Chemical equilibrium in mineral formation and diagenesis in the carbonate evaporite system: An assessment of chemical model application to site performance

Recently there has been considerable interest in predicting the long-range geochemical behavior of nuclear waste repository sites via thermodynamic modeling. Several models are discussed. The applications of well-defined chemical models to field settings have several important implications for applying models to natural systems in general. First, the authors show that detailed models of systems with very complex chemistry can be developed which are accurate to high concentration. Second, the Searles Lake application shows that the model not only could correctly predict the geochemistry of a complex natural system but also that the mineral formation is in equilibrium with the formation water. Third, (and perhaps the most important for assessing the feasibility of using models to reliably predict geochemical processes on a long-time scale) certain geochemical controls (e.g., late dolomitization and limited communication between units in the formation) are necessary to bring the model into good agreement with the chemistry of the real setting. These controls affected only the interpretation of field processes and in no way affected the parameterization or validation of the model. In the application to the Searles Lake borate deposit, these controls led to important information about possible geochemical processes involved in the formation of evaporative mineral deposits in lake beds. Prior to modeling the system, there was no a priori way to make these assumptions. This result has important implications when the application of less well-defined models to poorly defined settings is considered. In such situations, it may not be possible to separate effects due to model inaccuracies from the various unknown geochemical controls operating in the natural setting