Volatile organic compounds (VOCs), including tetrachloroethylene (PCE), trichloroethylene (TCE), cis-1,2-dichloroethylene (cis-1,2-DCE) and vinyl chloride were present in groundwater, soil, and/or soil vapor in a shallow groundwater aquifer comprised of saturated silty sand to poorly graded sand, approximately 18 to 24 feet bgs. The plume has approximately 1,000 to 4,000 μg/L of PCE and TCE. Cleanup goals were 530 μg/L for TCE, and 120 μg/L for PCE.
ISCO using Catalyzed Hydrogen Peroxide (CHP) had been selected as the remedy for this site to chemically degrade these VOCs. CHP consists of H2O2 + a chelated iron catalyst. CHP produces a suite of reactive oxygen species (hydroxyl radical, superoxide) which make it effective for many organic species. Limitations of using CHP include a short-lived hydrogen peroxide (H2O2) residual (a few hours to days) due to decay. After two rounds of injections, there was only a 18 to 29% reduction in TCE and PCE showed little to no change. These results cast doubt on future planned remedial actions at the site.
Mutch Associates was retained to diagnose why the first two rounds of ISCO were mostly ineffectual and to determine the number of future injections required to reach remedial targets. A three-dimensional reactive transport model was developed to help answer these questions. The model includes all of the relevant chemical reactions that govern CHP persistence and effectiveness including loss of residual H2O2. The model was constructed using MODFLOW + RT3D using six different chemical species including H2O2, bromide, and dissolved and sorbed TCE and PCE.