New data are given regarding to groundwater chemistry in the top hydrodynamic zone, including swamp waters, in West Siberian region. It was found that Fe is almost ubiquitous in the examined waters. Its maximal concentrations are typical of depths from a few to 200 m, where gley geochemical medium predominates and high concentrations of dissolved organic matter are recorded. Thermodynamic methods are used for the first time to calculate the equilibrium of different waters with minerals of water bearing rocks. Groundwater are shown to be in equilibrium with Al and Fe hydroxides; Ca, Mg, Fe carbonates; and clay minerals, including ferruginous. The swamp waters are also in equilibrium with blue iron earth and apatite. The conclusion is made that the sources of Fe, as well as other chemical elements, are minerals of primary aluminum silicate rocks, with which the waters are never in equilibrium, and which they actively dissolve.
