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Assessing performance and closure for soil vapor extraction: Integrating vapor discharge and impact to groundwater quality
Abstract
Soil vapor extraction (SVE) is typically effective for removal of volatile contaminants from higher-permeability portions of the vadose zone. However, contamination in lower-permeability zones can persist due to mass transfer processes that limit the removal effectiveness. After SVE has been operated for a period of time and the remaining contamination is primarily located in lower-permeability zones, the remedy performance needs to be evaluated to determine whether the SVE system should be optimized, terminated, or transitioned to another technology to replace or augment SVE. Numerical modeling of vapor-phase contaminant transport was used to investigate the correlation between measured vapor-phase mass discharge, MF(r), from a persistent, vadose-zone contaminant source and the resulting groundwater contaminant concentrations. This relationship was shown to be linear, and was used to directly assess SVE remediation progress over time and to determine the level of remediation in the vadose zone necessary to protect groundwater. Although site properties and source characteristics must be specified to establish a unique relation between MF(r) and the groundwater contaminant concentration, this correlation provides insight into SVE performance and support for decisions to optimize or terminate the SVE operation or to transition to another type of treatment.
... In addition to the measurement under SVE operational conditions, the contaminant mass flux from these persistent sources can be estimated under conditions with no SVE-induced flow [8]. This measure of contaminant mass flux can be used to estimate the impact of the persistent remaining source on ground surface or the groundwater if the SVE system is shut down [9]. ...
... Multidimensional numerical modeling provides a means to estimate contaminant transport in two or three dimensions to support selection of a remediation endpoint, although three-dimensional analysis is most appropriate for vapor phase transport [14]. Carroll et al. [9] recently conducted a predictive analysis demonstrating a correlation between vadose-zone volatile contaminant sources and groundwater contaminant concentrations. Their effort used a case study to evaluate the post-remediation relation between a persistent vadose zone source and groundwater contaminant concentrations, and was targeted for sites where SVE has been applied and has reached diminishing returns due to mass transfer limitations in some defined portion of the vadose zone. ...
... However, the method is applicable to other sites at this stage of SVE remediation that can be described using the general conceptual model shown in Figure 2 and basic hydraulic properties for the site. In the Carroll et al. [9] study, there was a linear function that described the groundwater contaminant concentration resulting from a vadose-zone volatile contaminant source as quantified by the vapor-phase mass discharge from the source. This vapor-phase mass discharge can be measured for an SVE system using the method described by Brusseau et al. [8]. ...
Soil vapor extraction (SVE) is a baseline remediation approach for volatile contaminants. While SVE is generally effective for removal of contaminants from higher permeability portions of the vadose zone, contamination in low-permeability zones can persist due to mass transfer processes that limit the removal effectiveness. Thus, a diminishing rate of contaminant extraction over time is typically observed, yet contamination may remain in low-permeability zones. Under these conditions, SVE performance needs to be evaluated to determine whether the system should be optimized, terminated, or transitioned to another technology to replace or augment SVE. Methodologies have been developed to quantify SVE performance over time and to evaluate the impact of persistent vadose zone contamination sources on groundwater quality. Recently, these methods have applied mass flux/discharge concepts to quantify contaminant source strength. Methods include field measurement techniques using the SVE system to quantify source strength and predictive analyses with analytical and numerical models to evaluate the impact of the contaminant source on groundwater.
... These two issues are generally the primary risk drivers for decisions regarding remediation of vadose-zone systems. Hence, the development and assessment of cleanup objectives for vadose-zone sources are typically based on evaluating the impact of vapor discharge from the vadose zone on groundwater quality or vapor intrusion (e.g., Brusseau et al., 2013a;Carroll et al., 2012;DiGiulio et al., 1999;EPA, 2001;Rosenbloom et al., 1993;USACE, 2002). ...
... A primary reason to conduct a vapor-phase MS-CMD test is to obtain a measured value of CMD under equivalent naturalgradient conditions that can be used in assessing the impact of vadose-zone contamination on groundwater quality or vaporintrusion potential. This application is illustrated using a recently developed vapor-discharge tool (Carroll et al., 2012;Oostrom et al., 2014). The tool is based on a data base relating long-term, maximum contaminant concentrations in groundwater to short-term vapor-phase mass discharges associated with vadose-zone VOC sources. ...
... A nomograph developed to simplify use of the discharge tool was presented by Carroll et al. (2012), and is adapted for use herein (Fig. 7). The CMD ng value of 32 g/d obtained for the TIAA site is used as input. ...
... Interest in its measurement and application has since grown during the past decade from these initial applications (e.g., Einarson and Mackay, 2001;Interstate Technology andRegulatory Council, 2002, 2010;Rao et al., 2002;Bockelmann et al., 2003;USEPA, 2003a;Newell et al., 2003;Brooks et al., 2004;Soga et al., 2004;SERDP, 2006;Brusseau et al., 2007Brusseau et al., , 2011aBrusseau et al., , 2011cDiFilippo and Brusseau, 2008). To date, research has focused on evaluating mass discharge for groundwater sources, and only a few studies have directly evaluated mass-discharge behavior associated with sources in the vadose zone (e.g., Rosenbloom et al., 1993;Poulsen et al., 1996;DiGiulio et al., 1999;Jellali et al., 2003;Truex et al., 2009;Brusseau et al., 2010;Oostrom et al., 2010;Stauffer et al., 2011;Carroll et al., 2012Carroll et al., , 2013. ...
... An illustration of establishing closure criteria for SVE systems in terms of future impact to groundwater was recently presented by Carroll et al. (2012). They used numerical modeling of vapor-phase contaminant transport to investigate the correlation between measured vapor-phase mass discharge (as characterized, for example, by the method of Brusseau et al., 2010) from a persistent, vadose-zone contaminant source and the resulting groundwater contaminant concentrations. ...
... The two examples are keyed to the "base case" type curve; MCL is maximum contaminant level (regulatory standard) (adapted from Carroll et al., 2012). ...
Contamination of vadose-zone systems by chlorinated solvents is widespread, and poses significant potential risk to human health through impacts on groundwater quality and vapor intrusion. Soil vapor extraction (SVE) is the presumptive remedy for such contamination, and has been used successfully for innumerable sites. However, SVE operations typically exhibit reduced mass-removal effectiveness at some point due to the impact of poorly accessible contaminant mass and associated mass-transfer limitations. Assessment of SVE performance and closure is currently based on characterizing contaminant mass discharge associated with the vadose-zone source, and its impact on groundwater or vapor intrusion. These issues are addressed in this overview, with a focus on summarizing recent advances in our understanding of the transport, characterization, and remediation of chlorinated solvents in the vadose zone. The evolution of contaminant distribution over time and the associated impacts on remediation efficiency will be discussed, as will the potential impact of persistent sources on groundwater quality and vapor intrusion. In addition, alternative methods for site characterization and remediation will be addressed.
... Data from a single rebound period (e.g., Brusseau et al. 1989Brusseau et al. , 2007Harvey et al. 1994;USACE 2002) or multiple rebound periods (Brusseau et al. 2010a) can serve as a source of information to help characterize mass-transfer constraints. SVE rebound data have also been analyzed to provide information on contaminant source location (e.g., USACE 2002; Switzer and Kosson 2007;Brusseau et al. 2010aBrusseau et al. , 2010bCarroll et al. 2012;Truex et al. 2013). These studies indicate that, when assessing SVE system performance due to diminishing contaminant removal, it is likely necessary to reevaluate the subsurface conditions to identify the distribution and strength of remaining sources and to develop a conceptual model for the site that is appropriate to support decisions to changes in remedial operations (Varadhan and Johnson 1997;Truex et al. 2009Truex et al. , 2012Carroll et al. 2012). ...
... SVE rebound data have also been analyzed to provide information on contaminant source location (e.g., USACE 2002; Switzer and Kosson 2007;Brusseau et al. 2010aBrusseau et al. , 2010bCarroll et al. 2012;Truex et al. 2013). These studies indicate that, when assessing SVE system performance due to diminishing contaminant removal, it is likely necessary to reevaluate the subsurface conditions to identify the distribution and strength of remaining sources and to develop a conceptual model for the site that is appropriate to support decisions to changes in remedial operations (Varadhan and Johnson 1997;Truex et al. 2009Truex et al. , 2012Carroll et al. 2012). ...
... Simulation results by Carroll et al. (2012) have shown that, for sites where vapor-phase transport is an important component of the overall contaminant fate and transport from a vadose zone source, a set of parameters control the resulting contaminant concentration in groundwater. The main objective of this work is to demonstrate the development of an approach applying a generalized conceptual model to estimate the 3D impact of vadose zone VOC sources on groundwater concentrations for sites where vapor-phase contaminant transport dominates (i.e., for relatively low recharge sites), and where a set of parameters control VOC groundwater concentrations. ...
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.
... Numerical modeling has become an important tool in the development of SVE technology by providing a better understanding of the SVE processes, and enhancement of SVE applications (Barnes, 2003;Barnes and White, 2006;Bradner and Murdoch, 2005;Harper et al., 2003) A proper numerical model can also assist in the design of an SVE system (Thomson et al., 1997), and help evaluate when SVE should be stopped (Carroll et al., 2012;Zhao and Zytner, 2008). Unfortunately, very few models incorporate transient mass transfer processes in a three-dimensional setting, which adversely affects performance. ...
... Some studies have focused on specific non-equilibrium mass transfer processes in one-dimensional (1D) column or two-dimensional (2D) horizontal experiments and numerical investigations (Abriola et al., 1997;Carroll et al., 2012;Høier et al., 2009;Rathfelder et al., 2000). The problem is, 1D/2D models cannot simulate the complex fluid flows and transport processes in subsurface soils with high heterogeneity. ...
... The ability of a 3D SVE model to match real field performance data is the most difficult, specifically within the tailing region (Barnes and White, 2006). Rate-limited mass transfer is an important factor that impacts tailing (Chai and Miura, 2004;Gidda et al., 2011), and the efficiency and speed of remediation (Carroll et al., 2012;Zhao and Zytner, 2008). Accordingly, the inclusion of rate-limited mass transfer coefficients within the 3D model is paramount to accurately simulating real field scenarios. ...
Mass transfer limitations impact the effectiveness of soil vapor extraction (SVE) and cause tailing. In order to identify the governing mass transfer processes, a three-dimensional SVE numerical model was developed. The developed model was based on Comsol Multiphysics® a finite element method that incorporates multi-phase flow, multi-component transport and non-equilibrium transient mass transfer. Model calibration was done against experimental data from previously completed lab-scale reactor experiments. The developed model, 3D-SVE, nicely simulates laboratory findings and allows for changes in the important governing mass transfer relationships. The modeling results showed that a single averaged mass transfer value is a poor representation of the entire SVE operation, and that a transient mass transfer coefficient is required to fully represent SVE tailing. Calibration of the lab scale model showed that the most important mass transfer occurs between the NAPL and vapor phase.
... In order to reduce the number and scale of experiments, the use of numerical simulations to analyze and predict the SVE process before formal testing has become an important way to develop SVE technology [14,15]. Numerical models can assist in the setting of soil parameters [16,17] and process flow design in SVE systems [18,19], as well as in the evaluation of the duration and effectiveness of SVE [20][21][22]. vacuum pump efficiency [12], which are used to predict VOC removal rates and required to complete cleanup goals. However, traditional pilot experiments r large number of trials of equipment that may be discarded later, which is time-con and costly [13]. ...
... In order to reduce the number and scale of experiments, the use o ical simulations to analyze and predict the SVE process before formal testing has an important way to develop SVE technology [14,15]. Numerical models can ass setting of soil parameters [16,17] and process flow design in SVE systems [18,19] as in the evaluation of the duration and effectiveness of SVE [20][21][22]. While soil parameters and process flow are important, the final evaluation al to consider the performance parameters of the equipment, which will affect whe equipment performs fully or whether it can achieve the VOC removal rate and required to complete the cleanup target as assessed by theoretical calculations a tests, which is important for government departments and project operators to before investing in the project [23]. ...
Soil vapor extraction (SVE) is one of the most commonly used technologies for soil remediation of contaminated sites, and the use of models to accurately predict and evaluate the operational effectiveness of SVE is a necessary part of site contamination treatment projects. A pneumatic model-based equipment model is proposed to comprehensively describe the SVE operation process. Though the numerical simulation, the influence of fan frequency, air valve opening, pressure, and total flow was analyzed, and an optimal extraction strategy was validated. Then, field experiments were carried out to verify the validity of the model. The proposed model and experimental results can provide a theoretical basis for the design and duration evaluation of SVE.
... While a large number of modelling studies were conducted to date, studies had been previously focused on mass transfer processes using a one-dimensional (1D) or two-dimensional (2D) model (Carroll et al., 2012;Høier et al., 2009;Kaleris, 2002;Rathfelder et al., 2000). The disadvantage of the usage of 1D/2D models, is that they may not completely simulate complex fluid flows and transport processes in soils with high heterogeneity. ...
... Other recent studies that had been conducted in the SVE field include the kinetic study on the calculation of degradation rate constant of petroleum hydrocarbons of up to 4 kg of contaminated soil (loamy sand, silt loam and sandy loam soil) in an intermediate scale experiment to obtain the degradation model of BV/SVE after a 30 day period (Khan and Zytner, 2013). Through numerical modelling of the SVE performance, the cleanup time can be estimated and hence providing a framework on the decision to continue or terminate soil vapour extraction via numerical modelling method (Carroll et al., 2012). The estimation of clean-up time through mathematical/analytical modelling had also been extensively studied (Alvim-Ferraz et al., 2006a;Alvim-Ferraz et al., 2006b;Kaleris and Croisé, 1997;Zhao and Zytner, 2008). ...
... Soil vapor extraction (SVE) tends to be highly effective for extraction of volatile organic contaminant (VOC) mass from the vadose zone during the early stages of remediation. However, mass-removal effectiveness declines over time, primarily due to mass-transfer constraints associated with contaminant mass residing within lower permeability, or high water content, portions of the vadose zone (e.g., Thomson and Flynn 2000;U.S. EPA 2001;US Army Corps of Engineers 2002;Switzer et al. 2004;Hoier et al. 2009;Truex et al. 2009;Yoon et al. 2009;Oostrom et al. 2010;Brusseau et al. 2010a;Carroll et al. 2012). A transition to less-efficient extraction and mass-transfer limited conditions may be due to organic liquid (i.e., dense nonaqueous phase liquids [DNAPLs]) mass transfer, desorption or gas/ water partitioning and diffusion, and mass transfer between lower-and higher-permeability media (e.g., Johnson et al., 1990;Brusseau 1991;Gierke et al. 1992;Armstrong et al. 1994;Conklin et al. 1995;Wilkins et al. 1995;Thomson and Flynn 2000;Yoon et al. 2008;Yoon et al. 2009). ...
... Recently, significant attention has been focused on direct measurement or estimation of contaminant mass discharge (CMD), and the use such measurements or estimates to evaluate risk and remediation performance (e.g., Freeze and McWhorter 1997;DiGiulio et al. 1999;Einarson and Mackay 2001;Stroo et al. 2003;Soga et al. 2004;Falta et al. 2005a;Falta et al. 2005b;Suchomel and Pennell 2006;Basu et al. 2006;Fure et al. 2006;Brusseau et al. 2007;Goltz et al. 2007;Annable 2008;Brooks et al. 2008;Falta 2008;DiFilippo and Brusseau 2008;Kaye et al. 2008;Basu et al. 2008;Basu et al. 2009;DiFilippo et al. 2010;Marble et al. 2010;Suthersan et al. 2010;ITRC 2010;Brusseau et al. 2011a;Brusseau et al. 2011b). To date, this prior research has focused on evaluating sources in groundwater, and only a few studies have evaluated mass-discharge behavior in the vadose zone (Rosenbloom et al. 1993;Widdowson et al. 1997;DiGiulio et al. 1999;DiGiulio and Varadhan 2001;Truex et al. 2009;Brusseau et al. 2010a;Carroll et al. 2012). ...
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.
... Physical treatment methods for oil-contaminated soil and groundwater encompass two key techniques: Soil Vapor Extraction (SVE) and Groundwater Pumping. SVE involves utilizing vacuum extraction to remove volatile organic compounds (VOCs) from soil, effectively reducing contaminant levels (Carroll et al., 2011). Conversely, Groundwater Pumping entails pumping contaminated groundwater to the surface for subsequent treatment, often employing techniques such as air stripping or activated carbon adsorption (National Research Council, 2003). ...
Oil contamination of soil and groundwater poses significant environmental and health risks, prompting this study to evaluate the effectiveness, costs, and environmental impacts of various treatment methods. A comprehensive review of 50 case studies and research papers reveals that biological treatment methods, specifically biodegradation and phytoremediation, achieve the highest contaminant removal rates (85-95%) at lower costs (100-500/m3) and physical methods (40-70%, high energy consumption). Furthermore, risk assessment indicates biological methods pose the lowest environmental risks, while chemical methods pose the highest. Sensitivity analysis underscores the importance of site-specific conditions and contaminant levels. Overall, the results suggest biological treatment methods, particularly biodegradation, as the most effective and cost-efficient option for oil-contaminated soil and groundwater remediation.
... Recently, many geophysical monitoring methods (Binley et al., 2015) for passive remediation have been applied such as spectral induced polarization and multi-frequency conductivity imaging (Bate et al., 2023;Hao et al., 2021;Moshe and Furman, 2022;Orozco et al., 2021). Performance-assessment guidance to support remedy decisions such as transitions to a new remedy or for site closure has also been developed (Carroll et al., 2012;Truex et al., 2017). For U.S. Superfund sites, monitored natural attenuation has increased to 30 % of recent remedy decisions for groundwater during 2018-2020, which was only 20 % in the prior three years (U.S.EPA, 2023). ...
... The difficulties in modeling the interaction of the vadose zone and groundwater are well described by Truex et al. (2009) As a result of the limitations in the early models and the complexity of the boundary condition at the water table, researchers continue to develop methods to examine the interaction of vadose zone contaminants with the groundwater in the context of SVE performance (Carroll et al., 2012;Oostrom et al., 2010;Truex et al., 2009). These approaches and those in U.S. ACE (2002) estimate the residual mass as well as the mass transfer coefficient (or utilize a onedimensional diffusion model) to assess the mass flux from a residual source and then assume various interactions with the underlying groundwater. ...
This document summarizes the state-of-the-science regarding the widespread use of SVE as a major treatment technology for removing VOCs from soil. SVE can be applied alone or as an integral component of more complex remedial technologies that volatilize subsurface contaminants (e.g., thermal remediation, air sparging). This EIP provides updated information since the issuance of the original Engineering Bulletin (U.S. EPA, 1991a) and two Engineering Forum Issue Papers (U.S. EPA, 1996a, 1997b) on SVE.
Download: https://cfpub.epa.gov/si/si_public_record_Report.cfm?dirEntryId=345171&Lab=NRMRL
... As equi- librium is reached over time, the maximum concentration impacts are attained. It depends on the contaminant mass and configuration of the vadose zone source in the subsurface, but generally a source would be expected to deplete and decrease in strength over time (e.g., Carroll et al. 2012;Truex et al. 2012). Thus, a constant-strength source would result in conservatively high concentration estimates over time, though the concentration estimates relative to a short time frame would merely be representative. ...
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 (VIETUS) software to provide soil gas concentration estimates for use in vapor intrusion (VI) 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 3–11 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 VOC remediation or technical justification for SVE termination, which can significantly reduce the cost to complete for a site. This article is protected by copyright. All rights reserved.
... However, great concern has been raised about the deterioration of groundwater quality related to rapid industrialization and civilization in the last decades [2,[6][7][8][9], and groundwater pollution events caused by human activities (the application of fertilizer and manure [10][11][12][13], the discharge of industrial effluents [14][15][16][17][18], and the leakage of landfill sites [19,20] have been reported round the world. Thus, the invention and implementation of groundwater remedial technologies is a hotspot issue for environmental experts nowadays [21][22][23][24], and numerous technologies involving physical, chemical, physical-chemical, biological and biochemical aspects have been invented and applied [25,26], e.g., the pump and treat method (contaminated groundwater is pumped by abstracted wells and further treated) [27][28][29][30], permeable reactive barriers (a subsurface barrier is constructed by reactive materials, and the polluted groundwater is remediated during flowing through the barrier) [31][32][33][34][35], soil vapor extraction (a vacuum is applied to the vadose zone to induce the controlled flow of air and remove volatile and some semi-volatile organic contaminants) [36][37][38], and monitored natural attenuation (based on efficient monitoring, pollutants are degraded to acceptable levels by natural physical, chemical and biochemical reactions) [39][40][41]. ...
: Due to the deficiency of fresh water resources and the deterioration of groundwater quality worldwide, groundwater remedial technologies are especially crucial for preventing groundwater pollution and protecting the precious groundwater resource. Among the remedial alternatives, bioelectrochemical systems have unique advantages on both economic and technological aspects. However, it is rare to see a deep study focused on the information mining and visualization of the publications in this field, and research that can reveal and visualize the development trajectory and trends is scarce. Therefore, this study summarizes the published information in this field from the Web of Science Core Collection of the last two decades (1999–2018) and uses Citespace to quantitatively visualize the relationship of authors, published countries, organizations, funding sources, and journals and detect the research front by analyzing keywords and burst terms. The results indicate that the studies focused on bioelectrochemical systems for groundwater remediation have had a significant increase during the last two decades, especially in China, Germany and Italy. The national research institutes and universities of the USA and the countries mentioned above dominate the research. Environmental Science & Technology, Applied and Environmental Microbiology, and Water Research are the most published journals in this field. The network maps of the keywords and burst terms suggest that reductive microbial diversity, electron transfer, microbial fuel cell, etc., are the research hotspots in recent years, and studies focused on microbial enrichment culture, energy supply/recovery, combined pollution remediation, etc., should be enhanced in future.
... The understanding of contaminant mass flux and/or discharge from NAPL source zones to groundwater plumes has developed over the past couple of decades (Freeze and McWhorter, 1997;Schwarz et al., 1998;Nichols and Roth, 2004;Soga et al., 2004;Falta et al., 2005a;Falta et al., 2005b;Basu et al., 2006;Fure et al., 2006;Annable et al., 2007;Brusseau et al., 2007;Goltz et al., 2007;Soga et al., 2007;Annable, 2008;Basu et al., 2008;Brooks et al., 2008;DiFilippo and Brusseau, 2008;Falta, 2008;Parker and Park, 2008;Basu et al., 2009;Tick and Rincon, 2009;Brusseau et al., 2010;DiFilippo et al., 2010;Suthersan et al., 2010;Brusseau et al., 2011;Carroll et al., 2012;Mobile et al., 2012). It has become a primary metric for assessing source-zone architecture and remediation performance, because it relates NAPL source-zone to dissolution dynamics. ...
... Soil contaminated with organic pollutants is commonly treated by conventional technologies, such as incineration, steam injection (Triplett Kingston et al. 2010;Tzovolou et al. 2011), soil vapor extraction and bioremediation (Lladó et al. 2013) where oxygen or nutrient amendments are injected through the soil. Most of these methods are highly energy-consuming (e.g., incineration) or have low removal efficiency in low permeability zones (e.g., soil vapor extraction, steam injection) (Nilsson et al. 2011;Carroll et al. 2012). The latter is due to the high permeability fluctuation of mineral soils (sand/clay/silt) that leads to preferential flow of the flushing remedial fluids through the high permeability soil layers Tsakiroglou 2008, 2009). ...
The remediation of soil, contaminated by organic pollutants, in a cylinder-to-plane dielectric barrier discharge reactor at atmospheric air pressure was reported. Two model organic pollutants were selected; a solid pollutant (2,6-dichloropyridine) and a liquid pollutant (n-dodecane). The effects of the contaminant’s initial concentration and state, the energy consumption, and the soil type on the pollutant removal efficiency were investigated. To that scope, various contaminated samples of both quartz sand and loamy sandy soil were treated by plasma for various treatment times and initial 2,6-dichloropyridine/n-dodecane concentrations. The results revealed that (1) the removal efficiency of 2,6-dichloropyridine was higher compared to that of n-dodecane at a given plasma treatment time and (2) the removal efficiency increased with the energy density increasing, but decreased as the soil heterogeneity, organic matter and pollutant concentration were enhanced. The main removal mechanism proposed is the evaporation of pollutant molecules coupled with their oxidation by plasma species in the gas and solid/liquid phase.
... The 92 mm/year estimate is within the range of recharge estimates reported by DePaolo et al. (2004) applying hydrogen isotopes to tank farm subsurface cores. The two recharge rates are part of official Hanford Site data sets (Last et al., 2006a;Fayer et al., 2010) for use in subsurface flow and transport modeling (e.g., Carroll et al., 2012;Oostrom et al., 2013). ...
For sites with a contaminant source located in the vadose zone, the nature and extent of groundwater contaminant plumes are a function of the contaminant flux from the vadose zone to groundwater. Especially for thick vadose zones, transport may be relatively slow making it difficult to directly measure contaminant flux. An integrated assessment approach, supported by site characterization and monitoring data, is presented to explain current vadose zone contaminant distributions and to estimate future contaminant flux to groundwater in support of remediation decisions. The U.S. Department of Energy Hanford Site (WA, USA) SX Tank Farm was used as a case study because of a large existing contaminant inventory in its deep vadose zone, the presence of a limited-extent groundwater plume, and the relatively large amount of available data for the site. A predictive quantitative analysis was applied to refine a baseline conceptual model through the completion of a series of targeted simulations. The analysis revealed that site recharge is the most important flux-controlling process for future contaminant flux. Tank leak characteristics and subsurface heterogeneities appear to have a limited effect on long-term contaminant flux into groundwater. The occurrence of the current technetium-99 groundwater plume was explained by taking into account a considerable historical water-line leak adjacent to one of the tanks. The analysis further indicates that the vast majority of technetium-99 is expected to migrate into the groundwater during the next century. The approach provides a template for use in evaluating contaminant flux to groundwater using existing site data and has elements that are relevant to other disposal sites with a thick vadose zone.
... local permeability, effective gas diffusivity, moisture), the NAPL composition and concentration and the biological activity of micro florae. For instance, due to the existence of strong heterogeneities in the pore space of mineral soils (sand/clay/silt), the pore length scales span five-to-six orders of magnitude [8], the local permeability fluctuates over a broad range [9,10], the fluid transport within soils is controlled by preferential flow pathways, and the efficiency of the conventional remediation techniques based on the flushing of fluids through the porous media becomes very low [11,12]. In addition, due to certain drawbacks of the conventional remediation technologies, such as energy/time requirements, and secondary pollution, it becomes widely accepted that the development of cost-effective, rapid and highly efficient methods for soil remediation is crucial. ...
A plane-to-grid dielectric barrier discharge (DBD) reactor operating with air at atmospheric pressure was used to investigate the removal of non-aqueous phase liquids (NAPLs) from soil layers. A mixture of n-C10, n-C12 and n-C16 was used as a model NAPL that polluted the soil at a very high initial concentration (100,000 mg/kg-soil). The effect of treatment time, energy consumption, and soil thickness on the NAPL removal efficiency was investigated, the plasma active species were identified, and the macroscopic gas temperature was determined. The NAPL remediation efficiency found to be as high as 99.9% after 60–120 s of plasma treatment, depending on soil thickness. The energy density required to remediate completely the NAPL was about 600 J/g-soil and was practically independent of the soil thickness, indicating that the DBD-based plasma has the potential to become a highly cost-effective technology for the remediation of NAPL-contaminated soils. N2+, N2∗, NOx and O3 were identified as plasma-induced reactive species, a maximum gas temperature close to 300 °C was recorded, and the total carbon detected in exhaust gases, in the form of CO and CO2, was ca 40% of that contained in the NAPL removed from the soil. The main mechanisms of NAPL removal by plasma found to be the evaporation of organic compounds coupled with their oxidation in liquid and gas phase. Using ATR-FTIR in combination with high-throughput organic profiling analysis by GC–MS, ketones and alcohols were identified as the main intermediate products of alkanes oxidation in soil matrix.
... Aqueous solutions may be injected for the purposes of flushing or to promote the in situ degradation of contaminants (Barcelona and Xie 2001;Devlin et al. 2004). Most of the soil remediation technologies have a limited NAPL removal efficiency due to the retention of pollutants in low-permeability zones (e.g., soil vapor extraction) (Brusseau et al. 2010;Carroll et al. 2012) or are C. A. Aggelopoulos (&) Á C. D. Tsakiroglou Institute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas, Stadiou Str., Platani, 26504 Patras-Rio, Greece e-mail: caggelop@iceht.forth.gr applicable to source zones composed of volatile organic contaminants (e.g., air sparging, supersaturated water injection) (Nelson et al. 2009;Adams et al. 2011). ...
Non-thermal dielectric barrier discharge plasma is examined as a method for the ex situ remediation of non-aqueous phase liquid (NAPL)-contaminated soils. A mixture of equal mass concentrations (w/w) of n-decane, n-dodecane and n-hexadecane was used as model NAPL. Two soil types differing with respect to the degree of micro-heterogeneity were artificially polluted by NAPL: a homogeneous silicate sand and a moderately heterogeneous loamy sand. The effect of soil heterogeneity, NAPL concentration and energy density on soil remediation efficiency was investigated by treating NAPL-polluted samples for various treatment times and three NAPL concentrations. The concentration and composition of the residual NAPL in soil were determined with NAPL extraction in dichloromethane and GC-FID analysis, while new oxidized products were identified with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The experimental results indicated that the overall NAPL removal efficiency increases rapidly in early times reaching a plateau at late times, where NAPL is removed almost completely. The overall NAPL removal efficiency decreases with its concentration increasing and soil heterogeneity strengthening. The removal efficiency of each NAPL compound is inversely proportional to the number of carbon atoms and consistent with alkane volatility. A potential NAPL degradation mechanism is suggested by accounting for intermediates and final products as quantified by GC-FID and identified by ATR-FTIR.
... These two issues are typically the primary risk drivers for decisions regarding remediation of vadose-zone systems. Hence, the development and assessment of cleanup objectives for vadose-zone sources are typically based on the impact of contaminant mass in the vadose zone on groundwater or vapor intrusion (e.g., Brusseau et al., 2013;Carroll et al., 2012;EPA, 2001;USACE, 2002). ...
A method termed vapor-phase tomography has recently been proposed to characterize the distribution of volatile organic contaminant mass in vadose-zone source areas, and to measure associated three-dimensional distributions of local contaminant mass discharge. The method is based on measuring the spatial variability of vapor flux, and thus inherent to its effectiveness is the premise that the magnitudes and temporal variability of vapor concentrations measured at different monitoring points within the interrogated area will be a function of the geospatial positions of the points relative to the source location. A series of flow-cell experiments was conducted to evaluate this premise. A well-defined source zone was created by injection and extraction of a non-reactive gas (SF6). Spatial and temporal concentration distributions obtained from the tests were compared to simulations produced with a mathematical model describing advective and diffusive transport. Tests were conducted to characterize both areal and vertical components of the application. Decreases in concentration over time were observed for monitoring points located on the opposite side of the source zone from the local-extraction point, whereas increases were observed for monitoring points located between the local-extraction point and the source zone. The results illustrate that comparison of temporal concentration profiles obtained at various monitoring points gives a general indication of the source location with respect to the extraction and monitoring points.
... The Pacific Northwest National Laboratory-continuum (PNNL-C) flow and solute transport simulations of the experiments were conducted with the water mode of the Subsurface Transport Over Multiple Phases (STOMP) simulator [58]. This fully implicit integrated finite difference mode of the simulator has been used to simulate a variety of single-and multiphase systems including (reactive) solute transport (e.g., [9,47]). The governing partial differential equation for water is discretized using the integrated-volume finite difference method by integrating over a control volume. ...
Four sets of nonreactive solute transport experiments were conducted with micromodels. Each set consisted of three experiments with one variable, i.e., flow velocity, grain diameter, pore-aspect ratio, and flow-focusing heterogeneity. The data sets were offered to pore-scale modeling groups to test their numerical simulators. Each set consisted of two learning experiments, for which all results were made available, and one challenge experiment, for which only the experimental description and base input parameters were provided. The experimental results showed a nonlinear dependence of the transverse dispersion coefficient on the Peclet number, a negligible effect of the pore-aspect ratio on transverse mixing, and considerably enhanced mixing due to flow focusing. Five pore-scale models and one continuum-scale model were used to simulate the experiments. Of the pore-scale models, two used a pore-network (PN) method, two others are based on a lattice Boltzmann (LB) approach, and one used a computational fluid dynamics (CFD) technique. The learning experiments were used by the PN models to modify the standard perfect mixing approach in pore bodies into approaches to simulate the observed incomplete mixing. The LB and CFD models used the learning experiments to appropriately discretize the spatial grid representations. For the continuum modeling, the required dispersivity input values were estimated based on published nonlinear relations between transverse dispersion coefficients and Peclet number. Comparisons between experimental and numerical results for the four challenge experiments show that all pore-scale models were all able to satisfactorily simulate the experiments. The continuum model underestimated the required dispersivity values, resulting in reduced dispersion. The PN models were able to complete the simulations in a few minutes, whereas the direct models, which account for the micromodel geometry and underlying flow and transport physics, needed up to several days on supercomputers to resolve the more complex problems.
... The water mode of the STOMP simulator (White and Oostrom, 2006) was used to simulate vadose zone aqueous phase flow and contaminant transport to examine the effects of perched-water conditions. The fully implicit, integrated finite difference code has been used to simulate several laboratory and field contaminant transport systems (e.g., Zhong et al., 2008;Oostrom et al., 2010;Carroll et al., 2012). The governing equations are the component mass-conservation equation for variably-saturated water flow and the solute transport equation, which is solved using a totalvariation diminishing scheme. ...
... igin. (c) PCE injection rate was increased to generate the NAPL phase of PCE. C1/4 denotes one-fourth of chlorine concentrations.Table 3 Simulation conditions for evaluating the effect of PCE source spill rates (m 3 /day), degradation rate constants (1/day), and enrichment factors (‰) on multi-phase contaminant transport and isotope fractionations. [6,9,24]. The simulation results shown inFig. 4a and b for case 1b are comparable to those from the verification example (Fig. 3a and b). It is noted that inFig. 4a , the mole fractions are normalized by the maximum PCE aqueous mole fraction (½X PCE;q max ffi 1:62 Â 10 À5 ). The concentration profiles inFig. 4a are different from those of ...
... Second, contaminant vapor from the vadose-zone source may migrate to the land surface and transfer into buildings, thereby causing vapor intrusion. The decision to require remediation of a vadose-zone source zone, as well as the development of associated cleanup objectives, is typically based on assessing the potential impact of the vadose-zone source on groundwater or vapor intrusion (e.g., Johnson and Ettinger, 1991;Rosenbloom et al., 1993;DiGiulio et al., 1998DiGiulio et al., , 1999EPA, 2001;Hers et al., 2002;USACE, 2002;Truex et al., 2009;Carroll et al., 2012). ...
Gas-phase transport experiments were conducted using a large weighing lysimeter to evaluate retention processes for volatile organic compounds (VOCs) in water-unsaturated (vadose-zone) systems, and to test the utility of gas-phase tracers for predicting VOC retardation. Trichloroethene (TCE) served as a model VOC, while trichlorofluoromethane (CFM) and heptane were used as partitioning tracers to independently characterize retention by water and the air-water interface, respectively. Retardation factors for TCE ranged between 1.9 and 3.5, depending on water content. The results indicate that dissolution into the bulk water was the primary retention mechanism for TCE under all conditions studied, contributing approximately two-thirds of the total measured retention. Accumulation at the air-water interface comprised a significant fraction of the observed retention for all experiments, with an average contribution of approximately 24%. Sorption to the solid phase contributed approximately 10% to retention. Water contents and air-water interfacial areas estimated based on the CFM and heptane tracer data, respectively, were similar to independently measured values. Retardation factors for TCE predicted using the partitioning-tracer data were in reasonable agreement with the measured values. These results suggest that gas-phase tracer tests hold promise for characterizing the retention and transport of VOCs in the vadose-zone.
Contamination of groundwater with chlorinated hydrocarbons has serious adverse effects on human health. As research efforts in this area have expanded, a large body of literature has accumulated. However, traditional review writing suffers from limitations regarding efficiency, quantity, and timeliness, making it difficult to achieve a comprehensive and up-to-date understanding of developments in the field. There is a critical need for new tools to address emerging research challenges. This study evaluated 1619 publications related to this field using VOSviewer and CiteSpace visual tools. An extensive quantitative analysis and global overview of current research hotspots, as well as potential future research directions, were performed by reviewing publications from 2000 to 2022. Over the last 22 years, the USA has produced the most articles, making it the central country in the international collaboration network, with active cooperation with the other 7 most productive countries. Additionally, institutions have played a positive role in promoting the publication of science and technology research. In analyzing the distribution of institutions, it was found that the University of Waterloo conducted the majority of research in this field. This paper also identified the most productive journals, Environmental Science & Technology and Applied and Environmental Microbiology, which published 11,988 and 3253 scientific articles over the past 22 years, respectively. The main technologies are bioremediation and chemical reduction, which have garnered growing attention in academic publishing. Our findings offer a useful resource and a worldwide perspective for scientists engaged in this field, highlighting both the challenges and the possibilities associated with addressing groundwater chlorinated solvent plumes remediation.
Background:
The National Institute of Environmental Health Sciences (NIEHS) Superfund Basic Research and Training Program (SRP) funds a wide range of transdisciplinary research projects spanning the biomedical and environmental sciences and engineering, supporting and promoting the application of that research to solving real-world problems.
Objectives:
We used a case study approach to identify the economic and societal benefits of SRP-funded research, focusing on the use of potentially hazardous substance remediation and site monitoring tools. We also identified successes and challenges involved in translating SRP grantees' research findings and advances into application.
Discussion:
We identified remediation and detection research projects supported by the SRP with the most potential for economic and societal benefits and selected 36 for analysis. To examine the benefits of these applied technologies, we interviewed 28 SRP-supported researchers and 41 partners. Five case studies emerged with the most complete information on cost savings-total savings estimated at >$100 million. Our analysis identified added societal benefits such as creation of small businesses, land and water reuse, sustainable technologies, exposure reduction, and university-industry partnerships.
Conclusions:
Research funded by the SRP has yielded significant cost savings while providing additional societal benefits. https://doi.org/10.1289/EHP3534.
There are complexity elements to consider when applying subsurface flow and transport models to support environmental analyses. Modelers balance the benefits and costs of modeling along the spectrum of complexity, taking into account the attributes of more simple models (e.g., lower cost, faster execution, easier to explain, less mechanistic) and the attributes of more complex models (higher cost, slower execution, harder to explain, more mechanistic and technically defensible). In this report, modeling complexity is examined with respect to considering this balance. The discussion of modeling complexity is organized into three primary elements: (1) modeling approach, (2) description of process, and (3) description of heterogeneity. Three examples are used to examine these complexity elements. Two of the examples use simulations generated from a complex model to develop simpler models for efficient use in model applications. The first example is designed to support performance evaluation of soil-vapor-extraction remediation in terms of groundwater protection. The second example investigates the importance of simulating different categories of geochemical reactions for carbon sequestration and selecting appropriate simplifications for use in evaluating sequestration scenarios. In the third example, the modeling history for a uranium-contaminated site demonstrates that conservative parameter estimates were inadequate surrogates for complex, critical processes and there is discussion on the selection of more appropriate model complexity for this application. All three examples highlight how complexity considerations are essential to create scientifically defensible models that achieve a balance between model simplification and complexity.
We examine the transient evolution of a negatively buoyant, laminar plume in an emptying filling box containing a uniform porous medium. In the long time limit, \unicode[STIX]{x1D70F}\rightarrow \infty , the box is partitioned into two uniform layers of different densities. However, the approach towards steady state is characterized by a lower contaminated layer that is continuously stratified. The presence of this continuous stratification poses non-trivial analytical challenges; we nonetheless demonstrate that it is possible to derive meaningful bounds on the range of possible solutions, particularly in the limit of large \unicode[STIX]{x1D707} , where \unicode[STIX]{x1D707} represents the ratio of the draining to filling time scales. The validity of our approach is confirmed by drawing comparisons against the free turbulent plume case where, unlike with porous media plumes, an analytical solution that accounts for the time-variable continuous stratification of the lower layer is available (Baines & Turner, J. Fluid Mech. , vol. 37, 1969, pp. 51–80; Germeles, J. Fluid Mech. , vol. 71, 1975, pp. 601–623). A separate component of our study considers time-variable forcing where the laminar plume source strength changes abruptly with time. When the source is turned on and off with a half-period, \unicode[STIX]{x0394}\unicode[STIX]{x1D70F} , the depth and reduced gravity of the contaminated layer oscillate between two extrema after the first few cycles. Different behaviour is seen when the source is merely turned up or down. For instance, a change of the source reduced gravity leads to a permanent change of interface depth, which is a qualitative point of difference from the free turbulent plume case.
Contaminants in vadose zone environments pose a long-term source and threat to groundwater resources, human health, and the environment. Several technical, regulatory, and policy challenges and opportunities are associated with contamination in vadose zone environments, particularly in remediation. In this special issue, ten papers present novel approaches to characterize, monitor, remediate, and predict the transport and fate of contaminants in vadose zone environments.
Soil vapor extraction (SVE) and bioremediation (BR) are two of the most common soil remediation technologies. Their application is widespread; however, both present limitations, namely related to the efficiencies of SVE on organic soils and to the remediation times of some BR processes. This work aimed to study the combination of these two technologies in order to verify the achievement of the legal clean-up goals in soil remediation projects involving seven different simulated soils separately contaminated with toluene and xylene. The remediations consisted of the application of SVE followed by biostimulation. The results show that the combination of these two technologies is effective and manages to achieve the clean-up goals imposed by the Spanish Legislation. Under the experimental conditions used in this work, SVE is sufficient for the remediation of soils, contaminated separately with toluene and xylene, with organic matter contents (OMC) below 4 %. In soils with higher OMC, the use of BR, as a complementary technology, and when the concentration of contaminant in the gas phase of the soil reaches values near 1 mg/L, allows the achievement of the clean-up goals. The OMC was a key parameter because it hindered SVE due to adsorption phenomena but enhanced the BR process because it acted as a microorganism and nutrient source.
Kd values obtained on sediment samples from 200-UP-1 and 10-ZP-1 contribute to a larger Kd database that exists for other Hanford sediments, and contains significant desorption data for CCl4. Adsorption results presented here validate the use of a linear adsorption isotherm (Kd) to predict short contact time CCl4 adsorption to sediments in 200-UP-1 groundwater plume for a distinct ranges in CCl4 concentration. However, this does not imply that values of Kd will be constant if the groundwater chemical composition at 200-UP-1 changes with space or time. Additionally, results presented here suggest the potential significance of slower intraparticle diffusion on the long-term fate of CCl4 within the subsurface Hanford environment. Such behavior could afford prolonged desorption of CCl4 and serve as a long-term source of contaminant CCl4 to the aquifer. Further evaluation of possible bimodal sorption behavior for CCl4 and the mechanism of CCl¬4 sequestration should be the subject of future investigations to provide a thorough, mechanistic understanding of the retention and long-term fate of CCl4. Comparison of previous data with new results (e.g., from this study) will allow inferences to be made on how the 200-UP-1 Kd values for CCl4 may compare with sediments from other Hanford locations. This site-specific sorption data, when complemented by the chemical, geologic, mineralogic, hydrologic, and physical characterization data that are also being collected (see Sampling and Analysis Plan for the 200-UP-1 Groundwater Monitoring Well Network, DOE 2002) can be used to develop a robust, scientifically defensible data base to allow risk predictions to be generated and to aid in future remediation decisions for the 200-UP-1 and 200-ZP-1 operable units.
Dispersive processes that diminish contaminant concentrations originating from an unsaturated source zone on the way to groundwater, were investigated. Simulations using the numerical model MIN3P were performed for a non-volatile, non-degrading contaminant from a persistent source after reaching a steady state. A 2-D vertical cross-section was used as geometry. Two different types of sandy sediment were simulated: a rather coarse sand with a capillary rise of 90% water saturation to 4 cm above the water table, and a silty sand showing a capillary fringe of 30 cm height (90% water saturation). Major dispersive fluxes were found to take place below the water table, thus dilution and concentration reduction at and above the water table is not very significant.
Site closure for soil vacuum extraction (SVE) application typically requires attainment of specified soil concentration standards based on the premise that mass flux from the vadose zone to ground water not result in levels exceeding maximum contaminant levels (MCLs). Unfortunately, realization of MCLs in ground water may not be attainable at many sites. The authors propose an alternative strategy based on evaluation of five components: (1) site characterization, (2) design, (3) performance monitoring, (4) rate-limited vapor transport, and (5) mass flux to and from ground water. Demonstration of closure is dependent on satisfactory assessment of all five components. The focus of this paper is to support mass flux evaluation. The authors present a plan based on monitoring of three subsurface zones and develop an analytical one-dimensional vertical flux model they term VFLUX. VFLUX is a significant improvement over the well-known numerical one-dimensional model, VLEACH, which is often used for estimation of mass flux to ground water, because it allows for the presence of nonaqueous phase liquids (NAPLs) in soil, degradation, and a time-dependent boundary conditions at the water table interface.
Groundwater modeling was performed in support of the Hanford Carbon Tetrachloride Innovative Treatment Remediation Demonstration (ITRD) Program. The ITRD program is facilitated by Sandia National Laboratory for the Department of Energy Office of Science and Technology. This report was prepared to document the results of the modeling effort and facilitate discussion of characterization and remediation options for the carbon tetrachloride plume among the ITRD participants. As a first step toward implementation of innovative technologies for remediation of the carbon tetrachloride (CT) plume underlying the 200-West Area, this modeling was performed to provide an indication of the potential impact of the CT source on the compliance boundary approximately 5000 m distant. The primary results of the modeling bracket the amount of CT source that will most likely result in compliance/non-compliance at the boundary and the relative influence of the various modeling parameters.
Three-dimensional modeling was conducted with layered and heterogeneous models to enhance the conceptual model of CT distribution in the vertical and lateral direction beneath the 216-Z-9 trench and to investigate the effects of soil vapor extraction (SVE). This work supports the U.S. Department of Energy's (DOE's) efforts to characterize the nature and distribution of CT in the 200 West Area and subsequently select an appropriate final remedy. Simulations targeted migration of dense, nonaqueous phase liquid (DNAPL) consisting of CT and co-disposed organics in the subsurface beneath the 216-Z-9 trench as a function of the properties and distribution of subsurface sediments and of the properties and disposal history of the waste. Simulations of CT migration were conducted using the Subsurface Transport Over Multiple Phases (STOMP) simulator. Simulation results support a conceptual model for CT distribution where CT in the DNAPL phase is expected to have migrated primarily in a vertical direction below the disposal trench. Presence of small-scale heterogeneities tends to limit the extent of vertical migration of CT DNAPL due to enhanced retention of DNAPL compared to more homogeneous conditions, but migration is still predominantly in the vertical direction. Results also show that the Cold Creek units retain more CT DNAPL within the vadose zone than other hydrologic unit during SVE. A considerable amount of the disposed CT DNAPL may have partitioned to the vapor and subsequently water and sorbed phases. Presence of small-scale heterogeneities tends to increase the amount of volatilization. Any continued migration of CT from the vadose zone to the groundwater is likely through interaction of vapor phase CT with the groundwater and not through continued DNAPL migration. The results indicated that SVE appears to be an effective technology for vadose zone remediation, but additional effort is needed to improve simulation of the SVE process.
Physical experiments were conducted to investigate the transport of a dissolved volatile organic compound (trichloroethylene, TCE) from shallow groundwater to the unsaturated zone under a variety of conditions including changes in the soil moisture profile and water table position. Experimental data indicated that at moderate groundwater velocities (0.1 m/d), vertical mechanical dispersion was negligible and molecular diffusion was the dominant vertical transport mechanisms. Under these conditions, TCE concentrations decreased nearly 3 orders of magnitude across the capillary fringe and soil gas concentrations remained low relative to those of underlying groundwater. Data collected during a water table drop showed a short-term increase in concentrations throughout most of the unsaturated zone, but these concentrations quickly declined and approached initial values after the water table was returned to its original level. In the deep part of the unsaturated zone, the water table drop resulted in a long-term decrease in concentrations, illustrating the effects of hysteresis in the soil moisture profile. A two-dimensional random walk advection-diffusion model was developed to simulate the experimental conditions, and numerical simulations agreed well with experimental data. A simpler, one-dimensional finite-difference diffusion-dispersion model was also developed. One-dimensional simulations based on molecular diffusion also agreed well with experimental data. Simulations which incorporated mechanical dispersion tended to overestimate flux across the capillary fringe. Good agreement between the one- and two-dimensional models suggested that a simple, one-dimensional approximation of vertical transport across the capillary fringe can be useful when conditions are appropriate.
A new investigation approach for the assessment of groundwater contamination based on the inversion of concentration time series measured within pumping wells is presented. Using the inversion approach, it is possible to investigate the mean pollutant concentration and the concentration distribution over a control plane perpendicular to the groundwater flow direction downstream of a pollutant source, as well as the mass flux over this control plane.
This guide describes the general use, input file formatting, compilation and execution of the STOMP (Subsurface Transport Over Multiple Phases) simulator, a scientific tool for analyzing single and multiple phase subsurface flow and transport. A description of the simulators governing equations, constitutive functions and numerical solution algorithms are provided in a companion theory guide. In writing these guides for the STOMP simulator, the authors have assumed that the reader comprehends concepts and theories associated with multiple-phase hydrology, heat transfer, thermodynamics, radioactive chain decay, and relative permeability-saturation-capillary pressure constitutive relations. The authors further assume that the reader is familiar with the computing environment on which they plan to compile and execute the STOMP simulator. Source codes for the sequential versions of the simulator are available in pure FORTRAN 77 or mixed FORTRAN 77/90 forms. The pure FORTRAN 77 source code form requires a parameters file to define the memory requirements for the array elements. The mixed FORTRAN 77/90 form of the source code uses dynamic memory allocation to define memory requirements, based on a FORTRAN 90 preprocessor STEP, that reads the input files. The simulator utilizes a variable source code configuration, which allows the execution memory and speed to be tailored to the problem specifics, and essentially requires that the source code be assembled and compiled through a software maintenance utility. The memory requirements for executing the simulator are dependent on the complexity of physical system to be modeled and the size and dimensionality of the computational domain. Likewise execution speed depends on the problem complexity, size and dimensionality of the computational domain, and computer performance. Selected operational modes of the STOMP simulator are available for scalable execution on multiple processor (i.e., parallel) computers. These versions of the simulator are written in pure FORTRAN 90 with imbedded directives that are interpreted by a FORTRAN preprocessor. Without the preprocessor, the scalable version of the simulator can be executed sequentially on a single processor computer. The scalable versions of the STOMP modes carry the -Sc designator on the operational mode name. For example, STOMP-WCS-Sc is the scalable version of the STOMP-WCS (Water-CO2-Salt) mode. A separate mode containing an evaporation model as a boundary condition on the upper surface of the computation domain has also been included. This mode, STOMP-WAE-B (Water-Air-Energy-Barriers) can be viewed as an extension of the STOMP-WAE (Water-Air-Energy) mode. Details of this particular mode are outlined by Ward et al. (2005)(a). STOMP V4.0 includes the reactive transport module ECKEChem (Equilibrium-Conservation-Kinetic Equation Chemistry) for the STOMP-W (Water) and STOMP-WCS (Water-CO2-Salt) modes. For this particular module, the -R designator is included in the operational mode name (e.g., STOMP-W-R, STOMP-WCS-R-Sc). This mode is described in detail by White and McGrail (2005)(b). For all operational modes and processor implementations, the memory requirements for executing the simulator are dependent on the complexity of physical system to be modeled and the size and dimensionality of the computational domain. Likewise execution speed depends on the problem complexity, size and dimensionality of the computational domain, and computer performance. Additional information about the simulator can be found on the STOMP webpage: http://stomp.pnl.gov. The website includes an introductory short course with problems ranging from simple one-dimensional saturated flow to complex multiphase system computations.
An experiment to investigate the natural attenuation of three volatile organic compounds, toluene, carbon tetrachloride, and tetrachloroethene (~1-10 mg L-1) was performed in a 3 m deep, sandy aquifer isolated within a 24 m long, 2 m wide, three-sided sheet pile alleyway (hereafter referred to as the gate). A constant flow was maintained in the test volume by pumping a well at the closed end of the gate at 130 mL min-1. The test compounds were introduced to the aquifer using diffusive emitters installed inside 25 cm diameter wells located at the open end of the gate. Monitoring was performed by sampling along six multilevel fences (consisting of 12 sampling points each) ranging in distance from 1 to 22 m from the source wells. A bromide tracer experiment established that there were no significant hydraulic leaks, nor was there any continuous channeling through the gate. Degradation of the test compounds was assessed by mass balance calculations between fences located 1 and 7 m from the source, and the results were compared with degradation rate estimates from snapshot analyses and the analysis of fluxes. There was reasonably good agreement between rates estimated by these different methods. Toluene degraded with a half-life of 58-62 days, carbon tetrachloride degraded with a half-life of ~11-13 days, and tetrachloroethene degraded too slowly for a reliable estimate of rate to be made. Transformation products identified in the gate included acetate, possibly from toluene degradation, chloroform, trichloroethene, and cis-1,2, dichloroethene. The latter two compounds only appeared in trace quantities and could not be assessed for continuing degradation. However, chloroform degradation was assessed with the snapshot data and using the flux estimates and was found to degrade with a half-life in the range of 10-34 days. No additional chlorinated methanes were detected in the gate, suggesting that the carbon tetrachloride was completely dechlorinated by natural processes within 10 m of the source wells. This experiment demonstrated that degradation of chlorinated solvents occurs naturally at the Borden site but that the ethenes are more resistant to biodegradation than the methanes. In addition, the flux calculations were found to be the most robust in terms of estimating degradation rates.
A two-dimensional flow cell experiment was conducted to study the removal of the carbon tetrachloride component of a DNAPL mixture from a layered porous medium through soil vapor extraction (SVE) with moist and dry air. A dual-energy gamma radiation system was used at various times to non-intrusively determine fluid saturations. The mixture, which contained the volatile organic carbon tetrachloride, mimics the DNAPL disposed at the Hanford Site in Washington State. The flow cell, which is 100 cm long, 75 cm high and 5.5 cm wide, was packed with two sloped coarse sand and two sloped silt layers in an otherwise uniform matrix of medium-grained sand. A V-shaped fine sand layer was placed at the bottom of the flow cell to prevent DNAPL from exiting the flow cell. The water table was located 2 cm from the bottom, creating variably saturated conditions. A 500-mL spill was introduced at the top of the flow cell from a small source area. It was observed that the DNAPL largely by-passed the silt layers but easily moved into the coarse sand layers. Residual DNAPL was formed in the medium-grained sand matrix. The DNAPL caused a distinct reduction of the capillary fringe. Most of the DNAPL ended up in a pool on top of the V-shaped fine sand. Through four treatments with moist air soil vapor extraction, most residual carbon tetrachloride was removed from the medium-grained matrix and the coarse sand layers. However, soil vapor extraction with moist air was not able to remove the carbon tetrachloride from the silt layers and the pool. Through a water table reduction and subsequent soil vapor extraction with dry air, the carbon tetrachloride in the silt layers and the pool was effectively removed. Based on gamma measurements and carbon tetrachloride vapor concentration data, it was estimated that after the final remediation treatment, almost 90% of the total mass was removed. Key Words: DNAPL; soil vapor extraction; desiccation; remediation
Methods are developed to use data collected during cyclic operation of soil vapor extraction (SVE) systems to help characterize the magnitudes and timescales 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 non-operation 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 a 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.
Releases of Dense Non-Aqueous Phase Liquids (DNAPLs) at a large number of public and private sector sites in the United States pose significant challenges in site remediation and long-term site management. Extensive contamination of groundwater occurs as a result of significant dissolved plumes generated from these DNAPL source zones that vary in size and complexity depending on site characteristics and DNAPL properties and distribution. Risk and liability management, consistent with regulatory compliance requirements, could involve remediation of the source zone as well as management of the dissolved plume. The source zone is defined here as the groundwater region (volume) in which DNAPL is present as a separate phase, either as randomly distributed sub-zones at residual saturations or "pools" of accumulation above confining units and includes the volume of the aquifer that has had contact with free-phase DNAPL at one time, but where all of the DNAPL mass is now present only in the dissolved or sorbed phases or diffused into the matrix in fractured systems. Over the past two decades, innovations in site characterization and remediation technologies have been developed and deployed at DNAPL sites.
Disposal quantities of organic wastes at the Brooklawn Site in Louisiana are suspected to equal nearly 1.45 × 10Kg, making this site one of the most contaminated dense nonaqueous phase liquid (DNAPL) sites in the world. Remedial activities at the site include groundwater and DNAPL extraction from recovery wells. DNAPL recovery has markedly declined in recent years, with many of the peripheral wells showing negligible recovery of organic liquids. Three-dimensional simulations of DNAPL movement in the subsurface were conducted using the STOMP simulator, including a new coupled-well model. The objectives of this modeling effort were to (1) determine the fate and transport of infiltrated DNAPL, and (2) measure the effects of active recovery through DNAPL pumping. A detailed three-dimensional geologic model of the Brooklawn primary DNAPL disposal area was developed and used as the framework for DNAPL simulations. Additionally, site-specific data were obtained to determine the most important hydraulic properties of the subsurface related to DNAPL movement and formation of entrapped DNAPL in the laboratory. In addition to a simulation using the best available subsurface information, several sensitivity simulations were conducted to assess the effects on DNAPL migration. These simulations include DNAPL pumping, well screen extension, an alternative geology, increased DNAPL density, lower DNAPL viscosity, and more-permeable sand and silt deposits.Results of the simulations were compared to field data that define the extent of DNAPL movement based on where DNAPL has been extracted in the site recovery wells. The model simulations show that pumping has a negligible effect on subsurface DNAPL saturations and movement. Pumped DNAPL volumes diminish rapidly due to the limited radius of influence of the wells and movement of the DNAPL out of the zone of influence of the wells with a maximum radius of influence of about 6 m. The numerical analysis also demonstrates that it is impractical to extend existing wells or install new wells to retrieve enough DNAPL to affect the overall extent of DNAPL movement.
A new framework for prioritizing environmental site cleanups considers the interaction of contaminant plumes with water supply wells
Contaminants may persist for long time periods within low permeability portions of the vadose zone where they cannot be effectively treated and are a potential continuing source of contamination to ground water. Setting appropriate vadose zone remediation goals typically requires evaluating these persistent sources in terms of their impact on meeting ground water remediation goals. Estimating the impact on ground water can be challenging at sites with low aqueous recharge rates where vapor-phase movement is the dominant transport process in the vadose zone. Existing one-dimensional approaches for simulating transport of volatile contaminants in the vadose zone are considered and compared to a new flux-continuity-based assessment of vapor-phase contaminant movement from the vadose zone to the ground water. The flux-continuity-based assessment demonstrates that the ability of the ground water to move contaminant away from the water table controls the vapor-phase mass flux from the vadose zone across the water table. Limitations of these approaches are then discussed with respect to the required assumptions and the need to incorporate three-dimensional processes when evaluating vapor-phase transport from the vadose zone to the ground water. The carbon tetrachloride plume at the U.S. Department of Energy Hanford Site is used as the example site where persistent vadose zone contamination needs to be considered in the context of ground water remediation.
Low-permeability layers of the vadose zone containing volatile organic compounds (VOCs) may persist as source zones for long time periods and may provide contamination to groundwater. At sites with low recharge rates, where vapor migration is the dominant transport process, the impact of vadose zone sources on groundwater may be difficult to assess. Typical assessment methods include one-dimensional numerical and analytical techniques. The one-dimensional approaches only consider groundwater coupling options through boundary conditions at the water table and may yield artificially high mass flux results when transport is assumed to occur by gas-phase diffusion between a source and an interface with a zero concentration boundary condition. Improvements in mass flux assessments for VOCs originating from vadose zone sources may be obtained by coupling vadose zone gas transport and dissolved contaminant transport in the saturated zone and by incorporating the inherent three-dimensional nature of gas-phase transport, including the potential of density-driven advection. This paper describes a series of three-dimensional simulations using data from the U.S. Department of Energy's Hanford site, where carbon tetrachloride is present in a low-permeability zone about 30 m above the groundwater. Results show that, for most cases, only a relatively small amount of the contaminant emanating from the source zone partitions into the groundwater and that density-driven advection is only important when relatively high source concentrations are considered.
Site closure for soil vacuum extraction (SVE) application typically requires attainment or specified soil concentration standards based on the premise that mass flux from the vadose zone to ground water not result in levels exceeding maximum contaminant levels (MCLs). Unfortunately, realization of MCLs in ground water may not be attainable at many sites. This results in soil remediation efforts that may be in excess of what is necessary for future protection of ground water and soil remediation goals which often cannot be achieved within a reasonable time period. Soil venting practitioners have attempted to circumvent these problems by basing closure on some predefined percent total mass removal, or an approach to a vapor concentration asymptote. These approaches, however, are subjective and influenced by venting design. We propose an alternative strategy based on evaluation of five components: (1) site characterization, (2) design. (3) performance monitoring, (4) rule-limited vapor transport, and (5) mass flux to and from ground water. Demonstration of closure is dependent on satisfactory assessment of all five components. The focus of this paper is to support mass flux evaluation. We present a plan based on monitoring of three subsurface zones and develop an analytical one-dimensional vertical flux model we term VFLUX. VFLUX is a significant improvement over the well-known numerical one-dimensional model. VLEACH, which is often used for estimation of mass flux to ground water, because it allows for the presence of nonaqueous phase liquids (NAPLs) in soil, degradation, and a lime-dependent boundary condition at the water table inter-face. The time-dependent boundary condition is the center-piece of our mass flux approach because it dynamically links performance of ground water remediation lo SVE closure. Progress or lack of progress in ground water remediation results in either increasingly or decreasingly stringent closure requirements, respectively.
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.
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.
Many emerging remediation technologies are designed to remove contaminant mass from source zones at DNAPL sites in response to regulatory requirements. There is often concern in the regulated community as to whether mass removal actually reduces risk, or whether the small risk reductions achieved warrant the large costs incurred. This paper sets out a proposed framework for quantifying the degree to which risk is reduced as mass is removed from DNAPL source areas in shallow, saturated, low-permeability media. Risk is defined in terms of meeting an alternate concentration limit (ACL) at a compliance well in an aquifer underlying the source zone. The ACL is back-calculated from a carcinogenic health-risk characterization at a down gradient water-supply well. Source-zone mass-removal efficiencies are heavily dependent on the distribution of mass between media (fractures, matrix) and phase (aqueous, sorbed, NAPL). Due to the uncertainties in currently available technology performance data, the scope of the paper is limited to developing a framework for generic technologies rather than making specific risk-reduction calculations for individual technologies. Despite the qualitative nature of the exercise, results imply that very high total mass-removal efficiencies are required to achieve significant long-term risk reduction with technology applications of finite duration. This paper is not an argument for no action at contaminated sites. Rather, it provides support for the conclusions of Cherry et al. (1992) that the primary goal of current remediation should be short-term risk reduction through containment, with the aim to pass on to future generations site conditions that are well-suited to the future applications of emerging technologies with improved mass-removal capabilities.
A large-scale permanganate-based in situ chemical oxidation (ISCO) effort has been conducted over the past ten years at a federal Superfund site in Tucson, AZ, for which trichloroethene (TCE) is the primary contaminant of concern. Remediation performance was assessed by examining the impact of treatment on contaminant mass discharge, an approach that has been used for only a very few prior ISCO projects. Contaminant mass discharge tests were conducted before and after permanganate injection to measure the impact at the source-zone scale. The results indicate that ISCO caused a significant reduction in mass discharge (approximately 75%). The standard approach of characterizing discharge at the source-zone scale was supplemented with additional characterization at the plume scale, which was evaluated by examining the change in contaminant mass discharge associated with the pump-and-treat system. The integrated contaminant mass discharge decreased by approximately 70%, consistent with the source-zone-scale measurements. The integrated mass discharge rebounded from 0.1 to 0.2 kg/d within one year after cessation of permanganate injections, after which it has been stable for several years. Collection of the integrated contaminant mass discharge data throughout the ISCO treatment period provided a high-resolution, real-time analysis of the site-wide impact of ISCO, thereby linking source-zone remediation to impacts on overall risk. The results indicate that ISCO was successful in reducing contaminant mass discharge at this site, which comprises a highly heterogeneous subsurface environment. Analysis of TCE sediment concentration data for core material collected before and after ISCO supports the hypothesis that the remaining mass discharge is associated in part with poorly accessible contaminant mass residing within lower-permeability zones.
A series of flow-cell experiments was conducted to investigate the impact of organic-liquid distribution and flow-field heterogeneity on the relationship between source-zone mass removal and reductions in contaminant mass flux from the source zone. Changes in source-zone architecture were quantified using image analysis, allowing explicit examination of their impact on the mass-flux-reduction/mass-removal behavior. The results showed that there was minimal reduction in mass flux until a large fraction of mass was removed for systems wherein organic liquid was present solely as residual saturation in regions that were hydraulically accessible. Conversely, significant reductions in mass flux occurred with relatively minimal mass removal for systems wherein organic liquid was present at both residual and higher saturations. The latter systems exhibited multi-step mass-flux-reduction/mass-removal behavior, and characterization of the organic-liquid saturation distribution throughout flushing allowed identification of the cause of the nonideal behavior. The age of the source zone (e.g., extent of mass removal prior to characterization) significantly influenced the observed mass-flux-reduction/mass-removal behavior. The results of this study illustrate the impact of both organic-liquid distribution and flow-field heterogeneity on mass-removal and mass-flux processes.
The purpose of this work is to identify the mechanisms that govern the removal of carbon tetrachloride (CT) during soil vapor extraction (SVE) by comparing numerical and analytical model simulations with a detailed data set from a well-defined intermediate-scale flow cell experiment. The flow cell was packed with a fine-grained sand layer embedded in a coarse-grained sand matrix. A total of 499 mL CT was injected at the top of the flow cell and allowed to redistribute in the variably saturated system. A dual-energy gamma radiation system was used to determine the initial NAPL saturation profile in the fine-grained sand layer. Gas concentrations at the outlet of the flow cell and 15 sampling ports inside the flow cell were measured during subsequent CT removal using SVE. Results show that CT mass was removed quickly in coarse-grained sand, followed by a slow removal from the fine-grained sand layer. Consequently, effluent gas concentrations decreased quickly at first, and then started to decrease gradually, resulting in long-term tailing. The long-term tailing was mainly due to diffusion from the fine-grained sand layer to the coarse-grained sand zone. An analytical solution for a one-dimensional advection and a first-order mass transfer model matched the tailing well with two fitting parameters. Given detailed knowledge of the permeability field and initial CT distribution, we were also able to predict the effluent concentration tailing and gas concentration profiles at sampling ports using a numerical simulator assuming equilibrium CT evaporation. The numerical model predictions were accurate within the uncertainty of independently measured or literature derived parameters. This study demonstrates that proper numerical modeling of CT removal through SVE can be achieved using equilibrium evaporation of NAPL if detailed fine-scale knowledge of the CT distribution and physical heterogeneity is incorporated into the model. However, CT removal could also be fit by a first-order mass transfer analytical model, potentially leading to an erroneous conclusion that the long-term tailing in the experiment was kinetically controlled due to rate-limited NAPL evaporation.
Laboratory experiments and mathematical modeling were conducted to examine the influence of a hydroxypropyl-beta-cyclodextrin (HPCD) solution on the dissolution of single- and three-component organic liquids. The results of batch experiments showed that HPCD-enhanced solubilization of the organic-liquid mixtures was ideal (describable using Raoult's Law), and that solubilization-enhancement factors were independent of mixture composition. Addition of the HPCD solution to columns containing residual saturations of the organic liquid enhanced the dissolution and removal of all three compounds in the mixture. The results of the column experiments and mathematical modeling suggest that solubilization was ideal for both water and cyclodextrin flushing. Concomitantly, the mass-flux reduction versus mass removal behavior was ideal for all experiments. Mass transfer was increased for HPCD solubilization relative to the water flushing due to solubility and concentration-gradient enhancement. Organic-liquid composition did not significantly impact mass transfer coefficients, and fractional mass removal behavior during HPCD solubilization was nearly identical for each compound whether present as a single component or in a mixture. Additionally, mass transfer coefficients for aqueous and HPCD solubilization for single and multicomponent mixtures were not statistically different upon normalizing by the solubility enhancement factor.
The efficiency of traditional soil venting or soil vapor extraction (SVE) highly depends on the architecture of the subsurface because imposed advective air flow tends to bypass low-permeable contaminated areas. Pneumatic SVE is a technique developed to enhance remediation efficiency of heterogeneous soils by enforcing large fluctuating pressure fronts through the contaminated area. Laboratory experiments have suggested that pneumatic SVE considerably improves the recovery rate from low-permeable units. We have analyzed the experimental results using a numerical code and quantified the physical processes controlling the functioning of the method. A sensitivity analysis for selected boundary conditions, initial conditions and parameters was carried out to examine how the method behaves under conditions different from the experimental set-up. The simulations show that at the laboratory level the pneumatic venting technology is superior to the traditional technique, and that the method is particularly efficient in cases where large permeability contrasts exist between soil units in the subsurface.
The impact of immiscible-liquid composition on mass transfer between immiscible liquid and vapor phases was evaluated for a complex mixture of chlorinated solvents and petroleum hydrocarbons. A mixture of tetrachloroethene and diesel was discovered at a site in Tucson, Arizona. Partitioning of tetrachloroethene into a layer of diesel has been observed, with resultant concentrations of tetrachloroethene up to approximately 15% by weight. The density, viscosity, surface tension, and interfacial tension were measured for tetrachloroethene-diesel mixtures with tetrachloroethene fractions ranging from 7% to 32%, and the results indicated that immiscible-liquid composition did impact the physical properties of the tetrachloroethene-diesel mixture. The results of batch phase-partitioning experiments were compared to predictions based on Raoult's Law, and the analysis indicated that immiscible-liquid/vapor and immiscible-liquid/water partitioning were both essentially ideal. Flow-cell experiments were conducted to characterize steady-state tetrachloroethene removal from the tetrachloroethene-diesel mixture via vapor extraction. The effluent concentrations for the experiment conducted with free-phase immiscible liquid were comparable to equilibrium values. Conversely, the effluent concentrations were significantly lower for the experiment wherein a residual saturation of immiscible liquid was distributed within sand. The lower concentrations for the latter experiment were attributed to dilution effects associated with a nonuniform distribution of immiscible liquid within the flow cell.
To protect water supply wells, a practical framework that can respond to the unprecedented risks posed by thousands of subsurface releases of chemical contaminants is urgently needed. Murray Einarson and Douglas Mackay describe a simple estimation method that could help policy makers address this problem.
Transverse dispersion is the most relevant process in mass transfer of contaminants across the capillary fringe (both directions), dilution of contaminants, and mixing of electron acceptors and electron donors in biodegrading groundwater plumes. This paper gives an overview on literature values of transverse vertical dispersivities alpha(tv) measured at different flow velocities and compares them to results from well-controlled laboratory-tank experiments on mass transfer of trichloroethene (TCE) across the capillary fringe. The measured values of transverse vertical dispersion in the capillary fringe region were larger than in fully saturated media, which is credited to enhanced tortuosity of the flow paths due to entrapped air within the capillary fringe. In all cases, the values observed for alpha(tv) were < 1 mm. The new measurements and the literature values indicate that alpha(tv) apparently declines with increasing flow velocity. The latter is attributed to incomplete diffusive mixing at the pore scale (pore throats). A simple conceptual model, based on the mean square displacement and the pore size accounting for only partial diffusive mixing at increasing flow velocities, shows very good agreement with measured and published data.
Flow of nonvolatile nonaqueous phase liquid (NAPL) and aqueous phases that account for mobile, entrapped, and residual NAPL in variably saturated water-wet porous media is modeled and compared against results from detailed laboratory experiments. Residual saturation formation in the vadose zone is a process that is often ignored in multifluid flow simulators, which might cause an overestimation of the volume of NAPL that reaches the ground water. Mobile NAPL is defined as being continuous in the pore space and flows under a pressure gradient or gravitational body force. Entrapped NAPL is defined as being occluded by the aqueous phase, occurring as immobile ganglia surrounded by aqueous phase in the pore space and formed when NAPL is replaced by the aqueous phase. Residual NAPL is defined as immobile, nonwater entrapped NAPL that does not drain from the pore spaces and is conceptualized as being either continuous or discontinuous. Free NAPL comprises mobile and residual NAPL. The numerical model is formulated on mass conservation equations for oil and water, transported via NAPL and aqueous phases through variably saturated porous media. To account for phase transitions, a primary variable switching scheme is implemented for the oil-mass conservation equation over three phase conditions: (1) aqueous or aqueous-gas with dissolved oil, (2) aqueous or aqueous-gas with entrapped NAPL, and (3) aqueous or aqueous gas with free NAPL. Two laboratory-scale column experiments are modeled to verify the numerical model. Comparisons between the numerical simulations and experiments demonstrate the necessity to include the residual NAPL formation process in multifluid flow simulators.
Analytical solutions are developed for approximating the time-dependent contaminant discharge from DNAPL source zones undergoing dissolution and other decay processes. The source functions assume a power relationship between source mass and chemical discharge and can consider partial DNAPL source remediation (depletion) at any time after the initial DNAPL release. The source functions are used as a time-dependent boundary condition in an idealized chemical transport model to develop leading order approximations of the plume response to DNAPL source removal. The results suggest that partial DNAPL remediation does not tend to have a dramatic impact on the maximum extent of the plume if very low concentration values are used to define the plume boundaries. However, the solutions show that partial DNAPL removal from the source zone is likely to lead to large reductions in plume concentrations and mass, and it reduces the longevity of the plume. When the mass discharge from the source zone is linearly related to the DNAPL mass, it is shown that partial DNAPL depletion leads to linearly proportional reductions in the plume mass and concentrations.
Analytical solutions, describing the time-dependent DNAPL source-zone mass and contaminant discharge rate, derived previously in Part I [Falta, R.W., Rao, P.S., Basu, N., this issue. Assessing the impacts of partial mass depletion in DNAPL source zones: I. Analytical modeling of source strength functions and plume response. J. Contam. Hydrol.] are used as a flux-boundary condition in a semi-analytical contaminant transport model. These analytical solutions assume a power relationship between the flow-averaged source concentration, and the source DNAPL mass; the empirical exponent (gamma) is a function of the flow field heterogeneity, DNAPL architecture, and the correlation between them. The DNAPL source strength terms can account for partial source remediation, either at time zero, or at some later time after the DNAPL release. The transport model considers advection, retardation, three-dimensional dispersion, and sequential first-order decay/production of several species. A separate solution is used to compute the time-dependent mass of each contaminant in the plume. A series of examples using different values of gamma shows how the benefits of partial DNAPL source remediation can vary with site conditions. In general, when gamma>1, relatively large short-term reductions in the plume concentrations and mass occur, but the source longevity is not strongly affected. Conversely, when gamma<1, the short-term reductions in the plume concentrations and mass are smaller, but the source longevity can be greatly reduced. In either case, the source remediation effort is much more effective if it is undertaken at an early time, before much contaminant mass has entered the plume. If the remediation effort is significantly delayed, the leading parts of the plume are not affected by the source remediation, and additional control or remediation of the plume itself is required.
An extensive site-characterization project was conducted at a large chlorinated-solvent contaminated Superfund site in Tucson, AZ. The project consisted of several components, including traditional site-characterization activities, tracer tests, laboratory experiments conducted with core material collected from the site, and mathematical modeling. The primary focus of the work presented herein is the analysis of induced-gradient contaminant elution tests conducted in a source zone at the site, investigation of the potential occurrence of immiscible liquid in the saturated zone, characterization of the relationship between mass flux reduction and mass removal, and evaluation of the impact of source-zone management on site remediation. The results of the present study, along with those of prior work, indicate that immiscible liquid is likely present in the saturated zone at the site source zones. Extensive tailing and rebound was observed for the contaminant-elution tests, indicating nonideal transport and mass-transfer behavior. The elution data were analyzed with a source-zone-scale mathematical model, and the results indicated that nonideal immiscible-liquid dissolution was the primary cause of the observed behavior. The time-continuous relationship between mass flux reduction and mass removal associated with the plume-scale pump-and-treat operation exhibited an initial large drop in mass flux with minimal mass removed, followed by a period of minimal mass flux reduction and a second period of large reduction. This behavior reflects the impact of both source-zone and aqueous-plume mass removal dynamics. Ultimately, a greater than 90% reduction in mass flux was achieved for a mass removal of approximately 50%. The influence of source-zone management on site remediation was evaluated by conducting two predictive simulations, one for which the source zones were controlled and one for which they were not. A plume-scale model was used to simulate the composite contaminant concentrations associated with groundwater extracted with the pump-and-treat system, which were compared to measured data. The information generated from this study was used to enhance the site conceptual model, help optimize operation of the pump-and-treat system, and evaluate the utility of source-zone remediation.
It is often difficult at contaminated sites to decide whether remediation effort should be focused on the contaminant source, the dissolved plume, or on both zones. The decision process at these sites is hampered by a lack of quantitative tools for comparing remediation alternatives. A new screening-level mass balance approach is developed for simulating the transient effects of simultaneous ground water source and plume remediation. The contaminant source model is based on a power function relationship between source mass and source discharge, and it can consider partial source remediation at any time after the initial release. The source model serves as a time-dependent mass flux boundary condition to a new analytical plume model, where flow is assumed to be one dimensional, with three-dimensional dispersion. The plume model simulates first-order sequential decay and production of several species, and the decay rates and parent/daughter yield coefficients are variable functions of time and distance. This new method allows for flexible simulation of natural attenuation or remediation efforts that enhance plume degradation. The plume remediation effort may be temporary or delayed in time, limited in space, and it may have different chemical effects on different contaminant species in the decay chain.
A series of flow-cell experiments was conducted to investigate aqueous dissolution and mass-removal behavior for systems wherein immiscible liquid was non-uniformly distributed in physically heterogeneous source zones. The study focused specifically on characterizing the relationship between mass flux reduction and mass removal for systems for which immiscible liquid is poorly accessible to flowing water. Two idealized scenarios were examined, one wherein immiscible liquid at residual saturation exists within a lower-permeability unit residing in a higher-permeability matrix, and one wherein immiscible liquid at higher saturation (a pool) exists within a higher-permeability unit adjacent to a lower-permeability unit. The results showed that significant reductions in mass flux occurred at relatively moderate mass-removal fractions for all systems. Conversely, minimal mass flux reduction occurred until a relatively large fraction of mass (>80%) was removed for the control experiment, which was designed to exhibit ideal mass removal. In general, mass flux reduction was observed to follow an approximately one-to-one relationship with mass removal. Two methods for estimating mass-flux-reduction/mass-removal behavior, one based on system-indicator parameters (ganglia-to-pool ratio) and the other a simple mass-removal function, were used to evaluate the measured data. The results of this study illustrate the impact of poorly accessible immiscible liquid on mass-removal and mass-flux processes, and the difficulties posed for estimating mass-flux-reduction/mass-removal behavior.
The magnitude of contaminant mass-flux reduction associated with a specific amount of contaminant mass removed is a key consideration for evaluating the effectiveness of a source-zone remediation effort. Thus, there is great interest in characterizing, estimating, and predicting relationships between mass-flux reduction and mass removal. Published data collected for several field studies were examined to evaluate relationships between mass-flux reduction and source-zone mass removal. The studies analyzed herein represent a variety of source-zone architectures, immiscible-liquid compositions, and implemented remediation technologies. There are two general approaches to characterizing the mass-flux-reduction/mass-removal relationship, end-point analysis and time-continuous analysis. End-point analysis, based on comparing masses and mass fluxes measured before and after a source-zone remediation effort, was conducted for 21 remediation projects. Mass removals were greater than 60% for all but three of the studies. Mass-flux reductions ranging from slightly less than to slightly greater than one-to-one were observed for the majority of the sites. However, these single-snapshot characterizations are limited in that the antecedent behavior is indeterminate. Time-continuous analysis, based on continuous monitoring of mass removal and mass flux, was performed for two sites, both for which data were obtained under water-flushing conditions. The reductions in mass flux were significantly different for the two sites (90% vs. approximately 8%) for similar mass removals ( approximately 40%). These results illustrate the dependence of the mass-flux-reduction/mass-removal relationship on source-zone architecture and associated mass-transfer processes. Minimal mass-flux reduction was observed for a system wherein mass removal was relatively efficient (ideal mass-transfer and displacement). Conversely, a significant degree of mass-flux reduction was observed for a site wherein mass removal was inefficient (non-ideal mass-transfer and displacement). The mass-flux-reduction/mass-removal relationship for the latter site exhibited a multi-step behavior, which cannot be predicted using some of the available simple estimation functions.
Engineering Forum Issue Paper: Soil Vapor Extraction Implementation Experiences
- U S Epa
U.S. EPA, 1996b. Engineering Forum Issue Paper: Soil Vapor Extraction Implementation Experiences. EPA/540/F-95/030.
Development of Recommendations and Methods to Support Assessment of Soil Venting Performance and Closure
- U S Epa
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