace were confirmed 14 .The study recommended measures to curb these effects but did not evaluate the radiation profile of the study area.Both Jibiri and Temaugee 15  In this current study, the radioactivity content of radionuclides in limestone and radiation hazard indices as well as the relationship between the radionuclides and hazard indices at the Tse-kucha mining and processing site, Gboko, Nigeria, has been determined along with appropriate recommendations.

MATERIALS AND METHODS Study area:
The study area is the Tse-kucha village limestone quarry site of Dangote Cement Company Gboko plant B. It is situated at kilometre 72 Makurdi, Gboko Road, on latitude 7E20 N and 7E30 N and longitude 8E56 E and 9E00 E within the Benue trough.
In situ measurements: A radiation alert inspector Exp + (Serial No: 24650), a Global Positioning System (GPS) and a measuring tape were used to carry out the in situ radiation measurements.
The ambient radiations in mR/hr and geographical location at 10 different points within the study area were carefully taken using the radiation alert inspector Exp + and GPS, respectively.

Laboratory measurement
Sampling: From the administrative office, ten samples of limestone were collected weighing 500-1000 g in intervals of 100 m in the study area and stored in thoroughly rinsed black polythene bags to avoid contamination.

Sample preparation for laboratory analysis:
The samples were dried and crushed separately into fine particles and stored in beakers for at least 30 days to attain secular equilibrium 1 .
Laboratory analysis: Using Sodium Iodide Thallium activated [NaI(Tl)] detector, the activity concentrations of 40 K, 238 U and 232 Th in Bq kgG 1 were carefully measured.

Computation of radiological risk parameters Radium equivalent index:
The radium equivalent index (Ra eq ) is an index for assessing activity concentrations and accounting for the radiation effects of radionuclide materials containing 238 U, 232 Th and 40 K.It is given by the Eq. 1 17,18 : External hazard index: It is denoted by (H ex ) and used to evaluate supplemental radiation hazards of natural gamma radiation.The H ex is given by Tawfik et al. 18 , in the Eq.2: (2) Internal hazard index: It is denoted by (H in ) and used to estimate radiation-based internal dangers such as asthma and other respiratory diseases.H in is given by: (3) Outdoor absorbed dose rate (D out ): Outdoor absorbed dose rate (D out ) is a measure of the energy deposited in a medium by ionizing radiation per unit mass.D out , measured in nGy hG 1 is given by Qureshi et al. 19 , in the Eq.4: Indoor absorbed dose rate (D in ): D in measured in nGy hG 1 , is given by 19 :

Annual effective dose equivalent of radiation based on absorbed dose rate of radiation in air (E):
It estimates the average effective dose equivalent received by a person.The annual effective dose is divided into annual outdoor and annual indoor effective doses given as 20 : Where, D out = Outdoor absorbed dose rate, D in = Indoor absorbed dose rate, OF = Occupancy factor/time of stay in the outdoor (20% of 8760 hrs = 1752 hrs) while the occupancy factor under the annual indoor effective dose is 80% of 8760 hrs = 7008 hrs and CF= Conversion factor (0.7 Sv GyG 1 ).

Annual Gonadal Dose Equivalent (AGDE):
Annual Gonadal Dose equivalent is a parameter used to monitor the radiation sensitivity of the reproductive organs such as the testis and ovaries.It also indicates the radiation dose level absorbed by the bone marrow.AGDE can be estimated using the Eq. 8 21 :

Excess Lifetime Cancer Risk (ELCR):
The excess lifetime cancer risk from the ionizing gamma rays is computed using the equation by Sayed et al. 18 : Where, E total (E in +E out ) is the annual effective Dose Equivalence, DL is the average duration of life estimated to be 70 yrs and RF is the risk factor (i.e.fatal cancer risk) given as 0.04 SvG 1 [4×10G 5 (mSv)G 1 ].

Pearson correlation coefficient analysis:
The correlation coefficient between variables ranges from -1 to +1.A positive correlation indicates a direct relationship between variables, while a negative correlation indicates an inverse relationship between variables.A correlation coefficient of 0-0.49indicates a weak relationship between the variables while a correlation coefficient of 0.5-1 indicates a strong relationship between the variables.

Note:
Where, A U , A Th and A K represent the specific activity concentrations of 238 U, 232 Th and 40 K, respectively.

RESULTS AND DISCUSSION
Figure 1 shows the area contour map which identifies the hilly/elevated areas with the higher radiation levels in the field represented by the closed loops which spread out through the low land areas with fewer radiation levels compared to the hilly areas.The sky blue colour represents the hilly/elevated areas with the highest radiation levels between 0.039 and 0.042 mR hrG 1 , while the deep blue represents low land areas with the lowest radiation levels between 0.019 and 0.023 mR hrG 1 .All radiation levels exceeded the 0.01 mR hrG 1 UNSCEAR standard.  and also less than the 4694.0±366.0 and 547.0±242.0Bq kgG 1 mean concentrations for 40 K and 238 U, respectively but higher than 0.0±32.0mean concentration for 232 Th obtained by Ajayi et al. 4 .The values obtained in this study are also seen to be less than the 212.41±0.64 and 22.97±0.20Bq kgG 1 mean concentration for 238 U and 232 Th, respectively but higher than 151.16±0.10Bq kgG 1 mean concentration for 40 K obtained by Fakeha et al. 7 16 ), but within acceptable limits for 232 Th (except Awodugba et al. 3 and Oyeyemi et al. 9 ) and 40 K (except Awodugba et al. 3 , Ajayi et al. 4 and Najam et al. 10 ).0.01±0.000.01±0.002.62±0.155.00±0.280.00±0.000.02±0.000.02±0.000.019 5.6 D-700 m 7.34±0.460.02±0.000.02±0.003.68±0.227.07±0.440.00±0.000.03±0.000.03±0.000.027 8.4 E-600 m 133.56±9.580.36±0.030.46±0.0464.53±4.50126.19±8.930.08±0.010.62±0.000.71±0.010.469 198.8 F-500 m 11.69±0.790.03±0.000.04±0.005.65±0.3711.08±0.740.01±0.000.05±0.000.05±0.000.041 14 G-400 m 102.44±6.70 0.28±0.020.31±0.0250.96±3.2598.16±6.320.06±0.000.48±0.030.54±0.030.373 151.2 H-300 m 33.33±1.860.09±0.010.09±0.0117.56±0.9833.67±1.870.02±0.000.17±0.010.19±0.010.13 53.2 I-200 m 14.52±0.880.04±0.000.04±0.037.17±0.4313.73±0.820.01±0.000.07±0.000.08±0.000.052 22.  20 of 0.029±0.00mSv yrG 1 for outdoor annual effective dose equivalent, 0.02±0.00-0.62±0.00mSv yrG 1 with an average value of 0.231±0.01mSv yrG 1 for indoor annual effective dose equivalent and 0.02±0.00-0.71±0.01mSv yrG 1 with an average value of 0.251±0.01mSv yrG 1 for total annual effective dose equivalent (E out +E in ).Similarly, these average values are within the worldwide average limit of 59 and 84 nGy hG 1 for outdoor and indoor absorbed dose rates, respectively, 0.07 and 0.41 mSv yrG 1 for outdoor and indoor annual effective dose equivalent and 0.48 mSv yrG 1 for total annual effective dose equivalent 3 .Annual Gonadal Dose Equivalent (AGDE) and Excess Lifetime Cancer Risk (ELCR) are also represented in the Table with values ranging from 0.019-0.469mSv yrG 1 with an average value of 0.032 mSv yrG 1 for annual gonadal dose equivalent and 5.6E-5-198.8E-5with an average value of 73.1E-5 for excess lifetime cancer risk.The Table shows that the average annual gonadal dose equivalent from this study is within the worldwide average limit of 0.3 mSv yrG 1 while the average value of excess lifetime cancer risk exceeds the accepted limit of 29E-5 6 , implying that long-term exposure to radiation from the study area will enhance the danger of suffering cancer.The mean excess lifetime cancer risk from the present study was compared with the reported mean value from Olanrewaju and Avwiri 16 , who assessed the hazard index in limestone using only the outdoor annual effective dose equivalent from the same study area.The analysis shows that excess lifetime cancer risk from this study exceeds the previous study (6.7E-5) by over 90%.However, the mean total annual effective dose equivalent from this study is above the 0.24 and 0.166 mSv yrG 1 values from Sadbakhma, Northern Iraq 10 and Ewekoro, SW Nigeria 9 , respectively but far greater than the 0.076 and 0.019 mSv yrG 1 obtained previously from Tse-Kucha, Nigeria (2013) 15 and Tse-Kucha, Nigeria (2017) 16 , respectively.This variation of annual effective dose equivalent results from Tse-Kucha, Nigeria, is because these previous studies evaluated only the outdoor annual effective dose equivalent while the present study evaluated the total annual effective dose equivalent (i.e., sum of outdoor and indoor annual effective dose equivalent).This change is also associated with the difference in the number of samples considered for the analysis.Figure 3 compares the total Annual Effective Dose Equivalent (AEDE) from the present study with the UNSCEAR accepted limit of 1 mSv yrG 1 for the general populace.It shows that the mean value of the total annual effective dose equivalent estimated from this study is within safe limits.

Fig. 4 :Fig. 5 :Fig. 6 :
Fig. 4: Correlation plot between 40 K and 238 U 16d Olanrewaju and Avwiri16had previously assessed the radiological hazard from limestone at various locations.Jibiri and Temaugee 15 reported mean activity concentrations of 124.81 and 12.30 Bq kgG 1 for 40 K and 232 Th, respectively, with 61.90 nGy hG 1 absorbed dose rate and 0.0761 mSv yrG1outdoor annual effective dose rate from limestone samples, while Olanrewaju and Avwiri16reported mean activity concentrations of 155.44, 11.81 and 12.43 Bq kgG 1 for 40 K, 238 U and 232 Th, respectively, 15.59 nGy hG 1 absorbed dose rate and 0.019 mSv yrG1outdoor annual effective dose rate.Both assessments were carried out with only two samples each within the study area.It is also observed that the correlation analyses which reveal the relationship between the radionuclides and hazard indices were not reported in the studies.

Table 2
compares the mean activity concentrations of 40 K, 238 U and 232 Th of the present study with other studies around the world.The values in this study are observed to be less than the 904.42±39, 296.86±12 and 171.14±6Bq kgG 1 mean concentrations for 40 K, 238 U and 232 Th, respectively reported byAwodugba et al.

Table 2 :
16mparison of mean activity concentration (Bq kgG1) of Radionuclides in the present study and other studies around the Qureshi et al.19values of this study are higher than the 35.86±7.06 and 5.75±2.57BqkgG 1 for 40 K and 232 Th mean concentrations, respectively but less than 91.30±2.33BqkgG1for 238tudy of radionuclide pollutants in bedrocks from Southwest Nigeria 5 and also higher than 18 and 0.7 Bq kgG 1 for 40 K and 232 Th mean activity concentrations, respectively but less than 26 Bq kgG 1 for 238 U from a study of natural radioactivity in rocks of the Modane-Aussois (SE France)8.The table also shows the comparison of the mean concentration of the present study to be much lower than 121.30±14.80 and 112.25±6.73BqkgG 1 values for 238 U and 232 Th, respectively estimated by Oyeyemi et al.9, but higher than 158.47±5.86BqkgG 1 for 40 K radionuclide.T mean concentration values from this study are also seen to be much lower than 578.43 and 51.94 Bq kgG 1 for 40 K and 238 U concentration values from Najam et al.10.Similarly, the mean concentrations from this study are seen to be lower than the mean concentration value of 12.30 Bq kgG 1 for 232 Th but higher than the value of 124.81 Bq kgG 1 for 40 K obtained by Jibiri and Tamaugee15and lower than the 155.44, 11.81 and 12.43 Bq kgG 1 for 40 K, 238 U and 232 Th, respectively reported by Olanrewaju and Avwiri16.The table also shows that the values are higher than the recommended values for 238 U (except Malczewski and Żaba 8 and Olanrewaju and Avwiri

Table 4 :
Comparing mean annual effective dose equivalent (mSv yrG 1 ) of radiation of the present study with other studies around the world

Table 4
20esents the comparison of the mean annual effective dose equivalent of the present study with other studies around the world.It is seen that the present study reports the least dose values except for Ewekoro, SW Nigeria 9 , Sadbakhma, Northern Iraq 10 , Tse-Kucha, Nigeria (2013)15and Tse-Kucha, Nigeria (2017)16.It is far less than 2.444 and 3.797 mSv yrG 1 measured at West African Portland Cement Plc, Nigeria3and Obajana, Kogi State, Nigeria4, respectively but slightly less than 0.270, 0.290 and 0.390 mSv yrG 1 , measured at Baathra, Northern Iraq 10 , Minia, Egypt20and Southwest Nigeria 5 , respectively.

Table 5 :
Pearson correlation coefficient among the estimated quantities

Table 5
presents the pearson correlation coefficients among the estimated quantities.The Tableshowsa strong, positive correlation between any two quantities estimated with the least coefficient of +0.727, a value obtained after the correlation analysis between232Th and 238 U.The highest correlation coefficient from this analysis has the maximum value of +1.This value is commonly seen from the analysis of the same parameter.From the Table, the outdoor absorbed dose rate (D out ) with the highest correlation coefficient of +1 followed by indoor absorbed dose rate (D in ), indoor annual effective dose equivalent (E in ), total annual effective dose equivalent (E total ), Annual Gonadal Dose Equivalent (AGDE) and Excess Lifetime Cancer Risk (ELCR), shows that it has a better relationship among estimated parameters than any other hazard parameter.Figure4-6show the correlation plot between the radionuclides.From the correlation plots, a closer interrelationship between 40 K and 238 U radionuclides was revealed in this study.