BEHAVIOR OF URANIUM, THORIUM AND RA-226 IN SURFACE WATERS FROM A RIVER PASSING THROUGH A GRANITIC REGION IN THE SOUTHWEST OF SPAIN

The distribution of natural uranium, thorium and Ra-226 has been studi ed in water samples from the Ortigas river, situated in a granitic reg ion in the west of Spain. Temporal and spatial variations were studied for a better understanding of the mobilization mechanisms in the stud ied system. The determination was made using alpha spectrometry. Two u ranium mines in the neighbourhood of the river and the granitic soils produce significant concentration of these radionuclides in the surfac e water under study. The U-234/U-238 activity ratio was greater than u nity for all the water samples. Direct recoil of Th-234 into the aqueo us phase and preferential leaching of U-234 Seem to be the mechanisms responsible for this effect. The Th-228/Th-232 activity ratio is great er than unity and higher than the values found for the U-234/U-238 act ivity ratio.