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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