Radiological Impact Assessment of Mining on Soil, Water and Plant Samples from Okobo Coal Field, Nigeria

Anthropogenic, including mining activities leaves the environment contaminated with potentially toxic substances and remote hazards if not properly checked. The quest in this study is the levels of naturally occurring radionuclides in Okobo coal, Nigeria and their possible distribution in coal mine vicinity soils, water and plants (cassava). Samples were characterized for levels of radionuclides and radiological detriments using high resolution gamma spectrometer, Gamma ray liquid scintillation and applicable radiological hazard indices. The range of mean activity concentrations (Bq·kg −1 ) for the environmental samples are as follows: 226 Ra (8.39 ± 1.0 to 77.6 ± 4.0), 232 Th (0.470 ± 0.4 and 77.8 ± 2), and 40 K (29.1 ± 0.4 and 289 ± 6), with their respective mean values of 32.7 ± 2.1, 54.0 ± 1.5 and 158.8 ± 3.1 (Bq·kg −1 ). Radiological detriments including radium equivalent activity (Ra eq ), external hazard index (H ex ), internal hazard index (H in ), radioactivity level index or gamma index (I γ ) and the ELCR for coal sample is 96.94 Bq·kg −1 , 0.26, 0.30, 0.69, and 1.56 respectively. Reported values were below the safety limits stipulated by UNSCEAR and implied that the environment is relatively safe with low levels of natural radioactivity. Over-all, this background study has demonstrated that Okobo coal mine is a less ra-dio-hazard contributor to environmental samples. Exceptions to this generalization are representative gamma index (I γr ) and annual effective dose equivalent values for some cassava and soil samples, which may call for future impact monitoring.


Introduction
In spite of its abundant deposits in Nigeria, Coal does not presently contribute to Nigeria's electricity generation. The problem with mining activities in Nigeria is the negligence of the stakeholders (miners, businessmen and the government) to provide best global mining practices.
The unsuspected populace that constitute these communities where coal mining activities take place, do not know their environmental obligations under the minerals and mining act. This affects their rights including right to education, infrastructures, life, security, health, adequate standard of living, liberty (Sambo et al., 2012).
Threats, linked to the residual and temporal side effects of coal mining on the environmental and agricultural dimensions are well documented (Bergh et al., 2011;Odunayo et al., 2016). There are strong indications of coal's impact on human health and the environment. Air pollution generated by coal mining and combustion in power plants (Din et al., 2013) can affect the environment and human health. Key pollutants from coal mining of adverse impacts on the environment and health include oxides of C, S, and N. Others include particulate matter, heavy metals (Ikwuagwu, 2017) and organic pollutants (PAHs, PCBs among others) even at low concentration pollute the environment.
Coal mining impact on groundwater quantity and quality, as it also impacts on river flows and consequential impact on agricultural land uses cannot be overemphasized (Ikwuagwu, 2017). Environmental impacts for coal mining range from mining subsidence, changes in ground water regimes and mining hydrology. There is also the release of methane into the atmosphere, the release of contaminated water and the generation of solid waste products.
Naturally occurring radionuclides are reported in varying proportions in rocks and soil of several geo-formations, including coal mine. These radionuclides may be deposited in sediment and dissolve into drinking water, thereby leading to human exposure (Raymond et al., 2013).
Uranium ( 238 U), Thorium ( 232 Th) and their decay products ( 226 Ra, 212 Pb, etc.) and Potassium isotope ( 40 K) are the natural radionuclides observed as inherent soil contents with known contribution to the radiation exposure and emission of gamma ray and subsequent ingestion through inhalation and the food chain (Mujahid & Hussain, 2011).
The potential radiological hazards parameters investigated in this study include the radium equivalent activity (Ra eq ), the annual effective dose rate (H R ), external hazard (H ex ) and internal hazard (H in ) indices, Representative Gamma Index (I γr ) and Excess Lifetime Cancer Risk (ELCR). Others (not considered in this study) are the activity utilization index (I), and the alpha index (I α ). The knowledge of radionuclides distribution and radiation levels in the environment is important for assessing the effects of radiation exposure due to both terrestrial and extra-terrestrial sources (Rajesh et al., 2013). If the values of the hazard indices are below the recommended levels; therefore, it is concluded that the environment is relatively safe.

Study Area
Okobo community, is a small town in Enjema district of Ankpa Local Government Area (7˚22'14"N, 7˚37'31"E) in Kogi state with reserves of up to 380 million tonnes of coal.
Studies on radiological hazards and detriments are widely documented (Bachama et al., 2017;Alausa et al., 2019;llori & Alausa, 2019). In this study, we focus on the levels of radiation exposure due to the natural radionuclides ( 238 U, 232 Th and 40 K) in Okobo coal and their vicinity soils, water and cassava plants.
The study, for the first time unveiled the radiological detriments associated with the impact of coal mine activities within the study area.

Materials and Methods
Analytical grade reagents were used in this study. Instrument employed for analysis include high resolution gamma spectrometer at the Center for Energy Research and Training (CERT), Ahmadu Bello University Zaria, consisting of high purity germanium (HPGe) detector, by Ortec Inc., connected to an Ortec series multichannel analyzer (MCA) through a preamplifier base and coupled to a personal computer. Water samples were analyzed using the liquid scintillation counter (Tri-CarbLSA1000).

Sampling
Water sampling at Okobo coal field was carried out from the months of November to December, 2018 and January, 2019. Method, described in Table 1 for the first stage of this study (Itodo et al., 2020) was followed. Water samples were collected at a depth of 2 -10 cm at the water surface using a 1litre plastic container with a screw cap, with respect to the coal mining operation site. Water samples were collected between 7 am and 10 am) in triplicates from 3 locations (50 cm upstream, midstream and downstream) with pre-cleaned glass bottles.
The samples in 1-L amber bottles were adjusted to pH < 2 using 6 M of hydrochloric acid (Adeniji et al., 2019). Dried samples were grinded and passed through a 2 mm sieve. 100 g of each sample were placed in a radon-tight plastic vessel. The vessels were weighted and sealed for 30 days to allow equilibrium in the 238 U and 232 Th with their respective progeny.
Coal collection was based on methods documented by Odunayo et al. (2016) and Querol et al. (1996) were carefully followed. A sample grid was established in which five samples of mass 20 g of coal was obtained from a split which was taken each 50 m away from the grid and then harmonized as one sample by means of a hand trowel and a hammer at the coal site. A gross sample of 60 g was obtained after homogenisation of the five samples. The gross coal samples were air-dried, milled and split carefully to obtain a representative 10 g sub-sample of particle size < 250 µm. Coal sample preparation involved crushing, screening and storage.
In crushing of coal, samples were reduced to smaller sizes by mechanical means, followed by pulverising in a rotary mill. The powdered samples were passed through a 210 micron sieve to prepare a representative 50 g sub-sample for analysis. The sample was tagged as Okobo coal (OC).
Cassava samples were randomly collected as donations based on farm basket approach. Control samples for the duo (soil and cassava) were collected from Ogaji district (6 -7 km away from Okobo), following similar protocol. Control water sample is a commercial and registered potable water.

γ-Ray Spectrometric NaI(Tl) Analysis
The soil, coal and cassava samples were dried and crushed to fine powder with the use of pulverizer. Packaging of the samples into radon impermeable cylindrical plastic containers was selected based on the space allocation of the detector vessel. To prevent radon-222 escape, the packaging in each case was triple sealed, which included smearing of the inner rim of each container lid with Vaseline jelly, filling the lid assembly gap with candle wax to block the gap between lid and container and tight-sealing lid container with masking adhesive tape. Radon and its short lived progenies were allowed to reach secular radioactive equilibrium by storing the samples for 30 days prior to gamma spectroscopy measurement.
The γ-ray spectrometer NaI(Tl) is equipped with NaI(Tl) detector that measures the natural radio-nuclides activity (count rate in the environmental samples). It was calibrated using known source such as 60 Co and 137 Cs point sources. In order to calculate the radionuclide activity concentration (activity per unit mass)

A. U. Itodo et al. Journal of Geoscience and Environment Protection
for each gamma ray photo-peak, we rely on the secular equilibrium between parents and daughters in the samples, the Equation (1) was used (Alashrah & Taher, 2017).
The analysis was carried out using a 76 × 76 mm NaI (Tl) detector crystal optically coupled to a photomultiplier tube (PMT). The assembly has a preamplifier incorporated into it and a 1 kilovolt external source. The detector is enclosed in a 6 cm lead shield with cadmium and copper sheets. This arrangement is aimed at minimizing the effects of background and scattered radiation. The samples were measured for a period of 29,000 seconds, for each sample. The peak area of each energy in the spectrum was used to compute for the activity concentrations in each sample by the use of the following equation: where: C = activity concentration of the radionuclides in the sample given in Bq·Kg −1 ; C n = count rate (counts per second).

Count per second cps Net Count
Live Time = (2) C fk = Calibration factor of the detecting system.
The measuring time for gamma-ray spectra was 43,200 s. To obtain the same gamma dose rate, the activity concentration from the three radionuclides assuming to be 370 Bq·kg −1 from 226 R, 259 Bq·kg −1 from 232 Th and 4810 Bq·kg −1 from 40 K. This is the definition of radium equivalent and is given as Equation (3): where A Ra , A Th and A K are the activity concentrations of 226 Ra, 232 Th and 40 K in Bq·kg −1 , respectively.

Liquid Scintillation Counting
Water sample (10 mL) was added into a scintillation vial containing 10 mL of installgel scintillation cocktail and analyzed using the liquid scintillation counter (Tri-CarbLSA1000). Initial Counting was carried out immediately the prepared samples were brought to the laboratory. Two litters of water sample treated with 10 mL of concentrated Nitric acid were used for each sample. Evaporation was achieved using beaker (500 mL) set on hot plate at a steady temperature below boiling point to avoid boiling of the water sample. This prevents loss of much residue. The volume evaporated was taken and recorded. After surface drying, residual volume was transferred into Petri-dish for further drying to give final dried residue. Residue was weighed and transferred into planchet, followed by addition of drops of acetone and vinyl-acetate. The vinyl-acetate helps in removing the moisture content and serves as a binder while the acetone was used as sterilizer. Then the sample is ready for counting.   and RTG-1 for Th-232 (Ti-208). The background count rate was done for 29,000

Calibration and Calibration Efficiency
seconds just as those of the samples.
Gamma ray spectroscopic systems were used to analyze the samples collected.
The radiation source was placed close to the detector in order to increase the intensity reaching the detector hence; the counting system may exhibit high detection efficiency (Bello et al., 2014). The net count was obtained using the expression: where N s = Sample counts (Net counts), N b = Background counts, N g = Gross counts.  The activity concentration (Bq·kg −1 ) of each radionuclide was obtained using: where ε = efficiency detector, y = gamma yield, m = mass of the sample, t = counting time.

Visual Inspection of Samples
The sampling sites for cassava, soil and water samples were presented in Figure 1. The physical outlay of cassava farm land from Okobo coal-mining areas physically shows varieties of stunted growth of cassava samples. Outburst of suspected polluted water from the coal mining regions in both downstream and upstream water samples from Okobo vicinity were well observed as coloured due to coal deposits and leachates. This is the only source of water for industrial, agricultural and domestic uses in Okobo.

Radiological Study
Results of radiological study for soil, coal and cassava samples from Okobo coal mine is presented in Tables 4-6. Table 4 shows the K-40 radiological data of water samples measured with liquid scintillation counter. K-40 counts do not follow a regular trend. Values recorded for upstream water samples (49.95 -111.32 cpm) exceed those reported for mid and down streams. This is an indication that the upstream water is more contaminated with K-40 radionuclide.

Activity Concentrations of K-40 in Water Samples
The radiological analysis of water samples by Gamma ray liquid scintillation at 60-minute count rate shown in Table 4 was converted to activity concentrations (Table 5)      and Th-232 were presented in Table 7. This study shows that the control soil from

Activity Concentrations of Radionuclides in Coal Sample
The range of activities obtained for the studies showed the mean concentrations    ples. This is shown in their respective radio hazard indices. A single parameter known as Radium equivalent activity (Ra eq ) in Table 8 is defined with respect to radiation exposure which compares the activity of materials containing different elements of primordial radionuclides. Its definition also considers external and internal effective dose from radon and its decay progeny (Huang et al., 2015).

Hazard Indices
Ra eq is measured in Bq·kg −1 and defined based on the assumption that specific activity of 370 Bq·kg −1 for 226 Ra uniformly distributed in any environmental sample can result in annual effective dose of 1 mSv at 1 m above ground level (Huang et al., 2015;Taskin et al., 2009). It is quantitatively expressed as (Beretka & Mathew, 1985;UNSCEAR, 2000).
( ) where A Ra , A Th and A K are the respective specific activities of 226 Ra, 232 Th, and 40 K.
The constants; 1, 1.43, and 0.077, represents the activity conversion rates for 226 Ra, 232 Th, and 40 K in sequence, which result in gamma dose rate at maximum permissible Ra eq of 370 Bq·kg −1 . The result of Ra eq (Bq·kg −1 ) in this study (Table 8) revealed the contributions of coal mining to vicinity environmental samples. The values reported for CSS (50.01 Bq·kg −1 ) is far less than those of the analytical soil samples (107.24 -175.67 Bq·kg −1 ) as well as their corresponding cassava samples (79.98 -150.17 Bq·kg −1 ). Figure 4 unveiled the difference between the Radium activity and detriment in cassava as higher compared to soils.

External Hazard Index (Hex)
Radiation hazard incurred due to external exposure to gamma rays is quantified in terms of the external hazard index (H ex ). The maximum permissible value for H ex is unity, which corresponds to Ra eq upper limit of 370 Bq·kg −1 (Huang et al., 2015;Oni, 2019 (Lu et al., 2006;Lu et al., 2012). Values of Hex (0.14 -0.47) computed for coal, soil and cassava samples in this study were below unity, hence within the permissible limit with sample tagged as non-hazardous with reference to radionuclides.
This parameter defined the risk factor as fatal cancer risk per Sievert, which according to Taskin et al. (2009) is assigned a value of 0.05 by ICRP 60 for the public for stochastic effects.

Absorbed Gamma Dose Rate (D)
This is a measure of the energy deposited in a medium by ionizing radiation. In the SI system of units, the unit of measure is joules per kilogram, and its special name is gray (Gy). Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection and radiology. It is also used to directly compare the effect of radiation on inanimate matter.
The measured activity concentrations of 226 Ra, 232 Th and 40 K were converted into doses by applying the conversion factors 0.462, 0.604 and 0.0417 for uranium, thorium and potassium, respectively. These factors were used to calculate the total dose rate (nGy·h -1 ) using the following equation:  (Diab, 2008). This calls for concern in some samples. On a mild note, an average of 68.098 mSv/year which poses no threat is more generalized for the cassava samples within Okobo coal field region.
Generally, the calculated values for Ra eq , external hazard index; H ex , internal hazard index and the H in values for Okobo coal sample itself were 96.94 Bq·kg −1 , 0.26 and 0.30, respectively. These values were below the safety limits stipulated by UNSCEAR (2000). These implied that radioactivity of Okobo coal is non-hazardous and within safe limits. Similarly, the calculated mean values ( Figure 6) for  Radium equivalent activity (Ra eq ), External hazard index (H ex ) and internal hazard index (H in ), for the Okobo Soil Sample, Control Soil Sample, Okobo Harmonised Cassava Sample, Control Cassava Sample Okobo, were all within the desired safe limits of less than 370 Bq·Kg −1 (for Ra eq ), less than unity (for H ex , H in , and I γr ) and below 0.05 for ELCR. Thus, none of these samples pose any radiation risk.
On the contrary, Representative gamma index (I γr ) and AEDE values for some samples calls for continual radiological impact monitoring.

Conclusion
The natural radiological activity concentrations of coal, cassava, water and soil samples within Okobo coal field have been estimated by gamma spectrometry.
Radiological detriment and hazard indices values for Ra eq , external hazard index H ex , internal hazard index H in , I γr and the ELCR for Okobo coal and neighbouring environmental samples were below the safety limits stipulated by UNSCEAR. This implied that radioactivity of Okobo coal is non-hazardous and within safe limits for now. Exceptions to this generalization are representative gamma index (I γr ) and annual effective dose equivalent values for some samples, requiring continual radiological impact monitoring.