## 3 Results and Discussion
### Oxygen and Deuterium Isotopes of Groundwater EDB
The statistical summary of the isotopes in precipitation data as presented in Table 2, is part of the continuous temporal record of monthly mean \(\delta^{2}\)H and \(\delta^{18}\)O for rainfall measurements between 2009 and 2018. The \(\delta^{2}\)H and \(\delta^{18}\)O isotopes in precipitation from these selected GNIP stations are plotted in \(\delta\)D/\(\delta^{18}\)O diagram (Figure 13) with a view to assess the possible influence of climatic and other effects such as altitude and continental factor on both \(\delta^{2}\)H and \(\delta^{18}\)O enrichment/depletion. Figure 12: Spatial distribution map of \(\delta^{2}\)H (%o) for the: (**a**) wet season; (**b**) dry season groundwater. The groundwater samples for wet and dry seasons plotted along with the meteoric water line for Douala (Cameroon), Cotonou (Republic of Benin) and Kano (Nigeria) and presented in Figure 14. The results show the groundwater samples clustering within the boundaries of the three Regional Meteoric Water Lines (RMWL) with slope values falling below the GMWL (Figure 12). This further reveals a large number of groundwater samples around the GMWL which indicates that meteoric water is a significant source of groundwater recharge in the study area. The Regional Meteoric Water Line (RMWL) characterising coastal precipitation from Cotonou (\(\delta\)D = 7 \(\delta^{18}\)O + 8.0) and Douala (\(\delta\)D = 7.2 \(\delta^{18}\)O + 9.3) are very close to the Global Meteoric Water Line (Figure 13). However, the stable isotopes data from the Kano station (\(\delta\)D = 7.1\(\delta^{18}\)O + 4.3) which has a slope closer to the GMWL but with lower \(\delta\)D intercept, indicates a climate effect of the region with relatively higher temperature and consequently higher evaporation rate with deuterium enrichment. \begin{table}
\begin{tabular}{c c c c c} \hline
**Isotopes** & **Minimum** & **Maximum** & **Average** & **Std Dev** \\ \hline \multicolumn{5}{c}{Douala GNIP Station \(n\) = 50} \\ \hline \(\delta^{18}\)O & \(-\)6.14 & 1.77 & \(-\)2.54 & 1.57 \\ \(\delta^{2}\)H & \(-\)38.80 & 17.68 & \(-\)9.05 & 11.89 \\ D-excess & 2.60 & 18.87 & 11.26 & 3.70 \\ \hline \multicolumn{5}{c}{Cotonou GNIP Station \(n\) = 50} \\ \hline \(\delta^{18}\)O & \(-\)5.53 & 1.82 & \(-\)2.86 & 1.63 \\ \(\delta^{2}\)H & \(-\)36.11 & 15.76 & \(-\)11.92 & 11.74 \\ D-excess & 0.30 & 17.44 & 10.99 & 3.30 \\ \hline \multicolumn{5}{c}{Kano GNIP Station} \\ \hline \(\delta^{18}\)O & \(-\)7.70 & 2.40 & \(-\)2.92 & 2.59 \\ \(\delta^{2}\)H & \(-\)58.30 & 22.30 & \(-\)16.30 & 19.36 \\ D-excess & \(-\)13.38 & 20.98 & 7.08 & 6.69 \\ \hline \multicolumn{5}{c}{Groundwater Samples Wet Season \(n\) = 97} \\ \hline \(\delta^{18}\)O & \(-\)5.2 & 0.3 & \(-\)0.3 & 0.6 \\ \(\delta^{2}\)H & \(-\)32.5 & 2.3 & \(-\)13.1 & 3.6 \\ D-excess & \(-\)0.3 & 13.8 & 11.0 & 1.7 \\ \hline \multicolumn{5}{c}{Groundwater Samples Dry Season \(n\) = 133} \\ \hline \(\delta^{18}\)O & \(-\)4.0 & 0.8 & \(-\)3.0 & 0.5 \\ \(\delta^{2}\)H & \(-\)19.7 & 7.5 & \(-\)12.4 & 2.8 \\ D-excess & 0.9 & 15.0 & 11.9 & 1.6 \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of the stable isotopes data from the three GNIP stations and groundwater samples. Figure 13: \(\delta\)D vs. \(\delta\)18O diagram for the selected regional precipitation data. The \(\delta^{2}\)H and \(\delta^{18}\)O data for the groundwater collected during the wet (\(\delta\)D = 6.18\({}^{18}\)O + 5.2) and dry (\(\delta\)D = 5.48\({}^{18}\)O + 4.1) seasons are presented in Figure 14. Groundwater from the EDB for both seasons falls almost along the same meteoric waterline with a slight difference in slopes and deuterium intercepts. The slopes and intercepts of the regression line are lower compared to the GMWL (\(\delta\)D = 8*\(\delta^{18}\)O + 10) indicating the effect of evaporative enrichment on groundwater driven by seasonal variations in precipitation and temperature.