United States Environmental Protection Agency Risk Reduction Engineering Laboratory Cincinnati, OH 45268 Research and Development EPA/600/S2-89/024 Jan, t990 Project Summary State of Technology Review: Soil Vapor Extraction Systems Neil J. Hutzler, Blane E, Murphy, and John S, Gierke Extracting vapor from soil is a cost-effective technique for the removal of volatile organic chemicals (VOCsf from contaminated soils. Among the advantages of the soil vapor extraction process are that it minimally disturbs the contaminated soil, It can be constructed from standard equipment, it has been demonstrated at pilot- and field- scale, it can be used to treat larger volumes of soil than can be practically excavated, and it has potential for product recovery. Unfortunately, there are few guidelines for the optimal design, installation, and operation of soil vapor extraction systems, A large number of pilot- and full-scale soil vapor extraction systems have been constructed and studied under a wide range of conditions. The major objectives of the Report summarized here are to critically review available documents that describe current practices and to summarize this information as concisely as possible. A typical vapor extraction system is briefly described, the experience with existing extraction systems has been reviewed, and information about each system is briefly summarized. Soil vapor extraction can be effectively used for removing a wide range of volatile chemicals over a wide range of conditions. The design and operation of this system are flexible enough to allow for rapid changes in operation, thus optimizing the removal of chemicals. Although a number of variables intuitively affect the rate of chemical extraction, no extensive study to correlate variables to extraction rates has been identified. This Project Summary was developed by EPA's Risk Reduction Engineering Laboratory, Cincinnati, OH, to announce k&y findings of the research project that is fully documented in a separate report of fne same title (see Project Report ordering Information at tne back). Introduction Soils may become contaminated in a number of ways with such volatile organic chemicals as industrial solvents and gasoline components. The sources of contamination at or near the earth's surface include intentional disposal, leaking undergrojnd storage tanks, and accidental spills. Contamination of groundwater Iron these sources can continue even after discharge has stopped because; the unsaturated zone above a groundwater aquifer can retain a portion or all of the contaminant discharge. As rain infiltrates, chemicals elute from the contaminated soil and migrate toward gioundwater. Alternatives for decontaminating unsaturated soil include excavation with on-site or off-site treatment or disposal, biological degradation, and soil flushing. Soil vapor extraction is also an accepted, cost-effective technique to remove volatile organic chemicals from contaminated soils. Among the advantages of the soil vapor extraction process are that it minimally disturbs the contaminated soil, it can be constructed from standard equipment, it has been demonstrated at pilot" and field-scale, it can be used to treat larger volumes of soil than are practical for excavation, and ------- it has a potential for product recovery. With vapor extraction, spiffs can be cleaned up before the chemicals reach Ihe groundwater table. Soil vapor extraction technology is often used with other clean up technologies to provide complete restoration of contaminated sites. Unfortunately, there are few guidelines for the optimal design, installation, and operation of soil vapor extraction system Theoretically based design equations defining the limits of this technology are especially lacking. Because of th-s. the design of these systems is moslSy empirical. Alternative designs can only be compared by the actual construction, operation, and monitoring of each design A large number of pilot- and full-scale soil vapor extraction systems have been constructed and studied under a wide range of conditions. The information gathered from these experiences can be used to deduce the effectiveness of this technology One of the major objectives o< the Report is to review available reports describing current practices critically and to summarize this information as concisely as possible, A brief description of a typical vapor extraction system is presented. The experience with existing extraction systems has been reviewed and information about each system is briefly summartEed in a standard forrr.. Th0 information is further summarized in several tables, which form the bases for a discussion of the design, installation, and operation of these systems. Because soil vapor extraction is a relatively new sod remediation technology, this Technology Review document will evolve as more information becomes available, Process Description A soil vapor extraction, forcad air venting, or in situ air stripping system fFigyre it revolves around the extraction of air containing volatile chemicals from unsafurated sort Fresh air is injected or flows into the subsurface at locations around a spill site, and the vapof-laden air is withdrawn under vacuum from recovery or extraction wells- System Components Extraction wells are typically designed to fully penetrate the unsaturated zone to the capillary fringe, Extraction wells usually consist of slotted elastic pipe placed in permeable packing System Operations During remediation, the blower is turned on and the air flow through trie soil comes to an equilibrium. The flows that are finally established are a function of the equipment, the flow control devices, the geometry of well layout, the site characteristics, and the air permeability of the soil. At the end of operation, the final distribution of VOCs in the soil can be measured to ensure decontamination of the site. Wells may be aligned vertically or horizontally, Vertical alignment is typical for deeper contamination /ones and for residue in radial flow patterns. If the depth of the contaminated soil or the depth to the groundwater table is less than 10 to 15 ft, it imay be more practical to dig a trench across the area o! contamination and install horizontal perforated piping in the trench bottom rather than to install vertical extraction wells. Usually several wells are installed at a site System Variables A number of variables characterize the successful design and operation ol a vapor extraction system: site conditions, soil properties, control variables, response variables and chemical properties. The specific variables belonging to these groups include. Site Conditions: distribution of VOCs, depth to groundwater, infiltration rate, location of helerogeneities, temperature, atmospheric pressure. Soil Properties: permeability, porosity, organic carbon content, soil struc- ture soil moisture characteristics, particle size distribution. Control Variables: air withdrawal rate, well configuration, extraction well spacing, vent well spacing, ground surface covering, inlet aw VOC concentration and moisture content, pumping duration, Response Variables, pressure gradients, final distribution of VOCs, final moisture content, extracted a»r concentration, exttacted air temperature, extracted air moisture, power usage. Chemical Properties: Henry's constant, solubility, adsorption equilibrium, diffusivity (air and water), density, viscosity Design and Placement Well spacing is usually based on some estimate of the radius of influence of an individual extraction well. In the studies reviewed, well spacing has ranged from 15 to 100 (t. Well spacing should be decreased as soil bulk density increases or the porosity of the soil decreases One of the major differences noted between systems was the soil bonng diameter Larger borings am preferred to minimize extracting liquid water from the soil In the simplest soil vapor extraction system's, air flows to art extraction we!) from the ground surface. To enhance air flow through zones of maximum contamination- it may be desirable to include air inlet wells in the installation. These injection wells or air vents, whose function is to control the flow of air into a contaminated zone, may be located at numerous places around the site. Typically, injection wells and air vents are constructed similarly to extraction wells tn some installations, extraction wells have been designed so they can also bo used as air inlets. Usually, only a fraction of extracted air comes from air inlets. This indicates that air drawn from the surface is the predominant source of ctean air. One study investigated the effects of air-flow rate and the configuration of the inlet and extract«on wells on gasoline recovery from an artificial aquifer. It was determined thai screening geometry only had an effect at the low arr-How rates. At tow flow rates, higher recovery rates resulted when the screen was placed near the water table rathe* than when the well was screened the lull depth of the aquifer. Woodward-Clyde made a similar assessment at the Time Oil Company site. Their engineers suggested that wells should be constructed with approximately 20 ft of blank casings between the top of the screen and the soil surface to prevent the short circuiting of air and to aid in the extraction of deep contamination. At most sites, the initial VOC recovery rates were relatively high then decreased asymptotically to zero with time. Several studies have indicated that intermittent venting from individual welfs is probably more efficient in terms of mass of VOC extracted per unit of energy expended, This is especially true when extracting from soils wfere mass transfer is limited by diffusion out of immobile water, Optimal operation of a sod vapor extraction system may involve taking individual wells m and out of service to allow time fur liquid diffusion and to change air f"ow patterns in the region being vented. Little work has been done to study this. -J» 4 , ------- Vapor Treatment & Inlet Weil i i Itif _ i i --» * I"! * i & ~* Si IS, traction Well Air/Water Separator 1 .+ -»-} ^ .+ _» _> T f t * T' t T t t f t E - ; i • u < 4. «-— f 4. ' ! ^ «•- Si %%%%%%£ IT -- - ^ -* ->| ^ 1 T^« $» f*-**f L'l Contaminated |-* £,* So« fy ,. J«,n»..™ «•».•« J ^ l-Sfil^&tt V., X» j *£^""^ t te "• iitukSsZa %% ''•' Tam figure 1. Soil vapor extraction system, One of the major problems in the operation of a soil vapor extraction system is determining when the site is sufficiently clean to cease operation, The design and operation of soil vapor extraction systems can be quite flexible; changes can be made during the course of operation with regard to well placement, or blower size, or air flows from individual wells. If the system is not operating effectively, changes in the well placement or capping the surface may improve it. Conclusions Based on the current state of the technology of soil vapor extracSion systems, the following conclusions can be made: 1, Soil vapor extraction can be effectively used for removing a wide range of volatile chemicals in a wide range of conditions. 2. The design and operation of these systems is flexible enough to allow for rapid changes in operation, tiius, optimizing the removal of chemicais. 3, Intermittent blower operation is probably more efficient in terms of removing the most chemical with the least energy, 4. Volatile chemicals can be extracted from clays and silts but at a slowerrate. Intermittent operation is cer-tainly more efficient under these conditions 5, Air injection has the advantage of controlling air movement, but injection systems need to be carefully designed. 6. Extraction wells are usually screened from a depth of from 5 to 10 ft below the surface to the groundwater table. For thick zones of unsaturated soil, maximum screen lengths of 20 to 30 ft are specified. 7. Air/water separators are simple to construct and shouid probably be installed in every system, 8. Installation of a cap over the area to be vented reduces the chance of extracting water and extends the path that air follows from the ground surface, thereby increasing the volume of soil treated. 9. Incremental installation of wells. although probably more expensive, allows for a greater degree of freedom n design. Modular construction where the most con- taminated zones are vented first is preferable, 10. Use of soil vapor probes in conjunction with soil borings to assess final clean up is less expensive tsian use of soil borings atone, Usually a complete materials balance on a given site is impossible because most sites have an unknown amount of VOC in the soil and in the groundwater, 11. Soil vapor extraction systems are usually only part of a site remediation system, 12, Although a number of variables intuitively affect the rate of chemical extraction, no extensive study to correlate variables to extraction rates has been identified. The full report was submitted in partial fulfillment of Cooperative Agreement No. CH-814319-01-1 by Michigan Techno- logical University under the sponsorship of the U.S. Environmental Protection Agency, ------- Neil J. Hutzter, Blane f. Murphy, and John S, Gierke are with Michigan Technological University, Houghton, Ml 49931. Paul ft, cte Percin is the EPA Project Officer fsee below), The complete report, entitled "State of Technology Review: Soil Vapor Extraction Systems," (Order No, PB 89-195 184IAS; Cost: $75.95, subject to change) wilt be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22W1 Telephone: 703-467-4650 The EPA Project Officer car be contacted at: Risk Reduction JEngineering Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 BULK RATE POSTAGE & FEES PAID EPA PERMIT No, G-35 Official Business Penalty for Private Use S300 EPA/600/S2-89/024 ------- |