The goal of this project was to demonstrate that the multistage vapor-phase contaminant mass discharge (MS-CMD) test and vapor-phase tomography (VPT) can effectively characterize persistent volatile organic compound (VOC) sources in the vadose zone and measure their associated mass discharge. It is anticipated that these technologies will improve evaluation of vadose zone source impacts on groundwater and vapor intrusion.

The U.S. Department of Defense (DoD) has focused significant effort on characterizing and treating chlorinated solvent sources in groundwater. There are two primary concerns associated with sites that contain vadose zone volatile organic compound (VOC) sources. First, discharge of contaminant vapor from the vadose zone source may impact the underlying groundwater. Second, contaminant vapor from the vadose zone source may migrate to the land surface and transfer into buildings, thereby causing vapor intrusion. The DoD manages thousands of sites wherein the vadose zone is contaminated by chlorinated solvents and other VOCs.

Currently, the decision to require remediation of a vadose zone source is typically based on assessing the potential impact of the vadose zone source on groundwater or vapor intrusion. Concomitantly, setting appropriate vadose zone remediation goals once a remedy selection is made, as well as evaluating attainment of these remediation goals, requires evaluating these persistent sources in terms of their impact on groundwater remediation goals or vapor intrusion concerns.

These issues are of particular relevance for soil vapor extraction (SVE), which is the presumptive remedy for vadose zone systems contaminated by chlorinated solvents. Standard practices guidance manuals developed by the EPA (2001) and the USACE (2002) outline procedures for assessing transition/closure of SVE systems using several types of analyses, including evaluating the impact of vadose zone source contamination on groundwater.

Characterizing the impact of vadose zone contaminant sources on ground-water or vapor intrusion requires determination of the contaminant mass discharge from the source. The standard approach for characterizing vapor-phase mass discharge is to measure static contaminant concentrations for vapor (soil gas survey [SGS]) or sediment (borehole cores) samples, and to use them as input for a mathematical screening model to estimate contaminant mass discharge. This approach has become widely used to evaluate the impact of vadose zone sources on ground-water or vapor intrusion. However, this approach can be subject to considerable uncertainty in the estimates obtained, depending upon the robustness of the input data as well as the simplifications employed in the development and application of the screening model.

Second, some portion of contaminant mass in the vadose zone is usually associated with regions that are poorly accessible (e.g., low permeability zones). Characterizing mass-discharge associated with these regions may often be problematic with the SGS method.

Third, the typical implementation approach for the SGS or sediment coring methods are not able to readily characterize the temporal variability of mass-transfer processes. The potential limitations associated with typical screening models for VOC transport are well documented. As a result of these and other issues, the current standard approach for characterizing vapor-phase contaminant mass discharge can be influenced by a large degree of uncertainty.

This research was conducted to demonstrate two vadose zone characterization technologies that can provide more accurate measures of vapor-phase contaminant mass discharge, characterize mass-transfer conditions, and provide a higher resolution characterization of the source distribution. These novel technologies will support improved assessment of vadose zone source impacts on groundwater and vapor intrusion. They will also support improved optimization of SVE systems, as well as

This work was done by Dr. Mark Brusseau of the University of Arizona for the Environmental Security Technology Certification Program. For more information, download the Technical Support Package (free white paper) below. ESTCP-0002


This Brief includes a Technical Support Package (TSP).
Use of Mass-Flux Measurement and Vapor-Phase Tomography to Quantify Vadose-Zone Source Strength and Distribution

(reference ESTCP-0002) is currently available for download from the TSP library.

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This article first appeared in the August, 2020 issue of Aerospace & Defense Technology Magazine.

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