Successful Biostimulation Treatment of TCE DNAPL

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Courtesy of Arcadis

The challenges posed by the presence of DNAPL are rooted in the unpredictable manner in which it spreads and moves through the subsurface, and the fraction of total contaminant mass it represents in an associated groundwater plume via sorbed, residual and drainable phases (typically greater than 99 percent). Under natural conditions, the dissolution of DNAPL is very slow, allowing groundwater plumes to persist for many decades if the dissolution rate cannot be enhanced. As a result of these challenges, the USEPA, Interstate Technology and Regulatory Council (ITRC), and the Department of Defense Strategic Environmental Research and Development Program (SERDP) have devoted significant resources toward evaluating various in-situ methods of DNAPL source depletion. In the studies, in-situ bioremediation techniques exhibited a real advantage, performing more efficiently than other in-situ technologies, and with very little post-remediation rebound. This approach has been the basis for the remediation of a TCE plume DNAPL source zone at Charleston Air Force Base in South Carolina where ARCADIS has been contracted to achieve closure of all environmental issues within the focus area for a firm-fixed price.

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.

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