Comparison of E_h and H₂ Measurements for Delineating Redox Processes in a Contaminated Aquifer

Measurements of oxidation−reduction potential (E_h) and concentrations of dissolved hydrogen (H₂) were made in a shallow groundwater system contaminated with solvents and jet fuel to delineate the zonation of redox processes. E_h measurements ranged from +69 to −158 mV in a cross section of the contaminated plume and accurately delineated oxic from anoxic groundwater. Plotting measured E_h and pH values on an equilibrium stability diagram indicated that Fe(III) reduction was the predominant redox process in the anoxic zone and did not indicate the presence of methanogenesis and sulfate reduction. In contrast, measurements of H₂ concentrations indicated that methanogenesis predominated in heavily contaminated sediments near the water table surface (H₂ 7.0 nM) and that the methanogenic zone was surrounded by distinct sulfate-reducing (H₂ 1−4 nM) and Fe(III)-reducing (H₂ 0.1−0.8 nM) zones. The presence of methanogenesis, sulfate reduction, and Fe(III) reduction was confirmed by the distribution of dissolved oxygen, sulfate, Fe(II), and methane in groundwater. These results show that H₂ concentrations were more useful for identifying anoxic redox processes than E_h measurements in this groundwater system. However, H₂-based redox zone delineations are more reliable when H₂ concentrations are interpreted in the context of electron-acceptor (oxygen, nitrate, sulfate) availability and the presence of final products [Fe(II), sulfide, methane] of microbial metabolism.