Field evaluation of in situ chemical oxidation through vertical well-to-well recirculation of NaMnO4

A field pilot test was conducted at a U. S. Department of Energy site to evaluate in situ chemical oxidation of trichloroethene (TCE) in a silty-sandy gravel aquifer located at 26 to 30 feet (8 to 9 m) below ground surface. A vertical well-to-well recirculation system was designed and implemented by placing four corner extraction wells in a square grid, with each well 45 feet (13.7 m) from a center injection well. Ground water was extracted from the four perimeter wells, combined aboveground in a piping network, and then amended inline with sodium permanganate (NaMnO4) before reinjection into the aquifer through a center injection well. During the pilot test, oxidant amended ground water (similar to250 mg/L NaMnO4) was injected at 18 gpm (68.1 L/m) for 10 days, during which time similar to354 pounds (160.6 kg) of NaMnO4 were delivered and approximately 240,660 gallons (911 kL) of ground water were recirculated and treated. NaMnO4 injection using vertical wells in a five-spot pattern was capable of providing sufficient hydraulic control to deliver oxidant throughout the permeable zones of the 4100 ft(2) (380 m(2)) test area within three days. TCE concentrations were reduced, from similar to2,000 mug/L to <10 mug/L, throughout all but the lower permeable eastern edge of the test region, indicating an apparent reduction in contaminant levels of similar to92% within three days and similar to97% within 10 days (two hours after the end of active recirculation). One month after system shut down, residual oxidant concentrations in the ground water had declined to <1 mg/L, while a gradual increase in TCE concentrations was observed. Oxidant consumption over time is expected due to a combination of factors, including oxidation of the natural organic material present, oxidation of TCE that diffused out from the finer-grained and less permeable zones, and advection of TCE into the test area from the upgradient plume. No adverse effects to system toxicity, generation of reaction intermediates and products (e.g., chlorinated organic acids or partially degraded chlorocarbons), or reduction in formation permeability were observed.