Front. Phys. Frontiers in Physics Front. Phys. 2296-424X Frontiers Media S.A. 1740296 10.3389/fphy.2025.1740296 Original Research The study of radiation contamination in roodepoort gold mine tailings using HPGe gamma spectroscopy Mvelase et al. 10.3389/fphy.2025.1740296 Mvelase Mashinga J. 1 2 * Writing - original draft Writing - review and editing Ntshangase Sifiso S. 1 Funding acquisition Supervision Writing - review and editing Majola Siyabonga N. T. 2 Funding acquisition Supervision Writing - review and editing Kheswa Bonginkosi V. 2 Supervision Writing - review and editing Masiteng Paulus L. 2 Supervision Writing - review and editing Maleka Peane P. 3 4 Supervision Writing - review and editing Department of Physics, University of Zululand, KwaDlangezwa, South Africa Department of Physics, University of Johannesburg, Doornfontein, South Africa SSC Laboratory, iThemba Laboratory for Accelerator Based Sciences, Somerset West, South Africa University of the Western Cape, Bellville, South Africa Correspondence: Mashinga J. Mvelase, emjay.mvelase@gmail.com 15 01 2026 2025 13 1740296 05 11 2025 16 12 2025 18 12 2025 Copyright © 2026 Mvelase, Ntshangase, Majola, Kheswa, Masiteng and Maleka. 2026 Mvelase, Ntshangase, Majola, Kheswa, Masiteng and Maleka https://creativecommons.org/licenses/by/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

In this study, a coaxial HPGe high-resolution γ -detector was used to measure the γ -signals in soil samples collected from the Roodepoort Gold Tailings. There have been complaints about dust from the gold tailings, and the study aims to determine the level of contamination. The activity concentration of 238U ranged from 132.88 ± 4.68 to 1,421.46 ± 13.38, with a mean of 464.96 ± 2.08 Bq/kg. The activity concentration of 226Ra ranged from 130.19 ± 4.48 to 1,359.27 ± 13.83, with a mean of 425.28 ± 8.02 Bq/kg. The range of 232Th was from 5.27 ± 0.67 to 19.37 ± 1.98, with a mean of 11.20 ± 1.03 Bq/kg, and 40K activity ranged from 65.23 ± 15.29 to 264.11 ± 25.66, with a mean of 127.50 ± 21.85 Bq/kg. The findings showed that 238U and 226Ra activities exceeded the global average of 35 Bq/kg, while 232Th and 40K were below their respective limits. The radiological hazard indices exceeded recommended limits, making the tailings soil completely unsuitable for use as building materials, as this may have deleterious health effects on residents in the future.

 AEDE ELCR HPGe γ - spectroscopy radioactivity radiological health hazards National Research Foundation 10.13039/501100001321 The author(s) declared that financial support was received for this work and/or its publication. This research was funded by the National Research Foundation (NRF) of South Africa under grant number CPRR23040388976. section-at-acceptance Radiation Detectors and Imaging Introduction

Since the formation of the Earth, natural background ionizing radiation has always emanated from the decays of 238U, 232Th, and 40K, which are present in the environment [1, 2]. Two isotopes make up most of the natural uranium found in Earth’s crust: 238U, which accounts for 99.3 % and 235U, which accounts for approximately 0.7 % [3, 4]. Radioactivity is significant in the mining and processing of ores other than uranium. These activities result in exposure to naturally occurring radioactive materials (NORM). Radiological exposures associated with radioactivity, such as absorbed dose rate, annual effective dose rate, and annual gonadal dose rate, should be evaluated. Most background ionising radiation originates from the decay of the 238U decay series, the 232Th decay series, and the non-series 40K [4–6]. In Soweto, environmental radioactivity originates from mining tailings, and radiation is transported as dust by the wind to distant locations, so no one is spared from radiation contamination.

Radiation affects the human body externally through γ -radiation and internally through α -radiation, which arises from inhaling dust and radon (222Rn) gas. Radon gas is the parent nucleus of both 218Po and 214Po, which are alpha emitters. Alpha particles can damage DNA by imparting a high density of ionizations due to their high linear energy transfer [7]. High linear energy transfer (LET) single hits can generate highly reactive oxygen species capable of inducing cellular damage by direct reaction with biological molecules [8], as well as nitrogen radicals, which may cause significant DNA damage. High LET radiation is more effective than low LET radiation at inducing cell death, apoptosis, mutation, transformation, carcinogenesis, chromosomal abnormalities, and chromosomal instability [9]. The highly reactive oxygen species (ROS) produced by high LET radiation consequently damage various cell components, such as lipids, proteins, and nucleic acids, resulting in several chronic and degenerative conditions, including ageing, dementia, kidney, cardiovascular and neurodegenerative diseases, cancer, respiratory disorders, rheumatoid arthritis, and other metabolic disorders [10]. Nuclear decays inside the body emit α -, β -, and γ -radiation [11, 12], which can damage DNA through cell ionizations. Alpha particles impart a high density of ionizations along their short path, as they have a high linear energy transfer, leading to DNA damage in human cells and potentially resulting in radiation-induced carcinogenesis [13]. Previous studies have reported that the gold mine tailings in the Witwatersrand Basin have a uranium concentration of 100 mg/kg U 3 O 8 , which is comparable to or higher than the concentrations of uranium mine tailings in Namibia, ranging from 45.9 ± 3.0 to 1752.1 ± 17.5 Bq/kg for 238U [14, 15].

The mining sector has had both positive and negative effects on the South African economy and gross domestic product (GDP) [16]. South Africa is endowed with numerous mineral resources, particularly metals, coal, and gold, with exploration beginning in the late 1800s [4] and mining commencing in 1886 [17]. Extensive mining activity has occurred, bringing soils and rocks that would otherwise have remained underground to the surface. These activities have left behind mountains of pollution known as mining tailings. Gold tailings are a source of dust, especially during the dry and windy season, due to their proximity to residential areas, affecting thousands of impoverished township residents and causing numerous health issues [18–20]. Inhaling toxic metals trapped in radioactive dust particles can cause various lung diseases [16]. In the context of climate change, gold tailings are even more hazardous due to unpredictable weather patterns and heavy rains that may cause overflow, allowing pollution to reach human settlements. Toxic metals such as As, Cr, and Ni, and radioactive metals such as 238U, 232Th, and 40K in tailings pose a synergistic health risk to residents [4]. Knowledge of the distribution of geogenic and anthropogenic radionuclides is recommended for assessing and managing public health risks and should inform any environmental interventions [21].

Most written reports are based primarily on the work of community activists and community-based non-governmental organizations (NGOs) campaigning against mining companies that have left pollution unaddressed. This study aims to measure radioactivity in gold mine tailings, which has been less frequently reported, providing valuable information on radioactivity concentrations. Based on the results, the corresponding probabilistic health risk will be calculated.

 Materials and methods Geology and climate of the study area

The study site has a subtropical highland climate with a warm, wet summer [22]. These tailings are from abandoned old gold mines. The research area’s land is mostly utilized for residential development, with very little employed for gold mining [22, 23]. The gold-bearing conglomerates in the sedimentary layers of the West Rand mining area consist of 10–30 % phyllosilicates and 70–90 % quartz SiO 2 . Uraninite UO 2 , brannerite UO 3 Ti 2 O 4 , arsenopyrite (FeAsS), cobaltite (CoAsS), galena (PbS), pyrrhotite (FeS), gersdorfite (NiAsS), and chromite FeCr 2 O 4 , sericite, KAl 2 AlSi 3 O 10 (OH)2, and minor minerals such as rutile TiO 2 , pyrite FeS 2 , chromite FeCr 2 O 4 , and uraninite UO 2 make up the bulk of the phyllosilicates [24, 25]. The West Rand Group, mainly composed of quartzite and shale, forms the sedimentary base. The Central Rand Group is characterized by quartzite and conglomerate, particularly gold-bearing conglomerates in its lower section, which are associated with basin-wide unconformities [26]. The highlands have long, hot summers that generally last from October to March and short, cold winters that last from June to August. Most of the annual rainfall, which ranges from 600 to 732 mm, falls during the summer months. Strong storms are frequent from October to March, and the average annual temperature is 16 ° C [25].

 Sample collection and preparation

Soil samples were collected from the gold mine tailings in Roodepoort, located at 26 ° 10′46″S 27 ° 52′03″E, and stored in plastic zipper bags labelled RDP. The study site has residential areas, businesses, and additional mine tailings in its vicinity, as shown in Figure 1. The samples were placed in polypropylene Marinelli beakers of known weight after being oven-dried at 105 ° C, crushed, pulverized, and sieved to remove uncrushed stones and organic matter.

The Roodepoort Google Map shows the gold tailings sampling location.

A satellite map view of an area showing locations such as Durban Deep, a quarry, and Durban Deep Rifle Club. Roads include Nick Toomey Boulevard, Hail Road, and Cemetery Road. Nearby places include ZCC Bramficher and JV's Corner Lounge. The map is labeled with Google Maps branding.

To achieve secular equilibrium between the gamma emitters in the 238U series (primarily 226Ra, 214Bi, and 214Pb) and the 232Th series (228Ra measured by 228Ac, and 228Th measured by 208Tl), white silicone was applied between the lid and brim of Marinelli beakers to prevent radon leakage, and the beakers were hermetically sealed for 42 days before γ - spectroscopic analysis, as shown in Figure 2. The gross weights were measured, and the difference between the empty and gross weights gave the sample weights. Most of these sample weights averaged approximately 0.250 kg. The prepared gold tailings soil samples are now in Marinelli beakers, where they are kept until they reach secular equilibrium. They will then remain sealed and analysed at a later stage, as shown in Figure 2.

The prepared gold tailings soil samples in Marinelli beakers.

Containers labeled RDP 1 to RDP 16 are arranged on a tray. Each container has a different sample inside, with varying shades of color. Analysis of soil samples

Gamma spectroscopic analysis of the natural radioactivity (238U, 232Th and 40K) in soil was carried out at the Environmental Radiation Laboratory (ERL) at NRF iThemba LABS. The radioactivity was measured using a p-type coaxial Canberra gamma-ray spectrometer detector, optimised for detecting gamma rays at low energies up to 2000 keV, Model No. BE2820 SN 8794, with 45 % relative efficiency and a resolution of ≤ 2.00 keV (FWHM) at the 1.33 MeV γ -ray line of 60Co. An electronic data acquisition system (Canberra DSA-1000 digital signal processing (DSP) system), interfaced with a Multichannel Analyzer (MCA) and Canberra Genie 2000 software (version 2.0), was used to acquire a spectrum.

The gamma spectrometry system was calibrated for energy and efficiency using a mixed radionuclide standard covering a wide range of gamma-ray energies (0.060–2.00 MeV) in a 500 ml Marinelli beaker. Both the samples and the background were counted for 25,200 s, and the background count was subtracted from the sample count to give the net count rate. For quality control, calibration for energy and efficiency was performed to maintain measurement quality. After the samples were hermetically sealed for 42 days in their respective Marinelli beakers, secular equilibrium was assumed to have been reached. The 186.22 keV γ -signal was used to quantify 226Ra. The 295.22 keV and 351.93 keV γ -signals for 214Pb and the 609.32 keV, 1,120.29 keV, and 1764.49 keV γ -signals for 214Bi were used to assess the activity concentration of 238U [27–31], while 911.21 keV for 228Ac and 583.1 keV for 208Tl were used for 232Th. The single 1,460 keV γ - signal was used to quantify 40K concentration. The specific γ - signals listed in Table 1 were tracked to measure each radionuclide in the soil samples. The information in the table was sourced from the literature [32, 33].

The data on NORM decay modes showing their specific γ - signals and intensity.

Parent nuclide Daughter Nuclear Decay mode γ -ray energy γ -intensity
(NORM) Nuclide Half-life ( % ) (keV) Yield ( % )
226Ra Ra-226 1,602 years α (94), α (6) 186 3.55
Pb-214 26.80 min β (100) 295 19.3
Pb-214 26.80 min β (100) 352 37.6
238U Bi-214 19.90 min α (0.021) 609 46.1
Bi-214 19.90 min β (99.98) 1,120 15.4
Bi-214 19.90 min β (99.98) 1764 15.4
232Th Ac-228 6.15 h β (100) 911 26.0
Tl-208 3.05 min β (100) 583 86.0
40K Ar-40 1.284 × 1 0 9 years EC (10.7) 1,461 10.7
The efficiency and energy calibration curves

When analyzing unknown samples, we rely on gamma spectroscopy; however, before we can use it effectively, we must calibrate the energy scale. To do this, we used several well-known radioactive sources that emit gamma rays at specific energies. These include 210Pb at 45.54 keV, 241Am at 59.5 keV, 109Cd at 88 keV, and 57Co at 122.1 keV, among others. We also use higher-energy sources such as 137Cs (661.65 keV), 60Co (which emits at both 1,173.2 and 1,332.4 keV), and 88Y (1836.1 keV). The emission probabilities for these radionuclides were obtained from previous research papers [34]. The efficiency equation depends on energy, as shown in Equation 1, and has been used in work published by the following authors [35–37]: ε E γ = 1 E γ ∑ i = 1 8 a i ln E γ E o i − 1

In this equation, E γ (keV) represents the peak energy of a particular radioisotope of interest, and E 0 = 1 keV [38]. The efficiency calibration curve for the HPGe detector was obtained using standard sources at NRF iThemba LABS, Gauteng. As there is a direct relationship between channel number and energy, we can convert our channel readings into energy measurements. The results of both the efficiency and energy calibration curves are shown in Figure 3.

Detector efficiency (left) and energy calibration for γ -ray energies (right).

Two graphs are shown. The left graph displays efficiency versus energy with data points and a curve showing decreasing efficiency as energy increases up to 2000 keV. The right graph illustrates energy plotted against channel number, with a linear fit indicated by a line, and the equation \(E_y = 0.1281C + 37.015\) with \(R^2 = 0.9997\), where \(E_y\) is energy. Minimum detectable activity and error calculations

The background count was conducted using the same geometry as the samples. An empty Marinelli beaker was placed on the detector, as with the sample measurements, and counted for periods similar to those used for the sample counts. The peaks generated from this procedure were subtracted from the corresponding peaks of the samples. In this way, the background count rate was manually subtracted from the measured samples. After determining the background radiation for the radionuclides of interest, the minimum detectable activity (MDA) was calculated using the following Equation 2: M D A B q / k g = 2.71 + 4.66 B I γ ε T m

Where MDA (Bq/kg) is the specific activity in a sample, B is the background activity of the sample, ε is the absolute detector efficiency of the specific γ -ray, T is the accumulation time, and m is the mass of the sample in kilograms. I γ is the emission probability of a specific energy photopeak. Using the background activities, the average MDA for 226Ra is 6.68 Bq/kg at 186.2 keV, 26.1 Bq/kg for 40K at 1,460 keV, and 1.68 Bq/kg for 214Pb at 352 keV.

For specific activity concentration, the error was calculated by taking the square root of the sum of the background and sample readings, then dividing by T, ε , I γ , and mass m. The resulting value was then multiplied by two, as 2 σ was used for our measurements. For the other radiological indices, error propagation was applied.

 Calculations of risk assessment of radionuclides

After calculating the detector efficiency and energy calibration, the samples were subjected to gamma spectroscopic analysis which resulted to the measurement of specific activity concentration. Radiological indices, including Radium Equivalent Ra Eq , Absorbed Dose Rate (ADR), Annual Effective Dose Equivalent (AEDE), Excess Lifetime Cancer Risk (ELCR), Internal Hazard Index H In , External Hazard Index H Ex , Annual Gonadal Dose Equivalent (AGDE), Alpha Index I α , and Radioactivity Index I γ , were estimated using their respective models from the literature.

Calculation of activity concentration in soil samples

All measurements were taken with the samples in contact with the detector housing for 25,200 s, and spectral analysis was performed using Genie 2000 software. The activity concentrations in the measured samples were calculated using Equation 3 [34, 39]: A B q / k g = N p I γ ε T m

Where A (Bq/kg) is the specific activity in a sample, N p is the activity of the sample with the background activity subtracted.

 The radium equivalent activity Ra<sub>Eq</sub>

The radium equivalent activity is an index introduced to represent the specific activities of 226Ra, 232Th, and 40K by a single quantity that accounts for the radiation hazards associated with them. The radium equivalent activity was estimated using Equation 4 [5, 39–41]: R a E q B q / k g = C R a + 1.43 C T h + 0.077 C K

This radiation index uses baseline measurements of 370 Bq/kg for 226Ra, 259 Bq/kg for 232Th, and 4,810 Bq/kg for 40K, as these levels produce equivalent gamma radiation doses [5]. To keep radiation exposure within safe limits, experts recommend that the index should not exceed 370 Bq/kg in soil [42]. This helps to assess how gamma radiation may affect living organisms and allows evaluation of potential health risks in different locations.

 The absorbed dose rate in air (ADR)

The absorbed gamma dose rate measures the rate at which ionizing radiation from gamma rays is deposited at a specific location, providing insight into the potential biological effects of gamma radiation. The total absorbed dose rate due to naturally occurring radioactive materials (NORM) in air 1 m above the ground is calculated using the following equations. Absorbed dose rates are estimated using Equations 5, 6 [40, 43–45]: D Ind n G y / h = 0.920 C R a + 1.100 C T h + 0.0810 C K D Out n G y / h = 0.462 C R a + 0.604 C T h + 0.0417 C K

Where 0.462, 0.604, 0.0417, 0.92, 1.1, and 0.081 are dose conversion factors in nGy/h per Bq/kg, and C T h , C R a and C K are the radionuclide concentrations for 226Ra, 232Th, and 40K, respectively [5, 41, 46, 47].

 The annual effective dose rate (AEDE)

The air absorbed dose rates found above are multiplied by a conversion factor F with a value of 0.7 × 1 0 − 6 Sv/y [5] to convert to the effective dose received by adults and 0.2 and 0.8 for the outdoor and indoor occupancy factors, respectively [46]. The effective dose rate per year should be less than a unity [48]. The indoor and outdoor annual effective dose equivalent is estimated using Equations 7, 8 [5, 49]: A E D E Ind m S v / y = D Ind × T × 0.8 × F A E D E Out m S v / y = D Out × T × 0.2 × F

Where T is hours in a year (365 × 24 h = 8,760 h), and F is the conversion factor with a value of 0.7 × 1 0 − 6 Sv/y.

 The excess lifetime cancer risk (ELCR)

Excess lifetime cancer risk (ELCR) is a term used in radiation protection to estimate the potential increase in a person’s cancer risk due to exposure to ionizing radiation that exceeds the baseline risk of cancer exposure without radiation exposure. The indoor and outdoor ELCR values should, on average, be less than or equal to the global average of 1.16 × 10 − 3 and 0.29 × 10 − 3 , respectively. The excess lifetime cancer risk ELCR Ind in the indoor environment was calculated using Equation 9 [5, 50–52]: E L C R Ind = A E D E Ind × D L × R F

The outdoor excess lifetime cancer risk ELCR Out was calculated using Equation 10 [51, 52]: E L C R Out = A E D E Out × D L × R F

Where DL is the life expectancy, which is about 70 years, and RF is the risk factor, which is given as 0.05 Sv − 1 . The indoor and outdoor ELCR values should, on average, be less than or equal to the global averages of 1.16 × 1 0 − 3 and 0.29 × 1 0 − 3 , respectively.

 The internal hazard index H<sub> <italic>In</italic> </sub>

The internal hazard index is a crucial concept in radiation protection and safety, assessing potential radiation exposure and hazards linked with radioactive material intake. It establishes tolerable intake limits for radioactive materials and assesses the need for extra precautions like radiation protection or medical follow-up. Inhaling radon and thoron gases can be hazardous to the respiratory organs [53] as these particles undergo alpha decay, thus releasing alpha particles, which can tear the epithelial cells of the lungs, and Equation 11 is used to calculate the hazard index [40, 54–56]: H I n = C R a 185 + C T h 259 + C K 4810

For the safe use of building materials in shelter construction, the index should be less than one.

 The external hazard index H<sub> <italic>Ex</italic> </sub>

External gamma radiation dose refers to the amount of ionising radiation a person is exposed to from gamma rays emitted by an external source, typically associated with radionuclides of concern. To limit this dose, an external hazard index H E x is calcutaled using Equation 12 [40, 54–57]: H E x = C R a 370 + C T h 259 + C K 4810

For the safety of individuals outdoors, the index should be less than unity.

 The annual gonadal dose equivalent (AGDE)

The annual gonadal dose equivalent (AGDE) is a measure used in radiation protection to estimate the potential dose to the reproductive organs (gonads) from a person’s exposure to ionising radiation over a year. The AGDE resulting from the specific activities of 226Ra, 232Th, and 40K was calculated using Equation 13 [40, 58]: A G D E μ Sv/y = 3.09 C R a + 4.18 C T h + 0.314 C K

The AGDE considers the type of radiation, the amount of radiation exposure, and the sensitivity of the gonads to radiation-induced damage.

 The alpha hazard index I<sub>α</sub>

The index estimates the risk of internal exposure to alpha radiation from a mixture of α -emitting radionuclides and expresses the total hazard in a single numerical value. Excess alpha radiation from inhalation of radon from building materials is estimated using Equation 14 [12, 59]: I α = C R a 200

The alpha index, I α ≤ 1 is equivalent to 200 Bq/kg of radium. Construction material with 226Ra exceeding 200 Bq/kg should be avoided in building because this may lead to 200 Bq/m3 of radon, exposing occupants to internal radiation. In the above equations, 226Ra, 232Th, and 40K represent the radionuclide concentrations.

 The radioactivity index I<sub>γ</sub>

This study examines the possibility of radiation exposure to human settlements near gold mine tailings dams, especially informal settlements. The plastering sand-like soil can easily be excavated for building purposes, and unaware of its toxicity, people may make bricks and use it as building sand, which will, in turn, expose the inhabitants to gamma radiation. The gamma radiations emitted by certain natural radionuclides in building materials are linked to this index by Equation 15 [50, 51, 60]: I γ = C R a 150 + C T h 100 + C K 1500

An increase in the gamma index beyond the worldwide acceptable limit may result in radiation risk, leading to the deformation of human cells, thereby causing cancer. I γ = 1 as an upper limit, I γ ≤ 1 corresponds to 0.3 mSv/y, and I γ ≤ 3 corresponds to 1 mSv/y. For materials used in bulk like bricks, the ranges of I γ are: 0.5 ≤ I γ ≤ 1 [61].

 Results and discussion Specific activity concentrations of natural radionuclides in samples

Samples from four different gold tailings were collected and analyzed at iThemba LABS. The activity concentration of 238U ranged from 132.88 ± 4.68 to 1,421.46 ± 13.38 with a mean of 464.96 ± 2.08 Bq/kg. The activity concentration of 226Ra ranged from 112.02 ± 9.09 to 1,195.80 ± 30.63 with a mean of 407.42 ± 4.23 Bq/kg, which is 12 the 35 Bq/kg recommended value by [5]. The range of 232Th was from 5.27 ± 0.67 to 19.37 ± 1.98 with a mean of 11.20 ± 1.03 Bq/kg, and 40K activity ranged from 65.23 ± 15.29 to 264.11 ± 25.66 with a mean of 127.50 ± 21.85 Bq/kg as presented in Table 2. The 238U was calculated from the concentrations of 214Pb and 214Bi, while radium was calculated from its signal of 186.20 keV. The findings showed that 238U and 226Ra activity exceeded the global average of 30 and 35 Bq/kg, respectively, while 232Th and 40K were below their respective limits [5].

The specific activity concentrations of 238U, 226Ra, 232Th and 40K in the tailings.

Gold tailings Sample ID Specific activity concentration in roodepoort samples (Bq/kg)
238U 226Ra 232Th 40K
Roodepoort RDP 01 444.81 ± 8.20 397.67 ± 16.6 10.66 ± 0.97 119.52 ± 18.18
RDP 02 584.28 ± 10.23 470.98 ± 18.73 12.76 ± 1.12 163.42 ± 20.92
RDP 03 483.32 ± 9.08 413.61 ± 16.26 8.96 ± 0.84 128.92 ± 17.53
RDP 04 480.67 ± 7.95 393.29 ± 17.04 12.60 ± 1.13 135.19 ± 19.61
RDP 05 441.44 ± 8.21 367.66 ± 16.82 11.84 ± 1.12 122.94 ± 19.64
RDP 06 279.51 ± 6.72 347.55 ± 15.788 10.12 ± 0.99 87.83 ± 17.17
RDP 07 247.96 ± 6,67 296.721 ± 14.26 8.73 ± 0.87 106.15 ± 17.28
RDP 08 362.03 ± 8.03 276.40 ± 13.06 8.64 ± 0.83 91.18 ± 15.40
RDP 09 499.35 ± 9.69 472.04 ± 20.27 19.36 ± 1.48 177.19 ± 23.81
RDP 10 1,421.46 ± 13.38 1,195.80 ± 30.63 18.81 ± 1.42 264.11 ± 25.66
RDP 11 456.98 ± 7.95 417.81 ± 18.55 14.71 ± 1.23 125.11 ± 20.81
RDP 12 437.17 ± 8.28 356.985 ± 15.54 10.86 ± 0.97 111.84 ± 17.48
RDP 13 547.86 ± 10.16 457.89 ± 18.85 10.49 ± 1.05 147.52 ± 20.89
RDP 14 373.40 ± 8.26 308.16 ± 14.29 8.02 ± 0.80 98.05 ± 16.56
RDP 15 132.88 ± 4.68 12.02 ± 9.09 5.27 ± 0.67 65.23 ± 15.29
RDP 16 237.16 ± 6.20 248.18 ± 13.70 6.82 ± 0.79 85.01 ± 17.46
RDP 17 474.11 ± 8.48 393.29 ± 18.04 11.82 ± 1.19 138.32 ± 21.36
Activity statistics Min 132.88 ± 6.20 112.02 ± 9.09 5.27 ± 0.67 65.23 ± 15.29
Max 1,421.46 ± 13.38 1,195.80 ± 30.63 19.37 ± 1.48 264.11 ± 25.66
Mean 464.96 ± 2.08 407.41 ± 4.23 11.20 ± 1.03 127.50 ± 21.85
STDev 274.12 222.56 3.76 45.58
Median 444.81 ± 8.20 393.29 ± 17.04 10.66 ± 0.97 122.94 ± 19.64

The concentration of 40K ranged from 113.34 ± 20.80 Bq/kg to 145.79 ± 22.51 Bq/kg with a mean of 126.15 ± 10.90 Bq/kg, which is 11 times the concentration of 232Th. The level of 40K is below the permissible limit of 400 Bq/kg, and 232Th ranged from 8.25 ± 1.31 to 11.20 ± 1.03 Bq/kg with a mean of 10.06 ± 0.68 Bq/kg. The mean concentration of 226Ra is 40.3 times that of 232Th, whereas it is 3.5 times the level of 40K in the samples, and in decreasing order they are: 238U > 226 Ra > 40 K > 232 Th.

In this analysis, 214Bi and 214Pb were used to estimate 238U, assuming secular equilibrium [29, 30], and radium was estimated directly from its 186 keV signal, as there were very few significant responses from 235U signals to indicate its presence. Both 214Bi and 214Pb are short-lived decay products of textsuperscript222Rn gas and are strong gamma-ray emitters. Their characteristic gamma rays—295 and 351.9 keV for 214Pb, and 609.3, 1,120.3, and 1764.5 keV for 214Bi–are easily detected by γ -ray spectrometry.

The use of these daughter products to determine the parent uranium concentration relies on the concept of secular equilibrium, which states that the activity of each intermediate product is proportional to the amount of uranium present because its rate of decay equals its rate of production. The activities of 226Ra and 214Pb were correlated to see if the analysis was indeed carried out at radioactive secular equilibrium samples for Roodepoort tailings. There was a good positive correlation between the activities of 226Ra and 238U in the samples, indicating the system was indeed at radioactive secular equilibrium for the Roodepoort tailings samples after 42 days.

The activity concentration of 232Th was detected, but it was within the background levels. The activity concentration of 40K was present at an intermediate level but below the permissible limit of 400 Bq/kg [5]. The specific activity concentration of 238U and 226Ra dominated all NORM activities in the soil samples from Roodepoort tailings, as shown in Figure 4.

The mean specific activity concentration.

Bar graph showing the specific activity of radionuclides in soil. Uranium has the highest activity at four hundred sixty-four point ninety-six becquerels per kilogram, followed by radium at four hundred fifty-two point twenty-eight, potassium at one hundred twenty-seven point fifty, and thorium at eleven point twenty.

Table 3 compares radium in this study, in the region, and globally in tailings of different mined resources. The high radium activity concentrations, compared with activities in other tailings except for gold from various countries, are presented in Table 3. The mean radium concentration in the CMR gold tailings is 451 ± 8.00 Bq/kg, which is lower than the 662 Bq/kg found in New Zealand’s phosphate rock ore [62], but higher than the 253.6 Bq/kg in Saudi Arabia’s phosphate ore [63]. Compared to phosphate tailings in Tanzania, which exceed 2,939.0 and 5,591.2 Bq/kg [64], the CMR concentration is significantly lower. However, it is higher than the concentration of 54 ± 3 Bq/kg around the Geita Gold Mine in Tanzania [65], and higher than 200.42 and 306.67 Bq/kg in silver mining areas of Brazil [66]. It is notable that phosphate rock waste have high radium and uranium activities. This is because the fertilizer processing focuses solely on extracting phosphates, leaving uranium concentrated in the waste.

The comparison of radium activity concentration in tailings of different mining wastes.

Tailings type Min Max Mean References
Phosphate rock Ore 107 1,649 662 [62]
Phosphate Ore 62.3 333.5 253.6 [63]
Phosphate tailings 1 311.0 3,945.8 2,939.0 [64]
Phosphate tailings 2 3,759.8 7,606.3 5,591.2 [64]
Geita gold mine 13 ± 9 99 ± 4 54 ± 3 [65]
Silver mining, zone A 124.26 303.45 200.42 [66]
Silver mining, zone B 105.76 596.26 306.67 [66]
West rand 12.35 941.07 53.09 [23]
Roodepoort 112.02 ± 9.09 1,195.80 ± 30.63 407.41 ± 4.23 This study
Radiological hazard assessment in soil samples

For RDP samples shown in Table 4, the Ra Eq has a mean of 433.28 ± 17.06 Bq/kg, a value 1.2 times the permissible limit of 370 Bq/kg [42]. The indoor and outdoor air absorbed dose rates have average values which exceeded their recommended values of 84 and 59 nGy/h, respectively. The indoor and outdoor AEDE are above their recommended values of 0.41 and 0.07 mSv/y, respectively [5, 67]. The values of the AEDE are higher than 0.23 ± 10.07 mSv/y for outdoor and 0.92 ± 10.29 mSv/y for indoor, as reported in a study in Cameroon [68]. Both the cancer risks (ELCR) are higher than their recommended values of 1.16 × 1 0 − 3 and 0.29 × 1 0 − 3 , respectively [67].

Comparison of radiological health indices with their globally recommended upper limits.

Site Radiological indices Minimum Maximum Mean Upper limit
RDP Ra E q (Bq/kg) 142.74 ± 9.24 1,242.98 ± 30.78 433.28 ± 17.06 370
ADR Ind (nGy/h) 114.15 ± 8.51 1,142.21 ± 28.32 397.42 ± 15.17 84
ADR Out (nGy/h) 57.68 ± 6.40 574.91 ± 14.23 200.35 ± 7.89 59
AEDE Ind (mSv/y) 0.56 ± 0.04 5.60 ± 0.14 1.95 ± 0.08 0.41
AEDE Out (mSv/y) 0.07 ± 0.01 0.71 ± 0.02 0.25 ± 0.01 0.07
ELCR Ind ( × 1 0 − 3 ) 1.96 ± 0.15 19.61 ± 0.49 6.82 ± 0.27 1.16
ELCR Out ( × 1 0 − 3 ) 0.25 ± 0.02 2.47 ± 0.06 0.86 ± 0.03 0.29
H I n 0.64 ± 0.06 6.59 ± 0.18 2.27 ± 0.10 ≤ 1
H E x 0.34 ± 0.07 3.36 ± 0.12 1.17 ± 0.09 ≤ 1
AGDE ( μ Sv/y ) 388.68 ± 28.75 3,856.56 ± 95.28 1,345.78 ± 52.90 300
I γ 0.42 ± 0.04 4.71 ± 0.12 1.46 ± 0.07 ≤ 1
I α 0.56 ± 0.05 5.98 ± 0.15 2.04 ± 0.08 < 1

The internal hazard index is 2.51 times higher than its recommended value, while the external hazard index is 1.3 times higher than its recommended value but is of the same order as the value found in the study in Cameroon [68] and higher than the value of 0.59 reported in Turkey [69]. The mean values of AGDE, I α , and I γ are 4.5, 2.0, and 1.5 times higher than their recommended values, respectively, and the average I γ is lower than the value of 1.60 reported in Turkey [69].

In the figure, the radium equivalent activity of the RDP gold tailings samples shows that only 4 out of 17 samples–RDP 06, 07, 15, and 16—with RDP 15 having the lowest values, fall below the recommended regulatory benchmark of 370 Bq/kg [42, 70], which corresponds to an external dose rate limit of about 1.5 mSv/y [70] for shelter occupants. The RDP samples with high radium levels–RDP 02, 03, 04, 05, 09, 10, 11, 12, 13, and 17—showed high Ra Eq values, with RDP 10 having the highest value, as shown in Figure 5; overall, the values exceeded the recommended limit. The mean radium equivalent was higher than the 189 Bq/kg found in a study of tailings in Tanzania [65].

The radium equivalent activity in samples.

Bar chart comparing permissible and measured radium equivalent values in Becquerels per kilogram for samples RDP01 to RDP17. Most measured values exceed permissible limits, with RDP10 showing a significant peak.

The relationship between the ADR and AEDE in Roodepoort gold tailings samples is shown in Figure 6. The internal and external absorbed dose rates exceeded their recommended limits of 84 and 59.9 nGy/h, respectively [5]. The indoor ADR is similar to the 413.50 nGy/h reported in the West Rand [23], but higher than the value found in Cameroon, which was 188.2 ± 59.4 nGy/h.The outdoor ADR is higher than 49.09 nGy/h and 70.12 nGy/h, which were found in the East Rand and Soweto, respectively [23]. The total annual effective dose equivalent exceeded the permitted value of 0.48 mSv/y in all samples, and they were higher than the global limit of 1.00 mSv/y [5, 71], and they were higher than the values in the East Rand of 0.51 mSv/y [23]. Figure 6 shows that a proportionality exists between the absorbed dose rate and the annual effective dose rate.

The comparison of ADR and AEDE in samples.

Bar chart comparing indoor and outdoor absorbed dose rates (ADR) in nanograys per hour and annual effective dose equivalent (AEDE) in millisieverts per year across 17 RDP locations. Blue bars represent indoor ADR, yellow bars represent outdoor ADR, black bars represent AEDE total, with limits marked by sky blue, red, and green bars. Peak values are observed at RDP10 for ADR and AEDE.

It is evident in Figure 6 that all samples follow a similar distribution trend to the absorbed dose rate, as the annual effective dose has a linear relationship with the absorbed dose rate. The relationship between ADR and AEDE in Roodepoort gold tailings samples is illustrated. The annual effective dose equivalent increases with an increase in the air absorbed dose rate; this relationship is confirmed by Figure 7, which shows that a proportionality exists between the absorbed dose rate and the annual effective dose rate.

The linear plot between ADR and AEDE.

Graph showing a linear relationship between total annual effective dose equivalent (AEDE_tot in mSv/y) and total ambient dose rate (ADR_tot in nGy/h). Data points form a line with the equation AEDE = 0.0037 × ADR - 0.001, and R² is 1, indicating a perfect fit. Error bars accompany the data points.

The radiological health indices in the Roodepoort gold tailings, for which the permissible maximum value should be unity, are shown in Figure 8, with RDP 10 exceeding all others and RDP 15 having the lowest values in H In , H Ex , I γ , and I α . These indicators suggest that, in the long term, people who reside permanently in these areas may face health issues.

The H In , H Ex , I γ and I α indices.

Bar chart showing radiological indices for RDP01 to RDP17. Bars represent HIn (blue), HEx (purple), Iγ (green), Iα (red), with permissible values (black). RDP10 has the highest indices.

Figure 9 shows a comparison of indoor and outdoor excess lifetime cancer risk factors calculated for these samples. For Sample RDP 10, the outdoor ELCR is nine times higher than its acceptable value. Both the indoor and outdoor ELCR exceed their respective limits of 1.16 × 1 0 − 3 and 0.29 × 1 0 − 3 [5, 67], as shown in the figure below.

The excess lifetime cancer risks and their limits.

Bar chart comparing Indoor ELCR, Indoor ELCR Limit, Outdoor ELCR, and Outdoor ELCR Limit for RDP01 to RDP17. Indoor and outdoor ELCR values are depicted as blue and green bars, respectively. Limits are shown as red and yellow bars. Indoor ELCR peaks at RDP10.

Figure 10 shows a plot of the ELCR and AEDE values for the gold tailings soil samples from Roodepoort. This plot indicates a linear relationship between radiation exposure and the likelihood of developing cancer during a human lifetime.

The linear plot between ELCR and AEDE.

Scatter plot showing a positive linear relationship between Annual Effective Dose Equivalent (AEDE) in millisieverts per year and Excess Lifetime Cancer Risk (ELCR). The red line represents the linear regression fit with the equation \(y = 3.296x + 0.568\) and \(R^2 = 1\). Blue data points with error bars are plotted along the line.

Figure 11 shows the relationship between the radium equivalent activity and the annual gonadal dose equivalent in the tailings samples from Roodepoort. The graph indicates that the AGDE is proportional to the activity present in the samples. The mean AGDE is 1,345.78 ± 52.90 μ Sv/y , which is 4.5 times the recommended value of 300 μ Sv/y [72] and is higher than the AGDE value of 678 μ Sv/y found in a study in Tanzania [65].

The AGDE and Rad Eq in Roodepoort samples.

Bar chart showing Radium Equivalent (RaEq) in blue and AGDE in black for samples RDP01 to RDP17. RaEq peaks significantly at RDP10. Permissible levels, marked in green, are constant and lower than both Radium Equivalent and AGDE across all samples.

Soil samples with high activity tend to release high levels of radiation; therefore, all radiation hazard indices for these samples are high, and the reproductive organs (gonads) of people exposed to such soil are at risk of significant radiation exposure, which can affect future generations. Radiation can cause genetic damage in the gonads, leading to mutations and hereditary diseases, as there is no minimum safe dose of radiation to the gonads [73]. No amount of ionizing radiation is considered negligible, as it is believed that any exposure may increase the risk of stochastic effects. It is assumed that these effects follow a linear model with no specific threshold; therefore, radiology specialists encourage adherence to the ALARA principle [74].

As radiation is invisible to the human eye but can be harmful at high levels, it is important to keep exposure as low as possible. The ALARA principle consists of three factors: time, distance, and shielding. If citizens live permanently near radioactive waste, these ALARA factors are continuously disregarded, which also violates the United Nations Sustainable Development Goals (SDGs), such as SDG 3: Good Health and Wellbeing and SDG 11: Sustainable Cities and Communities. If such radiation reaches water bodies, both SDG 14: Life Below Water and SDG 15: Life on Land are also not observed of [75].

 Conclusion and outlook

The Roodepoort gold tailings samples were collected, prepared, and analyzed for radioactivity using a high-energy resolution coaxial HPGe γ - detector. In this analysis, both uranium and radium exceeded their recommended global limits, while thorium and potassium were below their recommended limits.

The average radium equivalent was 1.2 times higher than its recommended value of 370 Bq/kg. The average indoor and outdoor absorbed dose rates were 4.7 and 3.3 times higher than their respective recommended values of 84 nGy/h and 59 nGy/h. The average indoor and outdoor annual effective dose equivalents were 4.8 and 3.6 times higher than their recommended values of 0.41 and 0.07 mSv/y, respectively. The average indoor and outdoor excess lifetime cancer risks were 5.9 and 3.0 times higher than their respective recommended values of 1.16 × 1 0 − 3 and 0.29 × 1 0 − 3 . The averages of internal and external hazard indices were 2.5 and 1.3 times higher than unity. The annual gonadal dose equivalent had a mean value 4.5 times higher than the recommended value of 300 μ Sv/y . The mean values of the alpha and gamma hazard indexes were 2.26 and 1.61 times greater than unity, respectively.

Although they originate from background ionising radiation, these findings may, in the long term, have deleterious health effects on residents in the vicinity of the tailings, as the recommended radiological health limits were exceeded. Additional oversight and regulatory control measures are required to ensure the safety of the environment and residents near the tailings. Our study was limited to measuring radioactivity using gamma spectroscopy and did not use alpha spectroscopy, even though radon gas is also an alpha emitter. Future studies may address this aspect.

 Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

 Author contributions

MM: Writing – original draft, Writing – review and editing. SN: Funding acquisition, Supervision, Writing – review and editing. SM: Funding acquisition, Supervision, Writing – review and editing. BK: Supervision, Writing – review and editing. PLM: Supervision, Writing – review and editing. PPM: Supervision, Writing – review and editing.

 Acknowledgements

The first author thanks the NRF-iThemba LABS for providing access to the facility. Special thanks are extended to Mr. A. Kwelilanga, an applied nuclear and radiation physicist at iThemba LABS, for his knowledge of gamma spectrometric analysis. Most importantly, the Department of Physics, University of Zululand, supported this study.

 Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

 Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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 Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Edited by: Mauro Menichelli, Istituto Nazionale di Fisica Nucleare di Perugia, Italy

Reviewed by: Belafrites Abdelfettah, University of Jijel, Algeria

Gebi Tuku, Mizan Tepi University, Ethiopia

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