Hydrogeologists and ground water geochemists have observed a decline in the measured (platinum electrode) potential, EH, of ground water as it migrates from upland recharge areas to lowland discharge areas under confined conditions. Such variations in EH and variations in concentrations of elements with variable oxidation states (e.g., oxygen, nitrogen, iron, manganese, sulfur, and carbon) can be accounted for by a sequence of oxidation–reduction reactions occurring in the flow systems, the sequence being based on thermodynamic principles as outlined by Stumm and the microbial catalysis of such reactions. This sequence of reactions results in the identification of three redox zones in ground water flow systems: (1) oxygen–nitrate, (2) iron–manganese, and (3) sulfide. The mobility and concentration of most transition metal and nonmetal ions vary according to zone—a matter of considerable significance in exploration geochemistry and in ground water pollution studies.

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