Mineralogical compositions of aquifer matrix as necessary initial conditions in reactive contaminant transport models

Mineralogical compositions and their spatial distributions are important in itial conditions for reactive transport modeling. However, popular K-d-base d "reactive" transport models only require contaminant concentrations in th e pore fluids as initial conditions, and minerals implicitly represent infi nite sources and sinks in these models. That situation results in a general neglect of mineralogical characterization in site investigations. This stu dy uses a coupled multi-component reactive mass transport model to predict the natural attenuation of a ground water plume at a uranium mill tailings site in western USA. Numerous ground water geochemistry data are available at this site, but mineralogical data are sketchy. Even given the well-defin ed pore fluid chemistry, variations of secondary mineral species and minera l abundances in the aquifer resulted in significantly different modeling ou tcomes. Results show that the amount of calcite in the aquifer determines t he distances of plume migration. The possible presence of jurbanite, an alu minum sulfate phase, can store acidity temporarily but cause more severe co ntamination on a later date. The surfaces of iron oxyhydroxides can store s ignificant amounts of sulfate and protons and serve as a second source for prolonged contamination. These simulations under field conditions illustrat e that mineralogical compositions are an essential requirement for accurate prediction of contaminant fate and transport. (C) 2001 Elsevier Science B. V. All rights reserved.