Overview of state approaches to vapor intrusion: 2018

Approximate analytical model for transient transport and oxygen-limited biodegradation of vapor-phase petroleum hydrocarbon compound in soil

Volatile organic compounds (VOCs) contamination may occur in subsurface soil due to various reasons and pose great threat to people. Petroleum hydrocarbon compound (PHC) is a typical kind of VOC, which can readily biodegrade in an aerobic environment. The biodegradation of vapor-phase PHC in the vadose zone consumes oxygen in the soil, which leads to the change in aerobic and anaerobic zones but has not been studied by the existing analytical models. In this study, a one-dimensional analytical model is developed to simulate the transient diffusion and oxygen-limited biodegradation of PHC vapor in homogeneous soil. Laplace transformation and Laplace inversion of the Talbot method are adopted to derive the solution. At any given time, the thickness of aerobic zone is determined by the dichotomy method. The analytical model is verified against numerical simulation and experimental results first and parametric study is then conducted. The transient migration of PHC vapor can be divided into three stages including the pure aerobic zone stage (Stage I), aerobic-anaerobic zones co-existence stage (Stage II), and steady-state stage (Stage III). The proposed analytical model should be adopted to accommodate scenarios where the transient effect is significant (Stage II), including high source concentration, deep contaminant source, high biodegradation capacity, and high water saturation. The applicability of this model to determine the breakthrough time for better vapor intrusion assessment is also evaluated. Lower first-order biodegradation rate, higher source concentration, and shallower source depth all lead to smaller breakthrough time.

A pilot study characterizing tetrachloroethylene exposure with exhaled breath in an impacted community

Martinsville, Indiana overlays four groundwater contamination plumes, including a U.S. Environmental Protection Agency (EPA)-designated Superfund site. The primary contaminants are tetrachloroethylene (PCE), trichloroethylene (TCE), and other volatile organic compounds (VOCs). Martinsville represents many similar communities facing the challenge of groundwater and soil contamination and vapor intrusion, where residents are often frustrated by the lack of help in understanding and addressing the problem. The objective of this study was to evaluate PCE in exhaled breath to identify and quantify exposure to PCE and to explore the extent and level of PCE exposure among community residents. We measured chlorinated VOCs in exhaled breath samples from 38 healthy individuals who lived either in a contamination area or outside any plume area. We also measured VOCs in indoor air and tap water samples collected from 10 homes. PCE was detected in all exhaled breath samples (mean: 6.6 μg/m³; range: 1.9-44 μg/m³) and tap water samples (mean: 0.74 μg/L; range: 0.39-0.92 μg/L). PCE was detected in six of nine (66%) homes with air concentrations ranging from 1.6 to 70 μg/m³, exceeding the EPA action level of 42 μg/m³. We did not detect TCE or any other chlorinated VOCs in these samples. PCE exposure occurred among individuals living on the EPA Superfund site, as well as among those living on other plume sites and those living outside any known plumes. Preventive measures should focus on identifying highly exposed groups and reducing their exposures, followed by addressing moderately elevated exposures in the community. Our results demonstrated that PCE in exhaled breath can be used as an effective tool in community engaged environmental health research to evaluate the extent and level of community exposure, increase awareness, and promote residents' participation in research and site cleanup decision-making.

Transient migration behavior of VOC vapor in layered unsaturated soils subjected to multiple time-dependent point pollution sources: Analytical study

Predicting the migration behavior of volatile organic compounds (VOCs) vapor is essential for the remediation of subsurface contamination such as soil vapor extraction. Previous analytical prediction models of VOCs migration are mostly limited to constant-concentration nonpoint sources in homogeneous soil. Thus, this study presents a novel analytical model for two-dimensional transport of VOCs vapor subjected to multiple time-dependent point sources involving transient diffusion, sorption and degradation in layered unsaturated soils. Two representative time-dependent sources, i.e., an instantaneous source and a finite pulse source, are used to describe the short-term and long-term leakage. Results reveal that soil heterogeneity can cause pollution accumulation, especially in low-diffusivity capillary fringe. The assumption of an equivalent plane source from multiple point sources would significantly overestimate the vapor concentration and the contaminated range. The previous single point source model is no longer inapplicable when the relative distance and/or the release interval between sources is small, giving a strong interaction between multiple sources. Moreover, a faster vapor degradation rate or a higher groundwater level will reduce the area of vapor plume linearly. Hence, close attention should be paid to the time-variation characteristics of multiple sources, the vapor degradation and the groundwater level fluctuation in practice to facilitate soil remediation. The proposed model is a promising tool for addressing the above issue.

Two-dimensional analytical solution for subsurface volatile organic compounds vapor diffusion from a point source in layered unsaturated zone

Although migration of subsurface volatile organic compounds (VOCs) from contaminant sources in unsaturated soil widely exists, the related analytical models are quite limited. A two-dimensional analytical solution is hence developed to simulate vapor diffusion from the subsurface contaminant source in the layered unsaturated zone. The contaminant source is simplified as a point source leaking at a constant rate. The influences of several important factors, including thickness of stagnant air layer, depth of groundwater table, source characteristics and soil layering characteristics, on vapor migration in subsurface soil are comprehensively investigated by the present model. Soil type does not affect the normalized vapor concentration profile for homogeneous soil, which is not valid for layered soil. The width and effective diffusivity of the upward diffusion pathway and downward diffusion pathway are favorable indexes to evaluate the intensity of subsurface vapor horizontal diffusion. The single-layer capillary fringe assumption overestimates the vapor plume, the assumption can give acceptable result for coarse soil while it is recommended to divide the soil into several layers based on the water-filled porosity profile for fine soil.

Observation of Conditions Preceding Peak Indoor Air Volatile Org Compound Concentrations in Vapor Intrusion Studies

Temporal and spatial variability of indoor air volatile organic compound (VOC) concentrations can complicate vapor intrusion (VI) assessment and decision-making. Indicators and tracers (I&T) of VI, such as differential temperature, differential pressure, and indoor radon concentration, are low-cost lines of evidence to support sampling scheduling and interpretation of indoor air VOC sampling results. This study compares peak indoor air chlorinated VOC concentrations and I&T conditions before and during those peak events at five VI sites. The sites differ geographically and in their VI conceptual site models (CSM). Relative to site-specific baseline values, the results show that cold or falling outdoor temperatures, rising cross slab differential pressures, and increasing indoor radon concentrations can predict peak VOC concentrations. However, cold outdoor air temperature was not useful at one site where elevated shallow soil temperature was a better predictor. Correlations of peak VOC concentrations to elevated or rising barometric pressure and low wind speed were also observed with some exceptions. This study shows how the independent variables that control or predict peak indoor air VOC concentrations are specific to building types, climates, and VI CSMs. More I&T measurements at VI sites are needed to identify scenario-specific baseline and peak related I&T conditions to improve decision-making.

Mathematical analysis and flux-based radius of influence for radon/VOC vapor intrusion mitigation systems

Volatile organic compounds (VOCs) and radon progeny pose potential health risks to occupants of certain buildings via subsurface vapor intrusion (VI) to indoor air. VI mitigation is usually performed using systems that extract gas from below the building, and the system performance is typically evaluated by measuring the distribution of applied vacuum below the floor. This article provides a new approach to assessing the radius of influence (ROI) for subslab venting systems based on mass flux instead of static vacuum distribution and includes an analyses of 121 pneumatic tests performed at 65 different suction points in 16 different buildings. The mathematical model represents a two-layer system with horizontal radial flow through transmissive material below the floor slab and vertical flow through discontinuities in the floor slab (which is simplified to approximate an equivalent porous medium). The analysis includes comparisons of the flux-based ROI to values calculated using the two-layer model for 1) vacuum, 2) velocity, and 3) travel time, which may be useful as alternative performance metrics for mitigation systems.

Two-dimensional analytical solution for VOC vapor migration through layered soil laterally away from the edge of contaminant source

A two-dimensional analytical solution is developed to simulate vapor migration in layered soil laterally away from the edge of contaminant source and has advantages in considering the vapor concentration profile in a functional form near the source edge. The analytical solution is validated against existing analytical solution, numerical model and experimental results. It has also proved to be an alternative screening tool to evaluate the vapor intrusion (VI) risk by compared with existing VI assessment tools. The influence of the characteristics of contaminant source and soil layer on the VI risk are investigated. The existence of capillary fringe effectively reduces VI risk. Among all the single-layer-soil cases, the lateral inclusion zone for sand is the widest due to the thinnest capillary fringe and the lowest effective diffusivity ratio between soil and capillary fringe. For layered soil, the lower effective diffusivity layer overlying the higher one enhances the horizontal diffusion and extends the lateral inclusion zone. The width of lateral inclusion zone increases logarithmically with increasing source concentration while it increases linearly with increasing source depth. Based on the calculation results, a simplified formula is proposed to preliminarily estimate the width of lateral inclusion zone for the typical single-layer-soil cases considering the capillary fringe.

Vapor Intrusion Investigations and Decision-Making: A Critical Review

At sites impacted by volatile organic compounds (VOCs), vapor intrusion (VI) is the pathway with the greatest potential to result in actual human exposure. Since sites with VI were first widely publicized in late 1990s, the scientific understanding of VI has evolved considerably. The VI conceptual model has been extended beyond relatively simple scenarios to include nuances, such as biological and hydrogeological factors that may limit the potential for VI and alternative pathways, such as preferential pathways and direct building contact/infiltration that may enhance VI in some cases. Regulatory guidance documents typically recommend initial concentration- or distance-based screening to evaluate whether VI may be a concern, followed by a multiple-lines-of-evidence (MLE) investigation approach for sites that do not screen out. These recommendations for detailed evaluation of VI currently focus on monitoring of VOC concentrations in groundwater, soil gas, and indoor air and can be supplemented by other lines of evidence. In this Critical Review, we summarize key elements important to VI site characterization, provide the status and current understanding, and highlight data interpretation challenges, as well as innovative tools developed to help overcome the challenges. Although there have been significant advances in the understanding of VI in the past 20 years, limitations and knowledge gaps in screening, investigation methods, and modeling approaches still exist. Potential areas for further research include improved initial screening methods that account for the site-specific role of barriers, improved understanding of preferential pathways, and systematic study of buildings and infrastructure other than single-family residences.

A conceptual model for vapor intrusion from groundwater through sewer lines

The role of sewer lines as preferential pathways for vapor intrusion is poorly understood. As a result, these pathways are often not considered when developing vapor intrusion investigation or mitigation plans. Neglecting this pathway can complicate data interpretation, which can result in repeated, and potentially unnecessary, rounds of sampling. Although a number of recent studies have highlighted the importance of sewers as preferential pathways at individual buildings, there is currently little specific technical or regulatory guidance on how to address it. The purpose of our study, therefore, was to conduct systematic testing to better understand the sewer vapor intrusion conceptual model. Through sampling at >30 different sites, the degree of interaction between impacted groundwater and the sewer lines were identified as the main factor when determining the degree of risk for sewer vapor intrusion at a given site. Higher risk sites are those with direct interaction between the subsurface volatile organic compound (VOC) source, such as groundwater, and the sewer line itself. This information can be used to prioritize sites and buildings to test for this particular exposure pathway.