Geochemical samples of waters along two hydrologic flow paths in four Upper Cretaceous aquifers of northeastern Mississippi and western Alabama indicate similar geochemical evolution of their respective waters. The waters of the Coker, Gordo, and Eutaw-McShan aquifers, noncalcareous sands, increase downgradient in dissolved solids and pH, and are dominated by sodium and bicarbonate ions, which generally result from a calcite dissolution-cation exchange process. Increases in dissolved iron from oxidation reduction reactions followed by decreases in total inorganic carbon from siderite precipitation occur along the flow paths. As the total inorganic carbon increases, carbon 13 (δ¹³C) generally is enriched in the moving waters, indicating the addition of a predominantly heavy source of carbon, most likely dissolving calcite. In the Coker aquifer δ¹³C values in the waters become more negative downgradient, resulting from lignite oxidation, followed by δ¹³C values becoming more positive, resulting from dissolving calcite and perhaps some mixing with brines. In northeastern Mississippi the Ripley aquifer, a calcareous sand, initially contains calcium-bicarbonate dominated water that evolves to a sodium- bicarbonate dominated water downgradient, primarily from the calcite dissolution-cation exchange process. Feldspar hydrolysis to kaolinite dominates aluminosilicate reactions in the upgradient parts of the aquifers. Authigenesis of smectite clay may be occurring in the deeper, downgradient parts of the aquifers.