THERMODYNAMIC MODELS FOR GROUNDWATER CHEMICAL EVOLUTION VERSUS REAL GEOCHEMICAL PROPERTIES OF GROUNDWATER - A REVIEW OF CAPABILITIES, ERRORS, AND PROBLEMS

Extensive research on the application of thermodynamic modeling to the oretical and applied hydrogeochemical problems and a survey of data ob tained in the United States are used to ascertain (1) the degree of ag reementor disagreement of thermodynamic models with the geochemical pr operties of real groundwaters and (2) the causes of disagreement. The principal causes of disagreement are a lack of correspondence between the models and the processes operating in real hydrogeochemical system s, inadequacy of thermodynamic models for groundwater chemical evoluti on (neglect of the redox processes and/or incorrect selection of the p otential-controlling systems; neglect of significant mobile species of chemical elements; and neglect of sorption processes or their incorre ct physicochemical interpretation), inaccuracy of the thermodynamic pa rameters incorporated into the software, and the use of insufficiently reliable analytical data on groundwater chemistry. Because it has a r igorous quantitative basis, thermodynamic modeling of groundwater chem ical evolution inherits all the qualitative character and uncertainty of the corresponding physicochemical models. This is responsible for t he high degree of uncertainty that is inherent in thermodynamic modeli ng of groundwater chemistry. The principal means for reducing this unc ertainty is improving the physicochemical models for groundwater chemi cal evolution. A special section of this paper analyzes the capabiliti es and limitations of computer programs developed in the United States and Russia for thermodynamic modeling of groundwater chemistry.