Complex Remediation of Residual VOCs

For decades, the project owner struggled to resolve residual VOCs that remained after successfully addressing radioactive contamination on the site. When another consultant’s in-situ solution failed to deliver necessary improvements, the client engaged Woodard & Curran to perform a cold-eye review and develop an alternative strategy.

A new approach

In 2019 another consultant conducted a pilot test of in-situ chemical oxidation (ISCO) at the site. Performance monitoring results indicate that reagent distribution and contaminant destruction was limited by the combination of a low permeability silty clay unit targeted for treatment and use of non-specialized injection equipment.

Based upon review of these results and other site characterization data, Woodard & Curran recommended an alternative, ISCR approach, utilizing specialized jet injection techniques to distribute a zero valent iron (ZVI) suspension and emulsified zero valent iron (EZVI). This injection method and equipment was developed and has been demonstrated to be more effective for reagent distribution within fine-grained soils. The ISCR approach provides several advantages including abiotic and biotic destruction of the contaminants, long reactive lifetime in the subsurface, and synergistic compatibility with ongoing natural degradation processes.

Timeline and results

For remedy execution and monitoring purposes, the project was divided into three main treatment areas: the residual dense nonaqueous phase liquids (DNAPL) area; a hot spot adjacent to a building on the site; and a permeable reactive treatment zone (PRTZ) area. Due to differences in contaminant levels, soil types, and hydrogeology, the project team implemented specialized programs for each area. All three have shown substantial reductions in contaminants throughout phase 1, and ongoing monitoring within each area provides validation that the remedies continue to perform as anticipated.

The graphs below chart the steady reduction in contaminant levels. PCE = Perchlorethelene; DCE = Dichloroethylene; TCE = Trichloroethylene; VC = Vinyl chloride

As the highly chlorinated parent compounds are dechlorinated by the remediation processes, ephemeral degradation products emerge. In this case, as Trichloroethylene is destroyed, we see an increase in degradation compounds Dichloroethylene and Vinyl Chloride. As time goes on, a common trend emerges with a sharp decrease in parent compounds followed by an increase and subsequent decrease in degradation compounds. The performance monitoring plan enables us to keep an eye on biogeochemical conditions as these transformations occur and continually reduce harmful contaminants to innocuous natural compounds.

The results from performance monitoring events over two years have provided strong evidence that the in-situ chemical reduction (ISCR) pilot treatment program is continuing to achieve significant in-situ degradation of  chlorinated volatile organic compounds within the targeted treatment areas.  The application of specialized injection equipment/methods and customized ISCR reagents within each treatment area has resulted in an innovative, effective and long-lived remediation solution at the site.  The ISCR approach has been recommended as the final groundwater remedy for this site.