Chemical reactions that occur in landfills are analogous to those reactions that occur in marine sediments. Lateral zonation of C, N, S, O, H, Fe and Mn species in landfills is similar to the vertical zonation of these species in marine sediments and results from the following reaction sequence: (1) oxidation of C, N and S species in the presence of dissolved free oxygen to HCO₃^-, NO₃^-and SO₄²; (2) after consumption of molecular oxygen, then NO₃^-is reduced, and Fe and Mn are solubilized; (3) SO₄^2-is reduced to sulfide; and (4) organic compounds become the source of oxygen, and CH₄ and NH₄⁺are formed as fermentation products. In a landfill in Delaware the oxidation potential increases down-gradient and the redox zones in the reducing plume are characterized by: CH₄, NH₄⁺,Fe²⁺. Mn²⁺, HCO₃^-and NO₃^-. Lack of SO₄^2-at that landfill eliminates the sulfide zone. Although it has not been observed at landfills, mineral alteration should result in precipitation of pyrite and/or siderite downgradient. Controls on the pH of leachate are the relative rates of production of HCO₃^-, NH₄⁺and CH₄. Production of methane by fermentation at landfills results in ¹³C isotope fractionation and the accumulation of isotopically heavy σ CO₂ (+10 to +18⁰/₀₀ PDB). Isotope measurements may be useful to determine the extent of CO₂ reduction in landfills and extent of dilution downgradient. The boundaries of reaction zones in stressed aquifers are determined by head distribution and flow velocity. Thus, if the groundwater flow is rapid relative to reaction rates, redox zones will develop downgradient. Where groundwater flow velocities are low the zones will overlap to the extent that they may be indeterminate.