Reactive Transport Modeling in Variably Saturated Media with MIN3P: Basic Model Formulation and Model Enhancements

MIN3P was developed as a general purpose multicomponent reactive transport code for variably saturated media. The basic version of the code includes Richard’s equation for the solution of variably-saturated flow, and solves mass balance equations for advective-diffusive solute transport and diffusive gas transport. Biogeochemical reactions are described by a partial equilibrium approach, using equilibrium-based law-of-mass-action relationships for fast reactions, and a generalized kinetic framework for reactions that are relatively slow in comparison to the transport time scale. MIN3P has been used to support multiple field and laboratory investigations involving the fate of inorganic and organic substances and has served as a platform for additional code development: MIN3P-Bubble, an enhanced version to simulate gas generation and exsolution in the saturated zone, as well as gas entrapment and release due to water table fluctuations; MIN3P-Dusty, a version of the code that includes gas advection and multicomponent gas diffusion based on the Dusty Gas Model (DGM); and MIN3P-Soil, a version that includes plant-soil interactions. The capabilities of the basic code and the follow-up developments are demonstrated by simulating the oxidation of pyrite in mine waste, associated metal release, and subsequent attenuation processes; the interactions between the formation of “excess air” and biogeochemical reactions in the vadose zone and below the water table; the evolution of vadose zone gas composition and transport processes at a petroleum hydrocarbon spill site undergoing natural attenuation; and the effect of plant-soil interactions on mineral weathering and secondary mineral formation in soils and surficial sediments.

Keywords: Vadose zone gas transport, gas entrapment and release, gas exsolution, dusty gas model, plantsoil interaction.