Pilots to enhance trichloroethene reductive dechlorination and ferrous sulfide abiotic transformation


Two field pilots are being conducted at Altus Air Force Base (AFB), Oklahoma in areas with high levels of sulfate and iron. An edible oil emulsion was injected into these pilots to promote biological reductive dechlorination of trichloroethene (TCE) and to stimulate ferric sulfide and ferric disulfide production and the abiotic reaction with TCE. Within the injection zone of the larger SS-17 pilot, TCE declined from 9.9 mM to <0.043 mM after 13 months with decreases in cis-1,2-dichloroethene (cDCE) [26%] and sulfate (99%), and increases in vinyl chloride (VC) to 0.75 mM and methane to 16.3 mg/L. In a well 7.6 m downgradient, TCE declined from 12.6 mM to <0.043 mM with decreases in cDCE (92%), and sulfate (42% to 247 mg/L). Increases in VC to 28 mM, ethene to 18.2 mM, and methane to 15.2 mg/L were observed at 13 months in this well. Although ferric sulfide and ferric disulfide were produced in the treatment zone by microbial activity, acetylene (an abiotic reaction product from ferric sulfide and ferric disulfide) was not detected at appreciable concentrations. Biological reductive dechlorination processes appeared to predominate in this area. A second smaller pilot was conducted in the OU1 plume with 1,700 mg/L of sulfate, but with no evidence for the presence of dechlorinators. In one injection well, TCE decreased from 43 mM to 0.33 mM after 13 months, cDCE increased to 7.1 mM, and little VC was detected. Both ethene and ethane were detected at high concentrations after 5 months, but then declined to trace levels at 13 months. Although acetylene was not detected, the abiotic pathway for TCE removal appeared to be favored because of the low levels of cDCE and VC that were produced. The results from these field pilots demonstrate that dechlorination of TCE can be stimulated by biological and abiotic process in areas with high sulfate and iron levels.

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