Factors Controlling Uranium and Thorium Isotopic Composition of the Streambed Sediments of the River Nile, Egypt

The streambed sediments of the River Nile in Egypt have been analyzed for their long-lived uranium and thorium radionuclides ( 238 U, 234 U, 230 Th) using Alpha Spectrometric Techniques. These sediments are dominated by sand size fractions with average heavy mineral contents of 2.65%. Carbonates and organic materials constitute considerable amounts of the sediments (averages 13.82 and 3.37%, respectively). The average contents of uranium and thorium in the streambed sediments are 2.06 and 6.58 ppm, respectively, which are comparable with the corresponding values of the world rivers. The 230 Th/ 234 U and 230 Th/ 238 U activity ratios of the sediments are greater than unity indicating preferential uranium leaching. During low water flow rate, the streambed sediments probably acquired some uranium from water with high 234 U/ 238 U activity ratios. This results in abnormally high 234 U/ 238 U activity ratios in the sediments. Heavy mineral contents, sorptive uptake of uranium by sediments of relatively high organic contents, geochemical behaviour of radionuclides and flow rates of the River Nile are considered as the most important factors controlling uranium and thorium isotopic composition of the streambed sediments. The sources of the radionuclides are probably the local granitic rocks and phosphate fertilizers used in the cultivated lands adjacent to the River Nile.