Human health risks from vapor intrusion related inhalation exposures to volatile organic compounds (VOCs) and radon are typically mitigated with subslab depressurization systems, the design and performance monitoring of which is typically based only on static vacuum measurements across the floor slab.

Subsurface vapor intrusion (VI) to indoor air of volatile organic compounds (VOCs) and radon pose potential health risks to building occupants through inhalation exposures. The most common method for mitigating risks is subslab depressurization (SSD), which is also known as active soil depressurization (ASD) or may be referred to as subslab ventilation (SSV) if the goal is to reduce concentrations below the floor slab instead of establishing a vacuum below the floor. Design and performance specifications were developed by radon researchers decades ago and were based mostly on achieving a measurable vacuum below the concrete floor slab. Revisions to guidance documents are in progress at the time of this report (ANSI/AARST RMS-LB, RMS-MF, RMS-SF for large buildings, multi-family residences and single-family residences, respectively). This poses an opportunity for advances to design and performance assessment, which was the motivation for this research.

The goal of this research was to demonstrate and validate a more rigorous and cost-effective process for design and optimization of systems for mitigating vapor intrusion for VOCs and radon to reduce the capital and long-term operating costs. This research was predicated on the conceptualization that an SSD/SSV or ASD system is essentially a “capture system” that could be designed and monitored using methods analogous to those used to contain the migration of a plume of contaminated groundwater. This demonstration shows how a test procedure analogous to a groundwater pumping test can be performed very quickly and efficiently for subslab gas flow characterization and can be used with tracer testing, mass flux monitoring and mathematical modeling to create several lines of evidence for performance assessment and monitoring to improve mitigation system design and operations. Specific objectives included:

• Reduce life-cycle costs associated with system design and installation, electricity to run fans, and energy loss due to conditioned (heated, cooled, humidified or dehumidified) indoor air being drawn into the subsurface and vented to outdoor air and quantify these costs and recognized savings for comparison to the costs of performing the optimization,
• Maintain protection of indoor air quality by maintaining indoor air concentrations below risk-based screening levels
• Maintain a protective mass removal rate, similar to or higher than the emissions of VOCs and radon through the building during building depressurization testing and/or ES-2 comparable to the mass removal rate of an existing mitigation system where the optimization is performed after the initial system commissioning,
• Demonstrate and validate the technology with sufficient data to allow a detailed review by third parties, include different buildings with a range of size and construction typical of DoD building stock, including residential buildings, engage a team of world-leading experts, and employ multiple lines of evidence, analytical modeling and long-term monitoring,

The scope included field testing at four buildings ranging from 1,200 to 64,000 ft² in footprint size. Testing methods included monitoring of:

• ambient cross-slab differential pressure to establish building-specific target subslab vacuum levels;
• subslab vacuum vs time in response to fan cycles (on/off) and subslab vacuum vs radial distance for matching to Equations 1 and 4 to characterize the transmissivity and leakance;
• subslab tracer testing to measure travel time from different radial distances to the point of suction to enable performance evaluation based on travel time and velocity;
• flow rate and VOC/radon concentrations in the vent-pipes under various operating conditions to assess the mass removal rate as a function of gas extraction rate for comparison to threshold values of mass loadings via building pressure cycling and the system performance during challenges imposed by a stress test, and;
• mathematical modeling using the Hantush-Jacob (1955) Leaky Aquifer Model, after making adjustments to account for different density between gas and water.

Analysis of these data yields information regarding the temporal distribution of ambient cross-slab pressure differential, the transmissivity of the material below the floor, the leakage of the floor, the thickness and effective porosity of the dominant zone of air flow beneath the floor, the radial profiles of vacuum, travel time, gas velocity and proportion of flow originating below vs above the floor, which provide lines of evidence for system design and performance assessment. These data can also be used to calculate a building-specific attenuation factor (AF) to support customized subslab screening levels.

Where the material below the floor is granular fill (which is usually specified in building codes) and the floor is relatively competent (e.g., few utility penetrations, epoxy sealants, sealed expansion joints), the optimal spacing between suction points can be very large, which reduces the capital cost of installation for a large building compared to designs with closer spacing between suction points. Appendix B of the Final Report provides an example of this scenario and shows that conventional design methods tend to underestimate the radius of influence in this scenario. The U.S. DoD has a Unified Facilities Criteria Program (http://www.wbdg.org/ffc/dod) to assure good and consistent construction practice and specifies 4 to 6 inches of reinforced concrete and 6 to 8 inches of ¾-inch granular fill that is gap graded to increase drainage rates. As a result, most new DoD buildings will fall into this category.

If the material below the floor is highly permeable, the flow velocity and induced ventilation below the slab can be sufficient to reduce subslab concentrations by SSV, even in areas where the induced vacuum is too small to reliably measure. If the ventilation rate below the slab is sufficient to reduce the VOC and radon concentrations to very low levels, then an occasional reversal of the cross-slab pressure gradient will not result in substantial VOC transport into the building, so the conventional minimum subslab vacuum design criteria of 6 to 9 Pa (ASTM E2121) may not be necessary to prevent unacceptable exposures due to vapor intrusion. In such cases, current standard practice generally results in unnecessary installation of larger fans and more suction points, which both increases capital and operation costs, but also results in wasted energy because conditioned indoor air is extracted and exhausted outdoors. The subslab tracer testing, mass flux monitoring and mathematical modeling developed during this research provide additional lines of evidence to support a protective design with smaller induced vacuums, a larger spacing between suction points and/or lower fan power which reduces capital and operating costs. The detailed example in Appendix B illustrates how conventional designs using a minimum specified static vacuum of 6-9 Pa result in a radius of influence estimate of 36 feet, whereas the methods developed in this research demonstrate the ROI is up to about 90 ft. For a building with these characteristics and a floor area of 100,000 ft² , a conventionally designed system would require 25 suction points drawing 2500 standard cubic feet per minute (scfm) of soil gas whereas an optimized system would require only 4 suction points drawing 400 scfm. Use of the optimized system would be much less costly to install, operate and maintain yet still meet all of the performance objectives. The permeability of the material below the floor and the leakance of the floor vary from building-to-building, so results will vary, but the test procedure includes building-specific measurement of these critical parameters as part of the optimized system design.

The rate of mass removal by the system also provides a useful performance metric that can be compared to the mass loading through a building via building pressure cycling as a means of demonstrating the adequacy of the mitigation system design and performance. It can also be used to support an exit strategy if the system is clearly capturing all the available mass and the rate of mass removal of the mitigation system is insufficient to pose an indoor air quality concern considering the building size and air exchange rate.

Where the material below the floor has a low permeability and the subslab vapor concentrations are very high (i.e., >~1E6 µg/m³), diffusive transport of VOCs through the floor slab can  potentially pose indoor air quality concerns even if there is an appreciable vacuum below the floor and supplemental measures such as increased building ventilation, floor coating or carbon filtration may be needed as interim measures until there is a reduction in subslab vapor concentrations. For new construction, the use of “radon ready” construction with subslab collection pipes or aerated floors enables subslab ventilation with minimal effort and may be feasible with passive ventilation (driven by thermal gradients, wind-driven turbines or solar-powered fans). Vapor barriers alone may be adequate in some cases, but they may also allow vapor concentrations to gradually increase to levels similar to those of the underlying source, and lead to diffusive transport across the barrier that could pose a threat to indoor air quality. Barriers will also inhibit the downward flux of oxygen, which is beneficial for natural degradation of petroleum hydrocarbon vapors and methane.

The field methods developed to support mitigation optimization are relatively fast and simple with readily-available equipment and can be mastered by practitioners skilled in the art and sciences of hydrogeology, vapor sampling and soil vapor extraction. The mathematical models are commercially available or readily programmed into a spreadsheet. As a result, the costs associated with implementing the lines-of-evidence developed in this research is modest compared to the potential savings in capital and operations, maintenance and monitoring, so a net savings is to be expected, particularly for larger buildings. The testing program demonstrated that conventional methods for determining a radius of influence may result in a much greater number of suction points being installed than are really needed, which is costly and disruptive. The testing also shows that total system flow rates may commonly also be overdesigned, which wastes electricity to run the fans and also incurs excess energy costs when conditioned indoor air is drawn through the floor and wasted by discharge to outdoor air.

Implementation issues will be minimal for most DoD buildings.  For rare cases where the floor slab rests on very low permeability native soil, subslab venting systems may not achieve sufficient flow or mass removal to be protective of indoor air quality, in which case, other options such as aerated floors, building ventilation and/or barriers may be needed. For new construction, aerated floors may provide a less expensive mitigation option.  Adoption of this technology into written standards may take some time.

McAlary, T., T. Gallinatti, G. Thrupp, W. Wertz, D. Mali, and H. Dawson. 2018. Fluid Flow Model for Predicting the Intrusion Rate of Subsurface Contaminant Vapors into Buildings. Environmental Science & Technology, 52(15):8438–8445.