What do radioactive equilibria say about the contamination of freshwater sediments in Bohemia with natural radionuclides?

134 samples of river and surface reservoir sediments in Bohemia were analyzed for uranium, Th-232, Ra-226 and Ra-228 using instrumental neutron activation analysis and gamma spectroscopy. Analysis of the extent of radioactive equilibria established in the samples enabled us to conclude that Ra-226 is present in uncontaminated sediments mainly in equilibrium with U-238, which indicates its position inside detrital particles. 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 contaminated with uranium in disequilibrium with radium. No sediments really contaminated with Ra-228 were found. Most of the analyzed sediments contained excess Ra-228 over equilibrium Th-232, indicating substantial adsorption of Ra-228 from water.