Uranium and other natural radionuclides in the sediments of a Mediterranean fjord-like embayment, Amvrakikos Gulf (Ionian Sea), Greece

The distribution of the natural radionuclides (238U, 232Th, 226Ra, 40K) and the artificial 137Cs was studied in sediment cores collected from Amvrakikos Gulf, a seasonal anoxic marine basin, using γ-ray spectrometry. The activity of radionuclides, along with the concentrations of Fe and Mn, were also studied in relation to the total organic carbon and the granulometric fractions of the sediments. The results obtained revealed higher 238U activity concentrations in all the examined sediment samples compared to the world and Greek average values for soil. The high activity values of 238U are attributed, besides the lattice-held fraction, to phosphate fertilizer inputs in the Gulf via major rivers and/or to alteration processes of phosphate ores located mainly in the drainage basin of the river Louros. The elevated activity values of 40K could be attributed to the mineralogical composition of the sediments and to phosphate fertilizers containing potassium. Organic matter seems to be a more efficient sorbent for U than clay minerals and amorphous Fe and Mn-oxyhydroxides. Scanning electron microscopy, together with qualitative analysis of some smectites, reveals the occurrence of U, suggesting a limited absorption of U onto clay minerals. The applied BCR sequential extraction procedure revealed that U was found mainly in the refractory phase or associated with organic matter and to a lesser extent as surface-coating oxides, with the exception of one sediment core which is characterized by high content of fresh marine organic matter and presents high percentage of U in the exchangeable fraction.

► We study the natural radionuclides and 137Cs in the sediments of Amvrakikos Gulf.
► We examine the fractionation of U and Fe in the sediments.
► The elevated activity of 238U could be attributed to phosphate fertilizers inputs.
► U was found mainly in the refractory phase or associated with organic matter.
► Fe was found associated with the refractory phase and as surface-coating oxides.