1 Introduction

The growing awareness of naturally occurring radioactive materials (NORMs) and their associated health implications has elicited global response from researchers. All natural materials and the environment are inherently radioactive and subject to ionizing radiation, leading to background radiation in the ecosystem [1,2,3]. NORMs mainly potassium (40 K) and those of the decay series of thorium (232Th) and uranium (238U) occur at significantly enhanced concentrations in both natural and anthropogenic sources [4, 5]. These primordial radionuclides are extensively dispersed in the environment through diverse activities, including mining, milling, mineral exploration, nuclear discharge, and industrial processes [6]. Radionuclides of this nature can accumulate within the ecosystem, including in soils, consequently elevating the background radioactivity in living environments. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) reported that primordial radionuclides in soil contribute about 40% (232Th), 35% (40 K), and 25% (238U) of the absorbed dose by humans [7]. A comprehensive understanding of the behavior and mobility of these NORMs is imperative for precisely assessing the potential risks they may pose to human health and the broader ecosystem.

Mining is a prominent anthropogenic activity that has substantially contributed to the global escalation of environmental radioactivity levels. According to a 2022 report from the World Health Organization (WHO), approximately 100 billion tons of raw materials enter the global economy yearly through anthropogenic mining-related activities [8]. Mining has been described as an age-long technique for extracting useful metals and minerals from the earth’s surface for income and employment generation and use in industries [9, 10]. There are presently 22,609 active mine sites and 159,735 abandoned metal mines across the globe [11]. It is estimated that metal mining impacts 164,000 km2 of floodplains, consequently affecting around 23 million people globally through various forms of concentrated pollutants released during the metal-mining process [12]. Gold, a rare earth metal with high value and industrial applications, is substantially mined worldwide, including in sub-Saharan African countries such as Ghana and Nigeria [13, 14].

In Nigeria, gold mining is predominant in the southwest and northwest regions, with significant deposits in the Iperindo area of Osun State [15]. The actions of artisanal gold mining in Nigeria have been consistently associated with heightened levels of environmental radioactivity, which is particularly evident at mining sites, as indicated by several studies [16,17,18,19,20,21,22]. For instance, Orosun et al. [19] reported average activity concentration values of radionuclides for 238U, 232Th, and 40 K that were above the global world average in soil samples taken from Moro gold mining fields in Kwara State, Nigeria. Additional studies conducted at Ijero Ekiti [17], Ile-Ife [20], and Itagunmodi [21] artisanal gold mines reported soil samples with average activity concentration of 238U and 40 K above the global average. Conversely, the average concentration of 232Th was found to be below the world mean value in these studies. The observed low value of thorium in these studies is attributed to its solubility in water, making it less susceptible to leaching. Tailings from mining operations are a major source of pollution, leading to the spread of NORM in air, waterways, and landfills [23].

Gold mining operations have been associated with the destruction of vegetation, land degradation, water quality deterioration, earth surface deformation, and soil erosion [16, 24,25,26]. In the specific context of Iperindo, artisanal gold mining, often conducted illicitly, is prevalent. Miners in this area employ basic tools such as pans, chisels, hammers, and shovels to extract gold from quartz-carbonate veins and altered gneisses, contributing to these environmental challenges. This study aimed to evaluate the concentration of radionuclides in soils from artisanal gold mining sites in Iperindo and assess the associated health risks. The results obtained will be benchmarked against globally accepted radiation standards [27, 28]. Control locations will be designated outside the mining area for a comparative analysis. The outcomes of this research will contribute to the understanding of environmental degradation and radiological exposures in Iperindo, providing crucial insights for government intervention through the Ministry of Mines and Steels Development (MMSD) in Nigeria to mitigate the risks linked to artisanal gold mining in the study area.

2 Materials and methods

2.1 Site description

The study was conducted at mining sites located at Iperindo, a small town situated in the Atakumosa-West and Oriade Local Government Areas of Osun, Southwest, Nigeria. The occupation of the town is mostly agrarian with the presence of small-scale businesses. The geographical coordinates of the study area, depicted in Fig. 1, are 7° 30′ 0" N latitude and 4° 49′ 0" E longitude. The town is well known for exporting fruits such as bananas, plantains, oranges, and cocoyam, amongt others, to other neighboring local government areas and states in Nigeria. Positioned within the Ilesha schist belt, the study area is characterized by amphibolite-facies biotite granite-gneisses of Proterozoic age, approximately 4 km east of the significant crustal 'break' identified as the Ifewara-Zungeru fault [16, 21, 25].

Fig. 1
figure 1

Map of Osun State showing the Iperindo study area

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Another common activity associated with Iperindo is the prevalent activities of large-scale and small-scale gold mining activities by artisanal miners. Iperindo has one of the largest gold deposits in Nigeria; hence, it is one of the few areas in southwest, Nigeria, previously under evaluation by the Nigerian Mining Corporation [26]. This evaluation became necessary owing to the worrisome environmental degradation caused by the activities of unregulated miners. As shown in Fig. 2, the youths in the community have equally learned the art of crude mining, an activity that continues to affect crop production within the town, thus putting nhabitants and miners alike at risk. The situation has worsened as the area has evolved into an informal, uncoordinated, and unmonitored mining site, leading to escalated and unregulated artisanal mining operations within the community.

Fig. 2
figure 2

Field screenshot of artisanal mining activities occurring at Iperindo and the associated impacts

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2.2 Sample collection, preparation, and processing

Soil samples were collected from three distinct mine sites within the study area, covering five randomly selected points. Moreover, the control samples were collected from five different locations in the living areas of Iperindo, which are situated approximately 15 km away from the mine sites. In all, twenty soil samples were collected using stainless-steel spades from the mine and control areas at well-distributed points and at varying depths between 2 and 4 m. The samples were appropriately labeled in zipped plastic bags for easy identification. The samples were processed according to the International Atomic Energy Agency (IAEA) standard methods [29]. The soil samples were air-dried at room temperature for 24 h to eliminate any moisture traces, followed by oven-drying at a consistent temperature of 110 °C to achieve dry weights. The samples were subsequently pulverized for homogeneity and radiochemical separation, after which they were securely sealed in appropriately labeled plastic containers and stored for a duration of 28 days to facilitate secular equilibrium for 238U and its short-lived progenies. Subsequently, gamma spectrometric investigations were conducted on the samples, via a counting process utilizing a sodium iodide (NaI) detector doped with Thallium (Tl), and each sample was counted for a period of 10 h.

2.3 Energy calibration and radionuclide counting

Gamma spectrometric investigations were conducted using a gamma multichannel analyzer (model GS-2000 Pro) equipped with NaI(TI) at the Radiation and Health Research Laboratory of Ladoke Akintola University of Technology, Ogbomosho, Nigeria. Energy calibration of the gamma spectrometer was carried out using standard gamma sources of known activity standards (137Cs and 60Co). This procedure was used to facilitate the straightforward identification and counting of specific radionuclides within the samples and to determine their respective quantities. To obtain an exact channel number for every pulse height, radionuclide sources with known energies were placed in the detector. The sensor's energy calibration was accomplished by employing the RSS8 gamma source kit, a primary reference material provided by Spectrum Techniques LLC, USA. The samples were conducted in accordance with the IAEA standard method [28].

To assess the efficiency of the NaI(TI) sensor, reference gamma radiation sources specifically designed for determining natural radionuclides in environmental matrices were counted for a duration of 600 min. The total energy peak efficiency (ε) was calculated, establishing a correlation between the spectrum peak point and the detected radioactivity level. The determination of the net peak count (Npc) and the activity concentration (Ac) for each radioactive isotope within the source, along with the consideration of the absolute gamma-ray emission probability (Pγ) of specific radionuclides of interest, the sample volume (Vs), and the acquisition time (TA), allowed for the estimation of the efficiency of the total peak energy, as described in [30]:

$$\varepsilon =\frac{{N}_{pc}}{{{A}_{c}\times P}_{\gamma }\times {V}_{s}\times {T}_{A}}$$
(1)

2.4 Determination of radionuclide activity concentration

The radionuclide activity concentration in the Iperindo soil samples was assessed using a meticulously shielded and accurately calibrated 3.0 cm by a 3.0 cm coaxial NaI (Tl) detector. This detector was enclosed within a 6 cm thick lead shield to effectively reduce background radiation. The counting of the samples was conducted in accordance with the IAEA standard method [28]. The Gamma energies of 1765 keV for 214Bi, 2615 keV for 208Tl, and 1460 keV for 40 K were used for determining the activity concentrations of 238U, 232Th, and 40 K in the Iperindo soil samples.

The activity concentrations of the samples were calculated in units of \({Bqkg}^{-1}\) by considering the total net count (CN), counting time (TC), mass of the sample (MS), efficiency (ε), and yield (γ) given by:

$${A}_{C}(Bq {kg}^{-1})=\frac{{C}_{N}}{{{T}_{C}\times M}_{S}\times \varepsilon \times \gamma }$$
(2)

The minimum detection limit (MDL) in Bqkg−1 of the detector estimated from the background count (Bc) for each NORM is given by [31, 32]:

$${M}_{DL}(Bq {kg}^{-1}=\frac{2.71+4.66\sqrt{{B}_{c}}}{{{T}_{C}\times M}_{S}\times \varepsilon \times \gamma }$$
(3)

2.5 Determination of radiological health indices

The concentration of radium activity equivalent \(({R}_{aeq})\) serves as an indicator of the gamma radiation dose output from natural radionuclide mixtures. In this study, the radium equivalent activity was estimated based on the estimated concentrations of 238U (\({A}_{U}\)), 232Th (\({A}_{Th}\)), and 40 K (\({A}_{k}\)) at 370 BqKg−1 238U, 259 BqKg−1 232Th, and 4810 BqKg−1 40 K, respectively using [33, 34]:

$${R}_{aeq}={A}_{U}+1.43{A}_{Th}+0.077{A}_{k}$$
(4)

The average absorbed dose rate, \(D\) in air at 1 m above ground level for naturally occurring radionuclides is determined by applying conversion factors to 238U, 232Th, and 40 K given by [28, 33]:

$$D \left(nGy{h}^{-1}\right)= 0.462{A}_{U} + 0.604{A}_{Th} + 0.042{A}_{K}$$
(5)

It is imperative to estimate the annual effective dose rate (AEDR) of radionuclides to the miners and people living within the mining sites. By considering an outdoor occupancy factor (OF) of 0.2, a dose conversion factor (DCF) of 0.7 SvGy−1, a total absorbed dose rate (D), and a total time (T) of 8760 h per year, the annual effective dose rate for outdoors was obtained using [28, 35, 36]:

$$AEDR \left(mSv{y}^{-1}\right)= OF \times DCF \times D \times T \times {10}^{-6}$$
(6)

The hazard indices are categorized into an external hazard index (Hex) and an internal hazard index (Hin). The external hazard index is used to estimate the health risk associated with the emission of gamma radiation by different natural radionuclides and is estimated using [37, 38]:

$${H}_{ex}=\frac{{A}_{U}}{370}+\frac{{A}_{Th}}{259}+\frac{{A}_{K}}{4810} \le 1$$
(7)

The internal hazard index on the other hand is an indication of the dangers posed by radon and its progeny to the internal exposures of living tissues and cells, and is estimated using [39, 40]:

$${H}_{in}=\frac{{A}_{U}}{185}+\frac{{A}_{Th}}{259}+\frac{{A}_{K}}{4810} \le 1$$
(8)

The alpha index, \({I}_{\propto }\) is a radiological parameter created to evaluate the safety of the environment caused by excessive radiation exposure through inhaling of soils used as building materials. The alpha index is determined using [41]:

$${I}_{\propto }=\frac{{A}_{U}}{200}$$
(9)

The gamma Index, \({I}_{\gamma }\), serves as a means of evaluating the danger of gamma radiation resulting from natural radionuclides in particular samples under investigation. For \({I}_{\gamma } \le 1\), the annual effective dose falls within the upper limit of 1 mSv, which indicates a lower radiological risk to the soil. This index is determined using [34]:

$${I}_{\gamma }=\frac{{A}_{U}}{150}+\frac{{A}_{Th}}{100}+\frac{{A}_{K}}{1500}$$
(10)

The effect of NORM on organs such as gonads, bone surface cells, and active bone marrow of people living within the study area was estimated. The annual gonadal equivalent dose (AGED) was determined using the specific activities of 238U, 232Th, and 40 K, and their respective activity concentrations as [21]:

$$AGED (\mu Sv{y}^{-1}) = 3.09{A}_{U} + 4.18{A}_{Th} + 0.314{A}_{K}$$
(11)

Miners face an elevated risk of develo** cancer over their lifetime due to continuous exposure to a specific level of carcinogens through daily interactions and the ingestion of NORMs. This measure is quantified as excess lifetime cancer risk (ELCR), and in this study, it was computed from the Annual Effective Dose Rate (AEDR), the average duration of life (DL) estimated at 70 years in Nigeria, and the risk factor (RF) of 0.05 Sv−1 for the general public, employing the following formula. [33, 42, 43]:

$$ELCR = AEDR \times DL \times RF \times {10}^{3}$$
(12)

3 Results and discussion

3.1 Activity concentration of radionuclides

The activity concentrations of 238U, 232Th, and 40 K in the soil samples from three gold mining locations and a control location in Iperindo are presented in Table 1. It was observed that 40 K had the highest NORM concentration for all the locations investigated. For all the mining areas investigated, the activity concentration of 238U varied from 33.40 ± 8.32 to 87.00 ± 19.44 Bqkg−1 with a mean value of 61.55 ± 13.97 Bqkg−1. For 232Th, the concentration ranged from 41.90 ± 12.57 to 105.10 ± 27.15 Bqkg−1 with an average value of 72.65 ± 16.83 Bqkg−1, while that of 40 K varied from 720.50 ± 85.04 to 1722.10 ± 200.41 Bqkg−1 with an average value of 1134.99 ± 142.24 Bqkg−1. In contrast, the concentrations of 238U, 232Th, and 40 K in the control areas were 10.90 ± 4.41 to 20.10 ± 8.41, with a mean value of 15.26 ± 6.96 Bqkg−1; 17.40 ± 5.49 to 25.30 ± 10.34 with a mean value of 21.46 ± 9.35 Bqkg−1; and 280.30 ± 35.67 to 511.70 ± 43.24, with a mean value of 381.04 ± 51.27 Bqkg−1. It is evident from the results that the activity concentrations of the radionuclides are greater in the gold mining areas of Iperindo than in the control locations outside the mining areas at 75.21%, 70.46%, and 66.43% for 238U, 232Th, and 40 K, respectively.

Table 1 Activity concentrations of NORMs in soil samples from the Iperindo mining and control areas
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The estimated average activity concentrations of radionuclides obtained from the gold mining sites in Iperindo are well above the worldwide average values of 33.0 Bqkg−1 for 238U by 86.51%, 45.0 Bqkg−1 for 232Th by 61.44%, and 420.0 Bqkg−1 for 40 K by 170.24% [28], while the average activity concentrations of radionuclides obtained from the control sites within the study area are well below the worldwide average values of 53.76%, 52.31%, and 9.28%, respectively for 238U, 232Th, and 40 K. A graphical illustration showing the comparison of the results obtained for the Iperindo mining areas is shown in Fig. 3. A possible explanation for these highly significant radionuclide values could be attributed to the dispersion of radioactive dust across the mine sites by the artisanal miners during the gold mining process. However, the main reason for the elevated concentrations of NORMs in the mining site stems from the excavation of tailings—soils close to gold deposits beneath the ground—known for enhanced concentrations of natural radionuclides. This occurs without proper reclamation and the handling of such tailings within the site, which can result in increased contamination risks of the ecosystem, dispersion of harmful substances, and long-term environmental degradation. The large increase in NORMs within the mining site is primarily due to the dispersion of the NORMs’ enriched tailings within the mining site and to other nearby locations. The geographical and geological formation of the earth’s crust has also been identified as a possible contributing factor to the observed high activity concentration of radionuclides in the mining sites [16].

Fig. 3
figure 3

Average activity concentrations of 238U, 232Th, and 40 K obtained at Iperindo mine sites compared with control and worldwide average values

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Average activity concentrations of 238U, 232Th, and 40 K obtained at Iperindo mine sites compared with control and worldwide average values.

A comparison of the average activity concentrations of radionuclides investigated in the soil samples from the Iperindo mine sites was performed with the activity concentrations reported from locations such as Ijero Ekiti [17], Itagunmodi [21], Shanono and Bagwai [18], Iperindo [16], Ile-Ife [20] and Moro [19] gold mines in Nigeria, and the results are presented in Table 2. The average activity concentration of 238U in the present study is consistent with that reported in previous studies conducted at all the other mine locations [16,17,18,19,20,21], which showed values above the permissible levels. For 232Th, the average activity concentration obtained in this study is consistent with that reported for Shanono and Bagwai [18], which are above the worldwide average. This is in contrast to measurements from the other remaining mines which are significantly less than the worldwide average permissible values [16, 17, 19,20,21]. The average activity concentration of 40 K identified in the present study is also consistent with the results of [16, 17, 19, 21], which are above the global average but differ from the results of [18, 20], which are below the global average. The significant increase in the concentration of 40 K is particularly worrisome considering that this radionuclide is a vital component of nutrient-rich soil and plays an essential role in promoting plant growth as well as human health through diet. Studies have linked high concentrations of 40 K to stomach cancer [44] and an increase in the risk of lung cancer among exposed smokers [45].

Table 2 Comparison of average activity concentrations of NORMs between this study and related studies in Nigeria
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3.2 Radiological health indices

The results of the radiological parameters estimations of the soil samples from the Iperindo mine sites and control locations are presented in Table 3. The radiological health indices and their associated risks were evaluated based on the activity concentrations of 238U, 232Th, and 40 K in the study areas. From the mine results presented in Table 3, the radium equivalent obtained varied from 161.16 to 321.10 at an average value of 252.83 Bgkg−1. The absorbed dose rate varied from 76.97 to 151.50, with a mean value of 119.98 nGyh−1, while the annual effective dose rate ranged from 0.09 to 0.19. with an average value of 0.15 mSvy−1. The external and internal hazard indices, respectively varied from 0.44 to 0.87; and from 0.55 to 1.03 with mean values of 0.68 and 0.85. The alpha and gamma indices also varied from 0.17 to 0.44 at a mean value of 0.31 and from 1.21 to 2.41 at a mean value of 1.89, respectively. The annual gonadal equivalent dose obtained ranged from 545.82 to 1074.91, with an average value of 850.23, while the estimated cancer risk ranged from (0.33 to 0.65) × 10–3 with an average value of 0.52 × 10–3.

Table 3 Radiological health indices estimated from Iperindo gold mines and control sites
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The average radiological health indices obtained from the Iperindo mine sites when compared with the results from the control areas showed that the radiological parameters at the mine sites are significantly higher than those from the control areas. A comparative analysis of the average radiological health indices obtained in this study with those from other mine locations in Nigeria, the control area, and the UNSCEAR global average is presented in Table 4. From the results, the radium equivalent, external and internal indices, and alpha indices obtained from the mine sites are below the global respective averages and are consistent with those reported by [17,18,19,20,21]. This was, however, in contrast with results obtained from the study’s mine locations for absorbed dose rate, annual effective dose rate, gamma index, annual gonadal equivalent dose, and excess lifetime cancer risk which were above the global respective averages. The readings were consistent with the results of [17,18,19] and differed significantly from those reported by [16, 20]. In contrast, all the radiological parameters estimated for the control areas were significantly lower than the UNSCEAR global average.

Table 4 Comparison of average radiological parameters between this study and other related studies
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The estimated radiological parameters at the Iperindo mines with values higher than those from the control areas and the global average suggest potential health hazards associated with the mine sites. Continued exposure and absorption of these NORMs by the miners puts them at a significant risk over the long term. For example, these results suggest a high likelihood of significant health risks associated with gamma radiation emissions and radon progeny, such as respiratory tract diseases occurring in the near future in Iperindo mining areas.

3.3 Statistical analysis

The descriptive statistical analysis results for the activity concentrations and associated radiological health indices obtained at the Iperindo mine locations and the control areas are presented in Table 5. The basic statistical parameters calculated included the mean, standard deviation (SD), range, kurtosis, and skewness coefficient. Table 5 shows that the relatively high SD value obtained for 40 K suggested a significant degree of variability in the activity concentration of radionuclides seen in the soil samples from the Iperindo mine sites. The low SD values obtained for 232Th and 238U suggest less variability, with the same result obtained for the control areas. It is important to note that for the Iperindo mine sites and the control areas, the SDs are lower than the mean values of the activity concentrations of the NORMs and the radiological parameters. This implies that there is a substantial degree of consistency in the estimated radionuclide levels and the radiological parameters within the soil samples from the Iperindo mine sites and the control areas.

Table 5 Descriptive statistics for radionuclide activity concentrations and radiological parameters obtained in this study
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Kurtosis, a measure of a distribution peakedness relative to its normal distribution, can be categorized as leptokurtic (for a relatively high peak), platykurtic (for a flat top), or mesokurtic (neither overly peaked nor underlay flat top) [31, 32, 39, 46]. For the activity concentrations, negative kurtosis values were observed for 238U and 232Th at the mining and control locations. This indicates a mesokurtic distribution for 238U at the mine locations considering its tail and peak shape, which are similar to those of a normal distribution. For 232Th, the distribution is platykurtic at mine sites with relatively flat distributions and lighter tails than those of the normal distribution. On the other hand, for 238U and 232Th, the control location exhibit a leptokurtic distribution with heavier tails and pronounced peaks than those of the normal distribution. Positive kurtosis values observed for 40 K at the mining and control locations indicate an asymmetric distribution. This suggests a mesokurtic distribution that is closer to normal, with tails and peakness resembling a standard normal curve. Figure 4 shows the frequency distribution curves of 238U, 232Th, and 40 K in soil samples from the Iperindo mine sites only.

Fig. 4
figure 4

Frequency distribution curves of the activity concentrations of 238U, 232Th, and 40 K in soil samples from the Iperindo mine sites

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Kurtosis values observed for radiological health indices were positive for all parameters at the mine and control locations, except for Iα which showed negative kurtosis values at the mine and control locations. These variables ranging from 1.78 to 2.10, exhibit leptokurtic distributions with heavier tails and pronounced peaks compared to a normal distribution. The implication for this positive kurtosis suggests that the data for these variables in the mining areas have heavier tails, implying a greater likelihood of extreme values. This can have implications for risk assessment, as extreme values could influence radiological health evaluations.

Skewness is crucial for gaining deep insight into the asymmetry of a distribution and helps to identify potential outliers in a dataset [32]. Positive skewness values suggest the possibility of extreme values on the right side of the distribution, while negative skewness values suggest the possibility of extreme values on the left side of the distribution. In the present study, for the activity concentration, 238U shows a slightly skewed positive distribution for the mine and control locations, suggesting a tail extending to the right with data slightly more concentrated on the left side of the mean. For 232Th, the mine data are slightly positively skewed toward the right side of the distribution, while the control data is slightly negatively skewed toward the left side of the distribution. The 40 K distribution data are moderately positively skewed with a tail extending to the right, indicating a concentration of data on the left side of the mean. The radiological parameter data showed negative skewness values ranging from − 0.72 to − 0.65, implying negatively skewed distributions with tails extending to the left and data concentrated at the right side of the mean.

4 Conclusion

The average activity concentrations of 238U, 232Th, and 40 K in the soil samples obtained from the Iperindo mine sites and control areas were 61.55 ± 13.97 Bqkg−1, 72.65 ± 16.83 Bqkg−1, and 1134.99 ± 142.24 Bqkg−1, 15.26 ± 6.96 Bqkg−1, 21.46 ± 9.35 Bqkg−1, and 381.04 ± 51.27 Bqkg−1 respectively. The measured activity concentrations at the mine sites are significantly greater than global average and significantly lower than the global average at the control locations. A high concentration of 40 K is particularly worrisome because it plays a significant role in promoting plant growth and human health through the diet. The radiological health indices estimated in the present study showed that the absorbed dose rate, annual effective dose rate, gamma index, annual gonadal equivalent dose, and estimated lifetime cancer risk are all above the worldwide thresholds for Iperindo mine locations. In contrast, the radium equivalent, external and internal hazard risks, and alpha indices, are all significantly lower than the global threshold limits. The estimated radiological parameters for the control areas were all below the worldwide average values. The implication of the results obtained from the present study suggest that mine workers are highly susceptible to health hazards from continued exposure to and absorption of NORMs from Iperindo mine sites. There is, however, no risk to the residents of the Iperindo community based on the results obtained for the control areas. Nonetheless, remediation of mine sites by the MMSD in Nigeria is recommended to commence as soon as possible, especially at abandoned mine sites within the Iperindo community.