Application of a chemical equilibrium model in the determination of pH of natural ground waters.

Abstract

Because of problems existing in determining a representative pH for natural ground waters, a model based on chemical equilibrium and utilizing thermodynamic relationships was developed to calculate in situ values of pH. Data from routine chemical analysis of water samples is utilized as input to the model, and, assuming the ground water is in equilibrium with any solid-phase calcium carbonate present in the aquifer, an equilibrium pH is calculated and compared with the field determined pH. Disagreement between the two values indicates nonequilibrium conditions at the well site, possibly due to the mixing of chemically dissimilar waters in the well bore, and possible encrustation or corrosion problems. Many of the calculated pH values in this study matched the field values within ±0.1 pH units. This indicates that the ground waters sampled are indeed in equilibrium with calcium carbonate, and that in situ pH values can be successfully determined by a chemical equilibrium model.