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The aim of this study is to assess the level of heavy metals (HMs) in surface sediments along the coastal estuaries of the Republic of Congo. A variety of approaches were employed, integrating elemental analysis (Atomique Absorption Spectroscopy), environmental, ecological, health risk assessment, and source identification techniques to evaluate the distribution, origin, environmental, ecological and health risks of potentially toxic element (PTE) contamination in sediments. The ascending mean concentration (mg/kg) of Cd (0.4) < Cu (18.6) < Pb (28.8) < Zn (62.1) < Mn (107) < Mg (569.6) complied with the local background, except for Cd and Pb. However, notable variations across sampling points and sites were observed, with a strong relationship between metals and organic matter. The comparing of PTEs with USEPA international standards and sediment toxicity guidelines showed compliance. The assessment of environmental risk parameters, based on the calculation of the geo-accumulation index, concentration factor and pollutant load indices showed low pollution of sediments. The ecological risk assessment, on the other hand, revealed a low level of pollution of biota and a low toxicity risk of ecosystems related to Cd and Pb and then to Cu in some sites. Regarding human health risk assessment, related to the PTEs, the cancerogenic effect indices were calculated for Cd and Pb through ingestion and inhalation pathways, and the non-cancerogenic effect indices were calculated for all HMs through ingestion, inhalation, and cutaneous contact routes, for children and adults, and showed that the results comply with the international standards set by USEPA, ranging from 10 -4 to 10 -6 and less than 1, respectively, where children were more sensitive and vulnerable than adults to the effects of the PTEs. Ultimately, this study revealed sediment lithology, anthropogenic activities, and mixed origins of HMs, with a large contribution from industrial activities. However, no significant impacts are discernible in the study area. In addition to the importance of this study, it forms part of the region database and could serve as a reference for future monitoring studies, since no data are available in th Congolese coastal environment., Competing Interests: Declarations. Ethical approval: All authors have read, understood, and have complied as applicable with the statement on "Ethical responsibilities of Authors" as found in the Instructions for Authors. Competing interests: The authors declare no competing interests., (© 2024. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)

Two sediment cores were collected from the Oualidia lagoon, on the Atlantic coast of Morocco, and analyzed for 210 Pb and 137 Cs activity by gamma spectrometry. The 210 Pb profiles were characterized by high activity at specific depths in each core, which were attributed to substantial increases in atmospheric 210 Pb input to the sediment. A modified CRS model was applied to develop age-depth relations (chronologies) for the cores and calculate sediment accumulation rates, taking into account changing unsupported 210 Pb delivery and specifying the year when the increase began. Calculated 210 Pb inventories (activity/area) and fluxes (activity/area/time) depend strongly on sedimentation rates and were much higher than mean values in similar coastal systems worldwide. We attempted to use 137 Cs as a time marker to support the modified CRS chronologies for both cores. The 137 Cs profiles, however, were affected by post-depositional cesium migration in the sediment which made it difficult to identify the 1963 atmospheric bomb-testing peak, especially in the core with low sedimentation rate. We conclude that the high activities of 210 Pb detected at specific depths in the Oualidia lagoon sediment cores are a consequence of decay of radioactive 222 Rn, which displayed periodic high concentrations in the overlying atmosphere.