Proposed plan offers tiering systems to determine preferred alternatives for vapor intrusion

EPA Region 9 has issued for public comment a proposed plan to mitigate VI in commercial and residential buildings at the Middlefield Ellis Whisman (MEW) Study Area, which includes three NPL sites in Mountain View and Moffett Field, CA. Formerly used for industrial and semiconductor activities, this area has undergone extensive soil and groundwater remediation to remove TCE and other contaminants in compliance with a 1989 record of decision (ROD).

As part of a supplemental remedial investigation from 2003-2008, over 2,800 air samples, including indoor, outdoor, and pathway air samples, were collected at 47 commercial and 31 residential buildings. TCE was found to exceed EPA Region 9's TCE interim action levels in several buildings. As a result, discrete mitigation methods were implemented (e.g., sealing conduits, enhancing ventilation, and installing sub-slab ventilation systems) to reduce indoor air concentrations.

The proposed plan presents EPA's preferred alternatives for protection of building occupants from potential long-term exposure to VI. Two tiering systems, one for existing and one for future buildings, are proposed. Detailed decision trees were developed to help assign each building a proposed action based on sampling results. Depending under which tier it falls, a building may require an engineered remedy, monitoring, and/or institutional controls.

Following consideration of all public comments extended through October 8, 2009, EPA will select the VI remedy in a ROD amendment and then work with the responsible parties to implement the remedy. For more information, see

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