Seasonal Variations of Groundwater Chemistry in the Basement and Sedimentary Rocks of Ibadan and Lagos Areas, Southwestern Nigeria

Hydrochemical investigation of forty boreholes drilled on the basement and sedimentary rocks in Ibadan and Lagos metropolis southwestern Nigeria respectively were carried out in order to determine the portability of the groundwater in both areas and to highlight differences in their chemical characters and variations with seasons. Data obtained indicated that the groundwater in Ibadan area is mainly the Na + Ca – HCO ­­ 3 type, while that of Lagos is Na + Ca – C1 + SO 4 type. The groundwater chemistry reflects weathering of sodic plagioclase feldspars in the basement rocks as well as arkosic sandstone in the sedimentary terrain. The higher chloride (ca. 124.2 mg/L) content of the Lagos water is probably due to salt-water intrusion along the coastal area. Elevated Na + (58.5-1021.2), Fe 2+ (0.3-2.8) and Mn 2+ (0.04-2.34) mg/L concentrations, particularly during the dry season, adversely affect the portability of the water from both localities. Apart from making the water unsuitable for irrigation, high Na + content is considered harmful to persons suffering from cardiac, renal and circulatory diseases. The Fe 2+ and Mn 2+ contents could also create staining problem. It is therefore desirable to remove these elements from the borehole water prior to consumption.