U-238 series radioactive disequilibrium in groundwaters: implications to the origin of excess U-234 and fate of reactive pollutants

Abstract

The concentrations of 238U, 234Th, 226Ra, 222Rn and 210Pb and 234U 238U activity ratios have been measured in several groundwater samples from Gujarat, India. In the aqueous phase the abundances of 234Th and 210Pb are grossly deficient relative to their parents, 238U and 222Rn respectively. The deficiency is ascribable to the impact adsorption of 234Th and 210Pb atoms onto particle surfaces which are very abundant in the groundwater regimes. The scavenging residence times for both these nuclides is about a day, suggesting that irreversible removal of ‘reactive’ metals and pollutants in groundwaters can occur on very short time scales. The fast removal of 234Th onto particles necessitates that in these groundwaters 234U ‘excess’ has to originate through leaching of soil grains rather than through in situ decay of dissolved 234Th in the water.