Multiphase Fluid Flow in the Unsaturated Zone

In addition to water, NAPLs, such as petroleum, oils, tars, and biological fluids, are often present in the subsurface. When more than one fluid is present, there is a need to describe how well they mix, referred to as their miscibility. Water and vegetable oil are immiscible fluids. Many of the NAPLs are immiscible with water and will occur as separate fluid bodies, droplets, zones, etc. in the subsurface environment. 

Migration of free-phase NAPLs in the subsurface is governed by numerous properties including density, viscosity, surface tension, interfacial tension, immisci-bility, capillary pressure, wettability, saturation, residual saturation, relative permeability, solubility, and volatilization. The two most important factors that control their flow behavior are density and viscosity. 

Density is the mass of a material per unit volume, which is the ratio of the density of a substance to that of some standard, notably water. Water has a density of about 1 g/cm3, whereas carbon tetrachloride (CC14), an important DNAPL contaminant, has a density of about 1.58 g/cm3 and will tend to sink through the water table. 

Viscosity is the ability of a fluid to resist deformation or flow, and is a measure of the tendency of a fluid to flow for example, molasses has a high viscosity relative to water. Viscosity is highly temperature dependent and has common units of cen-tipoise (cP). Water has a viscosity of 1.00 cP at 20°C, whereas carbontetrachloride has a viscosity of 0.97 cP at 20°C. Therefore, the two fluids will physically flow about the same. However, with respect to flow through porous media, surface tension is extremely important. 

FIGU RE 5.6 Graph showing similarities between moisture content vs. moisture potential and hydraulic conductivity vs. moisture potential. (After Fetter, 1980.) 

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