United States
            Environmental
            Protection Agency
Office of Solid Waste and
Emergency Response
(5102G)
EPA 542-F-97-007
December 1997
www.epa.gov
vvEPA  Analysis  of Selected

            Enhancements

            for  Soil Vapor  Extraction

                                      Introduction
                                      Soil vapor extraction (SVE) has been used at many
                                      sites to remove volatile organic compounds (VOC)
                                      from soil in the vadose zone. The effectiveness of
                                      SVE, however, may be limited for some contaminants
                                      and geologic conditions. In recent years, new ap-
                                      proaches have been developed to enhance removal of
                                      VOCs from vadose zone soils and VOCs dissolved in
                                      groundwater or adsorbed to saturation zone soils.
                                      These approaches use SVE as a base or "platform"
                                      for integration with other subsurface and groundwa-
                                      ter technologies.

            This report assists the site manager by providing an'evaluation of the current status of
            SVE enhancement technologies. The five technologies evaluated in this report are:

                             Q  Air Sparging
                             Q  Dual-Phase Extraction
                             O  Directional Drilling
                             Q  Pneumatic and Hydraulic Fracturing
                             Q  Thermal  Enhancement

            The report discusses the background and applicability; provides an engineering
            evaluation; evaluates performance and cost; provides a list of vendors; discusses
            strengths and limitations; presents recommendations for future use and applicability; and
            lists vendors and references cited for each SVE enhancement technology.

            Background
            SVE is an in situ remediation technique that applies a vacuum to vapor extraction wells
            and induces air flow through contaminated soil. As^the air migrates through the soil,
            VOCs are volatilized and transported  to the extraction wells where they are removed
            from the subsurface. The performance of an SVE system depends on properties of both
            the contaminants and the soil.  Enhancement technologies should be considered when
            contaminant or soil characteristics limit the effectiveness of SVE, or when containments
            are present in saturated soil. SVE by itself does not effectively remove contaminants in
            saturated soil. However, SVE can be used as an integral part of some treatment schemes
            that treat both groundwater and the overlying vadose zone.

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                                                    AIR SPARGING
                                                    Air sparging is a process during which air is
                                                    injected into the saturated zone below or
                                                    within the areas of contamination.  As the
                                                    injected air rises through the formation, it may
                                                    volatilize and remove adsorbed VOCs in soils
                                                    within the saturated zone as well as strip
                                                    dissolved contaminants from groundwater.
                                                    Air sparging also oxygenates the groundwater
                                                    and soils, thereby enhancing the potential for
                                                    biodegradation at sites with contaminants that
                                                    degrade aerobically.
Air sparging is most effective at sites with homogenous, high-permeability soils and unconfined aquifers.
Clogging of the aquifer, sparging probes, or well screens due to enhanced bacterial growth or
precipitation of metals under increased oxygen levels may occur. The report identifies factors affecting
the applicability of air sparging, summarizes published information on air sparging at 29 sites, and lists
nine vendors of air sparging technologies.


DUAL-PHASE EXTRACTION
Dual-phase extraction (DPE) removes both contaminated water and soil
gases from a common extraction well under vacuum conditions, jejiycing	
both the time and cost of treatment. By lowering the groundwater table at
the point of vapor extraction, DPE enables venting of soil vapors through
previously saturated and semisaturated (capillary fringe) soils.  High
vacuums typically associated with DPE systems enhance both soil vapor and
groundwater recovery rates.

DPE is most advantageous for sites with soils that have  low to moderate hydraulic conductivity.
Groundwater extraction rates required for effective operation in permeable soils may be prohibitively
expensive, and extraction drawdowns may be limited.  The report provides cost and performance
information for five case studies and lists 10 vendors of DPE technologies.
                                                  DIRECTIONAL DRILLING
                                                  Directional drilling employs the use of specialized
                                                  drill bits to advance curved boreholes in a
                                                  controlled arc (radius) for installation of horizontal
                                                  wells or manifolds for SVE and sparging
                                                  technologies.  Horizontal wells can be used for
                                                  enhancement of groundwater extraction, air
                                                  sparging, SVE, and free product removal systems.
Horizontal wells are best suited for silty sand, sand, and fine gravel lithologies.  Costs increase
dramatically in bedrock, clay, and glacial till or when cobbles or boulders are encounted.  Highly
fluctuating water tables can cause problems in horizontal well SVE systems. The report identifies 11
professional contacts and two information centers, as well as listing 21 vendors of directional drilling
technologies.

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                                       PNEUMATIC AND HYDRAULIC
                                       FRACTURING
                                       Pneumatic and hydraulic fracturing involve the injection of
                                       either gases (typically dir) or fluids (either water or slurries)
                                       to increase the permeability of the area around an injection
                                       well, thereby allowing increased removal or degradation
                                       rates of contaminants and potentially more cost-effective
                                       remediation. The typical application of pneumatic and
                                       hydraulic fractures is to improve the performance of wells
                                       used during SVE remediation.  Fracturing also can increase
                                       the recovery of free-phase fluids by increasing the
                                       production of recovery Wells.
Once horizontal fractures have been propagated, changes in soil vacuum can induce controlled vertical
or inclined fractures between horizontal fractures. Fracturing is ineffective in normally consolidated soils
and sediments in which the horizontal stress is less than the vertical stress. The report provides
information on remediation technologies at 24 sites enhanced by fracturing, and lists 13 vendors of
fracturing technologies.
                                                       THERMAL
                                                       ENHANCEMENT
                                                       Thermal enhancements for SVE involve
                                                       transferring heat to the subsurface to
                                                       increase the vapor pressure of VOCs or
                                                       semivolatile organic compounds (SVOC) or
                                                       to increase air permeability in the
                                                       subsurface formation by drying it out. The
                                                       removal of contaminants by SVE is
                                                       controlled by a number of transport and
                                                       removal mechanisms including gas
                                                       advection, chemical partitioning to the
                                                       vapor phase, gas-phase contaminant
                                                       diffusion, sorption of contaminants on soil
                                                       surfaces, and chemical or biological
                                                       transformation. Thermal enhancement
technologies raise the soil temperature to increase the reaction kinetics for one or all of these  removal and
transport mechanisms.  Thermal enhancement technologies include hot air or steam injection, radio-
frequency heating (RFH), electrical resistance (ER) heating, and thermal conduction heating.

Steam injection and hot air injection are limited to medium- to higlr-permeability soils; ER heating is
more appropriate for heating and drying lower-permeability soil in the vadose zone. Whereas ER
heating can be used to enhance bioremecliation, the high  temperatures associated with  RFH inhibit
biological activity. The report provides performance data for 18 sites and lists 11 vendors  of thermal
enhancement technologies.


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            Order Information.  This Report (EPA-542-R-97-007) is
            available free from the the EPA National Center for
            Environmental Publication and Information (NCEPI) at 800-
            490-9198 or 513-489-8190, or fax 513-489-8695.
            It is also viewable and downloadable from the Internet at
            http://www.clu-in.com

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