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Chemical equilibrium involving carbon dioxide, bicarbonate, carbonate, and pH influences the amount of ferrous iron in much natural water. Values of pH computed from assumed equilibrium involving calcite or siderite approximate measured values of pH in 13 of 20 samples of ground water from different geologic terranes; thus, some degree of equilibrium probably existed. Two samples were supersaturated and five were undersaturated with respect to both calcite and siderite. Failure to reach equilibrium may result from absence of the minerals in the aquifer, unrepresentative analytical data, lack of reaction time, or chemical complexing. At equilibrium, pH and iron content provide a basis for estimating the Eh (redox potential) of ground water. If much bicarbonate is present, however, the amount of iron dissolved may not change with change in Eh. For three samples from artesian wells, the Eh values measured in the field and believed to be valid ranged from +0.10 to +0.13 v (volt); they were no more than 0.03 v higher than the Eh values computed from the pH and bicarbonate and iron content. The available data are not complete enough to evaluate the differences. The measured Eh (0.28 v) of water from a pumped water-table well was 0.18 v higher than the computed value. This difference was probably due to the effect of oxygen from the atmosphere on the measurement.

Restraints on dissolved ferrous iron imposed by bicarbonate redox potential, and pH