Dense non-aqueous phase liquid (DNAPL) contamination represents a major environmental problem for many hazardous waste sites throughout the United States. Common DNAPLs include trichloroethene (TCE), tetrachloroethene (PCE), chloroform, and carbon tetrachloride. DNAPLs present a long-term problem due to their chemical properties, including low solubility and high density, which can result in release of high concentrations to groundwater over extensive time frames. Recent efforts have increasingly focused on source zone treatment of DNAPL contamination to reduce the volume and mass available for dissolution to groundwater. In situ chemical oxidation (ISCO) is one of several technologies that have emerged for remediation of DNAPL-contaminated sites.
The overall objective of this project was to advance the understanding of ISCO for DNAPL sites by quantifying the pore/interfacial scale DNAPL reactions and porous media transport processes that govern delivery of oxidant to a DNAPL-water interface and degradation of the DNAPL in situ. Specific objectives included: (1) determine the interphase mass transfer rates and degradation of DNAPLs as a function of oxidant type and concentration, interfacial cross-flow velocity, and system properties; (2) determine the effects of different types of porous media on DNAPL degradation; (3) determine the effects of DNAPL entrapment morphology on mass reduction and changes in mobile contaminants after ISCO; (4) assess coupling of ISCO with mass recovery and/or natural attenuation; and (5) determine if partitioning tracer test (PTT) methods can be used for performance assessment at ISCO-treated sites.
In situ chemical oxidation involves the introduction of chemical oxidants into the subsurface to destroy organic contaminants in soil and groundwater, with the goal being to reduce the mass, mobility, and/or toxicity of contamination. Depending on the contaminant properties, site conditions, and performance goals, ISCO systems can include different oxidants and methods of subsurface delivery.
In this project, an integrated set of tasks was carried out involving a comparative analysis focused on contrasting oxidant types (potassium permanganate and catalyzed hydrogen peroxide) and oxidant application methods (low to high dose concentrations and delivery densities) to treat PCE and TCE DNAPLs under conditions representative of a range of subsurface environmental settings. The research also addressed the potential secondary effects of ISCO, as well as the coupling of ISCO with pre- and post-ISCO treatment operations. A numerical model for ISCO, CORT3D, was developed. Results provide new insights into the effective application of ISCO for sites contaminated with DNAPLs as well as point out limitations.
Applications of ISCO to sites contaminated with dissolved and sorbed phase organics have been completed quickly and achieved performance goals (99% mass destruction) at costs that can be competitive with other technologies ($40/m3 of subsurface volume treated). Applications to DNAPL-contaminated sites could conceivably achieve similar performance, depending on site conditions, contaminant mass and architecture, and performance goals. Under Environmental Security Technology Certification Program (ESTCP) project ER-200623, the project team is now developing, implementing, and evaluating an ISCO Technology Practices Manual. (Project Completed – 2006)