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SERDP and ESTCP are organizations that fund research and technology development for environmental challenges, with a focus on groundwater management and remediation. They manage $211 million in program funds for projects in various areas, including Environmental Restoration. Recent projects have focused on PFAS and aqueous film-forming foams, with ongoing research on chlorinated solvents, perchlorate, and 1,4-dioxane. The organizations also host an annual conference, the DoD Energy and Environment Innovation Symposium, to showcase new environmental and energy technologies. [Extracted from the article]

The article discusses the slow pace of technology advancement and adoption in the remediation industry. The author highlights the challenges faced by various stakeholders, including Potentially Responsible Parties (PRPs), consultants, regulators, academics, and vendors. The author identifies two categories of technologies that tend to progress quickly and gain widespread acceptance: incremental technologies that can be applied at a large scale with predictable cost savings, and complex technologies that offer a significant advancement or exclusive solution to a specific challenge. The article also addresses the need for accelerated technology development in the PFAS remediation market and suggests increased support for collaborations, technology readiness levels, venture capital investment, knowledge transfer, and outcome-based contract models. [Extracted from the article]

Soil vapor extraction (SVE) can be applied for remediation, and also as an alternative to sub‐slab depressurization (SSD) for vapor intrusion (VI) mitigation. This study compares capital, operation, and treatment costs of SVE and SSD systems using data collected during a multi‐year demonstration project conducted at eight buildings in an urban setting. The capital cost of the SVE system is substantially less than the estimated total capital cost of individual SSD systems. The SVE operating costs are higher, especially in the early operating years when it is being operated for mass removal and treatment. As a result, the cumulative SVE system cost rises above that of the SSD systems in the sixth year of operation. A significant portion of the operations and maintenance cost advantage of the SSD systems comes from the assumption that off‐gas treatment is not required. Alternative cases show SVE costs are likely to be lower in scenarios where numerous small buildings requiring independent SSD systems overlie the SVE zone of influence. Conversely, SSD systems are less costly for cases with few small buildings overlying the SVE zone of influence. An additional benefit of SVE is continued mass removal. In a situation where an existing SVE can be repurposed for VI protection from residual volatile organic carbon (VOC) mass, the SVE cumulative costs over 30 years can remain lower than the cost of installing and operating SSD systems in multiple buildings. Article impact statement: Article is the first to compare soil vapor extraction (SVE) and subslab depressurization (SSD) costs for vapor intrusion (VI) mitigation. [ABSTRACT FROM AUTHOR]

Soil vapor extraction (SVE) is widely used to remove volatile organic compounds from the vadose zone. Design of SVE systems rely largely upon vacuum responses and limited vapor concentration data measured during short‐term soil gas extraction tests performed in single extraction wells. Interpretation of such vacuum data is often simply a rule of thumb as most field sites have layering complexity negating applicability of existing analytical models. This paper provides the derivation of an analytical model for steady, axisymmetric gas flow in heterogeneous (layered) soils from a single well. A general, variable flow boundary condition along the well screen represents actual conditions more closely than a uniform flow or uniform well pressure condition. Each soil layer is assumed homogeneous with anisotropic gas permeability. The solution is derived using the generalized integral transform technique and includes expressions for vacuum, velocities, and streamlines. The model is applied to the interpretation of multiple well tests at a field site and uses linear superposition to extend the flow model to multi‐well extraction. The demonstration site included an array of vacuum monitoring data collected during nine individual well flow tests. A method of normalizing the vacuum data is illustrated that allowed the full data set to be employed in a single calibration effort. The test site also included a surface cap with an apparent vertical permeability two to three orders of magnitude smaller than the sands of the vadose zone. This large permeability contrast posed no difficulties in evaluating the solution. Article impact statement: A new analytical model is derived for flow and vacuum in a multi‐layer subsurface with application to SVE and VI mitigation. [ABSTRACT FROM AUTHOR]

While this is a positive development to encourage remediation of Superfund sites that have not received adequate attention, EPA has also announced that it will use $1 billion from the IIJA appropriation to address the Superfund site backlog (USEPA 2021f). The Environmental Protection Agency (EPA) adopted a policy of accelerated Superfund site completions and subsequent deletion from the National Priorities List (NPL) of abandoned contaminated sites in 2017 (USEPA 2017). EPA Superfund Regulation EPA may delete an NPL site if it determines that no further response is required to protect human health or the environment. In April 2020, EPA issued an Interim Record of Decision (ROD) for additional cleanup of groundwater, soil, and soil gas contamination and resolve vapor intrusion issues while maintaining ICs (USEPA 2021e). [Extracted from the article]

Diminishing rates of subsurface volatile contaminant removal by soil vapor extraction (SVE) oftentimes warrants an in‐depth performance assessment to guide remedy decision‐making processes. Such a performance assessment must include quantitative approaches to better understand the impact of remaining vadose zone contamination on soil gas and groundwater concentrations. The spreadsheet‐based Soil Vapor Extraction Endstate Tool (SVEET) software functionality has recently been expanded to facilitate quantitative performance assessments. The updated version, referred to as SVEET2, includes expansion of the input parameter ranges for describing a site (site geometry, source characteristics, etc.), an expanded list of contaminants, and incorporation of elements of the Vapor Intrusion Estimation Tool for Unsaturated‐zone Sources software to provide soil gas concentration estimates for use in vapor intrusion evaluation. As part of the update, SVEET2 was used to estimate the impact of a tetrachloroethene (PCE) vadose zone source on groundwater concentrations, comparing SVEET2 results to field‐observed values at an undisclosed site where SVE was recently terminated. PCE concentrations from three separate monitoring wells were estimated by SVEET2 to be within the range of 6.0–6.7 μg/L, as compared to actual field concentrations that ranged from 3 to 11 μg/L PCE. These data demonstrate that SVEET2 can rapidly provide representative quantitative estimates of impacts from a vadose zone contaminant source at field sites. In the context of the SVE performance assessment, such quantitative estimates provide a basis to support remedial and/or regulatory decisions regarding the continued need for vadose zone volatile organic compound remediation or technical justification for SVE termination, which can significantly reduce the cost to complete for a site. Article impact statement: Soil Vapor Extraction Endstate Tool 2 provides quantitative estimates of vadose zone contaminant source impacts as key input to support soil vapor extraction system end point decisions. [ABSTRACT FROM AUTHOR]

Soil vapor extraction (SVE) is effective for removing volatile organic compound (VOC) mass from the vadose zone and reducing the potential for vapor intrusion (VI) into overlying and surrounding buildings. However, the relationship between residual mass in the subsurface and VI is complex. Through a series of alternating extraction (SVE on) and rebound (SVE off) periods, this field study explored the relationship and aspects of SVE applicable to VI mitigation in a commercial/light‐industrial setting. The primary objective was to determine if SVE could provide VI mitigation over a wide area encompassing multiple buildings, city streets, and subsurface utilities and eliminate the need for individual subslab depressurization systems. We determined that SVE effectively mitigates offsite VI by intercepting or diluting contaminant vapors that would otherwise enter buildings through foundation slabs. Data indicate a measurable (5 Pa) influence of SVE on subslab/indoor pressure differential may occur but is not essential for effective VI mitigation. Indoor air quality improvements were evident in buildings 100 to 200 feet away from SVE including those without a measurable reversal of differential pressure across the slab or substantial reductions in subslab VOC concentration. These cases also demonstrated mitigation effects across a four‐lane avenue with subsurface utilities. These findings suggest that SVE affects distant VI entry points with little observable impact on differential pressures and without relying on subslab VOC concentration reductions. Article impact statement: This paper provides new information demonstrating VI mitigation by SVE at distances beyond the primary sources for subsurface VOCs. [ABSTRACT FROM AUTHOR]

Abstract: 1,4‐Dioxane is a volatile organic compound that is fully miscible in water, allowing it to sequester in vadose zone pore water and serve as a long‐term source of groundwater contamination. Conventional soil vapor extraction (SVE) removes 1,4‐dioxane; however, substantial 1,4‐dioxane can remain even after other colocated chlorinated solvents have been remediated. A field demonstration of “enhanced SVE” (XSVE) with focused extraction and heated injection was conducted at former McClellan AFB, CA, achieving 94% reduction in soil concentrations. A screening‐level tool, HypeVent XSVE, was created to assist in system design and data reduction and to anticipate how operating factors affect XSVE performance (e.g., cleanup level, remediation time, etc.). It assumes well‐mixed conditions, and combines an energy balance, mass balances for water and contaminant, and a temperature‐dependent 1,4‐dioxane Henry's Law constant. User inputs include the target treatment zone size, initial 1,4‐dioxane and soil moisture concentrations, and ambient site and injection/extraction conditions (temperature, humidity). Projections based on inputs representative of demonstration site conditions adequately anticipated the observed macroscopic field results. Sensitivity analyses show that removal increases with increasing heated air injection temperature and relative humidity and decreasing initial soil moisture content. [ABSTRACT FROM AUTHOR]

Abstract: 1,4‐Dioxane is totally miscible in water, sequestering in vadose pore water that can serve as a source of long‐term groundwater contamination. Although some 1,4‐dioxane is removed by conventional soil vapor extraction (SVE), remediation is typically inefficient. SVE efficiency is hindered by low Henry’s Law constants at ambient temperature and redistribution to vadose pore water if SVE wells pull 1,4‐dioxane vapors across previously clean soil. It was hypothesized that heated air injection and more focused SVE extraction (“Enhanced SVE” or XSVE) could increase the efficiency of 1,4‐dioxane vadose treatment, and this new process was tested at former McClellan Air Force Base, CA. The XSVE system had four peripheral heated air injection wells surrounding a 6.1 m × 6.1 m × 9.1 m deep treatment zone with a central vapor extraction well. After 14 months of operation, soil temperatures reached as high as ~90 °C near the injection wells and the treatment zone was flushed with ~20,000 pore volumes of injected air. Post‐treatment sampling results showed reductions of ~94% in 1,4‐dioxane and ~45% in soil moisture. Given the simplicity of the remediation system components and the promising demonstration test results, XSVE has the potential to be a cost‐effective remediation option for vadose zone soil containing 1,4‐dioxane. [ABSTRACT FROM AUTHOR]

This study evaluates the theory, and some practical aspects of using temperature measurements to assess aerobic biodegradation in hydrocarbon contaminated soil. The method provides an easily applicable alternative for quantifying the rate of biodegradation and/or evaluating the performance of in situ remediation systems. The method involves two nonintrusive procedures for measuring vertical temperature profiles down existing monitoring wells; one using a thermistor on a cable for one-time measurements and the other using compact temperature data loggers deployed for 3-month to 1-year period. These vertical temperature profile measurements are used to identify the depth and lateral extent of biodegradation as well as to monitor seasonal temperature changes throughout the year. The basic theory for using temperature measurements to estimate the minimum rate of biodegradation will be developed, and used to evaluate field measurements from sites in California where biodegradation of spilled petroleum hydrocarbons is due to natural processes. Following, temperature data will be used to evaluate the relative rates of biodegradation due to natural processes and soil vapor extraction (SVE) at a former refinery site in the North-Central United States. The results from this study show that the temperature method can be a simple, cost effective tool for assessing biodegradation in the soil, and optimizing remediation systems at a wide variety of hydrocarbon spill sites. [ABSTRACT FROM AUTHOR]

Soil vapor extraction ( SVE) is a prevalent remediation remedy for volatile organic compound ( VOC) contaminants in the vadose zone. To support selection of an appropriate condition at which SVE may be terminated for site closure or for transition to another remedy, an evaluation is needed to determine whether vadose zone VOC contamination has been diminished sufficiently to keep groundwater concentrations below threshold values. A conceptual model for this evaluation was developed for VOC fate and transport from a vadose zone source to groundwater when vapor-phase diffusive transport is the dominant transport process. A numerical analysis showed that, for these conditions, the groundwater concentration is controlled by a limited set of parameters, including site-specific dimensions, vadose zone properties, and source characteristics. On the basis of these findings, a procedure was then developed for estimating groundwater concentrations using results from the three-dimensional multiphase transport simulations for a matrix of parameter value combinations and covering a range of potential site conditions. Interpolation and scaling processes are applied to estimate groundwater concentrations at compliance (monitoring) wells for specific site conditions of interest using the data from the simulation results. The interpolation and scaling methodology using these simulation results provides a far less computationally intensive alternative to site-specific three-dimensional multiphase site modeling, while still allowing for parameter sensitivity and uncertainty analyses. With iterative application, the approach can be used to consider the effect of a diminishing vadose zone source over time on future groundwater concentrations. This novel approach and related simulation results have been incorporated into a user-friendly Microsoft® Excel®-based spreadsheet tool entitled SVEET (Soil Vapor Extraction Endstate Tool), which has been made available to the public. [ABSTRACT FROM AUTHOR]

Methods are developed to use data collected during cyclic operation of soil vapor extraction (SVE) systems to help characterize the magnitudes and time scales of mass flux associated with vadose zone contaminant sources. Operational data collected at the Department of Energy’s Hanford site are used to illustrate the use of such data. An analysis was conducted of carbon tetrachloride vapor concentrations collected during and between SVE operations. The objective of the analysis was to evaluate changes in concentrations measured during periods of operation and nonoperation of SVE, with a focus on quantifying temporal dynamics of the vadose zone contaminant mass flux, and associated source strength. Three mass flux terms, representing mass flux during the initial period of an SVE cycle, during the asymptotic period of a cycle, and during the rebound period, were calculated and compared. It was shown that it is possible to use the data to estimate time frames for effective operation of an SVE system if a sufficient set of historical cyclic operational data exists. This information could then be used to help evaluate changes in SVE operations, including system closure. The mass flux data would also be useful for risk assessments of the impact of vadose zone sources on groundwater contamination or vapor intrusion. [ABSTRACT FROM AUTHOR]

Aerobic biodegradation can contribute significantly to the attenuation of petroleum hydrocarbons vapors in the unsaturated zone; however, most regulatory guidance for assessing potential human health risks via vapor intrusion to indoor air either neglect biodegradation in developing generic screening levels or allow for only one order of magnitude additional attenuation for aerobically degradable compounds, which may be overly conservative in some cases. This paper describes results from three-dimensional numerical model simulations of vapor intrusion for petroleum hydrocarbons to assess the influence of aerobic biodegradation on the attenuation factor for a variety of source concentrations and depths for residential buildings with basements and slab-on-grade construction. The simulations conducted in this study provide a framework for understanding the degree to which bioattenuation will occur under a variety of scenarios and provide insight into site conditions that will result in significant biodegradation. This improved understanding may be used to improve the conceptual model of contaminant transport, guide field data collection and interpretation, and estimate semi-site-specific attenuation factors for combinations of source concentrations, source depth, oxygen distribution, and building characteristics where site conditions reasonably match the scenarios simulated herein. [ABSTRACT FROM AUTHOR]

A series of laboratory experiments were conducted to evaluate the relationship between degree of tetrachloroethylene (PCE) mass removal and heating duration, initial dense nonaqueous phase liquid (DNAPL) saturation, and soil grain size. Data were collected to evaluate the impact of postheating sample temperature on PCE concentration and to calculate the thermal diffusivity of the soil. A linear relationship was found between initial DNAPL saturation and duration of the co-boiling plateau, which occurred at 89 °C ± 4 °C. PCE concentrations were reduced most in samples that were heated beyond dryout, but continued heating for 12 hours beyond that point did not lead to further decreases. Heating samples with initial DNAPL saturations between 4.9% and 39.9% pore space illustrated a positive relationship between initial saturation and final soil concentration for a fixed heating duration. Smaller grain size resulted in lower postheating soil concentrations. Cooling postthermal remedy soils to as low as 20 °C prior to sampling did not affect measured PCE concentrations. An analytical model fit to cooling data indicated that the soil diffusivity values ranged from 1.4 × 10−7 to 1.8 × 10−7 m2/s. [ABSTRACT FROM AUTHOR]

New semianalytical solutions for transient soil vapor extraction are developed for P (pressure) or P2 linearizations of the governing air flow equation. Our method treats flow rate and wellhead pressure as two distinct variables instead of customarily coupling them together. An extraction well can be either pressure or flow rate controlled with uniform applied pressure along the screen and nonuniform radial flux. The two linearizations yield different solutions for subsurface pressures; the difference increases with applied wellhead pressure. This increasing difference serves as a constraint on the magnitude of the wellhead pressure used in a test. The P equation is preferred for simulating the vadose zone pressure because it allows for more flexible implementation of boundary conditions. The solutions cover the common field test condition of unrestricted flow of atmospheric air into the vadose zone and can be readily modified for a leaky confining layer or time-varying pressure at the ground surface. Analysis of field test data showed that the new model can closely simulate transient subsurface pressure distribution and revealed mutual dependence of porosity θ, and permeability kr and kz for the problem analyzed. The estimated kz/ kr was greater than 1, higher than would be expected for natural soil in the absence of secondary effects on permeability, such as vertical cracks. [ABSTRACT FROM AUTHOR]

Over the past several decades, soil vapor extraction (SVE) systems have successfully reduced the mass of volatile organic compounds in contaminated soils at many sites. However, predicting when operation of these systems should end once mass removal rates decline and become asymptotic to some low value is still a difficult task. To help make this decision, a simple decision model has been derived from a more rigorous cost-risk-based objective function that requires extensive Monte Carlo analyses. Use of the simple model to make a decision on continued operation does not require the same extensive modeling effort. Yet, use of the model does require the contaminant removal rate to be asymptotic to some low value indicating that the removal process has become dominated by diffusion from relatively low permeable soil. At this stage of the removal process, the SVE system should be replaced with an alternative approach if the cost of this alternative approach is less than the capital cost of continued SVE system operation. Applicability of the simple model is discussed through a comparison of the results predicted by the simple model for an operating SVE system to the rigorous results determined with the objective function. Results from this study show that under most conditions the simple model can be used to make a decision of whether a SVE system should continue to operate. [ABSTRACT FROM AUTHOR]

An analytical solution for steady-state soil vapor extraction (SVE) is derived for both pressure-controlled and flow-controlled well with nonuniform radial flux along the well screen. Since the flow rate and wellhead pressure are not lumped into a single variable, the solutions developed here can predict wellhead pressures required to achieve desired extraction rates. Simulations based on the established P (pressure) or P2 linearization of the governing air flow equation yield practically indistinguishable results for steady SVE. Existing numerical models for ground water flow can therefore be used to represent SVE in the P linearization for more complex vadose zone geometry and heterogeneous soils. The maximum differences between pressure distributions predicted by the nonuniform and uniform flux models occur near the ends of the extraction well screen and diminish with radial distance. Application to field test data indicates that estimates of horizontal and vertical soil-air permeability by the nonuniform flux model become similar to those obtained with the uniform flux model as the extraction well screen is shortened. [ABSTRACT FROM AUTHOR]

In 1999, a pilot soil vapor extraction (SVE) system was installed at a waste area within the Department of Energy's Savannah River Site located near Aiken, South Carolina, to remediate trichloroethylene (TCE) contamination and to evaluate monitoring and operational strategies for SVE application in layered heterogeneous materials. The specific objectives of the results reported here were (1) to evaluate the use of rebound analysis of soil gas concentrations as the basis for operational strategies, and (2) establish the endpoint criteria for active remedial action. Three soil gas TCE concentration rebound tests were conducted over a period of 18 months to assess system performance and progress. For each rebound test, the system was shut down and allowed to equilibrate for two to four weeks. Soil gas TCE concentrations were measured several times during this equilibration period. Comparison of these rebound test results has been used for evaluating SVE system performance. A transient two-dimensional diffusion model has been used to convert soil gas TCE rebound concentrations to estimates from distance to source, and the model predictions correspond with observed dense nonaqueous phase liquid at the site. Also, these rebound tests can provide sufficient information about contaminant distribution and SVE mass transfer limitations to select a reasonable and appropriate endpoint for active remedial operations. [ABSTRACT FROM AUTHOR]

Simulations using a one-dimensional, analytical, vadose zone, solute-transport screening code (VFLUX) were conducted to assess the effect of water saturation, NAPL saturation, degradation half-life, and boundary conditions at the vadose zone/ground water interface on model output. At high initial soil concentrations, model output was significantly affected by input parameters and lower boundary conditions yet still resulted in consistent decision-making to initiate or continue venting application. At lower soil concentrations, however, typical of what is observed after prolonged venting application, differences in model input and selection of lower boundary conditions resulted in inconsistent decision-making. Specifically, under conditions of low water saturation, use of a first-type, time-dependent lower boundary condition indicated that the primary direction of mass flux was from ground water to the vadose zone, suggesting little benefit from continued venting application. Use of a finite, zero-gradient lower boundary condition, though, indicated continued mass flux from the vadose zone to ground water, suggesting a continued need for venting application. In this situation, sensitivity analysis of input parameters, selection of boundary conditions, and consideration of overall objectives in vadose zone modeling become critical in regulatory decision-making. [ABSTRACT FROM AUTHOR]

Abstract: Soil gas sampling for 1,4‐dioxane at elevated soil temperatures, such as those experienced during in‐situ thermal treatment, has the potential to yield low results due to condensation of water vapor in the ambient temperature sampling vessel and the partitioning of 1,4‐dioxane into that condensate. A simple vapor/condensate sampling apparatus was developed to collect both condensate and vapor samples to allow for determination of a reconstituted effective soil gas concentration for 1,4‐dioxane. Results using the vapor/condensate sampling apparatus during a heated air injection SVE field demonstration are presented, along with those of a comparable laboratory system. Substantial 1,4‐dioxane mass was found in the condensate in both the lab and field (as high as ~50% in field). As soil temperatures increased, less 1,4‐dioxane mass was detected in field condensate samples than expected based on laboratory experiments. Extraction well effluent sampling at the wellhead by direct vapor canister sampling provided erratic results (several biased low by a factor of 5 or more) compared to those of the vapor/condensate apparatus. Direct vapor canister sampling of extraction well effluent after the air‐water separator, however, provided results reasonably comparable (within 35%) to those using the vapor/condensate apparatus at the wellhead. Soil gas sampling at elevated temperatures using the vapor/condensate apparatus alleviates potential low sampling bias due to condensation. [ABSTRACT FROM AUTHOR]

Effective long-term operation of soil vapor extraction ( SVE) systems for cleanup of vadose-zone sources requires consideration of the likelihood that remediation activities over time will alter the subsurface distribution and configuration of contaminants. A method is demonstrated for locating and characterizing the distribution and nature of persistent volatile organic contaminant ( VOC) sources in the vadose zone. The method consists of three components: analysis of existing site and SVE-operations data, vapor-phase cyclic contaminant mass-discharge testing, and short-term vapor-phase contaminant mass-discharge tests conducted in series at multiple locations. Results obtained from the method were used to characterize overall source zone mass-transfer limitations, source-strength reductions, potential changes in source-zone architecture, and the spatial variability and extent of the persistent source(s) for the Department of Energy's Hanford site. The results confirmed a heterogeneous distribution of contaminant mass discharge throughout the vadose zone. Analyses of the mass-discharge profiles indicate that the remaining contaminant source is coincident with a lower-permeability unit at the site. Such measurements of source strength and size as obtained herein are needed to determine the impacts of vadose-zone sources on groundwater contamination and vapor intrusion, and can support evaluation and optimization of the performance of SVE operations. [ABSTRACT FROM AUTHOR]