In situ oxidation by fracture emplaced reactive solids

In low permeability but naturally fractured media. vertical leaching or vol atilization of toxic organic compounds can lead to high exposures and unacc eptable human health or environmental risk. A field test was recently compl eted to evaluate in situ remediation at such sites by using hydraulic fract uring to emplace iron metal (Fe-0) and permanganate (KMnO4) solids in the s ubsurface to chemically treat trichloroethylene (TCE). At an old land treat ment site, two test cells were installed in silty day 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 complete d 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 ir on-filled fractures formed a discrete reactive seam less than 1 cm thick, w herein the Eh decreased and reductive dechlorination could occur, but effec ts in the adjacent silty clay soils were negligible. Though the emplaced ir on exhibited some surface corrosion after extended emplacement in the subsu rface, its reactivity was unaffected. Iron from the fractures degraded TCE at efficiencies of as much as 36% after 24-48 ht of contact, which is consi stent with Fe-0 packed bed degradation half-lives of 1 to 2 hr. The permang anate-filled fractures yielded a diffuse reactive zone that expanded over t ime, reaching 40 cm in thickness after 10 mo. Throughout this oxidizing zon e, 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 loadin gs (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 efficienci es and rates are consistent with oxidation stoichiometry and previously det ermined half-lives of <2 min for permanganate oxidation of TCE. In both tes t cells there were no marked effects on the chemistry or contamination leve ls in the ground water beneath the cells. Though the results of this resear ch are promising for emplacement of horizontal treatment zones, further wor k is required to support full-scale application.