STRENGTHS AND LIMITATIONS on THE USE AND MISUSE OF AQUEOUS GEOCHEMICAL MODELS

Mineral saturation indices and speciation calculations are routinely used in water-chemistry investigations to interpret potential solubility limits controlling concentrations of aqueous constituents, the mobility of aqueous inorganic ions affected by complexing, water-rock interactions of both low-temperature and high-temperature, hazardous-waste investigations, remediation scenarios, toxicity effects of aqueous ions, and bioaccumulation in plants and animals. Although widely used, several important questions are rarely asked: Do equilibrium conditions apply? When they do apply, how reliable are they? What are the limitations in terms of ionic strength, temperature, accuracy of the thermodynamic data? A flow chart of major minerals likely to reach equilibrium saturation has been proposed and revised. Some analytical techniques are able to compare speciation by analysis with speciation by computation. For example, recent data on measured free-fluoride activities compare well with those calculated for a wide range of water compositions. Also, measured redox potentials compare well with calculated redox potentials based on Fe(II/III) determinations when detection limits for the platinum electrode and Fe(II/III) determinations are considered. The problem of hydrous iron oxide colloids and mixtures of iron phase precipitating from mine drainage going through most filter membranes and the range of Ksp values for ferrihydrite continue to hinder the interpretation of saturation indices. Although the ion-association model is often said to be limited to ionic strengths lower than 1 m, sylvite and halite solubility for temperatures of 0-100°C can be simulated with this model (3.8-7.5m). Saturation indices generally show equilibrium solubility for common minerals such as calcite, fluorite, barite, and gypsum lending confidence in aqueous speciation models. Higher-temperature data for these same minerals do not always compare as well. Much of the data necessary to refine and improve the thermodynamic properties for aqueous geochemical models are available but more evaluations of internal consistency are still needed to improve our quantitative interpretations for water-rock-biota interactions.