GEOCHEMICAL MODELING, PREDICTIONS, AND THE CHARACTERIZATION AND REMEDIATION OF MINE SITES

Geochemical modeling is applied widely to water-rock interactions in contaminated and uncontaminated areas, including the characterization, permitting, risk assessment, and remediation of mine sites. It may involve mass-balance, speciation, mass-transfer, and/or reactive-transport calculations, and may address surface waters, ground waters, issues of natural background, remediation scenarios, and future climate or seasonal hydrologic trends. Unfortunately, popular usage does not necessarily equate to knowledgeable and proper usage. All kinds of impressive model calculations can be produced, including contradictory outputs. Significance of the output depends on two prime requirements: (1) is the modeler well-experienced and educated in the processes that are being interpreted? and (2) is the model adequately constrained by reliable field data? Other important considerations are reliability and applicability of thermodynamic and kinetic data, adequacy of lithological and mineralogical data, adequacy of hydrologic data, and appropriateness of equilibrium, kinetic, and other assumptions. For mine sites, challenges to modeling include site size and complexity, ambiguity of leach tests results, difficulty of phase identification and characterization such as presence of mixed iron phases (schwertmannite, ferrihydrite, jarosite, goethite, hematite), obtaining water balances, poorly understood ground-water hydrology, and dynamic hydrologic events (storm events, snowmelt, seasonal changes, long-term climate change). Many of these complexities must be addressed for adequate risk assessment, but they cannot be addressed solely through modeling. They must be acquired through careful site characterization. Once a site has been properly characterized then it may be possible to use modeling to predict future scenarios and to help guide remediation decisions. However, it must be remembered that science and engineering only make logical predictions, not temporal predictions (i.e. assuming certain conditions and principles, a logical outcome is expected). Model calculations are often not unique whereas site conditions often are unique. Modeling can constrain possibilities but it cannot exactly predict the future.