The Site is a former dry cleaning operation where a release of perchloroethylene (PCE) to soil and groundwater had occurred. Hydrogen Release Compound® (HRC) is being used for source control to mitigate vapor intrusion to the existing building. The Site is located in the Connecticut River basin with PCE up to 250 mg/L in perched groundwater above a silt-layer aquitard. Site risk is driven by the soil vapor intrusion pathway into the commercial building. Soil vapor extraction was implemented to mitigate vapor intrusion, with no appreciable change in the perched groundwater conditions and rapid rebound of PCE in soil gas to pre-treatment levels in four months. Our evaluation of soil data following multiple HRC applications over an 8-year period into the perched groundwater on top of a Connecticut River basin silt deposit finds that treatment in the sandy unit above the aquitard achieved significant reduction of PCE in the silt layer below. This discovery changed the project Conceptual Site Model and led to further evaluation of the source of PCE feeding into soil gas. With decreased groundwater concentrations of PCE but persistent soil gas concentrations, Membrane Interface Probe (MIP) work was done to further assess the extent of additional area within the perched groundwater that required treatment. The results indicated that the extent of significant concentrations of PCE was in a peripheral area around the initial treatment zones. Following the MIP results, additional in-situ HRC treatment in the perched aquifer over a broader area than previous injections was implemented with the intent of removing a significant mass of PCE. As we expected, there was a PCE source in the silt layer below the treatment area. Subsequent data show further decreases in PCE concentrations measured in perched groundwater and soil gas.
Irwin, J. Andrew and Marsh, Daniel E.
"Removal of Perchloroethylene within a Silt Confining Layer Using Hydrogen Release Compound,"
Proceedings of the Annual International Conference on Soils, Sediments, Water and Energy:
Vol. 15, Article 20.
Available at: http://scholarworks.umass.edu/soilsproceedings/vol15/iss1/20