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IPEC 2020: Design Rational for Selecting Vacuum Blower Flow Rate and Inlet Vacuum for Active Sub-Slab Vapor Intrusion Mitigation Systems

Primary Author / Conference Presenter:
Robert Roth
Sr. Engineer
Terracon
Wheat Ridge, CO, USA
www.terracon.com

Co-Author:
Joe Yang, Terracon, Tustin, California

Sub-slab vapor intrusion mitigation system (VIMS) pilot tests were conducted at eight sites in six states with 44 sets of data that were used to estimate the sub-slab radius of vacuum influence (ROI) as affected by vacuum blower flow rate and inlet vacuum. Inlet blower vacuums ranged from 1 to 70 inches of water and ROI ranged from 9 to 36 feet. Correlation (R2) for vacuum vs. distance ranged from 0.79 to 0.95 depending on soil type. Based on the average sub-slab vacuum decay from the soil gas collector expressed by P = P0 e -0.34x and using distance for the ROI, the vacuum required at the inlet of the blower can be estimated. However, estimates were one to two orders of magnitude lower than blower vacuum measured during some pilot tests presumably due to sub-slab soil permeability and/or sub-slab structures that could have restricted sub-slab flow during the pilot tests. Since results of ROI vs. vacuum can be dependent on sub-slab soil permeability, sub-slab structures that can restrict sub-slab vapor flow, and short circuiting, additional pilot studies are needed for a larger data base that can be used by engineers during the design of VIMSs for proposed buildings.