A radiochemical sequential extraction procedure has been developed in our laboratory to determine 226Ra and 234, 238 U by a-particle spectrometry in environmental samples. This method has been validated for both radionuclides by comparing in selected samples the values obtained through its application with the results obtained by applying alternative procedures. Recoveries obtained, counting periods applied and background levels found in the alpha spectra give suitable detection limits to allow the Ra and U determination in operational forms defined in riverbed contaminated sediments. Results obtained in these speciation studies show that 226Ra and 234, 238 U contamination tend to be associated to precipitated forms of the sediments.

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