## 3 Results and Discussion
### Tracing the Origin of Groundwater within the EDB Using Deuterium Excess  
Deuterium excess (D-excess) is a useful measure to understand the source of groundwater. The D-excess is defined as D-excess \(=\delta^{2}\mathrm{H}-8\delta^{18}\mathrm{O}\)[20; 28; 50]. The D-excess is a function of prevailing conditions during primary evaporation, including variation in humidity, ocean surface temperature, wind speed and, thus, gives information on the sources of water vapour. The D-excess does not change with the change in equilibrium processes for any of the phases while non-equilibrium evaporation leads to a decrease in the D-excess, which is an indication of an increase in the vapour phase [18; 20; 28]. In this study, the D-excess values for groundwater in the wet season is slightly lower than that of the dry season. The higher D-excess in the dry season (Figure 15), is for samples collected between February and April 2017. During this period, a dry southward wind leads to the recycling of moisture from the continental recycling along with the rapid evaporation over warmer seawater responsible. This compares with plots of \(\delta^{18}\mathrm{O}\) vs. D-excess in Figure 16, which also showed the clustering of D-excess in relation to rain, sea and brackish water plot from the basin. The diagrams showed that most of the groundwater of the wet season plotted closer to the precipitation, which is the source of the groundwater recharge and forms a trend towards the brackish point as evaporation increases. The \(\delta^{18}\mathrm{O}\) vs. D-excess diagram shows most groundwater samples plot in a region between the recharge and evaporation zones reflecting young water of meteoric origin at an early transformation stage through evaporation. This suggests precipitation water moving towards groundwater storage through the vadose zone is rapid. That post-precipitation evaporation along these paths is a prolonged process, probably due to higher humidity and precipitation that characterised this region. Figure 15: Spatial distribution map of D-excess (%o) for the: (**a**) wet season; (**b**) dry season groundwater samples. Description of Conceptual Design of Hydrogeochemical Characterisation of the Shallow Coastal Aquifer of EDB  
The results from this study have established the hydrochemical dynamic and process prevalent in the eastern Dahomey Basin, which shows a subtle transformation as water moves from the recharge zones in the northern part towards the discharge area in the downstream along the river channels and flood plains. The clustering of groundwater samples between evaporation and recharge and along the seawater line \(\delta^{18}\)O vs. D-excess diagram (Figure 16) confirmed young groundwater in the basin. The slight variations in the distribution of the \(\delta\)D and \(\delta^{18}\)O values and clustering along the regional meteoric water line (RMWL) and Global Meteoric Water Line (GMWL) with a slight deviation of the dry season groundwater further confirmed the recent water's short residence time within the soil and vadose zones before recharging the shallow coastal aquifer of the basin. The information from (a) the geological field observations, (b) groundwater levels, (c) elevation data, (d) aquifer geometry, (e) hydrochemical evolution, (f) spatial variability of \(\delta^{18}\)O, \(\delta\)D, and D-excess, (g) river and static water level, and (h) line of a cross-section along representative profile were collected and integrated to develop a conceptual outlook for the prevailing hydrogeological processes within the eastern Dahomey Basin, with well-defined boundary conditions of impermeable basement rock at the north, and the ocean line of the Gulf of Guinea in the south, enclosed by the flow boundary of the Okitipupa ridge in the east, and extended sedimentary deposits [49]. The flow systems within this area can be categorised into mainly local, driven by gravity due to the topography of the basin [38].