&EFA
                          United States
                          Environmental
                          Protection Agency
                Office of Solid Waste
                and Emergency
                Response
Office of
Research and
Development
EPA/540/S-94/500
December 1993
Engineering
Forum   Issue
                          Considerations in  Deciding to  Treat
                          Contaminated Unsaturated Soils  In  Situ
Index

   1.0 Introduction
   2.0 Critical Factors in Technology Selection
      2.1 Technology Characteristics
      2.2 Generic Critical Factors for Feasibility
         Screening of In Situ Treatment
   3.0 Technology-Specific Factors
      3.1 Delivery and Recovery Systems
      3.2 In Situ Solidification/Stabilization
      3.3 Soil Vapor Extraction
      3.4 In Situ Bioremediation
      3.5 Bioventing
      3.6 In Situ Vitrification
      3.7 In Situ Radiofrequency Heating
      3.8 Soil Flushing
      3.9 Steam/Hot Air Injection and Extraction
   4.0 Acknowledgments
   5.0 References
1.0  Introduction

  This  Issue Paper was developed for the EPA national
Engineering Forum.   This group of  EPA professionals,
representing EPA's Regional Offices^ is committed to identi-
fying  and resolving the engineering issues related to the
remediation  of Superfund and RCRA sites.   The Forum
operates under the auspices of and advises EPA's Techni-
cal Support Project.
                            The purpose of this Issue Paper is to assist the user in
                          deciding if in situ treatment of contaminated soil is a poten-
                          tially feasible remedial alternative and to assist in the pro-
                          cess of reviewing and screening in situ  technologies.  The
                          definition  of an in situ technology is a technology applied to
                          treat the hazardous constituents of a waste or contaminated
                          environmental medium where they are located. Central to
                          the  definition of  in situ technology is the concept that the
                          contaminated material is not excavated.  The technology
                          must be capable of reducing the risk posed by these con-
                          taminants to an  acceptable  level (U.S.  EPA, 1990,  EPA/
                          540/2-90/002, p. 1).

                            Many biological, chemical, and physical mechanisms are
                          available to treat  contaminants in soils. These mechanisms
                          can  be either applied to excavated soil or used in situ.  The
                          costs,  logistical  concerns,  and  regulatory  requirements
                          associated with excavation, ex situ treatment, and disposal
                          can  make in situ  treatment an attractive  alternative. In situ
                          treatment entails  the iise of chemical or biological agents or
                          physical manipulations to  degrade, remove,  or immobilize
                          contaminants without requiring bulk soil  removal.   Contain-
                          ment technologies, such as capping, liners, and grout walls,
                          are not considered in this Issue Paper.

                            This Issue Paper is intended to assist in the identification
                          of applicable alternatives early in the technology screening
                          process.   The Issue Paper  discusses and lists important

                                                  tSS) Printed on R*ydtd Papw
                        Superfund Technical Support Center
                        for Engineering and Treatment

                        Risk Reduction Engineering
                        Laboratory

                        Engineering Forum
                                         Technology Innovation Office
                                         Office of Solid Waste and Emergency
                                         Response, U.S. EPA, Washington, DC

                                         Walter W. Kovalick, Jr., Ph.D.
                                         Director

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considerations for in situ technologies.  More detailed infor-
mation, is available on each  technology from  a variety of
reference sources. These references should be consulted
for all  of the technologies that are likely to  be useful at a
specific site.  In  addition to  the in  situ technologies dis-
cussed in this Issue Paper, technology screening also would
consider potentially useful ex  situ technologies.  Final tech-
nology  selection  typically will  be based on   site-specific
evaluation  and treatability testing (U.S.  EPA,   1992,  EPA/
540/R-92/071a).

   Applying the treatment agents to the largely  undisturbed
in situ geology give,s  in situ  treatment  unique  advantages
and challenges.   The obvious  advantage is that  no bulk
excavation  is required for  in situ treatment.   Preventing
excavation  eliminates  the cost and  environmental conse-
quences of moving the contaminated material.  The  condi-
tions of the subsurface will never be  as controlled  as in an
ex  situ reactor, however.  As  a result, in  situ treatment
requires more  extensive  site characterization  both  before
and after treatment, is harder to simulate in  the laboratory,
and must be designed and operated to  minimize the spread
of contamination.

   The principal  feature of in situ treatment is controlled
delivery and recovery  of  energy, fluids, or other treatment
agents to the subsurface.  The treatment agent usually is
water,  air, or steam but may be energy input by conduction
or radiation.  For both physical- and energy-based  in situ
treatment agents, controlled application  is a key to success.
Systems must be available to  apply treatment agents  and to
control  the  spread of  contaminants  and treatment agents
beyond the treatment zone.

   Several  in situ technologies also  rely on the  ability to
recover the treatment agent  and contained contaminants
from the subsurface. For example, recovery of flushing fluid
containing contaminants is an integral part of soil  flushing,
and the collection and treatment of steam and  condensate
are essential to steam/hot air  injection and extraction treat-
ment.

   Assessing the feasibility of  in situ treatment and selecting
appropriate  in situ technologies requires an understanding of
the characteristics of  the contaminants,  the site,  and the
technologies,  and  of  how these  factors and conditions
interact to allow effective  delivery, control, and  recovery of
treatment agents and/or the contaminants.

   This Issue Paper discusses established and innovative in
situ treatment technologies that are available or should be
available for full-scale  application by 1996.  Emerging tech-
nologies that are still being tested in the laboratory and are
not available for full-scale  implementation are not discussed.
Examples of emerging technologies include:  in situ  oxida-
tion or reduction,  electrokinetics, hot brine injection, polymer
injection, and soil freezing.
2.0 Critical Factors in Technology
     Selection

   This section describes critical factors to consider in the
selection of in situ treatment methods and during evaluation
of in situ technologies.  Factors to be discussed include the
general technology capabilities and generic critical factors
that  influence  the  general  suitability  of  in  situ treatment
when compared to  ex situ treatment.  Section 3.0  provides
more detailed technology descriptions and the technology-
specific critical factors.

   The process for screening  and selecting technologies is
described in Guidance for Conducting Remedial Investiga-
tions and Feasibility Studies under CERCLA  - Interim Final
(U.S. EPA, 1988, EPA/540/G-89/004).  The guidance docu-
ment describes preliminary screening of technologies based
on effectiveness, implementability, and cost.  The effective-
ness evaluation considers the protection of human  health
and the environment and reductions in  mobility, toxicity, and
volume of contaminant achieved by an  alternative.  The
implementability  evaluation  considers  the  technical and
administrative  feasibility  of  constructing, operating, and
maintaining a remedial action alternative.  The cost evalua-
tion considers the relative cost of alternatives.

   This Issue Paper will assist the user in prescreening  in
situ technologies for contaminated soil by determining whe-
ther  the technologies are technically feasible for a particular
site.   This paper is not meant to replace  Feasibility Study
Guidance.  Consideration and selection of remedial technol-
ogies is based on criteria that are defined by the  National
Contingency Plan.  This Issue  Paper describes the  potential
effectiveness of in situ technologies for treating the various
types of chemical groups,  and  reviews  both the general and
the technology-specific factors to consider during preliminary
evaluations of the effectiveness,  implementability, and cost
of in situ  approaches to treatment.   Although  this Issue
Paper describes only in situ treatment, the user should keep
in mind that selection  of in  situ technology candidates will
not necessarily eliminate  consideration of the ex  situ op-
tions.  At  many sites, both in  situ and ex situ  technologies
may be competing  candidates late in the technology selec-
tion process.

   Selecting  a  technology often  requires several  iterations
with  increasingly well-defined  data to  refine the selection.
As  the project progresses,  technology-specific and site-
specific information becomes  available.  This information
must be used to better define which technologies  are suit-
able for waste  materials and conditions at the  site.  As the
decision maker obtains more  information about site  condi-
tions, waste characteristics, and  treatability study results,
this Issue Paper can be used to help further refine selection
of candidate technologies.  However,  as the list of  candi-
dates is narrowed,  additional published sources and  expert
opinion should be sought to obtain more detailed information
about the candidate technologies.
                                                                                             Treatment of Soils In Situ

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2.7   Technology Characteristics

   The  applicability of the  technology to the contaminants
present,  the technology maturity, and the  ability of the
technology to operate in the unsaturated and/or saturated
zones should be considered in technology selection.  The
applicability of the technology to general types of contami-
nants is summarized  in Table 2-1.  The characteristics of
the technologies are summarized  in Table 2-2.  Preliminary
selection of technology candidates can  be based  on the
capabilities of in situ technologies  to treat chemical groups
present  at the site.

   The chemical contaminant groups considered are divided
into three general groups:  organics, inorganics, and  reac-
tives.   The types  of organics considered are halogenated
and  nonhalogenated  volatile organic  compounds  (VOCs),
halogenated and nonhalogenated  semivolatile organic  com-
pounds   (SVOCs),   polychlorinated   biphenyls   (PCBs),
pesticides,  dioxins and furans, organic cyanides, and organ-
ic corrosives^  Inorganics are subdivided into volatile metals
(and  metalloids),  nonvolatile  metals, asbestos, radioactive
materials,  inorganic corrosives,  and  inorganic cyanides.
Reactive species  may be either oxidizers or reducers.  The
types of materials in  these subgroups are outlined below.
More  detailed lists of  constituents  within each contaminant
group are given in Technology Screening Guide for Treat-
ment  of CERCLA  Soils and Sludges  (U.S.  EPA,  1988,
EPA/540/2-88/004, pp.  10-12).

   VOCs are  carbon  compounds with boiling points lower
than 200°C as analyzed  by EPA SW-846 method 8240.
SVOCs  are carbon compounds, other than those covered in
the more specific  subdivisions, analyzed by EPA SW-846
method  8270.  PCBs  are  any  of  several compounds pro-
duced by replacing hydrogen atoms in a biphenyl group with
chlorine. Pesticides are compounds other than PCBs  ana-
lyzed  by EPA SW-846  methods 8080 or 8150. Dioxins and
furans are  environmentally persistent,  toxic, heterocyclic
hydrocarbons.  Organic  cyanides  are carbon  compounds
with a CN  group  attached.  Organic corrosives are carbon
compounds that in aqueous solution have a pH less than or
equal to 2 or greater than or equal to 12.5,  or that exhibit a
strong tendency to dissolve materials.

   Volatile  metals are  metals or metalloids where the stable
species  in  an oxidizing atmosphere (metal  or oxide) has  a
boiling point less than  630°C. Nonvolatile metals are metals
where the stable species in an oxidizing atmosphere (metal
or oxide) has a boiling  point equal to or greater than 630°C.
Asbestos is any of several minerals  that readily separate
into long, flexible fibers. Radioactive materials are isotopes
that decay by particle  or energy release from the nucleus.
Inorganic cyanides are compounds  with a CN group at-
tached.   Inorganic corrosives are  compounds that in aque-
ous solution have  a pH less than  or equal to 2 or  greater
than or  equal to 12.5, or that exhibit  a  strong tendency to
dissolve materials.

   Substances with a strong affinity to acquire electrons are
called oxidizers, whereas substances with a  strong tendency
to donate electrons are called reducers.
   Treatment often requires  a sequence of operations to
deal with a combination of wastes.  When evaluating wastes
containing  contaminants from more than  one  chemical
constituent group,  each waste group  initially should  be
considered separately to develop a list of potentially appli-
cable treatment technologies for each chemical group pres-
ent in the soil.   The technology lists can be  compared to
determine if some candidate  technologies are able to treat
all of the groups present.

   If one technology is unable to treat  all  of the groups,
development  of a treatment  train may be  required.   For
example at a site with a combination of VOCs and metal
contaminants, soil vapor extraction (SVE) can be used to
remove  the VOCs followed by in situ solidification/stabiliza-
tion  to  reduce  the mobility of the metals.   The selected
treatment train also must be  reviewed for potential interfer-
ences or adverse effects.  For example, SVE may increase
the  proportion   of  hexavalent chromium,   increasing  the
mobility  and toxicity of the chromium.

   One  of the following three characteristics  is indicated for
each in  situ technology in Table 2-1:

1.  Demonstrated Effectiveness - The technology  has been
   shown to treat some contaminants in the chemical group
   to acceptable levels when applied to  contaminated soil.
   Treatment may  involve  removal, destruction,  immobili-
   zation, or toxicity reduction.   The demonstration  may
   have been at the laboratory, pilot, or production  scale.
2.  Potential Effectiveness - Literature reports indicate there
   is or  is not a mechanistic basis for the technology to re-
   move, destroy, immobilize, or otherwise treat some of the
   chemicals in the group when used to treat soil.
3.  Possible Adverse Effects - The contaminant is likely to
   interfere with the treatment technology or  to  adversely
   affect safety,  health,  or the environment.  Adverse effects
   may  occur only when the contaminant concentration is
   above a threshold level.  In many cases, the adverse
   effect may be alleviated by pretreatment to reduce the
   concentration of the adverse contaminant.

   Table 2-2 indicates the maturity of the  technology and its
applicability  for  saturated  and unsaturated  media.   The
maturity is indicated by the ranking shown below (U.S. EPA,
1992, EPA/542/R-92/011, p. 1).  Technology maturity is an
important factor in  the  cost  and timeliness of technology
implementation.

1.  Established Technology - The technology has been used
   on a  commercial scale and has been established for use
   in full-scale  remediations  (e.g.,  incineration,  capping,
   solidification/stabilization).
2.  Innovative Technology - The technology is an alternative
   treatment technology (i.e.,  "alternative" to land  disposal)
   for which use at Superfund-type sites is inhibited by lack
   of data on cost and performance.

   To further assist in the review  of technology candidates,
Table 2-2 indicates the  media typically treated, typical treat-
ment agents  or amendments,  and  delivery and  recovery
methods.  Figure 2-1 shows the approximate  range of in situ
Treatment of Soils In Situ

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                                      Table 2-1.  Effectiveness of In Situ Treatment on General Contaminant Groups for Soil
Contaminant Groups
Organic
Inorganic
Reactive
Halogenated Volatiles
Halogenated Semivolatiles
Nonhalogenated Volatiles
Nonhalogenated Semivolatiles
PCBs
Pesticides
Dioxins/Furans
Organic Cyanides
Organic Corrosives
Volatile Metals
Nonvolatile Metals
Asbestos
Radioactive Materials
Inorganic Cyanides
Inorganic Corrosives
Oxidizers
Reducers
In Situ
Solidification/
Stabilization
(a) (b)
Xd>
Y(2)
X
T
Y
Y
T
Y
•(3)
•
•
•
•
•
T
T
Soil
Vapor
Extraction
(b) (c)
•
T
•
•
Q
Q
Q
Q
Q
Q
Q
Q
U
Q
Q
Q
T
In Situ
Bioremediation
(d)
Y
T
Y
Y
Y
Y
T
Y
X
X<5>
X(5)
Q
X
X
X
X
X
Bioventing
(e)
Q
Y<4)
•
•
Q
a
Q
Q
X
X<5>
X<5)
Q
X
X
X
X
X
In Situ
Vitrification
(d) (0
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
•
Y
Y
Y
Y
Radio-
Frequency
Heating
(f)
Y
Y
•
•
Y
Y
Q
Q
Q
Q
a
Q
a
a
Q
a
a
Soil
Flushing
(b) (9)
•
Y
Y
•
Y
Y
Y
Y
Y
Y
•
Q
Y
Y
Y
Y
Y
Steam Injection
Stationary
System
(b) (f) (h)
•
Y
•
Y
Y
Y
Y
Y
Y
Y<6>
Y<6>
Q
Y<6>
YW>
YW>
Y<6>
Y<6>
Mobile
System
(b) (0 (h)
•
Y
•
Y
Q
Q
Q
Q
Q
Q
Q
Q
Q
a
Q
Q
Q
5T
31
•  Demonstrated Effectiveness: Successful treatability test at some scale completed,
T  Potential Effectiveness: Mechanistic basis indicating that technology will work.
Q  No Expected Effectiveness: No mechanistic basis indicating that technology will work.
X  Potential Adverse Effects.

(1) Vaporization and emission of volatile organic compounds may pose a hazard during mixing.
(2) Semivolatile organics are difficult to treat, but low concentrations of some compounds can be treated.
(3) Arsenic and mercury are difficult to immobilize with cement-based binder formulations.
(4) Possible to treat by cometabolism techniques.
(5) Metals can interfere with bioremediation or bioventing of organics; however, bioremediation methods for low
    concentrations of metals are being developed.
(6) Potential effectiveness only for water-soluble compounds.
                                                                                                                   Adapted from the following sources:
                                                                                                                    (a) U.S. EPA, 1993, EPA/530/R-93/012.
                                                                                                                    (b) Donehey et al., 1992, pp. 104-105.
                                                                                                                    (c) U.S. EPA, 1991, EPA/540/2-91/006, p. 2.
                                                                                                                    (d) U.S. EPA, 1988, EPA/540/2-88/004, p. 13.
                                                                                                                    (e) U. S. Air Force, 1992, pp. 5-10.
                                                                                                                    (f) Houthoofd et al., 1991, EPA/600/9-91/002, pp. 190-203.
                                                                                                                    (g) U.S. EPA, 1991, EPA/540/2-91/021, p. 2.
                                                                                                                    (h) U.S. EPA, 1991, EPA/540/2-91/005, p. 2.

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                                    Table 2-2.  Summary of In Situ Technology Characteristics
Technology/Maturity
Solidification/
stabilization (a)(b)
(See Section 3.2)
Established
Soil vapor extraction (b)(c)
(See Section 3.3)
Innovative
In situ bioremediation (d)
(See Section 3.4)
Innovative
Bioventing (e)
(See Section 3.5)
Innovative
In situ vitrification (d)(f)
(See Section 3.6)
Innovative
Radiofrequency heating (f)
(See Section 3.7)
Innovative
Soil flushing (b)(g)
(See Section 3.8)
Innovative
Steam/hot air injection
stationary system (b)(f)(h)
(See Section 3.9)
Innovative
Steam/hot air injection
mobile (auger) system
(b)(f)(h) (See Section 3.9)
Innovative
Media Typically Treated
Saturated or unsaturated soil,
sediment, or sludge
Approximate depth limits:
30 feet for auger system,
several feet for in-place mixing,
and not a major constraint for
grout injection
Unsaturated soil
Saturated or unsaturated soil,
sediment, or sludge
Unsaturated soil, sediment, or
sludge
Saturated or unsaturated soil,
sediment, or sludge
Approximate depth limit 20 feet with
possible extension to 30 feet
Unsaturated soil, sediment, or
sludge
Unsaturated or saturated soil
Saturated or unsaturated soil
Saturated or unsaturated soil
Approximate depth limit:
30 feet for auger system
Typical Agents or
Amendments
Cement, fly ash, blast furnace
slag, lime, or bitumen
Air
Aqueous solution containing an
electron acceptor (typically oxy-
gen), nutrients, pH modifiers, or
additives
Air
Electrical energy by conduction
Electrical energy by radiation
Water, acidic solutions, basic
solutions, chelating agents, or
surfactants
Steam and/or hot air
Steam and/or hot air
Delivery Methods
(see Section 3.1)
Auger mixing, in-place
mixing, or injection
Passive air inlet or
injection wells
Surface infiltration,
tilling, or water injec-
tion wells
Passive air inlet or
injection wells
Electrodes
Radiofrequency
antennae system
Extraction fluid
injection wells
Steam injection wells
Auger mixing
Recovery Methods
(see Section 3.1)
None required
Air extraction wells (off-
gas treatment may be
required)
None required
Air extraction wells may
be used (off-gas
treatment may be
required)
Off-gas collection and
treatment
Off-gas collection and
treatment
Extraction fluid recovery
wells
Condensate recovery
wells and off-gas
collection and treatment
Off-gas collection and
treatment
Adapted from the following sources:
   (a) U.S. EPA, 1993, EPA/530/R-93/012.
   (b) Donehey et al., 1992, pp. 104,105.
   (c) U.S. EPA, 1991, EPA/540/2-91/006, p. 2.
   (d) U.S. EPA, 1988, EPA/540/2-88/004, p. 13.
(e)U.S. Air Force, 1992, pp. 5-10.
(f) Houthoofd et al., 1991, EPA/600/9-91/002, pp. 190-203.
(g) U.S. EPA, 1991, EPA/540/2-91/021, p. 2.
(h) U.S. EPA, 1991, EPA/540/2-91/005, p. 2.
Treatment of Soils In Situ

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remediation costs.  The costs shown are based on  limited
data reported in the literature.  The sources rarely give full
characterization of elements included in the cost estimates.
The ranges should be viewed as preliminary indications of
approximate comparative costs of the various technologies.
                                                                                 Factors Increasing Cost
                    In Situ S/S
                SVE (Off-Gas Not Treated)


                SVE (Off-Gas Treated)
                Bloremedlatlon
                 Bioventing
                                                     In Situ Vitrification
               Steam/Hot Air Injection and Extraction
                                                                                  Difficult mixing
                                                                                  Small volume treated
                  Low air conductivity


                  Low air conductivity
                                                                                   Low hydraulic conductivity
                                                                                   Low ambient temperature
                                                                                  Low air conductivity
                                                                                  Low ambient temperature
                                                                                  High moisture content
                                                                                  High moisture content
                                                                                  High treatment temperature
                  Low hydraulic conductivity
                  Expensive solubility
                    enhancement additives

                  Low air conductivity
                              200              400


                                      Treatment Cost ($/ton)
                                                              600
                                                                              800
                                       Figure 2-1. Estimated Cost Ranges of
                                          In Situ Remediation Technologies
2.2 Generic Critical Factors for Feasibility
     Screening of In Situ Treatment

   Several critical factors apply to the evaluation of in situ
treatment at most sites.  These  generic critical factors have
broad application regardless of the specific technology. Five
categories have  been identified to assist  in  organizing
consideration of the potential feasibility of the in situ treat-
ment for  a  particular site.   This  evaluation relates to the
three screening  criteria named in  the National Contingency
Plan (NCP)  instituted by the Comprehensive Environmental
Response, Compensation, and  Liability Act  (CERCLA) of
1980 and described in the Guidance for Conducting Reme-
dial Investigations and Feasibility Studies under CERCLA -
Interim Final (U.S. EPA, 1988,  EPA/540/G-89/004):  effec-
tiveness, implementability, and cost. The five categories are
described in Table 2-3.

   These generic  factors  give an  overall  framework  for
evaluating the potential for using in situ technologies.  Site
conditions that give a poor ranking in one or even several
factors do  not necessarily indicate that  in situ approaches
are unlikely to succeed.  All of the generic and technology-
specific factors (see Section 3.0) of in situ and competing ex
situ technologies should be considered to indicate the gen-
eral  trend of  applicability of in  situ  treatment and to help
identify possible candidate treatment technologies.

   The generic critical factors are geologic and in situ waste
material characteristics that are significant in controlling or
affecting  the  effectiveness or implementability  of  in  situ
technologies.  Although these factors generally are of inter-
est at  all sites, some have more effect on the performance
of specific technologies.  The user must  not draw a conclu-
sion that in  situ treatment is inappropriate based on one or
two unfavorable factors.  The design features of a particular
technology may be able to eliminate or  avoid some  of the
limitations inherent with most in situ treatment technologies.
For example, in situ solidification/stabilization (S/S) technolo-
gies using mechanical mixing are less affected by the  initial
                                                                                              Treatment of Soils In Situ

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                          Table 2-3. Generic Factors Influencing Selection of In Situ Treatment
Factor Influencing Selection of
In Situ Treatment
Hydrogeologic flow regime
(See Section 2.2.1)
Regulatory standards
(See Section 2.2.2)
Time available for remediation
(See Section 2.2.3)
Removal logistics
(See Section 2.2.4)
Waste conditions
(See Section 2.2.5)
Conditions Favoring Selection of
In Situ Treatment
High or moderate conductivity uniformly distributed in
formation
or
Low-conductivity regions surrounded by regions of high or
moderate horizontal conductivity (a)
Deep water table and/or competent aquitard below
contaminated volume
Wastes that are difficult to treat in accordance with Land
Disposal Restriction (LDR) requirements
Completion time not critical
Large volume of waste
Waste not accessible due to existing structures
Excavation difficult due to matrix characteristics or depth
Poor transportation infrastructure
Large volume of waste
Low contaminant concentrations
Basis
Treatment reagents must reach contaminated
matrix by advective or diffusions! flow
Delivery and recovery of treatment agents must
be controlled
LDRs apply to excavated material, unless the
material is excavated and treated within a
Corrective Action Management Unit (CAMU)
In situ treatment requires more time to complete
than ex situ treatment
In situ treatment does not require excavation
It is not economical to excavate large volumes for
treatment of low concentrations
In situ treatment may reduce the need for capital-
intensive treatment equipment
(a) The low-conductivity regions must be "thin" with respect to diffusion path length, which can be feet or inches for gas-phase diffusion in dry soils and inches or less
   for water-phase diffusion. (See Table 2-2 for information on type of treatment agent.)
soil conductivity than are technologies that require delivery
of fluid flow (see Table 2-2).  Moreover,  technologies such
as steam injection,  in situ  vitrification, and radiofrequency
heating, although generally slower than conventional ex situ
methods, can proceed more quickly than in situ bioremed-
iation or soil vapor extraction.

2.2.1  Hydrogeologic Flow Regime

   The hydrogeologic flow regime characterizes the gas and
liquid flow  in the subsurface.  Examination of flow regime
characteristics is directed at answering questions such as
the following:

   •  Will contaminant removal be achieved at an accept-
     able rate?
   •  Will contaminant removal be complete  and uniform?
   •  Will contaminants or treatment agents  escape from the
     treatment area?

   The flow regime factor is controlled  mainly by the amount
of available primary and secondary fluid flow routes, the
magnitude  and homogeneity of hydraulic conductivity,  fluid
levels and pressures, and the proximity to a discharge loca-
tion.  Information  needed to define the hydrogeologic flow
regime  includes a complete understanding of the geologic
strata and how they were deposited, full characterization of
the fluids and deposits for fluid transmission properties, and
monitoring of soil moisture and water levels through at least
three seasons of one year.

   Geologic, hydraulic,  and fluid-behavior  data are needed
to evaluate the flow  regime.  Geologic data include, in part,
primary and secondary effective conductivity, history of geo-
logic strata  formation, and  the stratigraphic and structural
characteristics of the deposit.  Hydrologic data include both
surface water and groundwater flow,  level,  and  pressure
characteristics.  Surface water data, such as stream/lake
hydrographs and precipitation,  infiltration,  and  recharge
measurements, are needed  to understand the general water
balance of the system, whereas groundwater data,  including
pressure graphs, well hydrographs, and hydraulic conductivi-
ty and dispersion measurements,  are  needed  to  calculate
water and mass flux  through the system.

   Understanding the spatial variation of conductivity also is
essential to evaluate candidate in situ treatment technolo-
gies.   Preferred flow pathways develop in the  subsurface
Treatment of Soils In Situ

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due either to inhomogeneities in the conductivity or to geo-
logic facies.  Most soils have preferential flowpaths that are
responsible for much of the conductivity.  The preferential
paths  can arise  from a  number  of  causes such as root
intrusions, shrink/swell or wet/dry cycling, or uneven settling
(U.S. EPA, 1990, EPA/600/2-90/011,  p. 39).  These pre-
ferred  pathways  result in high  hydraulic conductivity con-
trasts that can diminish the reliability and efficiency of in situ
treatment  methods.    Geologic  deposits with  little  or no
vertical fracturing or  with  no  highly  developed bedding
planes and those containing hydraulic conductivity  contrasts
of less than an order of magnitude will be conducive  to in
situ methods.  Implementation time will be less and removal
will be more complete when the system tends toward homo-
geneity.

   A geology with uniformly distributed high conductivity is
most conducive to application of in situ treatment.  Hydraulic
conductivity of more than 10~3 cm/sec is most favorable to
technologies that require flow of water solutions (see  Table
2-2).   For technologies that  require air or vapor flow (see
Table 2-2), an air conductivity of more than 10"4  cm/sec is
most favorable (U.S. EPA, 1990, EPA/600/2-90/011, pp. 40
and 54).  In situ treatment still can be applied in  geologies
with much  lower conductivities.   However, contaminant
transport  in the  lower conductivity regions  will  occur by
slower diffusion processes rather than by bulk material flow.
Feasibility depends  on the  type  of  treatment agent,  the
contaminant transport  mechanisms, and the details of the
distribution of the primary and secondary flowpaths.

   Many  in situ  treatment technologies require injection of
treatment agents such as steam, chemicals, or nutrients.
Often the treatment agents must then be collected from the
subsurface for further processing.  The subsurface geology
should be amenable to containment of the treatment agents
in the  contaminated  area.  Containment will be maximized
when vertical and horizontal hydraulic  gradients are low or if
the treatment zone is bounded geologically by deposits with
low hydraulic conductivity.  Close  proximity to groundwater
discharge areas such as streams, lakes,  and seeps can
jeopardize containment of in situ treatment agents.

2.2.2 Regulatory Standards

   The regulatory standards factor characterizes the overall
regulatory climate  at the site based on federal,  state, and
local regulations.  Examination  of regulatory standards is
directed at answering questions such as these:

   • What contaminant cleanup levels are required?
   • Are  land-use restrictions consistent with the candidate
     technologies?
   • Will  in situ treatment cause unacceptable alteration of
     soil conditions?
   • Is injection  of treatment chemicals  consistent  with
     Land Disposal  Restrictions (LDRs) and other regula-
     tions, as required?

   If the site is a CERCLA site, 40 CFR 300.400(g) requires
that  any  remedial alternative must satisfy (or  provide a
waiver of) all Applicable or Relevant and  Appropriate Re-
quirements  (ARARs).    Applicable  requirements  include
federal  and state environmental standards,  cleanup stan-
dards, and  control standards  that  specifically  address a
hazardous  substance,   pollutant,  contaminant,  remedial
action, location, or other circumstance  at a  CERCLA site.
Relevant  and  appropriate requirements are standards that
are not "applicable" but that specifically address a problem
or situation  sufficiently  similar  to those encountered  at a
CERCLA  site  (i.e., their  use is well  suited to the particular
site).

   If the site is not a CERCLA site, it will not need to satisfy
a formal list of ARARs;  however, it is probable that certain
regulatory requirements  must still be met in the cleanup.  In
either case, these requirements will be specific  to the site
where treatment will occur and may vary from site to site.

   Cleanup  levels are one of the  most important  of the
regulatory requirements that will  determine whether in situ
treatment is potentially acceptable.  Treatability studies will
help to  determine if an  in situ treatment method can meet
the required performance levels.  For extraction technolo-
gies,  the  total  residual  contaminant levels must be deter-
mined to  demonstrate remediation.   For technologies that
reduce contaminant mobility, such as S/S or in situ vitrifica-
tion, the cleanup levels will be stated in terms of  leaching
resistance.  Leaching data such as results from the Toxicity
Characteristic Leaching Procedure (TCLP) or  other leaching
tests will be needed to demonstrate that the method immo-
bilizes the contaminants. The ability to demonstrate that an
in situ treatment method meets the regulatory performance
requirements will determine the acceptability of that type  of
treatment method. Thus, regulatory requirements should be
considered at the screening level to the extent that they are
known.   Although the  requirements may not have  been
finalized at the time screening is conducted,  the most cur-
rent list available should be used.

   In  situ treatment may require more  extensive sampling
than ex situ treatment to demonstrate  that required  treat-
ment  performance levels have been  achieved. With in situ
treatment, the  variation  in natural conditions and the distri-
bution of the contaminant must be determined.  This often
requires extensive sampling to build a  statistical basis for
evaluating whether or not analytical results represent in situ
conditions. In contrast, in a typical ex situ treatment system,
waste material is excavated, prepared, and homogenized as
part of the treatment operation.  These homogenized batch-
es can be represented  with a smaller number of samples
than corresponding in situ materials.

   In  the past,  regulatory  requirements favored  in  situ
treatment  in some cases because excavation of contami-
nated material  would have  caused  it to be treated  as  a
RCRA waste subject to the treatment standards and Best
Demonstrated  Available  Technologies (BDATs)  under the
LDRs.  Recently, however, the EPA published a final rule
allowing the use of  Corrective  Action  Management  Units
(CAMUs) at  RCRA sites  (58 FR 8658, February  16,  1993),
which can eliminate this advantage  for in situ  treatment.
Although these regulations were developed  for corrective
actions  at  RCRA facilities,  the  regulations  also may be
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  applied  as ARARs to CERCLA  sites,  particularly  where
  CERCLA remediation involves management of RCRA haz-
  ardous wastes.  A CAMU is defined as:

     "an area within a facility that is designated by the Re-
     gional Administrator under part 264 subpart S, for the
     purpose of implementing corrective action  requirements
     under section 264.101  and RCRA section 3008(h).   A
     CAMU shall only be used for the management  of reme-
     diation wastes pursuant to  implementing such corrective
     action requirements at the facility" (40 CFR 260.10).

  CAMUs  were designed to provide more flexibility in treat-
  ment of waste  generated during corrective actions.  An
  important provision of the new regulations is the  specifica-
  tion in 40 CFR 264.552(a)(1) and (2) that:

     "(1)  Placement  of remediation wastes into or  within  a
         CAMU does not constitute land disposal of hazard-
         ous wastes; and
     (2)  Consolidation or placement of remediation wastes
         into or within a CAMU does not  constitute creation
         of  a unit subject  to  MTRs  (minimum technology
         requirements)."

  As a result, an area or several areas at a RCRA facility (or
  CERCLA site) can be designated  as a CAMU  and the
  wastes can  be  removed  from  the  ground,  treated,  and
  replaced within the boundaries of that CAMU without being
  required to  comply  with  the  LDR  treatment standards.
  EPA's goal in issuing these regulations is to encourage the
  use of  more effective treatment technologies  at a specific
  site.  In situ treatment could still  be  the favored option at
  sites where the  Regional Administrator does not establish a
  CAMU and where the ex situ  treatment is subject to treat-
  ment standards and BOAT under the LDRs.

     Technologies  that accomplish the  treatment in  situ may
  reduce  or eliminate  point source  air emissions  or other
  discharges.  Many in situ treatment technologies, however,
  do have aboveground components. For example, materials
  are injected; groundwater is extracted, treated, and reinjec-
, ted; or vapors are captured and  treated.  The  aboveground
  portion  still may be  subject to  appropriate environmental
  regulations.  Technologies that  require  injection  of fluids
  may need to follow Underground Injection Control regula-
  tions.

  2.2.3 Time Available for Remediation

     The available time factor characterizes the amount of
  time allowed to  set up, operate, and  remove the  treatment
  technology.  Determining  the time  available  to  complete
  remediation is directed at answering questions such as:

     •  Can the cleanup be completed  in a time frame con-
       sistent with health, safety,  and  environmental protec-
      tion?
     •  Can the cleanup be completed  in a time frame con-
       sistent with end-use requirements?
   The time available for  remediation is  controlled first  by
the need to protect human safety and health and the envi-
ronment.   Remediation must proceed  quickly  if a toxic
contaminant is present, the  contaminant concentration is
high, or the contaminant is mobile and near a critical eco-
system. Time available may be controlled also by the value
or intended end use for the site.  It is undesirable to hold a
high-value site out of productive use for a long period.

   In situ remediation typically requires more treatment time
than the analogous ex situ treatment technology.   In situ
bioremediation, for example, typically requires about 4 to 6
years (U.S. EPA and U.S.  Air Force, 1993, p. 60). Excava-
tion allows essentially  immediate remediation  of the site.
However, the excavated  material often must be shipped and
stored before treatment.  Rapid remediation is needed if the
contaminant presents  an  imminent  danger due  to hazard
level,  mobility, or other  factors.   Rapid remediation of  an
imminent hazard  generally favors an ex situ  remediation
approach.

   The importance of the length of remediation time may  be
lessened if the time constraint is driven by economic or end-
use requirements.  Many in situ technologies can be applied
concurrently with other site operations.   For example, well
and injection/extraction equipment for bioventing,  soil vapor
extraction, or fixed-system steam injection  do not occupy the
full surface area of a site.  Depending on the technology
and the site use, it may  be possible to continue routine site
operations during an in situ remediation. However, the need
for rapid remediation still generally increases the favorability
of ex situ treatment technologies.

2.2.4  Removal Logistics

   The removal logistics factor characterizes the feasibility
of excavating, handling,  and transporting  the contaminated
soil.  Examination of removal logistics is directed at answer-
ing questions such as:

   •  Is the material accessible for excavation?
   •  Can the contaminated soil or water be moved efficient-
     ly  by  conventional bulk material-handling equipment
     and techniques?
   •  Will on-site (and if needed off-site) infrastructure sup-
     port transport of waste materials?

  . Removal logistics  are  determined  by  access to  the
contaminated  site for  excavation,  the  ability to handle
excavated materials, space for placement of ex  situ treat-
ment  equipment,  and the road  and  rail system  on  and
around the site.

   Data needed to evaluate the removal logistics include a
map of the site showing the general arrangement of struc-
tures and infrastructure and an approximate assessment of
the subsurface conditions  such as the location of contami-
nation and the location of major geologic and hydrogeologic
features such as surface water and aquifers.
  Treatment of Soils In Situ

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   Poor removal logistics favor  in situ  treatment.  In situ
treatment generally  is favored by conditions such as con-
tamination located under a building that is to remain after
remediation; presence of buried piping or utility lines in the
area; contamination  located at great depth or under a rock
formation;  poor  road or rail access;  nearby businesses,
schools, or heavy traffic areas; or site location in a remote
area distant from treatment facilities or  sources  of backfill.
Contamination  located deeper than 5 feet  or occupying a
volume of more than 1,000 m3 increases both the cost and
the complexity of excavation (U.S. EPA, 1990, EPA/600/2-
90/01 1, p.  60).  Specialized delivery and recovery systems
may be necessarylo overcome poor site logistics.

2.2.5 Waste Conditions

   The waste conditions factor characterizes the chemical
and physical form of the waste with respect to the ability to
effectively treat or remove the contaminant.   Examination of
waste conditions is directed at answering questions such as:

   •  Are the concentration and distribution of contaminants
     consistent with  effective in situ treatment?
   *  Does the waste distribution  or condition allow effective
     delivery of treatment agents to the contaminant?

   The waste conditions factor is controlled by  the in situ
conditions of the contaminant  and matrix.  The conditions
requiring  characterization include  the  concentration and
distribution of the contaminant,  the chemical form and spec-
iation of the contaminant and matrix, and physical properties
of the waste and matrix.

   Data needed to characterize the waste conditions include
a survey of the location, concentration measurements, and a
description of the form of contaminant, matrix, and debris in
the remediation site.   Some soil  sampling data  may  be
available, but assessment  of  the  waste condition at the
preliminary  evaluation stage typically will be based largely
on historical records.

   The understanding of waste conditions must be constant-
ly reevaluated as additional data are obtained.   In addition
to estimating the areal extent and concentration of contami-
nation, the assessment must address  the possibility of the
contaminant being  contained in  drums or tanks and the
potential presence   of  noncontaminant  debris  that  could
make excavation difficult or obstruct the flow  of in situ treat-
ment agents.

   In general, contaminants that are either highly  concen-
trated or spread over a relatively small area are best treated
by ex situ methods.  In particular, contaminants contained in
drums  or underground tanks are difficult  to treat with in situ
methods.  Dilute or widely distributed contaminants tend to
favor in situ treatment. When the contaminant is present at
low concentration, ex situ processing requires excavation,
handling, and,  processing of a  high  proportion of matrix
materials relative to a small amount of contaminant.
3.0  Technology-Specific Factors

   This section outlines the characteristics of in situ tech-
nologies and describes factors identified by current testing
programs as influencing the effectiveness, implementability,
and cost of specific in situ treatment technologies.  Review-
ing these technology-specific critical factors will help guide
planning of site characterization activities and identification
of technology candidates.   Where possible, specific values
are given to indicate what level of a factor is favorable for
application of in situ treatment.

   The user must consider all of the generic factors {see
Table 2-3) and technology-specific factors during evaluation
of technology alternatives.  The more important factors are
indicated in Tables 3-1 to 3-8 by an asterisk (*) to  assist in
the evaluation.  However, the evaluation must not focus on
only one factor or one technology.  All of the factors should
be evaluated for all technologies that are potentially effective
for the contaminants present at the site. After full consid-
eration of all the factors, the decision maker can  examine
the overall indications for favorable and unfavorable trends
to identify technologies with a high probability of being effec-
tive and  implementable.  The generic factors  will help indi-
cate if in situ approaches are generally favored for the site
and contaminants in  question.   If  an  in situ  technology
seems attractive, the technology-specific factors can help
guide selection of a group  of candidates for more detailed
testing and evaluation.

   The success or failure  of an applied technology often
depends  on site-specific  conditions  or design  features.
Selection of technology candidates should be based on site-
specific knowledge  and  requirements,  tempered  by  the
overall effect of all of the critical factors,  Treatability testing
typically will be required to support final technology selection
prior to completion of the feasibility  study (FS) or  prepara-
tion of the Record  of  Decision  (ROD) (U.S. EPA,  1992,
EPA/540/R-92/0713).

   Action  levels  are provided where  possible to give  a
starting basis for considering technology alternatives.  The
action levels give  an approximate "yardstick" to  use when
considering technology candidates.  However, these single-
value indications cannot characterize or summarize all of the
complex situations that occur in practice. There is no sub-
stitute for experience, site-specific knowledge, and treatabili-
ty testing.   The user must be aware of the limitations  of
giving a single value  to characterize complex interactions.

   Site-specific conditions can cause the action levels to be
different at a particular site  or with a particular combination
of contaminant and  matrix.  Design  or operating  features
may be applied to overcome technology limitations.   For
example,  at  a site  where  in situ  bioremediation  is  ideal
except for the condition of low soil temperature, a number of
methods  are available to improve the soil energy  balance.
Many of the technology-specific critical factors show thresh-
old effects.  The  factor may have  an  important effect  at
some level but have  no effect below the cutoff level.  For
example,  metals such as zinc are  trace nutrients at  low
levels but toxic to biological systems at higher levels.
10
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3.1  Delivery and Recovery Systems

   Efficient  delivery  and  recovery  methods  control  the
effectiveness, implementability, and cost of in situ treatment.
An array  of delivery  techniques are available to  apply or
inject treatment fluids into the  subsurface.  The  types of
delivery systems for in situ treatment can be classed gener-
ally as  gravity  driven, pressure driven, auger mixing, and
energy  coupling.   Recovery systems typically fall in  the
gravity-driven or  pressure-driven classification (U.S. EPA,
1990, EPA/600/2-89/066, p. 2).

   With the exception of radiofrequency heating and in  situ
vitrification, the in situ treatment methods discussed below
require delivery and control of liquid, slurry, gas, vapor, or a
combination in the  soil.  For some technologies (see Table
2-2), the fluids also must be recovered after passing through
the contaminated  in  situ volume.   Fluid delivery  may  be
accomplished  by  conventional  gravity infiltration  through
surface  or  trench application  or  by pressure   injection
through wells.    Conventional   recovery  methods include
trenches or wells.  The conventional methods rely on flow
patterns determined by the design  and placement of  the
drains or  wells and the subsurface  stratigraphy.   As  de-
scribed below, innovative techniques are available to modify
the subsurface conditions to improve  flow  rates or  flow
control.

   In  situ  treatment agents, summarized  in  Table 2-2,  in-
clude fluids delivered to the contaminated volume.  Possible
treatment  fluids include hot gasses or  vapors; water;  or
water-containing  nutrients,  surfactants,  anions,   cations,
bacteria, S/S  binder,  or  other  treatment  agents.   For  a
technology to be effective, implementable, and economically
competitive; the treatment agents must  be delivered in  a
well-controlled manner.  Conventional gravity- and pressure-
driven methods are available to deliver and  recover fluids.
Gravity-driven methods rely on infiltration and collection due
to hydraulic  gradients.   Typically delivery  is by surface
distribution and  collection  is by trench  or similar drains.
Pressure-driven  methods  rely on pressure gradients sup-
plied by a source pump, a blower or steam generator, or an
extraction  pump or blower.  A system of wells typically is
used  for delivery and recovery.   The conventional delivery
and recovery systems are highly dependent on the physico-
chemical environment in the subsurface.

   Innovative approaches are being developed and  tested to
improve   the   performance  of   delivery  and   recovery
technologies in low-conductivity  or ^heterogeneous  geologic
settings.  The innovative delivery and recovery technologies
may be  devised to increase the conductivity in the treatment
zone, decrease the conductivity below the treatment zone,
or improve the  efficiency of contact  between the treatment
agents and the material to be treated. Conductivity .modifi-
cation technologies include hydraulic fracturing, pneumatic
fracturing,  radial  well drilling,  jet  slurrying,  and  kerfing.
Technologies to  improve  the  distribution  or  application
efficiency  of treatment agents include colloidal gas bubble
(aphron) generation, ultrasonic methods, and cyclic  pumping
or steaming (U.S. EPA, 1990, EPA/540/2-90/002, p. 96).
   Auger-mixing  technologies  have  been  developed  to
deliver treatment agents with less  reliance on a favorable
existing geology. Auger mixing is applicable to delivery but
not to recovery of treatment agents.  The main examples of
auger delivery are steam injection and addition and mixing
of solidification/stabilization binders  with  augers.  One ven-
dor is  testing auger  mixing  for addition of  bioremediation
nutrients.

   Technologies to apply energy  rather than fluids also are
available  for in  situ  treatment.   Energy delivery systems
reduce dependence on  in situ conductivity  but are sensitive
to other in situ parameters.  The key  to energy delivery is
good coupling of the  electric or electromagnetic field to the
soil being  heated.  The electric  properties change  as the
moisture content changes.   The energy  input processes
vaporize water so the electrical coupling properties of the
soil must change as treatment proceeds.  The changing soil
properties  increase the challenge in designing an  efficient
energy application system.

   Systems for pneumatic fracturing and  hydraulic fracturing
to improve subsurface conductivity  and  a  system to inject
oxygen  microbubbles  to  remediate groundwater have been
accepted  in the  SITE Program.  The demonstration  of a
pneumatic fracturing system was completed at a site located
in South Plainfield, New Jersey (Mack  and Aspan, 1993, p.
321).   The Applications Analysis Report is  in preparation
(U.S. EPA, 1992, EPA/540/R-92/077, p. 5).

   For further information on delivery and recovery technol-
ogies, contact:

     Michael Roulier (513) 569-7796
                       or
     Wendy Davis-Hoover (513) 569-7206
       U.S. Environmental Protection Agency
       Risk Reduction Engineering Laboratory
       26 West  Martin Luther King Drive
       Cincinnati, OH 45268
3.2 In Situ Solidification/Stabilization

   In   situ  solidification/stabilization  refers  to  treatment
processes  that  mix or  inject  treatment agents  into  the
contaminated material in place to accomplish one or more of
the following objectives:

   • Improve the physical characteristics of the waste by
     producing a solid  from liquid or semiliquid wastes
   • Reduce the contaminant solubility
   • Decrease  the  exposed  surface area  across which
     mass transfer of contaminants may occur.

   In situ  S/S relies  on the delivery and  effective mixing of
binders with the contaminated  soil.   The critical factors
applicable for in situ  solidification/stabilization with inorganic
binders such as  cementitious materials (cements and pozzo-
lans), silicates, or lime are shown in Table 3-1.
Treatment of Soils In Situ
                                                                                                                     11

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       Table 3-1.  Solidification/Stabilization Critical Factors and Conditions for Cement-Based Treatment Systems'8'
Factor Influencing
Technology Selection
Presence of VOCs (*)
Contaminant
depth 0
Specific gravity,
viscosity, and
general mixing
properties (*)
SVOC content of
waste (*)
Oil and grease content
of waste (*)
teachability data (*)
Phenol content
Fine particle
Soluble inorganic salts
(e.g., chlorides) not tar-
geted by binder
formulation
Cyanide content of the
waste
Sulfate content of the
waste
Binder heat of
hyd ration
Conditions Favoring Selection
of In Situ Treatment
<50ppb{1)(2)
Varies with technology (1)
No action levels
specified (1)(3)
SVOCs <10,000 ppm (4)
No action levels
specified (1)
No action levels
specified (2)
<5% (5)
Limited amount of fine insoluble
paniculate (4)
No action levels
specified (1)
<3,000 mg/kg (4)
<1 ,500 ppm for Type I Portland
cement (6)
Various cement types can toler-
ate higher sulfate levels but no
action level specified (6)(3)
No action levels
specified (5)
Basis
« VOCs can vaporize during processing or curing;
therefore, low levels of VOCs are favorable
• Organic materials can interfere with bonding
• In-place mixing with conventional construction type
equipment is limited to near surface
• Auger systems demonstrated to 30 feet
• Grout injection depth typically is not a major limitation
« Good mixing is needed to ensure contact of the waste
and binder so a good S/S product is obtained
• Organic materials can interfere with bonding
» SVOCs can vaporize during processing or curing;
therefore, low levels of volatile compounds are favorable
(due to heat evolution in some processes, the favorable
limit can be much lower for some contaminants and S/S
binder combinations)
* Oil and grease can coat the waste particles inhibiting
setting or reducing the strength of the final product
• Mobile and soluble materials are more difficult to treat
• Phenol concentration greater than 5% can reduce the
compressive strength of the final product
• Fine particulates can coat the waste particles and
weaken the bond between the waste solids and cement
• Low concentrations are more favorable
• Threshold effects commonly occur
• Above some concentration levels, soluble salts can
reduce the physical strength of the final product, cause
large variations in setting time, or reduce the
dimensional stability of the cured matrix
• Cyanides interfere with bonding of waste materials
• Presence of sulfates can retard setting
» High sulfate levels in waste can cause treated waste to
spall during curing due to formation of expansive
hydrates
« Large amount of heat generated-by binder hydration
reactions, particularly in large mass treatment, can
increase temperature and volatilize organic
contaminants
Data Needs
• Analysis for VOCs
« Treataoility tests measuring
volatile emissions
* Waste composition and spatial
distribution
* Waste-specific gravity
• Waste particle morphology and
size distribution
* Waste viscosity
• Analysis for SVOCs and PAHs
« Treatability tests measuring
volatile emissions
• Analyse for oil and grease
• teachability testing
» Phenol content in waste
• Particle-size analysis, particularly
size fraction under 200 mesh
• Treatability testing
• Analysis of inorganic content
• Analysis for cyanides
• Analysis for sulfate
• Total and time-dependent heat
output due to hydration of binder
• Treatability tests measuring
volatile emissions
(a) Also see Table 2-3 for generic factors,
(*) Indicates higher-priority factors,
(1) Conner, 1990, pp. 189,205, and 464-477.
(2) U.S. EPA, 1993, EPA/530/R-93/012, pp. 4-51 and A-8.
(3) U.S. EPA, 1990, EPA/540/2-90/002, pp. 14-16.
(4) U.S. EPA, 1988, EPA/540/2-88/004, p. 93.
(5) U.S. EPA, 1991, EPA 540/2-91/009, p. 3.
(6) Amiella and Blythe, 1990, p. 93.
12
                                                                                                       Treatment of Soils In Situ

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  , The most common binders are Portland cement, pozzo-
lans (siliceous or aluminous materials that  can react with
calcium  hydroxide  to form compounds with cementitious
properties), and cement/pozzolan mixtures.  Inorganic binder
systems using sodium silicate, cement/silicate, or proprietary
binder systems also  are in use.   Solidification/stabilization
encompasses a wide variety of physical and chemical mech-
anisms to reduce contaminant mobility and/or impart other
desirable properties to the waste.   S/S treatment  using
inorganic binders ties up free  water by hydration reactions.
Mobility of inorganic compounds can be reduced by forma-
tion  of  insoluble  hydroxides, carbonates,  or  silicates; sub-
stitution  of  the metal  into a  mineral  structure;  sorption;
physical encapsulation; and other mechanisms.

   S/S treatment  of organic contaminants  with cementitious
formulations  is more  complex  than treatment of inorganic
contaminants.  Wastes where organics  are the primary
contaminant  of concern generally  are  not  suited to S/S
treatment because of the potential for volatilization of organ-
ics  and  reduced  S/S product quality when organics are
present.   This is particularly true  with  VOCs  where the
mixing process and heat generated  by cement hydration
reactions can increase organic vapor losses.  However, S/S
can be applied to wastes that contain  lower levels of organ-
ics   (particularly  when inorganics  are   present  at  high
concentrations) and/or the organics  have a low vapor pres-
sure, high  water solubility,  or both.   Furthermore,  recent
studies have indicated that addition of silicates or modified
clays to  the  binder system  may improve  S/S performance
with  organics (U.S. EPA, 1993, EPA/530/R-93/012, pp. 4-12
and 4-13).

   The most significant challenge in applying  S/S in situ for
contaminated soils is achieving complete and uniform mixing
of the binder with the  contaminated matrix  (U.S. EPA, 1990,
EPA/540/2-90/002,  p.  12).   Three basic approaches are
used for mixing the binder with the matrix:

   •  Vertical auger mixing
   •  In-place mixing
   •  Injection grouting.

   In vertical auger mixing,  a  system of augers is used to
inject and mix binder  into the  soil.  Auger-type mixing sys-
tems  developed by Novaterra (formerly Toxic Treatments
USA);  International Waste  Technologies  (IWT)/Geo-Con,
Inc.;  and S.M.W.  Seiko, Inc.  have been accepted in the
Superfund Innovative Technology Evaluation (SITE) Demon-
stration Program.  SITE demonstrations have  been complet-
ed for the Novaterra (U.S. EPA, 1991, EPA/540/A5-90/008)
and  IWT systems (U.S. EPA, 1990,  EPA/540/A5-89/004).
The  treatment  depth  is limited by the  length of available
auger equipment.  Current testing indicates a limit of about
30 feet.  Based on the SITE Program  test  of in situ S/S
using the IWT/Geo-Con auger system, estimated treatment
costs were $111/ton and $194/ton for 4-auger and 1-auger
systems, respectively.   The  costs  included  equipment,
startup and  fixed  costs, labor,  supplies, utilities, analytical,
facility modification, and demobilization (U.S. EPA,  1990,
EPA/540/A5-89/004, p. 26).   Note that some of the  auger
systems, particularly  the  Novaterra  system,  may  inject
steam (or steam and hot air) instead of binders to perform
steam stripping of organics.  These operations are discus-
sed in Section 3.9.

   In-place mixing involves spreading and mixing of binder
reagents with waste by conventional  earth-moving equip-
ment such as draglines,  backhoes, or clamshell  buckets.
The  technology is  applicable only  to surface  or  shallow
deposits of contamination.

   The reported cost of in-place mixing is $38/yd3.  The cost
includes labor, equipment, monitoring and testing, reagents,
and  miscellaneous  supplies.  Not included  are costs for
equipment mobilization and demobilization, engineering and
administration, and  health and safety (Arniella and Blythe,
1990, p. 101).

   For injection grouting, a  binder containing dissolved  or
suspended treatment agents  is forced into  the formation
under pressure and allowed to  permeate the soil.  Grout
injection can  be applied to contaminated formations lying
well  below the ground  surface.   The injected  grout then
cures in place to give an in situ treated mass. Grout injec-
tion is widely  used for soil stabilization. A grouting system
for very fluid wastes developed by Hazardous Waste Control
has been accepted for  testing in  the SITE  Program (U.S.
EPA,  1992, EPA/540/R-92/077, p. 100).

   For further information on in situ solidification/
stabilization technologies, contact:

     Patricia Erickson (513) 569-7884
       U.S. Environmental Protection Agency
       Risk Reduction Engineering Laboratory
       26 West Martin Luther King Drive
3.3 Soil Vapor Extraction

   In situ soil  vapor extraction (SVE) is the process  of
removing VOCs from the unsaturated  zone to the surface
for treatment.  Blowers attached to extraction wells alone or
in combination with air injection wells induce airflow through
the soil matrix.  The airflow  strips the volatile compounds
from the soil and carries them to extraction wells.   The
process is driven  by partitioning of volatile materials from
condensed  phases (sorbed  on soil particles,  dissolved  in
pore water, or  nonaqueous liquid phases) to the clean air
being introduced by the vacuum  extraction process.   Air
emissions from the systems typically are controlled ex situ
by adsorption of the volatiles  onto activated carbon, thermal
destruction  (i.e., incineration or catalytic oxidation), or con-
densation by  refrigeration (U.S.  EPA, 1991,  EPA/540/2-
91/006, p. 3).   Application of  soil vapor extraction  relies on
the ability to deliver, control the flow, and recover stripping
air. A decision  logic for treatability testing based on contam-
inant vapor pressure and air permeability of  the soil  has
been  described  in  the  literature   (U.S.  EPA,  1992,
EPA/600/K-92/003, pp. 4-8 and 4-9).

   The  critical  factors to consider during review of SVE
technology application are presented in Table 3-2.  The SVE
Treatment of Soils In Situ
                                                                                                                     13

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                           Table 3-2. Soil Vapor Extraction Critical Factors and Conditions*"'
Factor Influencing
Technology Selection
Contaminant vapor pres-
sure, Pv (*)
Air conductivity of soil (*)
Soil moisture content^*)
Clay content of soil (*)
Humic content in soil (*)
Soil sorption capacity (*)
Contaminant water
solubility {*)
Henry's law constant (*}
Dominant contaminant
phase
Soil temperature
Depth to groundwater
Conditions Favoring Selection
of In Situ Treatment
P¥ >0,5 mmHg at 20°C
(Reference describes decision
logic) (1)
>10~* cm/sec (2)
(Decision logic described) (1)
<10 volume % (2)
No action levels specified (1)
No action levels specified (3)
Specific surface area
<0.1 m2/g (2)
<100 mg/L (2)
>0,001 dimensionless (4)
Contaminant present as a sep-
arate phase (vapor or liquid) and
not sorbed to the soil (2)
>20°C (2)
Contaminant in the unsaturated
zone (2)(5)
Basis
• Contaminants of higher volatility are more easily
removed by air stripping
• High conductivity results in a large radius of
influence for extraction weiis
• Soil moisture inhibits airflow and can reduce
vapor pressure of soluble organics
* Low clay content is desirable
• Presence of clay increases sorption and inhibits
volatilization
« Low humic content is desirable
* Presence of humic materials increases sorption
and inhibits volatilization
• Contaminants held by sorption mechanisms are
more difficult to remove
* Dissolved orpnfcs are more difficult to remove
by air stripping
• Compounds that partition to the vapor phase are
more easily removed by stripping
• Vapors are more easily removed by air stripping
* Higher soil temperatures are more favorable to
volatilization
• Technology only effective in the unsaturated
zone
• Need to avoid water intrusion into extraction
wells
Data Needs
* Contaminant vapor pressure at
expected soil temperature
• Hydrogeologic flow regime
• Soil moisture content
• Soil composition
* Soil color
• Soil texture
* Soil composition
• Soil color
• Soil texture
* Soil-specific surface area
• Soil absorption isotherms
• Contaminant solubility at expected
soil temperature
• Henry's law constant
• Contaminant composition and
physical form
• Soil temperature
• Depth to groundwater
* Seasonal variation of groundwater
conditions
(a) Also see Table 2-3 for generic factors.
(*) indicates higher-priority factors.
(1) U.S. EPA, 1912. EPA/600/K-92/003, pp. 4-8 and 4-9.
(2) U.S. EPA, 1990, EPA/600/2-90/011, p. 40.
   (3) U.S. EPA, 1988. EPA/540/2-88/004, p. 89.
   (4) U.S. EPA, 1992. EPA/540/R-92/077, p. 175.
   (5) U.S. EPA, 1991. EPA/540/2-91/003, p. 52.
technology has  been  used in commercial operations  for
several years.  It has been chosen as a component of the
RODs at more than 80 Superfund sites (U.S. EPA, 1992,
EPA 542/R/92-011, pp. 31-46).

   Vertical  wells are the most widely used  SVE design
method.  Vertical wells are best used at  sites  where the
contamination extends far below the land surface.  Horizon-
tal wells or trenches may  be more practical  than vertical
wells where the  depth  to groundwater  is less than 12 feet.
Vertical  wells generally are inappropriate  for  sites with a
shallow  water table  due to the potential upwelling of the
water table that  may occur after application of  a  high vacu-
um (U.S. EPA, 1991, EPA/540/2-91/003, p. 52).
   SVE systems have been accepted in the SITE Program
(U.S. EPA, 1992,  EPA/540/R-92/077).  SITE demonstrations
of soil vapor extraction systems were completed at a Super-
fund site in Burbank, California (U.S. EPA, 1991, EPA/540/-
A5-91/002) and a Superfund site in Groveland, Massachu-
setts (U.S. EPA,  1989,  EPA/540/A5-89/003).  A reference
handbook on soil  vapor extraction (U.S. EPA, 1991, EPA/5-
40/2-91/003)  and screening computer software for an ap-
proach  to the design, operation,  and monitoring  of SVE
systems are available (U.S. EPA, 1993, EPA/600/R-93/028).

   Based on available data, SVE treatment cost estimates
typically  are  $50/ton for treatment of soil.   The  reported
estimates of  cost ranges are $15 to $60/yd3  (U.S.  EPA,
1990, EPA/600/2-90/011, p. 40) and $27 to $66/ton  (U.S.
EPA, 1989,  EPA/540/A5-89/003, p. 11).   The cost ranges
14
                                                                                            Treatment of Soils In Situ

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include consideration  of  site preparation; equipment pur-
chase,  installation,  and  operation;  residual  well  cuttings
disposal; analysis; and demobilization.  The high end of the
range  includes off-gas treatment, whereas the lower cost
does not. Off-gas treatment can  amount to more than 50%
of the total cost of an SVE system (U.S. EPA, 1990, 937.5-
06/FS, p. 3-141).

   For further information on soil vapor extraction technolo-
gies, contact:

     Michael Gruenfeld (908) 321-6625
        U.S. Environmental Protection Agency
        Risk Reduction Engineering Laboratory
        Building #10 (MS 104)
        2890 Woodbridge Avenue
        Edison, NJ 08837-3679
3.4 In Situ Bioremediation

   In biological processes, microorganisms degrade organic
compounds either directly to obtain carbon and/or energy, or
fortuitously in a cometabolic process  with  no significant
benefit to the microorganism. The ultimate goal of in situ
bioremediation  is to  convert  organic contaminants  into
biomass and  innocuous by-products of microbial metabolism
such as  carbon dioxide, inorganic salts, and  water.  Suc-
cessful in  situ  bioremediation can occur only if microbial
populations are  present that can be stimulated to degrade
the contaminants of concern. In situ bioremediation capital-
izes on  natural  biological  processes to  enhance in situ
degradation of organic contaminants.  Although biodegrada-
tion of organic contaminants occurs naturally in situ, often a
critical factor, such as oxygen, is limiting, thus limiting the
amount of biodegradation that can occur.

   To increase the amount of biodegradation that occurs, in
situ amendments often are necessary. These amendments
may include  electron acceptors  (such as oxygen), carbon
sources,  moisture, nutrients, or heat.  The critical factors to
consider  during review of  in situ  bioremediation applications
are presented in Table 3-3.

   Bacteria,  actinomycetes,  and fungi in the  subsurface
make up the  most significant group of organisms involved in
biodegradation.  These communities are diverse and adap-
table, capable of taking advantage of xenobiotic compounds.
Microbial populations at older sites generally are acclimated
to the  contaminants  of  concern.  Consequently,  lack  of
biodegradation  in situ  rarely is due  to  lack of populations
able to degrade the compounds, but more likely is due  to
environmental conditions  that limit  the  extent and rate  of
biodegradation.   Typically the most  important parameters
are electron  acceptor availability, moisture  levels, tempera-
ture, pH,  and nutrients.

   Another critical parameter affecting the extent of in situ
bioremediation  is  bioavailability   of  the contaminant(s)  of
concern.   Bioavailability is a general term to describe the
accessibility of  contaminants to  the degrading populations.
Bioavailability consists of (1) a  physical aspect related  to
phase distribution and mass transfer, and (2) a physiological
aspect related to the suitability of the contaminant as a sub-
strate (U.S. EPA, 1993, EPA/540/S-93/501,  p. 4).   Com-
pounds with greater aqueous solubilities and lower affinity to
sorb onto the soil generally are more bioavailable  to  soil
microorganisms  and are more  readily degraded.  Bioavail-
ability also depends on the suitability of the compound as a
metabolic substrate or cosubstrate.

   Aerobic (>0.2 mg/L oxygen)  or anaerobic conditions may
predominate  in  the  subsurface.   Mineralization of  many
organic compounds  occurs aerobically; therefore,  aerobic
bioremediation is the most developed  and most feasible in
situ  biotechnology.   In situ bioremediation under  aerobic
conditions involves  delivering oxygen  and nutrients  to the
subsurface through an  injection well  or infiltration system.
The oxygen and nutrients enhance the activity of indigenous
aerobic microorganisms that degrade  the contaminants of
concern.  In general, aerobic processes can be suitable for
remediation  of petroleum hydrocarbons,  halogenated and
nonhalogenated   aromatics,  polyaromatic  hydrocarbons,
halogenated   and  nonhalogenated   phenols,   biphenyls,
organophosphates,  and  some   pesticides and  herbicides.
Biodegradation rates are compound specific, so treatability
judgments should be based on literature  data  for the con-
taminants present or on treatability tests.

   Although  mineralization  of   many  compounds   occurs
aerobically,  some   halogenated  hydrocarbons  may  be
transformed under anaerobic conditions.  These halogenat-
ed  hydrocarbons include unsaturated  alkyl  halides (e.g.,
PCE  and TCE) and saturated alkyl  halides   (e.g.,   1,1,1-
trichloroethane and  trihalomethane).  In addition, supplying
nitrate as an electron acceptor  under  anaerobic conditions
may allow biodegradation  of some phenols,  cresols, and
lower-molecular-weight   polycyclic   aromatic  hydrocarbons
(PAHs).

   Anaerobic bioremediation is  at a much earlier stage of
development than aerobic bioremediation.  Establishing and
maintaining anaerobic conditions in situ is more difficult than
establishing  and maintaining aerobic conditions. Anaerobic
treatment systems can have undesirable secondary  effects
such as formation of volatile forms of metals (such as meth-
ylated  mercury or arsines), toxic  "deadend"  intermediates
such as vinyl chloride and hydrogen  sulfide,  or nuisance
odor compounds.

   Addition of amendments  to  promote in situ  biodegrada-
tion generally  relies on the  ability of  aqueous  solutions to
infiltrate  into the contaminated area.   Aqueous-supplied
amendments have met with limited success, as the electron
acceptor or  nutrient often is metabolized  before it reaches
the contaminated area.  Consequently, there is a high level
of microbial activity  near or in the infiltration  wells, often
resulting in plugging and poor flow.

   In extreme  environments, moisture or heat addition may
significantly  improve bioremediation  processes.   Surface
insulation, warm  water infiltration, and buried heat tape have
been used to increase the soil temperature.  Their use has
Treatment of Soils In Situ
                                                                                                                     15

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                                   Table 3-3. Bioremediation Critical Factors and Conditions'"'
Factor Influencing
Technology Selection
Spatial variation of
waste composition or
concentration (*}
Contaminant
biodegradability (*)
Oxygen content (*)
Available soil water (*)
Presence of elevated
levels of metals, highly
chlorinated organics, pes-
ticides and herbicides, or
inorganic salts (*)
In situ temperature
Soil nutrient content
Water solubility
pH
Redox potential
Organic carbon content
Conditions Favoring Selection of
In Situ Treatment
No action levels specified (1)
Ratio of biological oxygen demand
(BOD) to chemical oxygen demand
(COD) >0.1 (3)
Aerobic metabolism; dissolved 02
>0.2 mg/L (4)
Air-filled pore space of >10% (4)
Anaerobic metabolism; gas-phase
02 concentration <1% (4)
>25% and <85% of water-holding
capacity (4)
No action levels specified (1)
>10°C (3)
Optimum temperature typically
20°C to 40°C (5)
Carbon/nitrogen/phosphorus ratios
about 100:10:1 (4)
Carbon/sulfur ratio noted as
important but no action level
specified (1)
>1, 000 mg/L (2)
Between 5-9 pH units (5)
Aerobes and facultative anaer-
obes: >50 millivolts (mV) (4)
Anaerobes: <50 mV (4)
Total organic carbon (TOC) of
groundwater between 10 and
1000 mg/L (3)
Basis
• Homogeneous conditions are desirable
• Large variation in the contaminant concentration
causes variation in biological activity giving
inconsistent biodegradation
• Resistance to biological action inhibits
decontamination
• Oxygen depletion slows aerobic biological activity
• Oxygen is toxic to anaerobic systems
• High moisture content reduces bacterial activity by
limiting the transport of oxygen
» Low moisture content inhibits bacterial activity
• Materials can be toxic to microorganisms
• Lower levels are desirable
* Threshold effects commonly occur
• Optimum temperature range increases growth rate
• More diverse microbial populations are present in
optimum range
« Lack of adequate nutrients slows biological activity
• Contaminants with low solubility generally are more
difficult to degrade
• Toxic contaminants with high solubility, however, may
be more effective in suppressing bioactivity
• When pH is outside of range, biological activity is
inhibited
* Reflects oxygen availability in the soil
• Indicates the oxidation/reduction potential of the
matrix
• Low concentrations may cause organisms to favor
other food; high concentrations may be toxic to the
organisms
Data Needs
• Waste composition and spatial
distribution
* Waste composition
• Waste BOD and COD
* Presence of metals or salts
* Treatability testing
« Oxygen monitoring
• Percent water saturation of
pores
• Identification of specific com-
pound, oxidation state (metals),
and concentration
• Temperature history and/or
monitoring covering at least
three seasons
« Carbon/nitrogen/
phosphorus ratio
• Form of nitrogen (e,g., nitrate,
ammonia, organic nitrogen)
• Carbon/sulfur ratio
* Contaminant solubility in water
at treatment temperature
• Soil pH
* Soil redox potential
•TOC
(a) Also see Table 2-3 for generic factors.
(*) Indicates higher-priority factors.
(1) U.S. EPA, 1988, EPA/540/2-88/004, p. 114.
(2) U.S. EPA, 1990, EPA/600/2-90/011, p. 48.
(3) U.S. EPA, 1990, EPA/600/2-90/027, p. 85.
(4) U.S. EPA, 1993, EPA/540/S-93/501, p. 3.
(5) U.S. EPA, 1990, EPA/540/2-90/002, pp. 40 and 47.
                                                                                                          Treatment of Soils In Situ

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resulted in increased  microbial  activity and contaminant
degradation (Leeson et al., 1993).

   The reported costs for application of in situ bioremedia-
tion  range from  $14  to  $98/ton  (U.S.  EPA and  U.S.  Air
Force, 1993, p. 60).  The EPA Vendor  Information System
For Innovative  Treatment Technologies (VISITT), Version 2,
contains information from 11 vendors on in situ soil biorem-
ediation technologies.   The costs indicated by the vendors
typically range  from $8 to $250/yd3 (U.S. EPA, 1993, EPA/
542/R-93-001).

   A  variety of in  situ bioremediation systems have been
accepted  in the SITE Program.   Technologies include  the
use of naturally occurring microorganisms,  addition  of cul-
tured bacteria,  and addition of white-rot fungi.  Water and
nutrients generally are applied by well injection or infiltration.
However, one technology assembles a containment tank in
situ to form  a  controlled area for the  bioremediation, and
one technology uses vertical augers to distribute organisms
and  nutrients.   All of the  technologies stimulate  aerobic
biodegradation, except one, which combines anaerobic and
aerobic microbial  activity  (U.S.  EPA,  1992, EPA/540/R-
92/077, p. 208).

   For further  information on bioremediation technologies,
contact:

     Carl Potter (513)  569-7231
        U.S. Environmental Protection Agency
       Risk Reduction Engineering Laboratory
       26 West Martin Luther King Drive
       Cincinnati, OH 45268
      3.5  Bioventing

        Bioventing is the process of aerating subsurface soils to
      stimulate in situ biological activity and promote bioremedia-
      tion.   Although bioventing is related to SVE, their primary
      objectives are different.   SVE is designed  and operated to
      maximize  the volatilization  of low-molecular-weight com-
      pounds, with  some biodegradation  occurring.  In contrast,
      bioventing is designed to maximize biodegradation of aerobi-
      cally  biodegradable compounds, regardless of their molecu-
      lar weight, with some volatilization occurring.  Although both
      technologies involve venting of air through the subsurface,
      the differences in  objectives result  in different design and
      operation of the remedial systems.   Bioventing uses lower
      air flow.

        Bioventing  normally is applied to certain types of organ-
      ics, particularly petroleum hydrocarbons.  It generally is not
      considered useful  for treating  compounds such  as  PCBs
      and chlorinated hydrocarbons.

        The critical factors  to consider during review of biovent-
      ing are presented in Table 3-4.  The significant features of
      this technology include optimizing airflow to  reduce volatil-
      ization  while  maintaining  aerobic conditions for  biodegra-
      dation; monitoring local soil gas conditions to ensure aerobic
      conditions, not just monitoring vent gas composition; adding
      moisture  and   nutrients as required  to  increase  biodegra-
      dation rates at some sites; and  manipulating the water table
      (dewatering) as required to ensure air/contaminant contact.
      The in situ respiration test is  useful as a rapid screening test
      of the applicability of bioventing (Hinchee and Ong, 1992, p.
      1305).
                                 Table 3-4.  Bioventing Critical Factors and Conditions'"'
Factor Influencing
Technology Selection
Spatial variation of
waste composition or
concentration (*)
Initial soil gas
concentrations (*)
Soil permeability (*)
Presence of elevated
levels of metals, highly
chlorinated organics,
pesticides and herbi-
cides, or inorganic
salts (*)
PH
Conditions Favoring Selection of
In Situ Treatment
No action levels specified (1 )
Initial soil gas concentrations of 02
(<5%), C02 (>10%), and total petro-
leum hydrocarbons (-10,000 ppm) (2)
Soil air permeabilities >0.1 darcy (2)
No action levels specified (1)
Between 5-9 pH units (3)
Basis
• Homogeneous conditions are desirable
• Large variation in the contaminant concentration causes varia-
tion in biological activity giving inconsistent biodegradation
• These concentrations suggest high microbial activity due to
hydrocarbon degradation and generally indicate that bioventing
is feasible
• Low soil permeabilities restrict airflow through the soil,
decreasing the amount of air that can be provided for microbial
activity
• Lower levels are desirable
• Threshold effects commonly occur
• Materials can be toxic to microorganisms
• When pH is outside of range, biological activity is inhibited
Data Needs
• Waste composition
and spatial
distribution
• Soil gas monitoring
• Soil air permeability
testing
• Waste composition
• Soil pM
(a) Also see Table 2-3 for generic factors.
(*) Indicates higher-priority factors.
(1) U.S. EPA, 1988, EPA/540/2-88/004, p. 114.
(2) Hinchee et al., 1992.
(3) U.S. EPA, 1990, EPA/540/2-90/002, pp. 40 and 47.
Treatment of Soils In Situ
                                                                                                                       17

-------
   Understanding the distribution of contaminants is impor-
tant to any in situ remediation process.  Much of the hydro-
carbon  residue at a fuel-contaminated  site is found in the
unsaturated zone soils, in the capillary  fringe, and immedi-
ately below the water table.  Seasonal  water table fluctua-
tions  typically  spread  residues  in  the area  immediately
above and below the water table.  Any  successful bioreme-
diation effort must treat these  areas.   Bioventing provides
oxygen  to unsaturated zone soils  and can  be  extended
below the water  table  when integrated with  a  dewatering
system.

   Currently, conventional  enhanced   bioreclamation  pro-
cesses use water to carry oxygen or an alternative electron
acceptor to the contaminated zone.  This is common wheth-
er the contamination is  present in the groundwater or in the
unsaturated zone.  In most cases where water  is used  as
the oxygen carrier, the oxygen solubility is the limiting factor
for biodegradation.  If pure oxygen is used and  40 mg/L of
dissolved oxygen is achieved, approximately 80,000 Ib of
water must be delivered to the  formation to degrade 1  Ib of
hydrocarbon.  If 500 mg/L of hydrogen peroxide  is success-
fully delivered, then approximately 13,000 Ib of  water must
be used to degrade the same amount of hydrocarbon.  As a
result, even if hydrogen peroxide can be successfully used,
substantial volumes of water must be pumped through the
contaminated formation  to deliver sufficient oxygen.

   The  use of  an air-based oxygen supply for  enhancing
biodegradation  relies on airflow through contaminated soils
at rates and configurations  that will both ensure adequate
oxygenation for aerobic  biodegradation and minimize  or
eliminate the production of a hydrocarbon-contaminated off-
gas. The addition of nutrients  and moisture may be desir-
able to increase biodegradation rates;  however, field re-
search to date does not indicate the need for these addi-
tions (Dupont et al., 1991; Miller et al., 1991). A  key feature
of bioventing is the use  of narrowly screened soil gas moni-
toring points to sample  gas  in short vertical sections of the
soil.  These points  are required  to monitor  local oxygen
concentrations,  because oxygen levels  in the vent well  are
not representative of local conditions.

   Bioventing systems can be configured in either injection
or extraction mode, or a combination of the two to push or
pull air through the vadose zone.  A system using only air
injection  has the  advantage of not creating a point source
emission.  This technology relies on the ability to move air
through the contaminated soil.   Low-permeability soils  are
more difficult to treat with bioventingr

   Bioventing was accepted  in the SITE  Program in June
1991.  Treatability tests were  performed  at  the Reilly Tar
site in St. Louis Park, Minnesota, and the site was found to
be suitable for a test of the effectiveness of bioventing in
treating PAHs.  A single-vent system was installed and will
be operated for a 3-year test period (Alleman, 1993).  The
U.S. EPA has completed one field study of bioventing and is
conducting several others (Sayles,  1993).

   The  reported range  of costs for applying  bioventing is
$60 to $90/ton (U.S. EPA and U.S. Air Force, 1993, p. 61).
tact:
   For further information on  bioventing technologies, con-
     Gregory Sayles (513) 569-7607
        U.S. Environmental Protection Agency
        Risk Reduction Engineering Laboratory
        26 West Martin Luther King Drive
        Cincinnati, OH 45268
3.6 In Situ Vitrification

   In situ vitrification is a thermal treatment process that
converts contaminated soils to stable glass and  crystalline
solids.  It originally was developed to stabilize transuranic
contaminated wastes and is being extended to treatment of
other hazardous wastes.  For in situ vitrification processes,
high voltage  is applied  via electrodes placed  in the soil to
induce  current flow.  The current heats the soil to melt-
formation  temperature.    Heating  destroys  or  vaporizes
organic contaminants. After heating stops, the melt cools to
form a stable solid material.  Application of in situ vitrification
requires conduction of  electricity through the media to be
treated.

   One application of the technology is based on  electric
melter technology,  and the principle of  operation is joule
heating, which occurs when an electrical current is  passed
through a molten mass.  Field application requires insertion
of electrodes into  contaminated soils to  supply the  current
flow.   Because  unmelted soil  is not conductive,  a starter
path of flaked graphite and frit is placed between the elec-
trodes to  act as the initial flowpath for  electricity.  Resis-
tance heating in the  starter path creates a  melt to carry
more current. The melt grows outward and downward from
the starter path (U.S. EPA, 1990, EPA/540/2-90/002, p. 17).
The melt can grow to encompass a volume of  1000 tons.
The maximum treatment depth is about 20 feet with possible
extension  to  30 feet  as the technology develops.  Large
areas are treated in overlapping blocks.

   Critical factors to consider during review of in  situ vitrifi-
cation technology application are presented in Table 3-5.

   The  electric current  flow heats soil to temperatures as
high as 1370°C  (U.S.  EPA, 1991, EPA 540/2-91/009, p. 7).
If the silica content of the soil is high enough,  contaminated
soil  is  converted  into  durable glass.   The  combustible
wastes  are pyrolyzed and other contaminants are incorpo-
rated into the vitreous mass.  Off-gases released during the
melting process are trapped in an off-gas  hood.

   The main  requirement for the technology is the ability for
the soil melt to carry current during heating and then  solidify
to a stable mass as it cools.  Wet soils can be treated by in
situ vitrification, but highly permeable soils and the presence
of groundwater increase operating costs.  If the soil moisture
is recharged  by  groundwater, the electrical input needed to
vaporize the water increases costs. Buried combustibles or
containers such as tanks and drums introduce the possibility
of explosion.
18
                                                                                             Treatment of Soils In Situ

-------
   The  reported typical treatment rate is  3 to 5 tons per
hour (U.S. EPA, 1991, EPA  540/2-91/009,  p. 7).  In situ
vitrification is reported to  provide above average  long-term
effectiveness and  permanence, and reductions in toxicity,
mobility, and volume.

   In situ  vitrification has  been tested on a large  scale ten
times,   including  two  demonstrations   on  transuranic-
contaminated (radioactive) sites: (1) at Geosafe's test site,
and (2)  at the U.S. Department of Energy's (DOE's) Hanford
Nuclear Reservation. More than 130 tests  at various scales
have been performed on  a broad range of waste types in
soils and sludges.  The technology has been selected as a
preferred  remedy at several private, EPA  Superfund,  and
DOE sites but has not been implemented in full-scale appli-
cation.  In situ vitrification has been selected for  the SITE
Program (U.S. EPA,  1992,  EPA/540/R-92/077, p. 97).  Tests
are being  performed  at  the  Parsons/ETM  site  in Grand
Ledge, Michigan.
                                                                   There have been no full-scale applications to serve as a
                                                                basis for cost estimation.   A DOE life-cycle cost analysis
                                                                suggests the overall cost of in situ vitrification  would be
                                                                approximately $790/ton (U.S. EPA and U.S. Air Force, 1993,
                                                                p. 63).  A commercial vendor of the technology indicates an
                                                                estimated  cost  range of $300 to $400/ton  (Hansen  and
                                                                FitzPatrick, 1991).
                                                                   For further information on in situ vitrification technologies,
                                                                contact:

                                                                     Teri Richardson (513) 569-7949
                                                                       U.S. Environmental Protection Agency
                                                                       Risk Reduction Engineering Laboratory
                                                                       26 West Martin Luther King Drive
                                                                       Cincinnati, OH 45268
                             Table 3-5.  In Situ Vitrification Critical Factors and Conditions04
Factor Influencing
Technology Selection
Soil composition (*)
Contaminant
depth {•)
Organic liquid content of
contaminated material (*)
Presence of in situ
voids (*)
Conductive metal
content (*)
Presence of sealed
containers (*)
Presence of combustible
solids
Presence of
groundwater
Surface slope
Location of
structures
Conditions Favoring Selection of
In Situ Treatment
>30% Si02
>1.4%Na20+K20
on dry weight basis (1)
>6 ft and
<20ft(1)
<1 to 7% organic content depending on
the BTU content of the organic (1)
Individual void volume <150 ft3 (1)(2)
<5% to 15% of total melt weight and
continuous conductive path <90% of
distance between electrodes (1)(2)
None present (1)
<3,200 kg combustible solids per meter of
depth or average concentration <30% in
the soil to be treated (1)
Groundwater control required if contami-
nation is below the water table and soil
hydraulic conductivity is >10~4cm/sec (1)
<+/-5% (1)
Underground structures and utilities
located >20 ft from melt zone (1)
Basis
• Required to form melt and cool to stable
treated waste form
• Overburden assists in capture of volatile
metals
• Deep contamination requires surface excava-
tion to allow placement of electrodes
• Can generate excessive hot off-gas on
combustion
• Can generate excessive off-gas
• Can cause excessive subsidence
• Can create a conductive path resulting in
uneven current flow and uneven heating
• Containers can rupture during heating
resulting in a large pulse of off-gas generation
• Can generate excessive off-gas volumes on
combustion
• Water inflow increases energy required to
vaporize water
• Melt may flow under influence of gravity
• Items closer than 20 ft to the melt zone must
be protected from heat
Data Needs
• Weight loss on ignition
• Soil mineral composition as
oxides (x-ray fluorescence)
• Contaminant composition and
distribution
• Contaminant composition
• Heat of combustion of organic
materials
• Subsurface geology
• Subsurface matrix conditions
• Contaminant composition and
distribution
• Contaminant composition and
distribution
• Contaminant distribution
• Location of water table
• Seasonal variation of
groundwater conditions
• Site surface slope
• Contaminant composition and
distribution
• Subsurface conditions
(a) Also see Table 2-3 for generic factors.
(*) Indicates higher-priority factors.
                                                             (1) Pacific Northwest Laboratory, 1993.
                                                             (2) U.S. EPA, 1991, EPA 540/2-91/009, p. 3.
..^atment of Soils In Situ
                                                                                                                      19

-------
 3.7  In Situ Radiofrequency Heating

    Radiofrequency heating  is  a  technique  for  rapid  and
 uniform heating of large volumes of soil in situ.  This tech-
 nique heats the soil  to the point  where  volatile  and semi-
 volatile contaminants are  vaporized into the soil pore space.
 Vented  electrodes are then  used  to recover the  gases
 formed in the pores  during the heating process.   The  ex-
 tracted gases  can be incinerated  or subjected to other
 treatment  methods.  Application of  radiofrequency heating
 relies on efficient  electromagnetic coupling of the  radiofre-
 quency source and the media being heated.

    Radiofrequency heating is  accomplished  by  use  of
 electromagnetic energy in  the radiofrequency  band.   The
 heating process does not rely on the thermal  conductivity of
 the soil.  The energy  is  introduced into  the  soil matrix by
 electrodes  inserted into drilled holes.  The  mechanism of
 heat  generation is similar to that of a microwave oven.  A
 modified radio transmitter serves as the power source,  and
 the industrial,  scientific,  and  medical band  provides   the
 frequency at which the modified transmitter operates.  The
 exact operational  frequency is obtained from an  evaluation
 of the areal extent of  the contamination and the dielectric
 properties of the soil matrix.
                                            The critical factors to consider during review of radiofre-
                                         quency  heating technology  application are presented  in
                                         Table 3-6.  Full implementation of a radiofrequency heating
                                         system at a  contaminated hazardous  waste  site requires
                                         four major subsystems.

                                            • A radiofrequency energy depositions array
                                            • Radiofrequency  power-generating, transmitting,  moni-
                                              toring, and control systems
                                            • A gas and liquid  condensate  handling and treatment
                                              system
                                            • A vapor containment and collection system.

                                            Radiofrequency  heating originally was  developed  and
                                         tested  for  recovery of heavy oil.  Three treatability tests of
                                         radiofrequency  heating  on  contaminated soils have been
                                         performed.  The first test was conducted at Volk Air National
                                         Guard  Base,  Camp Douglas, Wisconsin.   The treated  vol-
                                         ume was 500 ft3 heated to a depth  of 7 feet.  The contami-
                                         nants were in a fire training  area where waste  oils, fuels,
                                         and other hydrocarbons had  been placed  and  ignited to
                                         simulate aircraft fires (U.S.  Air Force, 1989,  p. 1).   The
                                         second test, performed  at  Rocky Mountain Arsenal, heated
                                         a 1600-ft3 volume to a depth  of 13 feet  to treat organo-
                                         chlorine pesticides and organophosphorus compounds (U.S.
                            Table 3-6.  Radiofrequency Heating Critical Factors and Conditions13'
   Factor Influencing Tech-
      nology Selection
 Conditions Favoring Selec-
      tion of In Situ
        Treatment
                 Basis
        Data Needs
  Moisture content (*)
No action level
specified (1)
A low moisture content Is desirable
High moisture content increases cost due to energy
needed to vaporize water
Radiofrequency (RF) energy absorption properties
(dielectric constant and loss tangent) change as soil
dries, complicating design and operation of the RF
energy supply system
Soil moisture content
  Contaminant boiling point (*)
Boiling point below 300°C (2)
Approximate economic limit of radiofrequency heating
Contaminant boiling point or
vapor pressure as a function of
temperature
  Conductive metal content I
No action level specified (3)
Metals strongly absorb RF energy, creating uneven
heating
Subsurface matrix composition
  Soil dielectric
  constant
No action level specified (1)
Dielectric material is needed to couple with
radiofrequency fields for energy transfer
Change of properties with changing moisture content
is more important than actual magnitude of the
dielectric constant
Dielectric constant as a
function of moisture content
  Soil loss tangent
No action level specified (1)
Dielectric material is needed to couple with
radiofrequency fields for energy transfer
Change of properties with changing moisture content
is more important than actual magnitude of the loss
tangent
Loss tangent as a function of
moisture content
(a) Also see Table 2-3 for generic factors,
(*) Indicates higher-priority factors.
                               (l)Srestyetal., 1986, p. 88.
                               (2) U.S. EPA, 1990, EPA/540/2-9Q/QQ2, p. 83.
                               (3) Just and Stockwell, 1993, p. 248.
20
                                                                                                   Treatment of Soils In Situ

-------
Army, 1992, p. 2-1).  A demonstration of  a phased-acray
radiofrequency antenna  system to heat vadose  zone clay
deposits contaminated with  chlorinated hydrocarbons was
completed at the  DOE Savannah River Laboratory (Kase-
vich et al., 1993, p. 23).  Two radiofrequency heating tech-
nologies have been accepted in the SITE  Program.  The
demonstrations  are being  conducted  at  Kelly  Air  Force
Base, Texas and are scheduled for completion in 1994 (U.S.
EPA, 1992, EPA/540/R-92/077, p. 109).

   The vendor indicates that the approximate cost range for
application of radiofrequency  heating is $30 to $100/ton of
soil treated, depending on the moisture content (5% to 20%)
and the treatment  temperature (100°C to 250°C) (U.S. EPA,
1989, EPA/600/S2-89/008, p. 2)(Sresty et al., 1992, p. 363).

   For further information on radiofrequency  heating tech-
nologies, contact:

      Janet Houthoofd (513)  569-7524
        U.S.  Environmental Protection Agency
        Risk Reduction Engineering Laboratory
        26 West Martin Luther King Drive
        Cincinnati, OH 45268
3.8  Soil Flushing

   Soil flushing is a process whereby the zone of contami-
nation is flooded with an appropriate washing solution to re-
move the contaminant from the soil.  Water or an aqueous
solution is injected into or sprayed onto the area of contami-
nation.   The contaminants  are mobilized by solubilization,
formation of emulsions, or  a chemical reaction  with  the
flushing solutions.  After passing  through the contamination
zone, the contaminant-bearing fluid is collected by strategi-
cally placed wells and brought to the surface for disposal,
recirculation, or on-site treatment and reinjection.  Applica-
tion of soil washing relies on the ability to deliver, control the
flow, and recover the flushing fluid.

   The  critical factors  to  consider during review of soil
flushing technology application are shown in Table 3-7. Soil
flushing requires the identification of a flushing solution that
is available in sufficient quantity at a reasonable cost.

   Flushing  solutions may be water; acidic aqueous solu-
tions  (such  as sulfuric, hydrochloric, nitric, phosphoric,  or
carbonic acids); basic solutions (such as sodium hydroxide);
chelating or complexing agents; reducing agents; or surfac-
tants.   Water will  extract  water-soluble  or  water-mobile
constituents. Acidic solutions can be used to remove metals
or basic  organic materials.  Basic solutions may be used for
some metals such  as zinc,  tin, or lead and some phenols.
Chelating, complexing, and reducing agents may be needed
to recover some metals. Surfactants can assist in emulsifi-
cation of hydrophobic organics (U.S. EPA, 1991, EPA/540/2-
91/021, p. 2).

   Soil  flushing  to remove organic materials has  been
demonstrated  at both bench and pilot scale.  Several sys-
tems  are in  operation  and many systems are being de-
signed  for remediation of Superfund sites.   Studies  have
been conducted to determine the appropriate solvents for
mobilizing various classes and types of chemical constitu-
ents. Most of the applications involve remediation of VOCs
(U.S. EPA, 1992, EPA/542/R-92/011, pp. 26-29).

   The  soil flushing technology  may be easy or difficult to
apply, depending  on the ability to flood the soil with the
flushing solution and to install collection wells or subsurface
drains to recover all the applied  liquids.   Provisions also
must be made for ultimate disposal of  the  elutriate.  The
achievable level of treatment varies  and depends  on the
contact  of  the flushing solution  with the contaminants, the
appropriateness of the solutions for the contaminants, and
the  hydraulic conductivity  of the soil.   The technology is
more applicable to permeable soils.

   Water can be used  to flush water-soluble or water-mobile
organics and inorganics.   Hydrophilic  organics are readily
solubilized  in water.  Organics amenable to water flushing
can  be  identified according to their soil/water partition coeffi-
cients or estimated from their octanol/water partition coeffi-
cients.   Organics considered generally amenable  to  soil
flushing with water or  water and surfactants are those with
an octanol/water partition coefficient (K^) of less than about
1000.   High-solubility organics (e.g., lower-molecular-weight
alcohols, phenols,  and carboxylic acids) and other organics
with  a  coefficient less than 10 may already have been
flushed  from the site by natural processes.  Medium solubili-
ty organics (Koc = 10  to  1000)  that can be effectively re-
moved from soils by water flushing include low- to medium-
molecular-weight  ketones,  aldehydes,  and  aromatics and
lower-molecular-weight halogenated hydrocarbons, such as
TCE and tetrachloroethylene (PCE) (U.S. EPA, 1990, EPA/-
600/2-90/011, p. 50).

   Soil  flushing for inorganic treatment is  less well devel-
oped than soil flushing for organics.  Some  applications at
Superfund sites have been reported, however.  One system
is operational at a landfill with mixed organics and metals,
and  another is operational at a chromium-contaminated site
(U.S. EPA,  1992, EPA/542/R-92/011, pp. 27  and 29).

   Several  other inorganic treatment systems are in the de-
sign  or  predesign  phases  at Superfund sites.  Inorganics
that  can be flushed from soil with water are soluble salts
such  as the carbonates of nickel, zinc,  and copper.  Ad-
justing the  pH  with dilute solutions of acids or bases will
enhance inorganic solubilization and removal.

   Removal of  inorganic  contaminants  by  soil flushing
typically requires injection and recovery of a chemical leach-
ing  solution.   The leaching  solution  must  be selected  to
remove  the contaminant while not harming the in situ envi-
ronment.  Selection of the  leaching solution also may be
limited by Land Disposal Restrictions or Underground Injec-
tion Control regulations.

   Estimated costs for  application of soil flushing range from
$75 to $200/yd3, depending on the waste quantity.  These
are rough estimates and are not based on field studies (U.S.
EPA and U.S. Air Force, 1993, p. 56).
Treatment of Soils In Situ
                                                                                                                     21

-------
                                   Table 3-7, Soil Flushing Critical Factors and Conditions**'
Factor Influencing
Technology Selection
Equilibrium partitioning of
contaminant between soil
and extraction fluid (*)
Complex waste mixture (*)
Soil-specific surface area (*}
Contaminant solubility in
water (*)
Octanol/water partitioning
coefficient (*)
Spatial variation in waste
composition (*)
Hydraulic conductivity {*}
Clay content (*}
Cation exchange
capacity (*)
pHO
Buffering capacity (*)
Flushing fluid
characteristics (*}
Soil total organic carbon
content
Contaminant vapor pressure
Fluid viscosity
Organic contaminant density
Conditions Favoring
Selection of In Situ
Treatment
No action levels specified (1)
No action levels specified (1)
<0.1m2/g(2)
>1,OOOmg/L(2)
Between 10 and 1000 (2)
No action levels specified (1)
>10~3 cm/sec
No action levels specified (3)
No action levels specified (3)
No action levels specified (3)
No action levels specified (3)
Fluid should have low toxic-
ity, low cost, and allow for
treatment and reuse (1)
Fluid should not plug or have
other adverse effects on the
soil (1)
<1 wt% (2)
<10 mmHg (2)
<2 centipoise (cP) (2)
>2 g/cm3 (2)
Basis
* Contaminant preference to partition to the extractant is
desirable
* High partitioning of contaminant into the extraction fluid
decreases fluid volumes
• Complex mixtures increase difficulty in formulation of a
suitable extraction fluid
• High surface area increases sorption on soil
* Soluble compounds can be removed by water flushing
• Very soluble compounds tend to be removed by natu-
ral processes
• More hydrophilic compounds are amenable to removal
by water-based flushing fluids
• Changes in waste composition may require
reformulation of extraction fluid
• Good conductivity allows efficient delivery of flushing
fluid
• Low clay content is desirable
« Presence of clay increases sorption and inhibits
contaminant removal
« Low cation exchange capacity is desirable
* Cation exchange capacity increases sorption and
inhibits contaminant removal
• May affect treatment additives required, compatibility
with materials of construction, or flushing fluid
formulation
* Indicates matrix resistance to pH change
• Toxicity increases health risks and increases regulatory
compliance costs
• Expensive or nonreusable fluid increases costs
« If the fluid adheres to the soil or causes precipitate
formation, conductivity may drop, making continued
treatment difficult
• Soil flushing typically is more effective with lower soil
organic concentrations
• Volatile compounds tend to partition to the vapor phase
• Lower-viscosity fluids flow through the soil more easily
• Dense insoluble organic fluids can be displaced and
collected by soil flushing
Data Needs
* Equilibrium partitioning
coefficient
* Contaminant composition
• Specific surface area of soil
* Contaminant solubility
• Octanol/water partitioning
coefficient
* Statistical sampling of
contaminated volume
• Hydrogeologic flow regime
• Soil composition
* Soil color
» Soil texture
• Cation exchange capacity
* Soil pH
* Soil buffering capacity
* Fluid characterization
• Bench- and pilot-scale testing
• Total organic carbon content of
soil
* Contaminant vapor pressure at
operating temperature
* Fluid viscosity at operating
temperature
* Contaminant density at
operating temperature
(a) Also see Table 2-3 for generic factors.
(*) Indicates higher-priority factors.
                                                              (1) U.S. EPA, 1988, EPA/540/2-88/004, p. 79.
                                                              (2) U.S. EPA, 1990, EPA/600/2-90/011, p. 54.
                                                              (3) U.S. EPA, 1991, EPA/540/2-91/021, p. 3.
22
                                                                                                        Treatment of Soils In Situ

-------
   The Superfund site at Palmetto Wood, South Carolina,
cited  costs  of  $3,710,000 (capital) and $300,000  (annual
operation and maintenance). These totals, on a unit basis,
equal  $185/yd3 for capital costs and $15/yd3 per year for
operation and  maintenance (U.S. EPA, 1990, EPA/600/2-
90/011, p. 53).

   For further  information  on  soil  flushing technologies,
contact:

     Michael Gruenfeld (908) 321-6625
        U.S. Environmental Protection Agency
        Risk Reduction Engineering Laboratory
        Building #10 (MS 104)
        2890 Woodbridge Avenue
        Edison, NJ 08837-3679
3.9 Steam/Hot Air Injection
     and Extraction

   In situ steam injection/extraction removes volatile and
semivolatile hazardous contaminants from soil and ground-
water without requiring excavation.  Steam injection may be
supplemented by hot air injection.   In a few experimental
studies, hot air or hot combustion off-gas has been injected
to strip organics from in situ soil without steam (Smith and
Hinchee, 1993, p. 156).   Waste constituents  are removed
from the soil by this technology but are not actually treated.
Steam enhances the stripping of volatile contaminants from
soil and can be used to displace contaminated groundwater
under some conditions.

   Steam extraction  is effective for compounds with lower
vapor pressures than those  remediated  with ambient-tem-
perature  SVE systems.  By increasing the temperature from
initial conditions to the steam temperature, the vapor pres-
sure of the contaminants  increases, causing them  to be
more volatile.   Steam is  injected  to form a displacement
front by steam  condensation to displace groundwater.  The
contaminated liquid and steam condensate  are  then col-
lected for further treatment and/or recycling to the  steam
generator.
            Mobilized nonaqueous-phase liquid and groundwater also
         may be collected for treatment and disposal.  Application of
         steam/hot air injection and extraction relies on the ability to
         deliver, control the flow, and recover the heating fluid.

            The critical factors to consider during review of steam/hot
         air  injection  and   extraction  technology  application   are
         presented in Table 3-8.

            A  limited  number of commercial-scale in  situ  steam
         injection/extraction systems currently are in operation in the
         United States,  but  in  situ steam injection/extraction  is a
         rapidly developing  technology.   In situ  steam injection/
         extraction is being considered as a component of the reme-
         dy for only one Superfund site,  i.e., the  San  Fernando
         Valley in California  (Area 1) (U.S. EPA,  1991,  EPA/540/2-
         91/005, p. 6).

            There  are two  main types of  steam/hot air  injection/-
         extraction systems:   a mobile  system  and  a  stationary
         system.  The  mobile system consists of a unit that volatilizes
         contaminants in small areas  in  a sequential  manner by
         injecting  steam and hot air  through rotating cutter blades
         that pass through the contaminated medium. The stationary
         system uses  wells to inject steam into the soil  to volatilize
         and displace contaminants from the undisturbed subsurface.
         Examples of both types of steam injection technologies have
         been  accepted  in   the SITE  Program (U.S.  EPA,  1992,
         EPA/540/R-92/077).

            For the mobile   technology, the most  significant factor
         influencing cost  is   the treatment rate.  Treatment rate is
         determined primarily by the soil type (soils with higher clay
         content require longer treatment times), the waste type, and
         the on-line efficiency.  An evaluation of a  SITE demonstra-
         tion indicated costs of $67 to $317/yd3 for treatment rates of
         10 to 3  yd3/hr,  respectively.   These  costs  are based on a
         70%  on-line  efficiency and  include consideration  of  site
         preparation; equipment purchase, installation, and operation;
         and demobilization (U.S. EPA, 1991, EPA/540/A5-90/008, p.
         21).  Cost estimates for the general application of steam/hot
         air injection fall  in the range  of $50 to $300/yd3 (U.S.  EPA,
         1991, EPA/540/2-91/005, p. 6).
                   Table 3-8.  Steam/Hot Air Injection and Extraction Critical Factors and Conditions'8'
Factor Influencing
Technology Selection
Soil conductivity (*)
Humic content in soil (*)
Contaminant vapor
pressure (*)
Conditions Favoring
Selection of In Situ
Treatment
No action levels specified (1)
No action levels specified (1)
Boiling point below 250°C (2)
Basis
• Low soil conductivity inhibits vapor flow
• Low humic content is desirable
• . Presence of humic materials increases sorption
and inhibits volatilization
• More volatile contaminants are more easily
removed by air stripping
Data Needs
• Hydrogeologic flow regime
• Soil composition
• Soil color
• Soil texture
• Contaminant boiling point or
vapor pressure as a function
of temperature
(a) Also see Table 2-3 for generic factors.
(*) Indicates higher-priority factors.
(1) U.S. EPA, 1988, EPA/540/2-88/004, p. 89.
(2) U.S. EPA, 1990, EPA/600/2-89/066, p. 51.
Treatment of Soils In Situ
                                                                                                                     23

-------
   For further information on steam/hot air injection tech-
nologies, contact:

     Michael Gruenfeld (908) 321-6625
       U.S. Environmental Protection Agency
       Risk Reduction Engineering Laboratory
       Building f 10 (MS 104)
       2890 Woodbridge Avenue
       Edison, NJ 08837-3679
4.0  Acknowledgments

   This Engineering Issue Paper was developed for the U.S.
EPA Engineering Forum by the U.S. EPA Risk  Reduction
Engineering Laboratory (RREL) through Contract No. 68-CQ-
0003 with the Battelle Memorial Institute.  Battelle provided
primary authorship and layout of the document, while  many
other  people contributed in a significant way  by providing
direction, guidance, assistance, information, or review.

   The   EPA  Technical  Project  Manager  was   Janet
Houthoofd.  The Engineering  Forum  lead  contacts  were
Robert Stamnes, Region 10, and Paul Leonard,  Region 3.
The  Battelle  Work  Assignment  Manager  was Susan
Brauning, and the principal  author was Lawrence Smith.

   Other contributors or reviewers were Thomasine Bayless,
Joan  Colson,  Patricia  Erickson, Chi-Yuan (Evan)   Fan,
Michael Gruenfeld,  Carl Potter,  Teri Richardson, Michael
Roulier, Gregory Sayles, Laurel Staley, and Robert Stenburg
- EPA RREL;  Linda Fiedler - EPA Technology  Innovation
Office; John Matthews -  EPA Robert S. Kerr Environmental
Research Laboratory;  and Bruce Alleman,  Lynn Copley-
Graves,  Robert  Hinchee,  Andrea  Leeson, and Thomas
Naymik - Battelle.

   Acknowledgments are due also to the primary  Engineer-
ing Forum Superfund Contacts shown in the box below.
EPA Engineering
Region 1

Region 2
Region 3
Region 4
Region 5
Region 6
Region 7
Region 8
Region 9
Region 10
Headquarters
Forum Superfund
Lynne Jennings
Chet Janowski
Richard Ho
Paul Leonard
Jon Bornholm
Anthony Holoska
Deborah Griswold
Steve Kinser
Desiree Golub
Ken Erickson
Bob Stamnes
Richard Steimle
Contacts
(617) 573-9634
(617) 573-9623
(212) 264-9543
(215) 597-3163
(404) 347-7791
(312) 886-7503
(214) 655-6730
(913) 551-7728
(303) 293-1838
(415)744-2324
(206) 553-1512
(703) 308-8846
5.0  References

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   424-5715. Battelle Memorial Institute.  Columbus, Ohio.
Arniella, Elio F.  and  Leslie J. Blythe, 1990.   "Solidifying
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Conner, Jessie R., 1990.  Chemical Fixation and Solidifica-
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Donehey, Angela J., Reva A. Hyde, R.B.  Piper, M.W. Roy,
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   Treatments."   In  Proceedings of the  1992  U.S. EPA/
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Dupont, R. Ryan, William J. Doucette,  and Robert E. Hin-
   chee, 1991.   "Assessment of In  Situ Bioremediation
   Potential  and the  Application  of Bioventing  at a Fuel-
   Contaminated Site." In Robert E. Hinchee and Robert F.
   Olfenbuttel (Eds.),  In Situ  Bioreclamation: Applications
   and  Investigations for Hydrocarbon and Contaminated
   Site Remediation, pp.  262-282.  Butterworth-Heinemann.
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Hansen, J.E. and V.F. FitzPatrick, 1991. In Situ Vitrification
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Hinchee, Robert E. and Say Kee Ong,  1992. "A Rapid In
   Situ  Respiration Test for Measuring Aerobic Biodegra-
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Houthoofd, Janet  M., John H. McCready, and  Michael H.
   Roulier,  1991.   "Soil  Heating  Technologies  for In Situ
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   al  Hazardous  Waste Research Symposium. EPA/600/9-
   91/002.  Office of Research and [Development, Risk Re-
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Just, Sharon R. and  Kenneth J.  Stockwell, 1993.   "Com-
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Kasevich, Raymond,  Raymond Holmes, David Faust, and
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Leeson, Andrea, Robert E. Hinchee, Jeffrey A. Kittel, Grego-
   ry  Sayles,  Catherine Vogel,   and  Ross Miller,  1993.
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Mack,  James  P.  and Howard  N. Aspan, 1993.  "Using
   Pneumatic Fracturing  Extraction to  Achieve  Regulatory
   Compliance and Enhance  VOC  Removal from Low-
   Permeability Formations." Remediation. 3(3):309-326.
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Miller, Ross N., Catherine C. Vogel and Robert E. Hinchee,
   1991.  "A Field-Scale Investigation  of Petroleum Hydro-
   carbon Biodegradation in the Vadose Zone Enhanced by
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Sresty, Guggilam C., Harsh  Dev, Richard  H.  Snow, and
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Sresty,  Guggilam C., Harsh  Dev, and Janet Houthoofd,
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U.S. Environmental Protection Agency,  1991.  Superfund
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   Emergency Response.  Washington,  DC. and  Office of
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   Laboratory. Cincinnati, Ohio.
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                                 j.                                                       Treatment of Soils In Situ
                                 -KV.S. GOVERNMENT HUNTING OFFICE: MM - S5O-OOI/8O3S3

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