Monitored Natural Attenuation at Kelly Air Force Base, Former Building 2093 Gas Station, Texas

Site Name:

Kelly Air Force Base


Kelly AFB, Texas

Period of

July 1997 to July 1998


UST cleanup


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

Monitored Natural Attenuation
- Monitoring network not described

Cleanup Authority:
Texas Natural Resource Conservation Commission Petroleum Storage Tank Division

Site Contact:
Jerry Arriaga
301 Tinker Dr., Suite 2
Bldg. 301
Kelly AFB, TX 78241
(210) 925-1819

Air Force Contact:
Jim Gonzales
3207 North Rd., Building 532
Brooks AFB, TX 78235-5363
(210) 536-4324
State Contact:
Antonio Pena
P.O. Box 13087
Austin, TX, 78711-3087
(512) 239-2200

Gasoline constituents
- BTEX concentrations in groundwater measured as high as 2,807 ug/L in November 1997

Waste Source:
Leaking gasoline USTs and associated piping

Type/Quantity of Media Treated:
Soil, groundwater, and soil gas
- Source area plus dissolved plume covers 1.5 acres
- The site is underlain by silty clay; with a distinct clay unit from 35 to 40 ft bgs
- Groundwater occurs primarily in silt and possibly caliche seams that produce only small amounts of water; static groundwater levels range from 5 to 25 feet bgs, depending on location and season
- Hydraulic conductivity of the silty clay unit is 0.2 to 0.5 ft/day based on slug tests, and the estimated horizontal groundwater flow velocity is 31 ft/year

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

Regulatory Requirements/Cleanup Goals:
- TNRCC Plan A target concentrations for Category II aquifers, and TNRCC target concentrations for construction worker exposure are the cleanup goals for affected groundwater

Based on a Tier 1 screening, only the Plan A concentration for benzene of 0.0294 mg/L was exceeded, and benzene in groundwater and soil was identified as a contaminant of potential concern
Fate and transport modeling using the analytical code BIOSCREEN indicated that the maximum migration distance of dissolved benzene from the source area will be approximately 300 ft, and that dissolved benzene concentrations will be below groundwater quality standards within 10 years
Results of groundwater sampling events indicated that the dissolved contaminant plume is not increasing in areal extent, and that natural attenuation indicator parameters exhibit trends associated with a plume that is being naturally degraded.
The site was identified as a candidate for immediate closure according to TNRCC guidance
The Air Force will restrict use of the shallow groundwater at the site until all dissolved benzene concentrations decrease below TNRCC Plan A Category II criterion of 0.0294 mg/L
- Maximum-detected concentrations of BTEX in soil gas were compared to the chemical-specific OSHA 8-hour time-weighted average permissible exposure limits (PELs), and there were no exceedences

Cost Factors:
Not provided

As a result of UST integrity testing in 1989, the former Building 2093 Gas Station at Kelly Air Force Base, in Texas, was found to be leaking, and the UST and associated piping were removed in 1991. Site investigations found BTEX contamination in the groundwater. A 1-year-long bioventing pilot test was concluded in January 1995; the test results indicated that site soils were not sufficiently permeable to enable use of this in situ source reduction technique. Later in 1995, the dispensing islands and remaining below-grade piping were removed, and 2,750 cubic yards of soil in the area of the former tank pad and dispensing islands were excavated. Based on a RBCA analysis, the TNRCC issued a no-further-action memorandum closing the site based on plume stability, the occurrence of natural attenuation of fuel residuals, and the conclusion that site contamination will not pose a significant risk to potential receptors.