Mulch Biowall at Offutt Air Force Base, Nebraska

Site Name:

Offutt Air Force Base



Period of

Completed August 2000


Field demonstration

Mulch Biowall
- Biowall was 100 ft-long, 23 ft-deep, and 1 ft-wide; consisted of shredded mulch (generated on-site using shredded trees and leaves) mixed in a 50:50 ratio with coarse sand
- Biowall was installed using a continuous trenching machine, allowing simultaneous excavation and filling with mulch mixture
- Groundwater flows through the biowall under natural hydrualic gradient to promote contact with the slowly-dissolving organic matter; microbial processes in the subsurface degrade the mulch, generating breakdown products including metabolic acids, that provide secondary electron donors or fermentable substrates for hydrogen generation, the primary electron donor used in reductive dechlorination
- 8 groundwater monitoring wells (2 upgradient, 4 downgradient, and 2 control)

Cleanup Authority:
Not provided


Maj Ivette O'Brien, BSC, Ph.D., CIH
Chief, Technology Transfer
3300 Sidney Brooks
Brooks City-Base TX 78235-5112
Telephone: (210) 536-4329

Air Force Base:
Philip Cork
Offutt AFB
Telephone: (402) 294-7621
Technology Provider:
Carol Aziz
Groundwater Services, Inc.
Telephone: (713) 522-6300

Chlorinated Solvents
- TCE plume extends approximately 3,000 feet
- Concentrations of TCE as high as 2,200 µg/L where the biowall was installed

Waste Source:
Disposal of waste from manufacturing operations

Type/Quantity of Media Treated:

Purpose/Significance of Application:
Use of a mulch biowall to treat groundwater contaminated with chlorinated solvents

Regulatory Requirements/Cleanup Goals:
- No objectives for the field demonstration were identified

- After 31 months, TCE concentrations 20 ft downgradient of the biowall had been reduced by 70 percent
- The ratio of cDCE, a degradation byproduct, to TCE downgradient of the wall increased over 2 orders of magnitude relative to upgradient conditions after 5 months of treatment; this ratio subsequently dropped as cDCE was converted to vinyl chloride, ethene, and ethane; the control plot showed no decrease in TCE concentrations
- Sampling results showed evidence of that reducing conditions had been established including: depressed oxygen concentrations and oxygen-reduction potentials due to the consumption of organic matter and oxygen by aerobic bacteria; nitrate and sulfate levels also declined, while methane production was observed

Cost Factors:
- Costs for installing the biowall was $360/linear ft
- If mulch for applications had not been generated on-site (at no cost), the estimated cost was approximately $20/yd3

Offutt Air Force Base in Nebraska was used for the manufacture of aircraft. Wastes from operations resulted in groundwater at the site becoming contaminated with chlorinated solvents. TCE concentrations in groundwater were found as high as 2,200 µg/L where the biowall was installed, with a TCE plume identified at the site extending 3,000 ft.

A field demonstration of a mulch biowall was conducted at the site. The mulch biowall was installed using a continuous trenching machine to simultaneously excavate the trench and install the biowall materials. Data from the groundwater monitoring wells showed that, after 31 months, TCE concentrations had been reduced by 70 percent, and that reducing conditions had been established. Demonstration findings suggested that mulch biowalls are appropriate at sites with shallow (less than 8 ft) ground water and where the wall extends less than 30 ft below ground surface. Performance data from the demonstration test indicated that the biowall is a low-maintenance, cost-effective, in-situ treatment wall technology. Based on these results, a full-scale 500-ft biowall was installed at Offutt AFB in July 2001