<p>The submerged portion of the Campos Basin constitutes the sedimentary coastal aquifer system that serves as one of the most important sources of groundwater in the northern Rio de Janeiro State, Brazil. Additionally, the submerged portion of the basin also contains a significant share of Brazil’s oil reserves, representing one of the largest oil complexes in the world. In this context, this study investigates the hydrogeochemical evolution of groundwater salinization in the emerged portion of the Campos Basin. To achieve this, isotopic tracers were combined with classical hydrogeochemical analyses and diagrams. The findings suggest a possible progressive salinization of the Emborê Aquifer, with indications of onset in its deeper confined sections (Emborê Member) and potential influence extending toward the shallower Quaternary aquifer. This process is driven by both natural mechanisms, such as saline intrusion, and anthropogenic influences. The study offers significant insights into the dynamics of groundwater salinity and supports the development of improved water management strategies for the region.</p>