Assessments of seasonal variations and bedrock controls on groundwater chemistry in Lokoja Metropolis, Northcentral Nigeria

Abstract

Groundwater is the main source of potable water in Lokoja Metropolis, North-central Nigeria. Despite growing challenges related to demographic and anthropogenic pressures, bedrock heterogeneity, and recurring flooding episodes, the region lacks systematic seasonal quality and hydrogeochemical characterization. This study assessed the combined impact of seasonal dynamics and bedrock control on groundwater hydrochemistry to enhance sustainable water resource management in the region. A total of 129 groundwater samples were collected, comprising 65 samples from the dry season and 64 from the wet season. The groundwater samples were examined for physicochemical parameters such as temperature, pH, total dissolved solids (TDS), and electrical conductivity (EC), major cations (Ca 2 ⁺, Mg 2 ⁺, Na⁺, and K⁺), and major anions (HCO 3 − , NO 3 − , SO 4 2− , and Cl − ). The results revealed low to moderately mineralized groundwater, slightly acidic to alkaline, with notably higher electrical conductivity (EC up to 1730 µS/cm) and total dissolved solids (TDS up to 1273 mg/L) during the wet season, indicating significant solute leaching and seasonal recharge. The predominance of bicarbonate suggests CO 2 -enriched meteoric recharge, whereas elevated nitrate levels at certain locations indicate anthropogenic inputs. Hydrochemical fingerprints in the bedrock units showed lower ionic concentrations in the younger sandstone and alluvial units than in the migmatite–biotite gneiss and older granite aquifers. This distinction reflects silicate weathering within the basement complex as the primary geochemical control on the water chemistry. Overall, the findings demonstrate that the hydrochemistry of the aquifer system in Lokoja is influenced by both seasonal dynamics and lithological differences, underscoring the importance of bedrock geology in groundwater quality management and protection.