Geochemical dynamics and origin of natural radionuclides and heavy metals in current coastal sedimentary systems in the Republic of Congo

This study aims to characterize the geochemical distribution, levels, and origin of natural radionuclides ( ²³⁸ U, ²²⁶ Ra, ²¹⁰ Pb, ²³² Th, and ⁴⁰ K) and heavy metals (Cd, Pb, Zn, Mn, Mg, and Cu) in recent sediment samples from the coast of the Republic of Congo, an environment subject to significant human pressure but still understudied. Surface samples were collected in river and lagoon estuaries, then analyzed by gamma spectrometry (GeHP) for radionuclides and by atomic absorption spectroscopy (AAS) for heavy metals. Additional analyses of organic matter (LOI), Pearson correlations, and ACP allowed us to determine the origin and geochemical dynamics of the elements. The results show activities that are mostly below the UNSCEAR thresholds, except for notable enrichments in ²¹⁰ Pb at Mvassa and ²³² Th at Loukonzi lagoons. The metals show significant contamination in Cd and Pb, associated with industrial, port, and petroleum activities. The spatial distribution is controlled by grain size, hydrodynamics, organic matter, and atmospheric inputs. These results highlight the coexistence of lithogenic and anthropogenic sources and emphasize the need for enhanced environmental monitoring in sensitive areas to anticipate the growing impacts of industrial pressures and climate change.