Modeling of pyrite oxidation in saturated and unsaturated subsurface flow systems

Pyrite oxidation (FeS2) causes acidification and mobilization of metals. Ma thematical modeling of pyrite oxidation in variably saturated reactive flow systems is challenging because (1) it occurs through a complex interplay o f multi-phase flow and transport processes, and (2) aqueous concentrations of key species vary over tens orders of magnitude in different redox condit ions. Here, we present a general multi-phase reactive transport model for r edox processes. Two alternative implementations were made in the TOUGHREACT and TOUGH2-CHEM simulation codes which use sequential iteration and simult aneous solution, respectively. Both codes are used to simulate a fully and a variably saturated pyrite oxidation problem with simple 1-D flow and reac tion conditions. Results from both codes indicate that the effects of oxyge n partial pressure reduction due to reactions on the fluid flow is not sign ificant under ambient conditions. However, it must be noted that when fluid flow and chemical reactions are strongly coupled, such as when boiling tak es place in geothermal reservoirs, this could be essential. The fully simul taneous approach has a complete process description. The sequential iterati on approach is found to be more efficient computationally. The oxygen gas d iffusion process plays a dominant role in the chemical evolution for pyrite oxidation in unsaturated conditions. An example in 2-D fractured rock is p resented to demonstrate pyrite oxidation under complex flow and geochemical conditions. This example shows that pyrite oxidation exerts strong influen ce on hydrogeochemical evolution in variably saturated flow systems. The al teration of primary rock minerals and the development of secondary mineral assemblages predicted are consistent with field observations. This example serves as a prototype for oxidative weathering processes with broad signifi cance for geoscientific, engineering, and environmental applications.