## 1 Introduction  
The study of the neighboring environment of a nuclear power plant (NPP) has gathered interest from the scientific viewpoint to address the possible routes of release and diffusion of radionuclides in the environment. For this purpose, environmental campaigns are useful to verify the radiological impact of the activities of the NPP. This is performed using sensitive measurement methods capable of distinguishing the contribution from the fallout, due to nuclear tests in the atmosphere and major accidents at nuclear power plants (Chernobyl, Fukushima), from the possible impact of the NPP. The implementation of such measurement campaigns is necessary to safeguard the health of people and other living organisms. Moreover, it helps to lower the level of risk perception among the population, for which an objective and verifiable scientific investigation can be effective. Ultrasensitive accelerator mass spectrometry (AMS) turns out to be a powerful method for making such analyses. Indeed, AMS has an unparalleled sensitivity for rare isotope detection by single ion counting [1, 2, 3, 4, 5], which brings information about their origin: natural, weapon grade, reactor burn-up, fall out. Its sensitivity makes it possible to measure abundances and isotopic ratios of anthropogenic actinides (\({}^{236}\)U, \({}^{239}\)Pu, \({}^{240}\)Pu) below the common values due to fallout in the environment. These data may reveal the presence of irradiated U through the \({}^{236}\)U/\({}^{238}\)U ratio, while \({}^{240}\)Pu/\({}^{239}\)Pu gives further information on fuel composition, U enrichment, and burn-up [6, 7, 8, 9, 10, 11]. Over the years, several studies have been carried out near both active and spent NPPs, eventually under decommissioning, using different methods of analysis on various radionuclides. Environmental matrices, such as soil, air, water, vegetables, milk, meat, and fish, have been studied [12; 13; 14; 15; 16]. Riverine and sea waters and sediments near nuclear sites are interesting as possible transport routes for radionuclides released into the environment [17; 18]. Their study is crucial to assess the radiological status of the seabed and water and to identify any anomalies or variations from the environmental background values, to ascertain impacts caused by the NPP activities. Sediments are a reservoir of different pollutants and compounds that undergo physical and chemical processes such as absorption, ion exchange, co-precipitation, complexation, and chelation. The type of interactions between chemicals and sediment particles is affected by particle size distribution (content of the finest fraction), clay content, organic carbon content, cation exchange capacity, and pH. Metals and metalloids are preferably removed from surface water by adsorption on oxides or clay minerals or by forming complexes with organic ligands, while organic pollutants tend to absorb and chelate the organic carbon fraction of the sediments. The accumulation of pollutants in sediments constitutes a constant risk of secondary water pollution resulting from redistribution and resuspension processes, which can cause the exchange of pollutants between the lower sediments and the water column. The rate of migration of pollutants depends on the chemical form in which the pollutants are found in the substrate and in the water within the pores, as well as on the physico-chemical conditions at the sediment-water interface. The chemical characterization of sediments is thus a key point in marine coastal environmental studies and essentially concerns the study of the quality of surface sediments (sandy/pelitic). In particular, the fine sediment, by its very nature (high specific surface), constitutes a preferential environment for the accumulation of contaminants in general and its resuspension could lead to the reintroduction of contaminants in the water column and in the particulate cycle [19].