Mobility, activity ratio significance and risk assessment of radionuclides in sandstone of Magharet El Miah, Southwestern Sinai, Egypt

Abstract

Sedimentary rocks of Magharet El Miah were radiometrically analyzed for determination of radionuclides 238U, 232Th, 226Ra and 40K activity concentrations. The U-content and Th/U ratio in these rocks illustrated poor weathering and rapid deposition of rock detritus. Preferential dissolution of 234U due to α-decay related processes and its re-adsorption and precipitation are responsible for the observed radioactive disequilibrium in 234U/238U of Magharet El Miah sandstone. 234U mobilization could be related to the effect of surficial or underground water. 226Ra/238U disequilibrium could be revised to the difference in the geochemical characteristics of the two isotopes. 230Th/238U activity ratios were lower than unity in all samples (10 samples) except sample number 1, suggesting selective uranium accumulation in most samples or 230Th leaching by acidified water where the rocks contain high sulpher contents. Many of the 238U/235U activity ratios were shifted from the natural ratio due to the changes in redox conditions that leads to fractionation of higher isotope from lighter one. The plotted value of 234U/238U and 230Th/238U illustrated that the U-accumulation occurred in the period from <6 × 104–105 years. The studied sandstone had higher values for uranium (95.4 ± 7.1 to 130.3 ± 12.3 Bq.kg−1), thorium (65.8 ± 4.2 and 78.4 ± 6.7 Bq.kg−1) and radium (68.2 ± 4.4 and 80.3 ± 9.1 Bq.kg−1) but lower for potassium (38.4 ± 1.2 and 54.2 ± 2.3 Bq.kg−1) in comparison with international permissible levels. All the estimated environmental hazard indices illustrated that most samples lied in the permissible level ranges. Cluster analyses indicated that the main environmental health hazard parameters exist due to the activity concentrations of 238U and 226Ra.