Evaluation of errors in geochemical ion-interaction modeling of mineral solubilities

Abstract

The accuracy of the geochemical model PHRQPITZ (Plummer et al., 1988), which uses the ion-interaction method (Pitzer, 1973) for the calculation of activity coefficients in predicting mineral solubilities, is evaluated. Furthermore, a method for estimating computational error in saturation indices for minerals in natural systems is developed. The published experimental solubility data for celestite (SrSO₄), barite (BaSO₄), fluorite (CaF₂), and calcite (CaCO₃) are evaluated and compared to calculated solubility data for the minerals in simple binary (mineral-H₂O) and ternary (mineral-NaCl-H₂O) systems. The solubility modeling error determined for minerals in these simple systems is then extrapolated to several examples of natural systems. The error in magnitude and direction gives a good estimate of the computational uncertainty in the mineral saturation indices calculated by PHRQPITZ for the natural systems. The only adjustment to PHRQPITZ is the addition of polynomial-based log K(sp) equations for the minerals being evaluated.