Effects of size and composition of bitumen drops on intra-oil diffusion and dissolution of hydrocarbon solvents in froth treatment tailings ponds.

The rate of mass transfer of lower molecular weight hydrocarbons (naphtha) from bitumen drops in mature fine tailings of oil sand tailings ponds (OSTPs) may control their bioavailability and the associated rate of GHG production. Experiments were conducted using bitumen drops spiked with o-xylene and 1-methylnaphthalene to determine the mass transfer rate of these naphtha components from bitumen drops. The results were compared to simulations using a multi-component numerical model that accounted for transport in the drop and across the oil-water interface. The results demonstrate rate-limited mass transfer, with aqueous concentrations after 60 days of dissolution that were different than those in equilibrium with the initial drop composition (less for o-xylene and greater for 1-methylnaphthalene). The simulations suggest that mole fractions were unchanged at the center of the drop, resulting in concentration gradients out to the oil-water interface. Numerical simulations conducted using different drop sizes and bitumen viscosities also suggest the potential for persistent naphtha dissolution, where the time required to deplete 80% of the o-xylene and 1-methylnaphthalene mass from an oil drop was estimated to be on the order of months to years for mm-sized drops, and years to decades for cm-sized drops assuming instantaneous biodegradation in the aqueous phase surrounding the bitumen.

Forever no more: Complete mineralization of per- and polyfluoroalkyl substances (PFAS) using an optimized UV/sulfite/iodide system.

As the global issue of PFAS contamination in water continues to grow there exists a need for technologies capable of fully mineralizing PFAS in water, with destruction being measured as both a loss of the initial PFAS and a quantitative recovery of the resultant fluoride ions. This study investigates the use of sulfite and iodide in a bicarbonate-buffered alkaline system activated with ultraviolet (UV) light to destroy PFAS. The UV/sulfite/iodide system creates a reductive environment through the generation of aqueous electrons, which can degrade PFAS. The extent of degradation and defluorination was explored for perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), 6:2 fluorotelomer sulfonic acid (6:2 FTS), and perfluorobutane sulfonic acid (PFBS). An initial UV/sulfite/iodide system achieved 100 % degradation and > 90 % defluorination for PFOS, PFOA, and 6:2 FTS, but was not capable of completely degrading PFBS. Transformation product elucidation experiments were performed for PFOS under different UV systems, and 6:2 FtSaB using the initial UV/sulfite/iodide system. Several transformation products were identified including -nF/+nH PFOS (n = 1-13), -F/+H shorter-chain PFSAs, 6:2 fluorotelomer sulfonamidoamine (6:2 FtSaAm), 6:2 fluorotelomer sulfonamide, and 6:2 fluorotelomer unsaturated sulfonamide. Novel identification of -F/+H perfluoropropane sulfonic acid (PFPS) and -F/+H perfluoroethane sulfonic acid (PFES) following degradation of PFOS confirms CC bond cleavage, and different isomers of -F/+H PFOS confirms the potential for CF bond cleavage to occur throughout the perfluoroalkyl chain. Additional optimization experiments were performed aiming to fully degrade PFBS. The optimal protocol found in this study involved an elevated initial sulfite concentration and adding additional sulfite at regular intervals during UV-activation, achieving >99.9 % destruction and complete quantitative defluorination of PFBS.

Simulating field-scale thermal conductive heating with the potential for the migration and condensation of vapors.

Thermal conductive heating (TCH) is an in-situ thermal treatment (ISTT) technology for treating non-aqueous phase liquid (NAPL) source zones. Numerical models can be useful tools for improving remedial performance, but traditional multiphase flow models are rarely used to simulate mass recovery during ISTT applications at the field scale due to their computational expense. This study developed a 3D model based on macroscopic invasion percolation to simulate the vaporization of NAPL, and the subsequent vapor migration and potential condensation at the field scale. The model was used to simulate the mass recovery of trichloroethene (TCE) from a NAPL source zone under seven scenarios of different heater placements, including three scenarios with an undersized target treatment zone (TTZ). Simulation results showed that TCH was effective in removing NAPL within the TTZ, but the treatment zone did not extend far from the perimeter heaters. In addition, during heating, NAPL condensation outside the TTZ due to the escaping vapor was observed in all scenarios. Overall, the resulting mass recovery was lower in the three scenarios with an undersized TTZ (91-95%) than in the other four scenarios (≈ 99%). Moreover, the locations of unrecovered/condensed NAPL could be inferred by monitoring mass recovery tailing at individual extraction wells.

Peaks, pores, and dragon eggs: Uncovering and quantifying the heterogeneity of treatment wetland biofilm matrices.

Biofilms serve to house diverse microbial communities, which are responsible for the majority of wastewater constituent degradation and transformation in treatment wetlands (TWs). TW biofilm has been generally conceptualized as a relatively uniform film covering available surfaces. However, no studies attaining direct visual 3D representations of biofilm morphology have been conducted. This study focuses on imaging the morphology of detached, gravel-associated, and rhizospheric (Phalaris arundinacea) biofilms from subsurface TW mesocosms. Images obtained through both traditional light microscopy, environmental scanning electron microscopy (E-SEM) and Wet-SEM revealed that TW biofilms are structurally heterogeneous ranging from corrugated films to clusters of aggregates. Features such as water channels and pores were observed suggesting that pollutant transport inside biofilms is complex, and that the interfacial surface area between water and biofilm is much larger than previously understood. Biofilm thickness generally ranged between 170 and 240 µm, with internal biofilm porosities estimated as 34 ± 10 %, reaching a maximum of 50 %. Internal biofilm matrix pore diameters ranged from 1 to 205.2 µm, with a distribution that favored pores and channels smaller than 10 µm, and a mean equivalent spherical diameter of 8.6 µm. Based on the large variation in pore and channel sizes it is expected that a variety of flow regimes and therefore pollutant dynamics are likely to occur inside TW biofilm matrices. Based on the visual evidence and analysis, a new conceptual model was created to reflect the microscale TW biofilm dynamics and morphology. This new conceptual model will serve to inform future biokinetic modelling, microscale hydrology, microbial community assessment, and pollutant treatment studies.

Retention of PFOS and PFOA Mixtures by Trapped Gas Bubbles in Porous Media.

The transport of per- and polyfluoroalkyl substances (PFAS) in soil and groundwater is important for site investigation, risk characterization, and remediation planning. The adsorption of PFAS at air-water interfaces has been shown to significantly contribute to PFAS retention, with subsequent effects on concentrations and the time scales of transport. In this study, column experiments were conducted to investigate the transport of perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and 6:2 fluorotelomer sulfonate (6:2 FTS) individually and in binary mixtures in the presence of a trapped gas phase, using clean sands to isolate adsorption to air-water interfaces. Consistent with previous studies, the transport of PFOS, PFOA, and 6:2 FTS was retarded by adsorption at the air-water interface, with greater retention of PFOS due to its higher affinity for the air-water interface. Chromatographic separation occurred in the experiments using binary mixtures of PFOS and PFOA, with greater retention at lower influent concentrations. The mixture experiments also showed enhanced breakthrough of PFOA in the presence of PFOS, where effluent concentrations of PFOA were temporarily greater than the influent concentration prior to the breakthrough of PFOS. This enhanced breakthrough was attributed to competition between PFOS and PFOA for adsorption to the air-water interface.

Ebullition-facilitated mobilization of trapped dense non-aqueous phase liquid at residual saturation from sandy sediments.

Gas ebullition can mobilize dense non-aqueous phase liquids (DNAPLs) from sediments to the overlying water column, increasing the DNAPL-impacted area and posing serious challenges to the remediation and management of contaminated sediments. Despite this, there have been few laboratory studies focused on gas ebullition-facilitated transport of DNAPL. In this study, bubble-facilitated transport was investigated by injecting gas (air or nitrogen) at 1 mL/min through a creosote source zone (∼25% saturation) capped with sand layers of different thicknesses. Three short-term experiments (8.3-8.7 h) were capped with 11.4, 7.0 or 4.5 cm of sand to estimate DNAPL flux. One long-term experiment (30 days) was capped with 8 cm of sand to investigate DNAPL removal. Heptane placed on a layer of water above the sand was used as a solvent trap and analyzed for petroleum hydrocarbons (PHCs). Results showed that creosote travelled as thin coatings and films surrounding gas bubbles migrating out of the source zone. Gas invasion was dominated by capillarity in the 11.4 cm-thick sand layer and by fracturing in the 7.0 and 4.5 cm-thick sand layers. Migration through these fractures often led to the formation of creosote tails on mobilized bubbles that drained towards the rear end of the bubble. The mass released decreased exponentially with sand cap thickness. In the long-term experiment, images showed significant depletion of the source zone in 30 days. Linear regression analysis showed that relationships with high predictive capabilities for ebullition-facilitated fluxes of hydrophobic organic contaminants can be obtained by incorporating gas ebullition flux and source strength, based on results from this study along with others from the field and laboratory. To our knowledge, this is the first study to compile and integrate data collected from laboratory and field studies to develop an assessment tool to facilitate the management of contaminated sediments affected by gas ebullition.

Isotopic and Chemical Assessment of the Dynamics of Methane Sources and Microbial Cycling during Early Development of an Oil Sands Pit Lake.

Water-capped tailings technology (WCTT) is a key component of the reclamation strategies in the Athabasca oil sands region (AOSR) of northeastern Alberta, Canada. The release of microbial methane from tailings emplaced within oil sands pit lakes, and its subsequent microbial oxidation, could inhibit the development of persistent oxygen concentrations within the water column, which are critical to the success of this reclamation approach. Here, we describe the results of a four-year (2015-2018) chemical and isotopic (δ13C) investigation into the dynamics of microbial methane cycling within Base Mine Lake (BML), the first full-scale pit lake commissioned in the AOSR. Overall, the water-column methane concentrations decreased over the course of the study, though this was dynamic both seasonally and annually. Phospholipid fatty acid (PLFA) distributions and δ13C demonstrated that dissolved methane, primarily input via fluid fine tailings (FFT) porewater advection, was oxidized by the water column microbial community at all sampling times. Modeling and under-ice observations indicated that the dissolution of methane from bubbles during ebullition, or when trapped beneath ice, was also an important source of dissolved methane. The addition of alum to BML in the fall of 2016 impacted the microbial cycling in BML, leading to decreased methane oxidation rates, the short-term dominance of a phototrophic community, and longer-term shifts in the microbial community metabolism. Overall, our results highlight a need to understand the dynamic nature of these microbial communities and the impact of perturbations on the associated biogeochemical cycling within oil sands pit lakes.

Removal of trichloroethene from thin clay lenses by electrical resistance heating: Laboratory experiments and the effects of gas saturation.

The removal of dissolved volatile organic compounds (VOCs) from low-permeability lenses is important to limit back diffusion at sites impacted by dense non-aqueous phase liquids (DNAPLs). In situ thermal treatment (ISTT) technologies have the potential to treat DNAPL-impacted sites by enhancing diffusion from low-permeability lenses during heating. A series of two-dimensional laboratory tank experiments was conducted to investigate heating, gas formation, and trichloroethene (TCE) removal from a clay lens surrounded by sand. Results showed preferential heating of the clay and substantial TCE removal, with post-heating relative concentrations less than 0.06. The extent of TCE removal was not explained by only an increase in the aqueous TCE diffusion coefficient with increased temperature. Modelling estimates based on 1D diffusion from the lens showed that diffusion through both gas and water phases was required to match observations. Gas formation in the interior of the lens was also indicated by measured changes in bulk electrical conductivity of the clay during cool down, with gas saturations estimated to be greater than 0.21 at the end of heating. These estimates were larger than those needed to match the observed removal by diffusion, and suggest that connected gas pathways were created in the lens during heating, but that not all of the gas produced was part of those pathways. These results suggest that ISTT technologies may be effective in removing dissolved VOCs from thin clay lenses, and that gas formation within the clay should be considered when predicting the extent and rate of removal.

Mechanochemical remediation of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) amended sand and aqueous film-forming foam (AFFF) impacted soil by planetary ball milling.

Per- and polyfluoroalkyl substances (PFAS) are manmade, fluorinated organic chemicals which have been identified as persistent organic pollutants. PFAS have surface active properties that have made them suitable for applications in oil- and water-resistant products, as well as many firefighting foams. No on-site remediation strategies exist to treat PFAS impacted soils. Mechanochemical remediation of PFOS- and PFOA-amended sand via a planetary ball mill was studied. The effect of sand mass, KOH as a co-milling reagent, and water saturation on the degradation of PFOA and PFOS was evaluated. By 4 h of milling concentrations were reduced by up to 98% for PFOS-amended dry sand and 99% for PFOA-amended dry sand without the addition of a co-milling reagent. Water saturation was determined to be a significant hindrance on the mechanochemical destruction of PFOS and PFOA. A maximum of 89% of fluoride was recovered from PFOS-amended sand when KOH was used as a co-milling reagent. It is hypothesized that reactive particles generated from the fracture of sand grains react with PFAS molecules to initiate destruction, which can result in full defluorination. Milling experiments were also conducted on soils from a Canadian firefighting training area (FFTA), demonstrating that PFOS concentrations can be reduced by up to 96% in site soils. For the first time, ball milling for the remediation of PFAS in environmental media has been demonstrated using amended sand and legacy soils from a FFTA.

Intermediate-Scale Laboratory Investigation of Stray Gas Migration Impacts: Transient Gas Flow and Surface Expression.

Petroleum resource development is a significant contributor of greenhouse gas emissions to the atmosphere. A potential source of emissions may result from stray gas migration. However, its contribution to overall emissions and potential groundwater contamination is unknown, and quantification of flow and dissolution of stray gas is required. The environmental expression of stray gas was investigated using an intermediate-scale (150 × 150 × 2 cm3), two-dimensional flow cell packed in both homogeneous and heterogeneous sand configurations allowing for visualization and measurement of gas movement, collection of aqueous samples, and real-time measurement of gas fluxes escaping the surface of the sand. Results show that gas is transported to the surface of the system via varying dominant discontinuous conduits for flow dictated by geology, leading to surface expression that can be greater or less than the leakage rate of gas. This suggests that surface expression is not directly indicative of the expanse and magnitude of stray gas migration leaks. It was found that 35-39% of the methane was released to the aqueous phase and 61-65% to the atmosphere. The results underscore that subsurface characteristics and gas flow are the key drivers for the overall expression of stray gas in unconsolidated sand aquifers.

Modelling gas-phase recovery of volatile organic compounds during in situ thermal treatment.

In situ thermal treatment (ISTT) technologies can be used to remove mass from non-aqueous phase liquid (NAPL) source zones. Ensuring the vaporization of NAPL and the capture of vapors are crucial, and numerical models are useful for understanding the processes that affect performance to help improve design and operation. In this paper, a two-dimensional model that combines a continuum approach based on finite difference for heat transfer with a macroscopic invasion percolation (macro-IP) approach for gas migration was developed to simulate thermal conductive heating (TCH) applications at the field-scale. This approach simulates heat transport and gas migration, but is different than a traditional continuum multiphase approach. Mass recovery for 60 randomly generated realizations under three degrees of heterogeneity of the permeability field were simulated. The mass recovery curves had an overall similar shape for the various permeability fields. However, a wider range of completion times was observed for domains with a higher permeability variance. Results also showed that NAPL pools that were highly saturated, deep, and away from the heaters needed more heating time to be depleted, and that total NAPL mass was not a good indicator of completion time. The completion time was positively correlated with the maximum value of the mixed spatial moment of NAPL saturation about the heaters in the lateral and vertical direction, and the NAPL pool with the highest moment could increase the heating time by as much as 35%. This effect was most notable in simulations with a high permeability variance and suggests the potential to reduce heating time by locating the largest NAPL pools and placing TCH heaters accordingly.

Effects of hydrogen gas production, trapping and bubble-facilitated transport during nanoscale zero-valent iron (nZVI) injection in porous media.

The injection of nanoscale zero-valent iron (nZVI) can be an effective technique for the treatment of groundwater contaminants, including chlorinated solvents. However, its effectiveness can be limited by natural reductant demand (NRD) reactions, including the reduction of water resulting in the production of hydrogen gas. This study presents results from a series of laboratory experiments to investigate gas production and mobilization following the injection of nZVI solutions, along with sodium borohydride (NaBH4) that is used for nZVI synthesis. Experiments were performed in a thin, two-dimensional flow cell (22 × 34 × 1 cm3) to measure hydrogen gas volumes and local gas saturations, and to investigate the distribution of gas within and above the injection zone. An additional experiment was conducted in a larger flow cell (150 × 150 × 2 cm3) containing dissolved trichloroethene (TCE) to assess changes in aqueous flow pathways and enhanced vertical transport of TCE by mobilized gas. The results showed substantial gas production (60% to 740% of the injected solution volume) resulting in gas mobilization as a network of gas channels above the injection zone, with more gas produced from greater excess NaBH4 used during nZVI synthesis. Trapped gas saturations were sufficient to cause the diversion of aqueous flow around the nZVI injection zone. In addition, gas production and mobilization resulted in the bubble-facilitated transport of TCE, and detectable concentrations of TCE and reaction products (ethane and ethene) above the target treatment zone.

Intermediate-Scale Laboratory Investigation of Stray Gas Migration Impacts: Methane Source Architecture and Dissolution.

Stray gas migration as a result of hydrocarbon extraction has caused environmental concern and is receiving widespread attention. Natural gas migration in the subsurface can have environmental implications when gas components (e.g., methane, longer-chained hydrocarbons) dissolve into shallow groundwater or pass through groundwater systems to the atmosphere. Because of the complexity of the subsurface systems and the parameters affecting stray gas migration, systematic quantification is difficult, particularly in field studies. To focus on key processes of gas migration, laboratory experiments offer a controlled environment to collect data which can be applied to field and modeling efforts. In this study, methane was injected into an intermediate-scale (150 × 150 × 2 cm3) two-dimensional flow cell packed with saturated homogeneous or heterogeneous unconsolidated sands. The impact of active methane leakage versus stopping of leakage was investigated. High-resolution, visualization techniques coupled with high-frequency water sampling at multiple depth-discrete intervals allowed for understanding of coupled methane migration and mass transfer. Results show that methane dissolution is affected by heterogeneity, active versus inactive leakage, and multicomponent mass transfer, prolonging the longevity of both free- and dissolved-phase methane in the subsurface. Findings highlight the importance of considering geology, hydrogeologic conditions, and multicomponent mass transfer in gas migration systems at the field scale.

Low permeability zone remediation of trichloroethene via coupling electrokinetic migration with in situ electrochemical hydrodechlorination.

To address the challenge of trichloroethene (TCE) remediation in low permeability zone, an inexpensive Cu-Ni bimetallic cathode was proposed in electrokinetic (EK) remediation system to couple electrokinetic migration with in situ electrochemical hydrodechlorination. Aqueous phase TCE was originally added into the anolyte so that breakthrough curves through the low permeability porous soil compartment could be obtained to better understand TCE migration driven by electroosmosis flow using different cathodes. The Cu-Ni cathode resulted in more TCE migration of 7.64 mg compared to that of 5.99 mg with Ni and 4.22 mg with mixed metal oxide (MMO) cathode, suggesting that the Cu-Ni cathode was capable of driving more TCE flux out of the contaminated soil. With the Cu-Ni cathode, 98.4% of TCE flux that reached the cathode was electrochemically reduced on the cathode, which was much higher than that with MMO cathode (77.9%) or Ni cathode (59.6%). TCE mass that was transported by electroosmosis flow increased from 2.04 to 6.68 mg when the voltage gradient increased from 1 to 4 V cm-1, with the normalized energy consumption increasing from 0.06 to 0.16 kWh kg-1 per unit water movement, and from 0.54 to 2.55 kWh g-1 per unit TCE transport. For TCE that did reach the cathode compartment, > 98% degradation maintained at the Cu-Ni cathode with various voltage gradients. The coupled electrokinetic and electrochemical hydrodechlorination technology appears to be a promising strategy for the remediation of low permeability porous media.

Aqueous and surface expression of subsurface GHGs: Subsurface mass transfer effects.

Releases of greenhouse gases (GHGs) from the subsurface can result in atmospheric emissions and the degradation of water quality. These effects require attention in today's changing climate to properly quantify emissions, reduce risk and inform sound policy decisions. Flowing subsurface GHGs, including methane and carbon dioxide, present a risk in the form of two environmental expressions: i) to the atmosphere (surface expression) and ii) to shallow groundwater (aqueous expression). Results based on high-resolution observations in an analog experimental system and analytical modelling show that these expressions depend on the rate of gas flow and the velocity of the flowing groundwater. In deeper systems, the emission of flowing subsurface GHGs could be significantly limited by dissolution into groundwater, adversely impacting water resources without surficial evidence of an underlying issue. This work shows that mass transfer in the subsurface must be considered to quantify, monitor and mitigate risks of leaking subsurface GHGs.

A numerical model for estimating the removal of volatile organic compounds in laboratory-scale treatability tests for thermal treatment of NAPL-impacted soils.

Treatability tests can be carried out to assess the potential effectiveness of thermal treatment technologies under different site conditions and are important for specific technology selection and design. In order to reduce the costs for laboratory tests and expand the insights from previous treatability studies, a one-dimensional (1D) radial finite difference model was developed to simulate the removal of volatile organic compounds (VOCs) in laboratory thermal treatability tests. The processes considered in the model include heat conduction, co-boiling of single-component or multi-component NAPLs with water, and water boiling. An explicit approach is used to simulate the evolution of NAPL composition for multi-component NAPLs during heating. The developed model adopts only two fitting parameters and was calibrated and validated using previous laboratory experiments. In this paper, the developed model was first calibrated to three laboratory experiments using temperature measurements, which resulted in matches to the NAPL and gas saturations. After calibration, the model was able to predict the temperature, NAPL and gas saturations for the remaining seven experiments, including those with single and multi-component NAPLs, using the average value of each fitting parameter.

Laboratory study of creosote removal from sand at elevated temperatures.

In situ thermal treatment (ISTT) technologies have been applied at sites impacted by non-aqueous phase liquids (NAPLs). There is a need to establish expectations for the treatment of semi-volatile NAPLs, including those consisting primarily of polycyclic aromatic hydrocarbons (PAHs), and the potential benefits and limitations of partial NAPL removal. A series of laboratory experiments was conducted to investigate NAPL removal and soil concentrations during the heating of creosote-impacted sand, as well as aqueous concentrations during post-heating dissolution. The results showed co-boiling near the water boiling temperature due to the low volatility of most creosote components, with limited decreases in NAPL saturation (from 30% to 21% of the pore space). Decreases in soil concentration were more substantial than decreases in NAPL saturation (by a factor of 2-180), with greater removal for higher-volatility components at higher treatment temperatures. Results of the dissolution experiments showed mixed results, with decreases in the aqueous concentrations for 12 of 15 components, but increases in aqueous concentrations for phenanthrene, fluoranthene and pyrene after heating to 205 °C or 320 °C. Overall, the results illustrate the utility of bench-scale treatability tests in helping to establish ISTT goals and expectations.

Laboratory study of mass transfer from diluted bitumen trapped in gravel.

Diluted bitumen (dilbit) spilled to rivers has the potential to sink and become trapped in coarse bed sediments. Hyporheic flow through the river bed can then lead to the dissolution of hydrocarbons from this trapped oil, and subsequent risks to water quality and aquatic life. It is important to understand the concentrations of dissolved hydrocarbons in water, relative to aqueous solubility, that may result from mass transfer under these conditions, particularly under conditions where coarse sediments lead to faster hyporheic flow that could promote rate-limited mass transfer conditions. In this study, the dissolution of dilbit (Cold Lake Blend) trapped in gravel was measured using one-dimensional columns at flow rates representative of fast hyporheic flow. Dissolved concentrations in the column effluent were found to be less than 20% of effective solubility (equilibrium) concentrations and decreased with increasing flow rate, indicative of rate-limited conditions. These results show that risks posed by the contamination of gravel-bedded rivers by trapped dilbit may be lower, but persist for a longer period of time, than those estimated assuming dissolution at concentrations near solubility limits.

Bubble-Facilitated VOC Transport from LNAPL Smear Zones and Its Potential Effect on Vapor Intrusion.

Most conceptual and mathematical models of soil vapor intrusion assume that the transport of volatile organic compounds (VOCs) from a source toward a building is limited by diffusion through the soil gas. Under conditions where advection occurs, transport rates are higher and can lead to higher indoor air concentrations. Advection-dominated conditions can be created by gas bubble flow in the saturated zone. A series of laboratory column experiments were conducted to measure mass flux due to bubble-facilitated VOC transport from light nonaqueous phase liquid (LNAPL) smear zones. Smear zones that contained both LNAPL residual and trapped gas, as well as those that contained only LNAPL residual, were investigated. Results showed that the VOC mass flux due to bubble-facilitated transport was orders-of-magnitude higher than under diffusion-limited conditions. Results also showed that the mass flux due to bubble-facilitated transport was intermittent, and increased with an increased supply of dissolved gases.