Theme: 09. Vapor Intrusion

Topic Area: 9b. Vapor Intrusion Mitigation and Effectiveness

Formatted Title:

The Importance of Flow and Mass Removal Rates in Vapor Intrusion Mitigation Design

Background/Objectives:

Most active vapor intrusion mitigation systems (VIMS) are designed to induce a negative pressure or vacuum below the slab, an approach often called sub-slab depressurization (SSD), where downward pressure gradients are preventing or minimizing upward advection of soil vapors. As a result, performance criteria for SSD systems are typically expressed as a minimum vacuum level below the slab. Vacuum is induced by withdrawing air, which causes air flow below the slab. The relationship between the air flow rate and induced vacuum depends on the permeability of the material below the floor (the venting media) and the surrounding materials, including the overlying slab and underlying soil (the “air flow system”). Where the air flow system is highly permeable, air flow rates are high and the mitigation approach is often called sub-slab venting (SSV) because dilution and removal of compounds of concern (COCs) are the primary mitigation mechanisms. In reality, all active VIMS achieve both SSD and SSV to varying degrees, therefore VIMS design and performance monitoring should include flow, vacuum and mass removal to achieve the most cost-effective and sustainable approach. This presentation provides recommendations for doing this based on vapor transport principles, simple modeling, and experience.

Approach/Activities:

This study included evaluation of the contribution of airflow and mass removal to mitigation designs based on vapor transport first principles, simple mass flux equilibrium modeling, and case histories. First principles indicate that application of vacuum to the sub-slab media of any system will induce airflow and propagate vacuum. Application of mass flux equilibrium equations (e.g., as used by the Johnson and Ettinger Model) indicate that increased airflow results in and decreased sub-slab VOC concentrations, all else equal, which can be used to estimate mass flux and sub-slab vapor concentrations. Pilot and field scale case histories demonstrate how flow rate and mass removal rate measurements can indicate system radius of influence, effectiveness, and efficiency.

Results/Lessons Learned:

Current design practice includes steady-state measurement of both vacuum levels and air flow during mitigation diagnostic tests so that appropriate fans can be selected to meet vacuum level goals. This information can be used to target vacuum and air flow levels that optimize system performance for SSD and SSV modes of operation, respectively. In addition, simple mass flux equilibrium calculations or modeling based on these data allows estimation of the reduction in sub-slab vapor concentrations as a function of air flow rate, as well as degree of mass flux capture necessary to achieve both sub-slab vapor and indoor air target levels. Similarly, SSV conditions can be enhanced by installing air inlets, thereby increasing airflow rates, and reducing vacuum and energy requirements to achieve target sub-slab concentrations. A few additional measurements during diagnostic testing can provide more useful information at minimal cost. Specifically, measurement of the transient response of vacuum with radial distance to applied vacuum allows calculation of the transmissivity of the material below the floor slab and the rate of air leakage across the floor slab. This information provides additional lines of evidence for estimating the radius of influence of suction points, as well as site-specific slab attenuation factors and related sub-slab vapor concentrations protective of indoor air quality. In addition, the rate of mass removal can be quantified with sampling and analysis of the VIMS effluent and flow rate measurements – this provides one of the few reliable exit strategies when the rate of mass removal is too low, if no longer mitigated, to increase indoor air concentrations above levels of potential health concern.

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Eric Lovenduski