A Multi-Pronged Approach to Ⅵ Mitigation of a Large Mixed-Use Building

摘要

A large, mixed-use industrial manufacturing and office building impacted by vapor intrusion (Ⅵ) of chlorinated volatile organic compounds (CVOCs) presented several challenges for improvement of indoor air quality. These challenges included 1) the presence of a CVOC source zone beneath the building floor slab, 2) dissolved-phase CVOC presence in the relatively shallow groundwater beneath the building, and 3) building infrastructure and mechanical systems that provided multiple advective pathways and drivers for VI. Beginning with initial assessment and continuing to final mitigation, this paper shows how a combination of modern investigative tools and custom-designed measures were employed to illuminate the challenges and inform appropriate response actions. Using a field-portable gas chromatograph-mass spectrometer (GC-MS) and other tools, the magnitude and extent of the Ⅵ impacts to indoor air were rapidly assessed, and the principal Ⅵ pathways were identified. An evaluation of mitigation options included source remediation, adjustments to the air handling units, and both passive and active Ⅵ cutoff/interception methods. Due to the relatively high strength of the sub-slab source and presence of multiple Ⅵ pathways, passive mitigation measures, such as sealing of preferential pathways, were combined with active ventilation of subsurface utilities and sub-slab vapor extraction to provide redundancy and performance assurance. The portable GC-MS was used to evaluate the relative effectiveness of the implemented remedies and to refine the overall mitigation strategy. To reduce energy consumption associated with the active mitigation measures, the electric-powered blowers were equipped with variable speed drives, which provide operations flexibility to achieve lower operating costs as CVOC mass flux rates are gradually reduced over time. Post-startup monitoring and sampling indicates that the Ⅵ mitigation measures have been successful in reducing indoor air CVOC concentrations consistent with performance objectives.