Hydraulic and geochemical performance of a permeable reactive barrier containing zero-valent iron, Denver Federal Center

The hydraulic and geochemical performance of a 366 m long permeable reactiv e barrier (PRB) at the Denver Federal Center, Denver, Colorado, was evaluat ed, The funnel and gate system, which was installed in 1996 to intercept an d remediate ground water contaminated,vith chlorinated aliphatic hydrocarbo ns (CAHs), contained four 12.2 m wide gates filled with zero-valent iron, G round water mounding on the upgradient side of the PRE resulted in a tenfol d increase in the hydraulic gradient and ground water velocity through the gates compared to areas of the aquifer unaffected by the PRE. Water balance calculations for April 1997 indicate that about 75% of the ground water mo ving toward the PRE from upgradient areas moved through the gates, The rest of the water either accumulated on the upgradient side of the PRE or bypas sed the PRE. Chemical data from monitoring wells screened downgradient, ben eath, and at the ends of the PRE indicate that contaminants had not bypasse d the PRE, except in a few isolated areas. Greater than 99% of the CAH mass entering the gates was retained by the iron, Fifty-one percent of the CAH carbon entering one gate was accounted for in dissolved C-1 and C-2 hydroca rbons, primarily ethane and ethene, which indicates that CAHs may adsorb to the iron prior to being dehalogenated. Treated water exiting the gates dis placed contaminated ground water at a distance of at least 3 m downgradient from the PRE by the end of 1997, Measurements of dissolved inorganic ions in one gate indicate that calcite and siderite precipitation in the gate co uld reduce gate porosity by about 0.35% per year, Results from this study i ndicate that funnel and gate systems containing zero-valent iron can effect ively treat ground water contaminated with CAHs, However, the hydrologic im pacts of the PRE on the flow system need to be fully understood to prevent contaminants from bypassing the PRB.