Nonaqueous Phase Liquid Distribution Research Articles

Performance evaluation of surrogate models for simulating multiphase NAPL transport in heterogeneous aquifers

Abstract A better understanding of the distribution of nonaqueous phase liquid (NAPL) plumes is of great importance to groundwater pollution remediation and control. However, the efficiency of surrogate models in simulating the transport is still not well addressed. Selecting a leakage problem as an example, 50 sets of random permeability distributions are generated using the Monte Carlo method, and a numerical model is used to obtain benchmark data of NAPL transport. Four machine learning methods are employed to simulate dense NAPL transport under point leakage sources across spatiotemporal scales. The validation of the models demonstrate that the random forest model can also effectively capture the spatial-temporal distribution of the plume in heterogeneous aquifers, with a maximum mean absolute error and root mean square error smaller than 5.55 × 10−4 and 5.88 × 10−5, respectively. Meanwhile, the multiple phase outcome from the random forest model fits well with the numerical results under the scenarios of linear leakage sources and light NAPL transport. The total time consumed in the computation is reduced by over 150 times after using the surrogate models. The results suggest that surrogate models can provide a promising way to understand NAPL transport in heterogeneous aquifers.

An ice- air-water-NAPL multiphase model for simulating NAPL migration in subsurface system under freeze-thaw condition

Non-aqueous phase liquid (NAPL) leakage poses serious threats to human health and the environment. Understanding NAPL migration and distribution in subsurface systems is crucial for developing effective remediation strategies. Multiphase flow modeling is an important tool to quantitatively describe the NAPL migration process in the subsurface. However, most multiphase flow models are built for temperatures typical of warmer climates and above freezing conditions, only considering two phases (water-NAPL) or three phases (air-water-NAPL). To date, few studies simulate NAPL migration in a four-phase system (ice-air-water-NAPL), which would be more appropriate for cold regions. In this study, we developed a coupled non-isothermal multiphase transport model to quantitatively describe NAPL migration in a four-phase (ice, gas, water, NAPL) system. The ice phase was added in the continuity equations and the constitutive relationship between unfrozen water content and temperature was applied to solve the energy and flow equations. The developed mathematical model was evaluated using a two-dimensional experiment under freeze-thaw cycles (FTCs) with an R2 = 0.8803 between the simulated and observed NAPL saturation. Next, we evaluated the effect of freezing-induced changes in pressure and density between LNAPL and DNAPL on NAPL distribution under freeze-thaw condition. Simulation results show that ignoring the impact of ice formation and thawing during freeze-thaw cycles for LNAPL and DNAPL transport simulations can result in up to a 48% and 13% difference in model predictions of local NAPL saturations respectively, affecting model predictions of overall NAPL spatial distributions and potentially predicted remediation effectiveness.

Numerical Modeling and Data‐Worth Analysis for Characterizing the Architecture and Dissolution Rates of a Multicomponent DNAPL Source

AbstractA numerical solute transport model was calibrated to a high‐resolution monitoring data set to characterize a multicomponent source of nonaqueous phase liquids (NAPLs) and evaluate the uncertainty of estimated parameters. The dissolution of NAPL mass was simulated using SEAM3D with parameter zones including adjustable NAPL saturations and mass transfer coefficients, representing the heterogenous architecture of the source zone. Source zone parameters were simultaneously estimated using PEST from aqueous‐phase concentrations measured in a multilevel monitoring transect and from mass recovery rates measured at extraction wells during a controlled field experiment. Data‐worth analyses, facilitated by PEST ancillary software, linked maximum aqueous‐phase concentrations of all compounds to reductions in the pre‐calibration uncertainty of mass transfer coefficients. In turn, decreasing concentrations of the most soluble NAPL fraction constrained the source mass estimation. The accurate estimation of model parameters was possible by removing concentrations measured during early NAPL dissolution stages, identified as drivers of model bias using the iterative ensemble smoother PESTPP‐iES. Although uncertainty analyses highlighted model limitations for representing sub‐grid‐scale heterogeneity of NAPL distribution and mass transfer rates, final stages of NAPL dissolution measured at multilevel ports eliminated parameter bias and produced long‐term projections of multi‐stage source zone depletion. Including mass discharge rates for model calibration further improved the accuracy of estimated residual source mass, complementing multilevel monitoring constraints on the saturation distribution and mass transfer coefficients.

Emergence of non-aqueous phase liquids redistribution driven by freeze-thaw cycles in porous media based on low-field NMR

• Redistribution of NAPLs driven by freeze–thaw (FT) cycles was confirmed in a wide saturation range (14.34% to 72.05%). • Substantial remobilization of NAPLs occurred between macropores and mesopores during FT cycles. • Distribution of NAPLs in micropores remained almost unchanged during FT cycles. • Contribution of micropores to NAPLs redistribution was much smaller than that of mesopores and macropores. • A model related to freezing-induced pressure was proposed to explain the change in the redistribution rate of NAPLs. Increasing human activities in cold regions have led to serious environmental problems due to non-aqueous phase liquids (NAPLs) contamination. However, the NAPLs redistribution driven by freeze–thaw (FT) cycles is not well understood. In this study, the redistribution of NAPLs in porous media subjected to FT cycles in a temperature range from 20 °C to −15 °C was investigated using the low-field Nuclear Magnetic Resonance (NMR) technique. The redistribution of NAPLs in the prepared samples with NAPLs saturation ranging from 13.56% to 72.05% was confirmed using the transverse spin-spin relaxation time ( T 2 ) distribution and the magnetic resonance imaging (MRI). Experimental results revealed that substantial remobilization and distribution of NAPLs occurred between the macropores ( T 2 greater than 300 ms) and the mesopores (60 ms < T 2 < 300 ms) while the NAPLs in the micropores ( T 2 < 60 ms) remained almost unchanged during 30 FT cycles. The main reason is attributed to the fact that the redistribution rate in micropores is much lower than in macropores and mesopores. Furthermore, the NAPLs content and the number of FT cycles were positively correlated for macropores while negatively correlated for mesopores. For the samples with low NAPLs saturation (e.g., S N =13.56% and 13.56%), the redistribution rate of NAPLs versus FT cycles exhibited an overall linear relationship for macropores and mesopores while a non-liner relationship for micropores. For all pore categories, the increment in S N can cause a remarkable deviation extent. The balance between capillary pressure and freezing-induced pressure was evaluated by the newly introduced dimensionless pressure ratio Δ P , which could help determine the ultimate mobilization and redistribution of NAPLs during FT cycles. Experimental tests suggested that the Δ P was much larger in macropores than in mesopores and micropores. This can explain the dramatic difference in the redistribution rate among different pore categories. This fundamental study is helpful in understanding the mobilization and redistribution mechanisms of NAPLs in cold regions.

Mechanistic modeling and measurement of foamed gas flow resistance in fractures

Geologic heterogeneity is one of the important factors controlling migration and distribution of nonaqueous phase liquids in the subsurface. Heterogeneity, such as fractures, complicates the effectiveness of environmental remediation, and uncharacterized aquifer heterogeneity causes significant uncertainties in the removal of nonaqueous phase liquids. Foaming an injection gas is an effective method to alleviate the challenges in accessing heterogeneous media during remediation processes because foam is nonNewtonian and redirects gas and liquid flow. Foamed gas may also assist in increasing the storage of carbon dioxide in saline formations. This paper investigates foam flow behavior in microscale fractures and develops a mechanistic transient foam flow model using the population balance method. Microscale experiments in fractures with apertures of 25 and 88 μm are used to validate the model for pressure drop, gas saturation, and bubble texture. Key differences related to modeling foam in fractures are the potential for continuously varying gas–liquid curvature in fractures and the relationship of this curvature to apparent foam viscosity. Incorporation of a local foam flow resistance factor is important to representing flow physics accurately. Hence, the results of this study provide foundational understanding for upscaling foam rheology in different sized fractures to ensure successful operations in soil/aquifer remediation and CO2 storage in saline formations.

The mechanism of NAPL layer formation in a microfluidic device with dual-permeability: experiments and numerical simulation

In-situ remediation is an important technique for non-aqueous phase liquid (NAPL) contamination remediation in soils. Understanding the formation and distribution of NAPL contaminated layers in heterogeneous soils is essential to propose cost-effective remediation methods. Therefore, a two-dimensional microfluidic device with dual permeability zones was designed to simulate the soil-groundwater system and experimentally investigated the formation process of the NAPL contamination layer. Numerical modeling of phase field coupled with laminar flow was used to simulate the distribution of NAPL in soil-groundwater system, and the formation of NAPL contamination layer under typical groundwater flow rate and wetting angle was evaluated. The NAPL in the low permeable zone formed a stable contamination layer, while the NAPL in the high permeable zone was washed out to varying degrees and mainly resided in the junction of high-low-permeability regions of the chip, with residues of 42.2% and 23.3% in the chip high permeable region at groundwater velocities of 6.17 m/d and 10.16 m/d, respectively, the numerical simulation results were consistent with the experimental observations. This retention was diminished in the high permeable zone as the NAPL-to-wall wetting angle increased. In addition, when the contact angle was increased from 33° (lipophilic) to 108° (hydrophilic), the residual NAPL content in the high permeable zone decreased from 48.86% to 28.22%. This paper provides a reliable visualizable experimental platform for the study of NAPL pollution formation at micro-scale in heterogeneous groundwater system, and lays a foundation for the subsequent optimization of the remediation strategy.

Noninvasive survey technology for LNAPL-contaminated site investigation

Nonaqueous-phase liquids (NAPLs) usually move through the vadose zone not in a homogeneous spread manner but in the form of disconnected blobs, and they remain in the pores of porous media. A major challenge in remediating an NAPL-contaminated site is to detect and delineate the distribution of NAPLs. Geophysical technologies could act as investigating methods for sites with limited resources and time constraints. Based on the principle of magnetometric resistivity (MMR) method, in this study, we applied an electric current flow pattern and its resulting magnetic field to develop a new application for contaminated site investigation. The physical phenomena about the magnetic distribution, the flow pattern of electric current, and the influence of different media in packed sand were observed via sandbox experiments. A field test was performed at a light NAPL (LNAPL)-contaminated site by combining the magnetic field survey and a soil sample analysis. Two potential contaminated zones were determined according to the specific characteristics of the contour image plotted according to the magnetic field intensity collected on the ground. The quantitative results of the total petroleum hydrocarbons (TPHs) of 18 soil samples were used to verify the results of the magnetic field survey. The comparison results revealed that we obtained a high accuracy of 92.9% in the field test, which indicated that this technology could be used to investigate LNAPL-contaminated sites. Moreover, two phenomena, the scale effect led to poor resolution and the behavior of flow pattern disturbance distorted the survey images, were observed in both the sandbox experiment and the field test.

Rationale for Soil‐Gas Sampling to Improve Vapor Intrusion Risk Assessment in China

Contaminated site investigation and remediation are becoming increasing common in China since the implementation of the Action Plan for Prevention and Control of Soil Pollution (2016), the Soil Pollution Control Act (2019), and an overall desire by the national government to return >100,000 industrial sites closed since 2001 and >2 million hectares of brownfield land to productive use. Many of these sites are contaminated with volatile organic compounds (VOCs) that represent a potential risk for vapor intrusion (VI) (Ma 2019). To date, environmental guidelines and regulations for VOC and soil-gas sampling on a national level are lacking in China even though VI risks are predicted from soil concentration measurements assuming equilibrium partitioning relations (MEE 2014). Future VOC guidelines for VI risk assessment in China should consider building on experience and learnings from the United States on soil-gas sampling. A rationale and recommended methods for soil-gas sampling for VI risk assessment are provided in this editorial. VOC concentrations in soil gas cannot be adequately predicted based on soil concentration measurements (McAlary et al. 2011; Ma et al. 2018; Ma 2019). The authors are not aware of any field studies that clearly show meaningful correlations between paired VOC concentrations in soil and soil gas (Ma 2019). In particular, benzene has been observed at very high concentrations in soil gas (>2000 μg/m3) where corresponding soil concentrations are below detection limits (Zhang et al. 2019). VOC vapor concentrations can also be orders of magnitude less than those predicted based on soil concentrations and equilibrium vapor partitioning relations (Golder-Associates 2008). VOC concentrations in soil gas can also be overestimated in soil samples where nonaqueous phase liquid (NAPL) is present and unaccounted for in three-phase (soil, pore-water, vapor) partitioning relations (U.S. Environmental Protection Agency [U.S. EPA] 2015). The distribution of NAPL in soils can also be highly heterogeneous, contributing to the variability and poor correlation (Ma 2019). Volatilization (loss) of VOCs during soil sampling is difficult to avoid (Ma 2019). Based on these learnings, soil-gas sampling is preferred over soil or groundwater sampling for VI risk assessment. In addition, soil-gas sampling is preferred because the methodology provides a direct measure of the media (vapor) involved in the exposure pathway (Ma 2019). Depth-discrete soil-gas sampling is recommended to validate the variability of soil-gas data between vapor sources and building foundations, which is of particular importance at petroleum VI sites where biodegradation of VOCs is often significant. Soil-gas sampling from immediately below building foundations (subslab sampling) will also be needed in China to support the development of regionally specific risk-based screening and cleanup levels. U.S. regulatory values derived from empirical studies of vapor attenuation across building foundations will likely have limited applicability in China given the contrast in typical residential building types (single-family in the United States vs. high-rise in China) and transferability of empirically derived slab attenuation factors. Subslab sampling in China will also be affected by regulatory acceptance of institutional and engineering controls. Although soil-gas sampling is recommended over soil and groundwater sampling for VI investigations, it is important to understand that soil-gas sampling has certain limitations (Ma 2019). In particular, VOC soil-gas concentration data can exhibit substantial spatiotemporal variability, that will generally increase with increasing distance from a VOC source and proximity to building foundations. Soil-gas concentration data may also be a poor predictor of VI risks at sites with future building construction because of potential effects of buildings and building foundations on VOC distributions in the subsurface. Such scenarios are likely to be a common occurrence in China given the large number of redevelopment sites anticipated in the coming years. These sites will require predictions of VI risks based on soil and groundwater data. A variety of methods have been developed for soil-gas sampling, including Tedlar bags, canisters, and sorbent tubes. Tedlar bags are generally used for field screening but not suitable for laboratory analysis due to low recoveries (U.S. EPA 2015; Ma 2019). Canisters are commonly used in North America while sorbent tubes are more popular in Europe (Woolfenden 2010). Each method has its own advantages and disadvantages. Sorbent tubes are better suited for sampling less volatile VOCs and semivolatile VOCs, although some more volatile VOCs, such as vinyl chloride, can also be adequately trapped using proper sorbents (Ma 2019). Canisters are ideal for most VOCs of concern for vapor intrusion, including ultra-volatile reactive compounds such as H2S, but the recoveries of certain longer-chain (heavier) hydrocarbons (>C10 hydrocarbon) can be poor (Ma 2019). Based on this experience, sorbent tubes are generally the preferred methodology for soil-gas sampling in China because the method: (1) is less prone to false positive VOC concentrations caused by residual contamination from previous sampling events (McHugh et al. 2018); (2) provides a means to detect lower and wider ranges of VOC concentrations (Woolfenden 2010); and (3) is easier to handle, transport, and ship to analytical laboratories, which are still relatively scarce within the country. Guidance and training will be needed in China to promote best practices for sorbent tube sampling that include addressing inherent complexities of measuring flow rates before, during and after sampling (using high-precision flow meters), selecting appropriate sorbents, and screening soil-gas concentrations to prevent VOC breakthrough. Sorbent tubes may be less relevant at petroleum VI sites because associated analytical methods do not include the quantification of fixed gases (oxygen, carbon dioxide, methane, and nitrogen), which are key indicators of hydrocarbon biodegradation. These gases, with the exception of nitrogen, can, however, be analyzed in situ using landfill gas meters and generally at sufficient quality supporting vapor intrusion risk assessment (Ma 2019). In summary, we urge China to strongly consider the development and promotion of national and provincial guidelines for soil-gas sampling and analysis in an effort to improve VI risk at VOC-contaminated sites. The soil-gas sampling guidelines can largely be adopted from the United States and other countries where established practices and standard operating procedures already exist. It is however, important for China to adapt the guidelines to address a VI exposure model that is unique with respect to future redevelopment and building construction. This work was supported by Natural Science Foundation of China (21407180, 21878332), Beijing NOVA program (Z181100006218088), PetroChina Innovation Foundation (2018D-5007-0607), Science Foundation of China University of Petroleum-Beijing (2462018BJC003), and the Independent Project Program of State Key Laboratory of Petroleum Pollution Control (Grant No. PPCIP2017004), CNPC Research Institute of Safety & Environment Technology. J. Ma, PhD, corresponding author, is at State Key Laboratory of Heavy Oil Processing, Beijing Key Lab of Oil & Gas Pollution Control, China University of Petroleum-Beijing, Beijing 102249, China; ORCID ID: 0000-0003-3719-3814, email: rubpmj@sina.com M. Lahvis, PhD, is at Shell Global Solutions (US) Inc., Shell Technology Center, TX 77082.

Wettability Effects on Primary Drainage Mechanisms and NAPL Distribution: A Pore‐Scale Study

AbstractThe pore‐scale processes governing water drainage behavior in porous media have implications for geoscience multiphase scenarios including carbon capture and storage, contaminant site remediation, oil recovery, and vadose zone processes. However, few studies report directly observed pore‐scale water drainage phenomena in 3‐D soils. This knowledge gap limits our ability to verify assumptions underlying existing models and develop optimal solutions. This paper utilizes synchrotron X‐ray microtomography to present an experimental pore‐scale examination of nonaqueous phase liquid (NAPL)/water distribution along a primary drainage front as dense NAPL was injected upward into water wetting (WW) and intermediate wetting (IW) sand‐packed columns. Pore‐network structures were extracted from imaged data sets and mapped onto segmented NAPL/water data sets which allowed quantitative examinations of wettability impacts on (a) the extent to which NAPL fills individual pore bodies and (b) relationships between pore size and the phase occupying the pore, with both considered as a function of distance (and capillary pressure) relative to the NAPL front. These results revealed that several hypotheses treating IW sand similarly to WW sands are simplistic. IW systems exhibited a sequence of pore filling that deviated from traditional capillary pressure‐based model predictions: NAPL invades smaller pores, while larger, adjacent pores are bypassed leaving multipore residual water ganglia. NAPL pore saturations were close to 1 and did not change with capillary pressure in IW systems. Overall, the results illustrate how a relatively small change in operative contact angle alters NAPL distribution during water drainage, with important implications for geoscience multiphase flow scenarios.

Remediation of trapped DNAPL enhanced by SDS surfactant and silica nanoparticles in heterogeneous porous media: experimental data and empirical models.

The remediation of nonaqueous phase liquids (NAPLs) enhanced by surfactant and nanoparticles (NP) has been investigated in numerous studies. However, the role of NP-assisted surfactants in the dissolution process is still not well discussed. Besides, there is a lack of empirical dissolution models considering the effects of initial residual saturation Strap, NAPL distribution, and surfactant concentration in NAPL-aqueous phase systems. In this work, micromodel experiments are conducted to quantify mass transfer coefficients for different injected aqueous phases including deionized water, SDS surfactant solutions, and NP-assisted solutions with different levels of concentrations and flow rates. Observations reveal that silica nanoparticles (SNP) can significantly enhance interphase mass transfer, while SDS surfactant reduces the mass transfer coefficient. In addition, Strap and intrinsic interfacial area ai, as an indicator of dense nonaqueous phase liquids (DNAPL) distribution, influence the interphase mass transfer. The ai is also independent of DNAPL saturation SNAPL except for SNAPL < 7% when ganglia breakup occurs. Based on these observations, new empirical dissolution models are proposed in the presence and the absence of SDS surfactant and SNP in which ai, Strap, and surfactant concentrations are introduced as new parameters. The evaluated mass transfer rate coefficients using the proposed models show a significant improvement compared to available empirical models. The finding of this study might be attractive for application in field-scale simulations of surfactant-enhanced aquifer remediation (SEAR) and NP-assisted methods.

Effects of Pore‐Scale Heterogeneity on Macroscopic NAPL Dissolution Efficiency: A Two‐Scale Numerical Simulation Study

AbstractInterphase mass transfer or dissolution coefficient of nonaqueous phase liquids (NAPL) is an important parameter in predicting the transport of contaminant species in porous media. While the literature offers valuable insights into the dependence of this coefficient on different parameters at the continuum scale (e.g., contaminant saturation and Darcy velocity), effects of pore‐scale heterogeneity on macroscopic dissolution coefficient have received little attention. In this work a three‐dimensional pore‐scale model is developed to simulate interphase mass transfer over different synthetic pore network structures with various pore radii correlation lengths. The pore network modeling simulates dissolution of immobile NAPL into water (single phase) through diffusive throats for the water‐NAPL interface. The impacts of pore network spatially correlated heterogeneities, NAPL saturation/distribution, and aqueous phase velocity on NAPL mass transfer coefficient and water‐NAPL interfacial surface area are studied. These macroscopic properties are then employed in two‐dimensional continuum‐scale domains formed by concatenating 20 by 20 pore networks in x and y directions. The results highlight the impact of pore‐scale heterogeneity on the distribution of NAPL and subsequently on the dissolution rate (i.e., dissolution coefficient). An uncorrelated distribution of pore radii consistently leads to higher NAPL dissolution coefficient than spatially correlated heterogeneity. The results of continuum modeling show that NAPL dissolution rates are only different between domains formed by correlated and uncorrelated pore networks at very high flow rates and Darcy velocities. However, for typical values of Darcy velocity in groundwater systems, variation in mass transfer coefficient due to pore‐scale heterogeneity is minimal for efficient mass removal.

Experiments and sensitivity coefficients analysis for multiphase flow model calibration of enhanced DNAPL dissolution

Multiphase flow modeling is often used for the comparison and optimization of subsurface nonaqueous phase liquid (NAPL) remediation schemes. The calibration of such models is a challenging task due to the lack of detailed data describing the initial NAPL spatial distribution and the processes governing the fate and transport of NAPLs in porous media. In this study laboratory scale experiments were conducted to evaluate reagent-enhanced dense nonaqueous phase liquid (DNAPL) solubilization in saturated heterogeneous media. The DNAPL consisted of both pooled and residual saturation forms. To gain insight into the influence of various input parameters on effluent concentrations, the multiphase flow program was used to compute the sensitivity coefficients of key parameters, relating to the flow, flushing solution properties, soil parameters, NAPL distribution and mass transfer coefficient. The sensitivity coefficients were, in turn, used to aid in the model calibration and to underline the difficulties associated with the calibration of multiphase flow models, most notably the non-uniqueness of the calibration process when complete information is lacking. To alleviate this uncertainty and provide additional constraints, the conducted flushing experiments were jointly used to calibrate the multiphase flow model. The results of the model calibration suggest that the interphase mass transfer coefficient is dependent on the properties of the reagent aqueous solution used for DNAPL remediation, most notably the viscosity and interfacial tension.

Combining spectral induced polarization with X-ray tomography to investigate the importance of DNAPL geometry in sand samples

Although many studies have been performed to investigate the spectral induced polarization (SIP) response of nonaqueous phase liquid (NAPL)-contaminated soil samples, there are still many uncertainties in the interpretation of the data. A key issue is that altered pore space geometries due to the presence of a NAPL phase will change the measured IP spectra. However, without any information on the NAPL distribution in the pore space, assumptions are necessary for the SIP data interpretation. Therefore, experimental data of SIP signals directly associated with different NAPL distributions are needed. We used high-resolution X-ray tomography and 3D image processing to quantitatively assess NAPL distributions in samples of fine-grained sand containing different concentrations of tetrachloroethylene and link this to SIP measurements on the same samples. The total concentration of the sample constituents as well as the volumes of the individual NAPL blobs were calculated and used for the interpretation of the associated SIP responses. The X-ray tomography and image analysis showed that the real sample properties (porosity and NAPL distributions) differed from the targeted ones. Both contaminated samples contained less NAPL than expected from the manual sample preparation. The SIP results showed higher real conductivity and lower imaginary conductivity in the contaminated samples compared to a clean sample. This is interpreted as an effect of increased surface conductivity along interconnected NAPL blobs and decreased surface areas in the samples due to NAPL blobs larger than and enclosing grains. We conclude that the combination of SIP, X-ray tomography, and image analysis is a very promising approach to achieve a better understanding of the measured SIP responses of NAPL-contaminated samples.

Magnetic Resonance Imaging of Nonaqueous Phase Liquid Distribution in Unconsolidated Soils Using Discriminatory Freezing

AbstractOrganic contaminants present as nonaqueous phase liquids (NAPLs) in the subsurface often pose a long‐term risk to human health and the environment. Investigating the distribution of NAPLs in porous media remains a major challenge in risk assessment and management of contaminated sites. Conventional soil coring and monitoring wells have been widely used over past decades as the primary means of subsurface investigation to determine NAPL extent. Known limitations of conventional approaches have led us to explore an alternative or a complementary technique to provide high‐quality information of NAPL source zone architecture. This work advances an imaging tool for a variety of organic NAPL contaminants in unconsolidated soils through magnetic resonance imaging (MRI) of frozen cores. Using trichloroethylene (TCE) and o‐xylene as model species, we illustrate that discriminatory freezing of water, while keeping the NAPL in a liquid state, enables high‐resolution qualitative delineation of NAPL distribution within porous media. This novel approach may help improve site conceptual models and consequentially lead to highly tailored, more efficient remedial measures.

Non-aqueous phase liquids distribution in three-fluid phase systems in double-porosity soil media: Experimental investigation using image analysis

Over the last few decades, contamination of groundwater and soil by non-aqueous phase liquids (NAPLs) has become a serious and wide-spread problem for the environment In this research, a light transmission visualization (LTV) method was used to observe the migration of dense non-aqueous phase liquid (DNAPL) and light non-aqueous phase liquid (LNAPL) in double-porosity soil within a three-fluid phase system (air-NAPL-water). The double-porosity characteristics of the soil were created using a composition made up of local sand and sintered kaolin clay spheres arranged in a periodic manner. Toluene was used to simulate LNAPL while tetrachloroethylene (PCE) represented the DNAPL. Both NAPLs were dyed using Oil-Red-O for better visualization. For comparison purposes, the same experiments were carried out using just local silica sand acting as a type of single-porosity soil. A significant difference in the migration of the toluene and PCE was observed as both the NAPL migration rates in the double-porosity medium were much faster compared to the migration rates found in the single-porosity medium. This result is most likely due to the occurrence of inter-aggregate pores in the double-porosity soil that contribute to increasing velocity of fluids migration through porous media. Other factors such as the wettability of fluids and capillary pressure characteristics that exist in the soil pores were found to be influential factors in fluid migration within porous media. In addition, the results show that chemical properties have a significant influence on the NAPL migration in porous media. It was found that the migration velocity of toluene was much faster compared to the migration velocity of the PCE. This observation is most likely caused by the fact that the distribution coefficient of toluene was higher than that of PCE which in turn means that the retardation factor of toluene is lower than that of PCE in the same porous media. This paper proved that the LTV provides a non-intrusive and non-destructive technique for studying multiphase flow in double-porosity soil media where rapid changes in fluid distribution in the entire flow domain is not easy to measure using conventional tools.

Interpreting TDR Signal Propagation through Soils with Distinct Layers of Nonaqueous‐Phase Liquid and Water Content

Core Ideas The TDR response in the presence of NAPL transport was investigated in a homogeneous soil. The experimental framework was designed to simulate sharp NAPL fronts that moved in the soil. The TDR waveforms were analyzed to deduce the influence of NAPLs on TDR signal propagation. A general procedure to determine the location of an NAPL‐contaminated front was developed. Several studies have demonstrated that time‐domain reflectometry (TDR) has an enormous potential to detect and monitor nonaqueous‐phase liquids (NAPLs) in uniformly contaminated soils with reference to different‐textured soils and under different saturation conditions. Few attempts have been proposed to describe NAPL distribution by using TDR when a contaminated front propagates through a soil. In this study, the TDR response in the presence of NAPL‐contaminant transport processes was investigated in a homogeneous soil under confined conditions with a series of laboratory‐controlled tests. The laboratory procedure involved measurements of dielectric permittivity (εb) and electrical conductivity (ECb) and the acquisition of the reflected TDR waveforms. The experimental framework was designed to simulate sharp NAPL fronts that moved in the soil columns and generated zones of contrasting permittivity. During the experiments, different initial conditions were assumed, as well as different degrees of contamination, by varying the volumetric water (θw) and NAPL (θNAPL) contents. The acquired TDR waveforms were systematically analyzed to deduce the influence of NAPLs on TDR signal propagation. A general procedure to determine the location of an NAPL‐contaminated front within a soil column was developed. Equipment calibration, measurement accuracy, and error sources were investigated in relation to the experimental procedure setup and the column preparation conditions. The results show that there may be some difficulties when interpreting TDR signals to locate the NAPL front for two main reasons: first, the expected additional reflection at the interface is not always distinguishable; second, there may be problems even locating both the first peak and the reflection point at the end of the TDR probe, especially when the signal shifts from a low to a high impedance layer. The suggested methodology provides a tool to overcome such intrinsic difficulties in NAPL front detection during the propagation of the contaminant.

Effect of NAPL Source Morphology on Mass Transfer in the Vadose Zone.

The generation of vapor-phase contaminant plumes within the vadose zone is of interest for contaminated site management. Therefore, it is important to understand vapor sources such as non-aqueous-phase liquids (NAPLs) and processes that govern their volatilization. The distribution of NAPL, gas, and water phases within a source zone is expected to influence the rate of volatilization. However, the effect of this distribution morphology on volatilization has not been thoroughly quantified. Because field quantification of NAPL volatilization is often infeasible, a controlled laboratory experiment was conducted in a two-dimensional tank (28 cm × 15.5 cm × 2.5 cm) with water-wet sandy media and an emplaced trichloroethylene (TCE) source. The source was emplaced in two configurations to represent morphologies encountered in field settings: (1) NAPL pools directly exposed to the air phase and (2) NAPLs trapped in water-saturated zones that were occluded from the air phase. Airflow was passed through the tank and effluent concentrations of TCE were quantified. Models were used to analyze results, which indicated that mass transfer from directly exposed NAPL was fast and controlled by advective-dispersive-diffusive transport in the gas phase. However, sources occluded by pore water showed strong rate limitations and slower effective mass transfer. This difference is explained by diffusional resistance within the aqueous phase. Results demonstrate that vapor generation rates from a NAPL source will be influenced by the soil water content distribution within the source. The implications of the NAPL morphology on volatilization in the context of a dynamic water table or climate are discussed.

[Steam and air co-injection in removing TCE in 2D-sand box].

Steam and air co-injection is a newly developed and promising soil remediation technique for non-aqueous phase liquids (NAPLs) in vadose zone. In this study, in order to investigate the mechanism of the remediation process, trichloroethylene (TCE) removal using steam and air co-injection was carried out in a 2-dimensional sandbox with different layered sand structures. The results showed that co-injection perfectly improved the "tailing" effect compared to soil vapor extraction (SVE), and the remediation process of steam and air co-injection could be divided into SVE stage, steam strengthening stage and heat penetration stage. Removal ratio of the experiment with scattered contaminant area was higher and removal speed was faster. The removal ratios from the two experiments were 93.5% and 88.2%, and the removal periods were 83.9 min and 90.6 min, respectively. Steam strengthened the heat penetration stage. The temperature transition region was wider in the scattered NAPLs distribution experiment, which reduced the accumulation of TCE. Slight downward movement of TCE was observed in the experiment with TCE initially distributed in a fine sand zone. And such downward movement of TCE reduced the TCE removal ratio.

Field study of using naturally occurring radon to assess the dense non-aqueous phase liquid distribution in saturated zone

The concept of radon deficiency such as the ratios of radon concentrations to the maximum measured value of a sample batch was employed as the survey methodology for this study to investigate contamination sources in an industrial zone that was suspected of causing subsurface dense non-aqueous phase liquid (DNAPL) contamination. The results showed that radon concentrations in certain wells were significantly lower than that in uncontaminated regions. Radon concentrations in groundwater are influenced by the in situ bioremediation of vegetable oil, which causes abnormal reductions of the radon in groundwater because radon partitions into vegetable oil and results in more variable for the radon deficit method to showing the impacts of remediation. Six contaminated regions were identified by integrating radon concentration ratios (divided into low (L), middle (M), and high (H) levels) and DNAPL concentrations (divided into low (L) and high (H) levels). Contaminated regions in the LH, MH, and HH categories are located in the vicinity of the contamination source, and those in the HL category are located far from the source zone. The ML and LL categories indicate the involvement of unknown factors, and that additional analyses are required to uncover the facts that affect radon and DNAPL concentrations.

Influence of porous media heterogeneity on nonaqueous phase liquid dissolution fingering and upscaled mass transfer

The utility of existing models for describing upscaled mass transfer from nonaqueous phase liquid (NAPL) were examined when preferential dissolution pathways form in NAPL‐contaminated zones that extend over the scale of decimeters. Laboratory experiments were conducted in two well‐characterized, heterogeneous packings. Using data from these experiments and simulations, existing methods for upscaling the mass transfer rate coefficient for NAPL dissolution based on dissolution front length growth (LDF), aquifer heterogeneity and spatial moments of NAPL distribution, and the ganglia‐to‐pool ratio (GTP) were evaluated along with an equilibrium stream tube (EST) model for predicting contaminant flux. When the correlation length of permeability perpendicular to the mean water flow direction was 6.0 cm, greater than the scale of dissolution fingers, only 4.8% of the NAPL resided in pools. Dissolution fingers formed in this experiment, and the LDF, GTP, and EST models resulted in similar predictions of effluent concentrations, with root‐mean‐square errors (RMSEs) between 0.035 and 0.079 and the LDF‐heterogeneous model best. When the correlation scale was smaller (1.0 cm), 66.7% of the NAPL was in pools, and preferential dissolution pathways were dominated by channeling, preferential dissolution caused by spatial variations in aqueous phase permeability, and NAPL saturation. For this experiment the EST and GTP models performed well, with RMSEs of 0.055 and 0.103, respectively. Dissolution fingering was important when the permeability correlation length was sufficiently large that dissolution finger formation was not disrupted and NAPL pools were not dominant.