Hydrogeocheraical investigation of surface water and groundwater around Ibokun, Ilesha area, southwestern Nigeria

Abstract

In the Ibokun area of the Ilesha schist belt of southwestern Nigeria, the dominant rock types are granite gniess and amphibolite. The granite gneiss is medium to coarse grained, with lineation marked by alignment of biotite and microcline porphyroblasts. The amphibolite is commonly massive to weakly foliated, consisting to bluish-green to yellowish-green to sheaf-like aggregate of hornblende set in a fine grain matrix of tabular plagioclase. A total of 25 water samples, obtained from both groundwater and surface water sources in sites within the two dominant tock types, were analysed for their physico-chemical characteristics with ~ the aim of assessing their quality and usability. i Result of some physical and hydrochemical parameters show average values of 7.83pH, 177.40mg/l TDS; 30.86mg/l ca2*, ] 10.1 lmg/1 Na*; 14.79mg/l K \ 4.36mg/l Mg2*, 29.59mg/l, HCO,; 25.67mg/l CT, 0.17 mg/1 Fe2*, 4.74 mg/1 NO, and 318.22ms/cm conductivity. The water hardness and sodium absorption ratio (SAR) are respectively 11.71 mg/1 and 0.72. , The total hardness, HCO'; and Ca2* are higher in the groundwater than the surface water, while NO ,, Mg2* and conductivity are comparatively higher in the surface water than the ground water. ‘ ' Statistical analysis, using the product-moment coefficient of correlation, indicate positive correlation between the following pairs of parameters: Total Hardness and TDS (r = 0.70); Na* and K* (r = 0.69); TDS and Ec (r=0.60). Weaker correlations were obtained between Ca2* and HCOj' (r = 0.51); Ca2* and Mg2* (r = 0.36) while very weak correlations were observed between pH and TH (r = 0.22) and pH and C l- (r = 0.10). Four water groups were identified, based on characterization in the Piper trilinear diagram. These include Ca - (Mg) - Na - HCOj, Ca - Na - Cl - (S04) - HCO,, Ca - (Mg) - HCO, and Na - (K) - Cl - S04. They reflect diverse effects of bedrock lithologies, base exchange processes, precipitation and weathering. In general, both water sources are slightly alkaline. They have low sodium hazard, with SAR and other related parameters such as pH, NO', and Fe2* falling within the permissible limits for potable water. The sources are generally suitable for both domestic and agriculture uses requiring minor treatments