Case Studies - SC001 - Feroxsm
Cr+6 FEROXSM APPLICATION - FEDERAL FACILITY - CHARLESTON, SC
Project SummaryDuring late 2001 ARS was awarded a contract to apply its Feroxsm technology for source treatment at Charleston Naval Yard in South Carolina. The project focused on the source reduction of hexavalent chromium (Cr+6) to trivalent chromium (Cr+3). Levels of Cr+6 as high as 31,000 ppb were detected in the groundwater at the site.
Feroxsm was applied at depths ranging from approximately 15 to 31 ft bgs in a silty sand aquifer. The project required UIC permits from the SC DHEC.
Bench-scale treatability tests were conducted on the reduction of hexavalent chromium in groundwater and soils using ARS' proprietary ZVI powder were performed. The goal of the study was to derive field design parameters and dosage levels to reduce (Cr +6) contamination to its less toxic and soluble tri-valent (Cr+3) state. The studies evaluated several dosages of Feroxsm powder in reducing the Cr +6 to Cr+3 within 30 days. Groundwater and soil from the site had an initial lime-green color due to the high concentrations of (Cr +6). As Cr+6 was reduced to Cr+3 and precipitated out, the lime-green color of the water faded to clear.
Based on bench-scale results, it was decided to inject approximately 38,000 pounds of Feroxsm powder into the treatment zone. This is equivalent to an approximate 4000:1 iron-to-contaminant mass ratio. The slurry was injected through 15 separate injection points targeting various intervals from 11 to 31 feet below ground surface. The atomized powder slurry was injected using pressures ranging from 60 to 150 psig. During the injections, pressure response was measured in nearby monitoring wells screened at various depths. Pressure readings indicated that injections were influencing these wells at distances greater than 50 feet in some instances. Varying pressure readings made during injections at different depths indicated that powder was propagating horizontally outward from the injection points. Seven of the fifteen injection points were located within an occupied historic building, which made the project both logistically and technically challenging. Computer modeling of the building structure was first performed to ensure that the building was structurally sound when subjected to even the slightest of column movements. During injections, structural columns where closely monitored to ensure that the movement criteria, as determined from the computer modeling, was not breached. In addition, crack gauges were installed on the outside of the building to quantify any differential movement. The field operations were scheduled in a manner as to not disrupt the day-to-day operations of the tenant within the building. Working closely with the tenant, injection points were situated between equipment inside the building. The project was completed safely and on schedule, requiring a total of 18 workdays.
Post-injection groundwater sampling collected within the first five months of the application show Cr+6 concentration levels reduced by 71% over the next 20-month period, Cr+6 was observed to have been further reduced by more than 90%.
In addition, significant production of hydrogen gas was detected at levels as high as 73,000 nM, adding a secondary benefit of stimulating microbial activities.
The following figures show the reduction of the Cr+6 plume as a result of the Feroxsm injection.
Baseline
Post-Injection (20-months)
The following graphs show the changes in Cr+6 and total chromium concentrating as well as the generation of hydrogen gas
Chromium and Hydrogen Concentrations in Shallow Hot Spot Well
Chromium and Hydrogen Concentrations in Deep Hot Spot Well
Chromium and Hydrogen Concentrations in Secondary Hot Spot Well only post-injection data available
Case Studies