Fairchild Semiconductor Corp. (Mountain View Plant)

In June 1989, EPA issued a Record of Decision selecting the soil and groundwater cleanup remedy for the MEW Site. The soil remedy includes: excavation, with treatment by aeration; and soil vapor extraction with treatment by vapor-phase granular activated carbon. The groundwater remedy includes: slurry walls (barriers beneath the surface) to contain contaminants; and extraction and treatment systems to contain and clean up groundwater contamination using granular activated carbon and/or air-stripping systems.

Because the groundwater contamination at the MEW Site migrates northward and has mixed with contamination from sources at the NAS Moffett Field Superfund site, the groundwater remedy selected in the MEW Record of Decision also applies to the commingled regional groundwater contamination area on former NAS Moffett Field (the West-Side Aquifers), but not the entire former NAS Moffett Field facility.

515/545 Whisman Road and 313 Fairchild Drive (Former Buildings 1-4)

In 1986, Fairdhild installed a 40-foot deep slurry wall was installed in 1986 along the boundaries of these properties to limit migration of chemicals, and was keyed into the A/B aquitard. Groundwater extraction was initiated in the mid-1980s to control and clean up sources in the groundwater. The system was expanded and currently includes 12 source control extraction wells both inside and outside the slurry wall. The MEW Regional Program operates three wells outside the slurry wall. Treatment systems 1 and 3 have treated over an estimated 900 million gallons of groundwater and removed over 34,000 pounds of VOCs from the groundwater through December 2006.

401 National Avenue (Former Building 9)

In 1986, Fairchild installed a slurry wall along the boundaries of this property and that was keyed into the A/B aquitard at a depth of approximately 40 feet. Groundwater extraction began at this property in 1982 from well 65A. Since then, the groundwater system has been expanded to include four source control extraction wells within the slurry wall enclosure. Extracted groundwater from the five A Aquifer wells is treated at System 1. Three other source control extraction wells (GSF-1A, GSF-1B1, and GSF-1B2) have also been installed north of this facility and are the joint responsibility of Vishay/SUMCO and Fairchild/Schlumberger.

369 and 441 North Whisman Road (Former Buildings 13, 19, 23)

In 1986, Fairchild installed an approximately 40-foot-deep slurry wall along the boundaries of the 369 North Whisman Road property, keyed into the A/B aquitard. Groundwater extraction began in 1982 at this property, and was expanded to include seven A Aquifer source control extraction wells within the slurry wall enclosure; two A Aquifer source control extraction wells downgradient of the slurry wall to the north; three source control extraction wells in the B1 Aquifer; and two source control extraction wells in the B2 Aquifer. Groundwater extracted from these wells is piped to System 19, located at 369 North Whisman Road, for treatment. System 19 has treated over 800,000 gallons of groundwater and removed over 10,000 pounds of VOCs from the groundwater through December 2006.

644 National Avenue (Former Building 18)

One source control extraction well (RW-25A) currently operates in the A Aquifer northwest of the building. Groundwater from this extraction well is treated at System 1. System 1 also treats water from two basement-dewatering sumps at 644 National Avenue.

464 Ellis Street (Former Building 20)

No potential sources were found at this property. Raytheon installed and currently operates two source control extraction wells in the A and B1 Aquifers, respectively. These wells capture contaminants immediately downgradient of the Raytheon slurry wall. Groundwater from the two wells is conveyed to the Raytheon system at 350 Ellis Street for treatment.

EPA First Five-Year Review

EPA's 2004 Five-Year Review for the MEW Study Area determined that for the groundwater remedy to remain protective in the long-term, the following actions need to be taken: long-term protectiveness should continue to be verified by monitoring the extent of groundwater contamination along the estimated groundwater contamination plume boundaries. This evaluation should be accomplished through routine annual groundwater sampling events. Current data indicate that the remedy is functioning as required to meet the remedial action objectives; however, EPA recommends optimization of both the regional and facility-specific systems to enhance plume capture, evaluation of applicable technologies to potentially expedite contaminant mass removal and cleanup time, and evaluation of the potential need for institutional controls.

Current Actions

The MEW Companies, Navy and NASA have prepared a work plan to evaluate other alternate groundwater cleanup technologies to address the MEW Site-wide groundwater contamination.

EPA Evaluates Vapor Intrusion Pathway The existing soil and groundwater remedy at the MEW Site does not address risks from long-term exposure through the vapor intrusion pathway. Since the issuance of EPA’s 1989 Record of Decision, new information has been developed regarding the toxicity of TCE and potential vapor intrusion into buildings overlying shallow groundwater contamination. In 2003, as part of EPA’s Five-Year Review of the MEW Study Area, EPA began evaluating whether VOCs in shallow groundwater are potentially migrating upward through the soils and cracks in the floors or through plumbing conduits and other preferential pathways, and impacting indoor air.

Based on indoor air sampling of both commercial and residential buildings in the area conducted in 2003 to 2007, EPA has confirmed the presence of the subsurface vapor intrusion pathway into a number of structures overlying the shallow groundwater TCE plume. None of the samples taken to date indicate any immediate or short-term health threat to building occupants from this pathway. EPA’s main concern is whether the chemicals from the Site measured in indoor air pose an unacceptable risk of chronic health effects due to long-term exposure (25 years or more). It is EPA’s policy not to set cleanup levels or take action to reduce levels that are less than ambient background levels.

Some of the sampled buildings indicated indoor air contaminant concentrations that were elevated above background levels and above EPA Region 9’s TCE interim action level in indoor air for long-term exposure. In each of these buildings, the MEW Companies and NASA have taken voluntary interim measures (e.g., sealing cracks/conduits, upgrading/modifying ventilation systems, installing air purifying systems) to reduce the indoor air contaminant concentrations. In addition, many new buildings in the MEW Study Area have taken measures to help prevent or mitigate the vapor intrusion pathway. Although EPA has not yet determined what the long-term mitigation and monitoring strategy should be for all buildings overlying shallow groundwater contamination, the results of these interim measures have generally reduced the indoor air levels thus far and are protective of residents and workers.

EPA continues to evaluate this pathway and potential mitigation measures for impacted buildings overlying the shallow TCE plume. EPA is currently conducting and planning the following actions:

• Continue evaluation of buildings overlying the shallow TCE plume to identify potential pathways into buildings, and implement mitigation measures to reduce elevated levels of TCE in indoor air, as appropriate;
• Develop remedial action objectives to address the vapor intrusion pathway;
• Assess various alternatives and propose a remedy to address the vapor intrusion pathway.
• Amend the MEW Record of Decision to select a remedy that addresses potential long-term exposure of TCE and other VOCs at unacceptable levels through the vapor intrusion pathway.