In Situ Chemical Oxidation of RDX-Contaminated Groundwater with Permanganate at the Nebraska Ordnance Plant

Groundwater beneath the former Nebraska Ordnance Plant (NOP) is contaminated with the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The current pump and treat facility is preventing offsite migration but does not offer a short-term solution. Our objective was to quantify the effectiveness of permanganate to degrade RDX in situ. This was accomplished by performing laboratory treatability experiments, aquifer characterization, and a pilot-scale in situ chemical oxidation (ISCO) demonstration. Treatability experiments confirmed that permanganate could mineralize RDX in the presence of NOP aquifer solids. The pilot-scale ISCO demonstration was performed using an extraction-injection well configuration to create a curtain of permanganate between two injection wells. RDX destruction was then quantified as the RDX-permanganate plume migrated downgradient through a monitoring well field. Electrical resistivity imaging (ERI) was used to identify the subsurface distribution of permanganate after injection. Results showed that RDX concentrations temporally decreased in wells closest to the injection wells by 70% to 80%. Observed degradation rates (0.12 and 0.087/d) were lower than those observed under laboratory batch conditions at 11.5 °C (0.20/d) and resulted from lower than projected permanganate concentrations. Both ERI and spatial electrical conductivity measurements verified that permanganate distribution was not uniform throughout the 6.1-m (20 feet) well screens and that groundwater sampling captured both treated and nontreated groundwater during pumping. Although heterogeneous flow paths precluded a uniform permanganate distribution, pilot-scale results provided proof-of-concept that permanganate can degrade RDX in situ and support permanganate as a possible remedial treatment for RDX-contaminated groundwater.     

Modeling vertical stratification of CO2 injected into a deep layered aquifer

The vertical stratification of carbon dioxide (CO₂) injected into a deep layered aquifer made up of high-permeability and low-permeability layers, such as Utsira aquifer at Sleipner site in Norway, is investigated with a Buckley–Leverett equation including gravity effects. In a first step, we study both by theory and simulation the application of this equation to the vertical migration of a light phase (CO₂), in a denser phase (water), in 1D vertical columns filled with different types of porous media: homogeneous, piecewise homogeneous, layered periodic and finally heterogeneous. For each case, we solve the associated Riemann problems and propose semi-analytical solutions describing the spatial and temporal evolution of the light phase saturation. These solutions agree well with simulation results. We show that the flux continuity condition at interfaces between high-permeability and low-permeability layers leads to CO₂ saturation discontinuities at these interfaces and, in particular, to a saturation increase beneath low-permeability layers. In a second step, we analyze the vertical migration of a CO₂ plume injected into a 2D layered aquifer. We show that the CO₂ vertical stratification under each low-permeability layer is induced, as in 1D columns, by the flux continuity condition at interfaces. As the injection takes place at the bottom of the aquifer the velocity and the flux function decrease with elevation and this phenomenon is proposed to explain the stratification under each mudstone layer as observed at Sleipner site.     

Electrokinetic Migration of Nitrate Through Heterogeneous Granular Porous Media

This study investigates and quantifies the influence of physical heterogeneity in granular porous media, represented by materials with different hydraulic conductivity, on the migration of nitrate, used as an amendment to enhance bioremediation, under an electric field. Laboratory experiments were conducted in a bench‐scale test cell under a low applied direct current using glass bead and clay mixes and synthetic groundwater to represent ideal conditions. The experiments included bromide tracer tests in homogeneous settings to deduce controls on electrokinetic transport of inorganic solutes in the different materials, and comparison of nitrate migration under homogeneous and heterogeneous scenarios. The results indicate that physical heterogeneity of subsurface materials, represented by a contrast between a higher‐hydraulic conductivity and lower‐hydraulic conductivity material normal to the direction of the applied electric field exerts the following controls on nitrate migration: (1) a spatial change in nitrate migration rate due to changes in effective ionic mobility and subsequent accumulation of nitrate at the interface between these materials; and (2) a spatial change in the voltage gradient distribution across the hydraulic conductivity contrast, due to the inverse relationship with effective ionic mobility. These factors will contribute to higher mass transport of nitrate through low hydraulic conductivity zones in heterogeneous porous media, relative to homogeneous host materials. Overall electrokinetic migration of amendments such as nitrate can be increased in heterogeneous granular porous media to enhance the in situ bioremediation of organic contaminants present in low hydraulic conductivity zones.     

Laboratory Study of Polymer Solutions Used for Mobility Control During In Situ NAPL Recovery

The use of surfactant solutions for the in situ recovery of residual NAPL in aquifers is increasingly considered as a viable remediation technique. The injection of a few pore volumes of high concentration surfactant solutions can mobilize most of the residual NAPL contacted by the solutions. However, the washing solutions'physico-chemical properties (low density and high viscosity), combined with the natural porous media heterogeneity, can prevent a good sweep of the entire contaminated volume. From the petroleum industry, it is well-known that polymer solutions can be injected following a surfactant solution slug to act as a mobility buffer and increase the overall sweep efficiency. The objective of our laboratory study is first to select and characterize polymers that would be suitable for aquifer restoration. Our experiments showed that among several polymers, xanthan gum solution rheology was made in order to predict shear rates, xanthan gum concentrations, salinity, and temperature effects on solution viscosity. The second set of experiments were made with a sand box which was designed to reproduce a simple heterogeneous media consisting of layers of sand with different permeability. These tests illustrate the xanthan gum solution's ability to increase surfactant solution's sweep efficiency and limit viscous fingering. The tests established that: (1) the injection of xanthan solution behind a surfactant solution slug decreases fluid velocity in high permeability layers and increases it in low-permeability ones, thus increasing the sweep efficiency (2) xanthan solutions eliminate viscous fingering at the polymer/surfactant solution front; (3) a xanthan solution preflush is desirable to limit surfactant solution mobility and prevent surfactant adsorption on solids; and (4) depending on site heterogeneity injection strategies should be applied to limit overriding by low-density surfactant solution.     

轻非水相液体污染过程的高密度电阻率成像法室内监测

为探讨高密度电阻率成像法监测多孔介质中轻非水相液体迁移过程的有效性,本文通过三维砂槽进行了非饱和带中轻非水相液体的污染试验,并利用高密度电阻率成像法进行了同步的动态监测.试验之后,将砂槽层层挖开,通过数码成像,获取了污染区域的实际范围与形状.结果表明,由高密度电阻率成像法圈定的污染区域在范围与形状上都与实际的结果比较接近,并可通过三维电阻率相对值的时间变化明显的看出轻非水相液体的污染过程.这说明利用高密度电阻率成像法对非饱和多孔介质中轻非水相液体的空间分布范围进行圈定并监测其迁移过程是完全可行的.     

Hydraulic displacement of dense nonaqueous phase liquids for source zone stabilization

Hydraulic displacement is a mass removal technology suitable for stabilization of a dense, nonaqueous phase liquid (DNAPL) source zone, where stabilization is defined as reducing DNAPL saturations and reducing the risk of future pool mobilization. High resolution three-dimensional multiphase flow simulations incorporating a spatially correlated, heterogeneous porous medium illustrate that hydraulic displacement results in an increase in the amount of residual DNAPL present, which in turn results in increased solute concentrations in groundwater, an increase in the rate of DNAPL dissolution, and an increase in the solute mass flux. A higher percentage of DNAPL recovery is associated with higher initial DNAPL release volumes, lower density DNAPLs, more heterogeneous porous media, and increased drawdown of groundwater at extraction wells. The fact that higher rates of recovery are associated with more heterogeneous porous media stems from the fact that larger contrasts in permeability provide for a higher proportion of capillary barriers upon which DNAPL pooling and lateral migration can occur. Across all scenarios evaluated in this study, the ganglia-to-pool (GTP) ratio generally increased from approximately 0.1 to between approximately 0.3 and 0.7 depending on the type of DNAPL, the degree of heterogeneity, and the imposed hydraulic gradient. The volume of DNAPL recovered as a result of implementing hydraulic displacement ranged from between 9.4% and 45.2% of the initial release volume, with the largest percentage recovery associated with 1,1,1 trichloroethane, the least dense of the three DNAPLs considered.     

Variable-density groundwater flow and solute transport in irregular 2D fracture networks

Numerical simulations of variable-density flow and solute transport have been conducted to investigate dense plume migration for various configurations of 2D fracture networks. For orthogonal fractures, simulations demonstrate that dispersive mixing in fractures with small aperture does not stabilize vertical plume migration in fractures with large aperture. Simulations in non-orthogonal 2D fracture networks indicate that convection cells form and that they overlap both the porous matrix and fractures. Thus, transport rates in convection cells depend on matrix and fracture flow properties. A series of simulations in statistically equivalent networks of fractures with irregular orientation show that the migration of a dense plume is highly sensitive to the geometry of the network. If fractures in a random network are connected equidistantly to the solute source, few equidistantly distributed fractures favor density-driven transport. On the other hand, numerous fractures have a stabilizing effect, especially if diffusive transport rates are high. A sensitivity analysis for a network with few equidistantly distributed fractures shows that low fracture aperture, low matrix permeability and high matrix porosity impede density-driven transport because these parameters reduce groundwater flow velocities in both the matrix and the fractures. Enhanced molecular diffusion slows down density-driven transport because it favors solute diffusion from the fractures into the low-permeability porous matrix where groundwater velocities are smaller. For the configurations tested, variable-density flow and solute transport are most sensitive to the permeability and porosity of the matrix, which are properties that can be determined more accurately than the geometry and hydraulic properties of the fracture network, which have a smaller impact on density-driven transport.     

Analytical solutions of one-dimensional macrodispersion in stratified porous media by the quadrupole method: convergence to an equivalent homogeneous porous medium

In this article, the quadrupole method is implemented in order to simulate the effects of heterogeneities on one dimensional advective and diffusive transport of a passive solute in porous media. Theoretical studies of dispersion in heterogeneous stratified media can bring insight into transport artefacts linked to scale effects and apparent dispersion coefficients. The quadrupole method is an efficient method for the calculation of transient response of linear systems. It is based here on the Laplace transform technique. The analytical solutions that can be derived by this method assists understanding of upscaled parameters relevant to heterogeneous porous media.First, the method is developed for an infinite homogeneous porous medium. Then, it is adapted to a stratified medium where the fluid flow is perpendicular to the interfaces. The first heterogeneous medium studied is composed of two semi-infinite layers perpendicular to the flow direction each having different transport properties. The concentration response of the medium to a Dirac injection is evaluated. The case studied emphasises the importance in the choice of the boundary conditions.In the case of a periodic heterogeneous porous medium, the concentration response of the medium is evaluated for different numbers of unit-cells. When the number of unit cells is great enough, depending on the transport properties of each layer in the unit cell, an equivalent homogeneous behaviour is reached. An exact determination of the transport properties (equivalent dispersion coefficient) of the equivalent homogeneous porous medium is given.     

Issues and Options in the Delineation of Well Capture Zones under Uncertainty

The delineation of wellhead protection areas (WHPAs) under uncertainty is still a challenge for heterogeneous porous media. For granular media, one option is to combine particle tracking (PT) with the Monte Carlo approach (PT‐MC) to account for geologic uncertainties. Fractured porous media, however, require certain restrictive assumptions under this approach. An alternative for all types of media is the capture probability (CP) approach, which is based on the solution of the standard advection‐dispersion equation in a backward mode, making use of the analogy between forward and backward transport processes. Within this context, we review the current controversy about the correct form of the conceptual model for transport, finding that the advection‐diffusion model, which represents the diffusive interchange between streamtubes with differing velocities, is more physically realistic than the conventional advection‐dispersion model. For mildly to moderately heterogeneous materials, stochastic theories and simulation experiments show that this process converges at the field scale to an effective advection‐dispersion process that can be simulated with conventional transport models using appropriate macrodispersivity values. For highly heterogeneous materials, stochastic theories do not yet exist but there is no reason why the process should not converge naturally as well. Macrodispersivities appear to be formation‐specific. The advection‐dispersion model can be used for capture zone delineation in heterogeneous granular media. For fractured porous systems, hybrid equivalent porous medium and discrete fracture network or CP‐based approaches may have potential. In general, capture zones delineated by PT without MC will always be too small and should not be used as a basis for land‐use decisions.     

Analytical modeling of diffusion-limited contamination and decontamination in a two-layer porous medium

Diffusion of dissolved contaminants in multilayer porous media is an important phenomenon affecting both contamination and remediation in natural aqueous environments, including diffusion in groundwater aquitards and contaminated bed sediments. This study presents a new analytical solution for solute diffusion in a semi-infinite two-layer porous medium for arbitrary boundary and initial conditions. The solution was obtained by using the Green's function approach in the Laplace domain with the application of the binomial theorem to facilitate inversion back to the real time domain. Results based on this solution were found to be simple both in form and ease of calculation and to be in good agreement with those obtained using numerical calculations based on the Crank-Nicolson finite difference method. Applications of the solution are presented in the context of a contaminated groundwater aquitard to demonstrate how different boundary and initial conditions can greatly affect the contamination and decontamination of porous media, and to illustrate how diffusion modeling might be used in a forensic sense.     

Truncated plurigaussian simulations to characterize aquifer heterogeneity

Integrating geological concepts, such as relative positions and proportions of the different lithofacies, is of highest importance in order to render realistic geological patterns. The truncated plurigaussian simulation method provides a way of using both local and conceptual geological information to infer the distributions of the facies and then those of hydraulic parameters. The method ( Le Loc'h and Galli 1994 ) is based on the idea of truncating at least two underlying multi-Gaussian simulations in order to create maps of categorical variable. In this article, we show how this technique can be used to assess contaminant migration in highly heterogeneous media. We illustrate its application on the biggest contaminated site of Switzerland. It consists of a contaminant plume located in the lower fresh water Molasse on the western Swiss Plateau. The highly heterogeneous character of this formation calls for efficient stochastic methods in order to characterize transport processes.     

渗流方程的三维粗化算法

本文提出了地下流体渗流问题的三维解粗化算法,在粗网格内流体压强分布用直接解法求解三维渗流方程,用这些解计算粗网格的等效渗透率,在流体流速大的区域仍采用精细网格的计算方法.用所得等效渗透率计算了粗化网格的渗流场的压强分布,结果表明渗流方程的三维粗化解非常逼近采用精细网格的解,但计算的速度比采用精细网格提高了100多倍.     

随机弹性介质中地震波散射衰减分析(英文)

地震波衰减一直是许多学科研究的热点,因为可以反映介质的特性。导致地震波衰减的因素很多,如:传播过程中由于能量扩散导致的几何衰减,固体岩石内部晶粒间相对滑移导致的摩擦衰减,岩石结构不均匀引起的地震波散射衰减。本文主要从统计的观点出发,通过多次数值模拟的方法研究纵波散射在随机弹性介质中所引发的衰减。首先用随机理论建立了二维空间随机弹性介质模型,然后用错格伪谱法的数值方法模拟了波在随机介质中的传播,再通过波场中虚拟检波器的记录,用谱比法估计了弹性波在随机介质中的散射衰减。不同非均匀程度随机弹性介质中的数值结果表明:介质不均匀程度越高,散射衰减越大;在散射体尺寸小于波长的前提下,不同散射体尺寸的计算结果说明:散射体尺寸越大,弹性波衰减越明显。最后提出了一种不均匀孔隙介质中流体流动衰减的方法。通过对随机孔隙介质中地震波的总衰减和散射衰减分别进行了计算,并定量得出了随机孔隙介质中流体流动衰减,结果表明:在实际地震频段下,当介质不均匀尺度101米量级时,散射衰减比流体流动衰减要大,散射衰减是地震波在实际不均匀岩石孔隙介质中衰减的主要原因。     

Laboratory Study of Air Sparging: Air Flow Visualization

Laboratory flow visualization experiments, using glass beads as the porous medium, were conducted to study air sparging, an innovative technology for subsurface contaminant remediation. The purpose of these experiments was to observe how air flows through saturated porous media and to obtain a basic understanding of air plume formation and medium heterogeneity effects. The experiments indicate that air flow occurring in discrete, stable channels is the most probable flow behavior in medium to fine grained water saturated porous media and that medium heterogeneity plays an important role in the development of air channels. Several simulated scales of heterogeneities, from pore to field, have been studied. The results suggest that air channel formation is sensitive to the various scales of heterogeneities. Site-specific hydrogeologic settings have to be carefully reviewed before air sparging is applied to remediate sites contaminated by volatile organic compounds.     

Polymer-Enhanced DNAPL Flushing from Low-Permeability Media: An Experimental Study

Remediation of dense nonaqueous phase liquids (DNAPLs) is recognized as one of the most difficult problems associated with ground water pollution. The pump-and-treat technique, usually consisting of a continuous operation of extraction-injection wells, is widely used for ground water remediation. In a stratified or otherwise heterogeneous aquifer, however, this technique suffers from tailing and rebound problems, which limit its cleanup efficiency and result in higher operation costs. The tailing and rebound is usually due to slow diffusion of contaminants out of lower- permeability heterogeneities into the flow regime of the higher-permeability zone. In this study, we conduct bench-scale experiments to investigate a novel polymer system and injection method to improve the pump-and-treat technique for DNAPL trapped in a layer of porous media that has a relatively low permeability compared to the surrounding media. This technique might be useful, for example, to remove DNAPL from these low-permeability zones after removal of DNAPL from the higher-permeability zones by a more traditional remediation method. The polymer system consists of a mixture of anionic and cationic polyacrylamides in solution and the injection method is based on flow-induced polymer adsorption, called bridging adsorption. The study includes single and parallel-column experiments. The measured polymer penetration depths were compared with values predicted from a numerical simulation, which was developed previously by the authors of this paper. The experiments and simulations show that the polymer injection leads to a modification of the permeability contrast that favors a more efficient pump-and-treat process. These results suggest that additional research to upscale the technology to pilot scales is warranted.     

In Situ Measurement of Electro-Osmotic Fluxes and Conductivity Using Single Wellbore Tracer Tests

Electro-osmosis (EO), the movement of water through porous media in response to an electric field, offers a means for extracting contaminated ground water from fine-grained sediments, such as clays, that are not easily amenable to conventional pump-and-treat approaches. The EO-induced water flux is proportional to the voltage gradient in a manner analogous to the flux dependence on the hydraulic gradient under Darcy's law. The proportionality constant, the soil electro-osmotic conductivity or k_eo, is most easily measured in soil cores using bench-top tests, where flow is one-dimensional and interfering effects attributable to Darcy's law can be directly accounted for. In contrast, quantification of EO fluxes and k_eo in the field under deployment conditions can be difficult because electrodes are placed in ground water wells that may be screened across a heterogeneous mixture of lithologies. As a result, EO-induced water fluxes constitute an approximate radial flow system that is superimposed upon a Darcy flow regime through permeable pathways that may or may not be coupled with hydraulic head differences created by the EO-induced water fluxes. A single well comparative tracer test, which indirectly measures EO fluxes by comparing wellbore tracer dilution rates between background and EO-induced water fluxes, may provide a means for routinely quantifying the efficacy of EO systems in such settings. EO fluxes measured in field tests through this technique at a ground water contamination site were used to estimate a mean k_eo value through a semianalytic line source model of the electric field. The resulting estimate agrees well with values reported in the literature and with values obtained with bench-top tests conducted on a soil core collected in the test area.     

Influence of natural heterogeneity on the efficiency of chemical floods in source zones

Several technologies for cleaning up DNAPLs in source zones rely on solubilizing contaminants or destroying them in situ. Typically, these approaches employ an injection/withdrawal system to recirculate this treatment fluids. Our interest is in examining factors that influence delivery efficiency. Although many factors can affect this efficiency, this study looks at the combined influence of density-driven flow and porous media heterogeneities. The analysis is based on a series of numerical simulations of hypothetical chemical floods (e.g., potassium permanganate), which are highly resolved in space and time with a scale that is typical of field installations. Results indicate that the characteristics of convective mixing, i.e., natural, forced, and mixed convection, greatly affect delivery efficiency and patterns of mass transport. The ratio of the Grashof number (Gr) and Reynolds number (Re) proved useful in interpreting the patterns of flooding in a homogeneous porous medium. When higher-permeability layers are included in the domain, they act as conduits, effectively expediting the transport of treatment chemicals from injection to recovery wells. These large fluxes of chemicals in the high-permeability layers produce significant flooding inefficiencies. The problem is less severe in heterogeneous medium where the connectivity through the treatment zone is less well developed. Overall, this paper illustrates that density effects and high-permeability pathways need to be considered in designing chemical floods.     

Personal Sampler for Viable Airborne Microorganisms: Main Development Stages

A new personal bioaerosol sampler has been developed and verified as an efficient tool for monitoring of viable/non‐viable airborne microorganisms, including bacteria, fungi, and viruses. The operational principle of the device is based on continuous passage of an air sample through porous media submerged into a liquid layer. During motion along narrow and tortuous ways inside the porous media, the air stream is split into a large number of ultra small bubbles with the particulates are being scavenged by these bubbles and, thus, effectively trapped. The device was initially verified for monitoring of viable airborne bacteria and fungi, firstly, under controlled laboratory conditions and later in a field. It was demonstrated that bacterial recovery rates for these two groups of microorganisms were very high and the device was found to be fully feasible for such monitoring. The next step of the device investigation was performed in the laboratory on monitoring viable airborne viruses with a range of sensitivities to physical and biological stresses. As the result, the new personal sampler demonstrated a very high recovery rate even for viruses which are rather sensitive to environmental stress (Avian Influenza, SARS, Mumps, etc.). Some following field studies, undertook in a hospital and animal houses, also demonstrated an excellent performance of the new device for selective and reliable monitoring of viable airborne viruses even in environments highly contaminated by other microorganisms. This paper reviews the main development staged of the new personal bioaerosol sampler.     

Effects of In Situ Remediation Using Oxidants or Surfactants on Subsurface Organic Matter and Sorption of Trichloroethene

In situ remediation technologies have the potential to alter subsurface properties such as natural organic matter (NOM) content or character, which could affect the organic carbon‐water partitioning behavior of chlorinated organic solvents, including dense nonaqueous phase liquids (DNAPLs). Laboratory experiments were completed to determine the nature and extent of changes in the partitioning behavior of trichloroethene (TCE) caused by in situ chemical oxidation or in situ surfactant flushing. Sandy porous media were obtained from the subsurface at a site in Orlando, Florida. Experiments were run using soil slurries in zero‐headspace reactors (ZHRs) following a factorial design to study the effects of porous media properties (sand vs. loamy sand with different total organic carbon [TOC] contents), TCE concentration (DNAPL presence or absence), and remediation agent type (potassium permanganate vs. activated sodium persulfate, Dowfax 8390 vs. Tween 80). Results revealed that the fraction of organic carbon (f_oc) of porous media after treatment by oxidants or surfactants was higher or lower relative to that in the untreated media controls. Isotherm experiments were run using the treated and control media to measure the distribution coefficient (K_d) of TCE. Organic carbon‐water partitioning coefficient values (K_oc) calculated from the experimental data revealed that K_oc values for TCE in the porous media were altered via treatment using oxidants and surfactants. This alteration can affect the validity of estimates of contaminant mass remaining after remediation. Thus, potential changes in partitioning behavior should be considered to help avoid decision errors when judging the effectiveness of an in situ remediation technology.