Bubble-Facilitated VOC Transport from LNAPL Smear Zones and Its

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Bubble-Facilitated VOC Transport from LNAPL Smear Zones and Its Potential Effect on Vapor Intrusion Nicole C. Soucy and Kevin G. Mumford* Department of Civil Engineering, Queen’s University, Kingston, Ontario Canada, K7L 3N6 S Supporting Information *

ABSTRACT: 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.

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INTRODUCTION The migration of volatile organic compounds (VOCs) through soil gas and into buildings, referred to as vapor intrusion, is a recognized risk pathway associated with contaminated sites.1−6 The source of the VOCs can be a dissolved plume as well as light or dense nonaqueous phase liquid (LNAPL or DNAPL) in the vadose zone or saturated zone. In the common conceptual model for vapor intrusion, VOCs partition to soil gas, diffuse through soil gas toward a building, and enter the building through openings, such as cracks or gaps in the foundation, by diffusion and advection resulting from pressure differences between the building and the shallow subsurface.6,7 This common conceptual model for vapor intrusion forms the basis of many analytical7−11 and numerical12−18 models that can be used as part of a multiple-lines-of-evidence approach to investigate vapor intrusion at contaminated sites.6,19 Several important processes and conditions have been incorporated into these models, including oxygen-limited biodegradation of petroleum hydrocarbons 10,20,21 and spatial variation in permeability and water saturation.4,13,15 However, analytical and numerical models commonly assume diffusion-limited transport through the soil gas. That is, in these models, regardless of the conditions under which VOCs are transported through the building foundation or are removed from the building through air exchange, transport to indoor air cannot exceed the transport of diffusion through the soil gas toward the building. If advection through soil gas does occur, this would lead to faster VOC transport toward the building, and potentially higher indoor air concentrations and greater risk to building © 2017 American Chemical Society

occupants than predicted by diffusion-limited models. Therefore, if diffusion-limited models are used for screening-level risk calculations they could under-predict the risk associated with sites where advection plays a significant role. This possibility supports the need for site-specific sampling in combination with modeling, but also points to the need for the investigation of fundamental mechanisms that create advection-dominated conditions and the further development of models to accommodate these conditions. The potential for soil gas advection has been associated with fluctuations in barometric pressure and the presence of preferential pathways between the source and the building.6 It has also been associated with the methanogenic degradation of petroleum hydrocarbons, particularly ethanol blended fuels, where the production of methane and carbon dioxide can lead to advection in the vadose zone or gas bubble formation and ebullition in the saturated zone.22−27 Accelerated vertical gas transport has also been reported in the absence of biogenic gas production, and instead caused by the partitioning of VOCs to a trapped gas phase.28,29 In previous laboratory studies, the partitioning of both dissolved gases and VOCs from a DNAPL pool to trapped gas below the water table resulted in the expansion of that trapped gas. This expansion occurs when the sum of the partial pressures of the dissolved gases and VOCs exceeds the sum of the water pressure and capillary entry pressure of the porous Received: Revised: Accepted: Published: 2795

November 30, 2016 January 18, 2017 January 23, 2017 January 23, 2017 DOI: 10.1021/acs.est.6b06061 Environ. Sci. Technol. 2017, 51, 2795−2802

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Environmental Science & Technology medium.22,29,30 With continued expansion, capillary trapping forces were exceeded by buoyancy forces, resulting in the repeated fragmentation and vertical mobilization of gas bubbles that contained VOC.31 This bubble-facilitated VOC transport is faster than the rate of transverse vertical dispersion29 leading to higher vertical VOC fluxes. It is expected that this mechanism will also be active near LNAPL, particularly in smear zones, where VOC can partition from LNAPL residual and where dissolved gases can be readily supplied by flowing water or diffusion from the vadose zone. Smear zones are created by the falling and rising of the water table, which distributes LNAPL over a greater depth than that occupied by the original pool and traps it as a collection of immobile ganglia.32,33 Compared to an LNAPL pool that exists prior to any water table fluctuations, these smear zones are regions of increased dissolution, due to increased LNAPL-water interfacial area, and increased biodegradation, due to the increased aqueous permeability and the supply of oxygen from the flowing groundwater. Atmospheric gases are also trapped by fluctuations of the water table,34−36 and LNAPL and air can be trapped alongside one another creating an air-LNAPL-water smear zone. For example, Figure 1 shows images of an initially water-saturated

undergoing biodegradation, under either aerobic or anaerobic conditions, biogenic gas would also contribute to this process by supplying additional dissolved gases or generating gas bubbles.22,23 There is a need for the continued investigation of the fundamental processes that control risks associated with the vapor intrusion pathway.38 This includes bubble expansion and mobilization within an LNAPL smear zone, where there is the potential for VOCs to be transported in these bubbles to the vadose zone faster than by aqueous diffusion, and faster than they can be transported away from the source toward a building by diffusion in the soil gas of the vadose zone. This rapid transport would result in the advection of VOCs through soil gas, potentially increasing the mass transport rates that contribute to vapor intrusion. Therefore, the objective of this study was to investigate bubble-facilitated VOC transport and assess its effect on vertical VOC flux through soil gas. This was addressed using a series of laboratory column experiments to measure VOC mass fluxes from LNAPL-water and air-LNAPLwater smear zones.

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MATERIALS AND METHODS Proof-of-Concept Experiment. A preliminary experiment was conducted using a 20 × 40 × 1 cm3 glass-walled flow cell39 (details provided in Supporting Information (SI)) to visualize the mobilization of gas from an air-LNAPL-water smear zone. A 9 cm-high water-saturated packing of medium sand (Accusand 20/30, d50 = 0.7 mm40) was emplaced in the cell by continuously pouring it as a slurry. Water in the sand was drained through ports in the bottom of the cell, allowing the invasion of air from the top and down to 1 cm above the bottom of the cell. Following water drainage, 56 mL of pentane (Fisher Scientific, 99%) dyed red using oil red-O (SigmaAldrich, Inc.) was injected upward through the bottom ports using a gastight syringe. This was followed by the upward imbibition of water through the bottom ports by gravity flow, to displace excess pentane and create trapped pentane and air similar to Figure 1b. Pentane was used in all experiments in this study because of its high vapor pressure of 56.6 kPa at 20 °C41 to demonstrate this mechanism using shorter-term experiments. Excess pentane and water displaced from the top of the sand during water imbibition were removed, and an additional 10 cm-high water-saturated medium sand layer and 8 cm-high layer of dry 4 mm glass beads (Fisher Scientific, 11-312) were packed on top, to act as an occlusion zone.42 Following sand and LNAPL emplacement, water was pumped (Masterflex model 07528-10) through the cell using four ports in each side of the flow cell frame (spaced every 4 cm) at a total flow rate of 1.53 L/day (specific discharge of 76.5 cm/day). Images of the flow cell were captured every 5 min using a digital camera (Canon, EOS Rebel T3i). The camera was placed in front of the flow cell and a light source was placed behind the flow cell (Led Go, CN-1200H) in the configuration used for light transmission imaging.31,43 Images were processed by converting to grayscale and subtracting the image captured at the beginning of the experiment from each subsequent image using Matlab to highlight changes in fluid saturation,31 particularly the migration of gas into the occlusion zone above the smear zone. Column Experiments. A series of column experiments was conducted to measure pentane mass flux through soil gas from LNAPL smear zones (Table 1) using an LNAPL-water or airLNAPL-water smear zone (prefix LW or ALW) and water flow,

Figure 1. Near pore-scale images of (a) trapped LNAPL and (b) trapped LNAPL and air in an otherwise water-saturated sand, showing LNAPL ganglia outlined in black and air bubbles outlined in white.

medium sand (d50 = 0.7 mm) underneath an LNAPL pool (heptane dyed red using oil red-O) subjected to water drainage and imbibition from below. This was performed in a 20 × 40 × 1 cm3 glass-walled flow cell using either a 28 mm- or a 12 mmthick LNAPL pool.37 During drainage of the 28 mm pool, the water table was lowered from the top of the sand to 11 cm below the top, allowing heptane to invade the sand but maintaining an LNAPL pool at the top boundary. During drainage of the 12 mm pool, the water table was lowered to 22 cm below the top of the sand, which emptied the LNAPL pool and allowed air to invade the sand after heptane. Therefore, drainage of the 28 mm pool produced an LNAPL-water smear zone (Figure 1a), whereas drainage of the 12 mm pool produced an air-LNAPL-water smear zone (Figure 1b). As the intermediate-wetting fluid, LNAPL is likely present as films surrounding each trapped air bubble as well as in other pores in contact with the air, but those films cannot be resolved using these images. The conditions present in an air-LNAPL-water smear zone, as would be created by larger water table fluctuations and thinner LNAPL pools, are expected to promote bubble-facilitated VOC transport due to the presence of trapped gas, the proximity of volatile LNAPL to that trapped gas, and the supply of dissolved gases in the groundwater flowing through the smear zone. In the case of a source 2796

DOI: 10.1021/acs.est.6b06061 Environ. Sci. Technol. 2017, 51, 2795−2802

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allowing air to invade from above. Pentane was then injected through the bottom, and the water reservoir was raised to displace excess pentane, and trap pentane and air by double displacement.44,45 For the LW-NF experiments, a pentane pool above the sand was used to invade pentane rather than air during water drainage, which was followed by water imbibition to trap residual pentane. Water saturations at the end of this imbibition step were 0.86−0.93 across all experiments. The second 10 cm-high column section contained the occlusion zone, and was also packed with sand as a continuously poured slurry following LNAPL emplacement. Although LNAPL was not emplaced in this zone, the packing procedure likely resulted in some LNAPL being displaced from the first section and contamination of the water in the occlusion zone with pentane. The third 10 cm high section contained dry glass beads and acted as a vadose zone. The 4 mm glass beads were chosen to provide a sharp change in water saturation between the vadose zone (section 3) and saturated zone (section 2) (i.e., negligible capillary fringe). The fourth 2.5 cm-high section did not contain a porous medium and acted as a pathway for air flow over the top of the vadose zone (e.g., Kurt and Spain46). Immediately after column packing and smear zone emplacement, air and water flow were introduced to the column. Water was pumped (Masterflex model 07528-10) laterally through six ports (3.2 mm inner diameter) on each side of column sections 1 and 2 (Figure 2) from a 4 L reservoir containing water in equilibrium with the atmosphere to provide dissolved gases. Specific discharge values listed in Table 1 represent flow conditions midway through the column, with faster flow closer to the influent and effluent ports due to the diverging and converging flow field. Air was pumped (KD Scientific, model KDS-270) through ports (3.2 mm inner diameter) on each side of section 4, and the rates were adjusted between experiments (Table 1) to maintain pentane concentrations below 8500 mg/ m3 for analysis. Full details and photos of the column setup are presented by Soucy.47 Sample Collection and Analysis. Gas samples were collected from the outlet of section 4 using a 25 mL gastight syringe every 2−8 h throughout each experiment, and were analyzed directly and immediately using a gas chromatograph (GC) (Agilent Technologies type 7890B) equipped with a gas sampling valve and a flame ionization detector (FID). The modified GC method48 used a maximum temperature of 150 °C, a run time of 13.7 min and a PoraBOND Q 50 m × 0.53 mm column. Calibration was performed before and after each experiment, and was verified daily using a blank sample and a single calibration standard at a concentration of 150, 750 or 3000 mg/m3. The detection limit was 0.67 mg/m3, and no concentrations measured in this study were less than 3.8 mg/ m3. Mass flux from the column was calculated using the measured pentane concentrations, the air flow rate across the top of the column and the column cross-sectional area:

Table 1. Column Experiment Conditions experiment ALW-F-A ALW-F-B LW-NF-A LW-NF-B ALW-VF

trapped fluids in source zone

water flow (L/ day)

specific dischargea (cm/day)

pentane and air pentane and air pentane pentane pentane and air

2.2

21

59

13.8

1.9

17

60

14.0

0 0 8.5, 0.6

0 0 78, 5

11 11 106, 25

13.9 14.0 6.6

air flow (mL/min)

duration (days)

a

calculated as q = Q/(2r·h12) where q is specific discharge, Q is the water flow rate to the column, r is the column radius and h12 is the total height of sections 1 and 2 of the column.

no water flow, or variable water flow (F, NF, or VF). Duplicate experiments were denoted using the suffix A or B. The LW-NF experiments represent the conventional, diffusion-limited conceptual model for vapor intrusion, with no trapped air in the smear zone and no supply of dissolved gases from flowing groundwater. In each experiment, the column was composed of four sections of 7.25 cm inner- diameter stainless steel pipe, with a total length of 27.5 cm (Figure 2). Packing consisted of

Figure 2. Schematic of the stainless steel column containing an LNAPL smear zone in section 1.

20/30 Accusand (d50 = 0.7 mm40) and 4 mm glass beads (Fisher Scientific, 11−312). Additional experiments that contained a 2 cm-high layer of finer sand (30/40 Accusand,d50 = 0.5 mm40) are presented in the SI. The first 5 cm-high column section contained the LNAPL smear zone. Sand was first packed into this section as a continuously poured slurry to a porosity of 0.34 ± 0.02 (mean ± one standard deviation). For the ALW-F and ALW-VF experiments, the water-saturated sand was then drained to residual water saturation by lowering a water reservoir attached to the bottom of the column through hydrophilic membranes (10 μm Nylon Net Filters, Merck Millipore NY1002500) and

J=

CQ a πr 2

(1)

where J is the mass flux of pentane leaving the column (ML−2T−1), C is the concentration of pentane in the outlet air (ML−3), Qa is the flow rate of air leaving the top of the column (L3T−1), and r is the inside radius of the column (L). 2797

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Figure 3. (a−d) Original images and (e−h) processed images from the proof-of-concept experiment. The transition from the LNAPL smear zone to the water-saturated occlusion zone is indicated by the dashed line and the approximate locations of the gas fingers mobilized from the smear zone are indicated by the solid lines. Black regions in the processed images (f−h) represent regions of increased gas saturation.

Figure 4. Pentane mass flux based on concentrations measured at the top outlet of the column for the bubble-facilitated VOC transport experiments with an air-LNAPL-water smear zone (ALW-F-A and ALW-F-B) and diffusion-limited experiments with an LNAPL-water smear zone (LW-NF-A and LW-NF-B).

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RESULTS AND DISCUSSION Proof-of-Concept Experiment. Gas expansion and mobilization due to the partitioning of VOC to trapped gas was observed early in the proof-of-concept experiment (Figure 3). Gas expansion resulted in both fluid (water, LNAPL and gas) rearrangement in the smear zone, as well as the formation of gas fingers that extended up into the occlusion zone. These gas fingers were discontinuous, as expected in this size of sand and as reported in previous laboratory and modeling studies.31,49−51 The time scale of this transport in the occlusion zone is significantly faster than would occur by aqueous diffusion, with multiple gas fingers reaching a height of 7−9 cm after 60 min. Similar gas fingers have also been observed in

multiple laboratory studies, including those focused on gas injection.23,49,50,52−55 Based on previous investigations of gas expansion and mobilization above DNAPL pools28,29,31,56 these gas fingers are expected to contain VOC as well as oxygen and nitrogen from the initially trapped air and the dissolved air supplied by the flowing water, which drives the expansion of the trapped gas. Therefore, a VOC mass flux that is higher than by diffusion alone is expected as the VOC is transported by the gas fingers, which was further investigated in the column experiments. In addition, displacement of LNAPL up into the occlusion zone was also observed, for example the red color at the base of some gas fingers outlined in Figure 3d. LNAPL rearrangement, including ganglia mobilization by flowing gas 2798

DOI: 10.1021/acs.est.6b06061 Environ. Sci. Technol. 2017, 51, 2795−2802

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Environmental Science & Technology bubbles, has also been reported in applications of supersaturated water injection.57,58 While these results are qualitative, they provided the conceptual basis for the interpretation of the measurements in the column experiments. Bubble-Facilitated VOC Transport. The pentane mass flux due to bubble-facilitated VOC transport (ALW-F experiments) was orders-of-magnitude greater than that due to diffusion alone (LW-NF experiments) (Figure 4). The mass flux in the ALW-F-A and ALW-F-B experiments had a mean value of 9.8 × 100 g/m2·d and varied between 7.6 × 10−2 g/m2· d and 1.0 × 102 g/m2·d. In contrast, the mass flux in the LWNF-A and LW-NF-B experiments had a mean value of 9.7 × 10−2 g/m2·d and varied between 3.2 × 10−2 g/m2·d and 2.0 × 10−1 g/m2·d. Error bars represent a coefficient of variation of 12.4% based on repeated measurements of standards in the concentration range of these experiments. This difference between the results of the ALW-F and LW-NF experiments is substantial. The mean mass flux values differed by two ordersof-magnitude, and all but two (98.5%) of the mass flux values measured in the ALW-F experiments were higher than the highest value measured in the LW-NF experiments. This higher mass flux was attributed to VOCs being transported by mobilized gas bubbles through the occlusion zone, however upward LNAPL displacement from the smear zone to the occlusion zone (section 1 and section 2 of the column) could also increase mass flux by shortening the diffusive path length. These higher mass fluxes represent a significant increase in the concentration of VOCs that could potentially contribute to vapor intrusion and would increase the risk associated with that exposure pathway. Greater fluctuations in mass flux were also observed during bubble-facilitated VOC transport, with a standard deviation of 1.4 × 101 g/m2·d in the ALW-F experiments compared to 3.0 × 10−2 g/m2·d in the LW-NF experiments. Intermittent signals, including pressure and gas flow, are commonly observed in studies of gas flow in coarser porous media.31,52,59 The intermittent mass flux values measured here are consistent with expectations of gas bubble flow in coarser material, where initially trapped gas is mobilized by repeated expansion and fragmentation events, and mobilized bubbles can become retrapped due to pore-scale variability in capillary forces and remobilized due to bubble coalescence.31 This represents a dynamic steady-state, where the long-term temporal average mass flux consists of a combination of trapping periods punctuated by rapid mobilization events. It is expected that for a wider smear zone, with gas migration originating at more locations, and a deeper vadose zone, with more dispersion of VOC through the soil gas, this mass flux signal would show more steady behavior. However, these results demonstrate that for vapor intrusion dominated by advection (due to biogenic gas or VOC partitioning) rather than diffusion, the transient behavior of the mass flux should be taken into account when designing investigation strategies at a contaminated site. Importance of Dissolved Gas Supply. The magnitude of the pentane mass flux due to bubble-facilitated VOC transport varied with the water flow rate (Figure 5), and this effect is attributed to the supply of dissolved gases to the smear zone. While average mass flux in the ALW-F experiments (water flow rates of 1.9 and 2.2 L/day) was 9.8 × 100 g/m2·d, the average mass fluxes in the fast (8.5 L/day) and slow (0.6 L/day) periods of the ALW-VF experiment were 6.9 × 101 g/m2·d and 2.5 × 100 g/m2·d (Figure 6). The mobilized gas bubbles are multicomponent gas mixtures52 and the positive correlation

Figure 5. Pentane mass flux based on concentrations measured at the top outlet of the column for variable water flow through an airLNAPL-water smear zone (experiment ALW-VF). Water flow was changed from 8.5 L/day to 0.6 L/day after 4.3 days.

Figure 6. Average pentane mass flux from the ALW-F experiments and the fast and slow periods of the ALW-VF experiment showing increased mass flux with increased water flow rate, associated with an increased supply of dissolved gases to the LNAPL smear zone.

between the influent water flow rate and the pentane mass flux demonstrates the importance of both dissolved gases and VOCs on the sustained expansion and mobilization of trapped gas. In these experiments, VOCs are supplied by the partitioning of pentane from the LNAPL in contact with the trapped gas (Figure 1). The amount of other gases, in this case air, that correspond to these measured pentane mass fluxes can be calculated using the ideal gas law, assuming that the partial pressure of pentane in the mobilized gas bubbles is equal to its vapor pressure and that the mobilized gas is at a pressure of 103 kPa (the sum of atmospheric pressure, water pressure and the displacement pressure of this sand40). Based on these calculations, the pentane mass fluxes in the ALW-F experiments and the fast and slow periods of the ALW-VF experiment required 0.51 mmol/ day, 3.6 mmol/day and 0.13 mmol/day of other gases. 2799

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Assuming that influent water in these experiments was in equilibrium with the atmosphere and Henry’s law constants for oxygen and nitrogen of 8.0 × 104 Pa·m3/mol and 1.6 × 105 Pa· m3/mol, respectively,60 dissolved gases were supplied at rates of 1.7 mmol/day, 6.5 mmol/day and 0.43 mmol/day in the ALWF experiments and fast and slow period of the ALW-VF experiment, respectively, which exceed the required amounts. Biodegradation of the pentane may also have produced carbon dioxide, which would contribute to the dissolved gases available. It is expected that in the absence of water flow, dissolved gases initially present in the pore water during packing would allow gas expansion and mobilization for a short period of time after LNAPL emplacement, but that the supply of dissolved gases would soon be limited to diffusion from the vadose zone and that the mass fluxes of the magnitude measured here would not be sustained. In field applications, the supply of dissolved gases could be sustained by water flow through the smear zone, biogenic gas production (including under methanogenic conditions) or a combination of the two. Potential Implications on Vapor Intrusion. The results of these column experiments demonstrate the potential importance of bubble-facilitated VOC transport on vertical mass transport through the vadose zone, potentially increasing indoor air concentrations and vapor intrusion risk. It is not expected that this mechanism will be active at all sites, but that it will be most likely active in sources that contain volatile NAPL or high concentrations of dissolved VOCs, and are supplied with high concentrations of dissolved gases. Other site conditions are also expected to influence both the expansion of gas bubbles and their transport to the vadose zone. For example, this mechanism will be more active in shallower groundwater environments with coarser porous media, where lower water and capillary pressures result in lower gas pressures required for gas expansion and subsequent mobilization. Lower LNAPL saturations will result in less LNAPL-gas contact and fewer mobilization events over a given area. The presence of lower-permeability layers could serve as locations for gas accumulation and change the frequency of gas breakthrough to the vadose zone. Finally, the soil moisture profile in the vadose zone, as well as water and trapped air in the capillary fringe, will affect VOC transport from the smear zone toward the ground surface through partitioning between aqueous and gas phases and biodegradation along the transport pathway. As demonstrated in these experiments, LNAPL smear zones that contain trapped gas can provide the necessary conditions for bubblefacilitated VOC transport. However, other scenarios could also provide the necessary conditions. For example, shallow DNAPL sources could provide VOCs, and biogenic gas production from natural populations or as part of a bioremediation application could provide dissolved gases. Where trapped gases are in contact with volatile NAPL, the rate of bubble-facilitated transport will increase with an increasing supply of dissolved gases, making a characterization of that gas supply an important component of site assessment for this mechanism. Under such conditions, it is important that the limitations of many mathematical models, specifically the assumption of diffusionlimited transport through soil gas, is recognized and accounted for during risk assessment to avoid the potential underprediction of risk during screening-level calculations. Where this mechanism is active, models capable of simulating advective transport through the vadose zone coupled with gas bubble formation and mobilization in the saturated zone are required. Ref 51.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b06061. Figures S1−S3 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 613-533-6325; fax: 613-533-2128; e-mail: kevin. [email protected]. ORCID

Kevin G. Mumford: 0000-0002-8737-8158 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the American Petroleum Institute and the Natural Sciences and Engineering Research Council (NSERC) of Canada under Collaborative Research and Training Experience (CREATE) Grant 449311-2014. The technical assistance of Dr. Allison Rutter, Paula Whitley, and Stanley Prunster is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.est.6b06061 Environ. Sci. Technol. 2017, 51, 2795−2802

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DOI: 10.1021/acs.est.6b06061 Environ. Sci. Technol. 2017, 51, 2795−2802