Multicomponent reactive transport modeling in variably saturated porous media using a generalized formulation for kinetically controlled reactions

Abstract

[1] A generalized formulation for kinetically controlled reactions has been developed and incorporated into a multicomponent reactive transport model to facilitate the investigation of a large variety of problems involving inorganic and organic chemicals in variably saturated media. The general kinetic formulation includes intra-aqueous and dissolutionprecipitation reactions in addition to geochemical equilibrium expressions for hydrolysis, aqueous complexation, oxidation-reduction, ion exchange, surface complexation, and gas dissolution-exsolution reactions. The generalized approach allows consideration of fractional order terms with respect to any dissolved species in terms of species activities or in terms of total concentrations, which facilitates the incorporation of a variety of experimentally derived rate expressions. Monod and inhibition terms can be used to describe microbially mediated reactions or to limit the reaction progress of inorganic reactions. Dissolution-precipitation reactions can be described as surface-controlled or transport-controlled reactions. The formulation also facilitates the consideration of any number of parallel reaction pathways, and reactions can be treated as irreversible or reversible processes. Two groundwater contamination scenarios, both set in variably saturated media but with significantly different geochemical reaction networks, are investigated and demonstrate the advantage of the generalized approach. The first problem focuses on a hypothetical case study of the natural attenuation of organic contaminants undergoing dissolution, volatilization, and biodegradation in an unconfined aquifer overlaid by unsaturated sediments. The second problem addresses the generation of acid mine drainage in the unsaturated zone of a tailings impoundment at the Nickel Rim Mine Site near Sudbury, Ontario, and subsequent reactive transport in the saturated portion of the tailings.

A review of model applications for structured soils: a) Water flow and tracer transport

“…Gérard et al (2004) tried to predict 4 years of daily measurements of soil water content in a forest soil plot with PF in a capillary pore network. They utilized pedotransfer functions (ROSETTA, Schaap et al, 2001) and a bimodal hydraulic conductivity function (Mohanty et al, 1997) with the Richards' flow submodel of MIN3P (Mayer et al, 2002). However, soil water contents were under-predicted in the wet range.…”

Reactive transport modeling of geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN

“…Using inverse modeling, they demonstrated the importance of dissolution properties and oil saturation on BTEX loss from the oil and the relative unimportance of seasonal recharge variation. The most comprehensive previous modeling was by Amos [2006], who modified the unsaturated/saturated zone reactive transport model MIN3P [Mayer et al, 2002] to include bubble formation, contraction, and entrapment [Amos and Mayer, 2006]. A major contribution by Amos [2006] is the representation of redox-related gases in the saturated zone plume, including O 2 dissolution and CH 4 and CO 2 outgassing.…”

Influence of Uranyl Speciation and Iron Oxides on Uranium Biogeochemical Redox Reactions

“…The aim of this study is thus to provide a quantitative framework on bacterial reduction of U(VI) at varying concentrations of dissolved calcium and to determine the impact of Fe(III) (hydr)oxides, varying systematically in free energy of formation, on reduction rates. In addition to a kinetic analysis, we develop a quantitative biogeochemical model for U transformation using the reactive transport code MIN3P (Mayer et al, 2002), which is broadly applicable to geochemical problems involving kinetically controlled redox and mineral dissolution/precipitation reactions along with equilibrium hydrolysis, aqueous complexation, ion exchange, and surface complexation reactions. The results of this study provide an understanding of geochemical limitations on microbial uranium reduction imposed by Ca and modifications induced by Fe (hydr)oxides within the mineral matrix of soils and sediments, helping to predict the potential for migration in environmental settings.…”