In situ chemical oxidation (ISCO) employs strong oxidants (e.g. permanganate, persulphate, ozone, Fenton’s reagent) to oxidize organic contaminants in groundwater into benign end products. For example, permanganate and persulphate have both been used for transformation of perchloroethene to chloride ions and carbon dioxide. ISCO has a strong history in the field for remediation of source zones of chlorinated solvents, however it has been shown to be limited in its effectiveness when source zones contain heterogeneity or low permeability soils. Also, the technique has been shown to be effective for low saturations of NAPL in relatively homogeneous soils. At higher saturations it may be difficult to deliver enough oxidizing equivalents to the NAPL zone, and production of precipitates in the case of permanganate may hinder mass transfer to the NAPL-water interface. The most significant issue with ISCO has been rebound experienced at sites with heterogeneous soils, due the back-diffusion of contaminants from lower permeability regions once the oxidant is consumed or flushed away. As the diffusion of oxidants into low permeability layers is very slow, it is difficult to treat contaminants sequestered in these zones leaving them to cause back-diffusion related rebound in contaminant concentrations when active treatment has ceased.
A schematic representation of how this technology is applied is shown in the figures.
Significant opportunities exist to improve the effectiveness of this in situ groundwater remediation technology. Our research team will test two approaches for the treatment of rebound concentrations with bioremediation. In the first, bioaugmentation of the model aquifer with KB-1 culture, which is known to biodegrade chlorinate solvents. In the second, biostimulation of indigenous microbial communities that may have survived the oxidant treatment or migrated into the treatment zone from surrounding areas (Hrapovic et al, 2005) will be done. While many ISCO treatment sites may have become bioremediation sites post-ISCO (somewhat by accident, as the microbes passively recolonize the site), there has been little research to develop a better understanding of the effects of oxidants on microbial communities, nor to establish protocols for stimulating effective bioremediation after ISCO.