Several radionuclides of the 238U decay series ( 234Th, 234m Pa, 234U, 222Rn and 210Pb), 235U and 232Th have been measured through gamma spectrometry in surface water and four dug wells in two successive years, in order to study the effect of flash flood event of 18th January 2010 on radionuclides distribution in Wadi Naseib, southwestern Sinai, Egypt. Variation of 238U/235U activity ratio by flash flood is observed in surface water sample and increased with time. This is due to that the 238U activity was determined indirectly from the gamma rays emitted by its daughter products ( 234Th and 234m Pa), while 235U activity was determined directly by its own gamma ray peaks. The surface water sample reached secular equilibrium between 238U and its daughter products (234Th and 234m Pa) after about one hundred days. The water-rock interactions during the journey of flash flood (April 2010) in the studied area changed the chemistry and the specifications of the groundwater. The cations and anions results are varied with time of collecting water and place of the wells. The activity concentrations of 238U, 210Pb and 232Th in groundwater samples collected in 2011 are found lower than that collected in 2010 while that of 234U and 222Rn are higher in 2011 than that in 2010. The groundwater residence time of two samples, collected from (Well-Zeid) in two successive years (2010 and 2011) were 27.93 and 42.44 ky, respectively, which show the young date just after the flash flood and gain of uranium with the new added water while after one year with the decrease of U-concentrations, the residence time increased. The last date may be coincide with the last wet period in Egypt.
