CHANGE IN GASOLINE CONSTITUENT MASS-TRANSFER DURING SOIL VENTING

The mass transfer of gasoline constituents [benzene, toluene, ethylben zene, xylenes (BTEX), and naphthalene] to air during laboratory-simula ted soil vapor extraction (referred to here as soil venting) was deter mined. Soil venting was conducted on laboratory-packed columns brought to residual water then residual gasoline saturations. The columns con tained either a sandy, low organic soil (0.10% organic carbon), a sand y organic soil (1.65% organic carbon), or glass beads. Gasoline-air pa rtitioning at the start of soil venting was experimentally determined to be adequately described by Raoult's law for many gasoline constitue nts of interest. The application of the local equilibrium assumption u sed to describe the mass transfer of gasoline constituents from the re sidual gasoline to air during soil venting was investigated using: (1) Experimental techniques such as reduction of air flow rate: flow-inte rruption and discrete sampling of soil following soil venting; and (2) a local-equilibrium-based model employing Raoult's law to describe ga soline-air partitioning. The local equilibrium assumption was shown to be valid for describing the mass transfer of various gasoline constit uents until such time when the constituent was nearly depleted from th e non-aqueous-phase liquid gasoline (NAPL). Mass transfer of constitue nts became rate-limiting when the constituent was depleted from the re sidual gasoline. The effect of soil organic matter content was only sl ightly apparent at this time, as evidenced by the higher benzene and t oluene air concentration in the effluent of columns containing the san dy organic soil.