COMPARISON OF E(H) AND H-2 MEASUREMENTS FOR DELINEATING REDOX PROCESSES IN A CONTAMINATED AQUIFER

Measurements of oxidation-reduction potential (E(h)) and concentration s of dissolved hydrogen (H-2) were made in a shallow groundwater syste m contaminated with solvents and jet fuel to delineate the zonation of redox processes. E(h) measurements ranged from +69 to -158 mV in a cr oss section of the contaminated plume and accurately delineated oxic f rom anoxic groundwater. Plotting measured E(h) and pH values on an equ ilibrium stability diagram indicated that Fe(III) reduction was the pr edominant redox process in the anoxic zone and did not indicate the pr esence of methanogenesis and sulfate reduction. In contrast, measureme nts of H-2 concentrations indicated that methanogenesis predominated i n heavily contaminated sediments near the water table surface (H-2 sim ilar to 7.0 nM) and that the methanogenic zone was surrounded by disti nct sulfate-reducing (H-2 similar to 1-4 nM) and Fe(Ill)-reducing (H-2 similar to 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 H-2 concentrations were more useful for identifying anoxic redox processes than E(h) measurements in this groundwater syst em. However, H-2-based redox zone delineations are more reliable when H-2 concentrations are interpreted in the context of electron-acceptor (oxygen, nitrate, sulfate) availability and the presence of final pro ducts [Fe(II), sulfide, methane] of microbial metabolism.