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AN IN SITU PERMEABLE REACTIVE BARRIER FOR THE TREATMENT OF HEXAVALENT CHROMIUM AND TRICHLOROETHYLENE IN GROUNDWATER:VOLUME 2 PERFORMANCE MONITORING (EPA/600/R-99/095B)
Citation:
Blowes, D. W., R. Puls, AND R. W. Gillham. AN IN SITU PERMEABLE REACTIVE BARRIER FOR THE TREATMENT OF HEXAVALENT CHROMIUM AND TRICHLOROETHYLENE IN GROUNDWATER:VOLUME 2 PERFORMANCE MONITORING (EPA/600/R-99/095B). U.S. Environmental Protection Agency, Washington, D.C., 1999.
Description:
A 46 meter long, 7.3 meter deep and 0.6 meter wide reactive barrier was installed at the U.S. Coast Guard Support Center (USCG) in Elizabeth City, North Carolina, in June 1996. The reactive barrier was designed to remediate a hexavalent chromium [Cr(VI)] groundwater plume, in addition to treating portions of a larger and not yet fully characterized trichloroethylene (TCE) groundwater plume at the site. The barrier is composed of Peerless Metal and Abrasives of Detroit, Michigan (Peerless.) granular iron and removes Cr(VI) and TCE from the groundwater via processes of reduction and precipitation, and reductive-dechlorination, respectively. In addition to nine large-screen compliance wells, a monitoring network of approximately 150 small-screen sampling points was installed in November 1996 to provide detailed information on changes in porewater geochemistry through the barrier. This network was sampled seven times between November 1996 and December 1998 at 3 to 6 month intervals: November 1996, February 1997, June 1997, September 1997, March 1998, June 1998 and December 1998. Eh values decline from background values between 100 and 500 mV (vs. Standard Hydrogen Electrode, SHE) to values as low as -580 mV SHE within the barrier. Groundwater pH values rise from background values between 6 and 8 to values as high as 11.74 within the barrier. These extreme Eh and pH conditions within the barrier have a significant impact on the groundwater geochemistry. Concentrations of redox sensitive species such as sulphate (SO4) and nitrate (NO3) decline from background values of up to 140 mg/L and 5 mg/L to less than 20 mg/L and 0.05 mg/L, respectively. The decline of concentrations of Ca, Mg, Mn and alkalinity within the barrier may be the result of Ca, Mg, Mn carbonate mineral precipitation. Geochemical calculations indicate that the water within the barrier becomes supersaturated with respect to calcite [CaCO3], dolomite [CaMg(CO3)2] and rhodochrosite (MnCO3). Low Eh and high pH values indicate that conditions are suitable for the reduction of Cr(VI), the precipitation of Cr(III) oxyhydroxides and the reductive-dechlorination of TCE within the barrier. Sampling results indicate that upgradient concentrations of up to 5.1 mg/L Cr are consistently reduced to less than the maximum contaminant level (MCL) of <0.05 mg/L within the zero valent iron barrier. In addition, the upgradient concentration of TCE (up to 5,652 g/L) is being reduced to close to or less than the maximum contaminant level (5 g/L TCE) within the permeable barrier. Cr(VI) concentrations of less than the MCL value were consistently maintained downgradient of the barrier. TCE and cis-1,2DCE (cDCE) concentrations of less than MCL values were maintained downgradient of the barrier for most of the sampling sessions. High TCE concentrations (> MCL) were regularly measured in the deepest (7 m) downgradient monitoring points and in two downgradient compliance wells, one located at the western extent of the barrier and one located beneath the barrier. Due to the limited size of the barrier, this part of the TCE plume was not intercepted by the barrier. In the February and June 1997 sessions, TCE breakthrough at 17 and 6.8 g/L (respectively) was observed downgradient of the barrier at one location, suggesting the presence of a zone of lower granular iron density or thickness. Although there is localized breakthrough of TCE contaminated water, the results suggest that TCE and Cr(VI) contaminated water that flows through the barrier is successfully treated to MCL values. Vinyl chloride (VC) is also treated as the groundwater flows through the barrier. Occasionally VC concentrations downgradient of the barrier concentrations exceed the MCL (2g/L). This breakthrough of VC may result from inadequate residence time within the barrier, possibly due to higher than anticipated groundwater velocities within the barrier, or less iron thickness in the barrier than thedesign criteria.