Microbial Influence on Redox Geochemistry in Enugu Shale Aquifer: Implications for Iron, Manganese, and Arsenic Dynamics

Abstract

This study investigates the concentrations of redox-sensitive metals—iron (Fe³⁺), manganese (Mn²⁺), and arsenic (As)—in groundwater from selected locations within Enugu, Southeastern Nigeria. A total of 25 water and sediment samples were collected from hand-dug wells and streams across five geographical areas: Emene, Ugwuaji, New Artisan, Gariki, and Agbani Road. Samples were collected using sterile bottles, stored under cool conditions, and analyzed in the laboratory using Atomic Absorption Spectrophotometry (AAS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), following standard EPA analytical methods. All analyses were performed in triplicate, and results were reported as mean concentrations in milligrams per liter (mg/L). The results revealed widespread exceedance of World Health Organization (WHO) permissible limits for drinking water. In the Emene area, Fe³⁺ ranged from 0.280 to 0.350 mg/L, Mn²⁺ from 0.170 to 0.260 mg/L, and As from 0.018 to 0.035 mg/L. At Ugwuaji, Fe³⁺ levels varied between 0.100 and 0.300 mg/L, Mn²⁺ between 0.120 and 0.210 mg/L, and arsenic peaked at 0.050 mg/L, five times above the WHO limit of 0.01 mg/L. New Artisan samples recorded Fe³⁺ between 0.210 and 0.300 mg/L, Mn²⁺ between 0.180 and 0.270 mg/L, and As from 0.011 to 0.022 mg/L. In Gariki, iron concentrations were especially high, ranging from 0.360 to 0.553 mg/L, with manganese between 0.130 and 0.210 mg/L and arsenic reaching up to 0.074 mg/L. The Agbani Road area showed the highest contamination levels, with Fe³⁺ from 0.520 to 0.740 mg/L, Mn²⁺ from 0.600 to 0.780 mg/L, and arsenic values as high as 0.060 mg/L. The consistently elevated levels of these metals particularly arsenic and manganese, indicate the presence of reducing geochemical conditions in the subsurface, likely influenced by microbial activity and the reductive dissolution of metal-bearing minerals. These findings raise significant concerns regarding the safety of groundwater for domestic use in these areas and highlight the need for regular monitoring and remediation strategies to protect public health.