Non-aqueous Phase Liquid Content Research Articles

Study on numerical model of thermal conductivity of non-aqueous phase liquids contaminated soils based on mesoscale

The thermal conductivity of soils is an important factor affecting the efficiency of in-situ thermal desorption remediation of contaminated sites. Restrict the selection of in-situ thermal desorption repair methods and heating parameters. The existence of non-aqueous phase liquids (NAPLs) pollutants affects the original thermal conductivity of the soils. To obtain an accurate and efficient method to study the thermal conductivity of NAPLs-contaminated soils. In the paper, the NAPLs-contaminated soils models are established by the four-parameter random generation method, and the thermal conductivity is calculated based on the Lattice Boltzmann Method (LBM). Explore the impact of NAPLs-contaminated soil model on its thermal conductivity calculation results, and propose an optimized NAPLs-contaminated soil model. Finally, the numerical simulation results are compared with the experimental results, and a large number of calculations and statistical analyses are carried out on the thermal conductivity. The results indicate that it is feasible to combine the four-parameter random generation method with LBM to study the thermal conductivity of NAPLs-contaminated soils. The calculation accuracy of the two-dimensional model of NAPLs-contaminated soils is lower than that of the three-dimensional model, while the calculation of the three-dimensional model is too time-consuming. The optimized NAPLs-contaminated soils three-dimensional model has the characteristics of high computational accuracy and efficiency. Saturation and Nz have a great influence on the calculation time of thermal conductivity. The thermal conductivity of the two-dimensional model is more sensitive to anisotropy. With the decrease of model anisotropy, the increase of saturation, and the decrease of porosity, the calculation accuracy of thermal conductivity of two-dimensional and three-dimensional models is similar, otherwise, the calculation accuracy of two-dimensional model is lower. The influence of porosity and NAPLs content on thermal conductivity should be paid special attention to when in-situ thermal desorption.

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.

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.

Thermo‐time domain reflectometry to evaluate unsaturated soils contaminated with nonaqueous phase liquids

AbstractSoil and groundwater contamination by nonaqueous phase liquids (NAPLs) has become a serious environmental issue. This study applied the thermo‐time domain reflectometry (TTDR) technique to determine the effects of NAPL contamination on the thermal and electromagnetic properties of unsaturated soils. Furthermore, the applicability of TTDR to determine the NAPL content was evaluated, and a new calibration equation was established to estimate the NAPL saturation level in unsaturated soils. Multiple unsaturated sand and silt loam samples at a constant water content were mixed with various amounts of motor and sunflower seed oils. The TTDR probes were then applied to simultaneously measure the dielectric constant, electrical conductivity, thermal conductivity, and heat capacity. An independent experiment was conducted to compare the NAPL content estimated using the new calibration equation and the known content. The results show that the NAPL content had no effect on the dielectric constant, and the soil dielectric constant maintained a steady value at the same water content, which ranged from 5 to 20 with increasing water content. At a constant water content, the soil electrical conductivity slightly decreased as the NAPL content increased. The soil thermal conductivity generally increased with the addition of NAPLs. There was a linear correlation between the total liquid content in the soil and the volumetric heat capacity. A new calibration equation was established to determine the NAPL saturation in soils from the relationship between the soil saturation, volumetric heat capacity, and water content as measured by time domain reflectometry. Independent validation indicated that the calibration equation was able to provide accurate NAPL saturation data in the laboratory, with a RMSE of 0.042.

A soil non-aqueous phase liquid (NAPL) flushing laboratory experiment based on measuring the dielectric properties of soil–organic mixtures via time domain reflectometry (TDR)

Abstract. The term non-aqueous phase liquid (NAPL) refers to a group of organic compounds with scarce solubility in water. They are the products of various human activities and may be accidentally introduced into the soil system. Given their toxicity level and high mobility, NAPLs constitute a serious geo-environmental problem. Contaminant distribution in the soil and groundwater contains fundamental information for the remediation of polluted soil sites. The present research explored the possible employment of time domain reflectometry (TDR) to estimate pollutant removal in a silt-loam soil that was primarily contaminated with a corn oil as a light NAPL and then flushed with different washing solutions. Known mixtures of soil and NAPL were prepared in the laboratory to achieve soil specimens with varying pollution levels. The prepared soil samples were repacked into plastic cylinders and then placed in testing cells. Washing solutions were then injected upward into the contaminated sample, and both the quantity of remediated NAPL and the bulk dielectric permittivity of the soil sample were determined. The above data were also used to calibrate and validate a dielectric model (the α mixing model) which permits the volumetric NAPL content (θNAPL; m3 m−3) within the contaminated sample to be determined and quantified during the different decontamination stages. Our results demonstrate that during a decontamination process, the TDR device is NAPL-sensitive: the dielectric permittivity of the medium increases as the NAPL volume decreases. Moreover, decontamination progression can be monitored using a simple (one-parameter) mixing model.

Comparison of residual NAPL source removal techniques in 3D metric scale experiments

This study compared four treatment techniques for the removal of a toluene/n-decane as NAPL (Non Aqueous Phase Liquid) phase mixture in identical 1 cubic meter tanks filled with different kind of sand. These four treatment techniques were: oxidation with persulfate, surfactant washing with Tween80®, sparging with air followed by ozone, and thermal treatment at 80°C. The sources were made with three lenses of 26×26×6.5cm, one having a hydraulic conductivity similar to the whole tank and the two others a value 10 times smaller. The four techniques were studied after conditioning the tanks with tap water during approximately 80days.The persulfate treatment tests showed average removal of the contaminants but significant flux decrease if density effects are considered. Surfactant flushing did not show a highly significant increase of the flux of toluene but allowed an increased removal rate that could lead to an almost complete removal with longer treatment time. Sparging removed a significant amount but suggests that air was passing through localized gas channels and that the removal was stagnating after removing half of the contamination. Thermal treatment reached 100% removal after the target temperature of 80°C was kept during more than 10d.The experiments emphasized the generation of a high-spatial heterogeneity in NAPL content. For all the treatments the overall removal was similar for both n-decane and toluene, suggesting that toluene was removed rapidly and n-decane more slowly in some zones, while no removal existed in other zones.The oxidation and surfactant results were also analyzed for the relation between contaminant fluxes at the outlet and mass removal. For the first time, this approach clearly allowed the differentiation of the treatments. As a conclusion, experiments showed that the most important differences between the tested treatment techniques were not the global mass removal rates but the time required to reach 99% decrease in the contaminant fluxes, which were different for each technique.

Estimating Nonaqueous‐Phase Liquid Content in Variably Saturated Soils Using Time Domain Reflectometry

Core Ideas A TDR waveform interpretation method for NAPL content in soils was developed. Correlations between the dielectric response and the NAPL content were determined. The α dielectric mixing model for predicting NAPL presence in soils was expanded. This methodology can be used to obtain predictions of volumetric NAPL content. In recent years, several studies have been conducted in the detection and observation of nonaqueous‐phase liquids (NAPLs) in contaminated soils. Successful remediation of an NAPL‐contaminated site requires appropriate characterization of both the plume extent and the soil volumetric NAPL content (θNAPL). Noninvasive geophysical techniques, such as time domain reflectometry (TDR), may be used to discriminate between θNAPL and soil volumetric water content (θw). Accordingly, the main aim of this work was to develop a TDR waveform interpretation method based on soil dielectric permittivity measurement and observation of the change in the reflected TDR signal amplitude at relatively long times from the waveform source. We demonstrated that the asymptotic value of the reflection signal coefficient can be univocally related with θNAPL in an unsaturated soil. In the procedure adopted, a new formulation of a dielectric mixing model was also derived, introducing a fourth phase that takes the NAPL presence into account. Multiple samples of sandy, silt loam, and loamy soils, at predetermined different θw and θNAPL, were tested, each test consisting in emitting a TDR signal to the soil sample and receiving and analyzing the reflected electromagnetic wave. An empirical dielectric mixing model was calibrated and implemented to estimate the θNAPL. Equipment calibration, measurement accuracy, and error sources, related both to the experimental procedure setup and to the sample preparation conditions, are discussed. The results show that the suggested methodology can be used to obtain predictions of volumetric NAPL content (θNAPL) with acceptable accuracy (R2 = 0.95).

Integrating spatial and temporal oxygen data to improve the quantification of in situ petroleum biodegradation rates

Accurate estimation of biodegradation rates during remediation of petroleum impacted soil and groundwater is critical to avoid excessive costs and to ensure remedial effectiveness. Oxygen depth profiles or oxygen consumption over time are often used separately to estimate the magnitude and timeframe for biodegradation of petroleum hydrocarbons in soil and subsurface environments. Each method has limitations. Here we integrate spatial and temporal oxygen concentration data from a field experiment to develop better estimates and more reliably quantify biodegradation rates. During a nine-month bioremediation trial, 84 sets of respiration rate data (where aeration was halted and oxygen consumption was measured over time) were collected from in situ oxygen sensors at multiple locations and depths across a diesel non-aqueous phase liquid (NAPL) contaminated subsurface. Additionally, detailed vertical soil moisture (air-filled porosity) and NAPL content profiles were determined. The spatial and temporal oxygen concentration (respiration) data were modeled assuming one-dimensional diffusion of oxygen through the soil profile which was open to the atmosphere. Point and vertically averaged biodegradation rates were determined, and compared to modeled data from a previous field trial. Point estimates of biodegradation rates assuming no diffusion ranged up to 58 mg kg−1 day−1 while rates accounting for diffusion ranged up to 87 mg kg−1 day−1. Typically, accounting for diffusion increased point biodegradation rate estimates by 15–75% and vertically averaged rates by 60–80% depending on the averaging method adopted. Importantly, ignoring diffusion led to overestimation of biodegradation rates where the location of measurement was outside the zone of NAPL contamination. Over or underestimation of biodegradation rate estimates leads to cost implications for successful remediation of petroleum impacted sites.

Dielectric Response of a Variable Saturated Soil Contaminated by Non-Aqueous Phase Liquids (NAPLs)

In recent years, several studies have been conducted both in saturated and unsaturated soils to detect non-aqueous phase liquid (NAPL) hydrocarbon contamination in soils and groundwater by means of the time domain reflectometry (TDR) technique. This technique is widely used for measuring the dielectric permittivity and bulk electrical conductivity of multiphase systems. Only accurate knowledge of the dielectric response of soil matrix- water-NAPL (saturated condition) or soil matrix-air-water-NAPL (unsaturated condition) systems can allow the volumetric NAPL content (θNAPL) to be determined in the soil. This paper investigates the influence of NAPL contamination (corn oil, a non-volatile and non-toxic NAPL, was used) on TDR measurement in a volcanic soil, relating dielectric permittivity of the multiphase soil system to volumetric fluid content θf (i.e. water+NAPL). The soil samples were oven dried at 105°C and passed through a 2mm sieve. Known quantities of soil, water and oil were mixed and repacked into plastic cylinders (15cm high and 9.5cm in diameter); 40 different combinations of water and oil were tested, with θNAPL varying from 0.05 to 0.40 by 0.05cm3/cm3 increments. A volumetric mixing model with three (soil matrix-water-NAPL) or four (soil matrix-air-water-NAPL) phases permitted conversion from a dielectric permittivity domain into a θf domain. The results show that, the amount of contaminant in soil can be inferred if the total volume of pore fluid θf and the dielectric permittivity of the contaminated soil are known. Further work will be built on this initial study, concentrating on: i) enhancing the model linkage and validating it with new laboratory results; ii) validating the developed TDR interpretation tool with field results.

A novel method for simulating smoldering propagation and its application to STAR (Self-sustaining Treatment for Active Remediation)

This work presents the development and proof of concept of a phenomenologically-based numerical model capable of simulating the expansion of a smoldering front in a heterogeneous reactive porous medium. This practical tool has potential for investigating a range of large-scale smoldering phenomena including subsurface peat and coal fires. The model is applied to the smoldering of non-aqueous phase liquids (NAPLs) embedded in soil. NAPLs are industrial chemicals found extensively polluting the subsurface and their in situ destruction via smoldering combustion is a promising site remediation approach referred to as STAR (Self-sustaining Treatment for Active Remediation). The In Situ Smoldering Model was developed through the novel coupling of a multiphase flow simulator with a combustion front expansion model, linked at the local (i.e., nodal) scale by a calibrated expression for the forward smoldering front velocity that is a function of airflow velocity and NAPL content. Numerical testing revealed that the model is computationally efficient and capable of handling two-dimensional smoldering propagation for a range of challenging physical scenarios.

Assessment feasibility of in-situ capping and contaminant mobility in NAPL-contaminated sediments

In-situ capping (ISC) has been successfully used in numerous sites to separate contaminants such as polycyclic aromatic hydrocarbons (PAHs) in the sediment from the overlying water. The effectiveness of ISC over extremely soft sediment contaminated with nonaqueous phase liquids (NAPLs), however, is less well understood. Such sediments exhibit low bearing capacity and may be subject to significant consolidation increasing the potential for NAPL and dissolved contaminant expression in water. A series of experiments were conducted on lagoon and surge pond sediments from a site in south Louisiana contaminated with varying degrees of NAPL to predict the ability to cap the sediments without sediment failure and with containment of the NAPL. The methodology employed may prove useful in other sediment capping projects. The results indicated that placement of a cap would result in intermixing in cap sediment interface, and penetration of NAPL and dissolved contaminants into the cap layer due to consolidation. Sediments with higher NAPL content (up to 30% NAPL) exhibited a greater degree of intermixing of sediment and cap materials and greater penetration of contaminants. High NAPL content sediments were also more likely to be resuspended by the act of capping leading to contamination of the cap layer. The study demonstrated that traditional sand capping technology is feasible to cap NAPL contaminated sediment but identified the nature and extent of problems to be expected when substantial NAPL is present.

Estimating the Nonaqueous‐Phase Liquid Content in Saturated Sandy Soil Using Amplitude Domain Reflectometry

Groundwater contamination by nonaqueous‐phase liquids (NAPLs) has become a serious environmental issue. Therefore, it is necessary to estimate the NAPL content (θNAPL) in saturated soil to detect and monitor the NAPL contamination in groundwater. The objective of this study was to investigate three estimation methods for the θNAPL in saturated sandy soil, namely, the noncalibration method (NCM), the permittivity regression method (PRM), and the best‐fit α method (BFαM). These methods utilize the permittivity (K) measured by amplitude domain reflectometry (ADR) and two dielectric mixing models: a theoretical Maxell–de Loor (MD) model and an empirical α model. The NCM applies the measured K to the models directly. The PRM applies the measured K to the models indirectly using the regression equation between the measured and estimated K The RMSEs of the NCM for the α model were 0.038 m3 m−3 for light NAPL (LNAPL) and 0.015 m3 m−3 for dense NAPL (DNAPL) when the constant α value was set at 0.5. The RMSEs of the MD model, however, were 0.090 m3 m−3 for LNAPL and 0.070 m3 m−3 for DNAPL. Using the PRM, the RMSEs of the MD model became much better than those of the NCM. The RMSEs for θNAPL estimated by the BFαM were 0.007 m3 m−3 for LNAPL and 0.018 m3 m−3 for DNAPL. These results demonstrate that θNAPL was estimated easily and with sufficient accuracy with the α model and the PRM.

Spreading of a NAPL lens in a double-porosity medium

Spreading of a non-aqueous phase liquid (NAPL) denser than water (DNAPL) lens (mound) in the unsaturated zone of double-porosity aquifer above an impervious plane boundary is investigated. The double-porosity aquifer is conceptualized as a fracture network surrounding pervious blocks. Vertical gravity equilibrium is assumed to prevail in each one of the two media, fractures and blocks. Through vertical integration, two coupled partial differential equations for the DNAPL content in each medium are obtained. The mass exchange rate between high- and low-permeability media is considered as a function of NAPL content. The dominant effect is gravity, whereas capillary forces are negligible. Analytical solutions for one-dimensional and axisymmetric problems of mound spreading are obtained.

Groundwater contaminant flux reduction resulting from nonaqueous phase liquid mass reduction

This work presents a stream tube–based analytical approach to evaluate reduction in groundwater contaminant flux resulting from partial mass reduction in a nonaqueous phase liquid (NAPL) source zone. The reduction in contaminant flux, Rj, discharged from the source zone is a remediation performance metric that has a direct effect on the fundamental drivers of remediation: protection of human health and the environment. Closed form expressions are provided for analyzing remediation performance under conditions of joint spatial variability of both groundwater flow and NAPL content. The performance measures derived here are expressed in terms of measurable parameters. Spatial variability is described within a Lagrangian framework where aquifer hydrodynamic heterogeneities are characterized using nonreactive travel time distributions, while NAPL spatial distribution heterogeneity can be similarly described using reactive travel time distributions. The combined statistics of these distributions are used to evaluate the relationship between reduction in contaminant mass, Rm, and Rj. A portion of the contaminant mass in the source zone is assumed to be removed via in situ flushing remediation, with the initial and final conditions defined as steady state natural gradient groundwater flow through the contaminant source zone. The combined effects of aquifer and NAPL heterogeneities are shown to be captured in a single parameter, reactive travel time variability, which was determined to be the most important factor controlling the relationship between Rm and Rj. It is shown that as heterogeneity in aquifer properties and NAPL spatial distribution increases, less mass reduction is required to achieve a given flux reduction, although the overall source longevity increases. When rate‐limited dissolution is important, the efficiency of remediation, in terms of both mass and flux reduction, is reduced. However, at many field sites the combined effects of field‐scale heterogeneities and site aging will result in favorable relationships between mass reduction and flux reduction.

Radon‐222 as a Tracer for Nonaqueous Phase Liquid in the Vadose Zone: Experiments and Analytical Model

The potential use of the naturally occurring noble gas 222Rn as a tracer for vadose zone contamination by nonaqueous phase liquids (NAPLs) is studied experimentally and theoretically. n‐Dodecane was chosen as the model NAPL. In batch experiments containing unsaturated alluvial sand, a 2.9‐fold decrease of the steady‐state 222Rn activity in soil gas was measured as a consequence of the increase in the volumetric NAPL content from 0 to 0.074. A one‐dimensional analytical reactive transport model was developed that includes 222Rn production, gas‐phase diffusion, partitioning, and radioactive decay. Radon‐222 soil gas profiles were predicted for homogeneous and heterogeneous sandy profiles where NAPL contamination was restricted to selected depth layers. The resulting depth profiles document that the position of the NAPL has great influence on the 222Rn activity depth profile. An outdoor lysimeter experiment was performed using unsaturated alluvial sand contaminated by NAPL at depths from 1 to 1.2 m. In the lysimeter experiment, a spill of 2 L m−2 NAPL did not alter significantly the 222Rn profile, as predicted also by model calculations. We concluded that 222Rn can be used as a NAPL tracer in the vadose zone only at heavily polluted sites with uniform spatial 222Rn production.

Radon-222 as a Tracer for Nonaqueous Phase Liquid in the Vadose Zone: Experiments and Analytical Model

The potential use of the naturally occurring noble gas 222 Rn as a tracer for vadose zone contamination by nonaqueous phase liquids (NAPLs) is studied experimentally and theoretically. n -Dodecane was chosen as the model NAPL. In batch experiments containing unsaturated alluvial sand, a 2.9-fold decrease of the steady-state 222 Rn activity in soil gas was measured as a consequence of the increase in the volumetric NAPL content from 0 to 0.074. A one-dimensional analytical reactive transport model was developed that includes 222 Rn production, gas-phase diffusion, partitioning, and radioactive decay. Radon-222 soil gas profiles were predicted for homogeneous and heterogeneous sandy profiles where NAPL contamination was restricted to selected depth layers. The resulting depth profiles document that the position of the NAPL has great influence on the 222 Rn activity depth profile. An outdoor lysimeter experiment was performed using unsaturated alluvial sand contaminated by NAPL at depths from 1 to 1.2 m. In the lysimeter experiment, a spill of 2 L m −2 NAPL did not alter significantly the 222 Rn profile, as predicted also by model calculations. We concluded that 222 Rn can be used as a NAPL tracer in the vadose zone only at heavily polluted sites with uniform spatial 222 Rn production.

TDR detection of nonaqueous phase liquids in sandy soils using the eigendecomposition method

In this study, the presence of nonaqueous phase liquids (NAPL) in sandy soils are detected using a TDR probe system and the eigendecomposition method of analysis. As a demonstration, five NAPLs with different physicochemical properties (acetone, benzene, heptane, trichloroethylene, and xylene; Table 1) were used. Samples were prepared in such a way that the soil pore fluid has different contents of deionized water and NAPLs. For each experiment, a pulse signal with known characteristics was used and reflected signals were captured by an oscilloscope and analyzed using the eigendecomposition method. Autoregressive modeling and singular value decomposition were used to calculate the eigenvalues. The most significant eigenvalues were identified based on their power spectrum. The relative eigenvalue of the first mode (Eow), which is a measure of the power carried by the signal, was calculated and correlated to NAPL type and content, and octanol water partition coefficient (log Kow). The results indicated that for the same NAPL content, as log Kow increases, Eow decreases due to increase of hydrophobicity. For the same log Kow, as the organic content in soil pore fluid increases, Eow increases due to decrease of dielectric properties of the pore fluids.

Analysis of pore‐scale nonaqueous phase liquid dissolution in etched silicon pore networks

Predicting the dissolution rate of nonaqueous phase liquids (NAPLs) in groundwater is difficult, as the effects of variable pore and NAPL blob geometry are poorly understood. To elucidate these effects, fluorescence microscopy and digital image analysis were used to quantify the size and location of variably distributed NAPL blobs during dissolution in homogeneous and heterogeneous pore networks etched into silicon wafers. Results show that the dissolution rate constant (expressed as the Sherwood number, Sh) is relatively constant regardless of pore and NAPL blob geometry when the average mass transfer length scale remains constant during dissolution. Results also show that Sh increases with Peclet (Pe) between 2 and 26 and then levels off. The limiting value of Sh reached depends on the average diffusion length scale; this length scale was directly calculated and found to vary depending on the pore and NAPL blob geometry. For example, the average diffusion length scale decreases (and Sh increases) as the pore throat width to grain diameter increases. Last, results show that the volumetric NAPL content (θn) is linearly related to the specific NAPL‐water interfacial area (ait) over much of the dissolution process. However, this relationship depends on the pore and blob size distribution. For example, when multipore blobs control dissolution, the relationship between these parameters will change as smaller blobs dominate dissolution at low θn. These results are important because existing mass transfer correlations do not account for limiting values of Sh that can be obtained at high Pe for the effect of blob or pore geometry on the average diffusion length scale (and therefore on Sh) or for the effect of pore geometry and transient blob size distribution on the relationship between ait and θn.

Partitioning tracer transport in a hydrogeochemically heterogeneous aquifer

In this paper a stochastic model is developed to predict the behavior of partitioning tracers in a three‐dimensional aquifer subject to spatial heterogeneities of nonaqueous phase liquid (NAPL) and Darcy flux. Stochastic Eulerian perturbation methods are applied to derive a system of six coupled partial‐differential first‐order moment equations from the traditional advection‐dispersion‐retardation equation. These equations describe the transient behavior of the concentration moments that explicitly accounts for the effects of the considered heterogeneities on prediction uncertainty and spatiotemporal correlation between tracer concentration, Darcy flux, and NAPL distribution. A finite difference technique is used to solve these equations numerically. Three different cases are presented to evaluate the impact of various statistical relationships between the random fields of natural log conductivity and natural log volumetric NAPL content. It is found that strong negative correlation between NAPL content and hydraulic conductivity leads to increased prediction uncertainty, whereas strong positive correlation between NAPL content and hydraulic conductivity leads to decreased prediction uncertainty over the uncorrelated case. For all cases, cross correlations between tracer concentration and Darcy flux or NAPL saturation are more extensive in the direction of mean flow than transverse to the mean flow. Furthermore, the magnitude of these correlations is approximately constant over the duration of the tracer experiment, implying that all concentration measurements taken at a particular location contain approximately equal information about Darcy flux or NAPL distribution within the experimental domain. The cross correlation between tracer concentration and Darcy flux is relatively insensitive to the degree of the conductivity‐NAPL correlation. However, the correlation between tracer concentration and NAPL concentration is much larger for strongly (positively or negatively) correlated hydraulic conductivity and NAPL distributions. In a comparison paper [Zhang and Graham, this issue] these correlations, together with tracer measurements, are incorporated into a parameter estimation algorithm to produce site‐specific conditional estimates of NAPL and Darcy flux distribution.

Field test of high molecular weight alcohol flushing for subsurface nonaqueous phase liquid remediation

A pilot scale field test of non‐aqueous phase liquid (NAPL) removal using high molecular weight alcohols was conducted at Operable Unit 1, Hill Air Force Base, Utah. Petroleum hydrocarbons and spent solvents were disposed of in chemical disposal pits at this site, and these materials are now present in the subsurface in the form of a light non‐aqueous phase liquid (LNAPL). This LNAPL is a complex mixture of aromatic and aliphatic hydrocarbons, chlorinated solvents, and other compounds. The field experiment was performed in a 5 m by 3 m confined test cell, formed by driving interlocking sheet pile walls through the contaminated zone into an underlying clay. The test involved the injection and extraction of about four pore volumes (1 pore volume=7000 L) of a mixture of 80% tert‐butanol and 15% n‐hexanol. The contaminants were removed by a combination of NAPL mobilization and enhanced dissolution, and the results of postflood soil coring indicate better than 90% removal of the more soluble contaminants (trichloroethane, toluene, ethylbenzene, xylenes, trimethylbenzene, naphthalene) and 70–80% removal of less soluble compounds (decane and undecane). The results of preflood and postflood NAPL partitioning tracer tests show nearly 80% removal of the total NAPL content from the test cell. The field data suggest that a somewhat higher level of removal could be achieved with a longer alcohol injection.