Hydrogeochemical and salinity appraisal of surficial lens of freshwater aquifer along Lagos coastal belt, South West, Nigeria

In order to determine the hydrogeochemical processes and salinity status of the surficial lens of freshwater aquifer in the Lagos coastal basin, physicochemical parameters and hydrogeochemical tools were employed in this study. These tools include existing methods such as Piper diagram, Durov diagram, Gibb's diagram, statistical analysis and geochemical modelling, and the newly proposed cationic contribution evaluation and ionic ratio bivariate plots. The Piper plot revealed that chemical facies is rich in (Ca, Mg, Cl and HCO(3)) and (Ca and HCO(3)) for the groundwater in both wet and dry seasons while Mg–Cl and Na–Cl water types characterised the lagoon and the Ocean, respectively. The surficial aquifer is an essentially freshwater in the dry season and partly saline in the wet season due to flash flood, marine aerosol precipitation and infiltration from the surrounding brackish surface waters. The Ca(2+)+Mg(2+) vs total cation (TZ(+)); HCO(3)(-)/Ca(2+)vs Ca(2+)/Mg(2+) and HCO(3)(-)/Ca(2+) vs Ca(2+)/Na(+) plots as well as geochemical modelling indicated that carbonate weathering is the dominant chemical process controlling the groundwater chemistry for both seasons. In a descending order, water-rock interaction, ion exchange and evaporation are the geochemical processes controlling the groundwater quality in the area. Besides the natural processes, factor analysis revealed the imminent influence of anthropogenic activities (industrial and domestic) on the groundwater chemistry indicated by the presence of Mn, Fe, Ni, Cd, Pb, Cu and NO(3) above the permissible limit of drinking water standard, particularly in the wet season. In addition to the successful application of the proposed ionic plots, the study justifies the importance of multiple tools application in the hydrogeochemical deductions and recommends consistent monitoring of the shallow aquifers in the Lagos coastal basin.