Behavior of Chernobyl radiocesium in sediments of the Adriatic Sea off the Po River delta and the Emilia-Romagna coast

Coastal sediments from the northern Adriatic Sea were analyzed for 137Cs and 134Cs from the Chernobyl accident to better understand the transport and sedimentation processes in a marine area strongly influenced by the delivery of suspended matter from the Po River. In particular, the suitability of 137Cs as a tool for studying sediment accumulation rates was evaluated. Radionuclide distributions are determined by the influence of the river and by the grain size composition that is a function of the hydrodynamic processes. With respect to pre-Chernobyl values, 137Cs surficial activities were increased from 3 to 13 times, and inventories up to 3 times. Four types of 137Cs activity-depth profiles were found, being representative of different combinations of sediment accumulation and radiotracer displacement within the surficial layers. At some sites sediment accumulation rates of 1.6–4.8 cm y−1 caused the formation of a sub-surficial peak. At the other locations, the Chernobyl peak of 137Cs was still confined at the water–sediment interface with some downward penetration due to mixing and diffusion. In general, molecular diffusion is the prevailing mechanism but the incidence of mixing is very significant at three sites (58–71%).