CARBON SPECIATION IN REGOLITH AQUIFERS AT ENUGU, SOUTHEASTERN NIGERIA

This study investigates the carbon speciation in regolith aquifers within Enugu, southeastern Nigeria, with a focus on how redox conditions and microbial activities influence the distribution of carbon species such as aqueous carbon dioxide (CO₂(aq)), bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and aqueous methane (CH₄(aq)). A total of 30 groundwater samples from multiple locations such as Centenary, 9th mile, Ologo, Trans-Ekulu, New-artizan and Amechi from both hand-dug wells and stream sources and analyzed for pH and Eh (redox potential). Eh and pH were determined using the Standard Hydrogen Electrode and a pH meter respectively. The raw data were interpreted using Eh-pH (Pourbaix) diagrams for the CO₂/HCO₃⁻/CO₃²⁻/CH₄ system at 25°C. The results reveal that most of the sampled aquifers fall within the domains of bicarbonate and aqueous methane, indicating prevailing mildly reducing to strongly reducing conditions in the subsurface environment. These conditions are largely driven by microbial respiration and fermentation processes, particularly those involving sulfate-reducing bacteria, iron-reducers, and methanogens. Sites with more oxidizing Eh values, such as New Artisan stream and Premier Layout HDW, were associated with CO₂(aq) and HCO₃⁻ dominance, suggesting a higher oxygen availability or active organic matter degradation. Conversely, the dominance of CH₄(aq) in locations such as Egbunike Crescent HDW and Elshammal Estate HDW highlights anaerobic degradation pathways, likely driven by methanogenic archaea under anoxic conditions. This distribution of carbon species provides critical insight into the redox evolution of regolith aquifers in Enugu and underscores the significant role of microbial processes in shaping groundwater chemistry. These findings have implications for water quality, nutrient cycling, and the potential mobilization of redox-sensitive elements in shallow tropical aquifers.