Activity concentrations and distribution of radionuclides in surface and core sediments of the Neretva Channel (Adriatic Sea, Croatia)

The activity concentrations and the distribution of manmade Cs-137 and naturally occurring radionuclides K-40, U-238 and Th-232 in surface and core sediments of the semi-enclosed, river-dominated marine environment of the Neretva Channel were investigated in relation to the sedimentological characteristics and the total organic carbon (TOC) content. The activity concentrations of radionuclides were determined by gamma spectrometry. Distinct interrelationships between sediment properties and the spatial distribution of radionuclides were observed. The highest accumulation of Cs-137 occurs close to the river mouth, in the region of intensive deposition of organic matter of terrestrial origin. This discovery implies that the river-borne organic material and its deposition processes should be considered as the most important factor controlling distribution of Cs-137 in this transitional terrestrial-marine environment. Sediment accumulation rates, estimated from the distribution of Cs-137 in core sediments, were approximately 6 mm y(-1) in front of the Neretva River mouth and 4 mm y-1 in the channel area. The spatial distribution of natural K-40 and Th-232 radionuclides indicates their distinct association with fine-grained sediments. The interrelationship of U-238 with fine-grained particles was somewhat weaker but still present. The results obtained indicate that the accumulation pattern of natural radionuclides in the Neretva Channel sediments is mainly governed by the deposition of fine-grained material. This study scrutinizes the baseline level for the occurring radionuclides and should be used for monitoring and assessing the radionuclide pollution record in the investigated transitional terrestrial-marine environment of the Adriatic.