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

Site Name:

Castle Airport and Various Sites

Location:

Merced, CA and Various Locations

Period of
Operation:

March 1998 - October 1998 (Castle Airport); varying times for other locations

Cleanup
Type:

Field demonstration

Technology:
Natural Pressure-Driven Passive Bioventing
- Uses the force generated by normal daily fluctuations in atmospheric conditions for injecting air into the subsurface; primary advantage over conventional bioventing is that no electrical blower is needed
- 15 DoD site across the country were screened as possible demonstration sites for passive bioventing; screening criteria included evaluating suitability of lithology/stratigraphy, depth to groundwater, and natural air flow rates, with a minimum criteria for air flow into existing vent wells of at least 1 cfm; report includes summary of information by site and a description of selection process
- Report focuses on Castle Airport, which was selected for the demonstration
- One vent well - 4-in inside diameter, PVC casing, screened between 25 and 85 ft bgs, with three isolated 10-foot screened sections to evaluate airflow rates in three different lithologic zones
- 8 vapor monitoring points, installed at radial distances of 4, 8, 12, and 16 ft
- 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 cubic ft per day and averaged 3,400 cubic feet per day; peak airflow rates ranged from 5.1 to 15 cfm
- During the 6-month demonstration, six tests conducted to evaluate the technology, including establishing radius of influence and in situ respiration; conditions were varied, such as vent well open or closed

Cleanup Authority:
Not identified

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

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 12 mg/kg benzene, 80 mg/kg toluene, 40 mg/kg ethylbenzene, and 180 mg/kg total xylenes

Waste Source:
Spills and leaks of jet fuels and gasoline

Type/Quantity of Media Treated:
Soil
- Three main layers - upper 20 to 25 ft of subsurface comprised of silty sands/sand; underlain by sand to 35 ft; underlain by sand/silty sand
- Air permeability of sands below 25 ft, ranged from 38 to 200 darcies
- 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 9,300 cubic feet per day and averaged 3,400 cubic feet per day
- 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
- A cost comparison between the installation and operation of a full-scale passive bioventing and a
conventional bioventing system at Castle Airport suggests that the passive system would save
approximately $31,300; this cost saving would be significantly greater if electricity were not
already available at the site to operate electric blowers for a conventional bioventing system.

Description:
15 DoD site across the country were screened as possible demonstration sites for passive bioventing; screening criteria included evaluating suitability of lithology/stratigraphy, depth to groundwater, and natural air flow rates, with a minimum criteria for air flow into existing vent wells of at least 1 cfm. 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.