Outflow Methods for Evaluating the Soil Hydraulic Functional Relationships Between NAPL Pressure and Saturation in Porous Media

Remediation and cleanup of petroleum product contaminated ground water often require modeling of fluid transport processes when immiscible liquid phases (e.g., water and oil) are present. Modeling of such multiphase transport systems requires knowledge of the functional relationships between fluid pressures, saturations, and permeabilities. We evaluated the applicability of the multistep outflow method used in soil science to determine these functions for two porous media (loam and sand) using Soltrol 130 and water as wetting fluids.

The analytical retention and permeability functions of van Genuchten (1980) and Mualem (1976) were used, with an inverse method that has been shown to be reliable in estimating water retention and unsaturated hydraulic conductivity in soils (Eching and Hopmans 1993), to estimate soil hydraulic function parameters for Soltrol 130 and water. The water and Soltrol 130 cumulative drainage as a function of time and the equilibrium saturations were used as input to a numerical model (MLSTPM) to optimize, through an inverse solution of the Richards equation, the parameters needed for the hydraulic functions.

Optimizations were carried out for saturation paths corresponding to monotonically decreasing wetting phase saturations only. The functional relationships between oil pressures, saturations, and permeabilities in Oso-Flaco fine sand were accurately predicted from the optimized water retention curve parameters based on scaling by the ratio of interfacial tensions. However, this scaling procedure was inadequate to predict oil hydraulic function parameters from those of water in Yolo loam.