Effect of alteration processes on the distribution of radionuclides in uraniferous sedimentary rocks and their environmental impact, southwestern Sinai, Egypt

摘要

The contents of natural radionuclides in various types of sedimentary rocks in Um Bogma Formation and base of El Hashash Formation were determined by gamma-ray spectrometry. Three types of lower Carboniferous sedimentary rocks were investigated; sandstone (El Hashash Formation), dolostone and argillaceous sediments (Um Bogma Formation). The alteration processes are dolomitization, dedolomitization, karstification and lateritization. The specific radioactivity of ~(238)U, ~(226)Ra, ~(232)Th and ~(40)K determined in different samples, indicate that ~(238)U and its decay products contribute primarily to the high natural radioactivity of rocks. The maximum concentration of ~(238)U reached up to 2129.36 ppm in argillaceous sediments. The average concentrations of determined radionuclides (~(238)U, ~(226)Ra, ~(232)Th and ~(40)K) are 8.34 ppm, 7.88 ppm, 4.68 ppm and 0.3%, respectively in sandstone. In dolostones the average concentrations are 418.69 ppm, 808.75 ppm, 3.14 ppm and 0.29%, respectively. For argillaceous sediments are 276.88 ppm, 419.49 ppm, 11.47 ppm and 0.93%, respectively. The ~(238)U/~(226)Ra ratio in sandstone ranges between 0.89 and 1.25, while in dolostones and argillaceous sediments are 0.27-2.63 and 0.27-1.83, respectively. These variations in the concentrations of radioelements and their ratios are due to the action of the alteration processes affected these different sedimentary rocks in different times. Environmentally, the Ra_(eq) in dolostones and argillaceous sediments exceeds the permitted limits, while in the sandstone samples; it is within the permissible levels.