REDOX PROCESSES IN REGOLITH AQUIFERS AT ENUGU

Abstract

Redox reactions are the predominant controlling factors of groundwater chemistry in Enugu, Nigeria. Nitrogen, carbon, sulfur, and iron species stability at different sites is explored through Eh-pH diagrams focusing on how microbial activity influences water quality. A total of 20 samples were sampled. Eh and pH were measured then the results were used to plot an Eh-pH graph using geochemist work bench. From the result nitrate stability occurs under oxidizing conditions (Eh ≥ 0.5 V) like Gariki Stream and Precious Street HDW, where nitrification is carried out by Pseudomonas sp. Denitrification by Bacillus sp. and Geobacter sp. occurs under reducing conditions (Eh ≈ 0.1–0.2 V) in Promiseland, Oriental Estate, and Ihe Street HDW. Very reducing sites like Akwuke HDW and areas of Agbani prefer the accumulation of ammonium, implying dissimilatory reduction of nitrate. Carbon speciation is redox-dependent. Eh-rich sites favor CO₂ stability, and moderately reducing conditions favor bicarbonate dominance. Methanogenesis is observed in Akwuke HDW and Agbani-area wells, favored by Methanobacterium sp. Sulfur transformations reflect redox transitions. Bricks Estate HDW and Gariki Stream oxidizing conditions favor the stability of sulfate, and EmekaOgbodo HDW and Akwuke reducing conditions favor sulfate reduction to hydrogen sulfide. Iron speciation also follows in parallel. Oxidized ferric iron (Fe³⁺) dominates high-Eh zones, precipitating as hematite, while low redox conditions in Akwuke HDW and Agbani-area wells allow ferrous iron (Fe²⁺) precipitation. These findings demonstrate the deep relationship between redox conditions, microbial activity, and groundwater chemistry. The understanding of these relationships is crucial for controlling water quality and ensuring safe use of groundwater in Enugu.