Zero valent iron (ZVI) technologies have been incorporated into remedies at many contaminated groundwater sites since the mid-1990s. ZVI applications began with their use in permeable reactive barriers (PRBs); however, long-term monitoring data from has generally been sparse. This project involved the assessment of long-term performance of ZVI applications both as a source zone treatment and as a barrier treatment for chlorinated volatile organic compounds (VOCs). The overarching objective of this demonstration was to evaluate the long-term effectiveness of ZVI groundwater remedies with respect to reactivity, hydraulic performance, and mechanisms of action.
This project was completed through both desktop review and field investigations. The Final Report details the performance objectives, field activities and data evaluation that were conducted in support of the field study portion of the project. The recommendations in the report consider both phases of the project (desktop and field study).
Field data were collected at Allegany Ballistics Laboratory (ABL), located in Rocket Center, West Virginia, and the former St. Louis Ordnance Plant Operable Unit 1 (OU1), located in St. Louis, Missouri. Geochemical, contaminant concentration, mineralogical, reactivity, and hydraulic data were collected and evaluated for each site to determine the long-term efficacy of the ZVI treatments implemented at these sites and to assess the remaining active degradation mechanisms at each site.
Results of x-ray diffraction (XRD), x-ray absorption near edge structure (XANES) spectroscopy, magnetic susceptibility, magnetic separation, energy dispersive line scans, acidification and hydrogen generation, and resazurin dye testing indicated ZVI remaining at the two study sites is weathered with some passivation due to precipitation of coatings (e.g., calcium carbonate) and transformation of ZVI into less reactive minerals. Remaining iron identified was primarily magnetite and goethite. Despite passivation, evidence of primary and secondary reactivity (through iron sulfide minerals) was noted. Oxidation-reduction (ORP) values as low as -400 millivolts (mV) or lower were observed in some locations. Dye testing indicated higher reduction potential for treated area core material relative to background. Notable geochemical changes were observed across the PRB and mixing area including decreases in calcium, magnesium, and alkalinity and increases in methane, ethane, and ethene. A “clean front” was also observed on the immediate downgradient side of the PRB, indicating continued VOC treatment. Next Generation Sequencing (NGS or 16S sequencing) data at the PRB site indicated the presence of Sulfurimonas just downgradient of the wall, but not in other portions of the site, which may be a result of the release of reduced sulfur species from the PRB. At the mixing study site, halorespiring bacteria concentrations were one to three orders of magnitude higher within and downgradient of the mixing area indicating enhancement of biological degradation processes. Hydraulic evaluations at two sites indicated that mineral precipitation is not impacting hydraulic conductivity or remedy performance. Evaluating cost of implementation of ZVI remedies was not an objective of this project but has been evaluated in multiple previous studies.
This study demonstrated continued performance of ZVI remedies many years following treatment. The project identified best practices associated with pre-remedy selection, remedy implementation, performance monitoring, and optimization.