Vapor Courses

Gain extensive knowledge on VI with our pre-conference course on Wednesday, September 22 

Optimizing Vapor Intrusion Mitigation System Design: Advanced Mass-Flux and Pneumatic Assessment Tools for Improved VI Mitigation Implementation
Wednesday, September 22, 2021 • 1:00 pm - 5:00 pm 
Instructors: Paul Nicholson, Senior Engineer, Geosyntec Consultants; Todd McAlary, Senior Consultant, Geosyntec Consultants; Theresa Gabris, Project Geologist, Geosyntec Consultants

Vapor intrusion (VI) mitigation controls often consist of systems that extract gas from below the building foundation to create a sub-slab vacuum to prevent sub-surface vapor migration to indoor air. Performance of these systems is typically evaluated by measuring the distribution of the vacuum response below the floor, and a target minimum vacuum level in the range of 4 to 9 pascals (Pa), which are based on decades-old radon mitigation guidance documents and standards, is commonly used. Recent experience and consideration of mass-flux analysis of the vapor intrusion pathway indicates that in many cases effective mitigation may be achieved with a minimum applied vacuum as low as about 1 Pa. However, it can be difficult to reliably measure such a small applied vacuum because of baseline drift and noise in the cross-slab pressure from variable wind, temperature, mechanical fans and other factors. 

This short course will teach attendees two alternate data collection techniques that can be conducted for optimization of VI mitigation system design and operation: 

  • High volume sampling (HVS) – Extracting large volumes (100’s to 1000’s of liters) of sub-slab vapor to assess sub-surface VOC concentration distribution, while measuring pneumatic properties to evaluate building specific sub-slab vapor flow characteristics, and slab leakance
  • Building pressure control (BPC) – Control building pressure under positive and negative pressure conditions to assess background and preferential pathway (i.e., utilities) contributions to indoor air quality, and upper-bound mass-flux estimates
The course will review HVS data collection and analysis methods for the measurement of vacuum vs. distance and vacuum vs. time (transient data) to evaluate the radius of vacuum influence and slab leakance. Additionally, tracer testing with helium to evaluate sub-slab velocity measurements and system capture radius will be discussed. The theory behind HVS will be supported with a hands-on demonstration using a tabletop model floor slab with permeable fill. Challenges associated with a broad spectrum of sites (e.g., low or high permeability conditions beneath a slab, high water table concerns) will be discussed and which data should be collected to understand subsurface conditions to ensure successful mitigation.

HVS data analysis will be explained using a spreadsheet model to obtain system radius of influence. System design considerations will be discussed, such as if floor cracks should be sealed or remain unsealed, and the influence on system design. Sub-slab permeability will be discussed and the effects on measurable vacuum and the correlation with sub-slab flow rate. In addition, building specific attenuation factor calculations will be demonstrated using sub-slab pneumatic data.

The course will review BPC data collection and analysis methods to evaluate mass flux related to VI and determination of potential interior sources. The theory behind mass flux estimates will be presented and target mass flux for mitigation system design. A hands-on demonstration will also be conducted using a model house to show the influence of background indoor air concentrations and how to separate these factors from your VI assessment and mitigation. The influence of floor cracks and differential pressure will be demonstrated using the tabletop system. 

Attendees will gain a better understanding of how mitigation system effectiveness can be characterized through multiple lines of evidence, such as applied vacuum, sub-slab velocity, and pore volume exchanges that may be used to identify alternate performance targets for mitigation system. In addition, the hands-on demonstrations will provide attendees with insight on challenges associated with variability in building characteristics and the impact on successful mitigation.