Complete Dissolution of Chlorinated-Solvent Immiscible Liquid in Saturated Sandy Media

Dense immiscible liquids are commonly found in the subsurface beneath hazardous waste sites and can serve as a long-term source of contamination. Complete removal of the immiscible liquid is dependent on dissolution processes. A series of column experiments was conducted to examine the influence of porous-media texture and residual saturation on complete dissolution. Trichloroethene (TCE), a chlorinated solvent, was used as a representative dense immiscible liquid in these experiments. A residual TCE saturation was created by imbibing TCE into the column and then flushing with a saturated aqueous TCE solution to remove the mobile phase. Once a residual saturation was established, the dissolution process was started by flushing the column with an artificial groundwater solution (0.005M CaCl2). The effluent was monitored until the concentration reached 0.1 mg/L. The elution waves exhibited three distinct regions of behavior with respect to the rate of change of concentration. An initial steady-state stage was followed by a transient stage characterized by an exponential decrease in concentration. This was followed by an extensive low-concentration (0.4 to 0.1 ug/L) tail. The former two stages are associated with immiscible-liquid dissolution, whereas the latter is associated with rate-limited sorption/desorption. The results indicate that the elution waves exhibit an asymptotic profile for a portion of the transient stage. The extent of the asymptotic behavior appears to be a function of the uniformity coefficient (Uc), with the greatest extent for the most poorly sorted medium. The asymptotic transient-stage elution effect observed in these experiments can increase the amount of time required to reach concentrations below the maximum contaminant level (MCL).