Permeable Reactive Wall Remediation of Chlorinated Hydrocarbons in Groundwater at Moffett Field Superfund Site

Site Name:

Moffett Site

Location:

Mountain View, CA

Period of
Operation:

April 1996 - December 1997
(Monitoring data available June 1996 through December 1997)

Cleanup
Type:

Field demonstration

Remediation
Contractor:

Navy Contractor
Arun Gavaskar
Battelle
505 King Avenue
Columbus, OH 43201
614-424- 3403

Technology:
Permeable Reactive Barrier (PRB)
- Funnel-and-gate system; pea gravel added to gate to help distribute groundwater flow through reactive cell
- PRB is 10 feet long (6 feet of reactive material) by 10 feet wide; installed at depth from +5 feet bgs to - 14 feet bgs; keyed into low-permeability sediments (sand channel)
- Reactive material - iron (from Peerless Metal Products, Inc.); -8 to +40 mesh particle size range
- Groundwater monitoring well network includes wells within the PRB as well as upgradient and downgradient.

Cleanup Authority:
Installation Restoration Program

Navy Contact:
Charles Reeter
Naval Facilities Engineering Service Center
1100 23rd Avenue
Port Hueneme, CA 93043
805-982-4991

Stephen Chao
U.S. Navy, EFA West
900 Commodore Drive
San Bruno, CA 94066

Contaminants:
Chlorinated Solvents
- Groundwater contaminated with chlorinated volatile organic compounds (CVOCs) including TCE, cis-1,2-DCE, PCE, and 1,1- DCE; TCE is the most prevalent contaminant at the site
- CVOC plume, located in the near surface A aquifer, is more than 10,0000 feet long and about 5,000 feet wide
- TCE and PCE concentrations in the A aquifer reported above 20 mg/L and 0.5 mg/L, respectively

Waste Source:
Wastes from operations and waste management activities, including leaks from underground storage tanks, aboveground tanks, and sumps

Type/Quantity of Media Treated:
Groundwater
- The aquifer includes two units - A1 which is up to 20 feet thick and is overlain by a clayey surface layer of varying thickness; and A2 which is up to 20 feet thick and extends to 40 feet below mean sea level
- Aquifer contains multiple channels of sand and gravel; zone is not laterally homogenous due to the interbraided channel nature of the sediments
- Both units are contaminated; however, the pilot-scale PRB penetrates the A1 unit only
- A1 unit - hydraulic gradient ranges from 0.005 to 0.009; hydraulic conductivity ranges from 0.04 foot/day to 633 feet/day (due to lithographic variation); groundwater velocity ranges from 0.2 to 5.0 feet/day

Purpose/Significance of Application:
Field demonstration of PRB to remediate groundwater contaminated with chlorinated solvents

Regulatory Requirements/Cleanup Goals:
- Groundwater cleanup goals are the MCLs for PCE (5 mg/L), TCE (5 mg/L), cis-1,2-DCE (70 mg/L), and vinyl chloride (2 mg/L), as measured in the effluent from the PRB

Results:
- The PRB monitored on a quarterly basis from June 1996 to October 1997 (five quarters total)
- By October 1997, TCE, PCE, DCE, and VC were reduced to below the MCLs in the effluent from the PRB
- Data from two wells located within the reactive cell (one upgradient; one downgradient) were used to analyze trends in TCE and DCE degradation:
- TCE concentrations in both wells remained below the MCL every quarter except for June 1996; possible reasons for the elevated TCE levels in June 1996 included adsorption-desorption on the iron surfaces and residual contamination from construction activities attributed to the recent installation of the PRB (April 1996)
- DCE concentrations in both wells remained below the MCL for all five quarters
- Over the five quarters, TCE concentrations were relatively constant in both wells
- There was wider variation in DCE concentrations between the two wells; lower DCE concentrations were observed in the downgradient well, indicating that DCE degraded more slowly than TCE in the reactive medium.

Cost Factors:
- The total cost associated with the treatment of groundwater during the pilot-scale PRB demonstration was $802,375, including $652,375 in capital costs and $150,000 in O&M costs
- The projected capital cost for a full-scale PRB at Moffett Field was $4,910,942. O&M costs for a full-scale system were projected to be $72,278 in annual monitoring costs and $267,538 in barrier maintenance costs, incurred once every ten years, to replace part of the iron medium
- The projected full-scale costs assumed that the PRB would be constructed in two sections - the first section to capture and treat the groundwater; the second section, constructed downgradient from the leading edge of the plume to control further migration of the plume; both sections would extend to the base of the A2 aquifer zone, a depth of about 65 feet

Description:
The Naval Air Station, Moffett Field, located in Mountain View California, was selected by the U.S. Navy as part of the Installation Restoration Program for a field demonstration of a PRB. Groundwater at Moffett Field is contaminated with chlorinated solvents, and the site was placed on the National Priorities List in 1987. An area known as the West Side Plume, a chlorinated solvent plume (primarily TCE) located on the west side of Moffett Field, was used for the demonstration. Based on the results of laboratory testing, iron from Peerless Metal Powders was selected for the PRB.

The pilot-scale PRB, installed in April 1996, was a funnel-and- gate design, keyed into low-permeability sediments. The PRB was operated through October 1997, with groundwater monitored quarterly from June 1996 through October 1997 (five quarters total). By October 1997, TCE, PCE, DCE, and VC were reduced to below the cleanup goals in the effluent from the PRB. Additional data for TCE and DCE collected from wells located within the reactive cell showed that TCE and DCE concentrations within the PRB were generally below the MCLs, and that DCE degraded more slowly in the reactive cell than TCE. The projected cost for a full-scale PRB at Moffett Field was $4,910,942 in capital costs and $72,278 in annual monitoring costs. In addition, the projected O&M costs included $267,538 in barrier maintenance costs for iron medium replacement, incurred once every ten years.