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Three‐Dimensional Simulation of Volatile Organic Compound Mass Flux from the Vadose Zone to Groundwater
Abstract
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.
... When applying estimating techniques that relate vadose zone contaminant conditions to the groundwater, several overall considerations are important. For instance, Oostrom et al. [15] used numerical modeling to describe some key factors related to how vadose zone contaminants impact groundwater. As vapor-phase transport becomes more important (e.g., for arid sites with low aqueous recharge), three-dimensional gas transport and a coupled vadose zonegroundwater system need to be considered to effectively estimate the amount of contaminant mass transfer from the vadose zone to the groundwater. ...
... Oostrom et al. [15] also demonstrated that for sites with relatively low recharge rates, only a small amount of the contaminant emanating from the vadose zone source enters the groundwater. Most of the mass is transferred out of the domain into the atmosphere. ...
... Truex et al. [10], Oostrom et al. [15,16], and Carroll et al. [9] demonstrated that the groundwater velocity has a large effect on vapor-phase contaminant mass flux across the water table and resulting groundwater contaminant concentrations. At high groundwater flow rates, the mass transfer rate to the groundwater is larger compared to lower flow rates; as a result, the overall contaminant mass flux into the groundwater is larger. ...
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.
... Truex et al. (2009) compared the relative vapor phase and recharge-driven VOC flux into groundwater for a range of recharge and groundwater Darcy velocity conditions. They found that vapor-phase flux clearly dominates for recharge rates of 5 cm/year and lower, consistent with results of numerical simulations presented by Oostrom et al. (2010). Diffusion was the dominant vapor-phase transport process when the VOC vapor concentration is less than about 1500 ppmv and density-driven advection is minimal even at vapor concentrations up to 15,000 ppmv ). ...
... Thus, under conditions more likely to be relevant to evaluating SVE remedy decisions, vapor-phase diffusion can be considered to be the most important transport process in the vadose zone. For vaporphase VOC transport, mass loading into groundwater is a key process affecting the resulting groundwater contamination (Truex et al. 2009;Oostrom et al. 2010). Therefore, the estimation procedure described in this work focuses on diffusive vapor transport, mass loading into groundwater, and factors that control how these processes affect groundwater concentrations. ...
... This approach is consistent with a conservative evaluation of long-term impact after SVE has been terminated. Oostrom et al. (2010) and Carroll et al. (2012) showed that, at sites where vapor diffusion is the dominating VOC transport mechanism in the vadose zone, mass loading into the groundwater may be controlled by site-specific dimensions, vadose zone properties, and source characteristics. A listing of the considered parameters is shown in Table 3. Porosity is not included as one of these parameters as an analysis showed that predicted groundwater concentrations were largely insensitive to porosity changes for the considered conceptual model. ...
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.
... Soil vapor extraction (SVE) has been the presumptive remedy for volatile organic compounds (VOCs) in the vadose zone for approximately 15 years (U.S. EPA, 1996a). While initially SVE tends to be a highly effective method, it is recognized that SVE operational efficiency typically becomes limited over time primarily due to mass-transfer constraints associated with contaminant mass residing within lowerpermeability portions of the vadose zone DiGiulio et al., 1998;Hoier et al., 2009;Oostrom et al., 2010;Switzer et al., 2004;Truex et al., 2009;U.S. EPA, 1996b;Yoon et al., 2009). For most SVE systems, a decision point eventually develops regarding whether to continue under the reduced-efficiency conditions, to adjust the extraction protocol, or to cease operations and potentially switch to other remediation methods. ...
... Numerical simulations of a variety of scenarios were conducted using the three-dimensional (3D) water-oil-air operational mode of the STOMP simulator (White and Oostrom, 2006). This fully implicit, integrated finite difference numerical model has been used to simulate various multiphase systems (Oostrom et al., 2005(Oostrom et al., , 2007b(Oostrom et al., , 2010White et al., 2004). For the Hanford Site case study described in this paper, a multiphase numerical analysis is necessary, because vapor transport contributes significantly to contaminant movement. ...
... Hydraulic properties of the two porous media used in the simulations and other model parameters are listed in Table 2. The property values were primarily obtained from Hanford Site data bases, as reported by Oostrom et al. (2010) and Truex et al. (2009). The source zone was represented by a rectangular block centrally located in the CCU lower-permeability layer. ...
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.
... 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. ...
... One approach to characterizing contaminant mass discharge is based on the use of mathematical models. Using advanced, distributed-process, three-dimensional numerical models to simulate fluid flow and the transport and fate of contaminants is a powerful method that can support robust decision-making regarding risk assessment and implementation, optimization, and closure of remediation systems (e.g., Abreu and Johnson, 2005;Stauffer et al., 2007Stauffer et al., , 2011Bozkurt et al., 2009;Yu et al., 2009;Oostrom et al., 2010). The use of such models, however, involves significant data requirements and user expertise, such that their application may be impractical for many hazardous waste sites. ...
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.
... 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). ...
... The mean permeability of the Hanford and Ringold formations estimated via pneumatic testing were 5 × 10 −12 and 1 × 10 −12 m 2 , respectively. These values represent Hanford and Ringold hydraulic conductivity values of approximately 5 × 10 −3 and 1 × 10 −3 cm/s, respectively, which are consistent with the previously reported values (Oostrom et al. 2006Truex et al. 2009;Oostrom et al. 2010). The previously reported hydraulic conductivity for the CCU is 5 to 10 times lower (1.4 × 10 −4 cm/s). ...
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.
... 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
... Using the premodeled STOMP simulations results, SVEET2 estimates contaminant concentrations using lookups and interpolations for the user-specified set of input parameters. The STOMP simulations included vapor-phase processes, recharge-driven processes, and mixing into the groundwater, which have been demonstrated to be important for estimating contaminant transport (Truex et al. 2009;Oostrom et al. 2010;Brusseau et al. 2013). The simulations did not, however, account for attenuation mechanisms such as biodegradation or abiotic reaction. ...
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.
... For the gas phase, the relative permeability was computed using the Mualem (1976) permeability model. The longitudinal and transverse dispersivity values were 1 and 0.1 mm, respectively, based on values measured for the porous medium in prior research (Oostrom et al., 2010;Truex et al., 2009). ...
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 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. ...
Core Ideas
Soil vapor extraction (SVE) wells are an effective interim remediation technique.
Flow and transport modeling assists in evaluation of vapor plume behavior.
Plume simulations assuming drum failure and SVE provide a framework for remediation planning.
A small borehole subset could be monitored for detection of VOC releases due to drum failure.
Soil vapor extraction (SVE) has been used at sites across the Department of Energy complex, including sites where legacy subsurface wastes represent a potential source of groundwater contamination. At Los Alamos National Laboratory (LANL), leakage from waste drums buried at an inactive chemical waste site has created a subsurface vapor plume of volatile organic compounds (VOCs). Soil vapor extraction operation in 2015 and rebound testing through 2017 were successful in reducing the plume's mass and mitigating VOC migration toward the water table. However, the possibility that waste drums could fail and release VOCs could pose a challenge in the future. To explore the impacts of drum failure, as well as the capabilities of SVE remediation, we simulated hypothetical contaminant release scenarios and subsequent SVE remediation. Three‐dimensional subsurface VOC behavior, including advection, diffusion, and plume interactions with topography, were simulated using the porous flow simulator Finite Element Heat and Mass Transfer. Simulations of future site conditions have allowed identification of “sentry” boreholes that can be monitored for early detection in case of drum failure. Sentry boreholes can also be used to set concentration thresholds above which SVE should be initiated. For the LANL site, simulations show that SVE can be started 3 yr following drum failure and remain a viable remediation tool. More broadly, the principles outlined in this work can be used to support remediation planning at other subsurface waste sites. Predictive models of future releases can be analyzed to set concentration threshold values, guide selection of sentry boreholes, and increase operational efficiency.
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.
Colloid-facilitated transport of contaminants in the vadose zone has important implications to groundwater quality, and has received considerable attention. Natural organic matter (NOM) is ubiquitous in subsurface environments, and its influence on mineral colloids and solute transport has been well documented. However, research on the influence of NOM on colloid-facilitated transport is limited. The objective of this paper is to elucidate the effects of NOM on colloid-facilitated transport of a radioactive contaminant (Cs-137) within partially-saturated sediments. Measurements made with re-packed columns reveal that Cs-137 mobility was low when mineral colloids were absent and was unaffected by the presence of NOM. The addition of mineral colloids to influent increased Cs-137 mobility, and effluent Cs-137 was dominated by the colloid-associated form. When NOM was added to systems that contained mineral colloids and Cs-137, the mobility of Cs-137 further increased. A mathematical model simulating colloid-facilitated transport showed that NOM increases Cs-137 transport by increasing colloid mobility and reducing the rate of Cs-137 adsorption to the porous medium.
Copyright © 2015 Elsevier B.V. All rights reserved.
Although natural attenuation and phytoremediation can be cost-effective alternatives for the remediation of soils contaminated with volatile organic compounds (VOC), the risk of groundwater and plant contamination during the remediation process cannot be disregarded. In this study, the potential for subsurface contamination during natural attenuation as well as for plant contamination and recovery during phytoremediation in soils contaminated with 1,1,1-trichloroethane (TCA), trichloroethylene (TCE), and tetrachloroethylene (PCE) were investigated using a total of 56 polyvinyl chloride (PVC) columns filled with soil freshly contaminated by TCA, ICE, and PCE. Contaminant concentrations of soil gas, leachate, plant roots and shoots, and soil microbial biomass were monitored. The impact of easily degradable microbial substrate addition during natural attenuation on subsurface contamination was also examined. The results showed that VOCs migrated into deep soil or groundwater during natural attenuation and phytoremediation, and hence, the manner in which a site is initially managed is important for the effective application of natural attenuation and phytoremediation of VOC-contaminated soil. In addition, the use of plant species that are fast growing under local environmental conditions was an effective technique for reducing the release of VOC into deep soil. The addition of substrate during natural attenuation helped to reduce subsurface contamination for all three VOC-contaminated soil systems, particularly PCE. The VOC were first detected in root and then in shoot samples as the experiment progressed. The VOC in plant shoots, unlike those in roots, still remained after the soil was cleaned. Thus, careful plant management should be considered during VOC phytoremediation.
Groundwater organic pollutions are found in large amount of locations, and the pollutions are widely spread once onset; which is hard to identify and control. The key process to control and govern groundwater pollution is how to control the sources of pollution and reduce the danger to groundwater. This paper introduced typical contaminated sites as an example; then carried out the source identification studies and established groundwater organic pollution source identification system, finally applied the system to the identification of typical contaminated sites. First, grasp the basis of the contaminated sites of geological and hydrogeological conditions; determine the contaminated sites characteristics of pollutants as carbon tetrachloride, from the large numbers of groundwater analysis and test data; then find the solute transport model of contaminated sites and compound-specific isotope techniques. At last, through groundwater solute transport model and compound-specific isotope technology, determine the distribution of the typical site of organic sources of pollution and pollution status; invest identified potential sources of pollution and sample the soil to analysis. It turns out that the results of two identified historical pollution sources and pollutant concentration distribution are reliable. The results provided the basis for treatment of groundwater pollution.
Mass transport of chlorinated benzenes as found at the Petro-Processors of Louisiana, Inc. (PPI) Superfund site was characterized for a range of flow rates in small streams. At this site hazardous waste was historically disposed of in unengineered pits. Hexachlorobenzene and lesser chlorinated degradation products were found among other compounds. As waste was being disposed into unengineered pits, it seeped to lagoons and sediments of Baton Rouge Bayou (BRB), which flows through and nearby the former disposal areas. Characterization of the transport and fate of chlorobenzenes at PPI is an integral part of the Monitored Natural Attenuation (MNA) remedy currently underway. Laboratory experimental results and mathematical model predictions of the flux of 1,3-dichlorobenzene (1,3-DCB) from sediments into water are presented. 1,3-DCB was studied as individually and as part of a mixture of four contaminants with 1,2-DCB, chlorobenzene (MCB) and trichlorobenzene (TCB). Surficial sediments were collected, spiked with contaminants and leached to determine flux over time. Two advection-dispersion models were tested and the effect of low, cycling and fast stream flow on the contaminant flux was assessed. Model results suggest that tortuosity and effective diffusivity are related effective system predictors and descriptors. Statistical analysis supports the models' predictive capabilities.
A new and relatively simple equation for the soil-water content-pressure head curve is described. The particular form of the equation enables one to derive closed-form analytical expressions for the relative hydraulic conductivity, when substituted in the predictive conductivity models of N. T. Burdine or Y. Mualem. The resulting expressions contain three independent parameters which may be obtained by fitting the proposed soil-water retention model to experimental data. Results obtained with the closed-form analytical expressions based on the Mualem theory are compared with observed hydraulic conductivity data for five soils with a wide range of hydraulic properties.
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.
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.
An expression has been derived to describe both saturated and unsaturated permeability of porous media in terms of the pore size distribution as obtained from mercury-injection data or water-desorption isotherms. An interaction model has been adopted wherein both pore radius and effective area available for flow have been considered. The permeability values obtained using this expression have been compared with water and gas permeabilities of a variety of porous media. Satisfactory agreement is found between experimental and calculated values over a wide range of permeability.
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.
Numerical simulation has been applied in support of the U.S. Department of Energyâs (DOEâs) efforts to characterize the nature and distribution of carbon tetrachloride in the deep vadose zone at the Hanford site, near Richland, Washington. Three-dimensional computational domains were used, with layered and heterogeneous distributions of soil properties, in this numerical investigation into the vertical and lateral distribution of carbon tetrachloride beneath it release point (216-Z-9 trench) and the effects of soil vapor extraction process. The complexity of the modeled physical processes, namely, the nonlinearities associated with multifluid subsurface flow, including phase transitions and hysteresis in the relative permeability-saturation-capillary pressure functions, limits the grid resolution when executed using single processor computers. To achieve higher grid resolutions and acceptable detail in the subsurface distribution and remediation of carbon tetrachloride, execution on multiple processors was required. This paper describes and demonstrates a scalable implementation of a multifluid subsurface flow and transport with capabilities for volatile organic compounds, residual nonaqueous phase liquid formation in the vadose zone, and soil vapor extraction, using multiple wells. Developing scientific software for execution on parallel computers has unique challenges. The guiding objectives for developing this scalable code were to keep the source coding readable and modifiable by subsurface scientists, allow for both sequential and scalable processing, depend on domain scientists for code parallelization and scalable linear system solvers.
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
Vapor phase diffusion is an important transport process in the unsaturated zone affecting evaporation of volatile organic compounds (VOCs) from pure and multi component organic liquids. To evaluate some widely used empirical relationships for the estimation of effective diffusion coefficients in the unsaturated zone and to assess the validity of Raoult's Law during aging of organic mixtures, two series of laboratory-scale column experiments were performed using pure toluene, pure methyl tert-butyl ether (MTBE), and two multi component "kerosene-type" liquids containing four to seven compounds. The analytical one-dimensional solution of Fick's Second Law described the diffusion process of pure compounds very well in two sands with different water contents. The effective diffusion coefficients obtained correspond well to a recently published empirical relationship (Moldrup et al., 2000); the capacity factors fitted indicate equilibrium partitioning of the solute between gas phase and water. A one-dimensional numerical model based on the combination of Fick's Second Law and Raoult's Law was used to predict the volatilization and the diffusion process from multicomponent organic liquids. Both the vapor phase diffusion process of the VOCs and the aging of the organic mixtures were predicted very well solely on the basis of effective diffusion coefficients estimated from the empirical relationship and assuming an ideal mixture (e.g., an activity coefficient of 1 in Raoult's Law).
This guide describes the simulator`s governing equations, constitutive functions and numerical solution algorithms of the STOMP (Subsurface Transport Over Multiple Phases) simulator, a scientific tool for analyzing multiple phase subsurface flow and transport. The STOMP simulator`s fundamental purpose is to produce numerical predictions of thermal and hydrologic flow and transport phenomena in variably saturated subsurface environments, which are contaminated with volatile or nonvolatile organic compounds. Auxiliary applications include numerical predictions of solute transport processes including radioactive chain decay processes. 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 nonhysteretic relative permeability, saturation-capillary pressure constitutive functions. The authors further assume that the reader is familiar with the computing environment on which they plan to compile and execute the STOMP simulator. The STOMP simulator requires an ANSI FORTRAN 77 compiler to generate an executable code. 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. One-dimensional problems of moderate complexity can be solved on conventional desktop computers, but multidimensional problems involving complex flow and transport phenomena typically require the power and memory capabilities of workstation or mainframe type computer systems.
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.
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.
An experiment was conducted to evaluate whether vapor-density effects are significant in transporting volatile organic compounds (VOC's) with high vapor pressure and molecular mass through the subsurface. Trichloroethylene (TCE) was chosen for the investigation because it is a common VOC contaminant with high vapor pressure and molecular mass. For the investigation, a 2-m-long by 1-m-high by 7.5-cm-thick flow cell was constructed with a network of sampling ports. The flow cell was packed with sand, and a water table was established near the lower boundary. Liquid TCE was placed near the upper boundary of the flow cell in a chamber from which vapors could enter and migrate through the sand. TCE concentrations in the gas phase were measured by extracting 25-μl gas samples with an air-tight syringe and analyzing them with a gas chromatograph. The evolution of the TCE gas plume in the sand was investigated by examining plots of TCE concentrations over the domain for specific times and for particular locations as a function of time. To help in this analysis, a numerical model was developed that can predict the simultaneous movements of a gas, a nonaqueous liquid and water in porous media. The model also considers interphase mass transfer by employing the phase equilibrium assumption. The model was tested with one- and two-dimensional analytical solutions of fluid flow before it was used to simulate the experiment. Comparisons between experimental data and simulation results when vapor-density effects are considered were very good. When vapor-density effects were ignored, agreement was poor. These analyses suggest that vapor-density effects should be considered and that density-driven vapor advection may be an important mechanism for moving VOC's with high vapor pressures and molecular mass through the subsurface.
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.
A field experiment was performed in a sandy vadose zone, studying the fate of an emplaced fuel-NAPL source, composed of 13 hydrocarbons and a tracer. The UNIFAC model was used to testthe nonideal behavior of the source, and the numerical model MIN3P was used for assessing the effect of biodegradation on source evolution. The diffusive loss to the surrounding vadose zone and the atmosphere created temporary gradients in mole fractions of the individual compounds within the source NAPL. The evolution of the source composition corresponded in general with expectations based on Raoult's Law, with the exception thatthe mole fractions of aromatic compounds in the source NAPL decreased faster than fractions of aliphatic compounds of similar volatility. Calculation of activity coefficients (y) using the UNIFAC model implied nonideal conditions, with composition-dependent gammas different from 1. Positive deviations were calculated for the aromatic compounds. The effect of biodegradation on source depletion, evaluated by numerical modeling, was greater for the aromatic as compared to the aliphatic compounds. Hence, the faster depletion of the aromatic relative to aliphatic compounds of similar volatility is both a result of the nonideality of the mixture and a result of partitioning and biodegradation in the pore-water. Vapor concentrations of the compounds in the source were in reasonable agreement with predictions based on the modified Raoult's Law with the UNIFAC predicted gammas and the NAPL composition for the most volatile compounds. For the less volatile compounds, the measured vapor concentrations were lower than predicted with the largest deviations for the least volatile compounds. This field experiment illustrated that nonideal behavior and bioenhanced source depletion need to be considered at multicomponent NAPL spill sites.
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Carbon tetrachloride flow and transport in the subsurface of the 216-Z-9 trench at the Hanford site: Heterogeneous model development and soil vapor extraction modeling. PNNL-15914
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