In Situ Oxidation by Fracture Emplaced Reactive Solids

In low permeability but naturally fractured media, vertical leaching or volatilization of toxic organic compounds can lead to high exposures and unacceptable human health or environmental risk. A field test was recently completed to evaluate in situ remediation at such sites by using hydraulic fracturing to emplace iron metal (Fe⁰) and permanganate (KMnO₄) solids in the subsurface to chemically treat trichloroethylene (TCE). At an old land treatment site, two test cells were installed in silty clay soils with hydraulic fractures filled with either iron metal or permanganate solids at 1.8, 2.4, and 3.6 m depths. Fracture emplacement was monitored, and soil and ground water conditions were characterized. After 3, 10, and 15 mo of emplacement, continuous cores were collected and morphologic and geochemical data were taken across the fracture zones. Controlled degradation tests were completed using site ground water with TCE concentrations near 53, 144, and 480 mg/L, equivalent to 0.5, 1.2, and 4.1 g TCE per kg media, respectively. The iron-filled fractures formed a discrete reactive seam less than 1 cm thick, wherein the Eh decreased and reductive dechlorination could occur, but effects in the adjacent silty clay soils were negligible. Though the emplaced iron exhibited some surface corrosion after extended emplacement in the subsurface, its reactivity was unaffected. Iron from the fractures degraded TCE at efficiencies of as much as 36% after 24–48 hr of contact, which is consistent with Fe⁰ packed bed degradation half-lives of 1 to 2 hr. The permanganate-filled fractures yielded a diffuse reactive zone that expanded over time, reaching 40 cm in thickness after 10 mo. Throughout this oxidizing zone, the degradation efficiency was >99% after 2 hr of contact for dissolved TCE at 0.5 and 1.2 mg TCE per g of media. When exposed to higher TCE loadings (i.e., 4.1 mg per g), degradation efficiencies after 10 mo dropped to 70% as the TCE load exceeded the oxidant capacity remaining. These efficiencies and rates are consistent with oxidation stoichiometry and previously determined half-lives of <2 min for permanganate oxidation of TCE. In both test cells there were no marked effects on the chemistry or contamination levels in the ground water beneath the cells. Though the results of this research are promising for emplacement of horizontal treatment zones, further work is required to support full-scale application.