Fundamentals of In Situ Chemical Oxidation (ISCO)

In situ chemical oxidation (ISCO) is one approach for remediation that has been developed and applied for organic contaminant mass reduction in source areas as well as for control and treatment of plumes. Oxidants such as hydrogen peroxide (H₂O₂ with natural or supplemental Fe⁺²) and potassium or sodium permanganate (KMnO₄, NaMnO₄) are capable of achieving high treatment efficiencies (>99%) for common organic contaminants (e.g., PCE, TCE, benzene, PAHs) with very fast reaction rates (99% destruction in minutes). Oxidant delivery into the subsurface can be accomplished using wells, probes, fracturing, and mixing systems, the choice of which is site-specific and dependent on the subsurface hydrology and the geochemistry of the formation. The choice of ISCO over other in situ treatment methods has been motivated by the ability of chemical oxidation to be engineered to accommodate site specific conditions, to be implemented quickly with commercially available equipment and materials, and to yield measurable results in weeks to a few months.

Field-scale applications of ISCO have now occurred at more than 100 sites in the U.S., while the technology is still evolving and the standard of practice is still being established. It is clear that achieving performance objectives at a particular site requires careful matching of a chemical oxidant and delivery system based on the target contaminants and site conditions and the performance goals to be achieved. At any site, challenges for design and effective application of ISCO often revolve around the interaction of the oxidants with subsurface porous media and the oxidant-induced changes in the subsurface that can, in turn, affect the efficiency of ISCO treatment of the target organics as well as potentially lead to undesirable secondary effects (e.g., problems with ISCO-produced particles and permeability loss, gas evolution and fugitive emissions, mobilization of co-contaminant or naturally occurring metals). In addition, the application of ISCO for source control at sites contaminated by dense nonaqeuous phase liquids (DNAPLs) can be extremely challenging and the achievement of cost-effective source control and risk reduction is highly site specific, the full understanding of which is still evolving.

There is a theoretical basis for degradation of DNAPLs by ISCO. Chemical oxidants in the aqueous phase in contact with a DNAPL can enhance the interphase mass transfer rate from the nonaqueous to aqueous phase wherein the DNAPL organics are oxidatively destroyed through chemical reactions involving redox or free-radical processes. This of course requires that the oxidant can be transported through the porous media to the DNAPL surface and that the DNAPL compounds are susceptible to oxidation by the oxidant being applied. Moreover, it requires that oxidant interaction with the subsurface does not produce heterogeneities or other transport effects that prevent oxidant-DNAPL interaction, and that the interphase mass transfer resistance does not increase due to film formation or other interfacial effects. During this presentation, highlights of relevant theory, experimental results, and field observations will be given concerning the application of ISCO for source control and remediation at DNAPL contaminated sites.