Abstract

Groundwater compositions could be altered significantly due to geogenic and anthropogenic inputs from rock and human respectively. Such impacted water may be injurious to health if consumed. Studies of groundwater within the lateritic overburden on migmatite gneiss and pegmatised schist in Ibadan area were undertaken with the aim of determining their physico-chemical composition and portability. Groundwater within lateritic profiles over NE-SW trending migmatite and pegmatised schist were sampled. The temperature of the water ranged from 23.9 to 32.7°C, electrical conductivity from 60 to 580 µS/cm, pH from 5.5 to 7.3 and TDS from 55 to 422 mg/L. The concentrations of cations in mg/L ranged for Ca²⁺ (62.2-79.4), Mg²⁺ (7.5-15.3), Na⁺ (28.2-54.3), and K⁺ (55.0-85.1) while the anions are HCO₃^- (116.1-149.2), SO₄^2- (0.0-1.1), Cl^- (8.2-22.1) and NO₃^- (0.3-2.2) mg/L. These chemical data are characteristic of Na-(K)-HCO₃ water type. The cations, anions and the trace metals of the groundwater are generally within the WHO standards. The low level of chemical constituents in the groundwater reflects geogenic input arising from weathering of the basement rocks. The water is good for domestic, industrial and irrigation purposes.

Key words: Groundwater, water portability, migmatite, pegmatised schist, hydrochemistry.