DNAPL source depletion: linking architecture and flux response

The relationship between dense non-aqueous phase liquid (DNAPL) mass reduction and contaminant mass flux was investigated experimentally in four model source zones. The flow cell design for the experiments featured a segmented extraction well that allowed for analysis of spatially resolved flux information. This flux information was coupled with image analysis of the NAPL spatial distribution to investigate the relationship between flux and the up-gradient NAPL architecture. Results indicate that in the systems studied, the relationship between DNAPL mass reduction and contaminant mass flux was primarily controlled by the NAPL architecture. A specific definition of NAPL architecture was employed where the source zone is resolved into a collection of streamtubes with spatial variability in NAPL saturation along each streamtube integrated and transformed into an effective NAPL content for each streamtube. The distribution of NAPL contents among the streamtubes (NAPL architecture) controlled dissolution dynamics. Two simplified models, a streamtube model and an effective Damkohler number model, were investigated for their ability to simulate dissolution dynamics.     

Effects of source zone heterogeneity on surfactant-enhanced NAPL dissolution and resulting remediation end-points

The effectiveness of removal of nonaqueous phase liquids (NAPLs) from the entrapment source zone of the subsurface has been limited by soil heterogeneity and the inability to locate all entrapped sources. The goal of this study was to demonstrate the uncertainty of degree of source removal associated with aquifer heterogeneity. In this demonstration, source zone NAPL removal using surfactant-enhanced dissolution was considered. Model components that simulate the processes of natural dissolution in aqueous phase and surfactant-enhanced dissolution were incorporated into an existing code of contaminant transport. The dissolution modules of the simulator used previously developed Gilland-Sherwood type phenomenological models of NAPL dissolution to estimate mass transfer coefficients that are upscaleable to multidimensional flow conditions found at field sites. The model was used to simulate the mass removal from 10 NAPL entrapment zone configurations based on previously conducted two-dimensional tank experiments. These entrapment zones represent the NAPL distribution in spatially correlated random fields of aquifer hydraulic conductivity. The numerical simulations representing two-dimensional conditions show that effectiveness of mass removal depends on the aquifer heterogeneity that controls the NAPL entrapment and delivery of the surfactant to the locations of entrapped NAPLs. Flow bypassing resulting from heterogeneity and the reduction of relative permeability due to NAPL entrapment reduces the delivery efficiency of the surfactant, thus prolonging the remediation time to achieve desired end-point NAPL saturations and downstream dissolved concentrations. In some extreme cases, the injected surfactant completely bypassed the NAPL source zones. It was also found that mass depletion rates for different NAPL source configurations vary significantly. The study shows that heterogeneity result in uncertainties in the mass removal and achievable end-points that are directly related to dissolved contaminant plume development downstream of the NAPL entrapment zone.     

The significance of heterogeneity on mass flux from DNAPL source zones: an experimental investigation

Understanding the process of mass transfer from source zones of aquifers contaminated with organic chemicals in the form of dense non-aqueous phase liquids (DNAPL) is of importance in site management and remediation. A series of intermediate-scale tank experiments was conducted to examine the influence of aquifer heterogeneity on DNAPL mass transfer contributing to dissolved mass emission from source zone into groundwater under natural flow before and after remediation. A Tetrachloroethylene (PCE) spill was performed into six source zone models of increasing heterogeneity, and both the spatial distribution of the dissolution behavior and the net effluent mass flux were examined. Experimentally created initial PCE entrapment architecture resulting from the PCE migration was largely influenced by the coarser sand lenses and the PCE occupied between 30 and 60% of the model aquifer depth. The presence of DNAPL had no apparent effect on the bulk hydraulic conductivity of the porous media. Up to 71% of PCE mass in each of the tested source zone was removed during a series of surfactant flushes, with associated induced PCE mobilization responsible for increasing vertical DNAPL distributions. Effluent mass flux due to water dissolution was also found to increase progressively due to the increase in NAPL-water contact area even though the PCE mass was reduced. Doubling of local groundwater flow velocities showed negligible rate-limited effects at the scale of these experiments. Thus, mass transfer behavior was directly controlled by the morphology of DNAPL within each source zone. Effluent mass flux values were normalized by the up-gradient DNAPL distributions. For the suite of aquifer heterogeneities and all remedial stages, normalized flux values fell within a narrow band with mean of 0.39 and showed insensitivity to average source zone saturations.     

Evaluation of simplified mass transfer models to simulate the impacts of source zone architecture on nonaqueous phase liquid dissolution in heterogeneous porous media

Nonaqueous phase liquid (NAPL) dissolution was studied in three-dimensional (3D) heterogeneous experimental aquifers (25.5 cm x 9 cm x 8.5 cm) with two different longitudinal correlation lengths (2.1 cm and 1.1 cm) and initial spill volumes (22.5 ml and 10.5 ml). Spatial and temporal distributions of NAPL during dissolution were measured using magnetic resonance imaging (MRI). At high NAPL spill volume, average effluent concentrations initially increased during dissolution, as NAPL pools transitioned to NAPL ganglia, and then decreased as the total NAPL-water interfacial area decreased over time. Experimental results were used to test six dissolution models: (i and ii) a one-dimensional (1D) model using either specific NAPL-water interfacial area values estimated from MR images at each time step (i.e., 1D quasi-steady state model), or an empirical mass transfer (Sh') correlation (i.e., 1D transient model), (iii and iv) a multiple analytical source superposition technique (MASST) using either the NAPL distribution determined from MR images at each time step (i.e., MASST steady state model), or the NAPL distribution determined from mass balance calculations (i.e., MASST transient model), (v) an equilibrium streamtube model, and (vi) a 3D grid-scale pool dissolution model (PDM) with a dispersive mass flux term. The 1D quasi-steady state model and 3D PDM captured effluent concentration values most closely, including some concentration fluctuations due to changes in the extent of flow reduction. The 1D transient, MASST steady state and transient, and streamtube models all showed a monotonic decrease in effluent concentration values over time, and the streamtube model was the most computationally efficient. Changes during dissolution of the effective NAPL-water interfacial area estimated from imaging data are similar to changes in effluent concentration values. The 1D steady state model incorporates estimates of the effective NAPL-water interfacial area directly at each time point; the 3D PDM does so indirectly through mass balance and a relative permeability function, which causes reduced water flow through high saturation NAPL regions. Hence, when model accuracy is required, the results indicate that a surrogate of this effective interfacial area is required. Approaches to include this surrogate in the MASST and streamtube models are recommended.     

Laboratory investigation of flux reduction from dense non-aqueous phase liquid (DNAPL) partial source zone remediation by enhanced dissolution

This study investigated the benefits of partial removal of dense nonaqueous phase liquid (DNAPL) source zones using enhanced dissolution in eight laboratory scale experiments. The benefits were assessed by characterizing the relationship between reductions in DNAPL mass and the corresponding reduction in contaminant mass flux. Four flushing agents were evaluated in eight controlled laboratory experiments to examine the effects of displacement fluid property contrasts and associated override and underride on contaminant flux reduction (R(j)) vs. mass reduction (R(m)) relationships (R(j)(R(m))): 1) 50% ethanol/50% water (less dense than water), 2) 40% ethyl-lactate/60% water (more dense than water), 3) 18% ethanol/26% ethyl-lactate/56% water (neutrally buoyant), and 4) 2% Tween-80 surfactant (also neutrally buoyant). For each DNAPL architecture evaluated, replicate experiments were conducted where source zone dissolution was conducted with a single flushing event to remove most of the DNAPL from the system, and with multiple shorter-duration floods to determine the path of the R(j)(R(m)) relationship. All of the single-flushing experiments exhibited similar R(j)(R(m)) relationships indicating that override and underride effects associated with cosolvents did not significantly affect the remediation performance of the agents. The R(j)(R(m)) relationship of the multiple injection experiments for the cosolvents with a density contrast with water tended to be less desirable in the sense that there was less R(j) for a given R(m). UTCHEM simulations supported the observations from the laboratory experiments and demonstrated the capability of this model to predict R(j)(R(m)) relationships for non-uniformly distributed NAPL sources.     

Impact of nonaqueous phase liquid (NAPL) source zone architecture on mass removal mechanisms in strongly layered heterogeneous porous media during soil vapor extraction

An existing multiphase flow simulator was modified in order to determine the effects of four mechanisms on NAPL mass removal in a strongly layered heterogeneous vadose zone during soil vapor extraction (SVE): a) NAPL flow, b) diffusion and dispersion from low permeability zones, c) slow desorption from sediment grains, and d) rate-limited dissolution of trapped NAPL. The impacts of water and NAPL saturation distribution, NAPL-type (i.e., free, residual, or trapped) distribution, and spatial heterogeneity of the permeability field on these mechanisms were evaluated. Two different initial source zone architectures (one with and one without trapped NAPL) were considered and these architectures were used to evaluate seven different SVE scenarios. For all runs, slow diffusion from low permeability zones that gas flow bypassed was a dominant factor for diminished SVE effectiveness at later times. This effect was more significant at high water saturation due to the decrease of gas-phase relative permeability. Transverse dispersion contributed to fast NAPL mass removal from the low permeability layer in both source zone architectures, but longitudinal dispersion did not affect overall mass removal time. Both slow desorption from sediment grains and rate-limited mass transfer from trapped NAPL only marginally affected removal times. However, mass transfer from trapped NAPL did affect mass removal at later time, as well as the NAPL distribution. NAPL flow from low to high permeability zones contributed to faster mass removal from the low permeability layer, and this effect increased when water infiltration was eliminated. These simulations indicate that if trapped NAPL exists in heterogeneous porous media, mass transfer can be improved by delivering gas directly to zones with trapped NAPL and by lowering the water content, which increases the gas relative permeability and changes trapped NAPL to free NAPL.     

Evaluation of trichloroethene recovery processes in heterogeneous aquifer cells flushed with biodegradable surfactants

The ability of two biodegradable surfactants, polyoxyethylene (20) sorbitan monooleate (Tween 80) and sodium dihexyl sulfosuccinate (Aerosol MA), to recover a representative dense non-aqueous-phase liquid (DNAPL), trichloroethene (TCE), from heterogeneous porous media was evaluated through a combination of batch and aquifer cell experiments. An aqueous solution containing 3.3% Aerosol MA, 8% 2-propanol and 6 g/l CaCl(2) yielded a weight solubilization ratio (WSR) of 1.21 g TCE/g surfactant, with a corresponding liquid-liquid interfacial tension (IFT) of 0.19 dyn/cm. Flushing of aquifer cells containing a TCE-DNAPL source zone with approximately two pore volumes of the AMA formulation resulted in substantial (>30%) mobilization of TCE-DNAPL. However, a TCE mass recovery of 81% was achieved when the aqueous-phase flow rate was sufficient to displace the mobile TCE-DNAPL toward the effluent well. Aqueous solutions of Tween 80 exhibited a greater capacity to solubilize TCE (WSR=1.74 g TCE/g surfactant) and exerted markedly less reduction in IFT (10.4 dyn/cm). These data contradict an accepted empirical correlation used to estimate IFT values from solubilization capacity, and indicate a unique capacity of T80 to form concentrated TCE emulsions. Flushing of aquifer cells with less than 2.5 pore volumes of a 4% T80 solution achieved TCE mass recoveries ranging from 66 to 85%, with only slight TCE-DNAPL mobilization (<5%) occurring when the total trapping number exceeded 2 x 10(-5). These findings demonstrate the ability of Tween 80 and Aerosol MA solutions to efficiently recover TCE from a heterogeneous DNAPL source zone, and the utility of the total trapping number as a design parameter for a priori prediction of DNAPL mobilization and bank angle formation when flushing with low-IFT solutions. Given their potential to stimulate microbial reductive dechlorination at low concentrations, these surfactants are well-suited for remedial action plans that couple aggressive mass removal followed by enhanced bioremediation to treat chlorinated solvent source zones.     

Design of aquifer remediation systems: (2) estimating site-specific performance and benefits of partial source removal

A Lagrangian stochastic model is proposed as a tool that can be utilized in forecasting remedial performance and estimating the benefits (in terms of flux and mass reduction) derived from a source zone remedial effort. The stochastic functional relationships that describe the hydraulic "structure" and non-aqueous phase liquid (NAPL) "architecture" have been described in a companion paper (Enfield, C.G., Wood, A.L., Espinoza, F.P., Brooks, M.C., Annable, M., Rao, P.S.C., this issue. Design of aquifer remediation systems: (1) describing hydraulic structure and NAPL architecture using tracers. J. Contam. Hydrol.). The previously defined functions were used along with the properties of the remedial fluids to describe remedial performance. There are two objectives for this paper. First, is to show that a simple analytic element model can be used to give a reasonable estimate of system performance. This is accomplished by comparing forecast performance to observed performance. The second objective is to display the model output in terms of change in mass flux and mass removal as a function of pore volumes of remedial fluid injected. The modelling results suggest that short term benefits are obtained and related to mass reduction at the sites where the model was tested.     

Hot water flushing for immiscible displacement of a viscous NAPL

Thermal remediation techniques, such as hot water flooding, are emerging technologies that have been proposed for the removal of nonaqueous phase liquids (NAPLs) from the subsurface. In this study a combined laboratory and modeling investigation was conducted to determine if hot water flooding techniques would improve NAPL mass removal compared to ambient temperature water flushing. Two experiments were conducted in a bench scale two-dimensional sandbox (55 cmx45 cmx1.3 cm) and NAPL saturations were quantified using a light transmission apparatus. In these immiscible displacement experiments the aqueous phase, at 22 degrees C and 50 degrees C, displaced a zone with initial NAPL saturations on the order of 85%. The interfacial tension and viscosity of the selected light NAPL, Voltesso 35, are strongly temperature-dependent. Experimental results suggest that hot water flooding reduced the size of the high NAPL saturation zone, in comparison to the cold water flood, and yielded greater NAPL mass recovery (75% NAPL removal vs. 64%). Hot water flooding did not, however, result in lower residual NAPL saturations. A numerical simulator was modified to include simultaneous flow of water and organic phases, energy transport, temperature and pressure. Model predictions of mass removal and NAPL saturation profiles compared well with observed behavior. A sensitivity analysis indicates that the utility of hot water flooding improves with the increasing temperature dependence of NAPL hydraulic properties.     

Locally-calibrated light transmission visualization methods to quantify nonaqueous phase liquid mass in porous media

Five locally-calibrated light transmission visualization (LTV) methods were tested to quantify nonaqueous phase liquid (NAPL) mass and mass reduction in porous media. Tetrachloroethylene (PCE) was released into a two-dimensional laboratory flow chamber packed with water-saturated sand which was then flushed with a surfactant solution (2% Tween 80) until all of the PCE had been dissolved. In all the LTV methods employed here, the water phase was dyed, rather than the more common approach of dyeing the NAPL phase, such that the light adsorption characteristics of NAPL did not change as dissolution progressed. Also, none of the methods used here required the use of external calibration chambers. The five visualization approaches evaluated included three methods developed from previously published models, a binary method, and a novel multiple wavelength method that has the advantage of not requiring any assumptions about the intra-pore interface structure between the various phases (sand/water/NAPL). The new multiple wavelength method is also expected to be applicable to any translucent porous media containing two immiscible fluids (e.g., water-air, water-NAPL). Results from the sand-water-PCE system evaluated here showed that the model that assumes wetting media of uniform pore size (Model C of Niemet and Selker, 2001) and the multiple wavelength model with no interface structure assumptions were able to accurately quantify PCE mass reduction during surfactant flushing. The average mass recoveries from these two imaging methods were greater than 95% for domain-average NAPL saturations of approximately 2.6x10(-2), and were approximately 90% during seven cycles of surfactant flushing that sequentially reduced the average NAPL saturation to 7.5x10(-4).     

Parameters that control the cleanup of fractured permeable aquifers

This study develops a modeling approach for simulating and evaluating entrapped light nonaqueous-phase liquid (light NAPL-LNAPL) dissolution and transport of the solute in a fractured permeable aquifer (FPA). The term FPA refers to an aquifer made of porous blocks of high permeability that embed fractures. The fracture network is part of the domain characterized by high permeability and negligible storage. Previous studies show that sandstone aquifers often represent FPAs. The basic model developed in this study is a two-dimensional (2-D) model of permeable blocks that embed oblique equidistant fractures with constant aperture and orientation. According to this model, two major parameters govern NAPL dissolution and transport of the solute. These parameters are: 1) the dimensionless interphase mass transfer coefficient, K(f0), and 2) the mobility number, N(M0). These parameters represent measures of heterogeneity affecting flow, NAPL dissolution, and transport of the solute in the domain. The parameter K(f0) refers to the rate at which organic mass is transferred from the NAPL into the water phase. The parameter N(M0) represents the ratio of flow through the porous blocks to flow through the fracture network in regions free of entrapped NAPL. It also provides a measure of groundwater flow bypassing regions contaminated by entrapped NAPL. In regions contaminated by entrapped NAPL our simulations have often indicated very low permeability of the porous blocks, enabling a significant increase of the fracture flow at the expense of the permeable block flow. Two types of constitutive relationships also affect the rate of FPA cleanup: 1) the relationship between the saturation of the entrapped NAPL and the permeability of the porous blocks, and 2) the relationships representing effects of the entrapped NAPL saturation and the permeable block flow velocity on rates of interphase mass transfer. This study provides basic tools for evaluating the characteristics of pump-and-treat cleanup of FPAs by referring to sets of parameters and constitutive relationships typical of FPAs. The numerical simulations carried out in this study show that at high initial saturation of the entrapped NAPL, during initial stages of the FPA cleanup the contaminant concentration increases, but later it decreases. This phenomenon originates from significant groundwater bypassing the NAPL entrapped in the permeable blocks via the fracture network.     

Magnetic resonance imaging of nonaqueous phase liquid during soil vapor extraction in heterogeneous porous media

Soil vapor extraction (SVE) is commonly used to remediate nonaqueous phase liquids (NAPLs) from the vadose zone. This paper aims to determine the effect of grain size heterogeneity on the removal of NAPL in porous media during SVE. Magnetic resonance imaging (MRI) was used to observe and quantify the amount and location of NAPL in flow-through columns filled with silica gel grains. MRI is unique because it is nondestructive, allowing three-dimensional images to be taken of the phases as a function of space and time. Columns were packed with silica gel in three ways: coarse grains (250-550 microm) only, fine grains (32-63 microm) only, and a core of fine grains surrounded by a shell of coarse grains. Columns saturated with water were drained under a constant suction head, contaminated with decane, and then drained to different decane saturations. Each column was then continuously purged with water-saturated nitrogen gas and images were taken intermittently. Results showed that at residual saturation, a sharp volatilization front moved through the columns filled with either coarse-grain or fine-grain silica gel. In the heterogeneous columns, the volatilization front in the core lagged just behind the shell because gas flow was greater through the shell and decane in the core diffused outward to the shell. When decane saturation in the core was above residual saturation, decane volatilization occurred near the inlet, the relative decane saturation throughout the core dropped uniformly, and decane in the core flowed in the liquid phase to the shell to replenish volatilized decane. These results indicate that NAPL trapped in low-permeability zones can flow to replenish areas where NAPL is lost due to SVE. However, when residual NAPL saturation is reached, NAPL flow no longer occurs and diffusion limits removal from low-permeability zones.     

The effects of surfactant formulation on nonequilibrium NAPL solubilization

Surfactant-enhanced aquifer remediation (SEAR) involves the injection of surfactant solutions into aquifers contaminated with nonaqueous phase liquids (NAPL). Batch and column experiments were used to assess the effect of surfactant formulation on the rate of NAPL solubilization. The experimental variables were surfactant type, surfactant concentration, electrolyte concentration, and cosolvent concentration. Model equations were proposed and solved to describe solubilization under the conditions of each type of experiment. Using these models, a solubilization rate constant, kappa(b), and an overall mass transfer rate coefficient, kappa, were estimated from the batch and column experiments, respectively. The solubilization rate constant was consistently sensitive to surfactant type, surfactant concentration, and electrolyte concentration. The estimated solubilization rate constants varied over two orders of magnitude. The results of the column experiments also were sensitive to the surfactant formulation. Variations in the fitted mass transfer rate coefficient parameter, beta(0), were related to variations in the surfactant formulations. A comparison between the results of the batch and column experiments yields an apparent relationship between beta(0) and kappa(b). This relationship suggests that the mass transfer rate coefficient is directly related to the formulation of the surfactant solution.     

Surfactant enhanced recovery of tetrachloroethylene from a porous medium containing low permeability lenses. 1. Experimental studies

A matrix of batch, column and two-dimensional (2-D) box experiments was conducted to investigate the coupled effects of rate-limited solubilization and layering on the entrapment and subsequent recovery of a representative dense NAPL, tetrachloroethylene (PCE), during surfactant flushing. Batch experiments were performed to determine the equilibrium solubilization capacity of the surfactant, polyoxyethylene (20) sorbitan monooleate (Tween 80), and to measure fluid viscosity, density and interfacial tension. Results of one-dimensional column studies indicated that micellar solubilization of residual PCE was rate-limited at Darcy velocities ranging from 0.8 to 8.2 cm/h and during periods of flow interruption. Effluent concentration data were used to develop effective mass transfer coefficient (Ke) expressions that were dependent upon the Darcy velocity and duration of flow interruption. To simulate subsurface heterogeneity, 2-D boxes were packed with layers of F-70 Ottawa sand and Wurtsmith aquifer material within 20-30 mesh Ottawa sand. A 4% Tween 80 solution was then flushed through PCE-contaminated boxes at several flow velocities, with periods of flow interruption. Effluent concentration data and visual observations indicated that both rate-limited solubilization and pooling of PCE above the fine layers reduced PCE recovery to levels below those anticipated from batch and column measurements. These experimental results demonstrate the potential impact of both mass transfer limitations and subsurface layering on the recovery of PCE during surfactant enhanced aquifer remediation.     

Radon as a naturally occurring tracer for the assessment of residual NAPL contamination of aquifers

The noble gas radon has a strong affinity to non-aqueous phase-liquids (NAPLs). That property makes it applicable as naturally occurring partitioning tracer for assessing residual NAPL contamination of aquifers. In a NAPL contaminated aquifer, radon dissolved in the groundwater partitions preferably into the NAPL. The magnitude of the resulting radon deficit in the groundwater depends on the NAPL-specific radon partition coefficient and on the NAPL saturation of the pore space. Hence, if the partition coefficient is known, the NAPL saturation is attainable by determination of the radon deficit. After a concise discussion of theoretical aspects regarding radon partitioning into NAPL, related experimental data and results of a field investigation are presented. Aim of the laboratory experiments was the determination of radon partition coefficients of multi-component NAPLs of environmental concern. The on-site activities were carried out in order to confirm the applicability of the "radon method" under field conditions.     

Plume persistence caused by back diffusion from thin clay layers in a sand aquifer following TCE source-zone hydraulic isolation

This paper concludes that back diffusion from one or a few thin clayey beds in a sand aquifer can cause contaminant persistence above MCLs in a sand aquifer long after the source zone initially causing the plume is isolated or removed. This conclusion is based on an intensive case study of a TCE contaminated site in Florida, with the processes evaluated using numerical modeling. At this site, the TCE DNAPL zone formed decades ago, and was hydraulically isolated by means of an innovative system performing groundwater extraction, treatment and re-injection. Treated water is re-injected in a row of injection wells situated a short distance downgradient of the extraction wells, creating a clean-water displacement front to efficiently flush the downgradient plume. This scheme avoids the creation of stagnation zones typical of most groundwater pump-and-treat systems, thereby minimizing the time for aquifer flushing and therefore downgradient cleanup. The system began operation in August 2002 and although the performance monitoring shows substantial declines in concentrations, detectable levels of TCE and degradation products persist downgradient of the re-injection wells, long after the TCE should have disappeared based on calculations assuming a nearly homogenous sand aquifer. Three hypotheses were assessed for this plume persistence: 1) incomplete source-zone capture, 2) DNAPL occurrence downgradient of the re-injection wells, and 3) back diffusion from one or more thin clay beds in the aquifer. After careful consideration, the first two hypotheses were eliminated, leaving back diffusion as the only plausible hypothesis, supported by detailed measurements of VOC concentrations within and near the clay beds and also by numerical model simulations that closely represent the field site hydrogeologic conditions. The model was also used to simulate a more generalized, hypothetical situation where more thin clayey beds occur in a sand aquifer with an underlying aquitard. While there is no doubt that DNAPL source mass reduction can eventually improve downgradient groundwater quality, the magnitude and time scale over which the improvement occurs is the major uncertainty given current characterization approaches. This study shows that even one thin clay bed, less than 0.2 m thick, can cause plume persistence due to back diffusion for several years or even decades after the flux from the source is completely isolated. Thin clay beds, which have a large storage capacity for dissolved and sorbed contaminant mass, are common in many types of sandy aquifers. However, without careful inspection of continuous cores and sampling, such thin clay beds, and their potential for causing long-term back-diffusion effects, can easily go unnoticed during site characterization.     

Mass flux from a non-aqueous phase liquid pool considering spontaneous expansion of a discontinuous gas phase

The partitioning of non-aqueous phase liquid (NAPL) compounds to a discontinuous gas phase results in the repeated spontaneous expansion, snap-off, and vertical mobilization of the gas phase. This mechanism has the potential to significantly affect the mass transfer processes that control the dissolution of NAPL pools by increasing the vertical transport of NAPL mass and increasing the total mass transfer rate from the surface of the pool. The extent to which this mechanism affects mass transfer from a NAPL pool depends on the rate of expansion and the mass of NAPL compound in the gas phase. This study used well-controlled bench-scale experiments under no-flow conditions to quantify for the first time the expansion of a discontinuous gas phase in the presence of NAPL. Air bubbles placed in glass vials containing NAPL increased significantly in volume, from a radius of 1.0 mm to 2.0 mm over 215 days in the presence of tetrachloroethene (PCE), and from a radius of 1.2 mm to 2.3 mm over 22 days in the presence of trans-1,2-dichloroethene (tDCE). A one-dimensional mass transfer model, fit to the experimental data, showed that this expansion could result in a mass flux from the NAPL pool that was similar in magnitude to the mass flux expected for the dissolution of a NAPL pool in a two-fluid (NAPL and water) system. Conditions favouring the significant effect of a discontinuous gas phase on mass transfer were identified as groundwater velocities less than ~0.01 m/day, and a gas phase that covers greater than ~10% of the pool surface area and is located within ~0.01 m of the pool surface. Under these conditions the mass transfer via a discontinuous gas phase is expected to affect, for example, efforts to locate NAPL source zones using aqueous concentration data, and predict the lifetime and risk associated with NAPL source zones in a way that is not currently included in the common conceptual models used to assess NAPL-contaminated sites.     

SEAR技术修复土壤和地下水中NAPL污染的研究进展

就SEAR技术修复土壤及地下水中NAPL污染的原理及发展现状进行了综述.SEAR技术可以快速有效地去除土壤和地下水中的NAPL污染源,适于多种污染物.该技术通过增溶和增流2种途径提高NAPL污染物的去除率.表面活性剂的选择和微乳液体系的调配是SEAR技术实施的关键环节.将SEAR技术用于高浓度NAPL污染源的治理,并与生物修复和自然降解相结合,是经济高效的治理方案.     

Modelling of iron cycling and its impact on the electron balance at a petroleum hydrocarbon contaminated site in Hnevice, Czech Republic

Over a period of several decades multiple leaks of large volumes from storage facilities located near Hnevice (Czech Republic) have caused the underlying Quaternary aquifer to be severely contaminated with nonaqueous phase liquid (NAPL) petroleum hydrocarbons. Beginning in the late 1980's the NAPL plume started to shrink as a consequence of NAPL dissolution exceeding replenishment and due to active remediation. The subsurface was classified geochemically into four different zones, (i) a contaminant-free zone never occupied by NAPL or dissolved contaminants, (ii) a re-oxidation zone formerly occupied by NAPL, (iii) a zone currently occupied by NAPL, and (iv) a lower fringe zone between the overlying NAPL and the deeper underlying contaminant-free zone. The study investigated the spatial and temporal variability of the redox zonation at the Hnevice site and quantified the influence of iron-cycling on the overall electron balance. As a first step inverse geochemical modelling was carried out to identify possible reaction models and mass transfer processes. In a subsequent step, two-dimensional (forward) multi-component reactive transport modelling was performed to evaluate and quantify the major processes that control the geochemical evolution at the site. The study explains the observed enrichment of the lower fringe zone with ferrihydrite as a result of the re-oxidation of ferrous iron. It suggests that once the NAPL zone started to shrink the dissolution of previously formed siderite and FeS by oxygen and nitrate consumed a significant part of the oxidation capacity for a considerable time period and therefore limited the penetration of electron acceptors into the NAPL contaminated zone.