B Y C U R T I S C. T R A V I S JEAN M. MACINNIS apor extraction is an in situ soil-cleaning process designed to remove volatile or:anic c o m p o u n d s ,VOCs) from the unsaturated (vadose) zone of soil (the zone between the soil surface and groundwater) ( I , 2). Since the introduction of the vapor extraction system (VES) in 1984, its use has increased markedly; the VES now comprises 18% of selected remedies at Superfund sites, and this number continues to grow (3).The VES removes VOCs from the subsurface by providing a moving air stream that volatilizes contaminants and carries them to the surface. The typical VES is composed of one or more injection wells and one or more extraction wells. Air injection wells provide fresh air to the subsurface and enhance air flow through zones of maximum contaminant concentration. Extraction wells remove the contamination and transport it to emission control equipment, where it is processed before being released to the environment. As of August 1991 there were approximately 50 full-scale vapor extraction projects at Superfund sites ( 4 ) . The growth in VES implementation prompts questions concerning the effectiveness of this remediation technology. We recently reviewed vapor extraction operations at 13 sites (5)and found the vapor extraction technology to be very effective at removing large quantities of VOCs from the subsurface environment and significantly reducing soil concentrations. Measurements at the sites indicated removal efficiencies of 85-100%-all within a relatively short period (seven months to four years). For example: Savannah River, SC. Long-term leakage from an underground pipeline resulted in contamination of the underlying vadose and groundwater zones. More than 16,000 Ibs. of trichloroethylene (TCE) and tetrachloroethylene (PCE) were removed from the site during a oneyear VES pilot project. The VES uses a sparging design ( 6 ) , which 0013-936x/92/0926-1885$03.00/0 B 1992 American Chemical Socieiy
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consists of a horizontal air injection well traversing the length of the contaminated groundwater plume, and a horizontal extraction well in the vadose zone. The injection well releases air into the groundwater, forcing groundwater VOCs upward into the unsaturated zone where they are extracted by the VES. Twin Cities Ammunitions Plant, MN. More than 117,000 lbs. of VOCs have been removed from a site formerly used for open burning of solvents and fuels. A 99.55% reduction in TCE concentrations in soil samples taken to a depth of 35 A. has been reported. Custom Products, MI. Contamination at this former manufacturing facility resulted from discharge of PCE from a sludge tank.The VES at this site operated for 280 days and reduced PCE concentrations by
soil. In this example, the disparity in TCE removal times results solely from the higher air flow velocities (32 mlday) obtainable with vapor extraction, because TCE is twice as soluble in water (1.1g/L) as it is in air (0.46 g/L). Is the VES effective? There are two lines of evidence that indicate that the VES may not be as effective as it first appears. The first is the emerging view that when soils have been long contaminated, a significant fraction of the contamination is trapped inside the soil matrix and therefore is nearly inaccessible to removal by either vapor extraction or groundwater pumping (8, 1 2 , 12). The second is
Vapor extraction is more effective than groundwater pumping because the air flow rates it induces are higher than groundwater flow rates induced by pumping.
99.9997%.
Belleview, FL. More than 30,000 lbs. of VOCs were extracted during a nine-month period of VES operation at the site of a leaking underground storage tank. Concentrations of BTEX (benzene, toluene. ethylbenzene, and xylene) were reduced by 98.66%. Why does the VES work?
A general consensus is developing that groundwater pumping is ineffective for removing VOCs from the subsurface environment (7-9).If this is hue, why does the VES appear to be so effective? The primary factors controlling the effectiveness of VOC removal from the subsurface environment are the miscibility of the contaminant in the pumping medium (air or water) and the velocity at which air on water can be induced to flow through subsurface soils. Organic contaminants with large Henry’s law constants (a measure of the relative solubility of the compound in air vs. water) are amenable to vapor extraction because of their high solubility in air. Vapor extraction is more effective than groundwater pumping because air flow rates induced by vapor extraction are higher than groundwater flow rates induced by pumping. For example, EPA hydrologists estimate it would take 120 years for dissolution into groundwater of 30 L of TCE homogeneously distributed in 1 m3 of soil under natural groundwater flow conditions (0.03 mlday) (10).
Using the same conditions and methodology, we estimate that it would take only 89 days to remove 30 L of TCE from 1 m3 of vadose
an indication that EPA’s primary method for measurement of soil contaminant concentrations, purge and trap, is flawed (23).We discuss these issues in turn. The traditional view of organic contamination in subsurface soil environments is that of a rapidly obtained, completely reversible equilibrium between the organic fraction of soil and pore-space medium (air or water). The implication of this view is that organic contaminants can be easily removed from soils by using vapor extraction and groundwater pumping. As pumping (either air or water) reduces contaminant concentration in the pore space, the chemical rapidly desorbs from the soil to reestablish equilibrium. This simplistic view ignores contaminants that have diffused into the interior of the soil mahix. When organic contaminants have
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been in contact with soils for several years, a substantial portion of contaminant can be found in the inside of the soil matrix. Although pore space and soil surface-bound contamination can be removed by the VES, the contaminant bound in the soil interior must diffuse to the soil surface before it can be removed. For example, in a laboratory setting, passing approximately 750,000pore volumes of dry Ntrogen through a soil sample removed only 8% of 12-dibromoethanelongsorbed to agricultural soils (24-16). The EPA-preferred method for the determination of VOC concentrations in soils is purge and trap, in which an inert gas is passed through the soil, driving organic contaminants from pore spaces and external soil surfaces. The contaminant is then trapped, and concentrations are measured with a gas chromatograph. However, recent studies indicate that the purge-and-trap method does not remove contamination that has diffused into the internal micropores of the soil. In studies in which soil concentrations were measured by using both purge and trap and hot solvent extraction, Sawbney et al. (23)and Steinberg et al. ( 2 4 ) found that purge and trap identified less than 10% of the total soil contamination. In essence, purge and trap measures only the fraction of soil contamination that is easily removed by the purging gas flow (pore space and soil surface-bound contamination). In long-contaminated soils, approximately 90-99.9% of contamination may be found in the interior of the soil matrix (22, 15);thus, the purgeand-trap methodology measures only a small fraction of total soil contamination. Can the VES be used to reach health-based standards? Organic contamination can reside in five different locations within the soil matrix. In can be in a free-liquid phase between the soil particles, in the vapor phase, dissolved in soil moisture, adsorbed to the surface of soil particles and soil organic matter, or sequestered in the interior of the soil matrix. The first three phases of the subsurface contamination (free liquid, vapor, and surfacesorbed) are amenable to vapor extraction. Contaminants dissolved in groundwater are also potentially removable by vapor extraction, but more time is required. The soil contaminant concentration most difficult to remediate is that sequestered in the interior of the soil mabix.
in removing contaminants from the interior of the soil matrix. Available evidence indicates that the VES is very effective for removing those labile fractions of contamination located in the vapor and free-liquid phases or adsorbed to the external surfaces of the soil matrix. However, both theoretical considerations and field studies indicate that t h e VES will not be effective for removing contamination trapped in the interior of the soil matrix. Because the quantity trapped in the interior of the soil matrix may exceed surface contamination by 1-2 orders of magnitude, the VES can not be relied upon to return long-contaminated soils to their original pristine condition.
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Estimated distribution of 30 L (44,fOO 9) of trichloroethylene (WE) in 1 m3 of soila
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References (1) Thornton, J. S.:Wootan, W.L.1.Enviran. Sci. Health Port A 1982.A17.3144.
(2) Johnson, P. C. et al. Ground Water
Monit. Rev. 1990,IO,159-78. The Superfund Innovative Technalogy Evoluation Progmm: Technology Profiles: US. Environmental Protection Agency. Office of Solid Waste and Emergency Response. U.S. Government Printing Office: Washington, DC, 1990: EPA/540/5-90/006. 141 Innovative Treatment Technologies: Semi-Annual Status Report: U.S. Environmental Protection Agency. Office of Solid Waste and Emergency Response. US. Government Printing Office: Washington, DC, 1991;EPA/ 54012-911001. ( 5 ) Crotwell, A. T.et al. "An Evaluation of Vapor Extraction of Vadose Zone Contamination"; Center for Risk Management, Oak Ridge National Lahoratory: Oak Ridge, TN, 1992; ORNLl TM-12117. (6) Angell, K. G. Not]. Environ. 1. 1992, Jan.-Feb., 20-23. (7) Travis, C. C.: Doty. C. 8.Envimn. Sci. Technol. 1990.24.1464-66. (8) Ball, W. P.; Roberts, P. V. Environ. Sci. Technol. 1991,25. 123749. (9) Johnson, R. L.; Pankow. J. F. Enviran. Sci. Technol. 1992,26,896901. (10)"Basics of Pump-and-Treat GroundWater Remediation Technology"; U.S. Environmental Protection Agency. Robert S. Kerr Environmental Research Laboratory: Ada, OK, March 1990 EPA-600/8-90/003. (11)Pavlostathis, S.G.; Mathavan, G . N. Environ. Sci. Technol. 1992,26,53238. (12) Smith, J. M. et al. Enviran. Sci. Technol. 1990,24,67683. (13)Sawhney, B. L.: Pignatello, J, J,; Steinhem, S. M.I. Envimn. Qual. 1988.17, 14&52. (14)Steinberg, S. M.; Pignatello, J. J.: Sawhnev. E. L. Envimn. Sci. Technol. 1987.2i,'1201-08. (15) Pignatello, J. J. Environ. Toxicol. Chem. 1990,9,1107-15. (161 Pignatello, J. J. Environ. Toxicol. Chem. 1990,9,1117-26
(3)
Consider the case of TCE. We estimate that the TCE holding capacity of 1 m3 of soil (the maximum amount of TCE that the soil matrix will hold without the formation of free product) is 12.7 L (18,686g) or a purge-and-trap concentration (vapor phase, dissolved in water, and surface-sorbed) of 325 ppm [w/w). Below this concentration, the labile components of TCE in the soil matrix, although in equilibrium, are below saturation. Above this concentration, the holding capacity of the soil is saturated, and free product will form. When 30 L of TCE are homogeneously distributed in 1 m3 of soil, we estimate that 58% (17L) will be present as free product (Figure 1). As our previous calculations showed, this fraction will volatilize during the VES operation (in 89 days). This capability explains the large recovery rates observed during vapor extraction. However, after the labile fraction of TCE is removed, 18,150g of TCE will remain trapped in the soil matrix [at this point, purge-and-trap measurements will show zero TCE soil concentrations). Over time, the nonlabile fraction of TCE will diffuse out of the soil matrix, raising purge-and-trap soil concentrations back to preremediation levels. Crotwell et al. (5) found that soil
measurements at VES sites indicated removal efficiencies in the range of 85-100%. However, all of these studies used the purge-andtrap method to determine soil concentrations. Because purge and trap measures only surface and pore space concentrations, the measured drop in the concentrations determined by this method does not indicate that the VES has been effective
interests include hazordous woste cleanup and risk analysis.
Jean M. MacInnis is a research scientist with the Center far Risk Management a t Ouk Ridge National Laboratory. She holds a Ph.D. in moleculur biaphysi c s from F l o r i d a S t a t e University and is interested in the envii-anmentol behavior of hazardous materials.
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