Geochemical Modeling for Mine Site Characterization and Remediation
Nordstrom, Darrell Kirk
University of Colorado, United States of America
Geochemical codes are routinely used throughout the world for investigating a wide variety of water-rock interactions including mine site characterization and remediation. Unfortunately, there is an underappreciation for the knowledge that a code user must bring to bear on the interpretation of output and the limitations of using popular codes. An historical overview of geochemical code development combined with specific examples for acid mine drainage reveals strengths and weaknesses of our current modeling abilities. Speciation models can be partly corroborated by some analytical techniques that are sensitive to the activity of free ions, e.g. the strong correlation between measured vs. calculated Eh for acid mine waters and between measured vs. calculated free fluoride ion activity. Mineral saturation indices are corroborated by nearly constant values close to equilibrium for selected minerals such as calcite, gypsum, anhydrite, hydrous ferric oxides, anglesite, and fluorite in selected aquifers or surface waters. Other systems show supersaturation or apparent supersaturation. Sometimes the supersaturation effect is not caused by errors in the thermodynamic or analytical data and it appears real, other times it is caused by analytical or thermodynamic errors. For example, mine waters have typically high concentrations of sulfate which can lead to the formation of double sulfate or bisulfate complexes of trace elements such as Pb, Ba, and Sr or even for major elements such as Fe, Ca, and Mg. Stability constant data for these complexes are often missing from databases. Some have not been measured, others have been measured but the data is not in good agreement. When these data are missing from the database, apparent supersaturation can result. Hydrous ferric oxide supersaturation in acid mine waters can be shown to be caused by iron nanocolloids not being filtered out by stand filtering techniques. Sometimes the trend of cation concentration with anion concentration can reflect whether mineral saturation is being reached or a dominant mineral dissolution controls the chemistry of the water with or without speciation calculations. Results from the Questa Mine study shows these trends quite well. Modeling mass balances is even more important than accuracy in speciation because of its relation to the conceptual model. The results from long-term studies on the Miami Globe-Pinal Creek mine wastes in Arizona make clear the pivotal role of mass balances in interpreting water-rock interactions and why geochemical models are not unique.