Installation Date:
02/01/1995

Contaminants:
Trichloroethene, cis-1,2-Dichloroethene, Vinyl chloride, Freon

Reactive Media:
Fe⁰

Construction:
Funnel and Gate

Point of Contact:
Carol Yamane
Geomatrix Consultants, Inc.
Tel: 415-434-9400
Fax: 415-434-1365
Email: cyamane@
geomatrix.com
100 Pine Street
San Francisco , CA 94111

In 1995, after being approved by the California Regional Water Quality Control Board, a permeable reactive barrier (PRB) was installed at the Intersil Semiconductor Site in Sunnyvale, CA. Intersil had manufactured semiconductors at the site from the early 1970s until 1983. In 1972, the company had installed a concrete, epoxy-lined, in-ground system at the facility to neutralize acid in wastewater before discharge to a sanitary sewer. Soil and ground-water contamination from halogenated volatile organic compounds (VOCs) was identified near the neutralization holding tank site after it was removed early in 1987. Initial concentrations of contaminants were 50-200 µg/L of trichloroethylene (TCE), 450-1,000 µg/L of cis-1,2,-dichloroethylene (cDCE), 100-500 µg/L of vinyl chloride (VC), and 20-60 µg/L of Freon 113^®. Ground-water extraction and treatment, using an air stripper, began late in 1987. The in situ PRB system replaced the existing pump-and-treat system which was being maintained at a significant cost. The original system has been removed and the property has been restored to full economic use. The monitoring wells provide access to the in situ system for periodic monitoring compliance.

The contaminated area is in a semiconfined aquifer that is 2-4 ft thick. It is composed of interfingering zones of silty, fine-grained, fine- to medium-grained sand, and gravelly sand. The geometry of the aquifer is irregular, with a local presence of clay lenses. The aquifer is mostly confined by an upper silty-clay and clay layer, which ranges from 9-12 ft thick, and by a lower aquitard of clay and silty clay, which is about 65 ft thick.

The physical constraints of the site helped determine the geometry of the PRB and the construction technique used. To address historically changing ground-water flow directions, low permeability walls were installed upgradient and perpendicular to the PRB to contain affected ground water onsite prior to flow through the barrier. The treatment zone is sandwiched between permeable gravel layers to evenly distribute flow through the zone. The barrier itself is 4 ft wide, 36 ft long, and 20 ft deep. It is charged with 220 tons of granular iron (Fe⁰) to a depth of about 11 ft. A low, permeable "wing" that extends perpendicular from the treatment wall to about 20 ft downgradient was installed to reduce the impact on ground-water velocity through the wall due to variations in regional flow direction.

Installation cost for the in situ PRB system, including the slurry walls used to direct ground water toward the permeable reactive barrier, was $1,000,000. This includes the cost of construction, materials, and the iron. Design cost for this system is not available.

The cleanup goal established for the site is to reduce contaminant concentrations to levels below the Maximum Contaminant Level (MCL) set by the State of California and Primary Drinking Water Standards—5 µg/L for TCE, 6 µg/L for cDCE, 0.5 µg/L for VC, and 1,200 µg/L for Freon 113®. Since installation, VOC concentrations have been reported below cleanup goals from monitoring wells located within the iron wall. While seasonal hydraulic mounding has been observed above the PRB, it is not expected to affect long-term performance of the barrier. Affected ground water is contained onsite when mounding is present. When the mounding dissipates, ground water again flows through the barrier and is treated.

Lessons Learned

In addition to helping distribute flow through the PRB, the pea gravel zone placed upgradient of the PRB has resulted in precipitation of naturally occurring minerals and partial treatment of target chemicals upgradient of the iron treatment zone. Some mixing of the iron into the pea gravel zone is likely to have occurred during construction and resulted in chemical conditions favorable for some mineral precipitation (for example, higher pH, lower redox potential than ambient ground water). This is evidenced by inorganic chemistry data from wells within the pea gravel. While site managers did not anticipate this benefit, the result is expected to extend the life of the treatment zone relative to the potential negative effects of mineral precipitation.

SITE-SPECIFIC REFERENCES