CARBON SPECIATION IN REGOLITH AQUIFERS AT ENUGU, SOUTHEASTERN NIGERIA
DOI:
https://doi.org/10.58885/ijees.v10i1.36.ocKeywords:
Carbon speciation, Regolith aquifers, Redox conditions, Microbial activity, Groundwater chemistry, Eh-pH diagrams, Methane (CH₄), Enugu, Southeastern Nigeria.Abstract
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.
References
Appelo, C. A. J., and Postma, D. (2005). Geochemistry, Groundwater and Pollution (2nd ed.). CRC Press.
Boehme, S.E., Blair, N.E., Chanton, J.P., and Martens, C.S. (1996). A mass balance of C, N, and S in a shallow hydrocarbon seep, Gulf of Mexico. Geochimica et Cosmochimica Acta, 60(22), 3835–3845.
Chapelle, F.H. (2000). Ground-Water Microbiology and Geochemistry. John Wiley and Sons.
Chapelle, F.H. (2001). Ground-Water Microbiology and Geochemistry (2nd ed.). John Wiley and Sons.
Chapelle, F.H., and McMahon, P.B. (1991). Geochemistry of dissolved inorganic carbon in a coastal plain aquifer: 2. Origin of sulfate. Journal of Hydrology, 127(1-4), 69–86.
Christensen, T.H., Bjerg, P.L., Banwart, S.A., Jakobsen, R., Heron, G., and Albrechtsen, H.J. (2000). Characterization of redox conditions in groundwater contaminant plumes. Journal of Contaminant Hydrology, 45(3-4), 165–241.
Egboka, B. C. E., Uma, K. O., and Nwankwor, G. I. (1983). Principles and problems of environmental pollution of groundwater in Nigeria. Environmental Health Perspectives, 52, 161–170.
Ezeabasili, A.C.C., Anike, O.L., and Okoro, B.U. (2014). Geochemical assessment of groundwater quality in parts of Enugu town, southeastern Nigeria. International Journal of Water Resources and Environmental Engineering, 6(5), 130–142.
Ezeh, H. N., Ugwu, K. G., and Okeke, I. A. (2021). Hydrobiochemical and microbial assessment of shallow groundwater in Enugu metropolis, southeastern Nigeria. Journal of Water and Health, 19(4), 602–614.
Ezeh, H.N., and Anike, O.L. (2009). The use of electrical resistivity method in locating refuse dumps for sanitary landfill sites in Enugu, southeastern Nigeria. Journal of Applied Sciences and Environmental Management, 13(1), 85–89.
Ezeh, H.N., and Ugwu, G.Z. (2010). Hydrogeochemical and bacteriological analysis of groundwater in parts of Enugu town, southeastern Nigeria. Journal of Environmental Hydrology, 18(24), 1–12.
Ibe, K. M., Sowa, A. H. O., and Agwu, A. E. (2002). The impact of urbanization on the quality of groundwater in Enugu area of southeastern Nigeria. Environmental Geology, 42(1), 1–10.
Kotelnikova, S. (2002). Microbial production and oxidation of methane in deep subsurface. Earth-Science Reviews, 58(3–4), 367–395.
Langmuir, D. (1997). Aqueous Environmental Geochemistry. Prentice Hall.
Lovley, D. R., Holmes, D. E., and Nevin, K. P. (2004). Dissimilatory Fe(III) and Mn(IV) reduction. Advances in Microbial Physiology, 49, 219–286.
Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., and Stahl, D. A. (2021). Brock Biology of Microorganisms (16th ed.). Pearson.
Nwachukwu, M.A., Pulido-Bosch, A., and Buzoianu, M. (2010). Groundwater pollution in the Niger Delta: Impacts and options for sustainable management. Journal of African Earth Sciences, 56(3), 222–231.
Okeke, B.C. and Nwankwo, C.J. (2021). Assessment of Redox Conditions and Groundwater Quality in Selected Wells of Enugu Metropolis. Nigerian Journal of Environmental Sciences and Technology, 5(2), 120–130.
Okoro, B. U., Okoro, M. U., and Ukaegbu, V. U. (2012). Hydrogeochemical evaluation of the groundwater system in Enugu and environs, southeastern Nigeria. Journal of Mining and Geology, 48(1), 17–28.
Omeje, M., Okoro, A., Okoro, B., and Ocheli, A. (2020). Hydrogeochemical and health risk evaluation of shallow groundwater in Enugu, Nigeria. Environmental Earth Sciences, 79, 313.
Smedley, P.L. and Kinniburgh, D.G. (2002). A review of the source, behaviour and distribution of arsenic in natural waters. Applied Geochemistry, 17(5), 517–568.
Stumm, W. and Morgan, J.J. (1996). Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters. John Wiley and Sons.
Tesoriero, A. J., Liebscher, H., and Cox, S. E. (2000). Mechanism and rate of denitrification in an agricultural watershed: Electron and mass balance along groundwater flow paths. Water Resources Research, 36(6), 1545–1559.
Thauer, R. K. (1998). Biochemistry of methanogenesis: a tribute to Marjory Stephenson. Microbiology, 144(9), 2377–2406.
Umunnakwe, P.C., Ezeabasili, A.C.C., and Ezeh, H.N. (2021). Assessment of groundwater quality using integrated hydrochemical and multivariate statistical techniques: A case study of Enugu, southeastern Nigeria. Environmental Monitoring and Assessment, 193, 654.
Whitman, W.B., Bowen, T.L. and Boone, D.R. (2006). The methanogenic bacteria. In The Prokaryotes (pp. 165–207). Springer.
Zhang, G., Liu, G., Ma, J., and Wang, Y. (2020). Carbon speciation and microbial processes in the groundwater of a shallow aquifer affected by agricultural activity. Science of the Total Environment, 712, 136448.
Zhang, Y. and Wang, J. (2023). Redox-controlled carbon speciation and methane accumulation in deep aquifers of subtropical China. Chemosphere, 322, 138957.
Zinder, S. H. (1993). Physiological ecology of methanogens. In Ferry, J. G. (Ed.), Methanogenesis: Ecology, Physiology, Biochemistry and Genetics (pp. 128–206). Springer.
Downloads
Published
How to Cite
Issue
Section
License
Copyright (c) 2025 Authors
This work is licensed under a Creative Commons Attribution 4.0 International License.
