The RT3D software, one of the computer codes used in the Battelle Chlorinated Solvent Design Service, was used in the case study described here to evaluate bioremediation options for a contaminated plume located on the U.S. Department of Energy's Hanford site.  This case study was a Remediation Alternatives Evaluation, in which the bioremediation team determined if accelerated in situ bioremediation is cost effective when compared to other technologies for remediation of the contamination in the plume.  Battelle provides remediation alternative evaluations and other bioremediation services to clients around the country.

An estimated 20,000 to 70,000 kg of carbon tetrachloride (CT) is present in a 10-km² plume at the U.S. Department of Energy's Hanford site, located in southeastern Washington State.  The highest groundwater concentration of CT is approximately 7 mg/L.  Nitrate is a co-contaminant throughout most of the plume.  The aquifer is a heterogeneous mixture of sand, gravel, and silt with layered variations in permeability created by carbonate or silica based cementation and differences in silt content.  Regional-scale modeling of the plume indicates that remediation of contamination within the 100 µg/L contour may be sufficient to ensure contamination does not migrate to down-stream receptors.

Use the RT3D computer code to perform a cost/benefit analysis for remediation of CT contamination within 1) the 100 µg/L contour or 2) the 1000 µg/L contour.  Client-specified assumptions included equilibrium partitioning and a homogeneous aquifer.  Two remedial options were evaluated in this study:

• Groundwater extraction followed by air stripping and granular activated carbon adsorption.
• Accelerated in situ bioremediation of CT under denitrification conditions.

Accelerated in situ bioremediation of CT was less effective than pump-and-treat for the large dispersed CT plume at Hanford because volumetric treatment is not feasible at this scale.

Where is accelerated in situ bioremediation of CT cost effective?

• Plumes or portions of plumes small enough for volumetric treatment (maximum diameters in the range of 100 meters).
• Aquifers where plumes exhibit non-equilibrium contaminant partitioning (i.e., caused by preferential flow paths, layering, sorption in soil particles/micropores).
• High concentration (but below toxicity limit) portions of plumes with significant contaminant sorption.

 

For more information, contact:
Michael Truex Program Manager Field Hydrology and Chemistry Group	Battelle 902 Battelle Blvd., K6-96 Richland, WA 99352 USA	509-376-5461 509-372-1704 FAX mj.truex@pnl.gov