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.

Air sparging remediation of VOCs contaminated low-permeability soil based on pressure gradient control

Air sparging (AS) is deemed unacceptable for remediating VOCs contaminated soil with low-permeability. To improve air flow and contaminant removal in sparging process, an original approach, termed as pressure gradient-enhanced air sparging (PGEAS) approach, is proposed by controlling pressure gradient in soil. Then the remediation efficiency, mass transfer characteristics, and remediation mechanism are investigated. Results showed that, the PGEAS approach accelerates gaseous contaminant exhaust, reduces residue contamination in soil, and promotes total contaminant removal, finally results in an improved remediation efficiency compared to the conventional approach. Controlled by sparging pressure and flow distance, the pressure gradient is created in soil, and a critical value needs to be exceeded to enhance the VOCs removal and mass transfer characteristics. The measured results of pore pressure and liquid saturation confirm a notable pressure gradient and drainage behavior in soil, which indicate the massive air subchannel formation during air sparging. At a two-dimensional scale, discrete distributions of contaminant concentrations in exhaust air and soil are presented, the removal extent and area are both enhanced using the PGEAS approach with a pressure gradient higher than the critical value. The reached conclusions are of great importance to contaminant removal in heterogeneous stratigraphy at sites.

Zone of flow: A new finding on the characteristics of airflow within the zone of influence during air sparging in aquifers

Air sparging (AS) is a popular technology for the in-situ remediation of groundwater contaminated by volatile organic compounds. The scope of the zone within which injected air exists, i.e., zone of influence (ZOI) and the airflow characteristics within ZOI are of great interest. However, few studies have shed light on the scope of the zone within which air flows, namely, the zone of flow (ZOF) and its relation with the scope of ZOI. This study focuses on the ZOF characteristics and its relation with ZOI based on quantitative observations of ZOF and ZOI using a quasi-2D transparent flow chamber. The relative transmission intensity obtained by the light transmission method presents a rapid and continuous increasing near the ZOI boundary, providing a criterion for the quantitative determination of ZOI. An integral airflow flux approach is proposed to determine the scope of ZOF based on the airflow flux distributions through aquifers. The ZOF radius decreases with the growth of particle sizes of aquifers; while it increases first and then keeps constant with the increase of sparging pressure. The ZOF radius is around 0.55- 0.82 times of the ZOI radius, which depends on air flow patterns related to particle diameters d_p, that is, 0.55- 0.62 for channel flow (d_p < 1- 2 mm), while 0.75- 0.82 for bubble flow (d_p > 2- 3 mm). The experimental results show that the sparged air is entrapped with little flowing inside ZOI regions that are outside the ZOF, which should be considered carefully in the design of AS.

Mass transfer enhancement of air sparging on VOCs contaminated low-permeability soil by establishing pressure gradient

As one of the most effective methods for remediating VOCs contaminated site, air sparging technology is not suitable to low-permeability soil due to the poor remediation efficiency. To solve this problem, an improved approach aiming for mass transfer enhancement by establishing pressure gradient in soil is proposed in this study, and the remediation efficiency, removal mechanism, as well as the mass transfer characteristic are comprehensively investigated. Test results showed that, using the proposed approach significantly reduced the time for exhaust air contaminants reaching concentration equilibrium, and improved the contaminant removal zone and extent in soil, which were especially strengthened at sparging pressures higher than 40 kPa. The total contaminant removal rate was improved by introducing the proposed approach, with a maximum improved removal rate of 23.7% at 100 kPa sparging pressure. In mechanism analysis, the recorded changes in total pore pressure and average liquid saturation illustrated the pressure drop and discrete drainage phenomena, confirming the pressure gradient and air sub-channels formed in low-permeability soil. Finally, contaminant mass transfer characteristic was quantitatively analyzed using the lumped parameter model, in which the mass transfer coefficient and the air channel influencing fraction were enhanced almost fourfold and fivefold respectively by introducing the proposed approach. Compared to the conventional approach, the improved remediation efficiency using the proposed approach tackled the in-situ remediation challenge on low-permeability soil, and further expanded the application scope of air sparging technology on VOC contaminated site.

Effects of different permeable lenses on nitrobenzene transport during air sparging remediation in heterogeneous porous media

Air sparging (AS) is considered an effective remediation technology for groundwater contaminated by volatile organic compounds. However, the effects of AS remediation of heterogeneous aquifers with lenses of different permeability are still unclear, which limits the application of AS technology. In this study, the effects of different permeable lenses on nitrobenzene (NB) transport were quantitatively analysed by tracking the temporal and spatial evolutions of the NB concentration and using light transmission visualisation technology to observe airflow. Experimental results showed that the NB outside the airflow zone of the heterogeneous aquifer containing a gravel lens was rapidly removed, which is a special phenomenon. Through moisture content monitoring and colour tracer technology, the bubble-induced water circulation zone in a gravel lens was discovered during AS. At this time, the zone of influence (ZOI) included air flow zone and water circulation zone, while previous studies believed that the ZOI only contained the air flow zone. The presence of water circulation zone in the heterogeneous aquifer with a gravel lens increased the ZOI area and average contaminant removal flux by 5 and 2.3 times, respectively, compared with those in homogeneous aquifer. These findings have modified the conventional cognition about the ZOI and are conducive to an in-depth understanding of the remediation mechanisms and a better design of AS technology in heterogeneous aquifers with different permeable lenses.

Effects of gas sparging and mechanical mixing on sonochemical oxidation activity

The effects of air sparging (0-16 L min^-1) and mechanical mixing (0-400 rpm) on enhancing the sonochemical degradation of rhodamine B (RhB) was investigated using a 28 kHz sonoreactor. The degradation of RhB followed pseudo first-order kinetics, where sparging or mixing induced a large sonochemical enhancement. The kinetic constant varied in three stages (gradually increased → increased exponentially → decreased slightly) as the rate of sparging or mixing increased, where the stages were similar for both processes. The highest sonochemical activity was obtained with sparging at 8 L min^-1 or mixing at 200 rpm, where the standing wave field was significantly deformed by sparging and mixing, respectively. The cavitational oxidation activity was concentrated at the bottom of the sonicator when higher sparging or mixing rates were employed. Therefore, the large enhancement in the sonochemical oxidation was attributed mainly to the direct disturbance of the ultrasound transmission and the resulting change in the cavitation-active zone in this study. The effect of the position of air sparging and mixing was investigated. The indirect inhibition of the ultrasound transmission resulted in less enhancement of the sonochemical activity. Moreover, the effect of various sparging gases including air, N₂, O₂, Ar, CO₂, and an Ar/O₂ (8:2) mixture was compared, where all gases except CO₂ induced an enhancement in the sonochemical activity, irrespective of the concentration of dissolved oxygen. The highest activity was obtained with the Ar/O₂ (8:2) mixture. Therefore, it was revealed that the sonochemical oxidation activity could be further enhanced by applying gas sparging using the optimal gas.

Effects of air flowrate distribution and benzene removal in heterogeneous porous media during air sparging remediation

The decrease of remediation effect during air sparging (AS) remediation in heterogeneous porous media has attracted increasing attention. In this study, an improved light transmission visualization method was used to investigate the air accumulation, migration and flowrate distribution in benzene-contaminated heterogeneous porous media during AS. Experimental results indicated that the benzene removal rate in the porous media was mainly controlled by air flowrate distribution which could be used as a major factor to evaluate the remediation effect. Visualization of air migration showed that air accumulation occurred below the geologic heterogeneous interface when ΔPe > 0 kPa (ΔPe: the air entry pressure difference of the media above and below the interface), and the accumulation thickness and length presented exponential decay increases with increasing ΔPe and air injection rates. Air flowrate was monitored by gas flow sensors, and the flowrate distributions were found as Gaussian distribution when ΔPe ≤ 0 kPa, trapezoidal distribution when 0 <ΔPe< 0.3 kPa and fingered distribution when ΔPe ≥ 0.3 kPa. Fingered distribution of air flowrate resulted in extremely nonuniform benzene removal above the interface and reduced the overall benzene removal rate. These findings reveal the reasons for the poor performance of AS remediation in heterogeneous porous media, leading to a better understanding of the remediation mechanisms in heterogeneous aquifer.

Mechanism study of the air migration and flowrate distribution in an aquifer with lenses of different permeabilities during air sparging remediation

The poor performance of air sparging (AS) remediation in heterogeneous porous media is receiving increasing attention. However, understanding of the air migration and flowrate distribution mechanisms in heterogeneous aquifers is still lacking. In this study, for experimental purposes, a heterogeneous aquifer with lenses of different permeabilities was designed in the laboratory. The effects of the double interface between a lens and the background media on the air migration were visually observed for the first time, and four types of double interfaces and their related air flowrate distributions were identified. These were bimodal distribution (∆P_e ≤ -1.1 kPa, i.e., the air entry suction difference between the background media and the lens), fingered distribution for a low-permeability lens (-1.1 <∆P_e ≤ -0.3 kPa), Gaussian distribution (-0.3 <∆P_e < 0.4 kPa), and fingered distribution for a high-permeability lens (∆P_e ≥ 0.4 kPa). The experimental results indicated that double interface characteristics and air injection rates affected air accumulation behavior. A mathematical model was established to simulate the experimental data of the air flowrate distribution, and it could well describe the air flowrate distribution patterns in heterogeneous aquifers. These findings are significant for improving our understanding of the mechanisms of air migration and flowrate distribution in heterogeneous aquifers, leading to a better design and prediction of the AS remediation required for heterogeneous aquifer pollution.

Study on the effects of alcohol-enhanced air sparging remediation in a benzene-contaminated aquifer: a new insight

In this study, the effects of medium carbon chain alcohol (1-heptanol)-enhanced air sparging (AS) on the remediation of benzene-contaminated aquifers in different media (medium sand, channelized flow; gravel, bubbly flow) were investigated by comparison with a commonly used surfactant (sodium dodecylbenzene sulfonate (SDBS)). The results showed that the addition of 1-heptanol and SDBS significantly increased the air saturation in AS process under different airflow modes. Combined with water retention curves, 1-heptanol had the same effect on reducing the surface tension of groundwater and stabilizing bubbles as SDBS. In the study of benzene pollution removal, when the removal efficiency of the benzene pollutant exceeded 95%, the time required for surfactant-enhanced AS (SEAS) and alcohol-enhanced AS (AEAS) in medium sand was shortened by 28.6% and 52.4%, respectively, and the time required for SEAS and AEAS in gravel media was shortened by 16.7% and 58.3%, respectively, compared with the time required for AS. This finding indicated that the addition of SDBS or 1-heptanol could significantly increase the removal rate of benzene pollutants. Under the same surface tension conditions, the removal effect of 1-heptanol on the benzene pollutant was better than that of SDBS. This difference was due to the disturbance of the flow field during AEAS process causing the 1-heptanol on the gas-liquid interface to volatilize in the carrying gas, thereby inducing Marangoni convection on the interface, enhancing the gas-liquid mass transfer rate, and increasing the removal rate of benzene on the interface. Therefore, 1-heptanol is promising as a new reagent to enhance AS to remediate groundwater pollution.

Enhancement of sonochemical oxidation reactions using air sparging in a 36 kHz sonoreactor

The effect of air sparging on sonochemical oxidation reactions was investigated using a relatively large reactor equipped with a 36 kHz transducer module at the bottom. KI dosimetry and luminol techniques were used for quantitative and qualitative analysis of the reactions. The cavitation yield increased and then varied minimally as the liquid height increased from 1λ (42 mm) to 8λ (333 mm) with no air sparging. The flow rate of the air used for sparging and the position of the sparger significantly affected the extent of the sonochemical oxidation reactions. A significant enhancement in the sonochemical oxidation by air sparging was observed for higher liquid height and higher flow rate conditions at a constant input power. This enhancement is attributed to the violent mixing effect and the significant change in the sound field and cavitation-active zone in the liquid. Higher sonochemical activity was obtained when air sparging was applied closer to the transducer module at a higher flow rate. Imaging the motion of the liquid surface and sonochemiluminescence revealed that the instability of the liquid body was directly related to the sonochemical activity.

Electromagnetic waves' effect on airflow during air sparging

Air sparging is a popular, yet slow, remediation technology for soil and groundwater contaminated with volatile organic compounds (VOCs). This paper theoretically and experimentally studies the effect of electromagnetic (EM) waves on air-channel formation within a glass-bead medium-used as an analogy to soil-during air-sparging experiments. The impact of EM waves on cleanup is not the focus of this paper, and the impact on airflow may or may not positively impact resulting cleanup process using air sparging to remove VOCs through volatilization. The hypothesis is that dielectrophoretic forces by EM waves can be used to alter airflow. Air injection was performed at different pressures, in the presence of EM waves (referred to as EM-stimulated) of various power and frequencies and the absence of EM waves (referred to as unstimulated). Digital images of the airflow patterns were collected, processed, and analyzed for all tests. The shape of the zone of influence (ZOI) was observed, and the radius of the zone of influence (ROI) was measured, which showed a 16% increase in ROI due to EM stimulation. An experimentally validated numerical simulation of the electric-field component of EM waves was developed. The correlation between EM-wave and air sparging characteristics were then studied using the numerical simulation and acquired digital images of the airflow to investigate and validate that the dielectrophoretic mechanism is behind the EM effect on airflow.

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.

Increase in volatilization of organic compounds using air sparging through addition in alcohol in a soil-water system

This study developed a novel method to promote the remediation efficiency of air sparging. According to the enhanced-volatilization theory presented in this study, selected alcohols added to groundwater can highly enhance the volatilization amounts of organic compounds with high Henry's law constants. In this study, the target organic compounds consisted of n-hexane, n-heptane, benzene, toluene, 1,1,2-trichloroethane, and tetrachloroethene. n-pentanol, n-hexanol, and n-heptanol were used to examine the changes in the volatilization amounts of organic compounds in the given period. Two types of soils with high and low organic matter were applied to evaluate the transport of organic compounds in the soil-water system. The volatilization amounts of the organic compounds increased with increasing alcohol concentrations. The volatilization amounts of the test organic compounds exhibited a decreasing order: n-heptanol>n-hexanol>n-pentanol. When 10mg/L n-heptanol was added to the system, the maximum volatilization enhancement rate was 18-fold higher than that in distilled water. Samples of soil with high organic matter might reduce the volatilization amounts by a factor of 5-10. In the present study, the optimal removal efficiency for aromatic compounds was approximately 98%.

A mechanism study of airflow rate distribution within the zone of influence during air sparging remediation

In this study, an improved laboratory two-dimensional airflow visualization device was developed for the quantitative analysis of airflow distribution at different heights from the sparger (20, 30, and 40cm) within the zone of influence (ZOI). The results indicated that the measured airflow rate distribution appeared Trapezium when the height was 20cm; however, the airflow rate matched a Gaussian distribution when the heights became 30cm and 40cm. The conical shape of the ZOI was observed in the experimental processes. The experimental results verified that the airflow distribution within the ZOI conformed to turbulent jet theory. According to turbulent jet theory, the distribution of the airflow rate changes from Trapezium to Gaussian, and the jet boundary mixed layer is a linear extension in the processes of jets. Through our study, it was found that this theory could be applied to airflow distribution and predictive models for the ZOI in air sparging remediation. The shape of the ZOI should be cone-like and the boundary layer of the ZOI is a linear extension in air sparging process. All the results from this study can provide theoretical support for the design and prediction of air sparging remediation for groundwater pollution.

Applying short-duration pulses as a mean to enhance volatile organic compounds removal by air sparging

Application of short-duration pulses of high air pressure, to an air sparging system for groundwater remediation, was tested in a two-dimensional laboratory setup. It was hypothesized that this injection mode, termed boxcar, can enhance the remediation efficiency due to the larger ZOI and enhanced mixing which results from the pressure pulses. To test this hypothesis, flow and transport experiments were performed. Results confirm that cyclically applying short-duration pressure pulses may enhance contaminant cleanup. Comparing the boxcar to conventional continuous air-injection shows up to a three-fold increase in the single well radius of influence, dependent on the intensity of the short-duration pressure-pulses. The cleanup efficiency of Toluene from the water was 95% higher than that achieved under continuous injection with the same average conditions. This improvement was attributed to the larger zone of influence and higher average air permeability achieved in the boxcar mode, relative to continuous sparging. Mixing enhancement resultant from recurring pressure pulses was suggested as one of the mechanisms which enhance the contaminant cleanup. The application of a boxcar mode in an existing, multiwell, air sparging setup can be relatively straightforward: it requires the installation of an on-off valve in each of the injection-wells and a central control system. Then, turning off some of the wells, for a short-duration, result in a stepwise increase in injection pressure in the rest of the wells. It is hoped that this work will stimulate the additional required research and ultimately a field scale application of this new injection mode.

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.

Predictive models and airflow distribution associated with the zone of influence (ZOI) during air sparging remediation

Laboratory two-dimensional airflow visualisation model tests were conducted to assess the effect of particle size and air injection pressure on airflow patterns, physical characteristics of the zone of influence (ZOI) and the airflow rate distribution within the ZOI. The results indicate that the pattern transitions from chamber flow to channelized flow and then to bubbly flow occurred at effective particle sizes (D10) in the ranges 0.22-0.42 mm and 1.42-2.1mm, respectively. The ZOI is shaped like a conical frustum, and there exists a "stable ZOI" for each type of porous medium in channelised and bubbly flow during sparging tests. A formula for calculating the size of the ZOI radius was established based on the conical frustum-shaped results and the "stable ZOI", and comparing the calculated results with field data demonstrated that the formula has application value, except in large-scale heterogeneous aquifers. The distribution of the airflow rate within the ZOI, which is quite uneven, varies from the maximum rate (which occurred just above the sparger) to zero with the increase of the lateral distance from the sparger. Moreover, the airflow distribution can be fitted using a unified dimensionless Gaussian function under different sparging pressures for a given porous medium. All of the results described above provide valuable information for the design and theoretical modelling of air sparging for groundwater remediation.

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.

Evaluation of air sparging and vadose zone aeration for remediation of iron and manganese-impacted groundwater at a closed municipal landfill

High concentrations of iron (Fe(II)) and manganese (Mn(II)) reductively dissolved from soil minerals have been detected in groundwater monitoring wells near many municipal solid waste landfills. Air sparging and vadose zone aeration (VZA) were evaluated as remedial approaches at a closed, unlined municipal solid waste landfill in Florida, USA. The goal of aeration was to oxidize Fe and Mn to their respective immobile forms. VZA and shallow air sparging using a partially submerged well screen were employed with limited success (Phase 1); decreases in dissolved iron were observed in three of nine monitoring wells during shallow air sparging and in two of 17 wells at VZA locations. During Phase 2, where deeper air sparging was employed, dissolved iron levels decreased in a significantly greater number of monitoring wells surrounding injection points, however no radial pattern was observed. Additionally, in wells affected positively by air sparging (mean total iron (FeTOT) <4.2mg/L, after commencement of air sparging), rising manganese concentrations were observed, indicating that the redox potential of the groundwater moved from an iron-reducing to a manganese-reducing environment. The mean FeTOT concentration observed in affected monitoring wells throughout the study was 1.40 mg/L compared to a background of 15.38 mg/L, while the mean Mn concentration was 0.60 mg/L compared to a background level of 0.27 mg/L. Reference wells located beyond the influence of air sparging areas showed little variation in FeTOT and Mn, indicating the observed effects were the result of air injection activities at study locations and not a natural phenomenon. Air sparging was found effective in intercepting plumes of dissolved Fe surrounding municipal landfills, but the effect on dissolved Mn was contrary to the desired outcome of decreased Mn groundwater concentrations.

In situ treatment of arsenic-contaminated groundwater by air sparging

Arsenic contamination of groundwater is a major problem in some areas of the world, particularly in West Bengal (India) and Bangladesh where it is caused by reducing conditions in the aquifer. In situ treatment, if it can be proven as operationally feasible, has the potential to capture some advantages over other treatment methods by being fairly simple, not using chemicals, and not necessitating disposal of arsenic-rich wastes. In this study, the potential for in situ treatment by injection of compressed air directly into the aquifer (i.e. air sparging) is assessed. An experimental apparatus was constructed to simulate conditions of arsenic-rich groundwater under anaerobic conditions, and in situ treatment by air sparging was employed. Arsenic (up to 200 μg/L) was removed to a maximum of 79% (at a local point in the apparatus) using a solution with dissolved iron and arsenic only. A static "jar" test revealed arsenic removal by co-precipitation with iron at a molar ratio of approximately 2 (iron/arsenic). This is encouraging since groundwater with relatively high amounts of dissolved iron (as compared to arsenic) therefore has a large theoretical treatment capacity for arsenic. Iron oxidation was significantly retarded at pH values below neutral. In terms of operation, analysis of experimental results shows that periodic air sparging may be feasible.