Evaluating radiological impacts due to uranium mining in the Erongo Region, Namibia

Abstract

The unambiguous and continuously increasing exposures of human beings to terrestrial radiations is highly attributed to naturally occurring radioactive materials in the environments. Background radiation had been increasing gradually due to mining and milling activities worldwide to acquire nuclear materials needed for a response to electrical demands globally. The most significant health effects associated with the continued exposure to NORMs are lung cancer and leukemia. The main aim of this study was to evaluate the radiological risks associated with naturally occurring radioactive materials from uranium mining activities to critical groups in Erongo Region, Namibia. The study analyzed the radiological emissions of gamma rays in soil samples using high-purity germanium detector and determined the elemental and radiological concentrations in dust and water using inductively coupled plasma-mass spectrometry Equipment. The target radionuclides were 238U, 226Ra, 232Th and 40K. The weighted mean values of activity concentrations with standard deviations measured for 238U, 226Ra, 232Th and 40K in soil samples using high-purity germanium detector ranged between 104.77±5.23 and 4610.29±50.57 Bq.kg-1, 42.22±0.73 and 4909.40±18.13 Bq.kg-1, 91.02±1.09 and 436.74±5.16 Bq.kg-1, and 683.06±12.88 and 2225.00±52.65 Bq.kg-1, respectively. All activity concentrations calculated were found to be significantly higher than the world averages levels of 35, 33, 45 and 420 Bq.kg-1, respectively reported by United Nations Scientific Committee on the Effects of Atomic Radiation. Khan river was the only section of study with low radioactivity level. Mean values with standard deviation only for absorbed dose , radium equivalent, annual effective dose, internal and external hazard indices were ranging between 135.89±2.58 to 2290.70±23.53 nGy.h-1, 253.45±2.03 to 5271.51±19.28 Bq.kg-1 and 0.83± to 14.05± mSv.y-1, 0.86±0.01 to 27.51±0.01 and 0.68±0.01 to 14.25±0.01, respectively. Gamma and alpha indices were calculated ranging from 0.93±0.01 to 17.69±0.07, and 0.21±0.01 to 24.55±0.09, respectively. The probability of cancer development in a biological body was estimated by a human health related hazard known as excess lifetime cancer risk. The calculated cancer risks were ranging from 2.92x10-3 to 4.99x10-2 for the soil samples, values which were significantly higher than both the world average (0.29x10-3) and limit (1.45x10-3) documented by United

Nations Scientific Committee on the Effects of Atomic Radiation and International Commission on Radiation Protection, respectively. The elemental and activity concentrations for naturally occurring radioactive materials were also determined using inductively coupled plasma-mass spectrometry. The mean values in water from the tailings and boreholes were 17.18 and 3.17 Bq.l-1 for 238U, 1.72 and 0.024 Bq.l-1 for 232Th and 12.40 and 11.58 Bq.l-1 for 40K, respectively. These calculated radioactivity concentration values exceeded the drinking water quality guidelines levels recommended by World Health Organization for gross alpha screening and gross beta levels of 0.5 mBq.l-1 and 1.0 mBq.l-1, respectively. The results show that both tailings and borehole water were unsuitable for human consumption and therefore, contact with edible food must be avoided by all means. For the dust from mine 2, the average activity concentrations were 80.94 Bq.l-1 for 238U, 673.12 Bq.l-1 for 232Th and 27.95 Bq.l-1 for 40K. The results had shown high transferrable probability of thorium radionuclide in the atmosphere than uranium and potassium nuclides. In dust, 232Th was recorded with significant high value of radioactivity concentration at 673.12 Bq.l-1, which was higher than the world average level of 45 Bq.kg-1, documented by United Nations Scientific Committee on the Effects of Atomic Radiations. On excess life cancer risks, the RESRAD-OFFSITE model 4.0 was used with ICRP 107 based radionuclide transformations transfer factors and ICRP 60 external, inhalation and ingestion dose conversion factors to perform the analysis. The cancer morbidity risks modelled for naturally occurring radioactive materials in both mines’ samples had shown that 226Ra was the highest contributor. The RESRAD modelling was performed on water, plants, soil and atmospheric exposure pathways by external gamma, inhalations and ingestions and had shown risk factors in descending order as 226Ra ˃ 232Th ˃ 40K ˃ 238U. In RESRAD-OFFSITE model code, the total cancer morbidity risks were recorded with about 3 persons per 1 000 populations (3 x 10-3) by tailings soil (stockpiles) and surrounding soil samples of mine 1 less than 7 to 8 persons per 1 000 populations (7 x 10-3 – 8 x 10-3) by tailings (stockpiles) soil samples in mine 2 less than 7 to 9 persons per 1 000 populations (7 x 10-3 – 9 x 10-3) by uranium ore samples of mine 1 were at risks of

developing cancer. This could explicitly prove that the modelled cancer risks in the region were higher than the recommended level of 1 x 10-5 factor for a population and 1 x 10-3 for a subpopulation documented by the World Health Organization as well as the world average (0.29 x 10-3) documented by the United Nations Scientific Committee on the Effects of Atomic Radiation.