Monitored Natural Attenuation at Keesler Air Force Base, Mississippi

Site Name:

Keesler Air Force Base Service Station


Biloxi, Mississippi

Period of

September 1997 to April 1999


UST cleanup


John Hicks
Parsons Engineering Science, Inc.
1700 Broadway, Suite 900
Denver, CO 80290
(303) 831-8100

Monitored Natural Attenuation
- Bioventing and density-driven convection in-well aeration were used previously as source control measures
- Monitoring of 9 groundwater wells planned for five years
- Samples will be analyzed for aromatic volatile organics and geochemical parameters

Cleanup Authority:
EPA Region 4 and Mississippi DEQ

Air Force Contact:
Jim Gonzales
3207 North Rd., Building 532
Brooks AFB, TX 78235-5363
(210) 536-4324

State Contact:
Bob Merrill
Mississippi DEQ
P.O. Box 10385
Jackson, MS 39289-0385
(601) 961-5171
Site Contact:
Lisa Noble
508 L Street
Keesler AFB, MS 39534-2115

EPA Contact:
Robert Pope
USEPA Region 4
61 Forsyth St., SW
Atlanta, GA 30303-3104
(404) 562-8506

BTEX, Lead
- Soil concentrations measured as high as 166 mg/kg for BTEX and 8.7 mg/kg for lead
- Groundwater concentrations measured as high as 22,400 ug/L for BTEX and 21 ug/L for lead

Waste Source:
Gasoline USTs and associated piping

Type/Quantity of Media Treated:
Soil, groundwater, and soil gas
- Source area plus dissolved plume covers approximately 4.0 acres
Fine- to medium-grained sand to 20 ft bgs, underlain by a clay layer of unknown thickness
Groundwater present at 5 to 9 ft bgs
Average hydraulic conductivity of sand zone is 40 ft/day
Calculated horizontal groundwater flow velocity is 0.8 ft/day

Purpose/Significance of Application:
Monitored natural attenuation for a gasoline contaminated site

Regulatory Requirements/Cleanup Goals:
- Cleanup levels for BTEX was 100 ppm for soil and 18 ppm for groundwater
Risk-based screening levels for lead was identified as 400 ppm in soil and 15 ug/L in groundwater
OSHA PELs were used as screening levels for BTEX in soil gas

In February 1998, the only contaminant in soil to exceed the cleanup levels was BTEX (1 sample at 166 mg/kg); BTEX (1 sample at 22.4 mg/L) and lead (3 samples - 21, 21 and 16 ug/L) exceeded the cleanup levels in groundwater. Only lead in groundwater was identified as a chemical of potential concern for this site
Data from 1988 to 1998 indicated substantial oscillation in dissolved BTEX concentrations at the plume core since May 1993, but that the total BTEX plume appears to have been relatively stable

Cost Factors:
The estimated O&M cost for long-term monitoring was identified as $15,000 per event

In 1987, 10 USTs were removed from the Keesler Air Force Base, in Biloxi, Mississippi. During the removals, there was evidence that one or more of the tanks had leaked, and site investigations found gasoline components in the soil and groundwater, including BTEX and lead. A bioventing system was installed in 1993 and operated for three years. A density-driven convection (DDC) in-well aeration system was installed in 1996 and operated at least through February 1998. Based on a RBCA analysis, the recommended final remedial action was monitored natural attenuation. The recommendation was based on the finding that the site contamination does not currently (and will not in the future) pose a significant risk to potential receptors, the dissolved plume is stable and degrading, and institutional controls can be maintained with a high level of confidence. The RBCA analysis showed that concentrations of target analytes in all sampled media do not exceed applicable MDEQ RBSLs or OSHA PELs, and that detected concentrations of total lead in groundwater do not pose a risk to potential receptors.

Geochemical data indicated that biodegradation of fuel hydrocarbons is occurring at the site, primarily via the anaerobic processes of sulfate reduction, nitrogen fixation, and methanogenesis. Previous and current source removal efforts have reduced hydrocarbon concentrations in vadose zone and saturated zone soils, and the current system does not have an adverse effect on the natural attenuation processes at the site. A long-term monitoring plan was negotiated with the MDEQ and USEPA Region 4 that included monitoring of nine wells for five years. Monitoring will occur quarterly for the first year and annually for the second through fifth years. The purpose of the monitoring is to verify the effectiveness of naturally-occurring remediation processes at limiting plume migration and reducing dissolved contaminant concentrations.