Non-aqueous phase liquids (NAPLs), both denser than water (DNAPLs) and lighter than water (LNAPLs), also occur in the subsurface above and below the water table. The two most important properties of liquids which control their flow behavior are viscosity and density. Density is the mass of a material per unit volume. Water has a density of about 1?g/cm3, whereas carbon tetrachloride (CCl4), an important DNAPL contaminant, has a density of about 1.58?g/cm3 and will tend to sink through the water table. The viscosity is the ability of a fluid to resist deformation, and is a measure of the tendency of a fluid to flow, e.g., molasses has a higher viscosity than water. Viscosity is highly temperature dependent, and has common units of centipoise. Water has a viscosity of 1.00 centipoise at 20 degrees C, whereas carbon tetrachloride has a viscosity of 0.97 centipoise at 20 degrees C. Therefore, the two fluids will physically flow about the same. However, with respect to flow through porous media, surface tension is extremely important. At about 20 degrees C, water has a surface tension of 73.05 dynes/cm whereas CCl4 has a surface tension of only 26.95 dynes/cm. Therefore, water will be held in an unsaturated porous media by surface tension to a much greater degree than carbon tetrachloride, i.e., the permeability of porous media will be different with respect to each liquid. The ramifications are important for contaminant transport and remediation of mixed wastes.
Other NAPLs often present in the subsurface are petroleum, oils, tars, and biological fluids. 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.
It should be noted that the properties of any liquid can change as its composition changes. Salt water is much denser than fresh water, and they will not readily mix without agitation. Likewise, a specific oil composition will determine its density, viscosity and surface tension as well as its other properties.
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