In virtually every situation where the source of a hydrocarbon spill occurs at or near the ground surface, some quantity of hydrocarbon is retained in the soil as residual saturation within the vadose zone above the water table. Most unsaturated soils are capable of retaining hydrocarbons in a quantity equivalent to approximately 30% of their holding capacity (the amount of water that a particular soil can retain at saturation). In most cases, however, by the time remediation of the impacted soil is implemented, the residual hydrocarbon is less than maximum.