Natural Attenuation of Fuel Hydrocarbons at Multiple Air Force Sites

Site Name:

Multiple AF Sites

Location:

Multiple locations throughout U.S.

Period of
Operation:

Field demonstrations conducted between July 1993 and December 1998
Periods of operation were not provided for each site

Cleanup
Type:

Field Demonstration

Technology:
Monitored Natural Attenuation (MNA)
- Intrinsic bioremediation including sulfate reduction, methanogenesis, denitrification, and aerobic oxidation
- During the demonstrations, groundwater was sampled for contaminant concentrations, and other parameters including pH, temperature, conductivity, oxidation/reduction potential, dissolved oxygen, nitrate, nitrite, sulfate, sulfide, ferrous iron, total iron, methane, carbon dioxide, and alkalinity; geochemical trends and biodegradation rates also were evaluated

Cleanup Authority:
Not provided

Technical Contact:
Parsons Engineering Science, Inc.
1700 Broadway, Suite 900
Denver, CO 80290
Management Contact:
Jeff Cornell
Air Force Center for Environmental Excellence
Technology Transfer Division
Brooks AFB, TX 78235
E-mail: jeff.cornell@hqafcee.brooks.af.mil

Contaminants:
BTEX, Total Petroleum Hydrocarbons (TPH)
- BTEX concentrations were measured as high as 46,300 ug/L (benzene), 57,000 ug/L (toluene), 4,410 ug/L (ethylbenzene), and 68,000 ug/L (xylenes)
- TPH concentrations were measured as high as 120,000 mg/L

Waste Source:
Not provided

Type/Quantity of Media Treated:
Groundwater
- Depths to groundwater ranged from 0 to 48 ft bgs
- Plume areas ranged from 0.3 to 60 acres
- Average groundwater temperatures ranged from 5.5 to 26.9┬░C
- Aquifer matrices ranged from silty clays to coarse sand and gravel

Purpose/Significance of Application:
Field demonstrations of monitored natural attenuation for fuel hydrocarbons in groundwater at multiple Air Force sites

Regulatory Requirements/Cleanup Goals:
- Goals of the demonstration included evaluating the potential for fuel hydrocarbons to be naturally attenuated, the type of biodegradation processes taking place, and the effect on plume size

Results:
- Fuel hydrocarbons were undergoing natural attenuation at all 42 Air Force sites
- Key biodegradation processes were identified, in decreasing order of assimilative capacity, as sulfate reduction, methanogenesis, iron reduction, denitrification, and aerobic oxidation; the total BTEX assimilative capacity of groundwater averaged 64 mg/L
- With respect to plume size, 35 sites had plumes that appeared to be stable, 6 sites had plumes that were receding, and 1 site had a plume that was expanding
- For sites with measurable free-phase product, the average predicted time frame for dissolved BTEX to naturally attenuate to below cleanup standards was estimated at approximately 30 years; the addition of engineered source reduction reduced the estimate to 20 years or less, depending on type of source reduction used
- Regulatory authorities have approved the partial or full use of MNA with institutional controls at 17 of the 42 sites

Cost Factors:
- The average cost per site in this demonstration for completing site characterization using existing monitoring wells and a Geoprobe®, laboratory and data analysis, fate and transport modeling, and reporting was $125,000
- A recommended long-term monitoring program for MNA, including an average network of 11 wells, has a projected average annual cost of $192,000

Description:
In June 1993, the Air Force Center for Environmental Excellence (AFCEE), in cooperation with EPA/ORD, began an initiative to evaluate the effectiveness of MNA for remediation of groundwater contaminated with fuel hydrocarbons (also, refer to separate report about use of MNA for groundwater contaminated with chlorinated solvents). From 1993 to 1998, field demonstrations of MNA were conducted at 42 Air Force sites throughout the country. This included installing additional sampling points at the sites and collecting and evaluating data over a period of time.

The sites were evaluated for evidence that fuel hydrocarbons were being naturally attenuated, and to identify the degree and rate of attenuation. Data showed that fuel hydrocarbons were undergoing natural attenuation at all 42 Air Force sites, and that the degree and rate of intrinsic bioremediation was site-specific, involving processes such as sulfate reduction, methanogenesis, iron reduction, denitrification, and aerobic oxidation. The effect on plume size varied, with the plume stable at 35 sites, receding at 6 sites, and expanding at one site. For sites with measurable free-phase product, the average predicted time frame for dissolved BTEX to naturally attenuate to below cleanup standards was estimated at approximately 30 years; the addition of engineered source reduction reduced the estimate to 20 years or less, depending on type of source reduction used.