Concentrations of TCE detected in groundwater at Charleston Air Force Base have historically ranged up to 296,000 μg/L (just under 30 percent of saturation). Anaerobic in-situ reactive zone (IRZ) technology was selected for the remedy. Based on evidence of ongoing natural degradation, the remedy was designed to stimulate the natural microbial populations to access and treat the non-aqueous phase TCE mass through three pathways: 1) the production of more soluble intermediates, 2) an increase in concentration gradients at the water/NAPL interface, and 3) potential contributions in the form of donor and microbial-related surfactant effects. Initial implementation involved 21 reusable injection points for the delivery of a degradable organic substrate (carbohydrate-whey mixture).
The use of in-situ bioremediation at this site reduced TCE from baseline concentrations as high as 110,000 μg/L in May 2003 to less than 100 μg/L in certain areas of the plume by June 2005 (greater than 99.9% reduction), shrinking the total plume footprint by 30 percent. The decrease in TCE concentrations was accompanied by the sequential increase and decrease of cis-1,2-DCE and VC concentrations. Within 24 months, TCE degradation was successfully driven to completion, creating meaningful molar concentrations of ethene at nearly all of the monitoring wells in the treatment area, ranging from concentrations above 1,000 ug/L in many locations to as high as 11,000 ug/L. A strong correlation was observed between the onset of complete dechlorination (ethene production), the onset of ethanogenesis, and the availability of degradable organic carbon. The injection program will be continued with the goal of reducing contaminant concentrations to within the range of the targeted cleanup goals.