## 1 Introduction  
Rock-water interaction is a process that influences groundwater chemical evolution from recharge along the flow paths through the vadose zone to the phreatic zones. Aquifers' mineral solubility, residence time, and intrinsic chemical characteristics of original water determine how quickly groundwater quality changes [1,2]. The origin of water also plays a role before continuous evolution as it flows from the recharge zone downgradient within the geological unit of formations [3]. Sedimentary basins play a vital role as sources of water supply to meet water demand, especially in developing countries in Africa and Asia. Sub-Saharan African countries depend mostly on groundwater to meet their daily water demand due to infrastructural failure and poor water management [4,5]. Generally, groundwater resources are under stress from natural and human drivers. Coastal basins are more susceptible to these pressures due to their proximity to the sea, increased population, industrialisation, agricultural activities and the effect of global climate change [6,7]. For coastal basins, the stratigraphical characteristics make aquifers vulnerable to contamination and pollution [8]. Shallow coastal aquifers in some developing countries have been reported to be facing various challenges of groundwater quality deterioration, the causes of which are attributed to both geogenic and anthropogenic influence. Delineation of the hydrochemical status is usually a complicated and multi-source process. This is a common challenge in hydrological basins characterised with municipal, agricultural, and industrial activities. The activities associated with these areas often serve as a source of non-geogenic ions in groundwater [9]. These anthropogenic ions and metals influence the hydrochemical characteristics of groundwater. However, the geogenic processes such as saltwater intrusion, sea spray, and minerals dissolution from rock-water interaction remain the primary source of ions and metals in groundwater [1, 10,11]. In the near-ocean areas of coastal basins, Na-Cl is commonly the dominant water type, which is attributed to sea spray or seawater intrusion. In light of this, mineral dissolution and hydrochemical evolution of groundwater can be better understood using stable isotopes in conjunction with major ions. These will provide an insight into the hydrochemical transformation and residence time of groundwater within aquifers of such coastal basin [12, 13, 14]. Stable isotopes integrated with major ions in groundwater have been employed in regional groundwater studies at different locations across the world. The works of [2,15,16,17,18,19,15,19,20] have demonstrated effectiveness of this approach in understanding recharge pattern, origin and residence time of groundwater from specific sites to basin-scale hydrogeological investigations. Most of these studies have contributed vital information and knowledge that are useful in global groundwater resource management. The eastern Dahomey Basin (EDB) is one of the eight hydrogeological provinces of Nigeria, providing groundwater demand for about 30% of the country's population. Urbanisation, industrialisation, and agriculture, coupled with the dynamic geology of this basin, continue to pose a significant challenge to the understanding of its groundwater chemical dynamics [21,22]. In Nigeria, inadequate pipe-borne water leaves individual households to rely on groundwater from shallow boreholes and hand-dug wells to meet water demand for domestic, agriculture, and other usages [23,24]. A unique characteristic of this basin is its complex geology and relief with a drainage system controlled by topography and geology [8]. Most of the major rivers in the basin flow southward across different geologic units and formations and discharge water into lagoons and the ocean.