Vadose Zone Remediation Research Articles

Transient Simulation of 1, 4-Dioxane Fate and Transport During Enhanced Soil Vapor Extraction Treatment

1, 4-Dioxane is an emerging contaminant a that is fully miscible in water and resistant to biodegradation. The presence of 1, 4-Dioxane in vadose zone creates prolonged environmental issues and risk human health. To date, treatment technologies for 1, 4-Dioxane vadose zone remediation are still being developed. In this work, a numeric simulation tool was developed to investigate the fate and transport of 1, 4-Dioxane in the treatment zone of enhanced soil vapor extraction (XSVE) which utilizes heated air injection to improve contaminant removal efficacy. The results suggested identify source zone is critical for XSVE treatment, inaccurate heat injection point would prolong treatment duration.

Model fit to experimental data for foam-assisted deep vadose zone remediation

This study investigates how a foam model, developed in Roostapour and Kam [1], can be applied to make a fit to a set of existing laboratory flow experiments in an application relevant to deep vadose zone remediation.This study reveals a few important insights regarding foam-assisted deep vadose zone remediation: (i) the mathematical framework established for foam modeling can fit typical flow experiments matching wave velocities, saturation history, and pressure responses; (ii) the set of input parameters may not be unique for the fit, and therefore conducting experiments to measure basic model parameters related to relative permeability, initial and residual saturations, surfactant adsorption and so on should not be overlooked; and (iii) gas compressibility plays an important role for data analysis, thus should be handled carefully in laboratory flow experiments. Foam kinetics, causing foam texture to reach its steady-state value slowly, may impose additional complications.

Monitoring Vadose Zone Desiccation with Geophysical Methods

Soil desiccation was recently field tested as a potential vadose zone remediation technology. Desiccation removes water from the vadose zone and significantly decreases the aqueous‐phase permeability of the desiccated zone, thereby decreasing movement of moisture and contaminants. The two‐ and three‐dimensional distribution of moisture content reduction with time provides valuable information for desiccation operations and for determining when treatment goals have been reached. This type of information can be obtained through the use of geophysical methods. Neutron moisture logging, cross‐hole electrical resistivity tomography, and cross‐hole ground‐penetrating radar approaches were evaluated with respect to their ability to provide effective spatial and temporal monitoring of desiccation during a treatability study conducted in the vadose zone of the USDOE Hanford site in the state of Washington.

Foam-Assisted Delivery of Nanoscale Zero Valent Iron in Porous Media

AbstractFoam is potentially a promising vehicle to deliver nanoparticles for vadose-zone remediation because foam can overcome the intrinsic problems associated with solution-based delivery, such as preferential flow and contaminant mobilization and spreading. In this work, the feasibility of using foam to deliver nanoscale zero valent iron (nZVI) in unsaturated porous media was investigated. Foam generated using the surfactant sodium lauryl ether sulfate (SLES) showed excellent ability to carry nZVI. SLES and nZVI concentrations in the foaming solutions did not affect the percentages of nZVI concentrations in foam relative to nZVI concentrations in the solutions. When foams carrying nZVI were injected through the unsaturated columns, the fractions of nZVI exiting the column were much higher than those when nZVI was injected with liquid. The enhanced nZVI transport implies that foam delivery could significantly increase the radius of influence of injected nZVI. The type and concentrations of surfactants a...

CO2 Storage Contingencies Initiative: Detection, Intervention and Remediation of Unexpected CO2 Migration

High profile refinery, pipeline and well incidents over the past several years may have impacted stakeholder perception of the oil and gas industry's ability to prevent and control accidents. This perception may be translated to current and future CO2 storage projects, as this relatively new application has a limited track record at scale. To better understand unexpected fluid migration and responses, the CO2 Capture Project Phase 3 (CCP3) has developed the “CO2 Storage Contingencies” project. The project was initiated by a CCP3-sponsored workshop that brought together industry, national laboratory and academic experts in wells, reservoir engineering and geosciences. The goal was to systematically assess CO2 and displaced fluid migration scenarios with current versus needed capabilities for detection, intervention and remediation of damages. Three focus areas were addressed in detail: wells, conformance and seals/fractures. It was concluded that groundwater and vadose zone remediation strategies would be deferred owing to decades of related experience and that a focus on intervention might obviate their need. The underlying assumption of the workshop was that even if CO2 storage projects employ state-of-the-art site characterization, risk assessment, and monitoring systems, unexpected migration may nevertheless occur, particularly during the early stages of gaining experience with large scale deployment. Specific mechanisms for unanticipated CO2 and brine migration were identified with both established and novel mitigation approaches for remedying them proposed. The group concurred on a general approach to qualifying scenarios with potential mitigations. A forward plan was outlined to document relevant industry experience with a roadmap of needed research and development (R&D), including modelling, simulation, bench-scale experiments and field trial design through deployment. The initial phases of the study are ongoing with concurrent concept development of the latter phases entailing identification, assessment and possible deployment of a field trial of detection and intervention approaches.

Experimental Investigation of the Effective Foam Viscosity in Unsaturated Porous Media

Foam has the potential to effectively carry and distribute either aqueous or gaseous amendments to the deep vadose zone for contaminant remediation. However, the transport of foam in porous media is complicated because flow characteristics such as effective viscosity are affected not only by foam properties but also by the sediment properties and flow conditions. We determined the average effective foam viscosity via a series of laboratory experiments and found that the effective foam viscosity was significantly higher than the viscosity of gas or water and increased with the liquid fraction in foam, the injection rate, and sediment permeability. These impacts are successfully described with a mathematical expression, which is further demonstrated with data from the literature. These results suggest that, when foam is used in deep vadose zone remediation, foam flow is almost not affected by gravity and is capable of distributing amendments uniformly in heterogeneous sediments.

Effects of Porous Medium Heterogeneity on Vadose Zone Desiccation: Intermediate‐Scale Laboratory Experiments and Simulations

Soil desiccation (drying), involving water evaporation induced by dry gas injection, is a potentially robust vadose zone remediation process to limit contaminant transport through the vadose zone. A series of four intermediate‐scale flow cell experiments was conducted in homogeneous and simple layered heterogeneous porous medium systems to investigate the effects of heterogeneity on desiccation of unsaturated porous media. The permeability ratios of porous medium layers ranged from about 5 to almost 2 orders of magnitude. The insulated flow cell was equipped with 20 humidity and temperature sensors while a dual‐energy γ system was used to determine water saturations at various times. The multiphase code STOMP was used to simulate the desiccation process. For the layered systems, results show that injected dry gas flowed predominantly in the higher permeability layer and delayed water removal from the lower permeability material. For the configurations tested, water vapor diffusion from the lower to the higher permeability zone was considerable over the duration of the experiments, resulting in much larger relative humidity values of the outgoing air than based on permeability ratios alone. Acceptable numerical matches with the experimental data were obtained when an extension of the saturation‐capillary pressure relation below the residual water saturation was used. The agreements between numerical and experimental results suggest that the correct physics is implemented in the simulator.

Modeling foam delivery mechanisms in deep vadose-zone remediation using method of characteristics

This study investigates foam delivery mechanisms in vadose-zone remediation by using Method of Characteristics (MoC), a mathematical tool long been used for the analysis of miscible and immiscible flooding in porous media in petroleum industry. MoC converts the governing material-balance partial differential equations into a series of ordinary differential equations, and the resulting solutions are in a form of wave propagation (more specifically, for chemical species and phase saturations) through the system as a function of time and space.Deep vadose-zone remediation has special features compared to other conventional remediation applications. They include, not limited to, a high level of heterogeneity, a very dry initial condition with low water saturation (Sw), pollutants such as metals and radionuclides fully dissolved in groundwater, and a serious concern about downward migration during the remediation treatments. For the vadose-zone remediation processes to be successful, the injected aqueous phase should carry chemicals to react with pollutants and precipitate them for immobilization and stabilization purposes. As a result, foams are believed to be an effective means, and understanding foam flow mechanism in situ is a key to the optimal design of field applications.Results show that foam delivery mechanism is indeed very complicated, making the optimum injection condition field-specific. The five major parameters selected (i.e., initial saturation of the medium, injection foam quality, surfactant adsorption, foam strength, and foam stability) are shown to be all important, interacting with each other. Results also imply that although dry foam injection is generally recommended, too dry injection condition is found to hurt this process due to slow foam propagation.

Review: Technical and Policy Challenges in Deep Vadose Zone Remediation of Metals and Radionuclides

Contamination in deep vadose zone environments is isolated from exposure so direct contact is not a factor in its risk to human health and the environment. Instead, movement of contamination to the groundwater creates the potential for exposure and risk to receptors. Limiting flux from contaminated vadose zone is key for protection of groundwater resources, thus the deep vadose zone is not necessarily considered a resource requiring restoration. Contaminant discharge to the groundwater must be maintained low enough by natural attenuation (e.g., adsorption processes or radioactive decay) or through remedial actions (e.g., contaminant mass reduction or mobility reduction) to meet the groundwater concentration goals. This paper reviews the major processes for deep vadose zone metal and radionuclide remediation that form the practical constraints on remedial actions. Remediation of metal and radionuclide contamination in the deep vadose zone is complicated by heterogeneous contaminant distribution and the saturation-dependent preferential flow in heterogeneous sediments. Thus, efforts to remove contaminants have generally been unsuccessful although partial removal may reduce downward flux. Contaminant mobility may be reduced through abiotic and biotic reactions or through physical encapsulation. Hydraulic controls may limit aqueous transport. Delivering amendments to the contaminated zone and verifying performance are challenges for remediation.

Foam, a promising vehicle to deliver nanoparticles for vadose zone remediation

Foam delivery of remedial amendments for in situ immobilization of deep vadose zone contaminants can overcome the intrinsic problems associated with solution-based delivery, such as preferential flow and contaminant mobilization. In this work, the feasibility of using foam to deliver nanoparticles in unsaturated porous media was investigated. Carboxyl-modified polystyrene latex microspheres were used as surrogates for nanoparticles of remediation purposes. Foams generated from the solutions of six commonly available surfactants all had excellent abilities to carry the microspheres. The presence of the microspheres did not reduce the stabilities of the foams. When microsphere-laden foam was injected through the unsaturated columns, the fractions of microspheres exiting the column were much higher than that when the microsphere water suspensions were injected through the columns. The enhanced microsphere transport implies that foam delivery could significantly increase the radius of influence of injected nanoparticles of remediation purposes. Reduced tension at air–water interfaces by the surfactant and increased driving forces imparted on the microspheres at the interfaces by the flowing foam bubbles may have both contributed to the enhanced transport. Preliminary tests also demonstrated that foam can carry significant fractions of zero valent iron nanoparticles at concentrations relevant to field remediation conditions (up to 5.3gL−1). As such, this study demonstrates that surfactant foam is potentially a promising vehicle to deliver nanoparticles for vadose zone remediation.

Foam Delivery of Amendments for Vadose Zone Remediation: Propagation Performance in Unsaturated Sediments

Foam delivery of remedial amendments is potentially useful for vadose zone remediation because aqueous amendments are delivered while a low water content is maintained in the treated zones. The transport behavior of foam in the vadose zone has not been well studied. Benchtop‐scale column tests were conducted to investigate foam transport, foaming gas and liquid propagation, pressure buildup and distribution, the influence of foam quality and sediment permeability on foam injection pressure, and liquid uptake and distribution during foam flow in unsaturated sediments. Results indicated that under unsaturated conditions, the foaming gas front traveled much faster than the foam front where bubble rupture was significant. A liquid front was formed ahead of the foam flow front. The accumulated liquid in this front was mainly composed of the water injected with the foam. The foam injection pressure was primarily attributed to the resistance to foam flow by the sediment but not to the movement of the water ahead of the foam front. The foam had a high effective viscosity compared with water. A near‐linear pressure distribution within the foam‐occupied column section was observed. At a fixed injection rate, higher foam quality resulted in lower injection pressure. Higher permeability sediments did not necessarily have a lower foam injection pressure because other factors, such as the foam injection rate, foam bubble diameter, and sediment pore size, also influenced the pressure. In the foam flushed region, the liquid uptake and water content distribution in the sediment was negligibly affected by the initial water content and the foam injection rate within the ranges tested in this study. The implications of these findings to field remediation application were considered.

Desiccation of Unsaturated Porous Media: Intermediate‐Scale Experiments and Numerical Simulation

Soil desiccation (drying), involving water evaporation induced by air injection and extraction, is a potentially robust vadose zone remediation process to limit migration of inorganic or radionuclide contaminants through the vadose zone. Desiccation also has the potential to improve gas‐phase‐based treatments by reducing water saturation and therefore increasing sediment gas‐phase permeability. Before this technology can be deployed in the field, concerns related to energy limitations, osmotic effects, and potential contaminant remobilization after rewetting must be addressed. A series of detailed, intermediate‐scale laboratory experiments, using unsaturated homogeneous and heterogeneous systems, was conducted to improve our understanding of energy balance issues related to soil desiccation. The experiments were subsequently simulated with the multifluid flow simulator STOMP, using independently obtained hydraulic and thermal porous medium properties. In all experiments, the injection of dry air proved to be an effective means for removing essentially all moisture from the test media. Observed evaporative cooling generally decreased with increasing distance from the gas inlet chamber. The fine‐grained sand embedded in the medium‐grained sand of the heterogeneous system showed two local temperature minima associated with the cooling. The first one occurred because of evaporation in the adjacent medium‐grained sand, whereas the second minimum was attributed to evaporative cooling in the fine‐grained sand itself. Results of the laboratory tests were simulated accurately only if the thermal properties of the flow cell walls and insulation material were taken into account, indicating that the appropriate physics were incorporated into the simulator.

Estimating Persistent Mass Flux of Volatile Contaminants from the Vadose Zone to Ground Water

AbstractContaminants 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.

Air Permeability in Undisturbed Volcanic Ash Soils

Soil air permeability (ka) governs convective air and gas transport in soil. The increased use of soil venting systems during vadose zone remediation at polluted soil sites has created a renewed interest in ka and its dependency on soil type and soil air‐filled porosity (ε). Predictive ka(ε) models have only been tested within limited ranges of pore‐size distribution and total porosity. Andisols (volcanic ash soils) exhibit unusually high porosities and water retention properties. In this study, measurements of ka(ε) on 16 undisturbed Andisols from three locations in Japan were carried out in the soil matric potential interval from −10 cm H2O (near water saturation) to −15000 cm H2O (wilting point). Two simple power‐function ka(ε) models, both with measured ka at −100 cm H2O as a reference point, gave similar and good predictions of ka(ε) between −10 and −1000 cm H2O. For one location comprising finely textured and humic Andisols, both models largely underpredicted ka(ε) in dry soil (<−3000 cm H2O), suggesting a sudden occurrence of highly connected air‐filled pore networks during drainage. For the two other locations, the models satisfactorily predicted ka also in dry soil. Using recently published data for gas diffusivity and soil‐water retention together with the ka data in the Millington and Quirk (1964) fluid flow model, a plot of equivalent pore diameter as a function of soil matric potential was made for each soil. This plot, labeled a soil structure fingerprint (SSF), proved useful for illustrating effects of soil cultivation and high organic matter content on soil structure.

Correction to “Solutions to equations incorporating the effect of rate-limited contaminant mass transfer on vadose zone remediation by soil vapor extraction” by Jungi Huang and Mark N. Goltz

Correction to “Solutions to equations incorporating the effect of rate-limited contaminant mass transfer on vadose zone remediation by soil vapor extraction” by Jungi Huang and Mark N. Goltz

Modeling the Influence of Heat/Moisture Exchange during Bioventing

The presented modeling investigation examines the potential influence of advection-induced evaporation and condensation on bioventing, a vadose-zone remediation technology. Currently, few soil vapor extraction or bioventing models incorporate nonisothermal effects when considering system performance. Laboratory and field measurements suggest, however, that even small changes in temperature and moisture content can influence microbial activity and could thus affect the overall efficiency of a bioventing operation. The model here is a one-dimensional simulator that describes mass and energy transport under steady, gaseous phase flow conditions. The coupled mass and energy equations are solved using a sequential iterative solver with matric potential and temperature as primary variables. A literature-derived relation is used to quantify the combined effect of water potential and temperature change on biological growth rates. Simulations indicate that the injection of air at temperatures and/or water vapor concentrations different from the initial ambient soil conditions can induce changes in matric potential and local soil temperature, which could measurably impact biological activity.

Solutions to equations incorporating the effect of rate‐limited contaminant mass transfer on vadose zone remediation by soil vapor extraction

Soil vapor extraction (SVE) is a technique that is frequently used to remediate unsaturated soils in the vadose zone that are contaminated with volatile organic compounds. Laboratory research and field experience have demonstrated that rate‐limited mass transfer of a contaminant from aquifer solids and soil water to the gas phase significantly impacts the efficacy and speed of the remediation. An exact analytical solution to equations describing SVE controlled by rate‐limited mass transfer is presented. Processes modeled include contaminant advective‐dispersive transport in a gas phase converging on an extraction well and rate‐limited mass transfer of dissolved and sorbed contaminants into the gas phase, with the rate limitation modeled assuming first‐order kinetics. In addition to the exact solution an approximate solution, as well as an exact solution for plug flow (no dispersion), is presented. These analytical solutions may be useful in verifying numerical codes that are being developed to model SVE of volatile organic compounds.

Geochemical Reactions During Biodegradation/Vapor‐Extraction Remediation of Petroleum Contamination in the Vadose Zone

AbstractHydrocarbon concentrations in soil water and vapor are generally used to evaluate the progress of biodegradation and vapor extraction of petroleum contamination in the unsaturated zone. This study shows that changes in the inorganic composition of vadose zone water samples can be used to evaluate the reactions that occur in the unsaturated zone during such a remediation effort. Chemical analyses were completed on water samples collected from alluvial sediments contaminated with diesel fuel and gasoline at the Gallatin Farmers Cenex, Belgrade, Montana. The samples were collected from 7 suction lysimeters for 3 months after fertilization, but before vapor extraction, and then for 6 months following the start of vapor extraction. The geochemical reaction progress code SOLMINEQ.88 is used to calculate the aqueous equilibria in the samples and to simulate possible reaction pathways.Reduction in TPH and BTEX concentrations indicated that biodegradation of the petroleum began after fertilization, prior to vapor extraction, and continued after the start of vapor extraction. SO4=, HCO3‐, pH, and PCO2 show large systematic variations with both time and depth. These variations are independent of evaporation, mixing, sample extraction time, and soil moisture content and thus appear to be a direct consequence of bioremediation and vapor extraction. PCO2 is found to be a measure of the effectiveness of vapor extraction. The chemical mass transfer calculations also show that if vapor extraction occurs alone removing CO2 from solution without coupling of a process to buffer the solution pH, large amounts of carbonate minerals could precipitate, significantly reducing sediment permeability. These data suggest that analyses of inorganic compounds in lysimeter samples can be used to evaluate geochemical changes during vadose zone remediation and can be used to improve remediation design.