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Cometabolic Air Sparging at McClellan Air Force Base, OU A, Sacramento, CA

Site Name:

McClellan Air Force Base (AFB), OU A


Sacramento, CA

Period of

Air sparging initiated May 12, 1999; operted 540 days


Field demonstration

Cometabolic Air Sparging
- Demonstration system included sparge wells, SVE wells, multi-level groundwater and soil monitoring points, an air injection system, and SVE blower, propane gas storage and injection system, and an on-site field trailer; performed in two phases - Phase 1 focused on groundwater; Phase 2 on unsaturated zone
- Two test plots used for the demonstration - one to test cometabolic air sparging (injection of air and a gaseous organic growth substrate to promote in situ cometabolic degradation) and the other to serve as a control; the cometabolic test plot received propane and air and was identified as the “active zone”; the “control zone” test plot received air only; the air sparge wells were located 100 ft apart
- Each plot was equipped with an air injection well, air compressor (25 psi and 15 scfm), and six multi-level groundwater and soil gas monitoring points used to monitor the saturated and vadose zones
- Special safety considerations employed to reduce the risk of explosives hazard from propane
- SVE system - one vapor extraction well was installed next to each sparge well, each well was screened to a depth of 90 ft bgs; designed to extract 45 scfm per well; extracted vapor treated using GAC prior to discharge
- After 500 days of operation, propane degradation was not observed in the saturated zone; methane was substituted for propane to stimulate cometabolic degradation

Cleanup Authority:


Lt. Dave Kempisty
Air Force Project Manager
139 Barnes Drive, Suite 2
Tyndall AFR, FL 32403
Telephone: (850) 283-6126
Fax: (850) 283-6064
Dr. Andrea Leeson
Battelle Project Manager
Battelle Memorial Institute
505 King Avenue
Columbus, OH 43201
Telephone: (614) 424-5942
Fax: (614) 424-3667

Halogenated Volatiles
- Target chlorinated aromatic hydrocarbons (CAHs) for the demonstration included TCE, DCE, vinyl chloride, TCA, and lower ethene isomers
- TCE concentrations as high as 500 mg/L in the test area

Waste Source:
Former demolition and scrap material burning and burial pit

Type/Quantity of Media Treated:
Groundwater and soil
- 523 cubic yards treated during the demonstration

Purpose/Significance of Application:
Field demonstration to compare cometabolic and non-co-metabolic air sparging to treat CAHs in groundwater

Regulatory Requirements/Cleanup Goals:
- The purpose of the demonstration was to evaluate the effectiveness of cometabolic air sparging to remove CAHs from groundwater; the technology was also evaluated against the MCLs for the target CAHs

- In the saturated zone, CAH concentrations were reduced to near or below the MCLs after about 200 days of operation; reductions in the active zone were attributed to propane degradation and cometablism; volatilization was observed in the control zone and contributed to CAH removal
- In the vadose zone, after more than 500 days of operation, no CAH cometabolism through propane degradation was observed, indicating that propane-degrading bacteria were not stimulated during the demonstration; a possible reason for the lack of degradation was limited nitrogen in the vadose zone
- Methane was substituted for propane and was rapidly degraded; levels of CAH decreased after the addition of methane, suggesting cometabolic degradation by methanotrophs
- The propane was also degraded following the addition of methane, suggesting that either methane stimulated the direct degradation of the propane or that the propane was degraded cometabolically by the growth of methanotrophs in the vadose zone

Cost Factors:
- The costs for the demonstration were $891,800, or $1,705 per cubic yard based on 523 cubic yards treated
- Projected full-scale costs were $465,500, or $161 per cubic yard based on 2,888 cubic yards treated
- Results of the cost analysis indicated that treatment duration and vadose zone depth had the greatest effect on unit treatment costs

The Environmental Security Technology Certification Program (ESTCP) supported an 18-month field study to investigate use of a cometabolic air sparging (CAS) process at McClellan Air Force Base (AFB). McClellan was placed on the EPA National Priorities List in 1987 and was designated as a SERDP National Test Site in 1993. A demonstration of cometabolic air sparging was conducted to evaluate the effectivness and costs of the technology to remove CAHs from the groundwater. Two test plots were used for the demonstration - one control plot using air injection only and one active plot where air and propane were injected to test cometabolic air sparging.

Results from the demonstration showed that, after 200 days of operation, CAHs in the saturated zone were reduced to near or below the MCLs. However, after 500 days of operation, propane degradation had not been observed in the vadose zone and there was no evidence of CAH cometabolism. Methane was substituted for the propane and was rapidly degraded, along with the propane, and levels of CAHs were further reduced. These results suggested that a possible lack of nitrogen in the vadose zone limited propane degradation and that the addition of methane either stimulated direct propane degradation or that propane was degraded cometabolically by methanotrophs. The overall performance showed that cometabolic air sparging using propane as the growth substrate was effective in the saturated zone but did not meet expectations on the vadose zone.