Enhanced air sparging for groundwater remediation using alginate gel-based removable hydraulic barriers.

The objective of this study was to investigate the effect of a removable physical barrier on the air sparging performance using a lab-scale aquifer model was investigated. The barrier was installed in water-saturated porous media, prior to the air sparging, by injecting calcium chloride aqueous solution into the aquifer with pre-applied alginate solution. Changes in the air flow direction and air flux at the media surface during air sparging were evaluated. With a hydrogel barrier set at the center of the media, the airflow detoured the barrier resulting in a bimodal air flux distribution at the media surface. While employing two gel-formed barriers positioned away from the media's center, the airflow concentrated specifically on the gap between the barriers. The hydrogel was successfully removed using a sodium bicarbonate solution (1.0 mol/L). Using the hydrogel barrier, the performance of air sparging was significantly enhanced for removing contaminants [tetrachloroethene (PCE) and n-hexane mixture] due to increased air flux; 9.8% of PCE applied (7.8 g) was removed during 120 min air sprging for the gel barrier system whereas no PCE was removed for the control. Alginate gel did not show significant sorption capacity for PCE. It was stable in the contaminant up to 68 days with reasonable loss of its mass. Findings of this study present a promising option for air sparging process specifically targeting the contaminant source zone in the aquifer.

Temporary hydraulic barriers using organic gel for enhanced aquifer remediation during groundwater flushing: Bench-scale experiments.

This study presents the use of organic gel-forming material for the construction of hydraulic barriers in aquifer, which can be easily removed after use. Experiments on the performance of the temporary hydraulic barrier during NAPL removal (aquifer flushing) were also conducted. An aqueous solution of sodium alginate was injected into the horizontally oriented, 2-dimensional flow chamber packed with sand, followed by gelation using a calcium solution. The alginate gel formed in the porous media produced a circular shape barrier (24 cm diameter, 1.3 cm thickness) that was successfully removed using sodium bicarbonate solution (1.0 M) in 72 h, whereas the gel was stable for 7 days during simulated groundwater flushing at the same flow rate as the sodium bicarbonate solution. When circular hydraulic barriers (12 cm diameter each, 14 cm apart) were set on either side of the NAPL (n-hexane and PCE mixture)-contaminated zone, the increased water flux during water flushing resulted in significantly increased PCE removal by almost 108%. When a surfactant solution (sodium dodecyl sulfate, 0.037%) was applied, the influenced groundwater flow controlled by hydraulic barriers on the NAPL removal was amplified by 196% removal.

Aquifer remediation using surfactant-enhanced gas sparging applied to target the contaminant source.

The surfactant-enhanced gas sparging process designed to specifically target the source zone of an organic contaminant in an aquifer with minimal usage of injected additives was investigated using a physical model. Aqueous solutions of the anionic surfactant Sodium dodecylbenzne sulfonate (SDBS) and/or the thickener Sodium carboxymethylcellulose (SCMC) were applied in a contaminated horizontal layer in the simulated laboratory aquifer model followed by gas sparging. Fluorescein sodium salt (FSS) was added to the SDBS/SCMC solutions and represented the organic contaminant. Air and ozone were injected to generate gas sparging. A modified surfactant-enhanced ozone sparging method was also tested by applying additional air venting ports installed in the aquifer above the gas injection zone. Both non-aqueous phase liquid (NAPL) and water-dissolved TCA were applied to the SDBS-applied region to evaluate the removal of contaminants during gas sparging. A significant expansion of the de-saturated zone for the SDBS-applied region was observed during air sparging. During ozone sparging, the fluorescence by FSS in the SDBS-applied layer disappeared over a much wider range than that of the control experiment. SCMC application enhanced the performance of the SDBS-applied gas sparging process. The TCA mass removed by volatilization during air sparging from the SDBS-applied layer was 2.3 times the application in the absence of SDBS. Among five regions of injected NAPL contamination located above the single gas injection port, and during 2 h of ozone sparging, with SDBS applied, more than 50% of fluorescence in the NAPL was removed, whereas under the same conditions with no SDBS applied, less than 30% was removed. Diverted gas flow through the venting ports installed in the aquifer model induced a horizontally expanded oxidative reaction zone during ozone sparging. This study demonstrates enhanced gas sparging performance for the removal of contaminants from the aquifer with limited usage of additives applied specifically to the source zone.

Remediation of NAPL-contaminated porous media using micro-nano ozone bubbles: Bench-scale experiments.

Aqueous solutions of micro-nano bubbles (MNBs) containing ozone gas were injected through a NAPL-contaminated glass bead column. The glass column (15 cm × 2.5 cm) was packed with glass beads: the first 12 cm was packed with coarse glass beads while much finer glass beads were used to pack the remaining 3.0 cm of the column. Decane was used as the representative NAPL, to which an oil-soluble fluorescence tracer was added. The fluorescence tracer was considered as a constituent of the NAPL that readily reacts with ozone. Air and ozone-containing oxygen were used to generate MNB solutions, and injected through the column. In addition, H2O2 was introduced to the O3-containing MNB (O3-MNB) solution to investigate the effect of hydroxyl free radicals on the NAPL removal. An ozone gas sparging experiment was also conducted for comparison. After 72 h of O3-MNB application, a significant mass of n-decane (27.6% of the initial mass applied) was removed from the column. H2O2 injection into the column during O3-MNB application was effective in increasing the n-decane mass removal by 22%, compared to the O3-MNB experiment. The rate of NAPL removal during O3-MNB flushing was significant, although slower than ozone sparging. During O3-MNB application, fast decay of fluorescence was observed; whereas, during co-injection of H2O2 and O3-MNB solutions, only a slight change in the fluorescence was observed. This indicates that oxidative degradation of NAPL during H2O2 and O3-MNB injection takes place only at the NAPL-water interface due to the reactivity of hydroxyl free radical, whereas ozone diffusion into NAPL induced the decay of the fluorescence tracer in the bulk NAPL. The removal characteristics during MNB application and ozone gas sparging were investigated based on the analysis of NAPL using mass spectrophotometer. When O3-MNB and H2O2 were co-injected, only n-decane was detected in the NAPL; while when O3-MNB was used for flushing, oxidative products were found in the NAPL. More hydrophilic compounds were found in the NAPL after ozone sparging. This implies different removal mechanisms depending on the kind of oxidation agent, and the state of oxidizing fluid. Based on the findings in this study, the application of O3-MNB could be a feasible option for cleaning up NAPL-contaminated aquifers.

Effect of increased groundwater viscosity on the remedial performance of surfactant-enhanced air sparging.

The effect of groundwater viscosity control on the performance of surfactant-enhanced air sparging (SEAS) was investigated using 1- and 2-dimensional (1-D and 2-D) bench-scale physical models. The viscosity of groundwater was controlled by a thickener, sodium carboxymethylcellulose (SCMC), while an anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), was used to control the surface tension of groundwater. When resident DI water was displaced with a SCMC solution (500 mg/L), a SDBS solution (200 mg/L), and a solution with both SCMC (500 mg/L) and SDBS (200 mg/L), the air saturation for sand-packed columns achieved by air sparging increased by 9.5%, 128%, and 154%, respectively, (compared to that of the DI water-saturated column). When the resident water contained SCMC, the minimum air pressure necessary for air sparging processes increased, which is considered to be responsible for the increased air saturation. The extent of the sparging influence zone achieved during the air sparging process using the 2-D model was also affected by viscosity control. Larger sparging influence zones (de-saturated zone due to air injection) were observed for the air sparging processes using the 2-D model initially saturated with high-viscosity solutions, than those without a thickener in the aqueous solution. The enhanced air saturations using SCMC for the 1-D air sparging experiment improved the degradative performance of gaseous oxidation agent (ozone) during air sparging, as measured by the disappearance of fluorescence (fluorescein sodium salt). Based on the experimental evidence generated in this study, the addition of a thickener in the aqueous solution prior to air sparging increased the degree of air saturation and the sparging influence zone, and enhanced the remedial potential of SEAS for contaminated aquifers.

Neutrophil extracellular traps (NETs) in autoimmune diseases: A comprehensive review.

Neutrophil extracellular traps (NETs) are fibrous networks which protrude from the membranes of activated neutrophils. NETs are found in a variety of conditions such as infection, malignancy, atherosclerosis, and autoimmune diseases including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), anti-neutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV), psoriasis, and gout. Studies suggest that an imbalance between "NETosis," which is a process by which NETs are formed, and NET degradation may be associated with autoimmune diseases. Neutrophils, interleukin-8, ANCA and other inflammatory molecules are considered to play a key role in NET formation. Prolonged exposure to NETs-related cascades is associated with autoimmunity and increases the chance of systemic organ damage. In this review, we discuss the roles of various inflammatory molecules in relation to NETs. We also describe the role of NETs in the pathogenesis of autoimmune diseases and discuss the possibility of using targeted therapies directed to NETs and associated molecules to treat autoimmune diseases.

Enhanced removal of NAPL constituent from aquifer during surfactant flushing with aqueous hydraulic barriers of high viscosity.

This study examines the effect of controlled groundwater flow paths induced by hydraulic barriers on the removal of NAPL constituent. An aqueous solution of thickener [0.05% (w/v) sodium carboxymethyl cellulose, SCMC] was continuously injected into a horizontally set two-dimensional physical model (sand-packed), forming aqueous plume(s) of high viscosity. The water flux at the down gradient of the model was measured using a flux tracer (n-octanol) and passive flux meter (PFM, packs of granular activated carbon). A non-reactive tracer (pentafluorobenzoic acid, PFBA) was used to identify the plume of high viscosity (hydraulic barrier) and ambient groundwater. When the barrier of high viscosity was formed, the plume was separated from the background water with little mixing, which was confirmed by the concentration profile of PFBA; whereas, the measured flux of ambient groundwater showed a distinctive distribution, due to the hydraulic barrier. When two barriers were set, the ambient water flux was enhanced in the middle, and the removal rate of PCE from the non-aqueous phase liquid (NAPL), measured by PFM, was found to improve by 26% during three hours of water flushing. When an aqueous solution of surfactant [0.37% (w/v), sodium dodecyl sulfate, SDS] was applied instead of water into the domain with two barriers set around the NAPL-contaminated spot, the removal of PCE from the NAPL increased by 101% for a three-hour time period. Based on the observations made in this study, hydraulic barriers formed by continuous injection of thickener solution change the flow direction of groundwater, and may increase the flux of groundwater (or aqueous solution of remediation agent) through a NAPL-contaminated region, improving the removal of NAPL.

Enhanced removal of VOCs from aquifers during air sparging using thickeners and surfactants: Bench-scale experiments.

The effects of controlled air flow paths during air sparging on the removal of volatile organic compounds were examined in this study using a two-dimensional bench-scale physical model. An aqueous solution of sodium carboxymethylcellulose (SCMC), which is a thickener, was used to increase the resistance of water to displacement by injected air in a region around the targeted zone. At the same time, an aqueous solution of sodium dodecylbenzene sulfonate (SDBS), which is a surfactant, was used to reduce the air entry pressure to enhance the air flow through the targeted region. Trichloroethene (TCE), dissolved in water, was used to represent an aqueous phase volatile organic compound (VOC). A binary mixture of perchloroethene (PCE) and n-hexane was also used as a nonaqeous phase liquid (NAPL). Controlled air flow through the source zone, achieved by emplacing a high viscosity aqueous solution into a region surrounding the TCE-impacted zone, resulted in increased TCE removal from 23.0% (control) to 38.2% during a 2.5h period. When the air flow was focused on the targeted source zone of aqueous phase TCE (by decreasing the surface tension within the source zone and its vicinity by 28 dyn/cm, no SCMC applied), the mass removal of TCE was enhanced to 41.3% during the same time period. With SCMC and SDBS applied simultaneously around and beneath a NAPL source zone, respectively, the NAPL components were found to be removed more effectively over a period of 8.2h than the sparging experiment with no additives applied; 84.6% of PCE and 94.0% of n-hexane were removed for the controlled air flow path experiments (with both SCMC and SDBS applied) compared to 52.7% (PCE) and 74.0% (n-hexane) removal for the control experiment (no additives applied). Based on the experimental observations made in this study, applying a viscous aqueous solution around the source zone and a surfactant solution in and near the source zone, the air flow was focused through the targeted contaminant zone, enhancing the removal of VOCs from either an aqueous phase or a NAPL phase.

Changes in air flow patterns using surfactants and thickeners during air sparging: bench-scale experiments.

Air injected into an aquifer during air sparging normally flows upward according to the pressure gradients and buoyancy, and the direction of air flow depends on the natural hydrogeologic setting. In this study, a new method for controlling air flow paths in the saturated zone during air sparging processes is presented. Two hydrodynamic parameters, viscosity and surface tension of the aqueous phase in the aquifer, were altered using appropriate water-soluble reagents distributed before initiating air sparging. Increased viscosity retarded the travel velocity of the air front during air sparging by modifying the viscosity ratio. Using a one-dimensional column packed with water-saturated sand, the velocity of air intrusion into the saturated region under a constant pressure gradient was inversely proportional to the viscosity of the aqueous solution. The air flow direction, and thus the air flux distribution was measured using gaseous flux meters placed at the sand surface during air sparging experiments using both two-, and three-dimensional physical models. Air flow was found to be influenced by the presence of an aqueous patch of high viscosity or suppressed surface tension in the aquifer. Air flow was selective through the low-surface tension (46.5 dyn/cm) region, whereas an aqueous patch of high viscosity (2.77 cP) was as an effective air flow barrier. Formation of a low-surface tension region in the target contaminated zone in the aquifer, before the air sparging process is inaugurated, may induce air flow through the target zone maximizing the contaminant removal efficiency of the injected air. In contrast, a region with high viscosity in the air sparging influence zone may minimize air flow through the region prohibiting the region from de-saturating.

Standardization of the reducing power of zero-valent iron using iodine.

Because iron-based materials that are used for the permeable reactive barrier systems come in various shapes, sizes, and with various surface properties depending on the manufacturing sources, their reductive powers vary in a wide spectrum. A new experimental procedure to evaluate the reductive power of iron material was developed in this study. Tri-iodide (I3(-)) was used as the representative oxidizing agent that reacts with zero-valent iron (ZVI). Three iron-based materials (two scraps, two powders) and four chlorinated chemicals [perchloroethene (PCE), trichloroethene (TCE), 1,1,1-trichloroethane (TCA), and pentachlorophenol (PCP)] were used in this study. Redox reactions were conducted in glass vials containing aqueous solutions of chlorinated compounds or tri-iodide with known masses of iron material. After a predetermined reaction time each vial was opened and the solution was analyzed for the concentration of reduced compound. The apparent rate contant (k(i)(obs)) of iodine reduction reaction with ZVIs was found to be proportional to that (k(c)(obs)) of chlorinated contaminant. The surface area-normalized reduction rate constants (k(c)(nor)) for contaminants and tri-iodide (k(i)(nor)) were also proportional to each other. The ratio of rate constants, K(nor) (= k(c)(nor)/k(i)(nor)) was estimated for each contaminant; 3.29 × 10(-7), 5.86 × 10(-7), 6.70 × 10(-7), and 7.87 × 10(-10) M, for PCE, TCE, TCA, and PCP, respectively. The results of this study suggest that the reductive power of ZVI materials can be standardized using tri-iodide, and thus, can provide a good reference for the quantitative assessment of the reactivity of metallic reducing agents of environmental interest including ZVIs.

The N-terminal methionine of cellular proteins as a degradation signal.

The Arg/N-end rule pathway targets for degradation proteins that bear specific unacetylated N-terminal residues while the Ac/N-end rule pathway targets proteins through their N(α)-terminally acetylated (Nt-acetylated) residues. Here, we show that Ubr1, the ubiquitin ligase of the Arg/N-end rule pathway, recognizes unacetylated N-terminal methionine if it is followed by a hydrophobic residue. This capability of Ubr1 expands the range of substrates that can be targeted for degradation by the Arg/N-end rule pathway because virtually all nascent cellular proteins bear N-terminal methionine. We identified Msn4, Sry1, Arl3, and Pre5 as examples of normal or misfolded proteins that can be destroyed through the recognition of their unacetylated N-terminal methionine. Inasmuch as proteins bearing the Nt-acetylated N-terminal methionine residue are substrates of the Ac/N-end rule pathway, the resulting complementarity of the Arg/N-end rule and Ac/N-end rule pathways enables the elimination of protein substrates regardless of acetylation state of N-terminal methionine in these substrates.

Surfactant-enhanced ozone sparging for removal of organic compounds from sand.

An innovative surfactant-enhanced ozone sparging (SEOS) technique was developed in this study. The synergistic effect of simultaneous surfactant and ozone application on the removal of organic contaminants in an aquifer during air sparging was investigated. Using laboratory-scale one- and two-dimensional physical models packed with water-saturated sand, air sparging and ozone sparging were implemented either at high or low level surface tension of the groundwater. A water-dissolved chemical (fluorescein sodium salt) and a nonaqueous phase liquid (n-decane) were used as the representative contaminants. Sodium dodecylbenzene sulfonate was used for sparging experiments at low level surface tension. Ozone sparging at low surface tension (SEOS) was found to be the most efficient process for the removal of organic chemicals, among AS (air sparging at high surface tension), SEAS (surfactant-enhanced air sparging, air sparging at low surface tension), and OS (ozone sparging at high surface tension), based on the results from a one-dimensional column study. Two-dimensional model experiments also showed that SEOS is more efficient than conventional AS processes. The increased air saturation and sparging influence zone achieved by surfactant application, and the oxidative power of ozone are responsible for the enhanced removal of contaminants from the aquifer. Considering that the application of conventional AS is limited to volatile contaminants, and that OS has a very narrow influence zone, SEOS can be an useful option for the removal of contaminants of low vapor pressures from an expanded zone of influence.

Measurement of air and VOC vapor fluxes during gas-driven soil remediation: bench-scale experiments.

In this laboratory study, an experimental method was developed for the quantitative analyses of gas fluxes in soil during advective air flow. One-dimensional column and two- and three-dimensional flow chamber models were used in this study. For the air flux measurement, n-octane vapor was used as a tracer, and it was introduced in the air flow entering the physical models. The tracer (n-octane) in the gas effluent from the models was captured for a finite period of time using a pack of activated carbon, which then was analyzed for the mass of n-octane. The air flux was calculated based on the mass of n-octane captured by the activated carbon and the inflow concentration. The measured air fluxes are in good agreement with the actual values for one- and two-dimensional model experiments. Using both the two- and three-dimensional models, the distribution of the air flux at the soil surface was measured. The distribution of the air flux was found to be affected by the depth of the saturated zone. The flux and flux distribution of a volatile contaminant (perchloroethene) was also measured by using the two-dimensional model. Quantitative information of both air and contaminant flux may be very beneficial for analyzing the performance of gas-driven subsurface remediation processes including soil vapor extraction and air sparging.

Laboratory evaluation of surfactant-enhanced air sparging for perchloroethene source mass depletion from sand.

Surfactant-enhanced air sparging (SEAS) was evaluated in this laboratory-scale study to assess: (i) the removal efficiency of volatile contaminant from an aquifer model contrasted to conventional air sparging; and (ii) the effect of mass removal of dense non-aqueous phase liquid (DNAPL) during air sparging on the changes in aqueous flux of dissolved DNAPL. We conducted sparging experiments to remove perchloroethene (PCE) sources from laboratory flow chambers packed with sand. PCE was emplaced in rectangular zones at three locations within the flow chamber. The resident water was supplemented with the anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), to reduce the surface tension of water, and then sparged with nitrogen gas at a constant flow rate of 0.12 L/min. It was found that SEAS was significantly more efficient than conventional air sparging for removing PCE. For SEAS, about 78% and 75% of total PCE mass was depleted from the flow chamber at a surface tension of 52.2 dynes/cm (350 mg/L SDBS) and 63.1 dynes/cm (150 mg/L SDBS), respectively, whereas only 38% was removed at 72.5 dyne/cm (no SDBS added). Before and after sparging, PCE mass flux in the aqueous phase during steady water flow through the chamber was measured in the flow chambers. Post-SEAS PCE fluxes were reduced, but not in direct proportion to the reduction in PCE mass.

Measurement of gas-accessible NAPL saturation in soil using gaseous tracers.

In this laboratory study, a new experimental method involving the use of a set of four gaseous tracers, was developed for measuring the NAPL saturation directly accessible to the mobile gas as well as the total NAPL saturation in unsaturated sand. One tracer with low water solubility (n-pentane) was used as the tracerthat partitions into NAPL directly accessible to the mobile gas, and another (chloroform)tracer with moderate water solubility and NAPL-partitioning, was selected for detecting total NAPL saturation. Helium and difluoromethane were used as the nonreactive and water-partitioning tracers, respectively. A saturated hydrocarbon, n-decane, was used as NAPL. Column experiments were conducted attwo water saturations (Sw = 0.68 and 0.16). The total NAPL saturation and NAPL saturation not directly accessible to the mobile gas were also successfully measured using the combined results of tracer experiments. At Sw = 0.68, only 28% of the total NAPL was detected by n-pentane, whereas 87% of the total NAPL was accessible to n-pentane at Sw = 0.16, implying more NAPL was accessible to the mobile gas phase at lower water saturation.

Effect of surface tension reduction on VOC removal during surfactant-enhanced air sparging.

In this study, the effect of decreased surface tension of water on the removal efficiency of a volatile organic compound (VOC) during air sparing was evaluated using a laboratory-scale, two-dimensional physical model packed with sand and water containing dissolved toluene as a representative VOC. Two sets of air sparging experiments were performed: the first at a surface tension of 69 dyne/cm with no surfactant applied, and with a toluene concentration of 110 mg/L; the second at a surface tension of 50 dyne/cm due to 110 mg/L of anionic surfactant, and 99 mg/L of toluene. Under the experimental conditions used in this study, the sparging influence zone estimated at the lower surface tension was about 2.5 times that estimated at the high surface tension. Also the rate of toluene removal by air sparging was found to be much faster at the lower surface tension. The sparging time required for 50% removal of the initial mass of toluene was 16.8 hours at 50 dyne/cm, which was much smaller than 82.5 hours measured at 69 dyne/cm. The final mass recoveries at low and high surface tensions were 92.1% and 56.6%, respectively. Therefore it was concluded that air sparging at reduced surface tension (surfactant-enhanced air sparging) greatly improved the removal efficiency of VOCs from the porous medium compared to conventional air sparging with no applied surfactant.

Changes in air saturation and air-water interfacial area during surfactant-enhanced air sparging in saturated sand.

Reduction in the surface tension of groundwater, prior to air sparging for removal of volatile organic contaminant from aquifer, can greatly enhance the air content and the extent of influence when air sparging is implemented. However, detailed information on the functional relationship between water saturation, air-water contact area induced by air sparging and the surface tension of water has not been available. In this study, the influence of adding water-soluble anionic surfactant (sodium dodecyl benzene sulfonate) into groundwater before air sparging on the air-water interfacial area and water saturation was investigated using a laboratory-scale sand packed column. It was found that water saturation decreases with decreasing surface tension of water until it reaches a point where this trend is reversed so that water saturation increases with further decrease in the surface tension. The lowest water saturation of 0.58 was achieved at a surface tension of 45.4 dyn/cm, which is considered as the optimum surface tension for maximum de-saturation for the initially water-saturated sand used in this study. The air-water contact area generated in the sand column due to air sparging was measured using a gaseous interfacial tracer, n-decane, and was found to monotonically increase with decreasing water saturation. The results of this study provide useful design information for surfactant-enhanced air sparging removal of volatile contaminants from aquifers.

Surfactant-enhanced air sparging in saturated sand.

Air sparging as a subsurface remedial technique can be enhanced bythe addition of a surfactant. The effect of reduced surface tension of water on the extent of air intrusion and air saturation during air sparging in porous media was investigated. A sand column and a two-dimensional sand box were used for the experiments. The surface tension was controlled using an anionic surfactant, sodium dodecyl benzene sulfonate, and the concentration used was below the critical micelle concentration. Using the sand column, the air saturation was measured at different surface tensions and at different airflow rates. Initially water-saturated, the air saturation achieved in the column by air sparging at a surface tension of 3.42 x 10(-2) N/m was up to 5 times larger than that of water with no surfactant. Atthe same time, the rate at which the air saturation increased as a function of airflow rate was greater at reduced surface tensions. For box experiments with homogeneous sand, reduction of the surface tension caused a dramatic increase in the sparging area up to 5.2 times of that generated using water with no surfactant. A sand box experiment containing a vertical channel produced preferential flow of the air phase injected at the bottom of the channel when the surfactant was not applied. However, reducing the surface tension was found to promote airflow through the preferential channel and the finer sand surrounding the channel. These observations support the use of low concentration surfactants to improve air sparging swept zones.