Forty-five groundwater samples were collected from protected dug wells using a random sampling technique in parts of Lagos, Nigeria. Direct field measurements were recorded for pH, electrical conductivity and total dissolved solids. Fournier equation was employed in determining groundwater hardness. The cations and anions were analysed using standard procedure. The study aimed at identifying the processes of groundwater chemistry in the study area. Statistical techniques such as descriptive, multivariate and graphical method were employed in analysing the data used for this study. Piper’s diagram aided the characterization of the groundwater facies composition. The ArcMap software version 10 was employed to generate the map of the study. The result of the correlation shows that there is a significant relationship between TDS and major ions at p< 0.05. The linear regression model reveals that EC and Cl^- have a significant linear relationship with TDS obtained at 0.999 and 1.000 respectively. The factor analysis accounted for 84.52% of the total variance and indicate two major processes i.e. reverse cation exchange and carbonate mineral weathering controlling the groundwater chemistry of the study area while the sources of groundwater deterioration are linked to salinity and anthropogenic factors.The total cation index and the scatter plot between Mg²⁺ + Ca²⁺ against SO₄^2- + HCO₃^2- revealed that silicate weathering predominate. The Gibbs plot indicates rock-mineral weathering as the process responsible for the groundwater chemistry of the study area. The Revelle index shows that only 4.4% of the samples are strongly affected by salinity.The Piper diagram characterise four water types, namely Ca-HCO_3, Ca-Cl, Mg-HCO₃ and Mg-Cl accounting for 44.4, 31.1, 20 and 4.4 per cent respectively. The paper recommended a robust conceptual model for simulating groundwater in the aquifer system of the study area. Proper monitoring of groundwater abstraction, effective land use planning and handling of industrial effluent before being discharged into the subsurface was also recommended.