Bioventing at Multiple Air Force Test Sites

Site Name:

Multiple Air Force Test Sites


Multiple locations throughout U.S.

Period of

- Overall program: April 1992 to December 1995
- Each test: varied by site; typical operation about one year


Pilot scale


Not Provided

In Situ Bioventing
- Specific configuration varied by site for number, depth of vent (air injection) wells, number of monitoring wells, and blower size and type
- Typical configuration included vent wells (1 to 9 per site; depths -7 to 233 feet below ground surface); vapor monitoring wells (1 to 6 per site); blower (1 to 5 horsepower; either rotary vane or regenerative)
- Horizontal vent wells used at five sites

Cleanup Authority:
Sites are being addressed under CERCLA, RCRA, and state underground storage tank programs

Air Force Contact:
Lt. Col. Ross N. Miller
U.S. Air Force Center
for Environmental Excellence
Brooks, AFB
(210) 536- 4331

Benzene, Toluene, Ethylbenzene, and Xylenes (BTEX) and Total Petroleum Hydrocarbons (TPH)
- Data provided for average initial concentrations of BTEX and TPH in soil and soil gas (based on 328 samples from 100 test sites)
- Average BTEX constituent concentrations in soil (soil gas) - benzene - 106 mg/kg (88 ppmv); toluene - 250 mg/kg (13 ppmv); ethylbenzene - 276 mg/kg (64 ppmv); xylenes - 1,001 mg/kg (46 ppmv)
- Average TPH concentration in soil - 3,301 mg/kg; Total Volatile Hydrocarbons (TVH) in soil gas - 22,555 ppmv

Waste Source:
Leaks from underground storage tanks, including tanks used to store gasoline, JP-4, diesel fuel, heating oils, and waste oils

Type/Quantity of Media Treated:
- Quantities treated at each test site ranged from 200 to more than 270,000 cubic yards; based on radius of influence of vent well(s) at each site
- Soil gas permeability - about 20% of the test sites contained greater than 50% silt and clay fractions; the radius of oxygen influence from a single vent well was equal to or greater than the contaminated area at about 50% of the test sites
- Soil pH - pH ranged from 5 to 9 at the majority of sites
- Soil moisture - ranged from 5% to 20% at the majority of sites
- Total Kjeldahl nitrogen - ranged from <50 to 200 mg/kg at the majority of sites
- Soil temperature - not measured at each site; soil temperatures between 0°C and 25°C observed at test sites

Purpose/Significance of Application:
Major initiative to demonstrate the feasibility of bioventing to remediate petroleum-contaminated soil at 145 Air Force sites

Regulatory Requirements/Cleanup Goals:
- The objectives of the bioventing initiative included documenting the ability of bioventing to remediate petroleum-contaminated soils in a variety of conditions; and obtaining a significant set of bioventing data
- No specific cleanup goals were identified for the test sites

Results from data collected after one year of bioventing (328 sampling locations at 100 sites):
- Average reduction in BTEX concentrations of 97% in soil and 85% in soil gas; average TPH concentrations reduced by 24% in soil; average TVH concentrations reduced by 90% in soil gas
- Biodegradation rates measured at the test sites - at start of test ranged from <300 mg/kg/yr to >6000 mg/kg/yr; average 1,200 mg/kg/yr
- Average biodegradation rate decreased to 700 mg/kg/yr, as a result of the decreasing bioavailability of hydrocarbons over time
- Bioventing was effective in a variety of climate conditions, ranging from 0øC in Alaska to 25øC in California; higher biodegradation rates were observed on warmer soils
- A combination of high moisture content and fine-grained soils made bioventing infeasible at only two of the 145 test sites

Cost Factors:
- The average actual cost for design, installation, and 1-year of operation of pilot-scale bioventing at a single vent well site was $60,000
- The projected cost of full-scale bioventing generally ranges from $10 to $60 per cubic yard of soil treated
- At sites with more than 10,000 cubic yards of contaminated soil, costs are less than $10 per cubic yard; at sites with less than 500 cubic yards of contaminated soil, costs are greater than $60 per cubic yard
- Projected costs for a typical full-scale bioventing system (defined as an Air Force site with 5,000 cubic yards of soil contaminated with 3,000 mg/kg of JP-4 fuel; bioventing system consisting of four vent wells at a depth of 15 feet, operated for two years) - $92,300, including $27,000 for pilot testing and $27,500 for full-scale construction

In April 1992, the Air Force Center for Environmental Excellence (AFCEE), in cooperation with the Air Force Armstrong Laboratory and the U.S. Environmental Protection Agency, began an initiative to demonstrate the feasibility of using bioventing to remediate petroleum contaminated soils in a variety of climatic, soil, and contaminant conditions. Between April 1992 and December 1995, initial bioventing tests were conducted at 145 Air Force sites throughout the country.

The pilot scale systems included vent (air injection) wells, monitoring wells, and blowers. The specific configuration varied by test site, and horizontal vent wells were used at five of the sites. Concentrations of BTEX and TPH were measured in soil and soil gas from over 300 sampling locations at 100 sites at the start of bioventing operations and after one year of operation. Results showed that bioventing was effective in reducing concentrations of BTEX and TPH in soil and soil gas in a variety of site conditions. Soil BTEX and TPH concentrations were reduced by 97% and 24%, respectively. Soil gas BTEX and TVH concentrations were reduced by 85% and 90%, respectively. According to the Air Force, the reductions in BTEX are sufficient to meet the most conservative EPA risk-based cleanup criteria for soils, and regulatory acceptance of this technology was obtained in 38 states and the 10 EPA regions. The pilot-scale systems have been converted to full-scale systems at about half of the test sites, saving the Air Force an estimated $5 to $10 million in design and construction costs.