The Long-Term Viability of a Zero-Valent Iron Permeable Reactive Barrier

Zero-valent iron permeable reactive barriers (PRB's) hold significant potential as a tool for groundwater remediation. Uncertainties remain, however, as to the effective lifetime of Fe0 barrier technology under full-scale operational conditions. Potential limits on barrier lifetime include reductions in permeability due to clogging by precipitates and oxidation of all Fe0 prior to the exhaustion of the contaminant source. A 46 m long, 0.6 m wide, and 7.3 m deep Fe0 PRB was installed at the US Coast Guard Support Center, Elizabeth City, NC, in June 1996. The barrier was designed to remediate groundwater principally contaminated with Cr(VI) and trichloroethelyne. Extraction tests were performed on materials retrieved from the Elizabeth City site to allow characterization of the geochemical environment associated with the barrier and to estimate the rate of precipitate build-up within the barrier. Mass balance calculations were performed using extraction test and groundwater monitoring data to estimate the rate of oxidation of barrier iron and place an upper limit on barrier lifetime. Reduction of SO42- and NO3- accounts for 96 to 98% of total Fe0 oxidized in the barrier. Secondary phases are precipitating within the barrier and in the region up to 12cm downgradient. Estimates of porosity loss due to precipitation indicate decreases in barrier porosity of 1.0% to 2.5% per year. If whole-barrier porosity loss continues at the calculated rate(s) barrier clogging could occur 6 to 23 years after installation. Mass balance calculations using data from extraction tests and downgradient groundwater monitoring indicate complete oxidation of the barrier Fe0 in 13 to 78 years. Barrier failure, defined as breakthrough of contaminants at levels above drinking water standards, could occur in 10 to 59 years. Mass balance calculations utilizing influent oxidant concentrations and the reported mass of Fe0 in the barrier (280 tons) indicate complete oxidation of barrier Fe0 in 100 to 2.2x105 years. Barrier failure could occur in to 77 to 1.7x105 years. The variability in the results is directly attributable to the range of hydraulic conductivity values and influent oxidant concentrations used in the calculations. A best estimate for the upper limit of barrier lifetime is 490 years.