Season-induced variation in water table depth and selected chemical parameters of groundwater in Lagos Coastal Plain Sand Aquifer, Nigeria

Abstract

In-situ measurement of depth to aquifer and water table was undertaken in 45 protected dug wells over two seasons. Samples were also analyzed for pH, EC, TDS, Ca, Cl, K, HCO₃, CO₃, SO₄, Mg, K, and Na using universally accepted laboratory techniques. The study was aimed at examining seasonal variations in the chemistry of groundwater in the Lagos Coastal Plain Sand aquifer and its potability using the WHO standards. The study area covered parts of the Lagos Metropolis and the entire settlements of Ikorodu, Epe, and Badagry. The sample locations were mapped with ArcMap 9.3 software while the data were analyzed using descriptive statistics, paired sample T-test, correlation and multivariate statistical techniques using SPSS software 17.0 version. The result shows that about 60% of the samples had pH below the WHO minimum standard of 6.5 in both the dry and wet seasons. Mean EC and TDS levels were excessively high at Igando in both seasons. Some major ions, calcium, magnesium, potassium and sodium, and chloride exceeded the WHO limit at Oko-Oba, Ilogbo, Shomolu and Shogunle in the dry season. They were also excessively high in the wet season at Igando, Oko-Oba, Odo-Onosa, Ilogbo, Shomolu, and Shogunle. Correlation analysis shows that a significant relationship exists between the parameters at p<0.05 with the exception of Mg²⁺ and K⁺ in both seasons. The result of the paired sample T-test also shows significant variations among pH, Ca²⁺, HCO₃^- and Mg²⁺ with higher mean values in the dry than the wet season except for pH. Factor analysis identified salinity and anthropogenic activities as the two major sources of pollution. These account for about 74.48% and 84.21% of the total variances in dry and wet seasons, respectively. Consequently, protection of the recharge areas of the aquifer from environmental pollution and formulation and enforcement of appropriate policies that will stem the rate of indiscriminate groundwater exploitation and prevention of saline water intrusion in the coastal settlements are recommended.

Keywords: Aquifer; Coastal Plain Sand; Groundwater; Lagos; Seasonal variations, Water quality parameters