Permeable Reactive Barrier to Treat Contaminated Groundwater at Moffett Federal Airfield, Mountain View, California

Site Name:

Moffett Site

Location:

Mountain View, California

Period of
Operation:

Status: Ongoing
Report covers: 4/96 - 7/97

Cleanup
Type:

Voluntary pilot-scale study

Vendor:

Tim Mower
Tetra Tech EM Inc.
1099 18th Street, Suite 1960
Denver, CO 80202
(303) 312-8874

Chuck Reeter
Naval Facilities Engineering Service Center
1100 23rd Ave., Code 411
Port Hueneme, CA 93043-4370
(805) 982-4991

Technology:
Permeable Reactive Barrier (PRB)
- The PRB is a funnel-and-gate iron treatment wall system consisting of 2 sheet pile walls, permeable zones up- and down-gradient of the wall, and the reactive zone
- The PRB is composed of 100% granular iron, is 6 ft thick, 10 ft wide, and 18 ft high beginning 5 ft below ground surface
- Average flow rate through the wall was estimated as 0.5 ft/day (alternate estimates also provided)

Cleanup Authority:
Not Applicable

Navy Point of Contact:
Stephen Chao (Navy Project Manager)
Bldg. 210
Department of the Navy EFA-West
900 Commodore Drive
San Bruno, Ca 94066
EPA Point of Contact:
Lynn Suer
EPA Region 9
75 Hawthorne Street
San Francisco, CA 94105
(415) 744-2396

Contaminants:
Chlorinated solvents

- Maximum concentrations detected during 1991 investigations -include TCE (20,000 mcg/L) and PCE (500 mcg/L)

Waste Source:
Leaking underground and aboveground storage tanks, waste sumps; on-site migration of contaminants from Silicon Valley plume

Type/Quantity of Media Treated:
Groundwater
- 0.284 million gallons treated as of July 1997
- DNAPL suspected in groundwater on site
- Groundwater is found at 5 ft bgs
- Extraction wells are located in 5 hydrogeologic units, which include upward hydraulic gradients
- Hydraulic conductivity ranges from 0.3 to 400 ft/day

Purpose/Significance of Application:
Use of PRB technology in a pilot study for treatment of chlorinated solvents; included extensive sampling conducted at locations within the wall.

Regulatory Requirements/Cleanup Goals:
The objectives of the pilot project are to (1) demonstrate and validate the PRB technology in remediating groundwater contaminated with chlorinated hydrocarbons; (2) evaluate the long-term effectiveness of the barrier from a hydraulic stand point; and (3) develop cost and performance data.

Results:
- Data from sampling events in January, April, and July 1997 showed that chlorinated solvent concentrations were being reduced as the groundwater moves through the reactive zone. For example, TCE concentrations measured in upgradient wells during April 1997 were reduced to below the detection limit within the reactive zone. PCE and 1.2-DCE also were reduced to below the detection limit within the reactive zone.
- A tracer test performed in July 1997 showed that flow patterns within the wall are complex, with some lateral flow, and that flow velocities are lower than expected based on previous site characterization and modeling.

Cost Factors:
Actual costs for PRB use over one year at this site were $405,000 ($373,000 in capital and $32,000 in O&M), which correspond to $1,400 per 1,000 gallons of groundwater treated.

Description:
Moffett Federal Airfield is a former Navy facility providing support, training, operation, and maintenance associated with Navy aircraft. Aircraft engine repairs and aircraft maintenance have been performed on site for many years. Contaminant identification and cleanup activities have been underway at Moffett since 1987. Specific activities that contributed to the source at MFA included dry cleaning operations. The Navy and Department of Defense Environmental Security Technology Certification Program (ESTCP) are funding this PRB as a voluntary pilot study for treating a portion of a large plume that crosses the Moffett facility.

The PRB installed in 1986 is a funnel and gate iron treatment wall system. Components include two sheet pile walls, permeability zones up- and down-gradient of the wall, and the reactive zone. Analytical data showed that chlorinated solvent concentrations were being reduced as the groundwater moves through the reactive zone. A final technology evaluation report for this pilot study was planned to be completed by August 1998. Proposals are being presented to continue the sampling process annually or semi-annually.