Natural Attenuation of Chlorinated Solvents at Multiple Air Force Sites

Site Name:

Multiple AF Sites


Multiple locations throughout U.S.

Period of

- Field demonstrations conducted between July 1993 and August 1999
- Periods of operation were not provided for each site


Field Demonstration

Monitored Natural Attenuation (MNA)
- 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, and dissolved hydrogen; geochemical trends and biodegradation rates also were evaluated
- Four types of plume behavior were studies - Type 1 (anaerobic groundwater conditions with anthropogenic carbon); Type 2 (anaerobic groundwater conditions with native carbon); Type 3 (aerobic groundwater conditions with anthropogenic and/or native carbon); and mixed (different portions of the groundwater plume exhibiting different types of behavior)

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

Chlorinated Solvents
- Chlorinated aliphatic hydrocarbons, including TCE, cis- and trans-1,2-DCE, 1,1-DCE, and VC
- TCE was the most pervasive contaminant, followed by cis-1,2-DCE - found at 13 of 14 sites

Waste Source:
Not provided

Type/Quantity of Media Treated:
- Depths to groundwater ranged from 0 to 60 ft bgs
- Plume areas ranged from 1.6 to 210 acres
- Average groundwater temperatures ranged from 9.1 to 25.6┬░C
- Aquifer matrices ranged from clays to coarse sand and gravel

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

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

- Chlorinated solvents were undergoing natural attenuation at all 14 Air Force sites
- The degree and rate of intrinsic bioremediation was highly site-specific, and depended on the bio- and geo-chemistries of groundwater at the sites
- 11 of the sites exhibited mixed behavior, with nine exhibiting Type 1 coupled with either Type 2 or Type 3, and two sites exhibiting Type 2 behavior coupled with Type 3. Three of the sites exhibited primarily Type 1 behavior.
- With respect to plume size, three sites had expanding plumes, six sites had plumes that were either stable or expanding slowly, and five sites had plumes that were either stable or receding

Cost Factors:
- The average cost per site in this demonstration for completing supplemental site characterization using existing monitoring wells and a Geoprobe®, laboratory and data analysis, fate and transport modeling, and reporting was $122,000
- A recommended long-term monitoring program for MNA, including an average network of 17 wells and 3 surface water stations, has a projected average annual monitoring cost of $22,800

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 chlorinated solvents (also, refer to separate report about use of MNA for groundwater contaminated with fuel hydrocarbons). From 1993 to 1999, field demonstrations of MNA were conducted at 14 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 chlorinated solvents were being naturally attenuated, and to identify the degree and rate of attenuation. Data showed that chlorinated solvents were undergoing natural attenuation at all 14 Air Force sites, and that the degree and rate of intrinsic bioremediation was highly site-specific, and depended on the bio- and geo-chemistries of groundwater at the sites. The effect on plume size varied, with the plume expanding at three sites, remaining stable or expanding slowly at six sites, and remaining stable or receding at five sites. The study concluded that use of MNA for remediation of chlorinated solvents is highly site-specific, and that quantifying intrinsic bioremediation is difficult. The study states that engineered alternatives, such as source reduction, also should be evaluated to determine how they would limit plume migration and/or accelerate attainment of target cleanup levels.