Influence of Initial Water Saturation on the Residual Saturation of an Organic Liquid in the Vadose Zone

The influence of initial water saturation on residual organic saturation was studied by determining pressure-saturation relationships with a pressure cell apparatus. Initial water saturations varying from 0.02 to 0.40 were investigated by allowing gasoline to imbibe into the water wet soil, displacing the mobile air. Subsequently, air was forced back into the soil in a series of incremental steps that resulted in gasoline drainage and a measurement of residual gasoline saturation. The data indicate that water saturation must be considered in order to understand retention and movement of liquid organics in the vadose zone. It is concluded that residual organic saturation decreases by the amount that initial water saturation increases, as Leverett's assumption implies, for initial water saturations less than a critical value. For initial water saturations greater than the critical values the residual oil saturation remained constant, the total liquid saturation increased and Leverett's assumption was no longer valid.