Successful Rapid Response Mitigation of High Strength Vapor Plume Underneath a Large, Active Navy Building

摘要

Results from a base-wide vapor intrusion (VI) investigation at Naval Air Station North Island demonstrated that Building 379 had a complete VI pathway and that indoor air concentrations of trichloroethene (TCE) exceeded the EPA Region 9's urgent response action limit of 24 micrograms per cubic meter. Building 379 has a footprint of 172,000 square feet and overlies a light non-aqueous phase liquid (LNAPL) plume comprised of jet fuel and Stoddard solvent with a minor fraction of TCE and other chlorinated volatile organic compounds (cVOCs). The depth to the top of LNAPL is approximately 23 feet below ground surface. Volatilization of cVOCs from the LNAPL is believed to contribute to the cVOC vapor plume underneath the building. Several rapid response actions were taken to address noncancer TCE risk: communicating with stakeholders and the public, relocating employees if requested, modifying the operation of building's heating and ventilation (HV) system, operating portable air filtration units in closed rooms with lower ventilation, and conducting biweekly indoor air monitoring. In addition, a time-critical removal action (TCRA) is being implemented to mitigate VI risk. The TCRA consists of sealing VI pathways and installing and operating a horizontal well soil vapor extraction (SVE) system. Over 15,000 linear feet of expansion joint and floor cracks in the concrete slab were sealed, pipe penetrations in the bathrooms were sealed, a wall void space was filled, and the lunchroom floor tile was replaced with RetroCoat. Two horizontal wells were installed with extracted vapor treatment by cooling, compression, and condensation. The initial performance of the SVE system exceeded expectations. During the first 8 months of operation, over 300 gallons of CVOCs and 700 gallons of TPH were collected, sub-slab depressurization was greater than target levels up to 200 feet laterally from the SVE wells, and indoor air concentrations were reduced to below action limits. The system is currently being enhanced to include source reduction, including steam injection, LNAPL recovery, and additional SVE.