An improved equilibrium-kinetics speciation algorithm for redox reactions in variably saturated subsurface flow systems

Abstract

Reactive chemical transport occurs in a variety of geochemical environments, and over a broad range of space and time scales. Efficiency of the chemical speciation and water–rock–gas interaction calculations is important for modeling field-scale multidimensional reactive transport problems. An improved efficient model, REACT, for simulating water–rock–gas interaction under equilibrium and kinetic conditions, has been developed. In this model, equilibrium and kinetic reactions are solved simultaneously by Newton–Raphson iteration. The REACT speciation model was coupled with the multidimensional nonisothermal multiphase flow and mass transport code TOUGH2, resulting in the general purpose reactive chemical transport simulator TOUGHREACT. An application to supergene copper enrichment of a typical copper protore that includes the sulfide minerals pyrite (FeS 2) and chalcopyrite (CuFeS 2) is presented. The efficiency and convergence of the present model is demonstrated from this numerically difficult application that involves very large variations in the concentrations of oxygen, and sulfide and sulfate species. TOUGHREACT provides a detailed description of water–rock–gas interactions during fully transient, multiphase, nonisothermal flow and transport in hydrologically and geochemically heterogeneous media. The code is helpful for assessment of acid mine drainage remediation, geothermal convection, waste disposal, contaminant transport and water quality.