Change in Gasoline Constituent Mass Transfer during Soil Venting

The mass transfer of gasoline constituents [benzene, toluene, ethylbenzene, xylenes (BTEX), and naphthalene] to air during laboratory‐simulated soil vapor extraction (referred to here as soil venting) was determined. Soil venting was conducted on laboratory‐packed columns brought to residual water then residual gasoline saturations. The columns contained either a sandy, low organic soil (0.10% organic carbon), a sandy organic soil (1.65% organic carbon), or glass beads. Gasoline‐air partitioning at the start of soil venting was experimentally determined to be adequately described by Raoult's law for many gasoline constituents of interest. The application of the local equilibrium assumption used to describe the mass transfer of gasoline constituents from the residual gasoline to air during soil venting was investigated using: (1) Experimental techniques such as reduction of air flow rate; flow‐interruption and discrete sampling of soil following soil venting; and (2) a local‐equilibrium‐based model employing Raoult's law to describe gasoline‐air partitioning. The local equilibrium assumption was shown to be valid for describing the mass transfer of various gasoline constituents until such time when the constituent was nearly depleted from the non‐aqueous‐phase liquid gasoline (NAPL). Mass transfer of constituents became rate‐limiting when the constituent was depleted from the residual gasoline. The effect of soil organic matter content was only slightly apparent at this time, as evidenced by the higher benzene and toluene air concentration in the effluent of columns containing the sandy organic soil.