URANIUM AND THORIUM ISOTOPE COMPOSITION F ORMATION AND GEOCHEMISTRY IN THE UNDERGROUND WATER OF BAIKAL LAKE BASIN

Remarkable variations of uranium (0,04 to 1,77 mu g/l) and thorium (0, 004 to 0,02 mu g/l) has been found during the study of microelement co mposition of rivers' water of Baikal Lake basin. The great difference in uranium contents is caused by different U contents in the rocks of the river catchment area and soils forming on the rocks. Uranium and t horium isotope composition in the river water and Baikal Lake as well as in the bottom sediments shows remarkably nonequilibrium U-234/U-238 and Th-230/Th-234 ratios pointing to the U-234 excess and Th-230 defi ciency. The data obtained allows one to describe the U and Th accumula tion conditionsin the modern bottom sediments and the process of isoto pe composition formation in the Baikal Lake's water.