Natural Pressure-Driven Passive Bioremediation at Castle Airport, Merced, CA

Site Name:

Castle Airport

Location:

Merced, CA

Period of
Operation:

March 1998 - October 1998

Cleanup
Type:

Field demonstration

Technology:
Natural Pressure-Driven Passive Bioventing
- Uses the force generated by normal daily fluctuations in atmospheric conditions to replace a powered blower for injecting air into the subsurface
- A single vent well was installed to a depth of 65 feet with three isolated 10-foot screened sections to evaluate airflow rates in three different lithologic zones
- The radius of influence of the bioventing well was estimated at 42 feet after seven weeks
- The daily airflow rates ranged from 27 to 9300 ft3 per day and averaged 3,400 ft3 per day

Cleanup Authority:
Not identified

Site Contact:
Sherrie Larson
Project Manager
and Principal Investigator
Phone: (805) 982-4826
E-mail: larsonsl@nfesc.navy.mil

Treatment Technology Contact:
Michael B. Phelps
Parsons Engineering Science, Inc.
Phone: (510) 891-9085
E-mail: michael_phelps@parsons.com

Contaminants:
Total Petroleum Hydrocarbons (TPH), BTEX
- TPH concentrations in soil as high as 28,000 mg/kg
- BTEX concentrations in soil as high as 279 mg/kg

Waste Source:
Spills and leaks of jet fuels and gasoline

Type/Quantity of Media Treated:
Soil
- Upper 20 ft of subsurface comprised of silty sand overlying a laterally continuous silt layer between 20 and 25 ft
- Soil moisture - average about 6%

Purpose/Significance of Application:
Field demonstration of natural pressure-driven passive bioventing of petroleum-contaminated soil

Regulatory Requirements/Cleanup Goals:
- Goals of the demonstration included achieving consistent air flow rate to vadose zone greater than 1 cfm and 1,200 cubic feet per day and a radius of influence greater than 10 feet
- No specific cleanup levels were identified for the demonstration

Results:
- Air supply during demonstration consistently exceeded goals of 1 cfm and 1,200 cubic feet per day; ranged from 27 to 9300 cubic feet per day (cfd), and averaged 3,400 cfd
- The radius of influence was estimated to be 42 feet after seven weeks, exceeding the goal of 10 feet.
- As areas near the well are remediated and the oxygen demand is satisfied, the predicted radius of influence would be expected to be 85 feet, comparing favorably to conventional bioventing radius of influence of 110 feet

Cost Factors:
- The estimated cost of a full-scale passive bioventing system was $1.93 per cubic yard of soil treated. The cost of conventional bioventing was estimated at $2.09 per cubic yard
- Passive bioventing would require the use of 1.5 times as many wells as conventional bioventing, and a treatment time of 4 years instead of 3 years at the Castle Airport Site, however an overall reduction in costs would be achieved by eliminating the capital cost of blowers and the O&M cost of powering the blowers.

Description:
A demonstration of natural pressure-driven passive bioventing was performed at Castle Airport in Merced, CA The petroleum oil and lubricants fuel farm area was the bulk fuel storage and distribution facility for the former AFB located at the site. Soil and groundwater contamination resulted from leaking underground storage tanks and fuel distribution lines and surface spills. The Department of Defense Environmental Security Technology Certification Program (ESTCP), the Air Force Research Laboratory, and Naval Facilities Engineering Service Center, and the Air Force Center for Environmental Excellence (AFCEE) cooperated in conducting the demonstration.

Natural pressure-driven passive bioventing is similar to conventional bioventing with the exception that it uses the force generated by normal daily fluctuations in atmospheric conditions to replace a powered blower for injecting air into the subsurface. During the demonstration, six tests of natural pressure-driven passive bioventing were performed over a six month period. A single well installed to a depth of 65 feet achieved an average daily air flow rate to the vadose zone of 3,400 cubic feet and a radius of influence of 42 feet. As areas near the well are remediated and the oxygen demand is satisfied, the predicted radius of influence would be expected to be 85 feet, comparing favorably to conventional bioventing radius of influence of 110 feet. The projected cost of a full-scale passive bioventing system was $1.93 per cubic yard of soil treated, compared to $2.09 per cubic yard for conventional bioventing.