Thermodynamic and Microbial Controls on Redox-Driven Mobilization of Redox-Sensitive Metals and Metalloids in Shallow Aquifers of Enugu

Abstract

This study examines the thermodynamic and microbial processes influencing the redox-driven mobilization of selected redox-sensitive metal(loid)s iron (Fe), manganese (Mn), and arsenic (As) in shallow aquifers within Enugu metropolis, Nigeria. A total of 25 groundwater and sediment samples were obtained from hand-dug wells and surface water sources across five locations: Ologo, Trans-Ekulu, Amechi, Ajali River/9th Mile, and Centenary. Samples were collected in sterile containers, preserved under controlled conditions, and analyzed using Atomic Absorption Spectrophotometry (AAS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in accordance with standard EPA procedures. All measurements were conducted in triplicate, and results were expressed as mean concentrations in mg/L. The findings reveal notable spatial variations in metal concentrations, largely influenced by local hydrogeochemical and redox conditions. In Ologo, iron concentrations ranged from 0.300 to 0.390 mg/L, manganese from 0.154 to 0.300 mg/L, and arsenic from 0.029 to 0.068 mg/L. Amechi recorded iron levels between 0.250 and 0.355 mg/L, relatively stable manganese values (0.150–0.165 mg/L), and a narrow arsenic range of 0.043 to 0.049 mg/L. The Ajali River/9th Mile area exhibited comparatively higher iron (0.301–0.420 mg/L) and manganese (0.220–0.260 mg/L) concentrations, while arsenic ranged between 0.028 and 0.035 mg/L. At Centenary, lower iron levels (0.180–0.290 mg/L), moderate manganese (0.150–0.190 mg/L), and minimal arsenic concentrations (0.020–0.027 mg/L) were observed. Trans-Ekulu showed the greatest variability in iron (0.220–0.718 mg/L), consistent manganese levels (0.225–0.229 mg/L), and relatively low arsenic concentrations (0.013–0.016 mg/L). Overall, the results indicate heterogeneous redox environments across the study area, significantly affecting the mobilization and distribution of metal(loid)s. In some locations, iron concentrations exceeded recommended limits for potable water. These findings provide important insight into groundwater geochemistry in Enugu and highlight the need for ongoing monitoring to protect water quality and public health.