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Vapor extraction experiments with laboratory soil columns: Implications for field programs
WASTE MANAGEMENT, Vol. 11, pp. 231-239, 1991 Printed in thc USA. All rights rcserved.
0956-053X/91 $3.00 + .00 Copyright © 1991 Pergamon Press plc
VAPOR EXTRACTION EXPERIMENTS WITH LABORATORY SOIL COLUMNS: Implications for Field Programs Peter M. Kearl and Nic E. Korte* Environmental Science Division, Oak Ridge National Laboratory, Grand Junction Office, Grand Junction, Colorado 81503, U.S.A.
T. A. Gleason T. A. Gleason Associates, Cincinnati, Ohio 45208, U.S.A.
John S. Beale Allied Signal Aerospace Corporation, Torrance, California 90509, U.S.A.
A B S T R A C T . As part of a site remediation project, laboratory soil column experiments were conducted to evaluate the
effectiveness of a field vapor extraction system. Different soil types were placed in specially designed soil columns and saturated with 1,1,1-Trichloroethane and Jet-A fuel. The soil columns were connected to a vacuum pump and removal rates were monitored using mass balance, a portable photoionization detector, and a gas chromatograph. Results of the laboratory experiments indicated that the technique is useful for designing and monitoring field vapor extraction systems. Guidelines were developed for flow rate versus removal times, the effects of varying lithologies on removal rates and efficiencies, and the removal characteristics of organic mixtures consisting of varying volatile components.
INTRODUCTION
or higher (3). Examples of compounds that have been effectively removed include trichloroethene, 1,1,1-trichloroethane, tetrachloroethene, and most gasoline constituents (3). Although the site conditions and the nature of the contaminants may be conducive to remediation using vapor extraction, there are several questions concerning operational methodology that remain unanswered. These include:
Soil vapor extraction is an increasingly common remediation method for hazardous waste sites. If a spill of volatile organic compounds (VOCs) has penetrated the subsurface 5 to 15 m, has spread several hundred m at a particular depth, or has a total volume in excess of 500 cubic meters, then excavation and disposal costs may exceed those associated with a vapor extraction system (VES) (1). In fact, if groundwater is at a depth of more than 4.6 m and the contamination extends to the water table, the most cost effective method for removing volatile organic compounds (VCs) from the soil may be VES (2). Likewise, w h e n e v e r c o n t a m i n a t i o n e x t e n d s across a property boundary, beneath a building, or is located within an extensive utility trench network, vapor extraction should be considered (2). The VES technique is generally assumed to be useful for compounds that exhibit vapor pressures of 0.5 mm of Hg
• Reliability of available models for accurately predicting the effectiveness of VES. • The proper role for soil column studies in the design and operation of full scale vapor extraction systems. • Effects of varying lithologies on operating efficiencies. • Effects on removal rate of organic compounds with varying vapor pressures. • The time when there are no further benefits from operating VES in a conventional manner.
RECEIVED 18 JANUARY 1991; ACCEPTED 27 JUNE 1991. *To whom correspondence may be addressed. Acknowledgements - - The authors wish to express their thanks to Paul Arbesman of Allied-Signal Corporation for his support and suggestions, We also would like to thank Al Remetch and Jeff Price for their assistance with the laboratory experiments.
A technical demonstration of VES at a superfund site has been conducted (4). In addition, modeling studies to predict the effectiveness of VES in the laboratory and field sites have been performed ( 5 231
232
P. M. KEARL ET AL/
8). These modeling studies, however, have not addressed the problem of interbedded sediments consisting of permeable and low permeable units. This common field situation is important because these low permeable sediments, such as clays and silts, will contain the bulk of the organic contaminants in the subsurface. In order to resolve these questions, a series of laboratory experiments were conducted using soil columns. This paper describes those experiments, the equipment and testing methodology, the results, and their application to a full-scale field program. BACKGROUND
Soils underlying an industrial facility in southern California have been contaminated by several organic compounds, notably 1,1,1-trichloroethane, as a result of spills and leaking underground tanks. The geology beneath the site consists of alternate layers of coarse grain sands and fine grain units; dominantly silts and clays. The regional water table is located approximately 100 ft below the surface resulting in a substantial thickness of unsaturated soils. A vapor extraction system (VES) has been designed to remediate the contaminated soils. The VES consists of a surface blower connected to extraction wells completed in the unsaturated zone (Fig. 1). The wells are screened in the sand units and sealed with bentonite from just above the well screens to the ground surface. The wells are designed so that large volumes of air can be extracted from the permeable sands. As the organic vapor concentrations in the sand units
are reduced, organic compounds contained in the surrounding clayey silt units will diffuse into the sand units and eventually be removed by the VES. In order to evaluate the validity of the design and develop procedures for operating the VES, a series of laboratory experiments were conducted using soil columns containing 1,1,1-trichloroethane and Jet-A jet fuel. LABORATORY
APPARATUS
Two soil columns were designed for the laboratory extraction experiments (Fig. 2). Both columns are 6.3 cm in diameter and were designed to hold a 20 cm soil column. The first column was designed such that air flowed through the entire length of the soil (designated column #1). In the second column air flow was directed only through the upper portion of the soil (designated column #2). Sampling ports were located on the upstream and downstream sides of the columns. The upstream sampling port was used to monitor the influent gas concentration while the downstream sampling port was used to monitor the effluent. A schematic of the flow system is shown in Fig. 2. Both columns were connected to the extraction system by quick disconnect couplers. These couplers permitted rapid removal, weighing, and reconnection. Weighing of the soil column was accomplished using a top-loading balance with a range of 2000 g and an accuracy of 0.01 g. The soil columns were connected to a 25 liter per minute (lpm) electric vacuum pump. A flow meter, connected on the discharge side of the pump, was
.A¥
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,Y FIGURE 1. Vapor extraction well design and air flow/diffusionpatterns based on lithology.
VAPOR EXTRACTION EXPERIMENTS
233 COLUMN
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FIGL1RE 2. Laboratory apparatus illustrating different columns used in the experiment.
used to measure and control the air-flow rate. A vacuum gauge, located on the intake side of the pump, was used to monitor the system pressure. A gas chromatograph equipped with a flame ionization detector (Hewlett-Packard Model 5890) and a hand held Hnu model photoionization detector were used in conjunction with mass readings to determine the amount of VOC removed from the columns. PROCEDURES
A sandy soil and a fine-grain soil from the study site were used for the column experiments. The sandy soil was collected at a depth of 21 m and consists of medium grain, moderately sorted quartz sand. The moisture content of this sand is generally less than 2% by volume. Consequently, water content will have negligible effect on permeability. Based on air pumping tests, the permeability of the sand unit is 10 -5 cm 2. The fine-grain soil, from a depth of 4.6 m, consists dominantly of silt with minor amounts of clay. Moisture contents for this unit average approximately 10% by volume. Based on lithology, this unit has an estimated permeability of 10- ]<~cm 2.
Samples were collected using a ring sampler. The soils were dried in an oven at 105 °C for 24 hours prior to placement in the columns. Three types of column experiments were run. Column 1 (Fig. 2) was used for the sand only experiments. The sand was compacted as densely as possible in the column in order to simulate natural field conditions. In another set of experiments using the same column, a clayey silt soil core was removed from the 3.8 cm diameter ring sampler and carefully placed directly into the soil column. Sand was then poured around the clayey silt core and carefully compacted. For the final series of experiments, column 2 (Fig. 2) was used. The lower portion of the column is 5 cm in diameter the same size as the outer diameter of the soil core from the ring sampler. A 15 cm length of clayey silt soil was placed in the column such that the top of the core was just below the intake/outtake ports (Fig. 2). Sand was then placed on top of the finegrain soil but was not compacted to avoid compaction of the core. The soil columns were saturated using 1,1,trichloroethane (1,1,1-TCA) or Jet-A fuel. 1,1,I-TCA was chosen because it is the most common contam-
234
P . M . KEARL E T A L .
inant at the study area and because it has a lower vapor pressure than the other site contaminants. Compounds exhibiting the lowest vapor pressures will be removed at a slower rate and, therefore, will be the last contaminants removed from the soil. JetA fuel is not found at the site but was tested because it is composed of a wide range of compounds with a wide range of vapor pressures. Following saturation, the soil columns were allowed to drain for a period of 24 hours. The 1,1,1TCA and Jet-A that remained in residual saturation is the field capacity for that particular compound or mixture for the soils under study. The glass columns containing the soil samples were connected to the vacuum pump as illustrated in Fig. 2. The pump provided air of a known flow rate and temperature to the column. The removal of the organics from the soil column was monitored by means of mass measurements and measurements with a gas chromatograph and a hand held photoionization detector. To measure the mass of contaminant removed, the soil column was isolated using valves, detached from the pump, weighed, and reconnected to the system as soon as possible. This procedure usually took 15 to 30 seconds. Concurrently, air samples were collected from the sampling port using a gas syringe. Measurements with the photoionization detector were made by connecting the instrument to a tee in the effluent line and sampling the gas as it flowed from the column.
LABORATORY
RESULTS
Test results for several of the soil column experiments are presented in Figs. 3 through 6. These figures present plots of the mass of organic compound remaining in the soil column and effluent concentrations measured by the GC and Hnu as a function of time.
were extremely variable in contrast to the mass measurements. The difference between successive measurements was as much as 100/Lg/L. This variance is attributed to the sampling technique which consisted simply of removing a volume of gas from the effluent stream with a microsyringe. Turbulent flow in the gas stream and the high concentration of contaminant are believed responsible for the variations. However, once the concentrations of 1,1,1-TCA are in the tens of ILg/L range, the G C results declined in a uniform manner similar to the mass measurements. At this point in the experiment, concentrations in the gas stream are not changing so rapidly. Consequently, the turbulent flow in the effluent stream no longer exerts a significant effect on the sampling. Fortunately, it is the lower concentration range that is important in determining when the cleanup is complete. It is important to note that the high values measured by the GC appear reasonable considering the saturated vapor concentration of 1,1,1-TCA. Falta et al. (9) report a saturated vapor concentration of 890 itg/cc at 25 °C. The G C results were as high as 500 ltg/L - - a value which does not exceed the theoretical maximum. The measurements with the photoionization detector declined in the same relatively uniform manner as the mass measurements. The photoionization values, however, are not proportional to the GC results, While GC values decrease by over two orders of magnitude during the test, Hnu values only decline by a single order of magnitude. The inaccuracy at high concentration is due to the detector simply being overwhelmed by the high concentration of contaminants. The lack of accuracy at the low concentration range is simply due to the fact that the photoionization detector is calibrated with benzene and the response for 1,1,1-TCA is different.
Reproducibility Usefulness of Organic Measurement Methods" In order to evaluate and monitor the effectiveness of the VES system, it is also necessary to measure the concentration of the organic compounds in the effluent gas. Thus, measurements from the gas chromatograph and from the Hnu were compared with the mass measurements. Mass measurements were expected to be the most useful for column experiments but, of course, have no application for the field. The gas chromatograph is more sensitive but the photoionization detector is used by the State of California to determine compliance with Rule 1166 (a requirement of the field program) when evaluating organic emissions from a site. Thus, measurements were also obtained with an Hnu hand held photoionization detector. At high concentrations, the GC measurements
Three experiments were performed at one lpm with a sand column saturated with 1,1,1-TCA to evaluate the reproducibility of the technique (Fig. 3). In all three tests, approximately 100 minutes was necessary to remove 99 plus percent of the 1,1,1-TCA from the soil column. Two experiments were also performed with column 1 containing a clayey silt soil core surrounded with sand and saturated with I , I , I - T C A (see Fig. 4). These figures show that there is a distinct slope change in the plot of mass as a function of time. This slope change occurred in two experiments at 100 and 125 minutes, respectively. Based on the experiments with sand only, the change in slope suggests that 1,1,1-TCA has been removed from the sand at this point and that the clayey silt soil (Fig. 4A) contained 12 grams of 1,1,1-TCA after the sand has been
V A P O R EXTRACTION EXPERIMENTS 110 100
235
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FIGURE 3. I.I,I-TCA removal rates from sand columnpumped at 25 lpm (A), 5 lpm (B), and 1 lpm (C).
cleaned. An additional 150 minutes were required to remove the remaining 1,1,1-TCA. The same quantity of fine-grained material was used in Fig. 4B but 19 grams of I,I,I-TCA remained after the sand was cleaned. An additional 300 minutes were required to finish removal of the 1,1,1-TCA. Reproducibility was deemed adequate based on the similarity of the test results conducted on the sand and fine-grained soil columns. These experiments, therefore, demonstrated that the laboratory equipment and mass measurement technique are reliable for evaluating removal rates of organic compounds from soils in the laboratory.
Flow Rate Variations
Sand columns with residual saturations of 1,1,1-TCA were pumped at three different flow rates (Fig. 5). The mass measurements of the soil columns demonstrate that 20, 50, and 100 minutes of pumping at 25, 5, and 1 lpm, respectively, were needed to re-
move 99 plus percent of the 1,1,1-TCA from the soil columns. These results can be compared to a mass transport model utilizing equilibrium assumptions presented in Johnson et al. (8). These assumptions assume that equilibrium exists between vapor, free liquid, sorbed, and dissolved phases. The validity of this assumption can be assessed by calculating the distance (z) over which air becomes saturated with contaminant vapors using the following equation: z = 2/~L2/D
[1]
where /z is the pore vapor velocity, L is the pore throat diameter, and D is the molecular diffusivity of the compound in air. Using a typical pore throat diameter of 0.10 cm and a diffusion coefficient of 0.10 cm2/s (8), the equilibrium distances for the sand columns presented in Fig. 5 can be calculated. At 1 lpm (Fig. 5C), the pore vapor velocity for a sand column 6.3 cm in diameter exhibiting a porosity of
236
P . M . KEARL ET AL.
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FIGURE
MASS of TCA (grams) 0C SAMPLES ( u g / c c ) U (ppm)
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Reproducibility comparison of I,I,I-TCA removal rates from sand column pumped at 1 Ipm.
110-- --1100 100-- --1000
0.3 is 1.8 cm/s. Substituting these values into Eq. [1] yields an equilibrium distance of 0.36 cm. Since the sand columns are 20 cm in length, there is adequate distance for the air to become saturated with vapor. Results for the one lpm flow rate, shown in Fig. 5C, support this model. Because it takes approximately 100 minutes at a one lpm flow rate to remove all of the 1,1,1-TCA, 100 liters of air flows through the column. Using a saturated vapor concentration for 1,1,1-TCA of 0.89 g/cm 3 (9), the mass transport model predicts that 89 g of 1,1,1-TCA will be removed. Since there was 88.3 g of 1,1,1-TCA in residual saturation in the soil column, this is an excellent match with the predicted results. Consequently, experimental results for the one lpm flow rate indicate that the removal rate is flow dependent, Based on the above calculations, equilibrium conditions between liquid and vapor 1,1,1-TCA do not exist at the higher flow rates of 5 and 25 lpm. As illustrated in Fig. 5B, at a 25 lpm flow rate 250 liters of air flow through the sand column before the 1,1,1TCA is completely removed. Based on a saturated vapor concentration of 0.89 g/cm 3, the mass transfer model predicts that 222.5 g will be removed. This is nearly three times the original amount of 1,1,1-TCA present in the sand indicating that equilibrium conditions do not exist at the higher flow rate. This interpretation is supported by the GC measurements
E 8090-- 8 0 0 .~ ~ 70- ---700 == ~, 0o- - - 600 0-~ 5,o-6oo~ ~ L~,~t - - 400 ~ 30- - 300 -'oo [ 2O-1o__,oo o- - o , [ , i
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FIGURE 5. Reproducibility comparison of 1,1,I-TCA removal rates from silty clay column pumped at 1 lpm.
VAPOR EXTRACTION EXPERIMENTS
that indicate the effluent vapor is undersaturated with respect to 1,1,1-TCA. Equilibrium calculations for the 5 and 25 lpm flow rates indicate that equilibrium distances are 1.8 and 9 cm, respectively. Because the soil column is 20 cm in length, equilibrium conditions should occur. This is not, however, the case for the higher flow conditions. Consequently, rate of diffusion controls migration at these high flow rates. These test results indicate that for flow velocities (ll) at or below 2 cm/s (1 lpm flow rate), the removal rate of 1,1,1-TCA is flow dependent. At flow velocities between 2 and 9 cm/s (1 to 5 lpm flow rates), diffusion becomes the controlling factor for removal rates. For field applications, optimal recovery rates will occur in the soil where flow velocities are in the 2 to 9 cm/s range. In otherwords, higher flow rates will not increase the amount of 1,1,1-TCA removed but will result in a higher volume of air that will have to be treated. Consequently, inefficient recovery of 1,1,1-TCA will occur in regions where the flow velocity exceeds this range and partial removal will occur at flow rates below this velocity range.
Effects of Soil Type on Removal Rates Three sets of experiments were conducted to evaluate the effects of different soil types on the removal rate of 1,1,1-TCA. The first set of measurements was conducted on the sand columns as described previously (Fig. 5). These experiments simulated the field conditions where extraction wells are screened in the sandy zones. The wells are screened in the sand in order to induce high air flow rates. The results showed that 1,1,1-TCA is rapidly removed in a uniform manner from the sand. This rapid uniform removal is attributed to the permeable nature of the sand which permits large volumes of air to pass through the soil pores. Also contributing to rapid removal is the sand's low moisture retention characteristics. Due to the relatively larger pores in the sand, capillary forces are low resulting in less liquid retained in the soil matrix. Less liquid results in a larger air-filled volume in the pores allowing more effective mass transfer of volatile organic compounds. The second set of experiments was conducted using a core of clayey silt material surrounded by sand (Fig. 4). These experiments simulate the field circumstances in which thin clayey silt lenses are interbedded with the sand. These experiments were performed in order to estimate removal rates from the fine-grain lenses. Clayey silt materials exhibit higher capillary forces capable of retaining larger quantities of liquids compared to the sand. Thus, determining the removal rates from these thin clayey silt units was essential for a proper evaluation of the VES field program.
237
Both experiments conducted with a sand/clayey silt soil column showed a rapid removal of 1,1,1-TCA from the sand with a significantly reduced removal rate from the clay. The experiments showed, however, that 1,1,1-TCA is removed from the clayey silt material demonstrating that the VES will clean thin lenses of clayey silt material in the subsurface, if sufficient venting duration is provided. The final experiment conducted with 1,1,1-TCA consisted of a thick sequence of clayey silt material overlain by sand using column 2 (Fig. 6A). This experiment simulated a thick sequence of clayey silt material similar to the thick zones underlying the study site. By passing air through the overlying sand, field conditions where only the sand units are screened and pumped were simulated. This experiment was performed to evaluate the assumption that organic compounds will migrate from the clayey silt units due to the pressure gradient induced by the venturi effect of the air flow in the sand and by diffusion due to the reduced concentrations in the sand. Experimental results show that 1,1,1-TCA is removed from the clayey silt material at a substantially slower rate than was observed for the thin zone experiment (see Fig. 6A). Approximately 45 days of pumping were needed to remove over 90% of the initial mass of 1,1,1-TCA from the clayey silt material. Another 53 days of pumping yielded only one
300--
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FIGURE 6. I,I,I-TCA removal rate from column 2 (A) and removal rate of jet-A from column 1 (B).
238
additional gram of 1,1,1-TCA. This small amount of 1,1,1-TCA being removed at the end of the test indicates that conventional VES removes organic compounds very slowly from thick clayey silt units.
P . M . KEARL E T A L .
~°
Tests Using Jet-A Fuel The sand column was saturated with Jet-A fuel to evaluate the removal rate of less volatile organic compounds (Fig. 6B). Jet fuels typically consist primarily of saturated hydrocarbons. Alkylbenzenes are always present as well but in concentrations less than 10% of the total. After completing the laboratory extraction experiment, a portion of the soil was removed from the column, placed in a sample jar, and submitted to an analytical laboratory for analysis. The result indicated that benzene, toluene, ethylbenzene, and xylene were not detectable. The concentration of hydrocarbons remaining, however, was 38,000 mg/kg with a chromatographic fingerprint that still closely matched that of jet fuel. Results of the experiment (Fig. 6B) show that only slightly more than 50% of the organics are removed after 31 days of pumping. The initial portion of the curve indicates that the more volatile components (alkylbenzenes such as toluene and xylenes), are removed leaving behind the long chain organic molecules. It appears that only eight days of pumping were needed to remove the more volatile components. The heavier compounds are still being stripped but at a substantially reduced rate. These test results clearly show that the VES will remove the more volatile components of Jet-A within a few days but the heavier less volatile compounds will not be removed as rapidly. Consequently, secondary treatment or enhanced removal techniques, such as injection of heated air, may be required to complete treatment rapidly enough to satisfy economic and regulatory requirements. Based on the laboratory tests, this secondary treatment should begin early in the treatment process. RESULTS OF THE FIELD VES P R O G R A M
Vacuum extraction wells installed in highly contaminated areas were connected to surface blowers to initiate the soils cleanup. Total VOC concentrations were measured daily in the effluent gas stream and plotted as a function of days in which the system operated (Fig. 7). Increases in concentrations illustrated in the figure occur as the result of the system being shutdown for maintenance or repair. The curve for the field test is similar to that for the tests conducted using 1,1,1-TCA with the thick section of silty clay (Fig. 6A). Initial VOC concentrations of 30 Itg/cc rapidly decrease by nearly one half after four days of pumping. After 22 days of
0
5
10
15
20
25
30
35
40
45
50
55
60
65
RUN (DA¥$)
FIGURE 7. VOC concentrationsin the effluentgas fromthe field VES system. pumping, the VOC concentration had been reduced to 5 itg/cc. Based on interpretations from the soil column experiments, it appears that VOCs have been removed from the sand units. From 22 to 64 days, the VOC concentration only declined an additional 2/tg/cc. Once again, based on the laboratory study, it appears that VOCs are now present only in the fine grain units and are being removed at a substantially reduced rate. This observation is confirmed by soil samples that showed clean sands but significant VOC concentrations in the fine-grain units. Comparisons of the laboratory and field results indicate that operating the VES system in the present manner is of limited benefit in removing VOCs from the fine grain units. At this point during the field program, a decision should be made to cease soil remediation thus leaving a residual VOC concentration in the fine grain soils or several modifications to the VES operation should be implemented. Shallow soils (less than 2 m below land surface) that are contaminated may be more effectively cleaned using surface remediation techniques. Since the system is now diffusion controlled, the air flow rates can be reduced. In addition, different pumping strategies can be implemented to remove VOCs from the fine grain units. For VOCs in the middle silt and clay unit shown in Fig. 1, air can be injected into the upper sand unit wells and extracted from the lower sand unit wells. This will induce air flow across the silty clay increasing local removal rates. Heated air injected into the subsurface is another possibility to enhance the removal of VOCs in the deeper silty clay units. In summary, results from the field VES program indicate that VOCs are still present in the silty clay unit underlying the site. This interpretation is based on the results of the laboratory experiments.
CONCLUSIONS A N D RECOMMENDATIONS
The laboratory soil column experiments provided a useful guide for interpreting the subsequent field VES remediation program results presented in Fig.
VAPOR EXTRACTION EXPERIMENTS
7. The strategy of pumping the sand units and allowing VOCs to diffuse from the clayey silt material is shown to be an effective removal technique for thin beds of fine-grained material. The laboratory studies also suggest that the declining removal rate of VOCs in the pilot study indicates that a portion of the VOCs remain in clayey silt units. If the VOC concentrations in the effluent stabilize at a significant nonzero value, the laboratory data suggests that the sand units in the field are clean but that substantial concentrations may exist in the clayey silt material. Based on the results of the laboratory experiments, the following specific conclusions can be drawn: • The laboratory technique described in this report is useful for evaluating removal rates of various organic compounds in different soils. Comparisons of the field VES curve with the lab results from 15 cm section of clayey silt used in column 2 show similar responses. This data indicates that 1,1,1TCA spilled in the field has entered the clayey silt units and is being removed at a greatly reduced rate. Based on the lab results, a significant pumping time, perhaps years, will be necessary to remove the 1,1,1-TCA from the subsurface clayey silt unit at the field site. • At flow velocities of 2 cm/s, the flow rate controls the removal rate of 1,1,1-TCA from the soil columns. At greater flow velocities, diffusion begins to control the rate of removal. These results indicate that a mass transport model that assumes equilibrium conditions prevail under such flow velocities needs to be modified. • 1,1,1-TCA is rapidly removed from the sand. As expected, a longer period of time is required for the clayey silt sample. It is possible, however, to remove chlorinated solvents in thin units of clayey silt material (less than 2.5 cm in diameter) by forcing air through the sand portions and allowing the chlorinated solvent to diffuse from the clayey silt into the sand. • Even after extended periods of operation conventional VES will remove only a portion of 1,1,1TCA from thick sections (15 cm) of clayey silt material. • VES rapidly removes the volatile portions of JetA Fuel from sand. The heavier organic compounds can be stripped from the sand but at a substantially reduced rate. Because the laboratory studies showed that VES will not remove all of the VOCs in thicker sections of clayey silt material, an alternative pumping strategy may be necessary. For this field site, an alternate strategy may consist of injecting air, perhaps heated, into the lower sand zone and extracting air via the
239
VES from the upper sand unit. This would force air across clayey silt units and enhance removal efficiencies. In summary, this paper has shown that laboratory Column studies can be useful in designing and interpreting the data from field VES remediation programs. By using site specific soils and contaminants, the laboratory columns can provide information on optimal pumping rates, design and operation of the VES system, and guidance for assessing when there are no further benefits from operating the VES in a conventional manner. The low cost of the equipment, reliability of the mass measurements, and the ease of conducting the experiments result in a useful technique for conducting VES remediation programs at various field locations. The laboratory columns are not intended to replace soil sampling as a determination of cleanup. Soil sampling can, however, provide misleading or incorrect results. Because of analytical techniques, VOCs in soil can often be missed or underestimated. Additionally, subsurface VOC sources may be missed during field sampling. The laboratory columns provide additional information at low cost that can enhance the final determination of the VES's performance. REFERENCES 1. CH2M-HILL Inc. Remedial planning/field investigative team: Vcrona well field - - Thomas Solvent Company, Battle Creek, Michigan. Operable unit feasibility study. Reston, VA (1985). 2. Hutzlcr, N. J., Murphy, B. E., and Gierke, J. S. State of technology review soil vapor extraction systems. Environmental Protection Agency Hazardous Waste Eng. Res. Lab., Cincinnati, OH 11988). 3. Bennedsen, M. B., Scott, J. P., and Hartley, J. D. Use of vapor extraction systems in site removal of volatile organic compounds from soil. Proceedings of the National conference on hazardous wastes and hazardous materials, HMCRI. 92-95
11985). 4. EnvironmentalProtectionAgency.Technologydemonstration summary: Terra Vac in situ vacuumextractionsystemGroveland. Massachusetts. EPA/540/S5-89/003, Washington,D.C. (1989). 5. Wilson, D. E., Montgomery, R, E., and Sheller, M, R. A mathcmatical model for removing volatile subsurface hydrocarbons by miscible displacement. Water, Air, Soil Poll. 33: 231-255 (1987). 6. Massmann, J. W. Applying groundwater flow models in vapor extraction system design. J. of Environ. Eng. 115: 1, 129-149 (1989). 7. Baehr, A. L., Hoag, G. E., and Marley, M. C. Removing volatile contaminants from the unsaturated zone by inducing advective air-phase transport. Journal of Contaminant Hydrology 4 : 1 - 2 6 (1989). 8. Johnson, P. C., Kemblowski, M W., and Colthart, J. D. Quantitative analysis lor the cleanup of hydrocarbon contaminated soils by in-situ soil venting. Groundwater28:3,413-429 (1990). 9. Falta, R. W., Javandel, I., Pruess, K., and Witherspoon, P. A. Density-driven flow of gas in the unsaturated zone due to the evaporation of volatile organic compounds. Water Resources Research 25: 10, 2159-2169 (1989).
0956-053X/91 $3.00 + .00 Copyright © 1991 Pergamon Press plc
VAPOR EXTRACTION EXPERIMENTS WITH LABORATORY SOIL COLUMNS: Implications for Field Programs Peter M. Kearl and Nic E. Korte* Environmental Science Division, Oak Ridge National Laboratory, Grand Junction Office, Grand Junction, Colorado 81503, U.S.A.
T. A. Gleason T. A. Gleason Associates, Cincinnati, Ohio 45208, U.S.A.
John S. Beale Allied Signal Aerospace Corporation, Torrance, California 90509, U.S.A.
A B S T R A C T . As part of a site remediation project, laboratory soil column experiments were conducted to evaluate the
effectiveness of a field vapor extraction system. Different soil types were placed in specially designed soil columns and saturated with 1,1,1-Trichloroethane and Jet-A fuel. The soil columns were connected to a vacuum pump and removal rates were monitored using mass balance, a portable photoionization detector, and a gas chromatograph. Results of the laboratory experiments indicated that the technique is useful for designing and monitoring field vapor extraction systems. Guidelines were developed for flow rate versus removal times, the effects of varying lithologies on removal rates and efficiencies, and the removal characteristics of organic mixtures consisting of varying volatile components.
INTRODUCTION
or higher (3). Examples of compounds that have been effectively removed include trichloroethene, 1,1,1-trichloroethane, tetrachloroethene, and most gasoline constituents (3). Although the site conditions and the nature of the contaminants may be conducive to remediation using vapor extraction, there are several questions concerning operational methodology that remain unanswered. These include:
Soil vapor extraction is an increasingly common remediation method for hazardous waste sites. If a spill of volatile organic compounds (VOCs) has penetrated the subsurface 5 to 15 m, has spread several hundred m at a particular depth, or has a total volume in excess of 500 cubic meters, then excavation and disposal costs may exceed those associated with a vapor extraction system (VES) (1). In fact, if groundwater is at a depth of more than 4.6 m and the contamination extends to the water table, the most cost effective method for removing volatile organic compounds (VCs) from the soil may be VES (2). Likewise, w h e n e v e r c o n t a m i n a t i o n e x t e n d s across a property boundary, beneath a building, or is located within an extensive utility trench network, vapor extraction should be considered (2). The VES technique is generally assumed to be useful for compounds that exhibit vapor pressures of 0.5 mm of Hg
• Reliability of available models for accurately predicting the effectiveness of VES. • The proper role for soil column studies in the design and operation of full scale vapor extraction systems. • Effects of varying lithologies on operating efficiencies. • Effects on removal rate of organic compounds with varying vapor pressures. • The time when there are no further benefits from operating VES in a conventional manner.
RECEIVED 18 JANUARY 1991; ACCEPTED 27 JUNE 1991. *To whom correspondence may be addressed. Acknowledgements - - The authors wish to express their thanks to Paul Arbesman of Allied-Signal Corporation for his support and suggestions, We also would like to thank Al Remetch and Jeff Price for their assistance with the laboratory experiments.
A technical demonstration of VES at a superfund site has been conducted (4). In addition, modeling studies to predict the effectiveness of VES in the laboratory and field sites have been performed ( 5 231
232
P. M. KEARL ET AL/
8). These modeling studies, however, have not addressed the problem of interbedded sediments consisting of permeable and low permeable units. This common field situation is important because these low permeable sediments, such as clays and silts, will contain the bulk of the organic contaminants in the subsurface. In order to resolve these questions, a series of laboratory experiments were conducted using soil columns. This paper describes those experiments, the equipment and testing methodology, the results, and their application to a full-scale field program. BACKGROUND
Soils underlying an industrial facility in southern California have been contaminated by several organic compounds, notably 1,1,1-trichloroethane, as a result of spills and leaking underground tanks. The geology beneath the site consists of alternate layers of coarse grain sands and fine grain units; dominantly silts and clays. The regional water table is located approximately 100 ft below the surface resulting in a substantial thickness of unsaturated soils. A vapor extraction system (VES) has been designed to remediate the contaminated soils. The VES consists of a surface blower connected to extraction wells completed in the unsaturated zone (Fig. 1). The wells are screened in the sand units and sealed with bentonite from just above the well screens to the ground surface. The wells are designed so that large volumes of air can be extracted from the permeable sands. As the organic vapor concentrations in the sand units
are reduced, organic compounds contained in the surrounding clayey silt units will diffuse into the sand units and eventually be removed by the VES. In order to evaluate the validity of the design and develop procedures for operating the VES, a series of laboratory experiments were conducted using soil columns containing 1,1,1-trichloroethane and Jet-A jet fuel. LABORATORY
APPARATUS
Two soil columns were designed for the laboratory extraction experiments (Fig. 2). Both columns are 6.3 cm in diameter and were designed to hold a 20 cm soil column. The first column was designed such that air flowed through the entire length of the soil (designated column #1). In the second column air flow was directed only through the upper portion of the soil (designated column #2). Sampling ports were located on the upstream and downstream sides of the columns. The upstream sampling port was used to monitor the influent gas concentration while the downstream sampling port was used to monitor the effluent. A schematic of the flow system is shown in Fig. 2. Both columns were connected to the extraction system by quick disconnect couplers. These couplers permitted rapid removal, weighing, and reconnection. Weighing of the soil column was accomplished using a top-loading balance with a range of 2000 g and an accuracy of 0.01 g. The soil columns were connected to a 25 liter per minute (lpm) electric vacuum pump. A flow meter, connected on the discharge side of the pump, was
.A¥
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VAPOR EXTRACTION EXPERIMENTS
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used to measure and control the air-flow rate. A vacuum gauge, located on the intake side of the pump, was used to monitor the system pressure. A gas chromatograph equipped with a flame ionization detector (Hewlett-Packard Model 5890) and a hand held Hnu model photoionization detector were used in conjunction with mass readings to determine the amount of VOC removed from the columns. PROCEDURES
A sandy soil and a fine-grain soil from the study site were used for the column experiments. The sandy soil was collected at a depth of 21 m and consists of medium grain, moderately sorted quartz sand. The moisture content of this sand is generally less than 2% by volume. Consequently, water content will have negligible effect on permeability. Based on air pumping tests, the permeability of the sand unit is 10 -5 cm 2. The fine-grain soil, from a depth of 4.6 m, consists dominantly of silt with minor amounts of clay. Moisture contents for this unit average approximately 10% by volume. Based on lithology, this unit has an estimated permeability of 10- ]<~cm 2.
Samples were collected using a ring sampler. The soils were dried in an oven at 105 °C for 24 hours prior to placement in the columns. Three types of column experiments were run. Column 1 (Fig. 2) was used for the sand only experiments. The sand was compacted as densely as possible in the column in order to simulate natural field conditions. In another set of experiments using the same column, a clayey silt soil core was removed from the 3.8 cm diameter ring sampler and carefully placed directly into the soil column. Sand was then poured around the clayey silt core and carefully compacted. For the final series of experiments, column 2 (Fig. 2) was used. The lower portion of the column is 5 cm in diameter the same size as the outer diameter of the soil core from the ring sampler. A 15 cm length of clayey silt soil was placed in the column such that the top of the core was just below the intake/outtake ports (Fig. 2). Sand was then placed on top of the finegrain soil but was not compacted to avoid compaction of the core. The soil columns were saturated using 1,1,trichloroethane (1,1,1-TCA) or Jet-A fuel. 1,1,I-TCA was chosen because it is the most common contam-
234
P . M . KEARL E T A L .
inant at the study area and because it has a lower vapor pressure than the other site contaminants. Compounds exhibiting the lowest vapor pressures will be removed at a slower rate and, therefore, will be the last contaminants removed from the soil. JetA fuel is not found at the site but was tested because it is composed of a wide range of compounds with a wide range of vapor pressures. Following saturation, the soil columns were allowed to drain for a period of 24 hours. The 1,1,1TCA and Jet-A that remained in residual saturation is the field capacity for that particular compound or mixture for the soils under study. The glass columns containing the soil samples were connected to the vacuum pump as illustrated in Fig. 2. The pump provided air of a known flow rate and temperature to the column. The removal of the organics from the soil column was monitored by means of mass measurements and measurements with a gas chromatograph and a hand held photoionization detector. To measure the mass of contaminant removed, the soil column was isolated using valves, detached from the pump, weighed, and reconnected to the system as soon as possible. This procedure usually took 15 to 30 seconds. Concurrently, air samples were collected from the sampling port using a gas syringe. Measurements with the photoionization detector were made by connecting the instrument to a tee in the effluent line and sampling the gas as it flowed from the column.
LABORATORY
RESULTS
Test results for several of the soil column experiments are presented in Figs. 3 through 6. These figures present plots of the mass of organic compound remaining in the soil column and effluent concentrations measured by the GC and Hnu as a function of time.
were extremely variable in contrast to the mass measurements. The difference between successive measurements was as much as 100/Lg/L. This variance is attributed to the sampling technique which consisted simply of removing a volume of gas from the effluent stream with a microsyringe. Turbulent flow in the gas stream and the high concentration of contaminant are believed responsible for the variations. However, once the concentrations of 1,1,1-TCA are in the tens of ILg/L range, the G C results declined in a uniform manner similar to the mass measurements. At this point in the experiment, concentrations in the gas stream are not changing so rapidly. Consequently, the turbulent flow in the effluent stream no longer exerts a significant effect on the sampling. Fortunately, it is the lower concentration range that is important in determining when the cleanup is complete. It is important to note that the high values measured by the GC appear reasonable considering the saturated vapor concentration of 1,1,1-TCA. Falta et al. (9) report a saturated vapor concentration of 890 itg/cc at 25 °C. The G C results were as high as 500 ltg/L - - a value which does not exceed the theoretical maximum. The measurements with the photoionization detector declined in the same relatively uniform manner as the mass measurements. The photoionization values, however, are not proportional to the GC results, While GC values decrease by over two orders of magnitude during the test, Hnu values only decline by a single order of magnitude. The inaccuracy at high concentration is due to the detector simply being overwhelmed by the high concentration of contaminants. The lack of accuracy at the low concentration range is simply due to the fact that the photoionization detector is calibrated with benzene and the response for 1,1,1-TCA is different.
Reproducibility Usefulness of Organic Measurement Methods" In order to evaluate and monitor the effectiveness of the VES system, it is also necessary to measure the concentration of the organic compounds in the effluent gas. Thus, measurements from the gas chromatograph and from the Hnu were compared with the mass measurements. Mass measurements were expected to be the most useful for column experiments but, of course, have no application for the field. The gas chromatograph is more sensitive but the photoionization detector is used by the State of California to determine compliance with Rule 1166 (a requirement of the field program) when evaluating organic emissions from a site. Thus, measurements were also obtained with an Hnu hand held photoionization detector. At high concentrations, the GC measurements
Three experiments were performed at one lpm with a sand column saturated with 1,1,1-TCA to evaluate the reproducibility of the technique (Fig. 3). In all three tests, approximately 100 minutes was necessary to remove 99 plus percent of the 1,1,1-TCA from the soil column. Two experiments were also performed with column 1 containing a clayey silt soil core surrounded with sand and saturated with I , I , I - T C A (see Fig. 4). These figures show that there is a distinct slope change in the plot of mass as a function of time. This slope change occurred in two experiments at 100 and 125 minutes, respectively. Based on the experiments with sand only, the change in slope suggests that 1,1,1-TCA has been removed from the sand at this point and that the clayey silt soil (Fig. 4A) contained 12 grams of 1,1,1-TCA after the sand has been
V A P O R EXTRACTION EXPERIMENTS 110 100
235
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cleaned. An additional 150 minutes were required to remove the remaining 1,1,1-TCA. The same quantity of fine-grained material was used in Fig. 4B but 19 grams of I,I,I-TCA remained after the sand was cleaned. An additional 300 minutes were required to finish removal of the 1,1,1-TCA. Reproducibility was deemed adequate based on the similarity of the test results conducted on the sand and fine-grained soil columns. These experiments, therefore, demonstrated that the laboratory equipment and mass measurement technique are reliable for evaluating removal rates of organic compounds from soils in the laboratory.
Flow Rate Variations
Sand columns with residual saturations of 1,1,1-TCA were pumped at three different flow rates (Fig. 5). The mass measurements of the soil columns demonstrate that 20, 50, and 100 minutes of pumping at 25, 5, and 1 lpm, respectively, were needed to re-
move 99 plus percent of the 1,1,1-TCA from the soil columns. These results can be compared to a mass transport model utilizing equilibrium assumptions presented in Johnson et al. (8). These assumptions assume that equilibrium exists between vapor, free liquid, sorbed, and dissolved phases. The validity of this assumption can be assessed by calculating the distance (z) over which air becomes saturated with contaminant vapors using the following equation: z = 2/~L2/D
[1]
where /z is the pore vapor velocity, L is the pore throat diameter, and D is the molecular diffusivity of the compound in air. Using a typical pore throat diameter of 0.10 cm and a diffusion coefficient of 0.10 cm2/s (8), the equilibrium distances for the sand columns presented in Fig. 5 can be calculated. At 1 lpm (Fig. 5C), the pore vapor velocity for a sand column 6.3 cm in diameter exhibiting a porosity of
236
P . M . KEARL ET AL.
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Reproducibility comparison of I,I,I-TCA removal rates from sand column pumped at 1 Ipm.
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0.3 is 1.8 cm/s. Substituting these values into Eq. [1] yields an equilibrium distance of 0.36 cm. Since the sand columns are 20 cm in length, there is adequate distance for the air to become saturated with vapor. Results for the one lpm flow rate, shown in Fig. 5C, support this model. Because it takes approximately 100 minutes at a one lpm flow rate to remove all of the 1,1,1-TCA, 100 liters of air flows through the column. Using a saturated vapor concentration for 1,1,1-TCA of 0.89 g/cm 3 (9), the mass transport model predicts that 89 g of 1,1,1-TCA will be removed. Since there was 88.3 g of 1,1,1-TCA in residual saturation in the soil column, this is an excellent match with the predicted results. Consequently, experimental results for the one lpm flow rate indicate that the removal rate is flow dependent, Based on the above calculations, equilibrium conditions between liquid and vapor 1,1,1-TCA do not exist at the higher flow rates of 5 and 25 lpm. As illustrated in Fig. 5B, at a 25 lpm flow rate 250 liters of air flow through the sand column before the 1,1,1TCA is completely removed. Based on a saturated vapor concentration of 0.89 g/cm 3, the mass transfer model predicts that 222.5 g will be removed. This is nearly three times the original amount of 1,1,1-TCA present in the sand indicating that equilibrium conditions do not exist at the higher flow rate. This interpretation is supported by the GC measurements
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VAPOR EXTRACTION EXPERIMENTS
that indicate the effluent vapor is undersaturated with respect to 1,1,1-TCA. Equilibrium calculations for the 5 and 25 lpm flow rates indicate that equilibrium distances are 1.8 and 9 cm, respectively. Because the soil column is 20 cm in length, equilibrium conditions should occur. This is not, however, the case for the higher flow conditions. Consequently, rate of diffusion controls migration at these high flow rates. These test results indicate that for flow velocities (ll) at or below 2 cm/s (1 lpm flow rate), the removal rate of 1,1,1-TCA is flow dependent. At flow velocities between 2 and 9 cm/s (1 to 5 lpm flow rates), diffusion becomes the controlling factor for removal rates. For field applications, optimal recovery rates will occur in the soil where flow velocities are in the 2 to 9 cm/s range. In otherwords, higher flow rates will not increase the amount of 1,1,1-TCA removed but will result in a higher volume of air that will have to be treated. Consequently, inefficient recovery of 1,1,1-TCA will occur in regions where the flow velocity exceeds this range and partial removal will occur at flow rates below this velocity range.
Effects of Soil Type on Removal Rates Three sets of experiments were conducted to evaluate the effects of different soil types on the removal rate of 1,1,1-TCA. The first set of measurements was conducted on the sand columns as described previously (Fig. 5). These experiments simulated the field conditions where extraction wells are screened in the sandy zones. The wells are screened in the sand in order to induce high air flow rates. The results showed that 1,1,1-TCA is rapidly removed in a uniform manner from the sand. This rapid uniform removal is attributed to the permeable nature of the sand which permits large volumes of air to pass through the soil pores. Also contributing to rapid removal is the sand's low moisture retention characteristics. Due to the relatively larger pores in the sand, capillary forces are low resulting in less liquid retained in the soil matrix. Less liquid results in a larger air-filled volume in the pores allowing more effective mass transfer of volatile organic compounds. The second set of experiments was conducted using a core of clayey silt material surrounded by sand (Fig. 4). These experiments simulate the field circumstances in which thin clayey silt lenses are interbedded with the sand. These experiments were performed in order to estimate removal rates from the fine-grain lenses. Clayey silt materials exhibit higher capillary forces capable of retaining larger quantities of liquids compared to the sand. Thus, determining the removal rates from these thin clayey silt units was essential for a proper evaluation of the VES field program.
237
Both experiments conducted with a sand/clayey silt soil column showed a rapid removal of 1,1,1-TCA from the sand with a significantly reduced removal rate from the clay. The experiments showed, however, that 1,1,1-TCA is removed from the clayey silt material demonstrating that the VES will clean thin lenses of clayey silt material in the subsurface, if sufficient venting duration is provided. The final experiment conducted with 1,1,1-TCA consisted of a thick sequence of clayey silt material overlain by sand using column 2 (Fig. 6A). This experiment simulated a thick sequence of clayey silt material similar to the thick zones underlying the study site. By passing air through the overlying sand, field conditions where only the sand units are screened and pumped were simulated. This experiment was performed to evaluate the assumption that organic compounds will migrate from the clayey silt units due to the pressure gradient induced by the venturi effect of the air flow in the sand and by diffusion due to the reduced concentrations in the sand. Experimental results show that 1,1,1-TCA is removed from the clayey silt material at a substantially slower rate than was observed for the thin zone experiment (see Fig. 6A). Approximately 45 days of pumping were needed to remove over 90% of the initial mass of 1,1,1-TCA from the clayey silt material. Another 53 days of pumping yielded only one
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FIGURE 6. I,I,I-TCA removal rate from column 2 (A) and removal rate of jet-A from column 1 (B).
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additional gram of 1,1,1-TCA. This small amount of 1,1,1-TCA being removed at the end of the test indicates that conventional VES removes organic compounds very slowly from thick clayey silt units.
P . M . KEARL E T A L .
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Tests Using Jet-A Fuel The sand column was saturated with Jet-A fuel to evaluate the removal rate of less volatile organic compounds (Fig. 6B). Jet fuels typically consist primarily of saturated hydrocarbons. Alkylbenzenes are always present as well but in concentrations less than 10% of the total. After completing the laboratory extraction experiment, a portion of the soil was removed from the column, placed in a sample jar, and submitted to an analytical laboratory for analysis. The result indicated that benzene, toluene, ethylbenzene, and xylene were not detectable. The concentration of hydrocarbons remaining, however, was 38,000 mg/kg with a chromatographic fingerprint that still closely matched that of jet fuel. Results of the experiment (Fig. 6B) show that only slightly more than 50% of the organics are removed after 31 days of pumping. The initial portion of the curve indicates that the more volatile components (alkylbenzenes such as toluene and xylenes), are removed leaving behind the long chain organic molecules. It appears that only eight days of pumping were needed to remove the more volatile components. The heavier compounds are still being stripped but at a substantially reduced rate. These test results clearly show that the VES will remove the more volatile components of Jet-A within a few days but the heavier less volatile compounds will not be removed as rapidly. Consequently, secondary treatment or enhanced removal techniques, such as injection of heated air, may be required to complete treatment rapidly enough to satisfy economic and regulatory requirements. Based on the laboratory tests, this secondary treatment should begin early in the treatment process. RESULTS OF THE FIELD VES P R O G R A M
Vacuum extraction wells installed in highly contaminated areas were connected to surface blowers to initiate the soils cleanup. Total VOC concentrations were measured daily in the effluent gas stream and plotted as a function of days in which the system operated (Fig. 7). Increases in concentrations illustrated in the figure occur as the result of the system being shutdown for maintenance or repair. The curve for the field test is similar to that for the tests conducted using 1,1,1-TCA with the thick section of silty clay (Fig. 6A). Initial VOC concentrations of 30 Itg/cc rapidly decrease by nearly one half after four days of pumping. After 22 days of
0
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RUN (DA¥$)
FIGURE 7. VOC concentrationsin the effluentgas fromthe field VES system. pumping, the VOC concentration had been reduced to 5 itg/cc. Based on interpretations from the soil column experiments, it appears that VOCs have been removed from the sand units. From 22 to 64 days, the VOC concentration only declined an additional 2/tg/cc. Once again, based on the laboratory study, it appears that VOCs are now present only in the fine grain units and are being removed at a substantially reduced rate. This observation is confirmed by soil samples that showed clean sands but significant VOC concentrations in the fine-grain units. Comparisons of the laboratory and field results indicate that operating the VES system in the present manner is of limited benefit in removing VOCs from the fine grain units. At this point during the field program, a decision should be made to cease soil remediation thus leaving a residual VOC concentration in the fine grain soils or several modifications to the VES operation should be implemented. Shallow soils (less than 2 m below land surface) that are contaminated may be more effectively cleaned using surface remediation techniques. Since the system is now diffusion controlled, the air flow rates can be reduced. In addition, different pumping strategies can be implemented to remove VOCs from the fine grain units. For VOCs in the middle silt and clay unit shown in Fig. 1, air can be injected into the upper sand unit wells and extracted from the lower sand unit wells. This will induce air flow across the silty clay increasing local removal rates. Heated air injected into the subsurface is another possibility to enhance the removal of VOCs in the deeper silty clay units. In summary, results from the field VES program indicate that VOCs are still present in the silty clay unit underlying the site. This interpretation is based on the results of the laboratory experiments.
CONCLUSIONS A N D RECOMMENDATIONS
The laboratory soil column experiments provided a useful guide for interpreting the subsequent field VES remediation program results presented in Fig.
VAPOR EXTRACTION EXPERIMENTS
7. The strategy of pumping the sand units and allowing VOCs to diffuse from the clayey silt material is shown to be an effective removal technique for thin beds of fine-grained material. The laboratory studies also suggest that the declining removal rate of VOCs in the pilot study indicates that a portion of the VOCs remain in clayey silt units. If the VOC concentrations in the effluent stabilize at a significant nonzero value, the laboratory data suggests that the sand units in the field are clean but that substantial concentrations may exist in the clayey silt material. Based on the results of the laboratory experiments, the following specific conclusions can be drawn: • The laboratory technique described in this report is useful for evaluating removal rates of various organic compounds in different soils. Comparisons of the field VES curve with the lab results from 15 cm section of clayey silt used in column 2 show similar responses. This data indicates that 1,1,1TCA spilled in the field has entered the clayey silt units and is being removed at a greatly reduced rate. Based on the lab results, a significant pumping time, perhaps years, will be necessary to remove the 1,1,1-TCA from the subsurface clayey silt unit at the field site. • At flow velocities of 2 cm/s, the flow rate controls the removal rate of 1,1,1-TCA from the soil columns. At greater flow velocities, diffusion begins to control the rate of removal. These results indicate that a mass transport model that assumes equilibrium conditions prevail under such flow velocities needs to be modified. • 1,1,1-TCA is rapidly removed from the sand. As expected, a longer period of time is required for the clayey silt sample. It is possible, however, to remove chlorinated solvents in thin units of clayey silt material (less than 2.5 cm in diameter) by forcing air through the sand portions and allowing the chlorinated solvent to diffuse from the clayey silt into the sand. • Even after extended periods of operation conventional VES will remove only a portion of 1,1,1TCA from thick sections (15 cm) of clayey silt material. • VES rapidly removes the volatile portions of JetA Fuel from sand. The heavier organic compounds can be stripped from the sand but at a substantially reduced rate. Because the laboratory studies showed that VES will not remove all of the VOCs in thicker sections of clayey silt material, an alternative pumping strategy may be necessary. For this field site, an alternate strategy may consist of injecting air, perhaps heated, into the lower sand zone and extracting air via the
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VES from the upper sand unit. This would force air across clayey silt units and enhance removal efficiencies. In summary, this paper has shown that laboratory Column studies can be useful in designing and interpreting the data from field VES remediation programs. By using site specific soils and contaminants, the laboratory columns can provide information on optimal pumping rates, design and operation of the VES system, and guidance for assessing when there are no further benefits from operating the VES in a conventional manner. The low cost of the equipment, reliability of the mass measurements, and the ease of conducting the experiments result in a useful technique for conducting VES remediation programs at various field locations. The laboratory columns are not intended to replace soil sampling as a determination of cleanup. Soil sampling can, however, provide misleading or incorrect results. Because of analytical techniques, VOCs in soil can often be missed or underestimated. Additionally, subsurface VOC sources may be missed during field sampling. The laboratory columns provide additional information at low cost that can enhance the final determination of the VES's performance. REFERENCES 1. CH2M-HILL Inc. Remedial planning/field investigative team: Vcrona well field - - Thomas Solvent Company, Battle Creek, Michigan. Operable unit feasibility study. Reston, VA (1985). 2. Hutzlcr, N. J., Murphy, B. E., and Gierke, J. S. State of technology review soil vapor extraction systems. Environmental Protection Agency Hazardous Waste Eng. Res. Lab., Cincinnati, OH 11988). 3. Bennedsen, M. B., Scott, J. P., and Hartley, J. D. Use of vapor extraction systems in site removal of volatile organic compounds from soil. Proceedings of the National conference on hazardous wastes and hazardous materials, HMCRI. 92-95
11985). 4. EnvironmentalProtectionAgency.Technologydemonstration summary: Terra Vac in situ vacuumextractionsystemGroveland. Massachusetts. EPA/540/S5-89/003, Washington,D.C. (1989). 5. Wilson, D. E., Montgomery, R, E., and Sheller, M, R. A mathcmatical model for removing volatile subsurface hydrocarbons by miscible displacement. Water, Air, Soil Poll. 33: 231-255 (1987). 6. Massmann, J. W. Applying groundwater flow models in vapor extraction system design. J. of Environ. Eng. 115: 1, 129-149 (1989). 7. Baehr, A. L., Hoag, G. E., and Marley, M. C. Removing volatile contaminants from the unsaturated zone by inducing advective air-phase transport. Journal of Contaminant Hydrology 4 : 1 - 2 6 (1989). 8. Johnson, P. C., Kemblowski, M W., and Colthart, J. D. Quantitative analysis lor the cleanup of hydrocarbon contaminated soils by in-situ soil venting. Groundwater28:3,413-429 (1990). 9. Falta, R. W., Javandel, I., Pruess, K., and Witherspoon, P. A. Density-driven flow of gas in the unsaturated zone due to the evaporation of volatile organic compounds. Water Resources Research 25: 10, 2159-2169 (1989).