## 3 Results and Discussion
### Hydrochemical Characterisation  
Analysis of hydrochemical data using Geochemist's Work Bench revealed eight hydrochemical water types, Ca-HCO\({}_{3}\), Na-Cl, Na-HCO\({}_{3}\), Ca-Cl, B Na-SO\({}_{4}\), Ca-SO\({}_{4}\), K-Cl and Mg-SO\({}_{4}\) across the geologic units of the basin as presented in Figure 5 and supported by the piper diagrams (Figure 6). Na-Ca and Ca-HCO\({}_{3}\) water types are the dominant water type among the groundwater samples from the shallow aquifer of the basin. The groundwater types (Figure 5) along the flow paths within the basin provide clues to the prevailing hydrochemical processes of mixing as large portion of samples clustering in the mixing region of the piper diagram (Figure 6), especially in the wet season. Figures 7 and 8 present pie charts of water type across the geological units to possibly link the prevailing groundwater characteristics in each of the aquifers mineralogy as it plays a vital role in this regard. As these water types reflect the mineralogical composition of the basin sediment and rocks, there is no observed consistency in their pattern of distribution across the geologic units of the study basin. The lack of a defined distribution pattern could be attributed to the heterogeneous nature of any typical sedimentary basin. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{4}{c}{**Wet Season**} & \multicolumn{4}{c}{**Dry Season**} \\ \cline{2-9}  & **Min** & **Max** & **Aver** & **Std Dev** & **Min** & **Max** & **Aver** & **Std Dev** \\ \hline Ca\({}^{2+}\) & 0.3 & 374.0 & 16.5 & 41.2 & 0.2 & 448.5 & 21.6 & 55.2 \\ Mg\({}^{2+}\) & 0.0 & 1377.0 & 18.4 & 140.4 & 0.1 & 1125.0 & 16.1 & 102.2 \\ Na\({}^{+}\) & 0.1 & 8857.0 & 106.8 & 902.8 & 0.6 & 10,310.0 & 112 Figure 5: Spatial distribution of groundwater types across a different geological unit of the eastern Dahomey Basin, (**a**) wet season, (**b**) dry season. Figure 6: Piper diagram showing water types: (**a**) wet season groundwater samples; (**b**) dry season groundwater samples; (**c**) diamond chart for interpretation indicating mixed water types and (I = permanent hardness, II = temporary hardness, III = saline and IV = alkali carbonate). The hydrochemical process is complex and dynamic depending on many factors, ranging from the groundwater origin rock/minerals weathering status and mineral composition of the aquifer [9; 20]. The residence time between precipitation and aquifer recharge zones and impact from humans along the flow paths are compounding factors [19]. Meteoric water is generally dominated by Ca-HCO\({}_{3}\) or Ca-Mg-HCO\({}_{3}\) water types, [29], with Na\({}^{+}\) and K\({}^{+}\) additions when water interacts with rock-forming minerals, as shown in Equations (1) and (2). Equation (1). \[KAISi_{3}O_{8}+H_{2}CO_{3}+H_{2}O\ \rightarrow\ Al_{2}Si_{2}O_{5}(OH)_{4}+2K^{+}+ 4H_{4}SiO_{4}+HCO^{-}{}_{3} \tag{1}\]  
Equation (2). \[2 NaAlSi_{3}O_{8}+2H_{2}CO_{3}+2H_{2}O\ \rightarrow\ Al_{2}Si_{2}O_{5}(OH)_{4}+2 Na^{+}+ 4 SiO_{4}+2 HCO^{-}{}_{3}+H_{2}O \tag{2}\]  
Figure 8: Pie chart of percentage water type across the geologic units within the basin for the dry season water samples. Figure 7: Pie chart of percentage water type across the geologic units within the basin for the wet season water samples. The precipitation/infiltration water carries dissolved gases as it recharges, resulting in a weak acid of mainly carbonic with minor nitric or sulphuric acids (depending on anthropogenic pollution). Hydrolysis, acidolysis, and oxidation processes take place as aquifers are recharged through the infiltration of water into soils of the vadose zones [43; 44; 45]. Towards the northern part of the eastern Dahomey Basin, the Precambrian basement rocks are often characterised with highly weathered overburden, which outcropped in few locations around road cuts and mine sites.