Human health risk assessment of environmental radionuclides and heavy metals around a gold mining area in Gauteng Province, South Africa

Abstract

Naturally Occurring Radionuclide Materials (NORMs) and heavy metals are a common occurrence in the environment and have resulted in human exposure for the entire history of mankind. However, anthropogenic activities such as mining have resulted in elevated levels of these contaminants in the environment. A health risk assessment of NORMs and Heavy Metals from a gold mining area in the Gauteng Province of South Africa has been evaluated. In this study, a total of 136 soil, water and plant samples were collected from around the mine and from the control area for laboratory analyses. A broad energy germanium (BEGe) detector with a relative efficiency of 60 % and a resolution of 2.0 keV at 1332 keV gamma ray emission of ⁶⁰Co was used to measure the activity concentrations of NORMs. RESidual RADioactivity (RESRAD) OFFSITE modeling program (version 3.1) was then used to estimate excess cancer risk from NORMs for a hypothetical resident scenario. An Inductively Coupled Plasma Mass Spectrometer (ICP-MS) was on the other hand used to measure concentrations of heavy metals in all samples as well. The AlphaGuard Monitor was used to measure indoor radon gas. All the measurements were compared with those from the control area and reference standards. The average activity concentrations in Bq.kg⁻¹ for ²³⁸U, ²³²Th and ⁴⁰K in soil from the mine was found to be 574.3±39.5, 49.4±8.5 and 424.7±129.3, respectively. These results showed higher levels of ²³⁸U in soil from the mining area compared to the control area and the worldwide average. In plant samples, the average activity concentrations of ²³⁸U, ²³²Th and ⁴⁰K in Bq.kg⁻¹ were 17.4±3.1, 19.7±1.6 and 146.7±9.2, respectively. The values measured for ²³⁸U and ²³²Th in plant samples were higher than acceptable limits whilst that of ⁴⁰K was expected although it exceeded that of both ²³⁸U and ²³²Th. For water samples, the average values for ²³⁸U, ²³²TTh and ⁴⁰K in Bq.L⁻¹ were 0.66±0.03, 0.56±0.03 and 7.36±0.58, respectively. Compared with samples from the control area and South African Guidelines, average values for ²³⁸U, ²³²Th and ⁴⁰K in water samples were higher. Radiological hazards for soil, plant and water samples were also considered for the study area. The average values for radium equivalent activity (Raₑq in soil from the mining area was 677.7±43.8 Bq.kg⁻¹ compared to 86.9±1.4 Bq.kg⁻¹ from the control area. This value was above the worldwide average of 370 Bq.kg⁻¹ as recommended by the International Atomic Energy Agency (IAEA) under normal circumstances. For plant samples, the average value of Raeq in Bq.kg⁻¹ from the mining area was 56.8±4.0 compared to 44.1±2.1 Bq.kg⁻¹ from the control area, while in water samples it was 2.03±0.07 Bq.L⁻¹ compared to 1.39±0.08 Bq.L⁻¹ . These values for plant and water samples are lower than the worldwide average. The Annual Effective Dose Equivalent (AEDE) values in mSv y⁻¹ for soil, plant and water samples from the mining area were 0.38, 0.03 and 1.17×10⁻⁰³, respectively. All these values are below the worldwide average of 0.48 mSv for terrestrial gamma radiation. The annual effective dose of natural radionuclides in mSv through the ingestion of water and plant samples were estimated to be 1.24 ×10⁻⁰³ and 1.23×10⁻⁰⁴, respectively. Indoor radon from the mining area and the control area was also measured. The average activity concentrations of indoor radon from the mining area was 119.5 Bq/m³, compared to a 19.7 Bq/m³ from the control area. This translated to an average annual effective dose of 3.01 and 0.5 mSv for the mining area and control area, respectively. According to UNSCEAR, the worldwide average annual effective dose from inhalation of radon and its decay products is 1.26 mSv. The average value from the mining area studied was higher than the worldwide average. When all the samples were put together, the total annual effective dose from the measured samples was 3.42 mSv.y⁻¹, which is higher than the worldwide average of 2.4 mSv.y⁻¹ from natural radiation. This value is also higher than 1 mSv.y⁻¹, a limit recommended by ICRP for individual members of the public. Average external hazard indices (Hₑₓ) for soil, plant and water samples from the mining area were 1.8, 0.2 and 5.49×10⁻³, respectively, while corresponding internal hazard (Hᵢₙ) values were 3.4, 0.2 and 7.28×10⁻³, respectively. This shows that the average Hₑₓ and Hᵢₙ values for soil samples were higher than unity, posing a potential radiological threat to members of the public in the mining area. Plant and water samples were radiologically safe to members of the public as their values were less than one. The cancer risk for members of the public living in the mining area as a result of natural radionuclides was estimated using the RESRAD-OFFSITE Computer Code. The maximum total cancer risk for all the pathways was found to be 6.52 × 10⁻⁵. This was higher than the South African Individual cancer risk limit for the public of 5 x 10⁻⁶. Heavy metals in soil, plant, and water samples were also measured from the different locations of the gold mining area and from the control area. The average concentrations (mg.kg⁻¹) in soil decreased in the order of Cr>Ni>As>Zn>Cu>Co>Pb>Hg>Cd. The values were as follows: Cr (316.88); Ni (115.87); As (77.); Zn (68.01); Cu (50.79); Co (27.52); Pb (4.69); Hg (0.09); and Cd (0.05), respectively. These concentrations were higher compared to the soil from the control area. As and Cr were found to be higher than the maximum allowable limits. Average concentrations (mg.kg⁻¹) in plant samples decreased in the order of Cr>Ni>As>Zn>Cu>Co>Pb>Hg=Cd. Average values were as follows: Cr (6.90); Ni (2.89); As (2.50); Zn (0.44); Cu (0.32); Co (0.28); Pb (0.16); Hg and Cd (0), respectively. The results indicated that average concentrations of As and Ni in plant samples were higher than FAO/WHO and South African safe limits. Average concentrations of heavy metals in water samples (mg.L⁻¹) decreased in the order of Ni>Cu> Zn>As> Cr>Co>Pb>Hg=Cd. Average concentrations were as follows: Ni (0.39); Cu(0.38); Zn (0.33); As (0.19); Cr (0.14); Co (0.08); Pb (0.01); Hg and Cd (0), respectively. These average concentrations of heavy metals in water were generally higher than those from the control area. Compared with reference levels the average concentrations of As, Ni, Cr, and Zn in drinking water were higher than permissible limits. For non-carcinogenic risk of heavy metals in all the samples, a total HQ value of 2.62 was found. This value is greater than 1, which potentially causes a health risk to the public living in the gold mining area. The total carcinogenic risk from all the samples as a result of heavy metals was found to be 1.91×10⁻⁴. The sum total of the cancer risk due to natural radionuclides (6.52×10⁻⁵ ) and that of heavy metals (1.91×10⁻⁴) was found to be 2.56 ×10⁻⁴ (1 in 3906 individuals). This total value obtained was higher than the acceptable cancer risk limit. From the findings presented, it can concluded that natural radionuclides and heavy metal pollution in the mining area are an issue of health concern.