Abstract:
The remediation of groundwater contaminated due to NAPL (non-aqueous phase liquid) sources has been shown to be generally ineffective at reducing aqueous-phase NAPL concentrations to levels suitable for site closure. A novel, in-situ NAPL source-zone remediation method was tested in the laboratory using equilibrium batch tests and two-dimensional flow-cell experiments. The goal of this remediation method was to reduce the aqueous solubility and mass-flux of a target, toxic NAPL component through the in-situ creation of a NAPL mixture source-zone. Predetermined volumes of insoluble, benign n-hexadecane (HEX) or vegetable oil (VO) were injected into a toxic trichloroethene (TCE) source-zone through a simulated well within the flow-cell to form a NAPL mixture (TCE-HEX or TCE-VO). Initial NAPL-aqueous phase batch tests were conducted prior to the flow-cell experiments to evaluate the effects of various NAPL mixture ratios on equilibrium aqueous-phase concentrations of TCE and toluene (TOL) and to design optimal benign NAPL (HEX/VO) injection volumes for the flow-cell experiments. Variations in remediation performance between the different HEX injection volumes were relatively minor, and therefore inconsistent with Raoult's Law. This phenomenon likely resulted from non-uniform mixing of the injected HEX with TCE in the source-zone. VO injection produced a more favorable remediation result by causing TCE concentrations and mass-flux in the flow-cell to decrease more rapidly than with HEX injections. This phenomenon occurred because the injected VO was observed to mix more homogeneously with TCE in the source-zone due to a lower mobilization potential. Overall, this study indicated that the delivery of benign HEX/VO into the target, toxic TCE source-zone was effective in significantly reducing contaminant aqueous-phase concentration and mass-flux at intermediate scales. This novel remediation method may be feasible at larger scales, such as pilot field-scale studies, and may be a cost-effective solution to efficiently mitigate environmental pollution, attain regulatory compliance, and expedite site closure.
