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Groundwater quality and contaminant hydrogeology 217

Fig. 6.10 Comparison of fluid wetting
states for a porous sand containing water
and oil. In (a) water is the wetting fluid and
in (b) oil is the wetting fluid. After Fetter
(1999).

low solubility in water and partition preferentially in preference to another fluid. Thus, if fluid A has a
towards organic material contained in soils, sediments higher attraction to a given surface than fluid B, then
and rocks and in doing so demonstrate hydrophobic fluid A is the ‘wetting fluid’ with respect to fluid B.
sorption behaviour (see next section). For the purpose of considering groundwater pollu-
The presence of individual phases of water, NAPL tion by organic compounds within porous material,
and air (in the vadose zone) leads to the condition then water can always be considered the wetting fluid
of multiphase flow where each phase is competing with respect to NAPLs or air.
for the available pore space. In the presence of two If a porous material is water-wet and a compound
fluids, wettability is defined as the tendency for a such as oil is introduced, the water will continue to
given fluid to be attracted to a surface (solid or liquid) occupy the capillary space in preference to the oil