Lithological and Seasonal Controls on Trace Element Distribution and Groundwater Quality in Crystalline Basement Aquifers of Oyo Town, Southwestern Nigeria

Abstract

Groundwater in crystalline basement terrains serves as a critical water resource in southwestern Nigeria but is often influenced by lithological and seasonal controls that affect its quality. This study evaluates the lithological and seasonal influences on trace element distribution in groundwater from Oyo Town. A total of 80 groundwater samples were collected during wet (n = 40) and dry (n = 40), alongside representative rock samples for petrographic and geochemical analyses using X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). Results indicate that migmatite gneiss-dominated lithologies, rich in feldspars and ferromagnesian minerals, exert a primary geogenic control on groundwater chemistry. Elevated concentrations of Fe (0.02–4.52 mg/L in the wet season; 0.07–22.77 mg/L in the dry season), Mn (0.006–7.12 mg/L), Al (0.07–1.95 mg/L in the wet season; 0.49–42.7 mg/L in the dry season), and Pb (0.03–0.06 mg/L in the wet season; 0.01–0.23 mg/L in the dry season) are more pronounced during the dry season due to increased residence time and intensified water–rock interaction. Spatial distribution patterns and inter-element relationships further support a dominant lithogenic origin for most trace elements. The results demonstrate that groundwater chemistry is primarily controlled by lithology and seasonal variation, with limited evidence of anthropogenic influence. This integrated approach provides improved understanding of groundwater quality evolution in basement aquifers and highlights the need for continuous monitoring and sustainable groundwater management.

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