## 3 Results and Discussion
### Oxygen and Deuterium Isotopes of Groundwater EDB  
Information on precipitation, evaporation and origin of water can be deduced from the stable isotopes of oxygen (\(\delta^{18}\)O) and hydrogen (\(\delta^{2}\)H) values [2; 47; 48]. The relationship between \(\delta^{2}\)H and \(\delta^{18}\)O data is usually plotted in relation to the Global Meteoric Water Line (GMWL) defined by the equation: \(\delta^{2}\)H = 8\(\delta^{18}\)O + 10 [20; 28]. Stable isotopes of \(\delta^{18}\)O and \(\delta^{2}\)H have been used to understand the origin of groundwater in the eastern Dahomey Basin and suggested recent groundwater from precipitation [49]. In this study, \(\delta^{18}\)O and \(\delta^{2}\)H were used to explain the hydrochemical evolution of water as it flows from the surface as precipitation and infiltrates into the soil through the vadose zone to the aquifers. Results from isotopes data of groundwater samples during the wet and dry season show \(\delta^{18}\)O values range from \(-\)5.2 to 0.3\(\%\)\(\infty\) and \(-\)4 to 0.8\(\%\)\(\infty\) with an average value of \(-\)3\(\%\)\(\infty\), respectively, for both wet and dry seasons (Table 1). \(\delta^{2}\)H values range from \(-\)32 to 2.3\(\%\)\(\infty\) and \(-\)19 to 7.5\(\%\)\(\infty\) with an average value of \(-\)13.1 and \(-\)12.4\(\%\)\(\infty\) (Table 1), respectively, for wet and dry seasons. In this study, the 8D/\(\delta^{18}\)O diagrams (Figure 10) show that the groundwater for both seasons plotted along a line of lower slope slightly deviated from the Global Meteoric Water Line (GMWL) is indicative of evaporation. This implies recent water from precipitation with a slight influence of evaporation [13; 49]. A higher evaporation influence is observed in the dry season groundwater samples with lower slope and deuterium intercept values of 5.5 and 4.1 compared to those of wet season with values of 6.1 and 5.2, respectively, as presented in Figure 10. Figures 11 and 12 show the spatial distribution of \(\delta^{18}\)O and \(\delta\)D for wet and dry seasons, respectively. Figure 9: Weathering profile of Precambrian basement rocks underlying the oldest Abeokuta formation. (At the northern boundary of the basin from which aquifer recharge of the basin originates (**a**) road cut along Abiola Way, Abeokuta, (**b**) a road cut along Jiebu-Remo and Agowoye road. The weathered profile is known for its richness in kaolinite minerals). Figure 11: Spatial distribution map of \(\delta^{18}\)O (%\({}_{\rm o}\)) for the: (**a**) wet season; (**b**) dry season groundwater samples. Figure 10: \(\delta\)D/\(\delta^{18}\)O diagram for: (**a**) wet season; (**b**) dry season, groundwater samples. The wet season \(\delta\)D/\(\delta^{18}\)O diagram (Figure 10) showed a more extensive range along the Local Meteoric Water Line (LMWL), which indicated precipitation dominant, with rapid infiltration into the shallow coastal aquifers of the basin [49]. Compared to dry season samples plotted within a shorter/narrow range, which tend towards mixed water with significant influence of evaporation before infiltrating into the shallow phreatic aquifers of the basin. Integrating precipitation \(\delta^{2}\)H and \(\delta^{18}\)O isotopes data with groundwater results from the EDB can enhance understanding of the origin of groundwater given hydrodynamic complexities resulting from the influence of multiple factors from both geology and environment. Unfortunately, the only station of International Atomic Energy Agency (IAEA)/World Meteorological Organization (WMO) GNIP sited in Nigeria is in the northern part in the city of Kano in the Sahel savannah. Due to the different climatic condition around this station, it is not ideal, relating such data with the coastal zones groundwater isotopes data. \(\delta^{18}\)O and \(\delta\)D in precipitation from two selected stations, Doula in Cameroon, in the east of EDB, and Cotonou in the Republic of Benin, and located in the west of the studied basin, were used for evaluations and analysis.