WHAT DO RADIOACTIVE EQUILIBRIA SAY ABOUT THE CONTAMINATION OF FRESH-WATER SEDIMENTS IN BOHEMIA WITH NATURAL RADIONUCLIDES

134 samples of river and surface reservoir sediments in Bohemia were a nalyzed for uranium, Th-232, Ra-226 and Ra-228 using instrumental neut ron activation analysis and gamma spectroscopy. Analysis of the extent of radioactive equilibria established in the samples enabled us to co nclude that Ra-226 is present in uncontaminated sediments mainly in eq uilibrium with U-238, which indicates its position inside detrital par ticles. Contaminated sediments usually contain rather high proportion of Ra-226 adsorbed from water and thus in excess over the equilibrium amounts of uranium. However, several sediments were apparently contami nated with uranium in disequilibrium with radium. No sediments really contaminated with Ra-228 were found. Most of the analyzed sediments co ntained excess Ra-228 over equilibrium Th-232, indicating substantial adsorption of Ra-228 from water.