Prediction and minimization of vertical migration of DNAPLS using surfactant enhanced aquifer remediation at neutral buoyancy

Surfactant enhanced aquifer remediation (SEAR) had previously been thought to require a capillary barrier below the contaminated zone to prevent the vertical migration of a microemulsion containing solubilized DNAPL. This paper shows the vertical migration of the dense microemulsion is described and predicted by the value of three dimensionless scaling groups. Embedded within these scaling groups are four design parameters. The value of these parameters can be manipulated in order to reduce the amount of vertical migration anticipated for a given remediation design. Plots have been constructed that illustrate the relationship between vertical migration and the value of the scaling groups; such plots can be used to predict vertical migration and to determine appropriate screen intervals of extraction wells to ensure full capture of the contaminants. This predictive capability has been verified in laboratory experiments. Predicted migration of the microemulsion agreed within about 2% of that observed. Development of the scaling groups is presented, remediation design implications are discussed, and laboratory verification is shown. Additional discussion of the laboratory work is given in a companion paper [Kostarelos, K., Pope, G.A., Rouse, B.A., Shook, G.M., 1998. A new concept: the use of neutrally-buoyant microemulsions for DNAPL remediation, J. Contam. Hydro., this edition].     

Multiphase flow and transport in fractured clay/sand sequences.

A numerical model (Queen's University Multi-Phase Flow Simulator, QUMPFS) was used to assess the rate of trichloroethylene (TCE) dense, non-aqueous phase liquid (DNAPL) migration through fractured clay, with special attention focused on the influence of interbedded sand lenses. The presence of these sand lenses was found to increase the time required for the non-wetting phase to migrate through the full 30 m vertical extent of the clay sequence from a few days to several years. Applied vertical hydraulic gradients were found to be moderately influential in systems consisting solely of fractured clays, yet one of the dominant factors controlling speed of vertical migration when sand lenses were present. Larger displacement pressure of the sands relative to that of the fractures leads to slower DNAPL migration rates, due to the delays that occur during build-up of capillary pressures. Dissolution of DNAPL and subsequent matrix diffusion of the aqueous phase has little effect on the rate of DNAPL migration through systems consisting of fractured clay only, yet slows the rate of migration in systems containing sand lenses. In all cases examined, the rate of DNAPL loading to the lower aquifer far exceeded the rate of aqueous phase mass loading. It was also found that DNAPL reaches the lower aquifer at approximately the same time as the aqueous phase plumes even for systems experiencing downward groundwater flow due to the attenuation of the aqueous phase through matrix diffusion.     

Bench-scale visualization of DNAPL remediation processes in analog heterogeneous aquifers: surfactant floods and in situ oxidation using permanganate

We have conducted well-controlled DNAPL remediation experiments within a 2-D, glass-walled, sand-filled chamber using surfactants (Aerosol MA and Tween 80) to increase solubility and an oxidant (permanganate) to chemically degrade the DNAPL. Initial conditions for each remediation experiment were created by injecting DNAPL as a point source at the top of the chamber and allowing the DNAPL to migrate downward through a water-filled, heterogeneous, sand-pack designed to be evocative of a fluvial depositional environment. This migration process resulted in the DNAPL residing as a series of descending pools. Lateral advection across the chamber was used to introduce the remedial fluids. Photographs and digital image analysis illustrate interactions between the introduced fluids and the DNAPL. In the surfactant experiments, we found that DNAPL configured in a series of pools was easily mobilized. Extreme reductions in DNAPL/water interfacial tension occurred when using the Aerosol MA surfactant, resulting in mobilization into low permeability regions and thus confounding the remediation process. More modest reductions in interfacial tension occurred when using the Tween 80 surfactant resulting in modest mobilization. In this experiment, capillary forces remained sufficient to exclude DNAPL migration into low permeability regions allowing the excellent solubilizing properties of the surfactant to recover almost 90% of the DNAPL within 8.6 pore volumes. Injection of a potassium permanganate solution resulted in precipitation of MnO2, a reaction product, creating a low-permeability rind surrounding the DNAPL pools. Formation of this rind hindered contact between the permanganate and the DNAPL, limiting the effectiveness of the remediation. From these experiments, we see the value of performing visualization experiments to evaluate the performance of proposed techniques for DNAPL remediation.     

Removal of DNAPL contamination from the saturated zone by the combined effect of vertical upward flushing and density reduction

A common aspect of innovative remediation techniques is that they tend to reduce the interfacial tension between the aqueous and non-aqueous phase liquids, resulting in mobilization of the organic contaminant. This complicates the remediation of aquifers, contaminated with Dense Non-Aqueous Phase Liquids (DNAPLs), as they are likely to migrate downwards, deeper into the aquifer and into finer layers. A possible solution is the use of swelling alcohols, which tend to reduce the density difference between the aqueous phase and the DNAPL. To avoid premature mobilization upon the initial contact between the DNAPL and the alcohol, several researchers have proposed the use of vertical upward flow of the alcohol. In this paper, we present an equation, which describes the upward mobilization of both continuous and discontinuous DNAPLs and so the important parameters governing the upward controlled mobilization of the DNAPL. The need and required magnitude of this specific discharge was investigated by conducting four column experiments in which the initial density of the DNAPL and the permeability was varied. It was shown that the required flow velocities increase with the permeability of the porous medium and the initial density difference between the aqueous phase and the DNAPL. Whenever the specific discharge falls below the critical value, the DNAPL moves downward. A second set of column experiments looked at the impact of permeability of porous medium on the solubilization and mobilization of DNAPL during alcohol flooding. Columns, packed with coarse or fine sand, containing a residual trichloroethylene (TCE) or perchloroethylene (PCE) saturation were flushed with the alcohol mixture at a fixed specific discharge rate. The induced pressure gradients in the aqueous phase, which were higher in the fine sand, resulted for this porous medium in extensive mobilization of the DNAPL against the direction of the buoyancy force. The density of the first NAPL coming out of the top of the fine sand was close to that of the pure DNAPL. In the coarser sand, the pressure gradients were sufficient to prevent downward migration of the DNAPL, but upward mobilization was minimal. The predominant removal mechanism in this case was the much slower solubilization.     

Refinement of the density-modified displacement method for efficient treatment of tetrachloroethene source zones

A novel method to remediate dense nonaqueous phase liquid (DNAPL) source zones that incorporates in situ density conversion of DNAPL via alcohol partitioning followed by displacement with a low interfacial tension (IFT) surfactant flood has been developed. Previous studies demonstrated the ability of the density-modified displacement (DMD) method to recover chlorobenzene (CB) and trichloroethene (TCE) from heterogeneous porous media without downward migration of the dissolved plume or free product. However, the extent of alcohol (n-butanol) partitioning required for in situ density conversion of high-density NAPLs, such as tetrachloroethene (PCE), could limit the utility of the DMD method. Hence, the objective of this study was to compare the efficacy of two n-butanol delivery approaches: an aqueous solution of 6% (wt) n-butanol and a surfactant-stabilized macroemulsion containing 15% (vol) n-butanol in water, to achieve density reduction of PCE-NAPL in two-dimensional (2-D) aquifer cells. Results of liquid-liquid equilibrium studies indicated that density conversion of PCE relative to water occurred at an n-butanol mole fraction of 0.56, equivalent to approximately 5 ml n-butanol per 1 ml of PCE when in equilibrium with an aqueous solution. In 2-D aquifer cell studies, density conversion of PCE was realized using both n-butanol preflood solutions, with effluent NAPL samples exhibiting density reductions ranging from 0.51 to 0.70 g/ml. Although the overall PCE mass recoveries were similar (91% and 93%) regardless of the n-butanol delivery method, the surfactant-stabilized macroemulsion preflood removed approximately 50% of the PCE mass. In addition, only 1.2 pore volumes of the macroemulsion solution were required to achieve in situ density conversion of PCE, compared to 6.4 pore volumes of the 6% (wt) n-butanol solution. These findings demonstrate that use of the DMD method with a surfactant-stabilized macroemulsion containing n-butanol holds promise as an effective source zone remediation technology, allowing for efficient recovery of PCE-DNAPL while mitigating downward migration of the dissolved plume and free product.     

表面活性剂强化抽出处理含水层中DNAPL污染物的去除特征

为明确表面活性剂强化抽出处理含水层中DNAPL污染物过程中表面活性剂的增强修复效果,及DNAPL自身理化性质和介质孔径的影响,利用数码图像分析技术对1,2-二氯乙烷和四氯乙烯2种DNAPL在石英砂填充的二维砂箱中的抽取迁移过程进行了实验模拟研究,并对抽出水样中DNAPL的浓度进行了测试分析。结果表明,实验条件下加入低浓度(0.18%)的十二烷基苯磺酸钠(SDBS)大幅提高了对弱透水层截留的2种DNAPL聚集体的抽出处理效率。1,2-二氯乙烷在该表面活性剂溶液中的表观溶解度远高于四氯乙烯,因此其短时间内的绝对去除率更高。SDBS强化抽出处理DNAPL的作用机理以增溶作用为主,而其增流作用使DNAPL迁移流动后分布面积增大,增加了与表面活性剂溶液接触的面积,对增溶作用起到促进效果。细粒介质中DNAPL迁移后的最大分布面积较大,因此体系中DNAPL的溶解速率较高。在DNAPL聚集体质量与水力梯度固定的条件下,油水界面张力越低,DNAPL的密度越大,DNAPL垂向迁移的风险就越大。本研究为修复工程中如何依据DNAPL种类与场地多孔介质的情况选择表面活性剂提供了参考。     

蓖麻油衍生微乳液对菲的增溶和洗脱作用及其对菲污染土壤的修复机理

比较研究了蓖麻油硫酸盐(SCOS)与普通表面活性剂Triton X-100(TX100)、Tween 80(TW80)、Brij35、十二烷基苯磺酸钠(SDBS)和十二烷基硫酸钠(SDS)等对菲的增溶和洗脱作用.结果表明,菲表观溶解度与SCOS的浓度呈单一线性关系,SCOS微乳液对菲的增溶比SR=0.0314为最大,菲在微乳相和水相之间的分配系数logKem=4.44,大于菲在胶束相和水相之间的分配系数(logKmc).1:10土-水体系下,SCOS微乳液对菲污染土壤的清洗速率最快,清洗效率最高.SCOS有望成为土壤有机污染淋洗修复的增效试剂.     

Field study of TCE diffusion profiles below DNAPL to assess aquitard integrity

An area where a free-product accumulation of trichloroethylene (TCE) dense non-aqueous phase liquid (DNAPL) occurs at the bottom of a 10-m-thick surficial sand aquifer was studied to determine the integrity of the underlying, 20-m-thick, clayey silt aquitard formed of glaciolacustrine sediment. TCE concentration-versus-depth profiles determined from aquitard cores collected at five locations indicated penetration of detectable TCE 2.5 to 3.0 m into the aquitard. Two of the profiles show persistent DNAPL at the aquitard interface, while two others indicate that DNAPL, present initially, was completely dissolved away producing concentration declines at the aquitard interface. The fifth profile suggests shallow DNAPL penetration (<0.5 m) into the aquitard, however, this penetration, which was likely caused by cross-contamination during core collection or cone penetrometry (CPT) of the aquitard interface, did not increase the maximum depth of TCE penetration. Combining the field profiles with one-dimensional model simulations, downward migration of the aqueous TCE front, defined as the EPA MCL of 5 microg/l, which was below the analytical detection limit, was projected to a distance between 4 and 5 m below the top of the aquitard. Using a single set of estimated aquitard parameter values, simulations of aqueous TCE migration into the aquitard provided a good fit to four of the field profiles with a migration time of 35 to 45 years, consistent with the history of TCE use at the site. These simulations indicate aqueous TCE migration is diffusion-dominated with only small advective influence by the downward groundwater velocity of 2 to 3 cm/year or less in the aquitard due to pumping of the underlying aquifer to supply water to the facility in the past 50 years. The applicability of the parameter values was confirmed by in situ diffusion experiments of 1-year duration, in which stainless steel cylinders containing DNAPL were inserted into the aquitard. The diffusion-dominated nature of the profiles indicates that the aquitard provides long-term protection of the underlying aquifer from contamination from this DNAPL zone. Simulations of long-term migration of the TCE solute front indicate breakthrough to the lower aquifer at 1200 years for the no advection scenario and at 500 years if the strong downward hydraulic gradient persists. However, even after breakthrough, the mass flux through the aquitard to the underlying aquifer remains relatively low, and when considered in terms of potential impacts to pumping wells, concentrations are not expected to increase significantly above present-day MCLs. The use of contaminant profiles of different time and distance scales, in addition to hydraulic data, dramatically improves the ability to assess aquitard integrity, and provides improved transport parameter values for estimating contaminant arrival times and fluxes. The apparent lack of deep preferential pathways for TCE migration, such as open fractures, is probably due to the softness of the silty aquitard deposit and minimal physical or chemical weathering of the aquitard provides long-term protection of the underlying aquifer from contamination from this DNAPL zone. Simulations of long-term migration of the TCE solute front indicate breakthrough to the lower aquifer at 1200 years for the no advection scenario and at 500 years if the strong downward hydraulic gradient persists. However, even after the breakthrough, the mass flux through the aquitard to the underlying aquifer remains relatively low, and when considered in terms of potential impacts to pumping wells , concentrations are not expected to increase significantly above present-day MCLs. The use of contaminant profiles of different time and distance scales, in addition to hydraulic data, dramatically improves the ability to assess aquitard integrity, and provides improved transport parameter values for estimating contaminant arrival times and fluxes. The apparent lack of deep preferential pathways for TCE migration, such as open fractures, is probably due to the softness of the silty aquitard deposit and minimal physical or chemical weathering of the aquitard.     

Anionic surfactant remediation of soil columns contaminated by nonaqueous phase liquids

A variety of column experiments have been completed for the purpose of selecting and evaluating suitable surfactants for remediation of nonaqueous phase liquids (NAPLs). The various NAPLs tested in the laboratory experiments were tetrachloroethylene (PCE), trichloroethylene (TCE), jet fuel (JP4) and a dense nonaqueous phase liquid from a site at Hill Air Force Base, UT. Both Ottawa sand and Hill field soil were used in these experiments. Surfactant candidates were first screened using phase behavior experiments and only the best ones were selected for the subsequent column experiments. Surfactants which showed high contaminant solubilization, fast coalescence times, and the absence of liquid crystal phases and gels during the phase behavior experiments were tested in soil column experiments. The primary objective of the soil column experiments was to identify surfactants that recovered at least 99% of the contaminant. The secondary objective was to identify surfactants that show low adsorption and little or no loss of hydraulic conductivity during the column experiments. Results demonstrated that up to 99.9% of the contaminants were removed as a result of surfactant flooding of the soil columns. The addition of xanthan gum polymer to the surfactant solution was shown to increase remediation efficiency as a lower volume of surfactant was required for recovering a given volume of NAPL. Based on these experimental results, guidelines for designing highly efficient and robust surfactant floods have been developed and applied to a field demonstration.     

A theoretical model of air and steam co-injection to prevent the downward migration of DNAPLs during steam-enhanced extraction

When steam is injected into soil containing a dense volatile non-aqueous phase liquid contaminant the DNAPL vaporized within the heated soil region condenses and accumulates ahead of the steam condensation front. If enough DNAPL accumulates, gravitational forces can overcome trapping forces allowing the liquid contaminant to flow downward. By injecting air with steam, a portion of the DNAPL vapor remains suspended in equilibrium with the air, decreasing liquid contaminant accumulation ahead of the steam condensation front, and thus reducing the possibility of downward migration. In this work, a one-dimensional theoretical model is developed to predict the injection ratio of air to steam that will prevent the accumulation of volatile DNAPLs. The contaminated region is modeled as a one-dimensional homogeneous porous medium with an initially uniform distribution of a single component contaminant. Mass and energy balances are combined to determine the injection ratio of air to steam that eliminates accumulation of the contaminant ahead of the steam condensation front, and hence reduces the possibility of downward migration. The minimum injection ratio that eliminates accumulation is defined as the optimum injection ratio. Example calculations are presented for three DNAPLs, carbon tetrachloride (CCl4), trichloroethylene (TCE), and perchloroethylene (PCE). The optimum injection ratio of air to steam is shown to depend on the initial saturation and the volatility of the liquid contaminant. Numerical simulation results are presented to validate the model, and to illustrate downward migration for ratios less than optimum. Optimum injection ratios determined from numerical simulations are shown to be in good agreement with the theoretical model.     

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.     

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.     

Solubilization of DNAPLs by mixed surfactant: reduction in partitioning losses of nonionic surfactant

Efforts to remediate the dense nonaqueous phase liquids (DNAPLs) by mobilizing them face with risks of driving the contaminants deeper into aquifer zones. This spurs research for modifying the approach for in situ remediation. In this paper, a novel solubilization of DNAPLs by mixed nonionic and anionic surfactant, Triton X-100 (TX100) and sodium dodecylbenzene sulfonate (SDBS), was presented and compared with those by single ones. Given 1:40 phase ratio of DNAPL:water (v/v) and the total surfactant concentration from 0.2 to 10gl(-1), mixed TX100-SDBS at the total mass ratios of 3:1, 1:1 and 1:3 exhibited significant solubilization for the DNAPLs, trichloroethene (TCE), chlorobenzene (CB) and 1,2-dichlorobenzene (1,2-DCB). The solubilization extent by mixed TX100-SDBS was much larger than by single TX100 and even larger than by single SDBS at the ratios of 1:1 and 1:3, respectively. TX100 partitioning into the organic phase dictated the solubilization extent. The TX100 losses into TCE, CB and 1,2-DCB phases were more than 99%, 97% and 97% when single TX100 was used. With SDBS alone, no SDBS partitioned into DNAPLs was observed and in mixed systems, SDBS decreased greatly the partition loss of TX100 into DNAPLs. The extent of TX100 partition decreased with increasing the amount of SDBS. The mechanism for reduction of TX100 partition was discussed. TX100 and SDBS formed mixed micelles in the solution phase. The inability of SDBS to partition into DNAPLs and the mutual affinity of SDBS and TX100 in the mixed micelle controlled the partitioning of TX100 into DNAPL phase. The work presented here demonstrates that mixed nonionic-anionic surfactants would be preferred over single surfactants for solubilization remediation of DNAPLs, which could avoid risks of driving the contaminants deeper into aquifers and decrease the surfactant loss and remediation cost.     

Environmental features of two commercial surfactants widely used in soil remediation

One of the main limitations for a wider application of surfactants in soil remediation is the lack of knowledge about environmental fate and toxicity of surfactant itself especially for in situ application. Sorption behaviour, biodegradability, toxicity of parent compound and its metabolites are important processes that affect environmental fate of surfactants in site remediation applications. Tween 80 (poly(oxyethylene)(20)-sorbitane monooleate) and Aerosol MA+80 (dihexyl sodium sulfosuccinate) are surfactants that have been tested in laboratory and field scale remediation of soil and groundwater. In this work, the sorption and biodegradability of these surfactants were assessed to provide conditions and limitations for their use. The soil used in this experimentation was analysed for organic carbon content, soil bacteria, and size fraction and resulted to be a good model because is characterised by mean values for almost all considered parameters. Tween 80 showed high degree of biodegradability but a high affinity for soil matrix. Results suggest that Tween 80 could find its best application in ex situ solid phase remediation like ex situ bioremediation; its high affinity to soil could limit in situ applications. Biodegradation tests for Aerosol MA+80 show low degree of biodegradability and mineralisation. Biodegradation experiments, coupled with analysis of toxicity, could support the hypothesis that degradation of Aerosol MA+80 is not complete and leads to an accumulation of intermediates with at least the same toxicity of the parental compound. Therefore, aquifer remediation application with Aerosol MA+80 has to be conducted with necessary precautions to avoid product loss and excess surfactant should be flushed from the soil.     

In situ modification of an aquifer material by a cationic surfactant to enhance retardation of organic contaminants

Sorption of hexadecyltrimethylammonium chloride (HDTMA), a cationic surfactant, on aquifer material from Columbus AFB, Mississippi, U.S.A., was examined. Transport studies using flow-through columns and a box model aquifer showed that an almost stationary zone of HDTMA-modified aquifer material could be produced in situ without a significant decrease in hydraulic conductivity.Perchloroethylene (PCE) and naphthalene sorption isotherms on the HDTMA-modified aquifer material were linear, and sorption coefficients were increased by over two orders of magnitude relative to the unmodified material. The retardation of PCE by insitu emplaced HDTMA zones within a column was examined. Agreement between batch- and column-derived sorption coefficients and breakthrough curve symmetry indicates that local equilibrium was attained. Significant retardation of a naphthalene plume by an in situ emplaced surfactant zone was demonstrated in the box model aquifer system.The experimental results indicate that it is feasible to create in situ a sorbent zone within an aquifer using cationic surfactants. In most situations, the sorbent zone concept needs to be coupled with contaminant degradation processes for sorbent emplacement to be a practical tool in the remediation of groundwater contamination sites. Sorbent zones may be of benefit in the engineering of suitable environments for microbial or abiotic degradation reactions and by providing time slow reactions to occur.     

A controlled field experiment on groundwater contamination by a multicomponent DNAPL: creation of the emplaced-source and overview of dissolved plume development

A unique field experiment has been undertaken at the CFB Borden research site to investigate the development of dissolved chlorinated solvent plumes from a residual dense non-aqueous phase liquid (DNAPL) source. The "emplaced-source" tracer test methodology involved a controlled emplacement of a block-shaped source of sand containing chlorinated solvents below the water table. The gradual dissolution of this residual DNAPL solvent source under natural aquifer conditions caused dissolved solvent plumes of trichloromethane (TCM), trichloroethene (TCE) and perchloroethene (PCE) to continuously develop down gradient. Source dissolution and 3-D plume development were successfully monitored via 173 multilevel samplers over a 475-day tracer test period prior to site remediation research being initiated. Detailed groundwater level and hydraulic conductivity data were collected. Development of plumes with concentrations spanning 1-700,000 micrograms/1 is described and key processes controlling their migration identified. Plumes were observed to be narrow due to the weakness of transverse dispersion processes and long due to advection and significant longitudinal dispersion, very limited sorptive retardation and negligible, if any, attenuation due to biodegradation or abiotic reaction. TCM was shown to be essentially conservative, TCE very nearly conservative and PCE, consistent with its greater hydrophobicity, more retarded yet having a greater mobility than observed in previous Borden field tests. The absence of biodegradation was ascribed to the prevailing aerobic conditions and lack of any additional biodegradable carbon substrates. The transient groundwater flow regime caused significant transverse lateral plume movement, plume asymmetry and was likely responsible for most of the, albeit limited, transverse horizontal plume spreading. In agreement with the widespread incidence of extensive TCE and PCE plumes throughout the industrialized world, the experiment indicates such solvent plumes are likely to be highly mobile and persistent, at least in aquifers that are aerobic and have low sorption potential (low foc content).     

Removal of NAPLs from the unsaturated zone using steam: prevention of downward migration by injecting mixtures of steam and air

Steam injection for remediation of porous media contaminated by nonaqueous phase liquids has been shown to be a potentially efficient technology. There is, however, concern that the technique may lead to downward migration of separate phase contaminant. In this work, a modification of the steam injection technology is presented, where a mixture of steam and air was injected. In two-dimensional experiments with unsaturated porous medium contaminated with nonaqueous phase liquids, it was demonstrated how injection of pure steam lead to severe downward migration. Similar experiments, where steam and air were injected simultaneously, resulted in practically no downward migration and still rapid cleanup was achieved. The processes responsible for the prevention of downward migration when injecting steam-air mixtures were analyzed using a nonisothermal multiphase flow and transport model. Hereby, three mechanisms were identified and it was demonstrated how the effectiveness of these mechanisms depended on the air-to-steam mixing ratio.     

A field experiment on cation exchange-affected multicomponent solute transport in a sandy aquifer

A field experiment was performed in an aquifer in order to study multicomponent cation-exchange processes under natural flow conditions. The aquifer is a glacial outwash plain with sandy aquifer material having a cation-exchange capacity (CEC) of 1.0 meg/100 g. A continuous injection of groundwater spiked with sodium and potassium as chlorides was accomplished over 37 days to resemble leachate contamination from landfills. The plume was monitored by sampling in a dense spatial network (length 100 m, width 20 m) over a period of 2.5 years in order to obtain breakthrough curves and spatial contour maps of the chemical compounds. Na and especially K showed a substantial retardation caused by cation-exchange processes despite the low CEC of the aquifer material. The average velocity of K⁺ was only 10% of the velocity of chloride (0.7 m day⁻¹). The relative migration velocity of Na⁺ was not a constant in the plume, but apparently influenced by dilution. Ca²⁺ and Mg²⁺ were expelled from the cation-exchange sites of the aquifer material and subsequently transported with the same velocity as chloride. The breakthrough curves of the various compounds showed multiple peaks and low concentration zones. It was concluded by calculations with PHREEQE that changes in calcite equilibrium may occur in the lower part of the aquifer, while complexation processes seem to be of no importance. Cation exchange is then the most important process in this field experiment, and further evaluation of the data by a geochemical transport model including cation exchange is recommended.     

A compositional simulator for modeling surfactant enhanced aquifer remediation, 1 formulation

We describe a three-dimensional, multicomponent, multiphase compositional finite-difference simulator for application to the analysis of contaminant transport and surfactant enhanced aquifer remediation (SEAR) of nonaqueous-phase liquid (NAPL) pollutants. Mixtures of surfactant, water and NAPL can form many types of micellar and microemulsion phases with a complex and important dependence on many variables of which the dilute aqueous solution typically assumed in SEAR models is just one example. The phase behavior model is central to our approach and allows for the full range of the commonly observed micellar and microemulsion behavior pertinent to SEAR. The other surfactant related properties such as adsorption, interfacial tension, capillary pressure, capillary number and microemulsion viscosity are all dependent on an accurate phase behavior model. This has proven to be a highly successful approach for surfactant enhanced oil recovery modeling, so it was adapted to SEAR modeling. However, there are many significant differences between petroleum and environmental applications of surfactants, so many new features have been added to model contaminant transport and remediation and these are described and illustrated for the first time here.     

Field demonstration of surfactant-enhanced solubilization of DNAPL at Dover Air Force Base, Delaware

This study reports on a surfactant-based flood for tetrachloroethylene (PCE) removal from a control test cell at the Dover National Test Site. The surfactant formulation (sodium dihexyl sulfosuccinate (Aerosol-MA or AMA), isopropanol and calcium chloride) was able to achieve a high concentration of PCE in swollen micelles (supersolubilization) without vertical PCE migration. The hydraulic system included eight screened wells that were operated in both vertical circulation and line drive configurations. After 10 pore volumes of flushing, the overall PCE removal was 68% (65% of which corresponded to the surfactant flooding alone). In addition, the residual PCE saturation was reduced from 0.7% to 0.2%, and the concentration of PCE in the groundwater was reduced from 37-190 mg/L before the flushing to 7.3 mg/L after flooding. Recycling the surfactant solution reduced the required surfactant mass (and thus cost, and waste) by 90%. Close to 80% of the total PCE removal was obtained during the first five pore volumes which were operated in an upward vertical circulation flow scheme. No free oil phase was observed during the test. Further analysis of multilevel sampler data suggests that most of the trapped oil remaining in the cell was likely localized in secluded regions of the aquifer, which helps explain the lower PCE groundwater concentration after remedial activities. In summary, this field study demonstrated the feasibility of surfactant-enhanced remediation to reduce the mass in the source zone and significantly reduce the PCE aqueous concentration and therefore the risk associated with the contaminant plume.