Uncertainty Analysis of 1-D Multiphase Flow Calculations for Mixed Waste Systems Containing Surface-Active Solutes

Constitutive data such as the capillary pressure-saturation relationship for contaminated groundwater flow systems are typically difficult to obtain. A common approximation is to scale the capillary pressure of the pristine system by the surface tension of the contaminated system according to Leverett's theory. While this approach is reasonably accurate for strongly water-wet systems, it becomes less accurate when the wettability changes toward neutral-wet. For systems that do not exhibit strongly water-wet behavior, the Leverett function is often modified to include the contact angle formed at the solid surface by the two fluids within the porous medium. Inferring the operational contact angle from measured data appropriate for the modified Leverett scaling relationship is inherently difficult, however, when investigating systems that contain surface-active solutes that adsorb to solid surfaces. Treatment history, surface heterogeneities, and dynamic effects often lead to a wide variability in locally-measured contact angle. Conversely, measured capillary pressure-saturation data appear to show higher reproducibility. The study documented here investigates the sensitivity of 1-D flow model output to selection of scaling parameters derived from surface tension and contact angle measurements from contaminated systems, coupled with capillary pressure data from pristine systems. These results are then compared to model output using capillary pressure-saturation data from the same experimental systems. The uncertainty in model output attributable to uncertainties in choices for input constitutive data are reported and discussed.