The Suitability of Groundwater for Domestic and Irrigation Purposes: A Case Study of Ikere-Ekiti, SW-Nigeria

Abstract

Shallow groundwater in Ikere-Ekiti was assessed for potability and irrigation employing chemical and bacterial analyses. Twenty two groundwater samples were collected and analyzed using Atomic absorption spectrometer for cations and ion chromatographic method for anions determinations (˚C), pH and electrical conductivity (EC) (µS/cm) were measured in the field using pH Testr meter. The bacteriological analysis was carried out using nutrient agar medium to obtain plate count of living bacteria. Results of the analysis revealed that all EC values were less than 1000µS/cm indicating fresh water. The pH with average values of 9.48, 7.82 and 7.44 in migmatite, granite and charnockitic terrains respectively exceeded the approved standard (6.5 – 8.5) for drinking water in two samples from migmatite, one sample from granitic terrain and none from the charnockites. Sodium was the dominant cation with average concentrations (mg/L) of 95.65, 38.33 and 6.61 in migmatite, granite and charnockite respectively while K+ ions in the same order of rock units have average concentrations (mg/L) of 60.49, 32.33 and 15.77. The average concentrations (mg/L) of Ca2+ ions in groundwater located on migmatite, granite and charnockitic terrains were 36.67, 24.63 and 10.98 respectively while those for Mg2+ were 9.94, 7.48 and 4.57. The order of cation abundance was Na+> K+ > Ca2+> Mg2+. In respect of the major anions, Cl- was dominant with average concentrations (mg/L) in charnockites (187.20) within approved standard of 250mg/L while the average values (mg/L) in migmatite (475.2) and granite (340.62) exceeded the standard value. Following the same sequence of rock units, HCO3- average concentrations (mg/L) were 34.6mg/L, 27.07mg/L and 25.7. Sulphate and nitrate were less dominant ions and the order of anions abundance in the groundwater was Cl- > HCO3-> SO42-> NO3-. Bacteria evaluation revealed that all sampled groundwater tested positive to bacteria with TBC values (CFU/100ml) ranging from 1.76X108 to 1.78X109 in migmatite, 5.3x105 to 8.9x108 in granite and 2.55x107 to 8.2x108 in charnockite. Gibb's diagram revealed that chemical weathering of rock-forming minerals has contributed to solute source in the groundwater of the area. Water type on migmatite was mainly NaCl while granite and charnockite had NaCl and CaCl types revealing lithologic effects. Irrigation water quality assessment employing Sodium absorption ratio (SAR), Soluble sodium percent (SSP), Residual sodium bicarbonate (RSBC) and Permeability index (PI) revealed that the groundwater is suitable for irrigation purpose. Groundwater in the study area is low mineralized, chemically potable, suitable for irrigation but infected by bacteria pollutants. Differences in rock types affected the chemistry of the groundwater as reflected in their physico-chemical compositions, water facies and irrigation quality