Hydrogeochemical Processes of Groundwater from Basement Complex Rocks in Keffi, Central Nigeria (Published)

Hydrochemical study of groundwater was done in a typical Basement Complex terrain to identify geochemical evolution of the resource and possible quality issues. In addition to water samples, overburden and fresh rock samples were analyzed for major ion concentration. Relative abundance of these ions in groundwater was compared to their concentrations in the weathered and fresh rock. Conventional graphical plots (Gibbs, Piper, and ionic ratio plots) of ionic concentrations in the groundwater were used to characterize water Facies and identify major processes responsible for the ionic assemblage. The study area is underlain by two main rock types, schist and gneiss. Relative abundance of major cations in groundwater is in the order: Ca²⁺> Mg²⁺ > Na⁺> K⁺ while relative abundance of major anions is in the order: Cl^–> HCO₃^–> CO₃^2- >SO₄^2-. In comparison, a depletion in the concentration of calcium, magnesium and sodium was observed in the weathered zone while being enriched in the fresh rock. This indicates the possibility of the ions being leached by infiltrating/percolating groundwater. Physical parameters indicate that pH of groundwater ranges from 5.6 to 13.1; TDS is also high ranging from 146.10mg/l and 9146.00mg/l. The main water type identified is mixed Ca-Mg-Cl type; others are Na-Cl, mixed Ca-Na-HCO₃, and Ca-HCO₃ types. Results of the analysis indicate that the main sources of ions into the groundwater are from water rock interaction i.e. dissolution and leaching from silicate minerals in the study area. Ground water is a mixture of the multiple processes involved in its evolution but especially that mineralogy of the aquifers plays an important role in determining the water chemistry. Most sampling points were hand dug wells and boreholes for domestic use; the high TDS and alkalinity of the water therefore presents some quality concerns.

Keywords: Basement Complex, Groundwater, Keffi, hydrogeochemical