DISTRIBUTION OF URANIUM AND THORIUM IN SEDIMENTS AND PLANTS FROM A GRANITIC FLUVIAL AREA

A study of the presence of natural uranium and thorium isotopes in sed iments and plants belonging to a granitic fluvial region of the Ortiga s river (west of Spain) has been carried out. The existence of two ura nium mines in the neighbourhood of the sampled sites and the granitic characteristics of the zone produce significant concentrations of natu ral radionuclides. Temporal and spatial variations of uranium and thor ium concentrations and the activity ratios U-234/U-238, Th-228/Th-232 and Th/U were studied to better understand the mobilization;mechanisms such as leaching and transport at play in the studied system. These d eterminations were made using alpha-particle spectrometry with silicon detectors. The measurements were also compared with the results previ ously found for waters of this fluvial area. Uranium in sediments show ed variations due to changes in rainfall, but thorium content was near ly constant. Uranium and thorium concentrations in plants were lower a fter rainfall. Incorporation of uranium into the plants seemed to be m ainly from water, whereas incorporation of thorium seemed to be from b oth sediments and water. (C) 1997 Elsevier Science Ltd.