NONIDEAL SOLUBILIZATION OF TCE FROM A FUEL-BASED MIXTURE AND INFLUENCE ON ADVECTIVE-DIFFUSIVE TRANSPORT

Many subsurface sites contaminated by organic chemicals consist of multicomponent nonaqueous phase liquids (NAPLs). For example, chlorinated aliphatic hydrocarbons (CAH) commonly exist at small NAPL-phase fractions in waste-fuel mixtures. The CAH are often of primary importance because of their relative higher toxicity and solubility compared to the fuel components, combined with a relatively low potential for biological degradation. Thus, CAH contaminants pose a serious threat to groundwater quality, even at very low concentrations in the fuel. Small NAPL-phase mole fractions (<0.1) of CAH in fuels are typical. However, little published research describing the solubilization behavior of these compounds from fuel mixtures is available. Solubilization of hydrocarbon components from fuels is often considered to be “ideal” because the process approximately follows Raoult’s Law. However, when components differ significantly in size, structure, and hydrophobicity from the bulk mixture, and are present at small NAPL-phase mole fractions, nonideal solubilization may occur (i.e., equilibrium solubilization does not obey Raoult's Law). Batch-equilibrium solubility studies show that trichloroethene (TCE) present in a hypothetical fuel (n-decane) exhibits significant non-ideal solubilization at small NAPL mole fractions. Specifically, the measured aqueous TCE concentrations were greater than the values predicted using Raoult’s Law. Departure from ideality increased as the NAPL-phase mole fraction decreased. The UNIFAC method did not accurately predict NAPL-phase activity coefficients for TCE. Empirical models that describe the observed nonideal solubilization process were developed and incorporated into an advection-dispersion model. The model is used to assess the importance of nonideal dissolution on downstream transport of TCE in a low-permeability media wherein diffusion is important. Model predictions that assume ideal behavior result in much longer dissolution times; however, concentrations calculated at a downstream receptor are considerably larger at early times and significantly smaller at later times. Results indicate that the applicability of Raoult’s Law in describing fate and transport of CAH contaminants may not be appropriate for typical fuel-CAH mixtures.