High Purge Volume Sampling—A New Paradigm for Subslab Soil Gas Monitoring

Abstract

Subslab soil gas sampling and analysis is a common line of evidence for assessing human health risks associated with subsurface vapor intrusion to indoor air for volatile organic compounds; however, conventional subslab sampling methods have generated data that show substantial spatial and temporal variability, which often makes the interpretation difficult. A new method of monitoring has been developed and tested that is based on a concept of integrating samples over a large volume of soil gas extracted from beneath the floor slab of a building to provide a spatially averaged subslab concentration. Regular field screening is also conducted to assess the trend of concentration as a function of the volume removed to provide insight into the spatial distribution of vapors at progressive distances away from the point of extraction. This approach minimizes the risk of failing to identify the areas of elevated soil vapor concentrations that may exist between discrete sample locations, and can provide information covering large buildings with fewer holes drilled through the floor. The new method also involves monitoring the extraction flow rate and transient vacuum response for mathematical analysis to help interpret the vapor concentration data and to support an optimal design for any subslab venting system that may be needed.

Chlorinated vapor intrusion indicators, tracers, and surrogates (ITS): Supplemental measurements for minimizing the number of chemical indoor air samples—Part 1: Vapor intrusion driving forces and related environmental factors

“…Dye‐based methods are commonly used to detect dense nonaqueous phase liquids including chlorinated solvents in soil samples (Kueper et al., ) and boreholes (Riha, Rossabi, Eddy‐Dilek, Jackson, & Keller ,2000). More recently, methods that use a high‐volume soil gas sampling approach with continuous monitoring of FID or PID have been developed and employed at chlorinated hydrocarbon sites (Folson, ; McAlary, Nicholson, Yik, Bertrand, & Thrupp, ). Several publications have featured high temporal resolution, direct compound‐specific measurements of CVOCs with on‐site gas chromatographs (Holton et al., ; Hosangadi et al., ; Kram et al., ; U.S. EPA, , ).…”

Simplified Approach for Calculating Building‐Specific Attenuation Factors and Vapor Intrusion Mitigation System Flux‐Based Radius of Influence

“…The authors have conducted 176 tests of PFE in commercial/industrial buildings using methods described in Data S1. Each PFE test in the data set was analyzed using the two-layer Hantush-Jacob model to develop T and B values following protocols described by Thrupp et al (1996Thrupp et al ( , 1998 and McAlary et al (2010McAlary et al ( , 2018. Each high volume sample (HVS) test results in one unique pair of T and B values.…”

Subslab Depressurization Versus Subslab Ventilation: Insights from Recent Research

“…Consider the two‐layer conceptual model for flow in response to gas extraction beneath a floor (see Figure 1) and assume for the sake of comparison there are two buildings with conditions summarized in Table 1. The model formulation and assumptions are described in McAlary et al 2010, 2018, 2020 and ESTCP ER‐201322 (2019). Also assume: the floor slabs for both buildings are identical, with a leakance of 9 feet (2.75 m), the median value reported in McAlary et al 2018 (note: a more permeable/leaky slab will result in a lower leakance ( B ) value and a smaller radius of influence [ROI] and vice versa); the buildings are both large enough that more than one suction point will be required for the mitigation system; both buildings have the majority of gas flow beneath the slab in a layer compacted granular fill of 0.5 feet (15 cm) thickness and 25% air‐filled porosity; there are no preferential pathways through drains or pipes; and gas extraction occurs through a vertical pipe of 4‐inch (10 cm) diameter from a sump below the floor after removal of about 5 gallons (20 L) of soil/fill (i.e., similar to a conventional radon sump design).…”

“…Conceptual model for two‐layer flow in response to gas extraction below a floor (after McAlary et al 2010, 2018). …”