Iron Reactive Barriers at the Oak Ridge Y-12 Site

The concept

Barrier construction

PROJECT SUMMARY     

Two permeable iron reactive barriers (a continuous trench and a funnel-and-gate system) were installed in late November, 1997 at the U.S. Department of Energys Y-12 National Security Complex in Oak Ridge , Tennessee.  The overall goal of this research was to determine the effectiveness of the use of zero-valent iron (Fe0) to retain or remove uranium and other contaminants such as technetium and nitrate in groundwater.  The long-term performance issues were investigated by studying the biogeochemical interactions between Fe0 and groundwater constituents and the mineralogical and biological characteristics over an extended field operation. Results from nearly 3 years of monitoring indicated that the Fe0 barrier was performing effectively in removing contaminant radionuclides such as uranium and technetium. In addition, a number of groundwater constituents such as bicarbonates, nitrate, and sulfate were found to react with the Fe0.  Both nitrate and sulfate were reduced within or in the influence zone of the Fe0 with a low redox potential (i.e., low Eh).  An increased anaerobic microbial biomass was also observed within and in the vicinity of the Fe0 barrier, and these microorganisms were at least partially responsible for the reduction of nitrate and sulfate in groundwater. Decreased concentrations of Ca2+ and bicarbonate in groundwater occurred as a result of the formation of minerals such as aragonite (CaCO3) and siderite (FeCO3), which coincided with the Fe0 corrosion and an increased groundwater pH.  A suite of mineral precipitates was identified in the Fe0 barrier system, including amorphous iron oxyhydroxides, goethite, ferrous carbonates and sulfides, aragonite, and green rusts.  These minerals were found to be responsible for the cementation and possibly clogging of Fe0 filings observed in a number of core samples from the barrier.  Significant increases in cementation of the Fe0 occurred between two coring events conducted at ~1 year apart and appeared to correspond to the changes in an apparent decrease in hydraulic gradient and connectivity.  The present study concludes that, while Fe0 may be used as an effective reactive medium for the retention or degradation of many redox-sensitive contaminants, its long-term reactivity and performance could be severely hindered by its reactions with other groundwater constituents; and groundwater flow may be restricted because of the build up of mineral precipitates at the soil/Fe0 interface.  Depending on the site biogeochemical conditions, the rate of Fe0 corrosion may increase; therefore, the life span of the Fe0 barrier could be shorter than predicted in previous studies (~1530 years).