The Late Cretaceous Fox Hills Formation and the basal portion of the overlying Hell Creek Formation constitute an important aquifer in the Fort Union coal region. Throughout most of southwestern North Dakota and northwestern South Dakota the aquifer is at depths ranging from 1000 to 2000 ft, except for exposures along the Cedar Creek anticline. Water flows in the aquifer from southwest to northeast, with flow rates of a few feet per year. The recharge and discharge areas of the aquifer are separated by a north-south trending transition zone in which significant changes in water chemistry occur. Dissolved constituents in the recharge area (the western part of the study area) are Na⁺ = 18 mmol/l, Cl⁻ = 0.7 mmol/1, SO₄²⁻ = 2.7 mmol/1, and HCO₃⁻ = 13 mmol/l (δ¹³C = −12‰) with pH = 8.5. Ca²⁺, Mg²⁺, and K⁺ are each less than 0.1 mmol/l, dissolved O₂ = 0, and traces of H₂S and CH₄ are present. Computer modeling and carbon isotope data suggest the following reactions in the recharge area. CO₂ derived from lignitic carbon reacts to dissolve carbonate minerals, with cations then being exchanged for Na⁺ on clay minerals. The high pH in the aquifer is the result of buffering by carbonate-ion exchange equilibria. In the discharge area, pH values have declined to 8.3, Cl⁻ has increased from 0.7 to 5.5 mmol/l, with a parallel increase in Na⁺ SO₄²⁻ has essentially disappeared, HCO₃⁻ has increased from 13 to 21 mmol/l (δ¹³C = −9‰), CH₄ has attained concentrations greater than 0.5 mmol/l, and small amounts of He are present. Traces of H₂S are present, and Ca²⁺, Mg²⁺, and K⁺concentrations remain low throughout the aquifer: These changes can be accounted for by reactions in the aquifer: (1) sulfate reduction to pyrite with lignitic material as the carbon source and (2) continuous buffering of pH by the carbonate-ion exchange equilibria. Chemical and hydrologic data suggest that the increase in NaCl results from upward movement of small volumes of water into the Fox Hills aquifer from the transition zone eastward. Redox reactions in the aquifer are closely analogous to those observed in pore waters of reducing marine sediments. Reactions approach but do not achieve true thermodynamic equilibrium. Measurements of redox potential suggest a downgradient decrease in redox potential. The measurements are not amenable to quantitative interpretation.