MODFLOW SURFACT is a state-of-the-art simulator that utilizes vadose zone flow and transport equations to provide practical solutions to the analysis of flow and contaminant transport at various levels of complexity and sophistication as needed for site evaluation and closure. The variably saturated flow equation can be solved with standard retention functions or with bimodal or multimodal relative permeability curves for unsaturated flow in porous and fractured systems. The equation can further be solved with pseudo-soil retention functions for confined–unconfined simulations and for use in wellbore hydraulics. Finally, the equation can be cast in terms of air phase flow to analyze subsurface air flow behavior. The variably saturated transport equation can be solved for an unsaturated medium or can be used for confined–unconfined situations. The passive phase of flow can be included in the equation to include both air and water phases in the transport situation. An immobile multicomponent nonaqueous phase liquid (NAPL) phase can further be included in the transport simulation with equilibrium partitioning providing mass transfer between phases, which adjusts NAPL saturations. Dual domain equations can be condensed into the transport equation to provide capabilities for analyzing transport in fractured media. General reaction capabilities provide analyses of complex environmental and geochemical interactions. Two examples are provided to demonstrate the value of a comprehensive simulation capability for site investigations.

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