HYDROGEOLOGICAL CHARACTERISTICS AND QUALITY ASSESSMENT OF GROUNDWATER AQUIFERS IN SELECTED LOCATIONS IN ODEDA LOCAL GOVERNMENT, OGUN STATE, NIGERIA

Abstract

ABSTRACT Groundwater, a crucial and finite natural resource, plays a pivotal role in providing a reliable and economical water supply globally. This study characterized the water-bearing units within the basement complex environment of Odeda Local Government, Ogun State, Southwestern Nigeria for the assessment of hydrogeological characteristics and water quality for effective planning and development of groundwater resources. Vertical Electrical Sounding (VES) investigations were conducted across fifty-six data points using the Schlumberger array with a maximum half-current electrode separation of 132 m. The lithological and hydrogeological characteristics of the study area were derived from these VES investigations. The vulnerability index was calculated based on key aquifer parameters such as longitudinal conductance, overburden thickness, coefficient of anisotropy, and hydraulic conductivity. The potential groundwater spatial map was generated using basement resistivity, coefficient of reflection, transverse unit resistance, and transmissivity at each sounding point across the study area. Hydrochemical investigations covered Opeji, Itesi, Ilugun, Osiele, Obantoko, Olodo, Obete, Odeda, Alabata, and Isolu towns within Odeda Local Government, involving the collection and analysis of ninety groundwater samples from boreholes and hand-dug well and tested for physical, chemical, and heavy metal contents. The data were further analyzed using descriptive statistics, correlation analysis (CA), and hydrochemical characterization. The results showed diverse geophysical curve types such as H, KH, AH, AA, and A, with predominantly H types indicating the presence of a shallow crystalline basement rock. The inferred lithologies showed between three to five geoelectric layers, each characterized by distinct resistivity values and thicknesses. Protective capacity assessments at VES points indicated a varied vulnerability landscape, with approximately 48% exhibiting poor protection, 39% displaying a moderate level, and 13% showing robust defense against contamination. The potential groundwater spatial map showcased variations, with the Obantoko axis exhibiting mainly low and medium water yield, the FUNAAB axis demonstrating high groundwater potential, and the Odeda axis displaying a varied distribution of high, medium, and low potential zones. Hydrochemical analysis revealed slightly acidic groundwater across the ten locations, with other parameters generally conforming to recommended limits, except for elevated levels of calcium, lead, manganese, and copper. Piper plots illustrated a predominant CaCl2 water type, and Schoeller diagrams highlighted major ion abundance sequences, indicating the dominance of inorganic salts. While most physico-chemical parameters met permissible limits set by the World Health Organisation (WHO) and Nigeria Standard for Drinking Water Quality (NSDQW), elevated concentrations of calcium, lead, copper, and magnesium ions suggested potential signs of impairment. The findings identified optimal sites for proposed borehole installations based on evaluations of aquifer protective capacity, vulnerability and groundwater yield potential characteristics. Hydrochemical analysis showed generally compliant water quality, though some areas had elevated ion levels, requiring remediation to ensure safe drinking water standards. The study characterized the groundwater resources in the area, revealing a diverse hydrogeological landscape with varying vulnerability and groundwater potential essential for future borehole installations.