Experimental investigation of pneumatic soil vapor extraction

Journal of Contaminant Hydrology 89 (2007) 29 – 47 www.elsevier.com/locate/jconhyd

Experimental investigation of pneumatic soil vapor extraction C.K. Høier a , T.O. Sonnenborg b , K.H. Jensen c,⁎, C. Kortegaard a , M.M. Nasser a a

b

Environment and Resources DTU, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K., Denmark c Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K., Denmark Received 19 January 2006; received in revised form 5 July 2006; accepted 25 July 2006 Available online 20 September 2006

Abstract Soil Vapor Extraction (SVE) is a common remediation technique for removing volatile organic compounds from unsaturated contaminated soils. Soil heterogeneities can however cause serious limitations to the applicability of SVE due to air bypassing low permeable areas of the soil, leading to diffusion limitation of the remediation. To enhance removal from areas subject to diffusion limitation a new remediation technique, pneumatic soil vapor extraction, is proposed. In contrast to traditional SVE, in which soil vapor is extracted continuously by a vacuum pump, pneumatic SVE is based on enforcing a sequence of large pressure drops on the system to enhance the recovery from the low-permeable areas. The pneumatic SVE technique was investigated in the laboratory using TCE as a model contaminant. 2Dlaboratory tank experiments were performed on homogeneous and heterogeneous sand packs. The heterogeneous packs consisted of a fine sand lens surrounded by a coarser sand matrix. As expected when using traditional SVE, the removal of TCE from the low permeable lens was extremely slow and subject to diffusion limitation. In contrast when pneumatic venting was used removal rates increased by up to 77%. The enhanced removal was hypothesized to be attributed to mixing of the contaminated air inside the lens and generation of net advective transport out of the lens due to air expansion. © 2006 Elsevier B.V. All rights reserved. Keywords: Pneumatic soil vapor extraction; Remediation technology; TCE; Venting; Unsaturated zone; Gas expansion

⁎ Corresponding author. Tel.: +45 35 32 24 84; fax: +45 3314 8322. E-mail address: [email protected] (K.H. Jensen). 0169-7722/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2006.07.006

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1. Introduction Soil Vapor Extraction (SVE) is a widely applied technology for removal of volatile organic compounds (VOCs) from unsaturated soils. The efficiency of the technique depends on various factors including the geological settings of the contaminated area, the absolute permeability of the soil, the distribution of water saturation within the soil, the organic content and volatility of the compounds to be removed. In high permeable, homogeneous soils, the efficiency of SVE is generally high, although long tailing behavior of the concentration in the removed air is often found and attributed to mass transfer limitations. Armstrong et al. (1994) showed by modeling experimental data that air–water mass transfer was the limiting process in the removal of contaminant from homogeneous, fine sand. Gierke et al. (1992) found that mass transfer across the gas–water interface could be considered instantaneous whereas diffusion within intraaggregate pores caused non-ideal gas-phase transport. Fisher et al. (1996) found that diffusion in inter-particle water could cause tailing of concentrations during vapor extraction of homogenous soils. Other evidence indicates that removal efficiency can be constrained by rate-limited desorption (e.g., Brusseau, 1991; Croisé et al., 1994). Although the mentioned rate-limiting processes may hinder the removal of contaminants from the soil and water to a level that meets drinking water standards within an acceptable time scale, venting of high permeable homogeneous soils is usually less problematic than removal of VOCs from stratified or heterogeneous soils. Under such conditions the removal efficiency of SVE can be significantly reduced, due to air bypassing of low permeable areas. Thereby advective removal of the contaminant is prohibited within these areas and diffusion is the dominant mechanism responsible for bringing contaminants from the low permeable layer to the advective flow zones. Numerical studies of layered soils have shown that the efficiency of venting operations is highly sensitive to the magnitude and the distribution of soil permeability (Rathfelder et al., 1991). The effect of low permeable areas on SVE efficiency has been examined in the laboratory by Kearl et al. (1991), Ho and Udell (1992) and Smith et al. (1996). Ho and Udell (1992) studied the effect of soil heterogeneity in two-dimensional, two-layered laboratory systems using different permeability ratios. Permeability contrasts always resulted in a reduction of the SVE efficiency compared to homogeneous experiments and at a permeability ratio of 100:1 complete by-passing of the contaminated low-permeable area occurred. Smith et al. (1996) also studied twodimensional heterogeneous laboratory systems in which a low permeable lens was placed within a high permeable matrix. They showed that depending on the airflow velocity and the composition of the contaminant, the removal of contaminants from the low permeable zone could be restricted by the flow velocity in the adjacent high permeable zone (advective flow zone), diffusion within the low permeable area, or controlled by liquid resistance in a phase mixture. Pulsed pumping has been suggested as a method to increase the efficiency of the venting process in heterogeneous soils. Pulsed pumping involves intermittent periods of pumping and shutting down of the pump. The idea is that during periods of pumping the contaminated area is flushed until mass transfer limitations constrain the recovery. The pump is then stopped for some time to allow mass transfer to take place followed by restarting of the pump. Armstrong et al. (1994) tested the method numerically on a system, where diffusive mass transfer in water limited the removal efficiency of the venting process. They showed that pulsed pumping was less efficient than continuous pumping at an equivalent average rate. The reason was that continuous pumping maximizes the diffusive mass transfer of contaminant by maintaining the concentration gradient at a steady maximum state. Also Schulenberg and Reeves (2002) found that pulsed pumping was less efficient compared to constant pumping.

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An alternative new vapor extraction technique, pneumatic SVE, is proposed in this paper, as an attempt to improve removal of VOCs from low permeable areas in heterogeneous settings. To our knowledge the proposed method has not been presented previously in the literature. The technique is based on imposing large transient pressure fronts to the soil formation by repeatedly increasing the vacuum by periodically reducing the accessibility to the atmosphere. In this manner the method deviates from the pulsed pumping technique by which the pressure fluctuations are obtained by turning the pump on and off. Further, the rate of pulsing and the magnitude of imposed pressure drops are higher during pneumatic SVE compared to pulsed venting. Pneumatic SVE is hypothesized to take advantage of several simultaneous processes being active when alternating periods of enforcing and releasing partial vacuum are imposed. The compressibility of gas is the most important contributing mechanism. By generating a substantial pressure drop in the unsaturated zone the gas will expand and contaminated air is forced out of low permeable soil units. When pressure subsequently is released, air will tend to flow back into the low permeable soil units. However, if the contaminant forced out of the lens is removed from the adjacent high permeable area before the air re-enters the lens, net transport of contaminant from the less permeable to the more permeable soil is generated. Also mixing of the contaminated air inside the low permeable area arise from the pressure fluctuations, and contaminant concentrations near the high/low permeable boundary are expected to be higher than under diffusion-limited transport. An additional potential effect of pneumatic SVE is caused by the generation of non-stationary pressure field. The pressure fronts will continuously move through the contaminated soil and permanent no-flow stagnation points can therefore be avoided. In this study the effect of pneumatic soil venting was examined experimentally in a 2D tank with homogeneous and heterogeneous sand packs. Venting experiments were carried out for both constant and pneumatic SVE to allow for an analysis and comparison of the efficiencies of the two operations. 2. Materials and methods 2D-tank soil venting experiments were performed on homogeneous and heterogeneous sand packs contaminated with Trichloroethylene (TCE). The heterogeneous packs consisted of a fine sand lens surrounded by a coarser sand matrix. Three sets of experiments were carried out: (1) venting on homogenous sand packs using traditional SVE (constant airflow); (2) venting on heterogeneous sand packs using traditional SVE; and (3) venting on heterogeneous sand packs using alternating periods of SVE and pneumatic SVE. 2.1. Experimental setup 2.1.1. Overall description of setup 2D soil vapor extraction experiments were conducted in a tank with interior dimensions 106 cm × 74 cm × 8 cm (Fig. 1). Although the experiments were constrained to 2D the elimination of the third dimension will not have a major influence on the results as air is still able to flow around hydraulic obstacles. The tank was constructed of stainless steel with a glass front to allow visual inspection of TCE plume removal during the venting experiments. Teflon® and Viton® seals and stainless steal tubings were used to minimize NAPL interactions inside the tank. Water inlet and outlet were located at the bottom left and right side of the tank. Air entered the tank through two injection ports (18 cm × 8 cm) and exited the tank through one extraction port covering the entire width and depth of the tank (Fig. 1). The inlet and outlet ports were produced from stainless boxes covered by stainless steel meshes.

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Fig. 1. Laboratory setup for the 2D tank experiment.

A vacuum pump (Yasunaga, LP-30A) at the outlet was used to induce airflow through the tank. The air entered through a flow meter (AWM5104VN, Honeywell, NJ, USA) and passed through a humidifier that kept the humidity of the inlet air larger than 95% at all times. The flow meter continuously measured the flow through the tank during the constant flow experiments. During pneumatic venting the air pump was left running continuously, while the air inlet of the tank was repeatedly blocked and opened in 1-min intervals by use of an inlet valve. Therefore the inlet flow meter could not be used during these experiments. Instead the cumulated volume of air passing the tank was measured by two mechanical volume flow meters placed at the outlet. Two different set-ups were used for concentration measurements. For pneumatic venting, the cumulated mass of TCE removed from the tank was measured by an Activated Carbon filter (AC column, Fig. 1) placed in the outlet stream of the tank. For traditional venting TCE concentrations were measured by a Gas Chromatograph (GC). In this case TCE concentrations were monitored continuously throughout the entire venting period at an extraction port located at the outlet from the tank and at 11 locations inside the tank. Concentrations were not measured in the fine lens of the heterogeneous packs. Air and water pressures were monitored continuously at 14 and 15 locations, respectively, inside the tank. Further, the air pressure drop between the inlet and outlet ports was monitored. All pressure readings were obtained by logging signals from pressure transducers (type 395–279, 395–291, 286–686, Honeywell, NJ, USA). The uncertainty on these transducers is 1% of the measuring span (17.5, 70 and 350 cm H20, respectively). 2.1.2. Gas chromatograph measurements A gas chromatograph (GC) was used to withdraw and analyze air samples from the interior of the tank and from the effluent air during experiments with constant flow. The GC was mounted with a Thermal Conductivity Detector (TCD) and an Electron Capture Detector (ECD). A 25-m DB-1 fused-silica capillary column with an inner diameter of 0.53 mm and a 5-μm film thickness was used. The oven temperature of the TCD and ECD was held constant at 110 °C and 200 °C, respectively. High-purity Helium was used as carrier gas for both detectors as well as make-up gas for the TCD, whereas high-purity nitrogen gas was used as make-up gas for the ECD. The TCD was used for measuring TCE concentrations in the range of approximately 1500 ppm (∼ 8.2 mg/L, 20 °C) to saturation, whereas the ECD was used for measuring concentrations in the range of approximately 2 to 1000 ppm (∼ 0.01 to 5.5 mg/L, 20 °C). Consequently, TCE concentrations in the range 1000 to 1500 ppm were measured less accurately.

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A multi-position valve (16 position SD valve, Valco Instruments Co., Inc.) was mounted between the concentration outtakes and the GC to ensure a fully automatic and continuous switch between sample outtakes. A total of 13 samples constituted a measurement cycle: one fresh air sample, one sample from an extraction port placed in the effluent air from the tank, and 11 samples from extraction ports placed at different locations inside the tank (Fig. 1). Each sample was withdrawn and analyzed within a 3-min interval. Thus one measurement cycle lasted approximately 39 min which also corresponded to the time resolution of sample withdrawals at a specific sample port. Setting the sample interval to 3 min was a tradeoff between obtaining a short time lag between measurements at the same channel and proper flushing of valves, pipes and detectors between sample withdrawals. Failing to flush the system properly between measurements could result in an overestimation of the concentration due to rub-off effects. Tests of rub-off between sampling revealed that samples analyzed by the TCD showed 0% to 0.7% rub-off between consecutive sampling, while the rub-off for the ECD was in an order of 0% to 3% at low and high TCE concentrations, respectively. To minimize the effect of rub-off between measurements, the channels were placed so that the concentrations of TCE would increase along with the channel number in the measurement cycle. Further the effluent samples were withdrawn succeeding a clean air sample. The detection limits of the detectors, here defined by the concentrations having 10% uncertainty, were determined by tests involving six replicate samples of the concentrations 1, 5 and 10 ppm for the ECD, and 1000, 2000 and 5000 ppm for the TCD. The detection limits were determined to 1.3 ppm (∼ 7.1 μg/L, 20 °C) and 1563 ppm (∼ 8.5 mg/L, 20 °C) for the ECD and TCD, respectively. A total of four calibration curves, based on duplicate samples, were produced both for the ECD and TCD, respectively. The results revealed that the measurements analyzed by the ECD showed an uncertainty of approximately 3.5% at concentrations at 10 ppm (∼ 54.6 μg/L, 20 °C) and 1.1% in the range of 50 to 1000 ppm (∼ 0.3 to 5.5 mg/L, 20 °C). The corresponding numbers for the TCD ranged from approximately 5–6% for 5000 ppm (∼ 27.3 mg/L, 20 °C) to approximately 1% to 2% on the saturated concentrations. 2.1.3. Activated carbon measurements During pneumatic SVE experiments vacuum fluctuations were imposed by opening and closing the inlet valve (Fig. 1), while the pump at the outlet was left running continuously. Since high concentrations of TCE were expected immediately after opening the inlet valve, GC measurements would not be suitable for quantifying the mass leaving the system during pneumatic SVE. Instead a stainless steel column containing activated carbon (Reidel-de Haën, granulated purum, prod. no. 18002) was placed at the outlet of the tank. TCE leaving the system would adsorb to the AC, and thereby the cumulated mass of TCE leaving the tank could be determined by gravimetric measurements. As the mass of the AC column was relatively high (290 to 300 g) compared to the sorbed cumulated mass of TCE, a balance of high precision was used (Sartorius CP423S, weighing capacity 420 g, 1 mg precision). Test of the sorption capacity of the AC column (Nasser, 2003), revealed that an effective capture of TCE required a low airflow. To minimize errors the airflow out of the system was therefore split into two paths and the AC column was placed in the low flow path. Apart from sorption of TCE the AC column was also subject to sorption of water. As the air at the outlet end would have humidity higher than 95% this factor could potentially be a major

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Table 1 Properties of pure TCE at 20 °C (Mercer and Cohen, 1990) Density [kg/m3]

Surface tension [dyn/cm]

Interfacial tension [dyn/cm]

Viscosity [cP]

Molecular weight [g/mol]

Saturated vapor concentration [g/m3]

1.465

29.5

34.5 a

0.566

131.39

417

a

At 24 °C.

source of uncertainty. To minimize this effect the AC column was heated to about 80 °C. Nasser (2003) found that heating was beneficial to the reduction of water adsorption but at the same time it reduced the sorption efficiency towards TCE. A temperature of 80 °C secured an appropriate balance between these two opposing processes. Replicate tests with humid air flow through the AC column with the same flow as used in the experiments showed that water would adsorb to the activated carbon in the amount of 0.3 g/m3 air passing the column at a temperature of 80 °C. This adsorption rate was used as a standard correction factor for the gravimetric results. 2.1.4. Fluid and porous medium properties Trichloroethylene (TCE) was used as the contaminant because of its relatively high vapor pressure and its widespread occurrence at contaminated sites. In the experiments TCE was dyed with Sudan Red (0.8 g/L) to facilitate visual inspection of the spreading and recovery. The physical properties of pure TCE are listed in Table 1. Two different types of sands were used in the tank experiments. Coarse sand was used in all homogeneous packs, and in the heterogeneous packs to surround a rectangular lens of fine sand. The coarse sand consisted of equal amounts (by weight) of quartz sand #5F and #7 (Gebrüder Dorfner GmbH & Co., Hirschau, Germany). According to the manufacturer's specifications the sands are washed repeatedly and are free from humic substances and other impurities, and consist of 98.5% quartz with traces of feldspar, clay minerals and small amounts of various oxides. The fine sand used in the heterogeneous experiments was quarts sand Fine Vegas. The fractions of organic carbon (foc) were measured for both types of sand (Table 2) and found to be very low suggesting that sorption of TCE to the sand matrix would be negligible, and if any, TCE would sorb directly to the mineral grains of the sands (Piwoni and Banerjee, 1989). Characterization of the sands included measurement of hydraulic conductivity and air–water capillary pressure curves. The air–water capillary pressure–saturation relationships were Table 2 Properties of sands used in the tank experiments Property

Coarse sand

Fine Sand

Median grain size, d50 [mm] d10 [mm] d60 [mm] Intrinsic permeability, k [m2] Residual water saturation Swr (a/b)⁎ Residual air saturation after imbibition van Genuchten parameter n (a/b)⁎ van Genuchten parameter α [m− 1] Fraction of organic carbon foc [%]

1.2 0.7 1.3 4.70·10− 10 0.04/0.112 0.0 8.12/6.0 13.98/9.56 0.0

0.3 0.1 0.3 1.33·10− 11 0.145 0.15 4.5 3.3 0.01

Parameters n and α represent drainage characteristics. a: fit based on data from the tank. b: fit based on syringe flow pump method.

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measured on samples packed in cylinders, 5 cm height and 5 cm diameter, using the syringe pump method described by Wildenschild (1997). This technique may overestimate the residual saturation (Mortensen et al., 2001) and therefore additional measurements were made for the coarse sand on samples excavated from the tank after completion of the experiments. The samples were taken next to the water pressure transducers and the water contents were determined by gravimetric measurements. Thereby coherent data of water saturation and water pressure were obtained. The van Genuchten model (van Genuchten, 1980) was fitted to the drainage data for both sand types (Table 2). For the coarse sand the syringe pump data were combined with the tank data of the dry end of the drainage curve, and the parametric model was fitted to this curve. 2.2. Experimental procedure 2.2.1. Packing and wetting The tank was packed with air-dried sand by use of a funnel. Coarse steel meshes were inserted into the funnel to facilitate a homogenous packing. The packing of the tank was carried out in steps of 5 cm and after each filling step the tank was tapped at the outside to consolidate the matrix. The heterogeneous packs were constructed by inserting a lens of low permeable sand between levels 32 and 54 cm from the bottom of the tank. The length of the lens was 80 cm created by using thin metal plates, which were removed when the construction of the lens was completed. After the tank was packed imbibition of the porous material was performed at low water flow rates (15 mL/min) to keep air entrapment at a minimum. In the heterogeneous sand pack, injection of water was stopped for a period of several hours, when the water table reached the fine lens. Capillary forces were then allowed to undertake the imbibition of the fine sand, thereby minimizing air entrapment. When the imbibition procedure was completed, the tank was allowed to drain for a period of 5 days. During the first 24 h of drainage, the water table was fixed a few cm above the top of the fine lens, thereby allowing water from the coarse sand above the lens to drain through the high permeable, saturated coarse sand to the left and right of the lens. During the second 24-h period the water table was fixed a few centimeters above the bottom of the fine lens, to avoid effects of capillary barriers between the lens and the underlying coarse sand. Finally after 2 days of drainage, the water table was lowered to its final position 10 cm above the bottom of the tank. This procedure was followed to avoid different water saturations between the homogeneous and heterogeneous packs in upper part of the coarse matrix due to the presence of the fine lens in the heterogeneous pack. The volume of water removed from the tank was determined and used as an estimate of the air volume inside the tank. 2.2.2. TCE injection Dyed TCE was injected through 15 and 30 injection ports in the homogenous and heterogeneous experiments, respectively. The injection ports were placed 4 cm apart. In the homogenous experiments a total of 360 mL of TCE was injected through 15 injection ports located 12 cm below the top of the tank. In the majority of the heterogeneous experiments 180 mL of TCE was injected into the coarse sand (15 ports 12 cm below the top of the tank), and further 60 to 180 mL of TCE was injected directly into the fine lens (15 ports 22.5 cm below the top of the tank). In one of the pneumatic SVE experiments, however, TCE was injected into the fine lens only (60 mL). The experimental data including the injected volumes of TCE for the various experiments are listed in Tables 3 and 4. The TCE injection was performed manually in 8 positions over the entire 8 cm width of the tank to ensure that the TCE was evenly distributed. Injection rates were approximately 2 mL/min.

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Table 3 Experimental conditions for the tanks subjected to constant flow. ΔPair,

Sand pack Injected TCE [mL] Flow rate [L/min] ΔPair, tank [cm H2O] a b

tank

is the air pressure drop across the tank

Hom1

Hom2

Het1

Het2

Homogeneous 360 a 10.9 1.0

Homogeneous 360 b 10.1 0.8

Heterogeneous 360 a 10.4 1.2

Heterogeneous 360 a 9.5 1.0

Dyed TCE. Pure TCE.

The injection procedure was chosen based on preliminary injection tests on the coarse sand. In these tests the sand was excavated after injection and the TCE was observed to be homogeneously distributed within the sand pack. 2.2.3. Venting procedure After the injection procedure was completed, the tank was left to equilibrate for 36 h before the venting was started. In the homogenous experiments (constant airflow) venting was continued until the TCE concentration in the effluent air was almost constant and the tailing of the outlet concentration curve was established. In the heterogeneous experiments (constant airflow) the venting was stopped, when no TCE could be visually observed in the fine lens. In all experiments the airflow rate was approximately 10 L/min. The pneumatic pump scheme involved alternating periods of constant flow and pneumatic venting. Prior to applying pneumatic SVE air transducers with a larger measurement range (from 17.5 cm H2O to 350 cm H2O) were connected. As these transducers had a higher uncertainty (3.5 cm instead of 0.2 cm) air pressures could not be measured precisely during the open inlet periods of the pneumatic pumping scheme. In the first pneumatic experiment TCE was injected both in the fine and coarse sand. Constant flow venting was applied until the TCE was removed from the coarse sand, and the tailing of the outlet concentration curve was reached. At this point the GC instrument was replaced by the AC column, and the pneumatic pump scheme was applied for the remainder of the experiment. As the AC column was used for measuring outlet concentrations of TCE during both constant and pneumatic venting periods, direct comparison of outlet concentrations between these periods was possible. In the second pneumatic experiment TCE was injected in the fine lens only. In this case the pneumatic pump scheme and the measurement technique based on the AC column were used throughout the entire venting period. Table 4 Experimental conditions for the tanks subjected to pneumatic SVE Experiment

Pneum1

Pneum2

Sand pack Injected amount of TCE [mL] Flow rate during period of constant pumping [L/min] ΔPair, tank during period of constant pumping [cm H2O] Qcont, 1 a [L/min] Qcont, 2 a [L/min] Qcont, 3 a [L/min]

Heterogeneous 240 10.0 1.4 10.0 9.7 9.6

Heterogeneous 60 – – 9.2 8.8 8.7

a

Gas flow during constant flow periods of pneumatic SVE.

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When the pneumatic pump scheme was used, a clean preheated (80 °C) AC column was placed at the outlet of the tank. The temperature of the column was kept constant by continuous heating. Pneumatic SVE was imposed by opening and shutting the inlet valve with 1 min intervals, while the air pump was left running. Hereby large fluctuating vacuums were enforced on the tank. Every 5 h the AC column was dismantled for weighting purposes and replaced by a clean pre-heated column while the air pump temporarily was turned off. The pump scheme was continued until no TCE could be observed visually in the fine lens. During pneumatic SVE the airflow at the inlet was temporarily higher shortly after opening of the valve than during continuous pumping periods. When the valve was closed, partial vacuum was building up inside the tank, which caused air to be sucked in at very high rates, when the inlet was re-opened. Four to five seconds after the inlet valve was opened, the flow rate resumed the value set by the outlet pump. 2.3. Data analysis In the experiments performed using constant flow, concentration and flow data were obtained by direct measurements, whereas results had to be deduced indirectly from other data in the pneumatic experiments. The AC columns were placed at the effluent of the tank for certain periods of time, tAC (usually 5 h). In these periods the volume of air (VAC [m3]) passing the AC column and the volume (V [m3]) bypassing the column, were measured by two mechanical volume flow meters. The total volume of air passing the tank, VT [m3], is thus determined as VT;AC ¼ VAC þ V

ð1Þ

The average outlet concentration of TCE during the period tAC, CTCE [kg/m3], was calculated as CTCE ¼

MTCE;AC VAC

ð2Þ

where MTCE, AC [kg] is the mass of TCE sorbed on the AC column in the time period tAC. MTCE, AC was determined gravimetrically and correcting for water sorption (cf. Section 2.1.3) MTCE;AC ¼ MAC −MW;AC

ð3Þ

where MAC is the total mass change of the AC column and MW,AC [kg] is the cumulated mass of water sorbed on the AC column during tAC. MW,AC was calculated as MW;AC ¼ rw;ad VAC

ð4Þ

where rw,ad [kg/m3] is the sorption rate of water to the AC column. The total mass of TCE, MTCE,tot [kg], removed from the tank during tAC is thus MTCE;tot ¼ CTCE VT;AC

ð5Þ

3. Experimental results The results from a total of six tank experiments are presented in the following; four subject to constant flow SVE and two subject to pneumatic SVE. The overall experimental conditions for the six tank experiments are tabulated in Tables 3 and 4.

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3.1. Initial conditions and phase distributions The experiments were designed to examine the venting process in a sand matrix with stagnant water and NAPL. Drainage tests suggested that the water content profiles remained constant after 5 days of drainage and consequently all experiments were exposed to this drainage period before imposing the venting process. In addition the drainage tests suggested that water content profiles within the coarse sand of the homogenous and heterogeneous packs were identical. The initial water saturation and relative air permeability distributions within the packs were estimated based on the measured water pressure in the tank and using the capillary pressurerelative permeability-saturation relationships proposed by Parker et al. (1987) in combination with secondary drainage capillary pressure–saturation data for a core sample. In the coarse sand water saturation was estimated to be 0.04 in most of the pack except near the water table. In the fine lens water saturation varied from 0.72 in the upper part of the lens to 0.85 in the lower part. Thereby the relative air permeability of the lens varied from 5 10− 2 in the upper part to 9 10− 3 in the lower part. Thus even though the ratio between the intrinsic permeabilities of the coarse and fine sands is 35:1, the air permeability ratio is in the order of 1500:1. This ratio is well above the criteria of 100:1 that Ho and Udell (1992) found would cause complete air by-passing of the low permeable area. After equilibration of water in the tank, TCE was injected and allowed to equilibrate for 36 h. For the homogeneous pack visual inspection suggested that a uniform distribution of TCE was obtained in the area designed to be contaminated (Fig. 2A) and with an estimated saturation of 0.07. For the heterogeneous pack TCE was also observed to be uniformly distributed in the coarse sand (Fig. 2B) while it was not possible to attain the same degree of uniformity in the fine sand due to micro-layering. 3.2. Pressure distribution Fig. 3A shows the measured air pressure distribution during venting of tank experiment Hom1 (see Table 3 for experimental conditions). The pressure distribution is representative of both homogenous experiments, and was observed to be constant throughout the entire experiment. At the outlet the isobars were close to being vertical as the air was sucked out through an outlet port covering the entire width and height of the tank. Contrarily at the intake bending of isobars towards the inlet ports caused an upward flow of air after entering the tank (Fig. 3A). Fig. 3B shows the pressure distribution for the Het1 experiment which is also representative for the Het2 experiment. As the air is expected to bypass the lens, the isobar distribution is

Fig. 2. Distribution of TCE plume at different times in terms of pore volumes that have passed the tank for the homogenous (A) and the heterogeneous (B) experiments. The dark areas in the fine lens (B) indicate the placement of TCE after approximately 6400 pore volumes.

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Fig. 3. Isobars and flow lines during venting of the homogeneous experiment (A), and air pressure distribution (unit cm H2O) during venting of the heterogeneous experiment (B).

significantly more complex and it would be too uncertain to draw isobars on the basis of the available sampling grid. However the readings indicate that the pressure drop close to the upper inlet (A3, A4 and A5 on Fig. 1) is increased significantly compared to the homogeneous experiment. Due to the bypassing of the lens a larger amount of air was forced upwards in the area between the upper inlet and the lens, causing larger pressure drops in the vertical direction above the upper inlet. Simple Darcy based computations were made to corroborate the notion that air was bypassing the lens. The effective cross-sectional area of the homogenous pack was determined as the width of the tank (0.08 m) times the height of the unsaturated zone within the tank (0.58 m) determined as the height from the top of the capillary fringe to the top of the tank. Using measured pressure drop across the tank and measured flow the effective permeability of the coarse sand pack could be determined. For the heterogeneous pack the effective cross sectional flow area was subsequently derived based on measured pressure drop and flow for this setup and assuming that the same effective permeability applied for the coarse sand. In this manner a ratio of 1.4 between the effective cross sectional flow-areas of the two packs was found. This ratio corresponds to a reduction in height of 17 cm which compares closely to the lens height of 20 cm thus supporting the notion that the sand lens acted as a barrier to air flow.

Fig. 4. Air pressure readings during pneumatic SVE within (A8) and above the low permeable lens (A9).

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Table 5 Overall mass balances for tank experiments

Total volume injected [mL] Total volume recovered [mL] Mass balance error [%]

Hom1

Hom2

Het1

Het2

Pneum1

Pneum2

360 350.0 − 2.8

360 370.2 2.8

360 351.7 − 2.3

360.0 362.4 0.7

240 237.9 −0.9

60.0 59.7 − 0.5

When pneumatic SVE was used, large pressure drops were imposed on the system during the closed inlet periods. Fig. 4 shows the air pressure measurements in position A8 and A9 (cf. Fig 1). The air pressure was constant in the open inlet periods, and decreased when the inlet valve was closed. A fast initial decrease was observed, but as the maximum pump suction capacity was approached the pressure decrease was reduced. Upon re-opening of the inlet valve the air pressure increased instantaneously. The maximum pressure drop in both experiments was approximately 190 cm H20 corresponding to an absolute pressure inside the tank of 0.8 atm prior to the opening of the inlet valve. 3.3. Concentration and mass balances 3.3.1. Mass balances The mass balances for the experiments are tabulated in Table 5. As seen from the table excellent mass balances are obtained with errors in the range of − 2.8% to 2.8%. For the heterogeneous experiments mass balances for both the coarse sand and fine lens were calculated (Table 6). The volume of TCE recovered from the coarse sand was estimated by summing up the mass of TCE exiting the tank until the coarse sand was completely depleted by TCE as assessed from visual inspection. From this point in time the recovered TCE was assumed to originate from the fine lens only. From Table 6 it is seen that the estimated recovery of TCE from the coarse sand is less than injected. On the other hand a higher recovery is estimated for the fine lens and the deficit volume of the coarse sand corresponds well to the excess volume of fine lens. Thus it is likely that some of the TCE injected into the coarse sand has invaded the fine lens. The infiltrating amount is in the range of 20 to 23 mL. If this amount is subtracted from the TCE volume injected into the coarse sand, the residual TCE saturation of the coarse sand is estimated to be 0.07. This value is consistent with the residual saturation found for the coarse sand during the homogeneous experiments (cf. Section 3.1). For the Het1 experiment the difference between the injected and the recovered volume for the coarse sand was relatively high and higher than the estimated excess recovery from the fine lens. Table 6 Mass balances for the coarse and fine sand in the heterogeneous experiements Het1

Vinjec [mL] Vrecov [mL] MB error [%] Vrecov − Vinjec [mL]

Het2

Pneum1

Pneum2

Coarse

Fine

Coarse

Fine

Coarse

Fine

Coarse

Fine

180 149.0 −17.2 −31.0

180 202.7 12.6 22.7

180 159.0 − 11.7 − 21.0

180 203.4 13.0 23.4

180 157.9 − 12.3 − 22.1

60 79.9 33.2 19.9

– – – –

60 59.7 − 0.5 − 0.3

Vinjec is the injected volume of TCE, Vrecov is the volume of TCE recovered from the outlet of the tank.

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Fig. 5. Outlet concentration curve from the homogeneous experiment Hom1 (A) and both homogenous experiments Hom1 and Hom2 (B).

During injection it was observed that a small amount of TCE migrated towards the bottom of the tank below the water table and thus it is likely that the amount of TCE not accounted for (8.3 mL) in the experiment still resided in the tank upon completion of the experiment. 3.3.2. Concentration measurements in the homogeneous tank experiments The outlet concentrations from the Hom1 experiment are shown at Fig. 5A. When the venting was started the TCE concentration at the outlet of the tank declined moderately as a function of time up until the point (approximately 320 pore volumes) where the TCE phase was visually observed to be removed. At this time a steep decline occurred followed by a characteristic tailing caused by mass transfer limitations between the air and soil/water. Prior to TCE phase depletion, the slope of the curve depended upon the flow pattern inside the tank. The air pressure readings indicated that the pattern was very much influenced by the geometry of the inlet/outlet and outlet ports; however, the presence of water and TCE will also have an impact as air permeability is reduced accordingly. As shown in Fig. 2A the removal of TCE was faster in the lower part of the contaminated area than in the upper part at early stages of the venting experiment reflecting the non-uniform flow pattern present in the tank. Comparing Figs. 2A and 3A suggests that the flow field estimated on basis of the air pressure readings corresponds well with the observed removal of the TCE plume. The second homogeneous experiment (Hom2) was performed to investigate if the results from the first experiment could be reproduced when using pure TCE without dye. The outlet concentrations from both homogeneous experiments are compared in Fig. 5B. The results from the two experiments are consistent, and considering that two different sand packs were used suggests that dying of TCE did not influence the venting results. 3.3.3. Concentration measurements in the heterogeneous tank experiments (constant flow) The outlet concentrations from the heterogeneous experiments are shown in Fig. 6. When the venting was started the outlet concentrations declined relatively fast up until the point, where the TCE phase was removed from the coarse sand. This happened after approximately 930 pore volumes for both experiments. At this point the removal rate was reduced dramatically, and tailing concentrations were obtained in both experiments indicating that the removal of TCE from the fine lens was subject to diffusion limitations or significantly reduced advective transport. During the early stages of the tailing periods (1000–10000 pore volumes), the outlet concentration curves are in close agreement (Fig. 6A), whereas at later stages they differ (Fig. 6B). At early stages TCE was primarily removed from the upper part of the lens where a rather uniform distribution was

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Fig. 6. Outlet concentration curves from the heterogeneous experiments Het1 and Het2.

observed while at later stages removal took place from the lower part where the TCE distribution is much more non-uniform (Fig. 2B). These differences in NAPL distributions may explain the disparity in outlet concentrations between the two experiments at late stages. 3.3.4. Concentration measurements in the pneumatic tank experiments Fig. 7A shows the outlet concentration curve for the Pneum1 and Pneum2 experiments. Time zero is defined when the pneumatic pump scheme was imposed on the tank, i.e. when the AC column was connected. For the Pneum1 experiment this occurred after approximately 1100 pore volumes of constant pumping. At this point approximately 96 g (65.5 mL) of TCE remained in the tank. The pneumatic pumping scheme was started by firstly enforcing constant flow through the tank for approximately 1860 pore volumes. Then in the period between 1860 and 4020 pore volumes, the tank was subjected to pneumatic SVE. This change in operation clearly resulted in an initial large increase in the outlet concentration, followed by a steep decrease through the remaining pneumatic venting period. At the very end of the period the outlet concentration had dropped below the level obtained during the first constant flow period. When constant flow subsequently was enforced on the tank (4020 to 7620 pore volumes), the decline of the outlet concentration continued, but the slope of the outlet curve was less steep. After 7620 pore volumes pneumatic SVE was once more imposed. Again the system responded by an increased outlet concentration. This time, however, the outlet concentration remained constant throughout the pneumatic period. Fig. 7A also shows the outlet concentrations for the Pneum2 experiment. In general the two pneumatic experiments exhibited a similar behavior with the only difference that the second

Fig. 7. Outlet concentrations (A) and cumulated mass removal (B) for the pneumatic SVE experiments.

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Table 7 Removal data for various periods of the pneumatic SVE experiments Period

Pore volumes [−] TCE removed [%] TCE left in tank [%] Removal rate Increase (+)/Decrease(−) [mg/pore volume] in removal rate [%]

PNEUM1 Constant Pneumatic a Pneumatic b Constant Pneumatic Constant

1868 1198 954 3603 1006 1255

33.0 33.8 10.6 14.4 4.4 0.6

67.0 33.3 22.6 8.2 3.8 3.2

16.9 27.1 10.7 3.8 4.3 0.4

– +60.2 − 60.5 − 64.3 +11.4 − 89.6

PNEUM2 Constant Pneumatic a Pneumatic b Constant a Constant b Pneumatic Constant

2660 1192 770 1179 1169 1635 3786

38.7 30.7 11.4 9.1 3.6 6.0 0

61.3 30.6 19.2 10.1 6.5 0.5 0.5

12.8 22.6 14.8 6.8 2.7 3.2 0

– +76.7 − 34.5 − 53.8 − 60.4 +18.6 –

Last column represents the change in removal rate compared to the previous flow period. a Early stage of the individual periods. b Late stage of the individual periods.

pneumatic period of the Pneum2 experiment responded more in a manner of the first pneumatic periods. In general, however, both experiments documented a sharp increase in outlet concentrations when pneumatic SVE was applied. In Fig. 7B the cumulated mass removal for the two pneumatic experiments is plotted against the pore volume. A distinct increase in slope occurs during the initial part of the first pneumatic periods and then the slope gradually decreases at later times. Computed mass removal characteristics for the different periods of the pneumatic experiments are listed in Table 7. Removal rates are calculated for “early” and “late” stages of the individual venting periods. If the controlling transport mechanism in the tank was diffusion-limited transport between the lens and the coarse sand, the removal rate would decrease throughout the venting period. Table 7 indicates that this was not the case, since the removal rate increased, when pneumatic SVE was enforced on the tank. The increase was between 11% and 77% compared to the previous constant flow period. This clearly indicates that mechanisms other than diffusion were responsible for the TCE removal from the fine lens during the pneumatic SVE period. The results from both pneumatic experiments indicate that during the first period of pneumatic SVE the removal rates decreased with time by 35% and 60% at late stages of this period. During the subsequent constant flow periods the removal rates were further reduced (between 54% and 64%) indicating that even during periods of low efficiency of the pneumatic method, the removal rates were still higher than those obtained by traditional SVE. 4. Discussion and analysis Fig. 8 shows the outlet concentrations (Fig. 8A) and mass recovery (Fig. 8B) for the Hom1 and Het1 experiments, respectively. The curves demonstrate the dramatic effect that the presence of the low permeable lens has on the effectiveness of the traditional venting technique. In both

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Fig. 8. Outlet concentrations (A) and mass recovery (B) for the homogeneous (Hom1) and heterogeneous (Het1) experiments.

experiments 360 mL of TCE was injected into the tanks. In the heterogeneous pack, however, half of the injected volume was placed in the low permeable lens. When the TCE located in the coarse sand of the heterogeneous pack was removed, the presence of the lens caused a significant reduction in the removal rate for the remainder of the experiment. The processes responsible for the removal of TCE from the fine lens are horizontal advective transport through the fine lens and diffusive transport from the fine lens to the advective zone in the coarse sand. The diffusive transport needs to be the dominant process to benefit from pneumatic venting and to examine this we calculated the Peclet number defined as the ratio of advective mass transport to diffusive mass transport: Pe ¼

Jadv Kair JLd ¼ Jdiff hair Deff

ð6Þ

where Kair is the air conductivity, J is the hydraulic gradient across the lens (assuming horizontal flow only), Ld is the diffusion length which is set equal to half the lens height, θair is the air filled porosity, and Deff is the effective diffusion coefficient for TCE in the air fraction of the void space. Inserting approximate estimates of the parameters appearing in Eq. (6) yields Pe ¼

1:8d10−7 m=sd 6:3d10−3 d 0:1m ¼ 0:09 0:21d2d10−8 m2 =s

ð7Þ

This result indicates that vertical diffusive transport from the lens to the coarse sand is more dominant than horizontal advective transport through the lens. However, for areas close to the boundary between the fine and the coarse sand diffusive transport is expected to be even more dominant due to the small diffusion distance. The pneumatic experiments documented an enhanced TCE removal from the fine lens of the heterogeneous packs and evidently some mechanisms are introduced that facilitate this removal. Two mechanisms are hypothesized to be active when the transient pressure fronts were imposed on the system: (1) enhanced flow of contaminated air out of the low permeable lens due to expansion of air at low pressures, and (2) enhanced mixing of air due to fluctuations in airflow direction. Inside the fine lens the alternating expansion and compression of air create mixing of the air resulting in a more uniform concentration distribution as compared to the case where only diffusive mass transport occurs. As a result higher concentration of TCE will occur near the

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boundaries between the fine and the coarse sand and higher mass transfer rates between the lens and the advective flow field can be expected. Whether flow indeed occurred between the low permeable lens and the coarse sand during pneumatic SVE can be assessed by examining the air pressure readings inside and above the lens. Pressure readings from positions A8 and A9 (Fig. 1) from the pneumatic SVE experiment are shown in Fig. 4. In the open inlet periods, the pressure readings in the two positions apparently differed considerably (approx. 3.5 cm H2O), whereas the air pressure readings during constant flow venting of the heterogeneous pack showed that the difference between A8 and A9 was approximately 0.2 cm H2O (Fig. 3B). This apparent inconsistency may be due to change of pressure transducers and associated accuracy. Therefore it is assumed that the difference between the A9 and A8 pressure readings in the open inlet periods of the pneumatic experiments was caused by an off-set error. To compensate for this the measurements from the A9 transducer are corrected by subtracting this apparent off-set such that the A8 and A9 pressure readings coincide during the open inlet periods. In Fig. 9 the ratio between the corrected A9 readings (A9_cor) and the A8 readings is presented. It is seen that when the inlet valve was closed the ratio increased indicating that the absolute value of A9_cor increased faster than A8 and flow was thus promoted from the lens to the coarse sand. As time passed during the closed inlet period, the pressure difference outbalanced and the flow ceased until the inlet was opened again. At this point the ratio decreased indicating that air flowed from the coarse sand into the fine lens. As a result flow continuously changed direction to and from the lens during pneumatic venting. Assuming that the high flow velocities produced when opening the inlet valve would remove the TCE from the coarse sand above the lens before the air re-entered the lens, a net advective transport of TCE from the lens to the coarse sand would result. On the other hand, if the transport was too slow in the coarse sand, net advective transport would not occur, but the continuous change in flow direction would result in mixing of the contaminated air inside the lens. The effect of air expansion is evaluated by the following analysis. Assuming a mean air saturation of 0.21 in the fine lens (see Section 3.1), the air volume inside the lens can be estimated to be approximately 0.8 L at atmospheric pressure conditions. Assuming that the gas behaves as an ideal gas a pressure reduction of 0.8 atm will lead to a 25% increase in air volume for isothermal conditions. By geometrical reasoning it is calculated that air will be expelled from the fine lens within a distance of about 2 cm from the interface towards the coarse material during closure of the inlet valve. This mechanism will evidently facilitate the removal of TCE from the lens.

Fig. 9. Ratio of air pressure readings at location A9 (outside fine lens) and location A8 (inside fine lens).

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5. Conclusions Pneumatic SVE was tested on two different heterogeneous packs at two different stages of the venting process. During the first period the removal rate increased by 60% and 77%, respectively, for the two packs in comparison to the removal rates of the preceding constant flow period. During the second pneumatic period the increase in removal rate was 11% and 19%, respectively. At late stages of the first pneumatic period the removal rates decreased indicating that the pneumatic SVE cannot entirely overcome the problem of mass transfer limitation. The proposed processes responsible for the enhanced removal are (1) expansion of air forcing contamination out of the low permeable horizons, and (2) enhanced mixing due to the fluctuating pressure. The study documents that pneumatic SVE offers an enhanced removal compared to the traditional SVE technique at laboratory scale and for simple porous medium settings. The experiments have also shown that although the recovery of the contamination is enhanced, still the method does not entirely overcome the problem of mass transfer limitation. The laboratory experiments are very useful in developing the technology for full scale operations. On-going research involves numerical modeling of the presented experiments to quantify and confirm the proposed processes responsible for enhanced removal during pneumatic venting. Numerical modeling of full scale scenarios for examining the effect of the geological settings, imposed vacuum, location of vacuum pumps, and pumping intervals will be an obvious next step for further testing of the technique. A field demonstration experiment may then be designed as the following step and based on these results the technique may be implemented in practice. Perhaps the most important question is if it is possible to obtain the relative large pressure drops needed for the pneumatic venting technique to be effective at field sites. Air shortcutting from the surface may reduce the efficiency significantly. Also unknown geological settings and fragmental knowledge of the spatial location of the contamination may imply that the remediation is not designed properly but other technologies face the same problem. Acknowledgements We gratefully acknowledge Associate Professor Hans Mosbæk for his competent guidance and unfailing support in the design and operation of the monitoring systems. We would also like to thank two anonymous reviewers for very constructive comments. References Armstrong, J.E., Frind, E.O., MacClennan, R.D., 1994. Nonequilibrium mass transfer between the vapor, aqueous, and solid phases in unsaturated soils during vapor extraction. Water Resources Research 30 (2), 355–368. Brusseau, M.L., 1991. Transport of organic-chemicals by gas advection in structured or heterogeneous porous media: development of a model and application to column experiments. Water Resources Research 27 (12), 3189–3199. Croisé, J., Armstrong, J.E., Kaleris, V., 1994. Numerical simulation of rate-limited vapour extraction of volatile organic compounds in wet sands. In: Dracos, Th.A., Stauffer, F. (Eds.), Transport and Reactive Processes in Aquifers. Balkema, Rotterdam, pp. 569–575. Fisher, U., Schulin, R., Keller, M., Stauffer, F., 1996. Experimental and numerical investigation of soil vapor extraction. Water Resources Research 32 (12), 3413–3427. Gierke, J.S., Hutzler, N.J., MacKenzie, D.B., 1992. Vapor transport in unsaturated soil columns: implications for vapor extraction. Water Resources Research 28 (2), 323–335. Ho, C.K., Udell, K.S., 1992. An experimental investigation of air venting of volatile liquid hydrocarbon mixtures from homogenous and heterogeneous porous media. Journal of Contaminant Hydrology 11, 291–316.

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