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|a The present study aims to determine the levels of natural radioactivity and to assess the corresponding health risk in the region of elevated background radiation in Juban District, Yemen. The relationship between radionuclides concentrations and physicochemical properties in each geological formation of aquifers were estimated to determine the distributions of the radionuclides in groundwater. The mean external gamma dose rate was measured using portable survey meters, which was 374 ± 32 nGy h-1. Rock samples from different geological formations were measured to identify their crystal structures and quantitative determination of radionuclides using X- ray diffraction (XRD) and hyper pure germanium gamma spectrometer, respectively. The XRD results showed that monazite was the dominant radioactive mineral in all geological formations in Juban District. The mean activity concentrations of 232Th, 226Ra, and 40K were 1768 ± 918 Bq kg-1, 484 ± 230 Bq kg-1 and 1203 ± 186 Bq kg-1, respectively. The mean specific activity of 232Th and 226Ra were twenty one and six times higher than the world average, thus Juban district may be characterized as an elevated background radiation area. Natural radionuclides in groundwater were estimated using different methods depending on the chemical behaviour of each radionuclides and its kind of radiation. Inductively coupled plasma mass spectrometer, inductively coupled plasma-optical emission spectrometer, and atomic absorption spectrometer were used to analyse the concentration of uranium, thorium, iron and potassium, while the activity concentrations of 226Ra, 228Ra, 40K, 238U, 235U, 234U and 222Rn were measured using gamma spectrometer, alpha spectrometer and Rad 7, respectively. The measured concentration of uranium, thorium, iron, and potassium were 11.25 ± 2.65 μg L-1, 0.15 ± 0.04 μg L-1, 3.20 ± 0.37 mg L-1 and 17.02 ± 0.61 mg L-1 respectively. The mean activity concentrations of 226Ra, 228Ra, 40K , 222Rn, 238U, 234U and 235U were 94 ± 21 mBq L-1, 216 ± 32 mBq L-1, 3306 ± 356 mBq L-1, 226.4 ± 62.4 Bq L-1, 138.2 ± 26.9 mBq L-1, 234.0 ± 41.4 mBq L-1, and 7.2 ± 0.5 mBq L-1, respectively. The highest and lowest concentration of all radionuclides was found to be in the basement and sandstone aquifer, respectively. The mean value of 234U/238U activity ratios was 1.8. The relatively low 238U concentrations and high ratios of 234U/238U in the groundwater indicated the presence of younger waters with a stronger leaching of 234U from aquifer materials to the groundwater. The potential factors for high activity concentration of 222Rn 226Ra and 228Ra appear to be the presence of fault and shear. In contrast, the dominant factors affecting U concentrations were salinity of water and water table flow. The annual effective dose was 2.30 mSv, which was five times the world average. The external hazard index was ten times higher than recommended value, which further epidemiological studies of health effects relative to environmental radiation in Juban District need to be conducted.
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