Residual gasoline saturation in unsaturated soil with and without organic matter

The goal of this study was to investigate the influence of one variable, natural organic matter, on residual gasoline saturation in sandy soils. Capillary pressure-saturation (P_c−S) relationships (air-gasoline) were determined for three physically-similar sandy soils, with different organic carbon contents (0.086%, 0.89% and 1.65%) and residual gasoline saturations were compared. Two initial moisture conditions, residual water saturation and air-dry, were evaluated. One soil type was packed to two different bulk densities. Visualization of the soils using cryo-scanning electron microscopy was performed to aid in better understanding the role of the organic matter in the soil. The results showed that soils with higher organic contents had higher residual gasoline saturations when starting with an initially air-dry soil. Increasing the bulk density of the same air-dried soil resulted in an increase in residual gasoline saturation. In the presence of a residual water saturation, however, residual gasoline saturations were virtually identical for the three soils and independent of bulk density; approximately 5–10 times lower than in soil that was initially air-dry. The presence of the residual water effectively coated the surface of the soil thereby reducing or eliminating gasoline/soil interactions. Some residual water may also be occupying very small pore spaces, making these locations inaccessible to the gasoline.